EP0053530A1 - Electron multiplying photodetector tube for use in a colour video pick-up - Google Patents

Abstract

Tube permitting the detection of light. It comprises a photocathode (2) followed by a multiplying body (3) drilled with microchannels, then by a detecting anode (4). The anode comprises one or more elements depending on the flux number to be detected, and a mosaic of elements for detecting a video image. Each element constitutes a silicon diode biased in the blocking direction by an adjoining source (7). The invention applies to video pick-ups for colour images, among other things to border mapping indicators. <IMAGE>

Description

The present invention relates to an electron multiplication photodetector tube.

Electron multiplier tubes are known in which the multiplication of a primary electron beam is obtained by a series of secondary electronic emissions inside a set of rectilinear channels, of very small diameter, parallel and placed in a longitudinal electric field.

Multiplier tubes of this kind include an insulating body pierced with very small diameter channels, coated on their internal walls with a very thin resistive layer, endowed with the property of secondary electronic emission with a coefficient greater than unity, while a potential difference, established between the ends of the coatings creates in the channels a longitudinal electric field. With this arrangement, a beam of primary electrons, penetrating into the channels from various angles, gives rise to a series of secondary electronic emissions on the internal coatings so that at the exit of the channels the number of electrons is strongly increased compared to that of the primary beam.

Multiplier tubes of this type and their applications to devices known as "light amplifiers" or "image intensifiers" have been described in various publications, in particular in French patent 1,465,381 of March 24, 1965 relating to improvements according to which the electron multiplier body, or microchannel wafer, is produced by drilling a silicon diode polarized in the blocking direction.

The object of the invention is to change the function of the tube, in this case that it becomes a light detector by modifying its structure, light detection being understood as well as detection of a single luminous flux. , or a limited number luminous flux, or a relatively high number of flux corresponding to the different points of an image.

An object of the invention is to obtain such a photodetector tube by replacing the fluorescent screen normally placed at the rear of the microchannel plate by an anode forming a solid diode polarized in the blocking direction. The anode consists of one or more elements according to the intended use, in particular, a mosaic of elements allows the detection of a video image.

According to another object of the invention, the detector tube is produced with an anode composed of four detector quadrants, this configuration being advantageously usable in video image video player devices (slides, films, etc.) using an analyzer tube with mobile spot called "flying spot" and where we must extract the three channels by trichromatic optical separation followed by photodetection and multiplication of electrons.

• - Fig. 1, un schéma d'un tube détecteur à microcanaux conforme à l'invention ;
• - Fig. 2, un mode de réalisation d'un tube selon la Fig. 1 ;
• - Fig. 3, une réalisation selon la Fig. 2 où l'anode comporte plusieurs éléments ;
• - Fig. 4, un schéma d'une utilisation d'un tube selon l'invention dans un lecteur vidéo d'images couleurs à tube flying spot ; et
• - Fig. 5, une représentation partielle du montage lecteur de la Fig. 4, relative à la séparation optique trichrome et au tube détecteur conforme à l'invention.

The features of the present invention will appear in the description which follows, given by way of nonlimiting example with the aid of the appended figures which represent:

• - Fig. 1, a diagram of a microchannel detector tube according to the invention;
• - Fig. 2, an embodiment of a tube according to FIG. 1;
• - Fig. 3, an embodiment according to FIG. 2 where the anode has several elements;
• - Fig. 4, a diagram of a use of a tube according to the invention in a video image player with a flying spot tube image; and
• - Fig. 5, a partial representation of the reader assembly of FIG. 4, relating to the three-color optical separation and to the detector tube according to the invention.

Referring to FIG. 1, the electron multiplier tube comprises, in a known manner and placed in an enclosure 1 under vacuum, a photocathode 2 followed by a body or wafer 3 pierced with channels, then a third electrode 4. Externally at the enclosure, a high voltage generator represented by a DC voltage source 5 and a resistive divider 6, develops the various voltages supply to produce at the terminals of the electrodes of the tube to establish between the elements 2, 3 and 4, the potential differences desired to ensure operation, these voltages Ul, U2, U3, which can be from a few hundred to a few thousand volts . The ends of the channels facing the photocathode are brought to a higher potential than the photocathode and the opposite ends to a lower potential than the electrode 4.

