Editing Caesium standard (section)

=== Optical units ===
From 1967 to 1979 the SI optical units, lumen, lux, and candela are defined using the [[Incandescence|incandescent]] glow of [[platinum]] at its melting point. After 1979, the candela was defined as the [[luminous intensity]] of a [[Monochromatic radiation|monochromatic]] [[Light|visible light]] source of frequency 540&nbsp;THz (i.e {{sfrac|6000|1.02140353}} that of the caesium standard) and [[radiant intensity]] {{sfrac|1|683}} watts per steradian. This linked the definition of the candela to the caesium standard and, until 2019, to the IPK. Unlike the units relating to [[mass]], [[energy]], [[temperature]], [[amount of substance]], and [[electromagnetism]], the optical units were ''not'' massively redefined in 2019, though they were indirectly affected since their values depend on that of the watt, and hence of the kilogram.<ref>{{Cite web|url=https://www.bipm.org/en/history-si/candela|title=Candela - BIPM}}</ref> The frequency used to define the optical units has the parameters:
* Frequency: 540&nbsp;THz
* Time period: {{sfrac|50|27}} [[femtosecond|fs]]
* Wavelength: {{sfrac|14.9896229|27}} [[micrometre|μm]]
* Photon energy: {{val|5.4|e=14}} Hz × {{val|6.62607015|e=-34}} J s = {{val|3.578077881|e=-19}} J
* [[luminous efficacy]], ''K''<sub>CD</sub>, = 683 lm/W
* [[luminous energy]] per photon, <math>Q_\mathrm v</math>, = {{val|3.578077881|e=-19}} J × 683 lm/W = {{val|2.443827192723|e=-16}} lm s

This implies:
* 1 [[Lumen (unit)|lumen]], lm, = {{sfrac|{{val|e=6}}|{{val|2.246520349221536260971}}}} <math>Q_\mathrm v</math> {{math|Δ''ν''<sub>Cs</sub>}}
* 1 [[candela]], cd, = 1 lm/sr = {{sfrac|{{val|e=6}}|{{val|2.246520349221536260971}}}} <math>Q_\mathrm v</math> {{math|Δ''ν''<sub>Cs</sub>}}/sr
* 1 [[lux]], lx, = 1 lm/m<sup>2</sup> = {{sfrac|{{val|8.9875517873681764|e=2}}|{{val|1.898410313566852566340456048807087002459}}}} <math>Q_\mathrm v</math> {{math|Δ''ν''<sub>Cs</sub>}}/{{math|Δ''λ''<sub>Cs</sub>}}<sup>2</sup>