EP0075512B1 - Image-intensifying tube with memory, and operating method - Google Patents

Description

The invention relates to an image intensifier tube and its implementation.

The function of such a tube is to produce a bright image with high brightness. The image is formed on the exit screen of the tube, the screen of which is exposed to incident radiation. In between, a stream of electrons, emitted by a photocathode incorporated in the input screen, ensures the transfer of the signal from each point from one end to the other of the tube.

These tubes are widely known in the art, where they are used in particular in the medical field, with incident X-rays.

In this case, they also act as converters, in which the incident X-ray is transformed into visible radiation.

Generally, the output screen is a simple cathode-luminescent screen, capable of emitting light under the effect of bombardment by electrons, namely those of the flux in question. They then only allow operation in real time, where the image is visible on the output screen only when it is produced on the input screen, with the persistence near the traces on the 'output screen, of variable duration according to the provisions adopted, but, in any case, very limited in the current state of the art of these screens.

We know, on the other hand, another kind of screens, thin-film electroluminescent screens, which have the property of emitting visible radiation under the excitation of an electric voltage applied between their faces. There is a threshold voltage from which this effect manifests itself and, for some of them, continues for some time after this voltage has been reduced to a value below this threshold. The fall in luminance then occurs with hysteresis, and naturally gives rise to a memory effect. It is understood that by using a voltage it is then possible to obtain the storage, display and erasure of a signal, as is usually the case with the hysteresis phenomena.

This kind of screen is also sensitive to the action of electronic bombardment, like the cathode-luminescent screens to which reference was made first, but in a completely different respect: electronic bombardment adds its action here to that of applied voltage and is equivalent to an additional voltage which would be superimposed on it. In particular, it would make it possible to cross the threshold voltage for an applied voltage lower than the value of this threshold and, in general, various signal processing operations, all of which will be more fully specified below.

We have seen in the prior art the advantage which could be drawn from the use of light-emitting output screens in a certain number of types of electronic tubes to obtain a prolonged persistence of the image, and therefore a memory. of it, and various arrangements have been proposed which show tubes fitted with such screens (see in particular French patent 2431 184 (79.17428) and the patent granted in the United States of America under No. 3,908,148).

The embodiments described relate all, however, to cathode ray tubes (CRT), namely electronic tubes using an electron brush sweeping the display screen to form sequentially, point by point, an image thereon.

According to the inventors, this interest had to be even greater for tubes no longer of the TRC type, but in which, unlike the case of these tubes, the image on the output screen is formed by a flow of electrons whose l 'impact covers at once the entire surface of the exit screen, which is, so to speak, at a given moment, "sprinkled" entirely by the beam. This watering can also be permanent during the shooting, or applied by pulses, as we will see later.

This case is typically that of radiological image intensifiers (IIR), intended for the medical uses mentioned above, and bright image intensifiers (IIL) reserved, in other fields, for the collection of images with low illumination or nocturnal.

It is this type of tube, IIR or IIL, which the invention relates, which provides for the combination of an electroluminescent screen with the other elements of these tubes, which ensures that a certain number of advantages generally absent are obtained. tubes of the same type of the prior art, under the conditions which will be exposed. It concerns, in a generic way, all the tubes in which, as we said, the output screen is covered at all times at all its points by the flow of electrons carrying the signal coming from the input of the tube.

• - figure 1: une vue en coupe schématique d'un écran électrolumininescent;
• - figure 2: un diagramme montrant le cycle d'hystérésis de l'écran électroluminescent utilisé dans les tubes intensificateurs d'images de l'invention;
• - figures 3 et 4: une vue en coupe d'un tube intensificateur d'images de l'invention et un diagramme de ses utilisations, respectivement;
• - figures 5 à 8: des diagrammes de la tension appliquée dans differents modes de mise en oeuvre du tube de l'invention.

