# How do “holographic plates” work?

I asked a question about laser stage lighting over at Audio Video Production, and received an excellent answer that explained that laser clusters are generated from a single beam via something called a "holographic plate".

I'd imagine they're made up of a crystal-like structure of materials with different refractive indices, but I'm not exactly sure. If that's the case, surely such tricks with refraction would cause component colours to be refracted at different angles, but this doesn't seem to be the case.

How do these really work?

Update: There seems to be some confusion. Cluster laser lighting units seem to be able to produce arbitrary numbers of output beams, and have some rudimentary control their direction, all from a single laser diode. I'm not asking about the principles of holograms, but rather about the mechanism by which a single laser beam passing through the plate results in multiple distinct beams coming out of the other side.

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Effectively, such a holographic plate is a 2D diffraction grating (en.wikipedia.org/wiki/Diffraction_grating). –  Johannes Dec 11 '12 at 15:09
@Johannes That looks like the most accurate representation of what's happening. Post an answer, I'll give it an upvote! :) –  Polynomial Dec 11 '12 at 15:37
This image from this post was removed to comply with a DMCA request. Please do not edit it back in. –  Jaydles Mar 29 '13 at 21:11
A quick message to the person who issued the DMCA takedown request on the image: There was no need to go through a full takedown process. You could have just posted a polite comment, and I would have happily removed it. Instead, you made the assumption that I used the image in bad faith, and that I'd be uncooperative. Not exactly the best attitude to have. –  Polynomial Mar 29 '13 at 22:10

Effectively, such a holographic plate is nothing more than a 2D diffraction grating. A typical diffraction pattern of such a grating is shown here.

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So, just to be clear, it's a rough analogue of the double-slit experiment, except multiplied many times over and miniaturised into a mainly-transparent sheet? –  Polynomial Dec 11 '12 at 15:58
I added a link to my answer that highlights the orthogonal superposition of two diffraction gratings as an example of a 2D grating. By the way: in the picture that you provided the grating probably is a simple 1D grating (the beam appears to splits in only one direction). –  Johannes Dec 11 '12 at 16:02

The "holographic plates" are just a different word for "holograms". You should see an introduction to holograms, e.g.

http://en.wikipedia.org/wiki/Hologram

Holograms exist because of interference or diffraction - i.e. it depends on the wave optics. So your conjecture that the refractive indices are changing isn't valid. One can explain refraction in geometric optics: one doesn't need wave optics. But the wave character of light is totally essential for holography.

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Sorry Lubos. But, I strongly believe that the hologram is what the interference pattern which contains the information of the illuminated object. I'm working on the answer. –  Waffle's Crazy Peanut Dec 11 '12 at 12:32
I'm not sure I understand this answer. You seem to be talking about making a holographic recording, which I'm not trying to do. –  Polynomial Dec 11 '12 at 12:44
Apologies, I don't understand the objections although it may be my fault. –  Luboš Motl Dec 11 '12 at 16:28
Dear Lubos, I didn't say anything. That was just my suggestion. I also misunderstood the question. My English is bad ya know that. Please don't take it seriously :-) –  Waffle's Crazy Peanut Dec 12 '12 at 1:21
Your English is very good, better than mine. I am curious, and trying to find, whether there is some totally different object called "holographic plates" that has nothing to do with the plates on which holograms are made. –  Luboš Motl Dec 13 '12 at 7:59

## EDIT: (for my misunderstanding)

The Wikipedia article for Laser shows is far better for this question. As stated there, these holographic films use a passive holographic element which is normally diffractive. Anyways, the material (for any holographic film) is commonly coated with emulsion.

This process turns the conventional "laser pattern" into a random and seamless laser show where each new laser beam is scattered and dispersed over large area

It is difficult for me to delete this long post. Perhaps, users may ignore it...

## Holography mechanism:

Holography is generally a gift of LASER. It is basically a lensless photography where the phase of the reflected wave is also recorded along with the amplitude. Normal cameras for 2-D photography records only the amplitude. Whats the point of this?

To obtain a 3-D view of the object by interference and so, it looks like an awesome interference pattern which is called the hologram. When you look through the LASER-illuminated holographic plate, you'd experience the parallax (i.e) When you change the angle of your view, the image will appear differently (i.e) orient accordingly (which gives a realistic experience in 3-D). You may have a look at Wiki for the diagram or my naughty sketch below...

Construction: First of all, you will require a LASER beam to construct a sample holo. Because, laser is extremely coherent. It is made (doesn't diverge so easily) to diverge to fit accordingly with the recording plate. The beam is allowed to fall on the object to be 3-D'fied. Though it's a laser, it is some special form of light (amplified enough). So, it gets scattered from the illuminated object. Now, another beam of the same laser is made to fall on the holo-plate. Both the laser waves interfere each other and produce an interference pattern on the plate which is referred as hologram. This hologram (interfered fringes) contains the information (necessarily phase) of the object.

Now, the image has been recorded on the object. To view it, you'd need the same laser beam. An important thing to note: the orientation of this beam (relative to the plate) should be the same as that of the reference beam used while recording. Or else, distortion of image occurs. As you see through the plate, you could admire at the object as its been floated in colored space (depends on illuminated light).

Note: There are simpler holograms where laser is not mandatory to view atlast. The simplest one is the reflection hologram where normal light could act as a reference beam.

## Holographic Plate: (Oops... Sorry, this is what I should've told first)

This film records much finer resolution of light illuminated on it. So, it best suites our holography. Normally, these films use photosensitive emulsions, a kind of liquid-liquid colloid. By the word "photosensitive", I meant silver halide - the same thing used for photographic plates. But here, it captures the interference fringes more finely. The parts which receive more intense light stay somewhat darker while the others stay somewhat lighter. As it is in an emulsion, the scattered light doesn't directly fall on it. Differently oriented waves cause varied interference fringes on the AgX. To obtain the holo-image from it, we have to do a process called bleaching. Sadly, I don't know about it.

But, an article supports these facts.

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This explains how a hologram is generated, but doesn't really explain why the beams split out like they do. I'm not interested in recording anything onto the holographic plate, I'm just intrigued as to how they split the beam into various distinct parts. –  Polynomial Dec 11 '12 at 12:43
@Polynomial: Hi Polynomial, Could you clarify your statement: "beams splitout". I'm not pretty well in English (My bad). If you tell it somewhat good (not necessary to be broad), I'll be happy to tell you... –  Waffle's Crazy Peanut Dec 11 '12 at 12:49
One beam goes into the film, several come out the other side. It splits the incoming beam into several beams, which end up pointing in various different directions. –  Polynomial Dec 11 '12 at 12:52

the way I see it is that the recording of an interference pattern creates a diffraction grating pattern on the plate. This is not a pattern of just straight lines, the reflected light from the object has created a unique pattern. When light passes through this recorded interference grating it behaves like any wave hitting a wall with small gaps. At the other side of the gap the wave emenates in all directions. This sets up conditions for the the waves to interfere, recreating the interference pattern that was recorded

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