Cumulative line number counts per beam and resolution element. Numbers are shown at different wavelengths, as derived using SPRITZ. We do not expect FIRESS to be affected by line confusion down to at least 10⁻²⁰ W/m². The vertical dotted line shows the flux of 10⁻¹⁹ W/m², reachable with FIRESS with one hour of integration time. The horizontal dashed line and the shaded region show the limit of 15/beam/R, above which PRIMA could be affected by line confusion at the longest wavelengths.