Energy Efficiency and Power Consumption Improvement of IR Illumination for Surveillance Cameras CARLOS TORMO LLUCH KTH ROYAL INSTITUTE OF TECHNOLOGY INFORMATION AND COMMUNICATION TECHNOLOGY
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Energy Efficiency and Power Consumption Improvement of IR Illumination for Surveillance Cameras
CARLOS TORMO LLUCH
KTH ROYAL INSTITUTE OF TECHNOLOGY
I N F O R M A T I O N A N D C O M M U N I C A T I O N T E C H N O L O G Y
Master of Science Thesis
KTH School of Information and Communication Technology
SE-100 44 STOCKHOLM, SWEDEN
Energy efficiency and power
consumption improvement of IR
illumination for surveillance cameras
Carlos Tormo Lluch
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Master of Science Thesis
Energy efficiency and power consumption improvement
of IR illumination for surveillance cameras.
Carlos Tormo
Approved
Examiner
Supervisor
Commissioner
Contact person
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Abstract
The power and energy optimization of a device can lead to a reduced cost, smaller area, better temperature
performance, and higher lifetime. Furthermore, in systems that have limited power budget, it allows
running simultaneously more functionalities or using features that require higher power demand.
Therefore, both from the user and the company perspective, the value of a product increases as the energy
optimization improves.
For nighttime surveillance video recording, it is common to use infrared illumination to light the target
scene, which draws a significant portion of the total camera energy consumption. This master thesis
examines and discusses how stroboscopic infrared illumination can enhance the energy efficiency in video
recording cameras with rolling shutter image sensors. This report analyzes LED driver circuits,
recommends methodologies, and sorts the most relevant parameters to help to dimension and design the
illumination system for a light-strobing system. A promising field of use for this technique has been found
to be the license-plate recognition (LPR) scenario, for which this thesis dedicates a chapter in this
document.
This project has been developed at AXIS Communications, where a prototype has been built for one of
their network security cameras. The prototype has been tested for LPR for both strobing light systems
and conventional IR lighting systems. The results obtained prove that the energy efficiency of the
illumination system can be improved more than 95% when stroboscopic illumination is used.
Keywords: rolling shutter, energy efficiency, IR, stroboscopic illumination.
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Sammanfattning
Effektförbrukning och energioptimering av en produkt kan leda till lägre kostnad, mindre storlek, bättre
temperaturprestanda och högre livslängd. I system med begränsad effektbudget möjliggör detta dessutom
aktivering av fler funktioner samtidigt, eller användning av funktioner med högre strömförbrukning.
Därmed gör energioptimering att produktens värde ökar både för användaren och för företaget som
tillverkar den.
För videoinspelning med övervakningskamera nattetid är det vanligt att använda infraröd belysning för att
belysa scenen, vilket ofta förbrukar en betydande del av kamerans totala effektbudget. Detta examensarbete
undersöker och diskuterar hur blixtrande (Eng. strobed) infraröd belysning kan förbättra
energieffektiviteten vid videoinspelning med bildsensorer med rullande slutare. I denna rapport analyseras
LED-drivkretsarna, metodik rekommenderas samt att de mest relevanta parametrarna för att dimensionera
och designa ett belysningssystem baserad på strobed IR-belysning sorteras ut. Ett lovande
användningsområde för denna teknik har visat sig vara LPR-scenariot (License Plate Recognition), vilket
diskuteras i ett eget kapitel i denna rapport.
Projektet har genomförts på AXIS Communications, där en prototyp har byggts baserat på en av dess
nätverkskameror. Prototypen har utvärderats LPR-sammanhang med både strobed och konventionellt IR
belysningssystem. De erhållna resultaten visar att energieffektiviteten hos belysningssystemet kan förbättras
med mer än 95% när blixtrande belysning används.
Nyckelord: rullande slutare, energieffektivitet, IR, stroboskopisk belysning.
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Acknowledgments
I feel in debt with all the people that accompanied me along the duration of this project. First, to Steve,
with whom I have worked and discussed for many hours. Also, to Johan, Christian, and Anders. They
have all helped to turn problems into solutions, work into fun, and time into an outstanding experience.
As this project has been a small piece of a vast puzzle, I would like to sincerely thank all the other more
experienced puzzle builders that helped me: Ola, Jenny, and Andreas. Also, special thanks to Mark T.
Smith, my examiner, for his willingness to help and his excellent advice.
Sweet thanks to Malte, Fei, Karolis, Hang, and Martin, for sharing a few well-deserved fikas.
Finally, thanks to my always-supporting family and friends; and to Berta, who has stayed by my side even
in the darkest IR-light conversations.
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Abbreviations
CMOS Complementary Metal-Oxide-Semiconductor
DUT Device Under Test
EMI Electromagnetic Interference
EIT Extended Integration Time
LED Light-Emitting Diode
LET Long-Exposure Mode
LPR License-Plate Recognition
OF Overpower Factor
PAPR Peak-to-Average Power Ratio
PD Powered Device (Context: PoE)
PLD Programmable Logic Device
PoE Power over Ethernet
PSE Power Sourcing Equipment (Context: PoE)
IC Integrated Circuit
IR Infrared
IS Illumination System. The system composed of the LED driver circuit, LEDs, and
additional optics.
STS Shared-time Strobing
WFS Whole-frame Strobing
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Table of Contents
Abstract .......................................................................................................................................................................... iii
Sammanfattning ............................................................................................................................................................ iv
Acknowledgments ......................................................................................................................................................... v
Abbreviations ................................................................................................................................................................ vi
1. Introduction .......................................................................................................................................................... 9
1.1 Introduction: Surveillance cameras and IR illumination ....................................................................... 9
1.2 Background: The strobing light technique concept .............................................................................. 9
1.3 Goals and scope of the project ...............................................................................................................11
1.4 Methodology ..............................................................................................................................................12
2. Strobing-light technique and constant-light technique comparison ..........................................................13
2.1 Constant-light analysis ..............................................................................................................................13
2.2 Whole-frame strobing and Shared-time strobing ................................................................................14
2.3 WFS analysis ..............................................................................................................................................15
2.4 STS analysis ................................................................................................................................................15
Energy efficiency ..............................................................................................................................15
Power demand ..................................................................................................................................16
2.5 Extended Integration Time (EIT) technique .......................................................................................17
Introduction ......................................................................................................................................17
Overpowering the LED driving circuit ........................................................................................17
Long-Exposure technique (LET) ..................................................................................................18
2.6 Constant-light dimming at maximum integration times .....................................................................18
2.7 Light techniques comparison ..................................................................................................................19
2.8 LED Driver general specifications and design .....................................................................................19
Introduction ......................................................................................................................................19
Buck-converters overview ..............................................................................................................20
Current ripple in buck-converter LED drivers for video-recording illumination .................20
Edge times in buck-converter LED drivers ................................................................................21
Switching techniques: Series and shunt switching ......................................................................27
Buck-converter LED driver design ...............................................................................................28
2.9 Health regulations .....................................................................................................................................29
3. License Plate Recognition (LPR) .....................................................................................................................30
3.1 Introduction ...............................................................................................................................................30
3.2 Scenario overview .....................................................................................................................................30
3.3 LED Driver specifications for LPR .......................................................................................................31
3.4 LED Driver design guideline ..................................................................................................................36
3.5 Peak power reduction ...............................................................................................................................37
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4. Prototype .............................................................................................................................................................40
4.1 Overview ....................................................................................................................................................40
4.2 Image sensor ..............................................................................................................................................40
4.3 Serializer: Strobe signal generator ...........................................................................................................40
4.4 Processing unit...........................................................................................................................................42
4.5 LED Driver ................................................................................................................................................42
4.6 LED board .................................................................................................................................................43
4.7 Energy saving algorithms .........................................................................................................................43
Energy saving algorithm #1 ...........................................................................................................43
Energy saving algorithm #2 ...........................................................................................................44
5. Results ..................................................................................................................................................................45
5.1 LPR image examples.................................................................................................................................45
5.2 Power consumption results .....................................................................................................................47
5.3 Edge times results .....................................................................................................................................49
6. Conclusions and future work ...........................................................................................................................50
Bibliography .................................................................................................................................................................52
Appendix A: The ‘H’ unit ..........................................................................................................................................53
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1. Introduction
1.1 Introduction: Surveillance cameras and IR illumination
AXIS Communications is a Swedish company whose core-business is video-surveillance network cameras.
Video-surveillance applications often imply nighttime video recording, which usually uses IR illumination
to light the target scene without disturbing users and reducing camera’s intrusiveness. Since AXIS cameras
are connected to the network through ethernet cables, AXIS Communications has chosen to power them
via PoE, thus easing the installation of the security system. However, PoE implies a limited available power
for the cameras, which depends on their PoE class. While upgrading the PoE class is a possible solution to
high power demands, it increases the production cost of the camera from the company’s perspective, as
well as the installation cost from the user’s perspective, since the PSE´s higher power capabilities will be
translated into a higher cost.
According to AXIS Communications (personal communication, May 2018) IR illumination can represent
almost a half of the camera’s total consumption. Because of this, applications that require surveillance over
large areas can often require the installation of separate IR LED lamps for those cases where the camera
and the illumination system consumption exceed the power supply capabilities. Thus, optimizing the IR
illumination system of the cameras is desirable, since it can ease the surveillance system’s installation and
leave more room for additional image processing (more and/or more power demanding features in the
cameras). Furthermore, systems with optimized energy consumption dissipate less heat, which might mean
longer operation time when compared to systems that tend to overheat and need to reduce performance to
cool themselves.
1.2 Background: The strobing light technique concept
Video-surveillance cameras that use IR illumination for dark environments recording emit a constant beam
of light onto the area under surveillance. Hereafter, we will refer to this technique as constant-light
illumination technique.
However, cameras are only sensitive to light when their shutter is open, that is, when light flows into the
lens and excites the image sensor. The shutter time, which is the time that the shutter is open, is adjusted so
that the output stream of frames has the correct exposure and no blur. In very bright scenes or when fast
moving objects want to be precisely captured, the shutter time tends to be short. As can be seen in Figure
1, there is a fraction of the emitted light, that depends on the frame period and the shutter time, which is
not utilized.
The exposure of the resulting image is also affected by the ISO and the aperture of the lens, but the selection
of these parameters has multiple implications on the image quality other than the brightness of the image
and thus is out of the scope of this thesis.
Figure 1 For this example, the shutter is open only 5% (2/40) of the time. However, a traditional constant-light illumination system emits light for the entire 40ms period, thus wasting 95% of the luminous energy produced.
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Therefore, in that situation, if the constant-light technique were being utilized, 95% of the emitted light
would be wasted, since it would not be captured by the sensor. That is, all the energy required to emit light
during that closed-shutter time, is energy wasted.
This reasoning brings us to a conclusion, instead of emitting light constantly, it would be more efficient to
emit light only when the shutter is open. Hereafter, we will call this technique, strobing-light technique.
The implementation of the strobing-light technique will be affected by the type of image sensor used. There
are two types of shutter mechanisms for image sensors. The more intuitive one is the global shutter sensor, in
which all the pixels are exposed to light at the same time. The less intuitive one is called rolling shutter sensor,
in which each row of pixels exposure-time starts sequentially one after the other, delayed by 1H. ‘H’ units
measure time as a function of the configured output data-rate of the sensor (more information about this
can be read in “Appendix A: The ‘H’ unit”). In Figure 2, both shutter techniques are shown.
Figure 2 Rolling shutter and global shutter for a 6-row image (6-pixel height image).
As we can see in Figure 2, rolling shutter time extends the period in which one frame is being captured. This
does not affect the number of frames per second of the camera, since different rows’ exposure can be
overlapped in time, but affects the ratio of open-shutter time to frame period, and thus, the energy
improvement between the two techniques. For the example in Figure 2 and a frame period of 20H, the
strobing-light technique would be able to reduce the energy consumption by 20% for the rolling shutter and
by 45% for the global shutter when compared to the constant-light technique. Although the shutter is open
for the same amount of time per each row in both types of shutter techniques, the rolling shutter sensor
requires a longer pulse of light (16H) than the global shutter sensor (11H) to achieve the same image
brightness.
Also, rolling shutter image sensors are more sensitive to unsynchronized pulses of light, as can be seen in
Figure 3. Since different rows of pixels accumulate the light from different intervals of time, an
unsynchronized pulse of light can yield different levels of exposure at different rows in the same frame.
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Figure 3 Effects of an unsynchronized light pulse in rolling shutter and global shutter sensors. The resulting image (car) in the rolling shutter sensor looks darker at the bottom since those pixels have been exposed to light for a shorter time than those above them. In the global shutter sensor, an
unsynchronized pulse only means a darker image when compared to a properly synchronized one.
Stroboscopic illumination effects on video-recording cameras are discussed in [1] for the first time. The goal
of the report is to use synchronized pulses of light to remove the rolling shutter distortion, i.e., the distortion
caused by each row capturing the image at different intervals of time. Furthermore, the report aims to
synchronize the emitted light with several cameras. The method proposed in this report is used by
companies like “Photometrics” and “Qimaging” to avoid the rolling shutter distortion.
