Chapter 6

Chromatic-Adaptation Transforms and Colour Appearance

6.1 Introduction

The distinction between colour specification and colour appearance was touched upon in the review of the CIE system presented in Section 1.3. Whereas the CIE system of colorimetry (based upon XYZ tristimulus values) is clearly a system for colour specification, some advanced colour specification models such as CIELAB could arguably be described as models of colour appearance. The polar coordinates of CIELAB allow the description of a colour stimulus in terms of three terms, Lightness, Chroma and Hue, and these correlate quite well with the perceptual attributes of Brightness, Colourfulness and Hue. Furthermore, the normalisation procedures inherent in the transform from XYZ to CIELAB result in a* and b* values close to zero for a perfect reflecting diffuser (or any surface whose spectral reflectance does not vary with wavelength) irrespective of the illuminant. This is consistent with the fact that surfaces in general tend to retain their daylight colour appearance when viewed under a wide range of light sources and contrasts with the properties of the XYZ system. In some sense then, CIELAB is a colour-appearance space whereas XYZ is not. CIELAB is a relatively poor colour-appearance model, however, and this chapter describes several advanced colour-appearance models (CAMs).

The human visual system in most cases has a remarkable ability to maintain the colour appearance of an object despite quite large ...

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