Longman Dictionary of Contemporary English
The Collaborative International Dictionary
Photo- \Pho"to-\ A combining form from Gr. fw^s, fwto`s, light; as, photography, phototype, photometer.
Douglas Harper's Etymology Dictionary
word-forming element meaning "light" or "photographic" or "photoelectric," from Greek photo-, comb. form of phos (genitive photos) "light," from PIE root *bha- (1) "to shine" (see phantasm).
pre. 1 light, electromagnetic radiation 2 photography