adjective that, when applied to a quantity X pertaining to electromagnetic radiation, indicates:

either that X is a function of the wavelength λ, symbol: X(λ), or that the quantity referred to is the spectral concentration of X, symbol: 

NOTE 1 In the latter case, in French, "spectrique" is preferred to "spectral".

NOTE 2 Xλ is also a function of λ and in order to stress this, may be written Xλ(λ) without any change of meaning.

NOTE 3 The quantity X can also be expressed as a function of frequency ν, wave number σ, etc.; the corresponding symbols are X(ν), X(σ), etc. and Xν, Xσ, etc.

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