Cold Makes Photochromic Lenses Darker - and Slower to Clear
Cold Makes Photochromic Lenses Darker - and Slower to Clear
Photochromic lenses work in a Canadian winter, and the cold makes them darker, not weaker. Measured at 6 °C they sat at 23.1% light transmittance against 34.6% at 21 °C. The catch is the trip back: fading to 80% clear took 6.4 times longer in the cold.
Cold is not the enemy of a photochromic lens. It is the enemy of a photochromic lens clearing. That single fact explains almost every complaint we hear about these lenses in Edmonton, and it is why our honest answer for an Alberta winter is still two pairs rather than one lens doing both jobs.
Cold means darker
The dye holds its dark form when there is less heat around to close it again.
Cold means slow
6.4 times longer to fade back at 6 °C. You walk into the store still in sunglasses.
The windshield wins
Car glass removes almost all the UV that triggers a standard photochromic lens.
Tint is not glare
Snow glare needs polarisation. A photochromic lens controls brightness only.
Do photochromic lenses work well in Canada?
Yes, and the cold is on your side for the part most people worry about. A photochromic lens has to do two opposite things: darken when light hits it, then give the tint back when you go inside. Canadian winter improves the first job and damages the second.
Researchers at Kangwon National University measured exactly that. They put 12 photochromic lenses from five manufacturers through a spectrophotometer at a cold 6 °C and a warm 21 °C and published the results in PLoS One in 2020. In the cold, the darkened lenses transmitted 11.5% less light. They also took far longer to recover.
Note the temperatures. Their "cold" was 6 °C, which is an Edmonton October afternoon, not an Edmonton January. Our winter sits well below the coldest condition they tested, so treat their numbers as the floor of the effect rather than the ceiling.
Why does cold make them darker?
A photochromic lens is not a filter that switches on. It is a dye that exists in two shapes. Ultraviolet light knocks the molecule into an open shape that absorbs visible light, and the lens goes dark. Left alone, the molecule relaxes back to its closed, colourless shape and the lens clears.
The important part: the way back is thermal. It runs on the ambient heat in the lens, which is why the PLoS One team measured fading as a function of temperature at all. Take the heat away and you take away the energy driving the return. More molecules stay open at any moment, so the lens settles deeper, and the ones that do close take their time about it.
One dye, two directions
The darkening leg runs on light. The clearing leg runs on heat. Winter only slows one of them.
What the lab measured at 6 °C and 21 °C
Every figure in this table is from the PLoS One study. Nothing here is our own testing, and nothing is a manufacturer claim.
| What was measured | Cold vs warm | What you notice on the street |
|---|---|---|
| Light transmitted in the dark state | 11.5% lower | A visibly deeper tint in January than in July |
| Change in optical density, clear to dark | 1.4 times higher | A bigger swing between the two states |
| Change in transmittance, clear to dark | 1.2 times higher | More of the lens tint range actually gets used |
| Fading rate by half-life | 2.7 to 5.4 times slower | The tint comes off reluctantly |
| Time to fade back to 80% transmittance | 6.4 times longer | You are indoors, at the till, still wearing sunglasses |
Swipe the table sideways to see every column.
Read the last two rows together. Half-life is how long it takes to shed half the tint; the 80% figure is how long until the lens is close to clear again. The gap between 5.4 and 6.4 tells you the tail of the fade is the worst part. The lens loses the heavy tint reasonably quickly and then hangs on to the last of it.
“it is necessary to provide consumers with information on photochromic optical properties, including the transmittance in colored and colorless states, and the fading rates at temperatures corresponding to the summer and winter seasons for each product.”Moon, Kim and Yu, PLoS One, 2020 — the study's own closing line
The fade-back clock
A multiplier is abstract. Minutes are not. Set the tool below to however fast your lens clears on a summer afternoon and watch what the cold does to the same lens on one fixed fifteen-minute ruler.
The fade-back clock
You just stepped indoors. How long are you in sunglasses?
Two lanes, one fifteen-minute ruler. The winter lane applies the 6.4 times longer fade measured at 6 °C, and each lane starts at the darkness the study actually recorded.
21 °C Summer afternoon still dark 2:00
Dark state let 34.6% of light through — the lighter of the two.
6 °C Cold day still dark 12:48
Dark state let 23.1% of light through — it starts deeper and ends later. off the ruler
A lens that clears in 2:00 on a summer afternoon needs roughly 12:48 at 6 °C. That is long enough to walk into a store, find what you came for, and reach the till still wearing sunglasses.
