As everyone knows, sunglasses are made with darkened, usually tinted glass or plastic lenses to filter the quantity of light reaching the eyes. Their purpose is mainly to eliminate ultraviolet (UV) rays, to reduce direct light to the desired level of comfort and to eliminate or decrease glare.

But most of us are less familiar with the motivations of those who pioneered this concept two centuries ago. At a time when no one had heard about damaging UVs and everyone was naturally adapting to strong summer light with hats or visors, the aim of tinted glasses was initially to correct vision impairment (better techniques were soon developed), and later on to protect patients who had developed an abnormal sensitivity to light due to specific diseases. Not too surprisingly, the extended use of tinted glass by a much larger share of the world population came around  the 1920s, from the fashionable imitation of movie stars who had to protect their eyes from extremely bright lights on the sets (bright lighting was used to  make up for the low sensitivity of the first black & white films).

Not forgetting other occasional use, like hiding our emotions or physical defects, avoiding eye contact, or going unnoticed, we nowadays add outdoor eye comfort, style, and fashion benefits to the protection of our eyes from excessive ultraviolet radiation, which can lead to various diseases such as photokeratitis or cataract, as reiterated by health care professionals.

Both glass and plastic have the required properties to make tinted glass. The plastic materials mostly used for sunglasses are acrylic, nylon, and polycarbonate, but glass lenses have the best optical clarity and scratch resistance, and they remain the high-end market leader, even though they are slightly heavier than plastics.

In simple terms, polarized tinted glass eliminates most of the annoying glare coming from flat reflective surfaces (roads, snow, lawns, lake water, etc.). This effect is generally obtained by the presence of a polarizing film between two thin layers of glass, one being a tinted glass, and the other one being a neutral glass. As the glare from reflected light is by and large horizontally polarized, it will be blocked by the vertically oriented polarizing film. The result: an increase in tinted glassperformance, especially when driving, fishing, and sailing... This explains why, although more expensive, polarizing tinted galsses are one of the fastest-growing segments in eyewear.

Since 2010, Corning launched a new breakthrough polarized technology with Polarcoat ™ . Instead of using the standard sandwich technology described above, it is now possible to apply a surface treatment with 99% of polarization efficiency, bringing a long list of benefits such as process and inventory complexity reduction, no risk of delamination and many others.

To learn more on the Polarcoat ™ technology, please visit our Polarizing Solutions.

These lenses incorporate photochromic molecules which undergo a reversible chemical process when exposed to UV rays. This change of shape (gain/loss of an electron for silver atoms in photochromic glass, or opening/closing of the organic molecule in photochromic plastics) results in the absorption of visible light, causing the lenses to darken. These tinted glasses will darken when exposed to UVs and fade back indoors – ideal for changing environments. See our detailed photochromism page.

Finally, apart from the usual anti-reflective (AR) and hard-coating (HC) treatments, tinted glasses can also be coated with a complementary waterproof treatment (also active against condensation and stains) as well as with a very thin reflective coating (mirror effect), sometimes with a gradient effect so that added reflectivity/protection occurs on the upper half of the lens while better visibility is reserved for the lower half (ideal for driving in a sunny environment – reading the dashboard).

Example of a category 4 label for tinted glass (EN-1836) in French, with its self-explanatory pictograms.

• Choosing a lens category should take into account the local intensity of light, how sensitive the tinted glass wearer is to glare, and the need for UV protection (especially at high altitudes).
• The highest categories (cat 4 or less than 8% transmission for both European and Australian standards) being devoted to specific applications (such as mountaineering or protection of hypersensitive eyes), they are not suited to driving, an aspect which international standards also take into account (traffic signal vision tests data).
•  Regulators are actively working on a new, global standard.
• Corning has chosen to offer only medium or high levels of protection, thus not to address category 0 or 1 tinted glass.

Corning Specialty Glass offers a wide variety of glass solutions, including tinted lenses, photochromics, and transparent UV filter glass lenses for the manufacturing of polarized tinted glass. Our products, which you will find in some of the world’s leading tinted glass brands, comply with all international standards and beyond.

Outside on a sunny day, the brightness of the light ranges from about 1,000 lumens in the shade to more than 6,000 lumens. When the brightness of the direct or reflected light gets to about 4,000 lumens, our eyes begin to have difficulty absorbing the light. What we see when we try to look at these brighter areas are flashes of white; this is glare.
Source  http://science.howstuffworks.com/sunglass

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