Modulated-grating hologram.

By means of a multiple-exposure holographic recording method, a phase-and-amplitude, off-axis hologram, which we call a modulated-grating hologram, has been synthesized from three computer-generated transmission masks. Each mask controls one fixed-phase component of the complex hologram transmittance. The basic grating is generated optically, relieving the computer of the burden of drawing details the size of each fringe. The maximum information capacity of the computer plotting device can thus be applied to the generation of the grating modulation function. Large digital holograms (25 mm x 25 mm) have been synthesized by this method in dichromated gelatin. The method can be used with virtually any holographic recording medium. It was designed primarily for spatial filtering applications, in which a hologram with large dynamic range and large free-spectral-range is needed.