US2280191A - Cathode-ray signal-reproducing unit - Google Patents

Description

A ril 21, 1942. R. c. HERGENROTHER 2,280,191

CATHODE-RAY SIGNAL-REPRODUCING UNIT Filed Sept. 30. 1939 2 Sheets-Sheet l mm V at ill o c a 9 INVENTOR RU' LF 0. HERGENR ATTORNEY April 1942- R. c. HERGENROTHER CATHODE-RAY SIGNAL-REPRODUCING' UNIT Filed Sept. 30, 1939 2 Sheets-Sheet 2 C L W N 3 m F .D R W V LLF A .v l i1 4 $51 I O =2 Eu buu uamn m V=VoIIuqe of Primary Elecirons.

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INVENTOR RU LF 0. HERGENROTHER ATTORNEY Patented Apr. 21, 1942 CATHODE-RAY SIGNAL-REPRODUCIN G UNIT Rudolf C. liergenrother, Beechhurst, N. Y., as-

signor to l-Iazeltine Cor of Delaware poration, a corporation Application September 30, 1939, Serial No. 297,326

7 Claims.

This invention relates to signal-reproducing units for television receiving systems and particularly to such systems including a cathode-ray image-reproducing tube having a control grid upon one surface of which charge images are formed by a signal-modulated scanning ray in order to modulate another electron stream of substantially uniform density and of a crosssection comparable to the area of the grid, the latter electron stream being incident on the other surface of the grid and being utilized to produce a visible image.

The cathode-ray tubes conventionally utilized in television receivers are basically the same as the early Braun tube. The development of elec tron optics, resulting in eflicient electron lens designs, and the development of thermionic emitters have brought the design of the present-day cathode-ray tube almost to its theoretical limit of advancement. However, one of the problems to which a completely satisfactory solution has not yet been found is that of obtaining a large image of adequate brightness. The apparent brightness of an image to the eye is independent of the image size or its proximity to the viewer,

in accordance with well-known optical principles. This means that, as the image size is increased, the amount of light in the total image must be increased in proportion to its area, since the brightness must be kept the same for satisfactory reproduction regardless of size. In other Words, the power which must be provided by the electron beam bombarding the fluorescent screen must be increased in proportion to the rapidly.

the video-frequency voltage required to modu- It is possible to produce a television image on a fluorescent screen by means of an electron beam which is not directly subjected to modulation and scanning. This can be done by means of a tube containing a. type of electrode hereinafter called an image grid. The image grid can be regarded as a special kind of vacuum-tube amplifier grid. The image grid may take the form of a finely-perforated flat sheet or wire mesh which can receive and maintain on one surface thereof an electrical charge distribution representing a television image. This charge distribution is hereinafter called a charge image. The other surface of the image grid may be of conductive material and be utilized as an anode upon which is incidenta floodingelectron stream of substantially uniform density and of a crosssection comparable to the area of the grid. Electrons of the flooding stream, therefore, pass through the grid apertures but the density of the elemental electron stream passing through each small aperture of the grid is controlled by the electrical charge density of the charge image on the adjacent emergent surface. In this way, the electron stream leaving the grid surface is formed into an electron image analogous, for example, to that formed at a photocathode surface when an opticalimage is focused upon it. This electron image can be focused by means of electrostatic or electromagnetic lenses, or by a combination of these lenses, upon a fluorescent screen to produce a visible image. The charge image may be produced by scanning the imagegrid surface, which is usually of dielectric material, with a video-modulated electron beam. The term dielectric" as used herein is understood to include any surface of charge-retaining material including a mosaic on a dielectric backmg.

The use of an image grid of the type described introduces another degree of freedom in the design of imagereproducing,tubes and frees the tube from many of the limitations inherent in conventional tubes utilizing the Braun tube prin ciple. Among the more important advantages which the image-grid tube has over conventional tubes utilizing the Braun tube principle are the following:

1) The electron beam exciting the fluorescent screen need not be modulated or scanned. Only the relatively small part of the total power input to the tube which is used in the beam which builds up the charg image needs to be modulated and scanned. This means that the power required in the scanning circuits can be greatly decreased over that needed for operating conventional image-reproducing tubes of the same power output.

(2) The regulation of the voltage supply to the picture-producing electron beam is not nearly as critical as the regulation required for conventional picture tubes since, in the image-grid picture tube, this beam current varies only with the average picture brightness whereas, in the conventional image-reproducing tubes, the beam current varies over the full-modulation range when the beam passes from a dark picture element to a bright picture element. This lower voltage regulation requirement decreases the cost of the high-voltage power-supply unit.

(3) Direct current reinsertion, that is, control of the average picture brightness, can be effected by controlling the current in the pictureproducing beam by means of a control grid of its electron gun.

(4) In the image-grid lmage reproducing tube, a large number of elementary picture areas of the fluorescent screen are simultaneously excited. Therefore, although the average current to the fluorescent screen is the same for an image-grid tube as that for a conventional image-reproducing tube, the instantaneous current density is much less for the former. Since the space-charge effects which limit the theogrid over the whole surface simultaneously and continuously, either directly from the emitted photoelectrons or indirectly by electron-focusing of the electron image produced at the photoreticai maximum beam current density dependupon the instantaneous beam current density, the image-grid tube operation is not limited as severely by space-charge effects as is the conventional --image-reproducing tube.

(5)A much greater average-to-peak illumination is possible without burning the fluorescent screen.

