US2733292A - System fqb correcting-the-equilibrium - Google Patents

Description

Jan. 31. 1956 I J E. COPE ETAL 2,733,292

SYSTEM FOR CORRECTING THE EQUILIBRIUM POTENTIAL OF STORAGE TUBES Filed May 8, 1950 Attorney John'EdWard (lope and Richard Theile,

SYSTEM lFfiR CGRRECTING THE EQUILIB PQTEN'HAL F STORAGE TUBES Cambridge, England, assignors to Cathodeon Limited, Cambridge, England, a British company Application May 8, 1950, Serial No. 160,688 Claims. (Cl. 178-7.2)

;case of tubes of the image iconoscope type, .the storage surface is secondary emitting and the tube has a photocathode upon which the light image is focussed and which, in response thereto, releases photo-electrons which are accelerated and focussed onto the storage surface of the target to form the charge pattern by secondary emission from the storage'surface. In thecase of tubes of the iconoscope type, the storage surface is a photo-sensitive -rnosaic upon which the light image is focussed to form the charge pattern by photo-emission from the storage surface. In either case, the charge pattern on the storage surface is evaluated by a high velocity scanning beam of electrons the effect of which is to generate a train of signal pulses in the signal plate which constitute the output video signal of the tube. A collector electrode arranged in the tube near the storage surface collects electrons emitted from the storage surface under the action of the incident photoelectrons-in the case of the image iconoscope, or the incident light in the case of the iconoscope, and under the action beam.

of the scanning In the normal operation of such tubes, the storage surface is restored at each scanning to an equilibrium p0te ntial from which the charge storageprocess starts at each point of the storage surface in-accordance with the light value of the corresponding point .of the light image.

High velocity scanning-as employed in such tubes promotes the emission of secondary electrons from the storage surface in excess of, unity ratio to the number ofprimary bombarding electrons, and under these condi tions the storage surface assumes an equilibrium potential approximating the operating potential of the collector electrode.

Such video storage tubes suffer'fromanumber of disadvantages, and inparticular, low storage efficiency, generation of spurious signals, production of the visible effect .known as flare. in the reconstituted pictures, and lack of .average brightness" components in the generated video signal.

.An object of the present invention is to minimise the effects of the above mentioned disadvantages by providingan improved method and apparatus for producing a video signal utilizing a video storage. tube of the kind described, in which the operating point of the storage surface is modified. Another object is ,to provide an improved construction of video storage tube which is especially adapted for operation according tothe improved ;method.

" *atent 1.4. us ally of ,r nica,

2 According to the present.invention periodically during the flyback periodsof thescanningbeam, preferably .during the frame .flyback periods, the storage surface-ofa video storagetubeis. submitted to the simultaneousand combined actions. of an irradiation causing. the storage surface toemit electrons, and of afieldcausing the emitted electrons to return to thetstorage surface, .-SO- aS-;t0 cause the storage surface to be charged negativelyrelatively to its normal equilibriumpotential(normalcollectorpotential), whereby the equilibrium ,potentiaLor average .equilibrium I potential of the storage surface is changedfrom that which it would normally,attain.during the flyback periods, thus modifying "the operating point of the tube. .In this way, the storageesurfaceis made more negative to the collector electrode than it-would normally be, and consequently ,atgreater numberlofwthe electrons emitted. from. the storage surface in. response .to the light image or thescanning beam canbe-collectedby the collectorelectrode.

In carrying out the invention, therequiredelectron emission from the storage surface may be obtained,,in the caseof a tubeof the image iconoscope type, by irradiating the storage surface withapulse vofhigh velocity ,difiuse electrons to cause the surface to, emit secondary electrons, and the pulse of irradiating electrons maybe generated by illuminatingthe .photocathodeof the tube with a pulse of difiuselight. In thecaseofatube. of the iconoscope type, the photosensitive ,storage. surface thereof maybedirectly irradiated with pulses of electrons in .order to obtain the required electrons emissionirom the charge-storage surface. In either case, the required field for returningthe emitted electrons to the storagetsurface may be obtained by applying asuitable negative voltage pulse to the collector. electrode, or apositivevoltage pulse to the signalplate, while .the irradiation, of the storage surface is proceeding. Both the electron or light irradiation pulse and the voltage pulse shouldnboof the same duration and should not be of longenduration than the 'flyback period during which gt fiy are. applied. ,If

necessary, blankingpulses may be sirnultaneouslyapplied to the signal amplifier, connected to the output of the tube in order to avoid overloading and transientefiects.

The invention mayr be carriedout with theusenofvideo storage tubes of normal types of constructioniflwith the addition of only .simple ,apparatus forproducing the pulses.

in which afine meshtscreenis.arrangedbetween the-cob lector electrode and the. storage surface andcloseto-the latter, forobtaining increased sensitivity and other; advantages as will hereinafter be, more fully explained.

The invention is equallvapplicable,to,stationary.or portable equipment.

In order that the invention may be more clearly understood, reference willnowbe made to the accompanying schematic drawings, in which Fig. 1 depicts oneembodiment of the invention.

Fig. 2 depicts amodificatiomand 'Fig. 3 depicts a modifiedtube construction.

