US3439222A

US3439222A - Electronic zoom image intensifier tube

Info

Publication number: US3439222A
Application number: US564930A
Authority: US
Inventors: Bertrand M Driard; Lucien Francis Guyot
Original assignee: Compagnie Francaise Thomson Houston SA
Current assignee: Compagnie Francaise Thomson Houston SA
Priority date: 1965-07-19
Filing date: 1966-07-13
Publication date: 1969-04-15
Anticipated expiration: 1986-04-15
Also published as: NL6610130A; FR1468746A; DE1539792A1

Description

April 15,1969 I B. M.DRIARD ETAL 3,439,222

ELECTRONIC ZOOM IMAGE INTENSIFIER TUBE Filed July 13, 1966 Sheet or 4 I I l a o ru 2 5 ab inn '9 i l Hg L 7' m 0: E6070 4 fienxmgofifi DPIAIPD,

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April 15, 1969 a. M. DRIARD Er/u. 3,439,222-

ELECTRONIC ZOOM IMAGE INTENSIFIER TUBE Sheet Filed July 13, 1966 0.61 Uz/U1 ZHbId April 15, 1969 DRlARD ETAL 3,439,222

ELECTRONIC ZOOM IMAGE INTENSIFIER TUBE filed July 13, 1966 Sheet 5 OUTPUT IMAGE SHE manam sc L v Ima 1 5 I. u v (xv Fl 5 s s 10 11- 6/ 1 H/ I L I :l ,12 1e II II II H wr 2:, a

l m lgl April 15, 1969 B. M. DRIARD ETA!- 3,439,222

ELECTRONIC ZOOM IMAGE INTENSIFIER TUBE Filed July 13, 1966 .Sheet 4 of 4 United States Patent Int. Cl. riol 29/56 US. Cl. 315-31 9 Claims ABSTRACT OF THE DISCLOSURE To provide for variable magnification (electronic zoom), an image intensifier tube has a flat mesh electrode located spaced from the emissive cathode by a distance less than V and preferably by a distance of between and ,4 of the elfective diameter of the tube, the mesh electrode being provided with a potential which is adjustable, in accordance with desired magnification, over a wide range of value higher than the voltage applied to an annular electrode located opposite the mesh electrode (with respect to the cathode) and likewise closely spaced therefrom, so that an electron lens will be formed at the side of the mesh away from the cathode and bounded towards the cathode by a fiat equipotential transverse to the optical axis of the tube.

This invention relates to an improved electron-optical system of the electrostatic type, and it comprises as its chief feature an improved construction of an electron lens of optically positive, i.e. converging, character, for use in conjunction with an emissive electrode such as a photocathode.

The chief utility of the invention lies in connection with image converter tubes. Such a tube generally includes an emissive cathode on which a primary image is formed by means of visible or invisible radiation, so that the cathode emits electrons in a pattern corresponding to that. of the primary image. The tube further includes an electron-optical system which serves to accelerate the electrons and focus them as a secondary image unto a target electrode or anode. One important application of image converter tubes is the so-called brightness or luminance amplifier. In this case the primary image is a visible image, and the secondary image on the target is a similar visible image of greatly increased brightness.

It should be understood however that the emissive electrode is not necessarily a cathode, and correspondingly the charged particles handled by the system to convert the primary image into the secondary image are not necessarily electrons or negative particles. They may be protons which are positive particles, as is the case in certain important applications such as the proton microscope. While the invention will be described with main reference to its embodiments in systems using electrons as the charged particles operated on, it is fully applicable to systems using other types of particles such as protons, as will be apparent to those skilled in the art. In this connection it should be emphasized that expressions such as n is electron-optical system, electron-lens, and the like, as used in the specification and claims, are to be taken in their broad sense, and that the charged particles handled by the systems and lenses described by such expressions, are not necessarily electrons.

The present invention is based on the discovery that electron-optical systems having extremely interesting and practically advantageous characteristics can be obtained when the initial lens of the system, i.e. the lens positioned adjacent to the emissive electrode such as a cathode, is

3,439,222 Patented Apr. 15, 1969 constructed as the combination of a fiat mesh electrode and a tubular or annular electrode having one end closely spaced from a peripheral region of said mesh electrode and extending axially away therefrom.

