US3812396A

US3812396A - Circuit arrangement for producing pedestal current in a pyroelectric thermo imaging tube

Info

Publication number: US3812396A
Application number: US00313832A
Authority: US
Inventors: M Crowell
Original assignee: US Philips Corp
Current assignee: Philips North America LLC; US Philips Corp
Priority date: 1972-12-11
Filing date: 1972-12-11
Publication date: 1974-05-21
Anticipated expiration: 1991-05-21
Also published as: FR2210067A1; DE2360196A1

Abstract

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 nonconducting 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.

Description

United States Patent Crowell 1111 3,812,396 [451 May 21, 1974 Primary Examiner-Leland A. Sebastian Assistant Examiner- P. A. Nelson Attorney, Agent, or Firm-Frank R1 Trifari [5 7] ABSTRACT A pyroelectric thermo imaging tube employing an external circuit arrangement for the variable pulsing of the cathode duringhorizontal 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 nonconducting 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.

8 Claims, 5 Drawing Figures [75] Inventor: Merton I-I. Crowell, Mahopac, N.Y. [73] Assignee: North American Philips Corp., New

York, N.Y. 221 Filed: Dec. 11, 1972 [2]] App]. No.: 313,832

[52] US. Cl. 315/10, 250/332 [51] Int. Cl. H0lj 31/26 [58] Field of Search 315/10, 11, 12; 250/330, 250/332, 333; 252/629 [56] References Cited UNITED STATES PATENTS 3,056,062 9/1962 Garbuny et al. 3l5/ll 3,089,055 5/1962 Lehrer 315/12 3,144,578 8/1964 Sinclair 315/12.

l6 CATHODE ANODE PULSE PULSE AMPLIFIER AMPLIFIER l4 13 'U' CATHODE U ANODE PULSE PULSE AMPLITUDE AMPLITUDE CIRCUIT CIRCUIT HORIZONTAL AND VERTICAL TRIGGER AND H- SYNCHRQNIZING CIRCUITS FATENTEUMAYZI III I 3,812,396

' SHEU 1 III 3 .1 I CATHODE U ANODE U l:

PULSE PULSE 2 AMPLIFIER AMPLIFIER 1 1 HORIZONTAL AND U CATHODE U ANODE vERTIcAL PULSE PULSE TRIGGER AMPLITUDE AMPLITUDE AND 'cIRcuIT CIRCUIT SYNCHRONIZING n cIRcuITs Fig. I

VERTlCAL-- FLYBACK HORIZONTAL FLYBACK PER'OD PIERIOD I I I I L IL I I l ov A J j CATHODE 6OV POTENTIAL FATENTEU MAY 21 I974 SW] 2 Bi FATENTEDMAY 21 1974 3.812.396

sum 3 0r 3 ELECTRON BEAM Fig. 4

l9 2o 4 l l M I l M v \vvw l U M U I "h HI n "hi8 CIRCUIT ARRANGEMENT FOR PRODUCING PEDESTAL CURRENT IN A PYROELECTRIC THERMO IMAGING TUBE I BACKGROUND OF ru s 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 applicants 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 photoconducting 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 polarizationis formed on the nonconducting surface. Now once the charges on the nonconducting 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 -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 thevideo 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 isnot 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 Applicants 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 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 Applicants invention will be described in greater detail with reference to the following drawings in which:

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 nonconductive 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 vidicontype 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. I, 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 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 l6 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 I3 and 15 and the cathode pulse amplitude and the cathode pulse amplifier circuits l4 and 16. The input to resistor R1 is a positive pulse from the horizontal and vertical trigger and synchronizing circuits l2. 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 01 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 N PN 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 ofthe 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 fiyback 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 nonconducting 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 readout for processing by conventional means.

I claim: 1. A circuit arrangement for producing a substantially uniform pedestal current in a vidicon-type thersaid 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 one side of the 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 signals.

3. A circuit arrangement according to claim 2 further including means for inhibiting the negative pulses applied to said cathode when said horizontal trigger pulses coincide with said vertical trigger pulses.

4, A circuit arrangement as claimed in claim 1,

wherein said means for negatively pulsing the cathode of said pyroelectric tube comprises both amplitude and phase control circuits.

5. A circuit arrangement as claimed in claim 1, wherein said target scanned by the electron beam within a pyroelectric therrno 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 film.

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 obtaining said pedestal current from said leakage path.

7. A circuit arrangement as claimed in claim 6, further comprising means coupled to said resistive film to read-out the data representing the temperature changes of the received images.

8. A circuitarrangement for producing a substantially uniform pedestal current at the side of a nonconductive 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 thecathode ofsaid 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.

I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3.8124396 I Dated I May 21, 1974 Inv entorM/ MERTON H. CRGNELL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 13, delete (154-10-911) filed concurrently and I insert the following: -Serial No.

322,633, filed January 11, 1973, now

abandoned and Serial No. 45l,882, filed March 18, 1974 which is a continuation of said application-.

Signed and sealed this 5th day of November 1974.

(SEAL) attest: I I I I McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 12 39 at

d Ma

21, 1974 lnventorl MER'ION H. cRowELL It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 13, delete (154-10-911) filed concurrently/1'1 and insert the following: Serial No.

322,633, filed January 11, 1973, now

abandoned and Serial No. 45l,882, filed March 18, 1974 which is a continuation of said application.-

Signed-and sealed this 5th day of November 1974.

(SEAL) attest:

McCOY M. GIBSON JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents

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 one side of the target is greater than unity.

4. A circuit arrangement as claimed in claim 1, wherein said means for negatively pulsing the cathode of said pyroelectric tube comprises both amplitude and phase control circuits.

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 film.

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.