spectral distribution

spectral concentration

X_λ

density of a radiant or luminous or photon quantity, X(λ), with respect to wavelength, λ, at the wavelength λ

Note 1 to entry: The term "spectral distribution" is to be preferred to the equivalent term "spectral concentration" when dealing with the function X_λ(λ) over a range of wavelengths, not at a particular wavelength.

Note 2 to entry: Usually X_λ is also a function of λ and in this case, in order to stress this, can be written X_λ(λ) without any change of meaning.

Note 3 to entry: The spectral distribution of radiant flux is expressed in watt per nanometre (W⋅nm⁻¹), the spectral distribution of luminous flux is expressed in lumen per nanometre (lm·nm⁻¹), the spectral distribution of photon flux is expressed in nanometre to the power minus one (nm⁻¹). The units of spectral distributions of other quantities are expressed accordingly.

Note 4 to entry: The quantity X can also be expressed as a function of frequency, ν, wavenumber, σ, etc.; the corresponding symbols are X(ν), X(σ), etc. and the corresponding densities with respect to frequency, ν, wavenumber, σ, etc. are X_ν, X_σ, etc., in which case the expression of units will also change accordingly.

Note 5 to entry: This entry was numbered 845-01-17 in IEC 60050-845:1987.

Note 6 to entry: This entry was numbered 17-1213 and 17-1212 in CIE S 017:2011.