US3502928A - Image converter tube with a target screen assembly carrying cathode-forming evaporators and a fluorescent target screen spring-biased against tube window - Google Patents

Description

March 1970 F. GUYOT ETAL 3,502,928

IMAGE CONVERTER TUBE WITH A TARGET SCREEN ASSEMBLY CARRYING CATHODE-FORMING EVAPORATORS AND A FLUORESCENT TARGET SCREEN SPRING-BIASED AGAINST TUBE ,WINDOW Filed. Feb. 23, 1967 3 Sheets-Sheet 1 LQ/c/sN A 660 072 8527mm /4. DP #1 PD,

l/vmsw 7- 38 March 24, 1970 L. F. GUYOT ETAL 3,502,928 IMAGE CONVERTER TUBE WITH A TARGET SCREEN ASSEMBLY CARRYING CATHODE-FORMING EVAPORATORS AND A FLUORESCENT TARGET BIASED AGAINST TUBE WINDOW SCREEN srnme 3 Sheets-Sheet 2 Filed Feb. 23. 1967 AL 3,502,928 BEEN ASSEMBLY CARRYING CENT TARGET NDOW March 24, 1970 1 GUYOT ET v IMAGE CONVERTER TUBE WITH A TARGET SC CATHODE-FORMING EVAPORATORS AND A FLUORES SCREEN SPRING-BIASED AGAINST TUBE WI Filed Feb. 25, 1967 3 Sheets-Sheet 5 United States Patent 3 Int. Cl. H01," 29/08, 31/50 U.S. Cl. 313-82 13 Claims ABSTRACT OF THE DISCLOSURE Image converter tube having its target screen slidably connected with the annular anode and spring-biased outwardly so as to be pressed against the output end wall of the envelope when mounted in operative position. The screen thus forms a unitary sub-assembly with the anode and preferably also with another annular electrode rigidly and insulatedly connected with the anode. Photocathode-forming evaporator devices are mounted as a flat annular array in the space between said other electrode and the envelope wall.

Cross references This application is based under International Convention on French patent application No. 53,054 filed Mar. 11, 1966. The following common-assigned U.S. patent and application are referred to in the specifications: U.S. Patent 3,304,455 issued on Feb. 14, 1967 and U.S. Patent No. 3,378,714 issued on Apr. 16, 1968.

The invention relates to image converter tubes, such as brightness amplifiers, and more particularly to the arrangement of the output electrode or anode and the target screen in such tubes.

An image converter tube usually comprises a photocathode at one end of a sealed evacuated vessel or envelope, and a target screen, ordinarily phosphor-coated on one of its surfaces, at the other end. Arranged in the envelope between the photocathode and target screen is an electron-optical system consisting of a number of coaxial annular electrodes connected to suitable potentials for accelerating electrons emitted from the photocathode and focussing them on the target screen. When an input image is projected from an external source on the photocathode, the latter emits electrons from every point of its surface at a rate proportional to the brightness of the input image at the corresponding point. The emitted photoelectrons are accelerated and focussed by the electron-optical system so as to form an output image on the target screen, the output image being similar to the input image but of greatly enhanced brightness. Usually the output image is somewhat smaller than the input image but since its brightness is increased many thousandfold there is a great net gain in definition of detail. The output image on the target screen may be viewed directly, or it may be further picked up as for television transmission, cinematographic projection or other purposes. Such brightness amplifier tubes have many uses, one exemplary application being in the field of medical X-ray work.

Whenever the output image on the target screen is to be picked up by an optical system as in the applications just referred to, as well as in the case of direct viewing through a monocular magnifying lens or the like, it is desirable to use optics having wide-aperture character- "Ice istics in order to derive full benefit from the high-resolution of the output image of the brightness amplifier. This of course involves that the initial element of the pick-up optical system has a very short focal length, sometimes as short as half a centimeter or less. This in turn requires the said initial optical element, such as a lens, to be placed in direct proximity with the target screen of the brightness amplifier tube for proper focussing. Difiiculties have been encountered in this respect in the past.

