Sales of sunglasses soar during summertime as consumers rush to protect their eyes from the sun. While style is an important factor when deciding which sunglasses to buy, lens type is crucial. Photochromic lenses are clear in artificial light and darken when exposed to ultraviolet rays. Polarised sunglasses are dark all the time and reduce glare. Each lens achieves a different purpose and works best for different needs.
A substance called silver halide or silver chloride causes the lenses in photochromic sunglasses to change colours. Molecules of the substance are transparent before exposure to ultraviolet rays, but a chemical reaction occurs after exposure and the molecules change shape, absorb light, and darken. Once the UV rays are gone, the molecules revert back to their transparent state.
The lenses on polarised sunglasses filter light from certain angles to reduce glare, such as the reflection on bodies of water. Glare is caused by light molecules that reflect horizontally off of surfaces and hit the human eye at a certain angle. Polarised lenses have a surface containing vertical stripes that only allow vertically-reflected light into the eye, thus reducing glare.
Photochromic lenses are great for people with corrective eyewear. Instead of buying prescription sunglasses in addition to transparent corrective glasses, those without 20/20 vision can buy photochromic glasses that also work as sunglasses. Drivers, boaters, and those who play summer sports benefit most from polarised sunglasses. The reduction of glare allows fishermen to see below the surface of the water, and for drivers to see past the glare of the road.
The thicker a photochromic lens, the darker it turns when exposed to ultraviolet light. This can be a problem for those who wear corrective lenses with a different prescription for each eye. Polarised sunglasses can block some useful light, such as the light from LCD screens. The time of day affects the way light is filtered through the lenses; if the sun is very low or very high, the glasses will not prevent glare as effectively.
The Bottom Line
Photochromic sunglasses are a good choice for those who wear corrective lenses and want a stylish alternative to bulky sunglasses that fit over regular glasses. Also, some people suffering from vision loss can turn to photochromic lenses with certain colour tints to help enhance vision. Polarised sunglasses are best for drivers and fishermen, as long as they can still read important instrument panels through the sunglasses.