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Unformatted text preview: Chapter 11- The Visual System: Colour Perception A. Colour Science General: Newton showed in 1666 w/ prism experiments that the experience of colour has its origin in the nature of light entering our eyes. An animal is said to have colour vision is it has the ability to discriminate different lights only on the basis of its spectral (wavelength) content. It has been argued that colour vision developed to provide survival and reproductive advantages = 2 adaptive elements in Darwinian natural selection. Chromatic nature of light - Chromaticity: objective specification of the quality of a colour (defined by hue/saturation). Newtons breakthrough: - 1666: Newtons prism experiment- sunlight was passed through a small hole in a screen, through a prism, and fell upon a white screen, where a rainbow appeared. o Incident light refracts at both surfaces such that violet light is refracted the most and red the least. o Newton named the seven colours he noticed; ROY G. BIV. - Newton believed that sunlight is actually composed of different colours of light, which when combined produce white light; verified this by passing the rainbow of light through additional prisms until white light was produced. - He understood that the colours we perceive are not that of the light itself; that is, there is no colour inherent in light, but rather we see colour after a particular light interacts w/ our visual system. This was a highly controversial insight for centuries after Newton. Spectral nature of light sources: - Using a measuring instrument, a plot of the energy content of sunlight can be made at each visible wavelength (400- 700 nm band of electromagnetic radiation) = emission spectrum. - Natural sunlight contains relatively similar numbers of photons across the visible spectrum. o This allows sunlight through a prism to be refracted along a gradient of wavelengths. o Another light source that displays a continuous spectrum = tungsten light bulbs. o However, tungsten light is biased toward higher wavelengths (hence the yellowish appearance of household lights). - Outer light sources have different spectra, w/ energy contained only at certain wavelengths: o Fluorescent light bulbs contain a few discrete spikes of energy at different wavelengths along the visible spectrum (~400, 450, 550, 600 nm). o Most lasers have a single spike, e.g. a typical laser-pointer has a red colour (650nm). Additive colour mixtures: - Why does sunlight- a mixture of all visible light wavelengths- have a neutral colour appearance (white)? o Must consider perceptual result of superimposing 2 or more lights upon each other. - Newton undertook the first foray into this area. o He reasoned that the addition of 2 different coloured lights would produce an intermediate sensation from within the rainbow of colours he had discovered....
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- Fall '11