Apparatus having a television camera tube and a television camera tube for use in such an apparatus

Abstract

 A television camera tube which has an anti-comet-tail electron gun. In such tube a target plate (11) is line scanned by a focused electron beam which is at a cathode voltage of typically zero volts to stabilise the scanned area of the target plate (11) at that voltage and in so doing producing an output signal. During line flyback, the cathode voltage is increased to 5 V and the beam current is increased so that the target plate (11) is stabilised at that voltage. By this means the effects of moving high intensity lights can be mitigated.
However if the electron beam is focused during flyback then intense damage can be done to the photo-conductive layer of the target plate. A television camera tube is described in which the electron gun (14) is a multi-spot triode electron gun comprising a cathode (15), a multi-apertured control grid (16) and a multi-apertured accelerating anode (17). During line scanning a single low current electron beam is produced from the centre of the cathode (15) and during line flyback a plurality of low current electron beams are produced to form a large spot which does not damage by ion bombardment the photo-­conductive layer of the target plate (11).

Description

[0001] The present invention relates to an apparatus having a television camera tube and a television camera tube for use in such an apparatus.

[0002] More particularly the present invention is concerned with eliminating what is known as the "comet tail effect" which occurs when the electron beam is not sufficiently intense during the line sweep period to recharge or stabilise an anode target of the tube after a large discharge of the anode target has taken place due to a large intensity light source having been focused onto the camera picture tube.

[0003] United States of America Patent Specifications 3.548.250 (PHN 3036) and 3.883.773 (PHN 4217) disclose overcoming the comet tail effect by providing a lens element in electron gun of a television camera tube. More particularly this lens element is disposed about the longitudinal axis of the tube and is located between the accelerating anode and a diaphragm mounted at a second cylindrical anode. During a normal television line scan the electrons emitted by the cathode which is at 0 volts are formed into a cross-over close to the accelerating anode. The electron current between the cathode and the diaphragm is mostly collected by the diaphragm. Only the central part of the electron beam which passes through the aperture in the diaphragm is focused subsequently into a small spot for line scanning of the target which is stabilised substantially to cathode potential.

[0004] During line fly-back periods the cathode voltage is increased to 5V and the voltage applied to the lens element is made negative with the result that the cross-over is refocused at the opening in the diaphragm and a large current is available for stabilization of the photo-conductive layer to 5 volts.

[0005] Whilst the type of camera tube disclosed in these patent specifications is effective in eliminating comet tails it has some disadvantages in that during line scanning a great deal of the cathode current is lost to the diaphragm and in that during line flyback the impingement of the large current electron beam upon the target is intense which will lead to damaging of the photo-conductive layer and in consequence a reduced working life for the television camera tube.

[0006] It is an object of the present invention to mitigate these disadvantages in a television camera tube.

[0007] According to one aspect of the present invention there is provided an apparatus having a television camera tube comprising an envelope within which are provided a target for producing a potential image corresponding to a scene and an electron gun for producing an electron beam directed to the target, the electron gun comprising a cathode, a control grid having an inner aperture surrounded by a plurality of outer apertures, and an anode having an inner aperture of smaller cross-sectional area than that in the control grid surrounded by the same number of outer apertures as are provided in the control grid, means for focusing the electron beam onto the target and deflection means with the aid of which a periodic scanning of the target by the electron beam is obtained for stabilization of the potential of the target to cathode potential for supplying output signals which correspond to the potential image, the deflection means comprising a deflection member for line sweeps and line fly-backs and a deflection member for frame sweeps and frame fly-backs, means for applying during line sweeps a first voltage to the cathode and a second voltage to the control grid so that the electron beam comprises electrons drawn through the inner aperture of the control grid and means for applying during line fly-backs a third voltage to the cathode and a fourth voltage to the control grid so that electrons are also drawn from the cathode through the outer apertures of the control grid to provide a plurality of low current beams which serve to stabilise the target to the third voltage.