The difficulty arises out of the fact that it is not convenient to form the target screen directly on the output endwall of the glass envelope of the tube, as by depositinga phosphor coating directly on the inner surface of the tube endwall. Such deposition is generally effected by a sedimentation process or by cataphoresis. In either case the deposition process can be carried out much more conveniently, efficiently and economically if carried out on a separate strip of glass rather than on the wall of the large, heavy and fragile tube vessel. Nor is it practical toform the endwall of the vessel as a separate element and insert it in place after deposition of the phosphor coating thereon, because of the high risk of damage both to; the screen and the expensive tube vessel during the inserting step. For these reasons, in many otherwise good image converter tube constructions, the target screen is generally supported within the tube vessel in a metal frame secured to the output electrode, or anode, of the electron-optical system of the tube. Usually, the target screen with its mounting means is provided as a unitary sub-assembly with the said anode. The disadvantage of this construction, disclosed eg in U.S. Patent 3,304,455 issued on Feb. 14, 1967, is that because of manufacturing tolerances in the construction of the glass vessel and other causes, the target screen is necessarily spaced a substantial distance from the tube endwall, thereby making it impossible to use wide-aperture optics to pick up the image on the target screen, as indicated above.

Another difliculty that has been encountered in the construction of image converter tubes, and particularly the output sub-assembly of such tubes, relates to the evaporator receptacles that are placed within the tube vessel and contain charges of evaporable substances, including alkali metals and the like, which are vaporized after the vessel has been sealed in order to form the photoemissive layer on the tube cathodeby a vaporization-deposition process in vacuo. These evaporator devices must be connected to electrical conductors which provides the heating current necessary for evaporation. The connections, additional to those required for the electrodes of the electronoptical system in the tube, have seriously complicated the task of inserting and mounting the sub-assembly in the tube vessel.

Objects of this invention include the provision of improved image converter tubes and output sub-assemblies for image converter tubes, in which the target screen, formed as a separate element, will be conveniently and efiiciently supported at all times in direct proximity to the endwall of tube vessel, and to achieve this result regardless of tolerances in the manufacture of the tube. A further object is to provide such tubes and sub-assemblies having an improved construction of the evaporator devices therein, which will greatly simplify the assembling and electrical connecting operations.

In an image tube constructed according to the invention, the target screen is mounted for limited axial displacement relative to the anode and a spring acts between the anode and target screen to urge the latter into its abutted operative position closely proximate to the output endwall of the tube vessel.

FIG. 1 is a highly schematic view in longitudinal section showing one embodiment of an image converter tube according to the invention, with parts not directly relevant to the invention omitted;

FIG. 2 is a similar view, enlarged and more detailed, illustrating the output part of an improved tube in a slightly modified embodiment; and

FIG. 3 is a cross sectional view taken generally on the line A-A of FIG. 2.

The brightness amplifier tube shown in FIG 1 comprises a sealed evacuated glass envelope 5 which is cylindrical over its major length and has a reduced-diameter end section 501 at its output end, the lower end as here shown. A photocathode 4 (or input screen) is mounted in the input end of the tube, close to the input end face 6 of envelope 5. Photocathode 4 is in the form of a dished plate of transparent material having its concave face directed toward the output end, and peripherally supported through any suitable means not shown from the surrounding wall of envelope 5. The photocathode supporting means desirably include flanged annular means engaging the peripheral rim of the photocathode member 4 and arranged to seal off the space above the photocathode from the remainder of the space within envelope 5, as disclosed in the assignees US. Patent 3,304,455 issued on Feb. 14, 1967, The concave output side of photocathode member 4 is coated with a layer 401 of photoemissive composition, which may comprise antimony, plus a mixture of alkali and alkali-earth metals (sodium, potassium, caesium) deposited thereover. This photoemissive coating 401 is deposited on the photocathode by a process of vacuum-deposition within the tube itself, which process will be briefly described later.

At the output end of the tube is positioned a fluorescent target screen member in the form of a glass strip 3 mounted so as to have of its faces positioned closely adjacent to the output end face 7 of the reduced envelope section 501, in a manner later described in detail. The opposite, or input, face of the target screen 3 is coated with a fluorescent layer or phosphor 8, such as a composition comprising zinc sulfide plus activator, which in turn may desirably be over-coated with a reflective layer such as aluminium. The screen 3 is of course coated prior to its insertion into the tube envelope, as by a conventional sedimentation process.

