US2908835A

US2908835A - Pickup tube and target therefor

Info

Publication number: US2908835A
Application number: US459971A
Authority: US
Inventors: Paul K Weimer
Original assignee: RCA Corp
Current assignee: RCA Corp
Priority date: 1954-10-04
Filing date: 1954-10-04
Publication date: 1959-10-13
Anticipated expiration: 1976-10-13

Description

Oct. 13, 1'959 P. K. wElMER 2,908,835

PICKUP TUBE AND TARGET THEREFOR Filed oct; 4, 1954 4 sheets-stmt4 1 few mz ,5A/r meca/7' or# 5f/n.45 ryff 746657.'

f/f- TYPE 777/6657." Y

fg INVETOR.

PQI/L Wf/Mne Oct. 13, 1959 P." K. wElMER 2,908,835

' PICKUP TUBE AND TARGET mERsFoR Filed oet. 4, 1954 4 Sheets-Sheet 2 TTORNEY l,... l 'A Ott. 13, 1959 P. K. wl-:lMi-:R

PICKUP TUBE AND TARGET ITHEREFOR 4 Sheets-Sheet 3 Filed Oct. 4, 1954 OPAqz/f W50/Ame L a @NM1-mrc l 5.57 fa J7 14 f'/ C`Pl/arol 55 f 4 Hlamcawacrae Oct'. 13, 1959 RK. wElMER PICKUP TUB;` TARGET THEREFOR Filed Oct. 4, 1954 4 Sheets-Sheet 4 fij Teams/mmf 50p/mer ffm/P45 United States Patent Oliice 2,908,835 Patented Oct. 13, 1959 PICKUP TUBE AND TARGET THEREFOR Paul K. Weimer, Princeton, NJ., assignor to Radio Corporation of America, a corporation of Delaware Application October 4, '1954, Serial No. 459,971

19 Claims. (Cl. 313-65) This invention relates to television camera tubes and particularly to improved television camera, or pickup, tubes having a novel target structure.

Conventional television camera tubes of the Vidicon type normally consist of an elongated envelope enclosing an electron gun in one end of the envelope. In the other end of the envelope is provided a transparent electrode, which is referred to as a signal plate. On the transparent electrode there is deposited a thin layer of photoconductive material. the electron 'gun scans the photoconductor to establish a charge thereon. When exposed to light, electron flow occurs between the two faces of the target through the photoconductive layer at the illuminated areas in accordance with the light intensity. This action produces a signal which can be used to transmit the scene picked up. While this arrangement makes a fairly simple target for television camera tubes, certain Vimprovements result by using target structures made in accordance with this invention.

In the conventional television camera tubes which utilize a target including a photoconductive material, there is a limitation upon the materials which may be used as the photoconductor due to a requirement that the photoconductor bea material having a vrelatively high volume resistivity. This requirement occurs due to the fact that,

`when using the standard methods of operating camera tubes, the charge released by the photoconductor being exposed to light must be stored for the period of ytime occurring between successive scans by the electron beam. Since the scanning period is l/o of a second, the volume resistivity ofthe photoconductor must be high enough to store charges for this period of time. The lower limit on the volume resistivity of the photoconductive materials which can be used in conventional .targets is approximately 1011 ohm-centimeters. Several known photoconductive materials have extremely high sensitivities but halve resistivities that are lower than the 1011 ohm-centimeters requirement and thus cannot be used in conventional target structure. Since these highly sensitive materials cannot be used, prior to this time, a compromise has `been made between a lower -sensitivity and a higher resistivity in selecting a photoconductive material.

Furthermore, when using conventional target structures, a storage time as long as 1/30 of a second has certain disadvantages. As an example, in certain types of television transmission, it is often desirable to transmit pictures of rapidly` moving objects. When the storage time is 1/{30 of a second Vblurred images occur in attempting to televise these rapidly moving objects. y

Still further, in tubes utilizing conventional target structures and when utilizing a low velocity4 electron beam, the capacity of the target, i.e. the capacity between the signal plate and the photoconductor, must be kept below several thousand micro-micro-farads for the beamtodischarge the targetin one'scan. When objectionable capacity lag occurs, i.e. capacity lag longer than the lo ofv a second scanning period, signals from one frame are car- During operation, an electron beam from ried over into the next frame resulting in a poor reproduction of the scene. In other words, objectionable capacity lag occurs when the target is not discharged by one scan of the electron beam. In order to keep the capacity small in prior art targets, the photoconductor must be made thicker than would otherwise be desirable for maximum sensitivity.

It is therefore an object of this invention to provide a novel target structure for television camera tubes which permits the use of photoconductive materials having relatively low volume resistivities but which nevertheless provides the high sensitivity required in tubes of this type.

It is a further object of this invention to provide a novel target structure for television camera tubes which eliminates capacity lag effects and thus permits more effective transmission of rapidly moving objects.

A further object of this invention is to provide a new and more sensitive target for television camera tubes wherein it is not necessary for the electron beam to completely discharge the target in a single scan.

A still further object of this invention is to provide an improved camera tube having improved sensitivity.

