FR2494906A1 - ELECTRON MULTIPLICATION PHOTODETECTOR TUBE FOR USE IN A COLOR VIDEO READER - Google Patents

Abstract

TUBE ALLOWING LIGHT DETECTION. IT INCLUDES A PHOTOCATHODE 2 FOLLOWED BY A MULTIPLIER BODY 3 PERFORES MICROChannels, THEN A DETECTOR ANODE 4. THE ANODE HAS ONE OR MORE ELEMENTS DEPENDING ON THE NUMBER OF FLOWS TO BE DETECTED, A MOSAIC OF ELEMENTS TO DETECT A VIDEO IMAGE . EACH ELEMENT CONSTITUTES A POLARIZED SILICON DIODE IN BLOCKING DIRECTION BY AN ANNEX 7 SOURCE. THE INVENTION APPLIES IN PARTICULAR TO COLOR IMAGE VIDEO READERS, AMONG OTHERS TO ONBOARD CARTOGRAPHIC INDICATORS.

Description

TRO / BOR

  ELECTRONS MULTIPLICATION PHOTODETECTOR TUBE

  USEABLE IN A COLOR VIDEO PLAYER

  The present invention relates to a photodetector tube with

multiplication of electrons.

  Electron multiplying tubes are known in which the multiplication of a primary electron beam is obtained by a series of secondary electronic emissions within a set of parallel, very small diameter straight channels.

  and placed in a longitudinal electric field.

  The multiplier tubes of this kind comprise an insulating body, pierced with very small diameter channels, coated on their inner walls with a very thin resistive layer, endowed with the secondary electronic emission property with a coefficient greater than unity, whereas a potential difference, established between the ends of the coatings creates in the channels a field

  longitudinal electric. With this arrangement, a beam of electricity

  primary trenches, penetrating the channels from different angles, gives rise to a series of secondary electronic emissions on the inner coatings so that at the exit of the channels the number of electrons is greatly 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 No. 1,465,381 of March 24, 1965 relating to

  improvements according to which the multiplier body of

  trons, or microchannel slab, is produced by drilling a diode

  polarized silicon 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, the light detection being understood as well as the detection of a single luminous flux. , or a limited number of luminous flux, or a relatively large number of flows

  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 wafer by an anode forming a solid diode polarized in the blocking direction. The anode is constituted by one or more elements depending on 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 made with an anode composed of four detector quadrants.

  this configuration being advantageously usable in

  positive color image video players (slides, films, etc ...) using a mobile spot analyzer tube called "flyi-ig spot" and where we must extract the three channels by trichromatic optical separation followed

  of photodetection and multiplication of electrons.

  The features of the present invention will appear in the

  following description given as a non-restrictive example using

  annexed figures which represent - FIG. 1, a diagram of a microchannel detector tube according to the invention; - Fig. 2, an embodiment of a lounge tube FIG. 1; - Fig. 3, an embodiment according to FIG. 2 o the anode has several elements; - Fig. 4, a diagram of a use of a tube according to the invention in a color tube video player flying spot; and - Fig. 5, a partial representation of the reader assembly of FIG. 4, relating to the trichromatic optical separation and the tube

detector according to the invention.

  Referring to FIG. 1, the electron multiplier tube comprises, in known manner and placed in a vacuum chamber, a photocathode 2 followed by a body or pancake 3 pierced with channels, 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 establish between the elements 2, 3 and 4, the desired differences in potentials to ensure the 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 ends

  opposite, at a lower potential than the electrode 4.

  According to the invention, the electrode 4 is a planar anode consisting of a solid diode polarized in the blocking direction by a continuous source of polarization 7, this anode being

  itself as 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 N-type fine diffusion has been carried out, so as to obtain a reverse-biased PN junction by the source 7. The PN junction is preferable to the NP junction for questions of speed and quantum efficiency. The junction may also include a Schottky barrier. The detected video signal SV is recovered on the electrode

  corresponding across a load resistor 8.

  To treat a bright 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

  in the plane of the photocathode. The optics may consist of a dioptric or catadioptric lens, possibly followed by an optical fiber window. To treat a single luminous flux, or a

  limited number of distinct incident flows, Optics 9 is not required.

  sary. When a light image (or a flux) is projected on the photocathode 2, it emits electrons that penetrate the channels of the wafer 3. This primary electron beam causes electron emission channels on the walls of channels secondary schools

  their turn hits those walls, and so on. As the coef-

  ficient emission is greater than unity, the electrons are multiplied at the output of the channels and produce on the anode 4 an electronic image intensified with respect to the image (or flow)

  initial value 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

  have significant kinetic energy, in this case 5 KeV.

