Understanding Laser Tracker Targets
Carson Room 3 and 4, enter at Carson 3
Ken Steffey - Representing FARO
The quality of laser tracker measurements is reliant upon the precision of the target. This paper will explain in detail the types of laser tracker targets available and will allow the reader to understand when certain targets should be used, the critical properties of laser tracker targets and the impact they can have on their measurements.
First, the paper will establish how laser trackers measure distance (Interferometer and Absolute Distance Meter) and how the target’s characteristics can impact the measurements. Three types of targets (along with how they are constructed) will be covered including spherically mounted retroreflectors (SMRs), cateyes, and repeatability targets, with the most attention given to the open air SMR.
For solid glass optics, the impact of coatings to reduce ADM reflections and the impact of the glass index of refraction on the acceptance angle of the target will be explained. For open air retroreflectors, the advantages and disadvantages of the two surface types (protected silver and gold) as well as of the assembly variations (glass panel, single optic, and integrated optic) will be discussed.
It is very common for SMR manufacturers to place special emphasis on the centering of the optics in their specifications. Some users may not be aware of the impact that other SMR specifications such as sphere properties, dihedral angle error, maximum dihedral angle difference, wavefront distortion, and polarization can have on measurement performance. While all of these properties can impact the quality of measurement results, the dihedral angle error, wavefront distortion, and polarization can even more adversely affect the laser tracker’s performance (how well it tracks or measures the radial distance). Each of these properties will be discussed in detail to educate the reader on the important aspects of selecting a proper SMR based on application, environment and measurement goals. The paper will also discuss testing of the open air SMRs’ performance using interferograms (dihedral angles, wavefront distortion), beam profilers (beam shape), and laser power meters (reflectance of different wavelengths, polarization).
A real-world example will be given to illustrate the impact of different adhesives and their properties (including modulus elasticity) when mounting optics into the SMR. When making a break resistant target, you must balance the properties of the adhesive being too brittle while still being stiff enough to hold the optics in the nominal position.
Finally, the reader will learn how to determine if they have a good SMR, what kinds of errors and issues that a bad SMR can have on their measurement job (and how they can tell), as well as how to select the proper target for the job. They will receive SMR best practices, on how to obtain the best accuracy with the measurement system, along with proper cleaning procedures for the different types of targets.
