US2731580A - Target electrodes for use in television pick-up tube or the like - Google Patents

Description

G. S. P. FREEMAN TARGET ELECTRODES FOR USE IN TELEVISION Jan. l 7, 1956 PICK-UP TUBES OR THE LIKE Filed Oct. 22. 1949 i AVA GEORGE STANLEY PERCIVAL FREEMAN INVENTR.

HAS` AT TRNE Y United States Parent() TARGET ELECTRGDES FOR USE IY TELEVHSIN PICK-UP TUBE UR 'EEE LiKE George Stanley Percival Freeman, Chiswick, London,

England, assigner to Cinema-Television limited, London, England, a British company Application October 22, 1949, Serial No. 122,925

Claims priority, application Great Britain November 22, 1943 2 Claims. (Cl. 313-326) This invention relates to improvements in or relating to target electrodes for use in television pick-up tubes or the like and is more particularly concerned with such target electrodes of the double-sided type.

lt has previously been proposed to construct a target electrode, for use in a television pick-up tube, which comprises a thin metal ilm in contact with one face of a thin layer of insulating material. In the operation of a tube comprising such a target electrode, primary electrons from a photosensitive cathode penetrate the metal lm and release secondary electrons in the insuiating material which are accelerated away from the exposed face thereof. With such a construction, it is dicult to prevent primary electrons reaching the metal layer from passing through both the metal and insulating layers; such penetrating primary electrons, in conjunction with the released secondaries, have a deleterious effect on the image signal when the insulating face of the target is scanned with a low-velocity electron beam and the signal is derived from the return beam. In another similar arrangement, a thin metal film is juxtaposed to one face of a thin glass layer; in this arrangement also there is the possibility that primary electrons incident on the metal lm may pass completely through the target electrode and appear at the exposed glass face where the secondary electrons are liberated.

It is a primary object of the present invention to provide an improved double-sided target electrode, tor use in a television pick-up tube or the like, which provides efficient image conversion while preventing primary photoelectrons from entering the scanning section of the tube.

According to the invention there is provided a target electrode, for use in a television pick-up tube or the like, comprising the combination of athin metallic layer, a thin semi-conducting layer and a thin insulating layer, capable of emitting secondary electrons, interposed between the metallic and semi-conducting layers.

The present invention avoids the difliculty hereinbefore mentioned by virtue of the fact that electron velocities within the target electrode are reduced so that it is irnpossible for primary electrons from the photo-sensitive cathode to appear at the face of the target electrode which is scanned with the electron beam, although secondary emission is caused at an intermediate interface of the target electrode.

The features of the present invention which are believed to'be novel are set forth with particularity in the appended claims. 'The invention, together with further objects and advantages thereof, may best be understood, however, by reference to the following description taken in connection with the accompanying drawing in which:

Figure 1 is a sectional view, partly schematic, of a television pickup tube embodying a target electrode constructed in accordance with the present invention, and

Figure 2 is a sectional View of another embodiment of the invention.

With reference to Figure l, a television pickup tube comprises a continuous light-transparent photo-cathode 10 formed on the inside of the window 11 of an evacuated envelope 12. A target structure 13 is disposed in parallel relation with the photo-cathode 10 at a distance, for example, of about three millimeters. The target electrode 13 comprises a very thin electron-permeable metallic lm 14 and a thin plate 15 of semi-conducting material, constructed of glass for example, of the type used ina conventional image orthicon. A thin layer 16 of insulating secondary electron emitting material is interposed between metallic tilm 14 and semi-conducting plate 15; preferably, insulating layer 16 is alflxed to metallic lm 14, as for example, by depositing ilm 14 on layer 16 in a manner well known in the art. Metallic lm 14 may conveniently be made of aluminum, and insulating layer 16 may be constructed of aluminum oxide.;

On the other side of the target electrode 13 isdisposed an electron gun 17, which may be of conventional construction, and an electron multiplier systemv18 is arranged to intercept the return beam. A decelerating'systern 19, which may comprise a series of annular metal rings operated at progressively higher potentials in a direction away from electron gun 17, is provided, the purpose of which will be hereinafter described. Scanning coils 20 are provided for deecting the cathode-ray beam to cause it to scan semi-conducting plate 15, and a long solenoid 21 surrounds the entire tube envelope 12 to provide a long axial magnetic field. Further electrodes 22, Z3, and 24 are provided to control the cathode-ray beam; these electrodes may conveniently comprise conductive coatings on the internal walls of the tube envelope 12.

All of the electrodes may be supported within envelope 12 by conventional means well known in the art.

in operation, light from an image 25 is focussed on photo-cathode l@ by means of a lens 26, and primary electrons emitted from the photo-cathode 1i? penetrate metallic tilrn 1d and come to restin the insulating layer 16 just short of the surface thereof distant from the metallic ilm 14, thus releasing secondary electrons and imparting a local negative charge to the semi-conducting plate 1S. Thus, a white picture point produces a negative charge on the scanned face of the semi-conducting plate 15, while a black picture point produces no change of potential. With this arrangement, it is impossible for photoelectrons of secondary electrons to penetrate through the semi-conducting plate into the evacuated space beyond.

The scanning of the target electrode is performed in a manner similar to the scanning in an orthicon or image ort'nicon type tube, but instead of the beam being decelerated to zero volts velocity, it is allowed to strike the target with, for example, 205G volts velocity, thus releasing more than one secondary electron per primary, and so restoring the target to an equiiibrium potential near that of the final anode. The fact that the beam does not come to zero velocity improves the focus and also reduces the tendency for the beam to defocus at regions of high negative charge corresponding to high lights in the picture.

