1. Field of the Invention
The invention concerns television cameras having an electronic image pick-up tube.
2. Description of the Prior Art
The electronic image pick-up tube is a vacuum tube, a front surface of which is formed by a photosensitive target on which is focused, by lenses or other optical means, an image which is sought to be converted into an electrical signal known as a video signal.
The tube has an electron gun placed in the rear of the photosensitive target to produce a narrow electron beam, focusing means to focus this beam on the photosensitive target, and deflection means to make the beam (and consequently its point of impact on the target) scan 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. Most usually, the scanned surface is rectangular and the target is circular, with a diameter which is greater than the diagonals of the rectangle.
The electron beam focusing means may be electromagnetic (coils surrounding the electron gun) or electrostatic.
The electron beam deflection means may also be electromagnetic or electrostatic.
The electron gun generally consists of an emissive cathode from which there emerge electrons, or an accelerating electrode placed in front of the cathode and taken to a potential of a few hundreds of volts. There may possibly be different grids between the cathode and the accelerating electrode, in particular a control grid (Wehnelt) by which the intensity of the emitted beam can be adjusted.
The accelerating electrode is provided with a diaphragm perforated with a very narrow hole (a few hundreds of micrometers for example) limiting the diameter of the electron beam emitted in the tube.
Finally, the tube has a grid called a "field grid" placed in the vicinity of the target, taken to a high potential, for example, 1000 volts, enabling the creation, in the vicinity of the target, of a strong electrical field perpendicular at all points to the surface of the target, the latter being carried to a potential of a few hundred volts at the maximum. This field grid enables the electrons of the beam 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 great.
To supply a video signal representing the illumination of each point of the target, it is provided that the front face of the target should be coated with a transparent electrode connected to an output connection terminal at which the video signal will be read.
The tube works as follows: the image is focused, from the exterior, on the front face of the target, through the glass envelope of the tube and through the transparent front electrode, and is represented, at each point of the target, by a localized illumination which locally creates electrical charges (electron/hole pairs) proportionate to the illumination at this point. The electrical field in the material of the photosensitive target attracts positive charges towards the real face of the target, namely towards the inside of the tube, namely again, on the side where the target is struck by the beam of electrons. To produce this electrical field, it is seen to it that the mean potential of the front electrode is positive with respect to the tube cathode potential.
The electron beam scans each point of a rectangular zone of the target. At each point, it conveys electrons which compensate for the positive electrical charges that get accumulated at this point on the rear face of the target. A charge of current then flows from the output electrode towards the target to compensate for the localized charge modification thus produced. This charge current varies from one point to another, as a function of the illumination of the points. The result is an electrical signal that varies at the output terminal, said this signal representing the illumination of the target, line by line in a frame and a point by point in each line.
An irksome problem has been noted in certain camera tubes: the video signal collected at the output of the tube represents the superimposition of the real image, focused on the target, and a spurious image.
This spurious image phenomenon is pronounced in the case of a tube with electromagnetic focusing and electrostatic deflection. This is the case taken herein as an example.
The spurious image has been identified by its form: in practice, there are two spurious images. One of them is a precise representation, reduced by a factor approximately equal to two, of the accelerating electrode of the electron gun. The other spurious image represents, also reduced and rotated by about 30.degree., the scan rectangle of the electron beam when the scanning is rectangular.
In searching for the cause of these spurious images, the following conclusion has been reached: the electrons of the beam that reach the photosensitive target are not all absorbed by the target, since the absorption depends locally on the illumination. Those that are not absorbed set off again, accelerated by the field grid which is taken to 1000 volts. A proportion of these electrons again crosses this gate, which has a transparency to electrons of about 50%. These electrons strike the accelerating electrode that occupies the major part of the section of the tube in front of the electron gun. By reflection and by secondary emission of electrons, the accelerating electrode then behaves like an ancillary source of electron, that is, the electron gun no longer emits only one very narrow beam through the very small aperture of the diaphragm of the accelerating electrode. It also emits an ancillary beam from every point of the surface of the accelerating electrode. This beam goes back towards the target and gets focused and deflected by the focusing and deflection electrodes of the main beam.
This beam lands on the target and produces the same effect as the main beam, almost simultaneously since the period of time taken by the electrons to travel is negligible compared with the television scanning speed. Thus, a spurious video signal is produced and gets added to the main signal. The modulation of this spurious signal corresponds to the image of the accelerating electrode. Furthermore, the interaction between this ancillary beam and the target is weaker if the said beam lands within the scan rectangle than if it lands on the rest of the target, for this latter zone has a higher potential. This effect is responsible for the spurious image of the scan rectangle.
The spurious images are especially visible and irksome in electromagnetic focusing and electrostatic deflection tubes where there is excellent focusing of one plane on another, so that there is a perfect view of the image of the accelerating electrode (located, on the whole, in the plane transversal to the axis of the tube and going through the hole of the diaphragm) and the image of the scan rectangle. To put things clearly, these images correspond to a modulation of the video signal, the amplitude of which attains only a few nanoamperes, but they are distinctly visible on a television screen, for the geometrical contours have sharp contrasts.
Several means of preventing these spurious images have been proposed in the prior art. One of them is to coat the accelerating electrode with a layer preventing the re-emission of electrons when this electrode is struck by electrons. The proposed method, based on porous gold, is not wholly satisfactory and is difficult to implement, especially in tubes with high performance characteristics, which necessitate a de-gassing of the tubes at high temperature (about 800.degree. C.): at this temperature, the porous gold would get diffused in the metal forming the electrode and, at any rate, would not retain its porous structure.
It has also been proposed that an elongated tube, conveyed to the potential of the accelerating electrode, could be placed in the axis of the output electron beam of the electron gun. This tube axially surrounds the beam in front of the output diaphragm, on a length which is sufficient, in the axis of the image pick-up tube, to substantially deform the equipotential surfaces in the vicinity of the accelerating electrode. In this way, the electrons that strike the accelerating electrode are reflected in a direction that does not let them be again focused on the target so as to produce a spurious image.
This elongated tube is not entirely satisfactory and, moreover, it calls for an overall increase in the length of the image pick-up tube, whereas one of the advantages of tubes with electrostatic deflection (for which the spurious image is the most pronounced) is precisely the reduction in the overall length of the image pick-up tube.
Finally, it has been proposed that another electrode, called a repulsion electrode, should be placed in front of the accelerating electrode. Electrons of the return beam, coming from the target, come to this repulsion electrode. This electrode is electrically insulated from the accelerating electrode and is carried to a different potential. This potential causes the incident electrons to be reflected with a level of energy and in a direction such that they are no longer focused on the target when they set off again.
The drawback of this latter structure is evidently the need to provide for a mounting of an additional electrode, insulated from the accelerating electrode, and for a separate electrical supply for this electrode.
To avoid the drawbacks of prior art image pick-up tubes, the present invention proposes the placing, in front of the accelerating electrode, of a screen to mask this electrode, this masking screen having a smooth surface, without discontinuities, and having rounded edges, so that, from the target, neither any sharp-edged surface nor steps nor, again, any other discontinuities are seen.
As a matter of fact, the starting point of the invention is the observation that when a spurious image of the accelerating electrode gets superimposed, in the output video signal, on the real image projected on the target, this spurious image is particularly visible and irksome because is has transitions. Besides, this is generally the case, for accelerating electrodes have steps and discontinuities on the side pointed towards the target, and these steps get reproduced very clearly in the video signal.