According to the invention, the electrode 4 is a planar anode constituted by a solid diode polarized in the blocking direction by a continuous polarization source 7 annex, this anode itself being produced in the form of one or more distinct elements. In the version shown, a single detector element is considered for simplification. The anode is advantageously made with a P-type silicon substrate in which a fine N-type diffusion has been carried out, so as to obtain a PN junction reverse biased by the source 7. The PN junction is preferable to the NP junction for questions of speed and quantum efficiency. The junction can also include a Schottky barrier. The detected video signal SV is recovered on the corresponding electrode at the terminals of a load resistor 8.

To process a light image the tube must, in a conventional manner, be associated with a receiving optic 9 whose function is to produce, by focusing or otherwise, the light image in the plane of the photocathode. The optics can consist of a dioptric or catadioptric objective, possibly followed by a window in optical fibers. To process a single luminous flux, or a limited number of distinct incident fluxes, the optics 9 is not necessary.

When a luminous image (or a flux) is projected on the photocathode 2, this one emits electrons which penetrate in the channels of the wafer 3. This beam of primary electrons causes on the walls of the channels of the emissions of electrons secondary ones which in turn hit these walls, and so on. As the emission factor is greater than unity, the electrons are found multiplied at the outlet of the channels and produce on the anode 4 an electronic image intensified with respect to the initial image (or flow) corresponding to the primary electron beam.

After amplification by the wafer 3, the electrons emitted reach the anode 4. These electrons are accelerated by the voltage U3 of high value, for example 5 KV, so as to present a significant kinetic energy, in this case 5 KeV. By striking the anode 4, they create electron-hole pairs which are separated by the field prevailing in the space charge of the P-N junction, and a detected current IS can thus flow in the external circuit. To create an electron-hole pair by electron bombardment in silicon, an energy of the order of 3.5 eV per incident electron is required. In the present case, the energy of an electron can be of the order of 5 KeV, which means that the current gain can be of the order of 1,400. However, taking into account a layer without charge , called dead, a gain of 1000 can be obtained. The creation of the pairs takes place at a depth of penetration into the silicon of the order of 1 μm and the created pairs can be separated by the junction close to the receiving surface with a yield close to unity.

The gain G ₂ thus obtained by the detection 4 is added to the electronic gain G _l of the microchannels 3, so that the current IS which leaves the device is equal to G _l x G ₂ times the value IO of the electron current which attacks the microchannels. Taking into account the low efficiency of the photocathode, the total gain of the tube can, in the aforementioned concept, be between 1.5 10 ⁵ and 3 10 ⁵ , these values being indicated without limitation.

For the detection of several luminous fluxes arriving at the entry, as well as for the image detection, the tube is preferably arranged, upstream of the electron multiplier according to the assembly of FIG. 2 or 3. This arrangement is known in light image intensifier tubes incorporating a photocathode 11 arranged on an input window 12 in optical fibers to convert the input light image into an electronic image and a focusing anode 13 in the form of a cone acting as an electrostatic lens; a correcting electrode 14 can also be placed between the conical anode 13 and the multiplier wafer 3, in order to improve the linearity and the geometry of the image. As the electrostatic lens optically combines photocathode 11 and the input face of microchannels 3, the tube can be used to amplify separately and in parallel several optical signals incident simultaneously on photocathode 11. Figure 3 shows the path of two separate beams F1, F2, but is not limiting. The electrons emitted by photocathode 11 because of the light flux F1 strike the region A of the microchannel plate; similarly the distinct flow F2 reaches the distinct region B of the wafer. These two electron flows are amplified separately in the microchannel amplifier and it suffices to have two anodes 4-1 and 4-2 to obtain the currents IS1 and IS2 corresponding respectively to the amplified optical input signals F1 and F2.

In the case of an optical image projected on the photocathode 11, the anode 4 will consist of a detector mosaic, each element of which corresponds to a point of the electronic image and therefore to an elementary flux located at the level of the input. of the tube. With a mosaic of P = nxm elements distributed in n rows and m columns, we thus treat P incident elementary fluxes leading to the photocathode 11 at the conjugate locations of P detector elements. We could use for example a mosaic in charge transfer device, or CCD (charge coupled device).

Given the photodetector property presented by solid diodes not isolated from light, the planar anode 4 can be sensitized by residual light radiation which would have passed through the electrodes upstream. To eliminate and protect from stray light radiation, a very thin, opaque screen is interposed on the electronic path. This screen can be produced in the form of a deposit 15 on one of the lateral faces of the multiplier wafer, it being understood that it covers the entire face in question. The screen 15 may consist of a very thin layer of silica (SI0 ² ) or of alumina (AI 0) or of any other light metal oxide. In addition to the property of being both opaque for a flow of photons and transparent for a current of electrons, the screen 15 retains the ions which are torn from the microchannel electrode 3 and which would otherwise be transported on the photocathode, thus preserving the technical characteristics of the tube.

