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Foot-lambert, luminance
A foot-lambert or footlambert (fL, sometimes fl) is a unit of measurement of luminance in U.S. customary units. A footlambert equals 1 / π candela per square foot, or 3.4262591 candelas per square meter (nits). The luminance of a perfect lambertian diffuse reflecting surface in footlamberts 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.
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