Mesh electrodes have been known for many years in the art. Such an electrode comprises a screen or grid of very fine-mesh conductive wire, preferably finer than about 30 mesh per linear millimeter. When such a mesh is carried to a suitable potential and is traversed by charged particles, it defines an equipotential surface therefor. Mesh electrodes have been used for various purposes, such as for providing a positive termination of the electric field at the input and/or output of an electron-optical system; as a gate for controlling the flow of the particles; as a post-accelerating electrode; as a shield against stray and extraneous ions. While the invention in certain of its aspects may utilize a mesh electrode for the conventional purpose of gating the particles as will be later described, the invention in its main aspects puts the mesh electrode to an entirely different use. That is, the mesh electrode serves, in conjunction with the annular or cylindrical electrode associated therewith as herein described, as a converging or positive electron-lens to focus the particles passed through it. It will be shown that such a lens possesses certain unique properties which render it highly desirable in a system of the kind described.

The electron lenses used in conventional image converter tubes of the electrostatic type have usually been in the form of a pair of coaxial tubular electrodes separated by a small axial distance and carried to different potentials. As compared to such a lens, which may be termed the bicylindrical type, the improved lens using a flat mesh electrode and a tubular electrode positioned on one side thereof and separated by a small axial distance therefrom, with the two electrodes being carried to different suitable voltages, has many advantageous characteristics. Its convergence is considerably higher (focal length smaller), for a given voltage difference applied to the pair of electrodes constituting the lens. Its principal planes are more closely spaced, that is, the improved lens constitutes a thin lens as contrasted with the thick lenses heretofore available in electron-optics. Because of its inherent geometry, according to which the lens presents a substantial axial extent on only one side of the voltage discontinuity (as contrasted with the conventional lenses of the bicylindrical and similar types in which the lens necessarily shows notable axial length at both sides of the discontinuity), it can be positioned at an effective axial distance that is very short from the emissive electrode such as a photocathode. Also, since the mesh electrode defines an equipotential surface which is a transverse plane, it can be used in conjunction with, and placed very close to, a flat or planar emissive electrode. This has a number of theoretical and practical advantages that will be later described in detail.

Among these latter advantages is the fact that the intensity of the electric field adjacent the emissive electrode can be made uniform and high. This eliminates spherical aberration and other causes of image distortion.

The high convergence of the improved mesh-lens, noted above, makes it possible greatly to reduce the overall length of the electron-optical system in which such lens is incorporated as the initial lens of the system.

In certain applications of image converter tubes, it is system in order to alter their focal lengths in the correct manner to achieve the desired change in over-all magnification without loss of focus. The resulting arrangements have been complicated and unreliable, and there tended to be a loss of focus of the secondary image for certain magnifications with aging of the instrument. It has been established according to this invention that the properties of the mesh-type electron-lens used herein are such that when the voltage applied to one of its constituent electrodes is changed to change the focal length of the lens, there can be produced a large change in the size of the secondary image with only a minute axial displacement of said image so that its focus is not destroyed. The invention, therefore, provides a very simple and satisfactory way of accomplishing electronic zoom.

According to an important feature of the invention, means are provided for electrically decoupling the initial mesh-type lens of the system from the further electron lens of the system which is defined between the annular or tubular electrode associated with the mesh electrode, and a further electrode. This decoupling may be achieved by making the length of said first annular electrode sufficiently long relative to its diameter.

Objects, then, of this invention include the provision of improved electron-optic systems, and image-converter tubes embodying such systems, which will possess part or all of the following advantageous features: Greatly reduced axial distance between the primary image on the emissive electrode and the initial electron-lens; intense and uniform field adjacent the emissive electrode; use of a fiat emissive electrode assembly; reduced aberrations; reduced length of the optical system; and ability to effect a broad continuous variation in magnification (zoom) without loss of image focus, through action on but a single voltage in the electron-optical system. Other objects will appear.

Exemplary embodiments of the invention will now be described by way of illustration with reference to the accompanying drawings, wherein:

FIG. 1 is a schematic view in axial section of a brightness amplifier image-converter tube constructed in accordance with the invention, including means for varying the magnification therethrough;

FIG. 2 is a separate schematic view of an improved converging lens electrode used in the invention;

FIG. 2A is a chart showing the approximate variations of certain characteristics of the lens of FIG. 2 with applied voltage ratio;

FIG. 3 is a light-optical system diagram equivalent to the electron-optical system used in FIG. 1;

FIG. 4 shows a typical curve of variations of magnification with the voltage applied to the mesh electrode, when the tube of FIG. 1 is used for electronic zoom;

FIG. 5 is a similar to FIG. 1 and shows a modification; and

FIG. 6 is a highly schematic sectional view of another image converter tube according to the invention in which the chief purpose is to reduce the overall length of the tube.

