Introduction
What is a Display ?
(1) A complex optical device that renders an image, graphics and
text 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
Types of Displays
Direct-View
Backlight Emissive Reflective
display that
modulates
backlight
(light shutter)
display that
generates its
own light
display that
rejects/reflects
ambient light
active matrix, CRTs, FEDs, cholesteric LC,
STN, FLCD
LEDs, plasma STNs, MEMs,
FLCDs
EL, VPD
Projection
Transmissive SLM Reflective SLM
display (SLM) that
modulates projection
lamp (transmissive pixel)
active matrix
light valve
display (SLM) that
reflects projection
lamp (reflective pixel)
active matrix reflecting
pixel, digital micromirrors
Super-twisted nematic(STN); ferroelectric liquid crystal display(FLCD); cathode-ray-tube(CRT); light
emitting diode(LED); vacuum fluorescent display(VPD); field emitter display(FED); electroluminescent
(EL); micro-electro-mechanical(MEM); spatial light modulator(SLM); liquid crystal(LC)
Display Applications
Direct-View Displays
Projection Displays (3-pass)
white
light
dichroic mirrors
LCD
backlight
transmissive
room light
LCD
LCD
LCD
LCD
projection
optics
mirrors
Emissive - CRT
Emissive - CRT
Advantages:
Mature Technology (>100 years old)
Cheap to manufacture
Good Viewing Angle
Disadvantages:
Heavy
Bulky
Power hungry
Emissive – Plasma
L ig h t
O u tp u t
tra n sp a re n t
e le ctro d e
se a l
+
N e o n -A rg o n G a s
ca th o d e
•
•
•
•
-
N e o n g lo w d isch a rg e p rin cip le
N e o n o r N o b le g a s is io n ize d w h e n su fficie n t vo lta g e if a p p lie d
Io n iza tio n o f g a s re su lts in visib le g lo w (o ra n g e o r re d )
D C o p e ra tio n sh o w n ; A C sh e m e s a re a lso p o p u la r
Emissive – 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 - EL
se a l
p h o sp h e r la ye r
d ie le ctric film
V AC
e le ctro d e
g la ss
O ra n g e -ye llo w
lig h t o u tp u t fo r Z n S
A p p lica tio n o f a n e le ctric fie ld ca u se s visib le lig h t to b e
e m itte d fro m th e p h o sp h e r la ye r
Emissive - EL
• Metal electrode-insulator-phospher layer (EL), insulator, conducto
• 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.
Emissive - EL
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 - VFD
g la ss
g rid le a d
w ire g rid
fila m e n t
ca th o d e
a n o d e co a te d w ith p h o sp h e r
se a l
fila m e n t
le a d
g la ss tu b e
tip p e d o ffe va cu a te d
sp a ce
Emissive - VFD
• 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 bombardin 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
Emissive - VFD
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 Displays
fa ce p la te
IT O
d rive r
++
e va cu a te d
p h o sp e r
e le ctro n s
sp a ce r
+
e xtra ctio n
g rid
-
b a se p la te
In su la to r
e m itte r tip
e m itte r
e le ctro d e s
Emissive – Field Emitter Displays
• 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  120T exp( e / kT )
2
 work function
e charge
k Boltsman constant
T absolute temperature
Emissive – Field Emitter Displays
• 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 on
• 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:
j  6.2  10
6
( E f / )
1/ 2
Ef 
2
F e
( 6.810
73
 /F)
Ef is fermi energy
F is electric field
Emissive – Field Emitter Displays
ADVANTAGES:
Potentially high luminous
A lot of CRT phosphors
High speed addressing and response
No temperature sensitivity
Analog gray scale and full color possible
Limited photolithography requirements
DISADVANTAGES:
No low voltage phosphors developed yet
No manufacturing infrastructure
High driving voltages needed
High temperature fab equipment needed
Phosphors scatter sunlight (portable ?)
Cross talk of electrons in adjacent pixels
Still reseach projects for most
Emissive – Light Emitting Diodes
T o p E le ctro d e
L ig h t e m issio n
la ye r
B a tte ry
H o le
tra n sp o rt
la ye r
IT O
g la ss
L ig h t
Emissive – Light Emitting Diodes
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
Emissive – Light Emitting Diodes
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 LCD
Transmissive –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 LCD
Transmissive –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 – Electrophoretic
vie w e r se e s
w h ite
tra n sp a re n t
e le ctro d e
se a l
-
-
-
-
vie w e r se e s
b la ck
-
m e ta l
e le ctro d e
-
-
-
-
-
+
co llo id su sp e n sio n p a rticle s (su rfa cta n ts, so lve n t)
-
N e g a tive ly ch a rg e d w h ite p ig m e n t p a rticle s
B lu e d ye fo r e m a m p le
Reflective – 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-Gyricon
Reflective-Gyricon
Advantages
•Cheap
•Cool
Disadvantages
• High voltage
• needs active matrix
• sticky balls
Reflective - PDLC
P o lym e r D is p e rs e d L iq u id C rys ta l
(P D L C )
ne
•
•
•
•
•
E a sy to m a n u fa ctu re
G o o d vie w in g a n g le
B rig h t - n o p o la rize rs
N o ru b b in g la ye rs
G o o d p ro je ctio n d isp la ys
•
•
•
•
•
S lig h tly h ig h d rivin g vo lta g e s
C o n tra st o n ly 1 0 :1
P o o r re fle cta n ce
O ff-a xis h a ze
D ire ct-vie w (? )
no
V
np
ne
no
V
np
np ~ no
Reflective – H-PDLC
H o lo g ra p h ic P o lym e r D is p e rs e d L iq u id C ry s ta l (H -P D L C )
ne
no
np
V
ne
np
V
no
Reflective – H-PDLC
ADVANTAGES:
• High reflection efficiency
• Great color purity
• No polarizers
DISADVANTAGES:
• High driving voltage
• Still research
• Fabricate with laser scanning
Reflective – Cholesteric LCD
C h o le s te ric T e x tu re D is p la ys
P o ly m e r S ta b ilize d C h o le s te ric T e x tu re (P S C T )
•
•
•
•
H ig h co n tra st fo r re fle ctive
G o o d vie w in g a n g le
B ista b le m e m o ry
N o p o la rize rs - e a sy to m a n u fa ctu re
•
•
S lo w (n o t vid e o co m p a tib le )
B ra g g C o lo r S h ift
V
V1
V2
Flexible Displays
What technologies are adaptable
to a flexible type substrate ?
Threshold vs. Non-Threshold
Addressing: How do we supply voltages to
Render an image ?
Threshold vs. Non-Threshold
Examples of Threshold,
Non-Threshold Materials
Threshold
No Threshold
all LCD’s
electrophoretics
electroluminescent
plasma
light emitting diode
Gyricon
Direct 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 NM electrical connections
• For lower resolutions only <50 pixels inch
Direct Drive
Samples of Fixed Format Alpha
Numeric Matrices
7-bar
10-bar
13-bar
14-bar
Multiplexed Addressing
• Can address NM 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 Array
• Consider MN 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 is
ON state V=VS-(-VD) or V=VS-VD
Conditions for RMS
Responding Material
• Response time, governed by viscoelastic properties, must be
>> than period of driving waveform
• Interaction between LC molecule and applied electric field
must be a=E2 (induced polarization)
• In each multiplexing cycle, each row is selected on during 1/N
of the cycle time T
V
V
2
ON
2
O FF
=
1
N
=
1
N
+ VD

