Lamps carried into orbit on imaging sensors for the purpose of measuring the Responsivity of reflective band detectors as a function of time under the assumption that controlling or monitoring the current or spectral radiance of the lamp will result in a constant or known irradiance on the detectors. There were three such lamps on each of the Thematic Mapper (TM) sensors on the Landsat 4/5 satellites that were cycled in an 8-step sequence whenever the instruments were turned on. There were two lamps on the Enhanced Thematic Mapper Plus (ETM+) imager on Landsat 7; the Primary one was used on almost every acquisition, while the redundant one was only used a few times per year. (Source: Dr. John Barker)