Patent GB-1055670-A (GB)
Title: Variable magnification electron lens
IPC: H01J29/62
Filed: 1964-08-19
Abstract: 1,055,670. Electron lenses. GENERAL ELECTRIC CO. Aug. 19, 1964 [Sept. 16, 1963], No. 33926/64. Heading H1D. A variable magnification electron lens system, e.g. for an image converter tube, has two adjacent lens units arranged in series along the optical axis, each unit having two coaxial, axially spaced spiral electrodes physically separated and electrically connected by a conductive band, whereby each unit generates an electric field having a substantially quadratic variation along the axis, and a plurality of terminals connected to axially spaced points on the two lens units to permit control of the potential distribution with consequent variation of the magnification of the system. An electron image of a radiation image 11, incident upon the plane photocathode 5 of an image converter tube, is formed by a lens system 1 having a plane mesh electrode 6, a convex mesh electrode 7, and intervening electrically conductive annular bands 8 and 9, and thereafter traverses the variable magnification lens system 2, and a final accelerating system 3, before impacting a fluorescent screen or image orthicon target 37. Section 2 is formed by two similar units in tandem, having helical resistive bands 17, 19 and 26, 27, respectively, joined by conductive rings 18 and 28, helices 19 and 26 being connected to a common terminal 24, helix 17 being connected with the electrode 7 to terminal 22, and helix 27 being connected with a plane mesh electrode 33 to terminal 31. The uniform helical bands and separating conductive ring of each unit provides an approximately quadratic variation of potential along the axis of the tube; a quadratic variation may also be obtained by the use of a non-linear helix, or obtained to a higher approximation by replacing each uniform helix by two uniform helices of different pitch but equal resistance. Accelerating and target section 3 includes a semi-conductive titanium oxide film 35, which is coated on the glass support tube 34 mounted within the tube envelope 4, and is connected at one end to terminal 31 and at the other to terminal 36 adjacent the target 37, silver rings 38, which are coated on the film and float electrically, ensuring symmetry of the accelerating field. Ganged switches controlling the relative potentials applied to terminals 22, 23, 24, 30 and 31 permit adjustment of the magnification produced by section 2, without defocusing the image on the target. In three modes of operation providing different magnifications, terminals 22, and 31 are held at the same potential V and, for two modes, terminal 24 is also held at V while terminals 23 and 30 are held at potentials respectively less and greater than V or greater and less than V (Figs. 2A and 2C, not shown), while for the third mode terminal 24 is held at less than V and rings 18 and 28 allowed to float (Fig. 2B, not shown). The application of differing potentials to terminals 22 and 31 permits greater variation of the magnification. In modified apparatus (Fig. 3, not shown), section 3 is replaced by an accelerating and magnifying lens formed by a concave mesh electrode and a non-uniform helix giving a hyperbolic field, and arranged to provide such magnification that when section 2 is operating in the mode giving least magnification, the final image just fills the target area: in other modes, the image electrons overscanning the target are absorbed by the helical electrode.