3 ms·

I wouldn't be quite so dismissive of scratch holograms not being (a type of) hologram. See the reference in the article: http://amasci.com/amateur/holo3.html h

by mhalle 10y ago

I wouldn't be quite so dismissive of scratch holograms not being (a type of) hologram. See the reference in the article: http://amasci.com/amateur/holo3.html http://amasci.com/amateur/holo3.html Having studied with Steve Benton at the Media Lab and built some of the highest fidelity white light 3D displays of the time, I would agree with this article: scratch holograms behave very similar to rainbow ("Benton") holograms. They use a combination of diffraction, diffusion, and parallax to produce an HPO (horizontal parallax only) 3D image. Yes, it isn't made by interference. No, it isn't full parallax or does it form an image purely by diffraction. But a scratch hologram is the superposition of the equivalent of many zone plates, and the basic mechanics are the same as in a hologram (well, a holographic stereogram). It definitely isn't "just" parallax. All that said, I agree that what's happening is very different from a computed hologram that models interference mathematically and records the detailed pattern on a recording material. Computing and writing such a pattern requires very high resolution to get the high spatial frequencies needed to get large diffraction angles. That makes it very tedious to make any image, and even worse one made of lots of points. A scratch hologram (or a holographic stereogram) gets around that problem by essentially modulating details in the spatial domain (the image) onto a high frequency diffractive or diffusing carrier signal (the scratch, or a holographic diffraction pattern in a holographic stereogram).

dTal 10y ago
I find this reply confusing, and I think your citation is a little confused too. "This can be taken to ridiculous lengths: giant "holograms" composed of curved, polished metal rods become practical. The metal rods are the interference fringes!" We have obviously left the traditional definition of "interference fringes" well behind here. You say "They use a combination of diffraction, diffusion, and parallax". But the source you cite says, first thing, "They employ no diffraction in reconstructing images" (quite apart from the fact that diffraction is a form of interference). You then say "a scratch hologram is the superposition of the equivalent of many zone plates". But a zone plate, being the intersection of a planar wave and a spherical wave, only works for one wavelength. Scratch holograms have no wavelength-dependent effects. Furthermore, the geometry isn't even the same - in a zone plate, the ring spacing gets smaller as you get further from the center. This is not the case with the curved scratches on a scratch hologram: http://amasci.com/graphics/swatch.gif http://amasci.com/graphics/swatch.gif. And it's perfectly clear why - it's not about wave interference, but about maintaining curvature. It's much more akin to a fresnel lens. So, unless I'm missing something deep, zone plates and interference etc are a total red herring and not useful to understanding scratch holography at all.
- wbeaty 10y ago
  Key concept: what do the zoneplates of Benton holograms look like, as opposed to those conventional (off axis) holograms? A Benton white light hologram is composed of narrow horizontal slices of zoneplates, one for each reconstructed point. Something like this: http://amasci.com/graphics/swatch.gif http://amasci.com/graphics/swatch.gif My insight was this: if the above zoneplate-slice is squared off and increased in spatial frequency, it still reconstructs the same 3D image point. If we drew the nested fringes of that above zoneplate by scratching with a needle, it still produces the same glowing pixel floating in space. In other words, white-light holograms are frequency-independent, just as always claimed. Yes, if we change the frequency of the above zoneplate pattern, the rainbow coloration will shift, and may even appear white. Yet the reconstructed 3D image remains unaltered. The 3D image data is not contained in the spacing of the interference fringes. That's another way to say that the hologram still functions even if the spacing between fringes is randomized. It even keeps working if all fringes but one are removed. Here's the relationship between the zoneplates of Benton holograms, versus the 'zoneplates' of scratch holograms: http://amasci.com/graphics/holo/onestp2.gif http://amasci.com/graphics/holo/onestp2.gif Benton white-light holograms only use interference to record their zoneplates. They don't use interference to reconstruct the 3D image. (That's why they still function under white-light illumination.) Or, here's another approach. Benton holograms and scratch holograms are based on Poisson's Spot. They don't act like a fresnel lens, instead they act like a conical lens, causing an incoming beam to both converge and scatter into a long locus with no focal point. The "facets" of Benton holograms and scratch holograms need no be diffraction patterns or prisms, they can be random striations, parallel fibers, narrow cylinder lenses, reflective wires, etc. It's perfectly OK to insist that scratch holograms aren't genuine holograms. But to remain honest and consistent, we're also then forced to declare that Benton holograms aren't true holograms. Or, as I said in the linked articles, the bench setup for making Benton white-light holograms is actually an optical device for producing entire scratch holograms all at once, with no individual scratching needed. Heh, "metal rods are the interference fringes!" needs quotation marks, since I guess it wasn't obvious that I was being silly.

