The Collaborative International Dictionary of English v.0.48:

Polarimeter \Po`lar*im"e*ter\, n. [Polar + -meter.] (Opt.)
   An instrument for determining the amount of polarization of
   light, or the proportion of polarized light, in a partially
   polarized ray.
   [1913 Webster]

WordNet (r) 3.0 (2006):

polarimeter
    n 1: an optical device used to measure the rotation of the plane
         of vibration of polarized light [syn: polarimeter,
         polariscope]