For different purposes, J. C. Lee in [2] claims a similar method to the one presented by D. Bradley, B.
Atcheson, I. Ihrke, and W. Heidrich in [1]. However, the method is intended to allow using modulated light
for depth sensing in rolling shutter sensors.
Finally, US9854193[3] describes a method to improve the power efficiency and extend the useful life of
products based on strobing light. Additionally, the patent claims the use of high-speed CMOS sensors to
reduce the rolling shutter distortion.
The method in [3] consists of one or more sources of stroboscopic light, which are synchronized sequentially
to yield a wider light pulse. Furthermore, the patent proposes to light the scene temporarily and discard the
dark frames. To end with, the patent defines a region of interest as the part of the captured frame that contains
useful information. By knowing this, they propose to exclusively light this part, so that no energy is wasted
lighting useless parts.
This thesis focuses on the same purpose as [3] but with methods inspired by the idea proposed in [1]. In
addition to the power and energy analysis, this thesis also studies the design of strobing-light illumination
systems for rolling shutter sensors.
1.3 Goals and scope of the project
This master thesis is the result of AXIS Communications’ will to find how to increase the efficiency of the
IR illumination system. Of foremost priority was to investigate how strobing the IR light could be beneficial
when compared to constant-light illumination, i.e., how pulsing the light in a proper way could lead to better
quality image or better energy/power performance. While image quality is not the main concern of this
thesis, it is certainly a determining factor to guide the energy and power efficiency study.
The goals for this project as perceived by AXIS were:
- Determining the strobing-light technique advantages when compared to constant-light technique.
- Developing a prototype that could be used as a proof of concept.
- Evaluating how the strobing technique would affect the compliance of the health regulations.
Even though the strobing-light technique concept can be extended to any camera, this master thesis will
focus on a specific camera model, for which the prototype will be built. For non-disclosure agreement
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reasons, the model of the camera is not mentioned in this report and is referred to as the device under test
(DUT). Also, the LED driver design will be mainly oriented to fulfill its characteristics. The circuit was
chosen to operate at 12V, since it is a common available internal voltage in most of the cameras.
The image sensor on the DUT cannot either be disclosed. However, it is just necessary to know that it is a
rolling shutter sensor with different available frame rates and resolution modes.
The completion of this project should provide the necessary data to determine whether the strobing-light
technique could be useful or not, propose use cases, and provide a prototype that can be used as a proof of
concept.
1.4 Methodology
The accomplishment of the goals requires an initial research, techniques analysis, testing, and prototyping.
Since the most important goal is the prototype implementation, an iterative process will be followed, where
the first iteration will aim to deliver a basic but fully functioning prototype. This results in a short research
period at the beginning of project that sets the basic requirements for the prototype. After this short period,
an initial prototype can be implemented and the concept can be tested. The main reason to follow this
approach is to prioritize to have a working prototype by the end of this thesis. Since the development
platform is complex, diverse, and unknown at the beginning of the project, prioritizing to have an early
prototype reduces the risk of failing to achieve the main goal.
The initial research will include the state of the art of the technique and theory about the current image
sensor used in the DUT. Developing the prototype will require microcontroller programming in C language,
and PLD programming and testing with System Verilog. Also, it will be necessary to select or design an
electronic circuit that fulfills the requirements set during the initial research. Evidently, the DUT will need
to be reworked to include this new circuitry.
For testing purposes, the System Verilog hardware description code will be evaluated with testbenches. The
microcontroller code will be empirically tested since the functionalities required by it will be simple. The
proper functionality of the prototype will be tested at two levels: in the laboratory, by checking all the signals
with an oscilloscope; and in a real-case scenario, by evaluating the recorded video.
The discussion about the circuitry of the LED driver will be supported by electronic circuit design theory,
measurements on the prototype, and LTspice simulations, an electronic simulation tool from Analog
Devices, which is offered for free at its website (Windows and Mac operative systems versions).
Finally, the performance measurements of the prototype will be done with an oscilloscope, and voltage and
current probes.
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2. Strobing-light technique and constant-light technique
comparison
2.1 Constant-light analysis
In this chapter, we will compare the constant-light technique, the WFS technique, and the STS technique.
We will evaluate and extract equations to describe energy efficiency. For this purpose, we need to start by
evaluating the constant-light technique, so that we can later compare it to the strobing-light techniques. All
the analysis targets rolling shutter sensors, since this is the type which is used at AXIS. Some of the equations
might include specific sensor limitations of the image sensor of the DUT since they will be used later for
our scenario.
In Figure 4, time has been discretized in slots of 1H unit each. The shutter timing (red colored) is the time
in which the row exposure is being activated. The readout time (blue colored) is the time in which the
exposure has finished, and the data of all the pixels in the row is sent to the processing unit. The time in
between the shutter time and the readout time is the integration time and is set according to the image captured.
The longer the integration time is, the brighter the output image will become. On the other hand, long
integration times can produce blurry images if the recorded objects move too fast.
Figure 4 Constant-light technique example for two consecutive frames. For this example, the period (T) is 14H, and the integration time (Ti) is 5H.
From the example in Figure 4, we can see that light is emitted during the whole period (T=14H), while it is
only being used for 5H units of time per row (Ti=5H). Hence, the light utilization is 35%, which is calculated
by using Equation 1. The time unit used in this section is the ‘H’ unless specifically stated.
𝑈𝐶𝐿𝑇 =𝑇𝑖𝑇 [1]
Equation 1 shows that the utilization of light is directly proportional to the integration time and that, when
𝑇𝑖 ≅ 𝑇, the constant-light technique utilization is almost 100%. We can calculate the maximum utilization
for the constant-light technique (𝑈𝐶𝐿𝑇 𝑚𝑎𝑥) with Equation 2. The 2H units of time that cannot be utilized
are due to the time required for the shutter and readout (limitations of the DUT image sensor).
𝑈𝐶𝐿𝑇 𝑚𝑎𝑥 =𝑇 − 2
𝑇 [2]
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By using Equation 2, we can calculate the maximum utilization in a constant-light scenario when the
integration time is maximum (ti MAX). For this example, we will assume the sensor is working in Full HD
1080p operating mode, i.e., 1125 total number of vertical lines.
𝑈𝐶𝐿𝑇 =𝑡𝑖 𝑀𝐴𝑋1125𝐻
=(1125 − 2)𝐻
1125𝐻=1123
1125= 99,8%
2.2 Whole-frame strobing and Shared-time strobing
In the following sections, the strobing-light technique will be discussed and studied, so that we can expose
its advantages and disadvantages against the constant-light technique. Again, the analysis targets rolling
shutter sensors. Note that, for the analysis in this chapter, we refer to the shutter state as either “open” or
“closed” as an analogy to mechanical shutter systems. However, the DUT’s image sensor shutter is not
controlled mechanically, but electronically.
Two different types of strobing techniques have been identified as possible candidates: the whole-frame
strobing technique (WFS), and the shared-time strobing technique (STS). WFS consists of illuminating the
scene during the whole time that the shutter is open, while STS illuminates only during the fraction of the
time that all the pixels are sensitive to light (Figure 5).
Figure 5 Two different types of strobing are shown: whole-frame strobing (WFS), and shared-time strobing(STS).
The time slots which are exposed to light are shown in green, while those which are not, are white. The
addition of all the green and white time slots for one row is called integration time. To ease the analysis and
discussion, we have defined the shared time interval, which is a new term that gives its name to the STS
technique. The shared time is the interval for which all the rows (and all the pixels) are exposed simultaneously
to light. These terms can be seen in Figure 6.
Figure 6 Visual explanation of shared time and integration time in a 4-line frame. For this example, the shared time lasts for 6H units, while the integration time is 9H units.
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From Figure 5, we can see that the STS technique yields a higher utilization of the emitted light (more
efficient lighting) than the WFS technique. This is because all the light emitted in STS is being captured by
all the pixels of the sensor, whereas in WFS, this does not happen. We can also observe that STS might
require high peaks of power since the light must be squeezed into the shared time.
In the following sections, we will analyze the power and energy improvement for both WFS and STS when
compared to the constant-light technique.
2.3 WFS analysis
As we have already mentioned, WFS does not offer a better light utilization compared to STS, but this can
be the case when compared to the constant-light technique. This will happen in those configurations in
which there are time windows when no row is being exposed to light. Hereafter, we will call that time dead
time.
In Figure 7, we can see an example of WFS. In this example, the integration time is 5H (𝑇𝑖), the shared time
is 1H (𝑇𝑆), the dead time is 5H (𝑇𝐷), and the time period is 14H (𝑇). For this situation, the IR light is on for
9H and off for 5H units. The light utilization of the WFS technique can be calculated as in Equation 3,
which is 55%, which is greater than 𝑈𝐶𝐿𝑇 = 35%. Thus, the efficiency improvement compared to the
constant-light technique (𝜂𝑊𝐹𝑆) is 35%, which can be calculated as in Equation 4.
𝑈𝑊𝐹𝑆 =𝑇𝑖
𝑇𝑖 + (𝑁𝑅 − 1) [3]
𝜂𝑊𝐹𝑆 =𝑇 − (𝑇𝑖 + (𝑁𝑅 − 1))
𝑇=𝑈𝑊𝐹𝑆 − 𝑈𝐶𝐿𝑇
𝑈𝑊𝐹𝑆 [4]
𝑁𝑅: Number of rows.
Figure 7 Two consecutive frames illuminated with the WFS technique.
Note that for this technique, 𝑈𝑊𝐹𝑆 𝑚𝑎𝑥 = 𝑈𝐶𝐿𝑇 𝑚𝑎𝑥 when TD is null. Also, 𝑈𝑊𝐹𝑆 > 𝑈𝐶𝐿𝑇 when the frames do
not overlap in time with each other, while 𝑈𝑊𝐹𝑆 = 𝑈𝐶𝐿𝑇 = 𝑈𝐶𝐿𝑇 max when they do, since the WFS technique
will behave as the constant light technique (no deadtime).
2.4 STS analysis
Energy efficiency
As we already said, during the shared time, the light provided is more energy efficient since it is being used by
all the lines simultaneously. Instead, out of the shared time, the light is not as efficient because it is only used
by some of the lines (those that have the shutter open).
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While the constant-light technique and the WFS technique emit light outside the shared time, the STS does
not. Because of this, the light utilization of STS is always 100% (𝑈𝑆𝑇𝑆 = 100%) and the energy
improvement can be written as in Equation 5.
𝜂𝑆𝑇𝑆 = 1 − 𝑈𝐶𝐿𝑇 [5]
However, STS technique involves an important limitation: light can only be emitted during the shared time.
Because of this, to maintain a certain level of brightness, an IS using the STS technique might need to
increase the power, i.e., the exposure time reduction is compensated by a power increment.
Power demand
In this section, we will discuss how the power demand is affected when applying the STS technique. In
Figure 8, a simple 5-line frame illuminated by STS is shown. It can be concluded that, when the integration
time is equal to the number of rows, a 1H interval of shared time will be available.
Figure 8 Number of lines to achieve 1H interval of shared time.
Equation 6 is the general formula to calculate the available shared time (TS) in a scenario with ‘NR’ number of
lines and ‘Ti’ H units of integration time.
𝑇𝑆 = 𝑇𝑖 − 𝑁𝑅 + 1 [6]
For a 1110-row frame (1125 total lines1), the maximum allowed integration time is 1123H. Because of this,
the maximum available shared time (𝑇𝑆 𝑀𝐴𝑋@1125) will be of 14H units. This number does not depend on the
frame rate since in one frame there are always the same number of ‘H’ units. Also, we can conclude that
when 𝑇𝑖 ≤ 𝑁𝑅 − 1 = 1110 − 1 = 1109𝐻, there will be no shared time available and thus, the STS technique
cannot be used. For the power demand analysis, we will not consider those cases in which 𝑇𝑖 ≤ 1109𝐻,
since as we demonstrated, there’s no shared time available.
For each row, the STS technique must fit the same energy (E’) as the constant light technique (E) during TS
to achieve the same brightness. The power relation between the STS scenario and the constant-light one,
hereafter called overpower factor (OF), can be calculated as it is shown in Equation 7. In Graph 1, OF equation
is plotted over the valid Ti values.
𝑂𝐹 =𝑃′
𝑃=
𝐸′
𝑇𝑆𝐸𝑇𝑖
𝐸=𝐸′→
𝑃′
𝑃=𝑇𝑖𝑇𝑆 [7]
Where P’ and P are the power demand of the STS technique and constant-light technique, respectively.
1 For the image sensor of the DUT, 1110 pixel-rows contain useful/valid image information (valid lines), while the rest (15) do not.
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Graph 1 Overpower factor (OF) for STS against constant-light technique as a function of the integration time. The function is plotted for an image sensor configuration of 1125 rows.
For the full-HD capture mode (1125 total rows and 1110 valid ones), the worst-case scenario occurs when
TS is minimum (1H) at Ti=1110H, in which OF=1110. The best-case scenario occurs when the TS is
maximum at Ti=1123H, in which OF=80. From this graph and the previous calculations, we can see that
STS presents significant issues regarding the power demand.
