A system which automatically reduces change in effective angle and plane of incidence of a reflected focused beam of electromagnetic radiation entering a detector, via use of a detector with dimensions less than is the spatial spread of a reflected focused beam at a location distal to the location on said sample from which it is caused to reflect, preferably after passing through a collimating lens. The basis of operation is that the portion of a reflected focused beam intercepted by the detector changes with change in sample position and/or orientation.