US 2957097 A
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Oct. 18, 1960 Filed Jan. 22, 1958 P. SCHAGEN ETAL CATHODE RAY TUBE 5 Sheets-Sheet 1 Oct. 18, 1960 CATHODE RAY TUBE Filed Jan. 22, 1958 5 Sheets-Sheet 2 INVENTORS P SGHAGEN BIA. EASTWELL BY rum. CALDER P. SCHAGEN ETAL 2,957,097 I Oct. 18, 1960 P. SCHAGEN ErAL 2,957,097
CATHODE RAY TUBE Filed Jan. 22, 1958 s Sheets-Sheet 3 f FIGJS INVENTORS XII[ M U l x EI" H613 L0 1 1; i Z
Oct. 18, 1960 P. SCHAGEN ETAL 2,957,097
CATHODE RAY TUBE Filed Jan. 22, 1958 5 Sheets-Sheet 4 Tr Tb Oct. 18, 1960 P. SCHAGEN HAL CATHODE RAY TUBE 5 Sheets-She't 5 Filed Jan. 22, 1958 FIGZZ M ---------we r:; M E E!- w/ Fl (3.24
United States Pate t 2,957,097 Patented Oct. 18, 1960 CATHODE RAY TUBE Pieter Schagen, Salfords near Redhill, Bernard Albert Eastwell, Shenfield, Reading, and Nigel David Ritchie Calder, Crawley, England, assignors to North American Philips Company Inc., New York, N.Y.
Filed Jan. 22, 1958, Ser. No. 710,561 Claims priority, application Great Britain Jan. 30, 1957 16 Claims. (Cl. 313-76) This invention relates to cathode ray tubes for position-selecting, scanning and like operations and to devices comprising such tubes. Such devices may be applied for example to television display systems, information storage devices, switching systems and generally where it is required to select from a plurality of positions or elements or to scan a plurality of positions or elements.
According to one aspect of the invention, a cathode my device comprises an elongated evacuated envelope providing an elongated evacuated beam trajectory control space alongside of an elongated target, means for directing an electron beam into said trajectory control space from the region of one end thereof substantially in at least one trajectory plane intersecting the operative surface of said target on a rectilinear or approximately rectilinear line of intersection along the entire length of said operative surface, and means for setting up within said trajectory control space, and along the length of the envelope occupied by the operative surface of said target, a magnetic beam trajectory control field having lines of force intersecting orthogonally or substan tially orthogonally said plane and having a sense such as to bend the path of such an electron beam towards said target.
Although the field configuration may be orthogonal or substantially orthogonal to plane surfaces lying on either side of a principal or central trajectory plane so that more than one trajectory plane is available for use, the invention will be described as applied to unidimensional operation in a single trajectory plane except where otherwise stated.
The means for setting up the trajectory control field (referred to also more briefly as the control field) may comprise a permanent magnet system and/or a coil system. Such means may be symmetrical with reference to a plane used as the trajectory plane although this is not essential provided that the control field has the desired configuration in the region of the trajectory plane.
The means for directing an electron beam into the trajectory space may comprise an electron gun having its operative axis directed into said space from an end or edge portion thereof. Alternatively, said means may comprise an electron gun located in a dilferent manner but combined with deflection or mirror means (which may be external to the tube) from bending the beam from its initial direction into a path along which it can enter the trajectory space from an end or edge portion thereof The operative surface of the target is the surface over which it is possible to move the point of impact of the beam by varying the beam trajectory. The quality of the target will vary in accordance with the purpose of the apparatus. Thus, for information storage by electro-static charge deposition, the target may be of glass with a metal backing constituting the output electrode. However, the invention is mainly concerned with devices in which the target is constituted by luminescent material. Thus, a phosphor coating constituting the target may be provided on the inner side of a conductive surface or electrode held at an appropriate potential; in this case the screen is preferably viewed from its inner or impact side owing to the greater light efficiency obtainable, and also in view of the fact that the electrode can be made with a suflicient thickness of metal to cool the phosphor by dissipating heat generated locally by impact of the electron beam. Alternatively, the device may comprise a conductive coating permeable to electrons and provided on the inner or impact side of a phosphor target layer, in which case the screen ispreferably viewed from the side remote from the trajectory space. V
The position of impact of the beam on the target will depend on the initial velocity and direction of entry of the electrons and the intensity of the field. The position of the point of impact of the beam may, if desired, be varied solely by varying the initial velocity of the electrons, but this method is disadvantageous in the case of a displaydevice inasmuch as it becomes necessary to compensate for variations in brightness. Thus it is preferred to vary the intensity of the field (in time) and/ or the direction of entry of the beam into the trajectory control space .(referred to also more briefly as the control space). The choice of the actual method of controlling the range of the trajectory depends to a large extent on the focussing and spot-size requirements. In fact, as will be appreciated, with an elongated cathode ray tube it is diflicult to obtain a small spot at all points along the target when the beam enters the control space from one end thereof. This difficulty may be eased in various ways.
Thus, according to a further aspect of the invention, the cathode ray device is adapted to maintain the direction of entry of the beam into the trajectory control space at a constant acute angle away from the operative or inner surface of the target. 7 j t By this method, a greater angle of impact of the beam can be obtained at the target for a given maximum range and a given diameter or width of the cathode ray or substantially the same at any cross-section normal to the operative surface of the target and to the direction of elongation of the device. To effect scanning or position selection with such a device, the means for setting up the control field are such as .to permit the intensity of said field to be varied in time so as to control the position of the point of impact of the beam along the length of the target. For this purpose a control tfield coil system can be used, and a current, hereinafter referred to as the control current, may be supplied to the control coil system so that the control field is set up in the trajectory space such as to have a component urging the electrons of the beam towards the target. Thus, for example, a substantially rectilinear scan may be obtained by varying the control field so as to vary the range of the trajectory and thus cause the point of impacton the target to move along a line on said target, and this action does not re-.
quireany change in the initial electron velocity or direction.