2.5 Extended Integration Time (EIT) technique
Introduction
Although the STS technique looked promising from the energy efficiency perspective, there are severe issues
regarding the OF. Furthermore, we found that it can only be used under a very restrictive condition: 𝑇𝑖 >
𝑁𝑅 − 1. In this section, we propose and explore the Extended Integration Time (EIT) technique, which
aims to improve the performance of STS.
As it names indicates, the EIT technique extends the integration time to create a shared time interval long
enough to fit the strobing light pulse. In Figure 9, an example of the application of the EIT is shown. As it
can be seen, the new integration time (Ti’=5H) is longer than the initial one (Ti=1H). As we previously
calculated, we can only have 14H of shared time. Because of this, we can only apply EIT for initial integration
times of 1H to 14H, when Ti>14H, the integration time will already be at its maximum value.
Figure 9 EIT technique. Initially, the integration time is 1H and thus, every line is exposed to light during 1H. After applying the EIT technique, the integration time is increased to 5H and light is only emitted for 1H time interval, which yields the same image brightness but 100% of light utilization.
Overpowering the LED driving circuit
However, if the IR LED driving circuit is improved to be able to emit more light than in the constant light
scenario, and more light is emitted during the shared time, EIT can be used for longer integration times.
-18-
Hereafter, we will refer to this technique as the overpowering technique. For example, if the driving circuit can
provide four times more power (overpower factor of 4, 𝑂𝐹 = 4), the EIT can be used until the integration
time is 56H, i.e., 14H x 4. That is, the EIT applicability threshold has been increased to 56H.
This poses a technical challenge not only on the capability of providing the power but also on the capability
of storing that additional power so that the peak power consumption is not increased.
Long-Exposure technique (LET)
In the previous section, we have seen how to increase the EIT applicability threshold by overpowering the
LED driving circuit. In this section, we will look at how to increase even more the EIT threshold by using
the long-exposure mode of the image sensor of the DUT. This mode allows to have more integration time
at the cost of reducing the fps. However, the readout time is not increased. This results in more shared time
than if the frames per second were directly reduced.
When the target integration time is above our EIT applicability threshold, the longer-exposure mode can
be used. The new EIT applicability threshold (EITt) will be significantly increased and can be calculated
with Equation 8.
𝐸𝐼𝑇𝑡 = 𝑡𝑖 𝑀𝐴𝑋 · (𝑓𝑝𝑠
𝑓𝑝𝑠′− 1) + 𝑡𝑆 𝑀𝐴𝑋 [8]
𝑡𝑖 𝑀𝐴𝑋 [H]: maximum integration time that it is allowed in the new mode.
𝑓𝑝𝑠: frames per second of the previous mode.
𝑓𝑝𝑠′: frames per second of the new mode. 𝑓𝑝𝑠
𝑓𝑝𝑠′: frames per second ratio, which indicates the fps reduction after applying the long-exposure
technique.
𝑡𝑆 𝑀𝐴𝑋 [H]: maximum shared time of the previous mode (14H in the 1125-line mode).
We refer to this technique as the long-exposure technique (LET). For example, in a 1125-line scenario running
at 120fps, the EIT threshold would be 14H. By using EIT and LET at half the fps (𝑓𝑝𝑠′ = 60𝑓𝑝𝑠), the
EIT applicability threshold could be increased to:
𝐸𝐼𝑇𝑡 = 1123 · (120
60− 1) + 14 = 1137𝐻 = 1137 · 7,41𝜇𝑠 = 8,425𝑚𝑠
This new obtained EITt is much longer in time than the EITt at 60fps, which would be:
𝐸𝐼𝑇𝑡 60𝑓𝑝𝑠 = 14𝐻 = 14 · 14,82𝜇𝑠 = 0,207𝑚𝑠
2.6 Constant-light dimming at maximum integration times
Outside the strobing-light techniques repertoire, dimming a constant light source can be a very efficient way
of illumination. We have already seen that the efficiency of constant-light illumination is nearly 100% when
the integration time is close to the maximum value for its frame rate.
Using this fact to our advantage, if the integration time is kept maximum and the image brightness is
controlled by dimming the light source, the efficiency will remain almost 100% always. Also, the energy
consumed is spread over a longer time, which results in lower peak power.
This technique can be combined with the strobing-light techniques: low integration times can be tackled
with STS and WFS, while long ones with analog dimming and constant-light illumination.
We have assumed that we can increase the integration time to fit our energy and power improvement
algorithms. However this might not be true for all the applications. Sometimes, for image quality reasons,
we will require the integration time under a certain limit, case in which we will not be able to apply properly
or at all the presented techniques.
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2.7 Light techniques comparison
Figure 10 shows a summary of all the techniques placed in the initial integration time areas that they can be
applied. Also, the efficiency of the constant light technique is displayed. Every block represents a technique
that can be used to improve the efficiency in its range of initial integration times. Since the blocks represent
the strobing-light techniques, the efficiencies for the blocks are always 100%. Some concerns when using
each technique are shown inside brackets (e.g., increase of power and frame rate drop).
When two blocks overlap with each other, it means that both techniques can be used in that region. This is
the case of a the EIT+overpower region, where the EIT+LET can also be used. If the former technique is
used, an increase of power by a 𝑃𝑓 factor will be required, whereas, if the latter is used, a frame drop will
result.
Figure 10 Available techniques depending on the target integration time for Full-HD operating mode.
By properly using the techniques in Figure 10, we can translate any initial integration time into an equivalent
integration time that yields the same brightness but achieves virtually a 100% of light utilization. Note that,
as we highlighted in the previous sections, for integration times close or higher than the frame period, we
might decide to use constant-light illumination since the light utilization is almost 100%. For example, in
the example above, we could decide that above Ti=1080H, the constant-light technique should be used,
because the efficiency would always be higher than 96%.
Sometimes, applying the previously described techniques will impact the image quality. This is the case, for
example, when any source of light other than the one emitted by the camera’s illumination system is present
in the scene. In this situation, extending the integration time with the EIT technique will yield a brighter
image. In [10], the effects on the image quality when using this kind of techniques are evaluated, therefore,
we will not dwell on the details.
2.8 LED Driver general specifications and design
Introduction
From the previous sections analysis, we conclude that the strobing-light technique imposes additional
requirements on the LED driver when compared to the constant-light technique: fast switching capability
of the LEDs, and more peak-power demand if overpowering is used.
The edge times are limited by the minimum pulse of light that needs to be emitted, which depends on the
camera’s application. During our research, we have found that the shortest integration times considered at
AXIS are around 500µs, which are used for license plate recognition. Because of this, we set the rising and
falling time requirements one or two orders of magnitude smaller, from 5µs to 50µs.
For safety reasons, constant-light LED drivers have the capability to reduce the output current to reduce
the temperature in the circuit. For a strobing LED driver, this functionality could be achieved by PWM
dimming, therefore it is an optional feature. However, as we demonstrated in the previous analysis, the
temperature in the LED circuit might be significantly reduced due to the energy consumption reduction
achieved by the strobing-light technique.
-20-
On the one hand, the image quality can be enhanced in those cases in which temperature is considerably
reduced, as so will be the thermal noise in the CMOS sensor [4]. On the other hand, the switching of the
current of the LED driver will introduce additional noise into the circuit.
For those cases in which overpowering is used, we must make a proper selection of the components so that
they can withstand the maximum peak power during the longer output current pulse.
Buck-converters overview
Even though there might be multiple ways of designing an LED driver, one of the most versatile is to use
a switching converter LED driver IC. This solution is currently used at AXIS, and thus, it is interesting to
evaluate if it would still be appropriate for the strobing-light technique. Switching converter LED drivers
can offer output current control, stability control at the output, safety current limiting, overvoltage
protection, analog dimming, amongst other functions. Because of this, they are valuable components and
often it will not be worth the effort of designing an LED driver with discrete components.
In our case, where we have a 12V supply voltage (𝑉𝑆) available and a single LED, the LED driver IC must
be a buck converter, since the forward voltage of the IR LED (𝑉𝐹) is smaller than the supply voltage.
Current ripple in buck-converter LED drivers for video-
recording illumination
Current ripple is one of the most common constraints to consider when starting the design of a buck-
converter LED driver. For our scenario, we would like to evaluate the effects of current ripple regarding
image quality, LEDs forward current, component cost, and electrical noise.
To begin with, it is important to realize which are the consequences of ripple current on image quality. As
light hits the camera’s sensor, it is accumulated in every pixel of the sensor, yielding an image in which the
light has been integrated over the whole shutter time interval. Buck-converters usually work at switching
frequencies in the order of hundreds of kilohertz, while video-recording cameras frame-rate ranges from
25fps to 120fps. That is, for shutter times close to the camera´s frame-rate, the irradiance variation due to
the current ripple cannot be seen in the output image stream since the irradiance ripple will be integrated
over the whole shutter time. Fast shutter speeds are required when fast objects need to be captured, to avoid
blurry images. The fastest shutter speed used at AXIS is 500µs, for LPR purposes, which compared to a
slow switching period of 10µs (100KHz), is still 50 orders of magnitude greater. This means that 25 pulses
of light will be integrated per frame2, yielding no noticeable light exposure difference between frames. Below
200µs shutter time (10 pulses of light integrated per frame), the buck-converter design should take into
consideration the ripple regarding image quality, since the light exposure could significantly differ from
frame to frame. The value for the switching frequency (𝑓𝑆𝑊) that allows ‘𝑁’ waves of ripple per frame for a
minimum light pulse of ‘𝑇𝑝 𝑚𝑖𝑛’ seconds can be calculated with Equation 9.
𝑓𝑆𝑊 > 2 ·𝑁
𝑇𝑝𝑚𝑖𝑛 [9]
The LED forward current that the LED driver can supply also depends on the ripple. This is because the
components have maximum current ratings that cannot be exceeded, thus limiting the peak current that the
driver can withstand. For a constant maximum peak-current, the average forward-current decreases as the
ripple is increased. This can be tackled by selecting components that can tolerate higher peak currents, but
this generally means increasing the cost and area of the circuit.
2 Note that the period of the ripple is twice the switching period, since one ripple period consists of a rising edge (first switching period) and a falling edge (second switching period).
-21-
Finally, the ripple current is related to electrical noise generated by the driver circuit. This is due to the
change of current demand through the wires, that can generate conducted and radiated interference. As the
ripple increases, so will the noise, making EMI problems more likely to appear.
For the above reasons, we can conclude that, even though current ripple does not pose a problem regarding
image quality, it will determine the maximum peak current and thus, the cost and area of the driver. Also,
because of EMI, it is a smart choice to keep it low.
Edge times in buck-converter LED drivers
As we have seen in the previous sections, the LED driver has a strict requirement on the switching time of
the IR LEDs. In this section, we will study the switching time in buck-converter LED drivers. We will
assume that the current through an IR LED is proportional to the IR light emitted [5], [6], i.e., evaluating
the edge times of the current is equivalent to evaluating the edge times of the emitted light.
In Figure 11, a simplified circuit to evaluate the edge times of a buck-converter LED driver is presented.
This model consists of a voltage source (𝑉𝑆𝑊) that is not the supply power source of 12V, but the voltage
that simulates the initial state before the rise and fall edges. It also includes a voltage-controlled switch (𝑆1)
that simulates the switching transistor in the buck-converter, an 𝐿1 and 𝐶1 low pass filter (with ideal
components), a recirculating diode (𝐷2), and a light-emitting diode (𝐷1). As we said in the Methodology
section, we use the simulation tool LTspice to support the discussion presented in this chapter.
Figure 11 Simplified circuit of a buck-converter LED driver to evaluate rise and fall times.
Buck-converter LED drivers act as a controlled constant-current source. For this matter, since there are
supposed to be no current load transients, they can be designed without any output capacitor. Thus, the
design flow that we propose for the LED driver design is based on this fact: first, the LED driver will be
designed as if there was no output capacitance; later, the output capacitor will be added to improve the
performance.
For the falling-edge time, the initial voltage will be the one that makes the desired on-state current flow
through 𝐷1. For example, for the diode in the simulation with 550mA as forward current, the voltage drop
(𝑉𝐹1) is equal to 4V. Assuming no losses in the other components, 𝑉𝑆𝑊 must be of 4V to simulate properly
the starting conditions for the falling-edge time evaluation. The simulation will reach the steady state with
𝑆1 closed, that is, 550mA flowing through 𝐷1 (light-emitting state). When the switch is opened, the current
will stop flowing from 𝑉𝑆𝑊 and the storage elements will discharge through 𝐷1 and 𝐷2.
The current flowing through the inductor will decay almost linearly because it is contained in between two
diodes that fix a nearly constant voltage in between its terminals. In this situation, we can calculate the
current as:
𝑣𝐿1 = 𝐿 ·𝑑(𝑖𝐿1)
𝑑𝑡→ 𝑖𝐿1 =
1
𝐿∫ 𝑣𝐿1
𝑡2
𝑡1
𝑑𝑡 𝑣 𝑐𝑡𝑒→
𝑣𝐿1𝐿(𝑡2 − 𝑡1)
𝑡2, 𝑡1: two points in time where 𝑡2 > 𝑡1.
-22-
This means that the current in the inductor decays at the rate indicated in Equation 10.
