Calibration Methods for 3D Measurement Systems

This paper was published in Proceeding of SPIE Medical Imaging Conference and is made available as an electronic reprint with permission of SPIE. One copy or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. With the improving capabilities and acceptance of laser triangulation based systems, the natural extension of these systems to 3D measurement tools is gradually coming about. As with any metrology tool, calibration, including the ability to verify calibration in the field is becoming an important issue to potential users. There is currently no standard method of doing these calibrations. The methods used in Coordinate Measurement Machines, such as looking at polished spheres, were developed to account for some of the limitations of probing capabilities of the time such as touch probe lobing errors and machine axis squareness. Optical probes do not work the same as mechanical touch probes, and as such have new problems as well as capabilities. The ability to establish a calibration over an entire field at one time, using tens to hundreds of thousands of points rather than tens to a hundred points is both viable and necessary to effective 3D system calibration. This paper will discuss the basic calibration problems particular to triangulation based 3D systems that need to be addressed. The issues to be addressed include both optical aberrations such as field curvature and distortion as well as the effects of light distribution and surface reflectivity. Finally, we will describe and contrast a number of potential methods for performing effective full-field calibrations and discuss the pros and cons of each approach. Experimental results will be presented to illustrate these comparison points.