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I bought sunglasses in China that claims to have UV400 protection. I'd like to test this as my eyes' health is on the line :)

Is there any way to check the maximum UV amount these glasses can protect me against?

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    $\begingroup$ Easily - no AFAIK, but take them to a decent optician they may be able to help. Remember you get what you pay for - were they cheap? $\endgroup$ – Solar Mike Jun 16 '17 at 6:52
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    $\begingroup$ If your eyes' health is so much on the line, don't buy questionable sun glasses in the first place. $\endgroup$ – Olin Lathrop Jun 16 '17 at 11:25
  • $\begingroup$ @OlinLathrop well when you travel in China and buy glasses, you don't actually know which are questionable ;) and I canfind new sunglasses at around the same price anywhere in Europe ... (~10 euros) $\endgroup$ – dotixx Jun 16 '17 at 12:16
  • $\begingroup$ Can you access a spectrophotometer? If so, than this is easy. $\endgroup$ – Eric S Jun 16 '17 at 16:18
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Your question is a little naive: "Maximum UV protection" doesn't mean much.

What you want to know is the spectral attenuation values in the UVA and UVB wavelength bands. The glasses reduce the fraction of UV light that passes through them, so the stronger the source (e.g. noon Sun vs. sunset) the more photons per second will be transmitted. Since biological damage is cumulative (and nonlinear to boot), both the power level and the time of exposure matter.

Now, back to actual sunglasses. Quoting from allaboutvision.com,

Q: How can I find inexpensive sunglasses that are guaranteed to block 100 percent UV? Many are made in China and say 100 percent UV protection. But how do I really know? — M.D., North Carolina

A: The amount of UV protection that sunglasses provide cannot be determined by the color of the lenses, the country of origin or the price of the eyewear. The determining factor is the lens material and any additional UV-blocking coatings that may be applied. Uncoated plastic sunglass lenses block about 88 percent UV; polycarbonate lenses block 100 percent UV.

Many cheap sunglasses use a material called triacetate. This material absorbs only about 40 percent of the UV rays. Lenses may also be made of other acrylic materials, which will vary in how much UV protection they provide.

All sunglasses with polarized sunglasses block 100 percent UV, regardless of the lens material or price of the eyewear (though polarized sunglasses tend to be more expensive than sunglasses without this extra glare-blocking feature).

Personally, I far prefer polarized lenses in any case because of their glare-removal capability.

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  • $\begingroup$ I happen to hate polarized sun glasses as they make using some LCD screens impossible. $\endgroup$ – Eric S Jun 16 '17 at 16:20
  • $\begingroup$ @EricShain only screens produced by morons who never thought of the obvious use cases. But yeah, sadly those do exist. $\endgroup$ – Carl Witthoft Jun 16 '17 at 16:24
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The only real way to test this is to get a stable UV source and a diode that can detect that UV.

Measure the relative voltages from the diode with and without the sunglasses, and then you will know how much UV the sunglasses deflect.

This is pretty much the only way to test it yourself! Without these tools, you cannot test the sunglasses: what is transparent to UV is not always transparent in other parts of the spectrum. You cannot see UV rays, so merely looking at the glasses will not be enough.

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Someone proposed an original method. The result is binary (it doesnt give you a percentage) but it might gives you a rough idea if UV pass through the glass or not:

Testing can be one with a simple trick that will require access to a UV flashlight. You can find an affordable UV flashlight online or borrow one. Once you have it, go with it into a darkroom together with the sunglasses and paper money or a credit card. Turn on the flashlight and beam it on the credit card or money. Various strange symbols that you do not usually see in ordinary light will become visible. They include a line on the paper money and letters on credit cards. These are watermarks that the government and banks use to identify counterfeits.

source Ronak Furia, on quora

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