𝛾𝐿1 =𝑣𝐿1𝐿=𝑉𝐹1 − 𝑉𝐹2
𝐿≈𝑉𝐹1𝐿 [10]
𝑣𝐿1 at 𝑡1 can be calculated as 𝑉𝐹1 − 𝑉𝐹2 = 4 − 0,3 = 3.7𝑉. Because of this, in our example the decay rate is
𝛾𝐿1 =3,7
33µ= 0.1121
𝐴
µ𝑠 . This value is very close to the simulated value of the circuit in Figure 12, which is
0.1122 A/µs.
The approximation holds properly when 𝑣𝐿1 variation is small. For driving voltages (VSW) significantly
greater than the load’s conduction voltage, i.e., when the inductor voltage difference between the initial and
final states is high, the linear approximation will not hold as good anymore, and will introduce a considerable
error in the equation. For example, when 𝑉𝑆𝑊 = 8𝑉, Equation 10 predicts a decay rate of 𝛾𝐿1 =8−0,8
33µ=
0,218𝐴
µ𝑠, while the simulated one is 0,173
𝐴
𝜇𝑠. This situation though is not very common since the LED will
usually operate close to its conduction voltage, allowing us to consider the voltage drop across the inductor
to be constant.
Note that, this approximation does not include any effects from the output capacitor (𝐶1). This is because
of the LED driver design flow that we mentioned at the beginning of this subsection: the capacitor at the
output is neglected as it is a component that is added only to improve the performance after the driver is
designed. Therefore, its capacitance value is chosen so that the energy stored is neglectable when compared
to the inductor one.
From this analysis, we can conclude that the fall time will improve (shorten) if:
- The recirculating diode (𝐷2) forward voltage is lowered.
- The forward voltage of the load LEDs is increased.
- The inductor value is reduced.
For the rise time, the analysis can be performed the same way. In Figure 12 a simplified circuit for the
buck-converter rise time analysis is shown. Note that in the circuit, the output capacitor has not been
considered yet. In this situation, ‘S2’ has been added to ensure that ‘D1’ was completely discharged at the
beginning of the rising edge and helps no purpose other than setting the initial conditions for the simulation.
Figure 12 Buck-converter LED driver simplified circuit for the rise time analysis. ‘S1’ is the switch that controls the rise transient, while ‘S2’ is the switch that ensures that the LED is completely discharged at the beginning of it.
By considering the voltage across the inductor constant, we can approximate the increasing rate (𝛿) as in
Equation 11.
𝛿 =𝑣𝐿1𝐿=𝑉𝑆𝑊 − 𝑣𝐷1
𝐿 [11]
𝑣𝐿1: voltage-drop across the inductor.
-23-
𝑣𝑆𝑊: supply voltage seen by the inductor during the rising edge.
𝑣𝐷1: forward voltage-drop on the LED load (approximately constant).
For 𝐿 = 66𝜇𝐻, 𝑉𝑆𝑊 = 12𝑉, 𝑉𝐹 𝐷1 = 2,9𝑉, Equation 11 yields a 𝛿 = 0,137𝐴
𝜇𝑠, which is very close to the value
obtained by simulation: 0,133𝐴
𝜇𝑠 over the operating currents of the LED.
However, for the rise time analysis, it is important to consider the effects of the output capacitor. Figure
13 shows the simplified circuit of the rising edge scenario including the output capacitor (C1).
Figure 13 Buck-converter LED driver simplified circuit for the rise time analysis with output capacitor (C1).
For this circuit, the evolution of the most important signals are plotted in Figure 14 (𝐿 = 66𝜇𝐻, 𝐶 =
1𝜇𝐹, 𝑉𝑆𝑊 = 12𝑉, 𝑉𝐹 𝐷1 = 2,9𝑉). As it can be seen, the LED starts conducting after approximately 6µs
(point A in Figure 14), since the capacitor is storing all the energy. At point A, the capacitor’s voltage reaches
the LED’s forward voltage (𝑉𝐹 = 2,9𝑉) and thus, the LED starts to conduct. During the interval A→B, the
LED’s current increases rapidly (faster than the increase rate of the inductor). At point B, the current of the
LED and the one provided by the voltage source (the same as the one flowing through the inductor) are
virtually the same.
Figure 14 Most relevant signals in the buck-converter LED simplified driver circuit for the rise time analysis. Point ‘A’ in time indicates when the LED starts to conduct current, i.e., its voltage exceeds the conduction voltage. Point ‘B’ indicates when the LED’s current is virtually the same as the
inductor’s one.
From Figure 14 we can conclude that, for all the current values before point ‘B’, the time required for the
LED’s current to reach a certain current is always longer than for the buck converter without-capacitor
solution. This is due to the delay introduced by the capacitor (0→A time interval). The time difference
between the inductor and the LED currents to reach a certain value near 𝐼(𝑉𝐹) is close to the delay
-24-
introduced by the capacitor (𝑡𝑑 = 𝐴). However, in terms of current rise time, i.e., without including 𝑡𝑑, the
capacitor boosts the increase rate of the LED current, since it allows the current in the inductor to increase
steadily until its accumulated charge builds a voltage greater than the LED’s conduction voltage (point A).
In the 0→B time interval, the LED starts conducting, reducing its impedance as the voltage gets higher. At
point B, the diode will virtually behave like a very small resistor, whereas the capacitor will behave like an
open-circuit, since the ‘dv/dt’ will be almost null.
In the following paragraphs, we will find the parameters to which point ‘A’ is dependent on, this way we
will be able to predict the capacitor delay. We will also find a way to estimate the rise time and point ‘B’.
Hence we will be able to extract the current value for which the step appears (𝐼𝑠𝑡𝑒𝑝 = 1,3𝐴 in Figure 14).
For LED currents greater than 𝐼𝑠𝑡𝑒𝑝 the current-step shape will be observed, whereas lower current will not
and will yield a faster rise edge. Our ultimate goal is not to find exact values, but the relation of the variables
that influence those values, so that better design decisions can be taken when designing the driver.
To calculate the output-capacitor delay, we will assume that all the components are ideal and have no
losses. This way we can conclude that the energy stored in the capacitor (𝐸𝐶1) and the inductor (𝐸𝐿1) at
point ‘A’ (𝑡𝐴) is the total energy provided from 𝑡0→𝐴 by the power supply. The following 5 steps are used
to find 𝑡𝐴 and the current at 𝑡𝐴 (𝐼𝑡𝐴).
(I) Energy in the inductor (tA=A): 𝐸𝐿1 =1
2𝐿𝐼2 =
1
2· 𝐿 · (δL1 0→tA · 𝑡𝐴)
2
Where δL1 0→tA is the current increase rate through the inductor from t=0 to t=A. This
current rate has been approximated to the value shown in Equation 12. Care should be taken
when 𝑉𝑆𝑊 ≈ 𝑉𝐹, since this linear approximation will be less accurate.
𝛿𝐿1 0→𝑡𝐴 =(𝑉𝑆𝑊 −
𝑉𝐹2 )
𝐿 [12]
(II) Energy in the capacitor (tA=A):
𝐸𝐶1 =1
2𝐶𝑉2 =
1
2· 𝐶 · 𝑉𝐹
2
(III) Energy provided by VSW (0→A):
𝐸𝑆𝑊 = ∫ 𝑃𝑆𝑊
𝑡𝐴
0
𝑑𝑡 = ∫ 𝑉𝑆𝑊 · 𝐼𝑡𝐴
0
𝑑𝑡 = 𝑉𝑆𝑊∫ 𝐼𝑡𝐴
0
𝑑𝑡 = 𝑉𝑆𝑊∫ δL1 0→tA · 𝑡𝑡𝐴
0
𝑑𝑡 =
𝑉𝑆𝑊 ·δL1 0→tA · (𝑡𝐴)
2
2
𝑃𝑆𝑊: Power of the voltage supply VSW.
(IV) Energy conservation:
𝐸𝑆𝑊 = 𝐸𝐿1 + 𝐸𝐶1
(V) In Equation 13 we use (I) to (IV) to find 𝑡𝐴.
𝑡𝐴 = √
12 · 𝐶 · 𝑉𝐹
2
𝑉𝑆𝑊2 𝛿𝐿1 0→𝑡𝐴 −
12 · 𝐿 · 𝛿𝐿1 0→𝑡𝐴
2= √
2 · 𝐶 · 𝑉𝐹𝛿𝐿1 0→𝑡𝐴
= √2 · 𝐶 · 𝑉𝐹 · 𝐿
𝑉𝑆𝑊 −𝑉𝐹2
[13]
-25-
In Figure 15 the circuit has been simulated for VSW=12V, C1=1µF, L1=66µH and 1 LED load (VF=2,9V).
Since VSW is considerably larger than VF, the linear approximation for the current through the inductor will
be accurate (Equation 12).
δL1 0→tA =(12 −
2,92 )
66𝜇= 159848
𝐴
𝑠
The simulation yields δL1 0→tA = 165446𝐴
𝑠, that is, our equation has a 3,5% of error for this case. By using
(I) to (IV), we can extract tA:
𝑡𝐴 = √2 · 1𝜇 · 2.9
159848= 6.02𝜇𝑠
The value found in the simulation is 5.7µs, which means that our approximation introduced a 5,3% of error.
With both δL1 0→tA and 𝑡𝐴 we can find the current that flows through the inductor (and into the capacitor)
at point A. The approximated value is virtually the same as the simulated one (0,95A):
𝐼𝑡𝐴 = δL1 0→tA · 𝑡𝐴 = 159848 · 5,92µ = 0.94𝐴
Figure 15 Simulated ‘0’ to ‘A’ time interval for VSW=12V, C1=1µF, L1=66µH and 1 LED load (VF=2,9V). The increase rate of the current through the inductor is 165446 A/s.
At point A, the current will start flowing through the LED and will rapidly increase during the time interval
A→B. In the following paragraphs, we will propose a method to estimate with a linear approximation the
increase rate of the current for the time interval A→B. For our approximation, the starting conditions are
well known. However we need to determine the final conditions, i.e., conditions at point B in time. At that
point, we will consider that the current through the LED is much higher than the one through the capacitor
(𝐼𝐷1 ≫ 𝐼𝐶1), or equivalently, since both form a current divider, that the capacitor’s resistance is much higher
than the LED’s resistance (𝑍𝐶 ≫ 𝑅𝐷).
(I) Capacitor and LED resistance relation at point B: We will assume that a good
approximation is when the resistance of the LED is 10 times the resistance of the capacitor.
10 · 𝑅𝐷1 = 𝑍𝑐1
(II) Current divider:
𝐼𝐷1 = 10 · 𝐼𝐶1 → 𝐼𝐿1 =11
10𝐼_𝐷1
(III) Capacitor resistance at point B:
-26-
𝑍𝑐1 =𝑉𝐶1𝐼𝐶1
=𝑉𝐷1
𝐶1 ·𝑑(𝑉𝐷1)𝑑𝑡
=𝑉𝐷1
𝐶 ·𝑑(𝐼𝐷1)𝑑𝑡
·𝑑(𝑉𝐷1)𝑑𝐼𝐷1
=𝑉𝐷1
𝐶 ·𝑑(𝐼𝐷1)𝑑𝑡
· 𝑟𝐷1
𝑟𝐷1 =𝑑(𝑉𝐷1)
𝑑𝐼𝐷1: Dynamic resistance of the LED D1.
(IV) We use (I) to (III):
𝑑(𝐼𝐷1)
𝑑𝑡=𝑑 (1011𝐼𝐿1)
𝑑𝑡=𝑑 (1011(𝐼𝑡𝐴 + δL1 tA→tB · 𝑡))
𝑑𝑡=10
11δL1 tA→tB
𝑉𝐷1𝑅𝐷1 · 𝑟𝐷1
=100
11𝛿𝐿1 𝑡𝐴→𝑡𝐵 · 𝐶1 [14]
We can rewrite Equation 14 to ease its use by defining a new parameter (K) and rearranging
the variables of the LED:
𝐾 =100
11δL1 tA→tB · 𝐶1
𝑉𝐷1𝑅𝐷1 · 𝑟𝐷1
=𝐼𝐷1𝑟𝐷1
𝐼𝐷1𝑟𝐷1
≡ 𝐼𝐷1 · 𝑔𝐷1 = 𝐾 [15]
𝑔𝐷1 =1
𝑟𝐷1: Dynamic conductance of the LED D1.
Equation 15 contains the condition that fulfills the initial resistance relation (I). Thus, finding
the point in the ‘I/V’ curve of the LED (D1) that fulfills the equation yields the value of the
current 𝐼𝐷1 at point B. From this current and the increase rate of the current through L1, 𝑡𝐵
can be found (Equation 16). Also, the increase rate of the current through the diode in the
interval A→B can be written as: δD1 tA→tB =𝐼𝐷1 𝑡𝐵𝑡𝐵−𝑡𝐴
.
𝑡𝐵 =𝐼𝐷1 𝑡𝐵 − 𝐼𝐿1 𝑡𝐴𝛿𝐿1 𝑡𝐴→𝑡𝐵
+ 𝑡𝐴 [16]
𝐼𝐷1 𝑡𝐵: Current value of the diode (and inductor) previously found with Equation 15.
