Brightness
Perception of light level
Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and relies on the context of the viewing environment (for example, see White's illusion).
Brightness is a subjective sensation of an object being observed and one of the color appearance parameters of many color appearance models, typically denoted as 
The adjective bright derives from an Old English beorht with the same meaning via metathesis giving Middle English briht. The word is from a Proto-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. Brightness is an antonym of "dimness" or "dullness".
With regard to stars, brightness is quantified as apparent magnitude and absolute magnitude.
Two pictograms resembling the Sun with rays are used to represent the settings of luminance in display devices. They have been encoded in Unicode since version 6.0 (October 2010) in the Miscellaneous Symbols and Pictographs block under U+1F505 as "low brightness symbol" (🔅) and U+1F506 as "high brightness symbol" (🔆).
The United States Federal Trade Commission (FTC) has assigned an unconventional meaning to brightness when applied to lamps: on light bulb packages, brightness means luminous flux, while in other contexts it means luminance. Luminous flux is the total amount of light coming from a source while luminance is the amount of light per unit solid angle per unit of area on the source's surface. The table below shows the standard ways of indicating the amount of light.
| Quantity | Unit | Dimension |
Notes | ||
|---|---|---|---|---|---|
| Name | Symbol | Name | Symbol | ||
| Luminous energy | Qv | lumen second | lm⋅s | T⋅J | The lumen second is sometimes called the talbot. |
| Luminous flux, luminous power | Φv | lumen (= candela steradian) | lm (= cd⋅sr) | J | Luminous energy per unit time |
| Luminous intensity | Iv | candela (= lumen per steradian) | cd (= lm/sr) | J | Luminous flux per unit solid angle |
| Luminance | Lv | candela per square metre | cd/m2 (= lm/(sr⋅m2)) | L−2⋅J | Luminous flux per unit solid angle per unit projected source area. The candela per square metre is sometimes called the nit. |
| Illuminance | Ev | lux (= lumen per square metre) | lx (= lm/m2) | L−2⋅J | Luminous flux incident on a surface |
| Luminous exitance, luminous emittance | Mv | lumen per square metre | lm/m2 | L−2⋅J | Luminous flux emitted from a surface |
| Luminous exposure | Hv | lux second | lx⋅s | L−2⋅T⋅J | Time-integrated illuminance |
| Luminous energy density | ωv | lumen second per cubic metre | lm⋅s/m3 | L−3⋅T⋅J | |
| Luminous efficacy (of radiation) | K | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to radiant flux |
| Luminous efficacy (of a source) | η | lumen per watt | lm/W | M−1⋅L−2⋅T3⋅J | Ratio of luminous flux to power consumption |
| Luminous efficiency, luminous coefficient | V | 1 | Luminous efficacy normalized by the maximum possible efficacy | ||
| See also: | |||||
Sources and credits
This article is adapted from the Wikipedia article “Brightness”, written by its contributors and licensed under CC BY-SA 4.0. Fathomly has changed the layout, removed citation markers, navigation and maintenance notices, and adjusted punctuation. This adapted version is shared under the same license. For references, see the original article.
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