EP0338903B1 - Aufnahmeröhre mit einem Schirm zur Beseitigung von Geisterbildern - Google Patents

Aufnahmeröhre mit einem Schirm zur Beseitigung von Geisterbildern Download PDF

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Publication number
EP0338903B1
EP0338903B1 EP89401035A EP89401035A EP0338903B1 EP 0338903 B1 EP0338903 B1 EP 0338903B1 EP 89401035 A EP89401035 A EP 89401035A EP 89401035 A EP89401035 A EP 89401035A EP 0338903 B1 EP0338903 B1 EP 0338903B1
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EP
European Patent Office
Prior art keywords
target
screen
tube according
diaphragm
tube
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Expired - Lifetime
Application number
EP89401035A
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English (en)
French (fr)
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EP0338903A1 (de
Inventor
Jean-Luc Ricaud
Gérard Guilhem
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Thales SA
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Thomson CSF SA
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
    • H01J29/00—Details of cathode-ray tubes or of electron-beam tubes of the types covered by group H01J31/00
    • H01J29/84—Arrangements for removing or diverting unwanted particles, e.g. for negative ions or fringing electrons; Arrangements for velocity or mass selection

Definitions

  • the invention relates to television cameras comprising an electronic picture tube.
  • the shooting tube is a vacuum tube whose front surface is constituted by a photosensitive target on which one comes to focus, by lenses or other optical means, an image which one wants to transform into an electrical signal called signal video.
  • the tube comprises an electron gun placed behind the photosensitive target to produce a narrow electron beam, focusing means for focusing this beam on the photosensitive target, and deflection means for subjecting the beam (and consequently its point of impact on the target) a sweep of the surface of the target or a part of this surface.
  • the scanning is generally a line by line scanning, possibly by interlaced half-frames, in accordance with television scanning standards.
  • the swept surface is rectangular and the target is circular, with a diameter greater than the diagonals of the rectangle.
  • the focusing means of the electron beam can be electromagnetic (coils surrounding the electron gun) or electrostatic.
  • the deflection means of the electron beam can also be electromagnetic or electrostatic.
  • the electron gun is generally composed of an emissive cathode from which the electrons come out, and an accelerating electrode which is placed in front of the cathode and which is brought to a potential of a few hundred volts, with possibly different grids between the cathode and the accelerating electrode, in particular a control grid (Wehnelt) for adjusting the intensity of the emitted beam.
  • a control grid Wehnelt
  • the accelerating electrode is provided with a diaphragm pierced with a very narrow hole (a few tens of micrometers in diameter for example) limiting the diameter of the electron beam emitted in the tube.
  • the tube often comprises a grid known as a "field grid", placed near the target, brought to a high potential, for example 1000 volts, making it possible to create in the vicinity of the target a strong electric field perpendicular at all points to the surface of the target, the latter being brought to a potential of a few tens of volts at most.
  • This field grid allows the beam electrons to strike the target as perpendicularly as possible even when the overall deflection angle of the electron beam between the output diaphragm and the target is large.
  • the front face of the target is covered with a transparent electrode which is connected to an output connection terminal on which the video signal will be read.
  • the tube works as follows: the image focused from the outside on the front face of the target, through the glass envelope of the tube and through the transparent front electrode, translates into each point of the target by a point illumination which locally generates electric charges (electron-hole pairs) proportional to the illumination at this point.
  • the electric field in the material of the photosensitive target attracts the positive charges towards the rear face of the target, that is to say towards the interior of the tube, that is to say again on the side where the target is struck by the electron beam.
  • the average potential of the front electrode is positive compared to the cathode potential of the tube.
