Measurement of the speed of light II. Wavelength measurements and conclusion

Wavelength measurements have been carried out on the radiation at 9.3 $\mu $m from a stabilized CO$_{2}$ laser. The wavelength in vacuum was determined as 9 317 246 348 femtometres (fm), with a standard error of the mean of 11 fm (1.14 parts in 10$^{9}$) and a total systematic uncertainty of $\pm $ 13 fm ($\pm $ 1.4 parts in 10$^{9}$). An upconversion technique of optical mixing in a non-linear crystal was used, so that the interferometric wavelength measurement was carried out on visible difference-frequency radiation at 679 nm. Its wavelength was determined relative to that of an iodine-stabilized He-Ne laser at 633 nm, by using plane-parallel Fabry-Perot interferometers. When combined with the result of the frequency measurement described in part I, this wavelength measurement leads to a value for the speed of light in vacuum of 299 792 459.0 $\pm $ 0.6 m/s (Blaney et al. 1974). This result is in agreement (within the quoted uncertainties) with the value recommended internationally by the 14th General Conference of Weights and Measures in 1975.