An SPF number lets you know how effective sunscreen will be. The higher the number, the more effective it is.
According to the American Academy of Dermatology, dermatologists recommend using sunscreen with an SPF of at least 30, to block 97 percent of the sun’s UVB rays. Translated to window gets a little scientific.
SPF is a measure from 280 nm (or nanometers) to 400 nm of how much ultraviolet light is transmitted. A basic way to calculate the SPF of a product is to divide 100 by the UV transmission, for example 100/0.5 transmission = 200 SPF. This isn’t exactly accurate as SPF goes up to 400 nm, but it provides an estimate. Every film has a different SPF reading since each film is slightly different up to 400 nm.
For example, a clear film such as CL 800 by Madico® would have an SPF of 236 while a dark film like Sunscape® Starlite 8 by Madico would be 2,723. A film such as UV Gard by Madico, which is designed for UV protection, has a SPF of 1937 since it blocks further into the visible spectrum than a CL 800 type of film.
Those are some pretty huge numbers compared to that 30 SPF you put on yourself. SPF numbers are officially determined according to established testing protocols. No claims can be made about actual SPF numbers without having these tests performed by a qualified laboratory.