How & Why Photochromic Lenses Work

First I would like for you to know that this is my first blog and I am no English professor. So I hope you enjoy the content of the blog and over look the grammar errors. So with that said let's jump right into it.

You may ask why photochromic lens?

We will be touching on the history of the lens, how they work, and lastly advantages and disadvantages of photochromic lenses.

On the history side, at one time there was only one material available which was glass. Glass lens is very heavy and lot more likely to break if dropped also causing other problems if shattered pieces get into the eye. “Today there are many manufacturers of photochromic lens such as Sensity, Thin & Dark and PhotoFusion” (The Pros and Cons of Photochromic Lens, 2017) The most popular name for the lens you probably would recognize is “Transitions” and comes in plastic.

Photochromic comes from two words.

Photo - meaning light.

Chroma - meaning color.

Photochromic means something that changes color in response to light.

The earlier photochromic lens relied on silver chloride or silver halide to react to UV light and darken. Today’s photochromic lenses use proprietary dyes that undergo chemical changes that darken the lenses when exposed to UV light.

”William H. Armistead and Stanley Donald Stookey of Corning Glass Works (their US patent 3,208,860 describing the idea, titled Phototropic Material an article made therefrom, was filed on July 31, 1962)” .  In the early days of photochromic lens contained silver crystals and when light hits them they changed into microscopic bits of silver. As Armistead and Stookey explain in their original patent, a tiny bit of silver is needed (less than 0.1 percent by volume), and each crystal is less than 0.1 microns (one ten millionth of a meter—or about 100 times thinner than a human hair) in diameter. Unlike the photographic film that doesn't lighten up once it darkens, the photochromic lens does clear up. In the 1970’s British Pilkington glass company helped to popularize photochromic lenses by introducing brands called Reactolite and Reactolite Rapide (according to the US Patent and Trademark Office, these two trademarks were applied for in the United States in 1978 and both have since been cancelled). (See (Photochromic lens)

What makes Photochromic lens possible in plastic materials? These carbon base organic molecules called Naphthopyrans, they change when ultraviolet rays strike them resulting in darkening the lens. The reaction time is fairly quick. About half the darkening of the lens takes place in the first minute.

Now let's shift our focus onto the advantages and disadvantages of photochromic lens. First let's look at some disadvantages. Unfortunately they take longer to lighten up, you may find they darken more or less on cloudy days. This is due to the ultraviolet rays for that day. Temperatures will affect how dark they will get too. The colder the temperature the darker the lens will be and taking longer to lighten up when you go inside. This type of lens does not make good driving glasses due to most windshields block ultraviolet rays keeping the lens from darkening down. Also after about 3 years they lose their effectiveness of darkening down. So the solution for driving glasses would be a second pair of tinted sunglasses.

Some of the advantages of photochromic lens are they are clear inside but darken down like sunglasses outside. You only need one pair instead of a second pair saving you money. They come in several different styles, single vision, progressive, and line bifocal. They are a brilliant idea for someone that needs different glasses for different conditions and don't want to keep up with several different pairs of glasses. So If you are one of those people I would recommend you getting a pair photochromic lens.