气膜孔图像对焦评价函数的实验研究

为了使工业影像测头能够正确对焦于气膜孔的出口表面，开展了对焦评价函数选取的实验研究。首先，分析了各类对焦评价函数的特性、要求及优缺点，并确定选择灰度梯度函数用于图像序列的清晰度评价；然后，考察了Brenner梯度函数、Tenengrad梯度函数和Laplacian梯度函数等5种具有代表性的灰度梯度函数，通过实验获得了它们的函数曲线，并进行归一化处理和比较分析，最终确定Laplacian梯度函数作为系统的对焦评价函数，并进行了重复性验证。实验结果表明，Laplacian梯度函数可以正确判别出被测气膜孔的对焦位置，且对焦重复性小于0.005mm，满足气膜孔形位参数的检测要求。

Experimental Study on Focusing Evaluation Functions of Images of Film Cooling Hole

functions is executed. Firstly, characteristics and requirements which are necessary for a well qualified focusing evaluation function are analyzed, as well as the advantages and disadvantages of typical functions. In this procedure, the gray gradient functions are selected to evaluate the sharpness of image sequence. Afterwards, 5 representative gray gradient functions such as Brenner gradient function, Tenengrad gradient function and Laplacian gradient function etc are investigated by experiments, in which their function curves are derived and then normalized to carry on comparisons and analysis. Finally, Laplacian gradient function is chosen as the focusing evaluation function of the system, whose repeatability is verified by experiments. As the experimental results show, Laplacian gradient function could be utilized to determine the focusing position of the target film cooling hole and the focusing repeatability is smaller than 0.005mm, which could satisfy the inspecting requirements of the film cooling holes.