Foot-candle

A foot-candle (sometimes foot candle; abbreviated fc, lm/ft², or sometimes ft-c) is a non-SI unit of illuminance or light intensity widely used in the United States in photography, film, television, conservation lighting, greenhouse horticulture, the lighting industry, construction-related engineering and in building codes. The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away". As natural as this sounds, this style of name is now frowned upon, because dimensionally illuminance is not length times luminous intensity as the name would imply but instead luminous intensity per unit area.

The unit is defined as the amount of illumination the inside surface of a one-foot-radius sphere would be receiving if there were a uniform point source of one candela in the exact center of the sphere. Alternatively, it can be defined as the illuminance on a one-square foot surface of which there is a uniformly distributed flux of one lumen.

Thus one foot-candle is equal to one lumen per square foot or approximately 10.764 lux. In practical applications, as when measuring room illumination, it is very difficult to measure illuminance more accurately than ±10%, and for many purposes it is quite sufficient to think of one footcandle as about ten lux as is typically done in the lighting industry.