Eyes and Displays
© Dr. Zachary Wartell
3/23/2005
1
Light
• Vision is perception of electromagnetic energy
(EM radiation).
• Humans can only perceive a very small portion
of the EM spectrum:
Gamma X UV
Violet Blue
400
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
Infra Radar
Green
500
FM TV AM AC
Yellow
600
Red
700
Wavelength (nm)
3/23/2005
2
Radiant-Energy
Energy
or Power
Emission Spectrum
Wavelength
Energy [ joules]:
E photon , f  h f ,

Ef 
(h :Plank's Constant)
hf
all photons
Esource  
f

hf
all photons
Power[Watts=J/s] :

© Dr. Zachary Wartell
dE
dt
,
radiant flux, radiant power
3/23/2005
3
Light in real world
emission spectrum
medium
phototopic curve
(eye sensitivity)
reflection spectrum
© Dr. Zachary Wartell
3/23/2005
4
Light in graphics
RGB
emission spectrum
RGB
RGB
space “outside”
display typically
not computationally
modeled
Display
medium
reflection spectrum
RGB
pixels
RGB
© Dr. Zachary Wartell
RGB
3/23/2005
5
Light interactions
Light interacts with a surface in some combination of:
• emission
• reflection
– on surface : mirror, specular or diffuse
– suspended particles: random scattering
• transmission
– transparent, translucent, refraction
• absorption
© Dr. Zachary Wartell
3/23/2005
6
Eye Structure
• The eye can be viewed as a dynamic, biological camera: it has a
lens, a focal length, and an equivalent of film.
• A simple diagram of the eye's structure:
Cornea
Retina
Lens
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
3/23/2005
7
Lens Basics: Light Refraction
• Snell’s Law
i
sin  r  sin i
r
L
ηi
ηr
N
θi θi
R
reflected
T
θr refracted
• η index of refraction – light speed in vacuum
light speed in material
– complications: varies with material temperature, light
wavelength, anisotropic materials, double refraction
© Dr. Zachary Wartell
3/23/2005
8
Thin Lens Equation
o :object distance
s :image distance
1 1 1
 