[0008] By providing a multi-apertured control grid and a multi-­apertured anode then during line scanning electrons are drawn from a small central area of the cathode so that a relatively small current is available for line scanning. Although the central portion of the anode functions as a diaphragm and intercepts some of the cathode current it is considerably less than in the known television camera tubes with an anti-comet-tail electron gun. The outer apertures in the control grid and anode enable a large electron current in the form of a plurality of low current beams to be produced from the cathode to form a large spot at the screen thus enabling the required stabilisation to be achieved without damaging the photo-conductive layer of the target. The electron current during flyback is much greater than that produced during line scanning. This is made possible by the total cross-sectional area of the outer apertures in the control grid being greater than that of the inner aperture in the control grid.

[0009] If desired the cross sectional shape of the outer apertures in the control grid is different from that of the inner aperture. Additionally the inner apertures of the control grid and the anode may be disposed substantially co-axially of the longitudinal axis of the envelope. Further, the outer apertures in the anode may be of a different size to those in the control grid and/or the outer apertures in the anode may be offset radially with respect to the outer apertures in the control grid.

[0010] The focusing means may be electrostatic or electromagnetic.

[0011] According to another aspect of the present invention there is provided a television camera tube comprising an envelope within which are provided a target and an electron gun for producing an electron beam directed to the target, the electron gun comprising a cathode, a control grid having an inner aperture surrounded by a plurality of outer apertures and an anode having an inner aperture of smaller cross-sectional area than that in the control grid surrounded by a corresponding plurality of outer apertures as are provided in the control grid.

[0012] The present invention will now be described, by way of example, with reference to the accompanying drawings, characterised in that:

Figure 1 is a schematic diagram of an apparatus made in accordance with the present invention having an electrostatically focused television camera tube,

Figure 2 is schematic diagram of a television camera tube having electromagnetic focusing,

Figure 3 is a computer trajectory plot of an anti-comet-­tail electron gun during a line scanning mode, the abscissa represents measurements in millimetres from the cathode along the Z-axis and the ordinate the radius R in micrometres,

Figure 4 is a computer trajectory plot of an anti-comet-­tail electron gun during a flyback mode,

Figure 5 shows graphs of field strength in volts/metre x 10⁵ against radius (R) in microns for the line scanning mode (continuous line) and flyback mode (chain-dot line),

Figure 6 is a diagrammatic elevational view of one embodiment of a control grid for use in an anti-comet-tail electron gun, and

Figure 7 is a diagrammatic elevational view of a second embodiment of a control grid for use in an anti-comet-tail electron gun and in broken lines three different sizes and/or radial positions of the apertures in the anode are shown.

[0013] In the drawings the same reference numerals have been used to indicate the corresponding features.

[0014] The camera tube shown in Figure 1 comprises an evacuated cylindrical envelope 10 of glass. The tube comprises a target plate 11 which consists of a layer of substantially lead monoxide which is vapour deposited on a signal plate 12 which consists of a very thin layer of a readily conducting tin oxide provided on the inside of a window 13 formed by one end of the envelope 10. Near the other end of the interior of the envelope 10 is situated a rotationally-symmetric electron gun 14 which is coaxial with the envelope 10 and consists of a cathode 15, a multi-apertured control grid 16 and a multi-apertured acceleration anode 17. Between the acceleration anode 17 and the target plate 11 are a second cylindrical anode 18 and an electrically conductive mesh 20 provided on a cylindrical electrode 19. A focusing electrode 23 which, in cooperation with the second cylindrical anode 18, forms a focusing lens is situated within the second cylindrical anode 18. In the interests of clarity the connection means of the electrodes and various supply conductors are not shown in Figure 1. The tube is partly surrounded by line deflection coils and frame deflection coils, collectively denoted by the reference numeral 25.

[0015] The signal plate 12 is connected through a supply conductor 27 passed through the envelope 10 and a signal resistor 28 to one terminal of a voltage source 29, the other terminal of which is connected to earth. Pulse generators 30, 32, and 36 are connected by respective conductors 31, 33, and 37 to the cathode 15, the control grid 16 and the focusing electrode 23. A d.c. source 34 is connected by a conductor 35 to the accelerating anode 17.

[0016] The control grid 16 comprises a central (or inner) aperture 40 disposed coaxially of the envelope 10 and a plurality of outer apertures 42 surrounding the inner one. Two embodiments of the control grid 16 showing different outer aperture shapes are shown in Figures 6 and 7. Irrespective of the aperture shapes the total cross sectional area of the outer apertures 42 exceeds the cross sectional area of the inner aperture 40.

