US2173257A - Cathode ray tube - Google Patents

Description

Sept. 19, 1939. o. KLEMPERER CATHODE RAY TUBE Filed July 50, 1938 SHINE- Ill SECOND ANODE POTENTIAL INVENTOR. OTTO KL EMPERER BY WCzn ATTORNEY.

Patented Sept. 19, 1939 UNHTE ST CATHODE RAY TUBE Otto Klemperer, Iver, England, assignor to Electric & Musical Industries Limited, Hayes, Middlesex, England, a company of Great Britain Application July 30, 1938, Serial No. 222,210 In Great Britain September 11, 1937 2 Claims.

My invention relates to electron discharge devices of the cathode ray type and more particularly to cathode ray tubes having a fluorescent screen for the reconstitution of images such as used in television receiving systems.

Cathode ray tubes for television reception are usually provided with an electron gun for generating a beam of electrons and a fluorescent screen provided on the wall of the tube opposite the electron gun which is scanned by the electron beam to produce fluorescent effects. It has been found in the use of such tubes that the intensity of the fluorescent effect is approximately proportional to the velocity with which the electrons l8 impinge upon the fluorescent screen and it is,

therefore, desirable to use a high velocity electron beam in such tubes especially when the tubes are used in conjunction with an optical lens for projecting the fluorescent image upon a distant viewing screen. Where such a high velocity beam is employed, it is found that the fluorescent screen charges up negatively since with a high velocity beam the ratio of the number of secondary electrons emitted by the screen to the incident primary electrons impinging on the screen becomes less than unity. It is, therefore, necessary for the proper operation of cathode ray tubes employing high velocity beams to maintain the fluorescent screen at a highly positive voltage 80 or to neutralize the negative charge which is produced upon the screen by the high velocity electrons. It has been proposed to employ a thin translucent conducting film deposited on the end wall of the tube and beneath the fluorescent 85 material, which film is maintained at the desired positive potential. Considerable difiiculties are encountered in producing a conducting film of this nature and even when such a film is properly applied, it is undesirable in that the light output of the device is reduced because of the non-transparent properties of the film.

It is the principal object of my invention to provide a cathode ray tube wherein a high velocity electron beam is incapable of negatively charging the fluorescent screen with respect to the surrounding electrode structure. It is a further object of my invention to provide such a tube in which the fluorescent screen is maintained at full operating positive potential.

In accordance with my invention, I provide a cathode ray tube employing a high velocity electron beam wherein a source of electrons of low velocity is provided which is positioned to direct a spray or substantially uniform flood of low velocity electrons upon the fluorescent screen to cause thereby the emission from the screen of a greater number of secondary electrons than incident low velocity electrons.

Further in acure 1.

Referring to Figure 1, a tube made in accordance with my invention comprises a highly evacuated glass envelope or bulb l enclosing at one end a conventional type electron gun and at the opposite end a fluorescent screen 2 which is preferably deposited directly upon the flattened end wall of the bulb I so that it may be viewed directly through the transparent end wall.

The electron gun assembly is of the conventional type and comprises a cathode 3 from which an electron stream may be drawn, a control electrode 5 connected to the usual biasing battery and a first anode 5 maintained positive with respect to the cathode 3 by a source of potential such as the battery 6.

The electron stream leaving the first anode 5 is highly accelerated and concentrated into an electron scanning beam focused on the front surface of the fluorescent screen 2 by a second anode I.

My invention is particularly useful where the final accelerating potential such as applied to the anode i by the battery 6 is equal to or greater than 10,000 volts. For the purpose of deflecting the electron beam generated by the cathode 3 over the fluorescent screen 2, I provide electromagnetic coils 8 which are perferably located between the end of the anode l and the screen 2.

It is obvious, however,

that conventional electrostatic deflection plates may be substituted for one or both of the deflection coils if desired.

The fluorescent screen 2 may be of any desired composition depending upon the desired color of fluorescence, and may be of calcium tungstate, zinc sulphide, zinc silicate, or any other desired The fluorescent screen may likewise be deposited on the end wall of the bulb l by settling, spraying,

material.

method.

dusting, or by any other desired In accordance with my invention, I provide adjacent the fluorescent screen 2 but removed from the path between the electron gun and the fluorescent screen, an auxiliary source of electrons which, as shown in Figure 1, may comprise a thermionically activated cathode 9 and an auxiliary anode I6 positioned between the cathode 9 and the fluorescent screen 2 and so arranged that the electrons liberated from the cathode 2 are directed upon the fluorescent screen as a substantially uniform spray of low velocity electrons. The auxiliary cathode 9 in combination with the anode it may be termed as an auxiliary electron gun. The cathode 9 is maintained at a low negative potential, such as 200 to 400 volts with respect to the potential of the second anode 1, preferably by a source of potential such as the battery II. The anode I9 is preferably maintained at the same positive potential as the anode l and causes the electrons liberated by the oathode 9 to be continuously sprayed upon the fluorescent screen 2 at low velocity over the whole of the fluorescent screen with a susbtantially uniform intenstiy.

