“Aren’t polarized lenses just darker sunglasses?”
I’ve heard this at the counter more times than I can count. Both kinds of lenses make the world look a little dimmer, so it’s no surprise people treat them as the same thing. But they reduce light in very different ways: ordinary sunglasses dim all the light uniformly, while polarized lenses remove the light vibrating in one particular direction. The key word is “remove,” not “dim.”
Light vibrates in all directions; a polarizing film lets through only one
To explain that, you need one basic fact: light is an electromagnetic wave, and it vibrates in all sorts of directions.
- Natural (unpolarized) light: light from the sun or a light bulb vibrates every which way, with no pattern.
- Polarized light: light whose vibration is restricted to a single plane.
At the heart of a polarized lens is a polarizing film. Picture a fence with slats running in one direction only: light vibrating along the gaps gets through, and light vibrating in other directions is kept out.
Why glare off water and asphalt is especially easy to block
After reflecting off a horizontal surface, light vibrates mostly horizontally
When sunlight hits a flat, smooth, horizontal surface such as water, asphalt, a car bonnet or a glass tabletop, the reflected light no longer vibrates every which way; it vibrates mainly horizontally. Polarized lenses are designed with the film’s transmission axis set vertically, which blocks exactly that horizontally vibrating glare.
That’s why, looking through polarized lenses at water or a road after rain, the harsh white sheen fades noticeably while the background doesn’t get darker with it. The lens hasn’t darkened; the vibration direction of that glare simply matches the direction the film filters out.
Brewster’s angle: at one angle, reflected light is almost entirely horizontal
How strongly the reflection leans horizontal depends on the angle. Near a particular angle of incidence (known in physics as Brewster’s angle), reflected light is almost completely horizontally polarized, and the film blocks it most cleanly; the further you get from that angle, the more light of other orientations is mixed in, and the smaller the share that can be filtered. That’s also why the same polarized lenses often work more visibly on water in the distance than on a puddle at your feet.
Polarized lenses and ordinary sunglasses reduce light differently
| Item | Ordinary Tinted Sunglasses | Polarized Lenses |
|---|---|---|
| How they reduce light | Lower the brightness of all light | Remove light vibrating in a particular direction |
| Effect on water glare | Glare darkens along with the background; the contrast stays the same | Horizontally vibrating glare is largely filtered out |
| After rotating 90° | No change | Filtering changes (see the test below) |
| Key structure | Tinted lens body | Contains a stretched polarizing film |
How a polarizing film is made: a film stretched in one direction
An iodine-containing polymer film (PVA) is stretched in a single direction, so the long-chain molecules inside line up in parallel, forming countless molecular “fence slats” all pointing the same way. Vibration parallel to the slats is absorbed, and vibration perpendicular to them is let through. That’s where the film’s ability to pick a direction comes from.
A polarization test you can do yourself: cross two lenses
Stack two polarized lenses (two pairs of polarized sunglasses will do) and slowly rotate one of them:
- When both are aligned, you see clearly.
- When they’re at right angles (90°), it goes almost completely black.
The first lens restricts the light to a single direction, and the second blocks that direction, so of course nothing gets through. If you want to know whether a pair is really polarized, this is the fastest way.
Our Xinzhuang and Banqiao stores, and booking
Both our Xinzhuang and Banqiao stores make prescription polarized lenses. ZEISS and HOYA, the lens brands we carry, both have polarized ranges, available for myopia, hyperopia and astigmatism, and progressive lenses can be made polarized too; the ZEISS product names are listed on our lens range page. Prescription polarized lenses are corrective lenses, so an eye exam comes first; the exam itself takes about 45 to 60 minutes.
Whether you should choose polarized, photochromic or ordinary tinted lenses depends on where you’re out in the sun each week.
- Beyond Visual Optometry (BiYang Optometry Clinic), Xinzhuang store: No. 419-1, Zhongzheng Rd., Xinzhuang Dist., New Taipei City, near MRT Xinzhuang Station, +886-2-2206-6700, led by Optometrist YoYo
- Beyond Visual Optical (Beyond Visual Optometry Clinic), Banqiao store: No. 28, Ln. 47, Sec. 2, Shuangshi Rd., Banqiao Dist., New Taipei City, near MRT Jiangzicui Station, +886-2-2253-1246
Book a vision consultation, and bring the glasses you currently wear.
A lightly tinted polarized lens blocks water glare just as well
By now you can let go of the idea that polarized lenses have to be dark to work. The glare-blocking ability lives in the polarizing film, not in how deeply the lens is tinted. The film removes horizontally vibrating light regardless of whether the lens body is grey or brown, light or dark; so a lightly tinted polarized lens still blocks that white sheen on water, while a very dark tinted lens just darkens the glare and the background together, without reducing the glare’s share at all.
The flip side is worth remembering too: a lightly tinted polarized lens is still a tinted lens. The sunglasses buying guide from Taiwan’s Bureau of Standards, Metrology and Inspection (BSMI) requires sunglasses with a luminous transmittance between 8% and 75% to carry the warning “not suitable for twilight or night driving.” A light tint doesn’t mean you can drive at night in them; check the label when you buy.
Common Misconceptions / FAQ
Q: Can polarized sunglasses be made with a prescription?
Yes. Myopia, hyperopia and astigmatism can all be made polarized, and so can progressive lenses. Prescription polarized lenses are corrective lenses, so you need an eye exam before they’re made. If your prescription is on the stronger side and you want a curved wraparound frame, lens edge thickness and peripheral distortion need to be assessed first; our optometry staff will tell you whether that frame suits your prescription.
Q: Why do LCD screens or car dashboards look black or show rainbow patterns through polarized lenses?
Because many LCD screens emit polarized light themselves. If the screen’s polarization direction happens to be perpendicular to your lens’s transmission direction, that light gets blocked, so the display looks dim or even completely black; tilt your head and the angle changes, and colored patches may appear. The lenses aren’t defective; this is the normal optical result of two layers of polarization meeting.
Q: How can I check if a pair of glasses is genuinely polarized?
There are two quick ways. One is to cross two polarized lenses and rotate them: they go dark when perpendicular. The other is to look at an LCD screen through the lens and tilt your head: if the screen’s brightness changes with the angle, it’s polarized. If the lens is merely dark and you see none of these changes, it’s most likely an ordinary tinted lens.
Q: Can polarized lenses filter out all glare?
No. Polarization only works on reflected light that has already become polarized (mainly from flat, smooth horizontal surfaces), and it also depends on the angle. Light scattered back from rough, irregular surfaces has no fixed vibration direction to begin with, so the film can’t do much about it. Polarized lenses remove glare under specific conditions; they don’t switch off every bright light.
Polarized lenses filter glare off water because of a film that lets through only one direction of vibration, which happens to match reflections that vibrate mostly horizontally. When choosing polarized lenses, first ask whether they’re polarized, then ask how dark the tint should be, and keep that order.