The invention will be better understood by referring to the description which follows and to the attached figures which represent:

• - Figure 1: a schematic sectional view of an electrolumininescent screen;
• - Figure 2: a diagram showing the hysteresis cycle of the electroluminescent screen used in the image intensifier tubes of the invention;
• - Figures 3 and 4: a sectional view of an image intensifier tube of the invention and a diagram of its uses, respectively;
• - Figures 5 to 8: diagrams of the voltage applied in different embodiments of the tube of the invention.

We first recall the general constitution of electroluminescent screens and a number of their properties.

These screens consist of several layers.

Figure 1 shows in section an example of an electroluminescent screen structure.

There is a central layer 4 of zinc sulfide, doped with manganese, disposed between two layers 3 and 5 of dielectric material: for example aluminum oxide, tantalum oxide, silicon nitride, etc. These three layers have approximately equal thicknesses of the order of a few hundred nm; they are taken between two conductive layers 2, 6; layer 2, made of indium tin oxide, is transparent to light radiation emitted by the cell; layer 6 made of aluminum is opaque to these radiations, to avoid any disturbance of the photocathode. In the tube, layer 6 faces the electron bombardment. Between layers 2 and 6, a potential difference is established in operation; this difference in potential, as illustrated, is generally alternative and of any shape: sinusoidal, with pulses of various shapes, rectangular, triangular ..., of varied width and frequency. The whole rests, by the layer 2, on a thick glass support 1, which is not shown in proportions as to its thickness.

This type of screen has the property of emitting light radiation in the visible spectrum when it is excited by the applied voltage. This emission begins only above a certain threshold voltage V _s and increases rapidly in intensity beyond this value. To fix the ideas, it will be indicated that a current value of V _s is approximately 150 volts, the point of use, corresponding to the tention Vu, generally not exceeding 180 volts (Vu-V _s of the order of 30 volts ), under normal operating conditions: point at the extreme top of the curve in Figure 2 described below. The frequency of this very variable voltage is for example 5 kHz, all these figures being given by way of example and depending on the exact composition of the screen, in particular that of layer 4.

For certain compositions of this layer, and in particular for certain manganese contents, of between 1% and 5% of the total by weight, the same type of screen may have a memory effect linked to the hysteresis phenomena of which it is the seat (see Howard - Appl. Phys. Letters vol 31 page 399 September 1977).

Figure 2 showing the luminance L as a function of the voltage V, (L = f (V)), gives an example of one of these hysteresis cycles, where we distinguish the previous threshold voltage V _s and three values Ve _f , V _e and Va, the meaning of which will be given below.

The screen, or electroluminescent cell, is excited by the positive and negative pulses of a voltage of amplitude V _e called maintenance voltage. The cell is not illuminated.

The cell is then energized with an addressing voltage Va (always in + and - pulses). The operating point is located at A and the luminance of the cell is LA.

Again we apply pulses V _{e to it} . The operating point is located at B. The luminance is LB. The cell emits light, whereas in the first case for the same voltage, no light was emitted (V _e less than V _s ).

The amplitude of the pulses is reduced to a voltage V _ef . The cell turns off; no light is emitted: V _ef is the erasing voltage.

The amplitude of the pulses is reduced to V _e : (maintenance voltage). The cell emits no light.

These screens have, moreover, as we have said, the property of responding to an excitation by electron bombardment - and in general also by utraviolet radiation. A voltage being applied to them under the conditions indicated above, the additional contribution of such bombardment makes it possible to move on the hysteresis diagram, this contribution being equivalent to an additional voltage V applied to the screen. Such a contribution therefore constitutes a means of addressing a signal on the screen. This is what is done in the intensifier tubes of the invention in which the output screen consists of one of these electroluminescent screens. The quantity V is here that brought by the bombardment of electrons coming from the screen of entry of the tube.

With reference to FIG. 3, on which this tube bears the overall reference 10, we recall below the general structure of a widely known image intensifier tube.

