US3378714A - Image converter tubes with improved dust screen and diaphragm means - Google Patents

Description

P" 968 l F. GUYOT ETAL 7 IMAGE CONVERTER TUBES WITH IMPROVED DUST SCREEN AND DIAPHRAGM MEANS Filed Jan. 10, 1967 2 Sheets-Sheet 1 L UCIEN E 60707: BE)? THY/VP M. PEI/1P0, & FRANCIJ S/PO Apl'll 16, 1968 GUYOT AL 3,378,714

IMAGE CONVERTER TUBES WITH IMPROVED DUST SCREEN AND DIAPHRAGM MEANS Filed Jan. 10, 1967 2 Sheets-Sheet :1

United States Patent 0 f 3,378,714 IMAGE CGNVERTER TUBES WITH IME RUVPED DUST SCREEN AND DIAPHRAGM MEANS Lucien F. Guyotand Bertrand M. Driard, Paris, and Francis Siren, Epernon, France, assigncrs to Compagnie Francaise Thomson Houston-Hatchiriss Brandt, Paris, France, a corporation of France Filed Jan. iii, 1967, Ser. No. 688,314 (:iaims priority, application France, Han. 18, 1966, 46,245; Mar. 11. 1.966, 53,055 11 Claims. (Cl. Sid-65) AiifiTRAQT OF THE DILOSURE An image converter such as a brightness amplifier, having a fine-mesh conductive wire screen extending across the electron path beyond the lens and ahead of the viewing screen, for protecting the inner face of the viewing screen against dust particles generated in the major cavity of the tube. The dustshield screen is positioned to cause minimal distortion of the equipotential surfaces of the lens, and is mounted on a supporting diaphragm plate which affords protection against stray electrons.

Sumirmry of the invention This invention relates to image converter tubes, particulariy for use as bri htness amplifiers, as widely used in X-ray work and for other purposes. Such a tube generally comprises a photocathode at the input end of an evacuated glass vessel constituting the tube envelope, and an anode provided with a fluorescent screen at the output end. An electron-optical system including an electrostatic lens is interposed in the path of the photo-electrons from the cathode to the output screen. When suitable radiations, e.g., X-rays, are directed at the pho-tocatlrode to form a so-callcd primary image thereon, electrons are emitted by the photocathode in a pattern corresponding to said primary image. These electrons are accelerated and focussed by the eiectron-optical system towards the anode and form a secondary image on the output screen, the secondary image having a greatly increased brightness as compared to the primary image.

Inthe operation of this type of tube, considerable difficulties have been experienced owing to the unavoidable inclusion of small particles of foreign matter in the sealed evacuated tube envelope, for reasons that will be later described in greater detail. When such floating dust particles settle on certain active surfaces of the assembly, the quality of the image can be gravely impaired.

The invention comprises a dustshield member in the form of an extremely fine mesh screen of conductive wire, positioned beyond the electrostatic lens and ahead of the output image screen of the image converter tube, so as to prevent the passage of a major amount of foreign particles that may be present in the main-part of the tube cavity, towards the output screen, and thereby protect the surface of said output screen from such particles that would otherwise settle on it; the dustshield screen is, at the same time freely traversable by the photo-electrons. According to Patented Apr. 16, 1968 ice the invention, the dustshield screen is so positioned, at a predetermined axial distance from the plane of the electrostatic lens section of the electron-optical system; of the tube, that when the conductive wire screen is electrically connected to the anode of the tube, it will not appreciably distort the equipotential surfaces of the lens and will therefore not appreciably alter the optics of the tube. It has been determined that for this purpose, an optimal position for the dustshield screen is such that its plane is substantially tangential to the equipotential surface corresponding to about 0.95 times the potential. difference present across the lens, as measured in the absence of the dustshield screen of the invention.

According to a feature of the invention, the dustshield screen is mounted across the aperture of a diaphragm plate mounted at the position just indicated, said diaphragm plate serving both as a strong mechanical support for the delicate wire screen, and as a diaphragming means preventing the great majority of stray electrons from entering the useful beam of photoelectrons and forming undesirable spots on the image.

