Holography is the art, technology and science of creating and displaying holograms: three-dimensional images that appear in their full glory even without the use of special glasses. Though holograms are produced using lasers, some types of holograms can be viewed in ordinary white light.
Every hologram is a 3-D image, but not every image that appears to have three dimensions is actually a hologram. Some of these not-holograms can be spectacular in their own right, and one old technique for creating a 3-D illusion, called Pepper’s Ghost, has been used by stage magicians and other illusionists since the 1600s.
But the technique of holography has created such a stir that the prefix “holo-” is often attributed to any 3-D illusion, or simply to convey the idea of 3-D illusions, like the Holodeck from “Star Trek.”
(And just to complicate things, the word “holograph” has nothing to do with holograms or holography, but simply refers to a document entirely handwritten by its author.)
It’s All Done With Mirrors
If you toss a small pebble into a still pool, it will produce series of expanding circular ripples around the point where it fell. If you quickly toss in another pebble, it will create a whole new set of rings.
When the two sets of ripple rings expand and cross, they create striking, complex patterns as the ripples accentuate or cancel each other out. (If the pool is shallow, the effect of light on the pool bottom is even more dramatic.) These patterns are called interference.
Light also spreads out as microscopic waves or ripples, which can produce visible interference patterns. We don’t normally see these patterns because the ripples of ordinary light are “choppy,” like a pool full of people.
Lasers, however, produce “coherent” light with smooth ripple patterns, and the crossing of two laser beams will produce a clean, striking interference pattern. This doesn’t even require two separate lasers. You can “split” a laser beam with a mirrored lens that reflects part of the beam while still allowing part of it to pass through.
One part of the laser beam shines on an object while the other does not, then film exposed to both beams records the resulting complex pattern of interference.
It does more than that. Once the film is developed, shining another laser through the film will produce a 3-D image of the original object. This is a hologram. Some holograms, produced by so-called rainbow holography, do not require laser light to view, and can be seen in ordinary light.
This, in a nutshell, is what holography is all about. Northrop Grumman holographer, Fernando Catta-Preta, has described a hologram as “light that’s been sculpted.”
The Art of 3-D Illusion
Display a hologram of, say, a glass sitting on a table, and it is tempting to call the glass an illusion. If you reach in and try to pick up the glass, your hand closes on … nothing. But really the 3-D image is no more an illusion than an ordinary two-dimensional photograph — you couldn’t pick up the glass in a picture, either. We only think of holograms as illusions because we are not yet accustomed to them.
As Catta-Preta notes, there are other ways to produce apparent 3-D images that are genuine illusions. In the original Star Wars movie, Luke Skywalker viewed what looks like a hologram of Princess Leia. But the movie is a traditional 2-D film, and the image is “only” a special effect.
A classic not-really-a-hologram technique, Pepper’s Ghost, is perhaps most famous from its use in the Disney World “Haunted House” attraction.
Like a true hologram, this is done with a mirror. Behind an angled partially-reflecting mirror is a wax model of a young woman. To one side, apparently superimposed on her, is a skeleton. First a light shines on the model; it gradually dims while another light shines on the skeleton — and the lady seems to turn into a skeleton before the audience’s eyes.
Other techniques, such as autostereoscopy, exploit the separation between our eyes to present a slightly different view to each eye, creating a 3-D illusion. The special glasses you use to watch 3-D movies also cause each eye to see a different image, again tricking the eye into thinking that what you see on the screen is three-dimensional.
All of these optical effects can be impressive. What makes holography different is that you really are seeing a 3-D image.