GB2193595A

GB2193595A - Electron gun arrangements

Info

Publication number: GB2193595A
Application number: GB08715210A
Authority: GB
Inventors: Koji Ichida; Yoshifumi Nakayama; Hiromu Inoue
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 1986-06-30
Filing date: 1987-06-29
Publication date: 1988-02-10
Anticipated expiration: 2007-06-29
Also published as: FR2603737B1; FR2603737A1; GB8715210D0; KR880001022A; JPH0766751B2; GB2193595B; DE3721596A1; JPS6310440A; KR960004585B1

Description

GB2193595A 1

SPECIFICATION

Electron gun arrangements This invention relates to electron gun arrangements suitable for use in cathode-ray tubes, such 5 as, for example, a picture tube in a colour television receiver.

Fig. 6 of the accompanying drawings shows a previously-proposed multibeam single electron gun arrangement, by way of example, for use in a colour television receiver tube. This electron gun arrangement has cathodes K, KG and K, corresponding to electron beams for the colours red, green and blue, respectively. A first grid G, a second grid G, a third grid G, a fourth grid 10 G4 and a fifth grid G. are arranged in common for the cathodes K(K, KG and KJ. The cathodes K and the first to third grids G, to G. together form a cathode prefocusing lens, while the grids G, to G5 form a unipotential main electron lens. In this previously- proposed electron gun arrange ment, the electron beams respectively emitted from the cathodes KB, KG and K. intersect each other at a position substantially in the central portion of the main electron lens which meets the 15 so-called Fraunhofer conditions, namely at a position which provides conditions to eliminate coma aberration. A converging means C such as, for example, a set of electrostatic deflecting plates, is provided after the fifth grid G, to converge electron beams 13, B, and B, emitted from the cathodes KR, KG and KB, respectively a fluorescent screen (not shown). In such an electron gun arrangement, since the main electron lens exerts a common effect on all three of the 20 electron beams, the aperture of the main electron lens can be increased within a limited area of the neck section of the cathode-ray tube so as to reduce aberration.

On the other hand, in such a cathode-ray tube, focusing conditions are selected so that the electron beams respectively emitted from the cathodes K, K, and K, are focused on the fluorescent screen at optimum positions. Specifically, an optimum focusing voltage V, is applied 25 to a focusing electrode, for example the fourth grid G, of the main electron lens of the electron gun arrangement of Fig. 6. -However, the focusing conditions are different for the central portion and the peripheral portion of the fluorescent screen because the central portion and the periph eral portion have different distances from the main electron lens. Accordingly, it is a general practice to apply a dynamic focusing voltage synchronised with the horizontal and vertical 30 deflection of the electron beams on the fluorescent screen to the focusing electrode in addition to the fixed focusing voltage V, so that the electron beams are focused satisfactorily over the entire area of the fluorescent screen.

In an electron gun arrangement developed for the purpose of improvement of the arrangement shown in Fig. 6 in respect of aberration, a focusing voltage is applied to an electrode serving as 35 both the electrode of the main electron lens and the electrode for the cathode prefocusing lens.

In this improved electron gun arrangement, when a dynamic focusing voltage is applied to the cathode prefocusing lens in addition to the mixed focusing voltage V, the cathode current varies causing irregular brightness distribution on the fluorescent screen and making the peripheral portion of the fluorescent screen brighter than the central portion. 40 As mentioned above, in the-electron gun arrangement of Fig. 6, aberration is reduced by increasing the aperture of the main electron lens. However, the beam spot is liable to bloom due to increase in spherical aberration when the beam current is large. In the electron gun arrange ment -of Fig. 6, the ' effect of the focusing voltage for sharply focusing the electron beams at the same position on the fluorescent screen is different for a centre beam 13G travelling coaxially with 45 the axis of the main electron lens L, and side beams B, and B, travelling obliquely to the axis of the main electron lens L. That is, when the focusing voltage VF is appropriate for sharply focusing the side beams B,, and 13, the centre beam 13G is focused at a position before reaching the fluorescent screen and, when the focusing voltage V, is appropriate for sharply focusing the centre beam 13, the side beams 13, and B, are focused at a position effectively beyond the 50 fluorescent screen. Such a problem can be solved by disposing the central cathode K, for emitting the centre electron beam 13G away from the main electron lens L relative to the side cathodes K, and K, for emitting the side electron beams, as illustrated in Fig. 7 of the accom panying drawings. In the configuration shown in Fig. 7, since the second grid G2 and the successive grids are common to all the electron beams, a portion of the second grid G2 facing 55 the first grid G1G corresponding to the central cathode KG extends backwardly so that the respective gaps between the second grid G. and the first grids G,, G1G and G1E, res pectively corresponding to the cathodes K, K, and K, are substantially the same, which causes the effect of the main electron lens on all the electron beams to be the same. However, the equipotential surfaces relating to the central beam B. are curved as indicated by lines a in Fig. 7, and the 60 curved equipotential surfaces exert an additional focusing effect on the central beam B.. Conse quently, the crossover point of the central beam is varied, the substantial object point of the electron lens system relating to the central beam is moved and, in some cases, the spot of the central beam may be distorted. Such inconveniences may be avoided by curving the rear end of the third grid G3 facing the front end of the second grid G2 along the front end of the second 65 2 GB2193595A 2 grid G2 and by decreasing the gap between the second grid G2 and the third grid G, so that the equipotential surfaces are parallel to each other. However, since a high voltage is applied to the third grid G3, electrical discharges can occur between the second grid G, and the third grid G, when the gap between the second grid G2 and the third grid G, is too small.

