3. THE SCALE OF BRILLIANCE

A coloured surface will look greyed, pure-coloured, fluorescent or self-luminous depending on how its relative brightness (B) compares with that of a white surface in the same lighting.

Figure 10.6 Demonstration of perception of "brilliance". Three yellow shapes with relative brightness (B) of 53, 81 and 97, overlaid on photograph against backgrounds at three different levels of illumination, where white paper is represented by a grey of B = 53, 81 and 97 respectively. In each case the dot that has the same relative brightness (B) as the surrounding paper looks like a pure yellow, the less bright dots look olive-coloured, and the brighter dots look fluorescent or luminous. Photograph by David Briggs.

Evans (1974) investigated how a surface, as it gives off progressively more light compared to its surroundings, passes from looking greyish to pure-coloured, then fluorescent and finally luminous. He suggested the term brilliance for this scale and the term zero grey point for the point where colours exhibit neither greyness nor fluorescence. I argue below that this scale of brilliance has a direct relationship to relative brightness (B) in an image, and that a colour is seen to have zero greyness when it has the same value of B as what is perceived to be a white surface under the same illumination.

We've already seen that white, all of the full chroma colours, and all of the tints have a relative brightness (B) of 100. A consequence of principle 2 above is that in a relatively shaded area, where we use a grey with say, B = 50, to represent a white surface, all of the pure colours and tints under the same illumination will also be represented by colours with B=50 (Figure 10.6)

Figure 10.7. Aerial view of YCbCr space, showing the set of colours with B = 100 (left) and
B = 50 (right).

Putting this the other way around has a fascinating consequence. We can see that in a setting where a white surface is represented by grey with B of 50:

1. Any colour having B = 50 will read, depending on its saturation, as either a full-chroma colour, a pure tint or white.
2. Any colour with B < 50 will look like a dark surface colour, i.e. will exhibit a degree of greyness.
3. Any colour with B > 50 will look too bright to be simply reflecting light. If it has only moderate excess brightness, such as a patch of fluorescent paint might exhibit, it may have the appearance of a fluorescent surface colour. However if it has a large amount of excess brightness, it will read as being luminous, and will be seen either as an independent light source, or as a specular reflection of a light source.

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