2
V S
V S - V D

2
+
N -1
N
+
N -1
N
V
2
D
2
VD
RMS Responding Material
Alt and Pleshko
IEEE Trans. Electronic Devices ED-21,
146-155 (1974)
Using the previous equations, one can derive the
maximum number of rows
VO N
=
VO F F
For NMAX>>1
N MAX + 1
‘ Selection Ratio’
N MAX - 1
VO N
VO F F
= 1+
1
N MAX
Selection Ratio
Selection Ratio
2.5
2
1.5
1
0
200
400
600
NMAX
800
1000
Multiplexing: Practical Applications
VO FF  VTH
P 
VO N  VTH + Δ
Δ
VT H
Δ
VTH

1
N MAX
VTH: threshold voltage (turn on begins)
D: steepness parameter
Passive Multiplexing: Amplitude Modulation
time
pixel voltage
(row-column)
Frame 1
+S
S+D
T
+S
+D
Row Signals
+D
//
//
+S
+S
S-D
//
+S
S+D
+S
+S
+D
//
+D
+D
//
//
+S
D t
S-D
+D
+D
//
1
2
3 ...
+D
//
N
-D
+D
+D
//
-D
-D
-D
Column Signals
Passive Multiplexing: Pulse Width Modulation
time
1-f
Frame 1 Frame 2
Row Signals
+S
f
//
+S
//
+S
+S
//
//
+S
Dt
//
3
... N
1-f
+D
//
-D
f
Column Signals
S+D
S-D
+D
-D
+S
//
2
(row-column)
+S
T
//
1
pixel voltage
//
Examples of Multiplexing
N MAX
 VT H 
= 