With such arrangements the longitudinal scan requires control currents of sawtooth waveform which may have to be of considerable amplitude and thus may involve high potentials in the control coil system during the flyback periods. This diificulty can be avoided by adopting an alternative method of scanning or position-selection in which it is also possible to ease the focus and spotsize problems of an elongated cathode ray device according to the invention. Thus, according to a further aspect of the invention, the cathode ray device includes means for varying the direction of entry of the beam in the trajectory plane over a range including directions away from the operative surface of the target so as to vary the beam trajectory and thereby vary the position of the point of impact of the beam along said operative surface.
With such a device a longitudinal scan can be obtained by applying a relatively small amount of scanning energy to initial deflection means provided as the means for varying the direction of entry of the beam.
Such means for varying the direction of entry of the beam may, if desired, be used in conjunction with means for varying the control field (in time), in which case one method of control may be employed to provide the main component of a scanning movement of the beam while the other method provides a linearity or other correction. However, if control of the point of impact is efiected entirely by varying the direction, or angle of entry of the beam, then the control field configuration and intensity can be kept constant in time. Consequently the field, being static, can be set up accurately e.g. with the aid of permanent magnets and, as Will be explained hereinafter, it can readily be given a non-uniform or graded configuration such as to provide focussing additional to the initial focussing effected prior to entry of the beam into the trajectory space.
Thus, according to a further aspect of the invention, a cathode ray device includes means for varying the direction of entry of the beam in a trajectory plane over a range including directions away from the operative surface of the target so as to vary the beam trajectory and thereby vary the position of the point of impact of the beam along said operative surface, the configuration of said control field being non-uniform in the or each trajectory plane in such manner as to effect a degree of beam focussing. Since the direction of entry of the beam is varied to control the trajectory, the various trajectories will pass through different regions of the control space consequently the non-uniformity of the control field configuration may be made to differ from one region to another so that its focussing action varies in dependence upon the path taken by the beam, and such local variations in the field may be set up in such manner a to provide a substantially or at least approximately constant spot size at all points along the target.
Various non-uniform control-field configurations may be used to provide focussing etfects. Thus the control field may have a configuration such that the field strength increases away from the target over a region traversed by the beam. Alternatively, the control field may have a configuration such that the field strength increases away from the point of entry of the beam over a region traversed by the beam. As a further alternative, the control field may have a configuration which is curved in the region of the target in any plane normal to the length thereof and is concave as viewed from said target in order to effect transverse focussing of the beam.
The variants effect the form of the beam trajectory and the focussing conditions along the target as will be explained hereinafter, and it will be assumed that the lines of force are drawn in the conventional manner at equal increments or decrements of field strength.
Scanning or position-selection in one dimension may readily be obtained with devices according to the invention. In some applications requiring only movement of a spot along a line or a line trace, this is sufficient. Thus, for example, a line-scanning tube with elongated control field means may be used in a television display system employing mechanical means for the frame scan. When such a system is used for monochrome applications, a single electron gun may be provided to opcrate in a single trajectory plane. However colour television scanning and displayapplication are very important and, according to a further aspect of the invention, the target comprises a set of parallel luminescent strips adapted to luminesce with diflfering colour response. Such a device may employ a plurality of electron guns each operating in a separate trajectory plane, or it may employ a single gun together with means for selectively displacing the beam from one strip to another.
Before passing to the drawings, it may be observed that the use of a magnetic control field has the advantage over the use of an electro-static field that accumulation of charges on the envelope of the tube can readily be prevented by rendering the whole inner surface of the envelope bulb conductive without thereby interfering with the control field pattern. A suitable potential can be applied to such conductive surface, and a part thereof may be associated with the target when the latter comprises a phosphor layer.
The mode of operation discussed above will now be described in greater detail with reference to the diagrammatic drawings which show embodiments of the invention given by way of example and suitable for scanning in one dimension as may be required in television display systems as described in a copending United States application, Serial No. 683,321, filed September 11, 1957, now US. Patent 2,924,656, issued February 9, 1960, or in flying-spot scanners or films. The drawings employ the same reference numerals for the same or corresponding parts.
In the drawings:
Figures 1 and 2 show schematically in perspective two alternative arrangements of field coils applied to cathode ray tubes having cylindrical bulbs.
Figures 3 to 8 illustrate various control field patterns in which the configuration is, at any instant, the same or substantially the same at any cross-section normal to the operative surfaces of the target and to the direction of elongation of the device.
Figures 9 to 11 illustrate various beam trajectory and focussing conditions.
Figures 12 to 14 illustrate a further arrangement in which the field configuration varies, at any instant, along the length of the target.
Figures 15 to 20 show arrangements employing permanent magnets for setting up the control field, and
Figure 21 is the schematic crosssection of a tube suitable for colour display.
Figures 22 and 23 illustrate in cross-section further devices suitable for colour display.
Figures 24 and 25 are schematic plan views of particular target arrangements, and
Figure 26 shows schematically a device, with related circuit components, for use in colour or monochrome scanning or display.
In the arrangements illustrated it should be assumed that at least part of the interior of the bulb is preferably rendered conductive as aforesaid although conductive layers have not been shown in the drawings. Such a conductive surface may extend also in the region of the target and may act as a base on which phosphor is laid or as a backing on the inner surface of the phosphor. In addition, such a conductive surface may insome cases advantageously have optical properties such as to prevent spurious internal reflections, and for this purpose a colloidal suspension of graphite in water is suitable. Such problems will only be referred to in detail in particular cases.
Referring now to Figures 1 and 2, the arrangements shown are given in order to illustrate in an introductory manner some possible tube and control field'coil constructions, and it will be appreciated that the cylindrical form of envelope, though convenient, is in no way essential. Figure 1 shows an arrangement comprising. two
elongated generally rectangular" parallel control field coils L, L' located symmetrically on either side of a median this example located on the cylindrical wall of the en-' velope bulb B containing the control space.
In Figure 1, electron gun means may be located in a neck N1 if the beam is to enter the control space in a direction away from the target T. Alternatively, such gun means may be located in a neck parallel to the target and located above the axis of the bulb B, such alternative position being indicated at N2. As a further alternative, the neck may be co-axial with the envelope, in which case the tube preferably includes deflection means for varying the direction or angle of entry of the beam so as to move the spot along the target.