Which photochromic lenses get dark fastest?
Three things decide how fast a lens goes dark, and only one of them is the lens.
- How much UV actually reaches it. No UV, no reaction. Thin cloud passes plenty of UV; a windshield does not. Fresh snow throws UV back up at your face, which is why a bright winter field activates a lens hard.
- The temperature of the lens. Cold pushes the balance toward the dark form, so the same lens reaches a deeper tint outside in winter than it can manage in August.
- The dye package, the tint colour and the lens material. This is the part you buy. Transitions attaches its own footnote to every performance number: results are from grey lenses, and "photochromic performance may vary across colors and lens materials and is influenced by temperature and UV exposure."
On published speed, the current generation is genuinely quick. These are the makers' own figures, not ours.
| Lens | Darkening | Fade back | What it is for |
|---|---|---|---|
| Transitions GEN S | "Only 25 seconds to sunglasses dark (category 3)" | "Fadeback in less than two minutes", up to two times faster than the previous generation — measured to 70% transmittance at 23 °C | The fastest full cycle, dark and back again |
| Transitions XTRActive New Generation | "The darkest in hot temperatures & in the car"; filters up to 64% of blue-violet light behind a windshield | Not published on the page we checked | The one built to react behind glass |
| Any photochromic, in the cold | 11.5% lower transmittance at 6 °C than at 21 °C | 6.4 times longer to reach 80% transmittance | Winter does the darkening for you, then charges you for it |
Swipe the table sideways to see every column.
So the fastest lens to go dark is whichever current-generation lens you buy, in cold air, outdoors, in grey. The useful question in Alberta is not which one darkens fastest. It is which one lets you see again quickest when you step inside, and how many minutes you are willing to lose.
Do transition lenses work in the car?
A standard photochromic lens does almost nothing behind a windshield, and the reason is glass, not the lens. Laminated windshields are built to stop ultraviolet light. The lens is waiting for ultraviolet light.
The measurement comes from a 2016 JAMA Ophthalmology study of 29 cars from 15 manufacturers. Front windshields blocked an average of 96% of UV-A. Side windows blocked only 71%, and varied a lot from car to car. That is why drivers sometimes report a faint tint that seems to come from nowhere: it is the side window, not the windshield.
How much UV-A gets past car glass
Averages across 29 cars from 15 manufacturers, JAMA Ophthalmology, 2016. A UV-triggered lens has almost nothing to work with through the windshield.
If driving is the reason you want them
Say so before the lenses are ordered. Transitions XTRActive New Generation is the version built for it: Transitions calls it “the darkest in hot temperatures & in the car” and publishes filtering of up to 64% of blue-violet light behind the windshield. It is a different product from the standard photochromic lens, not a setting on it. Call (780) 490-0090 and we will tell you which options your prescription can be made in.
Will they handle snow glare?
Not the part that hurts. There are two different problems on a bright February morning and a photochromic lens only solves one of them.
Brightness is total light reaching your eye, and a tint fixes it. Glare is light bouncing off a flat surface such as snow, ice, a wet road or a windshield, and that reflected light is polarised. Cutting it takes a polarised filter, which a photochromic lens is not. You can darken the whole scene and still be squinting at the same white flare off the snowbank.
That is the honest limit of the technology, and it matters more here than in most of the country. Six months of snow on the ground is six months of reflected glare. See what a photochromic lens does and does not do on our transition lenses page, then compare it with the tinted and polarised options in the sunglasses collection.
One pair or two? The Alberta verdict
We fit both, and we will say plainly which one wins here. For a real Alberta winter, a dedicated tinted or polarised pair still beats one lens trying to do both jobs.
Three reasons, all of them from the numbers above. The fade-back is 6.4 times longer in the cold, so every building you enter costs you minutes of usable vision. The windshield stops 96% of the UV a standard photochromic needs, so the lens is closest to clear at 4 pm in December with the low sun straight in your eyes. And no photochromic lens is polarised, so the snow glare survives.
Photochromic lenses still earn their place. One pair to carry, UV protection on the days you would never have bothered with sunglasses, and a genuinely good answer for kids and for anyone who loses a second pair within a month. If that is you, a photochromic lens is the right buy and the cold will only make the tint better.
