Adaptation-colour constancyWhen we

Adaptation-colour constancy

When we go from light to dark, or vice verse, the eye adapts to allow us to see comfortably. Changes in colour sensitivity are less frequently noticed, but they are continually occurring and may be quite large. For example, white paper looks white, regardless of the illuminating source's spectral composition (as long as it cover the visible spectrum).
If it were not for adaptation, the white paper would look red under tungsten and blue under daylight. Nevertheless, exact adaptation rarely occurs and the colour distortion may be considerable. In light sources with irregular distributions, such as the fluorescent tube, with it spectral lines. Another factor which helps in this colour constancy is the relation to surrounding colours; if the colour of the source changes then all the colour will be distorted, which helps in the recognition of a colour.

Metamerism

Metamerism may be defined quite simply as the property of the eye and brain to receive the same colour sensation from two coloured samples with different spectral energy distributions. In other words, despite the fact that at many wavelengths in the visible spectrum the visible light energy emitted by the two stimuli may be different, both appear the same to the observer. Two objects matching in this way are known as metamers. The reason why this may occur relies on the fact that the eye has three receptors which are colour sensitive and means that the solo requirement for two colour to match is that the total light energy, with respect to the sensitivity of the receptors, is the same for both object.
With object metamerism we are concerned with differences between the spectral reflectance and yet for a given observer under a particular illuminant they match for colour. If we now change either the observer or the illuminant, it is likely that the match will break down and this is the situation which is of most interest.
In Figure 1.6, the spectral reflectance curves are shown of a metameric pair, which match when seen under daylight but not under tungsten light. It is apparent that the biggest difference between them is the spectral reflectance at the long wavelengths. Since the relative light output at the longer wavelengths of tungsten light is very high compared







Pic1.6




to that of daylight and, furthermore, the sensitivity of the eye at long wavelengths is fairly low, the combination of these effects is sufficient to ensure an approximate match under daylight, despite the large difference in spectral reflectance. The importance of metamerism rests on the fact that practically all colour reproductions exhibit the effect. Most methods rely on the mixture of three primary colourants to match a colour. In almost every case, the resultant match between original and reproduction is metameric. Without this very important property of the eye all methods of colour reproduction in common use would be impossible.