A mechanically generated holographic effect explored by William Beaty, a research engineer at the University of Washington (Seattle, WA), can be duplicated by anyone with a compass and some free time. Beaty, who described the effect at the Electronic Imaging Symposium (Jan. 20–24; Santa Clara, CA), refers to the images as "scratch holograms," and notes they replicate widely occurring natural phenomena that an alert observer can find in all sorts of glistening vistas, from moonlight crossing a still lake to the sparkle of polished aluminum knobs on an audio-system amplifier.
Beaty, who also hosts a heavily frequented amateur science website, first noticed the accidental holographic effect several years ago on a sunny day while walking through the parking lot of an engineering firm where he worked.1 He noticed that some of the glowing highlights on the hood of a black station wagon seemed to emanate from beneath the surface. Because he was working on optoelectronic sensors, Beaty said he was "optically primed" to notice such things, and "it wasn't long before I had half the engineering department out there acting like fools, moving their heads back and forth in front of this black station wagon."
Eventually Beaty figured out that the highlights were created by a polishing mitt that had "traced out millions of nearly parallel scratches in the black paint." He surmised that the resulting holographic effect was similar to the rainbow holograms—invented by Stephen Benton at the Massachusetts Institute of Technology (Cambridge, MA)—that are often used on the front of credit cards.
The randomly spaced scratches on the hood of the car seemed to be functioning holographically without the benefit of optical interference, said Beaty. So since rainbow holograms function regardless of illumination frequency by encoding depth information as variations in the orientation of interference fringes along a horizontal stripe, the concept could be interpreted as allowing holograms to function even if the fringe spacing was random and large enough to be visible to the unaided eye. "I had done some three-dimensional display work for the Boston Museum of Science several years before," Beaty said. "So I already knew how rainbow holograms worked. Otherwise I might never have noticed the connection."
The observation prompted Beaty to start creating his own hand-drawn scratch holograms using dividers and acrylic plastic (see figure). Posting the discovery on online science bulletin boards drew numerous responses, arguing pro and con as to whether these were real holograms. Based on this feedback, Beaty learned that the scratch hologram phenomenon had been used to create a wall-mural sized holographic artwork in Philadelphia in 1980.2
Hand-drawn "scratch holograms" can be made using a small piece of acrylic plastic and a set of dividers. For example, to make a "V"-shaped hologram, first draw a "V" as a guide (top inset). Then use the dividers to make quarter-arc scratches above the "V" (bottom inset). For each scratch, place the point on a different spot on the "V" while maintaining the distance between the metal points on the dividers. The scratches create the hologram, which can be observed in sunlight (above). The virtual depth of the hologram is determined by the spacing of the compass points.
Beaty also learned that the phenomenon had been observed and documented by optical engineers William Plummer and Leo Gardner a little more than a decade ago. Gardner (SSG; Waltham, MA) noticed ghost images of a lapping tool slightly above and below a plane nickel optical surface, after the tool had been used for wear tests on the surface and removed. In an Applied Optics paper published at the time by Plummer (Polaroid; Cambridge, MA) and Gardner, who also noted the similarity to rainbow holograms, Plummer offered "a nonholographic theory that accounts for the presence and character of the ghost images," and which is based on the geometry of light scattering from a fine, smooth scratch in an optical surface.
In applying the theory to Gardner's observations, Plummer reasoned that "a lap holding a multitude of sharp grains in any flat array and interrupted by voids such as carved channels or a missing center will be mapped by the orbital motion into an array of circles with displaced centers, and finally will be mapped by the preceding [binocular] viewing geometry into two apparent complete images of the original lap, apparently above and below the optical surface."3
One potential advantage of this holographic effect is the possibility of making huge three-dimensional images—conceivably as large as billboards in Times Square, Beaty said. "You'd probably need the sun to make it bright enough to see, though."
Hassaun A. Jones-Bey
- E. Garfield, Essays of Info. Scientist 5, 348 (1981–82)
- W. Plummer, L. Gardner, Appl.Optics 31(31) 6585 (November 1992)