Introduction. What is a Display ? (1)A complex optical device that renders an image, graphics and...

IntroductionIntroduction

What is a Display ?What is a Display ?

(1) A complex optical device that renders an image, graphics andtext by electrically addressing small switching elements (pixels)

(2) Serves as an interface between human being and machine

Let us survey some of the display technologies

display thatmodulatesbacklight(light shutter)

Types of DisplaysTypes of Displays

Direct-View Projection

Backlight Emissive Reflective Transmissive SLM Reflective SLM

display thatgenerates itsown light

display thatrejects/reflectsambient light

active matrix,STN, FLCD

CRTs, FEDs,LEDs, plasmaEL, VPD

cholesteric LC,STNs, MEMs,FLCDs

display (SLM) thatmodulates projectionlamp (transmissive pixel)

display (SLM) thatreflects projectionlamp (reflective pixel)

active matrixlight valve

active matrix reflectingpixel, digital micro-mirrors

Super-twisted nematic(STN); ferroelectric liquid crystal display(FLCD); cathode-ray-tube(CRT); lightemitting diode(LED); vacuum fluorescent display(VPD); field emitter display(FED); electroluminescent(EL); micro-electro-mechanical(MEM); spatial light modulator(SLM); liquid crystal(LC)

Display ApplicationsDisplay Applications

Direct-View Displays

backlight

LCD

LCDroom light

transmissive

Projection Displays (3-pass)

whitelight

LCD

LCDLCD

mirrorsprojectionoptics

dichroic mirrors

Emissive - CRTEmissive - CRT

Emissive - CRTEmissive - CRT

Advantages:

Mature Technology (>100 years old)Cheap to manufactureGood Viewing Angle

Disadvantages:

HeavyBulkyPower hungry

Emissive – PlasmaEmissive – Plasma

transparent electrode

cathode

seal +

-

Neon-Argon Gas

Light Output

• Neon glow discharge principle• Neon or Noble gas is ionized when sufficient voltage if applied• Ionization of gas results in visible glow (orange or red)• DC operation shown; AC shemes are also popular

Emissive – PlasmaEmissive – Plasma

ADVANTAGES:

• Established technology• Simplified driving schemes• Low cost, high volume because of simplicity• Color is feasible• Long lifetime

DISADVANTAGES:

• High voltage drivers• Low contrast ratio• Residual background glow

Emissive - ELEmissive - EL

VAC

seal

dielectric film

electrode

glass

phospher layer

Application of an electric field causes visible light to be emitted from the phospher layer

Orange-yellowlight output for ZnS


• Metal electrode-insulator-phospher layer (EL), insulator, conductor• All deposited by thin film techniques• Host material - zinc sulfide (ZnS) and activator manganese (Ms)• Manganese (yellow); terbium (green), cerium (blue)• High field is applied to phospher layer• Stack of insulators and phospher become charged, current flows in phospher layer• Resulting in ‘excitation’ of activator atoms raising them to higher energy level• Electric field is transferred to the electrons in activator atoms, raising them to higher energy level for short period of time.• Electrons relax to ‘ground state’ energy is released in the form of VISIBLE Light• The field in the phospher layer is then reduced and conduction stops until field is reversed.


ADVANTAGES:

•Thin and compact• Fast writing speeds (video compatible)• Good readability & brightness• Gray scale ability

DISADVANTAGES:

• High voltage drivers (170-200 volts)• Washout in bright ambient light (phosper layer scatters)• Color progressing but slow

Emissive - VFDEmissive - VFD

seal

filamentlead

grid lead

glass

filamentcathode

anode coated with phospher

glass tubetipped off-evacuated

space

wire grid


• Cathode filament is to 600oC to facilitate emission of thermal electrons• Anode voltage of 10-50 V is supplied to anode• At the same time voltage is applied to the grid of selected segments• Electrons from the filament are accelerated by the grid and sent to phospher coated anode• Activators in the phospher are ‘excited’ from the electrons bombard- in the phospher.• Energy from electrons transferred to phosphers raising the electrons to a higher energy level for a short period of time• When the electrons relax to their ground state, energy is released in the form of visible light• ZnS is often used as the phospher layer


