US3313977A - Image orthicon with improved beam modulation factor - Google Patents

Description

April 11, 1967 K. H. GEBEL IMAGE ORTHICON WITH IMPROVED BEAM MODULATION FACTOR Filed July 5, 1962 Few 2 me 7' S M m a a the second of the above factors.

United States Patent 3,313,977 IMAGE ORTHICON WITH IMPROVED BEAM MODULATION FACTOR Radams K. H. Gehel, Dayton, Ohio, assignor to the United States of America as represented by the Secretary of the Air Force Filed July 3, 1962, Ser. No. 207,441 3 Claims. (Cl. 315-1) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes without payment to me of any royalty thereon.

The modulation factor in an image orthicon, i.e., the ratio of the signal to the magnitude of the direct current component, is low, being of the order of 5 to percent. This greatly impairs the performance of the tube at low light levels. The reason for this is that the noise produced in the tube is approximately in proportion to the square root of the total current reaching the electron multiplier stage of the tube. Consequently, a low modulation factor, or, in other words, a relatively large direct current component, results in relatively high noise generation and a poor signal-to-noise ratio at low signal levels.

It has been experimentally determined that two factors are largely responsible for the low modulation factor. One is the wide spread of electron velocities in the scanning beam. At low light levels the information on the target plate of the tube consists of such a small charge that only the very fastest electrons reach the target and are effective as charge neutralizers. Since only the electrons reaching the target produce the beam modulation, the percentage modulation is low. A means for remedying this situation by limiting the electron velocities in the "scanning beam to a narrow range is described and claimed in my application S.N. 5,570, filed Ian. 29, 1960, now Patent No. 3,052,807. The other factor is the secondary emission produced at the edge of the beam aperture in the electrode that normally performs the dual function in the image orthicon of final grid in the electron gun and first dynode of the electron multiplier. These secondary electrons enter the electron multiplier where they increase the direct current component without increasing the signal, thus lowering the modulation factor.

It is the object of the invention to reduce the effect of Two methods are dis closed. Considered briefly, one method uses a separate dynode insulated from and at slightly higher potential than the 'last electrode of the gun. The separate dynode has a larger opening than the gun electrode and the wall of the opening receives the secondary electrons emitted at the edge of gun aperture. The other method provides an additional electrode surrounding the aperture and held at a slightly higher potential than the dynode to attract second electrons and prevent their entering the electron multiplier.

The invention will be described in more detail with reference to the accompanying drawings in which:

FIG. 1 illustrates the problem of secondary emission at the edge of the gun aperture in a conventional image orthicon,

FIG. 2 illustrates the separate dynode method of blocking secondary electrons from entering the electron multiplier; and,

FIG. 3 illustrates the use of a separate electrode for preventing secondary electrons from entering the electron multiplier.

Referring to the drawings, the electron gun and electron multiplier of a conventional image orthicon are 3,313,977 Patented Apr. 11, 1967 illustrated in FIG. 1. The construction and operation of this type of television pickup tube is well understood in theart and described in the literature, for example, in an article entitled The Image Orthicon by Rose, Weimer and Law appearing in the July 1946, issue of the Proceedings of the Institute of Radio Engineers. The electron gun comprises a thermionic cathode 1 having one end coated with a good emitter of thermal electrons 2 when heated by a heating winding 3 which receives current through leads 4. The cathode 1 is surrounded by a beam intensity control electrode 5 having an aperture 6 for the passage of electrons emitted by the cathode. An additional electrode 7, sometimes referred to as the 2nd grid of the electron gun, surrounds the electrode 5 and has an aperture 8 in alignment with aperture 6. Electrode 7 is made positive relative to the cathode so that the electrons are drawn away from the cathode to form a beam 9 which is controlled by adjusting the potential of electrode 5 which is normally negative relative to the cathode.

As is well known, the image orthicon has a target electrode (not shown) on which the picture information appears as a variation in positive charge. The beam 9, which is made up of electrons having just sufiicient velocity to reach the target, is caused to scan the target by appropriate beam deflecting apparatus (not shown). As the beam scans the target it neutralizes the positive charges, with the beam electrons in excess of those required to neutralize the charge at any point being returned toward the electron gun end of the tube as along path 10. The returning electron stream, which has been thus modulated in accordance with the picture information, strikes the face of electrode 7 which acts as the first dynode of an electron multiplier. The resulting secondary electrons travel to the second dynode 11, as along path 12, with further amplification by electron multiplication taking place in dynodes 13, 14 and 15. The final output current is collected by output electrode 16, which has connected to it a load impedance 17 and a coupling capacitor 18 for applying the output video signal to an external circuit.

It has been found that considerable secondary emission takes place at the outer edge of aperture 8 as the rc-v sult of primary electrons from the cathode striking electrode 7 at this edge. These secondary electrons have low velocities and are largely in directions transverse to the direction of beam 9. They therefore behave like the secondary electrons emitted from the face of elec trode 7 when this face is struck by electrons rejected by the target and, consequently, they travel to the second dynode 11 along with the secondary electrons resulting from the target return. This increases the total current in the electron multiplier, and therefore the noise generated, without increasing the signal and results in a reduoed signal-to-noise ratio.

