The elements of colour I: colour perceptions, colour stimuli, and colour measurement

Abstract

This paper presents an extended consideration of the question of what colours are from a scientific perspective by reviewing the connections between colour perceptions, colour stimuli, and colour measurement. The colour of an isolated light can be understood to be the way in which we perceive the overall balance of its spectral composition relative to that of daylight; “overall” here meaning at the level of its long-, middle- and short-wavelength components, as detected by the human visual system. Our ability to detect variations in this overall balance, first demonstrated by Newton, is now understood to rely on comparison of the responses of three receptor types by the process of cone opponency. The colour perceived as belonging to an object when it is freely examined in daylight, which we tend to think of as the (seemingly) intrinsic colour of the object, can similarly be understood to be the way in which we perceive its overall spectral reflectance, again at the level of its long-, middle- and short-wavelength components, as detected by the human visual system. Colorimetric measures are designed to quantify for practical purposes precisely these human-perceiver-dependent ”overall” properties of spectral distributions and spectral reflectances, by ignoring physical differences that we do not perceive as colour differences. In defining two senses of word colour, “perceived colour” and “psychophysical colour”, the CIE International Lighting Vocabulary in effect expresses a pluralist ontology of colour that acknowledges that we may wish to use the word “colour” either for our perceptions of colour, or for the measurable, human-perceiver-dependent properties that dispose physically different lights or objects to appear the same colour in the same context.