Hologram artwork in MIT Museum in Cambridge, Massachusetts An early display of 3-D effect of holography which is a series of geometric objects (cones, cubes, hoops, circles, etc.) set about a board; the word `HOLOGRAM,' made out of wooden stand-up letters, stretches diagonally into background.

Several types of holograms can be made. Transmission holograms, such as those produced by Leith and Upatnieks, are viewed by shining laser light through them and looking at the reconstructed image from the side of the hologram opposite the source. A later refinement, the “rainbow transmission” hologram allows more convenient illumination by white light rather than by lasers or other monochromatic sources. Rainbow holograms are commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission hologram are commonly formed as surface relief patterns in a plastic film, and they incorporate a reflective aluminium coating which provides the light from “behind” to reconstruct their imagery. Another kind of common hologram, the reflection or Denisyuk hologram, is capable of multicolor image reproduction using a white light illumination source on the same side of the hologram as the viewer.

One of the most promising recent advances in the short history of holography has been the mass production of low–cost solid–state lasers typically used by the millions in DVD recorders and other applications, but which are sometimes also useful for holography. These cheap, compact, solid–state lasers can under some circumstances compete well with the large, expensive gas lasers previously required to make holograms, and are already helping to make holography much more accessible to low–budget researchers, artists, and dedicated hobbyists.

Plane wave front Hologram

Point Source Hologram

Interference occurs when one or more wavefronts are superimposed. Diffraction occurs whenever a wavefront encounters an object. The process of producing a holographic reconstruction is explained below purely in terms of interference and diffraction. It is somewhat simplistic, but is accurate enough to provide an understanding of how the holographic process works.

A diffraction grating is a structure with a repeating pattern. A simple example is a metal plate with slits cut at regular intervals. Light rays traveling through it are bent at an angle determined by 'λ' (the wavelength of the light) and d (the distance between the slits) is given by sinθ = λ/d.

A very simple hologram can be made by superimposing two plane waves from the same light source. One (the reference beam)hits the photographic plate normally and the other one (the object beam) hits the plate at an angle θ. The relative phase between the two beams varies across the photographic plate as (2π y sinθ)/λ, where y is the distance along the photographic plate. The two beams interfere with one another to form an interference pattern. The relative phase changes by 2π at intervals of d = λ/sinθ. So the spacing of the interference fringes is given by d. Thus, the relative phase of the object and reference beam is encoded as the maxima and minima of the fringe pattern.

When the photographic plate is developed, the fringe pattern acts as a diffraction grating and when the reference beam is incident upon the photographic plate, it is partly diffracted into the same angle θ at which the original object beam was incident. Thus, the object beam has been re–constructed. The diffraction grating created by the two waves interfering has reconstructed the “object beam” and it is therefore a hologram as defined above.

A hologram of a point source

A slightly more complicated hologram can be made using a point source of light as object beam and a plane wave as reference beam to illuminate the photographic plate. An interference pattern is formed which in this case is in the form of curves of decreasing separation with increasing distance from the center. Refer figure at the bottom.

Photograph of a hologram

The picture at left is a photograph of a hologram recorded with photographic emulsion which was taken against a diffused light background. The area shown is about 8mm × 8mm. The holographic recording is the random variation in intensity which is a speckle pattern and not the regular lines which are likely to be due to interference arising from reflections in the glass plate on which the photographic emulsion is mounted. It is no more possible to discern the subject of the hologram from this than it is to identify the music on a gramophone record by looking at the structure of the tracks. When this hologram is illuminated by an appropriate light wave, the viewer will see the object used to make it (in this case, a toy van!!) because the light is diffracted by the hologram to re–construct the light which was scattered from the object.

Formation of a hologram

It should be clear from this why a hologram is not a 3D photograph. A photograph records the image of the recorded scene from a single viewpoint, which is defined by the position of the camera lens. The hologram is not an image, but an encoding system which enables the scattered light field to be reconstructed. Images can then be formed from any point in the reconstructed beam either with a camera or by eye. It was very common in the early days of holography to use a chess-board as the object, and then take photographs at several different angles using the reconstructed light to show how the relative positions of the chess–pieces appeared to change. The viewer will, of course, get depth information from the hologram if he/she is using both eyes (stereoscopic vision) in exactly the same way as when he/she is viewing the real scene.

Since each point in the hologram contains light from the whole of the original scene, the whole scene can, in principle, be re–constructed from a single point in the hologram. To demonstrate this concept, you can break the hologram into small pieces and you can still sees the entire object from each small piece. If you envisage the hologram as a ‘window’ on the object, then each small piece of a hologram is just a part of the window from which you can still view the object even if the rest of the window is blocked off. You do, however, lose resolution as you decrease the size of the hologram in the same way you do by reducing the size of the aperture in a camera – the scene becomes ‘fuzzier’.

Holographic recording media Holographic recording and reading processes. a, The data are digitized, formed into pages of data with which the signal beam is modulated to write on the storage medium. b, A reference beam is shone onto the storage medium and the modulation of the diffracted light is detected to reconstruct the original data.

The recording medium must be able to resolve the interference fringes as discussed above. It must also be sufficiently sensitive to record the fringe pattern in a time period short enough for the system to remain optically stable, i.e any relative movement of the two beams must be significantly less than λ/2.

The recording medium has to convert the interference pattern into an optical element which modifies either the amplitude or the phase of a light beam which is incident upon it. These are known as amplitude and phase holograms respectively. In amplitude holograms the modulation is in the varying absorption of the light by the hologram, as in a developed photographic emulsion which is less or more absorptive depending on the intensity of the light which illuminated it. In phase holograms, the optical distance (i.e. the refractive index or in some cases the thickness) in the material is modulated.

Most materials used for phase holograms reach the theoretical diffraction efficiency for holograms, which is 100% for thick holograms (Bragg diffraction regime) and 33.9% for thin holograms (Raman–Nath diffraction regime, holographic films of typically some μm thickness). Amplitude holograms have a lower efficiency than phase holograms and are therefore used more rarely.

To know more about the materials for holographic recordings click here.More Info

Holographic data storage

Holography can be put to a variety of uses other than recording images. Holographic data storage is a technique that can store information at high density inside crystals or photo polymers. The ability to store large amounts of information in some kind of media is of great importance, as many electronic products incorporate storage devices. As current storage techniques such as Blu–ray reach the denser limit of possible data density (due to the diffraction–limited size of the writing beams), holographic storage has the potential to become the next generation of popular storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.

Currently available SLMs can produce about 1000 different images a second at 1024 × 1024 – bit resolution. With the right type of media (probably polymers rather than something like LiNbO₃), this would result in about 1 gigabit per second writing speed. Read speeds can surpass this and experts believe 1 – terabits per second readout is possible.

DCG Holography

Since the beginning of holography, experimenters have explored the uses of holography. Starting in 1971 Lloyd Cross started the San Francisco School of Holography and started to teach amateurs the methods of making holograms with inexpensive equipment. This method relied on the use of a large table of deep sand (invented by Jerry Pethic) to hold the optics rigid and dampen vibrations that would destroy the image.

Many of these holographers would go on to produce art holograms. In 1983, Fred Unterseher published the Holography Handbook, a remarkably easy to read description of making holograms at home. This brought in a new wave of holographers and gave simple methods to use the then available AGFA silver halide recording materials.

To know more click here.More Info