by mhalle 10y ago

dTal 10y ago

I find this reply confusing, and I think your citation is a little confused too. "This can be taken to ridiculous lengths: giant "holograms" composed of curved, polished metal rods become practical. The metal rods are the interference fringes!" We have obviously left the traditional definition of "interference fringes" well behind here. You say "They use a combination of diffraction, diffusion, and parallax". But the source you cite says, first thing, "They employ no diffraction in reconstructing images" (quite apart from the fact that diffraction is a form of interference). You then say "a scratch hologram is the superposition of the equivalent of many zone plates". But a zone plate, being the intersection of a planar wave and a spherical wave, only works for one wavelength. Scratch holograms have no wavelength-dependent effects. Furthermore, the geometry isn't even the same - in a zone plate, the ring spacing gets smaller as you get further from the center. This is not the case with the curved scratches on a scratch hologram: http://amasci.com/graphics/swatch.gif http://amasci.com/graphics/swatch.gif. And it's perfectly clear why - it's not about wave interference, but about maintaining curvature. It's much more akin to a fresnel lens. So, unless I'm missing something deep, zone plates and interference etc are a total red herring and not useful to understanding scratch holography at all.

wbeaty 10y ago
Key concept: what do the zoneplates of Benton holograms look like, as opposed to those conventional (off axis) holograms? A Benton white light hologram is composed of narrow horizontal slices of zoneplates, one for each reconstructed point. Something like this: http://amasci.com/graphics/swatch.gif http://amasci.com/graphics/swatch.gif My insight was this: if the above zoneplate-slice is squared off and increased in spatial frequency, it still reconstructs the same 3D image point. If we drew the nested fringes of that above zoneplate by scratching with a needle, it still produces the same glowing pixel floating in space. In other words, white-light holograms are frequency-independent, just as always claimed. Yes, if we change the frequency of the above zoneplate pattern, the rainbow coloration will shift, and may even appear white. Yet the reconstructed 3D image remains unaltered. The 3D image data is not contained in the spacing of the interference fringes. That's another way to say that the hologram still functions even if the spacing between fringes is randomized. It even keeps working if all fringes but one are removed. Here's the relationship between the zoneplates of Benton holograms, versus the 'zoneplates' of scratch holograms: http://amasci.com/graphics/holo/onestp2.gif http://amasci.com/graphics/holo/onestp2.gif Benton white-light holograms only use interference to record their zoneplates. They don't use interference to reconstruct the 3D image. (That's why they still function under white-light illumination.) Or, here's another approach. Benton holograms and scratch holograms are based on Poisson's Spot. They don't act like a fresnel lens, instead they act like a conical lens, causing an incoming beam to both converge and scatter into a long locus with no focal point. The "facets" of Benton holograms and scratch holograms need no be diffraction patterns or prisms, they can be random striations, parallel fibers, narrow cylinder lenses, reflective wires, etc. It's perfectly OK to insist that scratch holograms aren't genuine holograms. But to remain honest and consistent, we're also then forced to declare that Benton holograms aren't true holograms. Or, as I said in the linked articles, the bench setup for making Benton white-light holograms is actually an optical device for producing entire scratch holograms all at once, with no individual scratching needed. Heh, "metal rods are the interference fringes!" needs quotation marks, since I guess it wasn't obvious that I was being silly.