US2249025A - Electron-optical device - Google Patents

Description

July 15, 1941.

G. A. MORTON ET AL 2.249.025 ELECTRON-OPTICAL DEVI CE Filed Feb. 28, 1933' 2 Sheets-Sheet 1 I afakaflz/vakron/ an 50mm an? ['96 l l'l F- fil llllq y 15, 1941- s. A. MORTON ETAL .2 943 5 ELECTRON-OPT ICAL DEVI OB Filed Feb. 28, 1938 2 Sheets-Sheet 2 Patented July 15, 1941 ELECTRON-OPTICAL DEVICE George A. Morton, Audubon, and Edward G. Ramberg, Haddon Heights, N. .L, assignors to Radio Corporation of America, a corporation of Delaware Application February 28, 1938, Serial No. 193,134

Claims. (01. 250-150) Our invention relates to electron-optical devices, and particularly to devices for intensifying an optical image as by converting the same into .a primary-electron image and, successively, into ,one or more secondary-electron images, and then .cants in the instant case) Serial No. 38,380, filed August 29, 1935. In that disclosure, the intensifying or multiplying or target electrodes and the screen or output electrode are arranged in planes successively more remote from the photo-sensitive cathode upon which the optical image is originally impressed. This standard arrangement of electrodes is objectionable in that the container or envelope must be larger, and the problems incident to its evacuation greater,

than we now find necessary or desirable.

Accordingly, an object of our present invention .is to provide a simplified and compact image intensifier and one having an improved electronlens system incorporated therein.

Another object of our invention is to provide an image intensifier device wherein the cathode,

vthe target electrodes and the output electrode are .radially arranged about a common center.

Other objects will be apparent and our invention itself, both as to its organization and method of operation, will be best understood by reference to the following specification and to the accompanying drawings, wherein Figure 1 is a partly diagrammatic plan view ,of an image intensifier device constructed and operated in accordance with the principle of our invention,

Figure2 is a side elevational view on a reduced scale of the device of Fig. 1 showing the device subjected to a magnetic field, as'in normal operation, and

Figure 3 is a partly diagrammatic plan view of a plural-stage image intensifier.

In carrying the invention into efiect, we provide a highly evacuated preferably globe-like envelope T which, in the embodiment of Figs.

1 and 2, has three "nec portions or extensions A, B, C, respectively. The vacuous space in the main or bulbous body of the tube and in the necks A, B, C, is continuous. The central axis a, b, c, of the necks A, B, C, preferably lie in a common plane. 'It will be noted that the angle formed by the intersection of axes a and b is velocity.

greater than the angle formed by the intersection of the axes b and 0. Considering each neck to comprise a stage of the device, the general rule is: the angular separation of successive stages (in point of electron travel) decreases to an extent dictated by the intensity of the electrostatic and electromagnetic fields employed.

The inner wall of the bulbous part of the tube is provided with a coating M of metal to which a potential may be applied through a lead m. This metal layer M, which constitutes an anode, extends tothe mouth of each of the extensions A and'B and into the extension 0 to a point adjacent the terminal end thereof.

Mounted in the neck portion A of the tube T is aphoto-sensitive cathode l which is adapted to release an entire primary-electron image in response to theimpress thereon of a light image from a source exemplified in the drawings by an optical lens L., Cathode I is preferably curved in a known manner to correct for image defects. A series of spaced rings 2 to 5, inclusive, and a short cylinder 6, arranged in the order named about the axis a, constitute an electron lens sys-' tem which, when supplied with suitable potentials from a source exemplified in the drawings by a voltage divider R, will project the electrons comprising the entire primary-electron image into the vacuous interior of the main portion of the, tube. Unless some force intervenes the electrons constituting such image will continue to advance along the axis a, and miss the extension B, which, as will more fully hereinafter appear, contains the image intensifying'means. For the purpose of directing the primary-electrons into the extension B, a magnetic field is required which is perpendicular to the plane containing the axes a., b and 0. Such magnetic field may be produced in the manner indicated in Fig. 2,

i. e., by a pair of coils F, F, suitably energized and supported exterior of the tube T.

