Introduction:

One of the innovative applications of lasers is in the progress of a novel method of photography, termed as holography. This term is basically the combination of two Greek words - holos (complete) and graphos (writing). That is, holography is the method of obtaining the complete picture (that is, as true as the object itself) of an object or a scene. In another words, it is a three-dimensional recording of an object or a scene. Well, you might be speculating as to what fundamentally distinguishes this method from the normal photography!

In normal photography, a (2-D) two-dimensional image of a (3-D) three-dimensional object is recorded on a photosensitive surface. The photosensitive surface records the intensity distribution of light falling on it after reflection from the object. As an outcome, we get a permanent record of the intensity distribution which existed at the plane occupied through the photographic plate if it was exposed. As the photosensitive surface is sensitive only to the intensity variation, the phase distribution existing in the plane of the photographic plate is totally lost and is responsible for the absence of the (3-D) three-dimensional character in it. Holography is that method of photography where not just the amplitude (and therefore the intensity) however as well the phase distribution can be recorded. As an outcome, pictures obtained through holographic method possess the three-dimensional form and are visually affluent.         

Holography was proposed by Dennis Gabor in the year 1948. He represented that one could certainly record both the amplitude and the phase of a wave by employing the Interferometry principles.

Holography: The basic principle

Holography is the procedure of recording the interference pattern generated by light waves reflected through an object and reference waves. This interference pattern of the object is exclusive and is termed as hologram (that is, total recording). Whenever you look at a hologram, you will realize that it doesn't even remotely look like the object. Though, when this recorded pattern is illuminated through a correctly chosen reconstruction wave, out of the many component waves emerging from the hologram, one wave fully resembles the object wave in both amplitude and phase. Therefore, whenever you look at this wave, you recognize the object still being in position even although the object might not be present there. As throughout the reconstruction (that is, image production), the object wave itself is emerging from the hologram, the image consists of all the effects of three-dimensionality. You can certainly shift your viewing position and look behind the objects.

Let us understand the fundamental theory comprised in holography by the help of a simple illustration.

Incident light, represented in the first part of the figure above, is diffracted via a point object. It gives rise to a sequence of bright and dark concentric rings. The pattern is recorded photographically and made into a transparency. This pattern, termed as a Gabor zone plate, is identical to a Fresnel zone plate. In the second part (top right) light is incident on the ring pattern (that is, the Gabor zone plate) and focused by it into a point, as focused through a zone-plate.

Now, observe the second portion of the image, in which the object comprises of two points (pixels). The diffraction pattern then comprises of two sets of concentric rings. If the pattern is illuminated, each of the two sets focus, and the image comprises of two points. As the object is an aggregate of numerous pixels, its diffraction pattern is represented in the third part of the image. The intermediate recording is a continuum of superposed zone plates - an unrecognizable multiplicity of lines and rings. Each and every pixel in the object made its own set of fringes. Within each and every set, the light interferes however between sets; there is no fixed phase relationship and therefore no interference. In order to make the different signals compatible in phase, the other wave termed reference is added.

Holography: The process

As illustrated above, the method of image formation through holography is a two-step process. In the initial step, the waves reflected from the object are recorded in such a manner that the complete information regarding the amplitude and phase variations is preserved. This recording of wavefront is termed as the hologram. The second step comprises the reconstruction of an image of the object through illuminating the hologram by light wave termed as reconstruction wave (that is, similar to the reference wave).

Production of a Hologram:

Holograms can be generated in several manners based on the relative orientation of the reflected (or scattered) and the reference waves. For illustration, Gabor's zone-plate that is nothing however a hologram, is the record of interference among the two waves travelling more-or-less in the similar direction. This is simply done with objects that encompass adequate open spaces between them, like a wire mesh or opaque letters on a clear background. Signal and reference, in another words, travel in the similar direction. Such a hologram is termed as Gabor hologram or in-line hologram. This was only after the invention of laser that this new method of photography became in fact practical. By using the help of lasers, N. Leith and Juris Upatnicks generated what is termed as off-axis hologram. In the off-axis hologram, the reference beam and the object beam arrive at the recording plate from substantially dissimilar directions. This made possible holography of solid (3-D) three-dimensional objects.

Now, the question occurs: How are holograms recorded? To comprehend this, refer to the figure shown below. A beam of coherent laser light (in which all the points on the wavefront are in phase) is split or divided into two beams. One beam illuminates the object to be recorded and the light reflected from this object falls on the photographic plate.

The other beam, termed as the reference beam, is reflected from the mirror to the similar photographic plate. Due to superposition of wave fronts of these two beams, an interference pattern is recorded on the photographic plate. The record on the photographic plate (that is, hologram) is simply a pattern of interfering wave fronts and exhibits no resemblance to the recorded object. The hologram, though, comprises all the information concerning the object.

Ordinarily, such interference fringes are much closely spaced and can't be seen by unaided eye. Therefore the hologram appears to be uniformly grey. Whenever seen by microscope, though, a hologram is found to include myriad of very small cells, each and every cell having a series of fringes of different lengths and spacing. Further, a laser is required for holography, merely because its coherence length surpasses the path difference due to inequality of the object.

Reconstruction of Image:

As illustrated above, the hologram of an object is the recording of the interference pattern, on a photographic plate, generated by the object and the reference waves. The hologram, if viewed by the unaided eye, doesn't even remotely resemble the object photographed. The method of obtaining the image of the object is termed as reconstruction. In the reconstruction method, as shown in figure above, the hologram is illuminated through the light beam (that is similar to the reference beam) alone and the reconstructed wave fronts appear to diverge from the image of the object.

Applications of Holography:

Holography is an extremely helpful tool in many areas, like in commerce, scientific research, medicine and industry. Some of the current applications which make use of holographic technology are as follows:

1) Holographic Interferometry is employed by researchers and industry designers to test and design numerous things, from tires and engines to the prosthetic limbs and artificial bones and joints.

2) Supermarket and department store scanners make use of a holographic lens system which directs laser light to the bar codes of the merchandise.

3) HOE's (or Holographic optical elements) are employed for navigation by airplane pilots.  A holographic image of the cockpit device appears to float in front of the windshield.  This lets the pilot to keep his eyes on the runway or the sky while reading the device.  This characteristic is available on a few models of automobiles.

4) Medical doctors can employ 3-D (three-dimensional) holographic CAT scans to make measurements devoid of invasive surgery. This method is as well employed in the medical education.

5) Holograms are employed in advertisements and consumer packaging of products to attract the potential buyers.

6) Holograms have been utilized on covers of magazine publications. One of the most memorable Sports exemplify covers was the December 23, 1992 issue featuring the Michael Jordan. Holograms have as well been utilized on sports trading cards.

7) The utilization of holograms on debit cards and credit cards give added security to minimize the counterfeiting.

8) Holography has been employed to form archival recordings of valuable and/or fragile museum artifacts.

9) Holography has been utilized by artists to make pulsed holographic portraits and also other works of art.

The Future applications of holography comprise:

1) Future color liquid crystal displays (or LCD's) will be brighter and whiter as an outcome of holographic technology.  Scientists at Polaroid Corporation have developed a holographic reflector which will reflect the ambient light to generate a white background. 

2) Holographic memory is a latest optical storage process that can store 1 terabyte (= 1000 GB) of data in a crystal approximately the size of a sugar cube. In contrast, current methods of storage comprise CD's which hold 650 to 700 MB, DVD's that store 4.7 GB, and computer hard drives which hold up to 120 GB. 

3) Optical computers will be able of delivering the trillions of bits of information faster than the most modern computers.

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