These and other objects, applicable to television ycamera tubesl of the monochrome type or the tri-color type, are accomplished in accordance with this invention by prosliding a television camera tube having a novel and an improved photoconductive type of target. In the various embodiments of the target shown and described, the photoconductive material functions in conjunction with a plurality of spaced apart signal output electrodes in a manner such that the resistance of the photoconductor forms one arm of a bridge while a fixed resistance forms a second arm of a bridge resulting in a bridge type equivalent circuit. The bridge type target may be incorporated into targets having the conventional transverse electron flow or in targets having lateral electron ow which will be described below.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself will best be understood by reference to the following description taken in connection with the accompanying four sheets of drawings and in which:

Fig. 1 is an equivalent circuit diagram of `a conventional series type target; i

Fig. 2 is the equivalent circuit diagram of Fig. l modified to illustrate a method of extracting the signal from the circuit of Fig. l;

Fig. 3 is an equivalent circuit diagram of a bridge type target made in accordance with this invention;

Figs. 4 and 5 are equivalent circuit diagrams of Fig. 3 modified to illustrate methods of extracting the signal from the circuit of Fig. 3;

Fig. 6 is a longitudinal sectional view of a camera tube utilizing a target in accordance with this invention;

Fig. 7 is an enlarged fragmentary sectional View of a target of the lateral current ow bridge-type in accordance with this invention for use in a'monochrome type pickup tube as-shown in Fig. 6; Y'

Fig. 8 is an enlarged fragmentary sectional viewofwa modification of a target of the transverse flow bridge type' accordance with this invention for use in a tri-color l pickup tube such as the tube shown in Fig, 6;

Figs. 12- and 13 are enlarged fragmentary `sectional views of modications of target structures in accordancewith this invention for use in a monochromev type of camera tube such as the tube shown in Fig.A 6;' and,

Fig. 14 is an enlarged fragmentary sectional view of la e Y Y 3 modification of a target structure in accordance with this invention for use in a tri-color type of camera tube such as the tube shown in Fig. 6.

Referring to Fig. 6 there is shown a pickup or camera, tube 10 comprising a vacuum tight envelope 11 having an electron gun 12 mounted in one end thereof. The electrodes of the electron gun 12 include the usual cathode, control electrode, and one or more accelerating anodes which are connected to lead-in means in the wellknown manner. An electron beam 14 from the gun 12 is directed upon a target 13 at the other end portion of envelope 11. Means are provided for focusing the electron beam 14, and scanning the beam 14 over target 13 to form a raster. These means may include a focus coil 17, a detiection yoke 15, and an alignment coil 18 as shown.v An electrode 16 is positioned adjacent to the target 13 and, in operation, together with focus coil 17 cooperates to insure that the electron beam 14 in its final approach to the surface of target 13 is normal thereto. A nal accelerating electrode 19 may take the form of a conductive coating on the interior of envelope 11. Fingers mounted on the gun 12, but insulated therefrom, serve to connect conductive coating 19 to one of the lead-in pins.

Target 13 is conventionally supported adjacent to a transparent window 20 and has terminal pins sealed therethrough as shown. The transparent window 20 is supported in the end of the envelope 11 by means of a hollow metallic member 21 Which is in turn welded to a similar hollow metallic member 21 that is sealed to the body portion of envelope 11.

The various elements of camera tube 10 that are referred to above are conventional. In a conventional tube the target 13 comprises a transparent conductive electrode, or signal plate supported adjacent to the window 20, and a photoconductive layer on the signal plate. The elements of a target electrode that are normally included in a conventional tube are not shown in detail.

Referring now to Fig. l there is shown schematically an equivalent circuit of a conventional photoconductive pickup tube. This circuit is shown for the purpose of more clearly explaining applicants invention and forms no part of this invention. In this circuit RPC and C represent the elemental photoconductive resistance and capacitance respectively, which exists between the scanned surface of the photoconductive layer and the signal plate. The light sensitive target Aelement is connected at point P, i.e. where the beam strikes the target, in series with a voltage supply V and an electron beam through a switch K which is normally open but which is closed momentarily each 1,630 of a second for a period of seconds, where N is the total number of picture elements. The beam resistance RB has a value of approximately 10 megohms for low velocity scanning.

Two conditions must be satised for charge storage i type of operation of conventional photoconductivepickup tube targets, ie. those of -a type schematically shown in Fig. l. These conditions are: (l) the time constant for storagepurposes, which is given by the product of RPC C, must exceedfl/go of a second for frame storage, ,or 1A5000 of Va second for line storage; and (2) the time constant for discharge purposes, which is given by RB times C, must be less than When -so yeiipressed, for frame storage, condition (l) above is RP01-.CT must be greater than lo of a second.

While condition (2) above, when so expressed is RECT- must be less than 176,0 of a second.

where: RPCT is the parallel resistance of the entire scanned surface.

Condition (l) above establishes a lower limit on the volume resistivity of the photoconductor which can be used in a conventional tube. For conventional targets, operating with transverse flow and frame storage, the volume resistivity must be 10n ohm-centimeters or greater. Lateral ow targets, i.e. where current ow is substantially parallel to the target, permit the resistivity to be as low as 109 ohm-centimeters. For line storage these values are each reduced by a factor of 500.

