Color Vision

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Color Vision
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The Physical Stimulus

• Light which is a form of electromagnetic
  radiation, part of the electromagnetic spectrum.
• Wavelength varies from 400 to 700 nm.
• Different wavelengths of light distinguish
  colors.
• The major spectral colors are red, orange,
  yellow, green, blue, and violet.

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The Physical Stimulus (contd)
The Electromagnetic Spectrum
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The Physical Stimulus (contd)
The Visible Spectrum
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The Physical Stimulus (contd)

• Color is a psychological phenomenon. Objects
  themselves are not colored.
• They appear colored because they reflect light
  from certain regions of the spectrum.
• This reflected light must be picked up by the
  right sort of visual system.

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The Physical Stimulus (contd)

• Light consisting of only one wavelength is called
  monochromatic. Considered pure.
• Natural light is generally broadband, i.e.,
  contains a number of wavelengths.
• Appears to be white. Least pure of light.

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The Physical Stimulus (contd)

• Light can be represented in a spectrum which
  displays how much energy there is at each
  wavelength (See Fig 14.1).
• Color consists of three psychological
  characteristics.
• Hue The essential color of a light. Influenced
  by wavelength.

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The Physical Stimulus (contd)

• Brightness The amount of light emitted by an
  object. Influenced by energy.
• Saturation Apparent relative amount of color
  (vs. whiteness) in a light. Influenced by
  purity.

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Color Mixture (contd)

• When two spectral colors are mixed, a new hue
  results.
• Additive Mixture Two or more colors are added to
  make new light.
• Different wavelengths of light are projected and
  superimposed on a screen.

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Color Mixture (contd)

• Two tiny color patches could be placed side by
  side. Cannot be resolved separately.
• Any color can be produced by a combination of
  three primary colors.
• I.e., red, green, and blue.
• E.g., yellow can be made from red and green.

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Color Mixture (contd)

• Subtractive Mixture Opposite of additive
  mixture. Light is successively removed from a
  mixture.
• Usually done by stacking filters which remove
  certain wavelengths (Fig14.3).
• Each filter removes a small portion of
  wavelengths.

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Color Mixture (contd)

• A series of filters can be compounded to remove
  all but a small range of wavelengths.
• Any colored light can be mixed by a combination
  of the primary colors.
• No primary color can be produced by the other two.

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The Psychophysics of Color

• Tested using a split field stimulus (see Fig
  14.5).
• The amount of each primary is adjusted until the
  halves are identical.
• Achieve a psychological match, i.e., not
  identical as the spectra are different.
• Called a metameric match.

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The Psychophysics of Color (contd)

• This can be carried out with colors which are not
  true primaries.
• E.g., Red can be matched with yellow, blue, and
  green.
• You can make a metameric match, but you need to
  use a negative light in the wrong half of the
  field.

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The Psychophysics of Color (contd)

• I.e., one of the primaries (green) must be added
  to the left side.
• Color is three dimensional in that any color can
  be created from three primaries.

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Color Theory

• If color is three-dimensional, then we must have
  three types of color receptors.
• Monochromacy one type of receptor (see Fig.
  14.10 of absorbance spectra).
• The receptor will be most efficient at catching
  quanta (photons) at a certain wavelength (505 nm).

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Color Theory (contd)

• This is twice as much quanta than absorbed at 550
  nm.
• If you double the amount of quanta available at
  550 nm, the same amount of quanta will be caught
  as was at 505 nm.
• Its impossible for the visual system to
  distinguish between a 505 nm stimulus and a
  stronger 550 nm stimulus.

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Color Theory (contd)

• Principle of Univariance A photoreceptors
  response can only signal the amount of quanta
  being caught NOT the wavelength of the light.
• Such a visual system cannot distinguish color.
  World is seen in shades of grey.
• Any two colors would be confused (including
  white).

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Color Theory (contd)

• Dichromacy two types of receptor types (see
  14.11, absorbance spectra).
• Each receptor obeys principle of univariance.
• But, signals from the two receptors can be
  compared, i.e., the visual system looks at the
  ratio of the quanta caught between the two.

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Color Theory (contd)

• Cant make one spectral light match another by
  just varying energy.
• Only two primaries are needed to match all
  spectral colors.
• There is one neutral point where a particular
  wavelength is confused with white light.

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Color Theory (contd)

• White light will excite both receptors at a
  certain ratio.
• Any monochromatic light that excites the
  receptors at this same ratio will be confused
  with white light.

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Color Theory (contd)

• Trichromacy Three receptor types (see absorption
  spectra, Fig 14.12).
• No monochromatic light excites all receptors
  equally. Thus, no neutral point.
• Visual system is three-dimensional, three
  primaries are required to match a given light.

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Trichromatic Theory

• Normal people possess trichromatic color vision
  (Young-Helmholtz theory).
• Young proposed that we had three fiber types to
  perceive color based on three primaries.
• Helmholtz elaborated an proposed hypothetical
  excitation curves.