FIGS. 2 and 3 illustrate two methods, in accordance with the invention, for overcoming the above problem. In FIG. 2, the solution is to use a first dynode 19 that is separate and electrically insulated from the 2nd grid 7 of the electron gun. The electrode 19 is maintained slightly positive relative to electrode 7 and has an aperture 20 large enough that primary electrons emerging from aperture 8 cannot strike the dynode. With this arrangement, the secondary electrons emitted at the edge of aperture 8 are attracted to and collected by the more positive wall surface of aperture 20, as shown, and are thus prevented from entering the electron multiplier.

The arrangement in FIG. 3 is similar in principle to that in FIG. 2 except that electrode 7 again performs the dual function of the 2d grid for the electron gun and first dynode [for the electron multiplier, as in FIG. 1,

with the secondary electron collecting rfun-ction being performed by an additional small annular electrode 21 surrounding aperture 8. The electrode 21 is insulated from and maintained slightly positive relative to electrode 7. Its inner diameter, as in the case of the diameter of aperture 20 in FIG. 2, is great enough that the electrode is out of the path of primary electron-s emerging from the electron gun through apertrue 8.

I claim:

1. In an image orthicon comprising an elongated electron gun having an accelerating electrode with an aperture concentric with the longitudinal axis of said gun for the emission of an axial beam of low velocity electrons and an electron multiplier having a plurality of cascaded circular dynodes and the second and succeeding dynodes of which surround said electron gun and are coaxial therewith and in which the first dynode is in the form of a fiat circular secondary emissive surface normal to and concentric with the axis of said gun and surrounding said aperture, means providing a conductive cylindrical surface of larger diameter than said aperture but of much smaller diameter than said first dynode, said cylindrical surface being placed close to said aperture so that electrons emerging 'from said aperture immediately enter the cylindrical space defined by said surface, the inner diameter of said cylindrical electrode being large enough that the surface be struck by electrons originating in said gun and means for maintaining said conductive cylindrical surface at a positive potential relative to said accelerating electrode.

2. In an image orthicon comprising an electron gun having an accelerating electrode with an aperture for the emission of a low velocity electron beam and an electron multiplier having a plurality of cascaded dynodes the second and succeeding dynodes of which surround said electron gun and are concentric with said aperture, a first dynode for said electron multiplier providing a secondary emissive surface located near said aperture and surrounding and normal to said beam, said first dynode having a central opening concentric with said aperture and pnoviding a cylindrical conductive wall adjacent to said aperture and concentric therewith, said opening having a diameter large enough that electrons originating in said gun cannot strike said first dynode, and means for maintaining said first dynode at a positive potential relative to said accelerating electrode.

3. In an image orthicon comprising an electron gun having an accelerating electrode with an aperture for the emission of a low velocity electron beam and an electron multiplier having a plurality of cascaded dynodes the second and succeeding dynodes of which surround said electron gun and are concentric with said aperture and in which said accelerating electrode has a flat circular surface surrounding said aperture and normal to said beam which serves as the first dynode of said electron multiplier, a cylindrical electrode having an inner diameter greater than said aperture and an outer diameter much less than the diameter of the flat surface of said accelerating electrode, said cylindrical electrode being concentric with said aperture and adjacent to but I electrically insulated from the flat surface of said accelerating electrode, the inner diameter of said cylindrical electrode being large enough that electrons originating in said gun cannot strike the cylindrical electrode and means for maintaining said clyindrical electrode at a positive potential relative to said accelerating electrode.

References Cited by the Examiner UNITED STATES PATENTS 2,747,133 5/1956 Weimer 3l365 X 3,197,665 7/1965 Cope 313-293 ELI LIEBERMAN, Primary Examiner. HERMAN KARL SAALBACH, Examiner.

S. CHATMON, 111., Assistant Examiner.

Claims (1)

1. IN AN IMAGE ORTHICON COMPRISING AN ELONGATED ELECTRON GUN HAVING AN ACCELERATING ELECTRODE WITH AN APERTURE CONCENTRIC WITH THE LONGITUDINAL AXIS OF SAID GUN FOR THE EMISSION OF AN AXIAL BEAM OF LOW VELOCITY ELECTRONS AND AN ELECTRON MULTIPLIER HAVING A PLURALITY OF CASCADED CIRCULAR DYNODES AND THE SECOND AND SUCCEEDING DYNODES OF WHICH SURROUND SAID ELECTRON GUN AND ARE COAXIAL THEREWITH AND IN WHICH THE FIRST DYNODE IS IN THE FORM OF A FLAT CIRCULAR SECONDARY EMISSIVE SURFACE NORMAL TO AND CONCENTRIC WITH THE AXIS OF SAID GUN AND SURROUNDING SAID APERTURE, MEANS PROVIDING A CONDUCTIVE CYLINDRICAL SURFACE OF LARGER DIAMETER THAN SAID APERTURE BUT OF MUCH SMALLER DIAMETER THAN SAID FIRST DYNODE, SAID CYLINDRICAL SURFACE BEING PLACED CLOSE TO SAID APERTURE SO THAT ELECTRONS EMERGING FROM SAID APERTURE IMMEDIATELY ENTER THE CYLINDRICAL SPACE DEFINED BY SAID SURFACE, THE INNER DIAMETER OF SAID CYLINDRICAL ELECTRODE BEING LARGE ENOUGH THAT THE SURFACE BE STRUCK BY ELECTRONS ORIGINATING IN SAID GUN AND MEANS FOR MAINTAINING SAID CONDUCTIVE CYLINDRICAL SURFACE AT A POSITIVE POTENTIAL RELATIVE TO SAID ACCELERATING ELECTRODE.

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