Improving calibration accuracy of structured light systems using plane- based residual error compensation

The performance of a structured light system (SLS) highly depends on its calibration especially in applications such as surface met-rology and product quality control where high accuracy is required. Motivated by building a real-time highly accurate flatness measurement system, we propose a plane-based residual error compensation algorithm for improving the calibration accuracy of SLSs. Following the highly accurate procedure of geometric calibration using circular control points, the proposed algorithm enforces the planar constraint on the three-dimensional reconstruction of the circular control points, which are projected on a perfect plane, to further reduce the residual calibration error. Our method compensates for the largest proportion of the residual error, in most cases being the model error of the lens distortion in the system. Experimental results show that the compensation elevates the calibration accuracy to a higher level—the planarity error is reduced by 70% and this accuracy is comparable to a well-reputed industrial mechanical measuring machine. 1 Introduction In industrial production, it is often necessary to examine the planarity of mechanical pieces for quality control. The traditional way of examining a batch of products consists of random batch sampling and testing samples by coordinate measuring machines (CMM). The drawback of this procedure is twofold: on one hand, random sampling does not guarantee the qualification of every piece in the batch; on the other hand, touch-trigger probes in measuring machines make the measurement process extremely slow. Therefore, motivated by building a real-time in-progress flatness measurement system for full quality control, we turned to the active stereo vision system, the structured light system (SLS). SLSs have been substantially used in high-precision three-dimensional (3-D) measurements, 1–3 such as shape measurement of turbine blades and precise planarity measurement , etc. A camera and a projector are two indispensable components of such systems. The projector projects some user-defined pattern onto the object in the field of view and the camera captures the object-distorted pattern, which reveals the texture and depth information of the object. According to the mechanism, the pattern is projected and the SLS can be classified into two broad categories: single shot and multiple-shot. The projection and acquisition of patterns in the single-shot SLS can happen in a fraction of a second, making this kind of SLS very suitable for performing on-line tasks. However, although being faster than CMM, the SLS could only provide a measuring accuracy that is comparable to that of CMM when proper …