The spectral sensitization of hardened dichromated gelatin to red light is carried out using methylene blue as sensitizer. The sensitized dichromated gelatin plate can be stored for over 1 month in an ammonia atmosphere without loss of photosensitivity. Holograms recorded in the sensitized dichromated gelatin using a He–Ne laser have high diffraction efficiency and low noise if they are swelled at a temperature near the gelatin gel melting point during development processing. Maximum diffraction efficiency has been found to be as high as 88%. The energy required to obtain 80% diffraction efficiency is 150–400 mJ/cm2.
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