FIG. 1 illustrates a brightness amplifier tube according to the invention comprising a generally cylindrical sealed envelope 5 made of glass or other suitable material in which a suitable degree of vacuum is maintained in a conventional way. Envelope 5 has an input window 13 formed in one end wall and an output window 14 formed in the other end wall, said windows being transparent to the radiation spectrum used, herein the visible spectrum. Mounted within enclosure 5 through conventional means not shown, are a series of element-s or electrodes which are enumerated as follows in the direction of ray propagation from the input to the output window: a photocathode 6, a mesh electrode 7, four accelerating and focalizing

electrodes

8, 9, and 11, and an anode 12 constituting a fluorescent screen on which, in this embodiment, the intensified image is directly observed.

The photocathode 6 is conventional, and may be formed in the usual way by depositing a layer of appropria e photoemissive substance over the inner surface of the tube endwall 13. It will be understood that depending on the desired application this construction may be varied. Thus, if the brightness amplifier tube is to be used for X-ray work, an X-ray sensitive fluorescent layer and an isolating layer may be interposed ahead of the photocathode 6.

The mesh electrode 7 which is an important feature of this invention comprises a peripheral conductive ring or diaphragm 15 and a fine-mesh conductive wire screen 16 mounted across the diaphragm aperture. The construction of such screen or mesh electrodes in the art of electronic optics is known, and the screen 16 may be constructed to any suitable finesness, preferably 200 mesh or 300 mesh per linear centimeter or higher, by any suitable technique e.g. electroforming.

Electrode 8 includes a cylindrical section terminating a short distance from, and generally opposite to, the diaphragm 15 of the grid electrode 7, and providing at its other end, that directed towards the anode, a diaphragm 17. It will be understood that the mesh electrode 7 in conjunction with the cylindrical electrode section 8 constitutes a first electrostatic lens of the electron-optic system of the invention.

The next electrode 9 is a simple annular cylindrical member aligned with the cylindrical part of electrode 8 and constitutes, together with the diaphragm section 17 of said electrode 8, a second electrostatic lens of the system.

This is followed by electrode 10, which has a diaphragm section 18 spaced from the adjacent end of electrode 9 and constituting a third electrostatic lens with it, and a cylindrical annular section 10 aligned with electrode 9.

The next electrode 11 is a simple cylindrical member aligned with the preceding electrodes and having its ends spaced from the end of electrode 10 on one side, and the anode 12 on the other.

The anode or target electrode 12 comprises in this embodiment a suitable photosensitive fluorescent layer deposited on the inner surface of window wall 14.

The various electrodes described above are supplied with suitable voltages from a direct current source 20 by way of a potentiometer assembly designated 19. As here shown this assembly includes a first potentiometer 31 having its resistance connected across source 20 and its adjustable tap connected to the diaphragm section 15 of grid electrode 7; and an array of

potentiometers

32, 33, 34 and 35 having their resistances connected in series across source 20 and their adjustable taps connected to

electrodes

8, 9, 10 and 11 respectively. The negative source terminal is connected to photocathode 6 and the positive source terminal is connected to anode 12. In a practical embodiment, the following voltage values were applied to the various electrodes, the voltage of cathode 6 being taken as the zero or reference potential:

Cathode 6 0.

Mesh electrode 7 Adjustable from 300 to 3000 volts.

Electrode 8 200 volts.

Electrode 9 1000.

Electrode 10 5000.

Electrode 11 10000.

Anode 12 20000.

It will be observed that the voltage applied to mesh electrode 7, while variable, is at all times higher than the voltage applied to tubular electrode 8 associated with it. This voltage relationship is important in that, as a consequence of it, the resulting electron-lens of the invention is optically positive, i.e. converging.

In the operation of the brightness amplifier tube, an image to be intensified is projected from any suitable source on the photocathode 6, causing the latter to emit photoelectrons from each point of its area, at a rate proportional to the amount of radiation striking each point. The photocathode 6 thus produces a primary electron image of the original scene or object, and this primary electron image will now constitute an object which is projected through the electron-optics of the tube upon the anode target Screen 12 to create thereat a secondary image having a brightness that is greatly intensified over that of the original image formed on photocathode 6.

More specifically, the initial electron lens of the system, formed by the mesh electrode 7 cooperating with the adjacent annular electrode 8, forms an enlarged virtual image of the photocathode image, which is positioned ahead (to the left in FIG. 1) of the photocathode 6. This initial lens acts in effect as a magnifying glass. The utilization according to the invention of a mesh-type electrostatic lens as the initial lens of the electron-optics system in a brightness amplifier tube of the type described, has certain unique characteristics which will be described later.

