A new operator to compute time differentiation in an image sequence is presented. It is founded on hybrid filters combining morphological and linear recur-sive operations. It estimates recursively the amplitude of time-variation within a certain interval. It combines the change detection capability of the temporal morphological gradient, and the (exponential) smoothing effect of the linear re-cursive average. It is particularly suited to small and low amplitude motion. We show how to use this filter within an adaptive motion detection algorithm.