DE2161186A1 - Circuit arrangement for a cathode ray tube - Google Patents

Description

December 9, 1971

A 40171 Ml / vs.

SANDERS ASSOCIATES, INC., Daniel Webster Highway, South, Nashua , New Hampshire Oj5O6O, V.St.A.

Circuit arrangement for a cathode ray tube

The invention relates to a circuit arrangement for a cathode ray tube, in particular for use with a cathode ray tube according to patent application P 21 16 288.9.

The circuit arrangement enables the electron beam of the cathode ray tube to be controlled, which thereby can be pointed to any of a certain number of places at which it appears on the screen of the Cathode ray tube should be deflected.

209825/1049

• According to the invention, a circuit arrangement for a cathode ray tube is created with first and second means for generating a beam deflection signal, first and second digitally operated control means which are connected to control the first and second beam deflection signal generating means a decoder which has its input with the output of a control means and its output with the input a function generator is connected, while at the output of the puncture generator a signal for modification a control signal for the beam of the cathode ray tube can be removed. The digital signals that influencing the control means can for example be coded in binary form or as groups of pulses occur that pass on the required information.

In one embodiment of the invention, the first and second beam deflection generating means generate Deflection signals for controlling the beam deflection in mutually perpendicular directions on the X and Y axes, and two decoders are provided, each with its input with the output of the corresponding Control means are connected and at least one functional

- 3 209825/1049

generator that emits a signal for modifying a control signal for the cathode ray and that with the Output of each decoder is connected.

The use of a digital control in the deflection of the cathode ray enables the exact Point in time to determine when the cathode ray hits each of a multitude of discrete points on the screen the cathode ray tube, and allows a control signal that according to a known law for each is derived from a number of parameters of the ray, a ray element at that very moment which is necessary to correct an undesired deflection or deviation of the beam.

In the preferred embodiment of the invention, the beam scans the screen surface of the cathode ray tube in a grid, as is generally used in television image reproduction, the discrete Points are given exactly by the X and Y coordinates. However, other types of scanning are also possible possible, such as a spiral path on the screen, and other methods of position

- 4 209825/1049

detection of the cathode ray such as polar coordinates r / θ can be applied.

The digitally operated control means are, in one embodiment of the invention, a pair of registers, controlled by a computer connected to a source for * the video signals. The computer gives a code, which is decisive for the desired coordinates of the position of the cathode ray on the screen, into the Register in sync with the video signal so that the beam is exactly in accordance with the video signal hits the screen.

In another exemplary embodiment, the Control means a pair of interconnected counters which are triggered by pulses from a timing pulse generator which is under the control of the synchronization pulses of the video signal.

The parameters that can be controlled include the linearity of the deflection signal, the focusing of the And the astigmatism of the ray.

- 5 209825/1049

According to a further feature of the invention, means are provided for controlling the beam deflection in such a way that that correction of any trapezoidal distortion of the image scan in the screen area is achieved.

In addition to the control means enumerated, the control of the beam scanning the screen get during the reproduction of an image, it is possible by means of a circuit part according to the invention to influence the deflection of the beam in such a way that it displays symbols on the screen of the cathode ray tube writes or draws. In addition, the control of the beam can be done so that on an area of the screen Characters or letters are written while a picture or a picture is on another area of the screen Characters generated by raster-shaped scanning will.

It has been shown in a specific embodiment of the cathode ray tube that the beam only requires a focus correction according to a law that is established for each line of the scan in X direction is the same. Also on this particular one

- 6 209825/1049

2167186

Embodiment the law symmetrical to the center of each of these scanning lines in the X direction. Naturally the focus correction can also be provided in accordance with the beam scanning in the Y direction, if this is necessary.

According to the invention, therefore, a circuit part is provided from which a correction signal emanates that is dependent from the position of the cathode ray on the screen to an electrode for cathode ray focus control is delivered.

One of these focus control circuit circuits used in the cathode ray tube mentioned at the outset comes, receives a series of output variables from a suitable decoder and thus generates what is required Focus correction signal corresponding to the respective scanning position of the electron beam, which is determined by the digitally actuated Control means is determined.

The focus control circuit circuit can be a resistor network, which according to the law of the required correction signal is graded and so

- 7 -209825/1049

is connected that it generates a signal on the one hand at its output and on the other hand with a voltage or Power source is connected via a number of switches which are operated by the output of the connected decoder that the voltage or the current at the signal output both correspond to the law of correction as well as the scanning position of the electron beam is changed.