The further electrostatic lenses defined by electrode sections 17-9, 9-18, and 10-11, act to deflect and accelerate the electrons passed by the initial mesh-lens 7, so as to focus the virtual image formed by the latter towards and upon the anode screen 12 in the form of a real image of considerable brightness. This general operation is conventional, it being sufficient to recall that the radial components of the voltage gradients created by the electrodes act to deflect the flight paths of the electrons transversely while the axial components of said gradients act to increase the energy of the electrons and hence accelerate them along their flight paths. Determination of the correct voltages to be applied to the electrodes in order to create the proper axial and radial gradients and achieve the desired accelerating and focussing effects can be effected by usual methods well known to those skilled in electron-optics. The set of exemplary voltage values given above was successfully used in a practical embodiment of the invention.

The use of a mesh electrode 7 as the initial electron lens in the system of the invention achieves a number of unique advantages. One fundamental property of a mesh electrode as contrasted with a conventional electrostatic electrode, for example of the bicylindrical type, is that the mesh materializes, so to speak, an equipotential surface which constitutes a transverse plane normal to the optical axis, and the resulting lens extends axially in one directoin only (rightward in FIG. 1) from such plane, but does not project axially to any substantial extent in the opposite (leftward) direction from said plane.

Because the grid represents a flat equipotential surface of the system, in the form of a transverse plane normal to the optical axis, it becomes possible to provide a photocathode 6 which itself is flat or substantially fiat, instead of having to make the photocathode convex as was generally necessary in conventional systems. One important advantage of a flat photocathode is that it becomes possible to provide a direct optical coupling of the photocathode with an optical objective lens system of wide-angle characteristics, a highly useful result in many applications.

Simultaneously, because of the property that the initial lens using a mesh electrode has no or very little axial extent as measured from the plane of the mesh towards the photocathode, the resulting lens can be (and is) positioned extremely close to the photocathode. In accordance with the invention, the mesh electrode 7 is positioned at an axial spacing from photocathode 6 which is less than about of the useful diameter of the grid, that is the diameter, indicated as d in FIG. 1, of the facing areas of the cathode and mesh. It will be evident that the resulting assembly including the flat photocathode 6 and closely adjacent mesh electrode 7 at a uniform and very short axial spacing from the photocathode, provides a much stronger uniform electric field adjacent the photocathode than could otherwise be obtained, using equal voltage values. This makes it possible to reduce the voltages for a specified overall resolution of the electron-optical system, or alternatively increase the voltage and hence the resolution, without introducing additional aberration and other difiiculties.

The high, uniform, electric field present in the space between the photocathode and the initial lens, made possible when the initial lens embodies a mesh electrode in accordance with the invention, almost completely eliminates spherical aberration and makes it possible to in crease the convergence (and thereby reduce the length of the tube) without increasing the ratio of the peripheral field to the central field, as is necessarily the case when a conventional form of electron lens is used as the initial lens. Field curvature and distortion is thus greatly diminished. Aberration is still further reduced because the high field intensity substantially reduces the angular spread or dispersion of the photoelectrons emitted from the cathode.

The unique properties of a grid-type electrode when used as the initial lens, in a system of the kind described, bring with them an unexpected and very advantageous feature in respect to so-called zoom operation.

In conventional electronic image converter'tubes, zoom, i.e., the ability to produce a wide change in magnification of the final image without loss of focus, can only be achieved by simultaneously varying in a correlated manner the focal lengths of all the electron lenses in the optical system, or in other words simultaneously varying by appropriate amounts the voltages applied to all the electrodes. It has not therefore been possible to vary but a single voltage in order to vary the overall magnification, without defocussing the final image on the fixed output screen of the tube. Consequently, zoom in conventional image converter tubes has required the provision of complicated devices, such as switches, for producing the necessary correlated voltage variations across resistive voltage dividers or the like. In such arrangements, unequal aging of the resistors has led to defocussing of the final image with time. In cases where continuous variations in magnification (true zoom) was desired rather than the discontinuous changes obtainable by switching, further complications in the form of quadratic spiral electrodes, square-law and other contour potentiometers, cam mechanisms, etc., have been necessary. Moreover, the obtaining of rapid variations in magnification has required the use of specially designed waveform generators for acting on the electrode voltages. I

It has been found that in an image converter tube using a mesh electrode as the initial electron lens therein according to this invention, successful zoom action can be achieved by the simple expedient of varying but a single voltage, the voltage applied to the mesh electrode (or to its associated tubular electrode 8). It is found that provided certain constructional precautions are taken, to be presently specified, varying the single voltage applied to the mesh electrode 7 does not result in defocussing the final image formed on the anode screen 12 of the tube, as would necessarily be the case were it attempted to vary but a single electrode voltage in a conventional tube of comparable kind.

