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This is a special paper in that it gives advice and instructions for the practical realization of the definition of the metre. It is thus of wide interest to all who need to make accurate measurement of length—for dimensional metrology, for spectroscopy, or for any other application. The definition of the metre is very simple: "The metre is the length of the path traveled by light in vacuum during a time interval of 1/299 792 458 of a second." While perfectly precise as a definition, it is not absolutely obvious how a practical measurement of length should be made. This paper is a formal document drawn up by the Consultative Committee for Length of the International Committee for Weights and Measures under the umbrella of the Metre Convention and gives the necessary practical information.

From the definition of the metre, the relation c = fλ allows the vacuum wavelength to be calculated if the frequency f of a radiation is known. Great advances are being made in optical frequency measurement. The invention of the femtosecond comb has led to the practical possibility of measuring the frequency of a visible laser radiation directly in terms of the caesum clock (which defines the SI second). In this way optical wavelengths can be determined directly. The paper also includes many optical transitions in the visible and infrared whose frequencies have been measured very accurately. It is worth noting that a few of these are now given with sixteen significant figures! The whole thing is a mine of information and embodies the results of the latest experiments in the field (up until 2002 despite it being labeled 2001!).