Physical Units
LUMEN (lm)

The lumen value is the derived unit of luminous flux. The luminous flux is a measure of the power of light perceived by the human eye. Only the visible light is measured in Candela (light intensity) multiplied by the solid angle.

Luminous flux of typical light sources

Light Source
Power consumption
typ. Light efficiency in lm
Candlelight
approx. 150W
15lm
1W - LED
1W
90lm
40W Bulb 230V
40W
470lm
60W Bulb 230V
60W
806lm
100W Bulb 230V
100W
1500lm
20W MR16 Low-Voltage Halogen Bulb 12V
20W
180lm
35W MR16 Low-Voltage Halogen Bulb 12V
35W
300lm
25W GU10 High-Voltage Halogen Bulb 230V
25W
125lm
50W GU10 High-Voltage Halogen Bulb 230V
50W
300lm
23W Compact Fluorescent Tube
23W
1300lm

 

Candela (cd)

The candela is the base unit of luminous intensity. A model of the sensitivity of the human eye to different intensities of radiation, also known as the luminous efficiency function.

Light intensity of some light sources

Light Source
Light Intensity
Candlelight
1000mcd
60W Bulb 230V
58cd = 58,000mcd
100W Bulb 230V
110cd = 110,000mcd
40W Compact Fluorescent Tube
180cd = 180,000mcd

 

Lux (lx)

Lux is the unit of illuminance. It is a derived quantity. 1 lx = 1 lm/m². The decisive factor is the distance to the light source.

Example:
A candle has a light intensity of 1cd (1000mcd). At a distance of 1m to the candle, the illumination is 1lx (1cd / 1m x 1m = 1 lm/m² = 1lx). If the spacing is 2m, the illumination is only 0.25lx (1cd / 2m x 2m = 0.25 lm/m² = 0.25lx) and only 0.11lx at 3m (1cd / 3m x 3m = 0.11 lm/m² = 0.11lx)

Kelvin (K)

The Kelvin is a unit of measurement for the thermo-dynamic temperature. In relation with the LED technology, the unit Kelvin means the colour temperature. It is important for the characterization of light sources. The colour temperature of white LEDs is particularly important for technical characteristics.

Colour temperature of white LEDs:
warm white ~ 3500°K
daylight ~ 5500°K
pure white ~ 7000°K
bluish white ~ 10000°K