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Atmospheric degradation of scene content: a new look

The usual answer to estimating the atmospheric degradation of a forward looking infrared (FLIR) system's performance is to use an absorption, or better yet, an extinction coefficient derived from LOWTRAN calculations. The actual quantity that is appropriate, however, is target contrast transmission that includes system and source spectral characteristics, path radiance, and inherent contrast changes. Until now only transmissometer systems have ben used to measure and compare absorption and extinction values and predictions for imaging system performance. The Mobile Imaging Spectroscopy Laboratory of the U.S. Army Atmospheric Sciences Laboratory was used to measure directly the target contrast transmission by comparison of calibrated closeup and distant IR imagery of a uniform temperature large area target board. Only through proper choice of target and background area dimensions are the target contrast transmission measurement values consistent with LOWTRAN predictions. The important additional parameter that is too often overlooked is the modulation transfer function (MTF) of the atmosphere which can severely change the effective magnitude of the contrast transmission value that actually applies to a specific target feature or hot spot. Measurements are presented that illustrate how dramatic this interdependence of target contrast transmission and atmospheric MTF can be.