Lecture one. Displays classified by technologies Cathode ray tubes (since 1900) Flat panel displays (FPD) Emissive Plasma display panels (PDP) Vacuum.

Lecture one

Displays classified by technologies

Cathode ray tubes (since 1900) Flat panel displays (FPD)

• Emissive• Plasma display panels (PDP)• Vacuum fluorescent displays (VFD)• Electroluminescent displays (ELD)• Light emitting diodes (LED)• Organic LED (OLED)

• Non-emissive (needs backlight or front light)• Liquid crystal displays (LCD)• E-ink, electrophoretic• Electro-wetting• MEMS (DLP, reflective grating)

Displays classified by driving techniques

Direct drive, simple multiplex• Segment displays

Graphics displays (dot matrix)• Passive matrix• Active matrix, high resolution

Displays classified by viewing

Direct view Projection Head mounted displays Holographic displays 3D displays

Early FPD market

FPD market now

Display market predictions

Hong Kong LCD market

HK LCD Production

0

1000

2000

3000

4000

5000

86 88 90 92 94 96 98

Year

Val

ue in

M$

Glorious history: Hong Kong had 20% of world LCD market in 1996

Hong Kong companies

Major companies: Truly (732), JIC Nam Tai (Nasdaq), Yeebo (259) and Varitronix (710)

Truly 2006 turnover = US$580M Varitronix 2006 turnover = US$148M Nam Tai turnover = US$250M Yeebo 2006 turnover = US$58M Total ~1% of world market. Used to be 20%

Observation

Semiconductor IC market is about US$250B Display market is about US$110B Every university has a semiconductor IC program Only a few universities have a comprehensive

display program The situation is changing. Many universities are

trying to establish display programs

Displays: major subfields

Ultimate display: flexible, colorful, light weight, low power, 3D etc etc

Ultimate displays

Thin film transistors: materials, device physics

Materials science, nanotechnology, manufacturing technology

Display modes: LCD and OLED science and technology

Video technology : drivers, signal processing, circuit design

Attributes of a good display

Low cost (capital cost and operating cost) High brightness (>300 nit) Large contrast ratio of at least 1024:1 (10 bit)  (cf: printed paper = 8:1) Lots of gray scales (8 bit) Large viewing angle (180o ideally) Excellent color saturation (100% NTSC color gamut) Large size/weight ratio Safe (no electrical hazard, radiation hazard) Low/no power consumption Flexible / rollable / foldable / durable

Common specs of LCD TV

• Resolution = 720p, 1080p• Brightness = 500 nit• 100% NTSC color gamut• Contrast ratio = 10000:1• Display size = 42” (diagonal)• Viewing angle = 170o x 100o

What are these unit?

Display resolution - monitors

CGA (Color graphic array) 320x240 VGA (video graphic array) 640x480 SVGA 800x600 XGA 1024x768 WXGA 1280x768 SXGA- 1280x960 SXGA 1280x1024 SXGA+ 1400x1050 UXGA (QVGA) 1600x1200 QXGA 2048x1536 QSXGA 2560x2048

Pixel = square elements of the display matrix

Display resolution - TV

PAL (Phase alternating by line) 625 lines interlacedused in Hong Kong and Europe

NTSC (National television system committee)used in USA 525 lines interlaced

HDTV :Format 1 (720p) 1280x720 progressiveFormat 2 (1080p) 1920x1080 progressive2kx4k (newest)  TV is going from analog to digital. Aspect ratio goes from 4:3 to 16:9. TV and data terminals are (not) merging.

