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Conceptual design and practical implementation of dichromated gelatin films as an optimal holographic recording material for large-format holograms

The commercial manufacturing of large format holographic optical elements (HOE) -- these are used in the fabrication of holographic solar concentrators or for daylighting applications in buildings -- requires inexpensive materials exhibiting high diffraction efficiency, bandwidth and controlled shift of the operating wavelength. Hence, the ideal recording material must possess adequate spectral sensitivity at the wavelengths of present day high power lasers and permit the desired shift of the operating wavelength by means of process control. The material should manifest a predictable diffraction efficiency as a function of the layer fabrication technique, of the exposure, and of the development process and display high spatial resolution and low noise. The properties of dichromated gelatin (DCG) as a recording material for volume holograms are close to ideal. It provides a large refractive index modulation, high resolution, negligible absorption, and low scattering. The holographic film is prepared in the laboratory and extensively tested. The processing of the film after exposure is a sequence of chemical reactions and physical treatments. We report in this paper our experience with large format DCG films on glass substrates and present the dependence of the holographic properties upon the layer preparation procedures and upon the exposure energy. The results for the film development and after-treatment are presented in a forthcoming paper.