Referring to Fig. 8 of the accompanying drawings showing another unipotential electron lens 5 system, a high voltage V, is applied to the third grid G. and the fifth grid G, of the main electron lens, and a focusing voltage VF is applied to the fourth gricl, G4 of the main electron lens. Ordinarily, in a unipotential electron lens system of this type, the grids G, and G, to which the high voltage V, is applied and the grid G4 to which the focusing voltage VF is applied are substantially the same in diameter, or as shown in Fig. 9 of the accompanying drawings, the 10 respective ends of the high-voltage grids G, and G5 facing the focusing grid G4 are reduced in diameter to shield the path of the electron beams from disturbances caused by an external electric field. In either case, the focusing grid G, is formed so as to meet the condition I/D=0.5 to 2.0 where 1 is the length and D is the diameter of the grid G,.

Fig. 10 of the accompanying drawings shows the calculated spherical aberration characteristics 15 of the unipotential electron lens system comprising the grids G3 to G. having the same diameter (as shown in Fig. 8) for various values of i/D=y in which the ratio f/D= (f=focal length, D=aperture)_is provided on the X-axis and the coefficient Cs ofspherical aberration is provided on the Y-axis. As is clear from Fig. 10, when the ratio is fixed, the coefficient Cs of the spherical aberration diminishes as the ratio y is increased. In practice, since the value of the 20 aperture D is limited by the diameter of the neck section of the cathoderay tube, when the focal length f is fixed, the Jongerthe length of the grid G,, the smaller the spherical aberration.

However, since the aberration is saturated when the ratio y is 2.0 or greater, it is desirable to reduce the focal length f when the ratio y is fixed. Nevertheless, in general, when the length 1 of the grid G, is increased, namely, when the ratio y is large, the focal length f cannot be 25 diminished. This problem will be described in detail with reference to Fig. 11 of the accompany ing drawings. In a unipotential electron lens system, suppose that lenses 1 and 2 are formed respectively between the third grid G3 and the fourth grid G,, and between the fourth grid G4 and the grid G,, respectively, f, and f2 are the respective object focal lengths of the lenses 1 and 2, f,' and f2' are the respective image focal lengths of the lenses 1 and 2, F, and F2 are the 30 respective object focal points of the lenses 1 and 2, F,' and F2' are the respective image focal points of the lenses 1 and 2, and the distance between F,' and F2' is C. Then, the composite focal length f' is expressed by f=fl'Xf2'/(-C) (1) 35 Generally, in an electron lens system, C<O, and hence f'>O. When the length 1 of the grid G4, hence the distance L between the lenses 1 and 2, is increased to diminish spherical aberration, the absolute value of C decreases and, as is apparent from Equation (1)., the composite focal length f' increases. Accordingly, significant increase of 1 and hignificant reduction of f for 40 satisfactorily reducing spherical aberration as explained with reference to Fig. 10 are incompati ble. Further, increase of f causes the focusing condition to change. Accordingly, to maintain f at a small value regardless of the increase of 1, as is apparent from Equation (1), the respective image focal lengths:of the lenses 1 and 2 need to be decreased However, the Variation of the distance Q between the after lens 2 and the fluorescent screen of the cathode-ray tube is 45 limited by the relation of the after lens 2 to the horizontal and vertical deflecting means provided at the base of the funnel of the cathode-ray tube, and the reduction of the focal length f2' of the after lens 2 is limited to a certain extent. Therefore, it is necessary to reduce the focal length f,' of the front lens 1.The focal length f,' of the front lens 1 can be reduced, for example, by increasing the ratio of the anode voltage V, applied to the grid G3 10 the focusing voltage VF 50, applied to the grid G4, namely, the ratio V,/V,. This method, however, requires an independent high-voltage circuit for applying a high voltage to the grid G3 in addition to the circuit for the fifth grid G3, which is troublesome in practice because the high-voltage circuit requires shielding.

