The Collaborative International Dictionary of English v.0.48:

Photochemistry \Pho`to*chem"is*try\, n. [Photo- + chemistry.]
   (Chem.)
   The branch of chemistry which relates to the effect of light
   in producing chemical changes, as in photography.
   [1913 Webster] Photochromic

WordNet (r) 3.0 (2006):

photochemistry
    n 1: branch of chemistry that deals with the chemical action of
         light