def GM(L_850, L_1660):
    """
        Goddijn-Murphy Index ====================================================
            From: "Proof of concept for a model of light reflectance of plastics
            floating on natural waters" by Lonneke Goddijn-Murphy and Juvenal
            Dufaur. - See Table 5

            Convert numpy arrays of radiance data to a Goddijn-Murphy map

        Inputs =================================================================
            L_850 : At-sensor radiance at 850 nm wavelength (numpy array)
            L_1660 : At-sensor radiance at 1660 nm wavelength (numpy array)

        Returns ================================================================
            Goddijn-Murphy Map
    """
    return L_850 - L_1660
