Review of the technology for the manufacturing of large format DCG-holograms for technical applications

Large format holograms are of potential interest to the energy and building sectors. Such holograms are considered at present for photovoltaic power generation and for daylighting or glazing in buildings. Dichromated gelatin (DCG) exhibits properties that are nearly ideal for these applications. Both sectors require large format holograms with accurately controlled properties, such as: spectral bandwidth, operating wavelength and diffraction efficiency. The required properties are attained by exactly controlling the thickness of the gelatin layer and the refractive index variation over the entire aperture as a function of layer depth. The information presented in this paper is based on ten years' efforts that includes layer deposition and film drying techniques with controlled thickness, development process and an inexpensive dry copying procedure for the industrial fabrication of the holograms on a flexible film substratum. Design criteria and experimental results for large format holographic gratings, lenses and mirrors are presented and discussed. The current technology facilitates the manufacturing of HOEs that operate between 400 nm and 1500 nm in various applications: photovoltaic solar concentrators for multicolor operation, hybrid collectors for simultaneous thermal and PV application, holographic stacks for white light (RGB) daylighting and wideband sun shading and holographic concentrators with spectral bandwidths that are adapted to particular photochemical reactions far solar chemistry.