How Holograms Work: Explaining 3D Image Reproduction

[Kazza_765]

I understand the principles behind recording the holograph, that we are using interference to record phase information as well as the usual intensity. What I don't understand is how recording this information enables us to reproduce a 3-Dimensional image.

So far I've reasoned that the only visual cue that could be tricking the brain into 'seeing' a 3D object is stereopsis. This would require, I think, that by recording an interference pattern we are actually recording more than one image of the object in our film. Therefore the effect of the hologram on light passing through it must depend on the angle of incidence. I'm not sure about this bit, I don't quite understand why the image perceived by each eye is different.

Actually, the more I think about this, the more I sort of get a feel for why it is happening. I ask because I'll be making transmission and reflection holograms in undergrad labs this week. I'm not expected to understand the maths, its supposed to be a qualitative report, but I know I'm going to go nuts trying to understand this. My lab book has a couple of lines of maths regarding transmission functions, but its been taken from a book I don't easily have access to, and its not really in any sort of context.

Anyway, the point of this post: There are plenty of sites explaining how holographs are recorded, but I can't find any that explain how they recreate a 3D object. Does anyone know of a source that gives a proper mathematical description of holographs?

 

[Mk]

I think they usually just enter in a bunch of numbers and variables to make a 3D graph, like in Mathematica, or your calculator.

 

[rcgldr]

Holograms aren't tricking the brain into seeing "3d". They're actual 3d images. I remember a "lamp shape" hologram that gave a 360 degree view of a person sitting on a stool as the hologram rotated. This was accomplished via a series of vertical stripes.

Holograms are recording interference patterns that represent all views of the target object(s) within range of the hologram frame. You look through the hollowgram to see the image, not at it. Even when looking at a hologram with a single eye, by moving around within the hollogram frame, you'll will see the target object(s) from different perspectives. It's truly a 3d image.

The only analogy I can think of is the image at the lens of the projector. The image redundantly passes through all parts of the lens. If you cover up half the lens, you just get a darker but complete picture, instead of half of the picture. Similarly, the target object(s) interference image is recorded in all parts of the hollogram, not just one part.

 

[PPonte]

How difficult is to make an hologram?

 

[rcgldr]

How difficult is to make an hologram?

There are kits you can get now. Bascially you need a laser, a prism to split the beam, a dark room, and the film.

Do a web search for hologram kit and you'll find sites like this.

http://www.litiholo.com/hologram_kits.htm

Quality and size will determine the overall cost. For large objects, you'll need a large dark room.

A true hologram is film you look through to see the image. There are also reflective holograms.

 

[scott1]

I think that's tricking the brain to see 3d images is what you do for video games or any 3d object on a 2d screen.

 

[rcgldr]

I think that's tricking the brain to see 3d images is what you do for video games or any 3d object on a 2d screen.

True, but in the case of holograms, you look through the 2d screen to see a virtual image. It's an interference pattern, no tricks involved, just your normal stereo vision at work. If you look at stereogram with just one eye, there's no way to see the 3d image, this is a trick. If you look at a hologram with just one eye, you can move the film around and view the virtual 3d object from different angles, in some cases a full 360 degree view.

Stereograms work when the viewer's eyes separate to look "through" the stereogram, merging repeating horizontal patterns (dots on the old ones, generic smoother patterns on newer ones) that are variably spaced (horizontally) to create the pseudo 3d image. This requires stereo vision of the viewer in order to work. It's a single fixed image, and if you move, every thing in the image moves as well, making the background appear to move faster than the foreground. It only works if the stereogram is aligned horizontally with the viewers eyes. This could be classified as a trick into fooling your brain into seeing a 3d object.

 

[Andrew Mason]

Anyway, the point of this post: There are plenty of sites explaining how holographs are recorded, but I can't find any that explain how they recreate a 3D object. Does anyone know of a source that gives a proper mathematical description of holographs?

You are right, there is not much information about the physics of the image reproduction. It may be one of those things that people discovered and then tried to figure out why it worked.

The principle is this: when you use the interference pattern (created by the interference of the light from the object and the light from the reference beam) as a diffraction grating, by bathing the hologram with (diverging) laser light at the same angle as the original reference beam, the light that emerges from the other side of the holgraph diffraction grating has the same characteristics as the light that originally came from the object (ie. to produce the interference pattern). Why that should be the case is something that is rarely explained, which I sense is your question.

It is explained by diffraction principles: consider the interference pattern from two coherent, monochromatic laser beams (the same beam, split into two and then each shone onto the hologram surface from two different directions). This will produce an interference pattern on the hologram surface consisting of alternating dark and light lines whose spacing will depend on the angles between the beams. This pattern on a high resolution photographic film that is developed, produces a series of 'slits'. These slits become a diffraction grating when illuminated with one of the original beams.

The line spacing in the diffraction grating related to the diffraction angle is the same as relationship between the angle between the interfering beams and the line spacing in the interference pattern. So the hologram grating has the ability to use one beam of light in order to re-construct the other beam.

One really neat thing about holograms is that you can cut a hologram up into little pieces and each piece can be used to recreate the original 3D image (with a reduction in resolution and detail depending on how small you make it).

AM

 

[rcgldr]

One really neat thing about holograms is that you can cut a hologram up into little pieces and each piece can be used to recreate the original 3D image (with a reduction in resolution and detail depending on how small you make it).

No reduction in resolution, the resolution is a function of how fine the film grain is (wavelength of the laser isn't significant). What you do get is a reduced perspective of the image. For example, the "view" from the upper half of a hologram remains the same regardless of whether you cut off the bottom half or not. I'm referring to holograms that you look through in order to see an image of the original 3d object. I'm not aware of a means to project a 3d image into open space (or steam filled space) like was done in Star Wars by R2D2 with a hologram.

 

[rcgldr]

More links:

http://www.holostudios.com/holohelper/singlebm.htm

This one includes what happens if you cut up a hologram:

http://www.holostudios.com/holohelper/faq.htm

A simple explanation from the cheap kit maker:

http://www.litiholo.com/what_are_holograms.htm

and of course wiki:

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

Regarding this comment: "Note that if the distance between the reflecting point and the plate changes just a quarter of a micrometer (about 10 millionths of an inch) the zones more exposed will be replaced by less exposed and vice versa and the hologram will be blurred and useless." It's referring to any movement during the film exposure. There's no set "focus point" for the target object, it's just that nothing can move during the exposure of the film or else the image get's blurry. This means that sources of vibrations between laser, object, and film must be eliminated (this was not explained at wiki). So what ever "shutter" mechanism that is used has to not create any vibrations.

One tricky hologram is the lamp shade hologram I mentioned that has a rotating view of a person sitting on a stool. In order to eliminate blurring caused by movement of the person, high speed photography was required, and the images were taken as a series of small vertical stripes. There's a limit as to how bright the laser can get without damaging the person's eyes, so the film speed had to make up the rest.

 

[rcgldr]

Here's a link to a site that includes a good windows movie of a 360 degree hologram in motion, but it could be "R" rated. I'll continue looking for more of these (hopefully more generic ones):

http://www.mccormackholography.com

 

[skynelson]

Re: Cutting a hologram and seeing a different, whole view in each piece: Do all types of holograms show that property well? If I want to demonstrate this in a presentation, can anyone recommend a decent sized, cutable hologram that can demonstrate this purpose and isn't too pricy?