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A foot-candle (sometimes foot candle; abbreviated fc, lm/ft2, or sometimes ft-c) is a non-SI unit of illuminance or light intensity. The foot-candle is defined as one lumen per square foot. This unit is commonly used in lighting layouts in parts of the world where United States customary units are used, mainly the United States. Nearly all of the world uses the corresponding SI derived unit lux, defined as one lumen per square meter.
The foot-candle is defined as the illuminance of the inside surface of a one-foot-radius sphere with a point source of one candela at its center. Alternatively, it can be defined as the illuminance of one lumen on a one-square foot surface with a uniform distribution. Given the relation between candela and lumen, the two definitions listed are identical, with the second one potentially being easier to relate to in some everyday situations.
One foot-candle is equal to approximately 10.764 lux. In many practical applications, as when measuring room illumination, it is often not needed to measure illuminance more accurately than ±10%; in these situations it is sufficient to think of one foot-candle as about ten lux.
In the motion picture cinematography field in the US, incident light meters are used to measure the number of foot-candles present, which are used to calculate the intensity of motion picture lights, allowing cinematographers to set up proper lighting-contrast ratios when filming.
Since light intensity is the primary factor in the photosynthesis of plants, US Horticulture often measure and discuss optimum intensity for various plants in foot-candles.
Indoor lighting for residences seeks to provide 5-40 fc for general spaces and 70-90 fc for work spaces.
Lighting requirements for commercial spaces range from 5 fc for storage spaces to 200 fc for visually intensive work.
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