Light and glare
Photochromic vs polarised lenses: how they work and which to choose
Both are sold as the answer to bright sunlight, and both are often confused with each other. A photochromic lens changes its darkness; a polarised lens changes which light gets through. They solve different problems, behave differently in a hot car, and can be combined.
How photochromic lenses work
A photochromic lens contains molecules that change shape when ultraviolet light hits them. In the open form they absorb visible light and the lens darkens; take the UV away and they relax back, and the lens clears. In modern plastic lenses the molecules are organic compounds embedded in or just below the surface; older glass versions used silver halide crystals.
The change is quick but not instant. Most lenses reach the greater part of their darkness in under a minute outdoors and take a few minutes to clear back indoors, with the last trace of tint fading more slowly. Because the return to clear is a thermal process, temperature matters: on a cool day the lens goes darker and clears slowly; in Gulf heat the molecules relax faster, so the same lens settles at a lighter shade. A photochromic lens that looks impressively dark in a European winter will look medium-grey on a July afternoon here.
Behind a windscreen
Car windscreens are laminated glass that blocks most ultraviolet light. Standard photochromic lenses therefore stay nearly clear while you drive, which surprises people who bought them for exactly that. Newer formulations respond to visible light as well as UV and reach a moderate tint in the car, but still not their full outdoor darkness. If driving is the main reason for the purchase, this is the first question to ask.
How polarised lenses work
Light reflected off a flat surface such as a road, a bonnet, water or sand comes off vibrating mostly in the horizontal plane, especially at the shallow angles you see when driving or looking across the sea. A polarised lens contains a thin film of aligned molecules that absorb horizontally vibrating light and pass the rest. The reflected glare is removed almost entirely while the scene itself stays visible, which is a different experience from an ordinary dark tint that simply dims everything.
Polarised lenses are a fixed tint. They do not clear indoors, and they have a few side effects worth knowing. Phone and dashboard screens, fuel pump displays and ATM screens use polarised light themselves and can dim or go black when you tilt your head. Tempered glass in car side windows shows faint rainbow patterns. Pilots are generally advised against them for these reasons. For everyone else, the glare reduction on roads and water is hard to give up once you have driven with it.
Head to head
| Feature | Photochromic | Polarised |
|---|---|---|
| What it does | Darkens in UV, clears indoors | Blocks horizontally reflected glare, fixed tint |
| Indoors | Clear or near-clear | Too dark |
| Behind a windscreen | Little or moderate activation | Works fully |
| Glare off road or water | Reduces brightness only | Removes most of it |
| Hot weather | Lighter shade, faster fading | Unaffected |
| Screens and dashboards | No effect | Can dim or black out at angles |
| With prescription | Yes, all common designs | Yes, all common designs |
| Best for | In-and-out days, convenience, children | Driving, water, outdoor work |
Combining both in one lens
Some lenses now do both. They are clear or lightly tinted indoors, darken outdoors in the usual way, and the darkening layer is engineered so that the lens also becomes polarised as it darkens. The best-known example is Transitions XTRActive Polarized, which its maker describes as reaching full darkness and polarisation outdoors and a moderate tint behind a windscreen. In the UAE, Transitions XTRActive Polarized lenses are offered by Optic One with prescription options, which makes them a practical one-pair solution for people who move between office, car and outdoors all day. The compromise is cost and the fact that a combined lens is never quite as dark or as fully polarised as a dedicated polarised sunglass.
Which to choose
Choose photochromic if you…
- Want one pair for indoors and out without swapping glasses.
- Walk or work outdoors in short bursts rather than for hours.
- Are buying for a child who will not manage two pairs.
- Accept that the lens will be lighter in summer and nearly clear in the car.
Choose polarised if you…
- Drive a lot, especially on bright roads with low sun.
- Spend time on or near water, at the beach or fishing.
- Work outdoors for long periods under full sun.
- Can keep a separate clear pair for indoors and screens.
Frequently asked questions
Why do my photochromic lenses not darken in the car?
The windscreen blocks most of the ultraviolet light that activates them. Visible-light-activated versions reach a moderate tint in the car; standard versions stay nearly clear.
Can I get polarised lenses in my prescription?
Yes. Polarised lenses are available in single vision and progressive designs in most materials, including thin high-index options for stronger prescriptions.
Why does my phone screen go dark with polarised sunglasses?
The screen emits polarised light, and at certain angles its polarisation crosses the filter in your lens. Tilting the phone or your head usually brings the picture back.
Do photochromic lenses wear out?
Their response slows and lightens gradually over a few years of use. Most people replace lenses with a new prescription before this becomes a problem.
Do either of them protect against UV?
Both types block UV in practice, and so does a good clear lens. Darkness and UV protection are separate things: a dark lens without UV blocking is worse than no sunglasses at all, so check the UV400 claim rather than the shade.