The holography technology can best be understood under the following two categories:

• Recording Process of Holography
• Reconstruction Process of Holography

Recording Process of Holography

The process of holography uses a reference beam to produce a recording of the phase of the light at every point in an image. The reference beam is combined with the light from the scene or the object which is referred as the object beam. Due to the superposition of the light waves, optical interference between the object beam and the reference beam produces a series of intensity fringes. These fringes can be recorded on standard photographic film and they form a type of diffraction grating on the film. This is called the hologram or the interference pattern.


It is also important to note that during holographic recording, rather than from just one point, each point on the hologram's surface is affected by light waves and these waves are reflected from all points in the scene. It means that during recording, each point on the hologram's surface acts like an eye which records everything it sees in any direction. After the hologram has been recorded, looking at a point in that hologram is similar to looking "through" one of those eyes. For example, you look at a small or cut out section of a recorded hologram. It is evident that from the same distance you see less than what you see in the full image. But still you can see the entire object by shifting your viewpoint laterally or by going very close to the hologram.

Reconstruction Process of Hologram

Once the film is processed, if there is illumination again with the reference beam, the original object beam in both intensity and phase is reconstructed with the diffraction from the fringe pattern on the film. The image appears three-dimensional because both the phase and intensity are reproduced. The image would rotate exactly as the original object would when the viewer move his or her viewpoint. Since there is an interference between the reference and object beams. There is the use of laser in holography. The light from the laser is divided into two beams-the reference beam and the object beam.

Difference between recording a hologram and taking a photograph

Recording a hologram is different from taking a photograph in various ways which are given below:

• In hologram, it is necessary that a coherent light source like a laser is used to illuminate the object.
  
  
• There is a second beam of coherent light which strike the film on which the hologram is to be recorded. This is called the reference beam. The reference beam and the object beam overlap at the surface of the film and they form an interference pattern.
  
  
• The recording material is capable of recording these fine patterns and a high-resolution photographic film is used.
  
  
• Because the lines that make up the hologram is usually less than a micron, holographic recording is more sensitive to movement and vibrations compared to photographic recording. This is actually a very serious restriction as it usually confines holography to the laboratory where vibration isolation is created.