In the embodimentdepicted in Fig. 1,;a.v ideo storage tube it? of the image iconoscope type is .utilized since a practical form of the invention maybe. easily; realised with this type oftube. The ,tubelt) is of.anorm al ,construction for this typeof. tube and essentially comprises an. evaeuated, generally cylindrical envelope .11r,having a homogeneous photo-cathode lZ :on,,the internalsurface or adjacent thereto at pneend of.the ..envelope,; and a charge storage target lS parallel ,to ,the photo-cathode and spaced ,therefrom towards the other, end of. theenvelope. flfhe target 13 consists of a thin insulating sheet wh is ba e by a ontinuou conductive layer forming the signal plate. The surface of the sheet 14 which faces the photo-cathode 12 constitutes the storage surface and may be treated so that it has a high secondary electron emissivity. The storage surface 14 may be regarded as consisting of a multiplicity of elementary condenser plates, each insulated from the others but each having capacity to the signal plate 15 which constitutes the common electrode of all the storage capacities and is connected externally of the tube to a load resistor 16 feeding into a signal preamplifier 17. Photo-electrons emitted from the photo cathode under the influence of a light image focussed thereon by a suitable lens system 18 are accelerated and focussed on to the storage surface by the field of an image focussing coil 19 surrounding the envelope 11. The storage surface 14 is scanned in a line raster by a high velocity electron beam generated by an electron gun 20 in a side tube 21 of the envelope 11, the side tube being surrounded by the necessary coils 22 and 23 respectively for focussing and deflecting the beam. A collector electrode 24 is arranged in the envelope 11, usually in the form of an internal conducting coating on the envelope wall, for collecting secondary electrons emitted from the storage surface 14 both by the incident primary photo-electrons from the photo-cathode 12 and by the incidental electrons from the scanning beam.

In the normal mode of operation of a tube of this type, at the equilibrium potential (collector potential) of the storage surface 14, the ratio of the number of secondary electrons reaching the collector 24 from the storage surface 14 to the number of primary electrons incident upon the storage surface from the photo-cathode 12 or the scanning beam is unity, the remainder of the secondary electrons liberated by the primary electron bombardment of the storage surface returning to and being redistributed over the storage surface, giving rise to the well-known spurious signals.

Present theory is that these spurious signals and other observed distortion effects are due to non-uniformities in this redistribution. The redistribution of the returning electrons is affected by the local potential conditions over the storage surface, variations in which are caused by the scanning process in which the storage surface is being bombarded only locally at any instant, and by the bombardment of the whole of the storage surface simultaneously and continuously by the photo-electrons liberated from the photo-cathode by virtue of the light image focussed thereon. The general effect of the bombardment with the scanning beam is to charge the area of the storage surface which is immediately and temporarily being bombarded, to the positive equilibrium potential (collector potential) by virtue of the liberation of secondary electrons, and as the scanning beam moves across the storage surface the area immediately behind the beam is left as the lost positive area on the surface, and consequently any free electrons in the neighbourhood migrate preferentially to this area and discharge it. Thus, as a result of the process of redistribution, there is a tendency for the free secondary electrons to migrate in the opposite direction to the motion of the scanning beam. The effect of the bombardment by photo-electrons from the photo-cathode is to charge the storage surface more positively, so counteracting the discharge effect of the redistributed secondary electrons which are, of course, liberated by the primary photo-electrons as well as by the scanning beam.

The most marked effects, which can be accounted for by non-uniformities in the redistribution of returning secondary electrons, are the development of an additive black signal from the earliest scanned portions of either line or frame, and the appearance of a white flare at the bottom of the viewed picture i. e., the area corresponding to that portion of the storage surface which is last scanned. The additive black signal, which is believed to be due to the migration of the redistribution electrons against the direction of scanning, can be bal-- anced out by compensation in the amplifying chain.- Flare, however, is more troublesome. This effect is believed to be due to the fact that when the scanning beam reaches the bottom of the storage surface it switches immediately to the top to recommence scan ning, and consequently there is no region of induced secondary electrons following this bottom area and in close proximity to it, with the result that the positive charge resulting from the scanning is less readily discharged by migration of redistributed electrons. The overall effect is that as this area is less discharged than the rest of the storage surface, it appears relatively white in the viewed picture and, for the same reason, the depth of picture modulation is reduced. The efiect is most noticeable when an area of black picture content reaches to the bottom of the frame, whereas when the bottom of the picture contains white areas (high light content) the effect is less noticeable due, it is believed, to the discharge of these positively charged regions being assisted by redistribution of secondary electrons induced by the arriving photo-electrons.

Due to the competing effects of the processes of positive charge by bombardment and discharge by redistribution, the overall storage efficiency is low and is not proportional to the current density of the photo-electrons. Moreover, since the maximum amplitude of charge is limited by the process described, it is not possible to increase the output of the tube above the level set by these processes, by increasing the light content of the scene to be transmitted.

A further disadvantage is that the output signal contains no component conveying intelligence regarding the mean value of light level of the picture. This condition arises from the requirement that the mean target current must equal zero over the frame period. Therefore, in order that any intelligence may be generated, it is necessary that the signal output waveform shall include both positive and negative departures from this mean value of zero. Since, therefore, the output signal is not unidirectional in relation to the zero value which is available as reference during the line blanking period, it is not possible to use this reference level for purposes of D. C. restoration. This limitation requires that during picture transmission it is necessary for the D. C. level of the outgoing signal to be manually adjusted for any appreciable change in the average light value of the transmitted scene.

