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)
    ---

def radianceSpectrum(Omegas,AbsorptionCoefficient,Environment={'l':100.,'T':296.},
                     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.
                      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)
    ---
    """
    # 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