ADVANTAGES:

• High brightness• Low cost for low information content displays• Full color available• Manufacturing is well established

DISADVANTAGES:

• Large screen & high resolution hard to do• Not for portable applications - high power• High voltage drivers needed

Emissive – Field Emitter DisplaysEmissive – Field Emitter Displays

electrons

baseplate

+

-

++

driv

er

phosper

ITO

spacer

faceplate

evacuated

emitterelectrodes

extractiongrid

Insulator emitter tip


• Original theory of Richardson (1934)• Electrons treated as substance that escapes from the solid state into a vacuum• Some electrons are reabsorbed into the surface• Equilibrium is established• Equilibrium changes with temperature• Increased temperature, electrons escape faster than they find themselves being reabsorbed by the surface• Electrons at the highest energy levels are allowed to escape (no very many)

Richardson-Dushman equation for current emission j:

j T e kT 120 2 exp( / ) work functione chargek Boltsman constantT absolute temperature


• Quantum mechanics - electron position viewed in terms of probability• Finite probability that electron will find itself outside energy barrier in spite of the fact if it has enough energy to ‘leap over’ the barrier• Tunneling• Small % of electrons will tunnel between emitter and vacuum• Increase % by narrowing the width of energy barrier• Higher probability that electrons tunnel through thin wall than thick one• Vary width with high electric field at surface of emitter• An electron that finds itself an infinitesimal distance outside emitter • escapes• High electric fields are needed 3-6 x 107 eV/cm

Fowler-Nordheim Equation for current emission j:

jE

EF e

f

f

F

6 2 106

1 2

2 6 8 107 3

.( / ) /

( . / )

Ef is fermi energyF is electric field


ADVANTAGES:Potentially high luminousA lot of CRT phosphorsHigh speed addressing and responseNo temperature sensitivityAnalog gray scale and full color possibleLimited photolithography requirements

DISADVANTAGES:No low voltage phosphors developed yetNo manufacturing infrastructure High driving voltages neededHigh temperature fab equipment neededPhosphors scatter sunlight (portable ?)Cross talk of electrons in adjacent pixelsStill reseach projects for most

Emissive – Light Emitting DiodesEmissive – Light Emitting Diodes

Bat

tery

Light

glass

ITO

Holetransportlayer

Light emissionlayer

Top Electrode


Mechanism of p-n Junction Operation

• When no voltage or reversed voltage is applied across a p-n junction, an energy barrier is formed preventing the flow of electrons and holes

• When a forward bias is applied across the p-n junction, the energy barrier is reduced allowing electrons to be injected into p regions and holes to be injected into n regions

• The injected carriers recombine with carriers of opposite sign resulting in the emission of light


ADVANTAGES:•Low voltage operation•Low cost for low information content•Multiple colors•Manufacturing well established•Large screen message screens available•Organic LED materials potentially easier to process• Organics now possible with flexible substrates

DISADVANTAGES:•High power consumption for portable products•High cost for high information content•Blue LEDs have low brightness•Full-color displays (?)

Transmissive –Twisted Nematic LCDTransmissive –Twisted Nematic LCD

Transmissive –Twisted Nematic LCDTransmissive –Twisted Nematic LCD

Advantages• Well established technology (early 1970’s)• Created the portable computer market• High resolution with active matrix• Excellent color purity

Disadvantages• Needs active matrix• backlight is the power sink• A lot of layers, both optical and electronic• Viewing Angle is said to be a problem but many solutions are practiced to fix it.

