The Collaborative International Dictionary of English v.0.48:

Brighten \Bright"en\, v. t. [imp. & p. p. Brightened; p. pr. &
   vb. n. Brightening.]

   Note: [From Bright, a.]
   1. To make bright or brighter; to make to shine; to increase
      the luster of; to give a brighter hue to.
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   2. To make illustrious, or more distinguished; to add luster
      or splendor to.
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            The present queen would brighten her character, if
            she would exert her authority to instill virtues
            into her people.                      --Swift.
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   3. To improve or relieve by dispelling gloom or removing that
      which obscures and darkens; to shed light upon; to make
      cheerful; as, to brighten one's prospects.
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            An ecstasy, which mothers only feel,
            Plays round my heart and brightens all my sorrow.
                                                  --Philips.
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   4. To make acute or witty; to enliven. --Johnson.
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The Collaborative International Dictionary of English v.0.48:

Brighten \Bright"en\, v. i. [AS. beorhtan.]
   To grow bright, or more bright; to become less dark or
   gloomy; to clear up; to become bright or cheerful.
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         And night shall brighten into day.       --N. Cotton.
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         And, all his prospects brightening to the last,
         His heaven commences ere world be past.  --Goldsmith.
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WordNet (r) 3.0 (2006):

brighten
    v 1: make lighter or brighter; "The paint will brighten the
         room" [syn: brighten, lighten up, lighten] [ant:
         darken]
    2: become clear; "The sky cleared after the storm" [syn: clear
       up, clear, light up, brighten] [ant: cloud,
       overcast]