3 ms·

I love this article. My first reaction is "no way!", but when you think about it it's obvious that it must be so. The hologram of a single point of light is a

by femto 10y ago

I love this article. My first reaction is "no way!", but when you think about it it's obvious that it must be so. The hologram of a single point of light is a "zone plate": concentric rings. Move a sharp point in concentric circles on a surface and the scratches will form a zone plate which focuses light to a point. An abrading object is just an array of sharp points, so the scratches formed by moving the object in concentric circles will focus light to an array of points, corresponding to the shape of the object: an image of the object. Simple! (but not initially obvious.)

dTal 10y ago
Sorry to burst your balloon, but this has nothing to do with zone plates, or even "holography" in the strict sense; no wave interference is taking place. The mechanism is purely geometric: a gleaming scintillation of reflected light in a scratch from a point source will exhibit apparent parallax if the scratch is horizontal and curved. The greater the curvature, the less the scintillation moves, and the less the apparent parallax.
- wbeaty 10y ago
  ...and that's how we prove that white-light holograms aren't holograms. (Credit-card holograms function just like scratch-holograms, and their 3D image is not created via diffraction.) [Are these REALLY holograms?](http://amasci.com/amateur/holo3.html http://amasci.com/amateur/holo3.html)
  - dTal 10y ago
    Interesting. I never quite grokked rainbow holograms, so perhaps there's something in that. But it's unfair to say all white-light holograms aren't holograms. A normal Denisyuk reflection hologram made with lasers and everything will replay perfectly well under a halogen lamp.
    - wbeaty 10y ago
      > I never quite grokked rainbow holograms, If you made a scratch hologram, but your compass-point had a small cluster of fifty tiny needles instead of a single big needle, that comes close to the fringe-structure of Benton white-light holograms. :) > But it's unfair to say all white-light holograms aren't holograms True. By "white light" I was referring to Rainbow / credit-card / Benton holograms.
  - mhalle 10y ago
    You are referring to stamped foil or embossed holograms such as the ones used as security marks on credit cards. Here's a description of the process: http://www.holoeast.com/holography/holograms_maiking.html http://www.holoeast.com/holography/holograms_maiking.html Inspired by the process for creating compact disks, stamped holograms took Benton's rainbow holograms and made them cheaply reproducible through a mechanical process. Mike Foster developed the process and Steve McGrew made it commercially viable. Other types of white light holograms, such as the two-step "rainbow" white light transmission hologram that Steve Benton created or the white light reflection hologram of Yuri Denisyuk are both holograms proper -- they are both made using a traditional holographic process. A transmission white light hologram has less controllable color because it uses broad spectrum diffraction. A white light reflection hologram uses a thick diffraction pattern to additionally filter white light down to a narrow spectrum, producing a more pure color. Full color reflection holograms can be produced by superimposing multiple diffractive patterns in the hologram layer, one for each primary color.
- 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.

by femto 10y ago

dTal 10y ago

Sorry to burst your balloon, but this has nothing to do with zone plates, or even "holography" in the strict sense; no wave interference is taking place. The mechanism is purely geometric: a gleaming scintillation of reflected light in a scratch from a point source will exhibit apparent parallax if the scratch is horizontal and curved. The greater the curvature, the less the scintillation moves, and the less the apparent parallax.

wbeaty 10y ago
...and that's how we prove that white-light holograms aren't holograms. (Credit-card holograms function just like scratch-holograms, and their 3D image is not created via diffraction.) [Are these REALLY holograms?](http://amasci.com/amateur/holo3.html http://amasci.com/amateur/holo3.html)
- dTal 10y ago
  Interesting. I never quite grokked rainbow holograms, so perhaps there's something in that. But it's unfair to say all white-light holograms aren't holograms. A normal Denisyuk reflection hologram made with lasers and everything will replay perfectly well under a halogen lamp.
  - wbeaty 10y ago
    > I never quite grokked rainbow holograms, If you made a scratch hologram, but your compass-point had a small cluster of fifty tiny needles instead of a single big needle, that comes close to the fringe-structure of Benton white-light holograms. :) > But it's unfair to say all white-light holograms aren't holograms True. By "white light" I was referring to Rainbow / credit-card / Benton holograms.
- mhalle 10y ago
  You are referring to stamped foil or embossed holograms such as the ones used as security marks on credit cards. Here's a description of the process: http://www.holoeast.com/holography/holograms_maiking.html http://www.holoeast.com/holography/holograms_maiking.html Inspired by the process for creating compact disks, stamped holograms took Benton's rainbow holograms and made them cheaply reproducible through a mechanical process. Mike Foster developed the process and Steve McGrew made it commercially viable. Other types of white light holograms, such as the two-step "rainbow" white light transmission hologram that Steve Benton created or the white light reflection hologram of Yuri Denisyuk are both holograms proper -- they are both made using a traditional holographic process. A transmission white light hologram has less controllable color because it uses broad spectrum diffraction. A white light reflection hologram uses a thick diffraction pattern to additionally filter white light down to a narrow spectrum, producing a more pure color. Full color reflection holograms can be produced by superimposing multiple diffractive patterns in the hologram layer, one for each primary color.
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.