Transmissive –Super Twisted Nematic LCDTransmissive –Super Twisted Nematic LCD

Transmissive –Super Twisted Nematic LCDTransmissive –Super Twisted Nematic LCD

Advantages• Well established technology• Great for inexpensive low-medium resolution displays•No need for active matrix, cheap passive solutions

Disadvantages• Poor color performance• Poor viewing angle• Medium resolution with passive addressing

Reflective – ElectrophoreticReflective – Electrophoretic

-- -- -- -- --

-- -- -- -- --

--

Blue dye for emample

Negatively charged white pigment particles

colloid suspension particles (surfactants, solvent)

transparent electrode

metalelectrode

seal

+-

viewer seeswhite

viewer seesblack

Reflective – ElectrophoreticReflective – Electrophoretic

ADVANTAGES:• Low power consumption - reflective• Adequate contrast• Wide viewing angle• High resolution possible• Inherent memory•New encapsulation techniques for stabilization (E-Ink)

DISADVANTGES:•Stability of suspension unclear•Higher drive voltage than available drivers•Slow switching speed•Complex chemistry

Reflective-GyriconReflective-Gyricon

Reflective-GyriconReflective-Gyricon

Advantages

•Cheap•Cool

Disadvantages

• High voltage• needs active matrix• sticky balls

Reflective - PDLCReflective - PDLC

non e

np

nonon en e

npnp

nono

n en e

npnp

V

V

Polymer Dispersed Liquid Crystal(PDLC)

• Easy to manufacture• Good viewing angle• Bright - no polarizers• No rubbing layers• Good projection displays

• Slightly high driving voltages• Contrast only 10:1• Poor reflectance• Off-axis haze• Direct-view (?)

np ~ no

Reflective – H-PDLCReflective – H-PDLC

V

V

Holographic Polymer Dispersed Liquid Crystal (H-PDLC)

np

np

no

ne

no

ne

Reflective – H-PDLCReflective – H-PDLC

ADVANTAGES:

• High reflection efficiency• Great color purity• No polarizers

DISADVANTAGES:

• High driving voltage• Still research• Fabricate with laser scanning

Reflective – Cholesteric LCDReflective – Cholesteric LCD

V1

V V2

Cholesteric Texture DisplaysPolymer Stabilized Cholesteric Texture (PSCT)

• High contrast for reflective• Good viewing angle• Bistable memory• No polarizers - easy to manufacture

• Slow (not video compatible)• Bragg Color Shift

Flexible DisplaysFlexible Displays

What technologies are adaptableto a flexible type substrate ?

Threshold vs. Non-ThresholdThreshold vs. Non-Threshold

Addressing: How do we supply voltages to Render an image ?

Threshold vs. Non-ThresholdThreshold vs. Non-Threshold

Threshold No Threshold

all LCD’s

electroluminescent

plasma

light emitting diode

electrophoretics

Gyricon

Examples of Threshold,Non-Threshold MaterialsExamples of Threshold,Non-Threshold Materials

Direct Drive AddressingDirect Drive Addressing

• Thresholdless nature of material is irrelevant

• Every pixel is independently addressed

• Every pixel has a connection for a N+M display, there are NM electrical connections

• For lower resolutions only <50 pixels inch

Direct DriveDirect Drive

Samples of Fixed Format AlphaNumeric Matrices

Samples of Fixed Format AlphaNumeric Matrices

7-bar 10-bar 13-bar 14-bar

Multiplexed AddressingMultiplexed Addressing

• Can address NM pixels using only N+M electrical connections

• Strict limitation on threshold voltage and T-V steepness curve

• Voltages applied to one pixel cannot be arbitrarily changed without affecting the applied voltage of the other cells

• For medium to high resolution ( 400 rows)

Multiplexing 2D ArrayMultiplexing 2D Array

• Consider MN Array, addressed with N rows and M columns

• The M elements in the first row can be turned ON or OFF depending on the voltages applied to each element. Let VS denote the row voltage and VD denote the column voltage

• The row voltage is always VS, and the column voltage can be VD

• The instantaneous drop at the pixel electrode isON state V=VS-(-VD) or V=VS-VD

• Response time, governed by viscoelastic properties, must be >> than period of driving waveform

• Interaction between LC molecule and applied electric field must be =E2 (induced polarization)

• In each multiplexing cycle, each row is selected on during 1/N of the cycle time T