Now, according to the present invention these disadvantages and their effects may be considerably reduced by shifting the mean potential of the storage surface 14 negatively sutficiently to improve the collection of secondary electrons by the collector 24 and so reduce their redistribution over the storage surface.

In order to effect the required negative shift of the storage surface potential, in the embodiment in Fig. l, the storage surface 14 is irradiated with pulses of diffuse electrons obtained by irradiating the photo-cathode 12 with pulses of diffuse light. A suitable light source for this purpose is shown as a miniature cathode ray tube 25 having a simple triode structure comprising a cathode 26, a control electrode 27 and an anode 28 which cooperate to direct an electron stream on to the fluorescent screen 29 of the tube so as to produce diffused luminescence thereof for illuminating the photo-cathode 12, the light output of the fluorescent screen 29 being pulsed by application of suitable voltage pulses e1 to the control electrode 27 from a square waveform generator 31 so as to obtain corresponding pulse emission of electrons from the photo-cathode. The square waveform generator 31 may be of any convenient form, such as a conventional multi-vibrator circuit arrangement. Since arrangements for producing the requisite waveform are very well known in the art, it has only been shown in block form on the accompanying drawings. The afterglow of the fluorescent screen 29 must be of short duration in order to avoid ar e-n prolongation of the light pulses into the picture scanning time. The pulses of electrons thereby released from the photo-cathode 12 are accelerated and imaged on to the storage surface 14 and impinge thereon in a diffuse stream at high velocity to cause secondary emission from that surface. Simultaneously with the application of the pulses e1 to the light source, suitable negative voltage pulses e2 are injected into a resistor 30inserted in series with the collector 24- of the tube by means of a square waveform generator 32 so as to cause these negative pulses to appear at the collector 24. The generator 32 may also comprise a conventional arrangement, as indicated for generator 31, and for the same reason has only been shown in block form. The pulses er and 22 are of equal duration, are applied during the frame flyback periods of the scanning beam from the gun 20, and do not exceed those periods in duration.

During the pulsing periods, the storage surface 14 is charged negatively relatively to the normal potential to which the collector reverts on the cessation of the pulses as, this charging apparently being brought about as follows. During each pulsing period, the retarding field at the storage surface due to the negative voltage pulse on the collector Z4 prevents the collection of the secondary electrons emitted from the storage surface 14 by the incident pulse of electrons and causes the secondary electrons to return to the storage surface, thus causing the latter to become progressively more negatively charged and continue charging either until equilibrium is established with the collector pulse potential, or until the cessation of the pulse, if the amplitude of the collector pulse potential should be too high for such equilibrium to be established within the pulse period. When the pulse ceases, the collector reverts to its normal potential and is, therefore, positive relatively to the negatively charged storage surface, thus achieving the required potential difference for obtaining improved secondary electron collection. Consequently, during the interval until the next pulse period, the tube It operates under the improved conditions required. During the next pulse period the charging process is repeated, and thus by successive repetition of the charging process, the improved operational conditions are maintained.

It the storage surface 14 is scanned in the absence of an image charge pattern thereon, the storage surface is discharged from its negatively shifted potential due to the pulse charging process, to its normal positive equilibrium potential (normal potential of the collector 24) by the action of the scanning beam, thus giving rise to a relatively high secondary emission current to the collector 24, which is of constant amplitude over the whole storage surface. f, however, a charge pattern is formed on the storage surface responsive to a light image on the photo-cathode 12, the surface is discharged from its negatively shifted potential due to the pulse charging process, to a greater or lesser extent from point to point over the storage surface according to the brightness values of the correspondingpoints of the light image, so as to form the charge pattern. Consequently, when the storage surface is scanned by the scanning beam from the gun 20, the action of the latter completes the process of discharging the storage surface to its normal positive equilibrium potential (normal collector potential), so that a video signal is generated at the signal plate in accordance with the charge pattern formed on the storage surface 14, which signal is of higher amplitude but of similar polarity to that obtained by operating the tube in the normal manner, i. e., without the pulse charging process.

in contrast to the normal mode of operation, when the tube is operated with the pulse charging process according to this invention, a greater proportion of the secondary electrons released from the storage surface 14 by the primary photo-electrons and the scanning beam is collected by the collector 24, thus giving rise to an increased signal output, and only a smaller proportion ofthe secondary electrons is returned to the storage surface, with a resultant reduction of the above-mentioned disadvantages effects due to redistribution electrons, and especially a reduction in. the amplitude of spurious sig nals and a reduction or avoidance of the flare effect. T he increase in efficiency of secondary electron collection by the collector also improves the storage efliciency of the storage surface by a similar ratio.

An additional and important advantage of the invention' is that the output video signal obtained during scanring is unidirectional in character, i. e., it contains D. C. components. This arises from the fact that the pulse charging process biasses the storage surface negatively during the frame flyback period but, as earlier mentioned, the mean target current must equal zero over the frame period. The principle of operation may, therefore, be described as ensuring the unidirectional nature of the signal nature of the signal current during a scanning (video transmission) interval by balancing it with a countergoing biassing pulse on the storage surface during a flyback (non-transmission) interval. Thus, the output signal contains information regarding the mean value of light level of the picture being transmitted. Consequently, the restoration of D. C. level is facilitated. These D. C. components may be retained during the passage through a resistance-capacity coupled amplifier by the use of a D. C. restorer referring to the line flyback periods. The unidirectional output and the reduction of flare are important advantages obtainable even if the storage surface is negatively biassed by the pulse char' ing to a lesser degree than maximum efficiency, for instance, only to such a degree that no substantial increase in sensitivity is obtained.

