Holograms

The word hologram comes from the Greek 'whole drawing' and is a photograph taken using light produced by a laser. A normal photograph records the intensity of light that falls onto film. It is made by one set of light waves and the image that the light waves create is two dimensional. A hologram records two sets of light waves. One set of wave is reflected onto the film by the object in the same as an ordinary photograph, the other set of waves arrive at the film from a different direction without meeting the object at all. At the point where the two sets of waves met each other, they produce interference fringes that are recorded in the film. The interference fringes produce a three dimensional image when viewed.
Hungarian Dennis Gabor outlined the principle of holography in 1948. He realised a beam of light could be split to produce a three dimensional image but needed a coherent light source to achieve this. This was not possible until the invention of the laser in 1960. In 1962, the first successful hologram was made by Juris Upatnieks and Emmet Leith.
There are two types of holography. Transmission holograms are created by splitting a laser beam. The 'object' beam passes through a lens and is reflected onto an object. Its light shines on to the holographic plate, coated with photographic emulsion. The 'reference' beam passes through a lens and is reflected onto the emulsion where it meets light from the object beam and produces interference fringes. A reflection hologram is created by shining a reference beam and an object beam onto thick film from opposite sides. The beams interfere to produce tiny areas of light and dark throughout the film. When the film is viewed, this pattern reflects light in a way that produces the holographic image.