US2295530A - Cathode ray device - Google Patents

Description

Sept. l5, 3942.. F. GRAY CATHODE RAY DEVICE Filed March 22 1940 2 Sheets-Sheet l V. [.m. H31 uztwwmm MV1 M l W M R Ak. WF., A l bmw, Nm, vr i NST 9v B n?, Nw mm 3 mm. r Mm. wm w ww ww s sv Q.

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/Nl/ENTOR F GRAY @Y ATTOR EV aiented Sept. l5, 1942 N l STAT S ATENT QFFICE CATHODE RAY DEVICE Application March 22, 1940, Serial No. 325,345

13 Claims.

This application relates to electron optical methods and apparatus and also to image production, and more specifically to electron optical arrangements for cathode ray tubes and circuits therefor.

An object of this invention is to provide novel methods of and apparatus for modulating a beam in a cathode ray device.

In Patent 2,217,198, issued Oct. 8, 1940, to C. J. Davsson, there is described an electrode system for a cathode ray television receiver tube in which modulation is achieved by defiecting a beam of electrons with respect to an aperture by means of a signal applied between a pair of modulating plates arranged on opposite sides of the axis of the tube. In the Davisson tube, a cross-shaped cathode is located in a uniform field which is not varied in accordance with signals, the only varying potential being applied to the modulating plates which are balanced with respect to a point of fixed potential. While signals other than the television signals, such as timing pulses or blanking pulses, may be appli-ed to the modulating plates in addition to the television signals, this is usually not desirable as the modulating plate circuits are frequently designed to have a certain impedance and this impedance is varied when other signals are mixed in with the television signals. It is, therefore, desirable to have means in addition to the modulating plates for modulating the beam intensity.

It is a further object of this invention to provide a cathode ray device in which two different types of modulating means are provided to simultaneously control the intensity of the beam which reaches the screen or target.

In the present invention means are provided for modulating the beam in the vicinity of the cathode and also for modulating it at a point more remote from the cathode by deflecting the beam with respect to an aperture in a diaphragm. Thus, signal voltages from a transmitting station can be appiied to the modulating plates to modulate the beam by deflecting it with respect to an aperture, and a control voltage, such as a timing pulse r a voltage to prevent the beam from reaching the screen when the sweep circuits are not operating, can be applied to the cylinder surrounding the cathode to change the field in the vicinity of the cathode. A negative voltage on this cylinder both defocusses the beam nil with respect to a small aperture adjacent the modulating plates and cuts down the emission of method of control in addition to that furnished by the modulating plates.

In an embodiment of this invention, shown by Way of example for purposes of illustration, a cathode ray tube is provided which employs two means for modulating the intensity of the beam, one of these means preferably being balanced and the other being unbalanced. The electron gun arrangement of this cathode ray tube preferably comprises a cathode, a control cylinder surrounding and negatively biased with respect to the cathode for focussing electrons from said cathode to a focal point, a three-part first anode comprising an apertured diaphragm member, a relatively long tubular member having two apertured diaphragms therein, the first one (the one nearer the cathode) being the focal point of the electron beam, and a relatively short tubular member, a second anode placed between the two tubular members ofthe first anode, a pair of modulating plates placed between the apertured diaphragm member and the rst apertured diaphragm 1n the long tubular member, and a conducting coating on the inner wall of the tube. 'I'he intensity of the beam can be controlled -by applying a signal voltage, balanced with respect to a point of fixed potential, between the modulating plates and also by controlling the potential of a negatively biased control cylinder surrounding the cathode. All members of the first anode and the conducting coating are preferably placed at ground potential as is also the average of the potentials applied to the defiecting plates. The average of the p0- tentials applied to the modulating plates is preferably made negative with respect to the rst anode to prevent a flow of secondary electrons emitted from the anode from influencing the television circuit.

The tube described in the preceding paragraph is adaptable for many types of circuits, two simple forms of which are illustrated in the drawings. In one of these circuits a. time scale is superposed on the image on the fluorescent screen of the tube by means of sharp periodic impulses which are applied to the focussing cylinder around the cathode. Each impulse defocusses and also suppresses the beam current, and the result is a. series of black lines across the image field, these lines constituting a time scale. By means of this circuit it is possible to study the distortion of television images introduced by the transmission channel, such as echo or ghost effects, it often being desirable to know the time electrons from the cathode, thus furnishing a intervals between different features of the distortion. The period between the black lines in the image is the reciprocal of the frequency of the pulses.