Condition (2) above establishes an upper limit of several thousand micro-micro-farads on the total capacity of the target in a conventional tube for a beam resistance of l0 megohms. A practical lower limit of the capacity is set by the amount of stored charge required to give a signal suiciently large for an adequate signal-to-noise ratio.

The signal current IS, which flows inthe series circuit shown schematically in Fig. l, could be measured and utilized in two ways. One method consists of inserting a load resistor RI, in series with the signal plate and coupling the resulting voltage fluctuation into an amplifier as shown in Fig. 2.

Alternatively, the load resistor may be omitted and the signal current fluctuation may be measured indirectly by collecting and amplifying the fraction of the electron beam which fails to be deposited on the target. Both of these methods of extracting a signal are conventional and form no part o-f the present invention, and are shown merely to illustrate the principles of this invention.

In principle,conventional series type photoconductive pickup tube targets wherein the photoconductor has a resistivity which is too low for charge storage operation can be operable. Howeversuch targets have never performed satisfactorily in the past. The reason for this is that the high dark current places severe demands on the uniformity on the secondary emission on the conductivity of the target. Thus, from a practical standpoint, charge storage for at least line time and preferably-for frame time is necessary. The present invention permits satisfactory operation without requiring charge storage.

Referring now to Fig. 3 there is shownA an equivalent circuit, for a picture element of a tube, such as tube 10 shown in Fig. 6, utilizing a bridge type target, -suchas the target shown in Fig. 7, and in accordance with this invention. In this instance, the photoconductive resistance RPC forms one arm of a potentiometer bridge circuit, instead of the series connected circuit of Fig. l. The other arm of the bridgecomprises a fixed resistance RF. A potenital difference Vfl-V2 is applied across the elemental bridge. The scanning beam makes contact, at point P, with the target by closing switch K at the junction of the light-sensitive resistance RPC and the fixed resistance RF. 'Ihe resistance and voltages are so chosen that, inthe absence of light, the bridge balances at the dark potential required by the scanning beam. Forlow velocity scanning the dark potentialmay equal the gun cath- 'ode potential. When light falls` Von the target, the variable resistance RPC decreases and the pointF assumes a different potential, which depends upon the new value of the ratio of RPC to. RF. 'For low velocity scanning the maximum light'potential may be several volts above the potential of the guny cathode. The fraction of .the beam deposited at point P, which depends upon the potential of point P, provides the signal current. As will be explained hereinafter both the fixed. resistance R1.- and photoconductive resistance Rp maybe obtained from the photoconductive material by shielding'portions of the photoconductor 4from the light to resultin a fixed resistance. Alternatively,4 theA xedsresistance can be obtained from anon-photoconductive semi-conductor.,

important feature of the bridge type targets in accordancewith this invention, is that the conditions (l) and (2) which have been set forth above, no longer have the same signicance. The charge storage time constant of the bridge type target is given by the expression RPCRF mima A signicant difference of the bridge type target, as compared to conventional targets, is that satisfactory output signals can be generated using photoconductors in which this time constant is considerably less than that necessary for storage purposes in conventional targets. For a very fast photoconductor, i.e. one having a time response much less than j/30 of a second, this means a slight reduction in sensitivity but a great improvement in the definition obtainable when televising rapidly moving objects. If the photoconductor has a time response of approximately JAG of a second, a low resistivity will not limit the sensitivity of the target nor will it have any elect on the sharpening of the resolution of moving objects. The lower limit on the resistivity of a photoconductor in a bridge type target in accordance with this invention, is probably established by the current carrying capabilities of the target structure, and by the sensitivity of the photoconductor which must be more sensitive for lower resistvities in order to provide adequate potential variations to effectively modulate the beam. The dark current through the bridge circuit does not appear in the output signal and does not have to be carried by the beam.

The condition, set forth above, for discharge purposes to avoid lag in conventional tubes, is not necessary in tubes utilizing bridge type targets in accordance with this invention, provided the frame storage time constant is less than 1/30 of a second. Under these conditions, the target is self-discharging when the light is removed and does not require the beam for discharge. A consequence of this feature is that the capacity of the target can be larger than would be permissible if it were necessary to satisfy the the second condition set forth above. Thus, the photoconductor can be made thinner than is possible when utilizing conventional series type targets. The thin photoconductor makes possible improved sensitivity and, when desired, larger target areas.

Referring now to Fig. 4 there is shown a method of inserting load resistors in the equivalent bridge type circuit shown in Fig. 3. The voltage uctuations produced by the load resistance RL are coupled to a video amplifier (not shown) by means of coupling capacitor C3. An altemative method is shown in Fig. 5. The load resisotrs RL1 and RL2 are connected to the opposite branches of the bridge circuit. The external capacitor C4 may be inserted to insure that the complete video signal is obtained. In. practice capacitor C4 may be unnecessary because its function is accomplished by the internal Aseries capacity C1 and C2. The total video signal is coupled to an amplifier (not shown) through the coupling capacitor C3. As an alternative of the methods of obtaining the output signal from a tube having a bridge type target, the signal may be obtained from the return beam which is collected and directed into an electron multiplier. In this case the load resistors RL1 and RL2 may be omitted.. It should be understood that in the equivalent circuit diagrams of Figs. 3-5 that the light sensitive resistance may be the negative yarm of the bridge, rather than the positive arm of the bridge as shown, in which case an inverted polarity signal results. Also, the mean potential of the bridge may be raised considerably above gun cathode potential for high velocity scanning.

