|Definition:|| qualifies or supplements the name of a quantity to indicate the quotient of that quantity by the wavelength or the frequency
NOTE 1 – The wavelength is used mainly in optics, and the frequency in radioelectricity, acoustics, and signal theory. Examples: power spectral density, spectral irradiance.
NOTE 2 – The adjective “spectral” is also used to designate quantities which are functions of wavelength or frequency, but which are not spectral concentrations, for example spectral emissivity. See ISO 80000-7.