the SI electromagnetic units, elementary charge, and the caesium-133 hyperfine transition radiation: 1 coulomb, C, = e 1 ampere, or amp, A, = 1 C/s = e 1 volt, V, = 1 J/C = /e 1 farad, F, = 1 C/V = e2/ 1 ohm, Ω, = 1 V/A = / e2 = h/e2 1 siemens, S, = 1/Ω = e2/h 1 weber, Wb, = 1 V s = /e = h/e 1 tesla, T, = 1 Wb/m2 = /e 2 = E/e c 1 henry, H, = Ω s = h /e2 Optical units From 1967 to 1979 the SI optical units, lumen, lux, and candela are defined using the Incandescent glow of platinum at its melting point. After 1979, the candela was defined as the luminous intensity of a monochromatic visible light source of frequency 540 Thz (i.e that of the caesium standard) and radiant intensity 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. The frequency used to define the optical units has the parameters: Frequency: 540 THz Time period: fs Wavelength: μm Photon energy: Hz × J s = J luminous efficacy, KCD, = 683 lm/W Luminous energy per photon, , = J × 683 lm/W = lm s This implies: 1 lumen, lm, = 1 candela, cd, = 1 lm/sr = /sr 1 Lux, lx, = 1 lm/m2 = /2 Summary The parameters of the caesium 133 hyperfine transition radiation expressed exactly in SI units are: Frequency = 9,192,631,770 Hz Time period = Wavelength = m Photon energy = J Photon mass equivalent = kg If the 7 base units of the SI are expressed explicitly in terms of the SI defining constants, they are: 1 second = 1 metre = c/ 1 kilogram = h /c2 1 ampere = e 1 kelvin = h /k 1 mole = elementary entities 1 candela = h 2 KCD/sr Ultimately, 6 of the 7 base units notably have values that depend on that of , which appears far more often than any of the other defining constants. See also Rubidium standard References External links Electronics