基于眼底图像成像原理及特征, 提出一种结合自动调焦窗口选择的眼底照相机自动调焦方法.根据改进阈值的局部信息熵调焦窗口选择算法实现调焦窗口的初步定位, 按照多方向步进算法实现调焦窗口的最终定位.针对眼底图像对比度低的问题, 结合区域对比度的清晰度评价函数与梯度向量平方函数的特点提出眼底图像清晰度评价函数.实验结果显示, 提出的自动调焦窗口选择算法可以保证调焦窗口的稳定性及准确性, 针对低对比度眼底图像的清晰度评价函数具有更好的灵敏度.

An auto focusing method based on auto focusing window selection was presented according to the imaging principle and characteristics of fundus images.The preliminary focusing window is extracted through the focusing window selection algorithm based on local entropy with improved threshold.The final window is located through multi-directional stepping algorithm.The image sharpness evaluation function combined with the characteristics of local contrast sharpness evaluation function and gradient vector square function was presented according to low contrast fundus images.Experimental results show that the presented auto focusing window selection algorithm ensure the stability and accuracy for the focusing window,and the image sharpness evaluation function has better sensitiveness for low contrast fundus images.