Holography is historically the art of recording optical data and reproducing the recorded information in a three dimensional display. Early holograms were produced by illuminating photosensitive material (photographic film) with coherent light reflected from real objects combined with a coherent reference beam from the same source. The two beams would interfere, and the interference pattern was recorded on the film. When the film was developed and later illuminated by a coherent beam, a reconstructed object beam resulted, which, when viewed by a person, would appear as a three-dimensional display of the real objects that were used as a basis for the hologram.
After the discovery and development of equipment for producing holograms, several alternative methods were developed. It was discovered, for example, that media other than photographic film could be used, and that interference fringes could be developed through the thickness of a medium rather than in a plane pattern. Holograms based on fringe recording through the depth of a medium are called volume holograms, and those using other than photographic material are known as phase holograms.
Phase holograms can be either of the transmission type, in which light is passed through the medium, or the reflection type, where light is reflected from the surface. One medium for recording phase holograms is thermoplastic. With a thermoplastic medium, high power lasers are used or long exposure times. Recording is accomplished by distorting the medium, producing a surface of varying topography according to the intensity of the interference pattern of light falling on the medium.
An interesting result of phase holograms on plastic film is that the three-dimensional display can be reproduced by reflecting ordinary white light off the film. Also, the plastic hologram can be mass produced quickly and inexpensively. The original hologram is used to produce a metal "master" plate, and the metal master is used to "stamp" reproductions in other thermoplastic media. These are the three-dimensional displays that are commonly used for greeting cards and credit cards. Each of the embossed reproductions of the original hologram has the topography of the original, and the three-dimensional "picture" is a result of light reflected from the irregular surface.
Typically holograms have been stationary three-dimensional pictures, and to make an original hologram typically requires that the object to be reproduced be held very steady over a period of time while the recording is made. Moreover, there are severe restrictions on the size of objects that can be "holographed". It has long been recognized, however, that a dynamic (moving) three-dimensional display would be very desirable and have many uses.
To make a dynamic three-dimensional display requires controlling the topography of a reflection phase hologram in a way that surface height may be altered over a relatively broad area. To produce the interference fringes necessary requires physical resolution in surface height on the order of less than the wavelength of visible light. The mean wavelength in the visible spectrum is about 5500 Angstrom units, or about 0.5 micron. Moreover differences of this order of magnitude need to be provided at relatively fine resolution in a planar matrix over a display surface to present a clear picture to the human eye.
Although attempts have been made and research continues, up to the present time no one has provided a dynamic hologram that is useful and reproducible to present moving pictures of real objects, particularly not of objects in real time. What is needed is a reflective surface as a basis for a dynamic three-dimensional display with a method of controlling the topography of the surface over a range of about 1 micron in an area matrix sufficiently small to present a dynamic three-dimensional picture to the human eye. The display surface needs to be amenable to known and tried production and control techniques. Such a display tool can become the basis for many, many advanced displays, such as for guiding surgeons during delicate surgery, for example arterial-splicing techniques. Another application among many, would be improved night vision apparatus. Still another would be theoretical modeling in many disciplines, both micro- and macro-scopic.