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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 widely used in the United States in photography, film, television, conservation lighting, 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 flux per unit area.[note 1]
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.[note 3] 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.
In the lighting industry, footcandles are a common unit of measurement used to calculate adequate lighting levels of workspaces in buildings or outdoor spaces. Footcandles are also commonly used in the museum and gallery fields, where lighting levels must be carefully controlled to conserve light-sensitive objects such as prints, photographs, and paintings, the colors of which fade when exposed to bright light for a lengthy period.
In the motion picture cinematography field, incident light meters are used to measure the number of footcandles 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, horticulturalists often measure and discuss optimum intensity for various plants in foot-candles. Full, unobstructed sunlight has an intensity of approximately 10,000 fc. An overcast day will produce an intensity of around 1,000 fc. The intensity of light near a window can range from 100 to 5,000 fc, depending on the orientation of the window, time of year and latitude.
- Some sources do however note that the "lux" can be thought of as a "metre-candle" (i.e. the illuminance cast on a surface by a one-candela source one meter away).
- Small candles molded in the shape of a foot were distributed by General Electric during the 1960s as advertising novelties. These visual puns promoted GE's line of fluorescent lighting and were an intentional reference to the unit of light; the sole of the "foot" had a label reading "GE makes the difference in light!" These were functional candles, had wicks, and could be lit. They were not, however, "standard candles" nor did they deliver any calibrated amount of illuminance.
- One lumen per square foot is equal to 0.3048−2 lumens per square meter. Since illuminance follows the inverse-square law a source that is farther away casts less illumination than one that is close, so one lux is less illuminance than one foot-candle.
- Photometry (optics) for more on the measurement of light.