A relatively new rating designation for sun protective textiles and clothing is UPF (Ultraviolet Protection Factor), which represents the ratio of sunburn-causing UV measured without and with the protection of the fabric. For example, a fabric rated UPF 30 means that, if 30 units of UV fall on the fabric, only 1 unit will pass through to the skin. A UPF 30 fabric that blocks 29 out of 30 units of UV is therefore blocking 96.7%. Unlike SPF (Sun Protection Factor) measurements that traditionally use human sunburn testing, UPF is measured using a laboratory instrument (spectrophotometer or spectroradiometer) and an artificial light source, and then applying a sunburn weighting curve (erythemal action spectrum) across the relevant UV wavelengths. Theoretically, human SPF testing and instrument UPF testing both generate comparable measurements of a product's ability to protect against sunburn.
From my understanding, this means that SPF should always be equal to UPF, i.e. that they measure the exact same thing.
However, both https://www.blockislandorganics.com/Blog/post/2018/07/11/Is-There-A-Difference-Between-SPF-UPF.aspx (mirror) and https://blog.coolibar.com/whats-the-difference-between-spf-and-upf/ (mirror) claim that UPF measures UVB and UVA protection whereas SPF only measures UVB (which contradicts Wikipedia, since the latter claims that both SPF and UPF are erythemally weighted). Also, if SPF is equal to UPF, then why having two metrics?
This confuses me. What is the difference between SPF (Sun Protection Factor) and UPF (Ultraviolet Protection Factor), if any?