Holograms

Holograms



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**What is a hologram?**
A hologram is a type of three-dimensional image that gives the appearance as a real object as the image has depth. The best way to think of a hologram is to visualize them as impressions on light waves. The patterns of light wave impressions are photographed or captured in the layer of emulsion. In other words, a hologram is an image or photograph of light wave interference. It is to be noted that not all 3D images are holograms and neither a hologram is required to be 3D. In technical terms, a hologram is the captured intensity and phase information which is produced by the interference or interconnection of two or more beams of light. People often consider a hologram as some type of a three-dimensional photograph. However, the only thing common between photography and holography is the use of a photographic film. The important difference is the way the image is produced in both the cases. A photographic image is produced by a camera lens and it is described almost accurately using a simple geometric or ray model for the behavior of light. On the other hand, the holographic image is formed depending on diffraction and interference, which are wave phenomena.

**How does it work?**
A laser beam is divided into two beams: The reference beam and the object beam. Using a lens or curved mirror, the width of the reference beam is increased conically and aimed directly at the film plate. The width of the object beam is also increased and aimed at the object which reflects some of the light on the holographic film-plate. The two beams form an interference pattern on the film by crossing each other. This leads to the forming of the hologram. To create a hologram, laser light is needed because it is made of coherent waves.

Video 1: How it's made
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History
The first hologram was conceived of, and produced in 1948 by Dr. Dennis Gabor, a researcher at the Imperial College of London. For his theories and work, he received the Nobel Prize in physics in 1971. Gabors early holograms were created without the use of a laser, since the laser wasn't invented until 1960. Therefore his holograms were only capable of showing the slightest amount of depth (about the thickness of a postage stamp). His light-producing instrument was a very highly-filtered, mercury-vapor lamp.

Different Types of Holograms:
There are a few different kinds of holograms, such as the rainbow holograms on credit cards and bank notes that change color depending on your viewing angle, and reflection holograms often seen in art galleries that display 3D images of an object when illuminated with a ray of white light. Transmission holograms are created through a different process and can only be seen when a laser shines on the holographic plate. Although there are different types of holograms, they all work by recording the interference pattern of light waves caused by an object and recreating that scene when illuminated. For example, to create a transmission hologram you need two beams of laser light. One beam bounces off of a mirror directly onto the holographic plate (the reference beam) and one illuminates the object. The light scattering off of the object toward the plate interferes with the reference beam, creating a unique interference pattern that is recorded on the holographic plate.

Properties:
Holograms have some very interesting properties such as follows:
 * Distributed information: The information in a hologram is always distributed. If it is cut into pieces, each piece contains enough information to reconstruct the entire scene.
 * Image projection: Under certain cases, merely shining or lighting a laser through the hologram projects an image on to a screen.
 * Viewing directly with a point source: A hologram reconstruction shows exactly the same light wavefronts to our eyes that we receive with the real object, using a point source of illumination. The hologram is not distinguished from the physical obje ct.

This is a rainbow hologram that is found in a credit card.

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