Environmental Signals

A basic problem for students of color vision is to relate the perceptual dimensions of color to physical features of the environment. Common experience suggests that color is an inherent property of objects: Apples are red; grass is green, and so on. However, is that the case, or is it rather that color is produced by active processes in a perceiver that are initiated by light? Philosophers and others have long debated these alternatives and their arguments continue to reverberate today. Most vision scientists, however, incline to some version of the second alternative and, in so doing, follow the lead of Isaac Newton, the great English scientist whose 16th-century observations caused him to conclude that "the Rays to speak properly are not coloured. In them there is nothing else than a certain Power and Disposition to stir up a Sensation of this or that Colour.''

Light reaching the eye from any location in space can vary in intensity (number of photons per unit time) and wavelength. People are sensitive to a narrow band of wavelengths, extending from about 400 to 700 nm (1 nm=10~9 m). For most ordinary viewing, the distribution of wavelengths and intensities reaching a viewer depends jointly on characteristics of the source of illumination, on the surface reflectance properties of an illuminated object, and on the geometric relationship between the object and the viewer (Fig. 1). For instance, a snowfield seen in full sunlight yields a very

400 500 600 700 Wavelength (nm)

Figure 1 Light reaching a viewer is dependent on the nature of the light source and on the reflectance properties of the object being viewed. In this illustration the spectral distribution of energy emerging from a light source (A) is indicated at the top. The spectral reflectance properties of the surface of the table (B) are given on the left. The spectral distribution of light reaching the viewer (C) that is shown on the right is the product of the spectral distribution of the illuminant and the reflectance property of the object.

400 500 600 700 Wavelength (nm)

Figure 1 Light reaching a viewer is dependent on the nature of the light source and on the reflectance properties of the object being viewed. In this illustration the spectral distribution of energy emerging from a light source (A) is indicated at the top. The spectral reflectance properties of the surface of the table (B) are given on the left. The spectral distribution of light reaching the viewer (C) that is shown on the right is the product of the spectral distribution of the illuminant and the reflectance property of the object.

different array of wavelengths and intensities for analysis from that provided by an expanse of lawn seen at twilight. Viewed across natural scenes there are usually substantial local variations in the wavelength and the intensity of light, and the number of possible combinations of the two is virtually infinite. All species with sight can exploit intensity variations as an aid to the discrimination of the form, location, and movement of objects. However, only those that also have a capacity for color vision can disentangle the effects of joint variations in wavelength and intensity and in so doing yield the experience of color.

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