Foot-Lambert is a measure of luminance, primarily in the US. The metric (SI) equivalent is candela per square meter (nits).
Foot-Lamberts, or ft-L, are measured with a luminance meter, either off a reflective surface such as a screen, or directly off a TV/display. In the US, movie theater screens are designed to ideally have 16 foot-Lamberts "open gate," or when the projector is on but there is no film. This is significantly lower with 3D. In the home, on a 100-inch screen, most projectors can do this amount or more. LCD and plasma televisions can do much, much more than this, with some LED models capable of 80 foot-Lamberts and higher.
A foot-Lambert equals 1 / π candela per square foot, or 3.4262591 candelas per square meter (nits).
The foot-Lambert is named after the Swiss scientist and mathematician Johann Heinrich Lambert, who's stylish mug you see here.
Foot-Lamberts should not be confused with lumens, which are an illuminance measurement, and are not convertible to ft-L (or vice versa).