Chapter 9 – Perceiving Color
The Focus of this chapter.
How do we perceive the various wavelengths light emanating from light sources and reflected
off objects?
Review of the two primary physical attributes of light
1. Intensity
The number of streams of particles entering the eye.
2. Wavelength / frequency
Wavelength: Conceptualized here as the distance between particles within a stream.
Problem Everything in the visual scene is a complex combination of intensities and wavelengths.
Describing complex lights: The Wavelength Spectrum
A plot of the intensity of light at each wavelength e mitted by a light source or reflected from
an object.
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Examples of spectra of various objects.
A plot high at the right
indicates that the object
appears red or yellow.
A spike in the graph, if
high enough, may
determine the
predominant hue of the
object.
.
Terms associated with experience of wavelength.
White, gray, and black
objects share the same
form of curve – flat. The
white flat curve is simply
higher than a gray flat
curve.
Hue – That aspect of experience which depends most directly on wavelength. What we
typically call “color”.
Saturation – The amount of hue relative to white / absence of white.
Saturated blue:
Unsaturated blue:
Brightness – The intensity of light (number of particle streams) emanating from what is being
viewed.
Low brightness:
Higher brightness:
Main points so far
1. How to read a spectrum.
2. Rough understanding of relationship of a spectrum to the appearance of an object.
3. Hue
4. Saturation
5. Brightness
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Toward an understanding of how we perceive intensity and wavelength . . .
What was known 300+ years ago - in 1672, for example. (From Blake & Sekular)
The belief in the 1600s was that white light was the purest light.
All other lights were “dirty”, like dirty clothes.
Isaac Newton discovered evidence challenging this belief in 1672 at age 29.
His evidence was based on experiments conducted in a shed on his farm, using prisms.
He found that white sunlight could be separated into colored components.
His ideas were greeted with such fury and disdain that he waited more than 30 years before
publishing his complete work on these topics in a treatise called Opticks, in 1704.
By that time Newton's genius was nearly universally acknowledged so he had little to fear.
Even so, Goethe, a philosopher and poet expressed shock at the patent absurdity of Newton's
prism demonstration. Goethe considered Newton a "Cossack" for denying the purity of white
light. Goethe argued that Newton's decomposition of white into colors implied that white was no
more special than any other color.
Goethe even wrote the following poem arguing against Newton’s position.
Friends, escape the dark enclosure,
where they tear the light apart
and in wretched bleak exposure
twist and cripple Nature's heart.
Superstitions and confusions
are with us since ancient times leave the specters and delusions
in the heads of narrow minds.
(Translated by Weisskopf and Worth, in Weisskopf, 1976)
Goethe obtained a prism and attempted to verify some of Newton's claims.
He never correctly repeated what Newton had done.
For example, Goethe looked directly through the prism but failed to see a spectrum of colors,
leading him to believe that Newton, in addition to being a Cossack, was a charlatan.
Goethe went further. He enlisted the services of a young man, Arthur Schopenhauer, who would
later become a famous philosopher in his own right. Goethe interested Shopenhauer in his ideas
about light and color, and persuaded him to continue the assault on Newton.
Using Goethe's own optical equipment, the younger man began to do experiments with light
(Birren, 1941, p. 214). Unfortunately for Goethe, though, these experiments convinced
Schopenhauer that Newton had been right after all. Sunlight does consist of many different
colors.
Main points . . .
1. Beliefs of everyone about basic experiences were once quite different from those we have
now.
2. Challenging accepted beliefs can be very difficult.
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Combining hues. - Additive vs. Subtractive mixtures of hues - G9 p 202
Additive mixing: The wavelengths striking the eye are those in all components.
Additive mixing is achieved by shining two or more lights on the same white surface.
For example, adding pure blue and yellow lights results in both short wavelengths (blue) and
longish wavelengths (yellow) striking the eye.
Eye
Lights
B
Experience is that resulting from stimulation by both short and
long wavelengths – white, as we’ll see later.
Y
White surface
Subtractive mixing: The wavelengths of white light reflected to the eye are those not absorbed
by either component.
Achieved by mixing paints, blue and yellow, for example.
Put blue paint which reflects short wavelengths and absorbs long on a surface.
Then put yellow paint which reflects long wavelengths on the same surface.
The only wavelength reflected is medium – perceived as green.
Short +middle wavelengths
reflected
400
500
600
700
Middle +long wavelengths
reflected
400
Yellow paint
on a surface
500
600
700
Only middle wavelength
reflected.
