Someone asked me on twitter which corresponded to the protection factors found in sunscreens, here is the answer.
The sunscreen factores ensures a level of protection against UVB rays (those who tan). The protection offered by an index will be identical whatever the product, its form or its country of marketing. Can be found next to these indices measure for UVA (those that give cancer) but no regulations oblige today.
The calculation is based on international recommendations Colipa. It is spread back portion of volunteer testers 2mg / cm ² of sunscreen, to expose them to a constant source of UVB during a specified period and increasing gradually to measure and test and measurement 24H later redness the epidermis. The sun protection factor equal to the ratio of the time it takes for a tester to blush in a protected area on the time it takes to get a sunburn on unprotected area.
For example, if a tester blushed after 2 hours (120 minutes) on a protected area when he did need 10 minutes to blush on an area not protected, the protection factor will be 12 (120 / 10 = 12). Such a tester who takes a sunburn in 5 minutes without protection, so he needs 3H (180 minutes) to take on a protected area, the index is 36 (180 / 5 = 36).
Finally, the IPS are indications that because all people are equal before the misdeeds of UVB, and pale skin will naturally protect themselves better than matte skin, but somehow as winter approaches, opt instead for a tube of self-tanner:)