The tolerance of high capacity and high dark current in the bridge type target in accordance with this invention makes this type of target especially suitable for use with .P-N junctions as the light sensitive resistance. Such photosensitive barriers have intrinsicallyu fast response side of the target.

and good sensitivity, but are difficult to make for the conventional series type targets because of their high dark current and high capacity per unit vof target area.

, Referring now to Figs. 6 and 7 there is shown a new and improved pickup tube 10 in accordance with this invention. Certain elements of tube 10, which are conventional, have been previously described. However, in accordance with this invention a target 13, which is shown more clearly in Fig. 7, comprises a support, or backing, plate 24 which may be of a material such as glass. The support plate 24 may be omitted when desired and the balance of the target 13 may be supported directly on the transparent window 20, shown in Fig. 6, which then also serves the same function as support plate 24. There is provided on the surface of the support plate 24, which is normally scanned by the electron beam 14, a plurality of spaced apart conducting signal strips 23 and 23. In contact with, and extending on each side of each of the signal strips 23", is a strip of photoconductive material 27. In contact with, and extending on each side of signal strip 23 is a non-photoconductive strip of semiconducting material 25. The semi-conductive material 25 extends along `the support member 24 and is in contact along its edges with the photoconductor strips 27. On the exposed surface of photoconductive material 27, i.e. the surface upon which beam 14 lands, there is provided a plurality of strips of insulating material 29. As can be seen from the drawing eaclr of the strips `of insulating material 29 is adjacent one of the signal strips; 23'. It should be understood that the insulating strips 29 are optional and may be omitted if the benefits of the coplanar grid effect are desired.

Connected to each of the plurality of signal strips 23 is a resistor 31 (RL.L of Fig. 5). Connected to each of the plurality of signal strips 23 is a resistor-capacitor circuit which includes a load resistor 32 (RI,2 of Fig. 5) and a capacitor 33 (C4 of Fig. 5), the latter of which is connected to the junction between resistor 31 and capacitor 30. As shown in the drawings, the other end of the resistors 31 is connected to the positive side of a potential source while the other end of resistor 32 is connected to the negative side of a potential source. The other side of `capacitor 30 is connected with the trst amplier stage (not shown) `after the camera tube.

In the embodiment shown in Fig. 7 the fixed resistance (RF of Fig. 3) is obtained by means of the resistance of the semi-conductor 25, while the variable resistance (RPC of Fig. 3) of the bridge comprises the photoconductor 27 which is exposed to light, i.e. the areas adjacent the signal strips 23.

As shown in the drawings, the lead-ins for the signal strips 23 and 23 extend through the support plate 24. This method of illustration is utilized merely for ease of illustrating the appropriate connections. In actual practice, tlhe ends of the conducting strips 23 and 23' would be connected together inside the envelope 11 and each set would have a common lead extending through the Walls of envelope 11.

The support plate 24 may lbe constructed of `any transparent material such as glass, mica, or quartz, and may be of any thickness ranging from more than 1/8 inch down to less than one-thousandth of an inclh. As an alternative to the materials mentioned for the support plate 24, the support plate may be made of a thin transparent membrane such as aluminum oxiderstretched on a rigid frame (not shown). Still further, the transparent window 20 may serve as the support plate. Alternatively-the photoconductor sheet or the signal strips may be made suiciently rigid that the entire target structure can be self-supporting inside the tube without contact to any support plate. A still different method of supporting the target, which is not illustrated, is to use a thin sheet of semi-conducting material or a support plate on the beam A thin sheet of lime glass about S microns thick has the proper resistivity to conduct the l23 and 23' for any given target 13 will be determined by the definition required lin the picture to be reproduced, and generally should be from about 500 to several thousand for a high quality picture. The signal strips 23 and 23 may be thin strips of conducting material ap proximately 100 to 1000 Angstrom units thick applied to the support plate 24 by Well-known means such as evaporation, or they may be metallic strips of metal heavy enough to support the `entire target.

The photoconductive material 27 may be any of the well-known highly sensitive photoconductive materials such as antimony sulfide, cadmium selenide, cadmium sulde, or selenium. The photoconductive material 27 may be of a thickness within the range of approximately 1000 Angstrom units thick to several microns. The resistivity of the photoconductive material 27 may vary over a wide range and may be as low as 106 ohm-centimeters. The photoconductive layer may be deposited by evaporation, crystallization, chemical reaction, or other Well-known means.

The insulatingstrips 29 may be any of the well-known goods insulators such as zinc suliide, magnesium fluoride or aluminum oxide. The thickness of insulator strips 29 should be of a thickness that is suicient to insulate the junction between a signal strip 23' and thephotoconductor sheet 27 from bombardment by the electron beam 14. An insulator of approximately 1 micron in thickness is usually sufficient. v

The semi-conductive strips 25 may be any serni-conductive material which is not a photoconductor. One example of such material is germanium. The thickness and the resistivity of the semi-conductor should be so chosen so that W-ith reasonable applied voltages the exposed areas of photoconductor. 27 will balance to ground in the dark, and further, that leakage along the semiconductor 25,'paralle1 to the strips 23, will not -be great enough to deteriorate resolution.

