This paper presents a brief description of a direct heat flux measurement system to measure the concentrated solar power delivered by a heliostat field onto the flat aperture of solar central receiver prototypes. The main advantages of this device are the low measurement uncertainty and the non-requirement of water-cooling. This system has been designed, mounted and used successfully on top of the SSPS-CRS tower at the Plataforma Solar de Almeria (PSA). The concentrated solar power and the spatial heat flux distribution, onto the aperture of a volumetric receiver prototype under evaluation, have been determined by the above-mentioned system. (C) 2002 Elsevier Science Ltd. All rights reserved.