Foot-lambert

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A foot-lambert or footlambert (fL, sometimes fl or ft-L) is a unit of luminance in U.S. customary units and elsewhere. A foot-lambert equals $1 / π$ candela per square foot, or 3.4262591 candelas per square meter (nits). It is named after Johann Heinrich Lambert (1728 – 1777), a German mathematician, physicist and astronomer.

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. The foot-lambert is rarely used by electrical and lighting engineers, in favor of the candela per square foot or candela per square meter.

The foot-lambert is still used in the motion picture industry for the luminance of images on a projection screen. The Society of Motion Picture and Television Engineers (SMPTE) recommends a light output of 16 foot-lamberts for commercial movie theaters with no film in the projector.[1]

The foot-lambert is also used in the flight simulation industry to measure the luminance of the visual display system. The peak display luminance (screen brightness) for Full Flight Simulators (FFS) is specified as at least 6 foot-lamberts by the European Aviation Safety Authority (EASA) and the Federal Aviation Administration (FAA) in the USA.