A second illustrative circuit is one in which the beam 1s suppressed in the absence of any sweep voltage on the television tube to prevent the screen from being burned. In this circuit a portion of the sweep voltage is applied to a discharge tube through a high input resistance so that very little power '1s drawn from the sweep circuit. This discharge tube is biased negatively so that it passes no current in the absence of a sweep signal. Assuming that the sweep signal is cut ofi?, then the voltage on the focussing cylinder around the cathode is made large enough to defocus the beam and to cut down the current from the cathode to such an extent that there is substantially no current in the electron beam of the television tube. When the sweep voltage is turned on, the discharge tube passes current and substantially shorts at least a portion of the biasing voltage, thus making it possible for a large beam current to pass to the screen. The television tube is thus in operation only when the sweep voltage is on.

Other and ancillary objects and features will be apparent from the following description, the accompanying drawings and the appended claims.

The invention will be more readily understood by referring to the following description taken in connection with the accompanying drawings forming a part thereof in which:

Fig. 1 is a perspective view of the cathode ray tube of this invention;

Fig. 2 is an enlarged longitudinal cross-section of a portion of the tube of Fig. 1 showing the electron gun arrangement in the tube;

Fig. 3 is a cross-section taken along line 3-3 of Fig. 2;

Fig. 4 is a schematic View of the electron gun of Fig. 2 with circuit connections for applying a timing potential to the focussing cylinder of the gun;

Fig. 5 shows schematically an equivalent optical system corresponding to the electron optical system of the electron gun of Fig. 2;

Fig. 6 is an end view of the fluorescent screen of the tube shown in Fig. l showing the timing scale introduced by the circuit shown in Fig. 4; and

Fig. 7 isa schematic diagram of a portion of the electron optical system of Fig. 2 together with a circuit for cutting off or at least substantially decreasing the intensity of the beam when the sweep voltage is oi.

Referring more specifically to the drawings, Fig. 1 shows a cathode ray tube preferably adapted for television reception and having an envelope I0 enclosing an electron gun arrangement II, two pairs of electrostatic delecting plates for producing deection of the beam so that it is possible to scan every elemental area of a field at the end of the tube, and a uorescent screen I2. A coating I3 of aquadag or any similar material covers the inside wall of a large portion of the tube.

Referring now to Fig. 2, wherein the electron gun arrangement is shown in greater detail, the electron optical system I I comprises a cathode I4, a control cylinder I5 surrounding the cathode I4 and having an apertured diaphragm I6 therein closely adjacent the end of the cathode I4, a iirst anode I1 comprising an apertured diaphragm I8, a long tubular member I9 having two apertured diaphragms 20 and 2I therein and a short tubular member 22, and a second anode 23 placed between the long cylinder I9 and the short cylinder 22. Within the member I9 is arranged a pair of modulating plates 36 and 31.

The cathode I4 is supported from a press 24 by means of a supporting wire 25. Supporting wires 26 and 21 are electrical contacts to a heater 12 for the cathode. Also supported from the press 24 are four rods 28 of any suitable conducting material, such as aluminum, which rods, in turn, support insulating discs 29 and 30 which hold the cylinder I5. the diaphragm I6, the tubular member I9 and the modulating plates 36 and 31 in place within the tube I0. The cylinder I5 is attached to the disc 29 by rivets or bolts SI which pierce a flange 32 of the cylinder I5. The cathode I4 is held in position within the in terior of the cylinder I5 by means of an apertured insulating disc 33 which is riveted to the disc 29. The long tubular member I9 is held in position within the tube I0 by means of a flange 34 which is attached to the insulating disc 30 by means of rivets 35. A pair of metallic modulating plates 36, 31 is held in position within the long tubular member I9 by means of langes 38 and 39 thereon which are attached to the insulating disc 30 by means of rivets 40. The short tubular member 22 is attached by means of rivets 4I to an insulating cylinder 42 which is in turn attached by means of rivets 43 to a flange 44 on the long tubular member I9. The second anode 23 is positioned within the insulating cylinder 42 and connection is made to the cylinder by means of lug 45 to which is attached a connecting wire 46 which passes through the glass insulating tubing 41 to the stem 24. Electrical contact is made between the long tubular member I9 and the short tubular member 22 by means of a contact wire 4B. A contact is made from the short tubular member 22 to the aquadag coating I3 by means of spring contact member 49 which is fas tened t0 the flange 50 on the tubular member 22 by means of rivets 4I. Connection between the long tubular member I9 and the diaphragm I8 is made by means of rivets 35. Electrical contact between the aquadag coating and the outside is made by means of a contact member 5I which passes out through a press 52 in the side of the tube. Deflecting plates (not shown) are attached to the electron gun structure by means of supports 53 and 54. The leads to the deiiecting plates and to the focussing or control electrodes have not been shown connected to these elements for the sake of simplicity in the drawings, but these leads 55 pass through the press 24. There are eight leads 55 in all, four of them being for the deflecting plates, two for the modulating plates, one for the second anode 22 and one for the focussing or control electrode I5. The lead 46 also passes out through the press.