The circuit arrangements used in the invention can be designed to either Output current or voltage values at the output and thus to both electrostatic and electromagnetic Deflection systems of the cathode ray tubes can be connected.

Exemplary embodiments of the invention will now be described in conjunction with the drawing for a precise explanation. Show it:

Fig. 1 is a schematic block diagram of a control circuit according to the invention,

Fig. 2 is a schematic diagram of an electrical circuit for generating the burning

- 8 209825/1049

point control signal,

Fig. 3 shows a cathode ray tube of the type mentioned, in which the invention is applied, and

FIG. 4 shows a section along the line IV-IV in FIG. 3 through the cathode ray tube.

The circuit arrangement shown in Figure 1 controls the deflection of the cathode ray of the cathode ray tube 1 and corrects the deflection signals for trapezoidal distortion, linearity and astigmatism. There is also automatic control over the focus of the Stralis while using the Cathode ray tube screen is deflected.

The operation of the circuit arrangement according to the figure is indicated by the digital output of a (not shown) Computer controlled, which sends its values to registers synchronously with a video signal. However, it is also possible to control the circuit of Figure 1 by other means such as a timing pulse emitting generator which is synchronized by the line and frame synchronization pulses of a video signal. In such an arrangement, the registers can be counters

- 9 5 / 1049-

that are connected to the time pulse generator and to each other and thus output pulses to digital-to-analog converters output and thus stepped X and Y deflection waveform signals produce.

The signals for controlling the circuit of Figure 1 are made up of a series of pairs of 8-bit words, where each pair corresponds to an X and Y coordinate position of the beam on the CRT screen, while load information consists of a single bit.

An input terminal for the 8-bit X coordinate word is provided at 2 and an input terminal for the 8-bit Y coordinate word is provided at j5. Load information that precedes the X and Y words and prepares the circuit arrangement for the next information words can either be binary Q or binary! Information and either directly to the circuit via an input terminal k or via an input terminal 5 and a Device 6 for sign reversal are supplied.

- LO -

'< ^fj · ^] « ^? ''' ^{] ()} ^ ⁹ «A" 0R / _{G / NAL}

The video signal is transmitted via an input terminal 7 which is synchronized with the incoming input signals at terminals 2 to 5.

The input terminals 2 and j5 are with the corresponding Inputs of X and Y registers 8 and 9 and the input terminals 4 and 5 also with inputs of these Register 8 and 9 connected.

Each of the 8-bit registers 8 and 9 can be set to one of 256 different code combinations, and a parallel output from the registers via lines 11 and 12 brings all of the codes set in the registers to 8-bit X and Y immediately -Digital / analog converter 1J5 and 14.

Part of the 8-bit codes in each of the 8 registers and 9 v / ird directly to the inputs via five parallel connections, each labeled 15 and Io directed by K and Ϊ decoders LY and l8.

DLe code combinations are fed to registers ο and ^v ) in n.aturl i rather binary sequence, and the connection

-Ll-

? η 9 s. 'f> / 1 0 u 9

BATH ORIGINAL

fertilize 15 and l6 direct the last five in the registers stored digits to the decoders with the result that the code supplied to the decoders with every eighth change the code combination in the registers changes what thirty-two changes of the decoders 17 and 18 supplied Code results in 256 changes of the digital-to-analog converter 13 and 14 supplied codes.

The digital-to-analog converters 13 and 14 convert the 256 code combinations which are fed to them into scanning waveforms of generally sawtooth shape with the parts which determine the cathode ray path and which consist of 256 discrete steps, the step jump corresponding to a specific one of 256 different code combinations . The sawtooth wave output of the digital-to-analog converters 13 and Ik is fed to the X and Y deflection plates of the cathode ray tube 1, denoted 20 and 21, through X and Y deflection amplifiers 22 and 23. It is thus possible to be 256 accurate Define defined deflection positions for the cathode ray both in the X and in the Y direction, which corresponds to a number of 65,536 discrete positions of the cathode ray spot on the screen.

- 12 . \ T. 3? 5/1 Q U 9

The X and Y deflection signals cause the beam from the coordinate position X = O and Y = O in the lower left corner of the screen, seen by the viewer, from the left distracted to the right and from the bottom to the top of the screen.

