为了对大型目标的平行度进行测量，设计并搭建了一套由双衍射光栅组成的平行度检测系统。采用两块衍射光栅正交组成光路，产生多条平行线作为基准线。通过照相机对参考线和待测目标进行拍照，对获得的图像进行处理，利用参考线的平行度偏差对目标所在平面的倾斜进行矫正，得到待测物的平行度测量结果。测量结果表明这种方法简单准确，满足测量精度高。

For parallelism measurement of large object, a parallelism measuring system which is composed of twodiffraction gratings is devised and established. The two gratings are placed orthogonal, and produce several parallel lines, which can be taken as baselines to measure the parallelism of the object. The picture taken by the camera is including the baselines and the object’s lines. Through this picture, it will get the coordinates of the baselines and the object’s lines. By the angle between the baselines, it can correct the angle between the object and the camera. By testing the angle of the large object in the new coordinate system, the parallelism of the object can be gotten. The result indicates that the method is simple with high accuracy.