Source : Webster's Revised Unabridged Dictionary (1913)

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.

Source : WordNet®

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}]