HISTORY Holography was first invented and discovered by a physicist Dennis Gabor from Hungary in 1947. He later won the Nobel Prize for his invention. Derived from two greek words: "Halos" meaning whole and "Gamma" meaning message.

Holograms are frequently used for security purposes. A Hologram is a type of photographic image having three-dimensional effect and appears to have depth. According to science, whenever an object is viewed, the human eyes each receive a distinct image. This image is received from slightly offset reference points and it is the human brain which combines the image into a three-dimensional one. The same effect is produced artificially by a hologram. A hologram is produced using the technique of holography, which can also be used to optically store, process and retrieve information. In short and simple terms, hologram is three-dimensional laser photography, containing depth and parallax, which mean the ability to see around the object.

How is a Hologram Created?

The process of creating a hologram is known as holography. In other words, holography is a form of photography-an advanced form-that allows an image to be recorded in three dimensions. Holographic process creates an image made up of two superimposed 2-dimensional images of the same object as seen from different reference points. Lasers are used in holography as it requires the use of light of a single exact wavelength. There are two laser beams and a photographic plate used to take an image of any object. In the process, the laser beams go through beam spreaders and like a flashlight the spreaders spread the laser light out . One beam illuminates the object from the side called the side beam while the other beam travels through the photographic plate. It hits the object head-on which is somewhat similar to the way in which a conventional camera takes a 2-D image. The second beam is known as reference beam and the reflecting light from this beam leaves an image on the photographic plate and at the same time the side beam also creates an image by its light.


The result is thus a photographic plate creating two images simultaneously. The image thus formed is a hologram. The hologram is made up of the interference pattern between the two original images and hence it is a three-dimensional image.

Difference between Photography and Holography

The only thing common between photography and holography is that they make use of photographic film. The difference between holography and photography is the information recorded and the way the image is produced. While a photograph contains only a single view point of an object, which is produced by a camera lens using a simple geometric or ray model for the behavior of light, the holographic image cannot be produced by a single ray model. The image is produced by diffraction and interference through wave phenomena. This difference in the mode of process makes a photograph two dimensional (2-D) image while a hologram three dimensional (3-D) images.

Characteristics of a Hologram

Holograms have certain unique characteristics. These are given below:

• Hologram Aberrations: One of the basic characteristics of holograms is that they suffer from aberrations which are caused by a change in the wavelength from construction to reconstruction. This is also caused by a difference in the reference and reconstruction beams. There are two types of aberrations-chromatic and non-chromatic-which are important even when there is a small difference between the reference and reconstruction geometry. One simple way to eliminate all the aberrations simultaneously is to copy exactly one construction beam in the reconstruction process.
  
  
• Orthoscopic and Pseudoscopic Images: A hologram produces two images, one which is real and the other a virtual image which is an exact replica of the object. However, to the appearance of the observer, the two images differ in appearance. The virtual image has the same appearance of depth and the parallax and produced at the same position as the original object. It appears that the observer is viewing the original object through a window defined by the size of the hologram. This virtual image is known as orthoscopic image. The real image is also formed in front of the hologram at the same time and at the same distance from the hologram. This real image is called pseudoscopic image where the scene depth is inverted.
  
  
• Off-axis Holography: It is possible to separate the twin images. This was demonstrated in a technique developed by two scientist Leith and Upatnieks in 1962. In this technique, during the recording process, a separate coherent reference wave falls on the hologram plate at an offset angle to the beam from the object. This technique is called an off-axis holography generating real and virtual images angularly separated from each other and also from the direct beam.
  
  
• Some Other Characteristics
  • It is possible to reconstruct the hologram of a diffuse object by a small portion of the hologram. In other words, if a hologram breaks into pieces, the entire image can be produced by each piece. However, as the size of the hologram reduces, a loss of image perspective, brightness and resolution result in the constructed image.
    
    
  • Another characteristic of hologram is that a contact print of a hologram will reconstruct a positive image which is not distinguishable from the image produced by the original.
    
    
  • A cylindrical hologram makes a 360 degree view of the object.
    
    
  • Without any cross-talk, more than one independent scenes can be stored in the same photographic plate and these can be viewed one at a time.

Applications of Holograms

Holography has today emerged as an important tool in science and technology. It is a well used method to produce pictures and represents one of the most prominent examples of recombining of scattered radiation to produce pictures. This process of producing holograms is now spreading from the research laboratory to various industries, and holograms find wider employment in communication and other engineering problems. A hologram is not only a three-dimensional image but also can store numerous quantities of information. In the computer technology, holograms can be used to store memories which are much larger and faster. Hologram has today become a very well known concept in credit cards, tickets or original covers on software computer programs or any objects to prevent falsification. An important area of application of hologram is bar-code readers in shops, warehouses, libraries etc. A code reader is based on the use of holographic components like optical gratings. Some other examples of the use of holographic technology is in the aircraft industry's head-up displays (HUD) or for making holographic optical elements (HOE) and so on. All these have contributed to make holography an industrial success.