Moore’s Law for displays? Moore’s law for semiconductors: number of transistors doubles

every 18 months in IC Moore’s law for displays? Total number of pixels? Improvement in quality such as viewing angle and color gamut

may not be describable by Moore’s law EGA VGA SVGA XGA SXGA UXGA QXGA … About 10x in 10 years, or 2x in 3 years

Tiling

10000

100000

1000000

10000000

100000000

1000000000

1980 1985 1990 1995 2000 2005 2010

Ergonomics of resolution

Ultimately, we want a smooth display Smoothness is related to sharpness of the

human eye Human eye has a resolving power of 0.15

mrad Angle is measured in radian = size/distance Typical viewing distance = 60cm. Therefore, a

1mm circle will sustain an angle of 1/600 rad or 1.67 mrad

Human vision There are 120M rod cells and 6-7M cone cells. Rods are sensitive

to light but not to colors - responsible for night vision. Rods are totally saturated in day vision

Cones are separated into RGB types and are responsible for normal vision and to provide high spatial resolution

Focal length of human eye (combining cornea and lens) is 17mm when relaxed.

It is a fantastic design: Much reduced signal transmission bandwidth and signal processing capacity needed for the brain

Optics of the eye

170000 cells per mm2 at center, corresponding to a spacing of about 2.5 m

Focal length of eye lens is 17mm. Thus resolution of the eye is thus 2.5/17000 or about 0.15 mrad and drops off rapidly to the sides

On the other hand, resolution of the eye lens is given by diffraction theory

Pupil = 3mm; thus a = 2.44 x 0.45 x 17 / 3 = 6.2 m Resolution of human lens = a/2f = 0.18 mrad The lens and the retina in the eye are perfectly matched – intelligent

design!

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-400 -300 -200 -100 0 100 200 300 400

x (degrees)

Sin

c(x)

df

442a

.

Apparent size of an object is determined by the angle it sustains at the eye. Resolving power is given in terms of angle, not absolute size

Calculation of display resolution - monitor

If we want the display to be smooth, we want the pixel to be <60cm x 0.0002 rad = 0.12 mm

What is the pixel size for a 17” diagonal LCD monitor with SXGA resolution?

d = 17 x 25.4mm x 4 / 5 / 1280 = 0.27mm This is 2.2x the requirement for smooth images Equivalent to 25.4/0.27 dpi or 94 dpi That is why UXGA or even higher resolution is

needed for monitors (Note dpi for printing has different meanings due

to RGBY subpixels)

Calculation of display resolution - TV

If we want the display to be smooth, we want the pixel to be 2m x 0.0002 rad = 0.4 mm

What is the pixel size for a 40” diagonal LCD TV with 1080p resolution?

d = 0.4mm Just right

Apple’s retinal display

i-Phone 3 (320x480) i-Phone 4 (640x960) is called retinal display. It has a pixel

density of 326 dpi or ppi (dots per inch or pixels per inch). It corresponds to a subpixel size of 78mx26 m – requires very small transistors on the pixel in order to have reasonable aperture ratio

i-Phone 5 (640x1136) also has 326 dpi Suppose we view the retinal display at 12” away, each pixel

sustains 25.4mm/326/305mm=0.26mrad Best human eye resolution is 0.15mrad Not quite retinal! Samsung S5 has 432 dpi (after subpixel rendering, thus not

real). HTC One has 469 dpi also not a real pixel density

Viewing angle – solid angle

Solid angle = angle sustained on a sphere

)cos(ddsind

r

A

12

0 0

2

0

2

0

2

where = cone angle, unit = steradian (sr)

means entire half sphere

means entire sphere

For FPD, the max viewing cone therefore is

Note analogy with a plane

Display optics - light

Light is electromagnetic waves, physically the same as radio waves, except that it can be seen

Waves = oscillating electric field and magnetic field

Intensity (brightness, Poynting vector)• e.g. intensity of bright sunlight is 1000W/m2.

