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Holography is an optical technology for storing visual information in the form of a very fine interference bangs using coherent laser light, appropriately projected; the image created by the interference bangs is characterized by an illusion of three-dimensionality. It is more properly a parallax effect in the perception of the image, i.e. the image is perceived differently depending on the point of vision; in the case of the two eyes, each of them perceives the image from a slightly different position than the other. This difference, called parallax, in normal vision determines the three-dimensional perception.
The etymology of the term “Holography” comes from the ancient greek ὅλος, holos, “all”, and γραφή, grafè, “writing” and literally means “I describe everything”.
Holography is a photographic technique that records the light scattered from an object and then presents it in a way that appears three-dimensional. A hologram is a physical structure that diffracts light into an image. The term “hologram” can refer to both the encoded material and the resulting image. A holographic image can be seen by looking into an illuminated holographic print or by shining a laser through a hologram and projecting the image onto a screen.
Theorized by Hungarian scientist Dennis Gabor, who made simple holograms using green light from the spectrum of a mercury vapor lamp, had no significant applications until the introduction of highly coherent laser light sources in the sixties. With the introduction of laser sources began the development of various holographic recording techniques following the contributions of Emmett Leith, Juris Upatnieks, Yuri Denisyuk, George Stroke and others. After the phase of the so-called holographic explosion (between 1963 and the mid-80s), silver halide holographic plates became almost unobtainable due to the decision by Kodak, Agfa and Ilford to stop their production.
The recorded information is the interference between a portion of light (from the same coherent source) reflected by a mirror and the wavefront reflected from an object on a very fine-grained photographic plate, called a hologram. The grain fineness of a holography plate allows the resolution of 3000 – 5000 lines/mm.
The holographic technique is based on the phenomenon of optical interference. When recording a hologram, the light from a laser is split by a semi-transparent mirror (beam-splitter). The two resulting rays are then expanded and conveyed through special mirrors: one of them goes to illuminate the subject (wavefront of the object), while the second directly affects a photographic plate (reference wavefront). On it the two wavefronts interfere and the recording of the interference bangs is the hologram. The plate is obviously developed and fixed as in an ordinary photographic process in black and white. When the reference wave is returned on the plate, the wavefront of the object is reconstructed point by point on the plate. This gives rise to the parallax effect typical of the virtual image that can be seen when looking through the holographic plate, exactly as if the object were seen point by point through a window.
With different techniques holograms with different properties are recorded:
- transmission hologram, in which the image is observed from the side opposite to that from which the light comes;
- reflection hologram, in which the image is observed from the same side from which the light comes.
The holograms on credit cards are white light reflection holograms (WLTO) mirrored to allow visibility by reflection. The holographic techniques aimed at the creation of holographic mass storage media are at an advanced stage of study (with the production of prototypes) and various announcements of imminent commercialization, by companies in the electronics industry such as Maxell (through the company InPhase Technologies), Optware, Fujifilm, CMC Magnetics.
The realization of a hologram does not require lenses, except for special techniques. For practical reasons, however, it is customary to use lenses as beam expanders to expand the laser beam so that it can illuminate the entire object and plate. The lenses must have a high surface quality in order not to compromise the characteristics of the coherent beam.
Holography has a wide range of applications: in industrial production controls, in the restoration of works of art, in computer science, etc.. The fundamental characteristic of a hologram is the enormous amount of information that can be stored in it. This has stimulated the development of holographic memories, a technique that can be applied both to the optical processing of data and their storage. Interesting are also the advances in the reproduction of holograms by illumination with white light.
Holography has uses also in metrology, and allows full-field displacement measurements with an accuracy of the order of magnitude of the wavelength of the ray used (typically fractions of microns for visible light). The technique used is called holographic interferometry, and is based on the superposition (even in real time) of the hologram of an object at rest and in a deformed state: the result is macroscopic interference bangs (visible to the naked eye) whose count allows to trace the displacement to be measured.
More recent techniques include: the combination of G. Lippmann’s color photography with the holographic technique of Y. N. Denisyuk; the rainbow holograms of S. Benton; the composite technique of R. V. Pole. With the first technique to produce the hologram are used 3 laser beams of wavelengths corresponding to 3 primary colors, in reproduction with white light is observed a three-dimensional color image.
In the case of rainbow holograms, moving the head vertically in the observation of the hologram reproduced in white light, it changes color continuously. In the composite holographic technique, holograms are the synthesis of a very large number of photographs, images of the same object taken from positions slightly different from each other. If Benton’s method is combined with the composite hologram method, the observation can be done in white light.