400
Blue paint
on a surface
500
600
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Blue + Yellow
paint on a
surface
700
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Additive vs. Subtractive tatoos
Practical applications of mixing rules . . .
Additive
Television displays are of 1000s of tiny dots of hue.
There are 3 sets of dots, intermixed.
At a typical viewing distance, the individual dots are not visible – only the additive combinations
of the hues.
Subtractive
Ink jet printers.
Three different inks are mixed using rules of subtractive mixing.
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Basic color phenomena that must be accounted for by a theory of color perception.
1. Color Matching. G9 p 204
Almost any hue experience associated with a single wavelength can be mimicked by the
appropriate additive mixture of 3 carefully chosen “primary” wavelengths.
Look carefully at the individual lights.
Each has a different wavelength.
Newton devised a graphical method that can be used to show the results of additive color mixing.
He found that a “wheel” representation of the colors was most appropriate.
The center of the wheel represents white.
Each point on perimeter of the wheel represents an experienced hue.
The farther a point is from the center, the more saturated it is.
Using the wheel for additive mixtures
The result of additive mixture of two
hues is represented on the wheel by
a) drawing a line on the color wheel
between the two existing hues
b) picking a point on that line that
represents the relative intensities of the
two hues
c) The location point on the circle is the
experienced hue.
This can be generalized to the combination
of 3 (or more) hues.
Note that equal mixtures of Blue and Yellow yield white, as do equal mixtures of Red and Bluish
Green. This is a characteristic of additive mixtures. Adding colors on opposite sides of the
wheel yields white.
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2. Color deficiencies. G9 p 208
1st Major Category: Most color deficiencies are associated with an inability to distinguish what
we call red from what we call green.
Protanopia – Looking ahead, we now believe this occurs when persons lack L cones
Deuteranopia – We now believe this occurs when persons lack M cones
A 2nd major category is an inability to distinguish various types of blue from yellow
Tritanopia – We now believe this occurs when persons lack S cones
A 3rd major category is the inability to distinguish color at all.
Rod monochromacy
Cone monochromacy – We now believe this occurs when persons lack two cone types.
A theory of color perception must explain why we find the three categories of color deficiency.
How things may look to persons with various types of color deficiency . . .
A dichromat is a person with only 2 types
of cones.
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Aside: Testing for color deficiency –
Problem – most objects that reflect different wavelengths also reflect different overall intensities.
Solution: Create figures in which different parts reflect different wavelengths but are of the
same brightness.
Typical stimulus – A figure of dots whose wavelengths are different from the background dots
is created, but the Intensities of the figure dots are equal to intensities of the background
dots.
Dvorine Pseudo-Isochromatic plates or Ishihara Color Vision Test. Y1 p 181.
Under the appropriate lighting conditions, persons with a red-green deficiency will see this plate
as a collection of circles all of the same hue.
Those with normal color vision will see the number 67.
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Trichromatic Theory of Color Vision – G9 p 204
Basic Premise: Color perception is the result of the activity of three different classes of
receptor, each one most sensitive to a different wavelength of light. The activity of the three
receptors is combined somehow and it is the combination that determines the experienced hue.
According to the theory, the experienced hue is like the total score on a test with multiple
choice and essay.
You can get 100% on the multiple choice and 80% on the essay and have a 90% total.
Or you can get 80% on the multiple choice and 100% on the essay and have 90%.
So, either collection of individual scores yields the same combination – 90%.
Originally proposed by Thomas Young in 1801.
Elaborated and quantified by Hermann Helmholtz in 1909.
Behavioral evidence supporting trichromatic theory
Color matching phenomena
Each experienced hue the result of the combination of activity of the three receptor types.
Any collection of wavelengths that could yield the same combination would have the
same hue.
Color deficiency phenomena
Difficulty in distinguishing red from green was due to a missing receptor for long or
medium wavelengths.
Difficulty in distinguishing blue was due to a missing receptor for short wavelengths.
Difficulty in distinguishing all colors was due to missing two or more receptors.
Physiological evidence for Trichromatic Theory G9 p 204
Evidence for the existence of 3 types of cones was provided by microspectro-photometry in the
early 1960s – 160 years after trichromatic theory was first proposed by Young.
This research found 3 classes of receptor in the retinas of humans
S cones: Most sensitive to light of 443 nm. Experienced as violet.
M cones: Most sensitive to light of 543 nm. Experienced as green.
L cones: Most sensitive to light of 574 nm. Experienced as greenish yellow.
An interesting historical note is that using carefully designed color matching experiments,
Helmholtz in the early 1900s proposed receptor wavelength sensitivities that were remarkably
near the wavelength sensitivity curves found 60 years later.
