The properties of relief phase holograms in unhardened dichromated gelatin have been investigated. Relief holograms are particularly attractive when it is desired to duplicate the stored information. Unhardened layers of gelatin were exposed with two collimated laser beams and developed by washing away the unexposed material. This resulted in high quality gratings with very low scattering. By varying the spatial frequency, it was found that the depth of modulation starts to decrease approximately linearly at a few microns grating spacing. This limited spatial resolution was shown not to be a property of the basic photochemical process of polymerization but due to a development step. Larger modulation and diffraction efficiencies can be produced at the higher frequencies by other development procedures, but they also result in increased scattering which is incompatible with high quality holograms.
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