Noise in Coherent Optical Systems:Minimum Detectable Object Contrast and Speckle Smoothing.

摘要

A theory is presented which relates the minimum detectable contrast level for an object in the presence of noise to the statistics of the speckle. Consideration is given to smoothing of the noise by multiple looks and by area. Measurements of the minimum detectable contrast are made for two types of speckle noise. First, a coherent, plane wave is added to an ideal diffuse wave and the threshold of detection is established as a function of the beam ratio. Second, these results are compared to the technique of speckle smoothing using an N-fold intensity superposition of fully developed speckle patterns. Good agreement of experiments with theory is observed. The equivalence of four different methods of N-look averaging is shown and it is demonstrated that fully developed speckle leads to an image degradation independent of light level over a very wide range of light levels. Detection probabilities and false alarm probabilities are determined from observations on random arrays of high- and low-contrast discs. (Author)