Effect of spatial distribution of impurity ions on the signal of ‘C/O monitor for Wendelstein 7-X’ - an indicator of plasma wall interactions

•A forward modelling was applied to investigate the response of the C/O monitor system;•Simulations were performed for Lyman-α lines of H-like ions of carbon and oxygen;•The analysis covered studies of impact of impurity transport on the output signals.•Systems' response on radial impurity distribution depends on the measured element.•Modelling should always be included during analysis of experimental data.