Color (wavelength, frequency)• Light is usually given in wavelength (nm)

Polarization • Direction of oscillation of electric field• Ordinary light source is not polarized due to random orientation

of many many many sources

n

c

3 important properties of light

Light

Light is a plane wave

Intensity (I), color () and polarization (E) completely defines the wave

2

rkjtjo

E8c

I

2k

eEtrE

,

W/m2

Photometric units

LC TV brightness = 300 or 500 nit (what is nit?) Radiometry – absolute units (W, W/m2,…) Photometry – units are related to human

perception (lumens, nit, lux,…) Wavelength (chrominance, color coordinate) Intensity (luminance)

Photometry

Measurement of light as perceived by human Brightness is measured in cd/m2 (=nit) Output of a light source is measured in lumens

(lm) Efficiency of light source is measured in lm/W E.g. incandescent light bulb = 15 lm/W,

fluorescent tube = 70 lm/W. Best efficiency white light source - HID lamp:

100 lm/W Goal for solid state lighting – 200 lm/W

Ocular response curve Photopic – bright environment (normal)

Scotopic (night vision)

K()

Where does this conversion come from?

Max = 673 lm/W

at 550nm

Light sources

Common lighting : fluorescent tube and incandescent lamp (and the sun)

LCD backlight – CCFL and LED Projectors – halogen lamps and arc lamps (HID and UHP) Light source efficiency = watt to watt electrical efficiency x

efficacy (related to spectral output) Light sources are also characterized by a color temperature

– equivalent blackbody Ultimately all light sources will be LED – so they wish

Efficacy and efficiency

Given any light source (radiant flux) P() in W. The luminous flux F in lumens is given by

The luminous efficacy is defined as lumens per Watt

The luminous efficiency is defined as normalized to the maximum of 673 lumens/Watt

Example: a fluorescent light tube has an efficacy of 70 lm/W or an efficiency of ~ 10%

For a light source, we have to take into account of electrical loss in terms of watt to watt efficiency:

dPKF

PF

efficacywattx

Blackbody – actually it is not black

102

103

104

100

102

104

106

108

spec

tral

rad

ianc

e (W

/m2/

um/S

r)

wavelength (nm)

Blackbody radiation at 5500K,3000K,2000K

)nm/m/W(1e

a)T,(W 2

T/b5

)/( 24BB mWTI

BB is very fundamental in the study of light. It should be called thermal radiation – radiation in thermal equilibrium with an atomic system

Planck’s law Stefan-Boltzman law

a = 3.742 x 10-16 W.m2

b = 0.01438 m-K

= 5.68x10-8 W/m2/K4

Einstein’s rate equation

BB was explained by Einstein using the concept of spontaneous and stimulated radiation from an excited state

1905 – a year to remember : Einstein published 3 papers in Annalen der Physik: special relativity, photon quanta (almost invented the laser) and Brownian motion. In the photon quanta paper, he derived the Planck spectrum and explained the photoelectric effect

He got the Nobel prize on explaining the photoelectric effect

2122 PPBAP

dtdP

BB spectrum derived

2122 PBPBAP

dtdP

kTh

1

2 eBA

BPP /

Spontaneous emission

Photon induced absorption

Photon induced emission

(stimulated emission!)

If the populations are at equilibrium with a photon bath then d/dt=0

Thus

(The population should be governed by Boltzmann statistics.)

Therefore

1e

BAkTh

//

BB efficacy

BB luminous efficacy

0102030405060708090

100

0 2000 4000 6000 8000 10000 12000

Temperature

lum

ens/

W

Other non BB light sources can have higher efficacy

Goal for SSL is 200 lm/W white light

How bright is 1 candle? – origin of lm/W

All of photometry is based on the brightness of a candle The illuminance of a candle at 1 ft is defined as 1 fc or 1 lm/ft2

The new candle with a luminous intensity of 1 cd is defined as 1/60 of the luminous intensity of 1 cm2 of a 2046K blackbody

Using Stefan-Boltzmann law for blackbody emission, this is equivalent to 1.659W of radiation or an intensity of 1.659/ W/sr

Thus 1-cd = 1.659/ W/sr or 1 lm = 1.659/ W Thus the conversion efficacy of 2046K BB is 1.659 lm/W or 1.82

lm/W This agrees exactly with the calculated BB efficacy of 1.92 lm/W

using the photopic curve. This confirms the lm/W conversion of the photopic curve

