Transmission
Hologram
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Properties of Display Holograms |
Some important properties of transmission hologram are as follows:
- When viewed with white light, transmission hologram looks like a
blurry rainbow image.
- These holograms are viewed as a sharp image using shining laser
light through the hologram.
- Material requirements for recording are more relaxed.
- Less resolving power is needed in materials.
- Transmission hologram can be formed in simple set-up.
- In such holograms, greater depth of the scene is possible.
- The scene in transmission hologram can be projected by shining a collimated laser beam through the hologram.
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Difference between Reflection Hologram
and Transmission Hologram |
The basic difference between the two is that while a reflection hologram is viewed when it is illuminated from the front, a transmission hologram is viewed by illuminating it from the back side. This is illustrated in the following diagram:
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Types of Transmission Hologram |
There are two main types of transmission hologram. They are as follows:
- Laser transmission holograms: Laser transmission hologram
is the earliest type of hologram developed by Leith and Upatniks way
back in 1962.These holograms are made with lasers and are lit with
lasers. Therefore, the images that are formed appear in the same
color of the laser which is used in illuminating them for viewing.
This color is usually red as in helium neon laser. In some cases,
other holograms use a laser transmission hologram as the master. It
is from this master, different copies are made.
- White light transmission holograms: These type of holograms are illuminated with white light, also known as incandescent light. The images thus produced contain the rainbow spectrum of colors. The colors of the hologram change as the viewer moves up and down and sometimes can also be referred as rainbow holograms. Holographers have developed considerable control over the colors displayed in white light transmission hologram to produce images in a near full, natural color or any specific color.
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Recording Process of Transmission
Hologram |
To make a hologram, two coherent light waves (laser light) are required. The first light wave is called object beam which is reflected from the object and carries information about the object. The second light wave is called reference beam and is a plane wave without information. An interference pattern is generated from the object beam (OB) and the reference beam (RB), which is recorded in the form of a hologram on film emulsion. During the exposure of the film, absolutely stable conditions are required. The type of hologram thus recorded is called transmission hologram because the light passes through the holographic plate. It is also to be noted that in transmission holograms, during the exposure, the object beam and the reference beam come in from the same side of the holographic film plate.
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Reconstruction Process of Transmission
Hologram |
In order to reconstruct the holographic image, the hologram is placed in its original position in the reference beam as during its recording. We see a replica of the object if we look along the reconstructed object beam. We see object from different perspectives as we shift viewpoints. Thus the object gives a three-dimensional (3D) effect. In reconstructing transmission hologram, the light does not actually pass through the image, but it creates a wavefront that makes it appear as though the light had been generated in the position of the object. This image thus formed is called virtual image. On the other hand, an image having light actually passing through it is called a real image.
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Applications of Transmission Hologram |
- Brouchers and catalogs
- Commercial display
- On packaging
- For security ID and labeling
- For identification purpose
- For printing jobs
- Food and drug packaging
