Abstract: To meet the fast and accurate requirement of image measurement in the auto-focusing system of image measurement instrument, gray contrast function was improved as following: by making use of the features that the focus image has smaller range of the gray scale transition than the defocused image and calculating the average change in gray value, focusing was reached with the number of changes in gray-scale. The comparison shows that the improved gray scale contrast function has simpler time computational complexity and higher focusing sensitivity. The improved function, with good stability, has faster focusing speed and higher focusing accuracy than other methods in the auto-focusing system of image measurement instrument.

Key words: image measurement, automatic focusing, gray contrast function, average value of gradient, number of changes in gray-scale

摘要： 在影像测量仪的自动对焦系统中，针对影像测量快速精确的要求，对灰度对比度函数进行改进。利用正焦图像比离焦图像灰度变化过渡范围小的特点，通过计算灰度平均变化值，结合灰度变化次数进行对焦。实验对比表明，这种改进的灰度对比度函数，计算时间复杂度较简单，对焦灵敏度高。在影像测量仪的自动对焦系统中，比其他方法具有更快的对焦速度和更高的对焦准确度，且具有良好的稳定性。

关键词: 影像测量, 自动对焦, 灰度对比度函数, 灰度平均变化值, 灰度变化次数