Comments on: Comments on 20432

Post number 20432

lagado — Mon, 30 Sep 2002 18:10:16 -0800

One sunny day, Bill Beaty was walking through a car park when he noticed a black car that appeared to have a series of interesting spots and highlights on its hood. On closer inspection, he also noticed several hand prints which had a curious property: they didn't appear to be on the surface of the paintwork at all but instead looked as though they were floating several inches below the surface. In some cases they even looked like they were floating above the surface. After thinking about this he came to realize that he looking at a kind of holographic effect but this kind of hologram didn't require all the usual paraphernalia nor was it caused by light wave interference. It was a kind of holography that could be used to draw pictures in 3D by hand. (More inside...)

By: lagado

lagado — Mon, 30 Sep 2002 18:12:27 -0800

A 3D image requires two images, a different one for each eye. The hand-prints looked different to each eye because they were being made up from scratches in the paintwork and scratches can look very different depending on the viewing angle. The owner of the car had evidently cleaned it with a gritty polishing mit which had left a series of long curved scratches on the hood of the car. In the process an image of his hand was preserved in 3D. To understand this, imagine that you're holding a sheet of black shiny plastic outside in the sun. Now with a compass, make an arc in the plastic. When the sun hits it you will get what is called a specular highlight (a bright spot or point of light) somewhere along the arc. If you close one eye the spot will appear at one place on the arc, if you switch to the other eye, the spot will appear at a slightly different place on the arc. This is because the angle of each eye is different relative to the reflective edge of the arc and the sun. As you vary the radius of this arc, the amount of displacement between these two spots will also vary. The smaller the radius of the arc the more the positions of the spots will differ from each other. Now when you view the arc with both eyes open, the two spots will merge to form a single spot and the amount of displacement between the two spots will combine to create a sense of depth, i.e. how far away the combined spot appears to be. So that's the basic idea but the interesting thing is that it is possible to use it to draw 3D images by hand. An image done this way is made up of many spots of light and is constructed by marking a sheet of black plastic with a great number of arcs. For example, to construct a simple "V" shape that floats a few inches below the surface of a black plastic sheet: First draw the "V" at the bottom of the sheet. Then take a compass and place one end at some point on the "V" and make an arc at the top of the sheet. Move to another point on the "V" and make another arc. Repeat this process over and over until you have covered as much of the "V" as possible and as a result you will have a series of arcs up at the top of the sheet. Now take it outside to view it under bright sunlight. The apparent depth of the "V" is set by the radius of the compass. You can find out more details about this cool effect on Bill Beaty's page, as well as tricks for drawing more elaborate images such as 3D polyhedra. Beaty spends a bit of time justifying his assertion that this is really a type of hologram and he compares it to other reflective holgrams like the ones you find on Visa cards. He also mentions the possibility of building large outside installations using chrome wire or tubing instead of scratches.

By: XQUZYPHYR

XQUZYPHYR — Mon, 30 Sep 2002 18:19:19 -0800

My cat's name is Mittens. Okay, seriously, this sounds really cool. I'm ready to go carve up the hood of my car now. Maybe I should practice first... off to find other people's cars outside!

By: dhartung

dhartung — Mon, 30 Sep 2002 19:45:21 -0800

Maybe we should try this ... on Mommy's car. This is really interesting; you'd think there would be some obvious commercial applications. I wonder if there's a patent. Is it really a hologram, though? I suppose that depends on whether you think stereograms count. In principle they're just extensions of the same phenomenon.

By: dg

dg — Mon, 30 Sep 2002 19:47:23 -0800

That is really interesting. Does the surface have to be black, or is that just to minimize reflections from the surface itself?

By: XQUZYPHYR

XQUZYPHYR — Mon, 30 Sep 2002 20:13:21 -0800

Actually, I once made stereograms by hand. Not the ultra-digital computer ones, with the hidden images, I mean, but just by drawing repeated objects at different distances from each other. For example I would draw 3 balls of the same size and shape exactly 3 inches apart from each other and then draw three squares about 3.25 inches apart, and so on with different patterns. When you started focusing your vision on two overlapping blurs, the others appeared at different distances. It's like forcing your own head to act like a viewmaster.

By: mikhail

mikhail — Mon, 30 Sep 2002 22:01:50 -0800

"An image done this way is made up of many spots of light and is constructed by marking a sheet of black plastic with a great number of arcs." I think they used to call them records. Seriously.

By: substrate

substrate — Tue, 01 Oct 2002 04:52:00 -0800

These aren't stereograms dhartung, they should be classed as holograms most likely. A hologram is a recording of the interference patterns of light waves interacting with an object. When light of the right wavelength is shined through/reflected off this interference pattern a three dimensional reconstruction of the original object is created. These hand made holograms are interference patterns, or at least recreations of potential interference patterns. They're kind of neat, there was a plastics shop in my hometown, when I was in university I talked them out of a few large sheets of black plexiglass in order to play with them. I scratched in increasingly complicated patterns just to see what could be done.

By: oissubke

oissubke — Tue, 01 Oct 2002 05:49:13 -0800

I was reading something in a book about M.C. Escher last night that said something about how he had tried these glasses that reversed the right and left eye views by means of prisms, and that it was a spectacular, bizarre experience because things that should have been close appeared far and vice versa. This just happened to remind me of that. Anyone know where I can get a pair of those suckers?

By: soyjoy

soyjoy — Tue, 01 Oct 2002 08:35:11 -0800

Excellent link. It reminds us how common and pervasive 3D experiences are in the modern world, if we know where to look. I remember as a kid sitting in the back seat of my parents' Karman Ghia staring at the interior of the convertible top and watching the little dots converge and create this depth illusion. This kind of thing (the basis of "Magic Eye") can be done on any grillwork or surface with a repeating pattern. Often there is a discrepancy at some point in the pattern, and that tends to give the impression of a shape. Hey! Is that a bunny?

By: lagado

lagado — Tue, 01 Oct 2002 17:05:31 -0800

dhartung: I wonder if there's a patent. Beaty does cite some prior examples: W. Plummer & L. Gardner, Applied Optics, V.31 No.31, Nov. 1992, pp. 6585-6588,A mechanically generated hologram?, E. Garfield, Essays of an Information Scientist, V5 pp348-354 1981-82 ISI's "World Brain" by Gabriel Liebermann: The World's First Holographic Engraving (3ft x 4ft scribed aluminum) Hans Weil UK Patent No. 37.208/34 in 1934 "Improvement in Advertising and the like Signs", describes methods of producing "directive reflections by grooving a metal surface or transparent sheet of glass" dg: That is really interesting. Does the surface have to be black, or is that just to minimize reflections from the surface itself? You're right, I should have simply said dark plastic but black is still probably the best. mikhail: I think they used to call them records. Seriously. Exactly, the grooves on an LP record work this way, as do CDs and those old fashioned brushed aluminium knobs that were common on 1970s-era stereos. dhartung: Is it really a hologram, though? I suppose that depends on whether you think stereograms count. In principle they're just extensions of the same phenomenon. substrate: These hand made holograms are interference patterns, or at least recreations of potential interference patterns. No they aren't caused by interference, each point is a distinct point of light and no light wave interference is necessary, just a reflective curve and a change of viewing angle. Beaty's main argument is one of definition, if these really aren't holograms then neither are other incoherent light holograms, the sort you see used on labels and credit cards. They work in a very similar manner, far more so than a simple stereogram (which only provides two images at a fixed viewing angle).