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Chapter 7: Perceiving Color
-The physical dimensions of color
-The psychological dimensions of color appearance
(hue, saturation, brightness)-The relationship between the psychological and physical dimensions of color
(Trichromacy Color opponency)- Other influences on color perception
(color constancy, top-down effects)
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Sir Isaac Newton
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Newtons Prism Experiment (1704)
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White light is composed of multiple colors
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Light
Monochromatic light: one wavelength (like a laser)
Physical parameters for monochromatic light
1. Wavelength2. Intensity
Heterochromatic light: many wavelengths (normal light sources)
For heterochromatic lightThe spectral composition gives the intensity at each wavelength
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Monochromatic light
Heterochromatic light
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Spectral composition of two common (heterochromatic) illuminants
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The spectral components of light entering the eye is the
product of the illuminant and the surface reflectance of objects.
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Reflectance of some common surfaces and pigments
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Surface reflectance of some common objects
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Spectral composition of light entering the eye afterbeing reflected from a surface =
XSpectral composition of theilluminant
Reflectance of the surface
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Consider a ripe orange illuminated with a bright
monochromatic blue (420 nm) light. What color will thebanana appear to be?
a. bright orange
b. dark/dim orangec. black
d. dark/dim blue
Wavelength
Relativeamountoflight
Blue (monochromatic)
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Spectral composition of light entering the eye afterbeing reflected from a surface =
XSpectral composition of theilluminant
Reflectance of the surface
Wavelength
Relativeamo
untoflight White light
Blue
(monochromatic)
Orange
(monochromatic)
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-The physical dimensions of color
-The psychological dimensions of color appearance
(hue, saturation, brightness)-The relationship between the psychological and physical dimensions of color
(Trichromacy Color Opponency)- Other influences on color perception
(color constancy, top-down effects)
Color Vision
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Color
Physical dimensions of color
Spectral composition
light source X reflectance
Psychological dimensions of color
1. Hue (e.g., red, blue, green)
2. Saturation (e.g., pastel, deep & rich)
3. Brightness (e.g., dim, bright)
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Newton, 1703
Newton noticed that the color spectrum could be represented in a circle.
Psychological dimensions of color
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1. hue
Psychological dimensions of color
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2. saturation
Psychological dimensions of color
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3. brightness
Psychological dimensions of color
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Psychological dimensions of color
The color solid.
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Graphics programs allow you to pick colors by hue,saturation and brightness from a color circle.
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How do we get from the physical properties of light:
To the psychological dimensions of color?
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Hue: peak (center) of spectral distribution
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Saturation: spread (variance) of spectral distribution
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Brightness: height of spectral distribution
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Bright, saturated blue Bright, partially desaturatedred (pink)
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Dark, desaturated green
These rules of hue, saturation and brightness are useful for simple (univariate)
spectral distributions:
But what about any arbitrary distribution?
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two primary colors
+ = ?
three primary colors
+ = ?+
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random
?
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Color mixtures
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Subtractive mixtures
Occur with paints (pigments) and filters
For pigments: In the mixture, the only wavelengths reflected by themixture are those that are reflected by all components in the mixture.
Cyan paint Yellow paint
White light Cyan reflected White light Yellow reflected
+ =
White light
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Subtractive mixtures
For filters: wavelengths in the mixture are those that are passed by every filter in use.
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The spectrum above is created on an RGB computer monitor byadditive mixing
Additive mixtures
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You can create additive mixtures by putting pixels veryclose together
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Thats how TVs and computer monitors work.
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And stadium scoreboards
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Georges Seurat, The Channel ofGravelines(1890)
Georges Seurat, the French Pointillist Painter knew about this!
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Closeup, Georges Seurat, TheChannel of Gravelines(1890)
Pointillism paintersmixed colorsadditively rather
than subtractively!
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TVs with three phosphors work because almost any color canbe generated by adding different amounts of the three primarycolors.
