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The most important function of sunglasses is to protect the eye against UV radiation. When they don't adequately filter ultraviolet (UV) light, it may even be worse to wear them than not to, because the dilation of the pupil will mostly be dictated by the amount of visible light falling into your eye.

I have several pairs of sunglasses lying around at home, but of most of them I don't know how well they block UV light. Is there any simple way to find out? More generally, is there any simple way to detect UV radiation? For example, to see infrared light (at least in some frequency range) you can use a digital camera, or (in an even more narrow band) an IR thermometer. Are there similar ways to detect UV?

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  • $\begingroup$ When you do find a method, be sure to test regular glasses too. (Both glass and plastic are generally rather opaque to UV, even without special treatment.) $\endgroup$
    – user10851
    Commented Jun 30, 2015 at 14:40
  • $\begingroup$ Stick the sunglasses onto 100 lab mice and see which ones grow cataracts first. [for the humor-impaired, this is a joke with a mild grain of reality included] $\endgroup$
    – Carl Witthoft
    Commented Jun 30, 2015 at 15:28

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If you or one of your friends has transition lenses for their glasses, then you can test the UV blocking ability with those. The transition to darker shades in these lenses is initiated by UV-light. So hold your sunglasses over some transition lenses and see if they start to turn darker.

You can try this with any photochromatic material really, but transition lenses were the first (and easiest?) example I could think of off the top of my head.

Side Note
If you want to test whether your sunglasses are really polarized, hold them between your eye and a cell phone display or any LCD. Rotate the sunglasses and if you do not see the cell phone display go black (or nearly black), then your sunglasses are likely not polarized. This is because all LCDs emit linearly polarized light (my old "flip phone" was at ~45$^{\circ}$ from horizontal of display when I last tried this) and all polarized sunglasses (of which I am aware) have a linear polarizing filter.

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Place the glasses between a UV counterfeit currency detector penlight, and the banknote. Best done at night.

If the banknote's special markings stop glowing, then at least you know the glasses block the 395 (or 365) nm of the penlight, which is in UV-A and near-UV.

However I found a cheap pair caused the banknote's protective markings to stop glowing, while with a pair marked "ANSI/ISEA Z87.1 CAN/CSA Z94.3 UV Filter Transmittance U6: .01% max effective far UV • .1% max near UV", the bill's marking glowed just as if I had not placed the glasses in the beam's path. So perhaps I am doing something wrong. I only tested at 395 nm though.

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you can buy self-adhesive UV dosimetry tags that change color from yellow to green to very dark blue upon exposure to UV light. These are used to test UV sources that are used to cure glues and inks on packaging production lines. These could be used to determine relatively how much UV is being blocked by each of your lenses.

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