Referring to Figure 2 which shows the secondary emission response of a conventional fluorescent screen with respect to second anode potential, it is seen that at low potential on the second anode which corresponds to low electron beam velocity from the cathode 3 that the number of secondary electrons liberated from the fluorescent screen with respect to the number of impinging electrons is less than unity but that as the voltage on the second anode is in-- creased, the curve rises to a point A where the number of secondary electrons liberated from the screen is equal to the number of impinging electrons. As the second anode voltage is increased still further the curve rises, indicating an increased ratio of secondary to primary electrons and as the second anode potential is still further increased the curve crosses the point B where the number of secondary electrons liberated is equivalent to the number of impinging electrons. A cathode ray tube having a fluorescent screen secondary emission characteristic, such as shown in Figure 2, will operate quite satisfactorily for anode voltages between the values represented by the points A and B but as the beam velocity is increased by increasing the anode potential beyond that represented by the point B, the fluorescence of the fluorescent screen does not materially increase because the fluorescent screen remains at the approximate positive potential of the point B. If, however, the screen is bombarded by auxiliary electrons having a velocity corresponding to apotential between those potentials represented by the points A and B, additional secondary electrons will be liberated from the fluorescent screen thereby increasing its potential in a positive direction notwithstanding the presence of the high velocity beam such as from the cathode 3 shown in Figure 1. It is desirable to maintain the velocity of the electrons from the cathode 9 as low as is possible commensurate with satisfactory operation to limit the fluorescence generated on thefiuorescent screen by these low velocity electrons. Thus, if the electronszfrom the cathode 9 are accelerated such as by potential of 200 volts on the auxiliary anode NJ with respect to the cathode 9 and the potential existing between the cathode 3 and the secondanode 1 is 20,000 volts, .the low velocity electrons .will produce approximately of the light intensity produced by the high velocity beam. This low value of background light produced by the low velocity electrons is not overly objectionable and becomes even less objectionable with further increase of second anode potential.

Although my invention has been described above as being applied to a cathode ray tube suitable for use in television receiving systems, it is to be understood that the invention may be applied to other forms of electron discharge devices employing fiuorescent screens such as electron telescopes, light transformers, and the like, without departing from the scope of the invention as set forth in the appended claims.

I claim:

1. A cathode ray tube and system comprising a tube having an electrically nonconducting screen base, an electrically nonconducting fluorescent screen on said base having a secondary electron emission ratio less than unity when impinged by electrons having a high electron beam velocity and a secondary emission ratio greater than unity at a lower beam velocity and an electron gun oppositely disposed from and wholly exposed to said screen, said gun comprising a cathode for generating an electron beam and an anode to focus said beam on said screen, a source of direct current potential connected between said cathode and said anode to apply a dirlcrence of potential sufiicient to accelerate the electron beam to a velocity in excess of the electron beam velocity which produces a greater than unity sec ondary electron emission from said screen, means to scan said electron beam over said screen, an auxiliary cathode adjacent and directly exposed to said fluorescent screen but outside the path of said electron stream from said electron gun, an auxiliary anode between said auxiliary cathode and said screen to direct a stream of electrons to said screen along a path outside the path of said beam, a source of direct current potential less than the potential between cathode and said anode connected between said auxiliary cathode and said auxiliary anode, said source being suiiicient to produce electron velocities at which the secondary electron emission is greater than unity from said screen but insufficient to excite the screen to a luminescence more than-a small fraction of the luminescence produced-by the said electron beam.

2. Aacathode ray tube and system comprising a tube having an electrically nonconducting base, anelectrically nonconducting fluorescent screen on said basehaving a secondary electron emission ratio less than unity at electron beam velocities greater than 10,000 volts and a secondary electron emission greater than unity at a beam velocity lower than 10,000 volts, and an electron gun oppositely disposed from and wholly exposed to'said screen, saidgun comprising a cathode for generating an electron beam and an anode to focus said-beam on said screen, a source of direct current potential exceeding 10,000 volts connected betweenisaidcathode and said anode to apply a difierenceof potential exceeding 10,000 volts and accelerate said electron beam to a velocity in excess of 10,000 volts, means to scan said electron beamover the surface of said screen, an auxiliary cathode adjacent and directly exposed to said fluorescent screen but outside the direct path between isaid electron gun and said screen, an auxiliary anode between said auxiliary cathode and said screen to direct a stream of electrons on the surface of said fluorescent screen along apath outside the; path of said beam, and a source of direct current potential of considerably less than 10,000 volts connected between said auxiliary cathode and said auxiliary anode to accelerate the electrons liberated by said auxiliary cathode and direct said electrons upon said screen at a velocity of bombardment under which the said screen has a secondary emission ratio greater than unity and a luminescence only a fraction of the luminescence produced by the electrons from said first-mentioned cathode.

OTTO 'KLEMPERER.

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