The science, technology and art of producing recorded images which appear three dimensional to the human eye.
Much like conventional photographs, holograms are light recordings on flat pieces of photosensitive material. However photographs are capable of recording either only the intensity of light (black and white photos) or both the intensity and the wavelength of light (colour photos). Holograms are capable of recording a third piece of information about light - direction. All holograms can record both intensity and direction of light and some can even reproduce wavelength as well - full-colour three dimensional images. The result is a flat image which can appear three dimensional or which can encode apparent motion.
Another difference between photographs and holography is that holograms rely on laser technology and lenses are not required between the subject and the recording medium (though lenses are typically used between the illuminating light source and the subject). The simplest form of hologram uses a mirror to split the light from a laser into two beams. One, a reference beam, illuminates the photosensitive film or plate directly. The other beam is reflected off the subject before hitting the film or plate. The two beams of light interfere with one another according to the reflective properties of the subject, thereby encoding directional information as well as intensity. The hologram is then viewed by shining laser light of the same wavelength used in the recording on it. The result appears as a three-dimensional monochromatic image of the original subject; a phenomenon known as wavefront reconstruction. Colour holograms require light from three lasers - red, green and blue.
The basic principles of holography were first devised by Hungarian-born physicist and Nobel laureate (1971, physics) Dennis Gabor (1900-1979) in 1947, while working on improving the design of electron microscopes. While he was unable to implement his work very effectively because there were no sources of coherent light available to him at the time (lasers were not invented until 1960), he laid down the groundwork for hologram technology and indeed coined its name, from the Greek holos (whole) and gramma (message).
The first laser holograms were made in 1962 by Emmett Leith and Juris Upatnieks of the University of Michigan. Over the years techniques and technology have been refined and now motion, white light and colour holograms are all possible.
Indeed its possible for amateur hobbyists to make basic holograms, though the more complex forms of colour holograms require extremely costly equipment.
cf. coherent light, light interference, laser.
Entry last updated 2002-04-05. Term 609 of 1487.
Previous term: hologram.
Next term: horizontal shutter.