CIRCUIT ARRANGEMENT FOR PRODUCING PEDESTAL CURRENT IN A PYROELECTRIC THERMO IMAGING TUBE
United States Patent 3812396
A pyroelectric thermo imaging tube employing an external circuit arrangement for the variable pulsing of the cathode during horizontal flyback periods that are not coincidental with vertical flyback periods to remove residual charges as well as opposite polarity charges and potentials generated on the non-conducting side of the pyroelectric target. Pulsing of the cathode provides secondary electron emission which produces a pedestal current that can be employed for the direct read-out from the target.
US Patent References:
Thermal image converter
Garbuny et al. - September 1962 - 3056062
Cathode ray tube
Lehrer - May 1962 - 3089055
Symbol displaying electron discharge tubes unsing secondary emission
Sinclair - August 1964 - 3144578
Storage tube with signal multiplication adjustment
Charles - March 1966 - 3240988
Thermal imaging device
Kazan - May 1966 - 3249757
Thermal image converter
Garbuny et al. - September 1962 - 3056062
Cathode ray tube
Lehrer - May 1962 - 3089055
Symbol displaying electron discharge tubes unsing secondary emission
Sinclair - August 1964 - 3144578
Storage tube with signal multiplication adjustment
Charles - March 1966 - 3240988
Thermal imaging device
Kazan - May 1966 - 3249757
Application Number:
05/313832
Publication Date:
05/21/1974
Filing Date:
12/11/1972
View Patent Images:
Export Citation:
Assignee:
North American Philips Corp. (New York, NY)
Primary Class:
Other Classes:
250/332, 313/388, 348/E05.090
International Classes:
H01J31/49; H04N5/33; H01J31/08; H01J31/26
Field of Search:
315/10,11,12 250/330,332,333 252/62.9
US Patent References:
Primary Examiner:
Sebastian, Leland A.
Assistant Examiner:
Nelson P. A.
Attorney, Agent or Firm:
Trifari, Frank R.
Claims:
1. A circuit arrangement for producing a substantially uniform pedestal current in a vidicon-type thermo-imaging tube having a target of non-conductive pyroelectric material, an electron gun with a cathode normally at substantially zero potential relative to the target and with a control electrode for directing an electron beam against one side of said target, and means for horizontally and vertically deflecting said electron beam according to a predetermined scanning pattern, said circuit arrangement comprising means for biasing said one side of the target relative to the cathode to a surface potential required for cathode potential stabilization during the forward scan of said beam, and means for negatively pulsing said cathode relative to the target during horizontal flyback periods not coinciding with vertical flyback periods, the amplitude of said negative pulses having a value at which the secondary emission ratio of said target for electrons impacting upon said
2. A circuit arrangement according to claim 1, wherein said negatively pulsing means include means for producing timing trigger pulses coinciding with flyback periods of horizontal and vertical deflection signals, respectively, and means for deriving negative pulses from the trigger pulses coinciding with flyback periods of the horizontal deflection
3. A circuit arrangement according to claim 2 further including means for inhibiting the negative pulses applied to said cathode when said
4. A circuit arrangement as claimed in claim 1, wherein said means for negatively pulsing the cathode of said pyroelectric tube comprises both
5. A circuit arrangement as claimed in claim 1, wherein said target scanned by the electron beam within a pyroelectric thermo imaging tube comprises a non-conducting substrate, a transparent conducting film covering the surface of said substrate, opposite to the surface facing the electron beam, a plurality of parallel conducting strips spaced apart from each other abutting the surface of said substrate, facing the electron beam the spacing between said conducting strips being determined by the resolution of the radiation of the received image of the pyroelectric tube, and means for producing D-C bias between the conducting strips and said conducting
6. A circuit arrangement as claimed in claim 5, further comprising a resistive film covering the surface facing the electron beam and abutting conducting strips to provide a leakage path for the target and means for
7. A circuit arrangement as claimed in claim 6, further comprising means coupled to said resistive film to read-out the data representing the
8. A circuit arrangement for producing a substantially uniform pedestal current at the side of a non-conductive pyroelectric target scanned by an electron beam within a vidicon-type thermo-imaging tube biased for a cathode potential stabilization, comprising means for negatively pulsing the cathode of said tube during horizontal flyback periods of said beam, the amplitude of the negative pulses having a value at which the secondary emission ratio of said target for electrons impacting upon said one side of said target is greater than unity.