[0017] The accelerating anode 17 has a generally similar arrangement of inner and outer apertures 44, 46, respectively, as are provided in the control grid 16. However the inner aperture 44 is of smaller cross-sectional area than that of the aperture 40. Also, as shown more clearly in Figures 3 and 4, the aperture 44 has a tubular extension 48 pointing towards the control grid 16 in order to provide more durchgriff. The central portion of the accelerating anode including the extension 48 functions as a diaphragm to limit the cross sectional area of the electron beam passing through the aperture 44.

[0018] The number of outer apertures 42 in the control grid 16 corresponds to the number of outer apertures 46 in the accelerating anode 17. The apertures 46 may be at the same radial distance as the apertures 42 - see aperture 46A (Figure 7). Alternatively the apertures 46 may be bigger than the apertures 42 - see the aperture 46B (Figure 7). In another variant the apertures 46 may be at a different radial distance compared to those in the control grid 16 - see the aperture 46C (Figure 7). Optionally the outer apertures 42, 46 in both the electrodes 16, 17 may be arranged in two or more concentric rings.

[0019] The electron gun 14 when operating as an anti-comet-tail gun has different operating voltages on the cathode 15, control grid 16 and focusing electrode 23 during the line scanning mode than when during the line flyback mode.

[0020] In the line scanning mode the cathode 15 is at zero volts (0 V), the control grid 16 has a mean voltage of -15 V and the accelerating anode 17 is at a voltage of 500 V. The scene to be recorded is projected, by means of an optical system diagrammatically denoted by a lens 50, onto the target plate 11 of the tube through the window 13 and the signal plate 12. During line scans the free surface of the target plate 11 is scanned according to a rectangular frame by the electron beam produced by the electron gun 14. Under the influence of the mentioned voltages an electron beam is formed by electrons emitted only from the central portion of the cathode, as shown in Figure 3, due to the enlarged field strength. This means that the electron beam has a small cross-sectional area. The electron beam cross-over is in the inner aperture of the accelerating anode so that little or no cathode current flows into the accelerating anode 17. The electron beam leaving the accelerating anode is focused at the target by the electrostatic focusing lens formed by the electrodes 18 and 23. The surface of the target plate is stabilised substantially at the potential (0 V) of the cathode, electric signals being formed which are derived from the signal resistor via a capacitor 52.

[0021] During line flybacks the pulse generator 30 supplies a positive pulse of 5 volts to the cathode 15. Simultaneously the pulse generator 32 supplies a negative pulse of 1 volt (that is -1 V) to the control grid 16. The accelerating anode 17 remains at 500 V. As a result electrons emerge through the outer apertures 42 of the control grid 16 as well as from the central aperture 40 to form a plurality of electron beams as shown in Figure 4. Although a central electron beam is present this is not essential and in any event its beam current is smaller say by an order of magnitude compared to the total beam current of the outer electron beams each one of which has a low beam current. The focusing voltage is adjusted by the pulse generator 36 so that at the target plate 11, the electron beams are merged to form a large diameter spot which stabilises the target plate at the cathode voltage of 5V. By the electron beams being merged into a large spot the desired stabilisation of the target plate can be achieved without the risk of damage by ions or x-rays to the photo-conductive layer of the target plate. During flyback the large spot may follow a path such that it covers the line which has just been scanned.

[0022] Figure 5 are graphs of field strength (ordinate) against radius R (abscissa) of the conditions prevailing during line scanning, curve A, and during flyback, curve B. During line scanning when the cathode is at 0 V and the control grid is at -15 V, there is positive field in the central aperture, that is over a radius 0 to 150 µm, but a negative field strength at the position of the aperture 42, that is between approximately 350 and 500 µm in Figure 5. Accordingly there is no electron emission through the outer apertures 42. Under line flyback conditions when the cathode is at 5 V and the control grid at -1 V, the field strength becomes positive at the position of the outer apertures so that electrons from the cathode are drawn through the apertured control grid.

