Several hologram plates.
(Obviously you cannot see the 3D effect in this flat photograph.)

Depending on the tilt of the plate with respect to the sun, you might accidentally discover the "pseudoscopic" image of the "V," and it may appear to float *above* the surface of the plastic. Tilt the plate to bring the far edge up and towards you and you'll then find the "orthoscopic" image floating deep within the plate.

If you had inscribed your entire name on the plastic, you'd now be seeing it down there within the surface. (hint hint!) It's also possible to draw complicated 3D objects by varying the compass spacing as you slowly draw glowing lines one point at a time. The distance between the compass points controls the depth of the hologram-dots you are making. I've been drawing cubes, pyramids, and holes with glowing stars at the bottom. As with conventional holograms, opacity effects can be achieved by controlling the location of the endpoints of the scratches. And many other sophisticated effects produced by conventional rainbow holography can be duplicated in black plastic and scratch patterns.

CHANCE FAVORS THE *CURIOUS,PLAYFUL* MIND

Hand-drawn cube hologram

The handprints in the above photo might appear to be on the surface of the hood, but in real life they look holographic and seem to float about one foot deep within the surface of the hood. Appears like white fishes in a dark pond.

With a start I suddenly I realized what I was seeing. It was incredible: each highlight was in the shape of a wool polishing mit, with matted fibers in the center surrounded by outwards-pointing wooly filaments! The matted part was in the shape of a human hand! There were several of them floating at various depths all over the car hood, with some of them even floating in space *above* the hood. They had been invisible to me, but then they weren't. Science is perception! It wasn't long before I had half the optosensor engineering department out there acting like fools, moving their heads back and forth in front of this black station wagon. I searched through the rest of the sunlit parking lot and found several other cars with glowing distorted handprints deep within various surfaces. Once you realized they were handprints, the shape was unmistakable.

Closeup of handprint pattern, 6" virtual depth

I proposed several crude theories to explain the phenomena, as well as numerous hoaxes which could be done via Elvis-shaped polishing mitts to encode miraculous images onto everyday objects. (The Hood of Turin?) It was about a week before I figured out what was really going on. The images were naturally-occurring holograms. The owner of the car had obviously polished the hood with a dirty lambswool mitt, and the millions of particles of grit in the mitt traced out millions of nearly-parallel scratches in the black paint. The particular hand motion had created a geometry of abrasion patterns which turn out to be nearly identical to the interference patterns which make up those embossed-foil whitelight Benton rainbow holograms.

Gabor Zone-plate, progressively simplified

But there was something extremely weird going on here (I mean even more weird than hood scratches causing images!) These scratches have random spacing. They seemed to be functioning as holograms without the benefit of optical interference. This is impossible of course, since holography is completely based upon interference effects. However, the "Rainbow" hologram technique invented by Benton at MIT allows a hologram to function regardless of illumination frequency. The optical bench for the classic single-stage "Rainbow" holography setup includes a horizontal slit which produces relatively large horizontal swatches of fine-line interference patterns on the film (figure A above.) The recorded interference pattern encodes the depth information as variations in orientation of the fringes across the stripe. In Benton's Rainbow Holograms, only the fringe orientation is important. The fringe spacing (wavelength) is not. Once I was clued in by the existence of the car-hood holograms, I realized that I could interpret Benton's white-light technique as allowing holograms to function regardless of *fringe spacing.* Frequency-independence leads to size-independence of fringes, figure B above. Nobody seems to realize that a Rainbow Hologram will still function even when the spacing of its fringes was made random. Or even if it were to be made immensely large.

(See:Are they *really* holograms?, and Scratch-holography FAQ)

No one seems to realize that the well-known "Rainbow Hologram" technique can also let you produce holograms which are not based on optical interference at all. This makes it possible for you to create holograms where the "interference fringes" are so large that they are easily seen by the naked eye; where they are more like the grooves of an LP record than like the patterns on a CDROM. And if the arrays of hyperbolic interference fringes in a conventional Rainbow Hologram are replaced by widely-spaced, hand-ruled scratches on a plastic plate, it becomes possible to draw complicated holographic images directly by hand with a sharp tool. Neolithic holography! Multiple parallel scratches aren't necessarily required, and circles can replace the hyperbolae. The circular-scratch technique is able to produce holographic images, and sweeping circular motions of gritty polishing mits can produce these holograms accidentally.

Hand-drawn cube hologram

Has anyone heard of this trick before? I've yet to find it mentioned in any (1995) holography article or text. It's starting to look like this is something new...

(AHA! Someone clued me in. A 1992 paper by Plummer & Gardner (ref. below) does detail just such a discovery. The authors completely analyse the math behind the reflections made by arrays of circular scratches produced by the orbits of a lapping tool on a metal mirror. -Bill B., 10/23/95

Noah Spurrier points out a SciAm article on phonograph record optics from 1989. And Nils Abramson points out a predecessor of the curved-scratch technique from 1930!)

Now I hear that the late Gabriel Liebermann was making scratch-holograms with an NC-machine back in 1981! See an article on his piece called "World Brain"

Conventional Rainbow Embossed holograms do trounce it badly in the brightness and sharpness catagories, but conventional techniques won't let you create a "physics educational" hologram with giant interference fringes easily viewed with the unaided eye. And conventional holography won't let you sit by the side of a lake in Vermont with plexi and dividers, drawing holographic virtual images of floating polyhedra by hand.

- Bill Beaty

P.S. The required tools are so simple that ancient peoples could have drawn these images in hardened sooty resin pools with wooden tools, had they but known the trick. Several ancient civilizations were heavily into geometry. I wonder if any artifacts exist in storage somewhere which appear to have some 'sandpaper marks' on their polished surfaces... ;)