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). 9th edition:117-216 ISBN 978-92-822-2272-0. - »Open Access pdf«
Base quantity Symbol for quantity Symbol for dimension Name of SI unit Symbol for SI unit [*] 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 amount of substance *,§ n = N·NA-1 N mole mol count *,$ N X counting unit x charge *,€ Q = N·e = n·F I·T coulomb C = A·s luminous intensity Iv J candela cd
- [*] »SI base units
- * For the quantities n, N and Q, the entity X has to be specified in the text or indicated by a subscript or in parentheses: nX; NX; QX.
- § 'Amount is a counting quantity, converting the counting unit [x] into moles [mol] using the Avogadro constant, NA.
- $ 'Count' is synonymous with 'number of counting entities'. 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)).
- € Charge is a derived SI quantity, included here for comparison. Charge is a counting quantity, converting the counting unit [x] into coulombs [C] using the elementary charge, e, or converting moles [mol] into coulombs [C] using the Faraday constant, F.
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