A photometric model includes at least one Gaussian model of a measurable physical magnitude in an image supplied by the camera and it is defined by the mean and the variance of the physical magnitude. A device comprises: means for computing the mean based on the current value of the physical magnitude, these means including a first summer mounted in a closed loop; means for measuring the difference between the mean and the current value of the physical magnitude, these means including a second summer; means for reducing the difference, these means including an automatic regulator. The first summer, the second summer and the automatic regulator are 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. Application: video surveillance, background subtraction.