The Collaborative International Dictionary of English v.0.48:

Lighten \Light"en\ (l[imac]t"'n), v. i. [imp. & p. p.
   Lightened (l[imac]t"'nd); p. pr. & vb. n. Lightening.]
   [OE. lightenen. See Light to kindle, illuminate.]
   1. To burst forth or dart, as lightning; to shine with, or
      like, lightning; to display a flash or flashes of
      lightning; to flash.
      [1913 Webster]

            This dreadful night,
            That thunders, lightens, opens graves, and roars
            As doth the lion.                     --Shak.
      [1913 Webster]

   2. To grow lighter; to become less dark or lowering; to
      brighten; to clear, as the sky.
      [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "lightened":
   ablaze, aglow, alight, bathed with light, bespangled, blanched,
   bleached, bleached white, brightened, candlelit, decolored,
   decolorized, drained, drained of color, enlightened, firelit,
   gaslit, illuminated, in a blaze, irradiate, irradiated, lamplit,
   lanternlit, lighted, lit, lit up, luminous, moonlit, spangled,
   star-spangled, star-studded, starlit, studded, sunlit, tinseled,
   whitened