Simple model for image-plane polychromatic speckle contrast

A simple geometrical model was developed for calculation of the contrast of a polychromatic image-plane speckle pattern from a source of light with hi gh spatial coherence. It is based on counting the number of independent spe ckle patterns that contribute to a given point in the image plane. This res ults in a simple equation for the contrast as a function of imaging geometr y; relative orientation of the projection direction, observation direction, and specimen normal; bandwidth of the light source; and surface roughness. Its validity was established by comparison with an exact solution: rms err ors in the calculated contrast were only 0.033 over a wide range of paramet er values likely to be encountered in practice. (C) 1999 Optical Society of America.