| 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
lumen is the total luminous flux of a light source of one candela of
luminous intensity:
- 1 lux = 1 lumen·m2 = 1sr·m2.
As with other SI units, SI prefixes can be used, for
example a kilolux (klx) is 1,000 lux.
A lumen defines the light energy emitted by
a light source. LUX on the other hand defines the "light density" over a
surface area and is effected by the structure of the light fixture. For
example, a parabolic reflector concentrates the light emission of a light
bulb and therefore increases the LUX output of the bulb.
In other words, 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 lumen, concentrated into an area of one square metre,
lights up that square metre with an illuminance of 1000 lux. The same 1000
lumen, 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 lumen 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 lumen.
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. |
