The Sunday Riley Ceo Rapid Flash Brightening Serum was designed to serve as a brightening and dark spot lightening serum. This product features key active ingredients including 15% THD ascorbate, which is a vitamin C ester. Using a vitamin C ester in skincare products, especially those that are designed to target hyperpigmentation and dark spots, can be a bit of a double edged sword. One the one hand, these esters have been stabilized, which allows them to remain active and usable for much longer than your traditional L-Ascorbic acid based serum. This stability also helps to minimize irritation. At the same time, most vitamin C esters don’t have the same amount of research behind them, so we don’t currently know if these types of ingredients work as well as pure vitamin C. Because of this, we’d recommend this product to those who are new to vitamin C and are concerned about possible irritation.