To obtain the most efficient operating conditions, it is necessary to establish the optimum required relationship between the pulse charging amplitude, the scanning beam current, the storage capacity, and the operating light range. Although from the viewpoint of obtaining the maximum advantage of saturated secondary emission from the storage surface, it would be desirable to make the amplitude of the negative pulse charge as high as possible, it is preferred not to operate the tube in this manner because limitations are placed upon this charge amplitude for other reasons. In particular, it is necessary to keep down the amplitude of the potential charge pattern developed across the storage surface in order to avoid the incidence of secondary effects, such as loss of definition due to chromatic aberration and the defiection of the emitted secondary electrons by transverse fields. Also, if the discharge is taken too far a saturation effect becomes apparent which gives rise to nonlinearity in the video signal circuit. it was found in practice that the permissible amplitude of the charge pattern was limited to the order of only a few volts.

The value of the storage capacity depends on the value of the charge which it is required to store. For operation at low light levels, small storage capacities are preferable so that they may be charged by small values of photo-electron current, and the evaluation of the charge pattern may be carried out by scanning with low intensity beams, which may be well focussed. On the other hand, if high light levels are available it is preferable to have relatively large storage capacities which can retain a larger charge. Under these conditions it is possible to obtain very strong output signals with negligible noise content. This feature of low noise working at high light levels has not been possible hitherto.

The method of operation according to the invention modifies the transfer characteristic of the tube 10 from that which is normal for that type of tube under normal operating conditions. According to the degree of the potential shift imposed on the storage surface, so is the initial (low light level) portion of the tube characteristic made more straight and the level at which output limitation occurs is extended to higher values. The half-tone values as seen on a normal television receiver are consequently different from those normally obtained, and it may therefore be necessary to dc-gamma the signal.

The average signal amplitude is considerably smaller than the charge pulse amplitude and since the charge pulses contain no useful information, it is desirable that they be eliminated at the amplifier, or at least restricted in amplitude to the limits imposed by the maximum excursion of the signal. For example, suitable means may be provided for simultaneously applying blanking pulses to the amplifier 17 as the charging pulses are applied to the tube so as to suppress the charge pulses in the amplifier and avoid overloading and transient effects. These blanking pulses may be derived from any suitable waveform generator, schematically illustrated at 35. The operating potentials for the cathode ray tube 25 may also be derived from existing sources. In addition, a pulse generator for producing the pulses er and as may derive its master pulses from the existing frame flyback pulses. Thus, it will be apparent that for carrying out the invention with the use of a normal construction of video storage tube 10 of the image iconoscope type, the additional equipment required is very simple.

With the use of a cathode ray tube 25 as the pulsing light source, the controlling pulses e1 may be modified to have a sloping trailing end for compensating for the decay time of the afterglow of the fluorescent screen 29.

During the transmission of continuously illuminated scenes, the processes of charging the storage surface 14 by photo-electrons released from the photo-cathode 12 and discharging the storage surface by the scanning beam occur simultaneously and their effects are superimposed. If the discharge of the storage surface is completed by a single scanning, then the video signal amplitude at the commencement of the scan may be lower than that obtained during a later part of the scan, due to the fact that the photo-electrons have had less time to form the charge pattern on the storage surface. This effect would be progressive across the storage surface and would give rise to a video signal amplitude modulation in the frame direction. This modulation may be compensated in the signal amplifier by means of a correspondingly modulated and balanced amplifier stage, such as a hexode modulated with a waveform of frame frequency. However, methods will now be described for reducing such video signal modulation in the tube itself, whereby a reduction of such modulation in the output signal taken off from the signal plate 15 is obtained. For this purpose, the methods to be described have for their object to obtain a useful amplitude of video signal at the commencement of each scan.

in one method, the intensity of the scanning beam is adjusted to such relatively low value that the storage surface does not reach the positive equilibrium potential (normal collector potential) during a single scan, but is only partially discharged, so that the charge pattern is not completely erased but the contours of a residual charge pattern remain on the storage surface at the end of each scan, from which to develop a useful amplitude of charge by the time the next scan commences. it has been found that this adjustment of the scanning beam is not critical and that the required value can easily be seen by viewing the resulting picture. Since, with the normal method of operating of an image iconoscope type of tube (i. e. without application of the pulse charging process) the charge pattern on the storage surface is also not completely erased by a single scan, the retention of a residual charge pattern, as now proposed, will not give rise to any deterioration in the definition of fast moving televised objects.

Now, if the pulse charging of the storage surface during the frame flyback period between successive scans were to be applied for the duration of that period and were to be of such amplitude as completely to erase the residual charge pattern retained from the previous scan, the advantage of limiting the discharging process of the scanning beam in order to provide that residual charge pattern would be lost. Therefore, in addition to limiting the discharging process of the scanning beam in this way, the pulse charging process is also limited so as to avoid elimination of the residual charge pattern retained from the scanning process and to ensure continuation of the charge pattern development after scanning has taken place. This is achieved, in the method under consideration, by limiting the amplitude of the negative pulse charge, which may be applied for the duration of the frame fiyback period, by pulse charging the storage surface indirectly. This may be performed by applying the pulse irradiation only to an area or areas of the storage surface adjacent to but not overlying the picture area (i. e., the scanned area) of the storage surface, thus avoiding direct irradiation of the scanned area. The irradiated area or areas may conveniently form a frame surrounding the scanning area, the storage surface being suitably enlarged for this purpose. Experiments have shown that this produces the desired negative charging of the scanned area of the storage surface, probably by a second order effect of indirect charging by redistribution over the scanned area of the slow moving electrons released from the adjacent irradiated area or areas. Experiments have also indicated that with this method of indirect charging, the contour of the charge pattern on the scanned area of the storage surface is sufficiently retained after the pulse charge has taken place.

