ULTRA-VOILET RAYS
In one of my previous post – Sunglasses for Pilots, we talked about why it is extremely important for a Pilot to be very careful about choosing proper quality and material for the Sunglasses.
The American Optometric Association recommends wearing sunglasses that incorporate 99 – 100% UVA and UVB protection. Fortunately, UVC, the most harmful form of ultraviolet radiation, is absorbed by the atmosphere’s ozone layer before it reaches the Earth’s surface. Some scientists believe, however, that depletion of the ozone layer may allow more ultraviolet to pass through the atmosphere, making 100% ultraviolet protection a wise choice when selecting eyewear.
LENS MATERIAL
The three most common lens materials in use today are optical quality “crown” glass, monomer plastic (CR-39®), and polycarbonate plastic (see Table 1). Lenses made from crown glass provide excellent optical properties (as indicated by the high Abbe value). Crown glass is more scratch resistant but heavier and less impact-resistant than plastic. Glass absorbs some ultraviolet light; however, absorption is improved by adding certain chemicals during the manufacturing process or by applying a special coating. Glass retains tints best over time; however, for higher refractive correction, the color may be less uniform, as parts of the lens will be thicker than others (see Figure 2).
CR-39® plastic lenses possess excellent optical qualities, are lighter in weight, and more impact-resistant than glass lenses, but are more easily scratched, even when scratch resistant coatings are applied. CR-39® lenses tint easily and uniformly, even for those requiring a great deal of refractive correction, but do not hold tints as well as glass. CR-39® plastic can be bleached and re-tinted if fading becomes excessive at some point.
High-index materials (i.e., index of refraction —1.60) are available in both glass and plastic for those who require a large degree of refractive correction and/or desire lighter, thinner lenses. High-index materials are not as widely available, require AR coats to improve optical clarity, and a scratch-resistant coating for durability. In addition, most high-index materials do not accept tints as easily and are less shatter resistant than polycarbonate.
In my next article (click here) you can read about other qualities that you should look for in a Pilot’s or an Aviator’s Sunglasses; like coatings, tints, polarization, frames etc.
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