Target identification performance as a function of spurious response: Aliasing with respect to the half sample rate

The sampling limitations associated with focal plane array (FPA) imagers cause an aliased signal that corrupts the image. The aliased signal is a function of pre-sample blur, sampling frequency, and post-blur or image reconstruction. Previous experiments at the U.S. Army Night Vision and Electronic Sensors Directorate (NVESD) have quantified the effect of aliasing on the task of infrared target identification. Based on data from these experiments, the MTF Squeeze model was developed. The degraded performance due to under-sampling was modeled as an increase in system blur or, equivalently, a contraction or "squeeze" in the MTF. This paper describes the results of numerous sampling experiments. The latest experiment relates the degradation in infrared target identification (ID) performance to the location and quantity of spurious response, or aliased signal, in the frequency domain. The spurious response locations are in-band, mid-band, and out-of-band relative to the half sample rate. The levels of spurious response are 0, 0.2, 0.3, and 0.4 for the identification task. An additional out-of-band spurious response experiment increases the level to 0.7. A number of integrated metrics of the sampled imager response function are then considered as descriptions for contraction, or "MTF Squeeze," functions. These functions are applied to a wealth of historical sampling experiments that were conducted at NVESD over the past three years. We maintain that in-band aliasing has little effect on target identification and we refine the measures of degradation seen for out-of-band aliasing.