Measurements of the intensity of the light režected and transmitted by a sample have been sources of information concerning the structure of matter for over a century. In recent decades, it has been found that measurement of the position of an optical beam that has scattered from a sample is an important and versatile means of characterizing materials and the motion of devices. Surely, the availability of a well-collimated beam from a laser has been crucial in the development of techniques and applications of optical beam de¦ection (OBD) sensing; however, the development and ready availability of various types of position-sensing detectors (PSDs) have also been important factors. OBD may be caused, for example, by propagation of a laser beam through a refractive-index gradient or by režection from a displaced surface. A PSD provides an electronic signal that is a function of the laser beam position on the detector.