The measurement of very high solar irradiance is required in an increasingly wide variety of technical applications. There is really only one commercial supplier of heat flux sensors that can be used for this purpose. These gages are calibrated using a black body as the radiant source. A systematic error has been detected when these heat flux sensors are later used to measure the irradiance from concentrated solar radiation; in this case, the calibration constants obtained for these gages in the laboratory must be systematically corrected. (C) 2003 Elsevier B.V. All rights reserved.