How photochromic lenses work
One lens from a 5am start to full daylight. How photochromic tint works, how fast it changes, and where it does not work.
The mechanism
Photochromic molecules sit inside the lens. Under ultraviolet light they change structure and the tint darkens. When UV drops they revert and the lens clears. The process repeats and does not wear out.
On DARA and NOA the range is category 0 to category 4. Clear, the lens transmits 86.6% of visible light. Fully activated, it transmits 7.29%. Both figures are measured and published on the product pages.
Category 4 is not permitted for driving in the EU, the UK or Australia. The photochromic lens reaches category 4 when fully activated.
Speed, and the asymmetry
The lens darkens in about 30 seconds. It clears more slowly, over several minutes. That difference is worth knowing before you buy, because walking indoors from bright sun means a few minutes of a lens that is still dark.
Heat also affects it. In high temperatures the lens runs slightly lighter than it would in cool conditions, which is relevant in Singapore and Hong Kong.
Photochromic solves a timing problem rather than an optical one.
What it replaces
The alternatives are carrying two pairs, swapping a lens mid session, or accepting a tint that is wrong for half the run. One photochromic lens removes the decision, which is most of what the extra SGD 26 over the standard DARA is paying for.
Where it does not work
Photochromic lenses react to ultraviolet light, and most car windscreens already block UV. Behind glass the lens stays close to clear, so it is not a driving lens. Choose a fixed tint for that.
It also does not replace polarization. Photochromic manages how much light gets through. Polarized manages reflected glare. They are separate problems. How polarized lenses work.
Common questions
Do photochromic lenses work while driving?
No. They react to ultraviolet light and most windscreens block UV, so the lens stays close to clear in a car. Use a fixed tint for driving. Fully activated the lens also reaches category 4, which is not permitted for driving in the EU, the UK or Australia.
How fast do they change?
About 30 seconds to darken. Clearing takes several minutes.
Which models have it?
DARA and NOA, both at SGD 155.
