An image detector (10) of a patient monitoring system is used (S6-1) to obtain a set of calibration images of a calibration sheet (100) or other calibration object at various orientations and locations. These obtained calibration images are then stored and processed to calculate camera parameters defining the location and orientation of the image detector and parameters identifying internal characteristics of the image detector and the calculated parameters are stored (S6-2). Subsequently the patient monitoring system is re-calibrated by using (S6-3) the image detector (10) to obtain an additional image of a calibration sheet (100) or calibration object. The additional image and the stored camera parameters are then used to detect any apparent change in the internal characteristics of the image detector (10) (S6-4). If a change in the internal characteristics of an image detector (10) is detected, a further set of calibration images are obtained using the image detector (10) and the camera parameters for the image detector are recalculated using the further set of calibration images. If no change in the internal characteristics of an image detector is detected, the recalibration of the patient monitoring system is achieved by recalculating (s6-9) the camera parameters for the image detector using the stored set of calibration images and the additional image of the calibration object obtained by the image detector.Selon l'invention, un détecteur d'image (10) d'un système de surveillance de patient est utilisé (S6-1) pour obtenir un ensemble d'images d'étalonnage d'une feuille d'étalonnage (100) ou d'un autre objet d'étalonnage dans des orientations et emplacements divers. Ces images d'étalonnage obtenues sont ensuite stockées et traitées pour calculer des paramètres de caméra définissant l'emplacement et l'orientation du détecteur d'image et des paramètres identifiant les caractéristiques internes du détecteur d'image, et les paramètres calculés sont stockés (S6-2).