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TVs with three colors (phosphors) work because almost anycolor can be generated by adding different amounts of the threeprimary colors.
Why? Because we have three types of photoreceptors.
Wavelength (nm)
Absorption(%)
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Hofer, H. et al. J. Neurosci. 2005;25:9669-9679
Physiology of color vision
The normal retina
contains three kinds of
cones (S, M and L),each maximallysensitive to a differentpart of the spectrum.
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Trichromatic theory of color vision
Young-Helmholtz Theory (1802,1852).
Our ability to distinguish between different wavelengths depends on theoperation of three different kinds of cone receptors, each with a uniquespectral sensitivity.
Each wavelength of light produces a unique pattern of activation in thethree cone mechanisms.
Perceived color is based on the relative amount of activitythe patternof activityin the three cone mechanisms.
The Principal of Univariance
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The Principal of Univariance
For example, the M cones will respond equally to a dim green light as a bright
red light. As far as the M cones are concerned, these lights look the same.
the absorption of a photon of light by a cone produces the same effect no matterwhat the wavelength.
A given cone system will respond the same to a dim light near peak
wavelength as a bright light away from the peak.
A given light will excite the L M and S cones differently
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S M L
A given light will excite the L, M, and S cones differently
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S M L
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S M L
W dd li ht t di t L M d S t diff t t f 3 i i
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We can add lights to predict L, M and S responses to different amounts of 3 primaries
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S M L
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S M L
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S M L
W l di L M d S diff l l f i
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We can also predict L, M and S responses to different levels of saturation
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S M L
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S M L
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S M L
Given almost any light, its possible to find intensities of the 3 primaries that
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y g p p
produce the same L, M and S responses
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S M L
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S M L
Because of the principle of univariance two different spectra that produce
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S M L
Because of the principle of univariance, two different spectra that produce
the same L, M and S cone responses will look exactly the same.
These pairs are called metamers. They make a metameric match
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S M L
So the theory of trichromacy explains why we only need three primaries to
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So the theory of trichromacy explains why we only need three primaries to
produce a variety of colors. But what does an arbitrary sum of primaries looklike?
In 1878, Hering argued that trichromacy wasnt enough. He asked:
Why dont we ever see yellowish blues? Or reddish greens?
Color aftereffects
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Color aftereffects
blue green
red yellow
Color aftereffects
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Why are red and blue opposites? Why are yellow and green opposites?
blue green
red yellow
Color aftereffects
Hering proposed the Opponent Process Theory
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g p p pp y
Color vision is based on the activity of two opponent-process
mechanisms:
1. A RED/GREEN opponent mechanism.
2. A BLUE/YELLOW opponent mechanism
The Red-Green opponent system subtracts M from L cone responses
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-1 1
S M L
pp y p
Red-Green
The Blue-Yellow opponent system subtracts S from L+M cone responses
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-1 1 1
S M L
pp y p
Blue-Yellow
Now with RG = L-M and BY = (L+M) S,
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we can predict the appearance of any arbitrary spectrum of light.
Yellow: 560 nm Blue: 450 nm Red: 650 nm Green: 520 nm
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S M L
R-G B-Y
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S M L
R-G B-Y
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S M L
R-G B-Y
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S M L
R-G B-Y
Now with RG = L-M and BY = (L+M) S,
we can predict the appearance of any arbitrary spectrum of light
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Sum of primaries:
we can predict the appearance of any arbitrary spectrum of light.
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S M L
R-G B-Y
Or more complex spectra:
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S M L
R-G B-Y
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S M L
R-G B-Y
Our experience of color is shaped by physiological mechanisms both in the
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Our experience of color is shaped by physiological mechanisms, both in thereceptors and in opponent neurons.