Δ


2
Display
Configuration
VTH
D
NMAX
(NMAX)2
TN
2 Volts
0.4 Volts
25
625
STN
4 Volts
0.2 Volts
400
1.6  104
PDLC
8 Volts
3 Volts
7
50
electrophoretic
none
undefined
0
0
Active 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 Circuit
Scan Line
Drain
Source
Liquid Crystal
Active Matrix: A Complex Device
Drain
Principle 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 TFT
1. At time 1, a positive voltage VG of duration T/N is applied to
gate 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 off
the source voltage VSD from VON to –VON. During the time
period 2 and 3, of duration (N-1)/NT, the pixel voltage VP remains
about >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 again
by applying a positive gate voltage of duration T/N. The LC
capacitor now sees a negative source-to-drain voltage VSD=-VON.
The pixel electrode is discharged from VP=VON at time 3 to
VP=-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.
TFT Addressing
VG
T/N
T
T
0
1 2
VSD
0
VON
time
Gate Voltage
time
Source Drain
Voltage
time
Pixel Voltage
3 4
VP
Notice that VP is not constant during the duration (n-1)T/N because
of a slight leakage current of LC cell. LC materials must have a
high voltage holding ratio (VHR) to minimize this.
Summary
Active Matrix
Multiplexed
LC Mode
TN
STN
Contrast
>100:1
10-15:1
Viewing (horizontal)
-600,+600
-300,+300
Viewing (vertical)
-300,+450
-250,+250
Response time
20-40ms
100-150ms
Addressable lines
>1000
~400
Gray-scale
>16
low
Basic Display Measurement
Display technology is a very interdisciplinary science, combining
basic 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 color
consideration is closely related to the frequency band pass
characteristics of devices and systems in electronics. Initially one
must decide if the spectral characteristics are to be considered for
the human eye (photometry) or power (radiometry).
Spatial Characteristics: The spatial characteristics are
geometric considerations affecting emission, reflection,
absorption, transmission, and sensing light.
Basic Display Measurement
Temporal Characteristics: Temporal considerations are
time related. Analogous to electronic devices, optical devices
have rise times and fall times and frequency bandwidths
associated with them.
Electromagnetic Spectrum: The electromagnetic spectrum
depicts the range of electromagnetic radiation. The region identified
as photometry corresponding to the visible spectrum- this is the
range where the human eye is sensitive.
Human Eye
Ultimate Reception for Displays
• 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 Eye
Ultimate 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.
Human Eye
Ultimate Reception for Displays
• 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, creating
design challenges for display engineers.
Radiometry
Radiometry is the basis for all light measurements. It is defined
by the Institute of Electrical and Electronics Engineers (IEEE) as
the measurement of quantities associated with radiant energy
Radiant Flux [W] - The watt (W) is the fundamental unit of
radiometry. All other radiometric units combine watts with units of
area, distance, solid angle and time.
Radiant Intensity [W/Sr] - A true point source is an isotropic
radiator. If we assume we have a 100W lamp, which is an
isotropic radiator then it radiate light into an imaginary sphere.
Radiometry
If we form a cone of 57.2960 (1 steradian, the unit of solid angle
which encloses a surface area on the sphere equal to the square
of the radius) with its surface of the sphere, the total radiation
flowing through the cone will be radiant intensity. A full sphere
contains, d Ω = s in  θ  d θ d  , 4  s r or 1 2 .5 6 6 s r :
Thus one sr will contain:
4
 8 W /S r
1 3 .5 6
The diameter of the sphere does not matter. As the sphere
diameter increases, the total radiation within the circle remains
the same.
Radiometry
Irradiance [W/m2] – is simply the amount of optical radiation
incident upon a specified surface area. The preferred unit is the
watt per square meter [W/m2]. The irradiance will change
inversely with the square of the distance. If the radiation source is
moved to twice the distance, the same amount o flight will be
spread over four times the area and the irradiance will be reduced
by 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 area
emitted, reflected, or transmitted by a 1m2 surface regardless of
direction.
Radiometry
irradiance
(1 Watt/m2)
1 square meter
Radiometric Units (SI)
radiant intensity
(1 Watt/sr)
radiant flux (Watt, power)
isotropic
radiation
radiance
(Watt/srm2)
1 steradian
1 steradian is the unit of solid angle that
encloses a surface area on sphere equal to the square of the radius.
Photometry
Photometry is a subset of radiometry. In radiometry, the detector
has 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 to
the spectral response curve of the eye.