  • the electron beam scans each point of a rectangular area of the target; at each point it brings electrons which compensate for the positive electric charges accumulated at this point on the rear face of the target; a charge current then flows from the output electrode to the target to compensate for the change in point charge thus produced.
  • This charge current varies from one point to another depending on the illumination of the points.
  • the result is a variable electrical signal on the output terminal, this signal representing the illumination of the target line by line in a frame and point by point in each line.
  • the stray image has been identified by its shape; in practice there are two parasitic images; one of them represents exactly, by a reduction of a factor approximately equal to 2, the accelerating electrode of the electron gun; the other represents, also in reduction and rotated about 30 degrees, the scanning rectangle of the electron beam when the scanning is rectangular.
  • the accelerating electrode which occupies most of the section of the tube in front of the electron gun; by reflection and by secondary electron emission, the accelerating electrode then behaves as an auxiliary electron source, that is to say that the electron gun no longer emits only a very thin beam at through the very small aperture of the diaphragm of the accelerating electrode; it also emits an auxiliary beam from any point on the surface of the accelerating electrode. This beam returns towards the target and is focused and deflected by the focusing and deflection electrodes of the main beam.
  • Spurious images are particularly visible and annoying in electromagnetic focusing and electrostatic deflection tubes in which the focusing from one plane to another is excellent, so that one can perfectly see the image of the accelerating electrode (located overall in the plane transverse to the axis of the tube and passing through the hole in the diaphragm) and the image of the scanning rectangle.
  • these images correspond to a modulation of the video signal whose amplitude reaches only a few nanoamps, but they are clearly visible on a television monitor because they are geometric contours that are well contrasted.
  • This elongated tube is not entirely satisfactory, and moreover, it requires an overall increase in the length of the shooting tube, while one of the advantages of electrostatic deflection tubes (for which the parasitic image is most marked ) is precisely the reduction in the overall length of the shooting tube.
  • the repulsion electrode on which the electrons of the return beam coming from the target come; this electrode is electrically isolated from the accelerating electrode and brought to a different potential; this potential is such that the incident electrons are reflected with an energy and a direction such that they are no longer focused on the target when they leave.
  • the present invention proposes to place in front of the accelerating electrode a screen for masking this electrode, this screen having a smooth surface, without discontinuity, and rounded edges, so that seen from the target, we do not see a surface with sharp edges, or steps or other discontinuities.
  • the invention indeed starts from the remark that when a spurious image of the accelerating electrode is superimposed in the output video signal on the real image projected on the target, this spurious image is particularly visible and annoying when it presents transitions; this is also the general case because the accelerating electrodes have steps and discontinuities on the side facing the target, and these steps are very clearly reproduced in the video signal.
  • the invention therefore provides an electronic shooting tube comprising an electron gun and a photosensitive target, the gun notably comprising a cathode emitting an electron beam, and, in front of the cathode, an accelerating electrode provided with a diaphragm pierced with a hole limiting the diameter of the electron beam, this tube being characterized in that it comprises, in front of the diaphragm, a screen for masking the accelerating electrode, the screen having an opening in view of the diaphragm hole, the screen further having a surface devoid of discontinuities or steep steps on a macroscopic scale on the side of the target and having rounded edges of convexity facing the target, both at the periphery and around of its opening opposite the diaphragm.
  • the masking screen is preferably brought to the same potential as the accelerating electrode, but it can also, in certain cases, be brought to a different potential modifying the re-emission energy of the secondary electrons which come to strike it, so that these electrons are not focused at new to the target.
