Hologram

A hologram is a physical recording of the light field that, when illuminated, diffracts light to reproduce a three-dimensional image of the original scene. Holography records both the amplitude and the phase of light by creating an interference pattern between light that has been reflected or scattered from the object (the object beam) and a separate reference beam. This interference pattern is stored on a photosensitive medium such as a silver-halide photographic plate or a photopolymer. When the hologram is later illuminated with light that matches the reference beam, the recorded wavefront is reconstructed, and a virtual or real three-dimensional image appears to occupy space in front of or behind the plate.

Holography was proposed by Dennis Gabor in 1947 and demonstrated experimentally after the invention of the laser in the 1960s by Emmett Leith and Juris Upatnieks, among others. It enabled the first true three-dimensional images by recording light fields rather than flat projections.

Holograms are broadly categorized as transmission or reflection types, depending on how the light is reconstructed. Transmission holograms are viewed with light passing through the hologram, while reflection holograms reflect most light toward the viewer. Computer-generated holograms and digital holography use computational methods and electronic sensors to create or reconstruct holographic data.

Applications span data storage, archival imaging, and security features on banknotes and identification documents; holographic displays and art; and scientific and medical imaging, where 3D information can be captured or presented. Limitations include the need for coherent light sources, precise optical alignment, and sensitivity to vibrations and environmental conditions.