During World War II, the development of the cavity resonance wavemeter for use in radar, together with precision timing methods, opened the way to laboratory-based measurements of the speed of light. In 1946, Louis Essen in collaboration with A.C. Gordon-Smith used a microwave cavity of precisely known dimensions to establish the frequency for a variety of normal modes of microwaves—which, in common with all electromagnetic radiation, travels at the speed of light in vacuum. As the wavelength of the modes was known from the geometry of the cavity and from electromagnetic theory, knowledge of the associated frequencies enabled a calculation of the speed of light. Their result, 299,792 ± 3km/s, was substantially greater than those found by optical techniques, and prompted much controversy. However, by 1950 repeated measurements by Essen established a result of 299,792.5 ± 1 km/s; this became the value adopted by the 12th General Assembly of the Radio-Scientific Union in 1957. Most subsequent measurements have been consistent with this value.

Measurement of distance at this time (before 1960)would have been by reference to a material standard.

The most accurate way of comparing and making replica standards was the use of optical interferometry. This compares lengths by using the wavelength of light. As interferometry progressed it became possible to measure the number of wavelengths of a given light in the length of the standard metre. Light sources became more stable and reproducible it was observed that the number of wavelengths of a given light in the standard metre never changed. This was not surprising as the two-way speed of light, as is used in interferometers, had been experimentally confirmed not to vary with motion by Michelson and Morley.