Substantiating claims: What does it mean to “brighten skin” or “reduce the appearance” of something? Looking at parameters, measuring, and testing (long post)

Oh wow, did I go down the rabbit hole for a question posed by Susan on the monthly Q&A quite a few months back. As with many of the awesome questions you ask there, this one took a while because it’s become a series that might end up being an e-book as well. Thank you for…