In 1947, Denis Gabor invented a different type of photography, know as holography. He named this new technique wave front construction. According to this technique both the phase and intensity attributes lf the wave are recorded and when viewed the photograph shows a three dimensional image of an object. For this work he was awarded Nobel prize in 1971. In holography, the light waves reflected from an object are recorded. This photographic record is called a hologram. The hologram has no resemblance with the object, although it contains all information about the object in a kind of optical code. When it is illuminated by a coherent light source, a three0dimensional image of the original object is formed. The formation of image from hologram is known as the reconstruction process.
Principle diagram Illustrates the generation of a hologram. As the light waves reflected form an object are characterized by their amplitude and phase. Gabor recorded the wave-pattern on a photographic plate by mixing two beams namely a reference beam and an object beam. The object is illuminated by highly monochromatic and coherent light. Light is reflected from the object to the entire photographic plate. A part of the incident laser beam is made to fall in a plane mirror which reflected it toward the photographic plate with the reference beam and produces an interference pattern on the plate. The photographic plate carrying the interference pattern is called a hologram. This id also known as Gabor zone plate in honour of Denis Gabor who developed the phenomenon of holography. In the reconstruction process, the hologram is illuminated by a collimated laser beam which undergoes diffraction phenomenon. One of the diffracted beam emerging from the hologram, when projected back, appears to diverge from an apparent object located in the position lf the original object. This beam forms a virtual image which is observed by looking through the hologram. The other beam produces a real image which can be recorded on a photographic plate. Thus, holography is a two stage process. Where in the first stage a hologram is recorded in the form of interference pattern and in the second stage the hologram acts as a diffraction grating for the reconstruction beam and the image of the object is reconstructed from the hologram.