The calculation of the incident solar radiation falling upon tubular collectors presents difficulties which do not exist when this is carried out for flat plate collectors. Due to their cylindrical shape, the solar incident radiation on their surface is a function of angle and time. This paper presents experimental results which were obtained by means of a device specially designed in order to measure the distribution of such radiation. The device's sensor element (a photodiode) was characterised, calibrated and subsequently used to measure the angular distribution of radiation incident on the inside surface of the inner tube of an evacuated solar collector. It was found that the experimentally measured distribution agrees with that obtained by the ray-tracing method which appears in the literature. © 1995.