HOW IS THE SUN PROTECTION FACTOR DETERMINED ?
The Sun Protection Factor, commonly known under the acronym SPF is the main measurement of the solar protection.
It has the advantage of having the same definition in every country and measured by the same International Sun Protection Factor Test Method.
Test method: 2 mg of sunscreen is applied per cm² of skin (on the back) of volunteers, who are then exposed to UV. 24h later, the Minimal Erythema Dose (MED) is determined, which is the smallest dose of UV light causing redness. The SPF will be a the difference of MED on protected and unprotected skin. This measurement comes down to the difference between amounts of UV required to cause sunburn with and without sunscreen.