The Collaborative International Dictionary of English v.0.48:

Brightness \Bright"ness\, n. [AS. beorhines. See Bright.]
   1. The quality or state of being bright; splendor; luster;
      brilliancy; clearness.
      [1913 Webster]

            A sudden brightness in his face appear. --Crabbe.
      [1913 Webster]

   2. Acuteness (of the faculties); sharpness 9wit.
      [1913 Webster]

            The brightness of his parts . . . distinguished him.
                                                  --Prior.
      [1913 Webster]

   Syn: Splendor; luster; radiance; resplendence; brilliancy;
        effulgence; glory; clearness.
        [1913 Webster]

WordNet (r) 3.0 (2006):

brightness
    n 1: the location of a visual perception along a continuum from
         black to white [ant: dullness]
    2: intelligence as manifested in being quick and witty [syn:
       brightness, cleverness, smartness]
    3: 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]

Moby Thesaurus II by Grady Ward, 1.0:

158 Moby Thesaurus words for "brightness":
   Munsell chroma, Technicolor, acuity, acuteness, adroitness,
   agility, alertness, aptitude, aptness, attention, attentiveness,
   auspiciousness, beneficence, benevolence, benignancy, benignity,
   blare, blaze, blazing light, blinding light, blitheness,
   blithesomeness, braininess, bright color, bright light,
   bright side, brilliance, brilliancy, brilliant light,
   burst of light, cheer, cheerfulness, cheeriness, cheery vein,
   chroma, chromatic color, chromaticity, clear thinking, cleverness,
   color, color quality, colorfulness, colorimetric quality,
   cool color, dazzling light, dexterity, docility, educability,
   effulgence, esprit, eupeptic mien, facility, fair weather,
   favorableness, flair, flamboyance, flood of light, fortunateness,
   fulgor, gaiety, geniality, genius, giftedness, gifts, gladness,
   gladsomeness, glamour, glare, glaring light, gloriousness, glory,
   good auspices, good cheer, good omen, gorgeousness, happiness,
   heavenliness, hopefulness, hue, impressionability, intelligence,
   intensity, keen-wittedness, keenness, lightness, luckiness, luster,
   malleability, mental alertness, mercurial mind, moldability,
   motivation, native cleverness, neutral color, nimble mind,
   nimble-wittedness, nimbleness, nous, optimism, plasticity,
   pleasantness, pliability, promptitude, promptness, propitiousness,
   prosperousness, punctuality, pure color, purity, quick parts,
   quick thinking, quick wit, quick-wittedness, quickness, radiance,
   radiancy, radiant splendor, ravishingness, readiness, ready wit,
   receptivity, refulgence, refulgency, resplendence, resplendency,
   richness, rosy expectation, sanguine humor, sanguineness,
   saturation, savvy, sharp-wittedness, sharpness, sleeplessness,
   smartness, smarts, splendidness, splendor, splendorousness,
   splendrousness, sprightly wit, streaming light, sublimity,
   sunniness, sunny side, susceptibility, talent, teachability,
   teachableness, tint, tone, trainableness, value, vividness,
   wakefulness, warm color, willingness, winsomeness



The Free On-line Dictionary of Computing (30 December 2018):

brightness
luminance
tone
value

    (Or "tone", "luminance", "value", "luminosity",
   "lightness") The coordinate in the HSB colour model that
   determines the total amount of light in the colour.  Zero
   brightness is black and 100% is white, intermediate values are
   "light" or "dark" colours.

   The other coordinates are hue and saturation.

   (1999-07-05)