Description The problem of floating particles of matter in the sealed envelope of an image converter tube is one that has seriously limited the effectiveness of high-power brightness amplifier tubes in recent years, and has grown more and more acute as the performance characteristics of the tubes have been heightened. The greater the factor of brightness amplification achieved, the worse will be the local impairment of the image caused by a minute foreign particle. In a copending US. patent application Ser. No. 335,857, filed Ian. 6, 1964, and which is now US. Patent No. 3,304,455, issued Feb. 14-, 1967 and assigned to the same assignees, means have been disclosed for sealing the input and output sections of the tube against the ingress of foreign particles. The means of that earlier application overcame an important cause of image-disturbance by foreign particles, in two critical regions of the image tube, but provided no safeguard whatever against particles settling on the input side of the output image screen. While this was of only minor consequence in many good tube constructions, at the time the earlier application was filed, the recent development of high-resolution brightness amplifier tubes using very fine pencils of photoelectrons has seriously increased the relative effect of particles settling on the input side of the output image screen.

It is an object of this invention to provide an image tube, such as a brightness amplifier, which will be more fully and efliciently protected against the scourge of floating particles than was heretofore possible. Another object is to provide a dust screen for screening the input side of the target screen of an image tube from a major number of contaminating particles, without at the same time affecting to an appreciable degree the normal operation of the tube; a specific object in this connection is so to position the dust screen that it will not substantially distort the equipotential surfaces of the electrostatic lens of the tube, whereby the over-all electron optical system of the tube can remain. unchanged. It will, for example, be possible to attach the novel dust-screen member of this invention to an existent brightness amplifier tube without any other appreciable modification to the tube construction.

A further object is to associate the novel dust shield or screen member with an apertnred diaphragm plate which, at the same time as it serves as a strong and convenient mount for the fine mesh dust screen, also acts to protect the output image against spurious electrons in a particularly critical region of the electron beam.

The invention will now be disclosed in detail in respect to exemplary embodiments thereof illustrated in the accompanying drawings, wherein;

FIG. 1 is a schematic view, in axial cross section, of a brightness amplifier tube illustrating the principle of the invention;

FIG. 2 is a large-scale diagrammatic view of the lens section of the tube illustrating the positioning of dust screen of the invention with respect to the equipotential surfaces of the lens;

FIG. 3 is similar to FIG. 1 but shows the dust shield member combined with a diaphragm according to a preferred embodiment of the invention; and

FIG. 4 is a large-scale view, in cross section, of one practical construction of the dust screen and diaphragm assembly of the invention.

A brightness amplifier tube as schematically shown in FIG. 1 comprises a sealed evacuated glass envelope 5 having a photocathode assembly generally designated 4 at its input end, herein the upper end of the tube, and a fluorescent screen 3 at its lower output end. An electronoptical lens system is interposed between the photocathode 4 and the output screen 3 for accelerating and focussing photo-electrons emitted by the photocathode 4 onto the screen 3. This electron-optical system includes, as shown the pair of coaxial annular electrodes 1 and 2., and preferably an axial accelerating electrode positioned ahead of the electrode 1, which accelerating electrode is of cylindrical shape as shown at 40. The entire system will generally present substantial symmetry of revolution about the vertical center axis of the tube. Operating potentials are applied from a suitable high-voltage source to the various elements of the system by way of connections passed through sealed nipples of the tube envelope 5. The connections include a connection wir 31 soldered to photocathode land passed through sealed nipple 10, wire 32 connected to first electrode 1 and passed through nipple 11, and wire 33 connected to the second electrode or anode 2 and passed through nipple 12. Electrode 40 has a connecting wire not shown. Some of the above enumerated elements will now be described in somewhat greater detail.