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Approximate numbers of the cones found in the 1960s . . .
S:
M
L
1 million/eye
2.5 million/eye
2.5 million/eye
But numbers of M and L cones differ across persons. Fig 5.12
Spatial distribution of the cones:
L&M cones: Distributed throughout the fovea.
S: Scarce in the fovea.
This explains the fact that letters written in blue or violet are most difficult to perceive.
New evidence on the Number of Cone Types
FROM THE JULY-AUGUST 2012 ISSUE of DISCOVER MAGAZINE
The Humans With Super Human Vision
An unknown number of women may perceive millions of colors invisible to
the rest of us. One British scientist is trying to track them down and
understand their extraordinary power of sight.
By Veronique Greenwood|Monday, June 18, 2012
http://discovermagazine.com/2012/jul-aug/06-humans-with-super-human-vision .
(I have a copy of this if it can’t be downloaded.)
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Why do we need two or more cone types? G9 p 206
Response of a single cone to different wavelengths . . .
100
Could be
480 or 550.
50
400
500
600
700
Problem: The single cone type gives the same response to multiple different wavelengths
combinations.
So they would be perceived as the same.
So a creature (like us) depending on the response of a single cone type could not disteinguish
different wavelengths and different intensities from each other.
But two cones, with slightly different wavelength preferences could distinguish different
wavelengths . .
Since 1 is at
10and 2 is at 2,
it must be 480.
Having two cone types would enable us to make some discriminations.
Having three types enables us to make many more.
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Color Vision facts that cannot be accounted for in terms of trichromatic theory G9 p 210
3. Afterimages and color contrast
Afterimages: Prolonged exposure to a single hue followed by viewing a gray field yields the
experience of an afterimage that is of a different hue.
Hue of the afterimage corresponds to the hue of stimulus.
Red stimulus -> Green afterimage.
Green stimulus -> Red afterimage
Blue stimulus -> Yellow afterimage
Yellow stimulus -> Blue afterimage.
Stare at this object for 60 seconds. Then stare at a blank wall.
View for 60 seconds, then look at a
blank surface.
Color Contrast: The apparent saturation of red is heightened when viewed next to green.
Same for blue/yellow combinations.
4. Lack of certain hue experiences:
Reddish Green
Blueish Yellow
5. The categories of hue experience . . .
This suggests that we must have 6 categories of response – six responses that are somehow
qualitatively different from each other. Perhaps 6 different neuron types?
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Opponent Processes Theory G9 p 210
Proposed by Ewald Hering who published between 1878 and 1920, around the time of
Helmholtz.
Proposed 3 color processes
1. A Red / Green system/process that responded in one way to light perceived as red and in the
opposite way to light perceived as green.
2. A Yellow / Blue system/process that responded in one way to light perceived as yellow and
in the opposite way to light perceived as blue.
3. A white/black or achromatic system/process that responded in one way to high intensity light
and in the opposite way to low intensity light. This system did not respond differentially to
different wavelengths.
He called these processes. They were called opponent processes because they were presumed
to respond in two ways, each way opposing the other.
The perceived hue of an object is determined by the strength and direction of response of
the three processes.
If the R/G process is responding strongly in the R direction, the object will be perceived
as having Red in it.
If the Y/B process is perceived as responding strongly in the Y direction, the object will
be perceived as having Yellow in it.
And if the W/B process is responding strongly in the W direction, the object will be
perceived as bright.
So here’s how the opponent process system would account for the 6 hues
Hue
Red
Orange
Yellow
Green
Blue
Violet
Red/Green
Red
Red
Neither
Green
Neither
Red
Blue/Yellow
Neither
Yellow
Yellow
Neither
Blue
Blue
Note that two, orange, and violet, of the 6 categories of hue are “combination” hues –
represented by responses of both the R/G and the B/Y system.
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Phenomena the Opponent Processes theory could account for
4. Afterimages. For example, prolonged viewing one hue causes fatigue of that response, so
that the response in the opposite direction predominates when a neutral surface is viewed.
For example, prolonged viewing of a red stimulus caused the R/G process to become “fatigued”
and caused the “green” response to predominate when a neutral surface is viewed.
