Spatial Pulse Modulation for Coherent Optical Densitometry

Abstract

Purely optical methods of spatial pulse width coding for coherent optical densitometry are described. The proposed coding scheme correlates the image intensity with the orientation of one edge of triangular or trapezoidal pulses. One encoding technique employs a contact screen similar to those used for half-tone printing. The image to be coded is copied through this contact screen onto hard-limiting film. This method is limited to the encoding of images with density distributions varying only slowly across the area of one pulse. A holographic encoding technique overcomes these drawbacks, utilizing the storage redundancy of holograms recorded with diffuse object beams. An array of identical holograms is reconstructed with a beam spatially modulated by the image transparency to be encoded. The array of reconstructed real images is recorded on hard-limiting film and renders a pulse-modulated version of the original image. The limiting conditions for the pulse size and the size of details in the image imposed by diffraction effects and speckle pattern are derived.