Zhao Youqin was a scientist in the late Song Dynasty (960-1279) and early Yuan Dynasty (1271-1368) who made great achievements in astronomy, mathematics and optics and wrote a series of books in those fields. Zhao studied on the linearity of the ray of light, pinhole image phenomenon and illuminance, and conducted large-scale equipment, which were unprecedented in physics across the world. The later research showed that Zhao’s qualitative law of illumination that the illumination is increases with the increase of the intensity of light source, and decreases with the increase of image distance was four centuries earlier than the law discovered by German scientist Lambert that the illumination is inversely proportional to the square of the distance. Moreover, Zhao explored the laws of nature based on subjective and large-scale experiments, which was also pioneering in the global scientific history of physics, and was two centuries earlier than world-renowned Italian physicist Galileo.