Color Vision, Part 2
Lecture 9. Thurs, 10/01
Jonathan Pillow
Perception (PSY 323), Fall 2009
The University of Texas at Austin
Basic Principles of Color Perception
energy
an illuminant with most power at medium
wavelengths (i.e., a greenish light source)
13 measurements of
power spectrum
(example)
Basic Principles of Color Perception
energy
an illuminant with power at all visible wavelengths (a
neutral light source, or “white light”)
13 measurements of
power spectrum
(example)
Basic Principles of Color Perception
Q: How many measurements of this same spectrum
does the human eye take (in bright conditions?)
Basic Principles of Color Perception
photoreceptor response
Q: How many measurements of this same spectrum
does the human eye take (in bright conditions?)
A: Only 3! One
measurement
from each cone
class
Basic Principles of Color Perception
photoreceptor response
Q: How many measurements of this same spectrum
does the human eye take (in bright conditions?)
A: Only 3! One
measurement
from each cone
class
Think of each of these
curves as a “receptive
field” (over wavelength)
for a single cone
• tell how to “add up” the
energy from different parts
of spectrum in order to
generate cone response
Basic Principles of Color Perception
photoreceptor response
Q: How many measurements of this same spectrum
does the human eye take (in bright conditions?)
could also call
this axis
“absorption” or
“sensitivity”
A: Only 3! One
measurement
from each cone
class
Think of each of these
curves as a “receptive
field” (over wavelength)
for a single cone
• tell how to “add up” the
energy from different parts
of spectrum in order to
generate cone response
More terminology:
photoreceptor response
absorption spectra - describe response (or “light absorption”)
of a photoreceptor as a function of frequency
Absorption
spectrum for
“L” (red) cone
A single photoreceptor doesn’t “see” color; it gives greater
response to some frequencies than others
single cone absorption spectrum
single cone absorption spectrum
• All the photoreceptor
gives you is a
“response”
• Cant tell which light
frequency gave rise to
this response (blue or
orange)
Problem is actually much worse: cant tell a weak signal at the
peak sensitivity from a strong signal at an off-peak intensity
single cone absorption spectrum
+0.5
spectral power
+1
• Response to a
+2
single-wavelength
light source is the
power times the
absorption spectrum
curve. (Just like the
receptive fields we
considered earlier)
• All three of these
lights give the same
response from this
cone
Problem of univariance: An infinite set of different
wavelength–intensity combinations can elicit exactly the
same response from a single type of photoreceptor
single cone absorption spectrum
+2
• Therefore, one
+0.5
spectral power
+1
type of
photoreceptor
cannot make color
discriminations
based on
wavelength
So a single cone cant tell you anything about the color of light!
Colored stimulus
Response of your
“S” cones
sensitivity
1
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
energy
illuminant #1
wavelength
cone responses:
40
175
240
sensitivity
1
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
energy
illuminant #1
wavelength
cone responses:
40
175
240
sensitivity
1
percept
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
energy
illuminant #1
wavelength
cone responses:
40
175
240
sensitivity
1
percept
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
energy
illuminant #1
#2
wavelength
cone responses:
40
175
240
sensitivity
1
percept
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
energy
illuminant #1
#3
#2
wavelength
cone responses:
40
175
240
sensitivity
1
percept
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
energy
illuminant #1
#4
#3
#2
wavelength
cone responses:
40
175
240
sensitivity
1
percept
0.8
0.6
0.4
0.2
0
400
450
500
550
600
650
700
illuminant #1
energy
Metamers
#4
#3
#2
wavelength
- Illuminants that are
physically distinct but
perceptually
indistinguishable
Implication: tons of things in the natural world have
different spectral properties, but look the same to us.
Implication: tons of things in the natural world have
different spectral properties, but look the same to us.
But, great news for the makers of TVs and Monitors:
any three lights can be combined to approximate any color.
