8 - Multicolor images  pp. 72-77

Multicolor images

By P. Hariharan

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In principle, a multicolor image can be produced by a hologram recorded with three suitably chosen wavelengths, when it is illuminated once again with these wavelengths. However, a problem is that each hologram diffracts, in addition to the wavelength used to record it, the other two wavelengths as well. The cross-talk images produced in this fashion overlap with, and degrade, the desired multicolored image. This problem has been overcome, and several methods are now available to produce multicolor images [Hariharan, 1983].

Multicolor reflection holograms

The first technique employed to eliminate cross-talk made use of the high wavelength selectivity of volume reflection holograms. If such a hologram is recorded with three wavelengths, one set of fringe planes is produced for each wavelength. When the hologram is illuminated with white light, each set of fringe planes diffracts a narrow band of wavelengths centered on the original wavelength used to record it, giving a multicolor image free from cross-talk [Upatnieks, Marks & Federowicz, 1966].

Higher diffraction efficiency can be obtained by superimposing three bleached volume reflection holograms recorded on two plates, one with optimum characteristics for the red, and the other with optimum characteristics for the green and blue. Brighter images can also be obtained if the final holograms are produced using real images of the object projected by primary holograms whose aperture is limited by a suitably shaped stop [Hariharan, 1980a].