DESIGN AND OPERATIONAL PARAMETERS OF IMAGE ORTHICON TUBES4½ INCH AND 3 INCH FIELD MESH TYPES

Eric D. HendryAssistant Manager, Photo Electric Tube Division

English Electrb Valve Company, Ltd.Chelmsford, Essex, England

Summary

Although the 5820 Image Orthiconreceived universal acceptance in theUnited States of America as the cameratube for studio and remote telecasts,itsprogress in Europe was impeded due tothe existance of other picture origina-ing equipment which produced picturessuperior in all respects other than sen-sitivity. As a result, considerablemodification of the image orthicon tubewas undertaken, culminating in the devel-oment of the 3 inch field mesh and 4½inch image orthicon tubes.

This development work has beeninstrumental in gaining acceptance forthe image orthicon tube in Europe. Theimproved performance characteristics ofthese tube types are now being recognizedand acknowledged in the United States ofAmerica as shown by their increasedusage.

The operational parameters of thevarious 3 inch and 4½ inch tubes willnow be discussed.

In the U.S.A. the first imageorthicons commercially obtainable werethe 2P23 type with the AgOCS photocath-ode. These were soon superseded by thetypes 5769, 5655 which both employed theSbCs photocathode with an improved colorresponse. The difference between thetwo types was in target spacing, andreflected the desire by the TV industryfor tubes with different characteristicsdesigned to suit respectively studio andremote applications. The studio tube wasclose spaced and was designed to givethe optimum performance when trans-mitting a scene of the relatively low

contrast normally obtainable in thestudio. The other type incorporated awider spaced target and was designedto be a more flexible tube for remoteuse and provide an adequate performancefrom scenes of widely differing bright-ness and contrast.

With the development of the SilverBismuth photocathode the 5769, 5655were replaced by the types 5820, 5826designed to fulfill similar functionstothe two preceding types. The 5826had a disappointingly short commerciallife. It was soon decided that theimprovement in picture quality, resul-ting from the use of the 5826, did notoutweigh the operational difficultiesassociated with the use of the closespaced tube. As a result of the widerspaced 5820's introduction for studiosby 1953 the demand for the 5826 wasnegligible.

The charge to the 5820, althoughdeplored by the purists, was inevitablesince the 5820 displayed two short-comings which in a curious way went partway towards compensating the deficiencesof the average American receiver of thattime. Both of these shortcomings wereassociated with the use of wide-spacedtargets and were respectively

1. Edge effect.2. Unstable transfer

characteristic.

The edge effect produced an arti-ficial sharpening of the transmittedpicture which improved the subjectivesharpness of the picture viewed on thereceiving set of inadequate bandwidth.Furthermore, its existence was appre-ciated by the viewers in the fringearea.

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The unstable transfer characteris-tic or A.V.C. characteristic as it issometimes called, not only simplifiedoperation of the tube but produced asignal which was very suited to the non-restored receiver which was common atthat time.

In Great Britain the image orthiconwas introduced for remotes during 1949,but four years were to elapse before itwas accepted for studio use. The reasonfor the delay was the existence in theB.B.C. of camera tubes, capable of pro-ducing pictures of better quality thanthose produced by the 5820 which was animproved orthicon, and the image icono-scope. It is not suggested that thesetubes could match the sensitivity, flex-ibility or operational simplicity, ofthe wide-spaced image orthicon, but whenused in studios, where the light leveland contrast range could be controlled,they were capable of producing picturesof a superior quality, and reducing thedifference between the live programsand those emanating from 35 m/m flyingspot equipment.

This was thus the situation in theearly fifties not only in Great Britainbut also in Western Europe where varioustypes of image iconoscopes held sway inthe studios. The dependence upon theiconoscopes with their required highlight levels was made easier by virtueof the fact that most studios wererelatively small.

The production of image orthiconsin Europe was first started in 1949/50by the English Electric Valve Co. Ltd.Initially 5820 tubes were manufactured,but due to the opposition to theiradoption in studios, ways of reducingthe deficiencies, which precluded itsuse as such, were investigated.

The major decision was to undertakethe commercial development of the 4½"image orthicon which had been previouslyinvestigated by R.C.A. but abandonedowing to lack of interest on the partof the TV networks.

In parallel with this development,modification to the 5820 was undertakendesigned to reduce the objectional pic-ture defects.