5. Lack of certain color experience. The R/G process cannot exhibit both a Red and a Green
response at the same time, so we have no experience of reddish green
6 The categories of color experience, e.g., red, green, blue, and yellow plus mixtures – Orange
= R+Y, Violet = R+B
Phenomena Opponent Processes theory could not account for
1. Matching.
2. Deficiencies.
Arguments against and evidence for the Opponent Processes Theory
Arguments against
The theory proposed that something in the brain responded in two different directions. There
was no evidence at the time that any nervous system component could do that. The belief at the
time was that nervous system components were either quiescent or active, so they could respond
in only one direction – becoming active. There was no evidence for an “opposite” response.
Physiological evidence supporting the Opponent Processes theory
Scientists studying the brain in the middle and late 1900s found that most neurons respond
continually, at what is called a base rate of activity. This opened the possibility for two types of
resonse of a neuron – 1) a decrease in rate or 2) an increase in rate.
DeValois and colleagues in the 1960s found neurons in the LGN that responded by increasing
firing rate to long wavelengths and decreasing firing rate to short wavelength lights.
His research found two general types of neuron - R/G neurons and B/Y neurons.
Response of B/Y neuron
Response of G/R neuron
Rate
Of
Activity
Wavelength
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Current Theory – G9 p 212
Now, virtually all researchers believe that
1) We have 3 types of cones and it is the output of these cones that begins the process of color
perception.
So trichromatic theory was correct in 1801 – 200+ years ago.
2) We have opponent processes that respond in opposite ways to R/G and B/Y.
So, opponent processes theory was correct – 100 years ago.
So how can this be?
The current belief is that the synaptic connections of receptors and bipolar and amacrine
cells in the retina result in retinal ganglion cell responses that are either Red vs. Green or
Blue vs. Yellow. Some ganglion cells are R/G. Others are B/Y.
Below is one possibility that is frequently presented.
Excitation is indicated by a (>). Inhibition is indicated by a ( | ).
G9 p. 212
S
M
Type of Cone
L
B/Y
R/G
Blue+ / YellowOpponent Process
Red + / Green Opponent Process
Response is
S – (M+L)/2
Response is
L-M
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Taking it to the extreme – why not have more than 3 receptor types? How about 300
types?
The above explorations have shown that the more types of receptors we have, the greater the
number of different wavelengths we can discriminate. Why not have 100s of receptor types?
Why stop at 3?
Suppose the visual system had 300 different types of receptor, each tuned to a different
wavelength.
402
403
405
404
406
700
407
401
400
Response of each
receptor type
Wavelength
Each receptor would signal a different wavelength.
The response of a given receptor would signal the intensity of the light at the wavelength to
which the receptor was most sensitive.
That is, the wavelength of light would be represented by which receptor was most active. The
intensity of the light would be represented by the level of activity of that one receptor.
The sum of the responses of all the receptors would signal the overall intensity of the light.
This system would be capable of analyzing the visual world into ALL its wavelengths and
intensities.
What would be good about having 300+ cone types?
1. The world would be incredibly more colorful.
Wavelengths that we now perceive as being the same, e.g., 620 and 625 are both
perceived as red, would instead be perceived as being quite different hues.
2. We could eat the time in order to feed all the receptors needed to process the wavelengths.
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What would be bad about having 300 receptor types?
1. To maintain acuity at all wavelengths, each type of receptor would have to be distributed
equally across the whole retina. This means the retina would have to be 100+ times bigger
than it is now to accommodate the 300 different types of receptor. Only then would we have
acceptable acuity at all wavelengths.
Current retina
with 100
million
receptors
Expanded retina
with 300*100 = 30
billion receptors
Also, there would be up to 100 times as many other types of cells – bipolar, ganglion,
horizontal, and amacrine – drastically increasing the nutritional demands of the retina.
So we’d have to have huge eyes, big enough to hold 300 times more receptors, bipolar cells,
horizontal cells, amacrine cells, and ganglion cells.
2. The LGN would also have to be 100 times bigger to process the information from the 300
receptor types. So we’d have huge bulges on the sides of our heads where the LGN is located.
3. There would have to be 100 times as many cortical cells to process the 300 times greater
amount of information form the 100 receptor types. The backs of our heads would have to be
huge, to contain all the neurons necessary to process information from 300+ receptor types.
Here’s how we might look.
Huge LGN from added
millions of neurons to
received added information
from retina - forming a bump
on the side of the head
Huge occipital
lobe hanging
over the back of
the neck.
Huge eyes required to contain
the added receptors and their
associated bipolar, amacrine,
and ganglion cells.
Brace to hold
up the huge
occipital lobe.
4. Every surface would be multicolored. We’d notice thousands of details that we don’t now
notice. We’d probably be driven crazy by the myriad colors of surfaces that we now perceive as
being all the same color.
