Testing sunglasses for UV protection

Question:

Sometimes you spontaneously buy cheap sunglasses during your holidays. But there is a risk that the UV protection will be lower than indicated on the glasses. By darkening the light, the iris opens in the eye and lets more UV onto the retina than without glasses. This can lead to damage. Is it easy to test the UV protection of sunglasses yourself?

Answer:

A very simple option is to buy a UV pen with an integrated UV LED. Sometimes also called "Invisible Ink Pen", "Secret Pen" or "Spy Pen". Costs so approx. 1-2 €/$, for example this one here on Deal Extreme (there are many such products, not meant as advertising).

If you have it, then paint a cross on a sheet of paper. Now let the LED shine through the sunglasses onto the cross. If the cross is not visible, the sunglasses block the UV. The LED is quite bright, so it's no problem even in a department store.

The nice thing about the pen is that you can have it with you at any time because it's small, light and cheap.

However, one should still consider: The UV LED probably only shines in a narrow frequency band close to the visible light. The test probably does not prove that the entire spectrum of harmful rays is blocked. Seen in this light, the test is probably incomplete. The assumption is obvious, however, that a manufacturer of UV-blocking sunglasses is trying to block all UV rays, not just a partial spectrum.

By the way: As a further application, you can also use the pen to find out whether a hotel room is being cleaned at all. For example, you can make a cross on your toilet seat and check if it is smeared when you come back.