otective lenses.Large-framed wraparound sunglasses can protect your eyes from all angles. Keep in mind that frames should not obstruct your vision, or side vision. However, the main criteria for frame style is wearer preference.Frame color is entirely at the discretion of the wearer. As long as a style is comfortable, functional, and does not block vision, then color has no real bearing aside from wearer preference.LENSESThe materials used to make sunglass lenses varies between glass, plastic and SR-91. In fact, there are many types of glass and plastic used as lenses in sunglasses. The most common materials are identified here. Finding a pair of sunglasses that are made of unique materials may make your decision a little more difficult. Consider doing your own research on variations of these more common materials.Glass lenses have the best optical clarity and provide a greater resistance to scratching than other materials. However, they are much heavier relative to the plastic or SR-91 lenses. Another advantage of glass lenses is they are better at retaining their shape in extreme temperatures. Specifically, leaving glass-lens sunglasses on the dash of your car indirect sunlight on a sweltering day will likely not warp or change their shape unlike plastic lenses. From a technical perspective, glass lenses tend to be thinner than plastic lenses due to their refractive index range. This is important when you are purchasing prescription sunglasses. Vision that requires greater correction requires more curvature of the lenses which results in thicker lenses. Relative to plastic lenses, glass lenses will likely be thinner due to their better refractive index range. Nonetheless, the glass lens will most likely be heavier than its plastic counterpart. One very important disadvantage of glass lenses is that they can shatter or break on impact. This is important to keep in mind depending on the application and use. As a general rule of thumb, you can expect to pay more for glass lenses than plastic lenses.SR-91 lenses are made from a proprietary resin-based material developed by Kaenon Polarized and are exclusive to their brand of eyewear. They are considered a luxury performance brand that has the highest rating of optical clarity and acuity. In addition, SR-91 lenses pass the high-mass impact ANSI1.1.1 testing. The SR-91 material is very light, much like plastic lenses, and are good for sporting and long wear applications. One can expect to pay more for these types of lenses than any other type of lens material including glass or plastic.Plastic lenses can be made from several types of plastic such as acrylic, polycarbonate, plastic polymer (CR-39), or polyurethane. In general, plastic lenses are lighter and provide greater shatter-resistance than glass lenses. However, they are more susceptible to scratches. Polycarbonate plastic lenses are the lightest of the plastics and are virtually shatterproof. This makes them exceptional choices for impact protection. The most common plastic used for eyewear lenses is CR-39. This material is light, has higher scratch resistance than other plastics, and low transparency for ultraviolet and infrared radiation.Note: Infrared wavelengths are invisible and produce heat. Sunlight has low levels of infrared rays, and the eye tolerates infrared well. Some sunglass manufacturers make health claims for their products based on infrared protection, but research has not shown a close connection between eye disease and infrared rays.Photochromic lenses can be made of glass or plastic. Also referred to as photosensitive lenses, they darken and lighten in response to the amount of available light or type of light. For sunglasses, this may be a valuable tool for situations where the amount of light varies. This would allow a lens to get darker in brighter light. The more direct sunlight they are exposed to affects how dark the lens will become. It is important for drivers to know that these lenses will not be as dark inside the vehicle because they are not exposed to as much UV light. This may cause eye fatigue on very bright days when glasses do not darken fully. Another important characteristic is that they darken more quickly than they lighten. This may create problems when moving from direct to indirect sunlight areas. While sunglasses by nature do have a specific lightness/darkness, this lens adds variability to a pair of sunglasses. This can be a very useful option for some wearers, but not very commonly found on the market making it difficult to find.