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.
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.
Wide Field-of-View
The field-of-view should be wide to allow a large parallax. The vertical field-of-view may be restricted (as in a rainbow hologram) but the horizontal field must be large.
High Resolution
The hologram should produce an image with excellent sharpness.
Low Image Aberrations The image must be aberration free and without distortions. The image should not move as the observer moves horizontally or vertically in the field. Low Background Noise
Background noise reduces the contrast of the image. The processing chemistry should not introduce scattering noise. Interference between light scattered by different parts of the body also introduces background noise (flare light).
True Colour
The hologram should produce the image with true colour. For monochrome object, while a display with only one colour may be satisfactory for many purposes, the addition of false colours may improve the display.
Ambience
hologram must display the image with proper contrast in high ambient light brightness. The image should be acceptably bright.
Hologram Durability
The hologram medium should be resistant to humidity, temperature, heat, abrasion, intense light, etc. Some of the techniques developed for holographic displays are discussed below.
The hologram reconstructs only that portion of the object whose scattered light is received by the photographic plate during recording. The front, sides and back of the object can be recorded on three, four or more photographic plates. Such holograms give 360 deg view of the object. A hologram recorded on a cylinder of film which surrounds the object gives complete 360 deg view of the object. A 360 deg hologram on a flat plate can be recorded by multiple exposure technique.
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.
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.
Transmission of holograms via television has been a natural desire of holographers due to its possible impact in the field of entertainment. The main difficulty has been due to the enormous information content of the holograms which cannot be handled by TV channels. The techniques can be employed to reduce the information content of the hologram. The transmission of holograms via TV was executed in 1966 but the actual holographic TV could not be realized in the absence of a suitable recording material.
Holographic cinematography (or cine holography) has not reached a stage of commercial exploitation so far although the demonstration of its principle was made quite early.
The difficulty arises from the requirement of creating and presenting a sequence of holograms of a scene to a large number of viewers. The recording of large size objects and scenes in their natural colours is another problem. Present pulse laser technology permits to record holograms of several cubic metres. The motion of the object also poses problem during recording. The velocity of the object must be low of the order of 1 m/s in the direction of the camera with a pulse duration of 20 ns. Komar in 1977 demonstrated for the first time a holographic movie of a few minutes duration which could be viewed by four persons at a time. The concept based on a projection technique could show the promise of holographic cinematography in the field of entertainment. A wide aperture lens (200 mm diameter) is used to record a series of image holograms on a 70 mm film. The processed holographic images can be projected through an identical lens onto a holographic screen which is a multiplexed holographic optical element serving as concave mirrors. The holographic screen forms multiple real images of the projection lens in front of the viewers. Looking through the pupil shows a full size three dimensional image.
Recording of Colour Holograms
Colour holograms are basically multiplexed holograms which produce multicolour images. They can be recorded with three wavelengths. When reconstructed with the recording wavelengths the hologram produces overlapping images in three colours producing a multicolour image. The behaviour of the reconstructed image depends on whether the hologram has been recorded in a thin medium or in a thick medium. Colour holograms recorded in a thin recording medium suffer from cross-talk. If the hologram is recorded with two wavelengths, then the two fringe patterns produced will be different, even though same angle between the object and reference waves for each wavelength are kept. When the hologram is illuminated with one wave, it will produce two images, one at its proper location and the other displaced. If the hologram is reconstructed with both the waves, four images are reconstructed.
Volume holograms effectively eliminate cross-talk images utilizing Bragg effect. Both transmission and reflection volume colour holograms can be recorded in thick media. The transmission volume holograms are reconstructed with the laser beams used to record it, while the volume colour reflection holograms are reconstructed with white light due to their inherent wavelength discrimination ability. In general, a colour volume hologram will not produce any cross-talk images if the bandwidth of the constituent gratings is smaller than the difference in any two of the recording wavelengths.
Pseudocolouring
It is possible to vary the thickness of the emulsion deliberately between exposure and viewing in order to obtain reconstruction wavelengths that are very different from the recording wavelengths. This ` pseudocolour' technique makes multicolour reconstruction possible with only one colour of laser light.
The shrinkage of the recording material is controlled with a proper combination of developers and bleaches. By varying the exposure energy and developing time combination, it is possible to vary the amount of silver removed which controls the thickness of the final emulsion. However, too much deviation from the optimum exposure/developer times would reduce the diffraction efficiency.
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