Since the Word For the Interaction of the Waves in a System Like This Is “Interference”, the Measurement Process Is Called _interferometry_ (Pronounced in Ter Fer Om E Try). Although Not New, It Can Now Be Applied For the First Time in Machine Tool Applications, Providing the Accuracy Needed in This Age of Space Technology and Microminiaturization. Measurements With a Laser Interferometer Can Be Made With an Accuracy of 0.5 Part Per Million at Distances Up to 200 Inches. Such Precision Was Previously Unheard of in a Machine Shop Environment, Having Been Limited to Laboratory Measurements, and Only at A Range of a Few Inches. Under Similar Laboratory Conditions, Measurements by Laser Interferometry Now Detect Movements of 10⁻¹¹ Centimeter, a Distance Approaching the Dimensions of an Atomic Nucleus. Now let us suppose we expand the laser beam as shown on page 22, and, with the aid of a mirror, direct part of it (the reference beam) at a photographic plate. The remaining portion of the diverging beam is used to illuminate the object to be photographed. Some of this light (the object beam) is reflected toward the plate and carries with it information about the object, as indicated by the wavy line. In the region where these two beams intersect, interference occurs, and a sample of this interference is recorded within the photographic emulsion. Where two crests meet a dark spot is recorded; where the waves are out of phase the processed plate is clear. The result is a hologram, a complex pattern of “fringes”, characteristic of the contour and light and dark areas of the object, as well as its distance from the plate. These fringes have the ability to diffract light rays. When light from a laser, or a point source of white light, is directed at the hologram from the same direction as the reference beam, part of the light is “bent” so that it appears to come from the place once occupied by the object. The result is a remarkably realistic 3-dimensional image. There, in a nutshell, is the incredible new technique of holography. The extreme order of laser light is illustrated by the regularity of the dots on the cover of this booklet. This strange kind of light provides us with yet other advantages. Indeed, one of the most important is the fact that the energy of the laser is not being sprayed out in all directions. All of it is concentrated in the narrow beam that emerges from the device. And it _stays_ narrow. Laser light has already been shone on the moon, the beam spreading out to only a few miles in traveling there from earth. The best optical searchlight beam would spread wider than the moon itself, thus dissipating its energy. It is for this reason, as well as its temporal coherence, that laser light is being considered for communications. A narrow beam is particularly important for space communications because of the long distances involved.