LY-A12 N:1.53 lens clock, radian apparatus

Instructions:
    Put 3 measuring hands on the center of the lens concave and convex surface, read out the graduations pointed: the negative value and the positive value. The difference between the absolute values of the negative and the positive is exactly the “degree” of the lenses. If the negative value is more than the positive, it means the nearsightedness. If the negative value is less than the positive, it means the farsightedness. By the same method testing the lens concave or convex surface, the difference between the maximum and the minimum graduations pointed in same surface is the “degree” of the astigmatism.
Specifications:
Measuring range: 0 ~ 20D, 0 ~ -20D
Each small graduation is 0.25D D=100 “degree”
Testing accuracy:
0 ~ 6 D; (-6) D ~ 0 D     △X <0.08 D
6 ~20 D; (-6) D ~ (-20) D      △X <0.20 D