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Slide 1
Overview of Modern Imaging Sensors: Proper Uses for CCD, EMCCD,
and CMOS Cameras
Slide 2
Charge-Coupled Device (CCD) Introduced in 1969 (Scientific
Imaging Standard for 40 years) Electron Multiplying Charge-Coupled
Device (EMCCD) Introduced in 2001 (Scientific Low-Light Imaging
Standard for 10 years) Scientific Grade Complementary Metal Oxide
Semiconductor (CMOS) Introduced in 2010 (Beginning to be
Adopted)
Slide 3
Same Basic Pixel Architecture e-e- e-e- e-e- e-e- e-e- e-e-
e-e- e-e- e-e- e-e- e-e- e-e- e-e- Electrical Connection Potential
Well Silicon Silicon Dioxide Polysilicon Gate Incident Light Light
when Back-Illuminated Peak Efficiency when Front-Illuminated: ~65%
CCD or ~57-65% scientific CMOS Efficiency when Back- Illuminated:
~95%
Slide 4
CCD Collecting Photons: Analogous to Catching Raindrops
Slide 5
Slide 6
Slide 7
CCD Camera Components Photons Electrons Electrons Voltage
Serial Register Gain A/D Converter Sensor Custom Electronics
Parallel Register Clocking and Other Control Electronics
Slide 8
CCD Image Acquisition Types Frame Transfer CCD Interline CCD
Full Frame
Slide 9
Standard Readout Amplifier & ADC Normal Voltage Serial
Register High Voltage Serial Register EM Readout Amplifier &
ADC EMCCD Cameras Back-illuminated, frame-transfer CCD (for >90%
QE) or standard front-illuminated (~65% QE) High Voltage EM
register multiplies signal electrons by up to ~1000x BEFORE
readout
Slide 10
EMCCD Cameras Normal Voltage Serial Register High Voltage
Serial Register Back-illuminated, frame- transfer CCD (for >90%
QE) or standard front- illuminated (~65% QE)
Slide 11
Photons Electrons Voltage CMOS Sensor Components Photosensitive
Diode Amplifier Photosensitive Diode Amplifier Photosensitive Diode
Amplifier Photosensitive Diode Amplifier Row Select Column Select
Electronics Built into CMOS Chip
Slide 12
CMOS Sensor Readout Modes Global ShutterRolling Shutter
Slide 13
In rolling shutter mode, each row starts and ends an exposure
at a different point in time, not simultaneously as in global
shutter mode. If an imaged object is moving, different parts of it
will be captured at different times by the rolling shutter, causing
possible distortion if object moves quickly enough. The distortion
will depend on which direction the sampled object is moving
relative to the rolling shutter direction. If rolling shutter
captures in this direction, Arrows Indicate Movement Direction
Observed object
Slide 14
CMOS Sensor Readout Modes Global Shutter Rolling Shutter (More
Common)
Slide 15
How to quantitatively compare performance: Signal-to-Noise
Ratio (SNR) Increasing SNR Larger SNR = easier to distinguish image
from noise = higher confidence in measurements Noise: Fluctuations
in signal that produces uncertainty in the signal
Slide 16
CCD Primary Noise Sources
Slide 17
EMCCD Primary Noise Sources
Slide 18
CMOS Primary Noise Sources
Slide 19
SNR Equation: CCDs
Slide 20
SNR Equation: EMCCDs
Slide 21
SNR Equation: CMOS
Slide 22
CMOS Random Telegraph Noise (Salt-and-Pepper Noise)
Slide 23
CMOS Random Telegraph Noise: Why it Happens Correlated double
sampling involves a reference being subtracted from the sample to
remove drift. This works great if the reference and sample are at
the same signal level but will cause a positive (bright pixel) or
negative (dark pixel) whenever the reference is at a different
level. a Normal signal Low signal due to e- trapped in pixel defect
SHR = Reference signal SHS = Sampled signal
Slide 24
Skewed CMOS Read Noise Compared to Gaussian CCD Read Noise 3%
outside of Gaussian fit 40% outside of Gaussian fit The CCD has a
read noise distribution close to Gaussian The CMOS read noise
distribution is skewed to much larger values due to the noisy
pixels (RTN). It is skewed from Gaussian.
Slide 25
Skewed Distribution of Read Noise: How does it behave compared
to Gaussian noise? Scientific CMOS ChipBias StackSubtractedCenter
Quadrant # Frames AveragedStdev Noise Reduction Expected Noise
Reduction%Difference 13.347645 = SQRT(Frame #) 41.6946790.510.501
91.1538240.340.333 160.8872850.270.256 250.7324520.220.209
360.6313510.190.1713 490.5620650.170.1418 Traditional CCD # Frames
AveragedStdev Noise Reduction Expected Noise Reduction%Difference
125.051838 = SQRT(Frame #) 412.4962250.4990.5000.2
98.3157340.3320.3330.4 166.2350330.2490.2500.4
254.9850570.1990.2000.5 364.1592810.1660.1670.4
493.5668150.1420.1430.3 Averaging frames is less effective in
reducing noise for CMOS. What it means: A researcher will need to
acquire more data with a CMOS (compared to CCD) to decrease the
error bars by the same amount for low light images.
Slide 26
Binning (in CCDs and EMCCDs) Binning provides the ability to
combine pixels into a larger super-pixel before digitization of
data. Boosts signal detection capability by increasing effective
pixel size Traditional Analog Binning Signal is combined before
digitization only 1x RN is applied For 2x2 binning: 4x pixels of
signal, 1x RN gives a 4:1 boost in signal to noise ratio
Slide 27
Binning in Scientific CMOS Binning with the scientific CMOS
sensor is slightly different when compared to CCDs. In CMOS
cameras, binning is applied after readout. So, read noise has
already been introduced to each individual pixel before combining
in software. Signal is combined after digitization and after RN has
been applied to each pixel. Adding 4 pixels together reduces 4x RN
to 2x RN. 4x pixels of signal, 2x RN gives only a 2:1 boost in
signal to noise ratio
Slide 28
Advantages of conventional CCDs: - Flexibility Conventional
CCDs (e.g. ICX 285) allow for greater flexibility and general
purpose imaging than sCMOS or EMCCDs. Available in a variety of
pixel sizes and formats Available in a wide variety of price points
Accessibility of flexible binning Availability of deep-cooled, very
low dark current variants
Slide 29
Advantages of EMCCDs: - Low-light fast EMCCDs offer the
greatest possible sensitivity, with accessibility of very high
speeds Available in back-illuminated frame transfer formats EM
multiplication offers the lowest effective read noise Availability
of binning Availability of deep-cooled, very low dark current
variants Generally at higher price points
Slide 30
Advantages of sCMOS cameras: - high speed, high resolution
sCMOS sensors allow for higher speed operation with similar noise
performance to conventional CCDs sCMOS readouts allow for high
speed operation (>30 fps) with large(er) pixel arrays than EM or
conventional CCDs Potentially lower (different ?) noise than
comparable ICX285 CCDs Moderate price points Novel technology with
room to grow
Slide 31
Summary Conventional CCDs flexible, workhorse Well established.
Good combination of speed, sensitivity and resolution EMCCDs low
light (fast) Offers the ultimate sensitivity coupled with high
speed when needed sCMOS Emerging technology, which offers high
speed large format images