For carrying out this indirect charging process, the embodiment of Fig. 1 may be modified in the manner depicted in Fig. 2, in which parts corresponding to those in Fig. l are denoted by similar reference numerals with the sufiix a. in the tube 10a in Fig. 2 the photo-cathode 120, corresponding to the photo-cathode 12 in Fig. l, is masked so that the pulsing light is allowed to fiood only an area of the photo-cathode surrounding the area on to which the picture to be transmitted is imaged by the lens system 518a. For this purpose, the tube 10a is fitted with an opaque funnel 40 which is inserted between the lens system 18a and the photo-cathode 12a and is of such shape that it frames the picture area of the photo-cathode and masks that area from illumination by the pulsed light source. The latter is shown as a plurality of cathode ray tubes 25a, each similar to the tube 25 in Fig. l, which are arranged around the funnel 40 so as to illuminate only the framing area of the photo-cathode 12a lying outside and surrounding the picture area framed by the funnel iii. The light output of the tubes 25a is controlled, similarly as is the case in the embodiment of Fig. l, by the pulses e1 applied to their control electrodes 27a from generator 31a, and the photo-electrons released from the outer framing area of the photo-cathode 12a in response to the pulse illumination thereof by the tubes 25a are focussed by the image focussing coil 1% of the tube 011 to a corresponding outer frame area of the storage surface 14:! which surrounds the scanned area thereof on to which are imaged by the coil 19a the pl1otoelectrons released from the picture area of the photo-cathode 112a in response to the light image. At the right hand of Fig. 2 is depicted a face view of the storage surface 14a on which the scanned area thereof is indicated by A, and the outer framing area from which the secondary electrons for indirectly charging the scanned area A are released under the influence of the light pulses on the outer area of the photo-cathode 12a is indicated by B, it being understood, of course, that there may not ordinarily be any visible demarcation between the two areas A and B, such as is indicated purely for the purpose of illustration.

By means of the described method of indirect charging, a useful amplitude of video signal may be obtained at the commencement of each scan and thereby the modulation of the video signal amplitude in the frame direction reduced. Any such modulation still remaining may .9 be compensated-in the amplifier in the manner already mentioned.

In another method, the duratic'nl'of the pulse charging process is limited to only a' portion of the frame flyback period and is made as short as possible, being timed to occur at the beginning of the frame flyback period. Thus, the latter is sub-divided into two intervals, during the first of which the pulse charge'is applied to obtain the desired negative shift" of the" storage surface potential, this first interval being immediately followed by the second interval which extendsto the remainder of the frame flyback period and during which the" storage surface is allowed to build up a video charge pattern before scanning commences. During this second interval,- referring to Fig. 1, the charge" storage pfoeessmay be accelerated by applying a positive voltage pulse to the collector 24, the potential of which is restored to its" normal value before scanning commences. Thus,- the cycle of potentials on the collector 2.4" would be as-follows. The collector would be pulsed negatively with the pulses ez' from generator 32a during the pulse charginginterval, then pulsed positively during the chargestorage interval, and then restoredto normal potential for the scanning process. Whilst with this method the char pulse development on the storage surface may still continue during the scanning period, the storage process is mainly concentrated to the second interval of the frame flyback period, so that a useful amplitude of video signal is obtained atthe commencement of each scan and modulation ofthe video signal amplitude in the frame direction is thereby reduced. Any such modulation still remaining maybe compensated in the amplifier in the manner already described. H

The methods above described for obtaining a useful amplitude of video signal at the commencementof each scan may be utilized individually or iii-combination.

In the case of a tube of the iconoseepe-type, theinvention may be applied in a similar manner to that above described in the case of s tube' of the image iconoscope type, except that" the light pulses irradiafe the photosensitive mosaic target of the ie'enescuse instead of the photocathode as in the case of the image iconoscope.

A further feature of the invention is that secondary electron multipliers may be incorporated within the tube to obtain an increased output, since with the improved method of operation aeeerding to the" invention, such multipliers operate with greater efficiency than they would do if incorporated iii high velocity scanned storage tubes operated by the normal metliod, owing to the higher velocity of the electrons leaving the storage; surface.

