In astronomy, the term radiometer is most commonly used for devices measuring EMR signal (radiant flux) from a band within the range of radio to submillimeter, i.e., including microwaves (microwave radiometer), into the shortest infrared wavelengths. The term radiometer has been used in sciences for devices measuring other EMR, but in astronomy, it is generally not used when the wavelength is short enough to be handled with optics.
In astronomy, the term radiometer is likely typically used for electronic devices that measure the radio signal by amplifying it and making comparisons, using with analog electronics, an approach key to radio astronomy for considerable time. Modern equipment using other methods is often identified with other terms that have come to be associated with their mechanisms, e.g., bolometers.
The term radiometer is used for space probe instruments, for observation of Earth and other solar system bodies.