Source : Webster's Revised Unabridged Dictionary (1913)

Radiometer \Ra`di*om"e*ter\, n. [L. radius radius + -meter: cf.
   F. radiom[`e]tre.]
   1. (Naut.) A forestaff.

   2. (Physics) An instrument designed for measuring the
      mechanical effect of radiant energy.

   Note: It consists of a number of light discs, blackened on
         one side, placed at the ends of extended arms,
         supported on an a pivot in an exhausted glass vessel.
         When exposed to rays of light or heat, the arms rotate.

Source : WordNet®

radiometer
     n : meter to detect and measure radiant energy (electromagnetic
         or acoustic)