Why do transition lenses go dark in sunlight when you’re outside but not when you’re in a brightly lit room?

Question

Why do transition lenses go dark in sunlight when you’re outside but not when you’re in a brightly lit room?

In: 12

Answer 1

First, a “brightly” lit room is FAR dimmer than direct sunlight. A typical brightly-lit indoor space is at an illumination of about 500 [lux](https://en.wikipedia.org/wiki/Lux). The direct light in a TV studio is perhaps 1,000 lux. Direct sunlight goes as high as *100,000* lux at noon in the tropics.

But it turns out the relevant part here is ultraviolet light. The sun is reasonably bright in near-UV, but indoor lighting is not, and it’s UV that triggers the lenses to darken.