The spatial uniformity of polar cap absorption events has been exploited to test the performance of riometer systems. In an imaging riometer the use of a fixed conversion factor for obtaining the equivalent zenithal absorption from that measured with an oblique beam is verified for most of the beams, and revised factors are suggested. The readings from the corner beams are found to be unreliable, however. Factors for converting the readings from a wide-beam antenna to true zenithal values are derived. The significance of antenna sidelobes is pointed out, and the upper limits to the absorption measurement due to the temperature of the mesosphere and to uncertainty of riometer calibration at the lowest signal levels are demonstrated.