(6) It is possible to operate the image-grid tube in such a way that thecomplete television image appearing on the fluorescent screen is at all times modulated point-by-point to change the brightness at such point from the value corresponding to one picture to that corresponding to the brightness of the next. This means completely flicker-less operation, even at low picture repetition rates, which is impossible with the ordinary type of image-reproducing tube because the decay of luminosity of the fluorescent material falls off exponentially.

Image grids have been utilized in signal-generating tubes of the prior art and tubes utilizing the image-grid principle have also been embodied in image-reproducing units. However, the modification of the principles involved in utilizing image grids in signal-generating tubes to those involved in their use in an image-reproducing tube is difiicult, inasmuch as the requirements and operating characteristics of the image-reproducing tube are radically different. For example, in a signal-generating tube, it is desired to transform an optical image into the appropriate electrical signal representing it. The method used is to cause the optical image to be focused on a photosensitive surface to produce an electron image. This electron image is cathode onto the image grid. The electrical signal is then produced by scanning this charge image with a sharply-focused electron beam, the passage of this beam through the image grid being governed by the local values of electrical charge of the charge image.

Furthermore, the image-grid type tubes which have previously been utilized for image reproduction have incorporated an arrangement by which the beam to be modulated and utilized for reproducing the picture is incident on the same surface of the image grid as that on which the charge image is formed. In such arrangements, electrons from the source utilized to effect picture reproduction have an undesirable discharging effect upon the charge surface of the image grid. This discharging efiect occurs because the sharply-focused electron beam produc-: ing the image charge, which is a focused, scanned, and modulated beam, and the wide uniform electron beam, which supplies the current going to the fluorescent screen, are both incident simultaneously on the same charge-holding surface. Theefiect of the wide beam predominates since it contains the greater current. This effect is to bring the whole charge-holding surface to a datum potential which may be positive or negative relative to the collector electrode 'depending on the secondary-emission characteristic of the charge-holding surface and the velocity of the electrons in the wide beam. The sharplyfocused electron beam can produce local changes a in the charge on the image grid at the point where it is incident at the moment, but this change is quickly erased and the potential brought to the datum value when the narrow beam moves away from a particular point in its scanning excursion. With such arrangements, it is, therefore, not possible to maintain a persisttent charge on the image grid. The luminous image appearing on the fluorescent screen is accordingly active over only a few points simultaneously, thus limiting the discharge by the space-charge effect in addition to limiting the made to produce the charge image on the image effective brightness of' this luminous image and producing substantially the same flicker effect as the conventional type of image-reproducing tube. A further disadvantage of such arrangements is that, when the wide electron beam is incident on the charge-holding surface, the potential of this surface may be unstable since the charge on it is nonuniform.

It is an object of the invention, therefore, to provide in a television receiving system a signal reproducing unit including a cathode-ray tube of the image-grid type in which one or more of the above-mentioned disadvantages of arrangements of the prior art are eliminated.

It is a further object of the invention to provide in a television receiving system a signalreproducing unit of the image-grid type described above, in which the image-forming beam is incident on the surface of the image grid opposite from that upon which the charge image is formed and in which the passage of electrons of the picture-beam source directly through the image grid is prevented.

It is still another object of the invention to provide in a television receiving system a signalreproducing unit including a cathode-ray tube of the image-grid type in which the unidirecing this iional component of the reproduced picture is in the picture-forming beam.

In accordance with a preferred embodiment of the invention, there is provided in a television receiving system a signal-reproducing unit comprising a cathode-ray image-reproducing tube including an apertured grid having one surface of dielectric material. There is further provided means for scanning this dielectric surface with a signal-modulated cathode-ray beam to produce a charge image thereon. There is also provided an electron source including means for directing against the other surface of the grid an electron flood stream of cross-sectional area comparable to the area of the grid, together with means for developing a source of low velocity electrons adjacent the other surface and of a crosssectional area comparable to the area of the grid, the lastnamed means comprising means for forming a virtual cathode or means for emitting secondary electrons. Means are provided for preventing the passage of electrons from the flood stream directly through the grid, whereby the density of the electron stream passing through the grid is space-modulated by the charge image on the one surface. Means are also provided for utilizlast-mentioned modulated electron stream to produce a visible image.

For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, and its scopewill be pointed out in the appended claims.

Fig. 1 of the drawings is a circuit diagram, partly schematic, of a complete television receiver of the superheterodyne type embodying a signal-reproducing 'unit in accordance with the invention; Fig. 2 is a fragmentary perspective view of the image grid utilized in the signalreproducing unit of Fig. 1; Figs. 3, 4, 5, and 6 are graphs utilized to explain the operating characteristics of the image-grid cathode-ray tube of Fig. 1; while Figs. 7, 8, 9, and are fragmentary crosssectional views of different arrangements of an image grid for the reproducing tube of the type shown in Fig. 1 to prevent the passage of electrons from the source of the picture-forming beam directly through the image grid.

Referring now more particularly to the drawings, the system illustrated comprises a. receiver of the superheterodyne type including, an antenna system H), II connected to a radio-fre quency amplifier H to which are connected in cascade, in the order named, an oscillatormodulator l3, an intermediate-frequency amplifier I'd, a detector I5, a video-frequency amplifier l6, and an image-producing device l1. A line-frequency generator l8 and a field-frequency generator l9 are coupled to an output circuit of detector l5 through a synchronizingsignal separator 20, the line-frequency generator l8 and the field-frequency generator I9 being coupled to line-scanning winding 2| and fieldscanning winding 2|, respectively, associated with image-reproducing device I l. The stages or units I045, inclusive, and I8, l9, and may all be of conventional well-known construction so that a detailed illustration and description thereof are unnecessary herein.

