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Safety Glasses USA
Polycarbonate (PC) is a thermoplastic, and is by far the most commonly used material in the safety eyewear industry. It is quickly growing in popularity for the prescription business as well. It possesses superior impact resistance (about 10 times that of glass and some other plastics) , it's inexpensive, and it also inherently blocks 99.9% of UVA/UVB rays without any extra or special coatings. PC possesses a higher refractive index at 1.59, which makes it useful for prescriptions. It also accepts add-on technologies to result in photochromic or polarized lenses. PC's biggest drawback is its low scratch resistance. Lenses are usually manufactured with a special scratch resistant hard coat to increase their tolerance. Although it is possible to find safety eyewear with uncoated lenses, they are usually not the preferred choice. Polycarbonate, a generic term, is sometimes referred to by a brand name (though rarely in safety eyewear), the most well known being Lexan.
Plutonite, in basic terms, is a purified polycarbonate. A proprietary material by Oakley, Plutonite is used in all Oakley-made lenses. Impact and scratch resistance are similar to those of PC, but the clarity is far superior, rivaling that of the clearest optical glass available. The material also provides 100% protection from UVA, UVB and UVC rays.
There are various types of plastics used in lower priced novelty and consumer sunglasses and, therefore, various levels of thickness, impact resistance, clarity, and overall quality. We won't elaborate too much on these types as they are not seen in the safety industry. One type, however, that is often grouped with the plastics is CR-39. This is still commonly used -- not for safety, but for sunglasses and prescription eyewear.
"CR-39" is simply the name of the resin, "Columbia Resin #39", developed in the early 1940's. The polymer has very good scratch resistance, but remains a relatively soft material -- too soft to pass ANSI Z87.1 impact tests. It also offers very good optical clarity and is still commonly used in sunglasses. It can be used for prescriptions, but its relatively low refractive index (1.498) requires thicker lenses for stronger magnifications. Its use in this application is decreasing with the emergence of the more impact resistant polycarbonate and, in particular, the Hi Index variety.
High Index
It is so called precisely because of its higher refractive index, usually 1.60 to 1.74. This higher index means that stronger prescriptions are made possible even with a thinner lens. This is appealing for wearers who prefer a smaller, lighter set. The material is basically a special polycarbonate, and pretty much only used for prescriptions, especially stronger prescriptions. Because of its thinner nature, this version of PC probably would not stand up to high impact as well as its thicker relative, and will likely never become common in the general safety market.
NXT/Trivex
NXT is a proprietary material made primarily of Trivex, an advanced polymer that possesses the benefits of polycarbonate, but also has superior optical clarity, scratch resistance and photochromic performance. Trivex is the newest player in the game and, if more eyewear manufacturers catch on, it has potential to be the star and MVP for a long time in the safety, fashion and prescription markets. Arguably the one area where Trivex will likely watch from the sidelines is in the case of stronger prescriptions. Its lower refractive index would require a thicker lens than that of PC or High Index.
Glass
Optical crown glass still exists for specific purposes, but is now rarely used in general eyewear applications since the emergence of optical quality "plastic" substitutes such as polycarbonate, CR-39, and Trivex. The two redeeming benefits to glass lenses are excellent clarity and superior scratch resistance. Unfortunately, its two biggest drawbacks, shattering and weight, tend to cancel out those advantages. Of the common lens materials, glass is by far the most prone to shattering, thus creating a significant safety hazard. To decrease the likelihood of a shatter, glass lenses are sometimes made thicker. Already more dense than plastic, a thicker glass lens becomes about twice as heavy as a plastic substitute, and is still not shatterproof. Additionally, the refractive index (1.52) is such that strong prescriptions also require a thicker lens, further compounding the unfavorable weight problem.