UV400 vs polarized sunglasses: what each does and when you need it

UV400 and polarized are not two versions of the same thing, so you rarely have to pick one over the other. UV400 means the lenses block light up to 400 nanometers, which covers all of UVA and UVB: that is protection for your eyes, and every pair should have it. Polarized means a filter that cuts glare reflected off flat, shiny surfaces such as water and wet roads: that is comfort and a clearer view, and it is not UV protection by itself. Many sunglasses have both.

So insist on UV400, then decide whether you want polarization for the places you spend time, and whether there is anywhere you would rather go without it. In mid-November the afternoon sun hangs low over Kyoto's western hills and the Kamo River turns into a strip of white light, a good season for noticing the difference between brightness that dazzles and light you cannot see at all.

Two features, two different jobs

UV400: protection you can't see

Ultraviolet light sits just beyond the violet end of what our eyes can see, so you can't tell by looking whether a lens stops it. UV400 means the lens blocks UV up to 400 nanometers (nm), taking in both UVA and UVB. The same promise is often written as "100% UV protection" or "100% UVA/UVB".

It matters because UV affects the eyes in two ways. A heavy dose in one day, on snow or water, can burn the cornea, the clear front surface of the eye. Years of smaller doses have been linked to cataract and to pterygium, a growth of tissue onto the cornea. The column on summer sun and your eyes covers these, along with hats and shade.

Polarized: comfort and clarity

Sunlight that bounces off a flat surface, such as a lake, a wet road or the hood of a car, comes back largely lined up in one direction, mostly horizontal. That is the hard white sheen that makes you squint. A polarized lens holds a filter set to block light lined up that way, so much of the glare is stopped while the rest of the scene stays in view, and you can see past the shine into the water or along the road.

Polarization does nothing about UV on its own. Most polarized sunglasses also block UV, but the two are separate features, so look for both on the label. How dark the lens is makes a third, separate choice: darkness is about comfort, not protection, and a dark lens without a UV filter can let more UV into the eye than wearing nothing, because your pupils widen behind it.

At a glance

Which to choose, place by place

Most people end up wanting UV400 everywhere and polarization for some places.

Driving

Glare off wet roads and the car ahead is exactly what polarized lenses handle well, and many drivers find them restful on a bright day. Check one thing first: polarized lenses can make some LCD screens, including car dashboards and navigation displays, look dark or patchy, sometimes only when you tilt your head. Sit in your own car with the displays on and look at each one before a long trip.

Keep the tint moderate, since very dark lenses make tunnels and shaded stretches hard to read, and take tinted lenses off as the light fades. No sunglasses are meant for night driving. If you need glasses to drive, your sunglasses have to correct your vision too. Autumn adds low sun at eye level and dappled light on wooded lanes, which the column on autumn leaves and glare describes.

Water, fishing and boating

This is where polarization earns its reputation. Water throws back a lot of glare, and people who fish like polarized lenses because the surface stops acting like a mirror and they can see into the shallows. UV also comes up off the water and in from the sides, so UV400 matters as much as the filter; choose a shape that follows the curve of your face and add a brimmed hat. On a clear winter morning by Lake Biwa, just over the hills east of Kyoto, the water can be brighter than the sky.

Snow and mountains

Snow reflects a large share of UV back toward the face, and UV rises as you climb, so UV400 comes first on any snowy day. For skiing and snowboarding, goggles that seal around the eyes protect better than sunglasses. Polarization is a matter of taste here: it calms the glare off a sunny slope, but some skiers prefer to see the shine that warns them of an icy patch.

Cycling, running and walking in town

For a walk around the city, UV400 is the part that counts, and polarization is a pleasant extra on rainy days when pavements and parked cars shine. On a bike it helps on wet roads, but some phone and bike computer screens can go dark at certain angles. A pair that stays put, and isn't so dark that curbs and shadows disappear, matters more than either label. Take them off when you step into a dim shop or a temple hall, where steps and raised thresholds vanish behind a tint.

Screens, pilots and people who operate equipment

Many phone and LCD screens give off light that is lined up in one direction, so through polarized lenses they may dim or go almost black at some angles. For most people that is a small nuisance. For anyone whose work depends on reading displays or instruments, it can matter more, and pilots and heavy-equipment operators are sometimes advised to avoid polarized lenses for that reason. If that is your work, follow the guidance from your training or employer.