The major change was the intro-duction of the field mesh, which wasdesigned to reduce the radial of elect-ric field in the neighborhood of thetarget. This had previously been inves-tigated as a means of avoiding the for-mation of an ion spot, with a consequentimprovement in tube life. During thecourse of these experiments it wasfound that if the separation of thefield mesh from the target was madesmall, the tube produced a picturewhich lacked the characteristic hardnessof the accepted image orthicon picture.The reasons for this change in perfor-mance was not really understood forseveral years, until 1957 when Theileand Pilz of R.T.I. Germany publishedthe results of their investigationsinto the generation of spurioussignals in the image orthicon. However,the advantage of the field mesh havingbeen experimentally established, thedevelopment of a 3" field mes tube wasinitiated.

The introduction of a field meshinto the image orthicon did not presentany major difficulties. Three mainrequirements had to be fulfilled.

1. The mesh must not be resolved bythe scanning beam.

2. The mesh must have a uniform hightransparency.

3. The tube should operate satisfact-orily within the range of poten-ials required by the existing imageorthicons of the non-field meshtypes.

Requirements 1 & 3 were not diffi-cult to meet. The optimum position ofthe field mesh could, in theory, be cal-culated, but it was found that theplacing of the mesh at an antinode ofthe beam was not necessarily satisfac-tory. Due to the fact that the beammakes two transits of the mesh, inter-ference between the two images of thefield mesh, so formed can arise appear-ing as strobe patterns of various fre-quencies. As a result the final posi-tion was determined empirically.

In order to ensure operation inexisting image orthicon cameras thefield mesh was mounted on the deceler-ator which was connected internally to

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the beam focus electrode. The deceler-ator control was thus inoperative, thegeometry and shading being satisfactorywithout the radial electric field corr-ection necessary in the case of the non-field mesh tube.

The combination of this tructureand a wide-spaced target, was the firstfield mesh image orthicon offered com-mercially being known as the E.E.V.P.807. This tube was the first imageorthicon accepted for studio use by theB.B.C., and was also adopted for remoteapplications.

The second requirement mentionedabove proved to be the most difficultto meet, requiring considerable advancesin mesh making technology.

This pioneer field mesh tube, ful-filled its initial promise, by producingpictures which lacked the characteristicimage orthicon look but did not receiveimmediate acceptance in all quarters.Two defects were apparent:

1. Low signal to noise ratio.2. Low amplitude response.

Similar defects had been noticed inthe early field mesh 4½" tubes, develo-ment of which was progressing at the sametime. It was established that the highnoise level and poor resolution were bothdue to the secondary electrons emittedfrom the field mesh under bombardment bythe beam. Those emitted as the resultof the impact of the forward beam con-tribute a general background noise. Thosereleased by the beam returning from thetarget produce an out of phase picture,the difference of phase being due to thedifference in transit time between theelectrons returning through the fieldmesh and the slow secondary electronsstarting at the field mesh. The effecton picture of the out of phase signalwas a trailing smear of short duration.This severely reduced the measured amp-litude response at 400 lines but alsoled to a further reduction in the "edgeeffect".

Various "chemical" methods of re-ducing the secondary emission wereinvestigated, that is coating the meshwith a material of low secondary emission.

However, it was decided to stop thesecondary emission contributing to thesignal, by electrical means. In the caseof the 4½" tube this was comparativelyeasy since no cameras had been manu-factured at this time. The secondaryelectrons were prevented from enteringthe multiplier by the simple expedientof operating the field mesh at a sligh-ly higher potential than that of thebeam focus electrode.

This remedy was not applicable inthe case of the 3" image orthicon sincethe bias potential required for thefield mesh was not readily available onthe many cameras already in the field.However, an elegant solution was devel-oped by the English Electric Valve Co.,Ltd. in 1957 and has since been adoptedby all other manufacturers of fieldmesh image orthicons. The basis of theinvention is the creation of a potentialminimum at the entrance to the multi-plier. The potential at this point isdepressed to approximately 90 voltswhich is intermediate between the targetpotential and the potential of the G4and hence the field mesh. As a resultsecondary electrons emanating from thefield mesh are unable to enter the mul-tiplier whereas the direction of theelectron beam to and fromthe target isunaffected, although the velocity ofthe beam as it passes through the neigh-borhood of the potential minimum.

