Spatial-frequency-filtering effect of multiphase holograms

A well-designed hologram can be used as a spatial frequency filter to remove moiré fringes in a digital imaging system. However, it also degrades the resolution of the image. We have used a homemade program to design holograms with better filtering characteristics. We have designed holograms with two, four, eight, and 16 phases and compared their diffraction efficiencies and modulation transfer functions (MTFs). The four-phase hologram showed a diffraction efficiency and MTF characteristics similar to those of the binary hologram. The eight-phase hologram, however, showed much better diffraction efficiency than the previous two, and thus higher MTF values. The 16-phase hologram was still better, but the improvement was marginal. We have manufactured binary and eight-phase holograms and measured their MTFs. We have also observed directly their filtering effects by attaching them to a digital camera and taking pictures of test charts. The Moiré-fringe-removing effects of the two holograms were similar, but the eight-phase hologram showed better resolution.