Real-time full-field range-sensing for three-dimensional surface-shape measurement

Abstract

Three-dimensional (3-D) surface-geometry measurement using structured light is commonly used for digital modeling of an object or environment when the geometry is unknown. Digital fringe projection with phase-shifting has become an increasingly common measurement method as it is a full-field method to improve acquisition speed over point-by-point and line-by-line methods, and it encodes all lines in the pattern to reduce the ambiguity problem that occurs with multiple-line projection methods. However, fringe-projection phase-shifting techniques are still limited in the speed of measurement, partly due to the use of projected light patterns that require complex computation, or that require several steps of repeated measurements with several images in order to reconstruct the 3-D object surface. This doctoral dissertation addresses the development of new methods for real-time full-field range sensing by fringe-projection phase-shifting techniques, to use more efficient algorithms. It also addresses the design, calibration, and measurement-accuracy testing of a new full-field real-time range-sensing system based on the developed methods. A novel 3-D shape measurement technique, called triangular-pattern phase-shifting profilometry, is developed in this research. The proposed method utilizes triangular gray-level-coded fringe patterns and newly developed intensity-ratio generation algorithms, essential to reconstruct a 3-D object surface. The minimum number of measurements required to reconstruct an unknown 3-D object is two, which is less than the minimum number of measurements required for the traditional sinusoidal-pattern and trapezoidal-pattern phase-shifting methods. Compared with the sinusoidal-pattern and trapezoidal-pattern phase-shifting methods, the new method involves less processing because of the simple intensity-ratio computation used and because fewer images or measurement steps are required to reconstruct the 3-D object. Extension of the method by increasing the number of phase-shifting steps was found to increase measurement accuracy. An optimal value of pitch of the projected triangular fringe pattern was found for each extended method. Two error compensation methods, repeated phase-offset and intensity-ratio error compensation, are proposed to reduce the measurement error, mainly caused by fringe projection non-linearity and image defocus. Error reductions of 24.0% and 28.5% were achieved, respectively. An off-line 3-D shape measurement system and a real-time 3-D shape measurement system have been developed with the proposed triangular-pattern phase-shifting method.