The eye chart is designed according to the principle of viewing angle. The so-called angle of view is the angle formed by the light from two points in the outside world through the nodes in the eye (that is, the angle formed when the two points of an external object intersect inside the eye). Under normal circumstances, the human eye can distinguish the minimum angle of view formed by the smallest distance between two points, that is, one point of view. The visual acuity chart is designed to effectively detect visual acuity based on one point of view.