In a vacuum envelope 20, there is in FIG. 3, an input screen generally by the reference 21, located at the left end of the envelope and, at the other end, an output screen 22 towards which converge (arrows) the electrons emitted by the photocathode incorporated in the screen 21, in which it is placed opposite another part of the screen, possibly in contact with it and called a scintillator, which converts the incident radiation, X-rays in the example, in photons for the use of the photocathode. A series of electrodes, designated overall by the reference 23, each represented with its passage, without reference, ensures the acceleration and the concentration of the electrons towards the exit screen of small size, compared to the entry screen; the output screen in question is represented, as is often the case, arranged at the bottom of an equipotential box without mark, the front face of which is pierced with a small hole at the point of the point of convergence of the electron beam. The object whose image we want to form, exposed to incident radiation (arrows on the left) has the mark 25. We have not shown in the drawing all the sources used, in operation, to supply the tube under the conditions known in the art; on the other hand, the source by which the voltage is applied to the electroluminescent output screen has been shown, with the mark S.

Figure 4 gives some of the possible uses of an intensifier tube according to Figure 3, which will be discussed below.

At the exit from the screen 22 the light beam 12 is taken up either by a photographic camera 26 for the presentation of films, or by a photographic camera 24 for the production of separate photos (radiophotos), or by a television camera 28, for viewing in real time, on a television monitor 30. A photon gain of 150,000 is commonly obtained with an image intensifier, namely 150,000 light photons emitted by the output screen for 1 incident X photon.

• - de la durée de ce bombardement;
• - de la densité des électrons de bombardement.

Finally, concerning the properties of these screens taken advantage of in the invention, it will be specified that the previous increase in voltage LN, resulting from the bombardment of the electroluminescent screen by the electron beam, depends:

• - the duration of this bombardment;
• - the density of the bombardment electrons.

This density is spatially modulated by the object whose image is sought to reproduce, because the intensity of the X-rays having passed through the object is a function of the point crossed.

From the above, it results from a certain number of new possibilities opened up by the intensifier tubes of the invention with an electroluminescent output screen developed below.

The electron beam bombardment induces at each point of the light-emitting cell an internal polarization, the field of which is added to the field created by the alternating voltage applied to the cell; this is equivalent to an increase in the voltage V applied to the cell.

The effects differ depending on the mode of implementation adopted, examples of which are given below.

In a first embodiment, the device operates in pulses: the X-ray is applied to the object in pulses.

During the whole duration of the pulse X, which can be from a few milliseconds, to 1 second, for example, the cell is addressed by the beam, spatially modulated, as we have said; it receives at its various points the image signal, that is to say the information; this information is only perceived on the condition that the voltage applied to the cell is sufficient so that, superimposed on the addressing signal, it is greater than the threshold voltage V _s . This is obtained with an applied voltage V, alternating function of time t of the form represented on figure 5, without stages with V = 0, whose maxima on both sides of the zero are of the order of the voltage d 'maintenance V _e between the values V _ef and V _s . This is also possible, by applying a constant voltage V equal to V _e ; the difference between the two cases being that in the first image is visible during addressing and that in the second it is not.

Reading of the information can then be done in real time during the whole time of addressing, part to the left of the line in the figure; it can be extended, immediately after the X pulse, for the entire desired time, maintaining the maintenance voltage V _e between the faces of the cell, part to the right of the line: it can also be stopped for the desired time and restored again; during this stop there is storage, that is to say memory of the information.

Figure 6 shows the voltage diagram corresponding to the latter case. This interruption can be obtained, as shown in this figure, by giving the applied voltage a value invariable in time but sufficient to prevent erasure, greater than V _ef . This voltage is advantageously the previous voltage V _e . During all the time r that the wide rung of the center of the figure lasts, the cell, under constant tension, gives no image, this being stored extinct; this reappears at the end of this step, when the alternating voltage V _e is again applied to the cell, and remains for the entire time r ₂ that this voltage V _e is maintained. There is therefore the possibility of storing the image for a given chosen time, after addressing, and before a new reading. The operation can be repeated several times, several successive readings of the same image being possible, separated by time intervals where it is stored.

This is a first result to the advantage of the invention compared to the prior art.