  By striking the anode 4, they create electron-hole pairs that are separated by the field that prevails in the space charge of the PN junction, and a detected current IS can thus flow into the

  outside circuit. To create an electron-hole pair by bombar-

  In silicon, it requires an energy of the order of 3.5 eV per incident electron. In this case, the energy of an electron can be of the order of 5 KeV, so that the current gain can be of the order of 1 400. However, taking into account a layer without load , called dead, a gain of 1,000 can be

  got. The pairs are created at a depth of

  in the order of 1 gym and the pairs created

  can be separated by the close junction of the receiving surface

  trice with a yield close to unity.

  The gain G2 thus obtained by the detection 4 is added to the electronic gain G1 of the microchannels 3, so that the current IS that leaves the device is equal to G1 x G2 times the value I0 of the electron current that attacks the microchannels. Taking into account the low efficiency of the photocathode, the total gain of the tube can, in the above concept, be between 1.5 105 and 3 105, these values

  being indicated in a non-limiting manner.

  For the detection of several luminous flux reaching

  the input, as well as for image detection the tube is preferably

  constructed upstream of the electron multiplier according to the arrangement of FIG. 2 or 3. This arrangement is known in the light image intensifier tubes incorporating a photocathode 11

  arranged on an input window 12 made of optical fibers for converting

  shooting the input light image into an electronic image and a cone shaped focusing anode 13 playing the role of a

  electrostatic lens; a correction electrode 14 can also

  be placed between the conical anode 13 and the

  3, to improve the linearity and geometry of the image.

  Since the electrostatic lens optically conjugates the photocathode II and the input face of the microchannels 3, the tube can be used to amplify separately and in parallel several optical signals incident simultaneously on the photocathode 11. FIG. 3 shows the path of two distinct beams Fl, F2, 1u but is not limiting. The electrons emitted by the photocathode 11 because of the luminous flux F1 strike the region A of the microchannel plate; in the same way the distinct stream F2 reaches the distinct region B of the slab. These two electron fluxes are amplified separately in the microchannel amplifier and it is sufficient to have two anodes 4-1 and 4-2 to obtain the currents IS1 and IS2 respectively corresponding to the optical input signals.

Fl and F2 amplified.

  In the case of an optical image projected on the photocathode 11, the anode 4 will be constituted by a detector mosaic in which each element corresponds to a point of the electronic image and consequently to an elementary flux located at the input level. of the tube. With a mosque of P = nxm elements distributed in n rows and m columns, we thus treat P elementary streams incident ending on the photocathode 11 at the conjugate locations of P elements.

  detectors. For example, a mosaic can be used

  load transfer device, or CCD (charge coupled device).

  Given the photodroptive property exhibited by uninsulated solid diodes of light, the planar anode 4 can be

  sensitized by residual light that would have

  poured the electrodes upstream. To eliminate and protect itself from parasitic light radiation, a very thin, opaque screen is interposed on the electronic path. This screen can be made in the form of a deposit 15 on one of the lateral faces of the multiplier slab, it being understood that it covers the entire face in question. The screen 15 may consist of a very thin layer of silica

  (SIO2) or alumina (AI 203) or any other light metal oxide.

  In addition to the property of being both opaque for a photon flux and transparent for an electron current, the screen retains the ions that are torn from the microchannel electrode 3 and which otherwise

  transported on the phortocathode, thus preserving the

technical characteristics of the tube.

  One of the possible applications of the tube according to the invention

  identifies the detection of signals from the trichromatic analysis of a

color image carried by a film.

  Fig. 4 represents a video player of color images corresponding to such an application and using a test tube to

  mobile spot, commonly called flying-spot, to sweep the film.

  It comprises the flying-spot tube 20, with its scanning circuits 21, an objective 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 substantially parallel and a separator optical trichrome 25. The

  separator consists of prisms with dichroic mirrors

  or interferential 26, 27 and reflecting mirrors 28, 29 to select the three beams each respectively carrying information contained in the blue, red and green spectral bands. These three beams emerge in parallel and reach a microchannel tube according to FIG. 3 and whose detector anode consists of four detector quadrants as shown in more detail in FIG. 5 with a particular prism separator arrangement for locating the beams in three of the quadrants. The separator assembly comprises, along the axis ZI of the condenser 24, an assembly of three prisms 31, 32 and 33. The prism 31 receives the luminous flux of axis Z1 on an input face and has a downstream face in contact with each other. with a corresponding face of the prism 32 to form by appropriate optical treatment the first dichroYque mirror 26. This mirror reflects one of the spectral bands to be filtered for example the red R, in a vertical direction to end on a reflecting face of a fourth prism 34; this reflecting face corresponds to the mirror 28. The prism 32 comprises 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 faces of

  output of the prisms 33, 34 and 35 are coplanar, delivering respectively

  the green luminous flux V, red R and blue B, along parallel axes. These flows are received by the optical fiber input window of the tube 1 and, taking into account the inversion produced by the

  electrostatic lens, the corresponding detec-

  if 4-1, 4-2, and 4-3 of the anode, together with

  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 standardized chromatic components R, V, B.