  • the potential is for example a much more positive potential than that of the accelerating electrode.
  • the front surface of the masking screen is given a structure with a low secondary emission of electrons, in order to reduce the throughput of the auxiliary beam and therefore reduce the amplitude of the modulation corresponding to the parasitic images of the accelerating electrode and of the scanning rectangle.
  • the front surface (facing the target), a rough texture on a microscopic scale.
  • This rough texture can be obtained for example by chemical attack with hot hydrochloric acid in the case where the front surface is made of stainless steel.
  • the surface of rough texture is covered with a microporous layer, preferably carbon, or possibly tungsten or "black” titanium, that is to say tungsten or titanium of high porosity.
  • the conventional shooting tube of FIG. 1 comprises a vacuum tube 10, the front face of which is a photosensitive target 12.
  • An electron gun 14 is placed at the rear of the tube.
  • Electromagnetic deflection coils 16 surround the tube.
  • Electrostatic deflection electrodes 18 are formed at the periphery of the tube.
  • the electron gun comprises an emissive cathode 20 and, in front of the cathode, an accelerating electrode 22 provided with a diaphragm 24 pierced with a small hole to allow a narrow beam of electrons to pass.
  • the video signal resulting from the scanning of the target by the beam is collected on an output terminal 26 connected to a transparent electrode on the front surface of the target.
  • FIG. 1 shows the typical path of the electrons.
  • the primary beam, FP coming from the electron gun through the diaphragm 24, strikes the target at a point 1.
  • a certain proportion of the beam is not absorbed because at the point of impact the stored charge has a intermediate value between the white level (maximum illumination) and the black level (zero illumination).
  • the unabsorbed electrons are returned towards the rear in the form of a return beam FR, part of which is absorbed by the field grid near the target and another part of which returns to the accelerating electrode 22.
  • the FR return beam returns all the better as the electrostatic deflection electrodes play a role both for the primary beam and for the return beam.
  • the electronic intensity of the return beam can reach, for example, 20% of that of the primary beam.
  • the impact of the return beam FR on the accelerating electrode 22 is designated by the reference 2. This impact obviously moves with the scanning of the main beam and of the return beam. The point 2 therefore moves over the entire surface of the accelerating electrode 22, including on the diaphragm 24 if the latter is placed in front of the electrode.
  • the accelerating electrode 22 emits secondary electrons with a secondary emission coefficient depending on the nature of its surface.
  • the secondary emission coefficient is 1.5 (for example for a stainless steel accelerating electrode), that is to say that for n electrons incident on the surface, 1.5xn electrons leave.
  • the quantity of electrons that leave is constant as long as the point of impact 2 sweeps over a uniform surface of homogeneous nature, but it changes suddenly if it encounters an irregularity such as the edge of a part or a sharp angle.
  • the quantity of electrons which leave is modulated according to the local state of the surface of the accelerating electrode, therefore carries in it the information of the image of this surface.
  • these electrons leave the accelerating electrode with the same speed as the main beam leaving the diaphragm hole, and are therefore focused and deflected with the same efficiency.
  • These electrons constitute a secondary beam FS whose origin scans the surface of the accelerating electrode; this beam is again focused on the target 12 by the focusing means 16, and it undergoes the scanning deflection generated by the deflection means 18.
  • the current passing through terminal 26 at a given instant is then the sum of the normal current corresponding to the charge at point 1 (corresponding to the actual illumination of an image point) and a stray current.
  • the interaction of the secondary beam FS is different depending on whether the point of impact 3 is located in the sweep rectangle or in the rest of the target.
  • the surface potential is periodically reduced to around zero volts (zero volts is conventionally the potential of the cathode) by the primary beam FP.
  • the second zone has a higher potential, therefore more favorable to the absorption of the secondary beam FS.
