This carries with it the danger that a regular pattern, such as a houndstooth design on clothing, or a chequer, may interact with the reseau of measurement points:
On clothing, such regular patterns are typically buckled:
So, at different parts of the CCD image sensor, different colour detection points fall into the shadow of the dark parts of the pattern, whilst others fall into the light. Here, on the right, for example, the red and blue are largely in shadow at the top, whilst the green is in the light. At the bottom right, however, the green is in darkness whilst the red and blue are illuminated:
This leads to coloured patterns appearing in the image, even though the original object was a black-and-white chequer. To avoid such colour effects, it is usual to arrange that the reseau of measurement points is at least three times smaller than the expected minimum circle of least confusion of the taking lens:
Another trick is to use a phase plate that makes some of the rays that lead to a fine pattern be out of phase with others. They therefore cancel each other. Many cameras use a combination of both techniques.
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