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)
    ---

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