In the particular embodiment of the invention which is described here, the circuit arrangement is used in connection with a cathode ray tube which is disclosed in the patent application mentioned at the beginning and in which the unaffected axis of the electron beam emitted by the electron beam gun is either parallel or at a very acute angle to the plane of the screen, then the X deflection near the top of the screen, which is away from the electron beam gun, is longer than the deflection near the bottom of the screen for the same angular deflection of the beam, which would result in a trapezoidal distortion of the scanned image. A circuit part 24 is therefore provided via which part of the output value of the Y amplifier 2J > is fed to the converter 1J> in order to modulate its analog signal at the output and

- 15 -

209825/1049

to achieve a correction of the trapezoidal distortion. The signal fed from the Y amplifier 23 via the circuit part 24 dea transducer 13 causes an increase in the X deflection signal as the size of the Y deflection signal decreases, with the result that the X deflection or line deflection causes a smaller angular deflection of the electrode beam when the The image spot is closer to the upper edge of the picture tube than when it comes close to the lower edge of the picture tube. This modulation of the X deflection with the help of an output value of the Y deflection amplifier, which results in a reduction in the deflection angle when the beam moves in the Y direction from the lower edge of the screen, i.e. from the edge closest to the electron beam wave, to the upper edge of the screen , compensates for the greater length of the scan path in the X direction when the beam becomes longer during the deflection in the Y direction. Since the circuit part described represents a DC voltage ^{coupling, it is necessary to transfer a 11} of eet "potential via a line 25 between Y-deflection amplifier 23 and X-deflection amplifier 22 in order to convert a possibly resulting parallelogram-like distortion of the described image into a rectangular scan If there is an alternating voltage coupling between the parts of the holding system, this "offset" potential is not necessary.

209825/1049, ^,

In order to be able to generate further correction signals, output signals from the X and Y registers 8 and 9 are sent to the X and Y decoders 17 and 18 via lines 15 and 16. In the exemplary embodiment described here, the last five bits of the coded signal in the controllers 8 and 9 in parallel on the lines 15 and 16 to the decoders 17 and 18 supplied. The coded signals in registers 8

^¥ and 9 change in a natural binary sequence, and since only the last five digits are fed to the decoders 17 and 18, the codes * on lines 15 and 16 are used when every eighth change in the code of registers ϋ and 9 occurs. Thus, for 256 code changes in registers 8 and 9, 32 codecs that are fed to decoders 17 and 18 result. The decoders 17 and 18 emit an output signal on one of 33 different output lines, from dentn at 30 baw. 31 only one is shown "and give the-. i se output signals to the corresponding X and Y linearity correction circle · 32 and 33 so! «automatic« aatigmatism correction circle »3? and 34 for di · X and Y deflection. Di «! The output value of the X correction circuits 32 and 34 go via connecting lines 36 and 37 to the X deflection amplifier 22, while the output values of the Y correction circuits 33 and 35 are fed to the Y deflection amplifier 23 via lines 38 and 39. This makes it possible, with 32 discrete deflection positions of the beam on the X or Y axis, which results in 1,024 discrete and identifiable coordinate positions for the cathode ray on the entire screen. Correction of the X and

209825/1041

- 15 -

Y deflection in terms of both linearity and astigmatism to be attached to each of these coordinate positions.

With the special cathode ray tubes used here, the deflection of the beam is electrostatic and the automatic astigmatism correction circuits 354 and 35 control the applied deflection voltages between the X and Y baffles 20 and 21 during the deflection of the cathode ray in the X and Y directions.

It has been found that in the cathode ray tube used here it is not necessary to change the automatic focus correction of the beam during the scan in the Y-direction and that the correction necessary for scanning in the X-direction for corresponding positions at each deflection is the same.

The output values of the X decoder are therefore via the lines 30 and 4l with an automatic focus function generator 42 connected, the output value in turn via a focus amplifier 4 ^ the focus electrons 44 of the cathode ray tube 1 is fed. It is thus in for 32 discrete positions of the cathode ray during its deflection X-direction possible to feed 52 certain control signals, to control the beam focusing according to known parameters of the tube.

- 16 209825/104 9

The video signal arriving at terminal 7 passes through a video amplifier 46 and from there to the control grid electrode 47 of the cathode ray tube.

