code: def radianceSpectrum(Omegas,AbsorptionCoefficient,Environment={'l':100.,'T':296.},
                       File=None, Format='%e %e', Wavenumber=None):
      
      # compatibility with older versions
      if Wavenumber: Omegas=Wavenumber
      l = Environment['l']
      T = Environment['T']
      Alw = 1-exp(-AbsorptionCoefficient*l)
      LBBTw = 2*hh*cc**2*Omegas**3 / (exp(hh*cc*Omegas/(cBolts*T)) - 1) * 1.0E-7
      Xsect = Alw*LBBTw # W/sr/cm**2/cm**-1
      if File: save_to_file(File,Format,Omegas,Xsect)
      return Omegas,Xsect
docs: INPUT PARAMETERS: 
          Wavenumber/Omegas:   wavenumber grid                   (required)
          AbsorptionCoefficient:  absorption coefficient on grid (required)
          Environment:  dictionary containing path length in cm.
                        and temperature in Kelvin.
                        Default={'l':100.,'T':296.}
          File:         name of the output file                 (optional) 
          Format: c format used in file output, default '%e %e' (optional)
      OUTPUT PARAMETERS: 
          Wavenum: wavenumber grid
          Xsect:  radiance spectrum calculated on the grid
      ---
      DESCRIPTION:
          Calculate a radiance spectrum (in W/sr/cm^2/cm-1) based
          on previously calculated absorption coefficient.
          Radiance spectrum is calculated at an arbitrary
          optical path length 'l' (1 m by default) and 
          temperature 'T' (296 K by default). For obtaining a
          physically meaningful result 'T' must be the same 
          as a temperature which was used in absorption coefficient.
      ---
      EXAMPLE OF USAGE:
          nu,radi = radianceSpectrum(nu,coef)
      ---