Divertor plasma opacity effects

•Opacity changes power and particle balance in tokamak divertor.•In carbon PFC tokamaks self-regulation imposed by chemical sputtering compensates for the opacity effects.•In metallic PFC tokamaks opacity shifts tokamak divertor operational window to higher densities or higher impurity content.•Opacity increases brightness of individual Balmer lines and change the ration between different Balmer lines .•Reliable interpretation of divertor plasma spectroscopic data requires corrections for the opacity.