US3249790A - Pickup tube electron gun - Google Patents

Description

May 3, 1966 n. L. SCHAEFER PICKUP TUBE ELECTRON GUN Filed Jan. 2. 1963 FIG.|.

INVENTORI DONALD L. HAEFER HI TTORNEY.

3.249.790 PICKUP TUBE ELECTRON GUN Donald L. Schaefer, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Jan. 2, 1963, Ser. No. 249,091

4 Claims. (Cl. 313-456) The. present invention relates to improvements in picture signal generating tubes such as image orthicons, and more particularly to an improved electron gun construction for image orthicons and the like" for reducing exposure of the photosensitive element of the tube to extraneous radiation.

A picture signal generating tube such as an image orthicon is provided with a photosensitive cathode which acts to transduce an optical or light radiation image to which it is exposed, to a pattern of electric charge from which can be produced a suitable electrical readout signal. A problem in achieving optimum performance with such lig-htimage pick-up tubes, and particularly those intended to have enhanced sensitivity at low light levels or enhanced sensitivity to radiation in the infrared region of the spectrum, is that extraneous radiation from the thermionic cathode heater is transmitted, within the tube envelope itself, into incidence on the photocathode or light-sensitive element of the tube, and thereby causes a false or extraneous electrical signal to be generated. In general the effect of such extraneous radiation is to increase the electrical noise output of the photocathode relative to the desired signal, and thereby reduce the capability of the tube to detect and resolve weak signals such as those from scenes of low illumination.

A major portion of such extraneous radiation from the cathode heater has been found to reach the photocathode by way of reflection from the interior surface of the adjacent portion of the tube envelope, after passage through a heater support ceramic. The ceramic is usually selected for its mechanical and electrical properties and is frequently made of a translucent material such as aluminum oxide. Attempts to block such extraneous cathode heater radiation from reaching the photocathode by using heater support ceramics made more opaque by addition of a dark material such as carbon have not proved satisfactory because additives such as carbon have a deleterious effect on the electrical properties of such ceramics.

A principal object of the present invention, therefore, is to provide an improved cathode-heater construction for picture signal generating tubes such as orthicons or vidicons, which minimize incidence of extraneous internal radiation on the photocathode without deleteriously affecting the electrical properties of the cathode-heater assembly.

This and other objects of the present invention will be apparent from the following description and the accompanying drawing wherein: 1

FIGURE 1 is apartially broken away fragmentary axial sectional view of a picture signal generating tube of the image orthicon type constructed in accordance with the present invention; and

FIGURE 2 is an enlarged fragmentary axial sectional view of the thermionic cathode-heater assembly portion of the tube of FIGURE 1, constructed in accordance with the present invention;

Referring to FIGURE 1, the image orthicon there shown includes an evacuated envelope 2 closed at one end by a stem provided with a base 4 having external leads 6. Envelope 2 also includes an image section 8 having an image receiving window 10 on the interior of which is supported a photocathode 12 for converting optical image radiation to a pattern of electric charge emitted from the back side of layer 12 and forming a stored ice charge image on a charge storage electrode 14, as is well known to those skilled in the art. Electrical readout of the charge stored on electrode 14 is provided by an electron beam formed from electrons emitted from thermionic cathode 16, (FIG. 2) the electron beam being suitably focused and deflected by means (not shown) external to the tube envelope, and the current of reading beam electrons returned from electrode v14 being suitably multiplied grade for 24 hours.

in electron multiplier dynode structure 18.

As best shown in FIGURE 2 the thermionic emissive material of the thermionic cathode 16 is supported on the metal end face 20 of a cylindrical cathode sleeve 22. The cathode sleeve is in turn carried by an annular ceramic insulator 24 supported within a cylindrical cathode and heater support casing 28. To facilitate support of some of dynode structure 18, casing 28 has a coaxial cylindrical extension 29. The sleeve 22 and insulator 24 and casing 28 are all arranged to be mounted within the envelope 2 coaxially withthe envelope axis 30. The rearward end of cathode sleeve 22, that is the end facing the tube base 4, is open so as to accommodate a coiled heater 32 which is carried by, and extends into'the sleeve 22 from a pair of relatively thick heater leads 36. Leads 36 extend through and are supported in spaced relation by a transversely arranged ceramic insulative disc 40 contained within and closing the rearward end of the casing 28.

The forward end of the casing 28 is completely closed by transverse metal electrode 50 .and control grid 52, except for the central openings 54 and 56 provided in electrodes 50 and 52 respectively to permit passage of electrons emitted from the cathode 16.

Since the forward end of the casing 28 is thus effectively closed by electrodes 50, 52 to passage of extraneous radiation from the heater 32 to the photocathode 12, it will be apparent that a principal source of extraneous radiation from the thermionic heater 32, to which the photocathode of the tube is sensitive, will be that emerging from the rearward end of the casing 28 (or extension 29) and reflected from the interior surface of the stern portion of the envelope and other reflective surfaces within the tube back up to incidence on the photocathode 12. In accordance with the present invention I make the insulative support 40 opaque without undesirably affecting its electrical properties by making the ceramic of a composition of alumina and about 10% to 15% by weight of the oxides of chromium, cobalt, or magnesium. The resulting ceramic is dark purple-red in color and transmits substantially no light in the visible spectrum, and very little light in the infrared portion of the spectrum.

In the making of such ceramic supports 40 I compound a homogenous mixture of about to by weight alumina, and about 10% to 15% by weight of one or a mixture of the oxides of chromium, cobalt, or magnesium. This homogeneous mixture is then pressed into shape with a suitable binder, such as carbo wax, and fired at a temperature of about 1400 to 1500 degrees centi- The resultant ceramic has satisfactory mechanical strength, excellent electrical properties, and is substantially completely opaque to extraneous radiation from a source such as heater 32. A cathode-heater assembly such as shown in FIGURE 2, equipped with such a ceramic 40, is highly effective for preventing extraneous radiation from heater 32 from reaching photocathode 12, and thus minimizes generation of extraneous electrical signals resulting from such radiation.