The cathode assembly 4 comprises a downwardly concave, dished element of transparent material, e.g. glass, having a photoemissive coating formed on its concave output side. Such a coating may comprise alkali metals such as potassium, caesium, as well as antimony and/ or other elements. One conventional and convenient method for applying this photoemissive layer to cathode member 4 is to use a process of vacuum-evaporation and condensation in situ during evacuation of the tube envelop 5. For this purpose, there is provided one or more so-called evaporator receptacles one of which is schematically shown at 14, suitably mounted in a side of the envelope. The receptacles 14 are small tubes made of sheet metal and containing the metallic and/or other substances of low boiling point which are to be evaporated and recondensed on cathode 4 to provide the photoemissive coating thereon. An electric heater resistance 35 is shown closely associated with receptacle 14 and is connected with a suitable source of voltage, not shown, through a wire 34 led out of the tube envelope by way of a nipple 34. Receptacle 14 initially contains a charge of suitable alkali metal and/ or other reagents, and during the manufacture of the tube heating current is applied over wire 34 to resistor 35 to evaporate the charge which then breaks out in vaporized form through gaps in the walls of receptacle 14 and the vapor recondenses on the surface of photocathode 4. A movable shutter device schematically shown .4 at 15 serves to prevent the vapours from entering the lower section of the tube envelope inside the electronoptics lens assembly and settling over the surfaces of the lenses and specially the secondary screen 3 where the resulting metal coating if allowed to form would impair the operation of the system. For this purpose the movable shutter device 15 may consist of a pair of generally semicircular shutter elements movable between the sealing position schematically shown, in which the elements overlap and provide a sealing partition across the top of annular electrode 1, and an open position in which the shutter elements are separated. A simple spring-latch mechanism, not shown, initially holds the shutter elements in the sealing position illustrated until the evaporation-condensation process described above has been completed. After that the shutter device is displaced to its final open position, as by placing the tube bodily upon a rotating jig and rapidly spinning the tube to move the shutter elements outwards by centrifugal force, the shutter elements then being permanently retained in their open position by suitable spring means or otherwise. A detailed description in the assignees German utility model specification No. 1,888,686, refers to this shutter means.

Annular electrode 1 has a larger-diameter upper section, across which the movable shutter device 15 just described is mounted, and a smaller-diameter lower section terminating in an inturned flange as shown. The second electrode or anode 2 is generally cylindrical and has an inturned flange at its upper end which is spaced from the inturned flange at the lower end of electrode ll. Anode 2 has an o-utturned flange at its lower end near the bottom end wall of the tube envelope 5, and has a cross wall 6 soldered across said outturned flange, the cross wall 6 being apertured at its center to support the secondary screen 3. In the construction here shown, the first and second electrode assemblies 1 and 2 are assembled into a sub-unit by means of a set of spacer posts such as 8 of insulating material, which may be three in number, and have their respective ends joined to a horizontal outer wall portion of electrode it, and to the outturned flange at the lower end of electrode 2, as shown. This sub-unit is held in position within the tube envelope 5 by means including a metallic ring 9 having its flanged lower end joined to a suitable shoulder section of the tube envelope 5, as shown, and having its flanged upper end soldered tothe horizontal wall section of electrode 1 around the spacer posts 8.

The secondary screen 3 may comprise a flat plate of transparent material, e.g. glass, having its upper or input face coated with a suitable fluorescent layer such as a phosphor composition of zinc sulfide and activator additions, as well-known in the art.

In the operation of the tube, the photocathode 4, electrode 1 and anode Z are connected by way of the conductors 31, 32 and 33 to respective potentials V V V such that the first electrode potential V is moderately positive relative to photocathode potential V and the anode potential V is strongly positive relative to first electrode potential V In these conditions the spaced adjacent end sections of the electrodes 1 and 2 define a positive electrostatic lens in respect to electrons issuing from the photocathode 4. When a primary image is formed by means of X-rays or other suitable radiations on the upper surface of photocathode 4, the photoemissive layer on the under surface of the photocathode emits photo-electrons whose rate of emission at each point of said surface area corresponds with the intensity of the incident radiations, i.e. the brightness of the primary image, at that point. The photo-electrons are accelerated and focalized by the electron-optical system, including the lens just referred to as defined between the adjacent ends of electrodes l and 2, and thus converge on to secondary screen 3. The electrons excite the fluorescent surface coating of screen 3 and thereby form on said screen a secondary image accurately corresponding in pattern to the original primary image. This secondary image is of somewhat reduced size as compared to the primary image, but is of greatly increased brightness. Thus, when the secondary image is magnified optically or otherwise to restore it to the initial size or to a size substantially increased over that of the primary image, a net gain in brightness of several thousand times can still be obtained.

