Kramida, Alexander Atomic Spectroscopy Group, Quantum Measurement Division, National Institute of Standards and Technology, Gaithersburg, Maryland.
Last reviewed:June 2020
- Lists of secondary standards
- Emission standards
- Determination of unknown wavelengths
- Reference wavelengths
- X-ray standards
- Related Primary Literature
- Additional Reading
Accurately known wavelengths of spectral radiation emitted from specified sources that are used to measure the wavelengths of other spectra. According to modern understanding, light consists of photons, which have the dual nature of particles and waves. As waves, photons are characterized by wavelength or frequency; and as particles, they are characterized by energy. The wavelength λ is related to frequency ν as λ = c/ν, where c is the speed of light. The photon energy E is related to frequency as E = hν, where h is Planck's constant. The science addressing wavelength measurement is called spectroscopy. The first wavelength measurements were made using grating spectroscopy in terms of the length standard, which was defined as a length of a certain artifact object stored at the International Bureau of Weights and Measures in Paris, France (BIPM). At present, the speed of light in vacuum is adopted as a constant equal to exactly 299,792,458 meters per second, and the standard unit of length, the meter, is defined as the length of the path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. Thus, the role of the length standard is now transferred to the frequency standard, which is presently defined as the frequency of the transition between two hyperfine levels of the ground state of the cesium-133 atom at rest at a temperature of 0 K. See also: Absolute zero; Diffraction grating; Light; Photon; Physical measurement; Spectroscopy
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