The Collaborative International Dictionary of English v.0.48:

Radiomicrometer \Ra`di*o*mi*crom"e*ter\
   (r[=a]`d[i^]*[-o]*m[-i]*kr[o^]m"[-e]*t[~e]r), n. [Radio- +
   micrometer.] (Physics)
   A very sensitive modification or application of the
   thermopile, used for indicating minute changes of radiant
   heat, or temperature.
   [1913 Webster]

WordNet (r) 3.0 (2006):

radiomicrometer
    n 1: radiometer that is extremely sensitive