An example of a co-axial neck and bulb arrangement has been shown in Figure 2. The arrangement of this figure comprises a single control field coil L located on the median plane M and having operative upper and lower limsb Lo and Li. Said limbs may, as shown, have their ends nearest the gun means bridged in such manner that half of the coil passes on either side of the tube neck N.
In the arrangements of Figures 1 and 2 the operative parallel upper and lower limbs of the or each coil have their ends bridged at a sufficient distance beyond the ends of the target T, or the operative portion thereof, to avoid distortion of the field pattern thereat.
Of course, a twin coil arrangement such as that of Figure 1 may be used with a co-axial neck and bulb arrangement as shown in Figure 2 and, conversely, a single central coil such as the coil L of Figure 2 may be used with a neck position such as is shown at N1 or N2 in Figure 1.
Both the arrangements of Figures 1 and 2 have operative coil limbs that are parallel to each other and to the target and thus are adapted to provide control field patterns in which the configuration is, at any instant, the same or substantially the same at any cross-section normal to the operative surface of the target and to the direction of elongation of the device. Examples of such parallel field patterns will now be described.
Figures 3 to 8 show three coil and control field configurations obtainable with arrangements similar to those generally indicated in Figures 1 and 2. In these figures the tube envelope will be omitted since its form is not relevant.
Figure 3 is a cross-section of a twin coil arrangement similar to that of Figure 1 so arranged as to provide a substantially uniform and orthogonal control field in the region of the part of the plane M traversed by the various trajectories. This is shown as a uniform spacing of the intersections I of the lines of force 7 with the median plane M. Figure 4 is a longitudinal section of the field pattern and target T of Figure 3 taken on the median plane M, and is drawn on the basis that a uniform spacing of lines represents a uniform distribution of lines of force and hence uniform distribution of field strength. Thus the uniform spacing of lines in both dimensions represents a median field pattern which is not only uniform in the direction normal to the target surface (as seen from Figure 3) but also has a configuration which is, at any instant, the same or substantially the same at any cross-section normal to the operative surface of the target and to the direction of elongation of the device.
When a beam is directed along the plane M, the forces acting on the electrons thereof lie in the direction of the plane M and have a sense such as to cause said electrons to strike the target T.
Figure 5 is a cross-section of a further control field pattern while Figure 6 is a longitudinal section thereof taken on the plane M. In this case a single central coil L such as that of Figure 2 has its lower limb Lo shielded e.g. by a permeable channel member P, so as to create a control field having an intensity which increases away from the'target T. This non-uniformity of the field is of a kind which can effect a degree of beam focussing as will be explained hereinafter. The non-uniform or graded field configuration is represented in Figure 6 by parallel horizontal lines, the density of the lines representing the local field strength. As in the succeeding Figures 8; 9 and 10, which are drawn on the same principle, the parallelism of the horizontal lines represents the fact that the configuration is, at any instant, the same or substantially the same at any cross-section normal to the operative surface of the target and to the direction of elongation of the device. The limb L1 may be divided into separate parts L1 and L1" for reasons which will be explained later with reference also to Figure 7.'
Figures 7 and 8 represent in a similarmanner a field pattern which may be obtained by a single coil arrangement such as that of Figure 2 without shielding. In this case the coil L provides at the median plane M a field pattern which exhibits increasing field intensity both towards the target T and towards the limb L1 of the coil remote from target T. In the ideal case of coil limbs L0, L1 having negligible thickness, the field intensity at any point on the plane M can be obtained from the formula 1 1 l (I) -v y( y) in which d is the distance between the axes of limbs L0, L1 and y is the distance of the point considered from a given limb.
Forms of beam trajectory and particular focussing conditions will now be discussed with reference to Figures 9 to 11.
Figure 9 is a sectionon the median plane of a field pattern similar to that of Figure 4. An electron gun G is shown directing a beam into the control space in a constant direction away from the target T. Gun G may be carried in a neck arranged in a manner similar to the neck N1 of Figure l, and the control field may be obtained by a coil system as shown in Figure 1 or Figure 3.
The control field has such polarity that the beam electrons are urged thereby towards the target T, Theposition of impact of the beam on the target will depend on the velocity of the electrons and the intensity of the field and, for a given velocity, such position can be deter mined by varying the control field intensity alone. Thus a substantially rectilinear scan may be obtained by varying the overall control field intensity so as to vary the range of the trajectory and thus cause the point of impact on the target to sweep along a line on said target, and this action does not require any change in the initial electron velocity. p p
This action is illustrated by the three trajectories bl-b3 shown in Figure 9. The electrons travel away from the target to a variable extent, depending on theintensity of the control field applied, before reaching the apex of the particular trajectory and being returned towards the target. I
The force acting tin the median plane and urging the beam electrons towards the target T is constant or substantially constant throughout the beam path, and therefore each trajectory will have the form of a circle in the ideal case of an exactly uniform field pattern, the range S of the trajectory being given by the equation 2 S=2R sin or where R is the radius of curvature of the path due to the" magnetic control field (such radius R being proportional to the velocity of the electrons and inversely proportional" to the field strength) and or is the angle of entry of the all trajectories if the beam enters the trajectory space through the target and through an entry point lying, as shown in Figure 9, in line, or substantially in line, with the line of intersection between the trajectory plane and the operative surface of the target.
As an alternative to the gun position of Figure 9, the beam may enter the control field parallel tothe target, e.g. from a gun in a neck located in a manner similar to the neck N2. of Figure l. The various trajectories are still circles tangent to the direction of entry of the beam. Although the various trajectories are circular or substantially circular, the angle of incidence on the target will no longer be constant.
Figure 10 is a section taken in the median plane of a field pattern similar to that of Figure 6 and obtainable e.g. with a shielded coil arrangement as shown in Figure 5. The beam enters in a constant direction parallel to the target T near an edge of the control space remote from the target T, and the beam may be obtained from a gun in a neck located as in the position N2 of Figure 1.
