Ecological optic is concerned with many-times-reflected light in the medium, that is, illumination. Physical optic is concerned with electromagnetic energy, radiation. Ambient light coming to a point in the air is profoundly different from radiant light leaving a point source. The ambient light has structure, makes available information about reflecting surface. The information that can be extracted from ambient light is not the kind of information that is transmitted over a channel. There is no sender outside the head and no receiver inside the head.