Comment: Absolute Measurement of Roughness and LateralCorrelation Length of Random Surfaces by use of the Simplified Model of Image-Speckle Contrast

This comment demonstrates several errors in the derivation of speckle contrast and corresponding measurements reported by Cheng et al. [Appl. Opt. 41, 4148 (2002)] for a 4f optical system. In particular, the theoretical derivation is wrong: It is used outside of its domain of validity, and the experimental results do not support the theoretical analysis.