The required electrical field isproduced by the introduction of a ringelectrode at cathode potential at theentrance to the multiplier. This elec-trode is connected internally to thecathode, so that no additional electri-cal supplies are required.

The first commercially availablefield mesh image orthicon with suppres-sor was the E.E.V. 7293 introduced in1958. This differed from the pioneerP.807 not only in the addition of thesuppressor but also in the design ofthe target field mesh region. The closespaced version of the P.807 was usedsuccessfully in the B.B.C. colour cam-eras during the period 1956/57. Althoughregistration was more easy than in thecase of non-field mesh image orthicons,it was decided that further improvementwould result from the introduction of a

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decelerator. As a result the E.E.V.7293 was provided with a deceleratormounted between the field mesh and thetarget. This was operated in the accep-ted manner, that is adjusted for optimumshading and geometry.

The basic design of the E.E.V. fieldmesh image orthicon, that is a suppressorand a decelerator hassince been used withminor modifications by all other manu-facturers of field mesh image orthicons.

Two types were made available com-mercially available the 7293 and the 7294.The latter incorporates a close spacedtarget and is intended for colour appli-cations and monochrome studio use.

The 7293 is the successor to theP.807 and incorporates a wide spacedtarget of the same capacity as that em-ployed in the 5820. As such it is opera-tionally interchangeable with the lattertube since its introduction in 1958, hasmade considerable progress at the expenseof it.

In Canada the 7293 is used in pre-ference to the 5820 by the majority ofthe C.B.C. and Commercial Stations, forboth studio and remote applications. Inthe United Kingdom, no non-field meshtubes have been used by the operatingcompanies for twelve years. In Europeand Australia field mesh tubes with closespaced targets such as the E.E.V. 7294and OS20F are preferred. These types arepreferred since it is generally acceptedthat these are capable of substantiallyreducing the normal difference betweenthe 4½" image orthicon pictures and thosefrom the 5820.

Since its introduction in 1958, the7293 has undergone further development,aimed at the elimination of the highlightghost and an improvement in image geometry.This has been accomplished by a radicalredesign of the image electrodes.

As mentioned above, the 7293 is op-erationally interchangeable with the 5820and it is of interest to compare the per-formance figures for the two types.

5820 7293Operational Sensi-tivity (Scene Brightnessat f5.6) 15 ft.L

Signal/Noise Ratio 32 db

Amplitude Response@ 400 lines. 40%

15 ft.L

31 db

40%

The only penalty paid for theremoval of the spurious picture charac-teristics of the non-field mesh tubeis a very slight increase in the noiselevel. This arises as a result of theattenuation of the signal during thepassage of the return beam through thefield mesh. It can be shown that thedegradation of the S/N is given by afactor T3/2 where T is the transmissionof the field mesh.

It has been possible to developuniform meshes of extremely high trans-parency. The resultant loss in signalto noise has thus been fractional.

The signal to noise ratio lost bythe inclusion of the field mesh can bereclaimed by a small increase of thetarget capacity. In the case of the7293 this has not been done, since suchchanges also produce changes in thetransfer characteristic of the tube.

Alternatively, the signal to noiseratio can be improved by running thetarget at a target voltage slightlygreater than the customary 2 volts. Thisremedy is not applicable to the non-fieldmesh types, as certain of the picturedefects are a function of the target vol-tage and operation at increased targetvoltage would accentuate the effects.

The additional benefits conferredby the adoption of a field mesh, areconsideraUe, and should be remembered ifthe protoganists of the 5820 type placeundue emphasis on the minute loss insignal to noise.

1. Improved black shading.2. Improved corner resolution.3. Improved white shading. In this

context, it should be mentioned,thatwhen the 7293 is operated at itscorrect beam focus of 130 volts,

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porthole shading is negligible,and as a result, thebbe can beoperated quite satisfactorily, incameras with yokes of doubtfulquality. This is not the case withnon-field mesh tubes which quiteoften give consistent bad landingin particular yokes.

The operation of the 7293 differsin only one respect from the 5820. Thealignment procedure which has been stan-darized for non-field mesh tubes, thatis observation of the dynode aperture,is no longer possible, as the apertureis not readily discernable. The recom-mended alignment procedure is to adjustthe alignment controls for maximum andmost uniform modulation.

