metre, m: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Until 1983 the metre was defined in terms of the wavelength of light emitted by a krypton spectral lamp. That definition limited the accuracy of length measurement to several parts in 108, which was insufficient for the highest accuracy applications. The change of definition in 1983, which defines the speed of light, allowed realisations based on a variety of lasers, which were developing rapidly at the time. Now the quality of lasers is such that there has been more than a million times improvement in the primary standards. Unfortunately this advance has enabled only moderate improvements in artefact length standards, such as gauge blocks, because of the limitations imposed by the properties of materials. Increasingly however lasers are finding their way into surveying equipment and high-grade measurement equipment, and it will not be long before the benefits of laser standards will be more widely available. The heaviest commercial pressure on length standards is currently at the extremes of lengths: the sub-micron lengths used in silicon chip manufacture; and the long geological scale lengths used in surveying and seismological studies.
Although the definition of the metre implies that length is measured by measuring the ‘time of flight’ of a light beam, very few length measurements are made this way. Instead lengths are measured by counting the wavelengths of light generated by a few approved light sources. The approved light sources and their wavelengths are specified in the mise en practique (translates as ‘put in practice) of the definition of the metre, which is published by the CIPM. The link to the definition of the metre is made through the relation
which relates the wavelength (λ) to the frequency (ν) and the speed of light (c). The best measurements of optical frequencies of some of the approved sources are accurate to about 1 part in 1015, close to the accuracy of the clocks used to define the second. Until very recently the experiments that determined the frequency of optical sources were monstrous affairs requiring many staff and years of effort, so only a few of the best performing sources have been measured this way. However recent advances in the development of very fast pulsed lasers may overcome many of the technical obstacles. In the future we will find ourselves using lasers more and more for measuring length.