Dimensional accuracy and repeatability of the NextEngine laser scanner for use in osteology and forensic anthropology

Abstract

The increasing accessibility of low-cost, desktop scanners has provided researchers with the means to acquire 3D geometric data that can be used to create digital archives of archaeological artifacts, such as bones, for geometric morphometric study in the fields of osteology and forensic anthropology. However, the process of digitizing and assessing in silico models is not straight forward. In this two-part study, we first compared the performance of the 2010 NextEngine model 2020i laser scanner (NE) and Hewlett-Packard Pro S3 structured light scanner (HP) to that of the Keyence VHX 6000 digital microscope (VHX), using a fabricated test block containing bas-relief features with offsets ranging from 3 mm down to 0.1 mm. The xyz coordinate data from the in silico models were also compared to xyz coordinate data obtained using a Microscribe MX 3D coordinate measurement machine (CMM), which showed no evidence of geometric distortion in the in silico models. We applied Bland-Altman analysis to quantify the coefficient of repeatability (CoR) for each instrument used in the study, and to determine agreement between the scanners and the VHX. Dimensional accuracy was assessed by comparing bas-relief offset dimensions on the in silico models derived from the scanners to the VHX measurements of the same features. Second, as an application to osteology we examined intra-observer scanning protocol variability, using the coefficient of variation (CV) to quantify surface area and volume variance between repeated scans of eight porcine capital femoral epiphyses with undulating mammillary processes on one surface with amplitudes covering the range of the test block bas-relief offset values. The CoR showed each test-retest measurement from each instrument differed by no more than their CoR: 0.010 mm, 0.137 mm, 0.068 mm, 0.193 mm for the VHX, NE, HP and CMM, respectively. There was agreement between the instruments, but each instrument (NE, HP and CMM) overestimated bas-relief features as reported by the VHX, on average (bias) by 0.046 ± 0.038 mm, 0.025 ± 0.033 mm, 0.026 ± 0.033 mm for the NE, HP and CMM, respectively. Both scanners captured surface features as small as 0.1 mm. For the intra-observer scanning protocol of the epiphyseal bones using the NE, the coefficient of variation (mean CV ± SD) was 0.69 ± 0.25% for the bone surface area and 0.77 ± 0.26% for bone volume. The scanner appears suitable for capturing bas-relief features in the range of 3 to 0.1 mm, while preserving the ratios of distances between measured points within the spatial volume surveyed. Examples of potential applications include scanning bone or pottery fragments for virtual reconstruction, where capturing dimensional detail and areas is important, reproducing the fine surface features of pottery, figurines, etc. for cultural preservation and for examination by scientists who cannot study the objects directly, as well as to provide access to the public by electronic means. The lower cost of these scanners may provide museums with small budgets with a means to begin to archive some items in their collection.