In Fig. 3 is shown the manner in which the modulating plates 36 and 31 are supported without making contact to the flange 34 of the tubular member I9. The flange 34 has a, gap 60 therein wiithin which the supporting members 38 and 39 for the plates 36 and 31 extend and are riveted to the insulating disc 3U. Leads 55 (not shown in this gure) are connected to the supporting members 38 and 39 to make contact with the modulating plates 36 and 31 which are preferably made of aluminum material 1/-inch wide and .01 inch in thickness, although it is to be understood that these dimensions are merely given by way of example of a practical embodiment of this invention.

In Fig. 4 a schematic representation of the electron gun arrangement iI is shown together with one of many circuit arrangements for which it is adapted. In this arrangement the first anode Il is preferably placed at ground or some other fixed or reference potential thus placing the diaphragm I8, the long tubular element I9 and the short tubular element 22 all at ground potential. The rst anode Il is made positive with respect to the second anode 23 by means of a source of potential 70. The second anode 23, in turn, is made positive with respect to the cathode I4 by means of the source ll. Heating current for the heater l2 of the cathode It is supplied from transformer 173 having a primary winding 7d and a secondary winding l5. The mid-tap 'i3 of the secondary winding is placed at the potential of the cathode M3. The primary winding le is connected to a suitable source of heating current ID3.

As an example of voltages actually used on a tube of the type being described, the second anode member 23 is made about 1500 volts negative with respect to the potential of the first anode member Il (ground) and the cathode is about 3000 volts negative with respect to ground. In operation electrons are drawn from the end of the equipotential cathode I6 by the relatively positive potential on the diaphragm I8 of the first anode located directly in front of the cathode and condensed by the negative cylinder I (usually placed 100 to 200 volts negative with respect to the cathode le) upon a small aperture 93 in the diaphragm 20. An electron image of this aperture is then projected on the iiuorescent screen I2 (shown in Fig. 1) by the lens system near the end of the long tubular anode i9. (See Fig. 5 for the substantially equivalent optical system for the electron gun system of this invention.) This lens system is in eiect constructed by cutting a section out of the long tubular anode I9 and replacing it with the tubing 23 at a lower potential. The diaphragm 2l with the aperture Illt therein confines the electron beam to the center of this lens system and reduces the aberration that would otherwise result from edge eect in the lenses. The intensity of the projected image may be controlled by a television signal applied to the pair of modulator plates 36, 3l which deflect the incident beam of electrons on or or the small aperture Q3 in the diaphragm 20. This aperture is preferably of the order of .006 inch square (all other apertures in the electron lens system being circular). The condensing cylinder I5 contains an apertured plate I6 primarily introduced to furnish a reference position for centering the cathode in the mechanical assembly of the tube. Having been introduced it plays a role in the electron condensing system and the required value of the negative focusing voltage is to a large degree determined by the distance the cathode is located behind this plate.

The television signal which is applied across i the pair of modulating plates 36 and 3T is shown in the drawings as being applied to the primary winding 'I8 of a transformer 19. The secondary winding 80 is connected to the input of a balanced amplier arrangement represented by the tubes 8! and 82, the output of which is connected through coupling eondensers 83 and 84 to the deflecting plates 36 and 3i. Connected across the plates 3E and 3l in series with equivalent batteries 85 and 86 are equivalent high resistances 8l and 8B. The common terminal of the batteries 85 and 86 is connected through a source of potential 89 to ground which is the potential of the first anode member. The source of potential 89 biases the pair of plates 36 and 31 as a whole negatively with respect to the first anode and thus prevents any now of secondary electrons back into the television circuit. For a more complete description of a circuit employing the feature of biasing the plates negatively with respect to a limiting aperture adjacent thereto, reference may be made to Patent 2,168,760, issued August 8, 1939, to C. J. Calbick. The sources of potential 85 and 86 are included in the circuit to partially deect the beam olf the aperture 93 so that the tube operates on the more linear portion of its characteristic. For a more detailed description of tubes employing balanced modulation by deiiecting a beam with respect to an aperture and characteristic curves of this type of tube, reference may be made to Patent 2,217,197, issued Oct. 8, 1940, to C. J. Davisson, and to the above-mentioned Calbick patent.

