Figure 2.

Contributions to luminance. Luminance values reaching the retina are modeled as a multiplication of illuminance from light sources and reflectance from the surfaces in the environment. The factorization of luminance is thus an ill-posed problem. One toy example of this degeneracy is shown here; the same stimulus can be produced by (at least) two possible combinations of illuminance and reflectance. Prior beliefs about the likelihood of these causes can be used to pick the most likely percept.