  The fourth quadrant of the anode is used as follows: one or more optical fibers, such as 36 and 37, have an end disposed upstream of the film, preferably near the imaging object 22. These fibers 36 and 37 thus collect a light energy proportional to that emitted by the spot of the tube 20. The output 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 401 to 40-3 of 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, V and B. By designating by SR, SV and 58, the respective currents of the channels R, V and B, and S0 the reference current from the fourth quadrant, the terminal signals are given by the reports:

  SR '= SR / S0, SV' = SV / SO, SB '= SB / S0

  the values obtained being thus independent of the fluctuations of the

  brightness of the analysis tube 20 and the total gain of the tube 1.

  The video player that has just been described has many advantages over photomultiplier solutions: - the entire detecting part 1 is in solid circuit; the device does not possess the microphonic sensitivity of the photomultipliers; - there is only one power supply THT for all detectors; this improvement is important because in a photomultiplier the gain varies proportionally to V n, where V is the value of the THT and n the number of amplifying dynodes. This results in the need for perfecting 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. The color of the analyzed point depends only on the ratio of the three RGB components to each other, a variation of the THT, even important, 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 due to the emission fluctuation of the phosphor of the analysis tube has been removed (no

  homogeneity of the layer; variation of THT, etc ...).

  Other benefits also result in volume and weight reductions of this video player device through the tube

  microchannel detector when the use requires

  corresponding data (high compactness, low weight). This is the case for cartographic indicators that make it possible to visualize a geographical map on board a vehicle, in particular on board

  plane, the map corresponding to the area overflown.

  The photodetector and electron multiplier tube with micro-

  Channels described above admit several variants in accordance with the characteristics exposed and which fall within the scope of the present invention. As has already been mentioned, the tube may or may not be equipped with an optical objective (9, Fig. 1), may or may not comprise an electronic optic (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 detector elements constituting the anode 4, are determined. depending on the intended application, four-quadrant detector as seen in any other form,

band for example.

Claims (10)

  1. Electron multiplication photodetector tube comprising   in a vacuum chamber (1), a photocathode (2) emitting a primary electron beam on receiving a light radiation, a microchannel slab (3) for multiplying said beam by a series of electronic transmissions secondary to the rectilinear parallel channels and 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 electrode downstream of the microchannel wafer (3) is an anode (4) formed of at least one element and constituting a polarized solid diode in the direction   blocking by an auxiliary voltage source (7).   Tube according to claim 1, characterized in that the anode is formed of a plurality of elements for detecting a plurality of   separate luminous 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 electrostatic lens upstream of the microchannel wafer, this assembly grouping a conical anode (13) preceded by the photocathode (11) deposited on a fiber optic input window (12) and possibly followed   an electrode (14) correcting linearity.   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 path electronics 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   performed on one side of the microchannel slab.   6. Tube according to any one of claims 1 to 5,   characterized in that it further comprises receiving optics (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- 1-4-4).   8. Use of a tube according to claim 7 in a color image video player comprising a flying-spot tube film reader (24), a trichromatic optical separator (25) and photodetection and detection means. amplification, characterized in that said means are constituted by said four detector quadrant tube followed by identical video amplifiers (40-1 to 40-4), the trichromic separator consisting of a prism arrangement (31 to 35)   with 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 exit faces corresponding to three quadrants.   9. Use of a tube according to claim 8, characterized in that it further comprises at least one fiber optic conductor (36, 37) for taking a fraction of the light energy emitted by the spot of the flying-tube. spot, said conductor being positioned at one end upstream of the film and coupled at its other end with the microchannel electron multiplier tube for cooperating with the fourth detector quadrant (4-4), the corresponding detected signal (SO) being applied after amplification respectively three analog divider circuits (41-1 to 41-3) interposed on the tracks   (R, V, B) detection downstream of the amplifiers.   10. Use of a tube according to claim 7 or 8 for   produce an on-board chart indicator.