Holography and Applications of Holography - Civil Engineering (CE) PDF Download

Holography
  is the technique of recording an image in which a light wave is a carrier of information and recording is done in terms of wave parameters like amplitude and phase.

Principle of Hologram Construction
When two highly coherent parallel light beams are incident on a photographic plate, one reflected from the object and the other incident directly, then an interference fringe pattern is recorded on a photographic plate due to interference which holds the 3D pictorial details of the object and the image of the of the object can be reconstructed by diffracting the light beam passing through the photographic plate.

Construction of a Hologram:
The Object Beam which is the Coherent beam of light from a 3-D object is incident on the photographic plate. The reference beam, which is a coherent laser beam, is incident directly on the photographic plate. Due to interference, fringes are recorded on the plate. The photographic plate is developed. If the Reference beam is incident on the photographic in the same direction as it was earlier, the 3-D view of the object appears even though there is no object beam. This is because of the diffraction of light by the interference fringes on the photographic plate.

Phase Recording:
Since the light waves which are reflected from different parts of the object travel different distances, the path difference will be different for such waves and during interference the fringes formed depend on the depth of the point on the object. Hence the phase recording is automatically achieved

Recording and Reconstruction of the image:
Recording of the image of the object.:
Recording is done either by wave front division technique or amplitude division technique.

Wave front division technique:
In this technique an expanded laser beam is obtained from the source. This is incident on the mirror and object simultaneously (as shown in fig.). The mirror and object are so placed that, the mirror and the object reflect a part of the beam (wave front division). The reflected beams from mirror (reference beam) and from the object (object beam) are made to incident on the photographic plate as shown in the figure. Due to which interference fringes are formed on the photographic plate. The photographic plate is developed.

Amplitude division technique:
In this Technique a Beam splitter is used to split the incident expanded laser beam into two parts (Amplitude division). One part is incident on the mirror and reflected onto the photographic plate (reference beam). The other part is incident on the object and reflected on to the photographic plate (object beam). Due to which interference fringes are formed on the photographic plate. The photographic plate is developed. Interference fringes are seen on the developed photographic plate.

Reconstruction of the image: 
The Original laser beam is directed at the hologram in the same direction as the reference beam in order to reconstruct the image. The reference beam undergoes diffraction in the hologram. Secondary wavelets originate due to diffraction. These secondary wavelets interfere constructively in certain direction and generate both a real and a virtual image of the corresponding point of the object on the transmission side of the hologram. It is as shown in the figure. The image can be photographed by keeping a photographic plate in the plane of the image formation of the convergent beam.

Applications of Holography
(1)Holographic Diffraction Gratings;
In a conventional grating rulings are made by the diamond tip fixed to the ruling engine. More uniform rulings can be produced by interference of two laser beams with plane wavefronts on a hologram.
(2)Information coding
Holograms are also used in the cases where the information has to be guarded. In this case an encoding mask is placed on the path of reference beam. This encoding mask generates special wave fronts which gets recorded on the photographic plate. In order to get back the information holograms has to be illuminated with the same kind of wave front only. If we keep the masking details as secret then the hologram is guarded.