o i f
f :focal distance
o
i
f
f
© Dr. Zachary Wartell
3/23/2005
9
Thin Lens Equation
o
i
f
f
• If the incident light comes from the object, we say it is a real object, and
define the distance from the lens to it as positive. Otherwise, it is virtual
and the distance is negative.
• If the emergent light goes toward the image, we say it is a real image, and
define the distance from the lens to it as positive.
• f = positive for a converging lens
• f often cited in measured in diopters (1/m)
• A light ray through the center of the lens is undeflected.
© Dr. Zachary Wartell, Dr. Larry Hodges
3/23/2005
10
Eye: The Lens
• The lens must focus (accommodation) on directly
on the retina for perfect vision:
• But age, genetic
factors, malnutrition
and disease can
unfocus the eye,
leading to near- and
farsightedness:
Nearsighted
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
3/23/2005
Farsighted
11
Eye: The Retina
•
The retina functions as the eye's "film".
•
It is covered with cells sensitive to light. These cells turn the light into
electrochemical impulses that are sent to the brain.
•
There are two types of cells, rods and cones
Retina
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
3/23/2005
12
Numbers of rods or cones per mm2
The Retina: Cell Distribution
180,000
Blind spot
140,000
rod
s
100,000
60,000
cones
“Blind Spot Trick”
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
Optic di
sk
Te
pe mpo
rip ra
he l
ry
Fovea
20,000
al ery
s
h
Na rip
pe
(Right Eye)
3/23/2005
13
Dark Adaptation
[Goldstein,pg 56]
© Dr. Zachary Wartell
3/23/2005
14
The Retina: Rods
• Sensitive to most visible frequencies (brightness).
• About 120 million in eye.
• Most located outside of fovea, or center of retina.
• Used in low light (theaters, night) environments,
result in achromatic (b&w) vision.
• Absorption function:
Cone
Rod
400
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
nm
3/23/2005
700
15
The Retina: Cones
• R cones are sensitive to long wavelengths (nm), G to
middle nm, and B to short nm.
• R: 64%, 32% G, 2% B
• About 8 million in eye.
• Highly concentrated in fovea, with B cones more evenly
distributed than the others (hence less in fovea).
• Used for high detail color vision (CRTs!), so they will
concern us most.
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
3/23/2005
16
The Retina: Cones
• The absorption functions of the cones are:
445 nm 535 nm 575 nm
B
G
R
400
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
700
3/23/2005
17
Color Constancy
• If color is just light of a certain wavelength, why
does a yellow object always look yellow under
different lighting (e.g. interior/exterior)?
• This is the phenomenon of color constancy.
• Colors are constant under different lighting
because the brain responds to ratios between
the R, G and B cones, and not magnitudes.
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
3/23/2005
18
Vision: Metamers
• Because all colors are represented to the brain as ratios
of three signals it is possible for different frequency
combinations to appear as the same color. These
combinations are called metamers.
This is why RGB color works!
• Example – [Goldstein,pg143]
mix 620nm red light with 530nm green light
matches color percept of 580 nm yellow
530 + 620
B
1.0
580
R
B
5.0 8.0
1.0
G
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky,Wartell
3/23/2005
G
R
5.0 8.0
19
Sensitivity vs Acuity
• Sensitivity is a measure of the dimmest light the
eye can detect.
• Acuity is a measure of the smallest object the eye
can see.
• These two capabilities are in competition.
– In the fovea, cones are closely packed.
Acuity is at its highest, sensitivity is at its
lowest (30 cycles per degree).
– Outside the fovea, acuity decreases
rapidly. Sensitivity increases
correspondingly.
© Kessler
Dr. Zachary
Wartell
©
, Watson,
Hodges, Ribarsky
3/23/2005
20
Displays: Pixel
• Pixel - The most basic addressable element in a image or on a
display
– CRT - Color triad (RGB phosphor dots)
– LCD - Single color element
• Resolution - measure of number of pixels on a image (m by n)
– m - Horizontal image resolution
– n - Vertical image resolution
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
21
Other meanings of resolution
• Dot Pitch [Display] - Size of a display pixel, distance from center
to center of individual pixels on display
• Cycles per degree [Display] - Addressable elements (pixels)
divided by twice the FOV measured in degrees.
• Cycles per degree [Eye] - The human eye can resolve 30 cycles
