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 photography, film, television, conservation lighting, and the lighting industry. The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away."

The unit is defined as the amount of illumination the inside surface of a 1-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 1-square foot surface of which there is a uniformly distributed flux of one lumen. This can be thought of as the amount of light that actually falls on a given surface. The foot-candle is equal to one lumen per square foot.

The SI derived unit of illuminance is the lux. One footcandle is equal to approximately 10.764 lux, although in the lighting industry, typically this is approximated as 1 footcandle being equal to 10 lux.

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.