[0023] The computer trajectory plots of Figures 3 and 4 have assumed that the radius of the inner aperture 40 is 185 µm and that the internal radius of the tubular extension 48 is 50 µm. The diameter of the outer apertures 42, 46 is 150 µm. The diameter of the cathode 15 is at least as large as the distance between diametrically located outer (or outermost holes. The distance between the cathode 15 and the control grid is 1.0 mm, between the cathode 15 and the nearer edge of the tubular extension 48 is 3.5 mm, between the cathode 15 and the accelerating anode 17 is 5.5 mm, and between the control grid 16 and the accelerating anode 17 is 4.0 mm. The length of the tubular extension 48 is approximately equal to its diameter. The operating voltages are as specified previously.

[0024] Figure 2 illustrates an embodiment of a television camera tube having electromagnetic focusing coils shown collectively by the reference numeral 60. Although it is a characteristic of electromagnetic focusing means that the electron beam remains focused during flyback, the fact that during flyback the electron beam comprises a multiplicity of low current beams avoids damage to the photo-conductive layer of the target plate 11.

[0025] Figure 2 also shows the electrical connections to the cathode heater 62, the cathode 15, the control grid 16 and the accelerating anode 17 to be by way of pins 64 extending through the envelope 10. As the operation of the television camera tube is substantially the same as for the tube shown in Figure 1, it will not be described.

Claims

1. An apparatus having a television camera tube comprising an envelope within which are provided a target for producing a potential image corresponding to a scene and an electron gun for producing an electron beam directed to the target, the electron gun comprising a cathode, a control grid having an inner aperture surrounded by a plurality of outer apertures, and an anode having an inner aperture of smaller cross-sectional area than that in the control grid surrounded by the same number of outer apertures as are provided in the control grid, means for focusing the electron beam onto the target and deflection means which the aid of which a periodic scanning of the target by the electron beam is obtained for stabilization of the potential of the target to cathode potential for supplying output signals which correspond to the potential image, the deflection means comprising a deflection member for line sweeps and line fly-backs and a deflection member for frame sweeps and frame fly-backs, means for applying during line sweeps a first voltage to the cathode and a second voltage to the control grid so that the electron beam comprises electrons drawn through the inner aperture of the control grid and means for applying during line fly-backs a third voltage to the cathode and a fourth voltage to the control grid so that electrons are also drawn from the cathode through the outer apertures of the control grid to provide a plurality of low current beams which serveto stabilise the target to the third voltage.

2. An apparatus as claimed in Claim 1, characterised in that the total cross-sectional area of the outer apertures in the control grid is substantially greater than that of the inner aperture in the control grid.

3. An apparatus as claimed in Claim 1 or 2, characterised in that the cross-sectional shape of each of the outer apertures is different from that of the inner aperture.

4. An apparatus as claimed in Claim 1, 2 or 3, characterised in that the outer apertures in the anode are of different size to those in the control grid.

5. An apparatus as claimed in any one of Claims 1 to 4, characterised in that the outer apertures in the anode are offset radially with respect to the outer apertures in the control grid.

6. An apparatus as claimed in any one of Claims 1 to 5, characterised in that the envelope is elongate and the inner apertures in the control grid and in the anode are disposed substantially co-­axially of the longitudinal axis of the envelope.

7. An apparatus as claimed in any one of Claims 1 to 6, characterised in that the focusing means are electrostatic.

8. An apparatus as claimed in any one of Claims 1 to 6, characterised in that the focusing means are electromagnetic.

9. A television camera tube comprising an envelope within which are provided a target and an electron gun for producing an electron beam directed to the target, the electron gun comprising a cathode, a control grid having an inner aperture surrounded by a plurality of outer apertures and an anode having an inner aperture of smaller cross-sectional area than that in the control grid surrounded by a corresponding plurality of outer apertures as are provided in the control grid.

10. A television camera tube as claimed in Claim 9, characterised in that the total cross-sectional area of the outer aperture in the control grid is substantially greater than that of the inner aperture in the control grid.

11. A television camera tube as claimed in Claim 9 or 10, characterised in that the cross-sectional shape of each outer apertures is different from that of the inner aperture.

12. A television camera tube as claimed in Claim 9, 10 or 11, characterised in that the envelope is elongate and the inner apertures in the control grid and the anode are disposed substantially co-axially of the longitudinal axis of the envelope.

Drawing