I wear spectacles in Thailand that I bought in the UK.

When I am in the UK, the lenses go quite dark in sunlight.

However when I am in Thailand, they do not darken.

A friend also has found the same with his spectacles.

Why do they not darken here?

Is it because the sun is more overhead?

Probably.

Or the UV is actually to HIGH!

I was wrong.. You were partially correct... Heat and height of the sun are factors!

http://superbeefy.com/how-do-photochromic-glasses-work-and-why-do-photochromic-lenses-turn-darker-in-the-winter-than-in-summer/

I wear spectacles in Thailand that I bought in the UK.

When I am in the UK, the lenses go quite dark in sunlight.

However when I am in Thailand, they do not darken.

A friend also has found the same with his spectacles.

Why do they not darken here?

Is it because the sun is more overhead?

Photochromic lenses work better at low temperatures.

As said heat is the reason - they do not work well in high temp - actually they do not work well anywhere in my opinion - a real danger driving through tunnels.

mine work fine

mine work fine

What brand.. I presume they work in Thailand!

I always buy photochromic lenses and work fine, get them from Modern Optical on Sukhumvit

Hmm...I always wondered why they turned extremely dark when it had snowed. The reflected light off the snow hits the lens at a right angle, and of course it's cold. Mystery solved.

The ones I brought with me from the UK don't work here, but the ones that I bought here work fine, so maybe different grades of photochromic???

I dont know, just guessing...

This is really interesting. I had no idea about photochromic lenses working less well in hot temperatures, so I've learnt something.

I recently bought some photochromic glasses from Zenni optical, the online seller. I've bought many pairs of prescription glasses from them before, and they have been really excellent as well as one fifth or less of the price of high street suppliers here.

But I was really disappointed with the photochromic lenses: going out into midday Bangkok they barely seemed to change, certainly not enough to reduce glare, in fact hardly enough to even be visible as a faint cosmetic tint. Useless as sunglasses.

After reading this I put them in the fridge for 30 minutes, then left them on the window ledge in the sun. They shifted to a much darker shade than I have seen them achieve before- very nearly sunglass dark. So this is true.

Doesn't help me much to use as sunglasses here but it's nice to know why!