Electron multiplying photodetector tube for use in a colour video pick-up.

p0001The present invention relates to a photodetector tube for electron multiplication.

p0002electron multiplier tubes are known in which the multiplication of a primary electron beam is obtained by a series of secondary electron emissions within a set of straight channels of very small diameter, parallel and placed in a longitudinal electric field.

p0003The photomultiplier tubes of this type include an insulative body, pierced with channels of very small diameter, coated on their inner walls with a very thin resistive layer, endowed with the secondary electron emission property with a coefficient greater than unity, while a potential difference established between the ends of coatings created in the channel a longitudinal electric field. With this arrangement, a primary electron beam, entering the channels from various angles, resulting in a series of secondary electron emission on the inner linings so that the output channels the number of electrons is strongly increased relative to that of the primary beam.

p0004The multiplier tubes of this type and their applications to devices called "light amplifier" or "image intensifiers" have been described in various publications, including in the French patent 1465381 of 24 March 1965 on the improvements according to which the body electron multiplier or microchannel plate, is produced by drilling a biased silicon diode in the blocking direction.

p0005The invention aims to change the function of the tube, in this case it becomes a structure modified by light detector, the detection light is heard as well as detection of a single light flux or a limited number of luminous flux, or a relatively large number of streams corresponding to different parts of an image.

p0006An object of the invention is to obtain such a tube photodetector replacing the fluorescent screen normally positioned at the rear of the microchannel plate by an anode forming a solid polariseée diode in the blocking direction. The anode consists of one or more elements according to the intended use, in particular, a mosaic of elements allows detection of a video image.

p0007According to another object of the invention the detector tube is formed with an anode consisting of four quadrant detectors, this configuration being advantageously usable in image color video players devices (slides, films, etc ...) using a pickup tube moving said spot "flying spot" and where one has to extract the three-way optical trichromatic separation followed by photodetecting and multiplying electrons.

p0008The features of the present invention will appear in the following description given by way of example using the attached figures which show:<ul><li>- Fig. 1, a diagram of a microchannel detector tube according to the invention;</li><li>- Fig. 2, one embodiment of a tube according to FIG. 1;</li><li>- Fig. 3, an embodiment according to Fig. 2 wherein the anode comprises a plurality of elements;</li><li>- Fig. 4, a diagram of a use of a tube according to the invention in an image reader video color flying spot tube; and</li><li>- Fig. 5, a partial representation of the drive assembly of FIG. 4, concerning the optical three-color separation and the detector tube according to the invention.</li></ul>

p0009Referring to Fig. 1, the multiplier electron tube comprises, in known and placed in a chamber 1 vacuum manner, a photocathode 2 followed by a body 3 or drilled channels wafer, and then a third electrode 4. Externally to the enclosure, a high voltage generator represented by a DC voltage source 5 and a resistive divider 6, develops the various supply voltages to be produced across the electrodes of the tube to be established between the elements 2, 3 and 4, the differences of potential desired to ensure operation, these voltages Ul, U2, U3, which may be a few hundred to a few thousand volts. The ends of the opposite channels of the photocathode are raised to a higher potential than the photocathode and the opposite ends, at a lower potential than electrode 4.

p0010According to the invention the electrode 4 is an anode constituted by a planar solid diode poled in the blocking direction by a DC bias source 7 Annex, this anode itself being designed as one or more separate elements. In the version shown, a single detector element is considered as simplification. The anode is preferably carried out with a-type silicon substrate P wherein a thin N-type diffusion has been performed, so as to obtain a PN junction reverse biased by the source 7. The PN junction is preferable to the NP junction for reasons of speed and quantum efficiency. The junction may also comprise a Schottky barrier. SV detected video signal is recovered at the corresponding electrode terminals of a load resistor 8.

p0011For treating a light image the tube must, conventionally, be associated with an optical receiver 9 whose function is to produce, by focusing or otherwise, the light image in the plane of the photocathode. The optics may be a dioptric or catadioptric objective, optionally followed by a fiber-optic window. To process a single lumen, or a limited number of separate incidents flows, optics 9 is not necessary.

