The radiometric response of an electronic detector that converts photons into voltage or current. Detector responsivity includes the end-to-end optical and electrical path, since radiometric calibration involves converting the digital signal into scientific units such as at-satellite radiance. Typically before launch, detectors within each spectral band of an imaging sensor are exposed to full aperture illumination from a uniform source such as a large integrating sphere, which itself has been calibrated against standards that are traceable to National Institute of Standards and Technology (NIST) standards. Absolute detector responsivity is in units of digital number per radiance unit (such as watts per square meter per steradian), or in units of digital number per spectral radiance unit (such as watts per square meter per steradian per micron). Once on-orbit, the detector responsivity is often monitored for possible change with time. Relative gain of the detector in comparison to a reference detector or to an average of detectors within a band can be made from image data by histogram equalization. Absolute gain can be monitored using internal lamps, or external sources such as the Sun, Moon or nearly homogeneous areas on Earth after correcting for atmospheric effects. Also see calibration lamps, NIST traceability, radiometric emissive band calibration, and radiometric reflective band calibration. .