SCI 200 Physical Science
Lecture 9
Color & Color Vision
Rob Daniell
July 21, 2011
Physical vs. Psychological Color
 Psychological Color
• Physical color
• Objective
• Directly measurable
• Based on
wavelength
• Any “color” can be
defined by the relative
intensity of light at
each wavelength
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 Subjective
 Indirectly
measurable
 Based on the
response of cones
and subsequent
processing
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 Ganglions
 Brain
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Physical Color
 Electromagnetic Spectrum
 Color vs. wavelength
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Physical Color
 Electromagnetic Spectrum
 Intensity vs. wavelength
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Physical Color
 Spectroscope




Light source
Entrance slit
Dispersive element (prism or grating)
Screen or detector
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Physical Color
 Simplified Grating Spectroscope
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


Project STAR Spectrometer
Transmission grating
Adjustable scale
Do not point directly at sun
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Physical Color
 Diffraction Grating
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Physical Color
 Simplified Grating Spectroscope
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
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Project STAR Spectrometer
Transmission grating
Adjustable scale
Do not point directly at sun
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Physical Color
 Wavelength spectra of various light sources
 Intensity units are relative
 Gilbert and Haeberli [2007] Am. J. Phys., 75, 313-319.
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Physical Color
 Wavelength spectra of fluorescent light bulbs
 As seen through Project STAR Spectrometer
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Physical Color
 Discrete spectrum
 Helium
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Physical Color
 Discrete spectrum: more examples
 Hydrogen, Sodium, Helium, Neon, Mercury
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Physical Color
 Continuous spectrum
 White light, sunlight, etc.
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Monochromatic vs.
Non-monochromatic Colors
 Monochromatic colors:
 Consist of a single wavelength
 Sometimes called “spectral colors”
 Non-monochromatic colors:
 1. A discrete spectrum
 several discrete wavelengths
 2. A continuous spectrum
Most colors in nature are nonmonochromatic
Example: sunlight
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Psychological Color
• Physical color as perceived by the
human eye and brain
• Color perception is mediated by the
cones in the retina
• There are (usually) three kinds of cones
operating
– Each cone type responds differently to a specific physical
color
– The signals from the cones are processed in a non-intuitive
way to produce the sensation of color
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Psychological Color
• Color specification systems:
– HSV: Hue, Saturation, Value
• Also:
– HSL (hue, saturation, lightness)
– HSB (hue, saturation, brightness)
• Corresponds most closely to human color perception
• Preferred by many artists
– RGB: (Red, Green, Blue)
• Used in additive color systems
• Used in many digital graphics applications
– Displays
– Software
– CMYK: (Cyan-Magenta-Yellow-blacK)
• Used in subtractive color systems
• Used for printing inks, etc.
– “Four Color Printing”
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Color Vision
• HSV: cylinder
 Hue:
perceived color
0°-240°
240°-360°
(“purples”)
 Saturation:
Purity of color
0-1
 Value:
Light intensity
0-1 or black to
white
(brightest)
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Color Vision
Another representation of HSL
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Trichromacy
History
Thomas Young (1773-1829)
Observed that it only takes three quantities
(Hue, Saturation, Value) to specify a color
Three output quantities require three input
quantities
Postulated three kinds of photoreceptors
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Trichromacy
 History (continued)
 Hermann von Helmholtz
(1821-1894)
 Suggested that Young’s
three photoreceptors were
 Short wavelength
 Intermediate wavelength
 Long wavelength
 Must overlap
 Monochromatic light of
different wavelengths
have different colors
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Trichromacy
Monochromatic light
would appear to
consist of exactly three
colors
Response
Suppose there were
no overlap:
400
S
I
500
L
700
600
 For example, (above) any monochromatic light source
between 400 and 500 nm would appear blue.
 Yet we know that 450 nm light is a very different shade of blue
than 475 nm light
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A. Overlap of Response Curves
 Example: six
monochromatic
emission lines from
atomic Helium
 Each a different color
 Conclusion: There
must be at least two
overlapping cones at
each wavelength in
the visible region
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 Line spectrum of helium
(He)






Blue-violet:
Blue:
Green:
Orange:
Red-orange:
Dark red:
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447.1 nm
471.3 nm
501.5 nm
587.5 nm
706.5 nm
728.1 nm
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Trichromacy
 Where do the curves
cross?
 This requires exploring
the properties of
psychological color
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Trichromacy
• Complementary colors:
• R+CW
• G+MW
• B+YW
• White can be produced by
• Broadband light (e.g.,
sunlight)
• Pairs of complementary
colors
• Stimulate the three kinds of
photoreceptors “equally”
• An infinite variety of other
combinations
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C = Cyan, M = Magenta, Y = Yellow
W = White
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Color Perception Mechanisms
If Helmholtz is right, how can we
determine the actual response curves?
