When photochromic lenses transform, it can look like a magic trick. One moment you’re wearing “regular” glasses with clear lenses… but as soon as the sun hits them, their lenses darken. Before you know it, you’ve switched to sunglasses without switching frames.
How does the magic trick work? This guide will explain the science behind glasses that turn into sunglasses, as well as their many benefits.
What Are Photochromic Lenses?
Photochromic lenses are lenses that stay transparent indoors, but darken when exposed to ultraviolet light from the sun. Essentially, they provide the benefits of glasses and sunglasses in a single frame.
The first photochromic lenses were patented in the 1960s and were made of glass. Plastic versions followed in the 1980s and ’90s, and are now the more popular choice among glasses wearers.
What Are Photochromatic Lenses?
Photochromatic lenses are the same thing as photochromic lenses: lenses that darken automatically in UV light. “Photochromatic” is simply a less common term for this type of glasses lens.
What Are Transition Lenses?
Transitions® lenses are a branded type of photochromic lenses. However, much like Kleenex and tissue, the Transitions® brand name (short for Transitions Optical) is often considered synonymous with the more general product. People also commonly omit the “s” from the end of Transitions® and call photochromic lenses “transition lenses.”
We’ll use “photochromic lenses” instead of “transition lenses” throughout this article to avoid confusion—we’re talking about all photochromic lenses, not just the ones sold by the Transitions® brand.
What Are Other Names for Photochromic Lenses?
Photochromic lenses go by many other names, including light-adaptive lenses, variable tint lenses, light intelligent lenses, and even “reactolights.” When we’re not using their scientific name, we prefer the term light-responsive lenses, because it neatly encapsulates exactly what these lenses do: they respond automatically to UV light.
If you can’t remember these terms, though, “lens that changes in the sun” usually gets the point across.
How Do Photochromic Lenses Work?
The light-sensitive molecules inside photochromic lenses shift their structure when they’re bathed in UV rays, allowing them to absorb more light and give the lenses a darkened appearance. Once the UV light goes away, the process reverses and the lenses become clear again.
These photochromic molecules are sensitive to both the presence and amount of UV light. They’ll change their structure more quickly when exposed to a lot of it, and more slowly if there’s only a little bit. That’s why you might notice your photochromic lenses darkening a bit less in the shade than out in the sun.
However, you don’t need to be standing directly in a sunbeam to get a dose of UV rays. UV light penetrates the clouds, and your photochromic lenses will still darken under overcast skies.
Want a closer look at how photochromic lenses work? Well, it’s a slightly different chemical reaction depending on the material of the lens.
How Do Plastic Photochromic Lenses Work?
Plastic photochromic lenses have carbon-based compounds within them that change their structure in the presence of UV light. These organic compounds are also known as photochromic dyes, and they have scary-long names like pyridobenzoxazines and indenonaphthopyrans.
You don’t need to know how to pronounce them—just know that they react to light more quickly than the chemical compounds in glass photochromic lenses. When the dyes are exposed to UV rays, a chemical bond inside them breaks, and the dye transforms into a different molecule that absorbs more visible light.
More light absorption equals darker lenses, and voila: your glasses are now sunglasses. When the UV light goes away, the dyes revert to their original form and the lenses clear up.
Plastic photochromic lenses are much more common than glass ones these days, and research is ongoing to see how they can be made even more advanced.
How Do Glass Photochromic Lenses Work?
Glass photochromic lenses have minute amounts of silver halide crystals (most commonly silver chloride) distributed inside them. When ultraviolet light hits these chemical compounds, the silver gains an electron and becomes elemental silver—the silver metal with which you’re probably familiar. The lenses appear darker as more and more silver molecules “appear” and absorb visible light.
Certain silver halides can also be used much the same way to make photographic film: exposure to light causes the image to darken and develop. However, unlike photos, glass photochromic lenses are able to revert back to their original clear appearance.
In the absence of ultraviolet light, a second compound (typically copper chloride) that’s also embedded within the glass takes the transferred electron back from the silver metal. The process reverses, and the lenses become transparent once more.
Glass photochromic lenses aren’t as prevalent in the eyewear world these days, but they paved the way for the light-responsive glasses of the future.
How Quickly Do Photochromic Lenses Work?
Photochromic lenses darken in 30–60 seconds on average when exposed to ultraviolet light. To clear, they take a bit longer: 2–3 minutes is normal.
Scientists are still working with the photochromic dyes to quicken the reactivity of the lenses. In the future, we could see faster and faster activation and clearing times.
How Does Temperature Affect Photochromic Lenses?
The rate at which photochromic lenses darken or clear up is impacted by temperature. When it’s colder, the lenses are apt to darken a bit more fully but take a longer time to clear up. When it’s warmer, they may not darken quite as much but will clear up more quickly, as the molecules within them are more reactive.
Do Photochromic Lenses Wear Out?
Eventually, your photochromic lenses will become less reactive to ultraviolet light and will take longer to shift between their two states. They may also take on a yellowish tint that signals their age. Most photochromic lenses will perform well for three years or more, however, so they’re still a long-term investment.
