US2887595A

US2887595A - Television camera tube

Info

Publication number: US2887595A
Application number: US583970A
Authority: US
Inventors: Tijs Willem Van Rijssel; Boers Martinus Will Augustinus; Maartens Johannes Hen Josephus; Boerman Jan Rigte; Kuhl Walter Heinrich
Original assignee: US Philips Corp
Current assignee: US Philips Corp; North American Philips Co Inc
Priority date: 1955-05-14
Filing date: 1956-05-10
Publication date: 1959-05-19
Anticipated expiration: 1976-05-19
Also published as: NL104633C; FR1153190A; DE1014155B; NL197262A; CH341862A

Description

May 19, 1959 T. w. VAN lRLJssEL ETAL 2,887,595

f TELEVISION CAMERA TUBE Filed May 1o, 195e INVENTOR rus WILLEM VAN mJssEL mrmus wlLLIsALous AuGusnNus soERs JOHANNES HENDRICUS JOSEPHUS MAARTENS JANLRIGTE BOERMAN AGENT TELEVISION CAMERA TUBE Tijs Willem van Rijssel, Martinus Willibaldus Augustinus Boers, Johannes Hendricus Josephus Maartens, Jan Rigte Boerman, and Walter Heinrich Kuhl, all of Eindhoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Application May 10, 1956, Serial No. 583,970 lClaims priority, application Netherlands May 14, 1955 9 Claims. (Cl. 31365) The invention relates to a television camera tube of the kind in which a charge pattern is produced by secondary emission. The tube comprises a photo-electric cathode. Electrons set free from this cathode by projecting a luminous image onto it are accelerated in the tube and focused onto an image electrode, where they produce secondary emission, owing to which a charge pattern corresponding to the luminous image is produced on the side of the image electrode facing the photoelectric cathode (image side) It is known that the image electrode of such a camera tube may be made of a substance which permits an electric charge to 4leak from one side of the electrode to the other. The specic resistance of this substance should be within the range from 109 to 1012 ohm-cm. To this end so-called conductive glass is particularly suitable, an example being the glass known in the United States of America as G-B glass, exhibiting a specific resistance of about 5 X 101 ohm-cm.

It is advantageous to have a high secondary-emission coefficient on the surface of the image electrode on the image side. It has therefore been suggested to provide the image electrode on the image side with a thin layer of an alkali oxide or alkaline earth oxide to increase this coefficient. However, it has been found that the improvement obtained by such an oxide layer on conductive glass is not always persistent and that in some of the known tubes the increase in efficiency obtained by secondary emission gradually falls olf.

This phenomenon becomes manifest with an increase in the collector accelerating voltage, i.e. the voltage at which the secondary electrons are conducted away as completely as possible to the collector. The invention provides means to obviate this disadvantage and to obtain a higher efficiency and longer lifetime of the tube.

In a television camera tube according to the invention, which tube comprises cesium and is provided with an image electrode of conductive glass, on which secondary emission produced by electrons emanating from a photo-electric cathode produces a charge pattern corresponding to a luminous image projected onto the photoelectric cathode and which is provided on the image side with a thin secondary-emissive layer (referred to hereinafter as the coating), this layer consists mainly of an inorganic sodium or lithium-halogen compound.

It is known that layers of an oxide or a halogenide of an alkali, an alkaline-earthor an earth metal on a metal electrode exhibit a comparatively strong emission.

It is furthermore known that the insulating image electrode of a camera tube operating with an electron beam focused on this electrode, where it sets free secondary electrons, may be constituted by an insulating layer of chloride or bromide of one or more alkali metals, for example KCl or NaCl, which is applied to a metal electrode. If necessary, an insulating layer for example of mica may be sandwiched between the said layer and the metal electrode.

Patented May l19, 1959 ice t A camera tube has furthermore been described, of which the image electrode is constituted by a thin insulating or semi-conductive layer, which may be made of an alkali halogenide and which is metal-coated on the rear side. This image electrode is scanned on the metalcoated side by rapid electrons which must render the image electrode conductive.

In none of these known cases, in w'hich reference is made to halogenide compounds, is there concern with the transport of electric charge through conductive glass, which is coated with 4a secondary-emissive layer, this being in contradistnctionto the tube according to the invention in which the said layer contains sodiumand/or lithium atoms or both. The fact that with this tube suchfsurprising results are obtained could be accounted for by the following theory, whichis not, however, to be considered as limiting.

It is presumably ions, emanating from the coating, that transport the charge through the glass. It might be that ions of the monovalent elements sodium and lithium are particularly suitable thereto, since, owing to their small size, they are suliiciently movable in the glass.