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Trichromatic Theory (contd)

• Physiological evidence came from
  microspectrophotometer studies conducted on
  humans and primates.
• Focused a beam of light onto a single cone and
  determined the amount of light absorbed by
  photopigments in the cones.
• Three types of cones were found.

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Trichromatic Theory (contd)

• Absorbance maxima were 450, 525, and 550 (see
  Fig 14.13).
• These cones are referred to as short wavelength
  sensitive (SWS), mid wavelength sensitive (MWS),
  and long wavelength sensitive (LWS),
  respectively.
• These cones dont completely correspond to the
  primary colors.

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Color Defects

• Trichromats (anomalous trichromats) Three cones
  but one or more of the pigments have abnormal
  absorbance spectra.
• Need three primary colors to make a match, but
  the relative amounts are different.
• Have difficulty making wavelength discriminations
  in the red and green regions of the spectrum.

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Color Defects (contd)

• Dichromats One of the three pigments is missing.
• Protanopes Lack the LWS cone pigment. A
  red-green defect.
• Confuses spectral lights from 550 - 700.

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Color Defects (contd)

• Deuteronopes Missing MWS pigment. Also a
  red-green defect. Confuse spectral lights from
  530 - 700 nm.
• These color defects are sex-linked hereditary
  defects located on the X chromosome.
• More common in males (8) than females

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Color Defects (contd)

• Tritanopes Missing SWS pigment. Confuse spectral
  lights from 445 - 480.
• Very rare color defect.
• In all these cases of dichromacy, subjects
  possess a neutral point.
• They can mix any color with two primaries.

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Color Defects (contd)

• Monochromats Most lack all three cone types.
  Possess only rods. Very rare.
• Can not make any wavelength discriminations.
• Another group of monochromats possess only SWS
  cones.
• Can not make wavelength discriminations except at
  night when rods contribute to color vision.

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Opponency

• There are visual phenomena that can not be
  explained by trichromatic theory.
• In color adaptation studies, subjects stare at a
  stimulus of a particular color.
• They then look at a white background and see an
  afterimage.
• Green - Red Blue - Yellow

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Opponency (contd)

• Complementary colors colors that can be mixed
  together to produce white.
• Red and bluish-green reddish-yellow and greenish
  blue and blue and yellow.
• Colors of the spectrum can not be described using
  only combinations of red, green and blue. Must
  include yellow.

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Opponency (contd)
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Opponency (contd)

• Similarly, when naming colors, certain pairs of
  colors tend to be exclusive.
• Red and green blue and yellow.

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Opponency (contd)

• According to trichromatic theory, we see the
  world in shades of red, green, and blue.
• Why do we need four color names to name the
  entire spectrum?
• These problems led Hering to come up with the
  Opponent-Process Theory.

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Opponent-Process Theory

• The three cone types are combined to produce
  red-green, blue-yellow, and black-white
  processes.
• The colors in each process oppose each other
  (i.e., opponent processes).
• If you present a red light, you should be able to
  add green to it to nullify the response of cells
  in this channel. The light appears neither green
  nor red.
• Same for blue and yellow.

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Opponent Process Theory (contd)

• Can explain color afterimages.
• Stare at a green light. The green portion of the
  red-green process becomes adapted and fatigued.
• When you stare at a white screen the red portion
  is more active, thus we perceive the white screen
  as being red.

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Physiological Evidence

• DeValois et. al. (1966) measured activity in LGN
  cells of monkeys.
• Found four types of cells 1) excited by red
  inhibited by green (R-G) 2) excited by green,
  inhibited by red (G-R) 3) excited by blue,
  inhibited by yellow (B-Y) 4) excited by yellow,
  inhibited by blue (Y-B).

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Physiological Evidence (contd)

• These opponent type cells have also been found in
  the monkey retina.
• These cells are are arranged with center and
  surround portion (Fig, 14.20).

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From Trichromacy to Opponency

• Initially, these were considered alternative
  theories for color vision.
• In reality, both apply.
• Trichromacy exists at the level of the cones.
  Opponency starts at the level of the ganglion
  cells.

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From Opponency to Trichromacy (contd)

• Cones Retinal/Ganglion cell
• R-G/G-R
• LWS Red
• MWS Green
• B-Y/Y-B
• MWSLWS Yellow
• SWS Blue

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Color Constancy

• Color constancy the tendency for a surface to
  appear a particular color despite the
  illumination conditions.
• The color of a surface is poorly related to the
  spectrum of light emanating from it.
• Land (1971) took a picture of a natural scene and
  then projected on a screen.

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Color Constancy (contd)

• He placed filters on the projector which
  controlled the wavelength of light emanating from
  the objects in the photo.
• If equal amounts of long, mid, and short
  wavelengths were reflecting from a red pepper, it
  still looked red.
• How can we explain color constancy?

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Color Constancy (contd)

• Our visual system performs some type of
  computation on the entire visual scene that
  allows us to determine apparent color.
• Cells in V4 respond to the apparent color of an
  object rather than spectral composition.
• Part of ventral stream (parvocellular).
• Also involves cells in the inferotemporal cortex
  (V8).