  A theoretical model of the influence of detection bandwidth properties on
observed line shapes in laser absorption spectroscopy is described. The model
predicts artificial frequency shifts, extra broadenings and line asymmetries
which must be taken into account in order to obtain accurate central
frequencies and other spectroscopic parameters. This reveals sources of
systematic effects most probably underestimated so far potentially affecting
spectroscopic measurements. This may impact many fields of research, from
atmospheric and interstellar physics to precision spectroscopic measurements
devoted to metrological applications, tests of quantum electrodynamics or other
fundamental laws of nature. Our theoretical model is validated by linear
absorption experiments performed on H2O and NH3 molecular lines recorded by
precision laser spectroscopy in two distinct spectral regions, near- and
mid-infrared. Possible means of recovering original line shape parameters or
experimental conditions under which the detection bandwidth has a negligible
impact, given a targeted accuracy, are proposed. Particular emphasis is put on
the detection bandwidth adjustments required to use such high-quality molecular
spectra for a spectroscopic determination of the Boltzmann constant at the 1
ppm level of accuracy.
