Photochromic lenses in Kathmandu: how fast they darken

Photochromic lenses darken in about 30 seconds of direct sunlight and take two to five minutes to go fully clear again indoors. That gap is the first thing people notice and the thing nobody warns them about. The second is that they stay nearly clear inside a car, because the trigger is ultraviolet light and a windscreen blocks most of it.

Here is how they actually behave through a Kathmandu year.

How photochromic lenses actually work

Millions of molecules sit inside the lens material. Ultraviolet light changes their shape, and the new shape absorbs visible light, so the lens goes dark. Remove the UV and the molecules relax back over a few minutes. Nothing electronic, nothing to charge, nothing to switch on.

Because the trigger is ultraviolet and not brightness, the lens does not care how bright a room is. It cares whether UV is reaching it.

The Kathmandu timing, season by season

Winter, December to February. Cold air favours the reaction, so the lenses go darker than in summer and take the longest to clear. Step from the street into a shop in Pulchowk at 11 am and you will be slightly tinted for two or three minutes.

Pre-monsoon, April and May. Heat works against the reaction. On a 32 degree afternoon the lenses will not go as dark as the same lenses did in January, which surprises people who expect the opposite.

Monsoon, June to September. Cloud cuts visible brightness far more than it cuts ultraviolet, so the lenses often sit at a medium tint on a grey day. That is correct behaviour, not a fault.

Where photochromic lenses fail

Driving is the big one. A laminated windscreen removes most UVA and almost all UVB, so the lenses stay close to clear while you drive, exactly when you wanted them dark. If your main reason for buying them is the Ring Road at 4 pm, buy sunglasses instead, or ask us about drive-specific photochromic lenses designed to react behind glass.

The second is depth of tint. A good photochromic lens reaches a medium to dark tint, but it rarely matches a dedicated category 3 sunglass. On a trek above the snow line that difference matters, as we explain in sunglasses for the Annapurna and Everest treks.

The third is age. Expect three to four years of daily wear before you notice the lens not going as dark as it used to. The prescription is unaffected; only the tint fades.

Who they suit

People who move in and out of buildings all day and do not want to carry two pairs. Students walking between blocks, shopkeepers, anyone on foot in Patan or Thamel for hours. One pair, no swapping, full UV protection either way.

They also suit anyone light-sensitive after an eye procedure, or with early cataract, where glare is the complaint rather than the prescription.

They suit riders less well than people expect, because a helmet visor cuts UV too. If you ride, look at what we wrote about UV400 versus polarised lenses before deciding.

Photochromic chasma in Nepal: what customers ask

"Ghaam ma matra kalo hunchha ki jahile?" Only in ultraviolet. Indoors under tube light or LED they stay clear, because those lights emit almost no UV.

"Gadi chalauda kaam garchha?" Mostly no, for the windscreen reason above. This is the single most common disappointment, so we say it before the sale rather than after.

"Power ko chasma ma milauna sakinchha?" Yes. Photochromic is a lens choice, so it goes with your prescription in almost any frame. A frame with enough lens area, like the Premium Rectangle Frame at NPR 1,500, shows the effect better than a very small one.

"Kati barsha tikchha?" Three to four years for the tint. After that the lens still corrects your vision perfectly; it just stops going as dark.

What we would tell you across the counter

Buy photochromic if you are on foot a lot and want one pair. Do not buy it as a substitute for sunglasses if you mostly drive. And if you want both, a clear multi-coated pair plus a separate UV400 sunglass such as the Navigator Round Aviator at NPR 1,800 often costs about the same as one photochromic pair and works better in both situations.

If you ride rather than walk, glasses that sit right under a helmet is the more useful page. Come in with your prescription and we will show you a demo lens under the UV lamp so you can watch the speed for yourself before you decide.