Single-frequency spectra
produced by (hypothetical)
monitor phosphors
illuminant #1
energy
Monitor phosphors
produce “metameric
match” to illuminant #1
(or any other possible
illuminant).
wavelength
Close-up of computer monitor, showing three phosphors,
(which can approximate any light color)
This wouldnt be the case if we had more cone classes.
hyperspectral marvel:
mantis shrimp
(stomatopod)
• 12 different cone
classes
• sensitivity extending
into UV range
This wouldnt be the case if we had more cone classes.
hyperspectral marvel:
mantis shrimp
(stomatopod)
• 12 different cone
classes
• sensitivity extending
into UV range
• No surprise that they never invented color TV!
Real vs. Conterfeit $$
Output of hyper-spectral camera
(colorized artificially)
Worth knowing about
if you end up seeking
employment in the
“informal” sector
following graduation.
(or law enforcement)
Color vision
photoreceptor response
Our color vision relies on comparing the responses of
three cone classes
Cone responses entirely determine our color percepts:
S
M
L
100
100
100
50
50
50
0
0
0
Cone responses entirely determine our color percepts:
S
M
L
100
100
100
50
50
50
0
0
0
100
0
0
0
100
0
0
0
100
Cone responses entirely determine our color percepts:
S
M
L
100
100
100
50
50
50
0
0
0
100
0
0
0
100
0
0
0
100
100
100
0
0
100
100
100
0
100
How did they figure this out?
-3 “primary” paints
-color mixture of lights:
any color can be made
three suitable lights...
How did they figure this out?
Late 17th Century: Isaac Newton
Late 17th Century: Isaac Newton
“The rays themselves, to speak properly, are not coloured”
Newtons Spectrum:
R
O
Y
G
B
I
V
Newtons Theory:
seven kinds of light -> seven kinds of photoreceptor
James Maxwell: color-matching experiment
Given any “test” light, you can match it by adjusting the
intensities of any three other lights
(2 is not enough; 4 is more than enough)
Trichromacy
Color space: A three-dimensional space that describes
all colors. There are several possible color spaces
(i.e., different coordinates for describing the same
space)
• RGB color space: Defined by the outputs of long,
medium, and short wavelength lights
• HSB color space: Defined by hue, saturation, and
brightness
Hue: The chromatic (color) aspect of light
Saturation: The chromatic strength of a hue
Brightness: The distance from black in color
space
•hue around the edge
•saturation increasing
from center to edge
Trichromatic color vision is called the Young-Helmholz
theory of color vision
• Thomas Young (1773–1829) and Hermann von
Helmholtz (1821–1894) independently discovered
the trichromatic nature of color perception
• This is why trichromatic theory is sometimes called
the “Young–Helmholtz theory”
• James Maxwell (1831–1879) developed a colormatching technique that is still being used today
Notice the variability between individuals!
Young-Helmholz theory of color vision
However, theory doesnt completely explain everything
(E.G. Why is red the after-image of green?)
response
Some Retinal ganglion Cells have center-surround receptive
fields with “color-opponency”
space
M
L
M
M
M
response
M
M
wavelength
Red-Green (L-M) Color-Opponent cell
Opponent Channels:
• L-M (red - green)
• S - (L+M) (blue -yellow)
• L+M - (L+M) (black - white)
Opponent Processes
Opponent color theory: perception of color is based
on the output of three mechanisms, each of them
based on an opponency between two colors: Red–
green, blue–yellow, and black–white
• Some LGN cells are excited by L-cone onset in
center, inhibited by M-cone onsets in their
surround (and vice-versa)
• Red versus green
• Other cells are excited by S-cone onset in center,
inhibited by (L + M)-cone onsets in their surround
(and vice-versa)
Blue versus yellow
Opponent Processes
Afterimages: A visual image seen after a stimulus has
been removed
Negative afterimage: An afterimage whose polarity is
the opposite of the original stimulus
• Light stimuli produce dark negative afterimages
• Colors are complementary. Red produces green
afterimages and blue produces yellow afterimages
(and vice-versa)
• This is a way to see opponent colors in action