The primary standard of length is defined in terms of the unperturbed 2<i>p</i><sub>10</sub> - 5<i>d</i><sub>5</sub> radiation of Kr-86. This paper reports new measurements of the perturbations due to the conditions of excitation necessary for its practical realization. The results confirm that the present recommendation of the International Committee of Weights and Measures yields the defined standard to an accuracy of better than 1:10<sup>8</sup> and they indicate how an improvement by about an order of magnitude may be realized by more exact specifications.
K. M. BAIRD and D. S. SMITH, "Primary Standard of Length," J. Opt. Soc. Am. 52, 507-513 (1962)