Editing Candela (section)

== Definition ==
The 26th [[General Conference on Weights and Measures]] (CGPM) redefined the candela in 2018.<ref>{{cite web |title=Convocation of the General Conference on Weights and Measures (26th meeting) |location=Versailles |date=13 November 2018 |publisher=Bureau International des Poids et Mesures |url=https://www.bipm.org/utils/en/pdf/CGPM/Convocation-2018.pdf#page=25 |access-date=10 February 2019 |archive-date=19 September 2019 |archive-url=https://web.archive.org/web/20190919050548/https://www.bipm.org/utils/en/pdf/CGPM/Convocation-2018.pdf#page=25 }}</ref><ref>{{Cite web|last=BIPM|date=22 March 2021|title=Mise en pratique for the definition of the candela in the SI|url=https://www.bipm.org/documents/20126/41489685/SI-App2-candela.pdf/1603b2e8-64a3-1fda-d9b4-7b748bad9c01?version=1.7&t=1637237944393&download=false|website=BIPM}}</ref> The new definition, which took effect on 20 May 2019, is:

<blockquote>The candela [...] is defined by taking the fixed numerical value of the [[luminous efficacy]] of monochromatic radiation of frequency {{val|540e12|u=Hz}},{{Refn|This frequency corresponds to a [[wavelength]] of 555&nbsp;nm in air, which is yellowish-green light approximately at the peak of human visual response. The color can be approximated on an [[sRGB]] display with [[CSS]] color value <code>rgb(120,255,0)</code> or hex <code>#78ff00</code> {{Colorsample|#78ff00|border=#55b900}}. |group=lower-alpha}} ''K''<sub>cd</sub>, to be 683 when expressed in the unit lm&nbsp;W<sup>−1</sup>, which is equal to {{nowrap|cd sr W<sup>−1</sup>}}, or {{nowrap|cd sr kg<sup>−1</sup> m<sup>−2</sup> s<sup>3</sup>}}, where the kilogram, metre and second are defined in terms of ''[[Planck constant|h]]'', ''[[Speed of light in vacuum|c]]'' and [[unperturbed ground state hyperfine transition frequency of the caesium 133 atom|Δ''ν''<sub>Cs</sub>]].<ref>{{SIbrochure9th|page=135}}</ref></blockquote>

=== Explanation ===
[[File:Linear visible spectrum.svg|thumb|right|upright=1.5]]
The frequency chosen is in the [[visible light|visible spectrum]] near [[green]], corresponding to a wavelength of about 555&nbsp;nanometres. The [[human eye]], when [[Adaptation (eye)|adapted]] for bright conditions, is most sensitive near this frequency. Under these conditions, [[photopic vision]] dominates the visual perception of our eyes over the [[scotopic vision]]. At other frequencies, more radiant intensity is required to achieve the same luminous intensity, according to the frequency response of the human eye. The luminous intensity for light of a particular wavelength ''λ'' is given by
<math display="block">I_\mathrm{v}(\lambda)= 683.002\ \mathrm{lm/W} \cdot \overline{y}(\lambda) \cdot I_\mathrm{e}(\lambda) ,</math>
where {{math|''I''<sub>v</sub>(''λ'')}} is the [[luminous intensity]], {{math|''I''<sub>e</sub>(''λ'')}} is the [[radiant intensity]] and <math display="inline">\textstyle \overline{y}(\lambda)</math> is the [[photopic]] [[luminous efficiency function]]. If more than one wavelength is present (as is usually the case), one must integrate over the [[spectrum]] of wavelengths to get the total luminous intensity.

=== Examples ===
* A common candle emits light with roughly 1&nbsp;cd luminous intensity. 
* A 25&nbsp;W [[compact fluorescent light bulb]] puts out around 1700&nbsp;[[lumen (unit)|lumen]]s; if that light is radiated equally in all directions (i.e. over 4{{pi}} [[steradian]]s), it will have an intensity of <math>I_\text{V} = \frac{1700\ \text{lm}}{4 \pi\ \text{sr}} \approx 135\ \text{lm}/\text{sr} = 135\ \text{cd}.</math>
* Focused into a 20° beam (0.095 steradians), the same light bulb would have an intensity of around 18,000&nbsp;cd or 18 kcd within the beam.