22 2OFF S D D

1 N -1V = V -V + V

N N

Conditions for RMSResponding MaterialConditions for RMSResponding Material

22 2ON S D D

1 N -1V = V +V + V

N N

RMS Responding MaterialRMS Responding Material

Alt and PleshkoIEEE Trans. Electronic Devices ED-21,

146-155 (1974)

Using the previous equations, one can derive themaximum number of rows

MAXON

OFF MAX

N +1V=

V N -1‘ Selection Ratio’

For NMAX>>1ON

OFF MAX

V 1=1+

V N

Selection RatioSelection RatioS

elec

tion

Rat

io

NMAX

0 200 400 600 800 10001

1.5

2

2.5

Multiplexing: Practical ApplicationsMultiplexing: Practical Applications

OFF THV V ON THV V + Δ

VTH: threshold voltage (turn on begins)

: steepness parameter

TH

ΔP

V

TH MAX

Δ 1

V N

Passive Multiplexing: Amplitude ModulationPassive Multiplexing: Amplitude Modulation

Frame 1

pixel voltage(row-column)

time

Column Signals

Ro

w S

ign

als

//

//

//

//

//

1 2 3 . . . N

t

//

//

S+D

S+D

S-D

S-D+D

-D

+D

+D

+D

//

//

+D

+D

+S

+S

T

+S

+S

+D

-D -D

+D

-D

+D

+S+S

+S

+D

+S

Passive Multiplexing: Pulse Width ModulationPassive Multiplexing: Pulse Width Modulation

pixel voltagetime

Frame 1 Frame 2

T

Column Signals

Ro

w S

ign

als

//

//

//

//

//

1 2 3 . . . N

t

S+DS-D

-D

(row-column)

+D

+S

+S

+S

+S

-D

+D

f

1-f

f

1-f

//

//

//

//

+S

+S+S

Examples of MultiplexingExamples of Multiplexing

2

THMAX

VN =

ΔDisplay

ConfigurationVTH NMAX (NMAX)2

TN

STN

PDLC

electrophoretic

2 Volts

4 Volts

8 Volts

none

0.4 Volts

0.2 Volts

3 Volts

undefined

25

400

7

0

625

1.6 104

50

0

Active Matrix DisplaysActive Matrix Displays

• Multiplexing is limited and not adequate for high resolutions (slow response, poor viewing angle, no gray scale)

• A non-linear element is build into the substate at each pixel, usually a thin-film-transistor

• Being isolated from other pixels by TFT’s, the voltage remains constant while the other pixels are being addressed

• Not subject to Alt-Pleshko Formalism

Active Matrix CircuitActive Matrix Circuit

Scan Line

SourceDrain

Liquid Crystal

Active Matrix: A Complex DeviceActive Matrix: A Complex Device

Drain

Principle of Operation-Active MatrixPrinciple of Operation-Active Matrix

• One line at a time addressing

• A positive voltage pulse (duration T/N, N # rows, T frame time) is applied to the line turning on all TFT’s

• The TFT’s act as switches allowing electrical changes to the LC capacitors from the columns (data or source)

• When addressing subsequent rows a negative voltage is applied to the gate lines thereby turning off the transistors for one frame time T, until ready to readdress it

• For AC drives schemes (LCD’s) the polarity is alternated on the data voltage

4 Basic Steps of TFT4 Basic Steps of TFT

1. At time 1, a positive voltage VG of duration T/N is applied togate to turn on TFT. The LC pixel (ITO) is changed to VON at time 2 within T/N, due to the positive source voltage VSD=VON.

2. At time 2, the gate voltage VG becomes negative, turning offthe source voltage VSD from VON to –VON. During the timeperiod 2 and 3, of duration (N-1)/NT, the pixel voltage VP remainsabout >0.9 VON as the LC capacitor is now isolated from data lines.3. At time 3 (the next addressing time), the TFT is turned on againby applying a positive gate voltage of duration T/N. The LCcapacitor now sees a negative source-to-drain voltage VSD=-VON.The pixel electrode is discharged from VP=VON at time 3 toVP=-VON within the time duration T/N.4. At time 4, the TFT is turned off by the negative gate voltage, and simultaneously the source voltage VSD changes from –VON to +VON.

