Steps 1 through 3
Corrective lenses can be made with glass or plastic, but nowadays, plastic is the most common. While several different types of plastic are used in making lenses, all of them follow the same general manufacturing procedures. Most of the steps outlined also apply to glass, although a few important differences are noted at the end.
A lab, even an automated one, follows 12 steps to make prescription lenses:
Step 1: The technician chooses a lens blank of the desired material with the proper base curve and, if needed, add power.
Step 2: If the prescription calls for a cylinder, a line is marked on the front of the lens to define 180 degrees, and then another line is drawn that matches the axis of the second curve. If there is a segment, the segment edge is used as the 180 degree line. Often the optical center of the lens is made slightly above the segment edge, and the line is marked the appropriate distance. (Note: When there is no segment or induced prism, the lens may be left unmarked and the cylinder axis determined after the lens is ground.)
Step 3: Since the front of the lens will be left as is, it is covered by a special tape to protect it.