A '''colour appearance model''' (abbreviated '''CAM''') is a mathematical model that seeks to describe the [[perception|perceptual]] aspects of human [[color vision]], i.e. viewing conditions under which the appearance of a colour does not tally with the corresponding physical measurement of the stimulus source. (In contrast, a [[color model]] defines a [[coordinate space]] to describe colours, such as the [[RGB color model|RGB]] and [[CMYK color model|CMYK]] color models.)
==ColorColour appearance==
[[ColorColour]] originates in the mind of the observer; “objectively”, there is only the [[spectral power distribution]] of the light that meets the eye. In this sense, ''any'' colorcolour perception is subjective. However, successful attempts have been made to map the spectral power distribution of light to human sensory response in a quantifiable way. In 1931, using [[Psychophysics|psychophysical]] measurements, the [[International Commission on Illumination|International Commission on Illumination (CIE)]] created the [[CIE 1931 colorcolour space|XYZ colorcolour space]]<ref>“XYZ” refers to a colorcolour ''model'' and a colorcolour ''space'' at the same time, because the XYZ colorcolour space is the only colorcolour space that uses the XYZ colorcolour model. This differs from e.g. the RGB colorcolour model, which many colorcolour spaces (such as [[sRGB]] or [[Adobe RGB color space|Adobe RGB (1998)]]) use.</ref> which successfully models human colorcolour vision on this basic sensory level.
However, the XYZ colorcolour model presupposes specific viewing conditions (such as the retinal locus of stimulation, the luminance level of the light that meets the eye, the background behind the observed object, and the luminance level of the surrounding light). Only if all these conditions stay constant will two identical stimuli with thereby identical XYZ [[CIE 1931 color space#Tristimulus values|tristimulus]] values create an identical colorcolour ''appearance'' for a human observer. If some conditions change in one case, two identical stimuli with thereby identical XYZ tristimulus values will create ''different'' colorcolour ''appearances'' (and vice versa: two different stimuli with thereby different XYZ tristimulus values might create an ''identical'' colorcolour ''appearance'').
Therefore, if viewing conditions vary, the XYZ colorcolour model is not sufficient, and a colorcolour appearance model is required to model human colorcolour perception.
