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I could find many vague definitions of UV400, but no precise definition.

Examples of vague definitions:

https://knockaround.com/blogs/journal/what-is-uv400-protection:

Investing in sunglasses that are labeled UV400 blocks 99 to 100 percent of UV light. This rating ensures that all light rays with wavelengths up to 400 nanometers, including both UVA and UVB rays, are blocked out.

https://www.leightons.co.uk/blog/eye-care/a-guide-to-the-uv400-protection-rating-in-sunglasses:

If you want to go one step further and block out 99-100% of UV rays (for example if you’re keen on yachting and want to protect your eyes from the sun), then look for sunglasses with UV400. ‘UV400’ refers to UV rays at the top end of the UV spectrum, up to the 400-nanometre wavelength.

https://www.walmart.com/ip/Aviator-Sunglasses-Shades-Driving-Men-Women-Outdoor-Sports-Eyewear-Glasses-UV400/120334638:

These sunglasses feature UV400 lens technology, absorbing over 99.9% of harmful UVA and UVB spectrum.

Questions:

  1. Does UV400 guarantee that at least 99% of all wavelenths between 290 and 400nm are blocked?
  2. How about wavelengths shorter than UVB rays (e.g., UVC)?
  3. Is that 99 or 99.9% protection?
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  • The phrasing "up to 400 nm" implies blocking of wavelengths shorter than 400 nm, so UV-C should be covered, given that its spectrum is 200-280 nm.
    – bob1
    Jun 27, 2022 at 2:06

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"Does UV400 guarantee that at least 99% of all wavelenths between 290 and 400nm are blocked?"

From wikipedia:

Medical experts advise the public on the importance of wearing sunglasses to protect the eyes from UV; for adequate protection, experts recommend sunglasses that reflect or filter out 99% or more of UVA and UVB light, with wavelengths up to 400 nm. Sunglasses that meet this requirement are often labeled as "UV400".

Note that "are often labeled" is vague, and not exactly the same as a standard, so I'd answer "NO", in absence of an actual standard mark on the glasses.

To be safe, standards: https://en.wikipedia.org/wiki/Sunglasses#Standards_for_sunglasses

"How about wavelengths shorter than UVB rays (e.g., UVC)?"

UVC are expected to be fully absorbed by the ozone layer and the atmosphere (see https://en.wikipedia.org/wiki/Ultraviolet#Subtypes)

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  • And if not absorbed in the atmosphere, those wavelengths are quite readily absorbed in either plastic or glass that is used for eyewear. (Source - way too much time on my hands in various research labs while wondering about UV transmittance with or without various coatings on my glasses).
    – Jon Custer
    Jun 28, 2022 at 18:42

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