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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

    350 400 450 500 550 600 650 700 750 800

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    Surface reflectance of some common objects

    350 400 450 500 550 600 650 700 750 800

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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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    350 400 450 500 550 600 650 700 750 800

    How do we get from the physical properties of light:

    To the psychological dimensions of color?

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    400 500 600 700400 500 600 700 400 500 600 700

    Hue: peak (center) of spectral distribution

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    400 500 600 700 400 500 600 700 400 500 600 700

    Saturation: spread (variance) of spectral distribution

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    Brightness: height of spectral distribution

    400 500 600 700400 500 600 700400 500 600 700

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    400 500 600 700 400 500 600 700

    Bright, saturated blue Bright, partially desaturatedred (pink)

    400 500 600 700

    Dark, desaturated green

    These rules of hue, saturation and brightness are useful for simple (univariate)

    spectral distributions:

    But what about any arbitrary distribution?

    400 500 600 700

    two primary colors

    + = ?

    three primary colors

    + = ?+

    400 500 600 700

    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(%)

    400 450 500 550 600 650 700 750

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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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    400 500 600 700

    300 400 500 600 700 800

    S M L

    A given light will excite the L, M, and S cones differently

    400 500 600 700

    300 400 500 600 700 800

    S M L

    400 500 600 700

    300 400 500 600 700 800

    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

    400 500 600 700

    300 400 500 600 700 800

    S M L

    400 500 600 700

    300 400 500 600 700 800

    S M L

    400 500 600 700

    300 400 500 600 700 800

    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

    400 500 600 700

    300 400 500 600 700 800

    S M L

    400 500 600 700

    300 400 500 600 700 800

    S M L

    400 500 600 700

    300 400 500 600 700 800

    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

    400 500 600 700

    300 400 500 600 700 800

    S M L

    400 500 600 700

    300 400 500 600 700 800

    S M L

    Because of the principle of univariance two different spectra that produce

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    400 500 600 700

    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

    400 500 600 700

    S M L

    So the theory of trichromacy explains why we only need three primaries to

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    400 500 600 700

    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

    400 500 600 700

    S M L

    R-G B-Y

    400 500 600 700

    S M L

    R-G B-Y

    400 500 600 700

    S M L

    R-G B-Y

    400 500 600 700

    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.

    400 500 600 700

    S M L

    R-G B-Y

    Or more complex spectra:

    400 500 600 700

    S M L

    R-G B-Y

    400 500 600 700

    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(%)

    400 450 500 550 600 650 700 750

    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.