For the previous circuit example, 𝐼𝑡𝐵can be calculated as:
𝐾 =100
11δL1 tA→tB · 𝐶1 =
100
11·12 − 2.9
66µ· 1µ = 1.25
The point in the LED’s ‘I/V’ curve in which 𝐼𝐷1 · 𝑔𝐷1 = 1.25 can be found by using its
electric model or can be approximated with the data from its datasheet. In Figure 16, an
example on how to find 𝐼𝑡𝐵 is shown. For the example, 𝐼𝑡𝐵 = 1𝐴 and 𝑉𝐷1 𝑡𝐵 = 3.8𝑉.
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Figure 16 'I/V' curve of the LED in the example circuit. The point in which ID·gD equals K=1.25 is shown (in red).
From the analysis of the buck-converter circuit and Equations 13 and 15, we can make some conclusions:
- The output-capacitor delay increases as the capacitance increases since more energy will be required
to reach the LED´s conduction voltage.
- The output-capacitor delay increases as the conduction voltage for the LED(s) increases since the
capacitor will need to charge up to a higher voltage for the LED(s) to start conducting.
- The output-capacitor delay also increases as the difference between the voltage source and the load
conduction voltage decreases. This is because the current through the inductor will be lower.
- The output-capacitor delay increases as the inductor’s value increases since a bigger inductor will
oppose more resistance to the change of current, thus providing less energy per unit of time to the
rest of the circuit.
- The rise-time decreases as the inductor value decreases since the rate of current change in the circuit
is higher, thus reaching faster the region in which the LED has less resistance.
- The rise-time decreases as the capacitor value decreases since it will oppose more resistance to the
current, thus making it flow through the load diode instead.
- The rise-time decreases as the difference between the voltage source and the load conduction
voltage increases, due to the increased current flowing through the inductor.
- The rise-time decreases the more abrupt the change of the ‘I/V’ curve is, since the low-resistance
region of the LED will be reached sooner.
Switching techniques: Series and shunt switching
We have only discussed the switching performance of a circuit that has the switching mechanism in series
with the LED (series switching). However, the switch can be placed in parallel to the LED (shunt switching).
In this new situation, when the switch is activated, the current flows through it and thus, the current through
the LED stops.
This technique allows faster falling edges since the current path for the LED and the inductor is not the
same anymore. Instead, the switch allows the current from the inductor to flow across it, leading to a sudden
drop of the current through the LED. This technique is described in the US11865695 patent[7]. For the
shunt-technique, the fall time is neglectable when compared to the rising edge, only limited by the transistor
switching capabilities and parasitic capacitances.
Figure 17 shows a simplified circuit to evaluate the rise time of the shunting technique. Initially, the current
is flowing through the shunting switch (‘SHUNT’), which is a voltage-controlled switch driven by
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‘V_SHUNT’. When the shunting device opens, capacitor ‘C1’ charges until 𝑉𝐹1 and then, current starts to
flow through the diode. For the shunting technique, the output capacitor delay is lower since the initial
current is higher than for the series shunting technique and thus, charges faster. For the same reason, the
rise time is also faster in the shunting technique than in the series technique. The results of the simulation
can be observed in Figure 18. We can see that the improvement of the rise time of this technique is not very
significant when compared to the fall time one.
Figure 17 Simplified circuit to evaluate edge times for the shunting technique.
Figure 18 Simulation results for the rising edge of the shunting simplified circuit. Initially 1A flows through the inductor and the ‘SHUNT’ switch, which is closed. At t=0us, the switch opens, the capacitor ‘C1’ charges, and current starts flowing through ‘D1’ at 2.41us.
The same equations derived in Section 2.8.4 can be used to approximate the behavior of the circuit if the
initial conditions are properly considered.
The edge time improvement in the shunting technique comes at the cost of a higher power consumption,
due to the additional power demand of the switching device during the LED off time. To minimize this
loss, the switching mechanism is chosen to have the minimum series resistance possible.
Buck-converter LED driver design
We have studied the parameters that affect the requirements of a buck-converter strobing-light LED drivers.
There are many ways and approaches to the buck-converter design, and it is out of the scope of this thesis
to create a guideline for it. In this section, we will discuss and propose when and how to consider the
particularities of a buck-converter for the strobing-light technique.
A common design flow for buck-converter circuits is to start by choosing the proper inductor value for
which the output ripple is within the required limits. In our case, the ripple is not the only main issue, but
also the edge times. Therefore, our starting point will be to find the range of inductor values that yield the
desired rise and fall times, while still meeting the ripple constraints. The switching frequency will be chosen
so that it fulfills Equation 9, and balances switching and conduction losses, while keeping the components’
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size and cost within the desired limits. As we already mentioned, explaining how to find the right values for
the buck-converter design is out of the scope of this thesis.
However, as we demonstrated in the previous section, the inductance value is not the only parameter that
determines the switching speed, but also the 𝑉𝑆𝑊 and the total 𝑉𝐹 of the load. This gives rise to another
design decision: should light emitted by the IS be increased by adding more LEDs or by increasing the
forward current in the LED driver?
From the formulas in the previous sections, we can conclude that to balance the rise and fall times, the
conduction voltage of the load must be half the supply voltage. If we consider the number of LEDs that
fulfill this condition as the starting point, adding more LEDs in series (higher forward voltage) will increase
the rise time, while removing LEDs will increase the fall time. If the shunting technique is used, it is
convenient to reduce the rising edge by having a low forward voltage since the falling edge is much faster.
Regarding area and cost, increasing the number of LEDs implies the adding the cost and area of the
additional LEDs. Also, the output capacitor must withstand the higher forward voltage. On the other hand,
increasing the forward current might imply selecting a new LED driver IC, transistors, L1, and C1.
If we need to achieve faster edge times or reduce the cost of the driver, we can iterate over the above
considerations after increasing the switching frequency of the buck-converter. Shorter switching periods
allow smaller components but decrease the efficiency of the converter.
2.9 Health regulations
According to [8], devices can be categorized in four risk groups depending on their infrared radiation, and
retina and near-infrared retinal thermal hazard duration. The hazard exposure limits for each group can be
calculated with the equations provided in that technical report.
As it is explained in [9], for infrared pulsed light-emitting devices, the exposure limit for each risk level is
calculated using the time averaged values of the emitted wave. Therefore, reducing the average power
consumption also reduces the photobiological hazard of the IS.
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3. License Plate Recognition (LPR)
3.1 Introduction
In Chapter 2 we concluded that very significant energy savings could be achieved by the strobing-light
technique for short shutter times. Generally, short shutter times are used to capture fast moving objects.
Thus, a suitable scenario for this technique is one that occurs in darkness and involves fast moving objects,
like LPR.
Since LPR is a perfect match for the strobing-light technique and a hot-topic in AXIS, special effort has
been put into determining how much can it outperform the constant-light technique. Furthermore, a real
scenario in which to apply this technique allows determining the specifications for a strobing-light LED
driver, that is, going one step further into understanding the problem and its solutions.
3.2 Scenario overview
For LPR purposes, cameras can be placed on the side of the road or above it (Figure 19). As we will see
later in this section, the position of the camera and the angle of its optical axis with the vehicle’s movement
direction are the most relevant parameters to consider, as they will determine the power and width of the
light pulses. This also means that the specifications of the LED driver have a direct connection with the
LPR system installation.
Figure 19 Different positions of cameras for LPR.
For simplicity, during the study of the LPR scenario, we will consider only the case in which the camera is
on top of the road. However, at the end of the explanation we will discuss how to derive the analogous
process for a camera on the side of the road, and for a camera that is both above and on the side. In the last
case, where the camera is both up and on the side of the road, we will also see how to consider both
simultaneous effects.
In an LPR scenario, vehicles move towards or away from the camera. From the camera´s perspective, that
movement can be described in two different components: a movement component in its optical axis, and
another perpendicular to it. In [10], these movements are analyzed, and it is concluded that the optical axis
movement generates neglectable blur when compared to the perpendicular one. Because of this, we will
derive the specifications of the LED driver considering that the only source of blur in the license plates
comes from the vehicle´s movement perpendicular component.
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A graphical representation of a vehicle´s velocity components for an LPR scenario is shown in Figure 20.
The sensor-plane movement (𝑣𝑠), as it is called the perpendicular component in [10], can be easily calculated
by trigonometric analysis from the vehicle’s speed (𝑣𝑣) and the angle of the camera (𝛼) as in Equation 17.
𝑣𝑠 = 𝑣𝑣 · 𝑐𝑜𝑠(𝛼) [17]
Figure 20 Vehicle speed components.
This means that, from the camera’s perspective, vehicles move faster the narrower the camera´s angle gets,
thus having stricter requirements to yield non-blurry images. The parameter directly affected is the pulse
width, which in the strobing-light technique is equivalent to the camera’s shutter time and will have an
impact on the peak power required during the strobe. This is because the amount of energy per unit of time
in the strobe increases (the peak power increases), to achieve the same brightness during a narrower strobe
time.
On the other hand, a wider angle means longer distance camera-to-car. This implies that the power of the
LED driver must be higher since the beam of light must reach the plate with the same intensity at a longer
distance.
The last effect we will consider for the analysis is the distortion perceived in the license plate’s letters due
to the angle of the camera. In this case, as opposed to the previous one, as α is reduced, the distortion is
more significant. In Section 3.3 we will propose a method to quantify this distortion, at this point we just
need to know that it is an effect that will require shorter strobes to reduce the blur as α gets smaller.
Sensor-plane speed, distance, and angle distortion are thus the variables that determine the characteristics
of the IR light pulses, width and peak power, which are needed to design and dimension the LED driver.
By finding the relation between these variables, an optimal LED driver can be designed that satisfies the
requirements for a specific scenario.
3.3 LED Driver specifications for LPR
To understand how the angle affects the LED driver pulse peak power, we analyze the situation represented
in Figure 21. In all the situations, we will consider that the resolution is kept constant (equal resolution
condition), i.e., if two situations with different camera-to-vehicle distances are compared, it will be assumed
that proper zoom is used to get the license plate with the same resolution.
First, to obtain the same brightness in the recordings for both positions (𝛼1 and 𝛼2), the amount of
irradiance (𝐸) at the plate must be the same: 𝐸1 = 𝐸2. For this to happen, the ratio between the power
radiated by the illumination system in position 1 (𝑃1) and position 2 (𝑃2) must be the one in Equation 18.
As it is shown in Figure 21, 𝑑1 and 𝑑2 are the distances from the camera to the license plates for 𝛼1 and
𝛼2, respectively.
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𝑃2𝑃1=𝑑22
𝑑12 = (
𝑑2𝑑1)2
[18]
Figure 21 Representation of two recording situations with different angles. Note that the car speed and the camera's height is the same for both cases.
To obtain the same blur (𝐵), the displacement of the vehicle during one frame must be the same. We will
consider both blur and displacement the same parameter, and it can be calculated as in Equation 19, as the
product of the speed in the sensor plane [pxl/s] and virtual shutter time [s].
𝐵 = 𝑣𝑠 · 𝑡𝑣𝑠 [19]
Considering 𝑡𝑣𝑠 as the virtual shutter time (strobe time), the relation between 𝑃1 and 𝑃2 can be found with
the following procedure:
𝐵1 = 𝐵2 → 𝐵1𝐵2= 1
𝐵1 = 𝑣𝑣 · cos(𝛼1) · 𝑡𝑣𝑠1 ; 𝐵2 = 𝑣𝑣 · cos(𝛼2) · 𝑡𝑣𝑠2
𝑣𝑣 · cos(𝛼1) · 𝑡𝑣𝑠1 𝑣𝑣 · cos(𝛼2) · 𝑡𝑣𝑠2
= 1
𝑡𝑣𝑠1𝑡𝑣𝑠2
=cos(𝛼2)
cos(𝛼1)
Since the peak power is inversely proportional to the virtual shutter time, we can rewrite the last equation
as:
𝑃2𝑃1=cos(𝛼2)
cos(𝛼1)
By trigonometry, we can translate the equation above as a function of the distance from the camera to the
car:
ℎ
𝑑1= cos(𝛼1) ;
ℎ
𝑑2= cos (𝛼2)
𝑃2𝑃1=𝑐𝑜𝑠(𝛼2)
𝑐𝑜𝑠(𝛼1)=𝑑1𝑑2 [20]
As we can observe in Equations 18 and 20, the ratios between distance are inverse to each other. Also, the
effect of distance is squared, that is, more significant than the effect of the speed increase due to the angle
of the camera. From both equations, we could conclude that increasing the distance is detrimental for the
LED driver since it will need to handle more power, that is, we should have 𝛼 = 90º to use the minimum
power. The resulting peak power ratio equation is shown in Equation 21.
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𝑃2𝑃1=𝑑2𝑑1 [21]
To exemplify graphically what this equation means we have added Figure 22. In it, it can be seen that a
certain driver will need to provide four times more peak power when the camera-to-vehicle distance doubles
(equal brightness condition). However, from the equal blur condition perspective, the requirements are laxer
since the same blur will be generated from a twice longer virtual shutter time. Both considerations yield the
result predicted by Equation 21: the resulting peak power ratio is the ratio between the distances.
