Simultaneous colour contrast

A further limitation of the original CIE system is illustrated by Fig. 4.1 where the two grey patches ate physically identical, create the same local rate of photopigment absorption, and give the same cone excitations according to Equation 4.1. Yet, they appear to be veiy different in lightness because of the difference in their excitations relative to nearby areas. Thus whereas colour constancy demonstrates that patches of colour that have different tristimulus defiiutions can have the same colonr appearance, simultaneous colour contrast demonstrates that patches of colonr that have the same tristimulus definitions... 

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