A foot-candle is a measurement of illumination. It is defined as the illuminance of a surface area that is one foot from the source of the light. One foot-candle equals one lumen per square foot. Full sunlight has a foot-candle rating of about 10,000 foot-candles. On an overcast, the number of foot-candles emitted drops to closer to 1,000. Foot-candle ratings in interior spaces vary depending on the source of the illumination (natural versus man-made) and whether the surfaces are also proximate to windows.
Industries ranging from lighting manufacturers to horticultural business to museums and art galleries use foot-candles in the course of doing business. Lamps and lighting fixtures are rated according to the number of foot-candles they emit, and architects and interior designers use those designations to figure correct lighting configurations for work areas and public spaces. The optimum level of foot-candle exposure for plants varies from species to species, so horticulturists must pay attention to this factor when planning gardens or adjusting light levels in greenhouses. Curators and art handlers in museums and galleries regulate light levels in their facilities because exposure to high levels of light can permanently damage art works and historical documents.
The level of foot-candles in any space is easily measured with a hand-held light meter, available from most camera stores or office-supply retailers who sell photographic equipment. Specific techniques for using light meters may vary somewhat, but generally speaking, the user points the light meter at the surface in question and the meter produces a reading of the intensity of the light on that surface in foot-candles.
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