US2223908A - Cathode ray tube - Google Patents

Description

Dec. 3, 1940. c.- s. BULL 2,223,908

GATHODE RAY TUBE Filed Aug. 19, 1958 INVENTOR. (4807' SEATON BULL ATTORNEY.

Patented Dec. 3, 1940 UNITED STATES PATENT OFFICE to Electric & Musical Industries Limited, Hayes,

Middlesex, England, a company of Great Britain Application August 19,

1938, Serial No. 225,679%

In Great Britain August 21, 1937 6 Claims.

This invention relates to cathode ray tubes such as are employed either for reconstitution of images in television reception or tubes that are employed in television transmission systems.

In cathode ray tubes used for the reconstitution of images in television reception it is usual to employ a circular cathode, sometimes of relatively large area which generates a beam of electrons of cylindrical or conical cross-section, the beam 10 being focussed to a point or circular spot, which is usually an image of a point called a cross over in front of the cathode, by the provision of spherical electron lenses.

It is the object of the present invention to provide an improved construction of cathode ray tubes possessing advantages over known types of cathode ray tubes particularly as regards sensitivity.

According to one feature of the invention a method of focussing an electron beam into a small spot is provided wherein electrons emanating from a cathode are constrained by a cylindrical electron lens into a sheet-like form, the sheet-like beam of electrons being subsequently acted upon by a further electron lens which focusses the sheet-like beam into a small spot.

According to another feature of the invention,

a cathode ray tube is provided comprising a cathode for generating a beam of electrons, means adjacent to said cathode and serving, when energised, to form a substantially cylindrical electron lens ior constraining said beam of electrons into a sheet-like form and a further electron lens disposed between said electrode system and a 85 screen upon which said sheet-like beam is focussed into a small spot. The said cylindrical lens will, in most cases, be of the electrostatic type but may in some cases be of the magnetic type. The further lensmay be a cylindrical lens 4 arranged substantially at right angles to the first-mentioned cylindrical lens or, alternatively, it may be a spherical lens produced either magnetically or electrostatically.

Preferably, the cathode comprises a straight rod-like element either in the form of a straight directly heated filament or of the indirectly heated type and the emission from the cathode may be controlled by the provision of a control electrode in the form of a pair of rods mounted one on each side of the cathode towhich suitable bias potentials or signals for controlling the electron emission for modulating the electron beam can be applied. The first cylindrical lens may be formed by the use of an open-ended electrode enclosing said cathode and control electrodes, said enclosing electrode being of elongated form. In 1 another form of the invention the control electrode may comprise an electrode surrounding the cathode and having an elongated opening through which the electrons from the straight cathode pass, the cylindrical lens being formed between the control electrode and an adjacent electrode.

In order that the said invention may be clearly understood and readily carried into efiect it will now be more fully described with reference to the, accompanying diagrammatic drawing in which:

Figure 1 is a perspective view of a. cathode ray tube and electrode structure embodying the invention,

Figure 2 is a cross-sectional view taken along the line 2--2 of Figure l,

Figure 3 is a perspective view of the electrode system of a cathode ray tube embodying a modi fied form of the invention,

Figure 4 is a perspective view of theelectrode system of a cathode ray tube embodying a further form of the invention.

In the form of the invention shown in Figures 1 and 2, the cathode ray tube comprises a cathode 8 which is in the form of a rod-like member either of the directly heated or indirectly heated type and arranged on each side of the cathode and in the same plane thereto is a pair of rods 9 which serves as the control electrode. The cathode and control electrodes are arranged at one end of an enclosing electrode IU of box-like form and of a rectangular form in cross-section, the end of the electrode I0 remote from the end in which the cathode and control electrodes are arranged being open to permit of the passage of electrons. The electrode I0 is in effect a first anode and consists of two portions, a principal portion of rectangular cross section and a minor portion likewise of rectangular cross section, with their major axes of cross section parallel and parallel with the longitudinal axis of the cathode 8. The electrode Ill, and the control electrode 9,

