15-06-2013, 03:48 PM
Holographic Television | Holographic Cinematography | Color Holography : Laser Holograms |
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INTRODUCTION
The striking three dimensional properties of holographic images make holograms ideal for displays. Holography can produce spectacular displays for scientific, educational, medical, artistic and commercial purposes. Holographic displays would avoid the risk of theft of art objects. The same object can be displayed at several places at the same time, thus permitting much wider exposure of rare items.
To day holographic studios exist in several countries for display holography. These holographic studios are equipped for recording of a wide range of objects and compositions including people and animals. A laser display hologram of the full size statue of Venus de Milo (height 2.18 m) had been made as early as in early 1970s on a 1.0x1.5 m size photographic plate by Tribillon and Fournier of Besancon, France. The holographic studio of State Optical Institute, Leningrad has recorded holograms of art work from Hermitage collection and museum articles.
Requirement of a Display Hologram
An ideal display hologram should meet the following requirements.
White Light Viewable Image
The holograms should be preferably viewed with a white light source. Finite size of the source produces blur in the image. A typical white light source is the sun. It is equivalent to the case of reconstructing a hologram with a light source of 9 mm diameter from a distance of one metre. The image blur is around 0.6 mm for an image point at 10 cm from the hologram. In other light sources, the apparent size of the source can be reduced.
Focussed Image
The image should appear to be well focussed, when the hologram is viewed by a white light source. The image is blurred due to the finite size of the source and due to its spectral bandwidth. The blurring increases with the increase in the distance of the image point from the hologram. The average distance of the image from the hologram can be minimized if the image straddles the hologram plane.
Rainbow Hologram
For display holograms it is desirable to use a white light source for hologram illumination. When a transmission hologram is reconstructed with a non-monochromatic source, the image is smeared due to dispersion property of the hologram. Benton in 1969 invented a special type of hologram called rainbow hologram in which parallax is eliminated in vertical direction to reduce the coherence requirements. The technique utilizes the full advantage of placing the image very close to the plane of the hologram. As there is no vertical parallax, colour smearing is minimized.
The absence of parallax in vertical direction does not affect the display as complete depth perception is preserved in the horizontal direction and the viewer normally moves his head in a horizontal direction to get different perspectives of the image.
Aberrations Control
For recording of a rainbow hologram it is essential that the primary hologram projects an undistorted real image into space which is achieved by illuminating the holo grams with the phase conjugate of the reference beam. The conjugate of a diverging reference beam is a converging beam which needs a large aperture convex lens. The conjugate of a plane beam is a plane beam propagating in reverse direction.
However, large aperture collimating lenses are needed to produce a large aperture plane reference beam. A diverging beam at a distance of 8-10 m may be approximated as a plane beam. However, perfect phase conjugation is difficult. The rainbow hologram process usually involves two conjugations, during the recording and viewing steps. Aberrations commonly arise in both the cases and affects the perception of the 3D image.
Resolution and Image Blur
The size of the slit is critical for achieving optimum image quality from white light holograms. It has been observed that if the slit is made narrower than 3 mm, the diffraction effects take place and the image becomes speckled and if it is made wider than 5 mm, the image becomes blurred. The image will be brighter if it is at a larger distance from the hologram. It may be pointed out that though the reconstructed images appear quite sharp, there is some blurring of the images due to the finite source size and the dispersion of the hologram. While viewing the rainbow hologram the finite diameter of the eye allows only a narrow range of wavelengths to form the image, thus in many circumstances, the image appears sharp.
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Holographic Stereogram
For making holographic stereograms, photographs of objects rather than the real objects are used. The two dimensional photographs are recorded in the form of a composite hologram such that the image appears to be three dimensional. The main advantage of this technique is that the images can be magnified or demagnified and it makes holography of outdoor scenes possible.