Source : Webster's Revised Unabridged Dictionary (1913)

Actinism \Ac"tin*ism\, n. [Gr. ?, ? ray.]
   The property of radiant energy (found chiefly in solar or
   electric light) by which chemical changes are produced, as in
   photography.

Source : WordNet®

actinism
     n : the property of radiation that enables it to produce
         photochemical effects