Normalised flux profiles as a function of projected radius R of gas being ionised by a central point source. The thick solid curve assumes that the gas is optically thin and distributed in an infinitesimally thin disk with a constant filling factor and constant density, so that the fall-off is the same inversely squared with radius as the point-source radiation. The thick dashed curve is when the gas is distributed in an axisymmetric disk of finite thickness with gas density both radially and vertically declining exponentially as ∝exp [−R/h_R] exp [−| z|/(q   h_R)] with fiducial values for the scale length of h_R = 3 kpc and for the flattening of q = 0.1, viewed at an inclination angle of i = 60°. The thin long/short dashed curves show the effect of a factor of two thicker/thinner disk, the dash-dotted curve is for when the scale length is much larger, and the dotted curve is when the gas distribution is spherical and more centrally concentrated.