THE PRINCIPAL TERMS USED IN ECOLOGICAL OPTICS
The environment of animals, as distinguished from the physical world, consists of a medium, substances, and the surfaces that separate the substances from the medium.
The medium for terrestrial animals is air. Air is insubstantial and thus permits locomotion. Locomotion is controlled by the information in the medium.
Information is provided by sound-fields, by odor-fields, and above all by illumination. Information, in this terminology, is not transmitted but is simply available.
Illumination is the steady state of reverberating radiant energy such that light is ambient at all points in the medium.
Substances are solids and liquids that vary in composition, and in resistance to change. Different substances have different affordances. Substances are generally opaque, that is, they reflect and absorb but do not transmit.
The surface of a substance has a characteristic texture, reflectance, and layout. The ambient light at any point in the medium is structured by the light reflected from surfaces so that these characteristics are specified.
Surfaces, substances, and the medium manifest both persistence and change, persisting in some respects and changing in others. The changes are environmental events. Animals need to perceive what persists and what changes. A surface goes out of existence when its substance evaporates or disintegrates; a surface comes into existence when its substance condenses or crystallizes.
Layout refers to the persisting arrangement of surfaces relative to one another and to the ground. Different layouts have different affordances for animals. The perception of layout takes the place of the perception of depth or space in traditional terminology.
The ground is the basic persisting surface of the environment. It is the surface of support, the terrain, the earth extending out to the horizon. It is normally cluttered.
Clutter of the environment refers to objects or surfaces that occlude parts of the ground and divide the habitat into semi-enclosures. Semi-enclosures provide vistas.
A detached object is a substance with a surface that is topologically closed and is capable of displacement. Animals are detached objects.
An attached object is a substance with a surface that is not wholly closed and is continuous with another surface, usually the ground. It cannot be displaced without breaking the surface.