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[rey-dee-om-i-ter] /ˌreɪ diˈɒm ɪ tər/

Also called Crookes radiometer. an instrument for demonstrating the transformation of radiant energy into mechanical work, consisting of an exhausted glass vessel containing vanes that revolve about an axis when exposed to light.
an instrument for detecting and measuring small amounts of radiant energy.
any instrument for the detection or measurement of radiant energy

1877, from radiometer “instrument to transform radiant energy into mechanical work” (1875), radiometry, from radio-, here indicating “radiant energy,” + -metric. Radiometric dating attested from 1906.

radiometer ra·di·om·e·ter (rā’dē-ŏm’ĭ-tər)

ra’di·om’e·try (-trē) n.
A device used to detect or measure radiation. Radiometers generally consist of a glass bulb containing a rarefied gas in which four diamond-shaped paddles are mounted on a central axis. Each paddle is black on one side and silvery on the other. When radiation such as sunlight strikes them, the black side absorbs radiation and the silvery side reflects it, resulting in a temperature difference between the two sides and causing motion of gas molecules around the edges of the paddles. This motion of the surrounding gas molecules causes the paddles to spin. Precision radiometers, which use a complete vacuum rather than a gas, exploit the difference in radiation pressure on either side of the paddles to cause them to spin. Radiometers measure the intensity of radiation by measuring the rate of spin of the paddles. Also called light mill.


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