For irradiating a target with a beam of energetic electrically charged particles comprising a plurality of beamlets, the target is exposed in a sequence of exposure stripes composed image pixels. These stripes (s1, s2) are, at their boundaries to adjacent stripes, provided with overlap margins (m12, m21) which are mutually overlapped, so nominal positions of image pixels in the overlap margin (m21) overlap, or substantially coincide, with image pixels in the corresponding overlap margin (m12). During the exposure of an overlap margin (m21), a first subset (n1) of image pixels in said overlap margin are exposed while those of a second subset (n2), possibly a complementary subset with respect to a desired pattern, are not exposed; contrariwise, during the exposure of the corresponding overlap margin (m12), image pixels corresponding to image pixels in the first subset are not exposed, but those corresponding to image pixels in the second subset are.