Source : Webster's Revised Unabridged Dictionary (1913)

Actinometer \Ac`ti*nom"e*ter\, n. [Gr. ?, ?, ray + -meter]
   (a) An instrument for measuring the direct heating power of
       the sun's rays.
   (b) An instrument for measuring the actinic effect of rays of
       light.

Source : WordNet®

actinometer
     n : an instrument for measuring the intensity of electromagnetic
         radiation (usually by the photochemical effect)