Abstract We measured the complex dielectric function from 30 to 680 cm −1 along the c axis of a large, high quality single crystal of La 1.87Sr 0.13CuO 4 at temperatures between 6 and 300K with a far-infrared Fourier-ellipsometer. We obtain the complex dielectric function directly, without Kramers-Kronig analysis. In agreement with findings of Tamasaku et al. [PRL 69, 1455 (1989)], our data indicate an almost complete blockade of c axis single-particle transport. We present accurate optical phonon data and discuss upper limits of the low temperature far-infrared c axis ac conductivity. We find an anomalously large strength of the 235 cm −1 optical phonon and discuss its possible origin.