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Fundamentally I want to know: How do holograms work?

The problem with that question is that normally you will end up with pages and pages talking about:

  • a laser
  • a beam splitter
  • a diffuser
  • the object being imaged
  • object beam
  • reference beam
  • mirror
  • holographic emulsion

Even Wikipedia is heavy on how to make a hologram, rather than how does a hologram work.

Other people have mentioned stereoscopic vision; how having two eyes gives the illusion of a 3d object. That is also irrelavent, since someone with one eye (or, in my case, one eye closed) can still experience a hologram.

What I am trying to figure out is how does a hologram work?. More to the point, how is it that rotating a flat holographic sticker allows the virtual object to change orientation - allowing me to see content that was not there a moment ago?

Wikipedia has an image that mentions reconstructing a virtual 3d object:

enter image description here

Some problems that that image, though, is that my credit card:

  • has no reference beam
  • is not being viewed at a 45 degree angle (meaning no interference can happen)

Assuming I have a holographic image of a simple cube. If I am looking at the holographic plate straight on, I will only see a square (i.e. the face of the cube closest to me):

enter image description here

If i rotate the holographic plate, so the right side of the plate is further away, the virtual cube will rotate, and I will actually be able to see the left face of the cube:

enter image description here

What is happening in the flat, 2-dimensional, holographic sticker that it can display continuously different information as I rotate it?

What is the mechanics of this holographic "paper" that it can present my eye different images?

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  • $\begingroup$ Short answer: think of the plate as a surface that, thanks to diffraction, bends/changes the direction of the light coming from your reference beam (~the source you use to illuminate your developed plate). The output directions are different in each plate small region. Now you can think of the hologram as a window through which you see the cube. When you change your relative position, the point of the glass through which you see, for example, the corner of the cube changes: that point is redirecting light to your new eye position but not to the former one. $\endgroup$
    – Andrestand
    Commented Sep 14, 2014 at 9:58
  • $\begingroup$ Long answer. You may like also this. $\endgroup$
    – Andrestand
    Commented Sep 14, 2014 at 10:00
  • $\begingroup$ I know SE doesn't like links, but here are two pages that helped me a LOT in understanding the basics - spie.org/Documents/Publications/00%20STEP%20Module%2010.pdf and holo.com/holo/book/book.html $\endgroup$
    – Dev Kanchen
    Commented Nov 1, 2014 at 15:44
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    $\begingroup$ This is odd that such an interesting subject has so very little information beyond the basic setup. How does the hologram looks in a microscope? Why can you still see the full object even if you break up the hologram to smaller parts? To what degree does that tear can continue? $\endgroup$
    – Eli
    Commented Sep 1, 2015 at 22:14
  • $\begingroup$ @Eli I would be happy if someone could explain the simple example! $\endgroup$
    – Ian Boyd
    Commented Sep 2, 2015 at 13:23

1 Answer 1

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What i am trying to figure out is how does a hologram work?. More to the point, how is it that rotating a flat holographic sticker allows the virtual object to change orientation - allowing me to see content that was not there a moment ago?

You see objects because your eyes register light that is coming from these objects. If you shift your head, your eyes will be subjected to a different "part" of reflected light waves from those objects, so you will see those objects differently -- for example you will be able to see objects that was obscured before.

Suppose that you have a techique that records completely all the light waves coming from your objects. And you also can "playback" those waves, so that they would exaclty reproduce the lightwaves from real objects. Then no one will be able to distinguish (using only one's eyes) the difference between the real objects and your "lightwave reproduction" of them. If you shift your head, your eyes will be subjected to a different "part" of "reproduced" light waves, so you will "see" those "objects" differently -- for example you will be able to "see objects" that was "obscured" before.

That is roughly what holograms do. They record the whole information about the incoming light by storing it in the interference patterns in a film. And when hologrms reflect (or, sometimes, pass through) light they reproduce exactly the lightwaves that was falling on them during recording.

...my credit card:

  • has no reference beam
  • is not being viewed at a 45 degree angle (meaning no interference can happen)

Well, first about interference. I think you misunderstand what does "interference" mean in that context. We are talking about wave interference between object wave and reference wave. The hologram records interference pattern, that is produced by wave interference of the two waves. And all that is relevant during creation of a hologram.

The interference patterns are very fine -- they are of a size of a wavelength. So there is a lot of information recorded on the hologram -- that is how all that information "fits" on one hologram.

Finally. When we reconstruct a hologram we use a reference beam, which diffracts on the recorded interference patterns and reproduces the object wave.

Now, speaking about reference beam. First of all -- you always need "reference beams" to see something. Light have to come from somwhere to get into your eye, right? On the other hand, for basic old-style holograms it is actually necessary to use a laser beam to recreate recorded waves. But there is a more "modern" approach called volume holograms, which work in ordinary light.

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  • $\begingroup$ Suppose that you have a techique that records completely all the light waves coming from your objects. And you also can "playback" those waves, so that they would exaclty reproduce the lightwaves from real objects. That's the question. How is a flat surface causing interference like that? $\endgroup$
    – Ian Boyd
    Commented Jul 27, 2012 at 13:24
  • $\begingroup$ @IanBoyd I've tried to expand my answer concerning your comment. To explain further one will need to do some math (actually there is some on wikipedia). Do you need that? $\endgroup$
    – Kostya
    Commented Jul 27, 2012 at 13:42
  • $\begingroup$ When i get some time i will update my question with examples of how lenticular holograms; that can serve as a starting point to explain how holographic holograms work. $\endgroup$
    – Ian Boyd
    Commented Jul 27, 2012 at 13:57
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    $\begingroup$ @IanBoyd I'd suggest to ask another question instead... $\endgroup$
    – Kostya
    Commented Jul 27, 2012 at 14:02

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