1. A device for updating a photometric model of a scene observed by a camera, the photometric model including at least one Gaussian model of a measurable physical magnitude in a pixel or in a block of pixels of an image supplied by the camera, the Gaussian model being defined by the mean and the variance of the physical magnitude, the device comprising: means for computing the mean based on the current value of the measured physical magnitude in the pixel or the block of pixels of the image, said means including a first summer mounted in a closed loop, and; means for measuring the difference between the mean and the current value of the physical magnitude, said means including a second summer, and; means for reducing the difference, said means including an automatic regulator; the first summer, the second summer and the automatic regulator being assembled in a closed-loop control of the first summer so as to update the model slowly in a period of stability of the observed scene and rapidly in a period of transition of the observed scene, so that the model remains true to the observed scene irrespective of the speed of fluctuation of the physical magnitude, the device being characterized in that the automatic regulator includes a PI or PD or PID regulator.