text: Let  be the effective absorption cross-section  be effective emission cross-sections at frequency , and let  be the effective temperature of the medium. The McCumber relation is
  where  
  is thermal steady-state ratio of populations; frequency  is called "zero-line" frequency;
   is the Planck constant and 
   is the Boltzmann constant. Note that the right-hand side of Equation (1) does not depend on .