{"pid":"https://si-digital-framework.org/SI/units/candela","unitId":"candela","label":"candela","symbol":"cd","startValidity":"2019-05-20","unitType":"SI base unit","unitTypeId":"https://si-digital-framework.org/SI#SIBaseUnit","unitDefinitionLabel":"candela2018","unitDefinitionId":"https://si-digital-framework.org/SI#candela2018","definition":"The candela, symbol $\\rm{cd}$, 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 ${540 \\times 10^{12}}\\ {{\\rm{Hz}}}$, ${K_{\\rm{cd}}}$, to be ${683}$ when expressed in the unit ${\\rm{lm}}\\ {\\rm{W}}^{-1} $, which is equal to ${\\rm{cd}}\\ {\\rm{sr}}\\ {\\rm{W}}^{-1}$, or ${\\rm{cd}}\\ {\\rm{sr}}\\ {\\rm{kg}}^{-1}\\ {\\rm{m}}^{-2}\\ {\\rm{s}}^{3}$, where the kilogram, metre and second are defined in terms of ${h}$, $c$ and $\\Delta\\nu_{\\rm{Cs}}$.","definingResolution":"CGPM26-Res1","eventName":"26th meeting of the CGPM","eventDate":"2018-11-16","resolutionLink":"https://doi.org/10.59161/cgpm2018res1e","unitDefinitionStatus":"current","previousVersionDate":"2019-05-19","hasPreviousDefinition":true,"previousDefinition":"https://si-digital-framework.org/SI#candela1979","resNbr":"1","resolutionBody":"CGPM","resolutionType":"Resolution","equation":"1\\;{\\rm{cd}} = \\left(\\frac{{K_{\\rm{cd}}}}{683}\\right) {\\rm{kg}}\\;{\\rm{m}}^2\\;{\\rm{s}}^{-3}\\;{\\rm{sr}}^{-1}","cstLabel":"luminous efficacy","cstHidden":"LuminousEfficacy","constant":"luminous efficacy","constantPid":"https://si-digital-framework.org/constants/LuminousEfficacy","prefixRestriction":false,"unitQuantities":[{"code":"LUIN","label":"luminous intensity"}],"notes":["This definition implies the exact relation ${{K_{\\rm{cd}}}} = {{683}}\\ {{\\rm{cd}}\\:{\\rm{sr}}\\:{\\rm{kg}}^{-1}\\:{\\rm{m}}^{-2}\\:{\\rm{s}}^{3}}$ for monochromatic radiation of frequency ${\\nu} = {540 \\times 10^{12}}\\ {{\\rm{Hz}}}$. Inverting this relation gives an exact expression for the candela in terms of the defining constants ${K_{\\rm{cd}}}$, ${h}$ and $\\Delta\\nu_{\\rm{Cs}}$: $$1\\;{\\rm{cd}} = \\left(\\frac{{K_{\\rm{cd}}}}{{683}}\\right)\\ {\\rm{kg}}\\ {\\rm{m}}^2\\ {\\rm{s}}^{-3}\\ {\\rm{sr}}^{-1}$$ which is equal to $$1\\;{\\rm{cd}} = \\frac{1}{(6.626\\:070\\:15 \\times 10^{-34})\\:({9\\:192\\:631\\:770})^{2}\\:{683}} (\\Delta\\nu_{\\rm{Cs}})^{2}\\:{h}\\:{K_{\\rm{cd}}} \\approx 2.614\\:8305 \\times 10^{10} (\\Delta\\nu_{\\rm{Cs}})^{2}\\:{h}\\:{K_{\\rm{cd}}}$$.","The effect of this definition is that one candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency ${540 \\times 10^{12}}\\ {{\\rm{Hz}}}$ and has a radiant intensity in that direction of ${1/{{683}}}\\ {\\rm{W}\\ \\rm{sr}^{-1}}$ The definition of the steradian is given below Table 4."],"notesIds":["https://si-digital-framework.org/SI#candela2018note1","https://si-digital-framework.org/SI#candela2018note2"]}