brightener

From brighten +‎ -er.

brightener (plural brighteners)

1. One who or that which brightens.
   • 1830, The Mirror of the Graces: Or, The English Lady's Costume, page 27:

     Country air in the fields, or in gardens, when breathed at proper hours, is an excellent bracer of the nerves, and a sure brightener of the complexion.

   • 2006, Linda Fritzinger, Diplomat Without Portfolio: Valentine Chirol, His Life and "The Times":

     Chirol's own advice was simple: get the Russian agreement signed. March, month of change, brightener of days and moods, brought that action notably closer.

2. A chemical agent that absorbs ultraviolet and violet light and re-emits blue light by fluorescence, used to make fabric or paper look whiter.
3. (metallurgy) Any substance added to an electrolytic bath to yield a brighter electroplated surface.