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An image output by a camera is generally not a faithful representation of the real scene, because it undergoes a series of radiometric disturbances during the imaging process. This study proposes a method for obtaining a more accurate measure of the light seen by a camera. The proposed method requires no specific calibration apparatus and only minimal supervision. Nevertheless, it is quite comprehensive, since it accounts for response function, exposure, vignetting, spatial non-uniformity of the sensor and colour balancing. This method works in two steps. First, the camera is calibrated off-line, in a photoquantity sense. Then, the photoquantity of any scene can be estimated in-line.The method is therefore geared to a wide range of computer vision applications where a camera is expected to give a measurement of the visible light. This study starts by presenting a photoquantity model of the camera-imaging process. It then describes the key steps of calibration and correction method. Finally, the results are given and analysed to evaluate the relevance of this approach.