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针对TFT-LCD面板尺寸大、厚度薄、光透过率高的特性,建立了基于激光三角测距法的自动对焦系统。采用光学回路和机械结构设计,对激光光斑中心位置准确求取以及自动对焦原理等算法进行了研究。首先,根据激光三角法测距原理进行了光学回路设计,并以纳米定位平台作为微小行程对焦驱动轴。其次,通过分析光斑图像特性,调节相机快门调整曝光量从而对图像进行噪声消除。另外,采用质心法快速提取光斑中心,在分析激光光斑中心位置与离焦关系的基础上,说明了自动对焦算法。最后,介绍了系统自动对焦时间的设定方法。实验结果表明:在行程为100μm的自动对焦范围内,5X物镜下,自动对焦时间为0.36 s,重复定位精度为±1.98μm。50X物镜下,自动对焦时间为0.41 s,重复定位精度为±0.26μm。该系统稳定性好、对焦精度高、抗干扰能力强,基本满足大尺寸玻璃基板光学检测的需要。
Aiming at the characteristics of TFT-LCD panel, such as large size, thin thickness and high light transmittance, an autofocus system based on laser triangulation is established. Using the optical circuit and mechanical structure design, the exact calculation of the laser spot center and the algorithm of autofocus are studied. First of all, based on the principle of laser triangulation ranging optical circuit design, and nano-positioning platform as a small stroke focusing drive shaft. Secondly, by analyzing the characteristics of the spot image, adjust the camera shutter to adjust the exposure so as to eliminate the noise of the image. In addition, the center of the spot is rapidly extracted by the centroid method. Based on the analysis of the relationship between the position of the laser spot and the defocus, the auto-focusing algorithm is described. Finally, the system autofocus time setting method is introduced. The experimental results show that under the 5X objective, the autofocusing time is 0.36 s and the repetitive positioning accuracy is ± 1.98 μm within the autofocus range of 100 μm. 50X objective lens, the AF time is 0.41 s, repeat positioning accuracy of ± 0.26μm. The system has good stability, high focusing accuracy and strong anti-interference ability, which basically meets the needs of optical inspection of large-size glass substrates.