The list of recommended radiations was first published by the CIPM in 1983 (see CI-1983, Recommendation 1) in the mise en pratique of the definition of the metre. This specified that the metre should be realized by one of the following methods:
For the full list of specifications, the reader is referred to the original Recommendation and to the various updates that have since been approved by the CIPM. Most of these updates have been recommended by the CCL, based on the work of the CCL Working Group on the Mise en Pratique (MePWG).
- by means of the length l of the path travelled in vacuum by a plane electromagnetic wave in a time t; this length is obtained from the measured time t, using the relation l = c0 · t and the value of the speed of light in vacuum c0 = 299 792 458 m s1,
- by means of the wavelength in vacuum of a plane electromagnetic wave of frequency f; this wavelength is obtained from the measured frequency f using the relation = c0/f and the value of the speed of light in vacuum c0 = 299 792 458 m s1,
- by means of one of the radiations from the list given here, whose stated wavelength in vacuum or whose stated frequency can be used with the uncertainty shown, provided that the given specifications and accepted good practice are followed.
In 2005 the MePWG and the CCL-CCTF Joint Working Group on Secondary Representations of the Second merged into a single CCL-CCTF Frequency Standards Working Group, which was charged with producing and maintaining a single list of Recommended frequency standard values for applications including the practical realization of the metre and secondary representations of the second. The drop-down menu at the top of this page gives access to the list being developed by this group. Where full data sheets are not yet available, links are given to relevant decisions of the CIPM or CC.