The Collaborative International Dictionary of English v.0.48:

densitometer \densitometer\ n.
   1. an instrument for determining optical or photographic
      density. A variety capable of measuring optical density of
      a sample at a number of different wavelengths, is called a
      spectrophotometer.
      [WordNet 1.5]

   2. same as densimeter.
      [PJC]

WordNet (r) 3.0 (2006):

densitometer
    n 1: a measuring instrument for determining optical or
         photographic density
    2: a measuring instrument for determining density or specific
       gravity [syn: densimeter, densitometer]