Interference Patterns

Holography does not record an image onto film the same way a camera does. Holography records the interference pattern of light, generated from a reference beam and reflected light from the subject (object beam), see figure 2. The light source required must be monochromatic (single light frequency) and coherent (wavelengths in phase). A Helium-Neon (HeNe) laser fits the bill.

Figure 2 illustrates a typical split-beam holographic setup. We don't need a set-up as complex as this illustration to generate holograms. It is possible to produce holograms using a single beam. In fact the holograms we will shoot are single beam set-ups, but in an effort to present a diagram that clearly illustrates the recording of the interference pattern created, the split beam set-up is better.

Figure 2

Figure 2

With print photography, the developed film becomes a negative. The negative is used to produce prints. A single negative can be used to create thousands of identical pictures. In holography there is no negative and the original piece of film is the hologram. While it is possible to make copies of holograms from a master hologram, the procedure is more complex than with photography.

When the finished hologram is viewed, it is a true, threedimensional image of the subject. Figure 3 demonstrates the parallax of the hologram. The vertical parallax is from top to bottom, and the horizontal parallax is from left to right. Parallax is a term referring to the viewable angles of the subject in the hologram.

Figure 3

Figure 3

Holographic Redundancy

If a hologram is broken into small pieces, the entire image would still be viewable through any of the broken pieces. This is easier to comprehend if we look at figure 3 again. Imagine that the holographic film becomes a window with memory when it is exposed. So any object behind the window, from any viewpoint, is faithfully recorded. So much so that if you covered the hologram with a black piece of paper with a peep hole in it you could still view the entire subject through the peep hole as if you were looking through a window. Where you placed the peephole on the window (hologram) determines from which perspective you would see the subject.

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