An electron-optical system of electrodes is positioned Within the tube envelope between the photocathode 4 and target screen 3, and serves to accelerate and focus a beam of photoelectrons as emitted from the photoemissive layer 401 of photocathode 4, upon the screen 8. As here shown, the electron-optical system includes the electrodes generally designated 1 and 2, positioned towards the output end of the tube. Usually, one or more additional electrodes, including a cylindrical pre-accelerating electrode extending axially from the photocathode 4, would be included in the electron-optical system. Such additional electrode or electrodes have not been illustrated since they have no direct bearing on the invention and may be conventional.

Electrode 1 is of generally annular shape and of stepped cross sectional profile, including the flange and plate portions 101, 102, 103 and 104 alternately axial and radial in extent, as shown. Electrode 2 is an anode or output electrode and is positioned in the reduceddiameter envelope section 501 and includes an inward radial flange 201, cylindrical body 202 and outward flange 203 at its output end. Both electrodes 1 and 2 are rigidly assembled together into an integral sub-assembly by means of spacer rods such as 9, e.g. three in number in an equiangular arrangement, and made of insulating material, such as ceramic. The spacer rods 9 have their respective ends secured by metal-ceramic bonds to the flanges 104 and 203 of the electrodes, and are of such length as to hold the adjacent flanges 104 and 201 of the respective electrodes, at a predetermined axial distance from each other so as to define an electron lens in the electron-optical system, as well-known in the art. The sub-asembly is held in position in the tube envelope by means of a flanged metallic mounting ring 13 having its respective flanges bonded to the radial plate portion 102 of electrode 1, and to the shoulder section which interconnects the large and small-diameter sections of envelope 5.

The target screen 3 is resiliently connected with the anode electrode 2 so as to form a part of the abovementioned subassembly. As shown, a cup-shaped annular retainer member 10 has a cylindrical end portion frictionally slidable in the cylindrical body part 202 of electrode 2, and has an inturned radial end flange 11 at its output end, upon which the screen member 3 rests. A coil spring 12 has its opposite end turns engaging the flange 201 of electrode 2 and the input (phosphorcoated) face screen member 3 so as to press the screen against flange 11 of retainer member 10, and press member 10 outwardly of electrode 2 and against the output end wall 7 of the tube envelope, in the assembled operative condition of the tube. In this condition the spring 12 is to some degree precompressed in order to exert a biasing force on the target screen throughout the service life of the tube. It will be understood that the target coating 8 is in electric connected relation with the anode 2, e.g. through spring 12.

Prior-to insertion of the sub-assembly into the tube envelope, spring 12 holds retainer ring 10 and screen 3 projected to a maximum extent out of anode electrode 2, the extent of projection being determined by the cooperation of an annular stop shoulder 21a of retainer ring 10 with a diaphragm plate 21b secured under the end annular shoulder 203 of electrode 2, as shown. To mount the subassembly including electrodes 1 and 2, retainer member 10 and screen 3 in the tube this sub-assembly is inserted into the tube envelope from the input end thereof prior to the sealing of the envelope by means of input end wall 6. The flanged mounting ring 13 has at this time already been positioned in the envelope with its one end flange bonded to the shoulder surface of the envelope as earlier described. The sub-assembly is thrust forward until the end flange 11 of retainer cup 10 engages the inner surface of end wall 7, after which the retainer 10 with the screen 3 therein is forced inward of electrode 2, compressing the spring 12. When electrode 1 has been advanced far enough for its flat plate section 102 to engage the flanged mounting ring 13, the said plate section is bonded to the ring flange in any suitable Way, as by screen not shown.

Suitable potentials are applied to photocathode 4 and to electrodes 1 and 2 by means of conductors -14, 15, 16 soldered there to and extending through sealed aperture in envelope 5, as shown. The slidable retainer cup 10 and spring 12 of the invention should be made from electrically conductive metal that is substantially non-magnetic, so as not to disturb the paths of the electrons as determined by the electric fields created by the electron-optical system. In selecting a suitable material for compression spring 12, it should further be remembered that the tube assembly is exposed to high temperatures during the preliminary treatments involving degassing and evaporation deposition of the photosensitive coating on photocathode 401, and the spring material should retain its resilient characteristics when exposed to such temperatures.

