The Collaborative International Dictionary of English v.0.48:

photocell \pho"to*cell`\ (f[=o]"t[-o]*s[e^]l`) n.
   An electrical device which changes voltage or resistance upon
   exposure to light, used to detect and measure light and other
   radiations.

   Syn: photoelectric cell, electric eye.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

photocell
    n 1: a transducer used to detect and measure light and other
         radiations [syn: photoelectric cell, photoconductive
         cell, photocell, electric eye, magic eye]