Another embodiment of the invent'ion', which incorporates secondary electron multiplication, is depicted in Fig. 3. This embodiment incorporates a video storage tube ltlb which is constructed similarly to a normal image iconoscope type of tube, but with modifications as will hereinafter appear. Accordingly, insofar asthe tube 10b conforms to a normal type of image iconoscop'e construction, the parts of the" tube 10]; corresponding to those of the image iconoscope 1 in Fig. 1 are denoted by similar reference numerals with the s'utfix' b. The difierenc es from a normal type of image iconos'c'o'pe tube, such as that in Fig. 1, are that in" front of the storage surface 14b of the target 1311 and closely spaced therefrom is a fine mesh wire gauze screen 50 and in place of the collector 24 in Pig. 1, there is provided a secondary-emitting collector ring 51, the inner: surface of which is preferably activated to have a high secondary electron emissivity. In this tube, the video signal is taken oif from the output electrode 52, after electron multiplication, instead of from the signal plate 155 o'f the' target 13b. In front of the inner surface ofthe collector 51 is provided a mesh or the like opeiiwor'k electrode 52 which constitutes the output electrode of the tube and is" connected externally of the tube to aload resistsrrea feeding into the signal amplifier'ldb.

In operation; the screen 50' is connected to ground po= tent'ia'l and the signal plate 15b is normally at the same potential as the screen, i.- e., at ground potential. The collector 51 is maintained at a positive potential and the output electrode 52 is maintained at a positive potential higher than that of the collector 51. The photo-cathode 12b is maintained at a negative potential to ground. Similarly as described in connection with Fig. 1, during the frame flyback periods of the scanning beam from the gun 2%, the photo-cathode 12b is illuminated with pulses of diffuse light from the cathode ray tube 251), the light output from which is controlled by the voltage pulses 21 applied toits control electrode 276 from a generator 311), so as to irradiate the storage surface 14b with pulses of diffuse electrons released from the photo-cathode 12b under the influence of the light pulses. Simultaneously with the application of the pulses e1 to the light source 255 suitable positive voltage pulses e3 from generator 32b are applied to the signal plate 15b so as to raise its potential for the duration of these pulses to a value approximating to the potential of the collector 51. This, it will be noted, is in contrast to the application of negative voltage pulses e2 to the collector 24 in the embodiment of Pig. 1.

During each pulsing period, the pulses of electrons bombarding the storage surface 14b release secondary electrons therefrom. Owing to the attracting field due to the positive pulsing of the signal plate 15b as described, and owing to the screen 59 now being at a lower potential than the signal plate, the secondary electrons released from the storage surface 14b are not attracted towards the Collector 51- but are returned to the storage surface and redistributed thereover, charging it negatively.

At the end of the pulsing period, the signal plate 35b is restored to its normal ground potential so that it is now negative with respect to the potential of the collector 51. Therefore, secondary electrons released from the negatively charged storage'surface 14b by incident photo-electrons arriving from the photo cathode 12. 1 in response to the light image are collected partly by the screen 50 and partly by the collector 51, while the video charge pattern is developed on the storage surface. When the storage surface is discharged to normal equilibrium potential by the scanning beam from the gun 2%, the secondary electrons emitted'from the storage surface as a result of the beam bombardment, are also collected partly by the screen 5% and partly by the collector 51. in each case, the secondary electrons emitted with suiiicient velocity to pass through the interstices of the screen Eli are accelerated to the collector 5 and bombard it to release secondary electrons from its secondary emitting surface, these electrons being drawn oii to the output electrode 52 and providing the output signal.

By arranging the screen fil -close to the storage surface 1%, the tube characteristic is given a high slope and consequently greater sensitivity. Less pulsing voltage is required and, due to the electrostatic screening action of the screen 553-, less interference is produced in the output circuit by the eficcts of the light and voltage pulses. The tube also provides an cliicient current multiplication by reason of the secondary emission possible, thereby providing an output signal of improved signal/noise ratio.

Of course, although for the purpose of illustration, only one stage of electron multiplication at the collector 51 has been shown, it will be apparent that any other suitable electron multiplier arrangement, which may incorporate several stages of multiplication if desired, may be asso ciated with the collector for increasing the output of the tube. it is not essential that the screen, and normally the signal plate, should be at ground potential, since they may be appropriately biassed to any other suitable potential value while still operating in the manner described. V

It will also be apparent that the methods earlier described herein for compensating for video signal ampliscribed in connection with Fig. 3, may be applied in a similar manner to a tube of the iconoscope type, by irradiating the photosensitive surface of the mosaic target of the iconoscope with pulses of electrons from any convenient source.

Although a cathode ray tube is depicted as the intermittent light source, and is preferred for this purpose as it may be easily controlled by voltage pulses applied to its control electrode, it will be apparent that other devices, such as a gas discharge tube or a light source obturated by rotating or vibrating shutters may be utilized to obtain the intermittent light pulses. Moreover, although with a video storage tube of the image iconoscope or iconoscope types the irradiating pulses may be conveniently generated by utilizing the existing photo-cathode or photo-sensitive mosaic, as the case may be, of the tube in conjunction with an intermittent light source, it will be apparent that as a further alternative an additional electron gun may be included in the tube from which irradiating electron pulses may be derived directly.

In the drawings, the photo- cathode 12, 12a or 1% in Figures 1, 2 and 3 respectively, which is pulsed with light, constitutes a large area source of electrons for biassing the charge storage surface.

