An intensity image is collected at each of a plurality of locations spaced apart in a propagation direction of a light beam. Information from the intensity images is combined using a Kalman filter which assumes that at least one co-variance matrix has a diagonal form. This leads to considerable reduction in computational complexity. An augmented Kalman filter model (augmented space state model) is used in place of the standard Kalman filter model. The augmented Kalman filter improves the robustness to noise.