per degree (20/20 Snellen acuity).
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
22
Basic Image Synthesis Hardware (Raster Display)
Peripheral
Devices
CPU
System
Memory
raster images
found here
System Bus
Framebuffer
Display
Processor
Display
Processor
Memory
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
Video
Controller
3/23/2005
23
Raster – Bit Depth
• A raster image may be thought of as computer memory organized
as a two-dimensional array with each (x,y) addressable location
corresponding to one pixel.
• Bit Planes or Bit Depth is the number of bits corresponding to
each pixel.
• A typical framebuffer resolution might be
1280 x 1024 x 8
1280 x 1024 x 24
1600 x 1200 x 24
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
24
Displaying Color
• There are no commercially available small pixel technologies that
can individually change color.
• spatial integration – place “mini”-pixels of a few fixed colors very
close together. The eye & brain spatially integrate the “mini”-pixel
cluster into a perception of a pixel of arbitrary color
• temporal integration - field sequential color uses red, blue and green
liquid crystal shutters to change color in front of a monochrome light
source. The eye & brain temporally integrate the result into a
perception of pixels of arbitrary color
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
25
CRT Display
CRT
Shadow Mask
Electron Guns
Red Input
Green
Input
Blue Input
Focusing
System
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
Deflection
Yoke
Red, Blue,
and Green
Phosphor Dots
3/23/2005
26
Electron Gun
•Contains a filament that, when heated, emits a stream of electrons.
•Electrons are focused with an electromagnet into a sharp beam and
directed to a specific point of the face of the picture tube.
•The front surface of the picture tube is coated with small phosphor dots.
•When the beam hits a phosphor dot it glows with a brightness
proportional to the strength of the beam and how often it is excited by
the beam.
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
27
Color CRT
G
•Red, Green and Blue electron
guns.
B
G
•Screen coated with phosphor
triads.
R
B
G
R
G
R
B
B
G
•Each triad is composed of a red,
blue and green phosphor dot.
•Typically 2.3 to 2.5 triads per pixel.
FLUORESCENCE - Light emitted while the phosphor is being struck by electrons.
PHOSPHORESCENCE - Light given off once the electron beam is removed.
PERSISTENCE - Is the time from the removal of excitation to the moment when
phosphorescence has decayed to 10% of the initial light output.
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
28
Shadow Mask
•Shadow mask has one small hole for each phosphor triad.
•Holes are precisely aligned with respect to both the triads and the electron guns, so
that each dot is exposed to electrons from only one gun.
•The number of electrons in each beam controls the amount of red, blue and green light
generated by the triad.
SHADOW MASK
Phosphor Dot
Screen
Red
Green
Blue
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
Convergence
Point
3/23/2005
29
Scanning An Image
Frame: The image to be scanned out on the CRT.
•Some minimum number of frames must be displayed each
second to eliminate flicker in the image.
CRITICAL FUSION FREQUENCY
•Typically 60-85 times per second for
raster displays.
•Varies with intensity, individuals,
phosphor persistence, room
lighting.
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
30
Interlaced Scanning
1/30 SEC
1/30 SEC
1/60 SEC
1/60 SEC
1/60 SEC
1/60 SEC
FIELD 1
FIELD 2
FIELD 1
FIELD 2
FRAME
FRAME
Time
•Display frame rate 30 times per second
•To reduce flicker at lesser bandwidths (Bits/sec.), divide frame into
two fields—one consisting of the even scan lines and the other of
the odd scan lines.
•Even and odd fields are scanned out alternately to produce an
interlaced image.
•non-interlaced also called “progressive”
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
31
Scanning
(0,0)
Device CS
(alternate conventions)
(0,0)
VERTICAL SYNC PULSE — Signals the start of the next field.
VERTICAL RETRACE — Time needed to get from the bottom of the
current field to the top of the next field.
HORIZONTAL SYNC PULSE — Signals the start of the new scan line.
HORIZONTAL RETRACE — Time needed to get from the end of the
current scan line to the start of the next scan line.
© Dr. Zachary
WartellZachary Wartell
©Larry
F. Hodges,
3/23/2005
32
Example Video Formats
NTSC – ? x 525, 30f/s, interlaced (60 fld/s)
