CR-39 (Plastic) Lenses
Plastic resin lenses are the most common material found in the ophthalmic industry and primarily utilized in the “dress” or “street-wear” market. However, these lenses must be thicker than polycarbonate to meet the safety standards, but still weigh much less than glass. They can also be tinted to most any color and shade desired. Plastic lenses do scratch more easily than glass but the scratch properties can be greatly enhanced by using various coatings. Plastic only comes with a “Basic Impact” approval from ANSI Z87.1-2003.
“High-Index” or “Mid-Index” (Plastic) Lenses
Very similar to regular plastic lenses but refract light differently so that higher RX corrections can be made with a thinner lens thickness. The other properties with this lens are identical to regular plastic; HOWEVER, there are no major advantages to ordering these lenses for occupational wear – ONLY “dress-wear” lenses take advantage of the higher index properties where safety is not a concern. The thinner lens concept does not apply to minimum thickness requirements under ANSI Z87.1-2003 (3mm Basic Impact) – Please make sure you are aware of this if someone recommends High-Index or Mid-Index lenses for your safety glasses.
The most impact resistant lenses available – This material is the only lens that comes with the ANSI Z87.1 “High Impact” rating and that standard can only be certified in a 2mm center thickness – ordering in a thicker style (as in 3mm for plastic and glass) is not longer tested or certified. Additionally, pediatric Optometrists and Ophthalmologists almost always recommend poly lenses for children due to the extra protective properties. These lenses are the lightest available material for safety and are frequently used with drill-mounted rimless frames for “dress-wear”. The main downside to Poly lenses is that scratch resistant properties are not as good as plastic or glass.
Glass lenses were the only lenses available for years and still have the best optical properties of all the available materials on the market today. These lenses also are the most scratch-resistant material; however, the weight of glass in safety or “dress” is a major disadvantage as well as that they will shatter if impacted hard enough. The only positive use for the safety market is for painters or chemical applications where mineral spirits are needed for cleaning.