WordNet (r) 3.0 (2006):

luminance
    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]

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)