The effects of coloured illumination follow the principles of subtractive mixing, since only wavelengths present in the light source and not absorbed by the surface can be reflected. Coloured lighting tends to neutralize and darken complementary-coloured surfaces, thereby tending to raise the relative lightness of all surfaces that reflect wavelengths present in the light, the latter surfaces also shift towards the hue of the coloured light (Figure 10.7). The range of colours seen is always less than seen under white light. In Photoshop the effects of coloured lighting can be suggested by an overlying coloured layer of variable opacity in multiply mode; the greater the opacity, the stronger the colour of the lighting.


Figure 10.8. Effects of coloured illumination. Left: Effects of green, red and blue coloured illumination, at two intensities, on the appearance of various coloured surfaces, simulated in Photoshop CS2 by an overlying coloured layer in multiply mode. Right: Effect of strong green illumination, shown as displacements in YCbCr space in side view (above) and in the CbCr plane. Colours lacking green (purple and red) are drawn directly towards black; colours containing some green drop in lightness, but less markedly, and shift in hue towards green; colours fully saturated with green (yellow, cyan and white) all converge towards bright green in hue and lightness.


Figure 10.9 shows the effect of coloured light on the colours of human skin. Under white light (10.9B), human skin shows a range of low chroma colours, often extending into slightly stronger colours in the direction of red (where capillaries are numerous) and orange (where pigment is denser). Incandescent light, being similar in hue to average skin colour, shifts these hues shift to exhibit higher chroma but less varied hue (10.9A). Under strongly bluish light, such as skylight, the colours become more neutralized, but may exhibit a full range of hues, including prominent crimson, greenish and bluish variants (10.9C).

Figure 10.9. Effect of different illuminants on human skin colours. Photograph taken in white (flash) illumination (B), and transformed in ColorSpace to simulate (A) warm incandescent illumination (illuminant A) and (C) cool illumination (D75); resulting colours shown on CbCr plane of YCbCr space.

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