Published April 7, 2022 | Version 1.0
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NPL MeP standard frequencies XML schema

  • 1. National Physical Laboratory


This is an example of a possible XML schema that could be used for transmission of Mise en Pratique data for radiations in the List of Recommended Radiations, maintained by the BIPM: (

The schema defines five elements: standard frequencies “stdfreq”, fixed uncertainty contributions “funcc”, implementation rules “rule”, optical transitions “transition”, and optical components “component”. The main element is the “stdfreq”, which contains the value of the reference frequency, administrative information such as labels and validity information, along with sets of the “rule”, “funcc” and “transition” elements. The “rule” elements contain information constraining the realisation of the standard frequency, such as cold-well temperature, by providing a description of the limit, the nominal, minimum and maximum values, and the sensitivity coefficient. A “funcc” element describes fixed uncertainty contributions, such as the effect of iodine purity, providing a description and a single value. A “transition” element defines the transition by target atom/molecule/ion, transition name and the offset from the nominal frequency for the transition. It also contains a set of “component” elements, containing names and offsets from the transition values. In this way the schema replicates the structure of the current BIPM documentation. To calculate the frequency of a specific component the end user simply sums the standard frequency, transition offset and component offset values from the file.


This work was funded by the UK Government's Department for Business, Energy and Industrial Strategy (BEIS) through the UK's National Measurement System programme.


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Additional details


  • Hutzschenreuter, Daniel, Härtig, Frank, Wiedenhöfer, Thomas, Hackel, Siegfried Gustav, Scheibner, Alexander, Smith, Ian, Brown, Clifford, & Wiebke Heeren. (2020). SmartCom Digital-SI (D-SI) XML exchange format for metrological data version 1.3.1 (1.3.1). Zenodo.