Post-optic MTF measurement using a reference optic

The Modulation Transfer Function (MTF) of an imaging device is a strong indicator of the resolution limited performance. The MTF at the system level is commonly treated as separable, with the optical MTF multiplying the postoptic (detector) MTF to give the system MTF. As new detector materials and methods have become available, and as the manufacturing of detectors has been separated from the optical system, independently measuring the MTF of the detector is of great interest. In this correspondence, a procedure for measuring the post-optic MTF of a mid-wave (3-5 micron) sampled imager is described. This is accomplished through a careful measurement of a reference optic that is later installed to allow for a final system MTF measurement. The key finding is that matching the chromatic shape of the illumination between the optic and system MTFs is critical, as in both measurements the effective MTF is scaled by the source and detector spectral shapes. This is most easily accomplished through the use of narrow bandpass filters. Our results are consistent across bandpass filter cut-on and F/number.