A.
B.
C.
D.
Overlap of response curves
Spectral complementaries
Hue discrimination
Microspectrophotometry
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Trichromacy
 Where do the curves
cross?
 Consider a
monochromatic color at
about 430 nm.
 Stimulates S with a little I
 Another monochromatic
color near 610 nm could
stimulate some I and more
L to produce white.
 Also, vice versa
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Trichromacy
 Where do the curves
cross?
 Note that in the region
where the Intermediate
photoreceptors dominate,
no single complementary
spectral (monochromatic)
color exists
 No one spectral color can
stimulate both the S and the
L photoreceptors equally.
 Empirically, this is the
region from 490 nm to 565
nm
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Trichromacy
 So 490 nm and 565 nm represent the crossover points between S
and I and between I and L, respectively
 Between these wavelengths, it takes two additional monochromatic
sources to combine with a “green” source to produce white
 A “blue” source and a “red” source - hence “purple” (or magenta)
 This has consequences for color mixing (Lecture 10)
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Trichromacy
 Hue discrimination:
 The difference in wavelength (Δλ,
pronounced “delta lambda”) at
which two monochromatic sources
are barely distinguishable
 Varies with wavelength
 Where Δλ is small, the
photoreceptor response must be
changing rapidly
 Further Details of the spectral
response curves required
microspectrophotometry
 The physical measurement of the
amount of light of each wavelength
absorbed by each kind of cone
 Although many cones have been
measured this way only three basic
types have been found
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Trichromacy
 Cone Mosaic:
 Simulation based on measured
cone densities
 No “blue” cones in the central
fovea!
 Visual acuity in blue light is less
than in green and red light
 Over the entire retina
 There are about 100 “red” and
“green” cones for every “blue”
cone
 There are about 150 “red” cones
for every 100 “green” cones
 However: Much variation among
individuals
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Trichromacy
 Spectral sensitivity of the
three types of cones in the
human eye
 Intensity of each wavelength
is the same
 There is considerable
overlap among the three
cone types
 Type II & Type III cones
have the same sensitivity
at about 560 nm
 Figure 6.4 from text
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Trichromacy
 Spectral sensitivity of
Type II (green) cones
 Two different wavelengths
can produce the same
response
 Figure 6.5 from text
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 Using all three types of
cones, the four colors can be
distinguished.
 Figure 6.6 from text
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Trichromacy
• Color vision:
– : 3 kinds of cones
• Type I: Short (S), beta (β), or blue (B)
• Type II: Intermediate (I), gamma (γ), or green (G)
• Type III: Long (L), rho (ρ), or red (R)
Note that the three kinds of
cones do not actually
correspond to blue, green,
and red. The RGB model is
merely a convenient means
of representing color.
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Color Vision
• RGB color system:
• Based (loosely) on the three cones of the human eye
• Z ~ blue, Y ~ green, X ~ red (even though it peaks shortward of red)
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Color Vision
• Additive color rules:
•
•
•
•
R+G+B=W
R+G=Y
G+B=C
R+B=M
• Complementary colors:
• R+C=W
• G+M=W
• B+Y=W
• Can any 3 colors be combined
to produce any other color?
• Can R, G, & B be combined to
produce any other color?
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C = Cyan, M = Magenta, Y = Yellow
W = White
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Color Vision
• Red, Green, & Blue can be
combined to produce most
colors, but some saturated
(or nearly saturated) colors
cannot be reproduced.
• Will be considered in more
detail in Lecture 10
C = Cyan, M = Magenta, Y = Yellow
W = White
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Color Vision
• Subtractive color
combination:
• Filters that absorb or block light
of certain colors
• Ink or pigments that reflect only
certain colors and absorb the
others
• Primary Subtractive Colors:
• Cyan, Magneta, Yellow
• Supplemented by Black in “four
color printing”
• Will be considered in more
detail in Lecture 10
C = Cyan, M = Magenta, Y = Yellow
K = Black
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Trichromacy
• Where does “yellow” come from?
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Trichromacy or Opponent Colors?
Statements:
Magenta looks like a mixture
of Red & Blue
Cyan looks like a mixture of
Green & Blue
Yellow looks nothing like a
mixture of Red & Green
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Trichromacy or Opponent Colors?
Based on the trichromacy theory
We should expect an additive mixture of red and green to give a
reddish green (or a greenish red).
Instead it gives yellow
In fact, it takes four psychological primaries to verbally
describe any color
Blue, green, yellow, and red
Orange looks yellowish red
Cyan looks bluish green
Purple looks reddish blue
Etc.