It will be apparent from the foregoing description that the fluorescent target screen 3 lies very close to end wall 7 of the tube, being only separated from the surface of said end wall by the thickness of the metal flange 11, which may be of the order of from 0.1 to 0.5 mm. thick. Said screen 3, at the same time, is separate and distinct from the tube end wall and can therefore be economically and efficiently constructed. Owing to the spring mounting of the target screen 3 in the output sub-assembly as described, there is provided a practical and economical method of tube construction in which the electrodes 1 and 2 and screen 3 can be prefabricated separately into a unit more easily than they could be thus assembled inside the tube envelope, and the resulting unit is then inserted bodily into the envelope and secured in position in a quick,simple operation. By this method, moreover, dimensional tolerances on the tube envelope can be broadened since minor differences in the axial dimensioning of different tubes will be taken up by compressional deformation of spring 12. While it is true that such dimensional variations will alter to a slight extent the axial distance between the lens defined by the electrodes 1 and 2, and the target screen, this is immaterial since the resulting out-of-focus condition can be easily compensated for by a suitable readjustment of the potential applied to electrode 1.

An additional advantage of the construction described is that the inturned flange 11 of the retainer member serves as an effective dust-shield for the small residual space between screen 3 and end wall 7. As explained in the afore-mcntioned US. Patent 3,304,455 minute particles of matter inevitably present in the tube envelope after evacuation tend to become electrostatically charged and attracted to high-potential surfaces of the tube in operation. If such particles were to cling to the outer or output face of the screen 3 they would appear as greatly magnified spots marring the image. Relatively complicated dust shielding means have been proposed, as described in the afore-mentioned US. Patent 3,304,455, to seal the output face of the screen from the remaining space of the envelope These means are made unnecessary when the herein disclosed construction is used. Preferably however, the flange 11 is arranged to be permeable to gases, as by leaving the under surface of the flange rough, in order to ensure proper de-gassing of the whole tube cavity during evacuation.

In the more detailed view of FIG. 2, parts corresponding to parts of FIG. 1 have been given the same references. Only the differences in construction will be described. Electrode 1 is seen to be secured to the end flange of mounting ring 13 by means of screws such as 17, which may be three in number. The other end of mounting ring 13 is not flanged, but instead is sealed in the end surface of a reentrant wall surface 502 forming an inward extension of the envelope section 501. The compression spring 12 is here shown of frustoconical shape with its diameter tapering towards the target screen 3. The reduced output end of spring 12 engages the screen 3 by way of an interposed seating ring member 18 which simultaneously with its spring seating function serves as a diaphragm covering the unused marginal area of fluorescent screen 3 and preventing objection-able radiation therefrom. The large-diameter inner end of spring 12 is seated by way of another diaphragm plate against an annular shoulder 19 formed within the electrode 2, substantially midway of its axial length. Diaphragm 20 is positioned and dimensioned to prevent a majority of electrons originating from sources within the tube other than the photo-emissive cathode 4, from striking the output screen 3 and displaying spurious bright spots on it. A diaphragm of this type and for this purpose has been disclosed in the aforementioned US. Patent 3,304,455. As also disclosed in that application, diaphragm plate 20 preferably supports a fine mesh wire screen (not here shown) across its central aperture. This screen serves to protect the input face of target screen 3 against small particles of matter floating about in the tube envelope as earlier mentioned herein, and which might otherwise be attracted to the fluorescent surface of screen 3 and mar the image. Such a screen does not appreciably interfere with the free passage of the photoelectrons through the tube as required for normal operation.

As earlier indicated, it is customary to form the photoemissive layer (such as 401, FIG. 1) on the photocathode of an image tube of the general type to which the invention relates, by a process of vacuum-evaporation and deposition in situ. A well-known procedure for this purpose is to seal one or more evaporator receptacles in the tube envelope, containing the necessary ingredients, including alkali metals and the like, in solid form. After the tube has been evacuated, the receptacles are heated electrically to evaporate their contents which thereupon deposits on the cool photocathode surface. Features of this invention relate to an improved construction and arrangement of the evaporators and associated components. This will now be described with reference to FIGS. 2 and 3.

