A specific type of lamp-based calibration system carried on-board an imager in space for the purpose of monitoring for possible radiometric changes in detector responsivity with time. The internal calibrators (ICs) on Landsat 4/5 TM consisted of 3 lamps that were sequentially cycled through 8 possible radiation levels. The IC on Landsat 7 ETM+ consisted of a single primary lamp, which was used during almost all acquisitions and a secondary or backup lamp. On both imagers, background and IC pulse values were taken on each scan as a shutter, which was synchronised with the scan mirror, obscured the ground image and projected a pre-launch calibrated radiance from the IC lamps onto the focal planes. Also see other on-board external full aperture and partial aperture solar calibrators. Also see calibration lamps.