The excessive time it takes for light to propagate in the reflecting prism will increase the measured distance by a certain value, that is to say, the propagation speed of light in glass is slower than that in air. We usually call this increased value the prism constant.

There are two types of prism constants, usually the domestic prism we use is -30mm, and the imported prism is 0mm. As for how to distinguish the prism constant, you can look at the bottom of the prism. If the anchor bolt of the prism is flat with the plastic shell, it is -30mm. If not, it is 0mm; in addition, I will teach you a little trick. Before the rear view is determined, the direction or angle can be seen as much as possible. The center of the prism cannot be taken, because you can't see the center of the prism if the distance is far In addition, the distance measurement does not need to be aimed at the center of the prism, as long as you look at any point on the reflective surface of the prism