One of the possible applications of the tube according to the invention relates to the detection of signals from the trichrome analysis of a color image carried by a film.

Fig. 4 shows a color image video player corresponding to such an application and using a mobile spot analyzer tube, commonly called a flying-spot, to scan the film. It comprises the flying-spot tube 20, with its scanning circuits 21, a lens 22 which forms the image of the front face of the tube on the film 23 to be analyzed, a condenser 24 which makes the emerging beam practically parallel and a separator three-color optics 25. The separator consists of prisms comprising dichroic or interference mirrors 26, 27 and reflecting mirrors 28, 29 to select the three beams each carrying information contained respectively in the blue, red and green spectral bands. These three beams emerge in parallel and reach a microchannel tube according to FIG. 3 and the detector anode of which consists of four detector quadrants as shown in more detail in FIG. 5 with a special prism separator assembly enabling the beams to be located in three of the quadrants.

The separator assembly comprises, along the axis Zl of the condenser 24, an assembly of three prisms 31, 32 and 33. The prism 31 receives the light flux of axis Zl on an inlet face and has a downstream face in contact with a corresponding face of the prism 32 to form, by an appropriate optical treatment, the first dichroic mirror 26. This mirror reflects one of the spectral bands to be filtered, for example red R, in a vertical direction to reach on a reflecting face of a fourth prism 34; this reflecting face corresponds to the mirror 28. The prism 32 has a second contact face this time with the prism 33 to form the second dichroic mirror 27 and reflect the blue spectral band B in a horizontal direction and therefore perpendicular to the previous one, so to be able to dispose laterally a fifth prism 35 which includes the reflecting face 29. The exit faces of the prisms 33, 34 and 35 are coplanar respectively delivering the green light flows V, red R and blue B, along parallel axes. These fluxes are received by the optical fiber entry window of the tube 1 and, taking into account the inversion produced by the electrostatic lens, the corresponding detection is carried out respectively if the qudrants 4-1, 4-2 and 4 -3 of the anode, conjugates of the exit faces of the prisms 34, 33 and 35.

The video signals delivered by the detector quadrants are then further amplified in circuits 40-1, 40-2 and 40-3 which deliver the three standard chromatic components R, G, B.

The fourth quadrant of the anode is used as follows: one or more optical fibers, such as 36 and 37, have one end disposed upstream of the film, preferably near the image forming object 22. These fibers 36 and 37 thus collect a light energy proportional to that emitted by the spot of the tube 20. The exit ends of the fibers illuminate through the tube 1 the fourth detector quadrant 4-4 which is followed by a similar amplifier 40-4 to those 40-1 to 40-3 of the other routes. The output of this amplifier is applied simultaneously to three analog divider circuits 41-1 to 41-3 interposed respectively on the three channels R, G and B.

By designating by SR, SV and SB, the respective currents of the channels R, G and B, and by S0 the reference current coming from the fourth quadrant, the terminal signals are given by the ratios:

SR '= SR / SO, SV' = SV / SO, SB '= SB / SO

the values obtained thus being independent of fluctuations in the luminosity of the analysis tube 20 and the total gain of the tube 1.

• - toute la partie détectrice 1 est en circuit solide ;
• - le dispositif ne possède pas la sensibilité microphonique des photomultiplicateurs ;
• - il n'y a qu'une seule alimentation THT pour l'ensemble des détecteurs ; cette amélioration est importante parce que dans un photomultiplicateur le gain varie proportionnellement à Vⁿ, où V est la valeur de la THT et n le nombre de dynodes amplificatrices. Il en résulte la nécessité d'une parfaite stabilisation des THT. Dans le lecteur proposé, le gain des trois canaux de couleur est le même, puisque l'amplificateur 1 est commun aux trois voies. La couleur du point analysé ne dépendant que du rapport des trois composantes RVB entre elles, une variation de la THT, même importante, n'affecte pas la chrominance du point ;
• - les quatre chaines d'amplification 41-1 à 41-4 sont identiques et délivrent des signaux vidéo normalisés dont on a enlevé le bruit dû à la fluctuation d'émission du luminophore du tube d'analyse (non homogénéité de la couche ; variation de THT, etc...).