0

0 time

time

time

T/N T T

1 2 3 4

VG

VSD

VP

VON

Gate Voltage

Source Drain Voltage

Pixel Voltage

Notice that VP is not constant during the duration (n-1)T/N becauseof a slight leakage current of LC cell. LC materials must have a high voltage holding ratio (VHR) to minimize this.

TFT AddressingTFT Addressing

Active Matrix Multiplexed

LC Mode

Contrast

Viewing (horizontal)

Viewing (vertical)

Response time

Addressable lines

Gray-scale

TN

>100:1

-600,+600

-300,+450

20-40ms

>1000

>16

STN

10-15:1

-300,+300

-250,+250

100-150ms

~400

low

SummarySummary

Display technology is a very interdisciplinary science, combiningbasic principles from all the sciences and engineering, and in addition, human physiology. Three basic concepts should be remembered when working with light measurement and displays-spectral, spatial and temporal.

Spectral Characteristics: The spectral, or colorconsideration is closely related to the frequency band pass characteristics of devices and systems in electronics. Initially onemust decide if the spectral characteristics are to be considered forthe human eye (photometry) or power (radiometry).

Spatial Characteristics: The spatial characteristics are geometric considerations affecting emission, reflection,absorption, transmission, and sensing light.

Basic Display MeasurementBasic Display Measurement

Temporal Characteristics: Temporal considerations are time related. Analogous to electronic devices, optical deviceshave rise times and fall times and frequency bandwidthsassociated with them.

Electromagnetic Spectrum: The electromagnetic spectrumdepicts the range of electromagnetic radiation. The region identifiedas photometry corresponding to the visible spectrum- this is the range where the human eye is sensitive.

Basic Display MeasurementBasic Display Measurement

• Secured by six muscles.

• Sclera is a dense white fibrous material,except where it becomes transparent (cornea).

• Transparent gel-like substance filb the eye (viteous humor).

• An elastic lens is situated in the viteous humor and secured by a muscle.

• The lens shape is controlled by muscle action to focus image.

• Outside in formation passes through cornea, lens, and the viteous humor, where the light is focused on a slight indentation on back wall, the fovea.

Human EyeUltimate Reception for Displays


• The inner wall of the eye is covered with a layer of light sensing cells (retina).

• Nerve fibers protruding from each cell form complex web networks, eventually forming the optic nerve.

• Between light sensing cells and their network of nerve fibers and the sclera, is another pigmented membrane, the choroid to absorb a residual light not absorbed by the light sensitive cells.

• The retina contains 120 millions photosensitive receptions, called rods and cones. The cones are concentrated in the fovea and responsive for color vision. There are 7 million cones.



• The rods are not present in the fovea, but populate other areas of the retina.

• The information created in the rods is funneled out through the optic nerve to the brain.

The human eye is not without limitations, creatingdesign challenges for display engineers.



RadiometryRadiometry

Radiometry is the basis for all light measurements. It is definedby the Institute of Electrical and Electronics Engineers (IEEE) asthe measurement of quantities associated with radiant energy.

Radiant Flux [W] - The watt (W) is the fundamental unit ofradiometry. All other radiometric units combine watts with units ofarea, distance, solid angle and time.

Radiant Intensity [W/Sr] - A true point source is an isotropicradiator. If we assume we have a 100W lamp, which is anisotropic radiator then it radiate light into an imaginary sphere.


If we form a cone of 57.2960 (1 steradian, the unit of solid anglewhich encloses a surface area on the sphere equal to the squareof the radius) with its surface of the sphere, the total radiationflowing through the cone will be radiant intensity. A full spherecontains, or : Thus one sr will contain:

dΩ = sin θ dθd , 4 sr

The diameter of the sphere does not matter. As the spherediameter increases, the total radiation within the circle remainsthe same.

12.566 sr

48W/Sr

13.56


Irradiance [W/m2] – is simply the amount of optical radiationincident upon a specified surface area. The preferred unit is thewatt per square meter [W/m2]. The irradiance will changeinversely with the square of the distance. If the radiation source ismoved to twice the distance, the same amount o flight will bespread over four times the area and the irradiance will be reducedby a factor of four.

