How are UV rays measured?

Answer 1

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

Answer 2

The oldest method is the use of Ion chamber. Ion chamber construction is based on two separated metal layers connected to the high voltage. If the X-ray beam is passing through them the current starts to flow and it's proportional to the amount of radiation.

There are also semiconductor detectors and they are based on some materials that change their resistance when irradiated.