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Chapter 12: Colour Vision
• Colour relates to wavelength, and is processed first by the three classes of cone receptors with different
spectral sensitivity curves. Visual neurons then use chromatic opponency to encode wavelength.
Traditional Colour Descriptions
• Hue: The colour itself, such as “red” or “blue”.
• Saturation: Purity of the colour, often described in terms of how much neutral colour (white) is
present, e.g. pink is desaturated red.
• Brightness: Corresponds to perceived intensity of light.
• These three attributes can be depicted in a 3-dimensional [HSV] perceptual colour space
• Hue varies around the circumference of a horizontal circle (neutral white light at centre), saturation as
distance from the center of the circle, and brightness being represented on the vertical axis.
• Four colours cannot be described as intermediates of other hues – red, green, blue, and yellow. They
define the cardinal directions of chromaticity, and there are two cardinal axes R-G and Y-B.
• Ewald Hering: Considered red, green, blue, and yellow to be elementary colour sensations – can
describe all other experiences of colour using these four, and they themselves could not be described by
any other colours.
• He proposed that red and green were opponent colours, in that the sensation of red and the sensation
of green never appeared to co-exist in the same colour. Likewise, blue and yellow are opponent, as are
light and dark.
• Subtractive Colour Mixing: Removal of
wavelength components from a stimulus by
absorption or scattering. This is the basis of mixing
• The primary pigments are cyan, yellow, and
magenta – each absorbing all wavelengths except
those in the region they appear. When mixed, the
result is determined by the light spectra that remain for reflection to the eye. As more pigments are
added, more wavelengths are subtracted.
• When mixed in varying proportions they offer the broadest range of colour sensations for
painted or printed image reproduction systems such as inkjet printers.
• Subtractive colour mixing has made little theoretical contribution to colour vision.
• Additive Colour Mixing: Superimposing spectrally different light sources adds
wavelength components. The result is determined by the cumulative spectra emitted from
the various sources. As more coloured lights are superimposed, more wavelengths enter
• The primary colours are blue, green, and red. When these lights are mixed at various
relative intensities, they offer the broadest range of colour sensations for image reproduction systems
such as televisions, liquid crystal displays, and projectors. Page 2 of 5
• Additive mixing can create metameric colours, colours that are perceptually indistinguishable despite
having different spectral compositions.
• A monochromatic source in the yellow region of the spectrum will appear identical to a dichromatic
source that emits appropriate intensities of 560 nm (green) and 700 nm (red).
Laws of Additive Mixture
• 1) Linearity: A linear system’s response to a stimulus containing x, y, and z is the same as the sum of
its individual responses to the separate stimuli x, y, and z. This has two consequences for metameric
o When the same wavelength component is added to two colours that are metamers, their
apparent colour may change, but they will remain metamers.
o An additive mixture of two light sources is perceptually equivalent to an
additive mixture of the primaries of their metamers.
• 2) Trichromacy: Normal observers never require more than three primaries to
match any colour experience by additive mixing (assuming none of the
primaries is a metamer of the other two).
Colour Matching: CIE Chromaticity Diagram
• CIE Chromaticity Diagram: Standard graphical representation of hue and
saturation attributes of colour, based on colour-matching data – 2D plotting proportions of light, since
colour matches are independent of intensity.
• Pure spectral colours are plotted along the perimeter with mixtures inside and white at the centre.
• Complementary: Wavelengths at each
end of a straight line across the space
are complementary when the line passes
• Any colour can be identified by its
location on the X- and Y-axes, as
• Display Phosphors: Primaries red,
green, and blue which can be seen in a
computer display. Colours outside have
more extreme levels of saturation.
Explanation of Additive Colour Mixture
• Additive colour mixing can be explained at the retinal level, with three classes of cones with different
absorption spectra peaking at short (S), medium (M), or long (L) wavelengths.
• Although an individual cone class is more likely to absorb certain wavelengths than others, this
information is lost once the light is absorbed. A cone’s response is governed by the principle of
univariance, only carrying information about one variable: the quantity of light absorbed.
• Wavelength information requires comparing responses from the three cone classes. If L cones are
highly active relative to S and M, the inference is that the incident light has a long wavelength = red.
• Metameric colours appear identical because the ratio of activity they create in S, M, and L cones is
• Trichromacy Theory: The trichromatic nature of additive mixing led Palmer, Young, Maxwell, and
Helmholtz to the conclusion that human vision was similarly trichromatic.
• Evidence: Colour cannot be blue and yellow, nor red and green; people who cannot see red cannot see
green and vice versa; after-images come in opponent colours Page 3 of5
• Three: Dimensions of colour (HSV), opponencies (Y-B, R-G, light-dark), primaries (red, green, blue),
• Hurvich & Jameson resolved the apparent discrepancy between theories of trichromacy and the
opponent pairing of colours, by proposing that an opponent stage of processing followed
photoreceptor trichromatic analysis
• Ganglion and LGN cells show red–green, blue–yellow, and light–dark opponent responses, based on
signals from the triad of retinal photoreceptors (S, M, and L cones):
Opponent channel Cone input Signal carried by
Red–green chromatic channel Opponent: L – M Midget ganglion cells
Blue–yellow chromatic channel Opponent: S – (L + M) Bistratified ganglion cells
Light–dark achromatic channel Nonopponent: L + M Parasol and midget cells
Simultaneous Colour Cont