2. A circuit arrangement according to claim 1, wherein said negatively pulsing means include means for producing timing trigger pulses coinciding with flyback periods of horizontal and vertical deflection signals, respectively, and means for deriving negative pulses from the trigger pulses coinciding with flyback periods of the horizontal deflection
3. A circuit arrangement according to claim 2 further including means for inhibiting the negative pulses applied to said cathode when said
4. A circuit arrangement as claimed in claim 1, wherein said means for negatively pulsing the cathode of said pyroelectric tube comprises both
5. A circuit arrangement as claimed in claim 1, wherein said target scanned by the electron beam within a pyroelectric thermo imaging tube comprises a non-conducting substrate, a transparent conducting film covering the surface of said substrate, opposite to the surface facing the electron beam, a plurality of parallel conducting strips spaced apart from each other abutting the surface of said substrate, facing the electron beam the spacing between said conducting strips being determined by the resolution of the radiation of the received image of the pyroelectric tube, and means for producing D-C bias between the conducting strips and said conducting
6. A circuit arrangement as claimed in claim 5, further comprising a resistive film covering the surface facing the electron beam and abutting conducting strips to provide a leakage path for the target and means for
7. A circuit arrangement as claimed in claim 6, further comprising means coupled to said resistive film to read-out the data representing the
8. A circuit arrangement for producing a substantially uniform pedestal current at the side of a non-conductive pyroelectric target scanned by an electron beam within a vidicon-type thermo-imaging tube biased for a cathode potential stabilization, comprising means for negatively pulsing the cathode of said tube during horizontal flyback periods of said beam, the amplitude of the negative pulses having a value at which the secondary emission ratio of said target for electrons impacting upon said one side of said target is greater than unity.
Description:
BACKGROUND OF THE INVENTION
The invention relates to pyroelectric thermo imaging camera tubes, and more particularly to methods of reading out extremely small variations of potentials and charges on non-conducting electrodes.
Pyroelectric thermo imaging tubes are well-known, such as disclosed in copending patent application (154-10-911), filed concurrently and assigned to the same assignee, and comprises means for periodically focusing heat radiated from an object to form an image onto an array of triglycine sulfate crystals (TGS), such as disclosed in copending patent application Ser. No. 137,174, filed Apr. 26, 1971, now U.S. Pat. No. 3,721,628, and assigned to the same assignee. These crystals have the property of converting the radiant heat energy into voltages representative of the temperature changes of the received images. Means are provided for transmitting voltages or charges from crystals to a non-conducting plate facing an electron gun that is used to sequentially read-out these voltages or charges. The problem solved by applicant's invention is the rapid removal of the residual charges that remain on the non-conducting plate after the electron beam has passed over it. Ordinarily, where a photo-conducting matrix is used, there is sufficient dark current flowing during the period of time that the shutter blocks the reception of radiation from the object. In the use of ferro-electric material, such as Triglycine Sulfate (TGS), there is no dark current produced, and other means must be resorted to provide for the removal of the residual charges before the shutter allows the next image to be received.