If you would rather have the right lens for each job, second-pair pricing is what makes it affordable: $99 for one single vision pair and $159 for two, or $350 for one progressive pair and $500 for two, down from a regular $700. Both packages include the frames, clear lenses and a scratch-resistant coating on both pairs, and the second pair is the one to have tinted — the tint or polarised lens is quoted on top of the package price. Details on our glasses deals page. If the pair you live in is mostly for screens, blue-cut lenses are available same day, which makes it a sensible everyday partner to a tinted pair.
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Wrap, for snow and wind
Pictured in Matte Black / Prizm Grey. A polarised colourway is listed for the same frame.
$140 CAD, polarised colourway $213
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Polarised, without the sport look
Pictured in Gold / Dark Green Polarised. Plain-tint colourways are listed lower.
$113 CAD polarised, from $88 plain
Ready-made sunglass prices, re-checked on 24 July 2026. Adding your prescription to a sun frame is quoted separately, and we ship across Canada with free shipping on orders over $100.
FAQ
Do photochromic lenses work in a Canadian winter?
Yes, and the cold actually helps the tint. In a 2020 PLoS One study of 12 photochromic lenses from five manufacturers, the lenses transmitted 11.5% less light in their dark state at 6 °C than at 21 °C, so they run darker in winter. The trade-off is recovery: the time to fade back to 80% light transmittance was 6.4 times longer in the cold.
Why do my transition lenses take so long to clear in winter?
Because the fade-back reaction runs on heat, not light. Ultraviolet light opens the dye molecules and darkens the lens, and ambient warmth is what closes them again. In cold air there is less heat to drive that return trip, so the lens holds its tint. Measured at 6 °C against 21 °C, the fading rate by half-life was 2.7 to 5.4 times slower.
Do transition lenses work in the car?
Standard photochromic lenses barely react behind a windshield, because they are triggered by ultraviolet light and the windshield removes it. A 2016 JAMA Ophthalmology study of 29 cars from 15 manufacturers found front windshields blocked an average of 96% of UV-A, while side windows blocked only 71%. Transitions XTRActive New Generation is the version made for driving: Transitions calls it the darkest of its lenses in hot temperatures and in the car, and publishes filtering of up to 64% of blue-violet light behind the windshield.
How fast do photochromic lenses clear?
It depends on the product and the temperature. Transitions publishes 25 seconds to sunglasses dark and a fade-back of less than two minutes for its GEN S lens, up to two times faster to fade back than the previous generation. Those numbers are measured on grey lenses at room temperature. In the cold, expect the fade-back to take several times longer.
Are photochromic lenses darker in cold or hot weather?
Colder is darker. The dark state and the clear state are two shapes of the same dye molecule, and warmth pushes the balance toward clear. That is why the same pair looks deeper on a January morning than on a July afternoon, and why the lab recorded a lower transmittance at 6 °C than at 21 °C.
Do photochromic lenses cut snow glare?
They cut the brightness, not the glare. Light bouncing off snow, ice or a wet road is polarised, and cutting it takes a polarised lens. A photochromic lens only controls how much light gets through, so it is not the same thing. If bright March mornings on the road bother you, a polarised pair is the tool for that job.
Should I get photochromic lenses or a separate pair of prescription sunglasses in Edmonton?
For an Alberta winter we usually suggest two pairs rather than one lens doing both jobs. A dedicated tinted or polarised pair is dark the moment you put it on, works behind the windshield, and can be polarised. Photochromic lenses win on convenience and on everyday ultraviolet protection. Two single vision pairs are $159 at Charm Optical and two progressive pairs are $500, both packages being clear lenses, so most people make the second pair the tinted one and have the tint or polarised lens quoted on top.
Pick the lens, then pick the tint
See what a photochromic lens does before you buy one
Our transition lenses page walks through the options, and the sunglasses collection is where the dedicated tinted and polarised pairs live. If you are not sure which side of this you fall on, bring your prescription to 5035 Ellerslie Road SW and we will show you both on your own face.
Questions about your own prescription? Call (780) 490-0090. Hours and directions on our visit us page. Prescription out of date? Eye exams are by appointment at see.charmoptical.ca.
Sources. Moon, Kim and Yu, Differences in the optical properties of photochromic lenses between cold and warm temperatures, PLoS One 15(5), 2020 — 12 lenses, five manufacturers, 6 ± 2 °C and 21 ± 2 °C. Transitions Optical trade pages for GEN S and XTRActive New Generation, read 24 July 2026. Boxer Wachler, JAMA Ophthalmology, 2016, on UV-A through car glass, summarised here. Charm Optical package prices verified against /pages/glasses-deal; frame prices re-pulled from the live product data on 24 July 2026.