When the tube is used as an oscillograph the modulating plates 3S and 37 are connected to ground with a variable voltage in one plate lead for controlling the intensity of the beam. In cases where it is desirable, this variable voltage may be an electrical signal for turning the beam olf and on, such as a time signal for giving a. time scale on the tube. Such a case will be described more fully below in connection with a television arrangement but the principle may be used for oscillographic purposes as well.

The tube is adjusted as follows: With a reasonable negative voltage of about volts on the condensing cylinder I5, high voltages of the values given above are applied to the anodes, and the spot on the screen I2 is roughly focused by adjusting the voltage on the anode 23. The Voltage on the condensing cylinder I5 and the biasing Voltage on the modulating plates are then adjusted to give maximum brightness. Under these conditions, the beam of electrons is usually centered directly on the small aperture 93. If, however, the tbue is slightly misalignedthe alignment can be corrected by placing a small magnet or other magnetic ield forming means outside the tube to hold the beam in place. The spot on the screen is then given a nal focus by adjusting the voltage on the anode member 23 to such a value that the spot on the screen remains sta- .tionary when its intensity is varied with the signals applied to the modulating plates. In tubes so far constructed thev mechanical alignment has been sufliciently accurate to center the beam directly on the small aperture and no external magnet has been necessary. Fig. 2 is drawn to scale except for the aperture sizes which are as follows: The aperture in diaphragm 4S, 0.080 inch, the aperture 7l in diaphragm I8, 11g inch, the aperture 93 in diaphragm 20, 0.006 x 0.006 inch and the aperture Imi in diaphragm 2l, 1A; inch. The modulating plates 36 and 3l are 1/8 inch wide x 0.010 inch thick. The tube I9 is 3% inches long.

It is an important feature of the present tube that the intensity of the beam can also be controlled by a voltage applied to the condensing cylinder I5. An increase in the negative voltage on this cylinder both defocusses the beam from the small aperture 93 and cuts down the emission of electrons from the cathode, thus furnishing a sensitive method of control. For a dei I fs'rptieng' iaintgtiiif arrangement: where; mada varying the zemissfijonfroin a ah renee: .made-tame Frankraylzicas vw i er' |15 te of f the Samplers source 94 biases the cylinder l5 negatively with respect to the cathode I4 to focus the beam on the small aperture 93. The pulses causing the formation of the time scale (such as is shown by the vertical lines 95 in Fig. 6 which are super- 45 imposed upon the image on the field of the fluorescent screen l2) are applied to the focussing cylinder l5 through the transformer 90. Each impulse defocusses the beam with respect to the aperture 93 and also suppresses the beam 50 current. The result is a series of black lines across the image field as shown in Fig. 6, these lines constituting a time scale.

The impulses may be generated by applying an alternating current voltage of known frequency from a suitable source 96 to the grid of a trigger tube such as the thyratron" 91 through a transformer 98. When the trigger tube 91 is not passing current, the capacity 99 in the cathode-plate circuit of the tube 91 charges up 60 through the resistance |99, it receiving charging current from a source of potential IUI. When the tube 98 breaks down on a peak of the alternating current voltage, the condenser 99 suddenly discharges through the transformer winding 9| and impresses a sharp voltage impulse on the focussing cylinder I5. The period between the black lines 95 in the image field of Fig. 6 is the reciprocal of the alternating current frequency and the lines thus constitute a background time scale superimposed on the image. In studying the distortions in television images introduced by the transmission channel, such as echo effects which produce ghosts in the television image, it is desirable to know the time n- 7b i #or .the :source es .a