It is peculiar to the pyroelectric effect that the electrical polarization, p, of a crystal displaying this effect varies with temperature. The camera pick-up tube described in the preceding paragraph operates in this mode; namely that a thin pyroelectric crystal is exposed to radiations from the object, these radiations cause local changes in temperatures, and thereby produce changes in polarization of the crystal resulting in producing electric charges. If one surface of the crystal is rendered electrically conducting and is held at a fixed potential, then a potential distribution corresponding to the changes in polarization is formed on the non-conducting surface. Now once the charges on the non-conducting side have been neutralized by the electron beam, the target temperatures must be restored to their former values before the appearance of new data. This is implemented by the use of a shutter between the target and the object to provide a period of time for restoring the target to a reference temperature. During this restoration period while the shutter blocks the object, charges remaining upon the target after the passing of the electron beam have to be removed because the target, approximating a perfect insulator provides no leakage paths for these residual charges. Although there are known techniques for achieving this result, as will be discussed in the following paragraphs, it is noted that these techniques have inherent disadvantages which limit the efficient use of the tube.
One prior art method of electron beam read out is known as "Cathode Potential Stabilization" (CPS) and is described by P. K. Weimer, S. B. Forgue and R. R. Goodridge in their article entitled "The Vidicon Photoconductive Camera Tube," published on pages 70-73 of vol. 23 of Electronics magazine in May, 1950. As the electron beam scans across the surface it functions as a commutator for the charges deposited thereon and restores the surface potential thereby resulting in a video signal being generated in the conducting plate. Since the video signal taken from the target is capacitively coupled to the scanning surface of the target, only electrons are deposited in this mode, and positive ions, attracted to the target for neutralizing are produced from the residual gas in the tube by scanning an electron beam across the target. The disadvantage of this method for a pyroelectric vidicon is the inherent lack of electrical conductivity in materials which are suitable for imaging applications.
Another prior art method is known as "Anode Potential Stabilization" (APS) and is described by J. Dresner in his article entitled "The High-Beam-Velocity Vidicon," published between pages 305 and 325 of vol. 22 of RCA Review (1961) wherein secondary emission of the anode is produced so that both positive and negative signals corresponding to the phase of the shutter result. This mode has the disadvantage that it is not as uniform as the CPS mode. In a known combination of these two modes, the target is first scanned while the shutter is between the target and the object to render the surface slightly positive and then discharged, by depositing electrons to attract positive ions produced from the residual gas in the tube for neutralizing the target. Then the target is exposed to the object and the video signal is read-out from the target electrode as described for the CPS mode in the preceeding paragraph. The process is then repeated for the shutter.
Although the two modes are known, the CPS mode, preferred from a signal to noise performance factor, is not practical because the required pedestal current is obtained by using ion currents resulting from a high residual gas pressure making it difficult to obtain a uniform flux of ions as well as providing a large number of ions that will bombard the electron gun cathode, thereby shortening the life of the cathode.
It is an object of the present invention to provide a circuit for removing residual charges and potentials generated in pyroelectric tubes which will result in signal to noise ratios of performance comparable to those of the CPS mode.
It is a further object of this invention to provide a method that will provide a much longer life for the cathode than those employing CPS modes depending upon ion currents.
It is a further object of this invention to provide a method for directly reading out the video data from the pedestal current obtained from the non-conducting face of an improved target structure.
SUMMARY OF THE INVENTION
Applicant's invention employs secondary electron emission, achieved by external circuit means for pulsing the cathode of the electron gun from zero potential to a negative potential during the horizontal flyback time periods that are not coincidental with the vertical flyback periods. The negative voltage is determined so that the secondary emission ratios for electrons striking the target is greater than unity. Due to the secondary emission, the successive surface portions of the target are uniformly charged to a positive potential during hoizontal flyback periods by means of the electron beam, and therefore a uniform pedestal current wll be generated during normal (CPS mode) forward scan.
BRIEF DESCRIPTION OF THE DRAWINGS
Applicant's invention will be described in greater detail with reference to the following drawings in which:
FIG. 1 is a block diagram of applicant's circuit arrangement operating with a pyroelectric thermo imaging tube.
FIG. 2 illustrates the time relation between horizontal and vertical deflection signals and the negative cathode pulses produced in the circuit of FIG. 1.
FIG. 3 is a schematic diagram for the block diagram of FIG. 1.
