Radiometer

A radiometer is a detector for measuring the irradiance. The term radiometer is not only used for a functional principle.

Invented by William Crookes Radiometer - a rotating impeller with lighting - does not measure the mechanical radiation pressure of light, but is driven by thermal molecular motion. The interior of the glass bulb, in which the impeller rotates, is substantially evacuated to a residual gas pressure of one to ten Pascal. The gas molecules are heated in the dark surface of the impeller and transferred a part of its momentum to the wheel, which consequently displaced in rotation. This effect is referred to in the literature as " radiometers " effect.

In contrast, a similar, about the same time incurred, but much more sensitive construction of Ernest Fox Nichols actually measures directly the mechanical radiation pressure of light. The same applies to a developed around this time experiment by Pyotr Nikolaevich Lebedev. In these experiments, the space around the impeller must be evacuated much stronger and the mechanical suspension must be very low friction.

Using a laser, mechanical radiation pressure effects in the laboratory are easily demonstrable.