A review of selected technological applications of DCG holograms

The subject matter of this presentation is to demonstrate the engineering applications of holographic optical elements (HOEs) fabricated in dichromated gelatin (DCG) films exhibiting enhanced properties. The composition, structure and the physical, chemical and optical properties of the DCG-film are briefly described in the introduction that is followed by detailed discussion of the developing processes used to achieve the necessary holographic characteristics required by the various applications. Other procedures are used to achieve explicit objectives, e.g., controlling the spectral characteristics of the HOE by inorganic and/or organic additives and using filler material to enhance the UV or IR performance. Stress induced by environmental forces, e.g., wind, or by mechanical strain also changes the performance of the HOE and may be exploited in engineering applications. The optical characteristics of the HOE are also modified by internally induced stress, such as changing the water content of the polymer or using cross-linking agents to modify and harden the gelatin matrix. The formation of thermal or density gradients in the gelatin film during the coating process or at some stage of the hologram development have also an effect on the holographic properties for they determine the conformational state and the mechanical strength of the gelatin film. The ratio between the coiled and the helical structures in the gelatin matrix determines the optical and mechanical properties of the holographic film. Multiple exposures are used to record up to four holograms in single DCG film that are used to reconstruct concurrently several monochrome or RGB beams.