The over-all shape of the electron beam within the tube from photocathode 4 to secondary screen 5 can be visualized in a general manner as constituting a cone 16 having an apex or cross-over point 0 substantially in the plane of electrostatic lens defined between electrodes 1 and 2. Hence, the secondary image on screen 3 is inverted relative to the primary image on photocathode 4.

In the use of brightness amplifier tubes of the general type described herein, considerable trouble has been caused by the presence of floating particles of matter scaled within the tube envelope. Such foreign particles may be due to many contaminating causes, including dust motes drifting in the atmosphere of the construction shop and which may become electrostatically charged so that they cling to the interior surfaces of the tube and cannot be removed during evacuation, but become sealed within the tube. Particles of matter are also liable to be detached from the surfaces of the various elements sealed inside the tube during processing of the tube and manipulation both in manufacture and in subsequent utilization.

In copending application Ser. No. 335,857 filed J an. 6, 1964 and assigned to the same assignce as the present application, dust-shield means have been described which prevent such floating particles from settling on the outer or output side of the secondary screen 3, as well as on the outer or input side of the photocathode i. As disclosed in the co-pending application, the dust-shield for preventing the floating particles from settling on photocathode 4 comprise an annular seal herein designated 39, and the dustshield preventing the particles from settling on the outer side of secondary screen 3 include a frustoconical annular sealing member herein generally designated 38, having one end sealed to the under surface of electrode cross wall 6 and its lower end sealed to the inner surface of the end wall of tube envelope 5. Such dust shield means have substantially reduced the adverse effects of foreign particles present within the tube on the final image provided thereby, but has not eliminated such effects since it does nothing to prevent particles from settling over the inner (here upper) surface of the secondary image screen 3. Particles present at this location do not affect the quality of the image too seriously in the case of image tubes in which the electron beam lid is relatively wide, because the wide range of electron incidence angles then ensures that there will generally be enough electrons to strike the phosphor layer of the screen beneath any settled particle. However, in recent high-performance tubes in which the electrode geometry and distribution of voltages are so selected as to create a fine pencil of electrons 16 in order to increase the resolution of the tube, this no longer holds true, and particles settling on the inner surface of output screen 3 assume a preponderant importance. Because of the size reduction of the secondary image even a minute fragment on the screen surface can appear as a spot of appreciable size marring the secondary image.

Because of the obvious necessity of keeping the inner space of the tube clear for the passage of the photo-electrons through it, it has not heretofore been possible to protect the inner surface of the secondary screen from floating particles, otherwise than through the exertion of utmost care to avoid the introduction of such particles in the manufacture of the tube. Since however the total absence of foreign particles introduced from the outside and/or detached from the interior of the tube is impossible to achieve in practice, their ill-effects have had to be accepted as a necessary evil.

It has been found in accordance with this invention that the inner surface of the secondary screen can be effective- 1y shielded against a major amount of foreign particles without substantially interfering with the flow of the photo-electrons through the tube, if an inner dust shield is provided in the form of a fine-mesh wire screen or grid of suitable construction, positioned in a manner that will now be described. As schematically shown in FIG. 1, the dust shield, generally designated 7, extends across the cylindrical wall of the second electrode or anode 2, a substantial distance below the upper end of the anode.

In order for such a screen to be effective as a dust shield, its mesh size should be small enough to arrest a majority of the particles capable of adversely interfering with the secondary image, and this object can be considered as attained when the mesh opening does not exceed about 80 microns.