A simple form of automatic focus control signal generator is shown schematically in FIG. 2, in which the generator 42 is indicated with dashed lines 49. the Output signals from the X decoder 17 arrive via lines 30 and 31 at 32 different input signals, those of the series are designated by 51 to 82, and run through a chain of resistance, consisting of a load resistor RL and the respective associated one of the resistors Rl - R32, which with an associated rectifier diode Dl - D32 is in series. The output value that can be tapped off at the resistor RL is fed to the amplifier 43 via ijde line 83. The resistors Rl R32 are chosen in their value so that the output signal at the resistor RL alg maturable output signal at any given moment Output signal of the decoder 17 corresponds, whereby the focus signal at the electrode 44 corresponding to the associated Position of the electron beam on the screen is modified.

In a modified embodiment, the load resistance is RL in series with a chain of resistance, and the corresponding points of the chain are in turn over individual Rectifier diodes through 33 outputs of decoder 17

209825/1049 " ^1? "

selected to generate the required sequence of output signals at the load resistor according to the combination of resistors in the selected part of the chain.

It can thus be seen that in the embodiment shown in FIG. 1, 256 binary digit codes, which are shown in the X and Y registers 8 and 9 in numerical order and synchronously with a video signal on input Y can be used to generate X and Y deflection fields that a scan of the screen by the beam of the cathode ray tube 1 results in 65,536 precisely determinable points. The electron beam can be in any desired sequence these points according to the opening in which the codes' are fed to the register, moved, and the signals fed to terminal 7 can be used to if necessary, the path of the electron beam on the To divide up or to darken Sehiriii. The digital Äiiälögwandler 13 and 14, which generate the Sägezähablenksigriäie ^ can be of any known design. The küppiuhgskreis 24 to correct the trapezoidal distortion, a suitably shaped fitting circle is required Own amplifier and can also be used together with the Gieiehspaftniirigs üöff-se't "- Pdtieriti ^ älkopplüngäweg 25 omitted werdeh when a cathode ray tube is used; the one Hient has such distortion.

-IB-2Ö982B / 1049

BATH ORIGINAL

The use of X and Y decoders 17 and 18 which give 32 separate output values during each Y and X sweep, indicated at 30 and 31, allow 32 signals to be evenly spaced during each X sweep of the beam to the linearity correction circuit 32 as well as to the circuit for the automatic astigmatism correction 34 and the automatic focus function generator 42, whereas these 32 signals are fed to the linearity correction circuit 33 and the circuit 35 for the automatic astigmatism correction during each Y deflection of this beam. The automatic focus function generator 42 is thus reached by signals on the lines 41 32 times during each X deflection, and corrected signals are fed to the focus electrodes 44 in Eritäsprechüiii each of the 32 output values per X-Äblehkurig from the generator 42, the outside of öler in. Äeii Generator 42 built-in law are dependent. If it is necessary, the Pöttiiskörrektür can of course also be foreseen in the Y-deflection of the cathode ray, as it is also aüoh iriöglidh to omit ate completely: Be the described Äiisfüitfuriisbeispiel thus performed.

Körrektürsiiriäie fourdeii outside of the X-Äblehfcverstäffcer 22. ^ 2 iiiäl during each period of time; äishMiiiig from Lihearitäts- and

- 19 2 D 9 8 2 5/1 Θ 4 9 BAD QFUGlNAl.

Astigmatism correction signals from circles 32 and J54 which are themselves the result of input variables which are supplied via the connections 30 from the decoder 17 will.

Thirty-two times during, every Y deflection or every eighth line, correction signals for linearity and astigmatism are obtained from the correction circles 33 and 35 to the Y deflection amplifier 23 as a sequence of input values supplied via lines 31. Corrections for these two parameters of the cathode ray thus result at 1024 coordinate positions over the entire screen.

The linearity and astigmatism correction circuits can take the form of known networks and be matrices, for example, which give the required previously determined output values, if a certain Line is selected.

Other combinations of coordinate positions at which corrections are to be made can accordingly special properties of the deceased cathode ray tube are selected, and as in previous ones

2 09825/1049 - 20 -

Point explained, the circuit arrangement described here has particular advantages when used in conjunction with the cathode ray tube according to the application mentioned above. This cathode ray tube should now will be briefly described again below in connection with FIGS. 3 and 4 of the drawing.