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Opponent Processing
When asked to name the
color of a spot of spectral
(i.e., monochromatic) light,
most people give
responses similar to those
at right
Note that there is no
“reddish green” or
“yellowish blue”
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Opponent Processing
Yellow and blue seem to oppose
each other
Red and green also seem to oppose
each other
How can the three kinds of cones be
wired together to produce this kind
of color opposition?
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Opponent Processing
S inhibits y-b and stimulates r-g & w-bk
I inhibits r-g and stimulates y-b & w-bk
L stimulates all three opponent systems
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Opponent Processing
Net stimulation of y-b makes the light
appear yellowish; net inhibition, bluish
Net stimulation of r-g makes the light
appear reddish; net inhibition, greenish
The w-bk channel conveys brightness
information
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Opponent Processing
 There are at least two rival theories for the details of
how the three kinds of cones get processed to produce
the opponent activity.
 One theory makes use of lateral inhibition in the form
of center-surround antagonism among the various
cones
 Another assumes some kind of filter that narrows the
wavelength range accessible to some cones but not
others.
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Genetics of Color Vision
 Review: basics of human
genetics
 Each cell in the human body
contains 23 pairs of
chromosomes
 The chromosomes are numbered 1
through 22 plus the X and/or Y
chromosome
 In each pair, one comes from the
mother, the other from the father.
 The gender is (mostly) determined
by the X and Y chromosomes
 Females have 2 X chromosomes, one from each
parent
 Males have an X chromosome from their mothers
and a Y chromosome from their fathers
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 Other primate species
have differing numbers
of chromosomes
 The Great Apes all have
24 pairs
 Gender is generally
determined in the same
was as for humans
 The genes controlling
color vision differ
among primate species
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Genetics of Color Vision
 Color Vision
 The number of cone types varies dramatically throughout the
Animal Kingdom
 Mammals
 Most mammals have only two types of cones – dichromats
 Short vs. long wavelength: Yellow vs. Blue
 Red-Green color blind
 Primates
 All new world primates are dichromats
 But see next slide
 Many old world primates are trichromats
 Especially monkeys, apes, and humans
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Genetics of Color Vision
 Scientific American, April
2009, The Evolution of
Primate Color Vision, pp.
56-63.
 Some Old World primates
(including humans) are
trichromats
 Gene for the short wavelength
(“blue”) cone resides on
chromosome 7
 Genes for the medium
wavelength (“green”) cone
and the long wavelength
(“red”) cone both reside on the
X chromosome
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Genetics of Color Vision
 Scientific American, April
2009, The Evolution of
Primate Color Vision, pp.
56-63.
 New World primates are mostly
dichromats
 Gene for the short wavelength
(“blue”) cone resides on
chromosome 7
 Gene for one of the longer
wavelength (“green”, “yellow”,
or “red”) cones resides on the
X chromosome
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Genetics of Color Vision
 Scientific American, April
2009, The Evolution of
Primate Color Vision, pp.
56-63.
 Some female New World
primates are trichromats
 One X chromosome has one of
the green, yellow, or red cones
 The other X chromosome has
a different “long wavelength”
cone
 These females can distinguish
colors that their dichromat
brothers and sisters cannot
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Genetics of Color Vision
 It appears that “new world”
dichromacy is the ancestral
condition:
 Among old world primates, a
recombination error resulted
in both “green” and “red”
genes appearing on every X
chromosome.
 Both males and females
became trichromats
 Strong selective advantage,
so this system became the
norm in Old World primates.
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Genetics of Color Vision
 Every cone cell contains the genes for every cone
type
 Dichromats have two types
 Trichromats have three types
 In any particular cone cell, only one gene is actually
expressed
 Mechanism for selection of which gene to express is not
known
 For the genes on the X chromosome, it appears that the choice
is random
 Matrix of “red” and “green” cones is a random distribution
 So an individual with both a red and a green cone gene
on the X chromosome would have both red and green
cones.
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Genetics of Color Vision
 Having both red and green cones would give
the individual a strong survival advantage
 It would be much easier to distinguish ripe fruits
(yellow, orange, etc.) from unripe fruits (green)
 It would be much easier to distinguish some
predators (e.g., a leopard with a tawny coat) from
the leaves or bushes (green) in which it was
hiding.
 The selection pressure was so strong that
trichromats have completely displace dichromats
among Old World monkeys and apes – and, of
course, humans
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Genetics of Color Vision
 Implications for “opponent processing”
 If the ancestral color processing was dichromacy:
 It probably involved the opposition of blue cones and the
“yellow” (i.e., longer wavelength) cones
 The advent of trichromacy with the simultaneous
appearance of red and green cones on the X chromosome
made a second color opposition possible
 Red vs. green
 Thus, the psychological colors - blue, green, yellow, and red
- may have arisen naturally from the basic distinction
between blue cones on the one hand and red, green, and
yellow cones on the other.