If the wire screen '7 has a mesh opening substantially larger than this value, it is found that it is liable to be traversed by particles which, on settling on the surface of output screen 3, will produce detectable dark spots in the output image; that is, the wire screen 7 will not then fulfill its dust-shielding function. It is to be noted that a wire mesh of the type here contemplated, and having a mesh opening of 80 microns as just indicated, has a mesh number, or fineness, of about 250 lines per inch. This linesper-inch value therefore constitutes a lower limit for the fineness of the wire mesh suitable for use in the invention.

On the other hand, an upper limit to the fineness of the wire screens usable herein is set by the condition that the wire screen 7 should not lower the efficiency of the tube by arresting too many of the electrons. In other words, the ratio of the clear surface area to the total surface area of the screen should not be too low. This ratio is defined as the optical transparency ratio, and it decreases sharply as the fineness of the mesh, in lines per-inch, increases or, in other words, as the mesh opening decreases. The invention uses wire screens having an optical transparency ratio not less than 5 0%, preferably not less than 60%. It is noted that a wire screen having an optical transparency ratio of or actually has an electron transparency ratio which is much higher, being of the order of to or more, so that the efficiency of the tube is not seriously impaired. In view of the optical transparency condition, a suitable upper limit for the fineness of the wire mesh usable herein is about 750 lines per inch, and correspondingly a suitable lower limit for the mesh opening is about 25 microns.

A preferred range of mesh fineness values used according to the invention is from 400 lines per inch to 500 lines per inch, the corresponding mesh openings being about 50 microns and about 40 microns respectively.

The wire mesh 7 may be of any suitable conductive material, such as gold, silver, copper, or nickel, the latter material being preferred. It is suitably constructed by the process known as electroforming, for example as manufactured by the Buckbee-Mears Company, 245 E. 6th St., St. Paul, Minn, U.S.A. The 500 lines per-inch nickel wire mesh having the Ruling Number 509A in Buckbee-Mears Electroformed Mesh List dated Mar. 25, 1965, has been used with special success.

The wire mesh 7 is mounted across the anode 2 in a manner that will be later described in detail and is electrically connected to said anode. According to an important feature of the invention, the dustshield screen '7 is positioned at a substantial axial distance from the upper end of anode 2 which constitutes the plane of the electrostatic lens, that is, at a position intermediate the plane of said lens, and the plane of the output image screen 3.

In this connection it will be understood that it would be desirable for the purposes of the invention to position the dustshield screen 7 as close as possible to the output screen 3 in order to protect the said screen against the foreign particles that are present in as large as possible a volume of the tube envelope. However, if the dust screen 7 is positioned too close the upper surface of the secondary screen 5, a shading effect is produced which prevents the screen areas underlying the mesh wires from being irradiated with photo-electrons, with adverse effect on the image. If on the other hand the wire mesh screen is positioned too close to the lens section defined between the ends of electrodes it and 2, ie. too close to the crossover point 0, the mesh will interfere with the traiectories of the electrons and will cause image aberration. A preferred position for the wire screen i of the invention, is such that the general plane of said screen is approximately tangent to an equipotential surface corresponding to not less than about 90%, and preferably about 95% of the potential difference between the electrodes 1 and 2, as measured in the absence of the wire screen '7.

