Comparison of LTE (dashed black line) and non-LTE (solid lines) modelling for the 324.8, 327.4, 510.6 and 578.2 nm Cu I lines in a metal-poor dwarf (top four panels) and a metal-poor giant (bottom four panels). The impact of different Cu + H collision data on the non-LTE profiles is shown, with red and blue lines using newly calculated rates, while the orange line represents rates from Belyaev et al. (2021). No macroscopic broadening (macroturbulence, instrumental, and rotational broadening) was added to the lines.