A Speckle Noise Model for Optical Heterodyne Line-Scan Imagery.

An imaging system that consists of a laser scanning a surface and a heterodyne receiver that measures the backscattered field is considered. When the scanned surface is rough compared to the wavelength of the incident laser beam the coherent properties of the laser beam are destroyed in the backscattered field. The result is a noise in the image that is referred to as speckle. The rough surface is modeled by multiplying the incident scalar field by a reflectance term and a random phase term. The random phase is modeled as a random process whose mean, variance, and correlation distance are functions of the rough surface. The statistics of the amplitude and amplitude squared of the current from the optical detector are developed in a manner similar to the well known narrowband noise model. Second moment models for detection of the amplitude or amplitude squared are presented. The mean and covariance functions of each model are related to the field correlation function, the reflectance of the surface, and the system parameters. The noise models describe both the average speckle cell size in the image and the contrast in the image caused by the speckle noise. Author