ABOUT LIGHT MEASUREMENT

From Wikipedia, the free encyclopedia

Lux is a derived unit based on lumen, and lumen is a derived unit based on candela.

One lux is equal to one lumen per square meter, where 4π lumens is the total luminous flux of a light source of one candela of luminous intensity:

1 lux = 1 lumin·m2 = 1sr·m2.

As with other SI units, SI prefixes can be used, for example a kilolux (klx) is 1,000 lux.

The difference between the units lumen and lux is that the lux takes into account the area over which the luminous flux is spread. A flux of 1000 lumens, concentrated into an area of one square metre, lights up that square metre with an illuminance of 1000 lux. The same 1000 lumens, spread out over ten square metres, produces a dimmer illuminance of only 100 lux. Mathematically, 1 lx = 1 lm/m2.

A single fluorescent light fixture with an output of 12000 lumens might light a residential kitchen with an illuminance of 500 lux. To light a factory floor with area dozens of times that of the kitchen would require dozens of such fixtures. Lighting a larger area to the same level of lux requires a greater number of lumens.

Lux versus footcandle

One footcandle ≈ 10.764 lux. The footcandle (or lumen per square foot) is a non-SI unit of illuminance. Like the BTU, it is mainly only in common use in the United States, particularly in construction-related engineering and in building codes. Because lux and footcandles are different units of the same quantity, it is perfectly valid to convert footcandles to lux and vice versa.

The name "footcandle" conveys "the illuminance cast on a surface by a one-candela source one foot away." As natural as this sounds, this style of name is now frowned upon, because the dimensional formula for the unit is not foot · candela, but lumen/sq ft. Some sources do however note that the "lux" can be thought of as a "metre-candle" (i.e. the illuminance cast on a surface by a one-candela source one metre away). A source that is farther away provides less illumination than one that is close, so one lux is less illuminance than one footcandle. Since illuminance follows the inverse-square law, and since one foot = 0.3048 m, one lux = 0.30482 footcandle ≈ 1/10.764 footcandle.

In practical applications, as when measuring room illumination, it is very difficult to measure illuminance more accurately than ±10%, and for many purposes it is quite sufficient to think of one footcandle as about ten lux.

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