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Chapter 5

PSYB51H3 Chapter Notes - Chapter 5: Subtractive Color, Scotopic Vision, Color Vision


Department
Psychology
Course Code
PSYB51H3
Professor
Matthias Niemeier
Chapter
5

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Chapter 5 - The Perception of Colour
Basic Principles of Color Perception
Color isn’t physical properly related to physical property
Most of light we see is reflected light
More light absorbed, darker surface will appear
Problem of Univariance
o Lights of 450 and 625 nm each elicit same response from photoreceptor
No way of distinguishing between 2 lights
o When seeing color, output of single photoreceptor is ambiguous
o Problem of variance: fact that infinite set of different wavelength-intensity combinations can elicit
exactly same response from single type of photoreceptor. One photoreceptor type can’t make color
discriminations based on wavelength
Trichromacy
Rods & Cones
Human retina contains 2 photoreceptors rods and cones
o Rods sensitive to low (scotopic) light levels
Scotopic: light intensities that are bright enough to stimulate rod receptors but too dim to
stimulate cone receptors
All rods contain same type of photopigment molecule rhodopsin same sensitivity to
wavelength
As consequence, though possible to tell light from dark under scotopic conditions, problem
of univariance makes it impossible to discriminate colors color is psychophysical
o Cone photoreceptors sensitive to higher, daylight levels
Phototopic: light intensities that are bright enough to stimulate cone receptors and bright
enough to “saturate” rod receptors
Come in 3 varieties, each containing slightly different photopigment
3 cone types named for where peak of sensitivity lies on spectrum
S-cone: 440 nm. Cone preferentially sensitive to short wavelengths; blue cone
M-cone: 535 nm. Preferentially sensitive to middle wavelengths; green cone
L-cones: 565 nm. Preferentially sensitive to long wavelengths; red cone
With 3 cones, can tell difference between lights of different wavelengths
Specific light produces twice as much M response as S response and twice as much S
response as L response
Trichromacy/trichromatic theory of color vision: theory that color of any light is defined in
visual system by relationships of 3 numbers, outputs of 3 receptor types now known to be 3
cones. Young-Helmholtz theory
Metamers
Nervous system knows only what cones tell it
If mixture of “red” plus “green” light produces same cone output as single wavelength of “yellow” light, mixture
and single wavelength must look identical
Metamers: different mixtures of wavelengths that look identical. More generally, any pair of stimuli that are
perceived as identical in spite of physical differences
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o 1. Mixing wavelengths doesn’t change physical wavelengths. Color mixture is mental event, not
change in physics of light
o 2. For mixture of “red” light and “green” light to look perfectly yellow, would have to have just right
red and right green. Other mixes look bit reddish or greenish
Lights, Filters and Finger Paints
Additive color mixture: mixture of lights. If light A and light B are both reflected from surface to eye, in
perception of color effects of those 2 lights add together.
Subtractive color mixture: mixture of pigments. If pigments A and B mix, some of light shining on surface will
be subtracted by A and some by B. only remainder contributes to perception of color
Three Numbers, Many Colors
Color vision is based on output of 3 types of cone photoreceptors.
Rods make small, important contribution, but only in dim light
Because we have 3 types of cone photoreceptors, light reaching any part of retina will be translated to 3
responses, one for each local population of cones
Rest of nervous system can’t glean anything more about physical wavelengths of light if light rays reflecting
off 2 surfaces produce same set of cone responses, 2 surfaces must and will appear to be exactly same color
metamers
Color space: 3 dimensional space, established because color perception is based on outputs of 3 cone types,
that describes set of all colors
Useful terms for describing perceived attributes of color are hue, saturation and brightness
o Hue: chromatic (colorful) aspect of color (red, blue, green, etc.)
o Saturation: chromatic strength of huge. White has zero saturation, pink is more saturated and red is
fully saturated
o Brightness: distance from black (0 brightness) in color space
History of Trichromatic Theory
Theory that color vision can be explained by responses of 3 mechanisms with different sensitivities to wave
lengths of light is known as “trichromatic theory of color vision”
Julie Schnaph managed to record activity of single photoreceptors
Jeremy Nathans found genes that code for different photopigments
In 1666, Newton understood that “rays to speak properly aren’t coloured. In them is nothing else than a certain
Power and Disposition to stir up a sensation of this or that color” knew it was mental event
3-D nature of experience of color was worked out in 19th century by Thomas Young and Hermann von
Helmholtz
o In honor, trichromatic theory is often “Young-Helmholtz theory”
James Clerk Maxwell developed color matching technique that was central to Helmholtz’s work on topic
Central observation from experiments was that only 3 mixing lights are needed to match any reference light
o 2 primaries aren’t enough and 4 are more than are needed
Opponent Processes
Repackaging the Info
Difference between L and M responses contains considerable info about color
Other info comes from differences between L and Mn responses, and S-cone responses
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