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How has mercury-198 been used as a wavelength standard?

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A periodic table element box for mercury shows its symbol Hg, atomic number 80, atomic weight 200.592, electron configuration, and name. Icons indicate mercury is a transition metal, liquid at room temperature, has a rhombohedral crystal structure, and is weakly basic. — mercury Properties of mercury.

Mercury-198 is an isotopically pure form of mercury prepared through the neutron bombardment of gold-197. Because it consists of a single stable isotope, it produces a highly defined spectral line, which has been used as a wavelength standard for precise scientific measurements.1