Speckle in rear-projection screens, such as those used in microfiche viewers, reduces resolution and can cause viewer fatigue. Many speckle-reduction techniques have been tried with varying degrees of success, including rapid movement of a single screen in its plane, the use of two stationary spaced screens, liquid-crystal screens, and the use of two closely spaced screens in slow relative orbital motion. We describe a simplified, two-screen support that permits only linear screen motion, and show that such motion provides total speckle elimination. An analysis of speckle in such screens is presented which accounts for the observed remarkable sensitivity of the speckle pattern to minute screen displacements. To confirm this analysis we have measured the speckle pattern’s correlation as a function of screen displacement using the speckle interferometry method of Françon. We consider the effects of a spatially and spectrally extended incandescent light source, and conclude that these effects are negligible in our application compared to the effect of screen motion. We show that these three speckle-reducing effects (screen motion, spatial extent of the source, and spectral extent of the source) can be considered together by introducing the notion of space-time correlation cells.
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