This last condition will be better understood from a consideration of FIG. 2. It will be noted that the equipotential surfaces, a few of which are shown, bulge outwardly in both directions in the manner known to be typical for electrostatic lenses of this type. The equipotentials are labelled in percentage values referred to the total potential difiference (V V of the electrodes 1 and 2. The potential gradient (or electric field intensity) is seen to drop rapidly on the output side from electrode 2, as is evidenced by the increased spacing of the equipotentials. If the wire mesh screen '7 of the invention is inserted across electrode 2 a sufficient distance beyond the lens section as shown, its presence will not appreciably distort the shape of the equipotential surfaces, because of the low field intensity in that region, and will, correspondingly, not appreciably disturb the paths of the electrons as determined by the equipotentials. As earlier said, a preferred position for dustshield screen 7 is on a plane that is substantially tangent to the 95% equipotential surface as measured in the absence of the screen. This last-mentioned surface is shOWn as the dotted curve E. When the screen of the invention is thus positioned, as shown in FIG. 2, its effect will be substantially merely to push the centre part of the 95% equipotential E a small distance upward toward the lens, to the full-line position shown. This is because the screen 7 of course represents the 100% equipotential surface, since said screen is connected to the same potential as electrode 2.

It will be understood that the optimal position for the wire screen dustshield member of the invention can be readily determined in the light of the above teachings, for any individual model of tube, as by a conventional rheographic testing procedure serving to determine the positions of the characteristic equipotential surfaces. After the positions of the equipotentials have been determined, by a rheographic or equivalent method, the wire screen 7 is positioned so that its plane is tangential to the center of the equipotential corresponding to at least 90%, and preferably 95%, of the voltage difference between the ultimate (anode) electrode, such as 2, and the preceding electrode, such as 1. If desired, the wire screen 7 may be positioned somewhat further away from the plane of the lens, as far down as the position of the 99% equipotential, for example.

In accordance with an important feature of the invention, the wire mesh dust shield disclosed above is associated with a diaphragm serving to prevent stray electrons from striking the secondary screen 3, and such diaphragm, in a preferred embodiment, is combined with said wire mesh dust shield to serve as an annular supporting flange therefor, as will be presently described with reference to FIGS. 3 and 4.

It should be understood that in image-converter or brightness amplifier tubes of the type to which the invention relates, a source of inefficient operation additional to that so far considered, is constituted by stray atomic particles, primarily electrons, generated within the tube from sources other than the photocathode, which particles are liable to strike the secondary screen 3 and excite bright spots thereon disturbing the secondary image. Stray electrons and ions can be produced in the tube from a number of sources. The residual gas molecules present within the imperfectly evacuated tube envelope become ionized and liberate electrons. Ions and electrons, including those produced in the way just referred to, can excite secondary emission on striking the electrode and other metallic surfaces in the tube, whereupon further electrons are emitted. Also, some parasitic photoemission is inevitably present within the tube. The chief reason for this is that some of the photoemissive metallic vapors from evaporator receptacles such as 14- unavoidably condense to some degree on surfaces other than the surface of photocathode 4; also, some of the photoemissive coating deposited on the cathode may subsequently become detached from it and settle elsewhere in the tube. Such parasitic photoemissive deposits will, during the operation of the tube, become excited by various sources of illumination, including the primary radiations transmitted and/ or diffused by the photocathode, as well as light from the glow discharges created by the electric field in the tube. Thus, stray photo-electrons are generated. A further source of stray electrons is that produced by field emission, especially from the sharp edges of certain electrodes, as well as any asperities that may inadvertently remain on said electrodes after manufacture. Of the various above sources of stray electrons, a particularly abundant source in many cases is constituted by the radial flange section of the first electrode 1, which is adjacent to and spaced from the radial flange of the second electrode 2 and cooperates with it to form an electrostatic lens as described above. This electrode section tends to emit a large number of predominantly slow electrons in all directions, both through the photoelectric and the secondary emissive actions described above, which are considerably promoted due to the high field intensity prevailing in that region.