This unexpected and seemingly paradoxical feature can be explained as follows. An electron lens formed from a mesh electrode such as 7 and an annular electrode 8 adacent thereto, in accordance with the invention, is shown separately in FIG. 2, with its components being designated by the same numerals as in FIG. I. The principal planes of the resulting electron-lens are indicated at H, and H and the corresponding foci are shown at F and P F being the object focus and P the image focus. It will be noted that when the voltage (U applied to mesh electrode 7 is higher than the voltage (U applied to annular electrode 8, as is the case in the tube of the invention, the positions of the principal planes are crossed with respect to the positions of the respectively related foci, as shown. In the figure, 2;;,, and Z designate the axial distances of the principal planes H and H from the origin defined by the plane of the mesh electrode, while f and f designate the focal lengths as measured from the respective principal planes. The diameter D is the useful diameter referred to earlier herein.

Such a lens system is characterized by the fact that the principal planes H and H are positioned close together and relatively close to the equipotential plane defined by the mesh 16, so that the lens is, in effect, thin in com trast with the thick lenses provided by the conventional annular electrode devices. Furthermore, it is shown that when the voltage ratio U U is varied, as by varying the voltage U of annular electrode 8, then the positions of the principal planes as defined by their abscissae Z Z are varied proportionately very much less than are the focal distances f f This is immediately apparent from the following tabulation in which the four values Z 2 f,,, f as measured for two different values of the U U 1 ratio are given:

This becomes even more apparent from a consideration of the chart of FIG. 2A, where it is to be understood that the straight-line curves are mere interpolations and do not actually represent the true variations of the four quantities between the points of measurement. The chart does, however, clearly show that the positions of the principal planes vary but little for a large variation of the focal lengths.

In order to demonstrate that by varying the focal length of the mesh-electrode lens 7-8 of the tube of the invention, it is possible to produce a large variation in image magnification without destroying the focus of the final image on anode screen 12, that is, to demonstrate that the improved converter tube of the invention is capable of providing electronic zoom through a variation of but a single voltage parameter therein, it is convenient to reason in terms of light-optics. Referring to FIG. 3, a light-optical equivalent of the electron-optical system of FIG. 1 is shown as consisting of two lenses L (focal length 3) and L (focal length g). Lens L represents the initial mesh-electrode lens 7-8 of the FIG. 1 system while lens L represents the optical assembly constituted by the remaining electron lenses of FIG. 1. The separation of lenses L and L is indicated as D. AB represents the object, i.e. the cathode image on photocathode 6, shown at a distance p from lens L A B is the virtual image of AB through L and is positioned at a distance p; from L The final image A B is the real image of A B through lens L and is at a distance q from L It is desired that A B he formed on the anode screen 12, which is fixed in position.

With the notation shown, the equations for lenses L and L; can respectively be written as follows:

The magnification M, which is the ratio of the size of the final image A B to the size of the object AB, can be written as AZBZAIBI q) P1 1. M 1 ig l Combining (3) with

Equations

1 and 2, we get f+ z ff 8 Maximum magnification is obtained when the focal length 3 of the initial lens L is so adjusted that the object-focus P of the lens is positioned in the object plane, i.e., f-=p. Equation 4 shows that this maximum magnification From this last equation it is evident that in order to increase the maximum magnification achievable with the system, it is necessary to reduce as far as possible the distance p between the object (photocathode 6) and the first lens of the system. Since the mesh-lens of the invention can be placed considerably closer to the photocathode 6 than can any other type of electronic lens for the reasons earlier indicated, it is apparent that the improved image-converter tube will achieve greater magnifications than a conventional one. This interim result of the analysis points out an additional, and important, advantage of the invention.

Returning to the main gist of the present analysis, it will be understood that when we alter the focal length I of lens L that is, when we vary the voltage applied to the annular electrode 8 in FIG. 1, the variation of the magnification M for a given change in f is given by the derivative a'M/df. Differentiating Expression 4 and putting for convenience S=the denominator of the second member in Equation 4, We get @g tDil df S (6) and similarly, the displacement of the final image A B along the optical axis is given by which represents the variation in position of the final image for a given variation in magnification.

It is manifest from Equation 8 that in order to cause as little axial shift as possible in the position of the final image with change in magnification, that is in order that dq/dM should be as small as possible, p should be as small as possible. Since the use of the mesh lens as the initial lens L according to the invention makes it possible to position the lens at an extremely small effective dis tance from the photocathode 6 as earlier explained, it is seen that it becomes possible herewith to reduce the axial displacement of the final image to an extremely small value, smaller in fact that the field depth of the electron-optic system represented by lens L so that there will be no apparent defocussing of the final image as observed on the anode screen 12. This means that the desired electronic zoom is achievable with the system of the invention by varying only a single voltage in the system.

