Modeled and Measured Partially Coherent Illumination Speckle Effects from Sloped Surfaces for Tactical Tracking

Abstract : The statistical properties of speckle relevant to short to medium range (tactical) active tracking involving polychromatic (partially temporally coherent) illumination are investigated. A numerical model is developed to allow rapid simulation of speckled images including the speckle contrast reduction effects of illuminator bandwidth, surface slope and roughness, and the polarization properties of both the source and the reflection. Regarding surface slope, Huntley's theory for speckle contrast, which employs geometrical approximations to decrease computation time, is modified to increase accuracy by incorporation of a geometrical correction factor and better treatment of roughness and polarization. The resulting model shows excellent agreement with more exact theory over a wide range. An experiment is conducted to validate both the numerical model developed here and existing theory. A short coherence length diode laser source is reflected off of a silver-coated diffuse surface. Speckle data is gathered for 16 surface slope angles corresponding to speckle contrast between about 0.55 and 1. Taking Hu's theory as truth, the measurements have -1.1% mean difference with 2.9% standard deviation, while the modified Huntley equation has 1.4% mean difference with 1.0% standard deviation. Thus, the theory is validated over the range of this experiment.