Figure 22 Visual exemplification of the equal brightness and equal blur conditions.
This does not resemble reality good enough yet, because in that situation the plate would be 90º inclined
with respect to the camera’s optical axis and the letter would not be visible. However, we still need to
consider the angle distortion, which we previously introduced in this chapter.
To consider the effect of the angle distortion we need to rewrite the blur condition. Regarding image
quality, the absolute amount of blur (B) is not relevant, but the amount of blur with respect to the letters
size (Br). This means that bigger letters can tolerate more blur than smaller ones, as it is shown in Figure 23.
Figure 23 In this picture two different sized letters suffer the same amount of blur (B), 4 pixels in the vertical direction. In green, the position of the letter at the start of the shutter time is shown; in grey, the final position; and in black, the result captured by the camera. While the absolute blur does not
represent the quality of the image, the relative blur (Br) does.
-34-
Angle distortion can be interpreted as a change of the size of the letters, i.e., the license plate and its letters
height, when looked from any 𝛼 < 90º, will shrink. The shrinking factor (𝑠) that the plate suffers can be
derived from Figure 24, as it is done in Equation 22.
ℎ𝑝′ = ℎ𝑝 · 𝑠 = ℎ𝑝 · 𝑠𝑖𝑛(𝛼) [22]
Figure 24 The height of the license plate (hp) shrinks to ‘hp'’ when looked from an angle ‘α ’.
If we consider 𝐵max @𝑟𝑒𝑠 as the maximum blur in pixels that we can tolerate at a certain resolution (𝑟𝑒𝑠), we can describe the blur as a relative variable to it as in Equation 23. Since we are always assuming to have
the same resolution, hereafter we will stop using ‘@res’.
𝐵𝑟 @𝑟𝑒𝑠 =𝐵
𝐵𝑚𝑎𝑥 @𝑟𝑒𝑠 [23]
𝐵max is a constant that could be extracted from image testing, qualitatively identifying the maximum blur
that a character can tolerate and still be readable, or by means of the analysis of the LPR algorithm results
under different amounts of blur. We will not find a method to extract values for this parameter.
The same blur condition can be rewritten using the relative blur instead of the absolute one, as the same
relative blur condition, as shown below.
𝐵𝑟1 = 𝐵𝑟2
𝐵1𝐵max 1
=𝐵2
𝐵max 2
However, the amount of permissible blur will depend on the angle of the camera (𝛼). For instance, if we
were looking frontally at the plate (𝛼 = 0º) we would tolerate the entire Bmax blur. In the most extreme case,
if we were looking the plate from the top (𝛼 = 90º), we would not see any character, thus the permissible
blur would be null. By trigonometric analysis, the perceived height of the plate can be found to be directly
related to the sinus of the camera position angle (𝛼). We can find the peak power relation by assuming that
the amount of maximum blur that can be tolerated (Bmax) is proportional to the height of the plate perceived
by the camera. The procedure is the one that follows:
𝐵1𝐵max sin(𝛼1)
=𝐵2
𝐵max sin(𝛼2)
𝐵1sin(𝛼1)
=𝐵2
sin(𝛼2)
𝑣𝑣 · cos(𝛼1) · 𝑡𝑣𝑠1sin(𝛼1)
=𝑣𝑣 · cos(𝛼2) · 𝑡𝑣𝑠2
sin(𝛼2)
𝑡𝑣𝑠1𝑡𝑣𝑠2
=𝑡𝑔(𝛼1)
𝑡𝑔(𝛼2)
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𝑃2𝑃1=𝑡𝑔(𝛼1)
𝑡𝑔(𝛼2) [24]
By basic trigonometry, Equation 24 can be rewritten as the power ratio for the absolute blur equation
(Equation 20) multiplied by the angle distortion ratio, as in Equation 25. Therefore, the angle distortion
ratio is sin(𝛼1)
sin(𝛼2).
𝑃2𝑃1=𝑠𝑖𝑛(𝛼1)
𝑠𝑖𝑛(𝛼2)·𝑑1𝑑2 [25]
We can extract the power ratio between two camera positions with different α by combining the same
relative blur and the same brightness conditions. The result is shown in Equation 26.
𝑃2𝑃1=𝑠𝑖𝑛(𝛼1)
𝑠𝑖𝑛(𝛼2)·𝑑2𝑑1 [26]
For our comparison purposes, it is only relevant to us the case in which the cameras are at the same height,
which yields Equation 27. Several curves for different 𝛼1 are plotted in Graph 2.
𝑃2𝑃1=𝑠𝑖𝑛(𝛼1) · 𝑐𝑜𝑠(𝛼1)
𝑠𝑖𝑛(𝛼2) · 𝑐𝑜𝑠(𝛼2)=𝑠𝑖𝑛(2 · 𝛼1)
𝑠𝑖𝑛(2 · 𝛼2) [27]
Graph 2 Power ratio between 2 cameras at different angle positions. Note that for α 1 it is only plotted the 0° to 45° range since their respective complementary angles yield the same curve (e.g., the a1=80° curve is equal to the a1=10° one).
In Graph 2 we can observe how the power ratio decreases for any initial angle ‘𝛼1’ until ‘𝛼2 = 45º’, thus
improving the efficiency of the driver (less power requirements for the same resulting quality image). After
that minimum, the power ratio increases beyond the initial power value because of the increase in the
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camera-to-vehicle distance, thus yielding worse efficiencies. This brings us to the conclusion that 45º is the
most efficient angle for the camera.
While this is an approximation, it provides us with a starting point to calculate the power that the LED
illumination system needs to provide. From the LED driver perspective, the main choice will be the peak
current and the output voltage that drive the LEDs, but this will be linked to the LED choice and any
additional optics. Again, optics and electronics are tightly related and need to be considered together,
evaluating the pros and cons for both at the same time.
As we mentioned at the beginning of this section, the analysis has been done for a camera which is
positioned above the road (Figure 19). However, it could also be placed on the side of it (at the license plate
height or at another height). The analysis for a camera which is on the side of the road is analogous to the
one we developed in this section, thus yielding 45º as the best angle regarding peak power versus image
quality performance. For a position in which the camera is biased from the license plate both in the
horizontal and vertical axis, the most restrictive regarding blur should be considered. For those cases in
which the blur is equally relevant in both axis, a more detailed study on how the LPR algorithm performs
should be done to determine how to treat both effects at the same time.
3.4 LED Driver design guideline
In this section, we present a guideline that aims to provide a method to find the specifications of an LED
driver circuit for the LPR scenario. This guideline helps us to structure all the acquired knowledge and
provides some rough values for the required peak power of the illumination system.
The guideline consists of several steps that need to be followed, which we present now framed. Before each
frame, we include a brief explanation for a better understanding.
First, the requirements for the LED driver arise from the setup of the camera and the targeted maximum
speed aimed to be captured. At this point, it is also required to know the number of frames that the LPR
algorithm needs.
Initial data:
a. Height of the installation (ℎ) and horizontal bias (𝑏).
b. Maximum speed required to capture (𝑣𝑣).
c. Minimum number of frames required by the LPR algorithm (𝑁𝐹).
The provided data allows selecting the most appropriate angle for the camera (𝛼). Starting from 𝛼 =45º, we
will evaluate ∀𝛼 ≥ 45º, until the camera is able to record 𝑁𝐹 number of frames for the fastest car that we
need to capture (𝑣𝑣). Once we have the 𝛼, the sensor-plane speed of the fastest vehicle (𝑣𝑠) can be calculated,
which is the main source of blur. As it is described in Equation 19, the virtual shutter time (𝑡𝑣𝑠) required
for the camera can be calculated from the maximum blur allowed (𝐵max ) and 𝑣𝑠.
(1) Find the angle of the camera (𝛼). This will determine virtual shutter time (𝑡𝑣𝑠), i.e., the IR light pulse width.
The proper lens can be found now. The diameter and focal length must be taken into consideration so that
the 𝑁𝐹 frames are properly focused and with the desired resolution. These are parameters that must be
considered by image quality engineers, and we will not dig into them any further in this thesis. For the
chosen lens characteristics and its settings, the amount of IR light energy that needs to reach the plate to
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get a proper brightness (𝑄𝑒) can be obtained. Once more, finding the proper 𝑄𝑒 for each lens is not part of
this thesis.
(2) Find lens characteristics and setup.
Find IR light energy (𝑄𝑒) in Joules that must receive the license plate to be correctly
exposed for the chosen lens and its settings.
Next, the angle beam for the IR LEDs (𝜃) can be chosen. The highest efficiency will be obtained by
choosing the narrowest possible angle that yields a proper exposure in the whole plate during 𝑁𝐹 frames.
The use of additional optics to modify the beam can help increase the efficiency, the benefits of it should
be evaluated for the final design in terms of cost and energy efficiency gained.
(3) Find minimum LED angle beam (𝜃).
Finally, the number of LEDs to use (𝑁𝐿) and the peak forward current that they should withstand (𝐼𝐿𝐸𝐷)
will lead to the selection of the proper LEDs to use. These variables can be found once the radiated power
that is required to achieve 𝑄𝑒 Joules at the license plate is calculated. However, more than a pair of values,
the result will be a range of possibilities.
Since the virtual shutter time is known, the pulsed current characteristics of the LEDs can be used to our
advantage, pushing the limits beyond the DC characteristics.
(4) Find the number of LEDs (𝑁𝐿) and peak current (𝐼𝐿𝐸𝐷).
Now, we can finish the LED driver design by choosing the right components as explained in Section
2.8.6, while taking into consideration the requirements we have found.
3.5 Peak power reduction
We have already mentioned that the strobing technique helps to significantly reduce the average power
consumption. However, the peak power demand keeps being equal to the constant-light technique one.
Devices powered with PoE have a limited instantaneous power available and, if the PSE detects that any
PD is exceeding its power quota for more than a predetermined time (in the range of 50-75ms) it will cut
its supply for several seconds[11]. Because of this, we conclude that in devices powered by PoE, it is
convenient to avoid high power demand peaks. Instead, systems with constant power demands will be
preferred.
As we mentioned in the introduction of this document, systems composed of devices that have a nearly
constant power demand can afford more simultaneous functionalities or more power demanding features
than those that don’t. In this section, we propose and discuss a solution to achieve this goal.
The analysis that follows evaluates the peak power consumption by comparing it to the average power
consumed. The parameter used is the Peak-to-Average Power Ratio (PAPR) defined as in
𝑃𝐴𝑃𝑅 =𝑃𝑝𝑒𝑎𝑘
𝑃𝑎𝑣𝑒𝑟𝑎𝑔𝑒
𝑃𝑝𝑒𝑎𝑘: Peak power consumed by the circuit.
𝑃𝑎𝑣𝑒𝑟𝑎𝑔𝑒: Average power consumed by the circuit.
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Ideally, the peak power could be reduced as much as the energy consumption (PAPR=1). However, due to
the current limiting circuit non-ideal behavior, the real PAPR will be greater than 1. Initially, the PAPR can
be calculated as in Equation 28.
𝑃𝐴𝑃𝑅 =𝑆𝑃𝐹
𝑡𝑣𝑠= (𝑡𝑣𝑠 · 𝐹𝑃𝑆)
−1 [28]
𝑡𝑣𝑠: virtual shutter time
𝑆𝑃𝐹 =1
𝐹𝑃𝑆: Seconds per frame
𝐹𝑃𝑆: Frames per Second
In Figure 25, a block diagram that illustrates a possible solution to reduce the peak power demand is shown.
The PAPR Reduction circuitry consists of an energy storing block and a current limiting circuitry. The
energy storing block contains enough energy to supply an entire strobe of the IS. While the LEDs are turned
off, the storage elements are charged at a controlled pace determined by the Current Limiting Circuit. As a
result, the PAPR perceived by the power supply is reduced.
Figure 25 Main blocks of a proposed Peak-Power Reduction circuit. The storage elements provide the demanded peak power by the IS when the LEDs are ON and get slowly charged while are not, thus reducing the PAPR.
A simple approach is to limit the current with a resistor and to store the energy into a capacitor (Figure 26).
This solution will be studied in the following paragraphs, where we will present a model of the circuit.
Figure 26 Peak power reduction circuit. The illumination system has been simplified as ‘S1’ and ‘B1’. ‘B1’ is a constant power load that draws 1W, and S1 is a switch that is closed for 2ms and open for 38ms.
First, the capacitor value must be chosen to retain enough energy for a complete light pulse. As a load we
have considered a 30º IR LED (from OSRAM Opto Semiconductors), operating at 350mA of forward
current (𝐼𝐹) and consuming 1W approximately for 2ms. This is a configuration that yields proper brightness,
as seen in Section 0. Thus, the energy that the capacitor must provide (𝐸𝐶) is:
𝐸𝐶 = 𝑃𝐶 · 𝑡𝑉𝑆 = 1 · 2𝑚 = 2𝑚𝐽
𝑃𝐶 : instantaneous power consumed by the LED at 𝐼𝐹 = 350𝑚𝐴.
𝑡𝑉𝑆: virtual-shutter time.
