The aperture depth of the diaphragm is designed to be 15 mm, the diaphragm aperture tension angle is designed to be 10° on the left and right, 6.5° on the top, and 3° on the bottom, and the diaphragm is installed behind a primary reflector, and the centerline of the diaphragm coincides with the corresponding main light. Through the specific aperture depth, tension angle and other structural design of the diaphragm so that the boundary inside and outside the amplitude of the distinction is obvious and the amplitude of the normal distribution, combined with the use of sensors will be light intensity and image source surface temperature in numerical display, easy to detect calibration. The design of the diaphragm is small in size, obvious detection effect, low cost, through the diaphragm and the logic of the sensor with the use of the car head-up display can be used for the detection of sunlight backflow, for the car head-up display industry to play a certain reference role.