Laser trackers work by measuring the radial distance from the tracker to the target and the zenith and azimuth angles.
The angles are measured by angular encoders and the radial distance measured with either an interferometer (IFM) or an absolute distance meter (ADM) or a combination of both.
Historically laser trackers started out using IFM, but as improvements in ADM technology occurred, many systems became ADM only and now only a small number of systems have both IFM and ADM in one device.
IFMs work by splitting the laser beam into two parts: one beam travels directly to the interferometer, while the other travels to a target device in contact with the component being measured.
The two lasers interfere with one another inside the interferometer (hence the name), resulting in a cyclic change, each time the target device changes its distance from the tracker.
ADMs use infrared light from a semiconductor laser which bounces off the target device and re-enters the tracker. The infrared light is then converted into an electrical signal for time-of-flight analysis.