The expression of the intensity of light reflected by the internal surface of an integrating sphere with an input power provided by a concentrated solar beam is established using a model of multiple reflection of photons. This intensity appears to be proportional to the input power. and thus makes viable the utilization of a photo-sensor interfaced with an integrating sphere to build a solar fluxmeter. The major parameters of the design of the fluxmeter are then identified, and an optimized design is proposed, An example of a practical instrument is given, and its performance is measured and discussed. The sensitivity of the fluxmeter to the spectral distribution of the solar radiation requires a careful calibration of the gauge in order to achieve a measurement error less than +/-5% of reading. (C) 2002 Elsevier Science Ltd. All rights reserved.