function as a cylindrical lens for producing a sheet-like beam of electrons of a parallel or convergent form the cathode 8 being parallel with the axis of the lens. The electron emission from the cathode 8 due to the disposition of the control electrode 9 emerges in a divergent sheet-like form, the electrode 10 then serving to converge the beam in a direction normal to the length of the cathode 8. Preferably the diameter of the cathode is small compared with its distance from either rod 9, and the length of the rods 9, long compared with their diameters, whilst the length of the electrode It] should be long compared with the distance between the sides thereof. In Figs. 1 and 2 the second lens is of the spherical type and comprises a cylindrical elongated electrode II and a further cylindrical electrode l2 which may constitute the second anode of the usual form of cathode ray tube. Both the cylindrical lens and the spherical lens formed by the electrodes II and [2 are of the electrostatic type. It will be seen from Fig. 1 that the electrode H) extends along the whole length of the cathode 9 and the open end of the electrode I0 is recessed so as to fit into the interior of the cylindrical electrode H. Adjacent to the end of the electrode l0 which projects into the electrode N there is provided a diaphragm [3 having a rectangular aperture M the width of which is slightly larger than the distance between the sides of the electrode I0, the purpose of this diaphragm l3 beingto intercept the upper and lower portions of the sheet-like beam, these portions containing electrons which are not travelling at right'angles to the cathode on account of the change in the cylindrical field at the ends of the electrodes 8 and 9 and would otherwise constitute aberrations. The diaphragm is is provided in a position which is substantially free from focussing fields. The electron beam, after passing through the aperture I4 isacted upon by the spherical lens generated between the electrodes l I and I2 which serves to focus the sheet-like beam of electrons travelling in the direction at right angles to the cathode into a small spot upon a screen or target it. The screen or target it may be of any suitable form, such as a screen adapted to be rendered luminous under the impact of electrons, or it may comprise a screen of the mosaic type or any other screen, such as'may be used for the generation of picture signals for television transmission purposes or any other form of screen as may befound desirable. The electrode structure is disposed in an evacuated envelope, as shown in Figurefl. Wherea cathode ray tube in accordance with the invention is designed for use in the reconstitution of television signals, the signals being employed to modulate the beam of electrons, suchsignals may be'applied to the control electrode 0 and may serve to vary the potential of the electrode 9 from a low positive value to a negative potential. The electrodes l0 and which are in contact with one another may have a positive potential of about 300-400 volts, whilst the electrode !2 may have a positive potential of about 1000 volts orrnore. The quantity of electrons emanating from the cathode 8 is varied by the signals applied to the rods 9. They are then accelerated by the increasing potentials applied to the electrodes 50, II and I2 and focussed into a small spot on the screen of 'the tube. The spot is then deflected over the surface of the screen to reconstitute the image, such deflection being accomplished in known manner by the provision of suitable defleeting plates 'or deflecting coils. It will, of course, be understood that the two lenses i. e., the cylindrical and spherical lenses should er.- fectively be spaced sufiiciently far apart so as to avoid interaction betweenthe fields produced by each lens.

In the embodiment of the invention shown in Fig. 3 the first cylindrical lens is formed between the control electrode and an adjacent electrode, but instead of the control electrode being in the foim of rodsfas shown in Figures 1 and2 it"is in the form of a box-like structure l5 enclosing the cathode 8 which, in this example, is of the indirectly heated type and of rectangular form in cross-section, the control electrode [5 having an elongated opening l6 extending along the length of the cathode 8 and opposite the emissive area thereof. Adjacent to the control electrode I5 is an open-ended box-like electrode ll, the first cylindrical lens being formed between the electrodes l5 and I1. The ends of the electrode H remote from the cathode are reduced in size so as to fit into the interior of a cylindrical electrode l I which together with a further cylindrical electrode l2 constitutes a spherical lens as in the arrangement shown in Figs. 1 and 2. A diaphragm I3 is also provided similar to the arrangement shown in Figures 1 and 2. The operation of the embodiment shown in Fig. 3 is similar to that described in connection with Figures 1 and 2, the main difierences between the two arrangements being the construction of the control electrode.

Figure 4 of the drawing illustrates a construction in which two cylindrical lenses are provided, the first cylindrical lens functioning in the manner described in connection with the preceding figures whilst the second cylindrical lens is arranged with its axis at right angles to the first lens and replaces the spherical lens formed by the electrodes II and I2. The cathode and control electrodes are of the form shown in Figures 1 and 2 comprising a cathode 8 disposed between a pair of rods 9. Adjacent to the cathode and control electrodes is an open-ended box-like electrode [9. As in the case of the arrangement shown in Figs. 1 and 2, the field between the electrodes 8, 9 and I9, serve to focus the beam into a parallel beam of electrons travelling at right angles to the cathode. Joined to the electrode l9 by an intermediate section 20 is a further box-like electrode 2! and spaced from the electrode 21 is a further box-like electrode 22. The electrodesZl and 22 constitute a further cylindrical lens the axis of which isarranged at right angles to the cylindrical lens formed by the electrodes 8, 9 and IS. The sheet-like beam of electrons thus produced by theelectrodes 8, 9, or N is then acted upon by the cylindrical lens formed by the electrodes 2| and 22 which accelerate the electrons and focus them into a small spot upon the screen.

