the quality of being bright.
Optics. the luminance of a body, apart from its hue or saturation, that an observer uses to determine the comparative luminance of another body. Pure white has the maximum brightness, and pure black the minimum brightness.

before 950; Middle English brihtnes, Old English beorhtnes. See bright, -ness Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
brightness (ˈbraɪtnɪs)
1.  the condition of being bright
2.  physics a former name for luminosity
3.  psychol the experienced intensity of light

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

O.E. beorhtnes "brightness, clearness, splendor, beauty;" see bright + -ness.
Online Etymology Dictionary, © 2010 Douglas Harper
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Computing Dictionary

brightness definition

(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.

The Free On-line Dictionary of Computing, © Denis Howe 2010
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Example sentences
Brightness is everywhere, indoors and out, in his first novel.
Photography is this miserable, weak little thing that can show you a print with
  a hundred-to-one brightness range.
It could snap a series of shots as the flash warms to full brightness and then
  drops back off.
Many paper manufacturers add fluorescent chemicals to enhance the apparent
  brightness and whiteness of their products.
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