UVA, UVB, UVC: wavelengths, UV index and UV400

UVA, UVB and UVC: Wavelengths, UV Index and UV400 Explained

Updated 28/10/2026 · 4 min read

UV radiation is invisible but measurable: UVA, UVB and UVC differ in wavelength and in how much of each reaches us. Here you can read which wavelengths belong to which type, what the UV index says and what to look for in sunglasses.

Key points at a glance

UVA (315–400 nanometres) is the long-wave range, UVB (280–315 nm) the medium range and UVC (100–280 nm) the shortest and most energetic.
The atmosphere absorbs UVC. At ground level it is mostly UVA that arrives, plus a small share of UVB.
The UV index shows how strong UV radiation is, from 1–2 (low) to 11 and above (extreme).
With sunglasses, UV400 (the lenses filter UV radiation up to 400 nm) and the CE mark count, not the tint.

UVA, UVB and UVC: the wavelengths at a glance

UV stands for ultraviolet. This radiation sits right next to the violet end of visible light in the spectrum, but we cannot see it and we do not feel it as warmth. By wavelength it is divided into three ranges (1 nanometre is a millionth of a millimetre):

UVA (315–400 nm): the long-wave range. It makes up most of the UV radiation at ground level.
UVB (280–315 nm): the medium range, more energetic than UVA. The ozone layer holds back a large part of it, a small part reaches the ground.
UVC (100–280 nm): the short-wave, most energetic range. The atmosphere absorbs it completely. Artificial UVC is produced, for example, in lamps used for disinfection.

Where UV radiation gets stronger

UV exposure is not the same everywhere or at every time:

Sun position: At midday and in summer the sun stands high, and UV radiation is at its strongest.
Region: The closer to the equator, the stronger the UV radiation. On a holiday in the south, check the labelling on your sunglasses.
Altitude: In the mountains UV radiation increases because the layer of air above is thinner.
Reflection: Snow, water and light sand reflect UV radiation. That is why there can be plenty of UV in the snow in winter, too. More in our guide to ski goggles.
Clouds: They let part of the UV radiation through. Even on cloudy days the exposure is not zero.

The UV index: how strong is UV radiation today?

The UV index is an internationally standardised scale for sunburn-causing UV radiation. The higher the value, the stronger the radiation. You can find it, including the forecast for tomorrow, in many weather apps and from your national weather service.

1–2: low
3–5: moderate
6–7: high
8–10: very high
11 and above: extreme

From a UV index of 3, health authorities advise protective measures such as shade, covering clothing and sun protection products for the skin. At high values it is also worth checking your sunglasses: do they have UV400 and CE?

How does UV radiation affect skin and eyes?

UV radiation acts on skin and eyes. How deeply it penetrates depends on the wavelength: UVA goes deeper than UVB. Too much UV radiation can damage the skin. The International Agency for Research on Cancer (IARC) classifies UV radiation as carcinogenic to humans. This applies to natural sunlight as well as to artificial UV rays, for example from sunbeds.

In the eyes, too much UV radiation can irritate the cornea and the conjunctiva. Specialists call this photokeratitis, or snow blindness when it happens in the snow. Symptoms often appear only hours later. In the long term, UV exposure is also named as a risk factor for clouding of the eye lens (cataract). If your eyes are red, painful or sensitive to light after a lot of sun, have them checked by a doctor.

UV400, CE and lens category: what matters in sunglasses

UV400: The lenses filter UV radiation up to a wavelength of 400 nanometres, which covers UVA and UVB.
CE mark: The sunglasses meet the European requirements for sunglasses (standard EN ISO 12312-1). This includes the lens category, usually on the inside of the arm, for example “Cat. 3”. In Great Britain the UKCA mark can appear instead.
Tint: It tells you how much visible light gets through, not how much UV radiation. Even light lenses can have UV400, and dark lenses without a UV filter let UV radiation through. The categories run from 0 (almost clear) to 4 (extremely dark). Category 4 is not suitable for driving; for everyday use, category 2 or 3 is usually right.
Prescription lenses: Ask your optician whether the UV filter still reaches 400 nanometres after the lenses have been made to your prescription.

If the labelling is missing, for instance on glasses from a beach stall or an unclear source, the UV protection is uncertain. An optician can measure with a device how much UV radiation a pair of glasses lets through. For more on quality and lens categories, see Cheap Sunglasses with Good UV Protection.

Our sunglasses currently include models from Ray-Ban, Oakley, Prada, Miu Miu and Gucci. Check the details on UV protection and lens category on each model’s page.

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