Radiant Exitance [W/m2] – measured in watts per square meter(as is irradiance) is used to indicate the total radiation per unit areaemitted, reflected, or transmitted by a 1m2 surface regardless ofdirection.


Radiometric Units (SI)irradiance(1 Watt/m2)

1 square meter

isotropicradiation

radiant flux (Watt, power)

radiant intensity(1 Watt/sr)

radiance(Watt/srm2)

1 meter

1 steradian is the unit of solid angle thatencloses a surface area on sphere equal to the square of the radius.

1 steradian

PhotometryPhotometry

Photometry is a subset of radiometry. In radiometry, the detectorhas a flat spectral response. In photometry, on the other hand,the spectral response useful to the visual system is considered.To accomplish that, the detector should be closely matched tothe spectral response curve of the eye.

The spectral sensitivity of the human eye, also known as thephoto-optic response curve.


It has been standardized at 683 lm/w. A standard 200W light bulbproduces a broad band radiation as well as heat in the form ofinfrared radiation. The radiant flux produced by the lamp is 100W.If all of its radiation were concentrated at 555nm, it would have anoutput of 200W 683 lm/w = 68,300

However, only 10% of the total radiant power radiated by thelamp is within the visible and even less (2%) is useful to thehuman eye because of the eye’s insensitivity to blue and redwavelengths. A typical output for a 200W bulb is 1750lm. Theluminous efficacy of the lamp is lumens per watt, 1750lm/100W = 17.5lm/W

Luminous Flux [lm] – The lumen is essentially a unit of poweruseful to the human eye. It is closely related to the watt as thespectral luminous efficacy (km) for monochromatic light at thepeak visual response wavelength of 555nm.


Luminous Intensity [lm/sr or candelas] – Assume the luminousflux is radiated in all directions, like a point source. If we form acone of 57.2960, or 1 steradian (the unit of solid angle thatencloses a surface area on the sphere equal to the square of theradius) with its origin at the lamp and extending to the surface ofthe sphere, the total visible light flowing through the cone isluminous intensity. Luminous intensity is expressed in lm/sr orcandelas. A full sphere contains 4 or 12.56 steradians. So alight bulb of 1750lm/12/56=136cd. Again, the diameter of thesphere is irrelevant. The luminous intensity in cd is the basic unitof photometry, all other units are derived by combining thecandelas with units of are, distance, solid angle and time.


Illuminance [lm/m2] – Illuminance is the amount of visibleradiation incident upon a specified surface area. The preferredunit is the lux (lumen per square meter). The deprecated Footcandle (lumen per square foot) is still used and can beconverted to lux by simply multiplying it by 10.764. The inversesquare law determines the illuminance.

Luminance [cd/m2] – Luminance is candelas per square meter,is the unit to indicate how much light is reflected, transmitted oremitted by a diffusing surface. The deprecated unit, thefootlambert () is still used.

Luminance Exitance – is also measured in lumens per squaremeter, analogous with illuminance, is used to indicate the totallight per unit area emitted, reflected, or transmitted by a surfaceregardless of direction.


Photometric Units (SI)illuminance(1 lm/m2=1 lux) 1 square meter

isotropicradiation

luminous flux (power)

luminous intensity(1 lm/sr=1 cd)

luminance(cd/m2, nit,lm/srm2)

1 meter


1 steradian

Lambertian Reflector


Photometric Units (English)illuminance(1 lm/ft2)

1 square foot

isotropicradiation

luminous flux (power)

luminous intensity(1 lm/sr=1 cd)

luminance(fL)

1 foot


1 steradian

Lambertian Reflector

Examples of Illuminance/LuminanceExamples of Illuminance/Luminance

Direct Sunlight

Daylight (excluding direct sunlight)

Overcast Sky

Heavy Overcast

Twilight

Full Moon

Overcast night sky (no moon)