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But, color perception depends on more than wavelength
Light/dark adaptive state (Purkinje shift)
Adaptation aftereffects
Simultaneous contrast effects
Color constancy
Examples:
Adaptation aftereffects
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Adaptation aftereffects
Simultaneous contrast effects
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Color Constancy is when the perceived color of objects does not vary
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The reflectance curve of a sweater (green curve) and the wavelengths reflected from the sweater when it is
illuminated by daylight (white) and by tungsten light (yellow).
Color Constancy , is when the perceived color of objects does not vary
much with changes in the illumination, even though these changes causehuge changes in the spectral light entering the eye.
Color Constancy, is when the perceived color of objects does not vary
much with changes in the illumination even though these changes cause
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Yellow light X blue surface =gray light entering eye
blue
Blue light X yellow surface =gray light entering eye
yellow
Example 1: same wavelengths entering the eye, different perceived color
much with changes in the illumination, even though these changes causehuge changes in the spectral light entering the eye.
Color Constancy
Somehow, the visual system knows the spectrum of the light source, and takes
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Yellow light X gray surface =yellow light entering eye
gray
Blue light X gray surface =blue light entering eye
gray
Example 2: different wavelengths entering the eye, same perceived color
that into account when determining the reflectance properties of a surface.
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Blue squares on the left are physically the same
as the yellow squares on the right!
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Blue squares on the left are physically the same
as the yellow squares on the right!
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Blue squares on the left are physically the same
as the yellow squares on the right!
Whats going on with this illusion?
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Remember, the light entering your eye is a combination of the light sourceand the reflectance properties of the object.
Whats important to you is the reflectance properties, not the light source.
The images on the left and right are drawn to look like the same object, justilluminated by two different lights (yellow on left, blue on right).
The blue checks on the left and the yellow checks on the right are both
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physically gray.
But with color constancy, the visual system knows that gray light under
yellow illumination must be caused by a blue surface (left),
and the gray light under blue illumination must be caused by a yellow
surface (right).
Another similar example. Center squares are physically the same, but look
different
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different.
The image is rendered to look like the two objects are illuminated by different
colored lights.
Chromatic adaptation supports color constancy
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Color Deficiency (Color Blindness)
Monochromat person who needs only one wavelength to match any color
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Monochromat - person who needs only one wavelength to match any color
Dichromat - person who needs only two wavelengths to match any color
Anomalous trichromat - needs three wavelengths in different proportions thannormal trichromat
Unilateral dichromat - trichromatic vision in one eye and dichromatic in other
Color Experience for Monochromats
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Monochromats have:
A very rare hereditary condition
Only rods and no functioning cones
Ability to perceive only in white, gray, and black tones
Univariance
True color-blindness
Poor visual acuity
Very sensitive eyes to bright light
Dichromats only two cone types
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Dichromats are missing one of the three cone systems, so there arethree types of dichromats.
Wavelength (nm)
Absorption(%)
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Protanopes missing L cones
Deuteranopes missing M cones
Tritanopes missing S cones
Dichromats only two cone types
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1. Protanopia affects 1% of males and .02% of females
They are missing the long-wavelength pigment
Individuals see short-wavelengths as blue
Neutral point (gray) occurs at 492nm
Above neutral point, they see yellow
Dichromats only two cone types
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2. Deuteranopia affects 1% of males and .01% of females
They are missing the medium wavelength pigment
Individuals see short-wavelengths as blue
Neutral point (gray) occurs at 498nm
Above neutral point, they see yellow
Dichromats only two cone types
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3. Tritanopia affects .002% of males and .001% of females
They are most probably missing the short wavelength pigment
Individuals see short wavelengths as blue
Neutral point (gray) occurs at 570nm
Above neutral point, they see red
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Color Processing in the Cortex
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There is no single module for color perception
Cortical cells in V1, V2, and V4 respond to some wavelengths orhave opponent responses
fMRI experiments on color vision show responses to color all over thevisual cortex, but particularly strong responses in area V4.
V4 seems to be necessary for color vision. Damage to
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V4 leads to cerebral achromatopsia complete colorblindness, even though the cones are normal.
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