Abstract: A cathode lens is formed between a gun electrode (8) and a specimen (9). An electron probe (11), produced as part of an electron column and suitably focused by lenses (1, 2 and 3) and scanned by suitable deflector/stigmator electrodes (2), is decelerated within the cathode lens field and its final landing energy is finely adjustable by the specimen negative bias. Emitted secondary electrons are re-accelerated within the same field and due to uniformity of this field, they increase their axial velocity only so that they are collimated into a narrow signal beam. The collimated signal beam passes mostly through an aperture (18) of electrode (8), where it enters the gun and a final lens consisting of a central earthed electrode (6) surrounded by two earthed electrodes (7) and (8). The signal beam approaches a special mirror electrode (4), the field of which decelerates and deflects the electrons further off the axis and returns them back towards the specimen (9).