Picture of the week : Bright
Brightness is an attribute of visual perception in which a source appears to emit a given amount of light. In other words, brightness is the perception elicited by the luminance of a visual target.
"Brightness" was formerly used as a synonym for the photometric term luminance and (incorrectly) for the radiometric term radiance. According to Federal Standard 1037C, "brightness" should now be used only for nonquantitative references to physiological sensations and perceptions of light.
Note, that the same target luminance can elicit different perceptions of brightness in different contexts. (See, e.g. White's illusion and Wertheimer-Benary illusion)
In the RGB color space, brightness can be thought of as the arithmetic mean μ of the Red, Green, and Blue color coordinates (although some of the three components make the light seem brighter than others, which, again, may be compensated by some display systems automatically)
Brightness is also a color coordinate in HSB color space.
With regard to stars etc. see apparent magnitude, absolute magnitude.
Source : wikipedia
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Article : Brightness