KEYWORDS: 3D image processing, Cameras, Digital image correlation, Calibration, 3D image reconstruction, 3D metrology, Speckle pattern, Digital electronics, Digital electronic circuits, Distortion
There is an urgent demand for high-accuracy, real-time three-dimensional (3-D) shape measurements in industrial production as an ideal tool for quality control. Based on 3-D digital image correlation (3D-DIC), in this study, we measured micro-stair-steps of solder paste and printed circuits on circuit boards. The 3-D shapes of the circuit boards were successfully reconstructed, and the shapes of both solder paste and printed circuits were identified with high accuracy. To evaluate the performance of the 3D-DIC method, a stylus profiler was used to conduct shape measurements for printed circuit boards (PCBs), and comparisons were made between the results of 3D-DIC and the stylus profiler. The experimental results demonstrate that: (1) printed circuits with a length of several tens of microns can be measured with an accuracy of 3 μm using 3D-DIC in a field of view of 14 × 12 mm2; (2) the time required for 3-D reconstruction was less than 1 s for a 500 × 500 image with a grid step of 3 and a subset size of 19 on a consumer-grade computer (i7 4790k CPU, 16 GB of memory), thus meeting the requirements of online and real-time shape measurements of PCBs; and (3) for printed circuit measurements, the subset size chosen should be close to or slightly larger than the width of the printed circuit.
Digital image correlation (DIC) is an optical measurement technique widely used in the field of experimental mechanics due to its practicality, flexibility, and reliability. The principle of DIC is to correlate the same point in the reference and deformed image. The performance of DIC is influenced by the image noise. Theoretical models have been presented to evaluate the random errors caused by image noise. However, these models are based on the assumption that the variance of image noise is uniform, while in practice the variance of image noise is non-uniform. In order to overcome this deficiency, in this paper, a formula of random errors of DIC corresponding to non-uniform noise is derived. The formula shows that the variance of image noise and image gradients has a coupling effect. In order to verify our theoretical analysis, actual experiments were carried out. The dependence between image noise and intensity was measured for two cameras; the DIC errors caused by image noise were measured experimentally; in order to take the effect of illumination into account, we measured the DIC errors in cases of both uniform and non-uniform illumination. The experimental results show good agreements with our theoretical analysis.
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