The record on which the light waves are “stored” is known as the “hologram.” It is essentially similar to the film for an ordinary camera but is of better quality and generally in the form of a photographic plate made of glass.
Some figure shows how the recording is done. An expanded beam of light from a laser is first divided into two parts by a special mirror. One part (called the “reference beam”) travels directly to the photographic plate, while the other part illuminates the object to be holographed. The complex pattern of light reflected from the object then also travels to the photographic plate. Light is thus arriving on the plate from two directions, producing a very detailed recording of the pattern on the plate.
Other figure shows how the playback process is done to give the 3-D image. The plate is first developed (as in ordinary photography) and the object removed. A single beam of light is now directed onto the plate. The light passes through the plate, but in so doing it is modified by the pattern embedded in the plate. The result is that the emerging light exactly duplicates the original light that came from the object, and so the object seems to reappear. To the viewer, the photographic plate is like a window through which the object is seen in full depth. By looking through the “window” in different directions, the object is seen from different angles. The image manifests such vivid realism that the viewer may be tempted to reach out and touch it, but, of course, nothing is there!
Holograms and the images they produce have many curious and fascinating properties. The hologram plate is equivalent, in holography, to the negatives obtained from an ordinary film. However, it is quite different in certain respects. For example, if you have some black-and-white negatives available, hold them up to the light and you will notice that they contain the picture (actually, in reversed form-the dark areas are light and the light areas dark). Hold the hologram plate up to the light and you will find that it bears absolutely no resemblance to any picture. Only under a microscope can the pertinent information be seen, but, even then, just as a highly irregular, unintelligible pattern of lines, blobs and whorls.
If part of an ordinary negative is damaged or cut away, then, obviously, that portion of the picture will be ruined or missing in prints made from the negative. Smash the glass hologram plate, however, and you will be surprised. The whole image can be reconstructed from any of the pieces! The quality will be impaired somewhat, depending on the size of the piece. Nevertheless, the image will always be complete!
The 3-D realism of the image produced from holograms is evident in several ways. If you change your viewing position through the “window” (the glass hologram plate), the perspective of the picture changes just as it would if you were looking at the original scene. If something in the foreground of the picture obstructs an object behind it, then by moving your head to the side you can look past it to see the hidden object. You will also find that the focus of your eyes will change when you look at near and far points in the scene and if you are nearsighted then your spectacles will help!
An interesting effect occurs if, say, a diamond ring is holographed. In the holographic image the diamond reflects glints of light from its facets and these appear and disappear as the viewer moves his head-exactly like the real diamond!
In short, the reconstruction has all the visual properties of the real thing.