The research shows that there are differences between the light path when wearing lenses and that when measuring lenses' degrees by focimeter, especially for progressive lenses.

When measuring lenses, the light path of the focimeter is a beam of parallel light (light 
source at infinite distance), which is perpendicular to the datum level of the measuring point after projection. It is obviously different from the actual use of the human eye.

Therefore, in order to make the actual degrees obtained by the wearer meet the requirements of prescription degrees, the outer progressive series lenses are designed according to the actual conditions of the human eye. The light path is used to correct the measured target value of the area.