angstrom or ångström

A unit of length originally used by spectroscopists and others studying light, defined by the CGPM in 1960 as exactly equal to 10⁻¹⁰ meter. Symbol, Å. One Å = 0.1 nanometer. This unit has also been called the tenthmeter (because it is 1.0 × 10⁻¹⁰ meter).

A unit of this length was employed by the Swedish spectroscopist Anders Jonas Ångström (1814-1874) in 1866 in his classic description of the solar spectrum. In 1907 the International Astronomical Union defined the international ångström by making the wavelength of the red line of cadmium in air equal to 6438.4696 international ångströms.¹ This particular value was chosen so that, within the limits of measurement at that time, the ångström would be 10⁻¹⁰ meter. The BIPM endorsed the unit in 1927. In 1961 the International Astronomical Union accepted the 1960 CGPM definition.

In 1978 the CIPM listed the ångström among those units “acceptable to be used with SI until the CIPM considers their use no longer necessary. However, these units should not be introduced where they are not used at present.”² In the same period, the American National Standard for Metric Practice (Z210.1) also discouraged use of the ångström.

In spectroscopy, the ångström has been displaced by the nanometer. However, in the early 21st century it continues to be used in various technical fields.

According to the current national standard in the United States³, the ångström is not to be used. The nanometer should be used instead.

Anders Jonas Ångström.
Recherches sur le spectre solaire.
Upsal : W. Schultz, 1868.

1. Transactions of the International Union of Solar Research, volume 20, page 28 (1907).

2. The International System of Units (SI).
NIST Special Publication 330, 1991 edition.

Page 13.

3. IEEE/ASTM SI10™-2002.
American National Standard for Use of the International System of Units (SI): The Modern Metric System.
New York: IEEE, 30 December 2002.

See Section 3.3.3.



The ångström is widely used by x-ray crystallographers and structural chemists because all chemical bonds lie in the range 1 to 3 ångströms. However, it has no official sanction from the CIPM or the CGPM.

U.S. Dept. of Commerce, National Institute of Standards and Technology.
The International System of Units (SI).
Barry N. Taylor and Ambler Thompson, editors.
NIST Special Publication 330, 2008 edition.
Washington: March 2008.
Page 35.


In particular, wavelengths should be expressed in metres with the appropriate SI prefix; e.g., for wavelengths in the visual range the nanometre (nm) should be used instead of the angstrom (Å), which is a source of confusion in comparisons with longer and shorter wavelengths expressed in recognised SI units. The notation of the form λ followed by a numerical value (which represents the wavelength in angstroms) should also be abandoned.

George A. Wilkins.
The IAU Style Manual (1989) The Preparation of Astronomical Papers and Reports.
Paris: International Astronomical Union, 1989.
Page S25.



Flux (10⁻¹⁸ erg cm⁻² s⁻¹ å⁻¹)

Legend for vertical axis of figure 1a, in
M. D. Lehnert, N. P. H. Nesvadba, J.-G. Cuby, A. M. Swinbank, S. Morris, B. Clèment, C. J. Evans, M. N. Bremer and S. Basa.
Spectroscopic confirmation of a galaxy at redshift z ≈ 8.6.
Nature, vol 467, pages 940-942 (20 October 2010). Letter.