We present three-dimensional simulations of the image formation process in
near-field optical microscopy, Our calculations take into account the diffe
rent components of a realistic experiment: an extended metal coated tip, a
subwavelength sample and its substrate. Mie investigate all possible detect
ion (transmitted, reflected and collected field) and scanning (constant hei
ght, constant gap) modes. Our results emphasize the strong influence of the
tip motion on the experimental signal. They also show that it is possible,
by controlling the polarization of both the illumination and the detected
field, to strongly reduce these artefacts.