The target 13 may be oriented in any desired manner within the tube 10. For example, the signal strips 23 and 23 may be arranged parallel or perpendicular to the direction of scan. The pickup tube may be operated in several Ways and both low and high velocity opera- .tion of the electron beam is suitable.

During operation of the tube 10, potentials are applied tothe various electrodes to form an electron beam 14 which is directedl toward the target 13. The electron beam 1-4 is scanned across the surface of target 13 in a rectangular raster by magnetic deflecting elds produced by the deiiecting yoke .15. The deflection yoke 15 normally consists of two pairs of magnetic coils with the coils of each pair connected in series and positioned on opposite sides of the envelope 11. The pairs of coils are arranged so that the iield produced by one pair is sub* stantially normal to the iield produced by the other pair of coils. Each pair of coils is connected to appropriate sources of voltages (not shown) to produce both horizontal deflection and vertical dellection of the electron beam 14 in a conventional manner to provide a rectangular scansion raster. The means for producing this type of scansion raster is well-known and is not considered further as it is not a part of this invention.

Electrons from the beam 14 will be ydeposited upon the exposed areas of the photoconductor sheet 27, and upon the strips of insulating material 29, until the povtential ofI the insulating strips is driven substantially to the potential of the'cathode of the electron gun 12. The electron beam 14 lands on the target 13 and the charge deposited is determined by the potential existing on the vstorage area ofthe photoconductor 27. These chargedy 8 storage areascomprise the portion of thephotoconductor sheet that is between an end of an insulating strip 29 and an adjacent insulating strip 25; The target 13 has two separate resistance elements. One' of these resistance elements is the resistance of the photoconductor which is beneath an insulator strip 29. The resistance of the photoconductor which is underneath an insulating strip 29 is the variable resistance which is described in connection with Figs. 3 through 5 as RPC. The other resistance of target 13 isithe resistance of the semi-conductor 25. The resistance of the areas of semi-conductor 25 is the fixed resistance which is described in connection with Figs. 3 through 5 as RF. When the elemental areas of the target 13 are not illuminated by a scene to be reproduced, the fixed and variable resistances are substantially equal. When an elemental area of the `target 13 is illuminated,` the variable resistance isv decreased and the potential of an elemental area of the photoconductor rises a few volts. When the electron beam 14 scans over an elemental area which has a potential that is slightly positive, electrons are deposited in the storage areas of the photoconductor and a video signal is generated in the' signal strips 23 and 23 by virtue of the |target capacity existing between the strips and the storage area. This signal is coupled to the irst tube of the video amplifier (not shown) by means of the

capacitors

30 and 33.

. As can be seen from Fig. 7, the capacity of the target 13 is a parallel combination of the capacity existing between the storage area and the adjacent signal strips 23 and 23. This parallel combination is illustrated by capacitors C1 and C2 in the equivalent circuit of Figs. 3, 4 and 5. Since these capacitors are shunted by the variable, or photoconductor resistance RPC, and by' the fixed resistance RF, the target capacity does not need to be discharged completely by a single scan of the beam 14 to avoid capacitive lag. In other words, when an image is removed from the target, the elemental storage areas return to zero or dark potential regardless of whether the elemental area is scanned or not simply by flow of current in the bridge formed by the fixed and the variable resistances.

There is tno objectionable lag in target 13 ifthe RC time constant of the target is substantially less than the 1/30 of a second scanning period. Since the RC timeA constant is the product of the 'parallel combination of the xed and the variable resistances and the parallel combination of the capacitors C1 and C2, the RC 'time constant may be varied by varying the resistivity and thickness of the photoconductor or by varying the spacing of the signal strips.

Although some photocurrent is Wasted by leaking off of the charge from the elemental storage areas, through the iixed resistor, this loss of photocurrent is more than compensated for by the freedom given the tube designer in being able to operate with a target time constant that is less than 1/30 of a second. Due to the fact that it is possible to operate with an RC time constant of less than 1/30 of a second, photoconductive materials having lower volume resistivities may be utilized, In fact, volume resistivities of the photoconductor' as low as yl06 ohm-centimeters are usable. Furthermore, due to the fact that the storage time may be reduced below 1/30 of a second, the images of rapidly moving objects will be sharp discreet images. v

Referring now to Fig. 8, there is shown a partiall sectional vieW of a modiication of this invention. The target =35 comprises a support plate 36 having on the beam side thereof, a plurality of spaced apart transparent conduct- 9 while the opaque conducting strips 40 are connected together and to a circuit comprising a coupling capacitor 33 and a load resistor 32 as was described above.

