Post-calibration dependence of the sensitivity of LOFAR on source elevation, measured during the observation of 4 pulsars (PSRs B0834+06, B1929+10, B1953+50 and B2217+47) at various hour angles, with the LOFAR core. Here, the sensitivity is expressed as the rms of the off-pulse flux density, which has been normalised to 0.5 mJy at elevation of θ = 90°; this flux density corresponds to the theoretical sensitivity of the instrument, assuming the parameters of our observations (see text). The best fit of a power law on 1 / sinθ to the data, as is expected by a simple signal projection, is shown with a solid curve. For comparison, we show the 1 / sinθ function (dotted curve), normalised in the same way, which is the expected theoretical dependence from Lambert’s law of a simple signal projection of unpolarised emission. The expected theoretical dependence of fully polarised emission, assuming an ideal dipole antenna follows Malus’s law, 1 / sin²θ (dashed curve).