Since the envelope of the tube is surrounded by a long soienoid providing an axial magnetic eid, the secondaries released by the scanning beam are substantially prevented irc-rn returning a rain or secondaries the target, and consequently the tilt and bend signals are of negligible proportions, if they are present at all. These secondaries, therefore, return to the gun end of the tube where they enter the electron multipiier 13.

if the tube receives an excess of light on the photocathode, the target may travel so negative in potential that the picture disappears because of the inability of the scanning beam to reach it, but the range of tolerance is very much greater in a device constructed in accordance with the invention than in the orthicon for instance, since the permissible average potential change of the target is greater. The remedy for such a disappearance Vof the picture is either to apply a negative pulse to the gun cathode, or to apply a positive pulse to the thin metal lm on the glass target. 'Y

To facilitate an understanding of the operation of the device, let it be assumed that point 27 of the image 25 is a black picture point, and point 2S isrra white picture point. When the image 25 is illuminated, a local negative charge is imparted to t'he'point 29 on semi-conducting plate corresponding to point 28 of the image, and point 30 of the semi-conducting plate 15, corresponding to point 27 ofV image 25,'is yunchanged in potential. Primary electrons from electron gun 17 scanning points 29 and 30.release secondary electrons from the surfaceV of semi-conducting plate lghowever, secondary electrons released from point'29 are accelerated to a greater velocity than those released from point 30. By adjusting the potentials applied to decelerating system 19, those secondary electrons released from black picture points may be prevented from reaching electron multiplier 1S, and are returned toa collecting electrode 31. On the other hand, secondary electrons released from a white picture point, being of Vhigher velocity, penetrate the decelerating eld and enter electron multiplier 18.V The electron multiplier may be of conventional construction, and a load irnpedance (not shown) may be coupled to the output electrode ofthe electron multiplier to develop an output signal. y Instead of deriving the picture signals from the return beam by way of the multiplier 18, the tube can be operated by drawing the picture signals capacitatively from the metal lm 14 on the glass target, using this as a signal plate, though the capacity to earth via the photo-cathode may be rather high.

In the preferred embodiment, an aluminum pellicle 14 is held parallel and in very close proximity to a very thin glass plate 15. The face of the aluminumnearest the glass plate 15 is coated with a layer 16 of materialV having a high secondary emission coefficient.

The aluminum pellicle faces the photo-cathode, and

kthe photoelectrons, travelling athigh velocity, penetrate the aluminum film and are halted at the boundary of the high secondary emitting coating, thus releasing a copious supply of secondaries. These strike the glass plate,-which is about .0002 distant from the secondary emitting coating so that the secondaries do not spread appreciably in travelling to the glass'plate. Thus va negative charge appears on the glass target where a light pattern exists on the photo-cathode.

In another embodiment, illustrated in Figure2, a glass plate 40 is coated on one side with a cellular amorphous layer 41 of aluminum oxide by known methods, and an aluminum iilm 42 is produced on this oxide layer as for example by methods similar to those used in producing a metal backing for a uorescent screen. The term amorphous as used in connection with the aluminum oxide is used in its technical correct meaning of having Vno crystalline structure, and not in the sense of signifying 4 spect than the target structure used in the conventional image orthicon. Y Y Y vA target electrode constructed in accordance with the present invention has the following advantages as compared with the type of target electrode used in the conventional image orthicon pickup tube.

(1) The full photo-current is utilized and there is not a 50% loss due to the opaque grid wires in the conventional image oithicon target. (The Figure 2 embodiment does not have this advantage for the reason stated.)

(2) The mechanism of releasing secondary electrons by halting primary electrons in theV surface boundary is very eiicient and so high multiplication factors may be obtained. Y Y

(3) Since the glass target is in close proximity to the secondary emitting surface, a volt or two potential diierence serves to saturate secondary electron emission and so prevent the white afterj black eifects noticeable in.

the image orthicon at high light levels. Hence the contrast range is greater and there is lessrspurious shading due to redistribution of electrons overthe target at high lighting than in the image orthicon. 'Y

The main disadvantage of the present invention is that there is less automatic control of the glass target potential relative to the capacitative electrode, and it may prove possible to drive the target so negative that the scanning beam cannot reach it.Y However, this is not a serious drawback since, if such a condition arises, the fact that the glass target is driven negative with respect to the aluminum layer tends to bias off the secondary emission caused by the photoelectrons and so helps the scanning mechanism to resume operation efciently. Y

While particular embodiments of the present invention have been shown and described, it is apparent that Various changes and modifications may be made, and it is therefore contemplated in the appended claims to cover all such changes and modifications as fall within theftrue spirit and scope of the invention.

I claim:

1. A target electrode for a picture-converting device comprising: a thinl electroni-permeable aluminum lm; a thin semiconducting glass plate supported parallel and in close proximity to said lm; and a thin layer of insulating secondaryfelectron-emitting material spaced from said plate and amxed to the surface of saidV lm facing said plate. Y

2. A target comprising: a thin metallic film; a thin plate of semi-conducting material; and a thin layer of insulating secondaryelectron-emitting material atxed to said iilm and spaced from said plate, the spacing between said layer and said plate being of the order of .0002 inch. v

References Cited in the le of this patent electrode forV a picture-converting device(

Claims (1)

1. A TARGET ELECTRODE FOR A PICTURE-CONVERTING DEVICE COMPRISING: A THIN ELECTRON-PERMEABLE ALUMINUM FILM; A THIN SEMI-CONDUCTING GLASS PLATE SUPPORTED PARALLEL AND IN CLOSE PROXIMITY TO SAID FILM; AND A THIN LAYER OF INSULATING SECONDARY-ELECTRON-EMITTING MATERIAL SPACED FROM SAID PLATE AND AFFIXED TO THE SURFACE OF SAID FILM FACING SAID PLATE.

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