In an Ordinary photograph of an object taken by a camera three-dimensional scenes is recorded as a two-dimensional scene. Since the emulsion on the photographic plate is sensitive to intensity variations only.
Therefore, the photograph records only the intensity distribution in a certain plane. In ordinary photographs, the phase distribution that prevailed at the plane of the photograph is lost. Therefore, the three-dimensional character of the object is lost and only two-dimensional recording of the three-dimensional scene is obtained on the photograph.
What is Holography? How does Holography Work?
1948, Dennis Gabor developed a technique of recording the amplitude as well as the phase of the light wave to get a three-dimensional image of an object. The technique of producing a three-dimensional image of an object is called holography.
The photograph showing the three-dimensional image of an object is called holograms (holos is the Greek word means ‘whole’). This technique becomes familiar after the invention of a highly coherent light beam of the laser.
Danish Gabor was awarded the Nobel Prize in 1971 for his invention and development of the Holographic method. After study how does holography work we have seen that there are many applications of holography.
How does Holography Work
The principle of holography is explained in two steps:
- Recording of the hologram.
- Reconstructing the image.
Recording of the Holograph
The laser beam is divided into two parts using a plane mirror. One part (2) of the laser beam illuminates the object whose Hologram is to be made. The other part (1) of the laser beam called reference Beam or wave is obtained on the photographic plate after reflection from the plane mirror.
Thus, the photographic plate is exposed simultaneously to both the reference Beam and The Light beam reflected from the object. These two waves or beams of light are coherent (i.e in phase).
The superposition of the two coherent beams of light gives rise to an interference pattern that is obtained on the holographic plate. When the photographic plate or film is developed, we get the Hologram. The Hologram contains information about the amplitude as well as the phase of the object wave.
Reconstructing the image ( viewing a hologram)
The technique used to reproduce the three-dimensional image of an object is called reconstruction. To see the image of an object, Hologram is again illuminated by a single beam of laser called the reconstruction wave. This reconstruction wave is identical to the reference wave used for recording the Hologram.
Both the real and virtual images of the object are formed as shown in the figure. When the observer moves his eye from one side to the other side, the near part of the object appears to move with respect to the far part of the object. Therefore, the observer sees the depth of the image and hence the three-dimensional effect of the image.
TYPES OF HOLOGRAMS
Holograms are divided into two types: Reflection Holograms and Transmission Holograms.
A hologram in which a three-dimensional image of an object is seen near its surface is the reflection hologram. It is the most common Hologram. This type of program is illuminated by a spot of the white light of the incandescent source located on the other side of the Hologram and held at a certain distance and angle. Therefore, the image consists of light reflected from the hologram.
A hologram in which a three-dimensional image of an object is viewed with laser light of the same source as was used while recording the Hologram is the transmission Hologram. The laser light is directed from behind the Hologram and the image is transmitted to the side of the viewer.
Applications of Holography
Holography has a number of applications now a day. The main application of holography are discussed below :
Microscopic particles or specimens floating in a Fluid are in continuous motion. When an ordinary microscope is used to study the size or other property of a specimen or a particle in motion, then it becomes difficult to locate it because it goes out of the focus in the microscope.
However, a particle or a specimen in motion can be freezed in a hologram if a holographic record of the entire volume containing these particles or specimens is made. Therefore, the reconstructed three-dimensional image can be studied easily at any time to determine the properties of a particle or specimen.
In a technique of double exposure photographic interferometry, first of all, an object is exposed to a photographic plate with a reference wave. Then, the object is stressed due to the application of the four force and this stressed object is again exposed to the photographic plate with the same reference wave.
The resulting Hologram is developed and illuminated by a reconstruction wave. The two objects wave to emerge from the Hologram. One wave corresponds to the unstressed object and the other wave corresponds to the stressed object.
These two waves Superimpose and interference fringes are produced. These interference fringes are the characteristics of the strain suffered by the object. Thus, the distribution of strain in the object under stress can be studied.
The ability to store data or information in a medium is of great importance. Holographic data storage is a technique to store data or information at high density inside a crystal or photopolymer.
The main advantage of holographic data storage is that the volume of the recording medium is used instead of the surface as used in another conventional recording medium. Hence, large data can be stored in a hologram.
It is becoming popular in the formation of 3D images in cinematography and television.
Holography is used to make security holograms. These holograms cannot forget easily as they are replicated from master Hologram that needs expensive and technologically advanced equipment. They are used in currencies of many countries, in credit and Bank cards in passports, in ID cards, and in books.
Holography vs Photography: A comparison
|A lens is required in photography to record the uniqueness.||In holography, light is scattered from the object directly onto the recording medium.|
|Record information regarding the light that comes from the One Direction.||Hologram record information regarding the light that comes from the original scene as scattered in a range of direction.|
|Photographs can be viewed in a wide range of lighting conditions.||Holographic recording requires a second light beam to the directed onto the recording medium.|
|When a photograph is cut in half, each piece shows half of the scene,||If Hologram is cut in half the whole scene can be seen in each piece.|