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new large scale facilities as for example the International Linear Collider~\cite{Himel_2007}.  An operation metrics should quantify the reliability of a particle accelerator.  If the objective is to assess the improvement of a specific facility over time,   then the operation metric metrics  should be very closely related to the specific user requirements for that facility~\cite{Luedeke:2014aa}.  But in order to compare the reliability of different facilities one needs a   common standard for the calculation of the operation metrics. 
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In particular beam availability is often used to compare the reliability of light sources.   While these statistics are published for most light sources,   few facilities do supply definitions on how these numbers are calculated.   A survey of several light sources revealed~\cite{L_deke_2009} that the calculation of this metric metrics  varies considerably. The conditions under which beam is considered available are often defined in common sense terms,  and even if there are formal definitions, they differ between facilities.   Furthermore a large variety of failure modes is often convoluted into ``beam not available'',  
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The metrics serve to clarify for each facility the beam parameter promised to the users,   and the statistical data reveal, how well the standards are met.  The authors are convinced that the application of this metrics will refine our ability  to learn from the reliability comparison of light sources.