What are the meanings of "UV Index" and "Sun Protection Factor" (SPF) marked on various UV protection articles?

UV index UV index UV intensity protection measures need to be taken: 1 level 0, 1, 2 is the weakest, so no protection measures need to be taken; Grade 2, 3 and 4 are weak, so some protective measures can be taken appropriately, such as applying protective cream. For grades 3, 5 and 6, wear a sun hat, sunglasses and umbrella when going out, and apply sunscreen with SPF index greater than 15, grades 4, 7 and 8. Besides the above protective measures, Top 9 should avoid going out from 10: 00 am to 4: 00 pm, or try to avoid outdoor activities in the shade, which is grade 5/kloc. When going out, all effective protective measures should be taken. Now all sunscreen products are specially marked with SPF. SPF and the following values indicate the SPF of the product. So, what does SPF mean? SPF value objectively reflects the protection ability of sunscreen products against ultraviolet UVB.