How sunglasses are made

Purpose for sunglasses O Sunglasses are eyewear designed to help protect the eyes from excessive sunlight. Eyes are extremely light sensitive and can be easily damaged by overexposure to radiation in the visible and nonvisible spectra. Bright sunlight can be merely a distracting annoyance, but extended exposure can cause soreness, headaches, or even permanent damage to the lens, retina, and cornea. Short term effects of sun overexposure include a temporary reduction in vision, known as snow blindness or welders' flash. Long-term effects include cataracts and loss of night vision. In both cases, the damage is caused by ultraviolet (UV) light, which literally burns the surface of the cornea.

Input O Sunglasses consist of a pair of light-filtering lenses and a frame to hold them in place. The vast majority of lenses are made of colorized plastic, such as polycarbonate. O Soluble organic dyes and metallic oxide pigments are added to the lens material to absorb or reflect light of certain frequencies.

Input (continued) O Sunglass frames are made from metal or plastic. Metal frames, particularly expensive ones, are often made of mixtures of nickel and other metals such as silver. These frames have precisely engineered features, such as sculpted and gimbaled nose-pads, durable hinges with self-locking screws, and flexible temples.

Process

Process (continued) O Lenses O 1 Colorant can be added to lenses 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, the colorant additives are incorporated into the lens while the plastic or glass is at high temperatures and still liquefied. Soluble organic dyes or metallic oxide pigments are added to plastic. Metallic oxide or metal particles are incorporated into glass. O 2 After the appropriate additives have been blended in, the molten plastic or glass is then cast into the general lens shape, or "puck." Inexpensive lenses are simple pucks that placed into frames. Expensive lenses are prepared in a manner similar to the method by which prescription lenses are made. First the appropriate lens puck is placed in a lensometer, an instrument that is used to find the optical center of the blank. O 3 The lens is then put in a curve generator, which grinds out the back of the lens according to the patient's 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. O 4 Lenses may now be coated with an anti-reflective material. 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. Such coating may be applied through a vacuum coating method, used to deposit an antireflective layer on the lens surface to reduce internal reflections. This faint bluish coating is also commonly used on camera lenses and binoculars.

Process (continued) O Frames O 5 The finished lenses are now ready to be mounted in eyeglass frames. Frames are constructed to hold the lenses in place using either a tension mount or screw mount design. Tension mounting is typically used in plastic frames. In this type of frame, the dimensions of the lens opening on the front surface of the frame are somewhat smaller than the lens itself. In this way the lenses can be pressed into their respective openings through the front edge of the frame without falling out its rear edge. A groove is formed in the periphery of each of the openings, and these mate with a ridge formed on the periphery of the respective left and right lenses. The plastic frame material permits sufficient stretching or elongation to allow the lenses to snap into these grooves. O 6 Metal frames use a screw mount design because metal tends to deform easily and cannot hold the lenses as well. The metal structure of the frames has thin extruded sections that are bent into desired shapes. The frame structure surrounding the lens openings forms an open loop into which the lens is inserted. After the lens is inserted, the loop is closed by attaching a screw to the two open ends.

Output O Beyond the regulations that ensure the glass and plastic used in lenses is shatterproof, there is little governmental regulation of sunglasses. Labeling of the absorbance rates of both types of UV light, UVA and UVB, is voluntary, but the American National Standards Institute (ANSI) has established transmittance guidelines for general purpose and special lenses. According to these standards, general purpose cosmetic lenses must block 70% of UVB, general purpose must block 95% of UVB and most UVA light. Special purpose must block 99% of UVB.

Output (continued) O There are no particular byproducts resulting from sunglasses manufacture. Waste materials include plastic, glass, and metallic scrap from grinding the lenses and making the frames.

Feedback O New coatings and colorants which deliver better protection against UV radiation continue to be developed. O New types of high performance sun- protective eyewear will be developed as advances are made in the fields of optics, surface chemistry, metallurgy, and others.

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