docs: def transmittanceSpectrum(Omegas,AbsorptionCoefficient,Environment={'l':100.},
                            File=None, Format='%e %e', Wavenumber=None):
      """
      INPUT PARAMETERS: 
          Wavenumber/Omegas:   wavenumber grid                    (required)
          AbsorptionCoefficient:  absorption coefficient on grid  (required)
          Environment:  dictionary containing path length in cm.
                        Default={'l':100.}
          File:         name of the output file                 (optional) 
          Format: c format used in file output, default '%e %e' (optional)
      OUTPUT PARAMETERS: 
          Wavenum: wavenumber grid
          Xsect:  transmittance spectrum calculated on the grid
      ---
      DESCRIPTION:
          Calculate a transmittance spectrum (dimensionless) based
          on previously calculated absorption coefficient.
          Transmittance spectrum is calculated at an arbitrary
          optical path length 'l' (1 m by default)
      ---
      EXAMPLE OF USAGE:
          nu,trans = transmittanceSpectrum(nu,coef)
      ---
      """
      # compatibility with older versions
      if Wavenumber: Omegas=Wavenumber
      l = Environment['l']
      Xsect = exp(-AbsorptionCoefficient*l)
      if File: save_to_file(File,Format,Omegas,Xsect)
      return Omegas,Xsect