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The candela

Definition

Cryogenic radiometer - the national standard for the measurement of the optical radiant power.

Cryogenic radiometer - the national standard for the measurement of the optical radiant power.

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 · 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian.

Realization

In principle, this definition is an instruction for the design of a lamp: Let us design a lamp which emits monochromatic light of frequency 540 ·1012 hertz (or with a wavelength of approximately 555 nm) and ensure that this lamp reaches a certain radiant intensity. This instruction is, however, little realistic for practical application, as a lamp with such a limited radiation spectrum is needed practically nowhere for illumination or as a signal lamp.

The advantage of the definition is that – for this wavelength – it links the photometric unit, luminous intensity, with the radiometric unit, radiant intensity. This shows that light is nothing else but optical radiation which becomes visible to the human eye in a certain wavelength range (from 360 nm to 830 nm). And the photometric units are nothing else but radiometric units which – through a weighting function – also take the properties of the human eye into account. To the human eye, yellow light, for example, appears to be brighter than red or blue light, although a radiation measuring instrument (e.g. a radiometer) would measure the same radiant intensity. In order to determine by measurement how bright or dark the light of a lamp appears to man, measuring instruments are needed which react to light of different wavelength in a similar way as the human eye does. Such detectors are referred to as photometers. At PTB, the unit "candela" is realized and maintained with a group of special incandescent lamps and photometers.

A method adopted in the past to realize the candela followed the above design instructions: Under specified conditions, a special "black-body radiator" (high-temperature cavity radiator) emitted "white" radiation and served as a luminous intensity standard. Optical radiation can, however, be measured more accurately by special thermal radiation detectors (cryogenic radiometers). In the case of these "absolute detectors", the radiant power is compared with electrical heating power (in watt). They are used for the calibration of photometers.

By the way: the stress in "candela" (the Latin word for candle) is on the second syllable.


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