Some degree of focussing of the spot in the scanning direction is given by such a control field pattern in which the field intensity increases away from the target in the region of the apices of the trajectories. The qualitative explanation of this effect is that the electrons farthest from the target experience the greatest curvature of their paths owing to the intensification of the field in that region. Thus the range S tends to become the same for parallel electrons of a beam entering at differing heights as illustrated at b2 in Figure 10. At shorter ranges, the range of an individual electron is less the greater its distance from the target, so that over-convergence can occur as is also shown in Figure 10 at b1.
If the direction of entry of the beam is arranged as in Figure 9, then the field configuration of Figure 10 will again give a constant or substantially constant angle of impact 7 owing to the longitudinally constant field configuration and the symmetry of the trajectories.
The field configuration of Figure 10 may be inverted so that the field strength increases towards the target. The field will then cause a degree of de-focussing in the longitudinal direction but it is possible to offset this by using curvature of the field near the target to obtain transverse focussing in the direction normal to the trajectory plane as will be explained below.
As aforementioned, a scan can also be obtained by varying the angle of entry of the beam by means of deflection means at one end of the control space. Such deflection means may be magnetic or electrostatic and an example of the latter arrangement is shown at D in Figure 11. The three trajectories b1-b3 shown are only illustrative and do not take into account the pattern of the control field. The latter may correspond to that of Figures 3 and 4, in which case the trajectories will again be of circular form. If the field pattern corresponds to that of Figure 6, focussing in the direction of scan can be obtained in the manner described with reference to Figure 10.
As a further alternative, the arrangement of Figure 11 may have a control field pattern of the type shown in Figures 7 and 8, in which case the field near limb L1 has a longitudinal focussing action while the progressive increase in field strength near the target applies a downward bend to the end portion of the beam thereby increasing the final angle of incidence 7 for all trajectories. This is illustrated in Figure 11 by the shift from paths b1b3 to b1'b3, and from angles of impact 1- 3 to 'y1'-'y3'. This action is accompanied by some de-focussing in the longitudinal direction, but this disadvantage may be more than offset by transverse focussing derived from the fact that the control field has a configuration which is curved in the region of the target in any plane normal to the length thereof and is concave as viewed from said target. The latter consideration also applies if the field configuration of Figures -6 is inverted by placing the target near the limb L1 instead of near limb L0.
In this connection, it should be noted that curvature of the field as shown in Figure 5 and Figure 7 near coil limb L1 would tend to de-focus the beam in the lateral direction if used with a target located as shown (i.e. near limb L0.) This can be obviated by flattening or reversing said field curvature e.g. by spreading out laterally the turns forming L1 or dividing the latter into two halves as indicated in dotted lines at L1'-L1 The control field patterns of the arrangements described with reference to Figures 1 to 11 are all characterized by a control fiel-d intensity which is constant (spatially) at the median plane along any line parallel to the target. Figures 12 to 14 show an arrangement in which the field pattern varies (spatially) along the length of the target.
Referring to Figures 12 to 14; two generally rectangular coils L, L are arranged at equal angles to the median plane M so as to converge beyond the end of the bulb B remote from the gun as shown in a plan view normal to plane M (Figure 12). Figure 13 is a cross section of the coils taken on the lines XIII-XIII of Figure l2, and at that point the field pattern has a crosssection resembling more or less that of Figure 3. On the other hand, at the opposite end of the tube the field pattern has a cross-section which tends to resemble that of Figure 7. Figure 14- is a longitudinal section of the field pattern taken in the plane M and the lines of this pattern, which are once more the lines of constant field intensity in the plane M, illustrate the gradual transition between the two types of fieldpattern (as before, line density represents field strength). Such a fieldconfi'guration may be arranged to give good focussing at all points along the target since the longer trajectories; for which focussing is more important, encounter greater overall field strengths and greater curvature of the field near the target.
The further examples illustrated in Figures 15 to 20 employ permanent magnet systems for providing. a control field constant in time and initial deflection means for varying the direction of entry ofthe beam so as to effect longitudinal movement of the point of impact of the beam along the target.
Referring to Figure 15, which is a schematic crosssection of an arrangement having a field patternwhich is substantially uniform in the direction normal to the inner surface of the target T, the control field is set up by permanent magnets or magnet systems m, m provided symmetrically on either side of a median trajectory plane M. The direction of magnetisation is indicated by the symbols N-S and the magnets may for examplebe of the material available under the registered. trademark Ticonal. The control field is rendered substantially uniform at each cross-section by the use of pole pieces P, P of soft magnetic material, for example mild steel. The presence of the pole pieces permits each of .magnet means m, m to be constituted, if desired, by a series of separate magnets spaced apart in the longitudinal direction of the device. In this arrangement the parts of a pole piece which are at a greater distance from the re spective magnet system are closer to the opposite .pole piece so as to compensate for such greater distance thereby providing the desired substantially uniform cross-sectional field distribution indicated by lines of force 1'. The form of the pole pieces is determined by the field configuration desired and not by the shape of the bulb of the envelope of the cathode ray tube. this figure a convenient arrangement is shown by way of. example which arrangement employs a cylindrical bulb B of circular cross-section with pole pieces having rightcylindrical pole-faces such as to fit the outer surface of the bulb. However, the right-cylindrical bulb form is in no way essential and this applies also to the following figures.
Referring now to Figure 16, there is shown in arrangement employing a single permanent magnet or magnet system m for producing a control field having substantially uniform intensity at any given cross-section. Once more, a cylindrical bulb is shown at B. In this arrangement two pole pieces of soft magnetic material such as soft iron or mild steel are provided at and P, and the cross-sectional form of the pole pieces is such as to provide the desired substantially uniform cross-sectional field distribution in the region of the trajectory plane M.
In the arrangement of Figures 15 and 16 a plurality of magnets may be distributed uniformly along the length of the pole pieces so as to render the field'strength substantially uniform along the length of the bulb as well as along its diameter in the plane M. Alternatively, the magnets may be graded in strength or in their spacing so as to cause the field strength to increase progressively from one cross-section to another away from the gun while remaining substantially constant at any given crosssection. The latter configuration can provide a degree of longitudinal focussing because, in a given beam, the electrons having the longer trajectories enter regions of greater field strength and thus undergo a greater degree of bending.
