A foot candle is a measure of luminance or light intensity. It is an imperial measurement unit, unlike the more common lux, which is based on the International System of Units (SI) or metric system. Foot candles are mainly used in film and photography, but can also be used in gardening.
One foot candle is the amount of illumination received on an area of a surface that is a sphere with a one foot radius, cast by a light source with an intensity of one candela. The candela is a unit of luminous intensity and is a fixed unit in the same way that a meter or kilogram works. The candela was originally known simply as "one candle," which is where the name foot candle derives from.
The foot candle can also be expressed in terms of lumen. This is the unit of how powerful light appears to be to the human eye, known as luminous flux. One foot candle is equivalent to the illumination cast by light with luminous flux of one lumen on a surface of one square foot.
The metric, or strictly speaking the "SI derived unit" equivalent is the lux. One foot candle is the same light intensity as 10.764 lux. Many people will simply use a one-to-ten conversion ratio, particularly with larger areas. This is partly for convenience and partly because most methods of measuring light intensity have enough of a margin of error that this less accurate conversion makes little or no practical difference.
There are a wide range of situations where the foot candle can be used. One example is in setting the lighting for a television or movie studio, both to make sure the lighting is adequate and to make sure the light used is consistent among scenes and takes shot on different days. Another example is in museums where old paintings and artifacts need to be stored under controlled lighting to avoid causing fading. In both these uses, the foot candle is a somewhat outdated unit, as people are more likely to use digital equipment such as a lux meter that is calibrated to use lux units rather than foot candles.
The measurement is also used in horticulture. It can be particularly useful in growing plants that can be particularly sensitive to light, such as orchids. People measuring light for these purposes are more likely to be using cheaper light meters, which in turn are more likely to be calibrated in foot candles, compared with expensive light meters that tend to use the lux.
By John Lister