“Objective” speckle arises from the uneven illumination of an object with a multiplicity of waves that interfere at its surface. “Subjective” speckle arises at rough objects even if they are illuminated evenly by a single wave. The noise in the image is caused by the interference of the point-figures, which have random phases. Subjective speckle cannot be reduced except by extending the aperture. On the other hand the “objective” speckle in a plane, for instance in the plane of a transparency, can be reduced, and in the limit made invisible, by a special type of wide-angle illumination. This consists of a one-parameter family of plane waves, which can be produced by diffraction at a special grating, or two crossed gratings, close to the object plane. This makes it possible to produce multiple holograms, with the same insensitivity to dust or scratches as diffused holograms, but without any visible speckle in the reconstruction.
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