A "foot-candle" is a unit of measure for quantifying the intensity of light falling on an object. A "lumen" is a unit of measure for quantifying the amount of light energy emitted by a light source. In other words, foot-candles measure the brightness of the light at the illuminated object, while lumens measure the power of the light radiated by the light source.
Both foot-candles and lumens are units of photometry, that is, measurements of electromagnetic radiation detectable by the human eye. Foot-candles are used to measure photometric "illuminance," the density of light energy reaching a reference surface at a given distance from one or more light sources. Lumens are used to measure photometric "luminous flux," the amount and rate of light energy radiated by a particular light source.
A foot-candle is a traditional unit of photometry based on the English system of measurements. A foot-candle equals 1 lumen per square foot. The international standard (SI) counterpart of the foot-candle is the "lux." A lux equals 1 lumen per square meter. The equation used to convert foot-candles to lux is: 1 foot-candle = 10.76 lux.
Lighting designers for residential, commercial and industrial must calculate the total number of lumens of various light sources and their distances from living spaces and work areas to ensure adequate illumination. Foot-candles are calculated using the formula: Foot-candles (fc) = Total Lumens (lm) ÷ Area in Square Feet. Lux are calculated using the formula: Lux = Total Lumens ÷ Area in Square Meters. Indoor light fixtures typically provide light outputs ranging from 50 to 10,000 lumens.
People sometimes confuse the wattage rating of a light bulb with the amount of light that the bulb will produce. In fact, it is the lumen rating which indicates the amount of light energy produced. The wattage rating indicates the amount of electrical energy that the bulb consumes. Because a bulb does not convert all of the electricity consumed into light energy---for example, heat is also generated---the lumen rating should be used to determine a bulb's light output capability.
The foot-candle continues to be commonly used by American and British lighting designers and photographers because many light meters used to measure illuminance are still calibrated in foot-candles instead of lux.
- Photo Credit Image by Flickr.com, courtesy of hobvias sudoneighm
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