Recommendation 2 of the 96th CIPM (2007)

On the value and uncertainty of unstabilised He-Ne lasers

The International Committee for Weights and Measures,


  • that most laser interferometers and many other measuring instruments used for length measurement are based on 633 nm He-Ne lasers;
  • that these instruments are often used at uncertainty levels that are large compared to the possible variation of the He-Ne laser vacuum wavelength;
  • that the vacuum wavelength of the unstabilized 633 nm He-Ne laser is restricted to within a narrow range by fundamental quantum phenomena;


  • that it would be necessary to provide guidance and documentary evidence concerning the value of the vacuum wavelength and its uncertainty that can be expected in the absence of calibration;
  • that such evidence could help to avoid unnecessary calibrations of these lasers in such applications;


  • that the values
    f = 473.612 7 THz
    λ = 632.990 8 nm
    with a relative standard uncertainty of 1.5 × 10–6, apply to the radiation in vacuum of a unstabilised helium-neon laser operating solely on the 3s2→2p4 transition, independent of the isotopic mixture of the neon;
  • that an entry for unstabilized helium-neon lasers, operating on the 633 nm (3s2→2p4) neon transition, be included in the second category of the list of standard frequencies, and that an accompanying paper with CCL authority be published in Metrologia.

reference The reader should note that the official version of this Resolution is the French text

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