What is the definition or description of: sun protection factor?

SPF measures UVB: Spf is the most familiar method of measuring sunscreen effectiveness, but it doesn't tell the whole story and new labeling regulations will be applied soon. Ultraviolet light from sunshine is divided into "a" and "b" wavelenths, with uvb causing burning and uva more linked to aging. And spf of 15 means that it takes 15 times longer to cause redness of the skin with uvb exposure.

Sunscreen SPF: Sun protection factor refers to the amount of protection a product gives you from getting a sunburn. Typically the higher the number the more protection is present. 50 spf is stronger than 20 spf.

SPF: Sun protection factor refers to the number of hours of sun exposure that would be equivalent to 1 hour of sun exposure without the sunscreen. Thus, for a sunscreen with an spf of 4, it would take 4 hours of sun exposure to equal 1 hour of sun exposure without this sunscreen. (which would be a burn for most of us.) the higher the spf, the better the protection and longer time ok in the sun!