A radiometer is a device for measuring the radiant flux, i.e., power of electromagnetic radiation. While other terms fit the same definition, the term radiometer is typically used for ultraviolet, infrared, and microwave radiation (microwave radiometer). It is typically used for devices designed to determine temperature at a distance through identification of enough of a black body spectrum to infer the temperature. Such devices are used in spacecraft for Earth science, e.g., determining temperatures of items on Earth or in the atmosphere, but have been used in radio astronomy and exploration of the cosmic microwave background.
Being designed to determine a temperature, radiometers often compare the signal (a type of noisy radiation) with a generated signal representing the black body radiation of a particular temperature and base conclusions on the expected sums or differences. Methods and procedures for calibrating these instruments so as to yield an accurate temperature measurement is an area of active research.