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left of line X;X is' the same 'as that' to the i'eft'- l of the line X-X in Fig. 4 and the sources of voltages and connections therefrom to the various electrode elements are the same as in Fig. 4 so will not be repeated. In this circuit a fraction of the power of one of the sweep voltage waves is applied to the grid of a tube I I0 through coupling condensers III. The input resistance H2 for the tube H0 is made high so that very little power is drawn from the sweep circuit. The tube H0 is biased negatively by means of a suitable source H3 so that it passes no current in the absence of a sweep signal. Connected between the anode and the cathode of the tube H9 is a condenser H4, the purpose of which will be explained below. Also across the anode-cathode path of tube H0 is a source H5 and a resistance H6. The anode of the tube H0 is connected to the focussing cylinder l5 through a source H1 and the cathode of the tube H0 is connected to the midpoint 16 of the secondary winding 15 of the transformer 13 and the cathode I4.

Suppose now that there is no sweep signal being applied to one pair of the deilecting plates |20 and also to the input circuit of the tube H0, then the voltage on the focussing cylinder l5 is approximately equal to the total voltage output of the sources H5 and H1. This is large enough to defocus the beam and suppress the current from the cathode` I4 to such an extent that there is substantially no current in the electron beam of the television tube. When the beam is defocussed the aperture |04 in the diaphragm 2l cuts off a large number of electrons in the beam. Thus only a relatively small central portion of the beam gets through theaperture IM to pass to the screen and this amount is further decreased because of the suppression of the beam by means of the voltage applied to the negative cylinder I5.

When the sweep voltage is applied to the plates |20 and the tube IIIl, the tube IIII passes current to substantially short out the source IIB from the cathode circuit, because the source II has a';1arge series resistance IIS. The bias on, the cylinder I5 is then about equal to the voltage from the source III. This voltage is adjusted so that it focusses the beam on the small aperture 93 in the television tube. There is then a large beam current to the screen I2. The television tube is thus in operation only when the sweep voltage is on. The capacity II4 smooths over the transient operations of the tube IIB when the sweep signal is on, and givcs a constant bias on the cylinder I I5.

It will be apparent from the description above that the tube of this invention is particularly adaptable to cases where it is desired to have the tube respond to two types of signals, one of them being balanced with respect to a point of fixed potential and the other of which is unbalanced with respect to said point. The tube is also adaptable to cases where it is desired to modulate the tube from an unbalanced circuit at some times and from a balanced circuit at other times, this tube being suitable for operation in both situations. Instead of applying a balanced circuit to theAmodulating plates, it is obvious that one of them can be placed at a fixed potential and the other one left free to vary in accordance with signals, in a manner well known to the art. Its use is thus very flexible. Various other modifications may be made in the .invention as above described, the scope of which is indicated by the appended claims.

What is claimed is:

1. In a cathode ray device, a cathode, an anode member having an apertured diaphragm therein, means for focussing a beam of electrons from said cathode on the aperture in said diphragm, a pair of modulating plates placed on opposite sides of the beam, means for applying modulating signals balanced with respect to a point of fixed potential to said pair of modulating plates to move the beam with respect to said aperture, and means separate from said first signal applying means for applying modulating signals which are unbalanced with respect to a point of fixed potential to said focussing means for varying the intensity of said beam and the focus thereof with respect to said aperture.

2. In a cathode ray device, an electron gun system, said gun having two means for modulating said beam, means for applying a source of balanced modulating signals to one of said modulating means, and means separate from said first signal applying means for applying a source of unbalanced modulating signals to the second of said modulating means.

3. In a cathode ray device. means for generating a beam of electrons, a screen or target, means for causing said beam to scan a field of view on said screen or target. a balanced means for modulating the intensity of said beam of electrons, means for applying a source of balanced modulating signals to said modulating means, a secpnd means for modulating said beam of electrons. and means separate from said first signal applying means for applying signals to said secaaeasso l ond means to periodically decrease the intensity of the beam'by a4 substantial amount.- 4. In a cathode ray device, means for gener-- ating a beam of electrons, a screen or target;

means for defiecting said beam across said screen or target in response to electrical waves, electrostatic means for modulating said ,beam of electrons, means for biasing said modulating means so that said beam has at most a very low intensity when deiiecting waves are not being applied to said -deiiecting means, and means to which defiecting waves are applied when and only when defiecting waves are applied to said deflecting means and which is responsive to said deflecting waves to decrease the bias on said modulating means and thus increase the intensity of said beam during the application of said waves to said defiecting means.

