The luminous intensity I of a light source specifies the radiation power emitted into a certain direction weighted by the impression of brightness generated in the human eye. The unit of luminous intensity is the candela (cd) (candela, Latin: candle). The candela is also one of the seven base units of the International System of Units (SI). The luminous intensity of a bulb is almost the same in each direction, (e.g. 100 W bulb: I ~ 110 cd), that one of a fluorescent lamp is essentially smaller in the longitudinal direction than vertical (40 W fluorescent lamp: I ~ 250 cd vertical to the longitudinal direction).
At BEV the realisation of the candela is carried out by standard lamps, the luminous intensity of which is calibrated regularly by the Physikalisch-Technischen Bundesanstalt (PTB, Germany).