A hologram is a recording made on a light sensitive medium, such as a photographic plate, of interference patterns formed between two or more beams of light derived from the same laser (as in Fundamentals of Physics by Halliday, Resnick and Walker). It is different from a normal photograph in the sense that whereas, a normal photograph records image on two dimensions, a hologram, on being viewed, gives a three dimensional image. The process of making holograms is known as holography. Holography today is based upon the works of three Nobel Laureates, Gabriel Lippmann, William Lawrence and Denise Gabor.
Gabor was one of the first scientists to realise that a beam of light could be split into two parts, which could be used to produce holograms. The following is the process of manufacture of a hologram:
First, a beam of white light is made incident on a semi-silvered mirror which splits the beam in two, maintaining them in phase. One part of this beam passes through a lens, after which it is reflected onto the holographic plate. The other part is also made to pass through another lens, after which it is reflected by a mirror onto the object to be shown on the hologram, which reflects a part of the beam, depending on its nature onto the holographic plate. The holographic plate contains a film made of glass coated with a special photographic emulsion, and has microscopic grains that are use to produce interference patterns of the light. These interference patterns are microscopic bright and dark areas, and form an image only when the hologram is placed before light of appropriate wavelength.
A holographic record diffraction grating can be used to combine independant colours so as to produce a full-colour hologram. Three different lasers, one for each primary colour are used for the production of such multicoloured hologram.
One of the most popular use of holograms today is in the financial sector - currency notes, cheques and credit cards have started having holographic stickers on them to ensure security. Most of these holograms are not transmission holograms, which can only be seen under lasers of specific wavelengths, but reflection holograms that can also be seen under sunlight. These reflection holograms are coloured, and are almost impossible to copy, thus ensuuring the security of the document on which they are stamped.
Holograms are also being used in head-up displays in aircrafts, to help the pilot to llok at the cockpit and out of the windows together. Head-up display technices are also being used in bifocal contact lenses, scanners and high-resolution spectrometers.
Holographic Optical Elements are being used in computers to ensure a faster chip-to-chip communication.
Other uses of holograms include the use of holographic interferometry in biomedical and engineering measurements and precision measurements involving high speeds near to that of light. Some people also adopt holography as their hobby.