Covering the exposed areas of the opaque conducting strips l40, and the transparent lconducting strips 38, is a sheet of photoconductive material 42. Covering the exposed surfaces of the photoconductive material 42, over each pair of transparent signal strips 38 and opaque signal strips v40, is a plurality of short conducting tabs 43. The photocurrent through the bridge circuit of target 35 is substantially from the transparent signal strips 38 through the photoconductor to the conducting tab 43, then parallel to the surface along the tab to an area above an opaque strip, then through the photoconductor to the underlying opaque strip 40. This flow of current in the Aphotoconductor is substantially transverse to the target, whereas in Fig. 7 the photocurrent was substantially parallel to the target surface. .The fixed resistance (RF of Figs. 3 through 5) of the bridge is the resistance of the photoconductor 42 in the areas over the opaque strips 40 while the variable resistance (RPC of Figs. 3 through 5) of the bridge is the resistance of the photoconductor in the illuminated areas.

:The short conducting tabs 43 may be of any highly conductive material such'as aluminum, gold, or tin oxide and each covers a rectangular area of one to three thousandth of an inch on a side and may be of a thickness of 100 to 1000 Angstroms. The other materials shown in the modification of the target structure described in connection with Fig. 8 are similar to those set forth above in connection with Fig. 7 and further description thereof is not deemed necessary at this time.

Referring now to Fig. 9 there is shown an enlarged fragmentary view of an embodiment cfa target structure in accordance with this invention for use in a tri-color, lateral flow, bridge type target. The target 44, which may be inserted in tube 10 shown in Fig. 6, comprises a support plate 45 having on the beam side thereof a plurality of spaced apart conducting signal strips '46 and 46' for the red, 47 and 47 for the green, and 48 and 48 for `the blue. Arranged on the surface of the support plate 45 between each respective pair of signal strips, and contiguous to one of the signal strips, is an insulating light filter strip 49 for the red, 50 for the green, and 51 for the blue. A plurality of opaque insulator strips 52 is provided each of which is arranged in between a respective filter strip and conducting signal strip, and on the surface of the support plate 45. Covering the exposed surfaces of the signal strips, the color filter strips, the opaque insulating strips, and the remainder of the support sheet 45 is a thin sheet of photoconductive material l53. Covering each of the areas of photoconductive sheet 53 which is adjacent to a junction of a signal strip and a color filter strip, i.e., signal strip 46 and red filter 49, for example, is a protective insulator strip 54.

The materials utilized in target 44 may be similar to those that have been described heretofore as may be the sizeand numbers of each. The color filter strips 49, 50 -and 51 are preferably made from some type of'multi- 'layer interference filter. These color filter strips are` normally made of alternate layers' of high index materials such aszinc sulfide, and low index materials such as magnesium -fiuoride with the thickness .of each layer .chosento .give the desired color. Othermaterials may. be'utilized as the insulating material in the multi-layer p Vinterferencefilter-such aspkryolite and zinc selenide. It

should be understood that other types `of insulating vcolor 'filterstrips may also be, utilized to derivefthe various color signals.

. rItnshould be notedthat/'theinsulating color filter strips V"49,50 and 51are"arranged in the conventional red, green,

the art.

The opaque insulating strips 52 may be of any opaque insulating material such as black aluminum oxide, or an opaque metal covered with an insulating film. The insulating strips 52 may be substantially the same thickness and width as the conducting signal strips 46 and 46.

During operation of the target 44, current flow is laterally through the photoconductor, i.e., `similar to Fig. 7, and occurs only when a light signal to be reproduced is passed through a particular color filter 49, 50 or 51. As can be seen from Fig. 9, the resistance and capacitance of the target 44 is a parallel combination of resistances and capacitances as has been described hereinabove. In target 44, the fixed resistance (RF of Figs. 3 through 5) ofthe target occurs in the areas of photoconductor directly over an opaque insulating strip 52; while the variable resistance (RPC of Figs. 3 'through 5) occurs in the area of photoconductor directly over a color filter strip 49, 50 or 51.

Referring now to Fig. l0, there is shown an enlarged fragmentary sectional view of a modification of a target structure of the transverse current flow, bridge type in accordance with this invention for use in a tri-color camera tube such as tube 10 shown in Fig. 6. The target 55 comprises a support plate 56 having on the beam side thereof a plurality of spaced apart opaque conducting strips 57, 57 and 57" for the red, green and blue signals respectively. Spaced intermediate cach of the opaque conductive strips is a conducting color filter strip 58, 58 and 58 for the red, green, blue respectively. Covering the opaque signal strips, and the conducting filter strips,

is a sheet of photoconductive material 59. Covering the areas of the vphotoconductive material 59 which are adjacent pairs of color filter strips and opaque conducting strips, as well as the photoconductor therebetween, is a plurality of short conducting tabs 60. Each of the color filter strips and itsassociated opaque conducting strip, is connected to a respective ouput circuit to provide an output signal in each of the primary colors as was described above.

The materials, and sizes, for the elements of target 55 are similar to those described in connection with Fig. 9. The conducting filters referred to here may be of the Fabry-Perot type consisting of two thin layers of metal separated by a thin transparent dielectric. Alternatively, the previously described insulating multi-layer interference filters may be used provided they are each covered with a different thin sheet of transparent conducting m'etal such as evaporated gold. The operational difference between target 55 and target 44 is that current ow occurs transversely through the target 55 which is similar to the current fiow in the monochrome type target described in connection with Fig. 8.

