The Collaborative International Dictionary of English v.0.48:

Photophone \Pho"to*phone\ (f[=o]"t[-o]*f[=o]n), n. [Photo- + Gr.
   fwnh` sound.] (Physics)
   An apparatus for the production of sound by the action of
   rays of light. --A. G. Bell.
   [1913 Webster]