It is clear that the disturbing action of the stray particles from the above-enumerated and other sources upon the secondary image formed on screen 3, could be very greatly diminished, if the beam of electrons striking the screen were restricted to the useful conical beam defined by the end paths 16 referred to earlier. According to the invention, this is achieved by diaphragming the beam by means of a diaphragm 17, positioned in the cylindrical anode 2 between the crossover point 0 and the secondary screen 3. The optimal position for such a diaphragm is the same as the optimal position for the dust-shield screen, as earlier determined herein. This can be shown as follows. On the one hand, it is apparent that the screening action of the diaphragm against stray electrons travelling in random directions at angles to the axis greater than the cone angle of useful beam 16, would be maximized if the diaphragm were positioned as close as possible to the crossover point 0 and were formed with an aperture of diameter equal to the diameter of the useful beam in the plane of the diaphragm, so as to screen off all but said useful beam. On the other hand, the closer the diaphragm is positioned to the crossover point 0 (of lens 1-2), the greater will be the distorting effect produced by it upon the equipotential surfaces of the lens, and hence also the aberration introduced into the final image. The optimal position for the diaphragm, as dictated by a compromise of these conflicting conditions, will therefore be on a plane tangential to on equipotential surface of the lens such as the equipotential earlier referred to, where the voltage gradient is low enough to avoid substantial distortion of the equipotential surface by the diaphragm, while yet being sufliciently close to the lens section to ensure that the diaphragm will achieve an efiicient screening effect.

The fact that the optimal position for the diaphragm coincides with the optimal position for the dust-shield screen, is an exceptionally fortunate circumstance, since it enables the dust-shield and diaphragm to be constructed as a unit, with the diaphragm serving as a supporting flange for the wire mesh. As earlier indicated, the wire mesh of the dustshield screen of the invention must be extremely fine if it is to be effective. The mesh wire will usually have a diameter of the order of magnitude of microns. The screen would therefore be fragile if it had a large area, and would impart fragility to the tube. However, by combining the wire mesh screen with the diaphragm and positioning the resulting unit at the common optimal position for both components as defined above, it is evident that the free area of the wire screen will simultaneously be minimized, since it will then be not substantially larger than the cross sectional area of the useful electron beam 16, which area is quite small at said optimal position. In this way, the composite screen-anddiaphragm unit will be no weaker than any of the other internal components of the brightness-amplifier tube, and a fully satisfactory assembly is obtained.

FIG. 4 shows in somewhat greater detail a preferred construction of the dustshield and diaphragm assembly. The diaphragm here designated 52 is a stainless steel plate formed with a central aperture 54 and having its periphery abutted against an annular shoulder 56 of the anode 2. Spring means, not shown, may be provided for holding plate 52 in its abutted position. A low annular ridge 58 having a rounded profile is formed on the upper side of plate 52 at a radial spacing from the periphery of aperture 54, and the electroformed wire mesh 7, is extended across the ridge 58, and has its outer margin pressed down against the surface of plate 52 by means of a spot-welded presser ring 60.

In one practical embodiment, the tube has an overall length of about 290 mm. and an inner envelope diameter of 195 mm. The output screen 3 has a diameter of mm. The anode 2 has its end plane (60, FIG. 4) at an axial distance of 30 mm. from the upper surface of output screen 3, and the diameter of its aperture (64, FIG. 4) is mm. The diaphragm plate 52, of 1 mm. gauge stainless steel, carrying the wiremesh dust screen 7 formed and mounted as described, was positioned with its under surface 13 mm. from the surface of output screen 3, and its center aperture 54 was 10 mm. in diameter, which is just larger than the cross section diameter of the useful electron beam 16 at that position. The plane of the wire mesh screen 7 was in this manner positioned about 15 mm. from the upper plane of anode 2. In many cases it is convenient to define the position of the wire screen 7 in terms of the ratio A/D of the distance A from said screen to the top of the anode, to anode aperture diameter D. Values in the range from 0.2 to 1.5 are generally found satisfactory for this ratio according to the invention. A preferred range is from 0.7 to 1.0.

