基于改进倾斜刃边法的CCD相机调制传递函数的测量

MTF measurement of CCD digital camera based on improved slanted-edge method

提出了一种在倾斜刃边法的基础上进行部分改进的方法。该方法由Canny算子检测采集到的图像中的刃边分割位置,结合刃边图像直方图呈双峰性的特点,利用类间方差最大化阈值分割算法(Otsu)计算对Canny算子性能具有决定意义的高阈值。以刃边区域中每行的质心为ESF中心位置,以0.1亚像素间距对Canny检测下每一行的分割位置及其刃边邻域像素进行离散采样,将获得的二维刃边信息数据沿着刃边方向整体投影成一维数据,得到提高采样率下平均处理的ESF。增加了测量结果的信噪比,运用该方法进行了实验室CCD相机整机系统的调制传递函数测试,对实验中影响MTF测试精度的因素进行了分析。实验测得在奈奎斯特频率处与理论值误差在5%以内。

An improved method was presented based on the slanted-edge method. It uses the Canny algorithm to detect edge in the acquired image, and calculates the high threshold which is significant to the Canny algorithm by using the Otsu algorithm combined with the bimodal characteristics of the edge image histogram. The centroid of each row is regarded as the center of edge spread function （ESF）, then the two-dimensional edge data sampled with subpixel spacing from a region along the direction of the detected edge is reprojected to a one-dimensional array of sub-pixel bins, the ESF is obtained after averaging under improving the sampling rate. It increases the signal-to-noise ratio of the measurement. We used this method to test the modulation transfer function （MTF） of charge couple-device （CCD） camera in the laboratory, and analyzed the influences of parameter settings on accuracy of the MTF measurement. Result shows that the relative errors between the theoretical values and the experimental results are within 5 %.