— 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: ${\displaystyle X_{\lambda }\equiv \frac{\text{d}X}{\text{d}\lambda }}$; in the latter case, in French, "spectrique" is preferred to "spectral"

X_λ is also a function of λ and in order to stress this may be written X_λ(λ) without any change of meaning

NOTE – The quantity X can also be expressed as a function of frequency v, wave number σ, etc.; the corresponding symbols are X(v), X(σ), etc. and X_v, X_σ, etc.