Alternatively the alignment controlsmay be adjusted for minimum rotation ofthe picture as the beam focus controlis rocked. This operation can be fac-ilitated by the use of a "wobbulator" oroscillator applying a low amplitude 30cycle modulation locked to field driveand applied to the beam focus electrode.In this case the alignment coils areadjusted to cause superposition of thetwo images so formed.

The operation of all the other tubeelements is similar but more definitethan in the non-field mesh types. Forinstance the operation of the multiplierfocus, is largely that of a gain controland has a very little effect on theuniformity of the black shading.

The lighting and staging practicesrequired for the 7293 do not differ fromthose which have been adopted for the5820. As mentioned above the capacityof the target is identical with that ofthe 5820 resulting in a similar contrasthandling ability. The standard 5820exposure adjustment, which relies on thelinearizing of the Retma low contrastlogarithmic wedge is equally applicable.

The major operational advantage tobe gained by the use of the 7293 is amore accurate and faithful reproductionofhigh contrast transitions. This re-duces the constraints which normally haveto be applied to the set and studio per-sonnel when non-field mesh tubes arebeing used.

As stated above the developmentof three inch field mesh tubes was theresult of one of the programs for elim-inating the undesirable features of thepicture produced by the wide-spaced 3"image orthicon.

The other program was the commer-cial development of the 4½" image orth-icon. In addition to the faults asso-ciated with the existence of a weakdecelerating field, the 5820 generatescertain signals which result from theuse of a low capacity target. Thiscapacity can be increased by the reduc-tion of the target spacing but a moreelegant solution is to obtain the in-crease in capacity by an increase intarget area. The use of the largertarget area besides reducing the defectsassociated with a low target capacity,gives additional benefits which may bebriefly enumerated as under:

1. Increased resolution.2. Reduced redistribution (Halo etc.)3. Extended life compared to 3" tube

with target of same capacity.

The subject of the 4½" image orth-icon has been covered more fully in anearlier paper and it is proposed torestrict discussion in the present paperto the application of the field mesh tothat type.

Non-field mesh 4h" image orthiconswere built in early stages of developmentby both R.C.A. and E.E.V. However, thedeflection of the beam by the targetpotential pattern, which had previouslybeen noticed in the 3" tube, appearedwith a severity which considerably re-duced the improvement in overall picturequality which had resulted from theadoption of the larger area high capacitytarget. Attempts were made to increasethe strength of the decelerating fieldby operating the beam focus electrodeat a high voltage but this introducedporthole shading.

The field mesh was therefore adop-ted, and in the first commercial versionsof the 4½" tubes was mounted between thetarget and the decelerator, as in thepresent three inch tubes.

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This arrangement was not found without sacrificing sensitivity. Thecompletely satisfactory owing to the E.E.V. 7295, and 7389 field mesh 4½"difficulty, when this arrangement was image orthicons have now been worldused, of placing the field mesh at an wide in operation for seven years andantinode of the beam. Further porthole this successful design is now beingshading could not be entirely eliminated, adopted by Companies who are just

entering this field.As a result the position of the

field mesh and decelerator were trans-posed, and the field mesh placed nextto the target. This resulted in aneven stronger decelerating field andpermitted operation at a higher beamfocus voltage without the introductionof porthole shading.

The additional benefits resultingfrom the use of the field mesh in thecase of the 4½" tubes, were even moremarked than in the case of the smallertube. Without a field mesb the scanon the dynode of the return beam is ofa very large amplitude. It thus becomesdifficult to control black shading andensure uniformity. In the case of thefield mesh version the scan is restrict-ed to a smaller area thus ensuring moreuniform collection from the firstdynode. More recent cameras have pro-vided for some adjustment of the pot-ential of the field as a means ofdefocusing any dynode background. Suchadjustment must not, however, removethe bias between the G4 and the fieldmesh designed to avoid collection ofthe secondary electrons emitted by thefield mesh.

The increased amplitude of scannecessary in the 4½" tube, would leadto a greater loss in corner resolutiondue to deflection defocusing. However,the incorporation of the field mesh isinstrumental in reducing this loss suchthat the resolution in the extremecorners is barely 40 db down on that inthe center, when the beam focus is ad-justed for optimum resolution in thecenter.

The combination of the field mesh andthe large area high capacity target hasresulted in the development of an imageorthicon which has matched the picturequality of the earlier camera tubes,

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