Frombrighten +‎-er.

brightener (pluralbrighteners)

1. One who or that which brightens.
   • 1830,The Mirror of the Graces: Or, The English Lady's Costume, page27:

     Country air in the fields, or in gardens, when breathed at proper hours, is an excellent bracer of the nerves, and a surebrightener 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. Achemicalagent thatabsorbsultraviolet andvioletlight and re-emitsblue light byfluorescence, used to makefabric orpaper look whiter.
3. (metallurgy) Any substance added to anelectrolyticbath to yield abrighterelectroplatedsurface.

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