PAL – ? x 625, 25f/s, interlaced (50 fld/s)
HDTV – 1920 x 1080i, 1280 x 720p
XVGA – 1024x768, 60+ f/s, non-interlaced
generic RGB – 3 independent video signals and synchronization
signal, vary in resolution and refresh rate
generic time-multiplexed color – R,G,B one after another on a
single signal, vary in resolution and refresh rate
© Dr. Zachary
Wartell
©Larry
F. Hodges,
Zachary Wartell
3/23/2005
33
Calligraphic/Vector CRT
Video
Controller
P0
P0
P1
P1
Line (P0,P1)
older technology
vector file instead of framebuffer
wireframe engineering drawings
flight simulators: combined raster-vector CRT
© Dr. Zachary Wartell
3/23/2005
34
Flat-Panel Displays
Flat-Panel
Emissive
Non-Emissive
LCD
DMD
Plasma
CRT
(90°deflected)
LED
ActiveMatrix
(TFT)
PassiveMatrix
Thin-Film
electroluminescent
© Dr. Zachary Wartell
3/23/2005
35
Flat-Panel Displays (Plasma)
Flat-Panel
Emissive
Non-Emissive
LCD
DMD
Plasma
CRT
(90°deflected)
LED
ActiveMatrix
PassiveMatrix
Thin-Film
electroluminescent
ToshibaTM, 42”, Plasma HTDV
$4,500 (circa 2005)
© Dr. Zachary Wartell
3/23/2005
36
Flat-Panel Displays (Plasma)
[Hearn&Baker,
pg 45]
© Dr. Zachary Wartell
3/23/2005
37
Flat-Panel Displays (thin-film electroluminescent)
[Hearn&Baker,
pg 45]
© Dr. Zachary Wartell
3/23/2005
38
Flat-Panel Displays (LED)
Flat-Panel
Emissive
Non-Emissive
LCD
DMD
Plasma
CRT
(90°deflected)
LED
ActiveMatrix
PassiveMatrix
Thin-Film
electroluminescent
© Dr. Zachary Wartell
BarcoTM “Light Street” (LED)
3/23/2005
39
Flat-Panel Displays (DMD)
Flat-Panel
Emissive
Non-Emissive
LCD
DMD
Plasma
CRT
(90°deflected)
LED
Thin-Film
electroluminescent
ActiveMatrix
PassiveMatrix
4 μm
Digital Micro-mirror (DMD)
© Dr. Zachary Wartell
3/23/2005
40
LCD
• Liquid crystal displays use small flat chips which change their
transparency properties when a voltage is applied.
• LCD elements are arranged in an n x m array call the LCD
matrix
• Level of voltage controls gray levels.
• LCDs elements do not emit light, use backlights behind the LCD
matrix
© Dr. Zachary
Wartell
©Larry
F. Hodges,
Zachary Wartell
3/23/2005
41
LCD nematic liquid crystal details
© Dr. Zachary Wartell
3/23/2005
[Hearn&Baker,
pg 46]
42
LCD Components
Small
Diffuser
LCD
Linear
fluorescent
Module
Polarizer
Linear
Color
tubes
Polarizer
Filter
© Dr. Zachary
Wartell
©Larry
F. Hodges,
Zachary Wartell
3/23/2005
Wavefront
distortion
filter
43
LCD Resolution
LCD resolution is occasionally quoted as number of pixel elements
not number of RGB pixels.
dot pitch
Example: 3840 horizontal by 1024 vertical pixel elements = 4M
elements
Equivalent to 4M/3 = 1M RGB pixels
"Pixel Resolution" is 1280x1024
© Dr. Zachary
Wartell
©Larry
F. Hodges,
Zachary Wartell
3/23/2005
44
LCD
• Passive LCD screens
– Cycle through each
element of the LCD
matrix applying the
voltage required for
that element.
– Once aligned with the
electric field the
molecules in the LCD
will hold their
alignment for a short
time
© Dr. Zachary
Wartell
©Larry
F. Hodges,
Zachary Wartell
• Active LCD (TFT)
– Each element contains
a small transistor that
maintains the voltage
until the next refresh
cycle.
– Higher contrast and
much faster response
than passive LCD
– Circa 2005 this is the
commodity technology
3/23/2005
45
LCD vs CRT
flat & Lightweight
low power consumption
always some light
pixel response-time (8-30ms)
view angle limitations
resolution interpolation
required
© Dr. Zachary
Wartell
©Larry
F. Hodges,
Zachary Wartell
heavy & bulky
strong EM field & high voltage
true black
better contrast
pixel response-time not noticeable
inherent multi-resolution support
3/23/2005
46
Eye Versus 1280 x 1024 Display
66 cm
1280 pixel =
640 cycles
28°
33 cm
Pictured: 22.8 c/d (cycles/degree
with Vres=600/x → 26.25→ 20/26.25 vision
(Snellen acuity)
Widescreen at 60° → 20/56.25 vision
© Dr. Zachary Wartell
3/23/2005
47
Measurements
• Radiometry
• Photometry
• “Brightness”
© Dr. Zachary Wartell
3/23/2005
48
Radiometry Measures
Quantity
SI unit
Abbr. Symbol Notes
radiant energy
Joule
J
Q
Energy
radiant flux/power
Watt
W
Φ
Energy per unit
time
radiant intensity
Watt per
steradian
W/sr
I
Power per unit
solid angle
radiance
Watt per
steradian per
meter2
W/(sr L
∙m2)
Power per unit
solid angle per unit
projected source
area
irradiance
Watt per
meter2
W/m2 E
power incident on a
surface
radiant
Watt per
emittance/exitance meter2
W/m2 M
power emitted from
a surface
© Dr. Zachary Wartell
3/23/2005
49
Intensity (Watts/steradian)
d
I
d
dω=dA/r2
dA
r
dΦ
N
θ
dω
y
x
© Dr. Zachary Wartell