p0012When a light image (or flow) is projected onto the photocathode 2, it emits electrons which penetrate into the channels of the wafer 3. The primary electron beam causes the walls of the channels of the electron emission side which in turn strike the walls, and so on. As the emission coefficient is greater than unity, the electrons are multiplied to the output channel and occur on the anode 4 an electronic image intensified with respect to the picture (or flow) of the corresponding initial beam primary electrons.

p0013After amplification by the wafer 3, the emitted electrons reach the anode 4. These electrons are accelerated by the voltage U3 of high value, for example 5 kV, so as to have a high kinetic energy, in this case of 5 KeV. By striking the anode 4, they create electron-hole pairs which are separated by the field prevailing in the area of ​​the PN junction load and a detected current IS can thus flow through the external circuit. To create an electron-hole pair by electron bombardment in silicon requires a power of about 3.5 eV incident electron. In this case, the energy of an electron may be in the range of 5 KeV, so that the current gain can be on the order of 1 400. Taking into account, however, a layer without load , read only, a gain of 1000 can be obtained. Creating pairs is at a penetration depth in the silicon of the order of 1 um and the created pairs may be separated by the junction close to the receiving surface with an efficiency of unity.

p0014The gain G₂ as provided by the detection 4 is added to the electronic gain G_l microchannels 3, so that the current IS which leaves the device is equal to G_l x G₂ IO times the value of the current of electrons that attack the microchannels. Considering the low yield of the photocathode, the total gain of the tube may, within this concept, between 1.5 10⁵, And 3 10⁵, These values ​​being given as non-limiting.

p0015For the detection of several luminous flux reaching the entrance, and to the image sensing tube is preferably arranged upstream of the electron multiplier according to the assembly of Fig. 2 or 3. This arrangement is known in the intensifier tubes light image incorporating a photocathode 11 disposed on an entrance window 12 optical fiber for converting the input light image into an electronic image and a focusing anode 13 tapering acting as an electrostatic lens; a correcting electrode 14 may also be disposed between the conical anode 13 and the wafer multiplier 3 in order to improve the linearity and the geometry of the image. As the electrostatic lens in the optical sense combines the photocathode 11 and the input face of the microchannel 3, it can use the tube for amplifying separately and in parallel several optical signals incident simultaneously on the photocathode 11. Figure 3 shows the path of two separate beams F1, F2, but is not limiting. The electrons emitted by the photocathode 11 because the light flux F1 strike the region A of the microchannel plate; so the separate stream F2 reaches the distinct region B of the plate. These two streams of electrons are amplified separately in the microchannel amplifier and just have two anodes 4-1 and 4-2 for the IS1 and IS2 currents respectively corresponding to the input optical signals F1 and F2 amplified.

p0016In the case of an optical image projected on the photocathode 11, the anode 4 will consist of a mosaic detector, each element corresponds to a point of the electronic image and hence a localized elementary stream at the input the tube. With a mosique P = n × m elements divided into n rows and m columns, and treated P elementary flux incident on the photocathode 11 leading to sites conjugates P detector elements. We can use such a mosaic of charge-coupled device, or CCD (charge coupled device).

p0017Since the photodetecting property presented by the solid diodes uninsulated light, planar anode 4 may be sensitized with a residual light radiation which would have passed through the upstream electrode. To remove and protect themselves from stray light radiation, a very thin screen, opaque, is interposed on the electronic route. This screen can be configured as a deposit 15 on one side face of the multiplier wafer, understanding that it covers the entire side in question. The screen 15 may consist of a very thin layer of silica (SI0²) Or alumina (Al 0) or any other light metal oxide. Moreover the property of being both opaque to a flux of photons and transparent to an electron current, the screen 15 retains the ions which are torn from the electrode microchannel 3 and which would otherwise be carried on the photocathode, thus preserving the technical characteristics of the tube.

p0018A tube of the possible applications of the invention relates to the detection of signals from the three-color analysis of a color image carried by a film.

