angstrom

History
Originally (in 1868 for solar radiations) and still used for expression of wavelengths in the  electromagnetic spectrum. After elaborate measurement of the wavelength of the red line of cadmium at 6438.4696 × 10¹⁰ m, it was agreed internationally in 1907 that the angstrom be defined by assigning the value 6438.4696 Å to that wavelength Barrell H. Proc. Roy. Soc. London Ser. A 186, 164–70 (1946)
(in dry air containing 0.03% carbon dioxide, at standard atmospheric pressure and a temperature of 15°C). This was the pioneer use of light to define length, presaging the application to the metre in 1960, using krypton. By that time the 1907 definition had been shown to have made the angstrom 1.000 000 2∼ × 10⁻¹⁰ m relative to the extant prototype metre; the 1960 re-definition changed the length of the metre to make it precisely 10¹⁰ Å as defined in 1907.

The 1978 decision of the CIPM considering it acceptable to continue to use the angstrom with the SI still stands.