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Chapter 5

psy 280- chapter 5.odt

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Chapter 5: the perception of color Basic Principles of color perception • electromagnetic spectrum between the wavelength spectrum between the wavelength of about 400 to 700 nanometers (nm) • most of the light we see is reflected • the more light that is absorb, the darker the surface will appear • the color of a surface depends on the mix of wavelengths that reach the eye from the surface Three steps to color perception 1. detection: wavelength must be detected 2. discrimination: we must be able to tell the difference between one wavelength length (or mixture of wavelength) and another 3. appearance : we want to assign perceived colors to lights and surfaces in the world. We want those perceived colors to go with the object and not to change dramatically as the viewing conditions changes (e.g. Blood should remain red in light or shadows) Step 1: Color Detection • we 3 types of cone ( S, M L- cones) they differ in their sensitivity to light of different wavelengths • s cone- sensitive to short wavelength “ blue cone” • M- cone sensitive to middle wavelength “green cone” • L- cone sensitive to Long wavelengths “red cone” • S cone is rare and is less sensitive compare to M and L • the combination of the three cones allow us to detect wavelengths from 400 to 700m • cone work at day time (photopic) and one type of rod works in dimmer (scotopic) light • photopic : light intensities that are bright enough to stimulate the cone receptors and bright enough to saturate the rod receptors (drive to their maximum responses) • Scotopic: light intensities that are bright enough to stimulate the rod receptors but too dim to stimulate the cone receptors Step 2: color discrimination The problem of univariance • a single photoreceptors cannot tell you the color, different combination of wavelength can get the same response • problem of univariance: infinite set of different wavelength intensity combination can elicit exactly the same response from a single type of photoreceptors. One photoreceptors type cannot make color discrimination based on wavelength In dimly lit scenes • there is only one type of rod photoreceptors • all rods contain the shame photopigment molecule: rhodopsin – same sensitivity to wavelength • we are unable to discriminate colors • e.g. Moonlit world appears drained color because we have only one type of rod photoreceptors transducing light under these scotopic conditions. We cannot make discrimination based on wavelength so we cannot see color - the same mix of wavelength that produces color perception during the day is the same as night but we fail to see color under dim illuminant like moonlight because dim light stimulate only rods The trichromatic solution • with three cone types, we can tell the difference between lights of different wavelengths • a specific light produces a specific set of responses from the three one types • if we increase the intensity of the light, the response size will change but the relationship will not • Trichromatic theory of color vision: the theory that the color of any light is defined in our visual system by the relationships of three numbers- the outputs of three receptors types now known to be the three cones Metamers • every light and surface that we see is emitting or reflecting a wide range of wavelengths Figure 5.7 • what is important is that red plus green produce mixtures that excites the L and M cone equally • Metamers: different mixtures of wavelengths that look identical. Or any pair that perceived as identical warning 1. mixing wavelengths does not change the physical wavelengths. It does not average or sum. Color mix is a mental event not a change in physics of light 2. for a mixture of red light and green light and to get perfect yellow. You need to exact red and green or it will look more reddish or greenish the mixture Generalize mixing of light • light reaching the retina from one patch in the visual field will be converted into three number by the three cone types • if those number are different from the number in another patch you will be able to discriminate those patches • if not , those patches are metamer • they will identical even if their wavelength are physically different The history of Trichromatic theory • basic theory was established by Isaac Newton • he discovered a prism could break up sunlight into the spectrum of hues and a second prism would put the spectrum back together into white • the trichromatic theory is often called “young helmholtz theory” • three missing lights are need to match any reference light. Two is not enough and four is too much Abrief digression of lights, filters and finger paints • addictive color mixture: a mixture of lights. If light a and light b are both reflected from a surface to the ey, the perception of color the effects of those two lights add together - added two wavelengths • subtractive color mixture: if pigment a and b mix some of the light shinning on the surface will be subtracted byAand some by B. Only the remainder contributes to the perception of color - color filters, e.g. Filters you put over stage lights Figure 10. • if we shine blue and yellow lights on the same patch of paper, the wavelength added up S,M and Lequals to white producing an additive color mixture From Retina to Brain: repacking the information To tell the difference between different lights, the nervous system will look at differences in the differences in the activities of the three cone types • computing differences between cone responses turns out to be useful way to transmit information to the brain • the difference between (L-M) contains most information about color • L and M are similar, a single comparison between S and L+m can capture the same information for L-S and M-S Cone- opponent cells in the retina and LGN • Lateral Geniculate nucleus( LGN): a structure in the thalamus, part of the midbrain, that receive input from the retinal ganglion cells and has input and output connections to the visual cortex • ganglion cells stimulate by spots of light • these cells have receptive field with characteristic centre surround organization . Excited when a light turns on in the central part and inhibited when lights turn on in the surround • Cone opponent cell: a cell type found in retina, lateral geniculate nucleus, and visual cortex- that in effect, subtracts one type of cone input from another. Step 3: color appearance • rods make contribution to color vision only i dim light • retina and LGN repacked information into cone-opponent difference signals that contains our ability to see differences between regions Three number, many colors? • We have exactly three different types of cone photoreceptors, the light reaching any part of the retina will be translated into three responses, one for each local population cone • going beyond the spectrum. We have three dimensional color space • color space: three dimensional space, established because color perception is based on the outputs of three cone types that describes the set of all colors • Achromatic: referring to any color that lacks a chromatic (hue) component black, white or grey figure 5.12 – describing color by Hue, saturation, brightness - different concept from red, green, blue • Hue: the chromatic (colourful aspect of color ( red, blue , green, yellow and etct) - each point on a spectrum • saturation : the chromatic strength of a hue. White has zero saturation pink more saturated and red is fully saturated • Brightness: the perceptual consequence of the physical intensit
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