Effects of both the local field and the image potential are taken into account in the determination of the ionization rates of the imaging gas in the field ion microscope. The model consists of a half-sphere superimposed on a flat surface and the results were obtained after the JWKB scheme. In comparison with the more usual cases, where only the external field and the atom potential are considered, the overall effect is to narrow the ionization zone and to decrease the ionization rate, except for the part of the ionization zone closest to the surface.