The word holography is derived from the greek root meaning entire picture. Holograms are a modern attempt at creating entire pictures, three-dimensional photographs that offer a viewer the same advantages of perspective, parallax, form and content as the original scene.
A hologram is in a sense a window with memories. It is actually a photographic emulsion in which the information about a scene is recorded in a very special way; holograms do not store visual images like ordinary photographs. Without the proper illumination the window appears apparently blank. However when illuminated properly an image can be recreated,in three dimensions, as if the object were being viewed through the window. If the viewer moves his/her head from one side, he/she will see the scene in the window from a new perspective. In effect the viewer can look around the corner of an object in a hologram. What is observed in not a psychological effect; this means that the light arriving from the hologram into the viewers eyes is physically the same as the light emitted from the original object. If the the window is partially covered (or the hologram broken), unlike a real photograph, the entire image is still visible through each section of the hologram. Thus, in some sense, all the information about the image is available in every piece of the hologram. Finally, if the window is rotated into a particular position and illuminated, a virtual image, in front of the window, free standing in air can be created of the original scene. These features are not possible with a conventional, flat photograph and add new dimensions to the holographic stage.
Most viewers, upon seeing their first high quality hologram, feel a certain puzzlement and disbelief. A common reaction is for the first time viewer to place his/her hand where the scene appears only to find nothing tangible is there. In this sense holograms are provocative and appeal to some basic sense of reality and imaging.
The basic theory of holography was described by Professor Dennis Gabor of England in 1947. However it was the discovery of the laser in the early 1960's that led to rapid advancement in this area. At that time, holography was heralded as the photographic technique that would revolutionize the motion picture media. That revolution has not yet arrived due to the technical difficulties in producing good holograms. However, recent developments have lead to the widespread use of holograms in the other areas such as credit cards to discourage counterfeiting; in checkout counters to scan bar codes; in laboratories to perform nondestructive testing; and in art to exhibit multicolor concepts. It is our contention that, given these advances, a well directed interdisciplinary research effort between Art and Physics can contribute significantly in this new technology adding dimension and depth to the Expanded Arts Program. Holography is particularly interesting for our purpose since its features, although quite complex, can be explained, understood, and used after learning simple physical principles. Moreover holograms can be made in various shapes with a variety of techniques and easily be integrated into complex art forms.
Holography is based on the interference of light. A beam of coherent light from a laser is split, with one portion, the reference beam, reflected by one or more mirrors until it illuminates a photographic plate. The other portion of the laser beam is reflected by a mirror to the object that will be hologramed. Some of the light scattered from the object illuminates the photographic plate and interferes with the reference beam. The interference pattern is recorded on the photographic plate and encodes the information about the object being photographed. When the photographic plate is developed it shows no discernible pattern, but when coherent light is sent through the plate, replacing the reference beam, the interference pattern on the plate recreates the pattern of light produced by the object. As the observer looks into the plate and receives some of the light from the pattern, he/she sees a virtual image of the object that had been photographed. When properly viewed, a quality hologram can be convincing enough to be mistaken for real objects. Like looking through a window, many viewers find it hard to believe they are not seeing real objects.
There are two basic types of holograms. The one described above is called a transmission-type hologram because when it is viewed, light passes through the photographic plate to the observer. One also can make reflection holograms, in which light is reflected from the plate as if it were a painting. In addition to these basic techniques there are numerous hybrids that combine techniques to serve particular functions. These include the ``white light" hologram, focused image hologram, cylindrical hologram, multiple image hologram, time lapse holograms, and computer assisted holograms. In the last case non-existent objects can be recorded via computer and viewed as three-dimensional images.
Successful holography requires stability of the platform and laser and all related components. Since the film is actually recording an interference pattern of light waves, a movement as small as the wavelength of light (approximately 1 millionth of a meter) can disrupt the pattern. It is this requirement that makes holography both difficult and interesting. To meet this requirement a typical holographic camera consists of a stable platform, usually a large mass on inner tubes, together with high quality laser, optical mirrors, lenses and mounts. The large mass makes the structure stable while the inner tubes isolate the structure and make it insensitive to external vibrations. Standard techniques (Michaelson Interferometry) may be used to determine the stability and relaxation or settling time of the system. In addition to the stability requirement, other techniques must be mastered to achieve high quality holograms. These include manipulation of the laser and optics, choice of exposure technique, lighting, and film processing. The mastering of these techniques requires exact alignment of the optical components for proper operation. Once these basics are achieved the holographer is free to apply the holography technique to his/her own direction. The creation of holgraphic art occurs by the exercise of expressive and aesthetic concerns combining art, physics, and photography.
عَـــادِل حَسَــب اـلّـه
