Figure 9.1. Same saturation, different chroma and "colorfulness". All four screen areas A-D emit light of the same saturation (pure red), but they differ among themselves in chroma, both when seen as surfaces in the subject (A[=B] > C[=D]) and, in a different way, when seen as surface colours in the image (B > A[=D] > C). Light from these four areas, though of  the same saturation, exhibits progressively more "colorfulness" in proportion to its brightness.

We saw in the introduction that a different set of dimensions applies to the visual appearance of light, as opposed to surfaces. Brightness is the perceived intensity of a light, and saturation is the perceived purity of colour or relative colour intensity of a light. "Colorfulness" - the absolute colour intensity of a light stimulus - is a function of brightness and saturation. Brightness is the perceptual correlative of the physical parameter of luminance, which in turn is the amount of light energy (radiance) weighted according to the relative sensitivity to each wavelength of human vision. Saturation is the perceptual correlative of physical parameter of spectral purity. Colours making up an image, which can be described in terms of lightness and chroma if looked at as surface colours, can also be described in terms of brightness, saturation and "colorfulness" if viewed as light coming from the image.

In earlier literature, brightness and saturation are often treated as essentially subjective parameters, unsuited to quantification, but more recent literature on colour appearance models is developing ways of treating these dimensions quantitatively (Moroney et al., 2002). However, absolute quantitative measures of these parameters, and of their physical correlatives, are not generally used by painters, and for most purposes they do not need to be. Digital artists are able to directly manipulate brightness and saturation values in their images in programmes such as Photoshop. This opens up enormous possibilities for emulating the effects of light from the imagination, as long as artists understand the basic principles of colour. Even so, for most such purposes, digital artists need ony concern themselves with the specific measures of brightness and saturation used in these programmes (defined in relation to the gamut of available colours), rather than with absolute quantitative measures of brightness, saturation, and their physical correlatives.

Tonal realist painters do systematically judge the brightness and colour intensity of the light coming to their eyes from their subjects, but in general they do not think in terms of absolute measures of these parameters, but only with relationships of these parameters between the different components of their subject. They typically (and often unconsciously) think of these relationships of brightness and "colorfulness" in terms of the value and chroma of the paint mixtures that they will use, sometimes in relation to an absolute framework such as the Munsell System.


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