EP0020234A1 - Pick-up target, tube provided with such a target and pick-up device comprising such a tube - Google Patents

Abstract

The invention relates to a shooting target. The signal plate (110) which covers on one of its faces the target proper (10) is divided into several elementary signal plates (101), (102), (103) ... electrically independent, in order to reduce the parasitic capacity when picking up the signal on this plate. In a preferred arrangement, these elementary plates are oriented like the lines of scanning by the reading electron beam. This reading is made either by a single beam or by a number of independent beams each covering a portion of the target. Same applications as in the prior art, especially taking infrared shots.

Description

The invention relates to a shooting target, as well as the shooting tube provided with such a target, and the entire device formed by the tube and its reading means.

The target of the invention can be of varied structure, photoconductive target in one of the usual materials, such as antimony sulfide (Sb ₂ s ₃ ), lead oxide (Pbo), etc., or mosaic target photodiodes-formed in a silicon substrate, or pyroelectric target ... The invention applies generally to all these kinds of targets used for taking pictures, without distinction as to their nature.

The target has on one of its faces a conductive plate, or signal plate, from which the electrical signal corresponding to the different points of the target is sampled, during point-to-point scanning of the other face thereof, by the reading electron beam. The beam deposits at each point a certain quantity of electrons to compensate for the effect produced at this point in the substrate by the incident radiation. This quantity, read in the circuit of the signal plate, constitutes the signal of the point.

One of the problems encountered in the technique of these targets, and in that of shooting in general, is that of the noise inherent in these systems, noise coming from various causes, and of the target itself in particular.

However, it appears that the capacity between the signal plate and the earth of the system is one of the causes of this noise.

The present invention relates to a target having a reduced signal plate capacity.

• - figure 1 : une vue schématique d'un dispositif de prise de vues de l'art antérieur ;
• - figure 2 : un schéma électrique équivalent se rapportant au dispositif de la figure précédente ;
• - figure 3 : une vue en perspective d'une cible de l'invention ;
• - figures 4 et 5 : des schémas montrant deux des systèmes de commutation utilisés dans les dispositifs de prise de vues 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 view of a shooting device of the prior art;
• - Figure 2: an equivalent electrical diagram relating to the device of the previous figure;
• - Figure 3: a perspective view of a target of the invention;
• - Figures 4 and 5: diagrams showing two of the switching systems used in the picture taking devices of the invention.

Below is given the general description of a prior art camera, shown schematically in Figure 1. In this figure we distinguish the target 1 and its two constituent parts, namely the target itself 10 , consisting of a plate of a photosensitive material, and, applied to one of the faces of this plate, the signal plate 11, from which, as we have said, the signals from the different points are taken in operation of the target. The incident radiation arrives from the right of the figure, on the side of the signal plate, which has good transparency to this radiation, represented by the wavy arrow.

On the other hand, there is a distinction between the picture tube, designated as a whole by the reference 2, and the vacuum envelope of which bears the reference 20. Inside thereof a cathode 21 provides , in operation, an electron beam, e (bent arrow), directed towards the target and scanning, as is known in the art, the target in question point by point; the means used to beam deflection to ensure this scanning have not been shown in this schematic view as known in the art. A grid 22 placed in front of the target is connected to the voltage source U _G.

Finally, in FIG. 1, a distinction is made between the preamplifier 3, at the output of which the signal, for example video, of the target is collected.

In a current arrangement, which will be taken as an example to fix ideas, the signal plate is polarized with respect to ground by the voltage source V _C , or target voltage, by means of a bias resistor Rp of 5 MQ. The preamplifier 3, with low input resistance, comprises two stages, the first of which is constituted, in the example, by a field effect transistor with junction 30, at low noise level, the source and the drain of which are represented in S and D respectively, and the grid in G; the second stage consists of an operational amplifier 31, the output A of which is that of the reading device. The signal plate 11 is connected to the transistor by a connection capacity C _L of the order of 10 nanofarads. The loop 40 includes a resistance R _F of the order of a few megohms; the drain of the field effect transistor 30 is polarized with respect to ground by the voltage source V and the resistance R _L.

