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A hologram is an image created by a photographic projection of a recording of a light field rather than an image formed by some sort of lens. It appears as a three-dimensional representation on a two-dimensional object, which can be seen without intermediate optics such as goggles or glasses. However these hologram images become unintelligible when viewed under diffused ambient light since they are not actual images. The photographic technique used to create these images is called holography.
Hologram refers to both the physical medium that diffracts the light to create the image and the resulting image itself. The first practical optical hologram that recorded a 3-D object was invented in 1962 by Yuri Denisyuk of the then Soviet Union and by Dennis Leith and Juris Upatnieks at the University of Michigan. Since its development in 1962, various hologram types have been developed.
One type is called a transmission hologram. These holograms are produced by splitting the laser light into an illumination beam and a reference beam. The illumination beam is projected directly on the object while the reference beam is projected directly onto the photographic medium, forming an interference pattern on the film; the result is a captured light field that was taken in a method similar to traditional photography processes.
Another type of hologram is the rainbow hologram, which is commonly used for authentication and security purposes. These are designed to be viewable under the illumination of white light rather than laser light like other types of holograms. The image is created using a vertical slit which removes vertical parallax in the resulting image, reduces spectral blur and preserves the three-dimensionality for most observers. These can usually be found on credit cards, product packaging and driver’s licenses.
Another common type is the Denisyuk hologram or reflection hologram. This type is seen in holographic displays and is capable of multicolor image reproduction.