R.L. Gregory: Stereoscopic Shadow Images

When I was a young film student, Sandy Mackendrick, Founding Dean of the CalArts School of Film, suggested that I should read R. L. Gregory‘s seminal book on visual perception, Eye and Brain.  I still have my well worn copy of that book.  A few years ago I was researching the history of stereoscopic shadow projection and I came across Gregory’s description of his experiments with twin polarized light source shadows.  This work, published in the journal Nature in 1964, was clearly an extension of the anaglyph and mechanical shutter shadowgraph process patented by Laurens Hammond in 1923, and discovered by Ken Jacobs in 1969.  Following is an abstract and a poor quality scan of R. L. Gregory’s full article in Nature:

“The following simple arrangement makes it possible to project, in stereoscopic depth, three-dimensional objects such as wire models of molecular or crystal structures. Small models may be presented enlarged in three dimensions, magnifications of ten or more times being possible. The optical arrangement consists of nothing but a pair of small bright light sources, separated horizontally by a few inches. The sources are placed behind ‘Polaroid filters’, set at orientations differing by 90°. The point polarized sources give a pair of shadow images of an object, such as a wire model, placed between them and a silver screen. Alternatively, back-projection with a ground-glass screen can be used; but the screen must not de-polarize the light. When the shadows are viewed through crossed ‘Polaroid’ glasses, they are fused by the brain to form a single stereoscopic shadow-image lying in space.”

This technique was devised for experiments supported by U.S. Air Force grant AF-EOAR 63–93, monitored by the European Office, Office of Aerospace Research.