FIG. 4 is an improved target structure for the pyroelectric tube of FIG. 1.
FIG. 5 is an improvement of the target of FIG. 4 for direct read-out from the face of the target scanned by the electron beams of the pyroelectric tube.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a pyroelectric thermo imaging television pick-up tube 1, is shown, having an electron gun 2, with a cathode 3, transparent conducting electrode 8, an array of ferrelectric crystals forming a non-conductive pyroelectric target 9, and an optical system including a lens 7 and a window 11. Conventional biasing voltages along with other potentials required for normal (CPS) mode of the operation of the vidicon-type pyroelectric tube are well known and therefore not shown. External to said pyroelectric tube are shown deflecting means 4, shutter 10 and source 6 of biasing potential for the electrode 8.
Horizontal and vertical trigger and synchronizing circuits 12 provides a means for producing the triggers both for negatively pulsing the cathode and for inhibiting the pulsing of the cathode during horizontal flyback periods that are within the vertical flyback periods, such as illustrated in FIG. 2. A schematical circuit embodiment for implementing this is shown in FIG. 1, where positive pulses are supplied by the horizontal and vertical triggers and synchronizing circuits 12 to the pulse amplitude circuit 13 and the cathode pulse amplitude circuit 14. Negative pulses from the pulse amplitude circuit 13 are then supplied to the control grid of the pyroelectric tube through pulse amplifier 15. The negative pulses from pulse amplifier 15 are delayed to occur at the same time as inhibiting pulses from the horizontal and vertical trigger and synchronizing circuits 12. Cathode pulse amplitude circuit 14 supplys pulse amplifier 16 and this in turn is coupled to cathode 3 of the pyroelectric tube. Although the video output is shown in FIG. 1 to be taken off electrode 8, it is noted that an improved target design to be described later provides another method of obtaining the pedestal current.
FIG. 3 shows schematic drawing for the anode pulse amplitude and anode pulse amplifier circuits 13 and 15 and the cathode pulse amplitude and the cathode pulse amplifier circuits 14 and 16. The input to resistor R1 is a positive pulse from the horizontal and vertical trigger and synchronizing circuits 12. Resistor R1 provides two paths; one of which is coupled to capacitor C1 and then through the parallel circuit of capacitor C2 and resistor R3 to the base of npn transistor Q1 operating to invert as well as amplify the input positive waveform to a negative pulse whose amplitude is determined by the adjustable voltage divider comprising resistors R4, R5 and R6. NPN transistors Q2 coupled to the output of the voltage divider operates as an emitter follower to match the output of the cathode pulse amplifier 16 to the input cathode circuit of the pyroelectric tube. The other path for the positive pulse of resistor R1 is coupled through capacitor C3 and the parallel circuit comprising capacitor C4 and resistor R9 to the base of NPN transistor Q3 operating to invert as well as amplify the input waveform to a negative pulse whose amplitude is determined by the adjustable voltage divider comprising resistors R10 and 11. This inverted pulse having a predetermined amplitude and delayed a predetermined interval time is connected to the base NPN transistor Q4 operating as an emitter follower which thereby couples this inverted pulse through capacitor C5 and diode D1 to the output of horizontal and vertical trigger and synchronizing circuits 12 as well as to the control grid of the electron gun of the pyroelectric tube. The video output of the pyroelectric tube is either taken off conducting electrode 8 which is coupled to capacitor C6 or as described later in the following paragraphs. Conducting electrode 8 is biased by battery, through resistor R14.
Operating of the above described circuit is as follows: Non-coinciding horizontal and vertical triggers are applied to horizontal and vertical trigger and synchronizing circuits 12 to produce negative pulses at the cathode of the pyroelectric tube during the horizontal flyback period, provided the horizontal flyback period does not coincide with the vertical flyback period. If the vertical and horizontal flyback times coincide, an inhibiting pulse is produced by the circuits 12 at the anode of the pyroelectric tube, negating the effect of the negative cathode pulse. Pulsing the cathode of the pyroelectric tube results in secondary emission in the tube which produces a positive potential on the non-conducting target and this will produce a uniform pedestal current during the normal forward scan.
