US2880338A - Television pick-up tube - Google Patents

Description

, Match 31, 1959 R. THEILEH 3 1 TELEVISION PICK-UP TUBE Filed Oct. 15. 1954 F/G/ v Inventor Richard Tr. e.

mMMMM Attorney Unite TELEVISION PICK-UP TUBE Richard Theile, Marburg an der Lahn, Germany, assignor to Pye Limited, Cambridge, England, a British company The present invention relates to television pick-up tubes (including monoscopes) of the kind in which a fine electron beam scans a picture area on a target in such man- 'zner that the beam electrons strike the target at a low 'velocity of between zero and a few electron volts, that is at less than the velocity at which secondary electrons ;are emitted in a ratio of 1:1. In such tubes the target is then carried negative until it reaches the cathode potential when it acts as an electron mirror and repels the electrons in the beam so that they are returned down the tube. Tubes employing such low velocity electrons are :also known as cathode potential stabilized tubes, in contradistinction to anode potential stabilized" tubes, in which the beam lands with a high velocity, thus causing more secondary electrons to be emitted than the electrons arriving in the clean current, so that the target will go positive until an equilibrium state is achieved with the target at the anode potential, when there is no accelerati'ng field to attract secondary electrons away from the target.

Scanning with low velocity electrons, such as it is employed in the so-called orthicon, image-orthicon, and

Vidicon tubes, offers the great advantage of avoiding any appreciable secondary emission from the target area and 'thus the generation so-called noise signals by stray electrons. On the other hand this mode of scanning intro- 'duces a certain difiiculty The present invention consists in a television pick-up tube of the kind above referred to, wherein the scanning beam is imaged on tothe target by electron-optical means producing a reduction in the cross-section of the beam between the cathode ray generating system (cathode ray 1 gun) and the target.

The electron optical means preferably comprises a long coil which surrounds the tube and extends from the cathode ray gun to the target, the ampere turns per unit of length of the coil, being such as to produce an imaging ratio less than lzl, and the arrangement being such that in the immediate vicinity of the target the lines of the field extend parallel to the axis ofthe tube.

Inorder that the invention may be more clearly understood, reference will now be made to the accompanying drawing, in which:

Fig. 1 illustrates the known electron-optical system for i the beam, and

Fig. 2 illustrates an electron-optical system according 1 to the invention.

as regards the attainable degree :of resolution, 1. e. of fineness (spot diameter) of the scanning electron beam.

rates, Patent H desired reduction of the producedvimage.

2,880,338 Patented Mar. 31, 1959 diameter of the emitting area. Although obviously in practice the loading of the cathode is increased to the permissible maximum, the definition hitherto obtainable with picture-scanning tubes in which scanning is efiected by low velocity electrons does not fully satisfy the high requirements of modern television, especially when a great number of lines are employed. This limitation is particularly noticeable in tubes which work with scan ning beams of relatively high intensity in the order of '1 microamp. (i.e. in tubes such as the orthicon, the CPS, Emitron, or the Vidicon) while being less critical in tubes in which the signal is derived through the medium of a built-in electron multiplier and in which considerably smaller beam currents can be used (smaller emitting area of the cathode).

Tubes of simple construction and having no multiplier are gaining increasing importance, more particularly in connection with the employment of semi-conductive photo layers as the storage target (Vidicon); in these tubes the lack of definition constitutes a very noticeable drawback, more particularly in the development of small tubes for industrial purposes, since in these the definition becomes poorer corresponding to the reduction of-thc itarget size. The reason is that with an imaging ratio of 1:1 resulting from the use of. the customary long focussing coil, the spot diameter of the scanning beam has a. constant absolute magnitude, while the size of the raster points of the storage area becomes smaller. The present invention has for its object to provide means for counteracting this drawback. I

According to the present invention an electron-optical device for the concentration of the scanning beam, when employing scanning with low velocity electrons, is constructed in such manner that the hitherto usual imaging ratio of about 1:1 between the beam size in the plane of the target and in the plane of the cathode ray gun, is reduced considerably, for example to a ratio of 1:2. Due to this feature it is possible to produce a scanning spot of very considerably smaller cross section and thus obtain a very greatly increased picture definition than has hitherto been possible, the definition being now ultimately limited only by the mutual repulsion of the electrons in the beam. This repulsion, however, plays no decisive role in the range of the current densities normally em- [ployed.

One embodiment of the invention will now be described with reference to the example illustrated in Fig. 2 which, similarly to Fig. 1, shows a television pick-up tube in which scanning is effected by low velocity electrons and in which the scanning beam is. concentrated with the help of a long coil. The tube is equipped with a cathode ray gun S and a target B on to, which an image of the :picture to be transmitted is projected by means of the lens 0. In order to carry the invention into efiect, the

adjacent the target than in the middle portion and the portion adjacent the cathode ray gun. This is achieved by providing the coil F, with a greater number of turns per unit length than the coil F This results in the v The dimensions of the coils are so chosen that in front of the target the field lines still extend parallel to the axis of the tube in order to ensure the required normal incidence of the scanning beam throughout the image field.