Figure 17 shows a further arrangement in which a single permanent magnet system or scrim of magnets in is provided near the target T. Means m may be constituted by a contiguous series of magnets without pole pieces as shown, or by a series of magnets spaced apart in the longitudinal direction between a pair of soft magnetic pole strips for distributing the field in the longitudinal direction. The resultant field has a cross-sectional distribution which has greatest intensity near the target and decreases at increasing distances from the target along the trajectory plane M. Such a field distribution may be used in the manner described with reference to Figure 11 so as to increase the angle of beam impact at the target, but it will be appreciated that the target T may alternatively be located at the opposite position T so as to permit operation similar to that described with reference to Figure 10. If the target is in the position T, the curvature of the field near the target T will, as in the arrangement of Figure 7, have a focussing action on the beam in the direction transverse to the plane M.
The lines of force 1 intersect orthogonally the trajectory plane M and are curved not only near the target but also in the remainder of the control space. The magnet system m may be arranged to provide substantially the same field strength at all cross-sections or it may cause the field strength to increase from one cross-section to another with increasing distance from the gun. In the latter event focussing will be applied by the control field in both the transverse and longitudinal directions.
Figure 18 shows an arrangement in which a cross-sectional field distribution is obtained which is approximately uniform in the plane M except for a region of increasing field strength near the target T. The arrangement employs a single magnet system or row of magnets m provided with pole pieces P, P' having cylindrical or substantially cylindrical pole faces. As in the arrangement of Figure 7 or Figure 17, the field curvature near the target T has a transverse focussing effect.
The arrangements of Figures 16 to 18 are particularly suitable for television display apparatus employing mechanical means for the frame scan. Such apparatus may employ a rotor having three cylindrical lenses and requiring an aperture of about 120' (as measured from the axis of rotation) through which the inner surface of the target may be viewed; this is readily attainable, particularly with the arrangement of Figures 17 and 18; in Figure 18 such viewing aperture is indicated at V on the assumption that the axis X of the bulb B coincides with the axis of rotation of the rotor.
equivalent of the limbs L0, L1 and L1" of Figure 7" may be obtained with a magnet system m as shown in Figure 17 combined with a pair of weaker magnet systems physically and magnetically spaced apart in positions equivalent to the positions of li'mbsL1L1" with their direction of magnetisation transverse to the plane M.
Referring now'to Figure 19, there is shown in side elevation a cylindrical envelope'N, B with a permanent magnet arrangement for setting up a magnetic control field which varies along the length of the target T as indicated by the lines representing, again, constant field intensity in the planeM. In this arrangement the pole pieces P may have a constant cross-section similar to that of Figure 18 in' which case the field pattern will be substantially of 'the same configuration at cross-sections taken at any point along the target, the field intensity varying, however, from one cross-section to another. The intensity of the field may be graded longitudinally by using a number of permanent magnets m spaced apart to a varying extent e.g. as indicated. This arrangement employs electrostatic'defiection means D, as shown, or electro-magnetic deflection means to vary the direction of entry of the beam in the plane M and thus efiect scanning as indicated by the beam paths b1b3. As will be seen, the general effect is to cause the longer electron paths to enter progressively stronger fields so that the angle of impact increases with range, and this helps to maintain a small spot size (in the scan direct-ion) at all points along the length of the target.
The arrangement of Figure 19 may be modified by inclining and shaping the upper edges p of the pole pieces P so as to provide a variation of the cross-sectional field pattern along the target.
The transverse focussing action of the curved field near the target of Figures 17, 18 or 19 may be combined with a complementary action in the longitudinal or scan direction so as to obtain a spot of rounder shape while maintaining a small spot size. This may be done by giving the cross-section of the beam an oval form with the major axis in the direction transverse to the plane M, or by applying astigmatism to the beam in the same direction. Such forms of correction may be applied in known manner and can provide a suitable compromise in the shape of the spot at various distances along the target.
Actually, a degree of astigmatism is in effect provided by the control field focussing action described with reference to .Figure 10. "On the other hand, astigmatism can readily be applied near the electron gun or in the neck of the tube by known means.
Alternatively, however, astigmatism may be applied by a permanent magnet e.g. as shown at A in Figure 20 which is a side elevation of an arrangement similar to that of Figure 19. The magnet A is located above the end of the bulb B nearest to the neck N, and its direction of magnetisation is the same as for magnets m. This arrangement is advantageous in that a gradual increase in field strength away from the target is obtained without obstructing the view of the target through the upper portion of the envelope wall as is suitable if the tube is to be employed in apparatus employing mechanical means for the frame scan.
Arrangements employing pole-pieces such as those of Figures 15, 16 and 18 to 20, are advantageous in that the pole-pieces provide screening against external fields and protection against X-rays. As will be appreciated, such pole-pieces may also be used with electro-magnets re placing the permanent magnets in cases in which it is desired to vary the beam trajectory by varying the overall intensity of the control field.
The arrangement of Figures 1 to 20 have been described with reference to a single selected trajectory plane M and a single scan path along the target. However, two or more trajectory planes may exist and be utilised, said planes intersecting the operative surface of the target on parallel rectilinear or substantially rectilinear lines of intersection along the entire length of said operative surface, and the control field having lines of force normal or substantially normal to each of said planes. A typical example arises in connection with colour television display. Thus in Figure 21 there is shown a bulb B having a target comprising three parallel phosphor strips Tr, Tg, Tb adapted to luminesce in differing colours, e.g. red, green and blue. The control field is indicated as having lines of force substantially normal to three trajectory planes M, M, M" each of which intersects one of the phosphor strips.
Three separate electron guns may be used, each with its operative axis in one of the trajectory planes, in which case simultaneous scanning of three beams may be caused by variation (in time) of the intensity of the common control field, or by common deflection means (such as means D of Figure 11) actuating the three beams. As an alternative, a single gun may be used with means for deflecting or displacing the beam transversely from one strip to another, e.g. at a spot-wobble frequency suitable for dot-sequential presentation. This transverse motion may, for example, be applied by a pair of electrostatic deflecting electrodes located near the target and running along the full length thereof, e.g. a pair of tensioned wires.
