Sunglasses come in a vast array of shapes and sizes to appeal to a variety of aesthetic sensibilities. However, aside from cosmetic differences, there is a large difference between the lenses of sunglasses. Specifically, whether the lenses are polarized or non-polarized.
Non-polarized sunglasses are designed to filter out all light and as a result everything looks darker. Polarized sunglasses are designed to filter out certain types of light while still allowing other types of light to show through. Specifically, they are designed to filter out light reflected off of horizontal surfaces while letting through light reflected off of vertical surfaces.
Polarized lenses are useful for any activity which requires a reduction of glare off horizontal surfaces. For example fisherman often use polarized lenses to reduce glare off the water, which in turn increases eye comfort and allows for greater visibility into the water. Similarly, polarized lenses are useful when driving or playing outdoor sports. There are some instances where polarized lenses might be unadvisable and the non-polarized option is the better choice. For example, skiers do not want to block the reflection of light off of icy patches because the glare off these patches alerts them to the hazard. Furthermore, polarized lenses can reduce the visibility of images produced by liquid crystal displays (LCDs) which are commonly used for instrument panels and GPS systems.
Cost and Availability
Polarized lenses are typically more expensive than their non-polarized counterparts. However brand name also plays a large role in the cost of sunglasses so it is possible to find off-label polarized glasses which are less expensive than non-polarized designer brands. Both types of lenses are readily available from sunglasses vendors.
Manufacturing and Durability
Polarized lenses can be produced in a variety of ways. The oldest form of polarization is called lamination, where the polarizing film is placed between two pieces of lens substrate. The separation of the polarizing film from the glasses is a major issue with this manufacturing process but the onset of plastic polarized lenses has all but eliminated the problem because the plastic is poured into a mold which already contains the polarizing film. As a result, the film become suspended in the plastic as it hardens, resulting in a lens which cannot be separated from the polarizing film. Non-polarized lenses are manufactured by pouring the plastic into the lens mold without the presence of the polarizing film.
Styling and Customization
There are a variety of shades available ranging from gray to brown for both polarized and non-polarized lenses. Not all polarized lenses have to be dark gray. In addition a mirror coating can be applied to either type of lens that reflects light in an effort to further reduce glare.