Resolution 2 of the 15th CGPM (1975)
Recommended value for the speed of light
The 15th Conférence Générale des Poids et Mesures,
considering the excellent agreement among the results of wavelength measurements on the radiations of lasers locked on a molecular absorption line in the visible or infrared region, with an uncertainty estimated at
considering also the concordant measurements of the frequencies of several of these radiations,
recommends the use of the resulting value for the speed of propagation of electromagnetic waves in vacuum
The relative uncertainty given here corresponds to three standard deviations in the data considered.