In the case of a television receiver required for blackand-white picture display as well as colour display, the target may comprise a fourth phosphor strip, adapted to luminesce substantially in white, provided alongside a set of red, green and blue strips for use when a monochrome signal is received. This can reduce the matching requirements of the colour phosphor system.
With a single-gun arrangement of this character, the tube may be rotated mechanically through a small angle when it is desired to change from colour to monochrome display or vice versa. The median plane M of the control field remains stationary and will intersect either the middle strip of the colour triplet or the white strip depending on the position to which the tube has been rotated.
if desired, one or more spare sets of phosphor strips may be provided on the inner Surface of the bulb, so that a fresh set may be brought into use by rotating the tube when the useful life of the first set has ended.
The arrangements illustrated hitherto have a control field which extends to the target surface. However, this is not an essential condition and the control field may extend along the length of the envelope occupied by the operative surface of the target without actually extending to the target. An example will now be described in which the control field is prevented from reaching the target by the presence of a magnetic shield.
This arrangement is shown diagrammatically in crosssection in Figure 22 wherein the cylindrical bulb B contains a plane transverse shield of permeable magnet material Pd with a slit Sd to permit the beam to escape into the further space between shield Pd and a tri-colour target TrTg--Tb parallel thereto. A pair of pole pieces P-P have cylindrical pole faces which embrace part of the bulb B and are operatively associated with a magnet or magnet system m. The field configuration is approximately as indicated by the lines of force f and does not extend beyond the shield Pd. An accelerating electric field may, if desired, be set up between shield Pd and the target. Alternatively, or in addition, this space may be devoted to a transverse deflection e.g. for the purpose of spot-wobble to provide dot-sequential colour display. Electrostatic deflection means Ds are shown for this purpose and comprise a pair of strips running along the full operative length of the target. The transverse deflection is indicated by the trajectories b.
Since the control field is constant in time, initial deflection means (not shown) operative in the plane M are provided to vary the direction of entry of the beam into the control space so as to effect longitudinal scanning of the beam along the slit Sd. In this example the electron gun may be located above the geometrical axis of the bulb B, for example in a neck such as the neck N2 of Figure 1. The beam will emerge from the slit Sd at a varying angle so that the path length of the beam between the shield Pd and the target will also vary; if, in spite of this, it is desired to maintain substantially constant the amplitude of the deflection applied by means Ds, constant or substantially constant deflection sensitivity may be ensured by varying appropriately the cross-section of the deflection plates Ds.
As will be appreciated, the permanent magnet system m of Figure 22 may be replaced by an electro-magnet system if scanning is to be efiected by varying the overall field intensity, in which case the beam may be made to emerge from the slit Sd at a constant angle. This can be done by adopting a longitudinally uniform field con figuration and causing the beam to enter the control field initially through the shield Pd in the manner adopted in relation to the target of Figure 9.
Whereas the target of the device of Figure 22 intersects the trajectory plane and is located alongside the exit slit Sd, a device according to a modification of the invention may have a target spaced from said plane. Such target need not be elongated, and a typical example arises when a unidimensional magnetic control-field system is used as the first scanning stage of a two-stage two-dimensional scanning or display device. The target may then be a generally rectangular and substantially plane luminescent screen associated with a pair of substantially plane and parallel control electrodes defining a secondstage control space. The magnetic device may have an exit slit such as the slit Sd located along one edge of the screen and the beam may be made to emerge from the slit at a substantially constant angle by employing a longitudinally parallel field configuration together with the scanning mode of Figure 9. Thus a raster can be set up by causing the magnetic system to effect beam control in one dimension while a sawtooth potential on the control electrodes eflfects displacement of the beam in a direction normal to or different from that of the slit so as to control the beam in a second dimension.
It is now convenient to revert to arrangements in which the control field extends to the target. When operating with three colour strips such as the strips T r-Tg---Tb of Figure 21, it is desirable in practice that any curvature of the control field configuration near the target be limited to relatively small values, e.g. to a curvature somewhat less than that obtained with the arrangement of Figure 18. This means that the transverse focussing action of such a curved field is not fully utilised. However, the full use of such action requires, as aforementioned, the introduction e.g. of appropriate astigmatism in the beam and, quite apart from the additional problems due to colour, there are circumstances in which it is desirable to render the system simpler and more economical by striking a compromise in which the field curvature is reduced to a level at which beam astigmatism can be dispensed with while retaining a smaller degree of transverse focussing.
A permanent magnet system providing such a compromise is shown in cross-section in Figure 23.
This arrangement differs from that of Figure 18 in that the or each magnet m is replaced by a magnet m. which is longer (in the transverse direction) so as to lessen the curvature of the lines of force 7 in the central region. The system is suitable for colour display and the target has therefore been shown as a tri-colour triplet of strips TrTg-Tb. The latter are shown backed by metal rods r of sufficient thermal conductivity to prevent undue local heating of the phosphors.
An elongated magnet may be used in a similar manner 13 in the arrangement of Figure 22 to reduce transverse defocussing due to field curvature.
In the longitudinal direction the field intensity of the device of Figure 23 may be graded by a variable distribution of magnets e.g. as shown in Figure 19. On the other hand it has been found in practice that arrangements such as those in Figures 18, 20 and 23 can operate satisfactorily with a graded field produced by a single magnet or pair of magnets located at or near the end of the bulb remote from the gun combined with a magnetic short-circuit bridging parallel-edged pole-pieces PP at or near the gun end of the bulb. In a practical example applied to a bulb having an operative target length of about 40 cm. and an outer diameter of about 10 cm., the single magnet for an arrangement such that of Figure 18 may be no larger than 2" in the longitudinal direction, in the transverse direction (between the pole-pieces PP) and in height.
The formation of three coloured traces may be produced by high-frequency spot-wobble action, which may be sinusoidal, applied to a single electron beam by transverse deflection means. When employing the apparatus to display colour images of which the colour information is transmitted in coded form as luminance, hue and saturation (e.g. so-called Y, I and Q signals), the I and Q signals being transmitted on a common sub-carrier with phases in quadrature, the frequency of the spot-wobble action is preferably the frequency of this sub-carrierto permit decoding of the composite colour signal to be effected at the electron gun. In this case the spot-wobble waveform is preferably of a stepped or sawtooth nature. In practice a sufficiently close approximation for this particular purpose may be obtained in known manner by using only the first two terms of the Fourier series for a sawtooth waveform, i.e. the fundamental and thirdharmonic components. With such high frequencies the energy requirements of the deflection means of the spotwobble system may be met more easily by tuning the circuit thereof.
