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The frequency (or, equivalently, the vacuum wavelength) of a red He-Ne laser is determined by comparison to an iodine-stabilized He-Ne laser. The attainable accuracy of this technique is ultimately limited by the uncertainty in the wavelength of the iodine stabilized laser. The relative standard uncertainty of the laser vacuum wavelength at NIST is 5 x 10-11 . The relative expanded uncertainty is 1 x 10-10 . The relative expanded uncertainty can be reduced to 5 x 10-11 on special requests. As a practical matter, however, the uncertainty of a comparison is usually determined by the limited reproducibility of the laser under test.Calibrations Phone: 301-975-5454, Fax: 301-975-2950 NIST,