Displacement Measurement System This device measures the linear displacement of a pair of position sensitive detectors.  A laser beam is split and sent to the two detectors.  Each detector puts out a voltage that corresponds to where the laser strikes it.  The voltages are scaled and summed in such a way as to eliminate any small angular differences between the two laser beams.  The movement of the sensors is shown on the LED display with a resolution of 10 um.  The bar graphs allow for quick alignment of the laser during installation.  This device is being used to measure tunnel wall movement at Yucca Mountain.