Influence of ambient light conditions on the accuracy of 3-dimensional optical scanner for face scanning

Abstract

Most 3D scanners use optical technology that is impacted by lighting conditions, especially in triangulation with structured-light or laser techniques. However, the effect of ambient lights on the accuracy of the face scans remains unclear. The purpose of this study is to investigate the effect of ambient lights on the accuracy of the face scans obtained from the face scanner (Einscan Pro 2X Plus, Shining 3D Tech. Co., Ltd. Hangzhou, China). A head model was designed in Rhinoceros 5 software (Rhino, Robert McNeel and Associates for Windows, Washington DC, USA) and printed with 200 microns resolution of polylactic acid and was dented with 2.0 mm of carbide bur to aid superimposition in software. The head model was measured by a coordinate measuring machine (CMM) to generate a reference stereolithography (STL) file as a control. The face model was scanned four times under nine-light conditions: cool white (CW), warm white (WW), daylight (DL), natural light (NL), and illuminant (9w, 18w, 22w). Scan data were exported into an STL file. The scan STL files obtained were compared with the reference STL file by 3D inspection software (Geomagic Control X version 17, Geomagic, Morrisville, NC, USA). To study the deviations, root mean square errors (RMSE) were selected for statistical analysis between the reference model (trueness) and within the group (precision). The statistical analysis was done using SPSS 20.0 (IBM Company, Chicago, USA). One-way ANOVA and post hoc with Tukey were done to analyze the results. For the trueness, the scanner showed the lowest RMSE under the NL group (77.18 ± 3.22) and the highest RMSE under the 18w-DL group (95.33 ± 6.89). There was a statistically significant between the NL group and the 18w-DL group (p<0.05) for the trueness. Similarly, for the precision, the scanner showed the lowest RMSE under the NL group (56.92 ± 4.56) and the highest RMSE under the 9w-CW group (78.52 ± 10.61). There was statistically significant between NL, 18w-WW, 18w-CW, 18w-DL, 22w-WW, 22w-DL group, 9w-CW, 9w-WW, and 9w-DL group (p<0.05) for the precision. Ambient lights affected the face scans. Under the natural light condition, the face scanner performed the best accuracy in terms of both trueness and precision. Contradictory, under 18w-DL and 9w-WW, 9w-CW, and 9w-DL conditions showed the least accuracy of trueness and precision, respectively.