Examples of modelled contrasts with various parameters. Heights of centres of modifications of atmosphere are given in the figure. Abscissa in arcsec covers one period in x, it is foreshortened for oblique viewing. Solid horizontal lines: zero contrast level; dotted: continuum contrast (at +20 Å from line minimum); solid: Hα_wing contrast with increased Planck function ΔB; dashed: solid plus increased line opacity Δκ_λ, dash-dotted: dashed plus increased coupling to Planck function Δε. See text for values of parameters.