But we will also note a certain number of other points.

Concerning in particular the gain, it will be noted that this, given the shape of the curves which generally make up a hysteresis diagram, like that given in FIG. 2 for the electroluminescent cell, has, in the tubes of the invention, a eigenvalue at each of the points of the image, contrary to what happens in tubes of the same kind of the prior art, where this gain, with near screen faults, is the same for all the points. It will be different for a point on the screen to which the states A and B correspond and for a point to which the states A 'and B' shown in FIG. 2 correspond, states which depend, according to the above, on the density electrons in the beam, corresponding to these points. The dynamics of the gain at the surface of the image will be very different from what it was in the prior art, as can be seen on the curves carrying the downward arrows in FIG. 2, which depend on them. -same of the slope of the right curve (rising arrows), characteristic of the selected cell. Thus, to obtain high luminances, it will be possible to choose operating points (V _e ) near the threshold voltage V _s and for high contrasts, values of V _e near V _ef on the contrary.

Another important point, and which constitutes another advantage of the invention, is that the gain in photons, ratio of the number of light photons emitted by a point of the cell to the number of X photons received by the corresponding point of the input screen, can reach extremely high values, much higher than that cited, thanks to the possibility of storage and successive observations for a very long total time; this is of great interest for the production of radiophotos (24, FIG. 4); conversely, with a given gain, it is possible to reduce the radiation dose.

This gain depends moreover, all other things being equal, on the frequency of the alternating voltage applied; it is multiplied by a factor of around 100 when going from 50 Hz to 50 kHz.

In the tubes of the invention, it can be easily adapted to the optimal value corresponding to each of the uses which is made of the image produced on the tube output screen (see FIG. 4), with incident irradiation intensity. fixed.

The integration achieved thanks to the possibilities of storage and repeated readings is also favorable to an improvement of the signal / noise ratio.

Operation in the vicinity of the saturation zone of the curve L = f (V) (FIG. 2) can possibly reduce the quantum noise.

This leads to specify that it is necessary to limit the duration of the addressing, because the tension induced by the bundle in the cell, increasing with each alternation, would risk causing this saturation.

Finally, the thin film structure of the electroluminescent cell, as shown by the thickness figures given above, allows high resolutions of the output images. The cells on their support commonly have the form of discs from 25 to 50 mm in diameter.

The erasure of the image is obtained simply by lowering the amplitude of the applied voltage below the value V _ef . We see the steps that correspond to this phase of the cycle in the right part of Figure 7, in which the part between the two mixed lines on the left corresponds to the reading, which here immediately follows the impulse of the incident radiation as in the case in Figure 5, and the one to the right of the last dashed line to address a new image.

In another embodiment, one operates in continuous X-ray radiation (FIG. 8).

In this operating mode, it operates in real time, that is to say without any information storage. The image is presented during the entire addressing period, then erased; then again, a second image is addressed, and so on.

• - un adressage par bombardement électronique et présentation de l'image en temps réel. Pendant toute la durée de l'adressage, la luminance croît à chaque échelon de la tension appliquée, car la polarisation interne de la cellule croit pendant toute la durée de l'adressage. Pendant cette période, la cellule est alimentée par la tension d'entretien V_e (partie de la figure entre les deux premiers traits mixtes);
• - la deuxième phase du cycle est la phase d'effacement de l'image qui peut se faire sous bombardement électronique; elle consiste simplement à diminuer l'amplitude de la tension appliquée à une valeur inférieure à V_ef, et éventuellement, cette fois, à séparer les échelons de la tension appliquée par des temps morts ou à amplitude nulle (partie de la figure entre les deux derniers traits mixtes).

An image cycle then comprises two phases:

• - addressing by electronic bombardment and presentation of the image in real time. During the whole duration of the addressing, the luminance increases with each step of the applied voltage, because the internal polarization of the cell increases during the whole duration of the addressing. During this period, the cell is supplied by the maintenance voltage V _e (part of the figure between the first two dashed lines);
• - the second phase of the cycle is the phase of erasing the image which can be done under electronic bombardment; it consists simply in reducing the amplitude of the applied voltage to a value lower than V _ef , and possibly, this time, in separating the steps of the applied voltage by dead times or at zero amplitude (part of the figure between the two last mixed lines).