5. Of course we’d have to have names for all the different colors. A whole grade in
elementary school would have to be devoted to just learning the color names.
ENOUGH COLOR, ALREADY!!!
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Constancies – G9 p 214
Constancy: The stability of experience in spite of changes in the external stimulation.
The hardest work that the nervous system does may be that done to create constancies – keeping
our experience of the world constant in spite of constantly changing external stimulation.
The latest DARPA challenge is to create a robot that could be of service in places (such
as nuclear power plant) that humans could not go. A major problem with the design of
these robots is making them so they don’t fall over all the time.
Types of constancy.
1. Shape/Form constancy.
The experience of shapes as being “the same” or unchanged even though the actual stimulation
on our retinas changes drastically.
See how all four views of the same object below are experienced as being “our text”.
2. Size constancy.
The perception of objects as having constant size in spite of large changes the actual size of the
image on the retinas. More on this in the chapter on depth/size perception.
3. Lightness Constancy.
The perception of objects as having constant lightness in spite of massive changes in the amount
of light reflected from them.
4. Color Constancy.
The perception of objects as having constant hue in spite of large changes in the spectrum
reflected from them.
Shape and size constancies are the only two we can “recognize”. We are essentially oblivious to
lightness and color constancies. Without careful thought and the use of instruments to measure
intensity and the spectrum, we wouldn’t even know that the last two existed.
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Color Constancy – G9 p 214-215
The perception of objects as having constant hue even though the spectrum of wavelengths
reflected by them changes drastically.
Examples
1. Spectral Reflectance Curve of a sheet of typing paper viewed in either sunlight or
incandescent light.
Curve in incandescent light – much
more long wavelength reflected.
Curve in sunlight– much more
short wavelength reflected.
Yet in either light, typing paper appears “white”. It doesn’t appear “blue” in sunlight and
“yellow or red” in incandescent light.
2. Spectrum of a blue sweater in sunlight and in incandescent light.
Reflectance curve of blue
sweater in Sunlight
Reflectance curve of same blue
sweater in Incandescent light
The appearance of the sweater appears essentially unchanged even though the amount of short
wavelength light reflected from it changes dramatically.
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Explanations for color constancy – how our visual system lies to us about wavelength
changes.
1. Chromatic adaptation. G9 p 215
The visual system adapts to prolonged exposure to a particular pattern of wavelengths, so if the
environment contains an excess of a particular wavelength, the visual system becomes less
sensitive to that wavelength.
So, for example, in incandescent light, in which there is a lot of long wavelength (orangish) light,
adaptation makes the visual system less sensitive to long wavelength light, make the
appearance of objects in that light look about the same as their appearance in sunlight which has
less long wavelength light than incandescent light.
This might explain why the insides of houses look normal when we’re in them but look
yellow when viewed from the street at night. Don’t do this.
When you’re in the street, your visual system has not adapted to the long wavelengths,
and hence, objects in the house appear yellow.
Chromatic adaptation certainly exists and does affect our perception of color. See
DEMONSTRATION, G9 p 215.
Research suggests that chromatic adaptation does play a role on color constancy. G9 p
215
2. Memory color. G9 p 217
Hue experienced is based on our memory of what the hue of the object was in other
circumstances.
An apple appears red in all illuminations because it’s been perceived as red so often.
There is evidence that memory has some, but not a huge effect on our experience of color.
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3. Effect of the surroundings. G9 p 216
This explanation assumes that the visual system automatically determines the average amount
of each wavelength reflected from all surfaces and uses this as an estimate of the spectral
distribution of the lighting in a particular situation.
Hue experience associated with an object arises from an automatic comparison of object
wavelengths with distribution of lighting in the situation.
This suggests that, for example, what is perceived as blue is that which reflects the shortest
wavelength of all the objects in a scene, regardless of the actual distrubtion of wavelengths.
The hue of an object is based on a comparison of its wavelengths with the wavelengths of
surrounding objects.
So, experienced hue is the result of a comparison of wavelengths across the visual scene.
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Using Habituation /Dishabituation to assess infant color vision
Q: How do we know whether or not an infant sees a newly presented object as being a different
hue from a previously presented object?
Present a stimulus repeatedly to an infant and record the amount of time the infant looks at the
stimulus.
Eventually, the infant habituates – quits looking at the stimulus.
Then present the test stimulus.
If the infant does not look at the test stimulus, then the conclusion is that it appears the same as
the habituating stimulus.
But if the infant dishabituates – looks at the new stimulus, that’s an indication that it the test
stimlulus is perceived as different.
4 month old infants.
510 nm
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