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In 2003, the International Committee for Weights and Measures (CIPM) recommended updated values of the frequency for certain optical frequency standards recommended for the practical realization of the definition of the metre. The text of this CIPM Recommendation and details of the updated radiations are given here. The complete updated set of recommended radiations, including frequencies, wavelengths, uncertainties and operating conditions where appropriate, is available on the BIPM website.
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R. J. Felder (2005) studied this question.
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