A radiometer or roentgenometer is a device for measuring the radiant flux (power) of electromagnetic radiation. Generally, a radiometer is an infrared radiation detector or ultraviolet detector.
- The name Radiometer is frequently used to refer to a Crookes radiometer ("light-mill"). This is an early device wherein a rotor in a partial vacuum spins when exposed to light.
- The Nichols radiometer is a newer and more sensitive radiometer than the Crookes type. It measures radiation pressure. It has two small mirrors suspended on a quartz fiber.
- A microwave radiometer operates in the microwave wavelengths. The radiometer contains argon gas to enable it to rotate.
- The MEMS radiometer can operate on the same principles of Nichols or Crooke radiometers. It can operate over a wide spectrum of wavelength and particle energy levels.