wavelength standards
Wavelength standards
Accurately known wavelengths of spectral radiation emitted from specified sources that are used to measure the wavelengths of other spectra. In the past, the radiation from the standard source and the source under study were superimposed on the slit of a spectrometer (prism or grating) and then the unknown wavelengths could be determined from the standard wavelengths by using interpolation. This technique has evolved into the modern computer-controlled photoelectric recording spectrometer. Accuracy of many more orders of magnitude can be obtained by the use of interferometric techniques, of which Fabry-Perot and Michelson interferometers are two of the most common. See Interferometry, Spectroscopy
The newest definition of the meter is in terms of the second. The wavelength of radiation from the cesium atomic clock is not used to realize length because diffraction problems at this wavelength are severe. Instead, lasers at shorter wavelengths whose frequencies have been measured are used. Frequency measurements can now be made even into the visible spectral region with great accuracy. Hence, when the 1983 Conférence Général des Poids et Mesures redefined the meter, it also gave a list of very accurate wavelengths of selected stabilized lasers which may be used as wavelength standards; these are shown in the table. Nearly ten times better accuracy can be achieved by using these wavelengths than by using the radiation from the krypton lamp which provided the previous standard. See Frequency measurement, Hyperfine structure, Laser, Laser spectroscopy, Length, Molecular structure and spectra, Physical measurement
The progress in laser frequency measurements since 1974 has established wavelength standards throughout the infrared spectral region. This has been accomplished with the accurate frequency measurement of rotational-vibrational transitions of selected molecules. The OCS molecule is used in the 5-micrometer spectral region. At 9–10 μm, the carbon dioxide (CO2) laser itself with over 300 accurately known lines is used. From 10 to 100 μm, rotational transitions of various molecules are used; most are optically pumped laser transitions. The increased accuracy of frequency measurements makes this technique mandatory where ultimate accuracy is needed. See Wavelength