The Collaborative International Dictionary of English v.0.48:

Bolometer \Bo*lom"e*ter\, n. [Gr. ? a stroke, ray + -meter.]
   (Physics)
   An instrument for measuring minute quantities of radiant
   heat, especially in different parts of the spectrum; --
   called also actinic balance, thermic balance. --S. P.
   Langley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

bolometer
    n 1: an instrument that measures heat radiation; extremely
         sensitive