Referring briefly, however, to the operation of the system described above, television signals intercepted by antenna circuit III, II are selected and amplified in radio-frequency amplifier l2 and coupled to the oscillator-modulator iii wherein they are converted into intermediatefrequency signals which, in turn, are selectively amplified in'intermediate-frequency amplifier I 4 and delivered to detector I5. The modulation components of the signal are derived by the detector l5 and the video components thereof are supplied to the video-frequency amplifier l6 wherein they are amplified and from which they are supplied in the usual manner to a brilliancycontrol electrode 22 of the image-reproducing device IT. The synchronizing-component output of detector I5 is supplied through synchronizingsignal separator 20 to generators I8 and [9. The intensity of the scanning ray of device I1 is thus modulated or controlled in accordance with the video-frequency voltages impressed upon the control electrode 22 in the usual manner Scanning waves are generated in the line-frequency and field-frequency scanning generators l8 and [9, which are controlled by synchronizing-voltage pulses supplied from detector l5, and are applied to the scanning elements 2|, 2| of the image-reproducing device I] to produce electric scanning fields, thereby to deflect the scanning ray in two directions normal to each other so as to trace a rectilinear scanning pattern on the image grid within tube l1 and thereby reconstruct the transmitted image as explained in de tail hereinafter.

Referring now more particularly to the portion of the system of Fig. l embodying the signalreproducing unit of present invention, there is provided in cathode-ray reproducing unit I! an image grid 25. The image grid 25 of tube ll consists of an apertured, a perforated or reticulated flat sheet one surface of which is made up of a thin dielectric charge film or plate 26 coated on a conductive sheet or backing plate 2! to form an opposed surface of secondary-electron emissive material, the dielectric surface being capable of holding a surface distribution of electron charge corresponding to a television image which is formed thereon by facing it toward the scanning beam modulated by electrode 22. A portion of the image-grid structure is shown in more detail in Fig. 2 of the drawings.

There is also provided within tube ll means for scanning the dielectric surface with a signalmodulated cathode-ray beam to produce a charge image on the dielectric surface. This last-named means includes an electron gun for developing the charge image upon grid 25 which comprises a cathode 28, control electrode 22, focusing and accelerating anodes 29 and 30, and a collector electrode 3|. Cathode-ray tube I! also comprises means for developing a source of low-velocity electrons of substantially uniform intensity and of a cross-sectional area comparable to the area of grid 25. This last-named means includes an electron gun for providing a picture-forming beam which includes a cathode 32, a control electrode 33, anode 34, and a grid 35, which is effective with the image grid 25, to develop a virtual cathode between the grid 35 and the conductive backing plate 21 and closely adjacent the latter. The gun for providing the picture-forming beam is thus effective to provide a source of low velocity electrons in the vicinity of the conductive surface 21 of image-forming grid 25 which is of substantially uniform density and of a cross-sectional area comparable to the area of the grid, whereby the density of the picture-forming electron stream passing through the grid 25 is spacemodulated by the charge image on the surface 26 of image grid 25.

In order to develop the background-illumination component of the received signal in the reconstructed image, there is provided means including a reinserter 31 having an input circuit coupled to detector l5 and an output circuit coupled to control grid 33 for controlling the electron stream derived from cathode 32. The electron stream which passes through the image grid is thus space-modulated by the charge image on surface 26 of image grid 25 and is timemodulated by successive charge images and by the unidirectional component of the received signal and this stream is focused on a fluorescent screen 38 of cathode-ray tube IT, by means of an electron lens system comprising focusing cylinders 39, to form a visible image.

For a successful reproducing unit of the type under discussion, the image charge on the image grid must be discharged during each scanning cycle. The charge image on the image grid may be discharged between successive image scansions by any one of several arrangements, an arrangement hereinafter called a chasing-beam scanning apparatus being illustrated in Fig. 1. The chasing-beam scanning apparatus includes an electron gun comprising a cathode 40, and focusing anodes ll and 42, for directing a stream of relatively low-velocity biasing electrons upon surface 25 of image grid 25, and a chasing-beam scanning generator M having output circuits ular elements of tube l1 and particularly the secondary electron emission characteristic of the surface 26 of image grid 25. The general nature of this characteristic is illustrated by the graph of Fig. 3 of the drawings in which the secondary' emission ratio R, which is defined as the secondary emission current divided by the primary electron current, is shown as a function of the total voltage V through which the incident or primary electrons fall in striking the grid 25. The curve starts from zero, rises to a maximum as V increases to a certain value, and then falls gradually and continuously as V increases beyond this value. It should be noted that the curve of Fig. 3 cuts the value of it =1 at two places having abscissae values V1 at the lowvoltage value and V1 at the high-voltage value. The curve of Fig. 3 has been divided into sections A, B and C which are defined by the lower and higher values of the unity secondary emission voltage V1 and V1, respectively. When the incident primary electron-beam voltage lies in the region A, the secondary emission ratio is less than unity, so that the target gains more primary electrons than it loses as secondary electrons and the net electron current flows to the target which is thereby charged negatively. This is also true when the incident primary-beam voltage lies in the region C. In the region B, the secondary emission ratio is greater than unity so that more electrons leave the target as secondaries than arrive as primaries and the net electron current is away from the target which is thereby charged positively.

