Measurement of polymers with 3D optical scanners: Evaluation of the subsurface scattering effect through five miniature step gauges

In this work, the interaction between 3D optical scanners and objects realized with different materials and colours was investigated. The interaction is mainly related to the translucency of some materials, such as polymers, which is a category of materials in ever-increasing demand by industry. At the same time, the increase of the geometrical complexity of manufactured parts raises the need for the use of 3D optical scanners thanks to their rapidity and effectiveness. Translucency causes a subsurface scattering effect that has been modelled for certain cases and affects the dimensional accuracy of measurements, as well as the quality of reconstruction. Recently, an investigation considered its effect in dimensional verification using a structured light scanner with fringe projection. In this work, the investigation is extended to other 3D optical scanners exploiting different measuring principles: laser, structured light based on phase shifting and gray code, and photogrammetry. This was done in order to study how the subsurface scattering effect can vary with the materials, colours, and also the measuring principle used. To this end, five miniature step gauges made of different polymers and colours were used, and the effects due to their different optical properties investigated. The analysis focused on dimensional verification of bidirectional lengths, first calibrated with a traceable coordinate measuring machine. The quality of the reconstruction was also evaluated through the flatness values. According to the results obtained, translucency of materials has a dual and significant effect: a measurement bias and an increase of the uncertainty due to the different quality of reconstruction retrieved. Thus, it has to be considered properly for each case.