  • the stray current is very reduced, so that the extent of the stray image of the scanning rectangle appears uniformly black.
  • the stray current is, as a first approximation, proportional to the intensity of the secondary beam FS, which itself is modulated according to the image of the accelerating electrode . Assuming that there is no image (zero illumination of the entire target), one should collect a constant video signal corresponding to black.
  • FIG. 2 represents in schematic form the parasitic image produced in the video signal by the secondary beam.
  • the scanning rectangle 28 of the normal image circles which are the images in reduction by a factor of 2 of the periphery of the accelerating electrode (circle 30) and of the periphery of other abrupt contours.
  • the accelerating electrode (the contour of the diaphragm 24 for example or any other steep step of the surface of the electrode 22); one of these contours gives rise for example to an image in the form of the circle 32; we also distinguish the image of a rectangle 34 which is the image, with reduced dimensions and rotated by about 30 degrees, of the scanning rectangle of the video image.
  • FIG. 2 also shows the shape of a line of the video signal (for example corresponding to an image line designated by the reference 36), assuming that the target is not illuminated; this video signal has large and abrupt variations when it should be constant between two synchronization pulses.
  • FIG. 3 shows the structural modification made by the invention.
  • a masking screen 38 is placed in front of the accelerating electrode 22; this screen receives almost all of the return beam FR; it masks the accelerating electrode, that is to say that it prevents the return beam FR from striking the front face thereof, or in any case the parts thereof which have steep steps; however, it has a central opening 40 for letting the primary beam pass at the outlet of the diaphragm; its edges are rounded, both at its periphery and around its central opening 40; the convexity of the rounded edges is turned towards the target.
  • the front surface of the screen that is to say the surface of the side facing the target, therefore does not have any discontinuity or steep steps on the macroscopic scale.
  • the screen is integral with the accelerating electrode 22 and it is preferably brought to the same potential as it, but it could also be envisaged that it is brought to a different, positive potential, to limit the re-emission of secondary electrons towards target.
  • Figure 4 shows in detail the mounting of the masking screen 38 in front of the accelerating electrode 22 and the diaphragm 24.
  • the front is the right side of the figure as in Figures 1 and 3.
  • the radius of curvature of the rounded edges can be from one to three millimeters.
  • the screen can be welded in front of the electrode 22 and the diaphragm 24 by means of spacers 39.
  • the screen is preferably made of stainless steel.
  • the front surface of the screen is rough (it can therefore include steep steps and irregularities), always to reduce the re-emission of secondary electrons.
  • the roughness is obtained for example by attacking stainless steel in an acid bath.
  • the rough surface is coated with a very thin layer (non-smoothing, that is to say not likely to make the roughness disappear) of a material with low secondary emission which is preferably carbon but which can also be titanium or black tungsten (metals deposited under conditions where they acquire a high porosity).
  • a material with low secondary emission which is preferably carbon but which can also be titanium or black tungsten (metals deposited under conditions where they acquire a high porosity).
  • FIG. 5 shows diagrammatically the appearance of the front surface of the screen 38 on a microscopic scale: the surface has roughnesses from several micrometers to several tens of micrometers in depth. This surface is covered with a microporous layer 42 of a few thousand angstroms of carbon for example.
  • FIG. 5 also shows an enlarged detail of the surface, showing the rough surface covered with a porous layer 42 of carbon and showing how an incident electron is absorbed by this porous layer, in the sense that the secondary electrons which it generates are trapped in porosities.
  • FIG. 6 represents the video signal corresponding to a scanning line with the structure according to the invention, in the absence of illumination of the target.
  • the variations in the black level have been reduced by a factor of 10 in amplitude and moreover, they are smoothed and therefore do not produce a contrasted parasitic image which is much more apparent and annoying than an image with little contrast.

