def calculate_ldf_value(self, r, phi, n_electrons=None, zenith=None,
                            azimuth=None):
        """Calculate the LDF value for a given core distance and polar angle

        :param r: core distance in m.
        :param phi: polar angle in rad.
        :param n_electrons: number of electrons in the shower.
        :return: particle density in m ** -2.

        """
        if n_electrons is None:
            n_electrons = self._n_electrons
        if zenith is None:
            zenith = self._zenith
        if azimuth is None:
            azimuth = self._azimuth
        return self.ldf_value(r, phi, n_electrons, zenith, azimuth, self._s1,
                              self._s2)