In the embodiment shown there are provided three evaporator receptacles 23 in the form of elongate, curved tubes or pods of thin-gauge high-resistivity metal sheet or metal foil, rolled into a tube pinched at its ends, and spot-soldered along its longitudinal seam so as to provide a path of egress for the vaporized contents of the tube. The tubes or pods 23 may contain any of the usual ingredients for a vapor-deposited photoemissive coating, such as antimony potassium, sodium, and or caesium. The three evaporator pods 23 are arranged to extend in a common plane transverse to the axis of the tube as will be apparent from FIG. 3. One end of each evaporator pod 23, which is positioned radially inward from the other end, is connected by solder to the inner end of a related conductor wire 26, 27, 28, which wires extend in a direction parallel to the image tube axis and are passed out of the tube envelope 5 through a sealed nipple located in the interconnecting region of the envelope between the large-diameter body section and the reduced-diameter output section 501. The outer ends of the evaporator receptacles 23 are all connected toa common conductor bus 25 in the form of a large-diameter wire ring supported through means not shown from the cylindrical wall of envelope 5, and connected to a common supply terminal not shown.

In addition to the evaporator pods just described there is provided an evaporable getter wire 31, e.g. of barium, having a similar elongate arcuate shape as the evaporator pods 23, positioned in the same transverse plane as they, and having its ends connected to an axially extending lead-in wire 29 and the annular common-bus wire 25 respectively. It will be understood that in order to perform the evaporation-depositing operation, electric current from a suitable source is applied through the lead-in wires 26, 27, 28 to the evaporator pods 23. Since the evaporator pods will usually contain different ingredients that are to be evaporated in a prescribed sequence, the plurality of different lead-in wires such as 26, 27, 28 are provided in this embodiment. The flow of current through the highresistance metal walls of the pods to the annular bus 25 heats the said walls by Joule effect and vaporizes the contents of said pods, such as antimony, caesium and/ or potassium. The vaporized metals break out of the evaporator pods 23 under their own pressure through the spot-welded seams of the pods, and. the vapours condense on the cooled photocathode 4. Heating current is likewise applied to lead 29 to vaporize the getter wire 31 for the usual gettering step.

It will be noted that as shown in FIGS. 2 and 3 and described above the evaporator devices are arranged in the annular space defined between the annular electrode 1 and the surrounding portion of the wall of the tube envelope 5 near the junction between the large-diameter envelope section and reduced section 501. The photocathode is thus shielded from the evaporators to the extent that the metals vaporised from the latter are obliged to follow circuitous paths in reaching the photocathode surface. This is desirable because it allows time for the chemical reagents to react completely and for heavy compounds to break down into the desired component metals before deposition on the photocathode.

Furthermore, the evaporator system arranged as disclosed will be seen to require essentially no additional space in the tube envelope beyond What is required by the dimensioning of the main tube components. This is due to the arrangement of the evaporator pods in a transverse plane of the tube and their positioning within the annular region defined around the annular electrode 1. It is thus possible to minimize the length-to-diameter ratio of the tube, and hence to derive full benefit from the reduction in said ratio resulting from the elimination of constructional tolerances due to the resilient mounting of the target screen 3 as earlier explained.

