Abstract: The SEM according to the invention is arranged to detect scattered electrons transmitted by the specimen (22). The detector is constructed so as to have a flat detector surface (26) for a large angular range. In order to select a given angular range, a mask (24) with an eccentric opening (40) is arranged between the specimen and the detector surface and it is also arranged so as to be rotatable. An optimum angle relative to a given direction in the specimen can thus be selected for a given type of contrast. The mask is preferably displaceable in the specimen direction, the detector surface being subdivided into a plurality of parts (26a, 26b) and the assembly formed by the specimen and the detector surface is tiltable through an angle relative to the optical axis (4). A high degree of flexibility is thus achieved for the selection of the electrons to be detected which are scattered by the specimen, so that adjustments can be made so as to achieve an optimum contrast situation.