A foot candle is a unit of measurement for light intensity, mainly used in film, photography, and gardening. It is equivalent to 10,764 lux and can be expressed in terms of lumens. It is used in TV and film studios, museums, and horticulture. Lux is a more common unit of measurement.
A foot candle is a measure of the luminance or intensity of light. It is an imperial unit of measurement, 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.
A foot candle is the amount of illumination received on an area of a surface that is a sphere with a radius of one foot, cast by a light source with an intensity of one candle. The candela is a unit of luminous intensity and is a fixed unit in the same way that a meter or a kilogram works. The candle was originally known simply as “a candle,” hence the name foot candle.
Foot candle can also be expressed in terms of lumens. This is the unit of measurement for the power of light that appears to the human eye, known as luminous flux. One foot candle is equivalent to the illumination thrown by light with a luminous flux of one lumen over an area of one square foot.
The metric, or strictly speaking the equivalent of the “SI derived unit” is the lux. A foot candle has the same luminous intensity of 10,764 lux. Many people will simply use a one to ten conversion ratio, especially with larger areas. This is partly for convenience and partly because most methods of measuring light intensity have enough margin of error that this less accurate conversion makes little or no practical difference.
There are a wide variety of situations where the foot candle can be used. One example is setting up the lighting for a TV or film studio, both to make sure the lighting is adequate and to make sure the light used is consistent between scenes and shoots on different days. Another example is in museums where old paintings and artifacts must be stored under controlled lighting to prevent fading. In both of these uses, the foot candle is a somewhat obsolete unit, as people are more likely to use digital equipment such as a luxmeter calibrated to use units of lux rather than foot candles.
Measurement is also used in horticulture. It can be especially useful in growing plants that can be particularly sensitive to light, such as orchids. People who measure light for these purposes are more likely to use cheaper light meters, which themselves are more likely to be calibrated in foot candles, than expensive light meters which tend to use lux.
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