instrument line shape 

Initialism: ILS
https://doi.org/10.1351/goldbook.08257
Idealized form of a feature in a spectrometer corresponding to a transition.
Notes:
  1. Ideal line shapes include Lorentzian, Gaussian, and Voigt functions, whose parameters are the line position, maximum height, and half-width.
  2. For Fourier-transform spectra, ILS varies with the apodization function used.
Source:
PAC, 2021, 93, 647. (Glossary of methods and terms used in analytical spectroscopy (IUPAC Recommendations 2019)) on page 662 [Terms] [Paper]