The technician first selects the proper lens blank, then mounts it to a carrier block made of wax or lead alloy and inserts it into a lens lathe. Based on your prescription, the specific information has already been entered in the computer system, the lathe then shaves layers of material off the back side to create the very precise curvatures and the desired thickness tailor made for your glasses. It is the combination of the front side curvatures and the back side curvatures that determine how the lens will bend the light rays to correct various degrees of myopia (nearsightedness), hyperopia (farsightedness), presbyopia, and astigmatism.
The technician places the lenses in a lensometer, an instrument used to locate and mark the "optical center" of the lens blanks. The "optical center" refers to the point that should be centered over the customer's pupil. Next, an adhesive tape is affixed to the front of each blank to keep it from being scratched during the "blocking" process. The technician then places one lens blank at a time in a "blocker" machine, which contains a heated lead alloy that fuses the block to the front of the blank. The blocks are used to fix the lens in position during the grinding and polishing processes.
The lens is then put in a curve generator, which grinds out the back of the lens to the appropriate optical curves for the specific prescription. An edge grinder then grinds the outer rim to its proper shape and puts a bevel on the edge, allowing the lens to fit properly into the frames.
When the lens comes out of the lathe, the back side is somewhat rough and gives a frosted appearance. The technician will need to use a fining machine to polish it. In a fining machine, there's something called a "lap;" it is a precisely tooled polishing block, and it is this block that is used to give the frosted lens a polished surface. The convex side of the lap is covered with a pad that is impregnated with a fine grit and is then mechanically agitated against the concave surface of the lens until the lens is totally clear.
Each lens is rubbed against an abrasive fining pad made of soft sandpaper. After a second, another fining pad made of a smooth plastic is placed over the original sandpaper pad, the lens is then polished again, as the fining machine rotates the pads in a circular motion while water flows over the lenses. When the process is done, the pads will be taken off and thrown away.
Lenses can be tinted in two primary ways, either by adding color to the molten lens material before the lens is formed, or by chemically post-coating the finished lens to achieve the desired hue. In the former method, high temperatures is the main concern as the colorant additives are added to the lens while the plastic or glass is still liquefied. The difference in handling plastic and glass is that: soluble organic dyes or metallic oxide pigments are added to plastic, while metallic oxide or metal particles are incorporated into glass. After the specific additives have been blended in, the molten plastic or glass is then cast into the general lens shape.
Lenses now can be coated with Anti-Reflective, Scratch-Resistant, UV and/or Blue Light Block Coatings. The demand for these coatings is rapidly growing. The post-coating method produces lenses that are more evenly coated regardless of the lens configuration. It also allows for the coating to be removed and recoated after the lens is made. In dust-free coating equipment, the lens typically receives up to 16 ultrathin layers of metal oxide coating. These various layers combine to dampen glare, repel water and sometimes repel oil.
The Additional Procedures of Manufacturing Sunglasses and Sportsglasses:
The prescription has been ground in and polished; the lens is much thinner than when it started, but it is still too big to fit into the frame. What's needed is to cut the lens down to the right size. Nowadays, most laboratories use a computerized lens edger. The newly surfaced lens goes into the edger together with the frame you have selected. The edger uses a digital tracer to capture an exact three-dimensional image of the frame and then, using the individual fitting measurements entered by the optician, the edger passes the lens over a diamond cutting wheel to have it reduced to the proper size and shape.