Single frame profilometry with rapid phase demodulation on colour-coded fringes

Abstract

Digital fringe profilometry is a non-contact surface profiling technique with huge potential at real-time dynamic whole-field measurement. However, this technique is usually bottlenecked at the phase demodulation and unwrapping during fringe analysis. This paper proposes a single frame profilometry system that used direct arccosine function demodulation on colour-coded sinusoidal fringes to simplify the fringe analysis process. Since the range of arccosine function output is restricted from 0 to π, the intensity gradient was used along with arccosine function to demodulate the fringe intensity levels into wrapped phase map (0–2π). The projected fringes were coloured in red, green and blue according to the De Bruijn's sequence. The fringe order was identified directly from the colours of three consecutive fringes by matching to the De Bruijn's sequence to unwrap the wrapped phase map into continuous phase map. The phase differences between the continuous phase maps of reference plane and object surface were then obtained and related to the equipment setup position using trigonometry to rebuild the 3D model. The proposed method was tested experimentally by reconstructing three physical objects. Although the reconstructed surface contained phase errors due to gamma non-linearity, the geometrical shapes of the objects can be reconstructed with reasonable accuracy and consistency. The percentage deviations of dimensions in x, y and z-axis were 1.24%, −1.96% and −2.30% respectively. Meanwhile the uncertainties of dimensions in x, y and z-axis were ±0.15%, ±0.24% and ±1.07% respectively at 95% confidence level.