The Collaborative International Dictionary of English v.0.48:

Sensitizer \Sen"si*ti`zer\, n. (Photog.)
   An agent that sensitizes.
   [1913 Webster]

         The sensitizer should be poured on the middle of the
         sheet.                                   --Wilis &
                                                  Clements (The
                                                  Platinotype).
   [1913 Webster]

WordNet (r) 3.0 (2006):

sensitizer
    n 1: (chemistry) a substance other than a catalyst that
         facilitates the start of a catalytic reaction [syn:
         sensitizer, sensitiser]