Ok, I know how a hologram is made, and I understand the behavior of light and interference patterns, but… How can any one piece of a hologram recreate the whole image? This would imply to me that the interference pattern is homogeneous; yet, the photographed subject is NOT homogeneous.
And, extrapolating the idea that an non-homogeneous object yields a homogeneous interference pattern, then wouldn’t ALL objects generate the same interference pattern? For, if the interference pattern is homogeneous, then it must be non-discriminatory about what surface the laser beam(s) bounce off of!
Last, a laser light is suppose to have the least amount of diffusion. In other words, it does not deviate from a straight line. So, if that is true, how can a one dimensional laser beam (i.e., no spread of the beam) ever capture the image a two-dimensional object…or 3D object???
I can only answer your last question since I’ve never heard that a fraction of a hologram can recreate the whole image so couldn’t explain how it works.
Laser beams can be split, diffused and focused just like regular light can. They simply diffuse far slower than regular light. The technology for splitting a laser beam has been around almost as long as lasers themselves.
First of all, you don’t use a laser “beam” to make a hologram - you use laser light.
Note how the beam is spread using a pinhole in this diagram.
The reason that any piece of a hologram can reconstruct the entire image is because the interference pattern covers the entire plate. It’s also not strictly true. As the size of the piece gets smaller and smaller, the resolution of the reconstructed image gets worse and worse. Once the piece is small enough, there is no image at all.
One way you can think about it is that each part of a normal photograph contains ALL the detail for a specific region of the image, When you cut out a piece of a photograph, all the detail for that region is gone (obviously).
Conversely, each part of a hologram contains some detail for ALL the image. When you remove a part of the hologram, the entire image loses detail, but it is all still visible.
IOW, The coherent light for creating holograms from a laser is made diffuse, after splitting the beam, so it can spread to light up the scene, yet remain coherent for the holography.
And to clarify, it’s not that any arbitrary piece of the holographic material contains the whole image, it’s that the bit that’s able to record any information, records the interference pattern of the entire holographed scene.
That’s why if you cut it to 1/16th the size, it’d still reproduce the illusion of the whole scene, but would be far fuzzier and lower fidelity because you’ve substantially decreased the amount of bits this interference pattern can reproduce—it’s resolution—so it has less to work with to recreate a crisp, stereoscopic image.
A hologram can be desribed as a window with memory. Make a window smaller. Can you still see “out” of the window? Yes. But what has changed? The range of various points of view you can look out of the window and still see the object. Very small window? You can only look at the object from one point of view. Very big window? You can look at it from over here to way over there.
Of course it is more complicated than that but its late and thats all I am typing for now.
It is worth noting that a laser doesn’t necessarily produce a non/minimally-divergent beam, at least in the case of semiconductor laser diodes:
(a laser diode from a DVD player (not burner) powered up by itself looks like a really bright LED (some high-brightness LEDs even have smaller viewing angles than typical laser diodes), the beam isn’t even dangerous either to look at from more than a few inches away)
People are saying that a trimmed-down hologram will be less crisp - but I don’t think that’s correct - it loses information, for sure, but it’s loss of viewing angles, not loss of resolution.
If I have a 10cm square hologram and I mask off three quarters of it with black tape (which is the same as cutting it), the quality/fidelity of the image visible in the remaining unmasked quarter isn’t reduced, however, I’ve only got a quarter of the original viewing angles for the whole image.
This is a loss of image information, but it’s not a loss of sharpness - the image becomes less 3D, not less clear.
Yes, because you’re trying to see the whole object through a quarter of the original view - so yes, I guess if you ‘zoom in’ (actually dolly in) so that you’re viewing the whole object again, through the one-quarter ‘window’, then you’ll see it at one quarter of the original resolution.
But cutting or masking off three quarters of a hologram (without changing the viewpoint) won’t suddenly make the remaining visible quarter look any different.
Actually, the loss of resolution is consistent with websites on the subject. Try www.holoworld.com (Unfortunately, these websites do not drill down far enough to answer my questions posted here.) One note, however: It could be that loss of resolution is the short story?
Thanks, all for your replies. I’ve learned some, but still unsure why the interference pattern should be homogeneous. Say you make a hologram of a bird. It’s “pieces and parts” that make up the surface of our subject would reflect light off at different angles. I was under the impression the angle of reflection also causes a phase shift in the reflected light. So, I am picturing a whole mess of reflected laser light at various phase angles interfering with the reference beam (I believe it is called), thus the interference pattern would not be homogeneous. Please correct me to help me better understand what is happening.
Also, re: the issue of cutting a hologram vs. masking a portion is a curious discussion. What would be the difference? Maybe they equate to the same thing? In either case, perhaps resolution is lost in either scenario for (apparently) the whole hologram is needed to have the best resolution.
Each point on the hologram contains the information about all parts of the image as visible from that point - if you pretend a hologram of an apple is actually an apple in a box, each spot on the front glass contains information about all direct lines of sight to the apple from that point.
Cutting vs masking the hologram should make no difference.
It’s loss of spatial resolution. If you destroy three quarters of a hologram, you destroy three quarters of the information about the 3D image as a whole - but the part you have still contains the same information, at the same quality, as it ever did.
If you mean less 3D because you can’t move your head/eye(s) around to view the recreated object from differing vantage points then yes it is less 3D (the small vs big window analogy).
If you mean less 3D in the sense that from one given point of view the object looks flatter and flater the smaller the hologram then that would be wrong (I think, I don’t start thinking well till sometime after noon).
I mean less 3D in the sense that the capacity to ‘look around’ the image is reduced.
In theory, a hologram trimmed down to a single point (impossible in practice) still contains the whole picture, but only from one angle - so such a thing would no longer be 3D
I don’t think that is true. Lets just make the hologram pretty darn small so it still works.
Lets say the hologram was made in this fashion. One chess piece was 2 inches behind the sheet of holographic film. Another one was 20 feet back. Expose film to laser light, process film, “replay” hologram.
Now, I take the hologram and cover it all up except a hole a few millimeters across. When I place my eye a few inches from that hole and peer through it, my eye will be able to tell that the close chess piece is just a few inches from my eye and the far one is 20 feet because I won’t be able to focus on both at the same time.
Or in other words, depth of field/focus considerations will still exist. So there is still 3 D information encoded even though for all practical purposes you have eliminated all but one perspective/point of view.