The embodiment of FIGS. 2 and 3 further shows improved means for connecting the desired electric potentials to the electrodes 1 and 2. The connection for electrode 1 includes a conductor wire 30 parallel to the axial direction of the tube, and desirably positioned on a common circumference with the other axial conductors 26- 29 earlier referred to. Clamped to the upper end of coniuctor 30 is a resilient contact member 32 in the form 3f a hairpin-bent spring-leaf or the like, which is adapted ;o be engaged by the underside of the flat section 102 sf electrode 1 when said electrode together with the subassembly of which it forms part is mounted in its final oosition as earlier described and establish a good elec- :rical contact therewith under the pressure of spring 32. The connection for anode electrode 2 is there shown as :omprising the lead-in conductor 16 extending transversely through the sidewall of the tube envelope sec- :ion 501, and having a coiled flexible conductor wire 22 rttached to its inner end. Wire 22 constitutes a coil ipring capable of being resiliently extended to a considerible length. Thus, prior to insertion of the sub-assembly ncluding electrodes 1 and 2 and target screen 3 mounted as described the free end of the coiled wire 22 can be :onnected through any suitable means, as by clamping )r solder, to the outer end of the electrode 2, as here shown to the annular stop plate 21b thereof. On inser- ;ion of the sub-assembly electrode 1 makes contact with he spring connector 32 as earlier described, and the :onductor coil 22 connected to electrode 2 contracts to assume a condition of minimum length. It will be Observed hat the electrical connections for the evaporator devices were all completed prior to the insertion of the output rub-assembly into the tube envelope. It will therefore )e apparent that the construction disclosed in addition its other advantages considerably simplifies the elecric connections and that the whole process of tube con- ;truction and assembly is made easier and more econonical.

Schematically indicated at 24 in FIGS. 2 and 3 is a mess in the tube envelope for mounting a conventional on pump, not shown, serving to maintain a vacuum n the tube envelope during the service life of the tube.

A releasable shutter device, not here shown, may be Jrovided for the purpose of protecting the phosphor- :oated surface of target screen 3 from the deposition hereon of the substances evaporated from the evaporttor devices described above, during the photocathode- Forming operation.

The invention has thus provided an improved image :onverter tube construction in which the target screen, vhile being in the form of a separate and distinct elenent, is positioned in direct proximity to the output end wall of the tube envelope, whereby large-aperture optical :ystems can be used for viewing, or transferring, the ugh-resolution image formed on said target screen. In tddition, the improved tube construction permits a substantial increase in tube manufacturing tolerances, as vell as rendering the tube construction and assembling )perations considerably easier, quicker and more econ- )mical to carry out.

What We claim is:

1. An image tube assembly comprising an envelope raving an output end wall, an electron-emissive electrode n the envelope, a target screen in the envelope adjacent :aid output end wall, and electron-optical means in the :nvelope for directing electrons from said emissive elecrode on to the target screen and including an output :lectrode adjacent the screen, wherein the improvement :omprises:

means mounting the target screen for limited axial displacement relative to the output electrode;

spring means in force-transfer relation with the output electrode and target screen for urging the screen axially outward from the output electrode;

supporting means in the envelope cooperating with the output electrode to determine an operative axial position thereof; and

an abutment surface on the mounting means cooperating with an inner surface of the envelope in said operative position 0 fthe output electrode for limiting the outward displacement of the screen relative to the output electrode whereby to position the screen at a prescribed close spacing from said output end wall in the operative position of the output electrode.

2. The image tube assembly defined in claim 1, wherein: the target screen mounting means comprises a generally cup-shaped annular retainer member having a side wall portion slidable in said output electrode and having an inwardly-directed flange at its outer end for seating said target screen, said flange having its axially outer surface abuttingly cooperating with the inner surface of said envelope end wall in said operative position, and said spring means comprising a compression coil spring positioned within the retainer member.

3. The image tube assembly defined in claim 1, including stop means on said output electrode and said screen mounting means for limiting the outward displacement of the target screen and preventing escape thereof from the output electrode whereby the output electrode and target screen form parts of a sub-assembly that can be handled as unit and inserted as a unit into the envelope, for placing the output electrode in said operative position thereof.

4. The image tube assembly defined in claim 3, wherein the electron-optical means includes another electrode positioned ahead of said output electrode, and rigid spacer means made of electrically insulating material mechanically interconnecting both said electrodes, whereby said other electrode forms a part of said subassembly.

5. The image tube assembly defined in claim 2, further including a diaphragm plate positioned in said output electrode, an annular shoulder formed in said output electrode generally midway of the axial length thereof and directed towards said output end wall, said diaphragm plate being seated on said shoulder, and said compression spring having its axially inner end engaging said diaphragm plate for applying it against said shoulder.

