# How can I measure the ability of sunglasses to block UV radiation?

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?

• 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.) – user10851 Jun 30 '15 at 14:40
• 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] – Carl Witthoft Jun 30 '15 at 15:28

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.