The Collaborative International Dictionary of English v.0.48:

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.
       [1913 Webster]

WordNet (r) 3.0 (2006):

actinometer
    n 1: an instrument for measuring the intensity of
         electromagnetic radiation (usually by the photochemical
         effect)