While the above analysis is only approximately true in that it was performed for simplicity in terms of the light-optics equivalent of the electron-optics system, its general conclusions are connected and are fully corroborated by experiment. It is found that the desired result is obtained in practice when the axial spacing between cathode 6 and mesh electrode 7 is not greater than about A (preferably not greater than about the useful diameter of the cathode. Even smaller spacings such as or said useful diameter, are preferred. The loss of focus of the secondary image on the target screen 12 is then found to be negligible. In order that this shall be the case however, an additional condition, peculiar to electron-optics analysis made above, should be met. This is that the electron lenses equivalent to the lenses L and L in FIG. 3, are electrically decoupled from each other. Specifically, it is necessary that the electric field created at the surface of mesh electrode 7 by a voltage difference applied between the annular electrodes 8 and 9 (which to igether constitute the second lens of the system), shall be comparatively small or negligible when compared to the field created at said mesh electrode by the same voltage difference when applied between the

electrode

7 and 8 which together constitute the initial electron-lens of the system. By the phrase comparatively small or negligible," it is here meant that the ratio of the first to the second field must be smaller than about 1 and preferably smaller than about or V where particularly good resolution is desired. This condition is fulfilled if the axial length (indicated as 1 in FIG. 1) of the annular electrode 8 as measured from the plane of the mesh electrode, is made long enough with respect to the diameter (indicated as d thereof. The diameter d if non-uniform, should be measured at the extremity of annular electrode 8 closest to the mesh. In practice, it is found that the ratio l/d of the axial length to the diameter of the annular electrode 8 should be larger than about 0.7 if the desired decoupling effect is to be achieved in a satisfactory manner for the purposes of the invention.

It may be noted in this connection that in the lightoptics analysis given above, the equivalent of increasing the l/d ratio is to increase the inter-lens spacing distance D. Equation 8 shows that an increase in D further reduces the axial shift of the final image for a given change in magnification, so that the condition just specified provides the additional benefit of improving electronic zoom action where this is desired. In the embodiment of FIG. 1, the l/d ratio is approximately unity. For high resolution, higher l/d ratios may be used, such as 2.5 or more.

As to the axial spacing between the diaphragm 15 of electrode 7 and the adjacent extremity of annular electrode 8, this spacing (indicated as 2 should be suitably small, e.g. smaller than /5, and preferably smaller than 1 the diameter d As already stated, the axial spacing between the mesh electrode and the photocathode (indicated as e) should also be small, and as will be understood from earlier explanations it is one of the outstanding advantages of the invention that the mesh electrode concept allows the axial distance between the photocathode and the initial lens of the system to be greatly reduced with respect to what was heretofore possible. While in some cases the ratio e/d may be taken as large as $5 with sufiiciently good results, said ratio should usually be about The graph of FIG. 4 illustrates the variations in size of the output image formed on anode screen 12 when the voltage applied to mesh electrode 7 is varied from about 0.3 to nearly 7 kilovolts and it will be seen that the size of the image is enlarged about three times over this range. Throughout the range, the image remains in excellent focus on the anode screen. When the range of variation of the grid voltage is restricted to the range from 0.3 to 3 kilovolts as in the practical embodiment described earlier with reference to FIG. 1, the magnification is seen to double over the extent of this relatively narrow range of mesh voltage adjustment. Since these adjustments are carried by acting on a single potentiometer (31, FIG. 1), it is evident that continuous zoom effect can be very easily and precisely accomplished, without the complication of having to correlate diiferent voltage distributions by intricate mechanical or electrical expedients, and without having to compensate for unequal changes in resistance characteristics with time.

It should be noted that because of the reverse voltage difference applied between the

electrodes

7 and 8 of the initial lens of the system, with the voltage on electrode 7 being higher than the voltage on electrode 8, any objectionable secondary emission from the mesh wires of electrode 7 is eliminated.

The relatively high voltage that is preferably applied to the last focussing electrode 11 is advantageous because it further increases the field depth of the system and provides increased tolerance for the position of the final image. This can be seen by writing the Lagrange-Helmholz equations yaU=a constant where y is the size of the image formed through any one of the lenses in the system, a the angle of convergence of the electron paths from the image to the center of the lens, and U is the voltage applied across the lens. The relation shows that if the voltage U on the last focussing lens is large, the corresponding convergence angle on is correspondingly small. This means that the electron trajectories intersect the optical axis at a small angle, giving good field depth.

