Radiometer

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A radiometer is a device used to measure the radiant flux or power in electromagnetic radiation. Although the term is perhaps most generally applied to a device which measures infrared radiation, it can also be applied to detectors operating any wavelength in the electromagnetic spectrum; a spectrum-measuring radiometer is also called a spectroradiometer.

Whenever describing a radiometer, the most important characteristics are:

  • spectral range (what wavelengths)
  • spectral sensitivity (what sensitivity versus wavelength)
  • field of view (180 degrees or limited to a certain narrow field)
  • directional response (typically cosine response or uni-directional response)

Radiometers can use all kinds of detectors; some are " thermal" that is absorbing energy and converting that to a signal, some sense photons (photodiode) having a constant response per quantum (light particle). In a common application, the radiation detector within a radiometer is a bolometer which absorbs the radiation falling on it and, as a result, rises in temperature. This rise can then be measured by a thermometer of some type. This temperature rise can be related to the power in the incident radiation.

An early detector of infrared and visible radiation (light) was the Crookes radiometer. A more sensitive device, employing a different principle, is the Nichols radiometer.

A Microwave radiometer operates in the Microwave region of the electromagnetic spectrum.

The term radiometer is occasionally used as shorthand for a Crookes radiometer, a device in which a rotor with dark and light vanes in a partial vacuum spins when exposed to light.

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