Eyeglass lenses are glass or plastic optical items that fit inside eyewear frames to enhance and/or correct the wearer's vision. The magnifying glass, invented in the early 1200s, was the first optical lens used for enhancing vision. Made from a transparent quartz and beryl lens, the invention revealed the critical discovery that reflective surfaces ground to certain angles could enhance vision. Following this invention, Alessando di Spina introduced eyewear to the general populace. Due to the increasing demand for eyewear, quartz and beryl lenses were virtually replaced by glass lenses. The convex lens was the first optical lens used in glasses to aid the correction of farsightedness, but other corrective lenses followed, including the concave lens for the correction of near-sightedness, and more complex lenses for the correction of astigmatism, as well as the invention of bifocals by Benjamin Franklin in 1784.
More than 80 percent of all eyeglasses worn today have plastic lenses, but plastic lenses have not always been the lens of choice. The glass lens remained dominant until 1952, when plastic lenses were introduced. The plastic lens rapidly grew in popularity because the lens was lighter and less prone to breakage. Today, the manufacture of plastic eyeglass lenses far exceeds the manufacture of glass lenses, but the process has remained much the same for both types. Plastic as well as glass lenses are produced by successive stages of fine grinding, polishing, and shaping. While the same process is used to produce lenses for telescopes, microscopes, binoculars, cameras, and various projectors, such lenses are usually larger and thicker and require greater precision and power. This article will focus on plastic eyeglass lenses.
In the past, opticians relied on separate optical laboratories to produce eyeglass lenses. Today, there are a number of full-service optical outlets that produce lenses for customers on-site. However, optical outlets do receive lens "blanks"—plastic pieces already formed to close-to-exact size with different curves ground into the front of the lens—from optical laboratories. Blanks with different curves are used for specific optical prescriptions.
The plastic blanks received from optical laboratories are round pieces of plastic such as polycarbonate approximately. 75 inch (1.9 centimeters) thick or thicker and similar in size to eyeglass frames, though slightly larger. Most finished eyeglass lenses are ground to at least. 25 inch (.63 centimeter), but this thickness may vary depending upon the particular optical prescription or "power" required. Other materials used to produce eyeglass lenses are:
Eyeglass lenses are designed in a variety of shapes to match eyeglass frames. The thickness and contour of each lens will vary
Convex and concave lenses, known as spherical lenses, require one ground curve per lens, while more curves are required to correct astigmatism. The degree and angle of the curve or curves in a lens determines its optical strength.
Various lens treatments and tints are added after the lenses are shaped but before they are inserted in frames. The coatings are added by dipping the lenses into heated metal bins filled with the treatment or tint. The treatments and tints available include various sunglass tints and colors, ultraviolet light tints, durability and impact-resistant treatments, and scratch-resistant treatments. Among the latest advances in tints is the light-sensitive tint, which combines the advantages of regular clear lenses with the protection of sunglasses. These lenses adjust to the amount of sunlight being radiated, thus providing sun protection when needed.
Various grades of plastic are used for eye wear, but the most popular is the "Feather-weight," an impact-resistant polycarbonate plastic. This type of plastic lens is more durable and 30 percent thinner and lighter than regular plastic lenses. It is also the more expensive lens. Other lens types include the standard "CR 39" trade name plastic lens—CR 39 is a monomer plastic—and the "High Index" plastic lens, which is 20 percent thinner and lighter than ordinary plastic lenses.
The following procedure assumes the plastic lenses are being made at an optical laboratory.
The plastic blanks have different curves already ground into the front of them; therefore, the technician must select the blank that corresponds to the optical prescription required for each lens. The rest of the optical prescription, or power, must be ground into the back of the lens.
Byproducts or waste from the manufacturing process include plastic dust or fine shavings and a liquid polishing compound consisting of aluminum oxide, water, and polymers. The waste material is placed in metal bins for 48 hours along with sanitation compounds (vermiculite of cat litter) before disposal.
Plastic eyeglass lenses must meet rigid standards set by the American National Standards Institute and the Food and Drug Administration (FDA). In addition, all licensed optical laboratories belong to the National Optical Association, which requires strict adherence to prescribed guidelines regarding quality and safety.
Throughout the normal production process, plastic lenses undergo four basic inspections. Three of these inspections occur in the laboratory and the fourth occurs at the optical outlet before the eyeglasses are given to a customer. Other periodic inspections may also be advised. The four inspections involve checking the optical prescription prior to the production process and verifying the optical center placement; visually checking lenses for scratches, chips, rough edges, or other blemishes; visually checking the optical prescription before the lenses are viewed in the lensometer, and verifying optics while the lenses are in the lensometer; and measuring and verifying frame alignment with a ruler.
Impact Resistant Lenses: Questions and Answers. U.S. Department of Health and Human Services, 1987.
"High-Speed Spindles Aid in Fabricating Plastic Eyeglass Frames with Special Finishes." Plastics Design & Processing. December 1983/January 1984, p. 21.
How Your Eyeglasses Are Made. Optical Laboratories Association.
Krasnow, Stefanie. "Athletic Specs: The Eyes Have It." Sport. August, 1987, p. 97.
More Than Meets the Eye. Optical Laboratories Association.
"Plastic Beats Acrylic for Lenses." Design News. August 18, 1986, p. 29.
— Greg Ling