Holography is a memorization technology of optic information under the form of an extra fine twine of disturbance edges by using laser coherent beam (both spatially and temporally). The so created image is characterized by the illusion of three-dimensionality.
Holography was theorized for the first time by the Hungarian scientist Mr Dennis Gabor. He realized simple holograms by using "green" light of a mercury vapours based lamp. It had no significant applications up to the introduction of highly coherent beam sources (LASER) in the sixties.
By introducing laser sources ,several techniques of holographic registration were developed, also thanks to contributions of Emmett Leith and Juris Upatnieks, Yuri Denisyuk, George Stroke and some others.
Hologram in modern age
The registered information represents disturbance between a part of light (coming from the same coherent source) reflected by a mirror and by a wave front reflected by an object on a very fine grain photographic sheet called hologram.
Grain subtlety of a holography sheet allows resolution of 3000 - 12000 lines/mm.
Holographic technique is based on phenomenon of optic disturbance. During hologram's registration, laser beam is divided by a half-transparent mirror (beam-splitter), the two resulting beams are so expanded and conducted by mirrors, one illuminating subject (object wave front) and the second one directly illuminating a photographic sheet (reference wave front): both fronts interfere and registration of disturbance edges on the photographic sheet is represented by hologram. The above mentioned sheet is obviously developped and fixed like in an ordinary black and white photographic process. When the reference wave is returned on the sheet, the object wave front is re-built in each point of the sheet. Hence parallax effect which is typical of virtual image which is seen looking through the holographic sheet, just like the object was seen through a window in each one of its points.
- Transmission hologram: Image is observed by the opposite side to the one where light comes from.
- Reflection hologram: Image is observed from the same side where light comes from.
Holograms on credit cards are reflection holograms with white light (WLTO) which are mirrored so as to allow its visibility by reflection.