5. In combination, a cathode ray tube having a cathode for generating a stream of electrons,

an anode, and an electrostatic focusing member adjacent said cathode, means for applying a source of potential between said cathode and said focussing member to bias said member negatively with respect to said cathode to such an extent that said stream has at most a very low intensity when substantially the full potential of said source is applied between said cathode and said member, and electron discharge means responsive to'signals for substantially short-circuiting a portion of said biasing source to thereby decrease said biasing voltage and thus increase the current in said stream.

6. The combination of elements as in claim `5 in which said last-mentioned means comprises an electron discharge device having an anode, a cathode and a grid, a resistance, and means for connecting a portion of said biasing source and said resistance in the anode-cathode path of said discharge device.

7. In combination, means including a cathode for generating a beam of electrons, a screen or target for said beam, means for scanning a field of view on said screen or target, means including signal applying means for modulating said beam with signals to reproduce an image on said screen or target, a second means for modulating said beam of electrons, and means separate from said first signal applying means for applying to said second modulating means a series of sharp impulses at a frequency greater than the line scanning frequency of said image to superinnpose upon said image a series of black, substantially parallel lines.

8. The combination of elements as in claim 7 in which said last-mentioned means comprises a condenser, means for periodically charging said condenser, means responsive to a source of fixed frequency for periodically and suddenly discharging said condenser, and transformer means for applying the impulses created by said discharges between said second modulating means and said cathode to periodically blank out said beam of electrons.

9. The method of modulating a beam in a cathode ray tube comprising the steps of varying the intensity of said beam by varying the intensity of a negative field near the source of the cathode ray beam in accordance with a varying voltage and simultaneously varying the intensity of said beam at a plane farther removed from said source by moving it with respect to an aperture in a diaphragm in accordance with a second varying voltage.

10. In combination, a cathode ray tube comprising an envelope enclosing a cathode. an anode, and a focussing member adjacent the cath-` ode, a rst source of potential, a second source of potential, an electron discharge device having an anode, a cathode, and a control means, means for 'connecting in potential-adding manner said two sources of potential in a series circuit including the path between the cathode and the focussing member in said cathode'ray tube to produce a negative bias in said tube sufilcient to at least materially restrict the ow of electron current in said tube in the absence of signals applied to the control means of said discharge device, a resistance member, means for connecting said rst source of potential and said resistance member in a series circuit including the anode-cathode path of said discharge device, a condenser connected across the anode-cathode path of said discharge device, and means for applying signals to the control. means of said discharge device to cause the voltage drop of said resistance member to be so increased that the negative bias in said cathode ray tube is reduced and the flow of electron current therein is correspondingly increased.

11. Television receiving apparatus comprising means for receiving television currents Vfrom a distant transmitting station, a cathode ray image reproducing device comprising means for generating a beam of electrons, a screen or target, and two separate means for modulating the beam, means for applying said received television current to `one of said modulating means to thereby modulate said beam and produce a television image yon the screen or target, said incoming television currents comprising periodic synchronizing pulses, means for locally generating a. voltage of a frequency bearing a multiple harmonic relationship to that of said pulses and which is less than the number of elemental plcture areas in the image field reproduced per second, and means for applying said locally generated voltage to the second of said modulating means to' control said image producing means to thereby perio'dicallymodii'y the tone value ofselected portions of the image ileld.

12. Television receiving apparatus comprising means for receiving television currents from s distant transmitting station, means for utilizing said received television currents to produce a television image, said last-mentioned means comprlsinga cathode ray device having means for generating a beam of electrons, a screen o1 target, and two separate means for modulating said beam, means for applying said received currents to one of said modulating means to produce a television image, and means for applying currents other than said received currents to the second of said modulating means and under control of the receiving operator for introducing ai will into the image ield, along with the structural forms produced by the incoming television currents, other visible structural forms different from those produced by the incoming image currents.

13. A television receiving apparatuscomprising means for receiving television currents from a distant transmitting station, a cathode ray image reproducing device, said cathode ray device comprising means for generating a beam of electrons, a screen or target, means for deiiecting said beam over said screen or target, and twc separate means for modulating said beam oi electrons, means for applying said received currents to the first of said beam modulating means to produce a television image on s aid screen o1 target, means for biasing the second of said modulating means so that said beam at most has a very low value in the absence of defiecting waves and means responsive to said defiecting waves tc decrease the bias on said second modulating means and thus increase the intensity of said beam.

FRANK GRAY.

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