In operation, the'electrodes I9 and 2! may be maintained at a positive potential of about 300 volts, and the electrode 22 at a positive potential of 1000 volts.

' In known typesof cathode ray tubes the oathode is of the indirectly heated type and is in the form of a circular electron emissive cap enclosing 'a heater. In the present invention the cathode may take the form of a straight filamentor a rod-like member and thus conform'substantially to the type of cathode employed in thermionic valves. These cathodes are more easily manufactured than the cap-like indirectly heated cathodes. With the construction according to the invention the mutual conductance may be at least of the same order'or even considerably higher than that of existing cathode ray tubes. The width of the'beam in the initial cylindrical lens'may be made relatively small without loss in mutual conductance or a decrease in thermionic emission. It is found also that the Width of the beam increases as the control electrode 9 in Figs. 1, 2, and l'is made more and more positive, but decreases as the potential of the accelerating electrode l0,'in Figs. 1 and 2, or IS in Fig. 4, is

made more and more positive. The second lens serves to focus relatively high velocity electrons and any correction for aberration which may be required due to the width of the electron beam when passing through such a lens may be accomplished by known methods. With the construction in accordance with the invention the power consumed by the cathode may be considerably reduced and if a directly heated filament is employed it may be heated by the high tension current employed for operating, for example, the receiving apparatus without the need for a separate source of cathode current. Alternatively, the cathode may be supplied with current intermittently during the black out periods of the cathode ray tube.

The particular construction of cathode and control electrodes described enables the input capacity to be made very small and reduced to such an extent that high frequency signals may be applied directly to the control electrodes without intermediate rectification by a diode. The bias is then chosen so that the grid voltage just swings to cut off when the carrier Wave of, for example, television signals has the amplitude corresponding to black.

The present invention is of course not limited in its application to cathode ray tubes for use in television receiving or transmitting systems, but also cathode ray tubes employed for other purposes.

I claim:

l. A cathode ray tube comprising an evacuated envelope enclosing an electrode structure including an elongated cathode to develop an electron stream of rectangular cross section, a target oppositely disposed and substantially parallel to the longitudinal axis of said cathode to receive electrons from said cathode, control means adjacent the cathode, a boxlike anode between said cathode and said target having a'portion of rectangular cross section and of extended length to generate with said control means a cylindrical focusing field to focus and converge the electron stream from said cathode in a direction normal to the length of said cathode and transverse to the minimum cross-sectional dimension of said target, an integral extension of rectangular cross section mechanically supported by and electrically connected to said anode, and means between said anode and said target for focusing the electron stream from said cathode in a direction normal to the first direction of focus and transverse to the maximum cross-sectional dimension of said stream to focus said stream on said target.

2. A cathode ray tube comprising an envelope enclosing an electrode structure comprising an elongated electron emissive cathode to generate an electron stream, a target oppositely disposed from said cathode, a control electrode adjacent said cathode, an elongated boxlike anode having a rectangular cross section normal to the axis between said cathode and said target, an extension on said anode of rectangular cross section, said anode and said extension being adapted to be maintained at a positive potential with respect to said cathode for converging the stream of electrons from said cathode, an electron limiting disk adjacent said extension having an elongated aperture, the longer dimension of said aperture being normal to the length of said cathode, and means between said anode and said target including two cylindrical members one of which overlaps the extension on said anode for focusing the electrons from said cathode which pass through said limiting disk on said target.

3. A cathode ray tube as claimed in claim 2 wherein the one of said cylindrical members which overlaps the extension on said anode is electrically connected to and mechanically supported by said anode.

4. A cathode ray tube comprising an evacuated envelope, an elongated cathode near one end of said envelope to generate a sheetlike beam of electrons, a target oppositely disposed from said cathode to receive electrons therefrom, a control electrode adjacent said cathode 'to control -the number of electrons flowing to said target,

an elongated boxlike anode having a principal section of rectangular cross section and having an extension of rectangular cross section, the major axis of which is perpendicular to the major axis of the principal section of said anode, and a second anode between said first-mentioned anode and said target to focus the electrons from said cathode passing through said anode on said target.

5. A cathode ray tube having an evacuated envelope and an electron gun comprising an electron emitting cathode, electrode means to control the electron emission from said cathode, an elongated anode having a principal portion of rectangular cross section and a minor portion of rectangular cross section, the major axis of said minor portion being substantially at right angles to the major axis of said principal portion, and a second anode more remote from said cathode than said first-mentioned anode, and a target oppositely disposed from said cathode and in the path of electrons issuing from said cathode.

6. A cathode ray tube as claimed in claim 5 wherein the said second anode is of rectangular cross section having its major axis parallel with the major axis of the minor portion of said firstmentioned anode.

CABO'I SEATON BULL.

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