On sun screen creams you will find a SPF, or sun protection factor figure, which attempts to give you some idea how long you can stay out in the sun without getting sunburn after applying it.

This figure is experimentally determined by taking a volunteer, placing them in the sun and timing how long it takes their skin to redden, with and without the sunscreen in question. The compound is applied in a standard dose of 2 milligrams per square centimeter of skin, and the SPF is given by the following equation :-

SPF = time taken for skin to redden with sunscreen / time taken for skin to redden without sunscreen

One major problem is skin reddening is caused by UVB radiation (280-320nm), no information is given about UVA protection. Most sun creams have this information given seperately now however. Also none of the creams as far as I now provide protection against UVC, as the ozone layer does that for us, at the moment.....

Source :- The Right Chemistry, Jeffery Hancock, Hodder & Stoughton ISBN 0-340-70194-3