The Collaborative International Dictionary of English v.0.48:

Photometer \Pho*tom"e*ter\, n. [Photo- + -meter: cf. F.
   photom[`e]tre.] (Physics)
   An instrument for measuring the intensity of light, or, more
   especially, for comparing the relative intensities of
   different lights, or their relative illuminating power.
   [1913 Webster] Photometric

WordNet (r) 3.0 (2006):

photometer
    n 1: measuring instrument for measuring the luminous intensity
         of a source by comparing it (visually or photoelectrically)
         with a standard source
    2: photographic equipment that measures the intensity of light
       [syn: light meter, exposure meter, photometer]