The deflection of the electron beam is obtained in the usual manner by two pairs of deflection coils A which, for example, may be arranged in the region in which the field intensity is relatively small. The deflection power required will be greater when employing image reduction than for equal size projectione-assuming the size of the scanning .areaiisthesamefibut this will not lead to any difficulties in practice in view of the high state of development of the art of sweep devices.

In a modifiediembodiment the-long coil may be subdivided into a number of coils, currentsof different 'intensities being fed to the individual coils to produce the stronger field adjacent the target.

In addition to the improvement of the picture definition, the invention ofiers still further advantages. Since the invention permits the diameter of the scanning beam to be reduced, the current density per picture element will be increased, thereby ensuring a more complete discharge of the picture elements in the storage area. This in turn noticeably decreases the 'remanent picture (so-called trailing or sticking eflect) which occurs with some kinds of storage-type pick-up tubes (e.g. with photo-conductive targets) and which is largely due to the fact that a single scan only produces an incomplete discharge .of the stored image.

I claim:

1. A television pick-up tube in which the target is scanned with low velocity electrons, comprising an evacuated tube envelope containing a cathode ray gun adjacentone end of the tube'envelope and a target adjacent the other end of said tube envelope and facing the cathode ray gun, said target comprising a-transparent signal .plate covered with a layer of photo-conductive material facing said gun and adapted to be illuminated by light :fallirig thereon through said signal plate and a transparent portion of the adjacent end wall of the envelope, magnetic beam deflecting means around said envelope the magnetic field produced by the focussing coils in the vicinity of the target extending substantially normal to the surface of the target.

3. A television pick-up tube in which the target is scanned by low velocity electrons, comprising an evacuated tube envelope containing a cathode ray gun adjacent one end of the tube envelope and a target adjacent the other end of said tube envelope and facing the cathode ray gun, said target comprising a transparent signal plate covered with a layer of photo-conductive material facing said gun and adapted to be illuminated by light falling thereon through said signal plate and a transparent portion of the adjacent end wall of the envelope, magnetic beam deflecting means around said tube envelope in the vicinity of the gun, and beam focussing means comprising at least two cylindrical electric coils arranged endto-end and surrounding the beam deflecting means and the tube envelope, the overall length of the coils extend- 1 ing from the region of the cathode ray gun to the region in-the vicinity of the gun and beam 'focussing means for concentrating and focussing the electron beam from the cathode ray gun onto said target, said beam focussing means comprising at least one electric coil surrounding the beam deflecting-means and the tube envelope, the ,overall length of said at least onecoil extending from the region of the cathode ray gun to the region of the target, and means 'for energising said at least one coil to produce a non-uniform focussing field with a field strength greatest in the vicinity of the target and producing a de-magnification of the beam diameter where it strikes thetarget to at least approximately one half of the gun diameter, the lines of the focussing field in the :immediate vicinity of the target extending normal to the surface of the target.

2. A television pick-up tube in which the target is scanned by low velocity electrons, comprising an evacuated tube envelope containing a cathode ray gun adjacent one end of the tube envelope and a target adjacent the other end of said tube envelope and facingthe cathode ray gun, magnetic beam deflecting means around said tlibelenvelope in the vicinity of the gun, beam tocussing means comprising at least two electric coils arranged endvto en'd and surrounding the beam deflecting means and the tube envelope, the overall length of the coils extending from the region or" the cathoderay gun to the region of the target, means energising the coil in the vicinity of the gun to concentrate the electrons emitted from the gun into a beam, and means energising the coil -in the vicinity of the target to produce a stronger magnetic field in the vicinity of the target further to concentrate the beam to produce a demagnification of the 'beam diameter where :it strikes the target to at least approximately one half the gun diameter, the lines of o f'the target, the coil in the vicinity of the target having a,-greater number of turns per unit of length than the coil in the vicinity of the cathode ray gun, means energisingthe coil in the vicinity of the gun to concentrate the electrons emitted from the gun into a beam, and means energising the coil in the vicinity ofthe targetto produce a stronger magnetic field in the vicinity of the target further to concentrate the beam to produce a de-magnification of the beam diameter where it strikes the target to at leastapproximately one half the .gun diameter, the lines of ,the magnetic field produced by the focussing coils in the vicinity of the target extending substantially normal to the surface of the target.

4. A television pick-up tube in which the target is scanned by low velocity electrons, comprising an evacuated tube envelope containing a cathode ray gun adjacent one end of the tube envelope and a target adjacent "the other end of said tube envelope and facing thecathode ray gun magnetic beam deflecting means around said tube envelope in the vicinity of the gun,-and beam focussingmeans comprising at least two electric coils arranged endto-end and surrounding the beam deflecting means and the tube envelope, the overall length of the coils extending from the region of the cathode ray gun to the region ,of the target, the coil in the vicinity of the target having a greater number of turns per unit of length-than the coil in the vicinity of the cathode ray gun, means energising the coil in the vicinity of the gun to concentrate the electrons emitted from the gun into a beam, and means energising the coil in the vicinity of the target to produce a stronger magnetic field in the vicinity of the target further to concentrate the beam to produce a demagnification of the beam diameter where it strikes the target ,to at least app oximately one half the vgun diameter, thelines of the magnetic field produced, by the focnssing coils in the vicinity of the target extending subtaatia orm t th s r a 9 the g "References Cited :in the file-of this patent UNITED STATES PATENTS

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