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The objective of this research program is the development of the technology for the industrial fabrication oflarge format holographic optical elements (HOEs) with predetermined spectral characteristics and angular selectivity. HOEs of this type are used in a variety of technical applications, such as: holographic concentrators for photo-voltaic energy conversion and solar photo-chemistry or as integrated holographic stacks comprising several holograms operating in different ranges of the solar spectrum for daylighting, glazing and shading in buildings. The latter are required for the effective control of the transmission of solar radiation through the windows or the glass curtain wall envelopes of buildings. The HOEs (reflective or transmissive) are recorded in dichromated gelatin layers (DCG) deposited on glass or plastic substrates. This material and the corresponding thermochemical development process facilitate the achievement of bandwidths, spectral ranges and angular selectivity that match accurately the design spectral and geometrical properties of a particular application1'2.The developed technology extends the applicability of DCG into the blue and red spectral domains and becomes a viable tool in the control and utilization of solar energy. The applicability of the technology is illustrated with design and test results obtained from various examples of HOE utilization. We will also report on the properties of asymmetric reflective holograms developed for window shading that permit unobstructed view through the window, but block the direct solar radiation for a particular angle of incidence. The angular selectivity, the bandwidth and the central wavelength can be adjusted to achieve the desired effect.

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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 will be presented in a forthcoming paper.

Dichromated gelatin layers (DCG) facilitate the design and fabrication of large format holographic optical elements (HOE) of high optical quality and diffraction efficiency. The HOEs are used for the fabrication of spectrally selective solar concentrators and as glazing materials for daylighting and passive sun control in buildings. The suitability of HOEs in these applications depends upon the achievable bandwidth, operating central wavelength, dispersion characteristics and low absorption losses. The HOEs are fabricated on glass or plastic film substrata in a DCG-layer of 5to 30 jim thickness. The layer thickness and the gradient are precisely controlled during the layer deposition and drying (±1 pm and 0.1 j.un/cm for standard layer of 10 p.m thickness). The production process is based on the fabrication of high quality master holograms that are copied by dry copying procedure. The current manufacturing facilities allow the fabrication of 1 m2 HOEs on glass substratum and a continuous production of HOEs on plastic substrawm with a width of 20 cm and length of 50 m. This technology is also used to fabricate holograms for instrumentation optics in metrology and for optical interconnects in multichip modules. The fabricated HOEs exhibit the desired operational characteristics: high diffraction efficiency, small Braggshift, large bandwidth and a central wavelength that may be freely selected over a wide spectral range. In this paper, we present the results from the experimental investigation and theoretical analysis of large number of holograms of the transmissive and reflective types. We discuss the attained angular and wavelength spectra, bandwidths, wavelength shifts and the diffraction efficiencies as functions of the holographic parameters. The HOEs are made for technical applications and are designed to operate in the 300 nm -1500 nm spectral range.