Introduction The measurements that an Agilent laser measurement system can make depend on the measurement optics (interferometers and retroreflectors) that are used. The basic measurement made by all Agilent laser measurement systems is a linear measurement of the relative movement between an interferometer and its associated retroreflector, along the path of the laser beam. In most cases, the interferometer is the fixed optic and the retroreflector is the one that moves.