How do you test to see if sunglasses truely block 100% of UV rays?

you can get kodak paper that they use for developing photos at a amera store. you have to put the glsses on the paper then release it to sunlight. make sure that the lens is entirely covering one part of the paper. take the lens off in a darker room after 20 minutes. if the paper is darker, that means uv passed through. if its the same color as before, then they protect against uv. there is a lot of sources of error in this experiment, like exposure to the sun before putting the glasses on, and also making sure that the paper is completely covered.

another thing: glass blocks 99.8% of UV radiation, but this experiment is for plastic.

Hey, I read about this on Digg yesterday.