The recording of holograms in which the reference beam enters through the supporting substrate involves attenuation factors which differ strongly between signal and reference. This paper addresses the issue of fringe contrast under these extreme conditions and provides quantitative data that are indicative of at least a partial solution to the problem. We define a spatial average for the fringe contrast and offer results which follow our theoretical model for Du Pont OmniDexTM photopolymers. The direct edge referenced technique for recording is re-visited with observation of a new phenomenon which we call self-induced index matching. An alternative fringe recording geometry is proposed and compared with direct recording.
An edge-lit holographic optical element has been developed which produces a high contrast and high resolution fingerprint image. Continuing work on the development of a holographic optical element for use in a compact fingerprint imaging system is described. The hologram is combined with an electronic imaging array into a package only several millimeters thick, using no image transfer lens system. Previously reported results with green holograms are reviewed. New results are presented for red holograms illuminated by a laser diode. Design issues and experimental results are discussed, relating to selection of hologram operating wavelength, angular and wavelength sensitivity of the hologram, and image quality versus packaging volume tradeoffs.