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

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

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