MTF testing algorithms for sampled thermal imaging systems

Abstract

The introduction of third generation thermal imagers brings a new challenge to the laboratory evaluation of the thermal imager resolution performance. Traditionally, the Modulation Transfer Function (MTF) is used to characterize the resolution performance of the thermal imager. These new third generation of thermal imagers can be categorized as sampled imaging system due to the finite pixel size of the elements comprising the focal plane array. As such, they violate the requirement of shift invariance required in most linear systems analyses. We present a number of approaches to measuring the resolution performance of these systems and conclude that source scanning at the object plane is essential for proper MTF testing of these sampled thermal-imaging systems. Source scanning serves dual purposes. It over-samples the intensity distribution to form an appropriate LSF and also generates the necessary phases between the thermal target image and the corresponding sensor pixels for accurate MTF calculation. We developed five MTF measurement algorithms to test both analog and digital video outputs of sampled imaging systems. The five algorithms are the Min/Max, Full Scan, Point Scan, Combo Scan, and Sloping Slit methods and they have all been implemented in a commercially available product.