The Collaborative International Dictionary of English v.0.48:

luminousness \lu"mi*nous*ness\ n.
   the quality of being luminous; emitting or reflecting light.

   Syn: luminosity, luminance, light.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

Luminous \Lu"mi*nous\, a. [L. luminosus, fr. lumen light: cf. F.
   lumineux. See Luminary, Illuminate.]
   [1913 Webster]
   1. Shining; emitting or reflecting light; brilliant; bright;
      as, the is a luminous body; a luminous color.
      [1913 Webster]

            Fire burneth wood, making it . . . luminous.
                                                  --Bacon.
      [1913 Webster]

            The mountains lift . . . their lofty and luminous
            heads.                                --Longfellow.
      [1913 Webster]

   2. Illuminated; full of light; bright; as, many candles made
      the room luminous.
      [1913 Webster]

            Up the staircase moved a luminous space in the
            darkness.                             --Longfellow.
      [1913 Webster]

   3. Enlightened; intelligent; also, clear; intelligible; as, a
      luminous mind. " Luminous eloquence." --Macaulay. " A
      luminous statement." --Brougham.
      [1913 Webster]

   Luminous paint, a paint made up with some phosphorescent
      substance, as sulphide of calcium, which after exposure to
      a strong light is luminous in the dark for a time.
      [1913 Webster]

   Syn: Lucid; clear; shining; perspicuous.
        [1913 Webster] -- Lu"mi*nous*ly, adv. --
        Lu"mi*nous*ness, n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

luminousness
    n 1: the quality of being luminous; emitting or reflecting
         light; "its luminosity is measured relative to that of our
         sun" [syn: luminosity, brightness, brightness level,
         luminance, luminousness, light]