The primary or photo-electrons leaving the cathode l are subject to the successively increasing accelerating voltages applied through adjustable leads 2a, 3a, etc., to the focusing rings 2, 3, etc., and also to the accelerating voltagesapplied to the focusing cylinder 6, and anode M through leads 6*, m, respectively, which, as indicated in the drawings are connected to the most positive point (say, 5000 volts) on the voltage divider R. The primary electrons thus approach the open end of the extension 3 at a relatively high Electrode adjacent the closed end of the neck B isv the intensifying? or electron-multiplying electrode. This electrode may comprise a silver or other metal plate treated with a substance which is the equivalent of caesium in order to enhance its ability to release secondary-electrons by impact excitation. The ring-like electrodes l2, l3, l4, l5, the cylinder I6 and the terminal cylinder I! constitute elements of an electron lens system which serves to focus the primary-electron image upon the multiplying electrode I I, and likewise serves to accelerate and to focus the secondary electrons from that electrode into the open central portion of the tube where they are subjected to the influence of the magnetic field and directed into the extension C which contains a fluorescent screen S for converting the secondary-electron image into a light image.

Considering the cathode I to be maintained atv ground potential, the focusing rings 2 to 5, inclusive, each at a potential 50 volts positive with respect to the next preceding electrode, and the focusing cylinder 6, anode M, to be maintained at, say 5000 volts positive with respect to the cathode, it will be apparent (since the magnet in no wise aifects the speed of the electrons) that the primary electrons will approach the extension B. with a velocity of 5000 volts since the outermost cylinder I! in that extension is maintained at that potential. The speed of the primary electrons passing through the cylinders I1 and i6, and the focusing rings I to l2 will be reduced as they approach the multiplying electrade I! and in striking that electrode their speed will be determined by the voltage applied thereto, in this case 1000 volts. The relative voltage distribution among the focusing rings. l2 to and cylinders l6 and H may be substantially the same as that of the rings 2 to 5 and cylinder 6 in the extension A, that is to say, each ring-like electrode may be maintained at a voltage 50 volts higher than its next preceding electrode while the outermost cylinder I1 is maintained at or near the anode potential, e. g. 5000 volts.

While the electrons constituting the primaryelectron image and those constituting the secondary-electron image both pass through an electrode (1'!) maintained at a voltage of 5000 volts positive with respect to the cathode, the departing secondary-electrons will actually be traveling much slower (i. e. 1000 volts slower) than the entering primary-electrons, because the potential drop between the electrode II and the anode M or the cylinder H is less than the drop between that electrode and the cathode I. Since the magnetic field has a greater efiect upon slower moving secondary electrons than upon the faster moving primary electrons it will'now be obvious why the angle formed by the intersection of the axes b and 0 must be smaller than the angle formed by the intersection of the axis a with the axis b.

The magnetic field (which, as has previously been pointed out, is perpendicular to the plane containing the axes a, b and c ofthe lens systems which produce the primary-electron image and the secondary-electron image) has a tendency to converge the electrons more in one direction in the-image field than in the-direction ance with the invention, by incorporating the electrical equivalent of a cylindrical-optical-lens among the electron-lens elements mounted in extension B. The tri-part element It constitutes such a cylindrical electron-lens and tends to increase the ccnvergence of the image electrons in the direction in which the converging effect of the magnetic field is minimum.

In Fig. 1 it will be observed that the several parts a, b and c of the lens elements l6 are so connected to the source R by variable leads and that the outermost part lfi is the most positive, electrically, of the three parts. In actual operation the relative voltage distribution may be different; for example, in certain instances better results may be achieved with both rings I6 and I5 at the same potential, and with ring 16 more positive than rings ['6 and It, all as will be readily determined by the operator as he observes the focus of the intensified image upon the fluorescent screen S.

The particular form of image defect caused by the magnetic field may be compensated for, at least. in part, before the primary-electrons are subjected to the magnetic field. That is to say, we may include a tri-part cylindrical electronlens element, similar to that shown at l6, Fig. 1, in the lens system through which the photo or primary-electrons pass immediately after leaving the photosensitive cathode. Such modification of the invention is shown in the plural-stage image intensifier of Fig. 3.

