Polarisation signal from a corona with τ = 1 and kT_e = 100 keV observed at 75° inclination with seed photons initially polarised according to Chandrasekhar calculations. The black line correspond to the extended slab corona covering the whole disc up to 1000 r_g. Coloured lines show the signal as we shrink the outer radius of the corona until we reach a compact spherical corona with radius 10 r_g (green line). Orange line represents a special case in which we reduced the ISCO to the value corresponding to a Kerr BH (see text for further detail).