Figure 2 gives the equivalent electrical diagram of the device object of Figure 1, for the AC component of the target current i (left arrow) flowing through the device. In this figure, C _p represents the parasitic capacitance of the signal plate (reference 11 on the overview of the previous figure) and which is of the order of 8 picofarads in the example, that is to say the capacity between the signal plate in question and the mass, and that connections with respect to the same ground. The capacitance C _L of the previous figure n ^t is not represented, because for the alternative, it is equivalent to a short circuit; C _S and C _D denote in the figure the capacities of the gate of the junction transistor with respect to the source and the drain thereof, of 2.5 and 1.5 picofarads respectively; the mark g represents the transconductance of the junction transistor; V _g represents the AC component of the voltage at the gate of the transistor.

In practice, among the significant noise sources of devices of this kind, one can cite the Schottky noise associated with the target current, the lower the lower the target current, the thermal noise associated with the resistors R _p and R _F , the lower as these resistances are high, and the noise associated with the noise voltage at the junction transistor. The noise associated with the target, generation and recombination noise in the case of a semiconductor target, thermal noise in the case of a pyroelectric target, is generally negligible compared to the other noise sources. The noise current associated with the first stage is also negligible in the case of a junction field effect transistor.

où C_T est égale à la somme des capacités parasites et où en désigne la tension de bruit du premier étage, c'est-à-dire du transistor à effet de champ dans l'exemple. On a C_T = Cp + C_s + C_D ; dans cette formule, B désigne la bande passante du dispositif, proportionnelle à la fréquence image et au nombre de points de la cible, c'est-à-dire à la résolution. Le courant i_B est de l'ordre de quelques centaines de picoampères.It can however be shown that, as a whole, the noise of such a device is equivalent to a target noise current which will be designated by i _a and of expression:

where C _T is equal to the sum of the stray capacitances and where denotes the noise voltage of the first stage, that is to say of the field effect transistor in The example. We have C _T = Cp + C _s + C _D ; in this formula, B denotes the bandwidth of the device, proportional to the image frequency and to the number of points of the target, that is to say to the resolution. The current i _B is of the order of a few hundred picoamps.

We see on expression (1) that the target noise i _B is a direct reason, of the total capacity C _T. For a given first stage, that is to say e _n , C _S and C _D given and with given bandwidth B, the noise is lower when the parasitic capacitance C _p is lower.

According to the invention, the reduction in capacity C is obtained by splitting the signal plate into several parts, electrically isolated from each other, under the conditions which will be specified.

To fix the ideas we reasoned in the above on a first stage of the preamplifier constituted by a field effect transistor with junction.

The preceding conclusion remains valid, in general, for any device for taking pictures using a target whose signal is taken by a signal plate, whatever the structure of the preamplifier stage to which it is connected. All other things being equal, the target noise decreases with the capacity of the signal plate.

Figure 3 shows in perspective a target of the invention generally designated by the reference 1, as in Figure 1, and composed as in the case of this figure of the target itself 10 and the signal plate. The latter bears the reference 110 and differs from that of FIG. 1 in that, in the invention, it is made up of several separate parts, electrically isolated from each other as shown in the drawing, and to which the references 101, 102, 103 have been given; in the figure, for clarity, the proportions of the elements, in particular their thicknesses, have not been respected.

The different portions of the target signal plate of the invention, or elementary plates, can have any orientation with respect to the direction of scanning of the target by the reading beam; however, in a preferred embodiment of the invention, they are arranged parallel to the direction of this scan.

The signal plate is thus divided into p elementary signal plates; p is equal to -, N being the number of lines of the scan, for example of television, and n the number of lines of this scan arranged opposite the elementary signal plate considered. The previous capacity C is divided by p. It goes without saying that the maximum value of p is N, that is to say the number of lines of the scan; in this case there are as many elementary plates as there are scanning lines.

Each of the elementary signal plates is connected to a preamplifier. A switching system makes it possible to switch at all times the output of the reading device to the preamplifier associated with the elementary signal plate which receives the single reading or analysis beam, according to known addressing techniques. The p preamplifiers as well as the addressing register can be, depending on the case, inside or outside the camera tube, which has the corresponding number of outputs.

