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| A '''foot-lambert''' or '''footlambert''' (fL, sometimes fl or ft-L) is a unit of [[luminance]] in [[United States customary units]] and some other unit systems. A foot-lambert equals 1/π [[candela]] per [[square foot]], or 3.426 [[candela per square meter]] (the corresponding [[SI unit]]). The foot-lambert is named after [[Johann Heinrich Lambert]] (1728–1777), a Swiss-German mathematician, physicist and astronomer. It is rarely used by electrical and lighting engineers, in favor of the candela per square foot or candela per square meter.
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| The luminance of a perfect [[Lambertian reflectance|Lambertian]] diffuse reflecting surface in foot-lamberts is equal to the incident [[illuminance]] in [[foot-candle]]s. For real diffuse reflectors, the ratio of luminance to illuminance in these units is roughly equal to the [[reflectance]] of the surface. Mathematically,
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| :<math>L_\mathrm v = E_\mathrm v \times R</math>,
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| where
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| :<math>L_\mathrm v</math> is the luminance, in foot-lamberts,
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| :<math>E_\mathrm v</math> is the illuminance, in foot-candles, and
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| :<math>R</math> is the reflectivity, expressed as a fractional number (for example, a grey card with 18% reflectivity would have <math>R = 0.18</math>).
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| The foot-lambert is used in the [[motion picture]] industry for measuring the luminance of images on a [[projection screen]]. The [[Society of Motion Picture and Television Engineers]] (SMPTE) recommended, in SMPTE 196M, a screen luminance of 16 foot-lamberts for commercial movie theaters, when measured "open-gate" (i.e. with no film in the [[Movie projector|projector]]). (Typical base density of 0.05 yields peak white of about 14 fL.) The current revision of SMPTE 196M specifies 55 [[candela per square meter]] ([[nit (unit)|nits]]).
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| The foot-lambert is also used in the [[flight simulation]] industry to measure the highlight brightness of visual display systems. The minimum required highlight brightness varies based on the type and level of [[Flight Simulation Training Device]] (FSTD), but is generally 3–6 foot-lamberts for most devices qualified under [[Federal Aviation Administration]] (FAA) or [[Joint Aviation Authorities]] (JAA) regulations.<ref>14 CFR Part 60</ref><ref>JAR-FSTD A</ref><ref>JAR-FSTD H</ref>
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| ==See also==
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| Other units of luminance:
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| <!-- in order of relevance in regard to foot-lambert -->
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| * [[Lambert (unit)|Lambert]] (L)
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| * [[Nit (unit)|Nit]] (nit)
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| * [[Stilb (unit)|Stilb]] (sb)
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| * [[Apostilb]] (asb)
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| * [[Blondel (unit)|Blondel]] (blondel)
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| * [[Skot (unit)|Skot]] (sk)
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| * [[Bril (unit)|Bril]] (bril)
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| {{SI light units}}
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| ==References==
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| <references/>
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| {{DEFAULTSORT:Foot-Lambert}}
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| [[Category:Photometry]]
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| [[Category:Units of luminance]]
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| [[Category:Customary units of measurement in the United States]]
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| {{optics-stub}}
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