Bayer滤波型彩色相机调制传递函数测量方法

A method to measure the modulation transfer function of Bayer filter color camera

Bayer滤波型彩色相机广泛应用于航天遥感、空间对地观测、环境监测等领域.由于Bayer滤波片造成彩色相机相比黑白相机在像质方面进一步退化,如何对Bayer滤波型彩色相机成像性能全频段综合评价是目前亟待解决的问题.调制传递函数(MTF)是相机成像性能综合评价的关键指标,传统调制传递函数测量方法无法实现对Bayer滤波型彩色相机MTF全频段高精度测量.为了解决这个问题,本文提出了一种采用旋转刀口靶测量彩色相机调制传递函数的方法.理论方面,推导了Bayer滤波型彩色相机调制传递函数测量理论模型,仿真分析了刃函数采样率和刀口刃边倾斜角度误差对调制传递函数测量精度的影响,并给出了计算算法.实验方面,对彩色相机R,G,B三基色调制传递函数权重因子进行了实验定标,并搭建了基于条纹板和旋转刀口靶的彩色相机调制传递函数测量试验装置.采用旋转刀口靶法和条纹靶板法测量彩色相机MTF结果在耐奎斯特频率f_c处极差为0.061,在空间频率f_c/2处极差为0.043,试验结果验证了所提方法的有效性.

With the development of optoelectronic technologies, color cameras have been widely exploited in space remote sensing, earth observations from space, environmental monitoring, urban construction, and many other fields. Currently,most commercial color cameras use a single charge coupled device（CCD） or complementary metal-oxide-semiconductor（CMOS） sensor that has a Bayer color filter array（CFA） on its pixel surface to obtain red（R）, green（G）, or blue（B）samples. As a way of evaluating imaging quality, modulation transfer function（MTF） can provide a comprehensive and objective metric for camera imaging performance. In the conventional knife-edge method for color camera MTF measurement, a linear uniform sampling of the edge spread function（ESF） must be completed before a fast Fourier transform（FFT） can be applied. As the sampling rate becomes large, the number of pixel points on the line which is parallel to the knife-edge become less. So taking average of the pixel points to obtain ESF can be strongly affected by the noise of sensor. Therefore it is necessary to balance the influences of sampling rate and sensor noise on the MTF measurement, and the recommended sampling rate is 4–6. When the tilt angle of knife-edge has an error, the nonuniform sampling ESF can be obtained by the slanted knife-edge method. This leads to a variation in the results of the camera MTF on a spatial frequency scale and early cut-off. The best MTF results of camera can be obtained by rotating knife-edge, calculating MTF power under different tilt angles of knife-edge, and finding the maximum MTF power. And we propose an algorithm for Bayer filter color camera MTF measurement. The algorithm processing includes extracting R, G, B colors of knife-edge images; projection; differential operation; Hanning window filtration; FFT; correction;weighting combination of R, G, B colors MTF; MTF power calculation; optimal tilt angle of knife-edge estimation. To verify the accuracy of the proposed method, the weighting response factors of R, G, B colors are calibrated and an experimental setup for color camera MTF measurement is established. The knife-edge target is rotated in angle steps of0.02？, and the MTF results are calculated under different tilt angles of knife-edge within ±0.1？surrounding the estimate position by the proposed algorithm. The maximum differences of MTF results between the proposed method and fringe target method are 0.061（Nyquist frequency f_c） and 0.043（f_c/2）, respectively. The results show that by searching the optimal tilt angle of knife-edge, the effect of non-uniform sampling on MTF result of color camera can be eliminated.Compared with the conventional method, the proposed method is superior for the measurement of the super-sampled MTF of color camera. Meanwhile, this method can also be applied to MTF measurements of radiographic systems, such as X-ray imaging system and other systems.