> In my imagination, the lab would have some a testing process that spreads a precisely-controlled volume over a standard surface area, textured to be similar to skin, then measures UV transmission percentage vs wavelength with a diffraction grating and photocell. Or something like that!

With this approach, how would you measure the effectiveness of the sunscreen when it's been absorbed by the skin (which is necessary for the sunscreen to work properly - that's why they always say to wait ten minutes after applying before going out into the sun)?

There's a reason in vitro and in vivo are both studied for clinical trials of medications. Sunscreen isn't any different: you're using a product making a specific claim about a clinical outcome, so that needs to be tested.

> With this approach, how would you measure the effectiveness of the sunscreen when it's been absorbed by the skin

You can eliminate the "can't possibly work" cases much faster and cheaper.

More importantly, it is cheap enough to be always used as a baseline verification when human testing is so expensive that it can only be used as a random sample double-check.

It's like unit testing vs full user acceptance testing. You can and should do both, but the latter isn't for every PR.

"that's why they always say to wait ten minutes after applying before going out into the sun"

They don't always say that. Some say explicitely that it provides instant protection. (there are different ways, that sunscreen provides protection)