Referring now to Fig. 11, there is shown an enlarged fragmentary sectional View of a modification lof atarget structure'fof the transverse current flow, vbridge type in accordance with this invention. The target 61, which may be utilized in tube A10, comprises .a support plate 62 having lon the beam side thereof a plurality of spaced apart color Vfilter strips 63, 63' `and 63 for the red, green and blue'respectively. Covering each ofthe color filter strips 63, 63' and 63 is a conducting strip of thin transparent metal 64. This layer may be omitted the filter strips themselves are conducting as'are the 'exposed surface'of 'the support plate A62. Individually covering each of the photoconductor-strips'65'isa strip 0f semiconductin'g material 66 which 'extends' over Vthe 13 v Av typical` semi-conductor would be tin oxide havinga resistivity of about 103 ohm-centimeters deposited in a layer less than 1 micron thick. Alternatively, a much thinner layer of evaporated germanium has been found to be suitable. The conducting strips may be gold or aluminum evaporated to a thickness of approximately .05 micron.

As can be seen from Fig. 14 there is provided a tricolor, transverse ow, bridge type target where in the variable resistance (RPC of Figs. 3 through 5) is obtained from the transverse flow of current through the photoconductor over the signal strips 87. The fixed resistance (RF of Figs. 3 through 5) is obtained from the lateral conductivity of the semi-conductor between conducting stn'ps 86 and the area over the photoconductor. Although certain examples of geometry of photoconductive materials, semi-conductive materials, and color filter materials are shown, these are merely examples and it should be understood that it is within the contemplation of this invention to utilize strips or tabs of photoconductive material or semi-conductive material in the various embodiments shown. Also, since this invention contemplates tubes having bridge type targets utilizing either lateral iiow, or transverse ow, for either the variable resistance or the fixed resistance, and certain examples of these targets are shown, it should be understood that the invention should not be limited to these eX- iamples, but should include all such targets as come Within thescope of the appended claims..

' What is claimed is:

1. An electron discharge device including an envelope containing means for supplying a stream of electrons along a path, a target positioned in said path, said target comprising a transparent supportv member, a plurality of conducting signal strips supported by means of said support member, alternate ones of said signal strips being electrically coupled to substantially fixed resistances in lsaid target, intermediate ones of said signal strips being electrically coupled to variable resistances in said target, the junctions between afixed resistance and a variable resistance being directly exposed to said stream of electrons, and the magnitude ofthe resistance of said variable resistances varying in response to light directed Onto said target. f

2. An electron discharge device as in claim l further comprising color filter means whereby the magnitude of said variable resistances varies in response to light of different colors.

3. An electron discharge device including an envelope containing means for supplying a stream of electrons along ,a path, a target positioned in said path, said target comprising a transparent supporting member, a plurality of signal electrodes supported by means of said supporting member, photo-conductive means electrically coupled to said signal electrodes, a part of said photoconductive means being shielded from light whereby the resistance of said part is substantially fixed, a second part of said photoconductive means being exposed to light whereby the resistance of said second part varies in response to light, and the junction between said parts being directly exposed to said stream of electrons.

4. An electron discharge device as in claim 3 further comprising color filter means.

5. An electron discharge device comprising an envelope containing means for supplying a stream of electrons along a path, a target positioned in said path, said target comprising a support member, a plurality of spaced apart conductive elements supported by said support member, a photoconductive means electrically coupled to the alternate of said conductive elements, semi-com' ductive means electrically coupled to the intermediate of said conductive elements whereby the alternate of said conductive elements are electrically coupled to resistances that vary in response to light and 4the intermediate of said conductive elements are electrically coupled to substantially fixed resistances, and the junctions between said resistancesl that vary in response t-o light and said fixed resistances being exposed directly to said stream of electrons.

6. An electron discharge device including an envelope containing means for supplying a stream of electrons along a path, a target positioned in said path and including a transparent support member, a plurality of transparent conductive elements supported by means of said member, a plurality of opaque conductive elements supported by means of said member, and photoconductive means electrically coupled to said pluralities of conductive elements with the opaque conductive members shielding saidy photoconductive means from light whereby the resistance of said photoconductor adjacent to said opaque members is substantially fixed.

7. A television pickup tube comprising an electron gun for producing an electron beam, a target electrode including a supporting member, a plurality of spaced apart conductive elements supported by means of said member, photoconductive means electrically coupled to said -conductiveelements a plurality of portions of said photoconductive means being shielded from light whereby said portions of said photoconductive means have a substantially fixed electrical resistance, a second plurality of portions of said photoconductive means being exposed to light whereby the resistance of said portions of said photoconductor varies in response to light, anda direct current path between said gun and a junction between a fixed resistance and a variable resistance.

8. A television pickup tube as in claim 7 further comprising color filter means.

9. A television pickup tube as in claim 7 wherein each of said fixed resistances is inelectrical parallel, relation With one of said variable resistances.

10. A television pickup tube comprising an electron gun for producing an electron beam, a target inthe path of said'bearn and comprising a support member, a plurality of spaced apart conductive elements supported by said support member, photoconductive means electrically coupled to the alternate of said conductive elements, semi-conductive means other than said photoconductive means electrically coupled to the intermediate of said conductive elements whereby the alternate of said conductive elements are electrically coupled to resistances .that vary in response to light and the intermediate of said conductive elements are electrically coupled to substancomprising a partof a direct current path between said gun and .a junction of a fixed resistance and of a variable resistance.