In Figures 3 and 4, a cathode ray tube is shown with a piston 101 through which a flat, transparent support screen 102 is visible. There is an electroluminescent screen on the inside of the carrier screen 103, which consists of a translucent, conductive layer and is coated with a material, which luminesces when it is bombarded with electrons. Directly behind and separate from the screen 102 is a beam direction electrode 104 in the form of a single foil, which, however, also of a plurality can be made of electrically interconnected foils. The support screen 102 on which the Iu-. Minescent material is applied, on the other hand, can be the translucent surface of the piston 101. In Another modification is the beam direction electrode made of a translucent material and

- 21 2 0 9 8 2 5/1049

the screen can thus be seen from both sides of the piston will. The beam direction electrode 104 has a greater height in the perpendicular direction of the beam than the screen 102, and along the edges of the electrode 104 extend outside the area of the screen 102 Strips 105 and 106 made of electrically conductive material, as shown in FIG. Terminals 107 and 108 on piston 101 are electrical to strips 105 and 106 tied together. An even resistive layer is on of electrode 104 is attached between strips 105 and 106. The conductive layer of the electroluminescent screen 103 is connected to a port 109 on the piston 101.

The piston 101 has a neck 110 in which an electron source 111 and a right-angled acting, deflection electrostatic electrode systems 112 and 113 are housed. An electromagnetic collimator lens device 114 is around the expanding part of the piston. Other corrective lens systems can to be used when requested. An electron beam, shown in two courses by solid and dashed lines Lines 115 and 116, respectively, is when hitting the

- 22 209825/1049

Screen shown at points which are far or close to the beam source 111.

In the application, in relation to the extremely high voltage potential that appears on the electroluminescent screen 103 across the terminal 109, a bias potential is placed between the terminals 107 and 108 and is via the electrode 104 between the strips 105 and 106 bridged. The bias potential on strip 105 has a higher value than that on strip 106, so that an electric field develops in the space 121 (Fig. 2) between the beam direction electrode 104 and the screen 103 expands kit to an intensity that is greatest in that part of the area of the space 121 which is furthest away from the electron source 111. the Even layer of resistive material between strips 105 and 106 causes the bias potential the beam direction in this way via the electrode 104 is distributed, that the field in space 121, although static, is not evenly distributed and that it is in its Intensity changed essentially linearly through space 121 in the direction from strip 106 to strip 105, where there is a maximum value in the range of the

- 23 209825/1049

Reached space 121, which is between the remote from the source edge of the screen 105 and the Strip 105 is located.

Deflection potentials are applied to the deflection electrode system 112 and 113 in a known manner in order to achieve the deflecting the electron beam emanating from the source 111 in two directions lying at right angles to one another and let it run through a grid or other selected image as it passes through space 121 reaches a certain point on the screen. When the electron beam has a lateral component of motion is given to its straight path before it enters the beam direction field in the space 121 between the screen 103 and the electrode 104 passes, its initial direction changes into space, and the distribution of the beam direction field changes in this space is such that the beam, as it passes through space 121, is at a given point on the screen 103, according to the angle at which it is deflected laterally from its rectilinear path before it entered the room 121, or in other words, according to its direction of entry into the Room 121. The electron beam can initially target the

- 24 209825/1049

Electrode 104 be directed when it enters the space 21 in order to possibly point to a certain point on the Strike screen 103. The real point at which the beam hits the screen 103 is determined by the Angle determined by which it has been deflected before it enters the beam direction field, further by the jet velocity and by the intensity distribution of the field in that part of the room 121, through which the beam passes.

The deflection fields generated by systems 112 and 113 determine the angle at which the beam is directed enters the area 121, and with a suitable formation of these fields, the shape of the surface of the screen 103 scanned by the beam. The collimator lens assembly 110 is for Correction of distortion, commonly known as "trapezoid" distortion, at which the width of a record which is closer to the source 111, otherwise it would be smaller than the width of a recording, which is further away from the source 107. However, as shown above, the keystone distortion on the other hand, through a suitable design of the deflection

- 25 209825/1 049

field can be corrected, for example by replacing of the electrode system 112 and 115 and by molding the Electrodes of the system 112 in such a way that the necessary correction field is created.

A scanning raster can be generated by the electrode system 112 and 113 in such a way that the scan lines on the screen 10J either in planes substantially at right angles to the axes of the Source 111 emitted electron beam or parallel to these axes.

