Holography is the “lens less” photography in which a 3-D image is created using laser beam. The recorded image is called Hologram (in Greek, Holos means ‘Whole’ and Gram the ‘message’). Holography was developed in 1947 based on the theory of Dennis Gabor. Hologram stickers are now widely used in commercial products and documents to prove authenticity.
In general photography, the image is created using the light reflected from the object but in holography, the image creation is entirely different. The 3-D image is produced by splitting the laser light into two beams. Of these, one beam reaches the film without hitting the object while the other beam hits the object before reaching the film. At the point of meeting the two beams, an interference pattern is produced which is recorded in the film. Each point on the hologram has information about the whole object.
The hologram records the intensity distribution of the reflected laser light and its phase distribution. The film distinguishes the laser beam reaching the light sensitive areas of the film and the beam reaching the surface due to difference in the wave amplitude. The Coherent laser light travel in phase with one another and the object is illuminated as a whole.
A portion of the laser beam is reflected by a mirror before reaching the film. This reflected beam is called “Reference beam”. It remains ‘Plane parallel’ and produce the interference with the laser reflecting from the object. The interference pattern will appear as ‘Concentric circles’ in the film. The space between the circles decreases as the radius increases.
When the hologram is observed in coherent light, the image becomes visible, but when viewed from a different angle, the image still appears from a different angle. The hologram reconstructs in space the wave fronts produced by the object so that a 3-D image is produced.
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