A popular buzzword in fashion eyewear these days is ‘photochromic’. Do you know what the word means? Do you know what photochromic lenses are? If not, this post is for you. Keep reading to learn all about one of the hottest trends in prescription eyewear and sunglasses.
A pair of photochromic lenses are designed to darken and lighten based on the amount of natural light they are exposed to. On a sunny day, you would expect the lenses to get darker outside. They should lighten up automatically when you go indoors. How fast they change depends on the amount of natural light and the temperature. Needless to say that photochromic lenses are not cheap.
How the Effect Works
Photochromic lenses have been around since the 1960s, explains Olympic Eyewear. Back then, the lenses of both prescription glasses and sunglasses were glass. Making lenses photochromic involved a process whereby microcrystalline silver halide was embedded in the glass substrate. As a photochromic substance, this halide would get darker when exposed to natural light.
The thing that triggers the darkening phase is actually ultraviolet light. When UV rays hit the halide, they absorb light at various wavelengths. This is what makes the molecules in the photochromic material darker. As the amount of natural light diminishes, there is less for the halide to absorb. Thus, it gets lighter.
Modern polycarbonate – and other plastic – lenses are not made with the same silver halide as this does not play well with plastics. Manufacturers have discovered other photochromic substances that they can use instead. Those substances react to natural light the same way silver halides react.
Temperature Makes a Difference
It is interesting to note that temperature has a significant effect on photochromic lenses. For example, high heat reduces how well the photochromic substance absorbs sunlight. On especially hot days, light absorption is limited. That’s why most photochromic sunglasses don’t get as dark as standard sunglasses on exceptionally warm days. On the other hand, darker lenses return to their lighter state more quickly in warm temperatures.
Cold temperatures affect photochromic lenses in the opposite way. The colder the air temperature, the darker the lenses will get. But it also takes them longer to return to their lighter state once indoors. So it’s a trade-off. Photochromic sunglasses work one way during the summer and another way during the winter.
A hidden benefit of photochromic lenses is that they offer 100% protection against both UVA and UVB light. UVA is ultraviolet light that presents at wavelengths ranging from 320 nm to 400 nm. They penetrate further into the skin than their UVB counterparts, which present at wavelengths from 290 nm to 320 nm.
When you buy a conventional pair of sunglasses without photochromic lenses, the lenses themselves feature an embedded layer that filters out UV light. You can tell how protective the glasses are by looking at the UV rating. A rating of UV 400 tells you that the lenses filter all ultraviolet rays of both types. That means 100% protection.
Note that the ultraviolet layer in a pair of non-photochromic lenses is clear. It does not add any tint and works by filtering out UV light. A pair of photochromic lenses is different. Rather than filtering out UV light, the photochromic substance embedded inside absorbs the light. Either way, you get 100% protection.
Now you know what photochromic sunglasses are. If you are willing to pay for them, they can be quite convenient. Photochromic lenses are especially beneficial to prescription lens wearers who do not want to purchase a separate pair of sunglasses.