105 lx

104 lx

103 lx

102 lx

1-10 lx

10-1 lx

10-4 lx

Examples of Natural Illuminance Levels

Examples of Illuminance/LuminanceExamples of Illuminance/Luminance

Sun’s disk

100W soft white light bulb

Fluorescent lamp surface

Overcast Sky

Blue Sky

White paper (in office)

CRT

1.5108 cd/m2

Examples of Luminance Levels

3104 cd/m2

104 cd/m2

103 cd/m2

3103 cd/m2

60-150 cd/m2

102 cd/m2

Radiometry/PhotometryRadiometry/Photometry

Radiometry Photometry

Radiant Flux (Watt)

Radiant Intensity (Watt/Steradian)

Irradiance (Watt/m2)

Radiance (Watt/Steradian m2)

Luminous Flux (lumen)

Luminous Intensity (lumen/Steradian)

Illuminance (lumen/m2, lux)

Illuminance (cd/m2, nit)

Conversion Factors between Photometric units in SI system andEnglish system.

Footlambert

Footlambert candela/m2

candela/m2

1

13.426

0.2919 Footcandles

Footcandles lux

lux

1

13.426

0.2919

Quantify ColorQuantify Color

• Most displays operate on color addition (red, green, blue), but a few do work on color subtraction (cyan, yellow, magenta).

• Need to stimulate the stimulus, or spectral power arriving at the back of the eye.

• Mathematical functions, called color matching functions that do just that.

• Color matching functions model the receptors responsible for color vision.

Deriving Color Matching FunctionsDeriving Color Matching Functions

The color matching functions are derived from a basic color matchingexperiment, to define a linear mapping from a test light spectral powerdistribution test lamp. The test light is set to unit energy at nm testwavelengths. The observer adjusts the primary intensities (RGB) until test andmixture fields match. The relative weights are termed tristimulus values, andthe color matching functions are spectral plots of the tristimulus values.

whitescreen

testlamp

screen mask

Observer adjusts RGBuntil the mix matchesthe test lamp

test

RG

B m

ix

The Color Matching FunctionsThe Color Matching Functions

y-axis (Relative Response) : x-axis (Wavelength in nm)

xz y

Tristimulus ValuesTristimulus Values

780

r,g,b r,g,b

380

X = k S λ xdλ

780

r,g,b r,g,b

380

Y = k S λ ydλ

780

r,g,b r,g,b

380

Z = k S λ zdλ

k=683 lm/watt (normalizing factor), Sr,g,b is the spectralpower distribution of source.

Chromaticity CoordinatesChromaticity Coordinates

rr

r r r

XX =

X +Y + Z

rr

r r r

YY =

X +Y + Z

rr

r r r

ZZ =

X +Y + Z

CIE 1976 Chromaticity CoordinatesCIE 1976 Chromaticity Coordinates

r

r r

4Xu =

-2X +12Y + 3

r

r r

9Yv =

-2X +12Y + 3

CIE 1931CIE 1931

780

r

380

X = k SPD λ R λ xdλ

780

r

380

Y = k SPD λ R λ ydλ

780

r

380

Z = k SPD λ R λ zdλ

SPD () is the spectral power distribution of sourcek is the normalizing factor

Reflective ObjectsDepend on Ambient Illumination

Reflective ObjectsDepend on Ambient Illumination

Photo-optic ReflectionPhoto-optic Reflection

SPD () R () y ()

Fluorescent Lamp CLC Theory Color Matching

%RP =

Standard Spectral Power DistributionStandard Spectral Power Distributiony-

axis

(R

adia

nce

in W

atts

/sr

m2)

x-axis (Wavelength in nm)

380 550 780

380 550 780

380 550 780

380 550 780

Examples ofSpectral Power Distribution

Examples ofSpectral Power Distribution

Flourescent Lamp

Sun P-LED

SylvaniaBulb

white

Blue

Magenta

Black

Cyan

Yellow

Red

Green

Magnified view of CRTpixels for the various colors

no output

white

Blue

Magenta

Black

Cyan

Yellow

Red

Green

Magnified view of CRTpixels for the various colors

no output

Generating ColorGenerating Color

An example of what you might see if you magnify a CRT screen.The primary and secondary colors are achieved by color addition.

