ISO 31-6

ISO 31-6 is the part of international standard ISO 31 that defines names and symbols for quantities and units related to light and related electromagnetic radiations. It is superseded by ISO 80000-7.

ISO 31-6 was initially published in 1980, and amended in 1985. It received a revision in 1992, which was amended in 1998. The standard was ultimately withdrawn in 2008, when it was replaced by ISO 80000-7.[1]


The definitions of ISO 31-6, as defined in 1980, included:[2]

Quantity Unit Remarks
Name Symbol Name Symbol Definition
frequency f, v hertz Hz 1 Hz = 1 s−1
circular frequency ω reciprocal second, second to the power of minus one s−1
radian per second rad/s
wavelength λ metre m
ångström Å 1 Å = 10−10 m 1 Å = 10 nm
wavenumber, repetency σ reciprocal metre, metre to the power of minus one m−1   The multiple cm−1 is often used.
circular wavenumber, circular repetency k
velocity (speed) of propagation of electromagnetic waves in vacuo c, c0 metre per second m/s   c = (2.99792458 ± 0.000000012) × 108 m/s.[a]
Sometimes c is used for the phase velocity in a medium, in which case c0 is used for the velocity in vacuo.
radiant energy Q, W,
(U, Qe)
joule J 1 J = 1 kg·m2/s2
radiant energy density w, (u) joule per cubic metre J/m3
spectral concentration of radiant energy density (in terms of wavelength), spectral radiant energy density (in terms of wavelength) wλ joule per metre to the fourth power J/m4
radiant power, radiant energy flux P, Φ, (Φe) watt W 1 W = 1 J/s
radiant energy fluence rate φ, ψ watt per square metre W/m2
radiant intensity I, (Ie) watt per steradian W/sr


  1. ^ This is the text of the note in the standard ISO 31-6 as published in 1980. At the time, the metre was not defined in terms of the speed of light. Today, the speed of light is defined as exactly 299,792,458 m/s.


  1. ^ "ISO 31-6:1980". International Organization for Standardization. Retrieved 2024-02-12.
  2. ^ International Standard ISO 31/6: Quantities and units of light and related electromagnetic radiations (PDF) (Second ed.). International Organization for Standardization. 1980-12-15. Retrieved 2024-02-11 – via iTeh.