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| Lumens, Footcandles, Lux |
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| Written by Andrew Thiess |
| Sunday, 17 May 2009 00:00 |
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The terms "Lumens", "Footcandles" and "Lux" are used to describe light fixtures, lamps, and lighting requirements. What do these terms really mean? The term "candlepower" was originally defined in England in 1860 as the light produced by a pure spermaceti candle of a certain size burning at a certain rate. Spermaceti was a substance found in the heads of sperm whales, used to make high-quality candles. The burn rate was measured in grains of candle wax per hour. Grains were a unit of mass based on a wheat or barley seed. Seeing the need for a more scientific definition, in 1948 scientists adopted the term "candela", currently defined as the emittance from a black body at a certain temperature and pressure, weighted for human perception. As opposed to the total amount of radiation coming from the candle including infrared (heat) and ultraviolet, the candela measures the light in a certain spectrum (actually about 555 nanometers, in the visible spectrum near green), a frequency representative of human perception of light. A common candle emits about one candela of light. A common 100-watt incandescent light bulb emits about 127 candela. A "lumen" is a unit of measure derived from the candela. It is defined as one candela, divided into a defined beam or angle of one "steradian", or about 1/12.57 of a sphere. One lumen equals 1 candela/12.57. Conversely, one candela equals 12.57 lumens. A common 100-watt incandescent light bulb emits about 1600 lumens. Candela or lumens can be thought of as the intensity of a light source. A "lux" is a SI (Systeme International - what we sometimes call metric) unit of measure, derived from the lumen. It is defined as one lumen over one square meter. One "lux" is perceived as about the brightness of a full moon in Barbados. A well-lit office area is about 400 lux, and full daylight is typically about 20,000 lux. Similar to the lux, a "footcandle" is the non-SI, or English, unit of measure, derived from the lumen. It is defined as one lumen over one square foot. Based on a simple conversion of square meters to square feet, one footcandle equals 10.76 lux. A generally accepted rule is that a good light level for reading is about 50 footcandles. Lux or footcandles can be thought of as the amount of light falling on your desk, or on the page of your book. A dim light one foot away can provide the same lux or footcandles as a very bright light ten feet away. For information on Overland Assessments, contact: Andrew Thiess, RAS, PMP, PE phone: (832) 250-2899 e-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it |
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