The scheme of such a switching system is given in Figure 4; four elementary nameplates are represented by rectangles without markers; each covers on the target, in the example, the surface of five scanning lines (broken lines). The p preamplifiers, limited to four in the example, p _l , P ₂ , P ₃ , P ₄ , are connected sequentially to the output A of the output amplifier a by switching transistors t ₁ , t ₂ , t ₃ , t ₄ . An address register R, the scanning of which is synchronous with the scanning of the target by the reading beam, allows the sequential addressing of the gates of the transistors.

The targets of the invention and their reading device can be produced in various ways, which can be classified into two categories, fully integrated or hybrid; in the first, the preamplifiers are integrated on the same substrate as the target. However, in the current state of the art of integrated circuits, it is difficult to obtain very low noise levels. The lowest noise voltage of an integrated operational amplifier is, in nanovolts, 4. √B, B being the bandwidth measured in hertz. This is why we will prefer the bybride type embodiment for the targets of the invention, in which the preamplifiers are produced in the form of separate "chips" bonded to a common substrate, which may be the window of the shooting tube, c that is to say the part of its envelope exposed to incident radiation: right-hand end face of this envelope in FIG. 1

faisceaux d'analyse. Chacun des P k faisceaux de lecture analyse en parallèle les k plaques signal élémentaires du groupe.Reading of the target of the invention, with a signal plate divided into several elementary signal plates, can be done by means of several reading beams, each of these being used to read the lines located opposite a plate or a group of elementary plates. Let k be the number of elementary plates per group; k is a sub-multiple of p, at most equal to p, which corresponds to the case of a single reading beam considered above, and at least equal to 1, which corresponds to an analysis beam per elementary plate . In the intermediate situation there is

analysis beams. Each of the P k reading beams analyzes in parallel the k elementary signal plates of the group.

In this case, a device for switching by analysis beam is used for reading, making it possible to connect sequentially to each of the outputs of the tube the k preamplifiers associated with each of the k plates of the group.

The pk electron beams required are obtained either from a single cathode and an electronic optic making it possible to divide the emitted beam into pk elementary beams, or a system of diaphragms arranged in the immediate vicinity of the cathode, or from pk elementary cathodes. Possibly the means of focusing and vertical and horizontal deflection are common to all the elementary beams.

Figure 5 shows the switching scheme in this case; the elementary signal plates, represented by the rectangles, six in the example, from the left of the figure, cover the space of n lines of the scan which comprises in all N; in the example n = 5 and N = 30. The same reading beam is used for a group of three elementary signal plates: on ak = 3; the preamplifiers have the references p ₁ , p ₁₀ , p ₁₁ and p _{2 '} p _20' p ₂₁ ; the figure shows only two of these groups, to which correspond the two address registers r _I and r ₂ and the two outputs A ₁ and A ₂ , each corresponding to a group of three transistors, mounted as in the example in FIG. 4, and without markers for clarity.

The advantage of using several beams of analysis is apparent from the following.

faisceaux élémentaires identique à la vitesse de balayage dans le cas d'un seul faisceau (bande passante inchangée). La période trame est alors T' =

où T est la période trame dans le cas d'un seul faisceau ; T' représente la durée séparant deux analyses successives d'un même point. Cette réduction de la période trame dans un rapport

est favorable notamment à la lecture d'une cible pyroélectrique dans laquelle la résolution spatiale est limitée par la diffusion latérale de la chaleur au sein du matériau pyroélectrique ; or la longueur de diffusion est proportionnelle à la racine carré du temps d'intégration, généralement confondu avec la période trame T'.In a first type of use, a scanning speed of the

elementary beams identical to the scanning speed in the case of a single beam (bandwidth unchanged). The frame period is then T '=

where T is the frame period in the case of a single beam; T 'represents the duration separating two successive analyzes of the same point. This reduction of the frame period in a report

is particularly favorable for reading a pyroelectric target in which the spatial resolution is limited by the lateral diffusion of heat within the pyroelectric material; however the diffusion length is proportional to the square root of the integration time, generally confused with the frame period T '.

This reduction is also favorable for reading a mosaic target of photovoltaic detectors, or MIS, sensitive to infrared radiation, and where the integration time is limited by the generation due to the continuous background.

