What is it?
Holography is a photographic technique for making three – dimensional pictures. It is a process by which these 3 -D images can be stored and reproduced using laser light. The medium that stores the image--called a hologram--is nothing more than an exposed, developed, fine-grained piece of photographic film that has been ´taken´ and viewed in special light. A photograph records an image as seen from one particular viewpoint. Because 3 – D object has depth as well as length and breadth, a hologram is a record of an image seen from many viewpoints. In fact, "holography" comes from the Greek word "holos" meaning "whole" and "graphos" meaning "message." A hologram does indeed record the "whole message" of an object.
Some of the descriptions of holograms are

·"image formation by wave front reconstruction.."
·"lens less photography"
·"freezing an image on its way to your eye, and then reconstructing it with a laser"
Consistent characteristics:
1. The images are true three-dimensional images, showing depth and parallax and continually changing in aspect with the viewing angle.
2. Any part of the hologram contains the whole image!
3. The images are scalable. They can be made with one wavelength and viewed with another, with the possibility of magnification.

How is it made?
To understand how a hologram is made and viewed, I am going to talk about two important properties of light: interference and diffraction. To describe these properties, think of light as a wave. Diffraction describes the ability of a wave to spread out when it meets an obstacle. When light waves meet the hologram's interference pattern, they spread out such that a perfect replica of the original object's wave front is created.Dennis Gabor’s theory was concerning interference patterns. Gabor theorized in 1947 that each crest of the wave pattern contains the whole information of its original source and that this information could be stored on film and reproduced. Basically, two light waves meet and interfere on a piece of film to produce a complicated pattern of tiny exposed and unexposed areas. This pattern is characteristic of the object being holographed. When two waves come into contact, they interfere with each other, the resulting interference pattern is called a standing wave. This is what is recorded onto a hologram. A pebble, dropped in a still pond, is the most basic example used to describe the wave interference process. If you drop a pebble into a pond, it creates an infinitely expanding circular wave pattern. If you drop two pebbles into a pond the waves' crests would eventually meet. The intersecting points of the waves' crests are called the points of interference. The interference of two or more waves will carry the whole information about all the waves.

To create such interference pattern, a light source that is monochromatic (has one wavelength) and coherent (all the crests and troughs are in line) must be used. A laser provides such a light source. Not until the laser was invented was it possible to create a hologram. The L.A.S.E.R. (Light Amplified by Stimulated Emission of Radiation) was invented to produce coherent light. Coherent light travels in the same frequency and in the same phase. (100% coherent light is rare) (Btw. just that you know, incoherent light is white light that travels in different frequencies and in different phases) Holograms need coherent light to record or playback the image clearly. It is important to use light that is coherent because the information is carried on the crest of each wave. For example, two waves are coherent if the crests of one wave are aligned with the crests of the other and the troughs of one wave are aligned with the troughs of the other. Otherwise, these light waves are considered incoherent. The more points of intersection there are, the more information. There are two basic kinds of holograms: transmission and reflection. Transmission holograms have light shining through them (the light source is behind the hologram) and reflection holograms have light shining on them (the light source is in front of the hologram).

A typical layout for producing a hologram – all components must be placed in a vibration - free environment. This is because any movement -- even one as small as 0.1 micron -- will completely obliterate the image. As I already said before, a hologram is made using special light produced by laser. It shows up best in the same kind of light. A hologram is created by first splitting a laser beam into two with a beam – splitter. One is called the object beam and the other is called the reference beam. The reference beam is directed towards the film with a mirror and expanded with a lens so that light uniformly covers the film. The other split beam, the object beam, is similarly directed with a mirror and expanded with a lens such that light uniformly covers the object to be holographed. The light reflected off the object produces a complicated wave front that contains information about the location, size, and shape of the object. At the film, the reference beam interferes with the object beam, creating an interference pattern, a standing wave, which is recorded onto the film. After the film is developed, the three-dimensional image can be reconstructed with light directed towards the holograms at the same angle as the reference beam. When a hologram is viewed with a coherent light source, the original light waves are reconstructed, creating a 3D image in space. This 3D image can be viewed from different perspectives, as if the object is actually there. Lenses are not essential in making a hologram. Since 1989, people work hard to create a "Holovideo" set that could replace your TV some day, just like Star Wars! A new visual medium -- electro-holography – was created; capable of producing realistic 3-D holographic images in real time.
One of the most interesting qualities of the hologram is that the whole contains the knowledge of each part, and that each part contains the knowledge of the whole. If you break a hologram into many pieces each piece will still contain the whole image, but with a limited perspective. The image stays the same size but you loose clarity and you loose perspectives. A hologram is like a window. If you make the window smaller, the objects don't get smaller but you loose some of your view.

Reflection Holograms
With a reflection hologram, the image is stored in a thick emulsion and can be viewed in white light. The simplest such hologram to make is the direct beam reflection hologram. In this case the direct beam through the film serves as the reference beam. Rainbow Holograms Rainbow holograms are holograms that can be viewed in white light. They are made by a double holographic process where an ordinary hologram such as a transmission hologram is used as the object and a second hologram is made through a slit.
Denisyuk Holograms
Denisyuk contributed to the development of the hologram by using Lippmann's photographic process to make reflection holograms that could be viewed in color if more than one coherent source was available. In his method the beam was passed through the photographic emulsion and back reflected from the object. The emulsion could then be mounted on a reflective backing for viewing by reflected light.

Hologram History
Historical notes:
Gabor was the discoverer of the hologram and received the Nobel Prize in Physics for it in 1971. This was pre-laser holography and he made holograms only of transparencies where his reference beam was the unobstructed light that went through the clear parts of the transparency. Leith and Upatnieks developed the off-axis reference beam method that is most often used today. It permitted the making of holograms of solid objects by reflected light. Benton is credited with the development of the rainbow hologram. Since it can be mass-copied and viewed with incoherent white light, it has become the most common type of hologram. Rainbow holograms have appeared on National Geographic's cover and on millions of credit cards as a deterrent to counterfeiting. Composite holograms are made as multiple strips so that you get a different image from different angles. They can be viewed in white light.