FIG. 4 shows a target design for improving gain of a pyroelectric tube comprising pyroelectric substrate 17 which has a transparent conductive film 18 covering one surface, and a number of parallel conducting strips 19, spaced a distance apart from each other on the opposite, surface of the substrate. The spacing between the conducting strips is very close; being determined by the resolution of the image desired. The surface of the substrate having the conducting strips would be positioned in the pyroelectric tube shown in FIG. 1. The operation of this target is similar to that of a coplanar triode; the conducting strips functions as the plate and the pyroelectric material between the conducting strips functioning as the grid.
FIG. 5 shows an improved target design of FIG. 4 having a resistive film 20 covering the conducting strip and surface of the pyroelectric substrate to provide leakage paths for the residual charges and the potentials generated during the restoration periods while the tube is being operated. A pedestal current is obtained by means of these leakage paths. Since such leakage current represents the temperature-voltage characteristics of the received image, this current can be read-out for processing by conventional means.
The invention relates to pyroelectric thermo imaging camera tubes, and more particularly to methods of reading out extremely small variations of potentials and charges on non-conducting electrodes.
Pyroelectric thermo imaging tubes are well-known, such as disclosed in copending patent application (154-10-911), filed concurrently and assigned to the same assignee, and comprises means for periodically focusing heat radiated from an object to form an image onto an array of triglycine sulfate crystals (TGS), such as disclosed in copending patent application Ser. No. 137,174, filed Apr. 26, 1971, now U.S. Pat. No. 3,721,628, and assigned to the same assignee. These crystals have the property of converting the radiant heat energy into voltages representative of the temperature changes of the received images. Means are provided for transmitting voltages or charges from crystals to a non-conducting plate facing an electron gun that is used to sequentially read-out these voltages or charges. The problem solved by applicant's invention is the rapid removal of the residual charges that remain on the non-conducting plate after the electron beam has passed over it. Ordinarily, where a photo-conducting matrix is used, there is sufficient dark current flowing during the period of time that the shutter blocks the reception of radiation from the object. In the use of ferro-electric material, such as Triglycine Sulfate (TGS), there is no dark current produced, and other means must be resorted to provide for the removal of the residual charges before the shutter allows the next image to be received.
It is peculiar to the pyroelectric effect that the electrical polarization, p, of a crystal displaying this effect varies with temperature. The camera pick-up tube described in the preceding paragraph operates in this mode; namely that a thin pyroelectric crystal is exposed to radiations from the object, these radiations cause local changes in temperatures, and thereby produce changes in polarization of the crystal resulting in producing electric charges. If one surface of the crystal is rendered electrically conducting and is held at a fixed potential, then a potential distribution corresponding to the changes in polarization is formed on the non-conducting surface. Now once the charges on the non-conducting side have been neutralized by the electron beam, the target temperatures must be restored to their former values before the appearance of new data. This is implemented by the use of a shutter between the target and the object to provide a period of time for restoring the target to a reference temperature. During this restoration period while the shutter blocks the object, charges remaining upon the target after the passing of the electron beam have to be removed because the target, approximating a perfect insulator provides no leakage paths for these residual charges. Although there are known techniques for achieving this result, as will be discussed in the following paragraphs, it is noted that these techniques have inherent disadvantages which limit the efficient use of the tube.
One prior art method of electron beam read out is known as "Cathode Potential Stabilization" (CPS) and is described by P. K. Weimer, S. B. Forgue and R. R. Goodridge in their article entitled "The Vidicon Photoconductive Camera Tube," published on pages 70-73 of vol. 23 of Electronics magazine in May, 1950. As the electron beam scans across the surface it functions as a commutator for the charges deposited thereon and restores the surface potential thereby resulting in a video signal being generated in the conducting plate. Since the video signal taken from the target is capacitively coupled to the scanning surface of the target, only electrons are deposited in this mode, and positive ions, attracted to the target for neutralizing are produced from the residual gas in the tube by scanning an electron beam across the target. The disadvantage of this method for a pyroelectric vidicon is the inherent lack of electrical conductivity in materials which are suitable for imaging applications.