If spot-wobble is applied near the target by means such as the aforesaid tensioned wires or the strips Ds of Figure 22, trapezium or equivalent distortion can occur only to a small extent.
On the other hand, if dot-sequential colour display is effected by spot-wobble action obtained by initial angular deflection of the beam in a direction transverse to the plane M, problems of trapezium or like distortion will arise. However, with spot-wobble at high frequencies of a few megacycles it is diflicult to apply conventional trapezium correction techniques by varying the angle of spot-wobble deflection during each line scan. In fact, such modulation of the high-frequency deflection waveform will normally introduce phase-shifts which, in practice, will disturb the colour rendering.
This difliculty may be overcome by taking advantage of the optical properties of display systems with optical de-magnification of the line thickness and this permits the use of a relatively course tri-colour line structure. Thus, in particular, it is possible to accept a modern trapezium or equivalent distortion on the basis that it will not be noticeable after the optical compression applied by the mechanical frame-scanning system.
Thus, in the absence of control field curvature, the amplitude of the spot-wobble motion at the target will tend to increase linearly with trajectory range, and this can be accommodated e.g. by setting the axes of the three colour strips at a small angle to each other and, if desired, giving them a tapered form as shown in Figure 24 which is a schematic plan view of the inner surface of the target. The gun is shown at G as set up for colour spot-wobble over the triplet TrTg-Tb. The alternative position of the gun (at G1) and median plane (at M1) is intended to represent rotation of the tube so as to align the white phosphor strip W with the median plane M when monochrome display is desired.
A control field having curvature near the target, e.g. as in Figure 18, would cause a distortion of modified trapezium type; spot-wobble amplitude would increase with range up to a certain range beyond which the amplitude would decrease again to some extent due to the fact that the beam is subjected to the curved part of the control field over a longer distance. This is true when the field is curved to a lesser extent also in regions remote from the target, and the effect is still more pronounced if the control field configuration is such that the field strength increases progressively towards the end of the bulb remote from the gun. Such variationscan readily be accommodated by modifying the plan form of the strips Tr, Tg, Tb of Figure 24 by reducing gradually the width of an end portion of the central strip Tg beyond a predetermined distance from the gun and curving inwardly the end portions of strips Tr and Tb to a corresponding extent as shown schematically in Figure 25.
A circuit arrangement for dot-sequential colour display is shown schematically in Figure 26. The tri-colour triplet of phosphor strips Tr-Tg-Tb may be as described with reference to Figure 24 or Figure 25. The control field is constant in time and may be produced by means (not shown) employing permanent magnets e.g. as described with reference to Figures 18-19. The bulb and neck may be right-cylindrical and co-axial as indicated in dotted outline. The longitudinal line scan is applied by deflection coils DL operative in the plane M and fed with sawtooth currents from a line time-base unit TB.
The coils DL are followed by a pair of coils DS which deflect the beam through asmall angle in a direct-ion normal to the plane M so as to apply a high-frequency spotwobble motion. Coils D5 are fed from a generator SW which may provide a waveform which includes a sinusoidal fundamental with the addition of a third harmonic component as aforementioned.
Thetube may be rotated through a small angle to bring into operation an optional fourth white strip W for monochrome display, the permanent magnet control-field system and the'coils DL andDS remaining stationary.
Having described particular examples of focussing action due to non-uniform control field configuration, some general observations may be made as regards focussing of the beam. First, the methods described may be sufficient in themselves to provide an adequately small spot size at all points along the target without the need for dynamic focussing; on the other hand, where the requirements are stricter, dynamic focussing may be added in known manner. From a different point of view it may be observed that, as is well known in the electron-optical art, a smaller spot'can be obtained with a large initial beam cross-section and with an initial focussing lens of wide aperture provided such lens has little or no aberration. However, if a large cross-section with a wide lens is used in the present invention, the consequent smaller depth of focus tends to require at the same time the use of dynamic focussing or an equivalent action by means such, for example, as the magnet A of Figure 20. On the other hand, where it is possible to accept a larger spot, a smaller initial beam cross-section may be used and-consequently dynamic focussing or the equivalent may more readily be dispensed with.
With the exception of the modifications previously described devices according to the invention have been defined as employing at least one trajectory plane intersecting the operative surface of the target along its entire length. Moreover, the said operative surface has been described as the surface over which it is possible to move the point of impact of the beam by varying the beam trajectory. These definitions need to be amplified in order to take into account slight alterations in the mode of application of devices that have been described and illustrated. I A
Thus a tri-colour triplet-of phosphor strips may be replaced by a target comprising an even number of strips, for example four strips of differing colour response. If such a target is disposed symmetrically in relation to a trajectory plane and the strips are spaced apart, such plane will pass between the two central strips. In such circumstances it is clear that the operative target surface may be regarded, for the purpose of the above definitions, as including imaginary sections extending between the operative surfaces of individual luminescent strips that are to be used as a single target system.
An analogous situation can arise in a device having elongated deflection means (such as the aforesaid tensioned wires) near the target for effecting transverse defiections such as spot-wobble motion. Taking the arrangement of Figure 22 as a specific example, it is possible to displace the tri-colour target to one side of the trajectory plane M while leaving the control field system and shield Pd unaltered. In fact, in operation, the spotwobble voltages applied to the deflection strips Ds may include a D.C. component which compensates for the offset position of the target. Moreover, a second target may be provided on the other side of the plane M and the polarity of the D.C. component may be reversed if it is desired to select such alternative target.
With these modes of operation there is, strictly speaking, no trajectory plane which actually intersects the target surface. Therefore, such arrangements may be regarded as being in accordance with a further modification of the invention in which the target lies substantially in the general direction of elongation of the device in a position alongside but spaced from the trajectory plane. In such an arrangement, the target, or an axis of symmetry thereof, may be parallel or substantially parallel to the line of intersection between the adjacent part of the envelope wall and the trajectory plane. As in the examples illustrated, the operative surface of the target may be plane or cylindrical, or it may be a partly plane and partly cylindrical surface generated by motion of a common rectilinear generatrix maintained at a constant orientation.
