1. Luminance Units :
1) The unit of Illuminance: Lux (lx)
2) The unit of Luminous Flux: Lumen (lm)
3) The unit of Luminous Intensity: Candela (cd)
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 ×1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
The energy density striking an object is given in lumens per square meter, generally known as lux.
This unit of invisible light in transit is the lumen. The official definition of the lumen, the unit of luminous flux, is: The luminous flux dF of a source of luminous intensity I (cd) in an element of solid angle dR is given by dF = IdR. In plain English: The flux from a light source is equal to the intensity in candela multiplied by the solid angle over which the light is emitted, taking account of the varying intensity in different directions.
2. Explanation of Luminance Units
Active illuminants like LED and incandescent lights adopt Candle Power（CD）as the unit of luminous intensity, while reflective or penetrating illuminants adopt Lumen (lm) as the unit of luminous flux. Lux is the SI unit of luminance and luminous emittance. It is used in photometry as a measure of the intensity of light, with wavelengths weighted according to the luminosity function, a standardized model of human brightness perception.
Lux versus lumen
The difference between the lux and the lumen is that the lux takes into account the area over which the luminous flux is spread. 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.
Achieving an illuminance of 500 lux might be possible in a home kitchen with a single fluorescent light fixture with an output of 12000 lumens. To light a factory floor with dozens of times the area of the kitchen would require dozens of such fixtures. Thus, lighting a larger area to the same level of lux requires a greater number of lumens.
Lux versus footcandle
One footcandle is approximately equal to 10.764 lux. The footcandle (or lumen per square foot) is a non-SI unit of illuminance. Like the BTU, it is obsolete but it is still in fairly common use in the United States, particularly in construction-related engineering and in building codes.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 meter 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.
Last but not least, the luminous intensity offered by the LED manufactures refers to the luminous intensity lightened under 20mA current. Single LEDs use Candle Power（CD）as the unit of luminous intensity. However, the luminous intensity has nothing to do with the color of LEDs. Generally speaking, the luminous intensity of single LEDs varies from several mCD to 5000mCD.
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