SPF is the sun protection coefficient, which refers to the ratio of the energy required to produce the smallest erythema on the skin protected by sunscreen to the energy required to produce the same degree of erythema on the skin without any protection. Simply put, it is the time multiple of skin's resistance to ultraviolet rays. The higher the SPF index, the greater the ability to protect the skin.
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SPF calculation method:
The value of the sun protection index is suitable for everyone. It is calculated by assuming that the intensity of ultraviolet rays will not change with time, and a person without any sun protection measures will turn red after staying in the sun for 20 minutes.
When he uses SPF 15 sunscreen, it means that it can be extended by 15 times, which means that it takes 300 minutes for his skin to turn red. Sun protection factor SPF is a test index to measure the defense ability of sun protection products against ultraviolet UVB in sunlight.
Sunscreen products can effectively resist the damage of ultraviolet UVA and UVB to the skin. If you want to know how long sunscreen can protect you, you should first know how long it takes for your skin to stay in the sun before it turns slightly red without sunscreen at all.
Then multiply the required time by SPF value to get the protection time of sunscreen. The SPF value marked on the sunscreen is the protection index it provides.
Baidu encyclopedia -SPF