3/23/2005
50
Irradiance (Watts/m2)
●sums radiance over hemisphere arriving on a
surface
dω
θ
N
x
© Dr. Zachary Wartell
y

3/23/2005
51
Radiant Emittance (Watts/m2)
●sums radiance over hemisphere leaving a
surface
dω
θ
N
x
© Dr. Zachary Wartell
y

3/23/2005
52
Radiance
●measure in specific direction
d 
L
d ds0 cos 
2
dΦ
N
ds0
x
© Dr. Zachary Wartell
θ
dω
y

3/23/2005
53
Planckian Radiator
• “black body” radiator
– radiate energy perfectly
– absorb light perfectly (no reflection)
– as the radiator temperature rises how does
the spectral power distribution change?
SPD
3000K
Φ (W)
2000K
1000K
λ (nm)
© Dr. Zachary Wartell
3/23/2005
54
Graph of Theoretic Planckian Radiator SPD
20000 K
10000 K
5600 K
3000 K
2000 K
1000 K
© Dr. Zachary Wartell
3/23/2005
55
Planckian radiator SPD in Visible Region
1000K
2000K
3000K
4000K
5000K
6000K
8000K
10000K
100,000K
• increasing temperature (1000K to 100,000K) yields red,
orange-red, yellowish-white, bluish-white
• tungsten bulb example
© Dr. Zachary Wartell
3/23/2005
56
Examples SPD’s
[McDonald]
© Dr. Zachary Wartell
3/23/2005
57
Examples SPD’s
[McDonald]
© Dr. Zachary Wartell
3/23/2005
58
Color Temperature
• if light source SPD is similar to black body radiator,
associate the source’s color with the temperature at
which a Planckian radiator will give similar color
Source
Temp.
candle
1200K
tungsten
lamp
2800K
bright midday 6000K
sun
heavy
overcast sky
© Dr. Zachary Wartell
10000K
3/23/2005
59
Photometry
45 W
retinal
stimulation
?
Conductors
60 W
Σ
-15 W
1.0
photopic curve
Vλ
X
W
© Dr. Zachary Wartell
? = 900 lm
nm
400
3/23/2005
700
60
Photometry & Lumens
Φ (W)
Σ
1.0
photopic curve
Vλ
X
λ (nm)
400
700
760
Φv  683     V
 380
1 W of 555nm yields 683 lm
© Dr. Zachary Wartell
3/23/2005
61
Photometry Measures
Quantity
SI unit
Abbr.
Symbol
Notes
luminous energy
lumen second
lm ∙ s
Qv
Energy
luminous
flux/power
lumen
lm
Φv
Energy per unit
time
luminous intensity
candela
Iv
cd
(=lm/sr)
luminance
candela/meter2 cd/m2
Lv
luminous flux per
unit solid angle
per unit projected
area
illuminance
lux (lm/m2)
lx
Ev
light incident on a
surface
luminous
lux (lm/m2)
emittance/exitance
lx
Mv
light emitted from
a surface
© Dr. Zachary Wartell
3/23/2005
lum. flux per solid
angle
62
Brightness Measures (!)
• very hard to define analytically (no simple Vλ)
• complications
– contrast effects
• Gelb’s (1929) experiment – breaks light constancy
– adaptation effects
• dark adapt one eye
• above effects can cause same SPD to be
perceived differently
• active area of psychophysics research
© Dr. Zachary Wartell
3/23/2005
63
Contrast Effects Example
© Dr. Zachary Wartell
3/23/2005
64
Colorimetry: Measuring Color
• Colorimeter: adjust primaries so that:
“C[C] = R[R] + B[B] + G[G]”
R[R]
white
screen
G[G]
B[B]
eye
black
partition
view hole
with surround
C[C]
© Dr. Zachary Wartell
3/23/2005
65
Negative tristimulus values
• very pure target color may be unmatchable
C[C] ≠ R[R]+ G[G]+ B[B] for any (R,G,B)
• all we can do is de-saturate the target color
C[C] + R[R] = G[G]+ B[B]
• this could be formulated as negative coordinates
C[C] = -R[R] + G[G]+ B[B]
• No set of real primaries will allow for positive
coordinates to match all real colors!
© Dr. Zachary Wartell
3/23/2005
66
An Early Experiment (1931)
• Primaries: [R]=700nm, [G] = 546.1, [B] = 435.8
• Determine tristimulus values (R,G,B) for set of target
stimulus {[Ci]} where [Ci] is a single spectral color (i.e.
SPD contains 1 wavelength)
• Use (R,G,B)λ to define
distribution curves
435.8 546.1
700
r, g, b
© Dr. Zachary Wartell
3/23/2005
67
Compute tristimulus value (R,G,B) for any SPD
• Multiply SPD by r , g , b and compute area under curve:
Let SPD be E 
E r