Another prior art method is known as "Anode Potential Stabilization" (APS) and is described by J. Dresner in his article entitled "The High-Beam-Velocity Vidicon," published between pages 305 and 325 of vol. 22 of RCA Review (1961) wherein secondary emission of the anode is produced so that both positive and negative signals corresponding to the phase of the shutter result. This mode has the disadvantage that it is not as uniform as the CPS mode. In a known combination of these two modes, the target is first scanned while the shutter is between the target and the object to render the surface slightly positive and then discharged, by depositing electrons to attract positive ions produced from the residual gas in the tube for neutralizing the target. Then the target is exposed to the object and the video signal is read-out from the target electrode as described for the CPS mode in the preceeding paragraph. The process is then repeated for the shutter.
Although the two modes are known, the CPS mode, preferred from a signal to noise performance factor, is not practical because the required pedestal current is obtained by using ion currents resulting from a high residual gas pressure making it difficult to obtain a uniform flux of ions as well as providing a large number of ions that will bombard the electron gun cathode, thereby shortening the life of the cathode.
It is an object of the present invention to provide a circuit for removing residual charges and potentials generated in pyroelectric tubes which will result in signal to noise ratios of performance comparable to those of the CPS mode.
It is a further object of this invention to provide a method that will provide a much longer life for the cathode than those employing CPS modes depending upon ion currents.
It is a further object of this invention to provide a method for directly reading out the video data from the pedestal current obtained from the non-conducting face of an improved target structure.
SUMMARY OF THE INVENTION
Applicant's invention employs secondary electron emission, achieved by external circuit means for pulsing the cathode of the electron gun from zero potential to a negative potential during the horizontal flyback time periods that are not coincidental with the vertical flyback periods. The negative voltage is determined so that the secondary emission ratios for electrons striking the target is greater than unity. Due to the secondary emission, the successive surface portions of the target are uniformly charged to a positive potential during hoizontal flyback periods by means of the electron beam, and therefore a uniform pedestal current wll be generated during normal (CPS mode) forward scan.
BRIEF DESCRIPTION OF THE DRAWINGS
Applicant's invention will be described in greater detail with reference to the following drawings in which:
FIG. 1 is a block diagram of applicant's circuit arrangement operating with a pyroelectric thermo imaging tube.
FIG. 2 illustrates the time relation between horizontal and vertical deflection signals and the negative cathode pulses produced in the circuit of FIG. 1.
FIG. 3 is a schematic diagram for the block diagram of FIG. 1.
FIG. 4 is an improved target structure for the pyroelectric tube of FIG. 1.
FIG. 5 is an improvement of the target of FIG. 4 for direct read-out from the face of the target scanned by the electron beams of the pyroelectric tube.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring to FIG. 1, a pyroelectric thermo imaging television pick-up tube 1, is shown, having an electron gun 2, with a cathode 3, transparent conducting electrode 8, an array of ferrelectric crystals forming a non-conductive pyroelectric target 9, and an optical system including a lens 7 and a window 11. Conventional biasing voltages along with other potentials required for normal (CPS) mode of the operation of the vidicon-type pyroelectric tube are well known and therefore not shown. External to said pyroelectric tube are shown deflecting means 4, shutter 10 and source 6 of biasing potential for the electrode 8.
