GB710960A - Improvements relating to magnetic lens arrangements - Google Patents

Abstract

710,960. Electron lenses. SIEMENS &HALSKE AKT.-GES. Aug. 29, 1951 [Aug. 29, 1950], No. 20402/51. Class 39(1) [Also in Group XXXV] A magnetic lens arrangement, preferably for use in an electron microscope, comprises a permanent magnet having at least one pair of pole shoes which are fixed relatively to the magnet but which are structurally separate members, a regulating member adjustable by external means to vary the refractive power of the lens connected in series in the magnetic circuit, and a magnetic casing connected to one pole of the magnet. The regulating member may be rotatable and/or axially movable. In one form, Fig. 1, the four lenses 1, 2, 3, 4 are energized by means of two annular permanent magnets 13, 14 co-operating with pole piece members 5, 8, 9, 10, 6 and enclosed within a jacket 7 of magnetic material, which may be constructed in two parts, as shown. Pole piece member 9 is held in position by a magnetic ring 40. An annular member 15 shunting the magnetic circuit can be moved along the axis to vary the focusing of the objective lens 1. 4 is the projection lens. The rotary regulator 17 for varying the image magnification comprises fixed teeth connected to the casing 7 and co-operating rotatable annular toothed member 26 mounted on magnetic ring 40. The ends of the teeth which may be in contact or separated by a small distance in the position of maximum flux flow may be bevelled or cylindrical, Figs. 2, 3 (not shown). In another form, Fig. 4, the two sets of teeth are replaced by arms 25 pivoting about axes 24. In another form, Fig. 5, permanent magnets 31, 32 are associated with three rotary regulators 33, 34, 35; 36, 37; 38, 39, the first two serving for adjusting the final image magnification and the third for focusing. 33, 34, 35 comprises two fixed and one movable toothed rings 34, 35; 33. The regulators'may have scales calibrated to indicate image magnification. Apertures 41 allow the cathode region to be exhausted if for example the vacuum pump is connected beyond the projection lens 4. In further modifications, an annular regulating member without teeth may be movable axially. The pole pieces may be composite i.e. separate pole shoes attached to supports. Prior art arrangements in which regulators are employed to adjust permanent magnetic lenses in electron microscopes are referred to, for example for adjusting the objective for focusing or the projection lens for varying the image magnification. They may consist of: (a) An adjusting mechanism operating by variation of stray flux external to the lens housing or (b) an axially movable part of a pole piece.