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PSYB51H3 (301)
Chapter 5

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University of Toronto Scarborough
Matthias Niemeier

Chapter 5 - The Perception of Colour Basic Principles of Color Perception Color isnt 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 cant 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 2 warningso 1. Mixing wavelengths doesnt 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 cant 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 brightne
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