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. 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. In old definitions of the photometric base unit, a specific value of the luminous intensity was assigned to a 'primary standard'. This unit is one of the seven base units in the SI system of units. In photometry, special units are used which are all derived from the unit of luminous intensity. 8 Medical Physics and Metrological Information Technology 4.01 Metrology for Functional Nanosystems.3 Chemical Physics and Explosion Protection Definition and realization of photometric units.Publications on the Medical Devices Act.Brochures of the International Cooperation.Brochures on the International System of Units (SI).Brochures and journals on the history of the PTR/PTB.Brochures on the structure of PTB: Info Sheets.
The 'Sonne' (sun), the moon and the stars.