` 1l. A target as in claim 10 further comprising color filter means.

12. A target for a television camera tube comprising, a transparent support member, a first plurality of spaced apart conductive strips on one surface of said support member, a second plurality of spaced apart conductive strips on said one surface of said support member, each of said first plurality of strips being spaced in between a pair of said second plurality of strips, a photoconductor supported by means of said support member, means ncluding each of said first plurality of strips for shielding portions `of said photoconductor from light whereby the resistance of said portions is substantially fixed.

13. A target for a television camera tube comprising a transparent support member, Ia plurality of transparent conductive strips spaced apart on one surface of said sup port member, a plurality of opaque conductive strips spaced apart on said one surface lof said support member and each spaced between an adjacent pair of said transparent strips, a continuous layer of photoconductive material covering said strips and the exposed areas of said support member therebetween, and a plurality of con'- ductive tabs spaced `apart on said photoconductive material, each of said tabs being disposed adjacent to one o f saidvtransparent strips and an -adjacent one of said opaque strips.v

14. A television camera tube `comprising an electron gun for producing an electron beam, a target in the path of said beam and comprising a transparent support member, Aa plurality of pairs of signal strips spaced apart on one surface of said member, a plurality of opaque insula'tor strips and a plurality of color filter strips supported on said one surface of said support member, one of said opaque insulator strips and one of said color lter strips being supported between each pair of said signal strips, a continuous sheet of photoconductive material covering all of said strips and the portion of said support member exposed therebetween the portions of said photoconductive material over said opaque strips comprising a' substantially xed resistance, the portions of said photoconductive material over said color lter strips comprising a variable resistance, the junctions between a ix'ed resistance and a variable resistance being directly exposed to said electron beam, and a plurality of strips of insulating material supported on said photoconductor and one adjacent to each of said color filter strips and the signal strip contiguous thereto. 15. A television pickup tube comprising :an electron gun for producing an electron beam, a target in the path of said beam and comprising a transparent support member, a-plurality of sets of signal output electrodes spaced apart on one surface of said support member, each'of said sets including a signal strip, a color filter strip, 'an opaque insulator strip and a second signal strip continguously arranged and in that order, a photoconductor spanning said sets and the exposed surface of lsaid support member therebetween,'the portions of said 'photoconductive material over said opaque' strips comprising a substantially xed resistance, the portions of said photoconductive material over said color filter strips comprising a variable resistance, the junctions between a xed resistance and -a variable resistance being directly exposed to said electron beam, `and a plurality of insulating strips each supported on said photoconductor adjacent to a color lter and the contiguous ysig- :nal strip. v 16. A target for a television pickup tube comprising a support member, a plurality of sets of signal output electrodes spaced apart on said support member, each vported on said photoconductor adjacent to one of `said sets.

17.'A `television camera tube comprising an electron of said sets including a conductive color lter strip and jj an opaque conductive strip, the strips in each of said sets being spaced apart, a photoconductor covering said gun for producing `an electron beam, a target in the path of said beam and comprising a transparent support member, a plurality of sets of signal output electrodes spaced :apart on one vsurface of said support member, and each of saidy sets comprising a layered unit of a color lter strip,` a signal strip, a photoconductive strip, a semi-conducting strip, a narrow semi-conducting strip, and a conducting strip. in that order each of said photoconductive strips comprising a variable'resistance, each of said narrow semi-conducting strips comprising a substantially fixed resistance, and each of said semi-conducting strips comprising la portion of a direct current path between said electron gun and a junction between a fixed resistance and a variable resistance. y

18. A television pickup tube comprising'an electron gun for producing an electron'beam, a target in the path of said beam and comprising a transparent support member, Va plurality 'of ypairs of conducting strips spaced` apart on one surface of said support member, each of said pairs including one transparent strip and one opaque strip, a photoconductor spanning said pairs and the exposed areas of said support member therebetween, the portion of said photoconductor over each of said opaque strips comprising a substantially fixed resistance, the por'- tion of said photoconductor over each of said transparent strips comprising a variable resistance a sheet of semi-conductive material on `said photoconductor and said sheet of semi-conductive material comprising a portion of a direct current path between said electronvgun and a junction between a fixed resistance and variable resistance.

19. A television camera tube comprising :an electron gun for producing yan electron beam, a target in the path of said beam and comprising a transparent insulating support member, a conductive layer on a surface of said support member,y a photoconductive layer on the exposed surface yof said conductive layer and comprisinga variable resistance, a plurality of insulating strips spaced apart on said photoconductive layer and each comprising a xed resistance', laplurality of conductive signal strips each arranged on one of said insulating strips, 'a

semi-conductive layer spanning said signal strips and the exposed areas of insulating stripsv and photoconductor therebetween, and said semi-conductive layer comprising a portion of a direct current path between said gun and a junction between a txed resistance and a variable resistance.

v1,747,988 sabbah Feb. 18,1930

y 2,816,954v Huffman c Dec. 17, 1957