A radiometer's characteristics are:
Radiometers detect and measure radiation as thermal (convert absorbed energy to a signal) and photodiode (photons absorbed at a constant response/quantum). The radiometer's radiation-detecting bolometer absorbs radiation falling upon it, raising its temperature, then is measured with a thermometer. The higher temperature might be related to the incident radiation's power.
The Crookes radiometer is an early-model infrared-radiation and light-detector. A variant type of is the Nichols radiometer that operates on a different principle, and is more sensitive than the Crookes type.
A Microwave radiometer operates in the Microwave wavelengths. The radiometer contains argon gas to enable it to rotate. Keely said that when you vibrate an atomic substance (e.g., argon) with an atomic vibration (e.g., microwave) you get rotation. When you put a radiometer containing an atomic elemental gas, such as argon, into a microwave, it causes it to spin, thus the microwave interaction with the argon gas is creating rotational raleigh waves.
As an eponym, radiometer usually denotes a Crookes radiometer, a device wherein a rotor (with dark and light vanes) spins when exposed to light in a partial vacuum.
Microwave Radiometer Calibration on Decadal Time Scales Using On-Earth Brightness Temperature References: Application to the TOPEX Microwave Radiometer
Dec 01, 2009; ABSTRACT A method is described to calibrate a satellite microwave radiometer operating near 18-37 GHz on decadal time...