The invention may be carried into effect by using as substance for the coating a chlorine compound, such as sodium chloride.

Further development of the invention has showny that particularly uorine may be selected as the halogen and that cryolite (sodium-aluminum uoride) is to be considered as an extremely favourable substance for the coating.

In accordance with a further aspect of the invention, atoms of a trivalent, ilumine-binding metal are applied to the coating containing a fluoride compound, the quantity of this metal being such that the surface conductivity and the secondary-emission coeicient of the coating are substantially not affected and that the majority of the fluorine atoms added initially to the sodium or lithium atoms in the coating are bound. It is thus ensured that during the operation of the tube free iiuorine atoms produced by secondary emission of the coating are bound with certainty in the coating, so that they do not emerge from the coating and Wander through the tube. Trivalent metals suitable to this end are, for example, antimony, bismuth, indium and arsenic. Use is preferably made of antimony. The atoms of this metal form, on the coating, a compound with the available cesium atoms, so that a compound having a comparatively low electrical conductivity is obtained. Even in the case of a comparatively large quantity of antimony on the coating the surface resistance thereof is little affected. A further advantage of antimony is that it hinders little the movement of the primary and secondary electrons striking the coating.

The invention will be described more fully with refer- 'ence to the drawing, which shows diagrammatically one for example, a cryolite coating 15 to enhance the secondary emission of the image electrode.

The part having the smaller diameter is closed by a at ,glass bottom 6, to the inner side of which is applied thephotorelectric cathode 7. The cathode may, as usual, be made of antimony activated by cesium. Reference numeral 8 designates an optical system, by means of which a luminous image can be projected onto the photoelectric cathode. Y

To the coating 1S is applied by vaporisation a small quantity of antimony. The antimony may be applied directly after or, if desired, partly simultaneously with the Vapplication 'by vaporisaton of the cryolite of the coating 15. Tlhe quantity of antimony applied is about 1% by weight of the applied quantity of cryolite in the coating 15. This quantity of antimony does not introduce an kappreciable change of the electrical surface re- `sistance of the coating 15, so that the image sharpness is not affected. The secondary-emission coeticient of thacoating appears to be `slightly higher owingto the antimony; this is believed to be due to a reduction in emission potential, owing tothe formation of an antimonycesium compound on the surface of the coating 1S, the cesium, of course, originating from the photo-cathode 7.

In the side tube 9 is arranged an electrode system 10, which serves to produce an electron beam of definite intensity. The axis of this system is directed to the centre of the image electrode 5. By means of the conventional deflection coils 11 and 12 the path of the electron beam can be controlled and by suitable energization of the coils 11 and 12 the complete image surface of the image electrode is periodically scanned bythe electron beam.

yThe wall of the tube has a conductive coating 13, which is electrically connected to an annular collector 14, ar-

ranged in the proximity of the image electrode.

`The image electrode is formed by a conductive glass layer of about 60p. applied to the metal supporting plate 4. The` specic resistance of this glass is about 1011 `ohm-cm. under the conditions of operation of the tube.

On the image side the conductive glass is coated with a layer of cryolite 15 of about 500 A. in thickness, precipitated from the Vapour phase.

V- On the wall inside the side tube 9 provision is made of an annular gold layer 16. This layer serves as a store for, or supply of, cesium, but provides at the same time that no excess of cesium vapour is contained in the tube, which wouldV be harmful to the sensitivity of the photo-electric cathode. The cesium is adsorbed by Vthe gold layer when cesium vapour is introduced into the tube during manufacture. The gold layer keeps the cesium away from the electrode system 10.

H The current conductor 3 is connected through a load resistor 17 to the negative terminal of a direct-current source 18, the positive terminal of which is connected `to the coating 13 and hence also to the collector 14, so

tial at the photo-electric cathode 7 with respect to the .coating 13 and the collector 14. The potential difference between the collector and the electrode system `with the photo-electric cathode is high with respect to the diierence between the collector andthe supporting plate 4, so that both the electrons of the scanning beam pand, the electrons emanating from the photo-electric cathode strike the cryolite layer with a suicient velocity to set freeA secondary electrons from this layer. lWhenstruclcby lthe scanning beam, each point of the surface of the image electrode is brought to a potential Vwhich is'approximat'ely equal to that of the collector.

During the time interval between two successive scanjnings (frame period) the potential of the point is reduced .since 'the image electrode is traversed by electric charge.