The secondary electron-emitting target of electron gun 28, 22, 29, 30 of cathode-ray tube I1 is eifectively the surface of the insulating film 26 of image grid 25 which forms one plate of a condenser, the other plate of which is conductive plate 21 connected to the collector electrode 3| so that it is maintained at a. proper operating potential. The grid 25 may take a variety of forms. For instance, the target may consist of a conductor over the surface of the dielectric or insulator, which conductor is not continuous but is broken up into a number of minute elements, as a mosaic. For such a target, any electrical charge produced on the target by the primary electron beam is localized at the region where the beam strikes the target and is not dissipated over the surface of the target. Alternatively, the target may'consist of a dielectric sheet or film 26. A primary electron beam incident on such a target releases secondary electrons from the outer layer of the target which behaves in much the same way as does the mosaic target described above; that is, any electrical charge produced by the incident primary electron beam remains localized where the beam strikes. The target of the mosaic type and the insulated target are thus similar in electrical behavior and the following discussion of the electrical charging properties of the insulated target is valid for the target of mosaic type.

In considering in detail the electrical prop= erties of the dielectric film 26, which is the target of the electron gun comprising cathode 28, it is assumed that the .beam current from cathode 28 is unmodulated and unscanned. The current to the backing plate 2?. is zero after a steady beam current from cathode 28 has been supplied for an appreciable time and the current surge has died out so that an equilibrium condition has been reached. For equilibrium, the beam current Ib must equal the collector current Ic. It will be assumed that the primary electron-beam voltage from the cathode 28 is in the A region of Fig. 3 having a value of, say, Va volts. It is alsoassumed that the dielectric surface 25 was initially uncharged so that its surface potential is the same as that of the backing plat 21. The primary electron beam striking the dielectric surface with the voltage Va has a secondary emission ratio less than unity, which simply means that fewer electrons are released as secondaries than arrive as primaries. These secondaries go to the collector electrode 3!. The portion of the surface of dielectric 26 upon which the electron beam is incident, therefore, becomes increasingly charged and its potential rises negatively with respect to the potential of th collector electrode 3! and the backing plate 21. This negative potential effectively reduces the voltage of the incident primary electrons from cathode 28and diminishes the beam current to the target. The potential of the dielectric surface 26 continues to increase negatively until the beam current to the target is reduced to zero, which represents an equilibrium condition of the tube. Under these conditions, elec- .trons from cathode 28 are repelled by the dielectric charge so that they are deflected directly to the collector electrode 3|. The potential of the dielectric surface, under these conditions, has a negative value relative to the collector electrode 3| which equals Va. In summary, therefore, it is seen that, under th conditions assumed, when the incident primary electron voltage Va is in the region A, the dielectric surface, upon which W tential.

the electron beam is incident, is charged to an equilibrium negative potential of Va relative to the collector electrode 3 i.

If, on the other hand, the voltage of the primary electron beam from cathode28 is in the region B of Fig. 3, for instance, 'has a value Vb,

the surface of the dielectric 26 loses more elec-- trons as secondaries than it gains as primaries so that it becomes positively charged relative to the backing plate 21 and the collector electrode 3|. Assuming, for example, that the dielectric is initially uncharged and the primary beam from cathode 26 is turned on, the building up of a posi. tive electrical charge on the surface of the dielectric 26, which begins when the primary beam from cathode 28 is turned on, has two effects: (1) The electrical field of the positive charge aids the electrons in the primary beam and causes them to strike the dielectric with increasing velocity, thus slightly increasing the secondary emission ratio, if Vb is on the rising part of the characteristic as assumed, or slightly decreasing this ratio if Vb is on the falling part of the curve; (2) the electrical field of the positive charge opposes the secondary electrons which leave the dielectric to go to the collector electrode 3!. This opposing field tends to suppress the secondary emission current to the collector electrode 3 I, turning back to the dielectric surface 26 those secondary electrons having an initial velocity of emission insuiiicient to overcome the electrical field. As the charge on the dielectric 26 becomes more positive, a greater portion of the secondary emission current is thus suppressed until a point of equilibrium is reached. This occurs when the r secondary emission current is suppressed to such an extent that the part of the secondary emission current which reaches the collector electrode (H is exactly equal to the primary beam current. The value of this limiting positive potential Vs depends upon the velocity distribution, that is, upon the voltage distribution of secondary electrons. The value of Vs has been measured for several conductors and is about three volts for caesium-oxygen-silver surfaces. V5 for insulators is of the same order of magnitude and may be as high as volts for an insulating surface such as that of dielectric 26. The suppressed secondary emission current is 1b (R1). In summary, therefore, it is seen that, under the present assumptions, when the incident primary electron-beam voltage Vb is in the B region. the dielectric charges up to an equilibrium positive potential of V5 relative to the collector electrode 3!, and that Vs, which is of the order of 10 volts for an insulator, is a characteristic of the dielectric material and depends upon the velocity distribution of the secondary electrons.

