Method of obtaining three-dimensional photographic images. These images are obtained without a lens, so the method is also called lensless photography. The records are called holograms derived from Greek holos, meaning whole and gram, meaning message. The theoretical principles of holography were developed by the British physicist Dennis Gabor in 1947. The first actual production of holograms took place in the early 1960s, when the laser became available. By the late 1980s, the production of true-color holograms was possible, as well as holograms ranging from the microwave to the X-ray region of the spectrum. Ultrasonic holograms were also being made, using sound waves.Production, A hologram differs essentially from an ordinary photograph in that it records not only the intensity distribution of reflected light but also the phase distribution. That is, the film distinguishes between waves that reach the light-sensitive surface while they are at maximum wave amplitude, and those that reach the surface at minimum wave amplitude. This ability to discriminate between waves with different phases is obtained by having a so-called reference beam interfere with the reflected waves.Thus, in one method of obtaining a hologram, the object is illuminated by a beam of coherent lighta beam in which all the waves are traveling in phase with one another. Such a beam is produced by a laser. Essentially, the shape of the object determines the form of the wave frontsthat is, the phase at which the reflected light arrives on each point of the photographic plate. Simultaneously, a portion of the same laser beam is reflected by a mirror or prism and directed toward the photographic plate; this beam is called the reference beam. The wave fronts of this latter beam, not having been reflected from the object, remain plane-parallel and produce an interference pattern with the wave fronts of the light reflected by the object. If the object is a point, for examp...
