The second stage in analyzing an affordance for an animal is very much a matter (for visually detected affordances) of mathematical optics: to describe the light patterned by an affordance-specific occurrent property and to characterize that patterning in terms of an optical variable, an optical property, that stands in strict correspondence to the occurrent property. (TSRM, 1981, p. 265).So, in comments on an earlier post, Andrew asked about where it is that TSRM talk about anchoring properties structuring light. It is in section 5. "The specification of intension: affordances and ecological laws", pp. 260-7. It is rough sledding.