It will now be assumed that the voltage of the primary beam from cathode 28 is in the C region of Fig. 3 as indicated by the value of Ve. In this region, as in the region A, the secondary emission ratio is less than unity so that the dielectric surface gains more electrons as primaries than it loses as secondaries and it acquires a negative po- This negative potential effectively reduces the voltage of the electrons in the incident primary beam and reduces their velocity as they strike the surface 26. This decrease in the voltage of the incident beam causes an increase in the secondary emission ratio. It should be noted that, under the conditions assumed, the secondary emission to the collector electrode 3| becomes saturated since it is aided by the negative potential of the surface of dielectric 26 upon which the primary beam current i incident. The negative charge or potential on dielectric 26 accordingly continues to increase until an equilibrium value is reached. This occurs when the potential of the dielectric surface has become sufiiciently neg ative so that the incident primary beam is slowed down to such an extent that it strikes the surface with a voltage of V1. Under this condition, the secondary emission coefficient is unity and the secondary emission current is exactly equal to the primary beam current. In summary, it is seen that, under the conditions presently assumed, when the incident primary electron-beam voltage Ve is in the C region, the dielectric 26 charges up to an equilibrium negative potential of Vc-Vl' relative to the collector electrode 3| The potential V1 is the higher value of beam potential for which the secondary emission ratio is unity.

Considering now the operation of the tube I! of Fig. 1 as an image-reproducing tube, it is seen that the electron gun structure, comprising cathode 28 and the image grid 25, is similar to a con-, ventional image-reproducing tube except that the fluorescent screen of the conventional tube is replaced by the thin sheet of dielectric 26 attached to the conductive backing plate 21. It is seen that the control grid 22 of this electron gun structure is connected, as in a conventional imagereproducing tube, to a television receiver and its operation will be considered during the scanning of exactly one frame of a television image starting with the dielectric 26 uncharged. It will be assumed that the secondary emission characteristic of the dielectric is that shown by Fig. 3 and that the scanning beam derived from cathode 28 v is operating in the B region of Fig. 3. The dielectric 26, under these conditions, is charged positively by the electron beam as set forth in detail above.

This results in a distribution of a positive electrical charge over the dielectric surface 26, that is, in the production of a charge image, which is an electrical replica of the received television image. This charge image re mains on the dielectric surface 26 for an appreciable length of time but eventually leaks off at a rate determined by the specific resistance of the dielectric 26.

In order to examine in detail the manner in which a charge image is built up on the dielectric 26 of the tube of Fig. 1, the operating characteristics of a small area of the dielectric surface will be examined while it is being bombarded with a fixed signal beam from cathode 28, that is, when there are no scanning potentials applied from generators l8 and I9. Let the signal beam voltage be equal to Vb. It is thus seen from the secondary emission characteristic of Fig. 3 that the secondary emission coeflicient has a definite value Rb. This means that if the beam current is Ib, the current leaving the dielectric film 26 initially will be equal to Is, where Ib'=;lb. It is furthermore assumed that the potential of the dielectric is equal to that of the collector electrode 3| and conductive backing plate 21 at the instant the beam is turned on.

The graph of Fig. 4 shows how the potential Vs of the dielectric surface 26 changes, under the conditions assumed, as a function of time after the beam has been turned on. The initial current which charges the dielectric is equal to In" Where Ib"=(Ri-1)Ib. As the dielectric becomes charged, the secondary emission becomes more and more suppressed until the charging current falls to zero and the dielectric has the equilibrium or saturation potential V The rate of charging depends upon the beam current, as shown by the various beam currents I1, Ia, and Is, in Fig. 4. If the beam is turned on only for a short time tp, the potential of the dielectric surface rises to a value Va which is dependent upon the beam current as shown in Fig. 4 by the points where the curves corresponding to various beam currents intersect the ordinate axis through the time tp. These points are plotted in Fig. 5 as a function of beam current Is.

The effect of the beam on a given area of the dielectric iilm is the same whether the beam is fixed (not scanned) and is turned on for time interval tp or whether the beam is operating continuously but is being scanned at such a rate that it requires the time interval t; to traverse the] given elemental area. This means that, .if the dielectric film 26 of the tube of Fig. 1 has initially a uniform zero potential relative to the collector,

and is then scanned for one complete frame with a television signal, the dielectric receives a pointfor-point charge distribution which is a replica of the television image, the electrical characteristic of which is shown by Fig. 5. From Fig. 5 it is seen that the beam current dielectric charge characteristic is linear only over a portion of the curve beginning at the origin and that the potential of the dielectric approaches a maximum saturation value as the beam current is increased. This means that the useful range of beam current modulation has a definite limit. It will be arbitrarily assumed that the upper limit of the beam current has the value Ismail) at which the beam current characteristic extrapolated linearly from the initial part of the characteristic intersects the limiting voltage value Vs as shown in Fig. 5.

The operating conditions for an elemental area A of the dielectric, equal to the area of the scanning beam, which is also equal to the area of each elemental picture element, will now be considered. For simplicity, it is assumed that this area is a square of side lengths /A. This part of the dielectric with its corresponding portion of the metal backing plate 2? can be regarded as forming a small condenser which is charged by the scanning beam. The capacitance C1 of this condenser is given by the relation:

where,

k=rlielectric constant,

a=area in square centimeters, d=thiclrness of dielectric in centimeters.