p0019FIG. 4 represents a video player color image corresponding to such an application and using a flying spot scanner tube, commonly called flying-spot to scan the film. It comprises the flying-spot tube 20, with its scanning circuits 21, an objective 22 that forms the image of the front of the tube onto the film 23 to be analyzed, a condenser 24 which gives the beam emerging substantially parallel and a séparaeur optical trichromatic 25. the separator is composed of prisms having dichroic or interference mirrors 26, 27 and reflecting mirrors 28, 29 to select the three carrier beams each information respectively contained in the spectral bands of blue, red and green. These three beams emerge parallel and reach a microchannel tube according to Fig. 3 and including detector anode consists of four quadrant detectors as shown in greater detail in FIG. 5 with a separator assembly in particular prisms for locating beams in three quadrants.

p0020The separator assembly comprises, according to the axis Zl of the condenser 24, an assembly of three prisms 31, 32 and 33. The prism 31 receives the light axis Zl of flow on an inlet face and comprises a downstream face contacting with a corresponding face of the prism 32 to form an appropriate optical treatment by the first dichroic mirror 26. This mirror reflects a spectral band filter for example red R, in a vertical direction to achieve on a reflecting face of a fourth prism 34; this reflecting face is 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 reflecting blue spectral band B in a horizontal direction and thus perpendicular to the previous, so can laterally dispose a fifth prism 35 having the reflecting surface 29. the prisms of the exit faces 33, 34 and 35 are coplanar respectively delivering luminal flow green G, red R and blue B, along parallel axes. These streams are received by the input fiber optic window of the tube 1 and, taking into account the reversal produced by the electrostatic lens, the corresponding detection is carried out if the qudrants respectively 4-1, 4-2 and 4 -3 of the anode, conjugates of the exit faces of the prisms 34, 33 and 35.

p0021The video signals output from the quadrant detectors are then further amplified in circuits 40-1, 40-2 and 40-3 which supply the three normalized color components R, G, B.

p0022The fourth quadrant of the anode is used as follows: one or more optical fibers, such 36 and 37 have one end disposed upstream of the film, preferably near the objetive 22 of image forming. These fibers 36 and 37 and collecting a proportional light energy than that emitted by the spot of the tube 20. The fiber outlet ends illuminate through the tube 1 the fourth quadrant detector 4-4 which is followed by a similar amplifier 40-4 40-1 to 40-3 in those other ways. The output of this amplifier is applied simultaneously to three analog divider circuits 41-1 to 41-3 respectively interposed on the three channels R, G and B.

p0023Denoting by SR, SB and SV, the respective current paths of the R, G and B, and S0 reference current from the fourth quadrant, the terminal signals are given by the relations:

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

p0024the values ​​obtained thereby being independent of fluctuations in the brightness of the scan tube 20 and the total gain of the tube 1.

p0025The video player which has just been described has many advantages over the solutions to photomultipliers:<ul><li>- All the sensor part 1 is solid circuit;</li><li>- The device does not have the microphone sensitivity of the photomultiplier;</li><li>- There is only one HV power supply for all sensors; this is important because in a photomultiplier the gain varies in proportion to V^notWhere V is the value of the VHV and n the number of dynodes amplifier. This results in the need for a perfect stabilization of THT. In the proposed reader, the gain of the three color channels is the same, since the amplifier 1 is common to all three channels. Color point analyzed depending only on the ratio of the three components of RGB together, a variation of the THT, even significant, does not affect the chrominance of the point;</li><li>- The four amplification chains 41-1 to 41-4 are identical and deliver standardized video signal which was removed the noise due to the phosphor emission fluctuation analysis tube (non-homogeneity of the layer; change THT, etc ...).</li></ul>

p0026Other advantages result further reductions in volume and weight of the video player device with the microchannel tube detector when the use requires corresponding characteristics (high compactness, low weight). This is the case for the map displays for displaying a map on board the vehicle, in particular airline flight, the map corresponding to the region overflown.

p0027The photodetector tube and multiplier microchannel electron described above allows for various possibilities conform to the exposed features and which fall within the scope of the present invention. As already mentioned the tube can be optionally fitted with an optical lens (9, Fig.l), or may not include an electronic optics (Fig. 1 or 2) and in the absence thereof the photocathode 2 will be located very close to the microchannel plate 3. the number and shape of the sensor elements constituting the anode 4, are determined according to the intended application, four-quadrant detector as seen or any other form tape for example.