It will be appreciated by those skilled in the art that the invention may be carried out in various ways and may take various forms and embodiments other than the illustrative embodiments heretofore described. Accordingly it is to be understood that the scope of the invention is not limited by the details of the foregoing description, but will be defined in the following claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A cathode-heater assembly for a picture signal generating tube having a photocathode comprising an opaque cathode-heater enclosure adapted to be disposed in spaced relation to the photocathode and having an aperture opposite said photocathode, a cathode disposed within said enclosure opposite said aperture and arranged to emit electrons through said aperture toward said photocathode, a heater within said enclosure adjacent said cathode, said heater having supporting and energizing leads extending through an opening in said enclosure remote from said aperture, a hard fired ceramic disc closing said opening and through which said heater leads pass, said ceramic disc being composed essentially of a mixture of aluminum oxide and about to by weight of material from the group consisting of the oxides of cobalt, chromium, magnesium, and mixtures of such oxides whereby said ceramic disc is opaque to and prevents extraneous radiation from said heater from passing out of said enclosure.

2. A cathode-heater assembly for a picture signal generating tube having a photocathode comprising a cathodeheater enclosure having an opaque generally cylindrical portion adapted to be disposed in spaced relation to the photocathode, said enclosure cylindrical portion being closed at one end by an end wall forming an electrode having an aperture opposite said photocathode, a cathode disposed within said enclosure opposite said aperture and arranged to emit electrons through said aperture toward said photocathode, a heater within said enclosure adjacent said cathode, a hard fired ceramic disc forming a transverse closure for the other end of said cylindrical portion of said enclosure, said ceramic disc being composed essentially of a mixture of aluminum oxide and about 10% to 15% by weight of material from the group consisting of the oxides of cobalt, chromium, magnesium, and mixtures of such oxides, whereby said ceramic disc is opaque to and blocks extraneous radiation from said heater from passage out of said enclosure into incidence on the photocathode, and supporting and energizing leads for said heater extending through said ceramic disc.

3. A cathode-heater assembly for a picture signal generating tube having a photocathode comprising a cylindrical metal casing adapted to be disposed with its axis extending generally normal to the photocathode, means including an electrode forming a closure for the end of said casing adapted to face said photocathode, said electrode having a central aperture, a hard fired ceramic discforming a transverse closure for said casing axially spaced from said apertured electrode, a cathode heater disposed within said casing between said apertured electrode and said ceramic disc and having supporting and energizing leads extending separately through said ceramic disc, said ceramic disc being composed essentially of a mixture of aluminum oxide and about 10% to 15% by Weight of material from the group consisting of the oxides of cobalt, chromium, magnesium, and mixtures of such oxides, and a thermionic emitter in said casing disposed in heat receiving relation with said heater and having an electron emitting surface facing said apertured electrode.

4. In a picture signal generating tube having a photocathode for transducing a light image to an electric charge pattern for reading by scanning with an electron beam, a cathode-heater assembly comprising a light-opaque casing adapted to be disposed in spaced relation with said photocathode and with one end facing said photocathode, means. including an electrode entirely closing said one end of said casing, said electrode having a central aperture for passage of an electron beam, a hard fired ceramic disc forming a transverse closure for the other end of said casing, a cathode heater disposed Within said casing between said electrode and said ceramic disc, energizing leads extending through said ceramic disc in spaced apart insulated relationship and fixedly supported by said disc, said ceramic disc being composed essentially of a mixture of aluminum oxide and about 10% to 15% by weight of material from the group consisting of the oxides of cobalt, chromium, magnesium, and mixtures of such oxides, and a thermionic emitter in said casing disposed in heat receiving relation with said heater and having an electron emitting surface facing the aperture of said apertured electrode, said thermionic emitter having leads connected to said energizing leads.

References Cited by the Examiner UNITED STATES PATENTS 2,015,172 9/1935 Wiegand 313340 2,443,916 6/1948 Keiar 3 l382.1 2,540,621 2/1951. Johnson 313-82.1 3,029,360 4/1962 Etter 313340 3,040,200 6/1962 Shaffer et al. 313-82 X JOHN W. HUCKERT, Primary Examiner.

A. J. JAMES, Assistant Examiner.

Claims (1)

1. A CATHODE-HEATER ASSEMBLY FOR A PICTURE SIGNAL GENERATING TUBE HAVING A PHOTOCATHODE COMPRISING AN OPAQUE CATHODE-HEATER ENCLOSURE ADPATED TO BE DISPOSED IN SPACED RELATION TO THE PHOTOCATHODE AND HAVING AN APERTURE OPPOSITE SAID PHOTOCATHODE, A CATHODE DISPOSED WITHIN SAID ENCLOSURE OPPOSITE SAID APERTURE AND ARRANGED TO EMIT ELECTRONS THROUGH SAID APERTURE TOWARD SAID PHOTOCATHODE, A HEATER WITHIN SAID ENCLOSURE ADJACENT SAID CATHODE, SAID HEATER HAVING SUPPORTING AND ENERGIZING LEADS EXTENDING THROUGH AN OPENING IN SAID ENCLOSURE REMOTE FROM SAID APERTURE, A HARD FIRED CERAMIC DISC CLOSING SAID OPENING AND THROUGH WHICH SAID HEATER LEADS PASS, SAID CERAMIC DISC BEING COMPOSED ESSENTIALLY OF A MIXTURE

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