If this condenser charges to a voltage Vs in a time t at a uniform rate of Ib"(max), the following expression results:

ClVs=Ib"(max)tp (2) Since the time t equals the time required to scan one picture element,

t, =52 seconds Upon substituting the values of Equations 1 and 3'in Equation 2. the following expression is derived:

However An==A' (5) where, A'=the area of charge image. Therefore, from Equations 4 and 5, the following expression may be derived:

As an example of the use of Equation 7, the maximum allowable beam current for a Cryolite film, 10 microns thick, utilized with a beam voltage of 1,000 volts where Vs==10 volts, A'=l00 square centimeters, {=30 frames per second, Ic=5, d=10- centimeters, and R=4"is found to be:

Ib(max)=4.4 microamperes In the operation of tube of Fig. 1, it is necessary to provide some biasing arrangement to bring the surface of the dielectric 26 to a uniform potential; that is, below the maximum value V. between successive picture 'scansions. If this is not done, the charge over the entire dielectric surface approaches a constant maximum value and the image disappears. This biasing may be effected either by electrical leakage of the charge through the dielectric 26 or by bombarding the dielectric 26 with electrons of sumciently low voltage that the secondary emission ratio is less than unity (see region A of Fig. 3) causing the charge on the dielectric to fall to the cathode potential of this bombarding beam. This cathode potential may have any convenient value.

If the image is discharged using only the leakage properties of the dielectric, the rate of leakage is determined by the specific resistance of the dielectric 26. If the discharge rate is such that, in the interval of time between successive scannings of the picture element by the signal beam, usually ,5 second, the electrical charge falls to a small fraction, for example 5 per cent, of its initial value, the dielectric surface biases itself to substantially the potential of collector electrode 3! between successive scannings and the charge image may be repeatedly built up during each scansion. The charge images in this case act in a manner analogous to that of a luminous discharge of a fluorescent screen, which has an exponential decay constant analogous to the discharge constant of the dielectric.

If the discharge rate is too slow, a blurring of moving objects of the transmitted picture will be apparent. In explanation of this, consider a small section of the dielectric surface, having an area A equal to that of a picture element which has received an electrical charge from the signal beam. If the thickness of the dielectric is small in comparison to the width of the area, sidewise leakage can be ignored and it can, therefore, be assumed that the charge will leak directly from the dielectric surface to the metal backing plate 21 which is at the potential of collector electrode 3|. The time constant te or time required for the charge to drop to l/e of its initial value is iven by the relation:

te=CR (8) where,

te=time in seconds required for the charge to fall to 0.368- (l/e) its original value, C=capacity in farads, R=resistance in ohms. However,

where, C=capacity in farads, k=dielectric constant, A=area of picture element in square centimeters,

d=thickness of dielectric in centimeters, and

R =pi where, R=resistance in ohms,

=specific resistance of dielectric in ohm centimeters. Therefore,

n lg-LllXlO (11) It is, therefore, seen that the time constant of the discharge is independent of the dielectric thickness but is directly proportional to the dielectric constant and its specific resistivity. In time t9, the charge falls to l/e or 0.368 of its original value; in time 2te, the charge falls to (l/c) or 0.135; and in time (its, it falls to (1/6) or 0.050 of its original value. For example, if the specific resistivity of the dielectric is to be found if the charge is to fall to per cent in one frame period ,6 second) assuming k=6,

p 2.1 X ohm centimeters It is also possible, as stated above, to provide the proper bias for dielectric 26 by bombardment with electrons of low velocity. From the secondary emission characteristic curve of Fig. 3, it is seen that if the dielectric surface is bombarded with an electron beam of voltage less than V1, the secondary emission ratio is less than unity and the surface becomes negatively charged since it receives electrons from the bombarding beam faster than it loses secondary electrons. The dielectric surface under these conditions continues to charge negatively until it reaches the cathode potential of the biasing beam. This cathode potential may be made negative relativeto the collector electrode 3| by a value equal to the biasing beam voltage if the anode of the biasing beam gunis connected to the collector electrode 3|. This allows the biasing beam to enter a fieldfree space. Of course, a negative bias for the dielectric surface 26 is preferable to a zero bias since it increases the linear range of the operating characteristic as shown in Fig. 5.

The image-producing beam produced by the electron gun including cathode 28 is a sharplyfocused beam which is modulated and scanned in the conventional manner. The biasing beam for wiping off the charge image may be completely independent of the image-producing beam and may be made to operate in a number of ways. For example, the biasing beam may be made to flood the whole dielectric surface with a uniform intensity beam. For example, this may be done in Fig. 1 by omitting scanning elements 45 and 46 and focusing the beam from gun 40, 4|, 42 into a wide angle beam to flood the dielectric 26.

If a flooding biasing beam is operated continuously, a given picture element, after having received a charge from the signal beam, is effective.- ly discharged at a constant rate until its potential approaches the bias potential V: when the rate of discharge decreases to zero. This is shown by the curves of Fig. 6, in which the time rate of change of potential of the element during discharge by the bias beam is illustrated for various given initial values of potential V1, V2, and V3 on the picture element. The biasing beam current should be so adjusted that a picture element having a maximum voltage V5 is substantially discharged in the time of one frame scansion second), Under equilibrium conditions, such a biasing beam current is equal to the average secondary emission current leaving the dielectric. It is apparent that an image element which has received only a small charge from the scanning beam is discharged sooner than another element which has received a larger charge. The complete history of the potential of an image element during one picture cycle can be obtained from the charge characteristic of Fig. 4, which is assumed to start from a negative volt-age Vi instead of the zero voltage as shown in Fig. 4 and the discharge characteristic of Fig. 6.

