Introduction
The ordinary photograph gives us only a two dimensional image of the object. In ordinary photography we make use of lenses to focus the image on the photographic plate. Laser Applications In Holography The focussing takes place only in a single plane and all other planes are out of focus. Thus, there is a two dimensional recording of a three dimensional object. The photographic plate records only the intensity variations while the phase distribution prevailing at the plane of photographic plate is completely lost.
Hence, the three dimensional character of the object is lost in recording. After the development of photographic plate, only two dimensional picture is obtained. If we examine the photograph from various directions, we do not see what is happening on the other side of the object.
A fundamentally new method of recording optical images known as holography is now available. The word "holography" originates from the Greek-word "hotes" meaning the whole and "graphy" meaning the writing. Le holography means "complete recording ".
In holography, the image of the object to be photographed is not recorded but the light waves reflected from the object are recorded. The photographic record is called as hologram. It should be noted that the holograph has no resemblance with the object, of course, it contains all information about the object in some kind of an optical code. When the holograph is illuminated by a coherent source of light, a three dimensional image of the original object is formed. is known as reconstruction process. So, holography is a two-step processes:Laser Applications In Holography
(i)Transformation of the object into hologram, i.e.. an object illuminated by coherent light is made to produce interference fringes in a photographie emulsion, and
(ii) retransformation or reconstruction of hologram into the image of the object, i.e. reillumination of the developed interference pattern by light of same wavelength to produce a three dimensional image of the original object.
IMPORTANCE OF HOLOGRAPHY
The hologram shows more information than can be seen by looking at the real scene. This occurs since the hologram "freezes" reality, i. e. it is time independent. Once the scene is recorded on the hologram, it can be observed throughly for a long time, which is not practical in moving scenes in reality. This can be called "Super Realistic Image". A good example is in holographic microscopy of bacteria or micro-organisms, where holograms enable researchers to study transient phenomena in details.
Seeing is a process that starts with light coming from any object into the eye.
Visual sensation of a 3 dimensional image is made when the light intensities from all the points of the object enter both eyes. The mechanism of seeing with two eyes gives us the direction from which each intensity enters the eye. The visual process is very complex and it involves the brain and processing of information at many levels. The retinas in the human eye are 2 - D . The parallax phenomenon is translated in our brain as depth perception.
The same visual sensation can be created by keeping all the information about the interference pattern. This enables creating a 3 - dimensional image of the original scene . Holography is about sceing. So an important aspect is to know that how do humans " see things .
To understand what holography really is , let 's first discuss the standard photography processes and then compare them to holography .
Regular Photography
Regular photography " freezes " a 2 - D image of the 3 - D world . Thus , enabling a 2 - D view of reality . Standard photographic film registers the total intensity which fall on each point of the film during exposure ( shutter open ) . The image is a 2 - dimensional mapping of irradiance , and does not contain more information (such as depth of field). Because of the imaging process, there is a point to point correspondence between the photographed object and the film. In standard photography, a lens (or pinhole) is used to create a real image on the film. After developing, a negative picture of the image is available on the film. From this negative, a positive print of the image can be made. Silver grains are created on the film according to the intensit y of light which fall on each area of the film.
Stereoscopic Photography
This is a photography technique which creates depth illusion by taking two images of the same scene with two lenses. These lenses are positioned at a distance similar to the distance between the eyes. By viewing the image through a special device composed of left and right lenses, each eye sees the picture that was filmed by the corresponding lens. The depth illusion is for a specific position of the head. It is not possible to see the picture from other angles. Also, it is not possible to see a body that is hidden by another body. Only the combination of looking simultaneously at the two photographs creates the optical illusion of a 3-D scene in our brain.
Holographic Recording
Holographic Recording writes on the hologram all the information from the scene. This information contains:
The intensity distribution which arrives from each point on the scene to the film, as in regular photography.
The direction from which each intensity arrives at the film. The direction is recorded by the phase of each wave when it arrives at the film. In order to record all the information, a reference is needed at every point on the film.
Reconstruction of the image is done at a separate stage, using special lighting conditions. The image is created at a different plane from the surface of the hologram.
Holographic image can only be viewed within specific viewing parameters:
(ii)Proper illumination.
In holography, each point of the scene is mapped onto all the points of the film. unlike the point to point mapping in standard photography. The light from each point of the scene strikes every point of the hologram. Thus, each point of the hologram contains all the information of the scene, as seen from this specific point. The depth information is recorded on the hologram by the phase difference of the wavefronts as they strike the holographic film.
