https://doi.org/10.1351/goldbook.08590
Spectrum of a material under the influence of an applied external perturbation represented by a variable , relative to a reference spectrum.
Note:
The reference spectrum may be chosen in many ways; e.g., it may be the average over , of the observed spectrum, or it may be zero.
The reference spectrum may be chosen in many ways; e.g., it may be the average over