I bet I’m not the only one here who would really love to understand holograms/holographs (and what the heck is the difference, anyway?) but the explanations I’ve read just have not given me a gut feel for what they are and how they work.
So this question invites anyone who wants to tackle the challenge, to go beyond or underneath the usual explanations so we can intuitively grasp what they are about, and how they are different from what we are used to thinking about. That’s valuable because supposedly so much of our reality would be more understandable if we understood these critters.
This is my second attempt – spent several hours on the first one, only to have chrome crash – (Grrr Mumble %@#$).
Lots of different aspects to this question.
Some of the explanations are easy to think of in terms of waves, some easier in terms of particles.
There is still much debate about the nature of light.
Light certainly has some properties that are particle like, and some that seem wave like, understanding the depths of the arguments is not really required for this explanation; and it seems clear to me that light is more particle like than it is wave like (at most scales – certainly most of the scales we are interested in for this explanation).
So if you think of light as particles travelling in straight lines, this will help with most of the understanding of what is going on in 3 dimensions. Interference is easiest to understand in terms of waves, and there is a particle equivalent explanation of interference, but it is a bit more complex – so I won’t worry about it at this time.
Holograms rely on the property of LASERs called coherence.
In coherence the phase relationship of all light particles is in step. If you use the wave analogy, think of the waves all lined up.
So without worrying about how LASERs generate coherence, just accept that they do.
When you have coherence, then interference can encode a lot of information.
Interference is when two positives add to create something bigger, and a positive and a negative add together to cancel each other out (and all shades in between).
The diagram below gives a visual representation:
On the left above, the two waves below are in phase (crests and troughs align), and add together to produce a larger wave.
On the right above, the two waves below are out of phase by 180 degrees, and cancel each other out – the crest of one always being with the trough of the other.
This property of interference is critical, as it allows the LASER to encode spatial information into the image.
Where ray paths are a whole number of wavelengths different, they add, if there is a half wave difference, they cancel out.
The diagram below gives a general setup for the sort of mechanism required to produce a LASER hologram.
While the diagram above gives the general idea, it does not really convey the complexity involved.
At every point on the photographic plate, whether or not the crystal develops is a very complex function that includes information about the reflectivity of and the spatial relationship to every point on the object.
From the point of that particular crystal of photographic emulsion, the way to compute whether or not it will develop is to first measure the path of the reference beam, then measure the path and intensity of all beams from all points on the object.
For every crystal in the photographic plate, all aspects of that function will be different.
Thus, every crystal is a simple binary, it either develops, or it doesn’t, but the function that controls that involves information about both the spatial relationship and the reflectivity, of every point on the object.
So every point contains information about the whole, but at the very lowest level, it is not much information (just a zero or a one).
It is only by having many such points, in their specific spatial relationship to each other, that we can start to see an image.
Cut a holograph in half, and you can still see the whole object, it just becomes less distinct, you have halved the amount of information available to construct a visual representation. If you keep on cutting the image in half, at some point it will become just an indistinct blurr, where you are unable to resolve anything.
Our ability to intuit is based on very similar principles.
The more information and experience we have, the clearer the images (or concepts) that form in our minds.
The more images and concepts we have in our library of experience, the more likely it is that the pattern match that our brains come up with will be a reasonably accurate analogy to reality.
There is a lot more to how our brains work (many other concepts and systems), and there is more to holography that is relevant to recall and conceptualisation, and this concept is key to understanding what Kant called “pure practical reason”.
The photographic plate is where the holograph is recorded.
For it to be redisplayed, it needs a LASER to give a coherent light source.
As the LASER light is reflected off the holographic plate, each eye gets to see something different, because the interference effect works in reverse, to produce a different image in each eye.
Thus, we get to see the original object in 3D, and we can move around it.
The illusion is created because each eye gets to see a different image, just as in the case of us looking at a real 3D image.
The “holograms” on credit cards are not “real” holograms in this sense, they are much more like the old images we used to get as kids behind a plate of plastic with a series of ridges on the surface. The ridges acted as lenses which allowed each eye to see a slightly different image, and thus, again, creates the illusion of three dimensions.
The technique used in real holograms is very different, and relies on coherent light.
Real holograms give a continuously varying image, as one moves one’s head. The others tend to move in discrete jumps, as they only have a few different sets of images to choose from.
Apologies for not filling in all the blanks – life has gotten very hectic here. Not as much time as I would like to put into these answers.
The LASER beam does not generate an image in the air – your brain does that.
All the LASER beam does is reflect off the holographic plate.
Each of your eyes receives this LASER light.
Because each eye is in a different place in space, the way in which the LASER light beams reflected from different parts of the holographic plate interact with each other (cancelling and enhancing depending of path lengths) creates slightly different images in each eye.
Your brain then interprets these different images as representing there being something in space in front of you (because that is what your eyes would see if there really was something in front of you).
There really isn’t anything there.
It is all just an illusion created by the interference effects that result from using a coherent LASER light (one of the beautiful “side effects” of coherence).