The candela, symbol cd, is the SI unit of luminous intensity in a given direction. It is defined by taking the fixed numerical value of the luminous efficacy of monochromatic radiation of frequency 540 × 1012 Hz, Kcd, to be 683 when expressed in the unit lm W−1.
Reference: Bureau International des Poids et Mesures 2019 The International System of Units (SI)
Quantity Symbol for quantity Q Symbol for dimension Name of abstract unit uQ Symbol for unit uQ [*] elementary entity *,$ UX U elementary unit x count *,$ NX = N·UX X elementary unit x amount of substance *,§ nX = NX·NA-1 N mole mol charge *,€ Qel = zX·e·NX I·T coulomb C = A·s length l L meter m mass m M kilogram kg time t T second s electric current I I ampere A thermodynamic temperature T Θ kelvin K luminous intensity Iv J candela cd
- [*] SI units, except for the canonical 'elementary unit' [x]. The following footnotes are canonical comments, related to iconic symbols.
- * For the elementary quantities NX, nX, and Qel, the entity-type X of the elementary entity UX has to be specified in the text and indicated by a subscript: nO2; Nce; Qel.
- $ Count NX equals the number of elementary entities UX. In the SI, the quantity 'count' is explicitly considered as an exception: "Each of the seven base quantities used in the SI is regarded as having its own dimension. .. All other quantities, with the exception of counts, are derived quantities" (Bureau International des Poids et Mesures 2019 The International System of Units (SI)). An elementary entity UX is a material unit, it is not a count (UX is not a number of UX). NX has the dimension X of a count and UX has the dimension U of an elementary entity; both quantities have the same abstract unit, the 'elementary unit' [x].
- § Amount nX is an elementary quantity, converting the elementary unit [x] into the SI base unit mole [mol] using the Avogadro constant NA.
- € Charge is a derived SI quantity. Charge is an elementary quantity, converting the elementary unit [x] into coulombs [C] using the elementary charge e, or converting moles [mol] into coulombs [C] using the Faraday constant F. zX is the charge number per elementary entity UX, which is a constant for any defined elementary entity UX. Qel = zX·F·nX
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- Entity, count, and number, and SI base quantities / SI base units
Quantity name Symbol Unit name Symbol Comment elementary UX elementary unit [x] UX, UB; [x] not in SI count NX elementary unit [x] NX, NB; [x] not in SI number N - dimensionless = NX·UX-1 amount of substance nB mole [mol] nX, nB electric current I ampere [A] A = C·s-1 time t second [s] length l meter [m] SI: metre mass m kilogram [kg] thermodynamic temperature T kelvin [K] luminous intensity IV candela [cd]
- Fundamental relationships
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- » Boltzmann constant k
- » elementary charge e
- » Faraday constant F
- » gas constant R
- » electrochemical constant f
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