For larger optics or optical system metrology, a laser tracker can provide sparse sampling points across the test surface or structures, which can allow for low-spatial-frequency surface metrology . The technique uses a laser tracker, a device that has two angular encoders and a distance measuring interferometer which measure the position of a spherically mounted retroreflector (SMR) in 3D space by aiming a laser beam at the reflector (i.e., corner cube centered in a precision spherical ball) and measuring the return signal. The distance-measuring interferometer provide accurate distance metrology, and new improvements, such as using a mode-locked laser, have further improved the measurement accuracy .

A laser tracker alone is sometimes not enough to measure the surface shape of a test optic under certain vibrations or fluctuations since the point-by-point surface scanning process takes time to finish the measurement. Four (or more) independent distance-measuring interferometers (DMIs) are positioned to measure retroreflectors at the edge of the test surface continuously. In doing so, the rigid body motion (i.e., 6 degrees of freedom) is well defined for the object under test for each sampling point measurement, and air refractive index variations can also be accounted for. The independent DMI measurements additionally serve to calibrate the laser tracker measurement.

In the actual data collection of the test surface, a SMR can be positioned at known points on the test surface. Ensuring dense enough sampling is taken across the full aperture is essential to guarantee an accurate full-aperture measurement is achieved without missing any key mid- to high-spatial frequency features.

Once all data points are measured, they can be combined and fitted modally to form a surface map of the surface under test. This technique has been used on the first 8.4-m off-axis segment of the Giant Magellan Telescope and was able to provide independent corroboration of low- to mid-spatial-frequency metrology results for the surface with an accuracy exceeding 1-mm RMS.

Laser tracker measurements, depending on the situation, can often require two people to achieve. One to control the laser tracker, which can be many meters away from the unit under test, and the other to carefully place the SMR and provide instructions for where to point the laser. Prior to starting any measurement, try to plan how many sample points are required for the surface shape frequencies you are trying to measure, and once you know the required sampling, take care where you choose to measure across the optical surface. Various laser trackers can be guided by automated programming, which can make acquisition of the data easier and more repeatable. If the surface is to be measured several times, or over the course of its fabrication life, it may well be worth spending the time creating an automated metrology process sequence.