Sunglass Lens Categories 0-4, Explained

There are three numbers that describe a sunglass lens and they are completely independent of one another. UV protection says how much ultraviolet the lens blocks. Polarization says whether it filters horizontally reflected glare. Filter category — the one on this page — says how much visible light gets through, which is another way of saying how dark it is.

Almost every expensive mistake in eyewear comes from treating those three as one number. People assume a dark lens must be a protective one; it need not be. They assume a category 4 mountain lens is the “best” one; it is close to unusable in a town. So here is the scale, what each band is for, and how to find yours.

The scale: VLT and the five categories

The underlying measurement is visible light transmission, or VLT: the percentage of visible light that makes it through the lens to your eye. A clear prescription lens is near 90%. A glacier lens might be 5%. Category is just VLT sorted into bands.

The exact band edges vary slightly between the standards used in different markets, which is why the figures above are given as approximate. The practical shape of the scale is the same everywhere: 3 is the normal dark sunglass, 4 is a specialist mountain tool, and 0 to 1 are barely sunglasses at all.

Where the marking is

Look on the inside of one temple arm, usually near the hinge, in very small print or laser etching. You are looking for something like CAT 3, Category 3, or a number alongside a CE mark. Sport and mountain brands often go further and print the exact VLT percentage, because their buyers care.

Some brands put it on a lens sticker instead, which is unhelpful because the sticker comes off in the first week. If a pair carries no marking anywhere, the honest position is that you do not know its category — and you should not infer it from how dark the lens looks, because tint color changes perceived darkness considerably.

The mistake that matters: category is not UV

This is the point of the whole page. Filter category is a measurement of visible light. Ultraviolet is not visible light. A lens can transmit 5% of visible light and still pass a large share of UV, and a lightly tinted category 1 lens can block 100% of it. There is no relationship between the two numbers except the one manufacturers create by making good lenses well.

The reason this is dangerous rather than merely confusing is pupil response. Your pupils open in dim conditions, and a dark lens is a dim condition as far as your eye is concerned. So a very dark lens with no UV filter lets more ultraviolet into the eye than no sunglasses at all — the darkness opens the door and there is nothing on it. The American Academy of Ophthalmology makes this point directly when it warns that lens darkness and price are not proxies for protection.

Practical rule: check for a stated UV figure first — UV400 or 100% UVA/UVB — and treat category as a comfort and suitability decision after that. Our UV protection guide covers what the labels mean, and whether expensive sunglasses are worth it covers what you get for the extra money once UV is off the table.

Category and driving

Category 3 is the darkest class normally considered appropriate for daytime driving. Category 4 is not: it transmits under 8% of visible light, and mountain eyewear carrying a category 4 lens is usually marked “not suitable for driving” for exactly that reason. If you buy glacier glasses, keep an ordinary pair in the car.

At night, no tinted lens of any category is appropriate. Yellow “night driving” lenses are a category 0 or 1 tint sold on a claim the AAO has repeatedly said does not hold up — they reduce the light reaching your eye and make things worse. Our driving roundup covers this in detail.

How category interacts with tint color

Two lenses of the same category can feel completely different, because category says nothing about which wavelengths are being removed. A category 3 neutral gray dims everything evenly and keeps color accurate. A category 3 copper removes more blue than red, which raises apparent contrast and makes greens and browns separate — that is why golf and baseball lenses are warm and general-purpose lenses are gray.

So pick category for the light level and tint for the task. Our lens colors guide has the tint-by-condition table, and for the third independent variable — whether the lens filters reflected glare — our polarized sunglasses explainer covers what polarization does that no amount of darkness can.

Photochromic lenses and categories

A photochromic lens does not have one category — it has a range, and good brands publish both ends. A lens described as “category 2-4” starts as a medium sunglass and darkens to a glacier lens as UV rises; a “0-3” lens starts effectively clear and ends as a normal dark sunglass. That published range is the most useful spec on a photochromic lens and the one to compare between products.

The limit to remember: photochromics are triggered by ultraviolet, so behind a car windshield — which blocks most UV — a photochromic lens sits near the bottom of its range regardless of how bright it looks. Our photochromic guide covers the chemistry and the practical limits.

Which category should you buy?

• Category 3 for essentially everything: bright sun, driving, the beach, most sport. If you buy one pair, this is it.
• Category 2 if you live somewhere with a lot of overcast days, or you find dark lenses uncomfortable and end up taking them off — a lens you wear beats a darker one you do not.
• Category 4 only for glaciers, high-altitude snow and sustained mountain use. Never for driving. Our skiing and snow roundup covers when it is genuinely needed.
• Category 0 or 1 for style, for low light, and for eye protection where you need a lens but not a tint — night riding, indoor sport, workshop use.

And in every case: confirm the UV figure first. The category tells you whether the lens suits the conditions. The UV rating tells you whether it is doing the one job that matters for your eyes over a lifetime.