# Brightness

Decreasing brightness with depth (underwater photo as example)

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light.[1] In other words, brightness is the perception elicited by the luminance of a visual target. This is a subjective attribute/property of an object being observed.

## Terminology

The adjective bright derives from an Old English beorht with the same meaning via metathesis giving Middle English briht. The word is from a Common Germanic *berhtaz, ultimately from a PIE root with a closely related meaning, *bhereg- "white, bright". "Brightness" was formerly used as a synonym for the photometric term luminance and (incorrectly) for the radiometric term radiance. As defined by the US Federal Glossary of Telecommunication Terms (FS-1037C), "brightness" should now be used only for non-quantitative references to physiological sensations and perceptions of light.[2]

A given target luminance can elicit different perceptions of brightness in different contexts; see, for example, White's 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):[3]

$\mu = {R + G + B \over 3 }$

Brightness is also a color coordinate in the HSB or HSV color space (hue, saturation, and brightness or value).

With regard to stars, brightness is quantified as apparent magnitude and absolute magnitude.

## Brightness of sounds

The term "brightness" is also used in discussions of sound timbres, in a rough analogy with visual brightness. Timbre researchers consider brightness to be one of the perceptually strongest distinctions between sounds,[4] and formalize it acoustically as an indication of the amount of high-frequency content in a sound, using a measure such as the spectral centroid.

## Notes

1. ^ Merriam-Webster.com Merriam-Webster Dictionary definition of bright
2. ^ Brightness” in Federal Standard 1037C, the Federal Glossary of Telecommunication Terms (1996)
3. ^ What are HSB and HLS?, Charles Poynton: "The usual formulation of HSB and HLS compute so-called "lightness" or "brightness" as (R+G+B)/3. This computation conflicts badly with the properties of colour vision, as it computes yellow to be about six times more intense than blue with the same "lightness" value (say L=50)."
4. ^ D. Wessel, Timbre space as a musical control structure, Computer Music Journal, 3 (1979), pp. 45–52.