June 1970

Terms Used in Ecological Optics

J. J. Gibson, Cornell University


The World Wide Web distribution of James Gibson's "Purple Perils" is for scholarly use with the understanding that Gibson did not intend them for publication. References to these essays must cite them explicitly as unpublished manuscripts. Copies may be circulated if this statement is included on each copy.

(A glossary to supplement chapters 10-12 in Gibson, The Senses Considered as Perceptual Systems)

Radiant light. The waves or photons radiating outward from a point in space, such as an atom.

Multiples reflected illumination. The steady state reached by radiant light when it is reflected back and forth, and when the reflecting surfaces are not mirror-like. This is characteristic of terrestrial spaces but not of outer space.

Ambient light. The light energy coming to a point in a medium from all directions around the point. It comprises the different intensities of light in different directions that are available for stimulation, or for producing photochemical affects.

Point of observation. Any point in a medium where an eye could be located. It is the convergence point of light-rays in geometrical optics, and the common apex of the set of all solid angles around a point in ecological optics. It is comparable to the station point in the theory of pictorial perspective except that there need be no picture-plane and a point of observation can move instead of being stationary.

Projection to a point of observation. The correspondence between some of the faces and facets of light-reflecting surfaces (components of an environment) and the set of all visual solid angles around a point (the components of an ambient optic array).

Ambient optic array. A structured arrangement of ambient light at a point of observation. The component of visual solid angles of an optic array can be chosen at any convenient level of angular size; the units are "nested" within larger units. The array is taken to be invariant under changes in the intensity of light, that is; its structure or arrangement is unaltered. When the array is unchanging in time it is said to be "frozen." The ambient array (as distinguished from ambient light) is assumed to contain stimulus information (as distinguished from stimulus energy).

Ordered set of points of observation (ordered family of ambient optic arrays). A series of adjacent points of observation is equivalent to a moving point of observation or a path of locomotion. Corresponding to every such ordered series of points is an ordered series of ambient optic arrays. The total manifold of all possible arrays are at

all possible points of observation constitutes the "permanent possibilities" of vision in the world.

Geometrical layout. The persisting arrangement of the rigid surfaces of the world with respect to abstract space. This layout is described by solid geometry in such terms as plane surfaces, curved surfaces, dihedral angles, and closed continuous surfaces (detachable objects). This description takes no account of the projection or non-projection of these surfaces to a point of observation.

Ecological layout. The persisting arrangement of the rigid surfaces of the terrestrial world with respect to points of observation and paths of locomotion in a medium. The surfaces are generally opaque. When layout is confined in this way the distinction can be made between projected and unprojected surfaces at a given point of observation, and this distinction is necessary for a theory of the perception of the environment as contrasted with theories of the perception of "space."

Occlusion. The non-projection of a surface to a point of observation. The surface in question is commonly said to be "hidden" or "concealed" or "out of sight." It may be either the "back" of an object or the "background" behind an object. The existence of occlusion means that some components of the layout do not have corresponding components in the ambient optic array at a fixed point of observation. (but cf change of occlusion). In general, one surface occludes another with respect to a point of observation if it is opaque and if it lies between the other surface and the point.

Change of occlusion. The adding of components to or the subtracting of components from an ambient optic array, contingent upon a movement of the point of observation or a motion of the occluding surface. The change in the array is reversed by an opposite movement (or motion).

Occluding edge. The apex of a dihedral angle (or the tangent along a curved surface) that separates a "front face" from a "back face," or an object from its background.

Selected sample of an ambient optic array. That portion of an ambient array registered by an eye at a given posture of the head-and-eye. In general, animals have to take successive samples of ambient light in order to perceive their surroundings, no animal having wholly panoramic vision. The sample in ecological optics is comparable to the retinal image in physiological optics. But the sampling theory of information-pickup is quite different from current theories of the processing of the inputs caused by a retinal image.

Delimited optic array. A solid angle of light coming to a point of observation from a display of some sort (often loosely called simply an "optic array"). It contains some of the stimulus information that might be contained in a natural ambient optic array, i.e., an unbounded array. But it should not be confused with a sample of the latter since its boundaries are not movable.

Display. An artifact providing a delimited optic array to a station point. The most familiar example is a picture or drawing in which case the array is frozen in time. Experiments on perception in the past have mostly been carried out with displays or drawings