National Physical Laboratory

Metre Realisation

The SI unit of length is the metre. Since 1983 the metre has been defined as

"the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second."

The definition of the metre fixes the speed of light in a vacuum, c0, at 299 792 458 ms-1.

For practical measurements of length any of the three following approaches may be used:

  1. The length l of the path travelled by light in a measured time t is given by l = c0t
  2. The wavelength λ of light of measured frequency f is given by λ = c0/f
  3. The vacuum wavelengths of a number of standard reference radiations have internationally recommended values, determined from frequency measurements.

The recommended radiation most commonly used for the practical realisation of the metre is the 633 nm light from an iodine-stabilised helium-neon laser. At NPL the metre is disseminated to UK industry by providing a direct, traceable link from our iodine-stabilised HeNe reference lasers to stabilised laser interferometer systems used in the measurement of length and displacement. This link also provides for traceability both through specialist NPL interferometer systems (such as the gauge block interferometer and the 30 m interferometer) and to commercial laser interferometer systems widely used for the calibration of CNC machine tools and co-ordinate measurement.

Last Updated: 25 Mar 2010
Created: 12 Aug 2007


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