We claim:

1. Television apparatus comprising a video storage tube having a charge-storage surface associated with a signal-plate electrode and adapted to form a charge pattern in response to an image incident upon said storage surface, means for scanning said surface with a highvelocity electron beam to discharge said surface to an equilibrium potential, a collector electrode for collecting electrons emitted from the said storage surface under the action of the scanning beam and of the incident image, a large-area source of electrons within the tube, an electron optical focussing device on said tube for forming an image of said large-area source on said charge-storage surface to cause the latter to emit electrons, means associated with said large-area source and operable during each frame flyback period of the scanning beam for pulsing said source, means to cause electron emission therefrom at a high velocity so that, as a result, secondary electrons are emitted from the charge-storage surface, the number of secondary electrons emitted exceeding the number of impinging primary electrons from said source, and a source of field-forming potential connected to one of said electrodes and operable simultaneously with said source-pulsing means for applying a field in the neighbourhood of said storage surface to cause the emitted secondary electrons to return to that surface, whereby to charge said storage surface negatively during the flyback period and thereby shift its potential negatively from said equilibrium valve attained by the discharging action of the scanning beam.

2. Television apparatus comprising a video storage tube having a charge-storage surface associated with a signal-plate electrode and adapted to form a charge pattern in response to an image incident upon said storage surface, means for scanning said surface with a highvelocity electron beam to discharge said surface to an equilibrium potential, a collector electrode for collecting electrons emitted from said storage surface under the action of the scanning beam and of the incident image, a large-area source of electrons within the tube, an electron-optical focussing device on said tube for forming an image of said large-area source on said charge-storage surface to cause the latter to emit electrons, means associated with said large-area source and operable during each frame flyback period of the scanning beam for pulsing said source, means to cause electron emission therefrom at a high velocity so that, as a result, secondary electrons are emitted from the charge-storage surface, the number of secondary electrons emitted exceeding the number of impinging primary electrons from said source, and pulsing means associated with said collector electrode for simultaneously applying to it negative voltage pulses to cause the emitted secondary electrons to return to said storage surface, whereby to charge said storage surface negatively during the flyback period and thereby shift its potential negatively from said equilibrium value attained by the discharging action of the scanning beam.

3. Television apparatus as claimed in claim 2, in which the video storage tube is of the image iconoscope type having a photo-cathode and in which said photo-cathode also comprises said large-area source of electrons within the tube, and comprising further a cathode-ray tube external of said video storage tube, said cathode ray tube having a beam-control electrode, pulsing means connected to said control electrode for controlling the light output of said cathode-ray tube, said light output being directed towards the said photo-cathode of said video storage tube.

4. Television apparatus comprising a video storage tube having a charge-storage surface associated with a signal-plate electrode and adapted to form a charge pattern in response to an image incident upon said storage surface, means for scanning said surface with a highvelocity electron beam to discharge said surface to an equilibrium potential, a collector electrode for collecting electrons emitted from the said storage surface under the action of the scanning beam and of the incident image, a large-area source of electrons within the tube, an electron optical focussing device on said tube for forming an image of said large-area source on said charge-storage surface to cause the latter to emit electrons, means associated with said large-area source and operable during each frame flyback period of the scanning beam for pulsing said source, means to cause electron emission therefrom at a high velocity so that, as a result, secondary electrons are emitted from the charge-storage surface, the number of secondary electrons emitted exceeding the number of impinging primary electrons from said source, and a source of field-forming potential connected to one of said electrodes and operable simultaneously with said source-pulsing means for applying a field in the neighbourhood of said storage surface to cause the emitted secondary electrons to return to that surface, whereby to charge said storage surface negatively during the flyback period and thereby shift its potential negatively from said equilibrium value attained by the discharging action of the scanning beam, said means for negative pulse charging of said storage surface being operable for a portion only of each frame flyback period commencing at the beginning thereof, whereby the remainder of the flyback period allows charge-pattern development on the said storage surface.

5. Apparatus as claimed in claim 4, comprising further, means for applying a positive pulse to said collector electrode during said remainder of the flyback period for accelerating the charge-pattern development.

6. Television apparatus comprising a video storage tube having a charge-storage surface associated with a signal-plate electrode and adapted to form a charge pattern in response to an image incident upon said storage surface, means for scanning said surface with a highvelocity electron beam to discharge said surface to an equilibrium potential, a collector electrode for collecting electrons emitted from the said storage surface under the action of the scanning beam and of the incident image, a large-area source of electrons within the tube, an electron optical focussing device on said tube for forming an image of said large area source on said charge-storage surface to cause the latter to emit electrons, means associated with said large-area source and operable during each frame flyback period of the scanning beam for pulsing said source, means to cause electron emission therefrom at a high velocity so that, as a result, secondary electrons are emitted from the charge-storage surface, the number of secondary electrons emitted exceeding the number of impinging primary electrons from said source, a source of field-forming potential connected to one of said electrodes and operable simultaneously with said source-pulsing means for applying a field in the neighbourhood of said storage surface to cause the emitted secondary electrons to return to that surface, whereby to charge said storage surface negatively during the flyback period and thereby shift its potential negatively from said equilibrium value attained by the discharging action of the scanning beam, and means for applying blanking pulses to a signal amplifier connected to the output of the tube simultaneously with the application of the charging pulses to said tube.