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The maximum capacitor voltage is limited by the 12V supply power source (fixed by specifications), while
the minimum capacitor voltage is fixed by the LED driver circuit. Even though 3V will drop in the LED,
we have decided to add some margin to account for the voltage ripple and increase the transient speed of
the pulses (as seen in Section 2.8.4), for what we decided to be able to provide until 5V. For the LM3406
TI driver circuit, the minimum input voltage (𝑉𝐼𝑁min ) that can provide 5V at the output (𝑉𝑂max ), can be
calculated as:
𝑉𝐼𝑁min =𝑉𝑂max
1 − 𝑓𝑆𝑊 · 𝑡𝑂𝐹𝐹min = 6𝑉
𝑓𝑆𝑊 = 700𝐾𝐻𝑧 : switching frequency of the LED driver (can be changed for the selected driver).
𝑡𝑂𝐹𝐹min = 230𝑛𝑠 : minimum off time of the LED driver (cannot be changed for the selected driver).
Since we expect to achieve a PAPR close to 1, we assume that the current through the resistor during the
capacitor discharging period is neglectable. Considering this assumption, we can simplify the charging and
discharging cycles with two circuits, Figure 27 and Figure 28, respectively.
Figure 27 Simplified peak-reducing circuit during the capacitor charging circuit.
Figure 28 Simplified peak-reducing circuit during the capacitor discharging circuit.
From Figure 27, we can calculate the rise time (𝑡𝐶) of the voltage at the output as in Equation 29.
𝑡𝐶 = −𝑅1 · 𝐶1 · 𝑙𝑛 (𝑉𝑓 − 𝑉𝑆
𝑉𝑖 − 𝑉𝑆) [29]
𝑉𝑓 , 𝑉𝑖, 𝑉𝑆: Final voltage, initial voltage, and supply voltage.
From Figure 28, we can calculate the discharging time (𝑡𝐷) of the capacitor as in Equation 30. We have
derived this equation from the already used one in Section 2.8.4 that describes the energy stored in a
capacitor.
𝑡𝐷 =1
2· 𝐶1 ·
(𝑉𝑓2 − 𝑉𝑖
2)
𝑃 𝑃=1𝑊 → 𝑡𝐷 =
1
2· 𝐶1 · (𝑉𝑓
2 − 𝑉𝑖2) [30]
𝑃: Power consumed by a constant power source (our model for the IS).
Substituting the charging and discharging times into the above equations yields the values of ‘R1’ and ‘C1’
for a desired input voltage ripple at the IS. It is convenient to choose a low ripple input of the IS since the
power dissipated in the current limiting resistor increases with the voltage drop across it. However, the
lower the ripple, the greater the ‘C1’ value.
For example, for a strobe length of 2ms at 25fps, we achieve an input ripple at the IS from 11.4V to 11.8V
with ‘C1=431µF’ and ‘R1=81Ω’. For this configuration, the maximum instantaneous power drawn from
the source is 88mW, which results from multiplying the maximum voltage drop and current at the limiting
resistor. Because of this, we can conclude that we have reduced the peak power by a factor of 11.36. In this
situation, the average power supplied to the circuit is 58mW, which is 8mW higher than the initial one.
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4. Prototype
4.1 Overview
The results presented in this chapter refer to the prototype built. The power consumption and performance
of the prototype were evaluated to prove the theoretical conclusions we made in previous sections. In
Section 0 we present the results obtained with the prototype in an LPR scenario.
The implemented IS prototype was integrated into the DUT, the main blocks that needed to be created or
modified are shown in Figure 29.
Figure 29 Prototype block diagram.
In Figure 29 two types of flow are shown: data-flow and control-flow. The data flow is generated at the
image sensor and contains the information regarding the scene being recorded; then it is transmitted to the
serializer component, which acts as a bridge to the processing unit. The control flow is generated in the
processing unit and the LED driver. The processing unit configures the image sensor parameters (II) and
sends the information for the serializer to know when to generate the strobing signal (I). When the serializer
detects that the strobe signal must be generated, it drives the LED driver control signal (III) for the duration
of the strobing pulse. Finally, the LED driver turns on/off the LEDs (IV) according to the received control
signal from the serializer.
Generating the signals from the serializer was found to be a convenient decision for the prototype for two
reasons: the output pins can be easily accessed, and the data from the sensor has a neglectable delay. Neither
of these conditions is true for the processing unit. In products where the processing unit handles the data
from the sensor directly, it could also control the LED driver. However, special care should be taken for its
processing delay.
4.2 Image sensor
No modifications had to be made to the image sensor. However, understanding its behavior has been a
primary source of complexity for the implementation of the other modules. The control flow from the
processing unit to the image sensor (II) was kept unmodified. To change the sensor parameters the camera´s
user interface was used.
4.3 Serializer: Strobe signal generator
The serializer is a PLD that receives the data stream from the image sensor and needs to correlate it with
the shutter operation. This can be done if the sensor’s configuration is known, as it shown in Figure 30.
Both WFS and STS techniques can be implemented in the serializer with just one parameter from the
processing unit, the ‘HOLD_STB’. This parameter tells the serializer how long does it have to hold the
strobing signal low before turning it on again (in ‘H’ units).
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For the STS technique, the LEDs need to be turned on at the beginning of the shared time. The serializer can
know when to do it by waiting the amount of ‘H’ units stored in ‘HOLD_STB’. The calculation of the
‘HOLD_STB’ value is made in the processing unit since it has all the data regarding the sensor configuration.
For the WFS technique, the process is analogous, but with a different ‘HOLD_STB’ value (Figure 30 does
not represent that case).
In the STS, the finalization of the shared time is always aligned to the beginning of the data lines, i.e., the valid
lines containing useful information. Therefore, when the serializer detects a valid data package, it turns off
the LED driver control signal. Whereas, in the WFS, the serializer control signal must be turned off at the
end of the valid data stream.
Figure 30 Variables involved in the strobing signal generation for the STS technique.
The hardware design of the serializer has been designed and implemented in collaboration with the master
thesis in [10]. In this master thesis report, we will describe in detail the WFS hardware architecture, while
the STS will be described in [10].
In Figure 31 we show a simplified hardware architecture of the strobing signal generator inside the serializer.
‘BL’ and ‘DL’ are both clock signals of 1H period. However, ‘BL’ is active during the blanking lines interval,
whereas ‘DL’ is active during the data lines interval. ‘HOLD_STB’ is the value transmitted by the processing
unit, stored in the PLD I2C registers, and ready to be used by the logic in Figure 31. The strobe signal
generator output is the ‘IR_STB’ signal that controls the LED driver.
Figure 31 Simplified hardware architecture of the strobing signal generator (‘IR_STB’) of the serializer.
Depending on the strobing technique (STS or WFS) and the switching technique (series or shunt) being
used, a different combination of blocks is generated for the strobe signal generator:
- STS Block: This block is always added. It contains the logic that makes the strobe signal generator
wait according to the ‘HOLD_STB’ parameter (rising edge generation) and the logic that waits for
the beginning of the data line period (falling edge generation).
To generate the rising edge, ‘HOLD_STB’ is compared with the number of times that ‘BL’ has
switched, which are stored at the output of the flip-flop. When the number of blanking lines exceeds
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the ‘HOLD_STB’ value, the comparator output goes high, indicating the beginning of the shared
time.
The falling edge is generated when the first data line is detected, which resets the flip-flop, turning
the output of the comparator to low.
- WFS block: This block is added only when the WFS technique needs to be applied. From an
implementation point of view, the WFS technique differs from the STS technique in the falling
edge generation.
The rising edge generation still follows the same behavior as the STS technique and thus, the same
hardware can be used. Note that the processing unit needs to generate different ‘HOLD_STB’
values depending on the technique used.
The falling edge generation is different than in the STS technique since the light pulse must be on
during the data line interval. To achieve this, a NOR gate detects when the counter is at ‘0’ (data
line interval) and sets the output high.
- Inverter block: This block is added depending on the logic of the output signal. When the series-
switching is used (positive logic), it is not added, whereas when the shunt-switching technique is
used (inverted logic) it must be added.
4.4 Processing unit
For what concerns the implementation of the prototype, the processing unit was only required to send the
necessary information to the serializer so that the strobing signal could be generated. The information was
sent through an available I2C bus that was already interconnecting both modules. This procedure involved
configuring new registers in the serializer to deal with the new I2C packages.
4.5 LED Driver
The LM3406 LED driver from Texas Instruments [12] was chosen for the implementation of the prototype
(Figure 32). The reasons that led to this decision are:
- Wide input voltage: The input voltage can range from 6 to 42V. Therefore, the LM3406 allows
not only to test the target 12V input voltage but also other cases.
- Comprehensive datasheet: The LM3406 and its evaluation board have comprehensive datasheets
that include useful information for the design and performance of the LED driver. The evaluation
board application note contains captures for the rising and falling edges, which gave us the certainty
that the LED driver would fulfill our requirements.
- Selectable output current: The LM3406 evaluation board allows to configure the output current
with ease. The selectable values are within the range of interest of this project (350mA to 1.5A).
- Two available dimming techniques: The LM3406 evaluation board includes the two types of
dimming techniques that we wanted to test: series and shunting techniques.
Figure 32 LM3406 Evaluation Board simplified schematic (based on the one provided in its datasheet [12]).
Figure 32 shows the schematic of the LM3406 Evaluation Board. The most relevant signals and components
are the following ones:
- Vin: Input voltage of the LED driver.
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- Vo/LED+, CS/LED-: Output pins to the LED load. The voltage sensed at ‘CS/LED-’ is used for
the feedback current sensing.
- D1, L1, Co: Recirculating diode, inductor, and capacitor of the buck-converter.
- Q1: Shunting-technique transistor (Section 2.8.5). This transistor is controlled by the ‘DIM2’ signal.
- DIM1: Series-switching technique control signal (Section 2.8.5).
- R.35, R.7, R1, R1.5: Output current control resistors.
- Ron: Resistor that sets the switching frequency.
4.6 LED board
The LEDs used for the prototype were mounted on a board and placed next to the lens. The selected LEDs
are from OSRAM and have 30º beam angle. The beam angle selection was done according to the LED
driver design guideline presented in Section 3.4. This beam angle yields an illuminated area of 5,35m of
diameter when the camera-to-vehicle distance is 10m, which fits the LPR test scenario.
4.7 Energy saving algorithms
This section covers several algorithms that aim to increase the energy efficiency of the IS by using the STS
technique. For this purpose, the integration time and other parameters of the image sensor configuration
will be changed to fit the light pulses inside the shared time. We call the initial integration time set by the
system Ti, and Ti’ to the new integration time that allows to fit a light pulse that yields the same brightness.
The algorithms are particularized for the usual operation mode of the camera: 25 frames per second at 60
frames per second data rate. This means that the sensor is configured at 60 frames per second, but the frame
rate is decreased to 25fps by using the long-exposure mode. The result of this configuration is shown in
Figure 33. Note that, as we predicted in Equation 8, the long-exposure mode has considerably increased the
shared time.
Figure 33 Time dimensions of a frame in H units.
In this mode, the ‘H’ unit (𝐻@60𝑓𝑝𝑠) can be calculated as:
𝐻@60𝑓𝑝𝑠 =
1601125
= 14.81𝜇𝑠
Energy saving algorithm #1
The first algorithm proposed aims to increase the energy efficiency at the cost of a frame rate drop (due to
the application of the EIT technique). This algorithm does not increase the power demand when compared
to the constant light algorithm. We can see the algorithm in Figure 34.
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Figure 34 Flow of the energy saving algorithm. This algorithm aims to generate a Ti’ from the initial integration time (Ti) to fit a pulse of light that yields the same illumination.
When the integration time set by the system is greater than 2565H, the algorithm keeps the constant-light
illumination technique. This is because the efficiency for those integration times is above 95% (Equation
1). For integration times in the range of 1589H and 2565H the shared time is not long enough to fit the light
pulse, hence the LET is used, which results in a frame rate drop. Finally, when the integration time is equal
or less than 1589H, the available shared time is enough to fit the light pulse. Therefore, Ti’ is kept at the
maximum value, and the light pulse width is value is set to the Ti value.
The minimum frame rate (𝑓𝑝𝑠𝑙𝑜𝑤 at 2564H) can be calculated as:
𝑓𝑝𝑠low = ((2564 + 1110) · 𝐻@60𝑓𝑝𝑠)−1= 18.37𝑓𝑝𝑠
Energy saving algorithm #2
The Energy saving algorithm #2 aims to achieve the same energy efficiency as #1 but avoiding the frame
drop. Therefore, in the 1589H to 2565H range, instead of using the strobing technique, it uses the constant-
light technique with analogue dimming.
For this purpose, the LED driver circuit must be able to analogically dim the current down to a 58% of the
initial one (1589H/2700H). This dimming factor allows to achieve the same brightness with constant-light
illumination and maximum Ti, as with constant-light illumination without dimming and Ti=1589H. The
advantage is that the efficiency is kept virtually at 100%.
An energy saving algorithm which only relied on analog dimming, i.e., the integration time is kept maximum
and the brightness is changed only by increasing or decreasing the dimming factor, presents several issues
that make it not suitable for our application:
1. Long integration times yield blurry images.
2. A very high dimming factor is required for short integration times (98% at 500µs). This dimming
factor increases the complexity of the LED driver and decreases the maximum current ripple
allowed at the output.