The video player which has just been described has many advantages compared to solutions with photomultipliers:

• - The entire detector part 1 is in solid circuit;
• - the device does not have the microphonic sensitivity of the photomultipliers;
• - there is only one THT supply for all the detectors; this improvement is important because in a photomultiplier the gain varies proportionally to V ⁿ , where V is the value of THT and n the number of amplifying dynodes. This results in the need for perfect stabilization of THTs. In the proposed reader, the gain of the three color channels is the same, since the amplifier 1 is common to the three channels. Since the color of the point analyzed depends only on the ratio of the three RGB components to each other, even a significant variation in THT does not affect the chrominance of the point;
• - the four amplification chains 41-1 to 41-4 are identical and deliver standardized video signals from which the noise has been removed due to the fluctuation in the emission of the phosphor from the analysis tube (non-homogeneity of the layer; variation THT, etc ...).

Other advantages also result from the volume and weight reductions of this video player device thanks to the microchannel detector tube when the use requires corresponding characteristics (high compactness, low weight). This is the case for the cartographic indicators making it possible to display a geographical map on board a vehicle, in particular on board an airplane, the map corresponding to the region overflown.

The photodetector and electron multiplier tube with microchannels described above admits several variants conforming to the characteristics set out and which fall within the scope of the present invention. As already mentioned, the tube may or may not be equipped with an optical objective (9, Fig.l), may or may not include an electronic optic (Fig. 1 or 2) and in the absence of this the photocathode 2 will be located very close to the microchannel wafer 3. The number and the shape of the detector elements constituting the anode 4, are determined according to the envisaged application, detector with four quadrants as we have seen or any other form, strip for example.

Claims (10)

1. Electron multiplication photodetector tube comprising, in a vacuum enclosure (1), a photocathode (2) emitting a primary electron beam on reception of light radiation, a microchannel wafer (3) for multiplying said beam by a series of secondary electronic emissions inside the rectilinear, parallel channels placed in a longitudinal electric field, a high voltage generator (5-6) for producing the supply voltages of the respective electrodes of the tube, the tube being characterized in that the electrode downstream of the microchannel plate (3) is an anode (4) formed of at least one element and constituting a solid diode polarized in the blocking direction by an auxiliary voltage source (7 ). 2. Tube according to claim 1, characterized in that the anode is formed of a plurality of elements for detecting a plurality of distinct light fluxes received at the entrance of the tube on the photocathode. 3. Tube according to claim 2, characterized in that it comprises a mounting known per se forming an electrostatic lens upstream of the microchannel plate, this mounting grouping a conical anode (13) preceded by the photocathode (11) deposited on a fiber optic entry window (12) and optionally followed by a linearity correcting electrode (14). 4. Tube according to any one of claims 1 to 3, characterized in that it further comprises a screen (15) opaque to light, transparent to electrons, and which is interposed on the electronic path reaching the anode . 5. Tube according to claim 4, characterized in that the screen is a deposit of light metal oxide, silica, alumina or other effected on one side of the microchannel pancake. 6. Tube according to any one of claims 1 to 5, characterized in that it further comprises a receiving optic (9) for focusing an image at the entrance of the tube and that the anode is a detector mosaic. 7. Tube according to any one of claims 1 to 5, characterized in that the anode and a four-quadrant detector (4-1 to 4-4). 8. Use of a tube according to claim 7 in a color image video player comprising a film player (24) with flying-spot tube (20), a three-color optical separator (25) and photodetection and d amplification, characterized in that said means are constituted by said tube with four quadrant detectors followed by identical video amplifiers (40-1 to 40-4), the three-color separator being constituted by a prism arrangement (31 to 35) comprising two dichroic mirrors (26-27) and two reflecting mirrors (28-29) to produce the separation of the red (R) green (V) and blue (B) channels according to output faces corresponding to three of the quadrants. 9. Use of a tube according to claim 8, characterized in that it further comprises at least one optical fiber conductor (36, 37) for taking a fraction of the light energy emitted by the spot of the flying tube- spot, said conductor being positioned by one end upstream of the film and coupled by its other end with the electron multiplier tube with microchannels to cooperate with the fourth detector quadrant (4-4), the corresponding detected signal (SO) being applied after amplification respectively with three analog divider circuits (41-1 to 41-3) interposed on the detection channels (R, G, B) downstream of the amplifiers. 10. Use of a tube according to claim 7 or 8 for producing an on-board cartographic indicator.

Images