What is claimed is:
l. A cathode-ray device comprising an elongated envelope defining an elongated trajectory control space, an elongated, electron-receiving target surface associated with said control space, means for launching an electron beam into the control space from the region of one end thereof and generally parallel to the target surface, and permanent magnet means for establishing substantially throughout the control space a magnetic field causing the beam to bend toward and strike the target surface at selected areas.
2. A cathode-ray device comprising a generally cylindrical elongated evacuated envelope defining an elongated trajectory control space, an elongated generally linear electron-receiving target associated with said control space, means for launching an electron beam into said control space from one end thereof along a selected trajectory making a given angle of entry of the beam relative to the target, magnetic-field-forming means for establishing a magnetic field within the control space to cause the path of said electron beam to be bent towards and thus strike said target, and means for varying the said angle of entry of the beam over a range including directions away from the inner surface of said target so as to vary the selected beam trajectory and thereby vary the position of the point of impact of the beam along the operative surface of the target.
3. A device as set forth in claim 2 wherein the target comprises plural, substantially parallel, phosphor strips luminescing in different colors.
4. A device as set forth in claim 3 wherein the plural phosphor strips include a white-luminescing phosphor.
5. A device as set forth in claim 3 wherein means are provided for wobbling the beam so as to impinge successively on different phosphor strips.
6. A cathode-ray device comprising an envelope of elongated form and defining an elongated trajectory control space, an elongated electron-receiving target associated with said control space, means for launching an electron beam into said trajectory control space from one end thereof along a selected trajectory, and magnetic-fieldforming means for establishing substantially throughout said trajectory control space a graded electron-deflecting magnetic field whose intensity increases in a direction away from the target for bending said selected trajectory to terminate on said target and for providing an additional focusing effect for the beam.
7. A cathode-ray device comprising an elongated envelope defining an elongated trajectory control space, an elongated electron-receiving target surface associated with said control space, means for launching an electron beam into the control space and generally parallel to the target surface, and means for establishing throughout the control space a graded electron-deflecting field that continuously increases in intensity along a given dimension of the device causing the beam to bend toward and strike the target surface at selected areas and providing an additional focussing effect on the beam.
8. A cathode-ray device comprising an elongated envelope defining an elongated trajectory control space, an elongated electron-receiving target surface associated with said control space, means for launching an electron beam into the control space from one end thereof along a selected trajectory, and magnetic-field-forming means for establishing substantially throughout the control space a graded electron-deflecting magnetic field whose intensity increases along the length of the control space in a direction away from the beam-launching means for bending said selected trajectory to terminate on said target and for providing an additional focussing effect for the beam.
9. A cathode-ray device comprising an elongated envelope defining an elongated trajectory control space, an elongated electron-receiving target surface associated with said control space, means for launching an electron beam into the control space from one end thereof along a selected trajectory, and magnetic-field-forming means for establishing substantially throughout the control space a graded electron-deflecting magnetic field whose intensity increases from a median point in directions toward and away from the target for bending said selected trajectory to terminate on said target and for providing an additional focussing effect for the beam.
10. A cathode-ray device comprising an elongated envelope defining an elongated trajectory control space, an elongated, generally line-type, electron-receiving target surface associated with said control space, means for launching an electron beam into the control space from one end thereof along a selected trajectory, and magneticfield-forming means for establishing substantially throughout the control space a graded curved electron-deflecting magnetic field that decreases in intensity along radial directions from the target and exhibiting, relative to the target, concave lines of force for bending said selected trajectory to terminate on said target and for providing an additional focussing effect for the beam.
11. A device set forth in claim 10 wherein means are provided for introducing astigmatism into the beam to counteract any detrimental effects on the beam shape due to the curved field.
12. A cathode-ray device comprising an envelope of elongated form and defining an elongated trajectory control space, an elongated electron-receiving target associated with said control space, means for launching an electron beam into said trajectory control space from one end thereof along a selected trajectory, permanent magnet means for establishing in said trajectory control space a graded electron-deflecting magnetic field that increases in intensity along the length of the control space in a direction away from the beam-launching means for bending said selected trajectory to terminate on said target 17 and for providing an additional focusing effect for the beam, and means for varying the angle of entry of the said beam into the control space thereby to cause the beam to scan a line along the elongated target.
13. A device as set forth in claim 12 wherein the envelope has a cylindrical form, and the target comprises a phosphor strip extending adjacent to the wall of the envelope and parallel to its length.
14. A cathode-ray device comprising a generally cylindrical envelope of elongated form and defining an elongated trajectory control space, an elongated electronreceiving target associated with said control space, means for launching an electron beam into said trajectory control space from one end thereof and along a selected trajectory, and plural permanent magnet means provided along the length of the device With a gradually decreasing spacing and including generally cylindrical pole pieces disposed on opposite sides of said control space to increase the field intensity at the end of the device farthest from the beam source for bending said selected trajectory to terminate on said target and for providing an additional focusing effect for the beam.
15. A device as set forth in claim 14 wherein the target comprises four generally parallel, adjacent, phosphor strips luminescing, respectively, in three primary colors and in white.
16. A cathode-ray device comprising an elongated cylindrical evacuated envelope defining an elongated trajectory control space, an elongated electron-receiving target comprising color-luminescing and white-luminescing, parallel, phosphor strips associated with said control space, means for launching an electron beam into said control space along a selected trajectory making a given angle of entry of the beam relative to the target, magnetic-fieldforrning means for establishing a control magnetic field to cause the path of said electron beam to be bent to wards and thus strike said target, and means for varying the said angle of entry of the beam over a range including directions away from the inner surface of said target so as to vary the selected beam trajectory and thereby vary the position of the point of impact of the beam along the operative surface of the target, said cylindrical envelope being rotatable relative to the angle-varying means to selectively locate the color-luminescing and white-luminescing strips in the path of the beam.
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