How are UV rays measured?

There are different ways to measure UV radiation. The mosty common one is by using an index number, which gives the approxiate danger level for humans. This index number is a result from taking different wavelengths into account, shorter wavelengths being more dangerous to cell structure than longer ones.

A better way however is to give the mW/(nm m2) (milliwatt per square meter per nanometer of wavelength).

For the exact calculations and more information, see:

http://www.epa.gov/sunwise/uvicalc.html

http://en.wikipedia.org/wiki/Ultraviolet_index#Definition