Consider a very simple measurement scheme, which, nevertheless, embodies all key features of a general position measurement. In the scheme shown in the Figure, a sequence of very short light pulses are used to monitor the displacement of a probe body M . The position x of M is probed periodically with time interval ϑ ϑ . We assume mass M large enough to neglect the displacement inflicted by the pulses regular (classical) radiation pressure in the course of measurement process.