用星形测试卡测试屏-片系统MTF的实验探讨与应用

Testing Screen/Film System MTF Using Star Testing Card

目的:探索用星形测试卡测试屏-片系统的调制传递函数(modulation transfe frunction,MTF)的方法,获得随空间频率连续变化的屏-片系统的MTF曲线。方法:把星卡的X射线照片用显微密度计分别沿黑、白楔条的角平分线方向进行扫描,将得到的数据用计算机处理,获得屏-片系统的MTF曲线。并将该方法得到的MTF曲线与用矩形波测试卡得到的MTF曲线进行比较。结果:通过星卡曝光—微密度计扫描的方法,测试得到了随着空间频率的增加而逐渐减小的屏-片系统的MTF曲线,曲线随着空间频率的变化而连续变化。结论:用星形测试卡可以获得随空间频率连续变化的屏-片系统的MTF曲线;并且用星形测试卡测试得到的MTF曲线,在空间频率的连续性上明显优于用矩形波测试卡得到的MTF曲线。

Objective： To explore a new method for measuring the screen/film system modulation transfer function （MTF） using star testing card, so as to get the MTF curve which varies continuously with spatial frequency directly. Methods： The star testing card was exposed, then microdensitometer sanned the image along the center of the wedge, the data files were processed by computer, the MTF curve obtained by the star was testing card compared with the MTF curve which was obtained by the rectangular testing card. Results： The screen/film system MTF curve that decreases continuously with spatial frequency can be obtained by this new method. Conclusion. We can get the screen/film system MTF curve directly using star testing card, the MTF curve obtained by this method is better than the MTF curve which was obtained by the rectangular testing card on the continuity of spatial frequency.