Children

For children, check the UV label first. The lens inside a child's eye is clearer than an adult's, so more UV reaches the back of the eye, and toy sunglasses often carry no UV statement at all. Polarization is a nice extra at the beach or in the snow, not a requirement. What children need most is a pair they will keep on, with a soft frame and a strap for play. A brimmed hat and shade at midday do much of the work, and babies are better kept in the shade. Don't keep children indoors, though: studies suggest around two hours a day outdoors helps lower the chance of becoming nearsighted.

If you wear glasses

There are three main ways to get UV400, and polarization if you want it, over a prescription.

• Prescription sunglasses are made with your prescription in a tinted lens, and can be ordered with UV protection and, if you like, polarization. The glasses prescription reader explains sphere, cylinder, axis and add before you order. Strongly curved wraparound frames suit only some prescriptions, so ask the optician.
• Clip-ons attach to the front of your everyday frame and are easy to keep in a bag. Polarized versions exist; check them for a UV statement like any other sunglasses.
• Fit-over sunglasses are larger frames that go over your glasses, one pair for the car and the riverbank without a second prescription pair. Try them with your own glasses, since large or tall frames may not fit underneath.

Light-reactive (photochromic) lenses darken in response to UV and clear indoors. Car windshields block most UV, though, so they often darken only a little while you drive.

Contact lens wearers can use any nonprescription sunglasses. Some contact lenses absorb UV, but a lens covers only the cornea, not the white of the eye or the eyelids.

What helps when choosing between UV400 and polarized

The choice is less about one feature against the other than about matching a pair to your days. Several of these options are habits rather than things to buy.

Start with the UV statement: UV400, 100% UV protection or 100% UVA/UVB. Then think about where you will wear the pair. If glare from water, wet roads or snow bothers you, polarized lenses on top of UV400 make sense. If you read screens outdoors for work, fly, or operate equipment, check whether polarized lenses suit you, and choose a UV400 pair without polarization if not. In a shop, look at your phone through the lenses and turn it slowly; if the screen dims at some angle, the lenses are polarized. Pick a size that covers the eye area with small gaps at the sides and stays put when you look down. Skip anything with no UV statement, novelty sunglasses for children, and the idea that a darker lens protects more.

When to see an eye doctor about light and glare

Never look straight at the sun, in any sunglasses. UV400 and polarized lenses are both far too weak for watching a solar eclipse, which needs special eclipse glasses or solar filters made for the purpose. Sun damage to the retina can happen without pain, so if you notice blurred central vision or a blind spot after looking toward the sun, see an eye doctor promptly.

New sensitivity to light, or eye pain, is a reason to see an eye doctor rather than to buy darker lenses. Go the same day if:

• bright light has suddenly become painful, especially with redness or blurred vision, or in one eye only (contact lens wearers: take the lenses out first)
• your eyes hurt, water and feel gritty some hours after a day on snow, water or a beach, which can be a UV burn of the cornea; don't rub them
• you notice new flashes of light, a sudden shower of floaters, or a shadow or curtain across part of your vision

Sudden loss of vision in one eye, even if it comes back, or severe eye pain with blurred vision, halos around lights, headache or nausea, is an emergency: go to an emergency department or call your local emergency number (such as 911, 999, 112, or 119 in Japan). Eye emergencies and first aid explains what to do.

Slower changes count too. If glare from sunlight or headlights has become harder to bear over months, or colors look faded, mention it at an eye exam, since it can be an early sign of cataract. Finding an eye doctor and planning a visit helps you book one. Light sensitivity for a few hours after your pupils are dilated is expected; the guide to how long dilation lasts covers getting home.

Questions people ask

Is polarized better than UV400?

Neither is better, because they do different jobs. UV400 protects your eyes from ultraviolet light, and polarization makes glare easier to live with. Look for UV400 in every pair, and add polarization if glare bothers you where you spend your time.

Do polarized sunglasses block UV?

Not because they are polarized. The polarizing filter works on glare, and UV protection is a separate feature of the lens. Most polarized pairs also block UV, but check the label for UV400 or 100% UVA/UVB.

How can I tell if my sunglasses are polarized?

Look at a phone screen through one lens and turn the phone slowly. With many screens, the display dims or goes dark at some angle if the lens is polarized. The test tells you nothing about UV, so still check the label.

Are UV400 sunglasses enough for kids?

UV400 or 100% UV protection is the essential part, and polarization is optional. A comfortable fit, a strap for play and a brimmed hat matter as much as the lens.

Sources

• American Academy of Ophthalmology eye health pages
• Prevent Blindness