 Yellow being a synthesis of the red and green cones
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Genetics of Color Vision
 The ability to distinguish between “blue” cones and the other
kinds of cones appears to be “hardwired” into the brain
 The ability to distinguish between “red” and “green” cones
appears to be “learned.”
 Female mice that have been genetically engineered to have a
“green” cone on one X chromosome and a “red” cone on the other
can learn to distinguish hues that are indistinguishable to their
dichromatic relatives
 There is evidence that the neural circuitry for distinguishing
“red” and “green” cones is the same as that used for spatial
vision
 Detecting boundaries, etc.
 If so, then “trichromacy can be viewed as a “hobby” of the
preexisting spatial vision system.”
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Genetics of Color Vision
• The trichromatic theory of color vision is
based on the three types of cones
• However, it has recently been discovered that
some people have a rare “yellow” cone
– Similar (identical?) to the “yellow” cone in New
World Monkeys
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Genetics of Color Vision
• For males, with only one X chromosome
– Standard trichromat: red, green, blue
– Non-standard trichromat (rare)
• Red, yellow, blue
• Yellow, green, blue
• For females with two X chromosomes
– Standard trichromat: red, green, blue
– Tetrachromat: red, green, yellow, and blue
• Still rare, but different
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Genetics of Color Vision
 Implications for “tetrachromacy” in some
women.
 If distinguishing between “red” and “green” cones
is learned, perhaps distinguishing among “red,
yellow, and green” cones is also learned
 Unfortunately, so far vision tests have not
produced conclusive evidence for true
tetrachromatic vision, but research is ongoing.
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Genetics of Color Vision
 Genetics of and evolution of color vision
 Subject of ongoing research
 Very complex system with lots of threads to unravel
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Color Vision Problems
• There are various kinds of color vision
“anomalies” or “deficiencies”
• Some are sex linked, since they involve the red
and green (and yellow?) cones on the Xchromosome
• Some are more general genetic anomalies
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Color Vision Problems
• Monochromats: People who see only one
color
• Relatively rare
• Two main types:
• Cone monochromats have cones, but only one type is
actually functional
• Can see under photopic conditions
• Rod monochromats lack all cone function
• Have difficulty seeing in bright light
• Poor visual acuity (no foveal rods)
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Color Vision Problems
• Dichromats: People who see only two colors (and
their combinations)
• Two subtypes:
• People with only two kinds of functional cones
• Three classes, depending on which cone type is nonfunctional
• Protanopia: lacking L (red) cones
• Deuteranopia: lacking I (green) cones
• Tritanopia: lacking S (blue) cones
• People for whom one of the opponent color systems is not
working
• Two classes, depending on which of the two opponent color
systems is nonfunctional
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Color Vision Problems
• Trichromats: People who see all three colors (and
their combinations)
• Normal trichromats
• Slight variations in the cone pigments
• Anomalous trichromats
• Large variations in cone pigments
• Connections between one type of cone and the nerve cells is
defective
• Protanomalous, deuteranomalous, tritanomalous variations
recognized
• No sharp boundaries, however
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Color Vision Problems
•
•
•
Ishihara “Test for Colour-Blindness” (2 sample plates)
On left: “Normals” see “26”; Protanopes and some protanomalous
observers see only the “6”. Deuteranopes and some deuteranomalous
observers see only the “2”
On right: Many color deficients can see a serpentine path between the
two x’s; Normals cannot.
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Summary
 Three variables or quantities are sufficient to
describe any color
 Trichromacy theory:
 Developed during 19th century
 Confirmed and quantified in the 20th century
 Three kinds of cones:
 Long, Intermediate, Short
 Red, Green, and Blue
 Not the whole story: Opponent Color
processing
 Signals from the three kinds of cones are
processed to produce Yellow-Blue and Red-Green
opponents
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Summary
 Original Opponent Color processing seems to involve
Blue vs. Yellow (Short vs. “Long”)
 Still the dominant form of color vision in New World monkeys
 Also most mammals
 In Old World Primates
 A recombination error enabled Red-Green opponent colors
 Apparently “learned” behavior
 Also enabled three color processing and the rich colors
visible to humans
 There appear to be three kinds of “intermediate” and
“long” cones
 “yellow” cones are relatively rare
 Some women have four kinds of cones
 May be able to sense an even richer range of colors
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Summary
 Various kinds of color vision “anomalies” or
“deficiencies”
 Trichromats: normal color vision
 Except some people have deficient cones of one or more
colors
 Dichromats: can see only two colors
 Monochromats: only a single color
 Some have no cones, so have poor visual acuity and
difficulty seeing in bright light
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Homework & Lab
Read Chapter 6 in textbook
Homework Packet 9:
Due Thursday, July 28
Next Lab: Lab 6: Water Prism
Thursday, July 21
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