Modulation transfer function measurement on QWIP focal plane array

Abstract

Modulation Transfer Function (MTF) is an important quantitative measure of imaging quality of an infrared camera system. It provides quantitative description on how infrared camera system transfers contrast from object to image space. The higher the MTF values as a function of spatial frequency, the better the reconstruction of fine detail of the object space. Therefore, MTF is a good metric of performance measure for infrared camera systems. However, for QWIP Focal Plane Array (FPA) there is little comprehensive MTF result that has been reported. The over-sampled tilted knife-edge technique to generate the MTF plot is the most common method to measure MTF. Thus we have developed a method to construct the edge function from the image of tilted knife-edge. This construction is reversible since the tilted knife-edge image can be reconstructed from the edge function. Unfortunately, the knife-edge technique requires knowledge of the lens' MTF. The lens MTF is removed from the total system MTF to derive the detector and electronic MTF. In order to validate the knife-edge technique, an interferometric and small spot illumination method of MTF measurement was conducted. In the interferometric method the QWIP FPA was placed at the pupil plane, and therefore diffraction limitation was avoided. The interferogram also provides a high-resolution plot of the spectral response of the detector. The spot illumination mimics the point-spread function. Since spot illumination requires re-imaging of point source by lenses, it also requires lens' MTF that needs to be divided out. The objective of this paper is to report on an extensive MTF characterization of large format QWIP FPA.