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Dichromated gelatin is a photosensitive material that has been used to make volume holographic elements for over 50 years. The film is an ideal material for many applications in volume holography, such as display technologies and photovoltaic systems. The material has high transparency, low scatter, high index modulation capacity, and has bandwidth broadening processes that can be controlled to extend the angular and spectral bandwidth beyond conventional films. The film is suitable for high-volume production since it is inexpensive and can be manufactured using roll-to-roll techniques. The film consists of gelatin interspersed with light-sensitive ammonium dichromate. After exposing with an interference pattern of light, the film selectively bonds in regions with bright fringes. A hologram is formed after a series of chemical baths removes unbonded chromate from the film and forms a modulation in the index of refraction. Unfortunately, the material has a reputation for inconsistency that has driven many researchers and engineers away from using the material more extensively. While many have commented that it is critical to control the environmental humidity and temperature conditions to achieve consistent hologram formation, little work has been done to systematically address the problem or report findings in literature. In this paper we use data taken from hundreds of hologram samples to quantify the “inconsistency” by measuring the variation in diffraction efficiency and Bragg wavelength. We use a partially controlled facility in which the humidity is controlled during the film drying process, but not in any other stage of the process. We show a strong correlation between drying humidity and repeatability and show that 55-65% drying humidity is optimal. Drying the film at 65% humidity compared to 25% improves the repeatability in both Bragg wavelength and diffraction efficiency by nearly 10X.
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