7. Television apparatus comprising a video storage tube having a charge-storage surface associated with a signal-plate electrode and adapted to form a charge pattern in response to an image incident upon said storage surface, means for scanning said surface with a high-velocity electron beam to discharge said surface to an equilibrium potential, a collector electrode for collecting electrons emitted from the said storage surface under the action of the scanning beam and of the incident image, a largearea source of electrons Within the tube, an electron optical focussing device on said tube for forming an image of said large-area source on said charge-storage surface to cause the latter to emit electrons,'means associated with said large-area source and operable during each frame flyback period of the scanning beam for pulsing said source, means to cause electron emission therefrom at a high velocity so that, as a result, secondary electrons are emitted from the charge-storage surface, the number of secondary electrons emitted exceeding the number of impinging primary electrons from said source, a source of field-forming potential connected to one of said electrodes and operable simultaneously with said sourcepulsing means for-applying a field in the neighbourhood of said storage surface to cause the emitted secondary electrons to return to that surface, whereby to charge said storage surface negatively during the flyback period and thereby shift its potential negatively from said equilibrium value attained by the discharging action of the scanning beam, a signal amplifier connected to the output of said tube, said signal amplifier including an amplifying stage, and means for modulating said amplifying stage to compensate for video-signal-amplitude modulation in the frame direction in the output signal from said tube.

8. Television apparatus comprising a video storage tube having a charge storage surface associated with a signal-plate electrode and adapted to form a charge pattern in response to an image incident upon said storage surface, means for scanning said surface with a highvelocity electron beam to discharge said surface to an equilibrium potential, a collector electrode for collecting electrons emitted from said storage surface under the action of the scanning beam and of the incident image, a large-area source of electrons within the tube, an electron-optical focussing device on said tube for forming an image of said large-area source on said charge-storage surface to cause the latter to emit electrons, means associated with said large-area source and operable during each frame fiyback period of the scanning beam for pulsing said source, means to cause electron emission therefrom at a high velocity so that, as a result, secondary electrons are emitted from the charge-storage surface, the number of secondary electrons emitted exceeding the number of impinging primary electrons from said source, pulsin means associated with said collector electrade for simultaneously applying to it negative voltage pulses to cause the emitted secondary electrons to return to said storage surface, whereby to charge said storage surface negatively during the flyback period and thereby shift its potential negatively from said equilibrium value attained by the discharging action of the scanning beam, and when means associated with said video tube so that only an area of said charge-storage surface adjacent to but not overlying the area thereof which is scanned by the scanning beam is irradiated by the electron pulses, whereby to avoid direct irradiation of the scanned area and to efiect the negative charging of the scanned area indirectly by electron emission from said irradiated area.

9. Television apparatus comprising a video storage tube having a charge-storage surface associated with a signal-plate electrode and adapted to form a charge pattern in response to an image incident upon said chargestorage surface, means for scanning said surface with a high-velocity electron beam to discharge said surface to an equilibrium potential, a collector electrode for collect ing electrons emitted from the storage surface under the action of the scanning beam and of the incident image, a large-area source of electrons within the tube, an electron optical focussing device on said tube for forming an image of said large-area source on said charge-storage surface to cause it to emit electrons, means associated with said large-area source and operable during each frame flyback period of the scanning beam for pulsing said large-area source to cause electron emission therefrom at a high velocity so that, as a result, secondary electrons are emitted from the charge-storage surface, the number of secondary electrons emitted exceeding the number of impinging primary electrons from said source, a fine-mesh screen closely spaced in front of said chargestorage surface, and pulsing means connected to said signal-plate electrode for applying a positive voltage pulse during each fiyback period to the signal plate to cause the return of emitted secondary electrons to the chargestorage surface, said screen and signal plate being arranged to operate at other times at the same potential, the collector electrode being arranged for collecting electrons emitted from the storage surface which pass through said screen, and said collector electrode being secondaryemissive and forming part of an electron multiplier, the output of which constitutes the output of the tube.

10. Television apparatus comprising a video-storage tube having an image photo-cathode and a charge-storage surface associated with a signal plate electrode and adapted to form a charge pattern in response to a light image incident upon said photo-cathode, means for scanning said storage surface with a high-velocity electron beam to discharge said storage surface to an equilibrium potential, a collector electrode for collecting electrons emitted from said storage surface under the action of the scanning beam and of the incident image, an electron-optical focusing device on said tube for forming an image of said photocathode on said charge-storage surface to cause the latter to emit electrons, means for masking a picture area of said photo-cathode on to which said light image is projected from an area of said photo-cathode area surrounn ing said picture area, said electron-optical focusing device being operable to focus photo electrons released from the picture area of the photo-cathode on to the scanned area of said charge-storage surface and to focus photo electrons released from said surrounding area of said photo-cathode on to a corresponding area of said chargestorage surface which surrounds the scanned area thereof, means for illuminating only said surrounding area of the photo-cathode with periodical light impulses during each frame fiyback of the scanning beam for pulsing said surrounding area, means to cause electron emission from said surrounding area at a high velocity so that, as a result, secondary electrons are emitted from the said corresponding area of the charge-storage surface, the numher of secondary electrons emitted exceeding the number ofimpinging primary electrons from said surrounding area of the photo-cathode, and pulsing means associated with said collector electrode for simultaneously applying to it negative pulses to cause the emitted secondary electrons to return to said storage surface, whereby to charge said storage surface negatively during the flyback period and thereby shift its potential negatively from said equilibrium value attained by the discharging action of the scanning beam.

References Cited in the file of this patent UNITED STATES PATENTS 2,345,282 Morton et al Mar. 28, 1944 16 Schade Feb. 6, Schade Feb. 27, Mayle June 19, Wiemer et a1 Oct. 12, Snyder Nov. 23, Schade Dec. 25, Weighton Nov. 25,

FOREIGN PATENTS France Oct. 13,

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