Analysis of measurement sensitivity to illuminance and fringe-pattern gray levels for fringe-pattern projection adaptive to ambient lighting

Fringe-projection measurement systems are susceptible to image saturation and measurement error if ambient illuminance is increased during measurement after earlier calibration. This paper presents analyses of fringe-projection measurement-accuracy sensitivity to object illuminance and fringe-pattern gray levels for an adaptive approach to fringe-pattern projection. Calibrations of a measurement system with no ambient light using eight different maximum fringe-pattern gray levels between 175 and 255 were performed utilizing the two-step triangular-pattern phase-shifting method. Measurements using the same patterns and corresponding calibration parameters performed with 0 and 300 Ix ambient illuminance had mean RMS errors over depth below 0.5 and 0.7 mm, respectively, for maximum gray levels ranging from 195 to 255. The optimal tradeoff of lower maximum gray level to tolerate ambient lighting and avoid image saturation and higher maximum gray level for greater robustness to image noise occurred at 215, where the minimum error was 0.44 mm. In an adaptive fringe-pattern projection approach, the maximum gray level would be adjusted based on images acquired during measurement, and calibration parameters determined in earlier calibrations would be utilized. The approach is applicable to single or multiple-step measurement methods. (C) 2009 Elsevier Ltd. All rights reserved.