A new measuring system is proposed that can measure the motions of arbitrary rigid bodies moving in 6 degrees of freedom. The measurement principle is based on the detection of laser beams reflected from a specially fabricated three-facet mirror that looks like a triangular pyramid having an equilateral cross-sectional shape. The mirror is mounted on the object to be measured. It reflects a laser beam, generated from a source, in three different directions, depending on the motion of the object of interest. The reflected beam is then detected by three position-sensitive detectors (PSDs). From the signals of the PSDs, we can calculate the 3-D position and orientation of the three-facet mirror, thus enabling us to determine the 3-D position and orientation of the object. We model the relationship between the 3-D position and orientation of an object in motion and the outputs of three PSDs. A series of experiments are performed to demonstrate the effectiveness and accuracy of the proposed method. The experimental results show that the proposed sensing system can be an effective means of obtaining 3-D position and orientation of arbitrary objects and provides reasonable measurement accuracy. © 2002 Society of Photo-Optical Instrumentation Engineers.