The function of high quality x ray collimator is to control the irradiation field of X-ray, so that the X-ray needed for human diagnosis can be controlled in a certain range and position, thus avoiding the harm of too much X-ray to human body. Therefore, to ensure the high quality of beam limiter is what we must do. What is the specific standard? In the technical standard of YY0007-0009-90 for 200-500m medical diagnostic X-ray machine, the performance requirements of high quality beam limiters are described in Article 4.19 as follows: the starting and closing of high quality beam limiters should be light, flexible and reliable, and the rectangular irradiation field of any size can be adjusted. When the gastrointestinal examination camera is pulled farthest from the bed surface and the beam limiter is closed to the minimum, the irradiation field on the fluorescent screen should be no more than 20 mm20. The deviation between the center of X-ray irradiation field and the center of light irradiation field and their edges should be within 2% of the distance between the focus of X-ray tube and the image plane.