def get_rad_factor(rad, emission, solar_irr, emissivity=None):
    """
    Return radiance factor (I/F) given observed radiance, modeled emission,
    solar irradiance and emissivity. If no emissivity is supplied,
    assume Kirchoff's Law (eq. 6, Bandfield et al., 2018).

    Parameters
    ----------
    rad (num or arr): Observed radiance [W m^-2 um^-1]
    emission (num or arr): Emission to remove from rad [W m^-2 um^-1]
    solar_irr (num or arr): Solar irradiance [W m^-2 um^-1]
    emissivity (num or arr): Emissivity (if None, assume Kirchoff)
    """
    if emissivity is None:
        # Assume Kirchoff's Law to compute emissivity
        emissivity = (rad - solar_irr) / (emission - solar_irr)
    return (rad - emissivity * emission) / solar_irr