Sketch of two lines of sight (dashed lines) through a spherically symmetric clump (gray area) with impact parameter x, crossing (bottom panel) and not crossing (top panel) the central cavity of radius R_i (white central area). The thick circles denote the minimum and maximum radius of the region contributing to the brightness along the given l.o.s., while the thin circles are template annuli defined by Eq. (27), to be used for the numerical integration of the radiative transfer equation.