Color AdditionColor Addition

The simple color addition scheme for electric displays. ExamplesInclude R+G+B=W, R+G=Y, and B+G=C, where Red (R), Blue (B),Green (G), Yellow (Y), Cyan (C), Magenta (M), and White (W).

Ways to Perform Color AdditionWays to Perform Color Addition

0<time<T1

t1<time<T2

t2<time<T3

Full Color Displays Color Addition-RGBTemporal Synthesis

0<time<T1

t1<time<T2

t2<time<T3


0<time<T1

t1<time<T2

t2<time<T3

0<time<T1

t1<time<T2

t2<time<T3


Full Color DisplaysColor Addition-RGB

Spatial Color Synthesis


Spatial Color Synthesis

Full Color DisplaysColor Addition -RGB

Color Additive Intraged Stack


Color Additive Integrated Stack

Color TemperatureColor Temperature

Many times in the center of a chromaticity diagram (white region)you will see temperatures listed. An object to any temperatureabove 650-800K will produce a spectrum emission with its colorrelated to temperature. This is known as blackbody radiation.The color progresses from a very deep red, through orange, yellow,white, and finally bluish white. This path is often plotted on thechromatic diagram, and is known in the literature as the Plankianlocus. Most natural light sources, such as the sun, stars and firefall close to this locus of points. Displays are often designed tomeet these criteria.

Color Temperature (K)

6500

~6500

Light Source

Daylight, fluorescent lamp

CRT Computer Displays

ContrastContrast

CR = on

off

L Luminance of on - pixel=

L Luminance of off - pixel

Lon: Luminance of the on-pixelLoff: Luminance of the off-pixel

The derivation of PCR is intuitive and can be performedheuristically. The display row lines must be strobed sequentially when refreshing the display image. The pixel in a row will have a luminance of Lon and all pixels intended to be off, Loff will experience a partial signal. When the next row is addressed, the previous row will experience a partial signal and will be stimulatedto Loff for the remaing M-1 rows. Over the entire frame is the sum of individual light pulses, therefore the pixel has a luminance of Lon+(M-1)Loff.

ContrastContrast

The contrast ratio is a measure of the ratio of luminance betweenan on and off pixel. A more sophisticated approach is toincorporate both luminance and chromaticity contrast, where thetotal contrast is the root mean square of chromaticity andluminance contrast. To arrive at the chromaticity contrast, therehave been many empirical studies to ascertain a normalizedchrominance index. An empirical chrominance ratio u:

1/22 2Δu + 2.224Δv

0.027

Chromaticity ContrastChromaticity Contrast

Where u’ and v’ are the difference in chrominance between thetwo regions (a pixel) as plotted on the CIE chromaticity diagram.The 0.027 is an empirical factor based on just perceivabledifference. The total contrast ratio, which includes both chrominance and luminance, can be combined as a root meansquare.

2 2CR total = chrominance + Luminance Contrast

Chromaticity ContrastChromaticity Contrast

Resolution: is the ability to delineate (resolve) picture detail.The smallest discernible and measurable detail on a visualpresentation. This is not a quantifiable definition.

Possibly the best way to quantify resolution is pixel density (PD),i.e. pixels per linear distance, how close pixels are together. Thestandard is # pixels per inch.

‘Ball Park’ definition:Ultra-high PD > 120High 120 > PD > 70Medium 70 > PD > 51Low PD < 50

ResolutionResolution

SummarySummary

• Display Technologies• Threshold vs. Non-Threshold• Direct Drive Addressing• Multiplexing Addressing• Active Matrix Addressing• Radiometry• Photometry• Chromaticity Coordinates• Contrast• Resolution

• Display Technologies• Threshold vs. Non-Threshold• Direct Drive Addressing• Multiplexing Addressing• Active Matrix Addressing• Radiometry• Photometry• Chromaticity Coordinates• Contrast• Resolution

Introduction. What is a Display ? (1)A complex optical device that renders an image, graphics and...

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