A biasing flood beam may be operated intermittently during the line-retrace period, each line being discharged immediately after it has been charged by the signal beam. Under these conditions, each line of the image is luminous only during a single line-scanning period and the individual image elements of any line or active for various durations. If the biasing flood beam is operated intermittently during the frame-retrace period, the individual picture elements of the entire picture are active for various durations. Thus, the first element scanned is active for the length of a full line-scanning or framescanning period, whereas the last element scanned is active for onl a minute fraction of such period, resulting in a nonuniform image illumination over the lines or over the complete picture. To avoid this effect, the charge image must be actively made use of only during a fraction of the respective retrace period prior to the initiation of the biasing flood beam. For example, this can be done in the circuit of Fig. 1 by omitting scanning elements 45 and 46, focusing the beam from the gun 40, 4|, 42 into a wide angle beam to flood the dielectric 26, and periodically suppressing the electron stream from cathode 40,

The preferred method of biasing the image grid is that illustrated in Fig. 1 which utilizes a bias beam from the electron gun 40, M, 42 which is sharply focused to a scanning beam and scanned over the surface of dielectric 26. This bias beam is scanned in the same manner as the signal beam but the phase of the saw-tooth fieldscanning current is retarded by nearly a full cycle, or advanced by a relatively small angle, so that the bias beam follows or chases the signal beam by almost a full field-scanning period. Therefore, the charge image is retained at full intensity for almost the entire field-scanning period. This leads to an ideal condition giving a substantially completely flickerless image. Since it is impracticable to utilize an arrangement in which the biasing beam of cathode 40 is focused anywhere nearly as sharply as that of the image-forming beam from cathode 28, it is necessary to have a gap of appreciable width, for example, 20 lines, between the image-forming beam and the bias beam. This phase relation can be obtained by advancing the phase of fieldscanning current of the bias beam developed by chasing beam-scanning generator 44 by the equivalent of 20 lines relative to the fieldscanning current developed by generator l9. Under these conditions, flicker can appear in only 20 lines so that the resultant average flicker is extremely low.

Up to this point in the discussion of the operation of the system of Fig. 1, only the arrangements for forming a charge image on the dielectric film 25 and the process of wiping ofi or discharging the charge image between scanslons has been considered. In order to provide a visible image by means of the charge images developed on image grid 25, an electron stream is directed against the surface of conductive backing plate '2? by means of the electron gun arrangement comprising cathode 32. This electron stream is of substantially uniform density and of a crosssectional area comparable to the area of grid 25 and, neglecting the effect of electrode 35, would also produce a cloud of secondary electrons of uniform density adjacent the surface 21 of grid 25. The density of the electron stream through the holes of any particular region of grid 21 is determined by the potential of the dielectric immediately surrounding the holes of that region, just as the grid potential of a vacuum-tube repeater determines the density of the electron current passing through the grid. The charge image on the perforated dielectric screen or charge plate 26, therefore, acts as a vacuum-tube grid having a two-dimensional charge distribution corresponding to a television image. The passage of electrons of the picture-forming beam from cathode 32 through the grid 26 thus forms an electron stream, the density of which is spacemodulated and which may be focused by means of the focusing electrodes 3% upon fluorescent screen 38 to provide a luminous television image.

Since, under the conditions assumed, the picture-forming beam from cathode 32 is directed against the conductive backing plate Zl to produce secondary electrons, a part of the beam may pass directly through the holes of the grid 25. In the regions of the image grid 25 which are charged positively relative to the backing plate 2?, the secondary electrons are pulled through the holes by the dielectric field, as indicated in Fig. 7. A part of the picture-forming beam produces secondary electrons which, in turn, are controlled in their passage through the apertures by the charge image to form a picture beam of the secondary electrons emitted by conductive backing plate 27. The velocity distribution of these secondary electrons is roughly Maxwellian with an average velocity of about 3 volts so that the beam can readily be focused on the fluorescent screen by means of the electron lens system 39 illustrated. The distribution of initial vectorial velocities of the electrons causes a spreading out of the component elements of the electron beam focused at the fluorescent screen 38, but this effect (analogous to chromatic aberration) decreases with increased accelerating voltage in the electron-optical system, in a well-known manner, and determines the lower limit for this voltage in accordance with the resolution required in v the electron image focused on the fluorescent screen 38.

That part of the picture-forming beam which passes through the apertures directly has an undesirable effect superimposed on the desired picture on fluorescent screen 38. If the velocity of the electrons in the picture-forming beam is high, that part of the beam passing directly through the apertures of the image grid 35 will not be appreciably modulated by the charge image and will not be focused sharply on the fluorescent screen 38, giving rise to a diffused background illumination of the fluorescent screen 38 of substantially constant value and independent of the charge image. The efiect of this diffused background illumination is to increase the minimum brightness level and, therefore, to decrease the contrast ratio, which determines the perceptibility of the picture. Therefore, it is desirable to prevent these electrons from reaching the fluorescent screen and this is accomplished in the embodiment illustrated in Fig. 1 by means of electrode 35 in conjunction with the image grid 25 which is biased highly negative relative to electrode 35 and is, therefore, efiective to form a virtual cathode closely adjacent the conductive backing plate 21, thereby to prevent high velocity electrons from passing through the image grid 25, while the virtual cathode has the effect of producing a cloud of electrons similar to that produced by the secondary electrons, as described above, if electrode 35 is not used.

Another arrangement which may be utilized to prevent direct passage of electrons from cathode 32 through the image grid 25, in case electrode 35 is not used, is that illustrated in Fig. 8 of the drawings wherein there is shown a portion of grid 25 together with a portion of an auxiliary apertured electrode 60 disposed closely adjacent to the backing plate 2! of the image grid in such a way that the openings in the grid Bit register with the imperforate part of the image grid, this arrangement comprising means for physically blocking the direct path through the grid 25 of electrons from cathode 32. Such a battle may also consist of small tabs 6!! attachedto or upset from the edges of the apertures of the conductive backing plate 2! as illustrated in Fig. 9.

