The function of an eyeglass lens is to bend rays of light so that when they enter the eye they focus perfectly on the eye's retina. This idea of using lenses to improve faulty vision goes back many centuries. While the technology, equipment and precision used in the process of making lenses have changed exponentially in recent years, the basic steps in creating a prescription lens have remained largely the same for the past 50 years. Here's how eyeglass lenses are made:
Start with Lens Blanks
The process of creating a prescription eyeglass lens begins with large buttons of lens material that look like clear hockey pucks. These lens blanks are typically injection molded and mass produced using a variety of plastic resins. The front side of these lens blanks have set curvatures that define the nature of the lens. Single-vision lenses, bifocals, trifocals and the myriad of progressive addition lenses each have a unique front surface. Lens manufacturers often put millions of dollars into engineering the precise front curvatures of their progressive addition lenses (just in case you were wondering why your eyeglasses cost so much).
Step 1 - Surfacing
Once the technician selects the proper lens blank, he mounts it to a carrier block made of wax or lead alloy and inserts it into a lens lathe. Using computer settings dictated by your eye doctor's prescription, the lathe shaves layers of material off the back side to create very precise curvatures and the desired thickness. It is the combination of the front side curvatures and the back side curvatures that determines how the lens will bend the light rays to correct various degrees of myopia, or near sightedness, hyperopia, or far sightedness, presbyopia, or reading vision, and astigmatism.
Step 2 - Fining
When the lens comes out of the lathe, the back side is somewhat rough. This gives it a frosted appearance. The technician now puts the lens on a fining machine, which uses a precisely tooled polishing block, called a lap, to polish the back side of the lens. 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. To picture this, make a fist with one hand, and then drape your other hand over the top of it and rotate your palm. That's the fining process.
Step 3 - Edging
Now that the prescription has been ground in and polished, the lens is much thinner than when it started, but it is still the diameter of a hockey puck. The next step is to cut the lens down to size. In most labs today this is done with a computerized lens edger. The newly surfaced lens goes into the edger along 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 fitting measurements provided by your optician, the edger passes the lens over a diamond cutting wheel until it is reduced to the proper size and shape.
Step 4 - Coatings
Now the lens is ready for finishing touches. For example, lenses can be specially coated to resist scratching, to block ultraviolet light or to reduce reflections. Anti-reflective coatings are a rapidly growing segment. 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 Future of Eyeglass Lenses is Getting Clearer
While the vast majority of eyeglass lenses made today follow the procedures above, new technology is entering the market. Knowledge developed in the refinement of LASIK eye surgery is rapidly making its way to lens laboratories. The same concepts that doctors use to custom "carve" a patients cornea during LASIK surgery can be applied to plastic lenses. This emerging science, known as wavefront technology, will ultimately make lens blanks a thing of the past. New wavefront lens lathes will custom carve the front and back surfaces of lenses, resulting in greatly reduced visual distortion and improved lens clarity.
To learn more about eyeglasses, eyeglass lenses, proper eye care, eye wear and Shawnee Optical, please visit our website at www.shawneeoptical.com.