4. Wavelength standards

4.1 Primary standard

In 1960, the iridium-platinum standard meter was abandoned and the meter was redefined as equal to 1,650,763.73 times the wavelength in vacuum of the radiation corresponding to the transition between the 2p ₁₀ and 5d ₅ levels of the krypton 86 atom.

Since then, the wavelength of this orange-red line is the primary wavelength standard, equal to 605.780 211 nm in vacuum or 605.612 525 nm in air.