Instead of generating a raster image, the beam can also be deflected in this way by the systems 112 and 113 that alphanumeric information is written directly on the screen. This latter type of recording is particularly suitable for using quantitative information in the record, for example when information is displayed on an aircraft dashboard. In modern aviation technology it is necessary to display a large number of different data encoders and, since these encoders can be scanned relatively easily electrically; information about exceeding given limits

- 26 2 0 9 8 2 B / 1 0 k 9

2161188

from "any donors can be selected and on special Parts of the ad are written at the same time, to emphasize the fact that a limit has been crossed.

It can be understood from the particular embodiment of the invention described above Deviations are made within the scope of the concept of the invention, such as an increase in the number of parameters, which are to be corrected, or the use of aids other than binary coded digital signals for the Controlling the circuitry.

209825/1049

Claims (9)

PATE NT CLAIMS Circuit arrangement for a cathode ray tube with first and second beam deflection signal generating Circuit parts, characterized in that first and second digitally operated control circuits (8, 9) are provided which control the first and second deflection signal generating circuit parts (Ij5, 14) Decoder (17, 18) is connected with its input to the output of one of the control circuits (8, 9) and a Puncture generator (32-35, 42) with its input with the Output of the decoder is connected and on the output side a signal for modifying a control signal for a cathode ray generated by the cathode ray tube. 2. Circuit arrangement according to claim 1, characterized in that that the input value to the decoder from the control circuits (8, 9) has a series of digital signals, which are supplied with a sequence that is proportional to the sequence with which the signals from the control circuits (8, 9) the circuit parts (13,! 4) for generating the beam deflection signals but less than this. 3.- circuit arrangement according to claim 1 or 2, da- 2 0 9 8 2 5. / 1 04 9- ■ '- 28 - characterized in that the first and second circuit parts (I3,! 4) for generating the beam deflection signals Generate X-Y deflection signals for a raster scan and a feedback connection is provided to a dem Y-deflection signal proportional signal to that of the X-deflection signal generating circuit to modify the X-deflection signal according to the Y-deflection signal, thereby to obtain a trapezoidal distortion correction of the image on the screen. 4. Circuit arrangement according to one of the preceding claims, characterized in that the first and the second control circuit (8, 9) is formed by a counter and this counter is formed by a time pulse generator receive a series of timing pulses under the controlling influence of the synchronizing pulses of a video signal, whereby the scanning of the screen of the cathode ray tube takes place in synchronism with an image to be displayed thereon. 5. Circuit arrangement according to one of claims 1 to 4, characterized in that the function generator (42) with an input with the output of the first decoder (17) and with its output with a focus control electrode is connected so that a correction signal of the focus 2 0 9 3 2 Γ, / 1 0 A 9 - 29 - Control electrode (44) both dependent on the deflection of the beam by signals generated by the first beam Circle (I3) are evoked as well is possible depending on the output of the puncture generator (42). 6. Circuit arrangement according to claim 5, characterized in that that the puncture generator (42) contains a load resistor (R_ J, at which the output signal can be tapped to modify a control signal, a plurality of function-determining resistors (R ₁ to R, o) is provided, all of which are connected in parallel with the load resistor (R _T ) in series, Yy with each function-determining resistor (Ri-RZ ₂ ) a diode (D1-DJ52) is in series and a connection is made for each signal to each diode in series, whereby a large number of output signals, all of which correspond to a certain cathode ray position, at the load resistor (R ₁ ) detachable 1st. 7. Circuit arrangement according to claim 6, characterized in that the function-determining resistors (R ₁ -R ₇ C ₂ ) are in series with one another. - 30 - 209825/1 04 9 8. Circuit arrangement according to one of the preceding claims, characterized in that the control means operated by digital signals in binary form. 9. cathode ray tube device with a circuit arrangement according to claim 1, characterized by a piston (lol) with an electron beam source (111), an electroluminescent Screen (105), a beam direction electrode (Io4) at a distance from the screen (103), the electron beam source is arranged so that the electron beam (115, 116) in the space (121) between screen (103) and beam-directing electrode (Io4) is shot in, so that, when a beam-directing potential is applied between the screen (103) and the beam-directing electrode (Io4), the beam-directing field in the space (121) between the beam-directing electrode becomes effective in the direction of the screen, and a beam deflection arrangement (112, 113) for deflecting the The beam acts in two directions on the beam before it enters the space (122). 209825/1 049