The Collaborative International Dictionary of English v.0.48:

densitometry \densitometry\ n.
   measuring the optical density of a substance by shining light
   through it and measuring the intensity of the transmitted
   light.
   [WordNet 1.5]

WordNet (r) 3.0 (2006):

densitometry
    n 1: measuring the optical density of a substance by shining
         light on it and measuring its transmission