My introduction to Specular holography was in the 90s through William Beatty’s amazing science hobbiest site, amasci.com, an incredibly influential site for many of today’s STEM’ers. Beatty noticed an odd effect on a freshly waxed car, where ghostly handprints seemed embedded below or floating above the hood.
After working out the mechanism of the effect, Beatty began creating hand-drawn holograms of increasing complexity by etching circles on plexiglass (demonstrated below). He called these “Scratch Holograms
Beatty & I have since learned this was a rediscovery of a known, but largely ignored phenomena.
Specular holography is a technique for making three dimensional imagery by controlling the motion of specular glints on a two-dimensional surface. The image is made of many specularities and has the appearance of a 3D surface-stippling made of dots of light. Unlike conventional wavefront holograms, specular holograms do not depend on wave optics, photographic media, or lasers.
The principle of operation is purely one of geometric optics: A point light source produces a glint on a curved specular (shiny) surface; this glint appears to travel on the surface as the eye or light source moves. If that motion is projectively consistent with binocular disparity, the viewer will perceive — via stereopsis — the illusion that the glint occurs at a different depth than the surface that produces it. A specular hologram contains many such curved surfaces, all embedded in a host surface. Each produces a glint and the brain integrates the many 3D cues to perceive a 3D shape.