Example, if a certain flame has a temperature of 1550K, and has an area of 2 cm2, then P = 65 W = 8.2 lm. Efficacy = 0.13 lm/W

But not that bright This is a lot of power ! (mostly IR)

Conversion between photometric and radiometric units

Total flux Areal intensity

Angular intensity

Specific intensity(Brightness)

Radiometry W W/m2 W/sr W/sr-m2

Photometry Lumen (lm)

lumen/m2 (lux)

lumen/sr (candela, cd)

cd/m2 (nit)

Illuminance and luminance

Define the illuminance by a candle at 1 foot away as 1 foot-candle and is defined to be 1 lm/ft2

1 foot-candle = 1 fc = 1 lumen/ft2 of illuminance Thus 1 candle generates 4 lumens of light output (How many

Watts of total radiation does a candle emit?) Lambertian reflector (scatterer)

Angular intensity = 1 lumen/sr = 1 cd

4lumens

Illuminance S = 1 lumen/m2 = 1 lux

1 m

Luminance at normal = 1/ cd/m2 = 1/ nit

cosS

)(L

Luminance

Illuminance

Luminance of a flat light source

Same formula

P = Lumens of light emitted A = area of light source = emission solid angle If the emission is Lambertian, Sometimes the emission angular distribution is non-Lambertian, e.g. microcavity OLED

cosAP

L

Why ?

Observed area of emitter = A

Observed area of emitter = A cos

A

PP

ddP

dAdLoutputlumensTotal

LL o

2/

0

22

0

2/

0 2

sin2sincos)(__

cos)(

Interpretation: Total emission solid angle is , even though the half space has a cone solid angle of 2.

Brightness (luminance) = lumen output from the light source / illuminated area /

AP

L

English system

Brightness is in foot-Lambert (fL) Illuminance is in foot-candle (fc) 1 fc = 1 lm/ft2 = 10.76 lm/m2 = 10.76 lux 1 fL is defined as the luminance of a Lambertian

surface upon illumination by 1 fc Thus 1 fL = 1 fc/ sr = 10.76/ lux/sr = 3.426 nit

Vision ergonomics

Best reading brightness is 50-150 nit CRT TV ~ 300 nit LC TV (large size) ~ 500 nit Light box for viewing x-ray ~ 1700 nit Backlight unit for LC TV ~ 7000 nit Bright sunlight on snow

• Illuminance = 1000 W/m2 = 920000 lux• Luminance = 920000/ = 290000 nit • can cause snow blindness

Typical sunlight is perhaps 3000 nit. Thus sunlight readability of display is an important issue.

Moonlight ~ 10 nit Thus natural light has huge dynamic range – can affect

human behavior and mood – psychophysical

Example: photometry of a desk lamp

30W lamp at 0.5m away Light output = 30x15 lm =

450 lm Suppose said lamp

concentrates light into a cone of 900, thus illuminated area = (0.5m)2 = 0.78m2

Brightness = lumen / area /

Hence brightness = 450/0.78/ = 180 nit, perfect for reading

0.5m

0.5m

LCD monitor : backlight analysis

LCD consists of backlight + LCD panel Backlight of LCD monitor ~ 12W CCFL Light output = 12x70 lm = 840 lm 17” monitor has an area of 10.2”x13.6” = 0.09m2

Formula : Brightness = lumen / area / Hence brightness of LCD backlight = 840/0.09/= 2972

nit Transmission of active matrix LCD panel = 7% Hence LCD monitor will have a brightness of 2972x0.07

= 200 nit, just right For TV need higher brightness of 500 nit due to larger

viewing distance

LED BLU for cell phone

Backlighting unit for mobile phone LCD consists of several LEDs mounted on the edge of a piece of plastic waveguide. Structures on the plastic deflect light to the surface of the plastic to illuminate the LCD.

Suppose 2 LEDs are used with a total power of 10mW to illuminate an area of 6cm2. Suppose that the efficacy is 35 lm/W. Assume further that the packaging and optical efficiency of the backlight is 60%. What is the brightness of this backlight?