In another embodiment of the invention, direct passage of electrons from cathode 32 through image grid 25 may be prevented by projecting the electrons of the picture-forming beam onto the backing plate 2i at a sufficiently small angle relative to the surface of plate 2! such that the tunnel effect of the aperture does not allow the direct passage of the beam. Such an arrangement is illustrated in the modification of the invention shown in Fig. 10.

While there have been described what are at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. In a television receiving system, a signalreproducing unit comprising, a cathode-ray image-reproducing tube including an apertured grid having one surface of dielectric material and an opposed surface of secondary electronemissive material, means for scanning said. surface with a signal-modulated cathode-ray beam to produce a charge image on said surface, means including an electron source for directing against the other surface of said grid an electron stream for developing a source of lowvelocity electrons of substantially uniform density and of a crosssectional area comparable to the area of said Y grid, means for physically blocking the direct path of electrons of said stream through said grid, whereby the density of the electron stream passing through said grid is space-modulated by said charge image on said one surface, and means for utilizing said last-mentioned modu- N lated electron stream to produce a visible image.

2. In a television receiving system, a signalreproducing unit comprising, a cathode-ray image-reproducing tube including an apertured grid having one surface of dielectric material and another surface forming a conductive back- ,ing plate therefor, means for scanning said one surface with a signal-modulated cathode-ray beam to produce a charge image on said surface, an electron source including means for directing against said other surface of said grid an electron stream for developing a source of secondary electrons of substantially uniform density and of a cross-sectional area comparable to the area of said grid, said backing plate including elements forming means for physically blocking the direct path of said electrons through said source from said grid, whereby the density of the electron stream passing through said grid is spacemodulated by said charge image on said one surface, and means for utilizing said last-mentioned modulated electron stream to produce a visible image.

3. In a television receiving system, a signalreproducing unit comprising, a cathode-ray image-reproducing tube including an apertured grid of appreciable thickness having one surface of dielectric material, means for scanning said surface with a signal-modulated. cathode-ray beam to produce a charge image on said surface, an electron source including means for directing against the other surface of said grid an electron stream for developing a source of lowvelocity electrons of substantially uniform density and of a cross-sectional area comparable to the area of said grid, the electrons from said first-mentioned source being incident on said surface with a relatively small angle, whereby the tunnel effect of said apertures effectively prevents the direct passage, of electrons from said source through said grid and whereby the density of the electron stream passing through said grid is space-modulated by said charge image on said one surface, and means for utilizing said last-mentioned modulated electron stream to produce a'visible image.

4. In a television system for receiving a carrier-wave signal including video and unidirectional background-illumination modulation components, asignal-reproducing unit comprising, a cathode-ray image-reproducing tube including an apertured grid having one surface of dielectric material, means for repeatedly scanning said surface with a cathode-ray beam modulated by said received video components to produce successive charge images on said surface, an electron source for directing against the other surface of said grid an electron stream for developing a source of secondary electrons of substantially uniform density and of a cross-sectional area comparable to the area of said grid, means for modulating said electron stream with said unidirectional background component of said received signal, whereby the density of the electron stream passing through said grid is space-modulated by each charge image on said one surface in accordance with the video components of the received signal and is time-modulated in accordance with the unidirectional background component of the received signal, and means for utilizing said last-mentioned modulated electron stream to produce a visible image.

5; In a television receiving system, a signalreproducing 1m? comprising, a cathode-ray image-reproducing tube including an apertured grid having one surface of dielectric material, means for repeatedly scanning said surface with a signal-modulated cathode-ray beam to produce successive charge images on said surface, means for biasing said surface substantially to a predetermined potential, an electron source including means for directing against the other surface of said grid an electron stream for developing a source of secondary electrons of substantially uniform density and of a cross-sectional area comparable to the area of said grid, whereby the density of the electron stream passing through said grid is space-modulated by each charge image and time-modulated by successive charge images on said one surface, and means for utilizing said last-mentioned modulated electron stream to produce a visible image.

6. In a television receiving system, a signalreproducing unit comprising, a cathode-ray image-reproducing tube including an apertured grid having one surface of dielectric material, means for repeatedly scanning said surface with a signal-modulated cathode-ray beam to produce successive charge images on said surface, a biasing flooding beam for biasing said surface substantially uniformly to a predetermined potential, an electron source including means for directing against the other surface of said grid an electron stream for developing a source of secondary electrons of substantially uniform density and of a cross-sectional area comparable to. the area of said grid, whereby the density of the electron stream passing through said grid is space-modulated by each charge image and time-modulated by successive charge images on said one surface, and means for utilizing said last-mentioned modulated electron stream to produce a visible image.

7. In a television receiving system, a signalreproducing unit comprising, a cathode-ray image-reproducing tube including an apertured grid having one surface of dielectric material, means for repeatedly scanning said surface with a signal-modulated cathode-ray beam to produce successive charge images on said surface, means for biasing said surface substantially uniformly to a predetermined potential between successive scansions, an electron source including means for directing against the other surface of said grid an electron stream for developing a source of secondary electrons of substantially uniform density and of a cross-sectional area comparable to the area of said grid, whereby the density of the electron stream passing through said grid is space-modulated by each charge image and time-modulated by successive charge images on said one surface, and means for utilizing said last-mentioned modulated electron stream to

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