The modified embodiment of the invention shown in FIG. 5 is largely similar to that of FIG. 1 and will not be described in full. It differs from the first embodiment in that an additional mesh elect-rode 21 is interposed between photocathode 6 and the mesh electrode 16 first referred to. Auxiliary mesh electrode 21 is connected by way of 0. capacitor 23 to the output of a conventional pulse generator 22 which generates negative pulses. The mesh 21 is also connected through a resistor 24 to an adjustable tap of potentiometer resistance 31, so as normally to apply to mesh 21 a potential intermediate between that of primary mesh 7 and that of cathode 6. Thus, in the absence of a pulse from pulse generator 22 the auxiliary mesh electrode 21 is inoperant, while the application of negative pulses from the generator will cut off the beam of photoelectrons from cathode 6 and block the operation of the image tube.

The modified image converter tube of FIG. 5 is advantageous in those cases where it is desired to control the electron beam through the tube, e.g. in television work and other applications, since it enables this function to be accomplished without having to alter the voltages present on the accelerating and target electrodes, as was usually necessary for a similar control function in conventional tubes.

FIG. 6 illustrates a further modification of improved image-converter tube embodying a mesh-electrode in its initial electrostatic lens unit. In this modification the properties of the mesh electrode are put to use in order to construct a satisfactory image converter tube of substantially shorter length than was heretofore possible. As earlier indicated, the lens unit of the type disclosed herein has considerably higher convergence, that is, a shorter focal length, than conventional types of electron lenses, for a given voltage drop. This, coupled with the flat, thin-lens characteristics of the improved lens, makes possible the reduction in length just mentioned. Further, the reduction in length does not lead to any objectionable increase in image distortions or aberrations, because inter alia of the intense uniform field created by the mesh electrode adjacent the photocathode.

The image converter tube very schematically shown in FIG. 6 includes generally the same components designated by the same reference numerals as the tube of FIG. 1, including envelope 5, fiat photocathode 6, flat mesh electrode 16, spaced a short distance from the cathode, e.g. a distance less than d/20, where d is the useful diameter of the photocathode. Further, a cylindrical annular electrode 8 is positioned with its extremity adjacent to the mesh electrode and spaced therefrom an axial distance which is preferably less than about A the diameter d of the annular electrode and defines an initial electron lens with the mesh electrode 16. This annular electrode 8 has its end remote from the photocathode terminating in a transverse wall 17 defining a diaphragm aperture 43 of diameter d A further annular electrode 9, here shown as of smaller diameter than electrode 8, is spaced from the diaphragm wall 17 to define therewith the second electron lens of the system. Annular electrode 9 is provided at its end near the anode with a transverse diaphragm wall 41 defining the aperture 44. A final accelerating electrode 42 includes a diaphragm 45 spaced from wall 41 and followed by an annular wall portion which terminates around the target electrode or anode 12.

In operation, mesh electrode 16 is carried to a positive potential with respect to photocathode 6, and annular electrode 8 is carried to a positive potential somewhat lower than that of the mesh electrode. The

subsequent electrodes

9, 42 and 12 are carried to successively higher positive potentials.

By selecting the voltage applied to annular electrode 8 at a value relatively close to the voltage value applied to mesh electrode 16, the resulting electron lens can be made to have a very short focal length; that is, the paths of the electrons will converge on the optical axis at a point quite close to the photocathode. This allows the overall length of the optical system and tube to be considerably reduced, without increased image distortion.

A satisfactory set of relative dimensions for a short image-converter tube of the kind shown in FIG. 6 is found to be the following. The diameter d of annular electrode 8 is in the range from about l.ld to about 1.2d (d being the input or useful diameter of the photocathode). The length l of annular electrode 8 may be from about 0.401 to about 0.6d, although if desired said length I may be selected greater than 0.6d, in order to improve the de-coupling between the first and second lenses as described with reference to FIG. 1. The diameter d of diaphragm 43 is in the range from about 02d to about 03d. The purpose of diaphragm 43 is, essentially, to reduce the field created by the anode at the mesh electrode 16.

It will be understood that the actual dimensions should be adjusted in relation to the characteristics of the electrodes, the applied voltages and the general specifications of the tube considered. Such determination is Well-known in the art, and may involve the usual computations and analog tests, e.g. by means of an electrolytic tank set-up.