The spectral sensitivity of the human eye, also known as the
photo-optic response curve.
Photometry
Luminous Flux [lm] – The lumen is essentially a unit of power
useful to the human eye. It is closely related to the watt as the
spectral luminous efficacy (km) for monochromatic light at the
peak visual response wavelength of 555nm.
It has been standardized at 683 lm/w. A standard 200W light bulb
produces a broad band radiation as well as heat in the form of
infrared radiation. The radiant flux produced by the lamp is 100W.
If all of its radiation were concentrated at 555nm, it would have an
output of 200W  683 lm/w = 68,300
However, only 10% of the total radiant power radiated by the
lamp is within the visible and even less (2%) is useful to the
human eye because of the eye’s insensitivity to blue and red
wavelengths. A typical output for a 200W bulb is 1750lm. The
luminous efficacy of the lamp is lumens per watt,
1750lm/100W = 17.5lm/W
Photometry
Luminous Intensity [lm/sr or candelas] – Assume the luminous
flux is radiated in all directions, like a point source. If we form a
cone of 57.2960, or 1 steradian (the unit of solid angle that
encloses a surface area on the sphere equal to the square of the
radius) with its origin at the lamp and extending to the surface of
the sphere, the total visible light flowing through the cone is
luminous intensity. Luminous intensity is expressed in lm/sr or
candelas. A full sphere contains 4  or 12.56 steradians. So a
light bulb of 1750lm/12/56=136cd. Again, the diameter of the
sphere is irrelevant. The luminous intensity in cd is the basic unit
of photometry, all other units are derived by combining the
candelas with units of are, distance, solid angle and time.
Photometry
Illuminance [lm/m2] – Illuminance is the amount of visible
radiation incident upon a specified surface area. The preferred
unit is the lux (lumen per square meter). The deprecated
Footcandle (lumen per square foot) is still used and can be
converted to lux by simply multiplying it by 10.764. The inverse
square law determines the illuminance.
Luminance [cd/m2] – Luminance is candelas per square meter,
is the unit to indicate how much light is reflected, transmitted or
emitted by a diffusing surface. The deprecated unit, the
footlambert () is still used.
Luminance Exitance – is also measured in lumens per square
meter, analogous with illuminance, is used to indicate the total
light per unit area emitted, reflected, or transmitted by a surface
regardless of direction.
Photometry
illuminance
(1 lm/m2=1 lux)
1 square meter
Photometric Units (SI)
luminous intensity
(1 lm/sr=1 cd)
luminous flux (power)
luminance
(cd/m2, nit,
lm/srm2)
isotropic
radiation
1 steradian
Lambertian Reflector
1 steradian is the unit of solid angle that
encloses a surface area on sphere equal to the square of the radius.
Photometry
illuminance
(1 lm/ft2)
1 square foot Photometric Units (English)
luminous intensity
(1 lm/sr=1 cd)
luminous flux (power)
isotropic
radiation
luminance
(fL)
1 steradian
Lambertian Reflector
1 steradian is the unit of solid angle that
encloses a surface area on sphere equal to the square of the radius.
Examples of Illuminance/Luminance
Examples of Natural Illuminance Levels
Direct Sunlight
105 lx
Daylight (excluding direct sunlight)
104 lx
Overcast Sky
103 lx
Heavy Overcast
102 lx
Twilight
1-10 lx
Full Moon
10-1 lx
Overcast night sky (no moon)
10-4 lx
Examples of Illuminance/Luminance
Examples of Luminance Levels
Sun’s disk
1.5108 cd/m2
100W soft white light bulb
3104 cd/m2
Fluorescent lamp surface
104 cd/m2
Overcast Sky
3103 cd/m2
Blue Sky
103 cd/m2
White paper (in office)
102 cd/m2
CRT
60-150 cd/m2
Radiometry/Photometry
Radiometry
Photometry
Radiant Flux (Watt)
Luminous Flux (lumen)
Radiant Intensity (Watt/Steradian)
Luminous Intensity (lumen/Steradian)
Irradiance (Watt/m2)
Illuminance (lumen/m2, lux)
Radiance (Watt/Steradian m2)
Illuminance (cd/m2, nit)
Conversion Factors between Photometric units in SI system and
English system.
Footlambert
candela/m2
Footlambert
1
0.2919
candela/m2
3.426
1
Footcandles
lux
Footcandles
1
0.2919
lux
3.426
1
Quantify 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 Functions
screen mask
white
screen
RGB mix
test
lamp
test
Observer adjusts RGB
until the mix matches
the
test lamp
The color matching functions are derived from a basic color matching
experiment, to define a linear mapping from a test light spectral power
distribution test lamp. The test light is set to unit energy at lnm test
wavelengths. The observer adjusts the primary intensities (RGB) until test and
mixture fields match. The relative weights are termed tristimulus values, and
the color matching functions are spectral plots of the tristimulus values.
The Color Matching Functions
z
y
x
y-axis (Relative Response) : x-axis (Wavelength in nm)
Tristimulus Values
780
X
r,g ,b
= k