The duration of a cycle can be 20 ms, thus making the presentation of the image compatible with a shooting by photo camera or television camera (marks 26 and 28 in FIG. 4).

The frequency of the maintenance voltage, its amplitude, the duration of addressing, the image frequency (or duration of a complete cycle), the dose X, the acceleration voltage of the addressing electrons are all parameters which allow the presentation of the image to be adapted to the use made of it.

• - une analyse en temps réel, avec reprise par caméra de télévision pour toute image évolutive, suivant le processus qui vient d'être décrit (continu), et
• - un arrêt sur une image fixe, par suppression simultanée de l'adressage, par coupure des X, et de la phase d'effacement du cycle image. La dernière image est alors mémorisée et présentée pendant le temps nécessaire à l'observation suivant le premier mode de fonctionnement, en impulsions.

A mixed operation, combining the two previous modes, can also be envisaged, allows:

• - an analysis in real time, with recovery by television camera for any evolving image, according to the process which has just been described (continuous), and
• - a stop on a fixed image, by simultaneous suppression of the addressing, by cutting of the X, and of the erasing phase of the image cycle. The last image is then memorized and presented for the time necessary for observation according to the first operating mode, in pulses.

Claims (5)

1. An image intensifier tube comprising an input screen exposed to incident radiaton and an output screen, and means for quiding the flow of electrons emitted by a photocathode incorporated in the input screen in the direction of the output screen, in order from thereon the image of the incident radiation, characterized in that the output screen is formed by an electroluminescent cell,

- which is able to emit visible light when an alternating voltage with an amplitude greater than a given threshold V_s is applied between its surfaces,

- which possesses a hysteresis, this light being erased only when the voltage amplitude drops below an erasure value V_ei, which is less than V_s,

- and which is able to be addressed by electron bombardment, i. e. to emit visible light when subjected to such bombardment for a maintenance voltage V_e, which is less than V_s.

2. A method for operating an image intensifier tube as defined in claim 1, characterized in that this cell, when working, comprises means for applying an electric voltage V between its surfaces, this voltage being one of the following:

- a direct voltage V, between V_ef and Vs;

- an alternating voltage V₂ composed of positive and negative rectangular pulses with amplitude between V_ef and V_s, without levels at V = between the pulses;

- an alternating voltage V₃ composed of positive and negative rectangular pulses with amplitude below V_ef

- an alternating voltage V₄ composed of positive and negative rectangular pulses with amplitude below V_el, and with levels at V=0 between the pulses.

3. A method for operating the image intensifier tube as defined in claim 2, characterized in that the voltage V is applied to the cell in accordance with a predetermined cycle, using the voltages V₁, V₂, V₃ and V₄.

4. A method for operating the image intensifier tube as defined in claim 3, characterized in that, if the incident radiation is applied to the object in the form of separate pulses, the cycle comprises the following steps for each such pulse:

A) throughout the duration of the incident radiation pulse, application of voltage Vi or V₂, to ensure the addressing of the cell during this pulse;

B) immediately after the end of the preceding phase, application of V₁ or V₂ for given durations chosen in advance and possibly alternate application of either of these voltages, to maintain the image in the cell in a visible state when voltage V₂ is applied, and in an invisible state when voltage V, is applied;

C) immediately after the end of the previous phase, and before application of the subsequent incident radiation pulse to the object, application of V₃, to erase the image corresponding to this radiation pulse.

5. A method for operating the image intensifier tube as defined in claim 3, characterized in that, if the incident radiation is applied continuously to the object, the cycle comprises the following alternating steps:

A') application of voltage V₂, to address the cell and cause a visible image to appear on it;

B') application of voltage V₄ to erase the image.

Images