Chapter 20, Additional Exercise 8: Identifying Techniques in an Extended Definition

Identify the techniques used in writing the following extended definition, intended for the general reader:

Holography, from the Greek holos (entire) and gram (message), is a method of photography that produces images that appear to be three-dimensional. A holographic image seems to change as the viewer moves in relation to it. For example, as the viewer moves, one object on the image appears to move in front of another object. In addition, the distances between objects in the image seem to change.

Holographs are produced by coherent light, that is, light of the same wavelength, with the waves in phase and of the same amplitude. This light is produced by laser. Stereoscopic images are created by incoherent light—random wavelengths and amplitudes, out of phase. The incoherent light, which is natural light, is focused by a lens and records the pattern of brightness and color differences of the object being imaged.

How are holographic images created? The laser-produced light is divided as it passes through a beam splitter. One portion of the light, called the reference beam, is directed to the emulsion—the "film." The other portion, the object beam, is directed to the subject and then reflected back to the emulsion. The reference beam is coherent light, whereas the object beam becomes incoherent because it is reflected off the irregular surface of the subject. The resulting dissonance between the reference beam and the object beam is encoded; the beam splitter records not only the brightness of the different parts of the subject but also the different distances from the laser. This encoding creates the three-dimensional effect of holography.