S r,g ,b
 λ  xd λ
S r,g ,b
 λ ydλ
S r,g ,b
 λ  zd λ
380
780
Y r,g ,b = k

380
780
Z r,g ,b = k

380
k=683 lm/watt (normalizing factor), Sr,g,b is the spectral
power distribution of source.
Chromaticity Coordinates
X
r
X
=
Yr =
Zr =
X
r
r
+ Yr + Z r
Yr
X
r
+ Yr + Z r
Zr
X
r
+ Yr + Z r
CIE 1976 Chromaticity Coordinates
u =
4X r
-2 X r + 1 2 Y r + 3
v=
9 Yr
-2 X r + 1 2 Y r + 3
CIE 1931
Reflective Objects
Depend on Ambient Illumination
780
X
r
= k

SPD
 λ R  λ  xd λ
SPD
 λ R  λ  y d λ
SPD
 λ R  λ  zd λ
380
780
Yr = k

380
780
Zr = k

380
SPD (l) is the spectral power distribution of source
k is the normalizing factor
Photo-optic Reflection
780
 SPD(l ) R (l ) y (l )dl
%RP ==
380
780
 SPD(l ) y (l ) dl
380
SPD (l)
R (l)
Fluorescent Lamp
CLC Theory
y (l)
Color Matching
y-axis (Radiance in Watts/sr m2)
Standard Spectral Power Distribution
380
550
780
380
550
780
380
550
780
380
550
780
x-axis (Wavelength in nm)
Examples of
Spectral Power Distribution
Flourescent Lamp
Sun
Sylvania
Bulb
P-LED
Generating 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 Addition
The simple color addition scheme for electric displays. Examples
Include 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 Addition
F u ll C o lo r D is p la y s
C o lo r A d d itio n -R G B
S p a tia l C o lo r S y n th e s is
Full Color Displays
Color Addition-RGB
Addition -RGB
Color Additive Integrated
Intraged Stack
Stack
F u ll C o lo r D isp la ys C o lo r A d d itio n -R G B
T e m p o ra l S yn th e sis
0 < tim e < T 1
t 1 < tim e < T 2
t 2 < tim e < T 3
Color Temperature
Many times in the center of a chromaticity diagram (white region)
you will see temperatures listed. An object to any temperature
above 650-800K will produce a spectrum emission with its color
related 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 the
chromatic diagram, and is known in the literature as the Plankian
locus. Most natural light sources, such as the sun, stars and fire
fall close to this locus of points. Displays are often designed to
meet these criteria.
Light Source
Color Temperature (K)
Daylight, fluorescent lamp
6500
CRT Computer Displays
~6500
Contrast
CR =
Lon
L o ff
=
L u m in a n c e o f o n - p ix e l
L u m in a n c e o f o ff - p ix e l
Lon: Luminance of the on-pixel
Loff: Luminance of the off-pixel
Contrast
The derivation of PCR is intuitive and can be performed
heuristically. 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 stimulated
to 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.
Chromaticity Contrast
The contrast ratio is a measure of the ratio of luminance between
an on and off pixel. A more sophisticated approach is to
incorporate both luminance and chromaticity contrast, where the
total contrast is the root mean square of chromaticity and
luminance contrast. To arrive at the chromaticity contrast, there
have been many empirical studies to ascertain a normalized
chrominance index. An empirical chrominance ratio u:
 Δu
2
+ 2 .2 2 4 Δ v 
0 .0 2 7
2

1 /2
Chromaticity Contrast
Where Du’ and Dv’ are the difference in chrominance between the
two regions (a pixel) as plotted on the CIE chromaticity diagram.
The 0.027 is an empirical factor based on just perceivable
difference. The total contrast ratio, which includes both
chrominance and luminance, can be combined as a root mean
square.
C R  to ta l  =
 c h ro m in a n c e 
2
+  L u m in a n c e C o n tra s t

2
Resolution
Resolution: is the ability to delineate (resolve) picture detail.
The smallest discernible and measurable detail on a visual
presentation. 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. The
standard is # pixels per inch.
‘Ball Park’ definition:
Ultra-high
High
Medium
Low
PD > 120
120 > PD > 70
70 > PD > 51
PD < 50
Summary
• Display Technologies
• Threshold vs. Non-Threshold
• Direct Drive Addressing
• Multiplexing Addressing
• Active Matrix Addressing
• Radiometry
• Photometry
• Chromaticity Coordinates
• Contrast
• Resolution