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5. Results
5.1 LPR image examples
In this subsection, the results from a real use case for the strobing light technique are shown: the LPR
scenario. Three different images are shown that belong to three different videos of the same car traveling at
the same speed (110 kmh). The configuration of the camera is the same for the three videos, except for the
IS strobing configuration. In Figure 35, the STS technique is applied, in Figure 36 the constant-light
technique, and in Figure 37 the WFS technique. To perform this experiment, the camera was placed on a
bridge on top of a highway and the same car was driven at the same speed repetitively under it. The area
was chosen to be as dark as possible, so that the only source of light was the IS LEDs. The camera, IR
illumination system, and test equipment were powered with a portable battery.
It can be observed that the brightness in all the images is the same since the current pulse provided to the
LEDs yielded the same amount of luminous energy per row. However, the power consumed in each case
was measured to be 100mW (STS), 0.6W, and 1.1W (see Section 5.2 for more details about the power
consumption measurements). Therefore, it can be concluded that the strobing technique achieved a
significant average power reduction while maintaining the same image quality.
Figure 35 One frame of a video from an LPR scenario with strobing-light technique (STS).
Figure 35 (STS) differs from the other captures from the other lighting techniques (Figure 36 and Figure
37) in the perceived brightness of the headlamps of the vehicle and the amount of details captured in the
road. As we introduced in Section 2.7, this is due to the superior ambient light sensitivity of this technique,
which is a result of its longer shutter times after applying the EIT technique. In the LPR scenario, this
effect is mostly detrimental for two reasons: firstly, the headlamps look much brighter and thus reduce
the contrast between the license plate and its surroundings, making it more difficult for the LPR algorithm
to decode the characters on the plate; secondly, other external lights might illuminate the plate (e.g. the
sun at dawn) and produce additional blur during the whole integration time. As it is discussed in [10], the
former effect is the main concern of the EIT technique, while the blur produced by the later effect is
usually too dim to have a significant impact on the quality.
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Figure 36 One frame of a video from an LPR scenario with constant-light technique.
Figure 37 One frame of a video from an LPR scenario with strobing-light technique (WFS).
The results in Figure 36 and Figure 37 are completely identical. This is due to the fact that the WFS technique
does not change any parameters of the image sensor configuration but only modifies the light strobing
pattern. However, as we predicted in our theoretical study, the energy consumption improvement of the
WFS technique is much lower than the STS one.
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5.2 Power consumption results
In this section we present the measurements of the power consumed by the LED driver when using the
Energy Saving Algorithm #1. The average power consumption of the strobing technique is compared to
the constant-light power consumption, and the ideal power consumption. The ideal power consumption is
a reference value calculated by considering the LED power consumption as the only source of power
dissipation.
The power measurement setup is shown in Figure 38. The power was measured by measuring both the
voltage and the current at the IS with an HRO 66zi LeCroy oscilloscope. To measure the current, a AP015
current probe was used (50MHz bandwidth and 10mA/div sensitivity). Then, the values of 1000 frames
were averaged. The camera modes were configured through and external computer.
Figure 38 Power measurement setup. The voltage at the input of the IS and the current it consumes are measured during 1000 frames and averaged to obtain the average power consumption per frame. The camera is configured and powered through a computer via an ethernet cable.
In Graph 3 and Graph 5 the average power consumption of the series and shunting technique are shown,
respectively. As we can see, the shunting technique values differ from the ideal curve, since the driver
consumes a significant amount of power during the off-state of the LED. Graph 4 and Graph 6 show the
power improvement achieved by the series and the shunting technique, respectively. The series-switching
technique achieves 96% of improvement, while the shunting technique only 62%.
Some relevant power consumption improvement values for the LPR scenario are shown in Table 1. We
have chosen 500µs, 1000µs, and 2000µs since they are typical values for LPR mode recording.
Integration
time (Ti) [µs]
Series-switching
technique
Shunt-switching
technique
500 95% 60%
1000 94% 59%
2000 91% 58%
Table 1 Measured average power consumption improvement of the strobing-light technique when compared to the constant-light technique.
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Graph 3 Average real power consumed by the LED driver circuit when using the Energy Saving Algorithm #1. Series-switching is being used.
The maximum difference between the ideal and measured powered consumption is found to be 100mW,
which is due to the non-idealities of the components of the LED driver circuit (Graph 3). Even when we
consider these non-idealities, as can be seen in Graph 5Graph 4, the efficiency improvement is significantly
improved. For an LPR scenario, in which the strobe length is below 135H (2ms), the efficiency improvement
is always greater than 86%.
Graph 4 Average real power consumption improvement between constant and strobing-light technique (Energy Saving Algorithm #1). Series-switching is being used.
When the shunting technique is used, the ideal and measured power consumption differ significantly. The
greatest difference is found to be 0.6W in the shortest integration time setting. This is because this setting
has the longest deadtime, i.e., has the greatest losses due to the shunting technique.
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Graph 5 Average real power consumed by the LED driver circuit when using the Energy Saving Algorithm #1. Shunt-switching is being used.
As a result, the power improvement is so adversely affected that it might not be useful to use the shunting
technique. The major drawback is that it is during short integration times that the losses are more significant
(Graph 6).
Graph 6 Average real power consumption improvement between constant and strobing-light technique (Energy Saving Algorithm #1). Shunt-switching is being used.
5.3 Edge times results
In this section we present the measured rising and falling edge times of the LM3406 for our prototype
configuration. The measured values will be compared to the estimated values obtained using the equations
derived in Section 2.8.4. From the edge times measurements, we can conclude that buck-converter
topologies are suitable for strobing-light LED driver circuits.
In Figure 39 the results obtained for the series-switching are used (‘DIM1’ switching according to the LED
driver evaluation board). For the prototype component values L=33µH, C=470nF, VF=1.9V VSW=12V, the
measured and calculated values are collected in Table 2.
Series-switching Shunt-switching
Rising
Edge
Measured 1.3µs 920ns
Calculated 1.04 µs (Eq. 15, K=1.37) 710ns
Falling
edge
Measured 13.6µs 100ns
Calculated 13µs (Eq. 10) -
Table 2 Measured and calculated values for the edge times in the prototype.
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Figure 39 Current pulse through one LED driven by the LM3406 (series-switching technique).
Figure 40 Current pulse through one LED driven by the LM3406 (shunt-switching technique).
For this experiments, the current through the LED was measured with an AP015 current probe, like the
one used for the power consumption measurements.
6. Conclusions and future work
In this thesis we have presented an analysis from the power consumption and energy efficiency perspective
of the strobing-light technique. From the analysis we have been able to propose two energy saving
algorithms applicable to nighttime recording. We identified the LPR scenario as a potential candidate for
this technique, for which we have proposed an LED driver design guideline that relates the physics-related
requirements with an optimal LED driver design. We also addressed the most important parameters to
consider when designing a buck-converter LED driver circuit for this technique. A prototype was
implemented to demonstrate the applicability of this technique in an LPR scenario, for which we have been
able to prove its advantages when compared to constant-light illumination. Finally, from the photobiological
safety point of view, the strobing-light technique has been found to be beneficial.
In Chapter 2, we performed an in-depth analysis and proposed equations for the energy improvement of
the strobing-light technique when compared to the constant-light technique. For this analysis, two different
types of strobing-light techniques were considered: STS and WFS. Thanks to this evaluation, we could
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explain why short integration times yield the best energy efficiencies when STS is applied. We also proposed
the EIT and LET techniques, which allow us to apply the strobing technique for any desired integration
time. However, we saw that for long integration times, it might be more convenient to illuminate constantly
since the energy efficiencies are already high.
Later in this chapter, we discussed an optimal flow for a buck-converter LED driver design that could fulfill
the requirements extracted from our previous theoretical analysis. The discussion focuses on how to achieve
the edge times, forward current, and ripple constraints of the LED driver. We concluded that buck-
converter topologies are suitable for the implementation of strobing techniques.
Chapter 3 does an in-depth study of the strobing-light technique for the LPR scenario. This scenario has
been identified as a potential candidate for this technique due to the short integration times required, which
maximizes the amount of energy saved by the STS technique. In this chapter, we correlate motion blur and
image brightness in a specific LPR scenario to the LED driver design, so that the optimal specifications can
be found. This analysis led us to the conclusion that, placing the camera at an angle subtended by the vehicle
and its own position of 45º yields the optimal power consumption. We also identify the need of using a
peak-power reduction circuit for peak-power consumption limited systems, and we propose a circuit suitable
for the LPR scenario.
Finally, in Chapters 4 and 5, we described the implemented prototype and showed the obtained results in
an LPR scenario. These results prove that the strobing-light technique can significantly reduce the power
consumption of the cameras while keeping the same output image quality. In the tested scenario, we
achieved efficiency improvements of more than 95% with the STS technique when compared to the
constant-light technique. For the WFS technique, we could reduce 55% of the energy consumption,
however the output image quality was exactly the same as in the constant-light technique. Instead, we
observed that STS increases ambient light sensitivity, which reduces the contrast between the plate and its
surroundings, and might be a source of blur.
For future work, we propose to evaluate the interaction between several strobing-light cameras. In that
situation, all the cameras’ shutter times must be synchronized with the emitted pulses of light. Otherwise,
as we saw in the introduction, different pixel-rows in the same frame might be subjected to different amount
of light levels.
From the electronic design point of view, peak-power reducing circuits need to be more carefully studied.
In this report, we proposed a circuit based on a resistor that acts as a current limiting device, and thus
dissipates part of the power. For the evaluated LPR situation, due to the short length and low current pulses,
the dissipation is not very significant, but it could become for more power demanding scenarios.
We also propose to study the noise reduction that can be achieved because of the temperature reduction in
the camera when the efficiency is improved by the strobing-light techniques. This has been presented as an
advantage as the temperature-noise relation is well known to exist; however, the effects on the camera
performance could be studied to determine their importance.
Although we presented an LED driver design guideline, it does not address how to choose the
components to optimize its consumption. The optimal LED selection, load configuration (series or
parallel), and optimal point to drive the LED, are amongst the main points to consider that could be
added to the guideline.
Our analysis on the electronic circuits refers only to buck-converter topologies due to the requirements of
this project. However, similar strobing-light projects could require other topologies, for which the
guidelines should be re-evaluated and written. Also, switching LED drivers are not the only choice. More
cost-effective solutions could be addressed in the future.
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Master thesis, KTH Royal Institute of Technology, 2018. [11] IEEE, “IEEE Standard for Ethernet,” IEEE, Technology standard IEEE 802.3-2015, Mar. 2016. [12] Texas Instruments, “LM3406 1.5-A, Constant Current, Buck Regulator for Driving High Power
LEDs,” Technical Datasheet, Sep. 2008.
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Appendix A: The ‘H’ unit
The ‘H’ unit is defined as the time required to read a whole pixel-row of the image sensor. Thus, faster
readout rates yield shorter ‘H’ values, and vice versa. We can calculate the frame period by multiplying the
total number of rows times the 'H' value. Note that, the total number of rows does include not only those
that contain useful information (valid rows) but also those that do not (blanking rows). This unit was
adopted from the DUT image sensor datasheet, as it eases the analysis of shutter timing diagrams and can
help future designers to directly implement strobing-light solutions for all the cameras that use the same
sensor. However, when another sensor whose shutter timing is not described in ‘H’ units needs to be used,
the analysis, discussion, and conclusions made in this report will still be valid.
At ‘fps’ frames per second and for the selected DUT image sensor (1125 pixel-rows in total), the 'H' unit
can be calculated as in Equation 31. The equation can be generalized for any total number of rows (R) as in
Equation 32.
𝐻 =1
𝑓𝑝𝑠 ∙ 1125 [31]
𝐻 =1
𝑓𝑝𝑠 ∙ 𝑅 [32]
However, we can increase the total number of rows per frame by adding more blanking lines, which reduces
the value of the ‘H’ unit for the same frame rate. In the image sensor's datasheet, this is referred to as the
Long-Exposure Mode and implies that:
- we are extending the amount of available integration time per row.
- we are decreasing the frame rate.
These effects can be seen in Figure 41. From the figure, we can also observe that the available shared time
is dependent on the number of blanking rows (B), as we express in Equation 33.
𝑇𝑆 = 𝐵 − 1 [33]
Figure 41 This example shows how the maximum available integration time can be extended by adding blanking lines. At the same time, we can see how adding blanking lines implies a longer shared time.
Some of the techniques we discuss in the report rely on the available amount of shared time. Therefore, it
is relevant to mention that to maximize it while keeping a specific frame rate and level of brightness, i.e.,
the same integration time, it is the best choice to select the minimum 'H' unit and add the amount of blanking
lines that yield the desired integration time. Figure 42 shows how short ‘H’ units help increasing the shared
time.
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Figure 42 This example shows how the shared time can be increased by selecting a short 'H' unit. The example shows 2 different shutter patterns with the same frame period. However, the bottom one, has more available shared time since it has a shorter 'H' unit.
TRITA-EECS-EX-2018:449
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