Returning to the dust-shielding function of the wire screen of the invention, it will be observed that owing tt. the presence of the wire screen 7, only those floating fragments and dust particles of appreciable size that may be present in the relatively small space defined between said wire screen and the secondary-image screen 3, are able to settle on the surface of said image screen and mar the image thereon. This represents only a small fraction of the total amount of particles floating about in the envelope, and which would be liable to settle on the image screen 3 in the absence of the dust-shield screen of the invention. Further, inasmuch as a chief source of objectionable foreign particles originating during the use of the tube are residual fragments of the contents of the evaporator receptacles 14, that failed to vaporize during the evaporation step earlier described, and which later become detached during handling of the tube in service, it is seen that this source of trouble is eliminated. The same applies to fragments of photoemissive coating liable to be detached from the photocathode 4. An auxiliary useful function of the dustshield screen 7 is to prevent fragments broken off from the phosphor coating on the upper side of image screen 3, if such phosphor is provided, from landing on the underside of the photocathode 4 where they would seriously impair emission.

It should be understood that, while a typical construction of a particular form of brightness amplifier tube has been shown and described for illustrative purposes, the teachings of this invention are largely independent from the constructional details of such tube, including the geometry, relative dimensions and shape and number of electrodes used in the electron-optics of the tube, and other tube characteristics.

What we claim is:

1. An image tube having a sealed evacuated envelope defining a path for an electron beam, a photocathode at one end of the path, an output image screen at the other end of the path, and electron-optical means for directing photo-electrons along said path from the photocathode to the image screen, said means including an annular anode adjacent said image screen and a further electrode positioned ahead of the anode and defining an electrostatic lens therewith, wherein the improvement comprises:

a dust shield element in the form of a fine mesh, conductive wire screen extending across said anode and electrically connected thereto, and so positioned at a substantial spacing from the general plane of said lens as to leave the equipotential surfaces of the lens generally undisturbed.

2. An image tube according to claim 1, wherein said dust shield element is so positioned that its general plane is substantially tangent to an equipotential surface of said lens corresponding to not less than about and not more than about 99% of the potential difference present across the lens, as determined in the absence of said element.

3. An image tube according to claim 1, wherein said dust shield element is so positioned that its general plane is substantially tangent to an equipotential surface of said lens corresponding to about of the potential difference present across the lens as determined in the absence of said element.

4. An image tube according to claim 1, wherein said element has a mesh opening less than about 80 microns in width, and an optical transparency ratio not less than about 50%.

5. An image tube according to claim 4, wherein said element has a mesh size in the range of about from 250 to 750 lines per inch.

6. An image tube according to claim 1, wherein said element has a mesh opening in the range of about from 40 to 50 microns, and a mesh number of in the range of about from 400 to 500 lines per inch.

7. An image tube according to claim 1, which further includes a diaphragm made of electrically conductive material extending across said anode adjacent to said element and electrically connected to said electrode and element, said diaphragm having an aperture not substantially larger than the cross sectional area of the useful electron beam.

8. An image tube having a sealed evacuated envelope defining a path for an electron beam, a photocathode at one end of the path, an output image screen at the other end of the path, and electron-optical means for directing photo-electrons along said path from the photocathode to the image screen, said means including an annular anode adjacent said image and a further electrode positioned ahead of the anode and defining an electrostatic lens therewith, wherein the improvement comprises:

an annular, electricallyconductive, dustshield-and-diaphragm unit mounted across said anode and electrically connected therewith, said unit comprising:

a transverse apertured plate mounted across the anode and having a central aperture not substantially larger than the cross sectional area of said useful electron beam; and

a finemesh wire screen stretched across said aperture and bonded to said plate;

said conductive unit being so positioned at a substantial axial spacing from the general plane of said lens 1 1 as to leave the equipotential surfaces of the lens generally undisturbed.

9. An image tube according to claim 8, wherein said unit is so positioned that its general plane is substantially tangent to an equipotential surface of said lens corresponding to not less than about 90% and not more than about 99% of the potential difference present across the lens, as determined in the absence of said unit.

10. An image tube according to claim 8, wherein said unit is positioned at an axial spacing from said lens in the range of about from 0.2 to 1.5 times the diameter of the lens aperture.

11. An image tube according to claim 8, wherein said References Cited UNITED STATES PATENTS Rotow 313-65 Niklas 313-65 X Stoudenheimer et al. 313-65 Mesta 313-94 JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner.

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