Effects of subsurface scattering on the accuracy of optical 3D measurements using miniature polymer step gauges

We present a study on the effects of sub-surface scattering quantified by means of uni- and bidirectional distance measurements on miniature step gauges, which were manufactured in four different polymer materials (POM, PEEK, PPS and ABS), and calibrated using a coordinate measuring machine. Measurement were performed using an own developed structured light 3D scanner, which provides detailed knowledge of the scan process and allows access to raw data and unbiased evaluation of the sub-surface effects. Analysis was carried out with the CMM measurement strategy adapted to optical data in the software GOM Inspect. Results show bidirectional deviations (optical measurement compared to CMM) in the order of 100u.m to 800|im (material dependent) and consistently higher than corresponding unidirectional measurements, indicating a systematic error induced by the light-material interaction. We hypothesize that part of these effects can be accounted for, enabling optical measurements of a wider range of materials.