Landsat-5 thematic mapper cold focal plane characterization

The gain response of the detectors in the cold focal plane of Landsat-4 and Landsat-5 Thematic Mapper (TM) is known to fluctuate temporally. This is believed to be caused by an interference effect due to a film build-up on the cold focal plane dewar window. This paper analyses the effect using a thin-film interference model. This model relates detector gain to film thickness through indices of refraction of the materials involved. The dewar window material is zinc selenide (ZnSe) with a known index of refraction, thus the index of refraction of the film in TM band 5 and band 7 can be determined by fitting the model to the gain data from each detector. The film is found to be quite absorptive in band 7, but only slightly absorptive in band 5. Film thickness has been determined to be as much as 1400 nm from the data sample used. This model is also used to remove gain variation for gain characterization over instrument lifetime. An accurate model of this effect can improve the calibration of Landsat-4 and Landsat-5 by 3-4% in band 5 and band 7. It is also applicable to Landsat-7 Enhanced Thematic Mapper Plus (ETM +) if this thin-film build-up is present.