In one satisfactory practical embodiment of the short tube shown in FIG. 6, the absolute dimensions referred to above have the following values: d=16 cm.; d =l8 cm.; [=9 cm.; and d =4 cm. The overall diameter 4: of the tube was less than 19.6 cm. and the overall length L=23 cm. Conventional image converter tubes of comparable characteristics and performance are usually constructed with an overall length of from 30 to 35 cm. or more. The reduction in length of about 25% achieved by the invention is a direct consequence of using the improved electron lens including a mesh electrode, as the initial lens in the electron-optical system.

The invention is, of course susceptible of a great number of modifications in addition to those expressly shown herein. In cases where the particles emitted by the emissive electrode are positively rather than negatively charged, for instance are protons as in a proton microscope, the polarities of the voltages applied to the electrodes would be reversed, although their relationships would, essentially, remain unchanged.

The expression mesh electrode, as used in the specification and claims, should be understood broadly as referring to any electrode having a conductive surface permeable to the charged particles and defining an equipotential surface. Thus, instead of a fine wire mesh as disclosed above, the mesh electrode may be provided in the form of a continuous conductive film thin enough to exhibit substantial permeability to the electrons or other particles involved. Such thin film electrodes have been prepared in the form of membranes or films of beryllium, or other suitable metals, having a thickness of the order 12 of a few millimicrons. The use of such film electrodes should be understood as lying in the scope of the present invention as a substitute. for the wire mesh electrodes specifically referred to earlier herein.

What we claim is:

1. A variable focal length optical system providing selected image magnification by adjustment of a single voltage level comprising:

a substantially flat emissive electrode positioned transverse to the optical axis of the system and adapted to emit charged particles of a determined polarity;

a substantially flat mesh electrode positioned parallel to and spaced from the emissive electrode by a distance less than one-twentieth of the effective distance of the emissive electrode;

an annular electrode having an end closely spaced from a peripheral region of the mesh electrode and extending axially therefrom away from the emissive electrode; and

voltage means connected for ap lying to said mesh electrode and said annular electrode respectively a first and second voltage each having a polarity relative to the emissive electrode which is opposite to the polarity of said particles;

said first voltage being adjustable in accordance with selected image magnification over a wide range of values all higher than the value of said second voltage to vary the focal length of said lens;

whereby said mesh and annular electrodes will together constitute a thin converging variable focal length lens for focalizing said particles.

2. The system defined in claim 1, wherein the axial spacing from the emissive electrode to the mesh electrode is in the range of from to of the effective diameter of the emissive electrode.

3. The system defined in claim 1, wherein the axial spacing from the mesh electrode to said end of the annular electrode is not more than about one fifth the axial length of the annular electrode.

4. The system defined inclaim 1, including an additional mesh electrode positioned between the emissive electrode and the first mentioned mesh electrode, and means for controlling the voltage difference between the mesh electrode to control the fiow of particles through said lens.

5. Variable focal length image converter tube providing selected image magnification under control of a single voltage level comprising:

a generally fiat emissive cathode transverse to the tube axis;

a generally flat mesh electrode parallel to and spaced from the cathode by a distance less than one twentieth of the effective diameter of the emissive electrode;

a first annular electrode having one end closely spaced from the mesh electrode and extending axially away therefrom and defining a first electron lens therewith;

at least one further electrode having one end closely spaced from the other end of said first annular electrode and extending axially away therefrom;

said further annular electrodes cooperating to constitute at least one further electron lens and being spaced from said mesh electrode by a distance sufficient to be essentially decoupled from the first electron lens defined by said mesh and said first annular electrode;

a target electrode positioned to receive the electrons,

and

voltage means connected to apply successively increasing positive voltages to said first and second annular electrodes and said target relative to said cathode, for accelerating the electrons from the cathode to the target; including adjustable means applying to said mesh electrode a positive voltage adjustable over a wide range of positive values all higher than the voltage 13 applied to said first annular electrode to vary the size of said projected image on the target, said first electron lens acting to converge said electrons and produce a virtual image of the cathode for projection by said at least one further electron lens as a real image on the target.

6. The image-converter tube defined in claim 5, wherein the electric field produced by said second annular electrode adjacent said mesh electrode is not greater than about one tenth the electric field produced by said first annular electrode adjacent said electrode.

7. The image-converter tube defined in claim 5, wherein the axial length of the first annular electrode is not less than about 0.7 times the diameter thereof.

8. The image-converter tube defined in claim 5, including further electrode means positioned beyond the second annular electrode and defining a third electron lens with the end of said second annular electrode directed toward the target. A

9. The image-converter tube defined in claim 5, including an additional mesh electrode positioned between the cathode and the first-mentioned mesh electrode.

References Cited RICHARD A. FARLEY, Primary Examiner.

M. F. HUBLER, Assistant Examiner.

US. Cl. X.R. 3 l3-81