, G

energy
360
780
E g


360

, B
Eλ
λ
R
E b



r
λ
Eλ r
Yuck! (R,G,B)
can be < 0!
λ
© Dr. Zachary Wartell
780
360
tristimulus
R
780
3/23/2005
68
Transform tristimulus values between primaries
If C[C] = Rc1[R1]+ Gc1[G1] + Bc1[B1] and
1[R1] = RR2[R2]+ GR2 [G2] + BR2 [B2]
1[G1] = RG2[R2]+ GG2[G2] + BG2[B2]
1[B1] = RB2[R2]+ GB2[G2] + BB2[B2]
then
C[C]=Rc1 (RR2[R2]+ GR2 [G2] + BR2 [B2])+
Gc1 (RG2[R2]+ GG2[G2] + BG2[B2])+
Bc1 (RB2[R2]+ GB2[G2] + BB2[B2])
=(Rc1RR2 +Gc1 RG2 +Bc1 RB2 ) [R2] +
(Rc1GR2 +Gc1 GG2 +Bc1 GB2 ) [G2] +
(Rc1BR2 +Gc1 BG2 +Bc1 BB2 ) [B2] +
= Rc2[R1]+ Gc2[G1] + Bc2[B1]
© Dr. Zachary Wartell
3/23/2005
69
Use “Imaginary” primaries to allow > 0 (R,G,B)
• CIE primaries defined by:
C700  0.73467[X]  0.26533[Y]  0.00000[Z]
C546  0.27376[X]  0.71741[Y]  0.00883[Z]
C435  0.16658[X]  0.00886[Y]  0.82456[Z]
• Solve for (X,Y,Z) over spectrum yields:
z
y
© Dr. Zachary Wartell
x
3/23/2005
Yey! (X,Y,Z)
always > 0!
70
CIE XYZ Space
Y
Features:
- X,Y,Z > 0
-[Y] follows Vλ
-equal energy SPD has
coordinate (k,k,k)
X
Z
© Dr. Zachary Wartell
3/23/2005
71
Chromaticity
• chromaticity – independent on amount of luminous
energy, dependent on dominant wavelength and
saturation
X
Y
Z
x
, y
, z
X Y  Z
X Y  Z
X Y  Z
• Note:
– x+y+z=1
– x , y , z are on X+Y+Z=1 plane
– if we know x , y, z is just 1- x – y, so we define
chromaticity by just x , y.
– use 2D plot of x , y by projecting X+Y+Z=1 plane onto
X,Y plane
– if we need full X,Y,Z we need to know (x , y ,Y) where
Y encodes the luminous energy
© Dr. Zachary Wartell
3/23/2005
72
Chromaticity Diagram
C - white light
B – dominant wavelength of A
D & E – complementary
AC/BC – excitation purity
F – nonspectral (G)
dom. wave. is complement
of H
B
A
H
E
D
© Dr. Zachary Wartell
C
F
G
3/23/2005
73
Color Gamut of Displays
[G1]
[G2]
[B1]
© Dr. Zachary Wartell
[B2]
[R1]
[R2]
3/23/2005
74
Correlated Color Temperature
Planckian locus
(labeled in Kelvin)
∞
© Dr. Zachary Wartell
(x,y) of colors with c.c.t
of 10000K
3/23/2005
75
Color perception more complicated
• Jameson (1985) – SPD of blue chip under
tungsten light = yellow chip under sunlight, yet
its still blue (color constancy)
• color constancy only approximate
• chromatic adaptation
– red adapt one eye & compare perception
between eyes
– eye adapts to tungsten light long wv’s indoors
(white paper appears white), but if adapted to
nighttime looking inside light appears yellow
© Dr. Zachary Wartell
3/23/2005
76
White Point
• visual system scales response from
photoreceptor according to illuminant color
– white point of scene is reference point for
perception
• display white point – result of maximum output
on all 3 channels to display
– described by c.c.t. or chromaticity (x,y)
– sometimes called c.c.t. or c.t. (yuck!)
– adjust internally (bias & gain) or digitally in
graphics card
© Dr. Zachary Wartell
3/23/2005
77