Effects of Cylindrical Chopper Geometry on Calculating Power Coupling Efficiency and Noise Equivalent Temperature Difference

This report explores the geometrical effects of a cylindrical chopper wheel on calculations of power coupling efficiency and noise equivalent temperature difference (NETD) of array elements. The overall efficiency of the cylindrical chopper wheel is found to be about 53% from geometrical arguments. In the limit that a detector beam fills the chopper aperture, if this efficiency is not accounted for, it can lead to detector NETD values that are a factor of 1.9 too high when calculated using standard double-modulation measurement techniques. For the particular geometry used with this setup, a power coupling efficiency of 78% was calculated for typical detector elements, resulting in values of detector NETD that were too large by a factor of 1.3 when the coupling efficiency was unaccounted for. This factor plays a role in accounting for some of the discrepancy between NETD values calculated based on diode sensitivity estimates and NETDs measured for individual array elements using double-modulation measurements.