The vacuous envelope T of Fig. 3 is provided with four radially extending neck portions which are, designated A, B, C, and D. Extension A contains a photosensitive cathode here designated 3|, focusing rings 32 and 33, a tri-part cylindrical electon-lens 34, and a terminal accelerating electrode in the form of a short cylinder 35. In each of. extensions B and C, a secondary-electron emissive electrode, designated 4| and 5|, respectively, is provided. The lens elements 4| to 4'! and 57, inclusive, may be similar in all respects to those in extension B, described in connection with Fig. 1. Extension D is allotted to the fluorescent screen or output electrode S.

The path of the electrons through the device of Fig. 3 may be traced from the cathode 3| to the first multiplying electrode M. The secondaryelectrons emitted from 4| by the impact of primary electrons are drawn to the second multiplying-electrode 5!, where the image is further intensified, and the secondary-electrons there released are directed into the extension D where they impinge upon the willemite or other screen S and produce a visible image thereon. It would needlessly complicate the drawing to show the circuit connections for each electrode and lens element of the device of Fig. 3. Suffice it to say that the relative voltage distribution between the several elements in each extension may be relatively the same as that described in connection with Fig. 1.

As in the earlier described embodiment of the invention, the electrons leaving a given lens system (say the lens system in extension C) travel less rapidly than the arriving electrons, and. hence their paths are subject to a greater degree of curvature, by the magnetic field than the faster moving entering electrons. Consequently, and as previously described, the angular separation between any twosuccessive stages (in point of electrontravel) must be less than the angular separation between the next preceding stages. This is visually indicated in Fig. 3, wherein the angle between the axes a and b is shownto be 155; the angle between by axes b and 0, 145, and the angle between by axes c and d, 135. The exact angles will be dependent upon the potentials to be applied to the image-intensifying electrodes.

Certain other modifications of the invention will suggest themselves to those skilled in the art. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense, except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. An image intensifier comprising an evacuated envelope containing a photosensitive image cathode, a secondary electron emissive image intensifying electrode and an image target electrode, said cathode, intensifying, and target electrodes being mounted in spaced relation about and facing a common reference point which is determined by the intersection of lines drawn normal to the respective electrodes from their respective midpoints, the angle formed by the intersection of the line from the cathode to said reference point and the line from said reference point to said intensifying electrode being substantially greater than the angle formed by the intersection of said last mentioned line with the line drawn from said target electrode to said reference point. r

2. An image intensifier comprising an evacuated envelope containing a photosensitive image cathode, a secondary electron emissive image intensifying electrode and an image target electrode, said cathode, intensifying and target electrodes being mounted in spaced relation about and facing a common reference point which is determined by the intersection of lines drawn normal to the respective electrodes from their respective midpoints, the angle formed by the intersection of the line from the cathode to said reference point and the line from said intensifying electrode to said reference point being substantially greater than the angle formed by the intersection of said last mentioned line and the line drawn from said reference point to said target electrode, and a magnet exterior of said envelope for directing a primaryelectron image in a curved path from said cathode to said intensifying electrode and for directing a secondary ated envelope having a central globe-like por-' tion and a plurality of neck portions, extending radially therefrom, a photosensitive image cathode and a plurality of image-forming electronlens elements in one of said neck portions, a secondary-electron emissive surface and a plurality of image-forming electron-lens elements in another of said neck portions, and an image-target electrode in a third neck portion, and magnetic means exterior of said envelope for directing the photo-electron image formed by the first-mentioned electron lens elements in a curved path to said secondary electron emissive surface and for directing the secondary-electron image formed by the second-mentioned electron-lens elements in a curved path to said target electrode.

5. An image intensifier device comprising an image cathode, a secondary electron-emissive image intensifying electrode and an image target electrode circumferentially arranged about a common center, electrostatic means adjacent each of said cathode and intensifying electrodes for respectively projecting a primary electron image from said cathode and a secondary electron image from said intensifying electrode in the direction of said common center, the spacing between a given one of said electrodes and its next succeeding electrode, along the path of electron travel, being less than the spacing between said given electrode and its next preceding electrode along the path of electron travel, and magnetic means for directing said primary electron image away from said center in the direction of said intensifying electrode and for directing said secondary electron image away from said center toward said target electrode.

GEORGE A. MORTON. EDWARD G. RJlMIBEiEtG.

Images