Landscapes

  • Image-Pickup Tubes, Image-Amplification Tubes, And Storage Tubes (AREA)
  • Cathode-Ray Tubes And Fluorescent Screens For Display (AREA)

Claims (9)

1. Elektronische Bildaufnahmeröhre mit einer Elektronenkanone (14) und einer lichtempfindlichen Anode (12), wobei die Kanone insbesondere eine Kathode (20), die einen Elektronenstrahl aussendet und vor der Kathode eine Beschleunigungselektrode (22) mit einer Blende (24) aufweist, deren Blendenöffnung den Durchmesser des Elektronenstrahls begrenzt, dadurch gekennzeichnet, daß sie vor der Blende einen Maskenschirm (38) der Beschleunigungselektrode aufweist, der einen Öffnung (40) gegenüber der Blendenöffnung besitzt, wobei der Maskenschirm außerdem eine von Unstetigkeiten oder plötzlichen Stufen im makroskopischen Maßstab freie der Anode zugewandte Oberfläche und abgerundete Ränder besitzt, deren konvexe Seite der Anode zugewandt ist, und zwar sowohl am Umfang als auch um die Öffnung gegenüber der Blendenöffnung herum.
2. Röhre nach Anspruch 1, dadurch gekennzeichnet, daß der Maskenschirm auf dem selben Potential wie die Beschleunigungselektrode liegt.
3. Röhre nach einem der Ansprüche 1 und 2, dadurch gekennzeichnet, daß die vordere Oberfläche des Maskenschirms eine Struktur mit geringer Emission von Sekundärelektronen besitzt.
4. Röhre nach Anspruch 3, dadurch gekennzeichnet, daß die zur Anode zeigende vordere Fläche des Maskenschirms eine im mikroskopischen Maßstab rauhe Textur besitzt.
5. Röhre nach Anspruch 4, dadurch gekennzeichnet, daß die vordere Oberfläche des Schirms aus rostfreiem Stahl besteht und eine rauhe Textur besitzt, die durch chemischen Angriff mit erhitzter Chlorwasserstoffsäure erhalten wird.
6. Röhre nach einem der Ansprüche 4 und 5, dadurch gekennzeichnet, daß die Tiefe der Rauhigkeiten zwischen etwa zehn und einigen zehn Mikrometer liegt.
7. Röhre nach einem der Ansprüche 3 bis 6, dadurch gekennzeichnet, daß die vordere Oberfläche des Schirms mit einer mikroporösen Schicht eines Materials bedeckt ist, das einen niedrigen Sekundäremissionskoeffizienten besitzt.
8. Röhre nach Anspruch 7, dadurch gekennzeichnet, daß die mikroporöse Schicht aus Kohlenstoff oder ggfs. "schwarzem" Wolfram oder "schwarzem" Titan besteht, d.h. aus Wolfram oder Titan mit sehr hoher Porosität.
9. Röhre nach Anspruch 8, dadurch gekennzeichnet, daß die mikroporöse Schicht eine Dicke von einigen tausend Angström aufweist.
EP89401035A 1988-04-22 1989-04-14 Aufnahmeröhre mit einem Schirm zur Beseitigung von Geisterbildern Expired - Lifetime EP0338903B1 (de)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
FR8805348 1988-04-22
FR8805348A FR2630586A1 (fr) 1988-04-22 1988-04-22 Tube de camera avec ecran de suppression d'image parasite

Publications (2)

Publication Number Publication Date
EP0338903A1 EP0338903A1 (de) 1989-10-25
EP0338903B1 true EP0338903B1 (de) 1992-05-27

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EP89401035A Expired - Lifetime EP0338903B1 (de) 1988-04-22 1989-04-14 Aufnahmeröhre mit einem Schirm zur Beseitigung von Geisterbildern

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US (1) US5218443A (de)
EP (1) EP0338903B1 (de)
JP (1) JPH01313843A (de)
DE (1) DE68901630D1 (de)
FR (1) FR2630586A1 (de)

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Publication number Priority date Publication date Assignee Title
KR950000047Y1 (ko) * 1990-08-30 1995-01-07 주식회사 금성사 코일형상의 고저항체를 가지는 음극선관
JP3147227B2 (ja) * 1998-09-01 2001-03-19 日本電気株式会社 冷陰極電子銃

Family Cites Families (7)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
CA724782A (en) * 1965-12-28 V. Duerr Melvin Electron discharge device
US2719243A (en) * 1951-07-03 1955-09-27 Du Mont Allen B Lab Inc Electrostatic electron lens
NL7109140A (de) * 1971-07-02 1973-01-04
US4075533A (en) * 1976-09-07 1978-02-21 Tektronix, Inc. Electron beam forming structure utilizing an ion trap
JPS59215639A (ja) * 1983-05-24 1984-12-05 Nippon Hoso Kyokai <Nhk> 反射電子除去電極
NL8401824A (nl) * 1984-06-08 1986-01-02 Philips Nv Televisiekamerabuis.
JPH069132B2 (ja) * 1985-12-11 1994-02-02 松下電子工業株式会社 静電偏向型撮像管

Also Published As

Publication number Publication date
US5218443A (en) 1993-06-08
DE68901630D1 (de) 1992-07-02
EP0338903A1 (de) 1989-10-25
FR2630586A1 (fr) 1989-10-27
JPH01313843A (ja) 1989-12-19

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