Sunglasses lens types are classified based either on style, the lens material itself, the tints and coatings applied on them, or the lens technology built-in to the lens. How sunglasses work for you depends on the material, styling, and coatings used on the lenses.
BASED ON STYLE
Style-wise, lens are classified mostly based on the shape of the lens frame, hence they range from the common geometric shapes (e.g. circle, square, rectangle, heart, star) to less common, fanciful shapes (there are frames shaped like a playing card). The style names can also come from the purpose for which the designs were originally intended (e.g., ‘shield’, ‘biker’, ‘aviator’, ‘wayfarer’)
BASED ON LENS MATERIAL
Usually, sunglass lenses are either made of optical grade glass or shatter-resistant plastic (e.g. polycarbonate, polyurethane, and acrylic). Sport sunglasses (to reduce glare in skiing or surfing, for example) are usually made of plastic.
Glass, especially optical grade ones, afford the best distortion-free finish and scratch resistance (some glass lenses are virtually scratch proof). Naturally this makes them more expensive than plastic lenses. But, since glass can break, glass lenses are usually unsuitable for rough sport – or sports that, even if not rough, may still hit you in the eye with a tough object (golf, for example).
Plastic lenses usually have the ability to block 100% of the sun’s ultraviolet (UV) rays without the need for a special coating. Here are the most common plastic lenses available.
For shatter-proof sunglasses, polycarbonate lenses are the material of choice (even aircraft windshields are made out of them). Polycarbonate isn’t as scratch resistant or optically clear as glass though, although the next generation of lens plastics combine the best qualities glass and polycarbonate. Polycarbonate lenses block all UV rays without the need for special coating
NXT lenses are the NeXT generation of sunglass lens materials. The polyurethane material is trademarked as Trivex NXT and is manufactured in a patented process. NXT lenses have the clarity and scratch resistance of glass and the lightness and impact resistance of plastic. NXT lenses are even tougher than polycarbonate but are also costlier than glass (Apache military helicopter windshields are now made from NXT).
Acrylic is a classic sunglass material. It has less optical clarity and durability than polycarbonate. However, acrylic is an inexpensive choice, and is usually the material found in very affordable sunglasses for fashion and eye protection.
BASED ON TINTS/COATING
Coatings are either for giving your sunglasses the desired color or to reduce glare.
Lens tints not only gives your lenses a desired color, they also afford the eye certain advantages. Depending on your purpose, they may give excellent contrast or depth perception at the expense of showing you the true colors of what you see (color distortion).
Brown tints enhance contrast and depth perception but distort the colors in exchange. This could result in eyestrain when worn for long periods.
These lenses amplify available light so are best for low light conditions. They also give good contrast and depth perception but, since they amplify light, they don’t give much protection from bright sunlight. Good for snow sports and other activities that have low lighting.
Amber and rose/vermillion lenses are also called ‘blue-blocking’ lenses since they absorb blue light which, some studies say, can increase the risk of eye damage like macular degeneration. Rose lenses also give excellent contrast and cause very little eye eyestrain when worn for longer periods. Good choice for skiing, hunting, boating, and when flying aircraft.
Most blue/purple coated lenses are just for fashion purposes and don’t really enhance contrast or depth perception. Moreover, they let in blue light, which many believe can harm the eyes over time.
These enhance contrast without distorting colors. They cut glare and also enhance depth perception without causing eyestrain. These are good for driving. Most ‘black’ tinted glasses only look black because they are tinted very darkly. They are actually tinted a very dark grey or green.
Gradients merely refer to the deliberately uneven tinting of any color – some parts are tinted darker and some parts lighter. Most gradient lenses are tinted darkest from the top and lightest below. This compromise protects drivers from overhead sun and glare while allowing them to clearly see their dashboards in low light. Some sunglass lenses have double gradients: the lightest areas are in the middle part while the top and bottom are darkest. This protects the eyes from glare both from above and below. Good for spending a lot of time around snow, sand, water, or concrete but you won’t want this for downhill skiing since you need to see where you’re going better.
Aside from style, material and color, what to look for when buying sunglasses is UV protection. Polycarbonate lenses have built-in UV protection while others need a UV coating. Go for lenses that are labelled as giving 100% protection from UVA and UBV rays. Photochromic lenses already have UV protection although they turn dark slowly than already dark lenses. For best UV protection, go for sunglasses that protect your eyes from every angle (like ones with large lenses with wraparound frames) – this can impact style, however.
Glare reduction can be achieved in lenses by having polarized coating or an anti-reflective coating. Polarized lenses reflect the glare off water or solid objects. They are therefore popular for water sports like boating and fishing. They however, tend to also polarize the light coming from LCD displays like smartphones or certain car dashboards.
Anti-reflective coated lenses reduce reflection coming from the back of your lenses, giving greater comfort when using the computer for long periods and sharper vision when driving at night. Anti-reflective lenses can be tinted but they are usually found in clear glasses.
Mirror coatings (also called ‘flash coatings’) reduce the amount of light entering the eyes. They also prevent people from seeing your eyes. They are therefore good for activities in very bright light sun. The metallic coatings can be of any color – silver, gold, blue, pink, for example. The actual colors that get into your eyes depend on the color of your tinted lens – which are usually of lighter tints because the addition of mirror coating makes objects appear darker than they are. These lenses are good for eyes sensitive to bright light.
Also called ‘transition lenses’, Photochromic lenses are clear (or almost clear) indoors then slowly darken when exposed to sunlight (specifically the UV rays in sunlight). They save you from having to buy clear glasses for indoor use and sunglasses for outdoor use. However, since most car windshields already reflect UV light, most photochromic lenses do not darken inside cars. However, some modern photochromic lenses can do this since they are also sensitive to visible light.
WHERE TO BUY SUNGLASSES
This question is usually asked when buying either prescription glasses or sunglasses online. There are thousands of online retailers, and to be honest they are all much the same. The only caveat is to choose a decent seller who guarantees the product is real and accept returns, apart from that just look for the cheapest! While the prices of sunglasses sold in retail stores have been going up and up, online they work to give you the best prices available, and with their low overheads they usually do!