def _reduced2physical_deflection(self, alpha_reduced, idex_lens):
        """
        alpha_reduced = D_ds/Ds alpha_physical

        :param alpha_reduced: reduced deflection angle
        :param z_lens: lens redshift
        :param z_source: source redshift
        :return: physical deflection angle
        """
        factor = self._reduced2physical_factor[idex_lens]
        #factor = self._cosmo_bkg.D_xy(0, z_source) / self._cosmo_bkg.D_xy(z_lens, z_source)
        return alpha_reduced * factor