Owing to this potential drop secondary electrons set free byjthe photo-electrons during the frame period can be attracted and collected by the collector 14. Consehas a certain amount of conductivity.

The secondary-emission coeflicient of the cryolite layer 15 is particularly high.` Values between 9 and 11 are found. These high valies are also ascribed to the presence of cesium in the tube. Cesium `adheres readily to antimony and `halogenide ions, particularly to those of fluorine, which substance is contained in the cryolite. For a large part the favourable properties of the tube according to the invention may be due to these conditions.

It is assumed that `cesium atoms adsorbed at the surface of the coating could then bind halogenide atoms, while forming ions, these halogenide atoms being produced wherrhalogenide ions give off electrons by secondary emission.` This bond could prevent the halogenide atoms from leaving the coating and from spreading in the tube, and from alecting the photo-electric cathode.

Consequently, the `tube should contain cesium. In the Vembodiment described this substance is provided in the photo-electric cathode. If this is not the case, a sepa- `rate supply ofcesium` can be provided. This provision may, otherwise, also be made when the photo-electric cathode already contains cesium, since experiments have shown that an excessively small supply of cesium in the tube may imply that after some time such a quantity of cesium may be withdrawn from the photo-cathode that' the effect of this cathode falls ctt. In the embodiment described a cesium supply is formed by the gold layer 16 inthe side tube `When thecoating of the tube according to the invention contains fluoine, as is the case with the example described, the spreading of fluorine atoms affecting the photo-electric cathode in the tube `is avoided with certainty by applying to the coating by vaporisation a small quantity of atoms of a trivalent, ilumine-binding metal. Such a metal may, for example, be antimony, bismuth, indium or arsenic: use is preferably made of antimony, as is indicated above in the description of the example.

Camera tubes for television exhibit rather often a troublesome phenomenon, referred to as inscription This phenomenon consists inthat during the use of the tube the coeflicient of the secondary emission is changed at those areas ofthe image electrode which have been loaded most. This drawback may sometimes be obviated or at least reduced by taking the tube out of operation for some time.

Cryolite has the favourable property that it does not or substantially not exhibit the aforesaid phenomenon of inscription Since it is, moreover, readily precipitated from the vapour phase in the tube itself on the con ductive glass, it is an excellent material for the manufacture of the secondary-emissive coating.

What is claimed is:

l. A television camera tube comprising an envelope and inside said envelope the combination comprising a photo-electric cathode and image electrode cooperating to establish a charge pattern on said image electrode corresponding to a televised object, said image electrode including a conductive glass member and a thin, secondary-electron-emissive coating thereon comprising essentially an inorganic halogen compound containing an alkali metal selected from the group consisting of sodium and lithium, Said tube also containing cesium.

2. A` television camera tube as set forth in claim l wherein the halogen compound is a uorine compound.

3.V A television camera tube as set forth in claim 2 wherein the emissive coating consists essentially of sodium-alu`rninum fluoride.

4. A television camera tube comprising an envelope and inside said envelope the combination comprising a photo-electric cathode containing cesium and an image electrode cooperating to establish a charge pattern on -saidirnage electrodecorresponding to a televised object; `said image electrodeincluding a glass member having a resistivity within the range from 109 to 1012 ohm-cm..

an electrode on one side of said glass member, and a thin, secondary-electron-emissive coating on the other side of said glass member and constituted of an inorganic halogen compound containing an alkali metal selected from the Igroup consisting of sodium and lithium; a supply of cesium within said envelope and spaced from said photo-electric cathode; and means for scanning said other side of the image electrode with an electron beam.

5. A camera tube as claimed in claim 2 wherein the emissive coating contains on a surface atoms of a trivalent, uorine-binding metal in an amount such that the surface conductivity and the secondary-emission coeficient of the coating are substantially not aiected.

6. A camera tube as claimed in claim 5 in which the trivalent metal is antimony.

7. A television camera tube comprising a photo-electric cathode containing cesium and an image electrode, said image electrode being constituted of a glass member having a resistivity in the range of 109 to 1012 ohm-cm., and a thin cryolite coating on said glass member.

8. A tube as set forth in claim 7 wherein antimony is present onthe cryolite coating.

9. A tube as set forth in claim 8 wherein the antimony constitutes about 1% by weight of the cryolite, and the cryolite coating has a thickness of about 500 A.

References Cited in the tile of this patent UNITED STATES PATENTS 2,535,817 Skellett Dec. 26, 1950 2,547,638 Gardner Apr. 3, 1951 FOREIGN PATENTS 510,784 Canada Mar. 8, 1955