6. A sub-assembly for use in an image tube, comprismg:

an annular electrode member;

a retainer member axially slidable in the electrode member;

a target screen supported in the outer end of the retainer member;

a compression spring positioned to act between the retainer and electrode members so as to urge the retainer member and target screen axially outward from the electrode member; and

cooperating stop means on said members to limit said outward displacement to an amount substantially greater than the amount by which said members are relatively displaced when said sub-assembly is mounted in operative position in said image tube, whereby said target screen will be retained under substantial spring bias in said operative position.

7. The sub-assembly defined in claim 6, wherein said retainer member is flanged at its outer end for supporting the target screen said image tube having a sealed envelope comprising and output end wall, and said flanged end of the retainer member is adapted to seat against an inner surface of said output end wall in said operative position, and said target creen has a phosphor coated inner surface.

8. The sub-assembly defined in claim 6, further comprising another annular electrode member, and spacer means of electrically insulating material rigidly interconnecting both electrode members in coaxial relation with said other member axially spaced from the first electrode member in the direction away from said target screen.

9. The sub-assembly defined in claim 6, including a diaphragm plate axially slidable in said electrode member and positioned to be urged by said spring in a direction away from the target screen, and a stop surface in said electrode member engageable with said diaphragm plate to determine the position thereof in said electrode member.

10. An image tube assembly as defined in claim 1 wherein:

said envelope includes a main body section and a reduced-diameter output end section;

said electron-emissive electrode includes a photo-emissive layer on a surface thereof; and

said electron-optical means includes a first annular electrode positioned adjacent the junction of said main body and output end sections and defining an annular space with the surrounding envelope wall portion, and a second annular electrode positioned in said output end section and axially spaced from the first electrode; further comprising:

evaporator receptacle means containing a charge of vaporiza-ble substance capable of contributing to the formation of said photo-emissive layer; means supporting said receptacle means within said annular space substantially on a common transverse plane of the tube; an electrical heating conductor externally connectable to a heating current source, extending through the envelope wall and internally connected in a heating circuit for heating said receptacle means and vaporizing the contents thereof; and a high-voltage electrical conductor externally connectable to a high-voltage source, extending through the envelope wall and a connector member on the inner end of said last conductor engageable by said first annular electrode on engagement thereof with said supporting means in operative position; whereby said first annular electrode can be secured in operative position and simultaneously connected said said high-voltage conductor after said evaporator receptacle means has first been connected with said heating conductor. 11. The image tube assembly defined in claim 10,

wherein siad evaporator receptacle means comprises a plurality of elongate receptacles disposed in a part-circumferential array on said common transverse plane within said annular space and made of high-resistivity material, said receptacles having radially extending inner ends connected to said heating conductor and a bus conductor in the form of a wire-ring supporting coaxially with and adjacent said envelope wall and means connecting the radially extending outer ends of said receptacles with said bus conductor.

12. The image tube assembly defined in claim 10, further including rigid spacer means of electrically insulating material mechanically interconnecting said first and second annular electrodes, and means resiliently supporting the target screen from said second annular electrode, whereby said first and second annular electrodes and target screen form a unitary sub-assembly.

13. The image tube assembly defined in claim 12, wherein said screen supporting means comprises a retainer member axially slidable in said second annular electrode and having means supporting the target screen at the axially outer end of said retainer member, spring means positioned to urge the retainer member and screen axially outward from said second electrode, and stop means on said second electrode and retainer member for limiting the outward displacement of the retainer member and preventing escape thereof prior to insertion of the sub-assembly into the envelope, said retainer member having an axially outer end surface cooperating with said output end wall of the envelope to limit the outward displacement of the retainer member and position the screen closely adjacent to said output end wall when said sub-assembly has been inserted with the first electrode engaging supporting means in said operative position thereof.

References Cited UNITED STATES PATENTS 2,310,147 2/1943 Dailey 31318O 2,661,437 12/1953 Beckers 313-'71 2,894,163 7/1959 Orthuber et a1 313-149 3,240,976 3/ 1966 Driard 313237 3,304,455 2/ 1967 Mesta 31394 3,378,714 4/1968 Guyot et a1 313- JAMES W. LAWRENCE, Primary Examiner V. LAFRANCHI, Assistant Examiner US. Cl. X.R.

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