Horizontal and vertical trigger and synchronizing circuits 12 provides a means for producing the triggers both for negatively pulsing the cathode and for inhibiting the pulsing of the cathode during horizontal flyback periods that are within the vertical flyback periods, such as illustrated in FIG. 2. A schematical circuit embodiment for implementing this is shown in FIG. 1, where positive pulses are supplied by the horizontal and vertical triggers and synchronizing circuits 12 to the pulse amplitude circuit 13 and the cathode pulse amplitude circuit 14. Negative pulses from the pulse amplitude circuit 13 are then supplied to the control grid of the pyroelectric tube through pulse amplifier 15. The negative pulses from pulse amplifier 15 are delayed to occur at the same time as inhibiting pulses from the horizontal and vertical trigger and synchronizing circuits 12. Cathode pulse amplitude circuit 14 supplys pulse amplifier 16 and this in turn is coupled to cathode 3 of the pyroelectric tube. Although the video output is shown in FIG. 1 to be taken off electrode 8, it is noted that an improved target design to be described later provides another method of obtaining the pedestal current.
FIG. 3 shows schematic drawing for the anode pulse amplitude and anode pulse amplifier circuits 13 and 15 and the cathode pulse amplitude and the cathode pulse amplifier circuits 14 and 16. The input to resistor R1 is a positive pulse from the horizontal and vertical trigger and synchronizing circuits 12. Resistor R1 provides two paths; one of which is coupled to capacitor C1 and then through the parallel circuit of capacitor C2 and resistor R3 to the base of npn transistor Q1 operating to invert as well as amplify the input positive waveform to a negative pulse whose amplitude is determined by the adjustable voltage divider comprising resistors R4, R5 and R6. NPN transistors Q2 coupled to the output of the voltage divider operates as an emitter follower to match the output of the cathode pulse amplifier 16 to the input cathode circuit of the pyroelectric tube. The other path for the positive pulse of resistor R1 is coupled through capacitor C3 and the parallel circuit comprising capacitor C4 and resistor R9 to the base of NPN transistor Q3 operating to invert as well as amplify the input waveform to a negative pulse whose amplitude is determined by the adjustable voltage divider comprising resistors R10 and 11. This inverted pulse having a predetermined amplitude and delayed a predetermined interval time is connected to the base NPN transistor Q4 operating as an emitter follower which thereby couples this inverted pulse through capacitor C5 and diode D1 to the output of horizontal and vertical trigger and synchronizing circuits 12 as well as to the control grid of the electron gun of the pyroelectric tube. The video output of the pyroelectric tube is either taken off conducting electrode 8 which is coupled to capacitor C6 or as described later in the following paragraphs. Conducting electrode 8 is biased by battery, through resistor R14.
Operating of the above described circuit is as follows: Non-coinciding horizontal and vertical triggers are applied to horizontal and vertical trigger and synchronizing circuits 12 to produce negative pulses at the cathode of the pyroelectric tube during the horizontal flyback period, provided the horizontal flyback period does not coincide with the vertical flyback period. If the vertical and horizontal flyback times coincide, an inhibiting pulse is produced by the circuits 12 at the anode of the pyroelectric tube, negating the effect of the negative cathode pulse. Pulsing the cathode of the pyroelectric tube results in secondary emission in the tube which produces a positive potential on the non-conducting target and this will produce a uniform pedestal current during the normal forward scan.
FIG. 4 shows a target design for improving gain of a pyroelectric tube comprising pyroelectric substrate 17 which has a transparent conductive film 18 covering one surface, and a number of parallel conducting strips 19, spaced a distance apart from each other on the opposite, surface of the substrate. The spacing between the conducting strips is very close; being determined by the resolution of the image desired. The surface of the substrate having the conducting strips would be positioned in the pyroelectric tube shown in FIG. 1. The operation of this target is similar to that of a coplanar triode; the conducting strips functions as the plate and the pyroelectric material between the conducting strips functioning as the grid.
FIG. 5 shows an improved target design of FIG. 4 having a resistive film 20 covering the conducting strip and surface of the pyroelectric substrate to provide leakage paths for the residual charges and the potentials generated during the restoration periods while the tube is being operated. A pedestal current is obtained by means of these leakage paths. Since such leakage current represents the temperature-voltage characteristics of the received image, this current can be read-out for processing by conventional means.