  The problem of conversion of experimentally measured luminescence spectrum
into the absorption cross section is revisited. The common practice of using
the van Roosbroeck-Shockley (or Kubo-Martin-Schwinger or Kennard-Stepanov)
relation in this context is incorrect because luminescence from semiconductors
is essentially all due to the spontaneous emission component of the
recombination of carriers distributed far-from-equilibrium. A simple,
physically consistent, and practical prescription for converting the
luminescence spectra into absorption is presented and its relation to the
so-called nonequilibrium generalization of the van Roosbroeck-Shockley
relationship is discussed.
