A foot-lambert or footlambert (fL, sometimes fl or ft-L) is a unit of
luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...
in
United States customary units
United States customary units form a system of measurement units commonly used in the United States and U.S. territories since being standardized and adopted in 1832. The United States customary system (USCS or USC) developed from English units ...
and some other unit systems. A foot-lambert equals 1/π or 0.3183
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, who prefer the candela per square foot or candela per square meter units.
The luminance of a perfect
Lambertian diffuse reflecting surface in foot-lamberts is equal to the incident
illuminance in
foot-candles. For real diffuse reflectors, the ratio of luminance to illuminance in these units is roughly equal to the
reflectance of the surface. Mathematically,
:
,
where
:
is the luminance, in foot-lamberts,
:
is the illuminance, in foot-candles, and
:
is the reflectivity, expressed as a fractional number (for example, a grey card with 18% reflectivity would have
).
The foot-lambert is used in the
motion picture
A film also called a movie, motion picture, moving picture, picture, photoplay or (slang) flick is a work of visual art that simulates experiences and otherwise communicates ideas, stories, perceptions, feelings, beauty, or atmosphere ...
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
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 (
nits).
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.
[JAR-FSTD H]
Military specifications for illuminated switches, panels, and displays, such as MIL-PRF-22885 and SAE AS7788, also require luminance measurements in foot-lamberts. Luminance levels can vary from hundreds of foot-lamberts for sunlight readable switch displays per MIL-PRF-22885 to only a few foot-lamberts in night conditions for panels in accordance with SAE AS7788.
See also
Other units of luminance:
*
Lambert
Lambert may refer to
People
*Lambert (name), a given name and surname
* Lambert, Bishop of Ostia (c. 1036–1130), became Pope Honorius II
*Lambert, Margrave of Tuscany ( fl. 929–931), also count and duke of Lucca
*Lambert (pianist), stage-name ...
(L)
*
Nit (nit)
*
Stilb (sb)
*
Apostilb (asb)
*
Blondel (blondel)
*
Skot (sk)
*
Bril
Bril is a surname. Notable people with the surname include:
*Ben Bril, Dutch boxer
*Joel Bril, pen name of Joel Löwe
*Matthaeus Bril, Flemish painter
*Paul Bril, Flemish painter, brother of Matthaeus
See also
* Brill (disambiguation)
* Brühl (d ...
(bril)
References
Units of luminance
Customary units of measurement in the United States
{{optics-stub