Now assumes that the LCD transmits only 20% of the backlight (a CSTN display), what is the brightness of the final display? (70nit)

Projector brightness analysis

Typical projector uses a 120W UHP arc lamp with an efficiency of 70 lm/W

Thus available light is 8400 lm Suppose one can use F/2 optics and collect 50% of the light

onto the DLP panel, that is 4200 lm DLP has color wheel for time sequential color- loss of 1/3 of

light Thus output ~ 1400 lm (check newspaper advertisement) Suppose we want the luminance of the screen to be 500 nit,

this output can only project an image of size 0.89 m2

If we turn off the room light and allow a luminance of 200 nit, then the screen can be 2.23 m2

Full color (16 million colors)

Each pixel is divided into 3 sub-pixels (RGB) 8 bit grey scale for each sub-pixel Thus each color has 28 or 256 grey levels Hence total possible colors = 24 bit = 

28 x 28 x28 = 16,777,216

Color temperature

BB spectrum is universal Every light source (display) gives a spectrum Fit the spectrum with a BB curve – the corresponding

temperature is called the color temperature Sun has a color temperature of 5500K Displays are characterized by a color temperature as well Different cultures prefer different color temperature for

displays. Japanese prefers 10000K, US and Europe prefer 6000K

Color science

Red – 650nm Green – 550nm Blue – 450nm R(), G(), B() are the response curves of the three types

of cones in the human eye Any light is characterized by a spectrum L() (radiometry).

Its perceived color is given by the tristimulus values R, G, B. We can normalize them by requiring R+G+B=1. Thus only 2 variables are needed to specify chromaticity

780

380

)()( drLR 780

380

)()( dgLG 780

380

)()( dbLB

RGB - XYZ

The CIE 1931 color-matching functions

0

0.25

0.5

0.75

1

1.25

1.5

1.75

400 450 500 550 600 650 700

Wavelength (nm)

Tri

stm

ulu

s va

lue

x yz

`

780

380

)()( dxLX

780

380

)()( dyLY

780

380

)()( dzLZ

x(), y() z() are called color matching functions

Color coordinate (CIE chart)

Need only two values to define color (chrominance values)

ZYX

Xx

ZYX

Yy

White light = (0.33, 0.33)

Edge of chart = pure color

Color mixing

All colors can be generated by mixing RGB – additive color mixing

R+G=yellow; G+B=cyan; B+R=magenta Subtractive color mixing is used in paints Color display – each pixel is divided into three

sub-pixels (spatial color, as in printing) Possible to use temporal color as well Color gamut = triangle formed by RGB points Color saturation = percentage of NTSC standard

color

Color saturation

NTSC standard Saturation of a particular

color = distance from white point / edge of CIE chart (pure color)

Color saturation of a full color display = ratio of area of color gamut / NTSC

Notebook – 60% NTSC (thin color filter is used for saving power)

LCTV with CCFL – 80% NTSC LCTV with LED backlight –

100% NTSC LCTV with LED backlight and

quantum dots color conversion – 110% NTSC

White light

White light can be obtained by an infinite number of combinations of RGB

Some are good for light (same as sunlight), some are not so good – measured by color rendition index

Solid state lighting applications – to replace lamps Approaches : R+G+B, B+Y, R+C (yellow=R+G, magenta=B+R, cyan=B+G) Color coordinate = (0.33, 0.33)

Color rendition index

CRI is a measure of the quality of a light source in reproducing natural color

CRI is calculated by comparing with a perfect white light source at the same color temperature for eight standard colors

New development for color displays

Color mixing with 4 or 5 primary colors – better than 3 primary colors in color rendition

Time sequential color with LED backlight – this will be a major trend. Same pixel can be used for RGB

Advantages : better resolution, better light throughput, savings on color filters and processing

Disadvantage : need to switch backlight, need very fast LCD mode since sub-frame is <2ms only.