Measuring the modulation transfer function of an imaging spectrometer with rooflines of opportunity

Measuring the modulation transfer function (MTF) of digital imagers focused at or near infinity in laboratory or field settings presents difficulties because the optical path is longer than a typical laboratory. Also, digital imagers can be hindered by low-resolution detectors, result- ing in the resolution of the optics surpassing that of the detector. We measure the MTF for a short-wave infrared hyperspectral imager devel- oped by Resonon, Inc., of Bozeman, Montana, which exhibits both char- acteristics. These difficulties are overcome with a technique that uses images of building rooflines in an oversampled, tilted knife-edge-based MTF measurement. The dark rooftops backlit by a uniformly cloudy sky provide the high-contrast edges required to perform knife-edge MTF mea- surements. The MTF response is measured at five wavelengths across the imager's spectral band: 1085, 1178, 1292, 1548, and 1629 nm. The MTF also is observed at various distances from the roof to investigate performance change with distance. Optimum imaging is observed at a distance of 150 m, potentially a result of imperfect infinity focus and atmo- spheric turbulence. In a laboratory validation of the MTF algorithm using a monochrome visible imager, the roofline MTF results are similar to re- sults from point-source and sine-card MTF measurements. C 2010 Society