, de même que la bande passante. Le signal est également divisé par

. Par contre, puisque le temps d'analyse d'un point donné se trouve de ce fait multiplié par

, on peut avoir la même efficacité de lecture des points de la cible avec une résistance de faisceau plus élevée d'un facteur

, donc avec un courant de cible plus petit d'un facteur

, la résistance de faisceau étant inversement proportionnelle au courant de cible. Le bruit Schottky associé au courant de fais- ceau proportionnel à

(i_c désignant le courant de cible et B la bande passante) est donc globalement divisé par

, puisque i et B sont chacun séparément divisés par ce facteur. Le bruit associé au préampli-ficateur,

, est divisé par

à cause de la réduction de la bande passante, et par un terme supplémentaire à cause de la réduction de la capacité C_{T =} C_p + C_S + C_D due au fractionnement de la plaque signal. Globalement, en conservant la période trame T et en utilisant

faisceaux d'analyse en parallèle, on obtient donc un gain sensible du rapport signal sur bruit.In another type of use, on the contrary, the frame period T is kept; the scanning speed is then divided by

, as well as the bandwidth. The signal is also divided by

. On the other hand, since the analysis time for a given point is therefore multiplied by

, you can have the same reading efficiency of the target points with a beam resistance higher by a factor

, so with a target current more small by a factor

, the beam resistance being inversely proportional to the target current. Schottky noise associated with the beam current proportional to

(i _c denoting the target current and B the bandwidth) is therefore globally divided by

, since i and B are each separately divided by this factor. The noise associated with the preamplifier,

, is divided by

because of the reduction in bandwidth, and by an additional term because of the reduction in capacity C _{T =} C _p + C _S + C _D due to the splitting of the signal plate. Overall, keeping the frame period T and using

analysis beams in parallel, we therefore obtain a significant gain in the signal to noise ratio.

The applications of the target of the invention are the same as those of the targets of the known art, in particular taking infrared shots.

Claims (8)

1. Shooting target, composed of a flat substrate and a signal plate, electrically conductive, applied to one side of the substrate, the other side of which is scanned point by point, in operation, by an electron beam, which brings the quantity of charges necessary to compensate for the effect produced by the incident radiation on the substrate, which quantity of charges, read in the circuit of the signal plate, constitutes the signal corresponding to the point scanned, characterized in that the signal plate is divided into several elementary electrically independent signal plates. 2. A shooting target according to claim 1, characterized in that the elementary signal plates consist of bands parallel to each other and parallel to the direction of scanning of the substrate by the electron beam. 3. A target for taking pictures according to claim 2, characterized in that each of the signal plates faces several scanning lines. 4. Camera tube, comprising, inside a vacuum envelope, means producing the emission of electrons and a target towards which these electrons are directed, and means focusing these electrons and deflecting them from so that they produce on one of the faces of the target substantially punctual impacts, moving from one point to the other of the target in a network of rows and columns, characterized in that the target in question is a target according to claim 1. 5. Tube of shots according to the claims tion 4, characterized in that the electrons are emitted in a single brush sweeping successively all the points of the target. 6. An image pickup tube according to claim 4, characterized in that the electrons are emitted in several brushes, each scanning simultaneously, all of the points of the target situated opposite a group of elementary signal plates . 7. Camera, composed of: - an image pickup tube comprising a target and means for scanning one of the faces of this target point by point with an electron brush produced in the tube and depositing electrons therein at each of these points; - a device for reading the charges circulating in the circuit of the signal plate covering the opposite face of this target, characterized in that the image pickup tube is a tube according to claim 5, and characterized in that the reading device includes a preamplifier by elementary signal plate and a switching system ensuring sequentially the switching of each preamplifier to the single output of the device. 8. Camera, composed of: - an image pickup tube comprising a target and means for scanning one of the faces of this target point by point with an electron brush produced in the tube and depositing electrons therein at each of these points; - device for reading the charges circulating in the circuit of the signal plate covering the opposite face of this target, characterized in that the image pickup tube is a tube according to claim 6, and characterized in that the reading device comprises a preamplifier for each signal plate and as many switching systems as there are groups of elementary signal plates, each system sequentially ensuring the switching of each preamplifier at an output of the device common to the plates of the same group.

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