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

• Published in: Optical Engineering Vol. 49; no. 10; p. 103201
• Main Authors: Nugent, Paul W, Shaw, Joseph A, Kehoe, Michael R, Smith, Casey W, Moon, Thomas S, Swanson, Rand C
• Format: Journal Article
• Language: English
• Published: 01.10.2010
• Subjects: Detectors; Digital; Infinity; Knife-edge; Modulation transfer function; Optical communication; Optical transfer function
• ISSN: 0091-3286; 1560-2303
• DOI: 10.1117/1.3497051

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, resulting in the resolution of the optics surpassing that of the detector. We measure the MTF for a short-wave infrared hyperspectral imager developed by Resonon, Inc., of Bozeman, Montana, which exhibits both characteristics. 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 measurements. 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 atmospheric turbulence. In a laboratory validation of the MTF algorithm using a monochrome visible imager, the roofline MTF results are similar to results from point-source and sine-card MTF measurements.