A pretty well-known and often used ingredient with the magic ability to fade brown spots. It's used traditionally in Japan and can be found naturally in a couple of plants, including the leaves of pear trees, wheat and bearberry.
Arbutin seems to work its magic and hinder the pigmentation process at the second step of it. An enzyme called tyrosinase is needed to create melanin (the pigment that causes the brown spots) and while several other skin lightening agents work to inhibit the synthesis of tyrosinase itself (like vitamin C or licorice), arbutin lets tyrosinase be and rather hinders the melanin-forming activity of the enzyme. (So it might be a good idea to combine arbutin with some direct tyrosinase inhibitors for more skin lightening effect.)
All in all, arbutin is one of the better-known skin brightening agents, that's probably worth a try if pigmentation is an issue for you.
- Journal of Pharmacology and Experimental Therapeutics February 1996, 276 (2) 765-769; Arbutin: mechanism of its depigmenting action in human melanocyte culture
- Leslie Baumann, MD, Cosmetic Dermatology, 2nd edition, Arbutin - page 280