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Definition and realization of photometric units

Luminous intensity

In photometry, special units are used which are all derived from the unit of luminous intensity. This unit is one of the seven base units in the SI system of units. In old definitions of the photometric base unit, a specific value of the luminous intensity was assigned to a "primary standard". Until 1979, this primary standard was a cavity radiator (black-body radiator) at the temperature of freezing platinum, whose luminance was converted into a luminous intensity by indicating a specific area [E1]. The current definition is:

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 THz and that has a radiant intensity in that direction of 1/683 watt per steradian.

While the quantity luminous intensity is a spectrally weighted integral quantity by definition, the definition of the unit candela does not contain any spectral weighting. Any Practical application requires, however, a weighting function. Therefore, the CIPM (Commission International des Poids et Mesures) prescribed in 1982 the application of the definition together with the functions of the spectral sensitivities of the human eye V(λ)  for photopic vision and V '(λ)  for scotopic vision [E3]. These relations are discussed in detail in [E4], for example.

To include the spectral weighting of the measured quantity, the unit of luminous intensity - the candela (cd) - is today realized on this basis using incandescent lamps - which are calibrated with photometers - as luminous intensity standards. The photometers act as transfer standards for the radiant power which is measured radiometrically by means of a cryogenic radiometer. At PTB, a whole network of lamps and photometers is used for the maintenance of the photometric base unit [E5].

Luminous flux

The photometric quantity which is most important for a national economy is the luminous flux with the unit "lumen" (lm). The luminous flux of a lamp indicates how much light it emits into all directions of the surrounding room. The unit of luminous flux is derived at PTB goniophotometrically from the base unit of luminous intensity.


The illuminance is the metrologically most important quantity - it is indicated by the unit "lux" (lx). For its measurement, photometers (here: luxmeters) are used which have been adapted to the spectral luminous efficiency function
V(λ and which are characterized by their photometric sensitivity. Their directional dependence can also be adapted to specific geometric weightings (e.g. planar, spatial) [E6].


The luminance is increasingly gaining in importance, its unit is cd·m-2 and has no name of its own. It serves to characterize, for example, displays and is measured e.g. with a spatially resolving receiver (CCD camera).


[E1] Comptes Rendus des Séances de la 9e Conférence Générale des Poids et Mesures (CGPM), Paris 1948, 53,  BIPM, F-92310 Sèvres, France
[E2] Comptes Rendus des Séances de la 16e Conférence Générale des Poids et Mesures (CGPM), Paris 1979, 100, BIPM, F-92310 Sèvres, France
[E3] BIPM Com. Cons. Phot. Radiometrie, 1982, 10
[E4] Sauter, G., Die Candela: Erläuterungen zum Verständnis der Definition und der Realisierung; PTB-Mitteilungen 107, 1997
[E5] W. Erb, G. Sauter; PTB network for realization and maintenance of the candela; Metrologia, 1997, 34, 115-124
[E6] DIN 5032, Teil 6; Begriffe, Eigenschaften und Kennzeichnung von Photometern