November 1967
Note on Illumination and Space (Draft)
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 References to these essays must cite them explicitly as unpublished manuscripts.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.
The principles of ecological optics as distinguished from physical optics depend on the notion of an illuminated space (Senses Considered, p.12 ff and 191 ff). This is taken to be different from the notion of physical space through which light is propagated. It needs elaboration and criticism.
One way of describing an illuminated space is to say that it consists of an infinite number of photons endlessly reflected back and forth in an enclosed medium. Whenever there is multiple reflection of light there will arise “an infinitely dense network of rays consisting of ray pencils intersecting at every point in the medium (p. 191).”
This abstract geometrical conception seems to be realized by the interior of an optical integrating sphere (which has a very smooth surface). It would also be realized inside an airspace surrounded by a shell of haze or fog. The network can be thought of as a “steady state.”
Now, the trouble with the infinitely dense network within an optical sphere or a shell of fog is that it yields an illuminated space but not a visible space. That is, it does not yield conditions adequate for visual perception. There is nothing to see. An ocular positioned anywhere in this space could not make any of the adjustments that I would call perceptual: it can only make adjustments of the kind calledsensory (that is, activity of the pupil and of the rods and cones). The evidence suggests that in fact there is no perception in such a situation.
But now know let us suppose, instead of a sphere or a shell of fog, a “layout” of reflecting surfaces, that is, an arrangement of plane forms in a hierarchy of sizes from very large (called “faces”) to very small (called “facets”). All faces have facets (except for the rare type called mirrors). Let us further assume that different faces (and even facets) usually have different reflectances (being composed of different substances). An entirely different kind of steady state will be formed in this illuminated space than in the former.
The infinitely dense network of hypothetical rays, the concept of the photon-flux, is supplanted by a network of projections on the faces and facets of the layout. Instead of mutually intersecting lines there exists a bested hierarchy of solid angles at each station point. The ambient light at each point is now structured by “contrasts;” it is now an array where formerly it was not. A contrast is here considered only as a transition, that is, without reference to the quantities of light within the solid angles. An ocular system positioned anywhere in this space can now be established, it can accommodate, it can fixate, explore, pursue, converge, or in general adjust with whatever activities the evolution of that system has developed.
The laws of the structuring of ambient light by the differential facing and reflectance of surfaces can now be studied directly. The laws need to be elaborated (Senses Considered, p.208-220) but this can only be done if the notion of a stationary projective network is employed along with the notion of a reverberating flux of energy. The “texturing” of the array of projections can be considered at any level of “fineness, “as analysis requires, but it will never consist of rays; it will consist of the aspects of facets. Aspects contain information about surfaces whereas photons transmit (or propagate) information only about points, that is, about the atoms of a surface.
Note that the speed of the propagation of light becomes irrelevant when the steady-state projective network is established. All station points in the illuminated space become contemporary or simultaneous. The aspects of objects are simply available at all positions; they are not radiated or broadcasted to those positions. This fact is in contrast with the propagation of energy that carries information in the kind of space conceived by physicists, “outer” space.
Note also that the intensity of illumination becomes irrelevant by definition if the steady state of projections is assumed. Once formed, the steady state of projected faces and facets is unaltered by any change in the amount of illumination, so long as it does not to fall to zero. This network is never established, of course, in outer space where light does not reverberate by multiple reflection.
The optics of multiple reflection has not been as fully studied as the optics of radiation has been, and the concept of a structural ambient array is not as familiar as the concept of radiant light. But the illuminated space needs to be studied, for the seeing of an obstacle in the path of an observer is not at all the same as the seeing of a star in the night sky. The former reflects; the latter radiates. Seeing the obstacle does not depend on the intensity of the light; seeing the star does. The states of terrestrial objects are contemporary in the optic array; the states of celestial objects in the Universe are not. There is a great deal of information about obstacles in the light to an eye; there is very little information about stars. Our primitive ancestors saw obstacles very exactly; they saw the stars in the sky as gods. Modern observers see automobiles on earth with good discrimination; they see things like flying saucers in the sky with poor discrimination.
The theory of vision based on physical optics can explain the detection of radiating objects with considerable success on the assumption that excited atoms radiate energy. It can also explain sensations of glare, after sensations, dark adaptation, rainbows, the seeing of stars with and without telescopes, and the dependence of visual acuity on level of illumination. Such a theory need only assume that rays of energy in the dark are stimuli for a photoreceptive mosaic. But it cannot (or has not yet been able to) explain the visual perception of terrestrial objects as distinguished from the excited atoms of their surfaces. But that a different level of optics with a correspondingly different level of mathematics treatment seems to be required. This new kind of optics cannot explain the adjustment of the pupil, or dark adaptation, or glare, or any of the curious phenomena seen in the sky. But those phenomenon are not its province.