In the box method, it is the difference between the horizontal size of the circumscribed rectangle. Therefore, LSWF should indicate whether it is the box method. Determine the LSWF, find the geometric center of the frame or the cut lens that conforms to the shape of the frame, which is the M point in the reference line method; measure the maximum distance from the center of the collection to the edge of the frame or lens; twice the distance is the minimum The diameter of the uncut lens; when measuring the condition or the length of the reference line of the lens; find the difference between the minimum uncut lens diameter and the length of the reference line, which is LSWF.

The maximum distance from the geometric center (M) of the lens to the edge of the lens is 29mm, so 58mm is the minimum diameter of the lens required. If the length of the reference line (AA’) is 48mm, the LSWF is 10mm. For LSWF, you can choose an appropriate size of raw film for cutting, which is especially important when the lens needs to be shifted. If the manufacturer can mark the frame LSWF on each pair of frames, it will enable us to easily determine the minimum size of the raw film needed before cutting the lens that needs to be shifted.

The thickness of the lens can be measured with a thicknesscaliper. Place the lens between the jaws J, and the pointer P will move a certain distance d on the arc facet, which indicates the thickness of the measured lens. The scale can be accurate to 0.1mm. It can be obtained by the principle of similar triangles: moving distance d=lens Thickness×CP/CJ Generally, the length and short arms of the caliper are designed as a 4:1 ratio, that is, if the CP/CJ is 4, the lens thickness is d/4mm, and the arc scale can be determined from this.  