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PSYC 215 (33)
Niko Troje (24)


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PSYC 215
Niko Troje

Page 1 of 5 Chapter 12: Colour Vision COLOUR SPACE • 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. Opponent Colours • 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. COLOUR MIXTURE Subtractive Mixtures • Subtractive Colour Mixing: Removal of wavelength components from a stimulus by absorption or scattering. This is the basis of mixing pigments. • 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 Mixtures • 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 eye. • 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 matching: 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 through white. • Any colour can be identified by its location on the X- and Y-axes, as chromaticity coordinates. • 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 identical. Trichromacy Theory • 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), cone classes DUAL-PROCESS THEORY • 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 COLOUR INTERACTIONS Simultaneous Colour Cont
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