To reduce the focal length f,' of the front lens system without encountering such problems, a front electron lens 1 of a d - eceleration type is formed of a first electrode (namely the third grid 55 G,) and a second electrode (namely the fourthgrid GJ, and an after electron lens 2 of an acceleration type is formed of the second electrode (namely the fourth grid GJ and a third electrode (namely the fifth grid G,) as shown in Fig, 12 of the accompanying drawings. In this arrangement, the length 1 of the grid G, is determined so that the respective electron lens regions of the front electron lens 1 and the after. electron lens 2 are separated from each other, 60 and the front electron lens 1 and the after electron lens 2 are designed so that the aperture of the front electron lens 1 is smallerthan that of the after electron lens 2. That is, the respective opposite ends of the third grid G, and the fourth grid G, are designed so that the aperture D, thereof is smaller than the aperture D2 of the respective opposite ends of the fourth grid G, and t he fifth grid G Namely, the grids are designed so as to comply with the inequality: 65 3 GB2193595A 3 DJ1D,=k<l. To separate the respective electron lens regions of the front lens 1 and the after lens 2 from each other, the grids are designed so as to comply with the inequalities: 1,>D1, 12>D, and I>lDil+D2, where 1, is the length of the reduced section of the grid G,, 12 is the enlarged section of the grid G,, D, is the diameter of the reduced section of the grid G,, and D2 is the diameter of the enlarged section of the grid G,. 5 Suppose that the front electron lens 1 and the after electron lens 2 are the same in diameter, namely k= 1, so as to form an optical system as shown by continuous lines in Fig. 13 of the accompanying drawings, and the electron beams are focused on the fluorescent screen S of the cathode-ray tube. In Fig. 13, PO is an object point, namely a cathode image formed at the cross over point of the electron beams focused by a cathode prefocusing electron lens, P, is a virtual 10 image formed by the front electron prefocusing electron lens, P, is a virtual image formed by the front electron lens 1, namely the object point of the after lens 2, and P2 is an image focused on the fluorescent screen S by the after electron tens 2. To reduce the focal length f,' of the front electron lens 1 by decreasing the diameter D, without varying the focusing system, namely to maintain the focusing system so that the image is focused on the fluorescent screen S, the front 15 electron lens 1 and the object point PO are shifted to positions indicated by respective broken lines, in Fig. 13. Optically, reducing the diameter of the front electron lens 1 is eqivalent to reducing the focusing system of the lens 1 without varying the magnification, because the respective magnifications of the lenses 1 and 2 are fixed. That is, the amount of aberration attributable to the lens 1 is expected to decrease according to the degree of reduction of the 20 focusing system. More specifically, if the aperture D, of the front lens 1 is decreased, the distance between the lenses 1 and 2 is increased, and the cathodes K are shifted. Supposing that M= 12, Q=50xD2, and MVA is fixed, where M is the magnification of the lens system, and Q is the distance between the lens 2 and the image point P2, 0 is the distance between the after lens 2 and the object point PO, and L is the distance between the lenses 1 and 2, the 25 variations of the distance 0 and L with the aperture ratio k are indicated by a continuous line and a broken line, respectively, in Fig. 14 of the accompanying drawings. In this case, the aperture D, of the after electron lens 2 is 6 mm. In Fig. 14, the distances 0 and L are measured on the Y-axis using the aperture D, as a unit, namely D2 = 1.

Fig. 15' of the accompanying drawings shows the calculated results of the relation between 30 the coefficient of spherical aberration and VJV, for the aperture ratio, where M= -8 and Q=50xD2, In Fig. 15, curves 10, 11, 12 and 13 represent the variations of the coefficient Cs of spherical aberration with W/VA for k=1.0, 0.8, 0.6 and 0.4, respectively. Values in paren theses in Fig. 15 are the values of the distance L using D2 as a unit. In Fig. 15, the ratio Cs/D2 is provided on the Y-axis. ' - 35 Fig. 16 of the accompanying drawings in similar to Fig. 15, except that M= 10, Q= 50 X D,, and curves- 20, 21, 22, 23 and 24 are the variations of Cs with MVA for k= 1.0, 0.8, 0.6, 0.4 and 0.3, respectively.

As is apparent from Figs. 15 and 16, the smaller the aperture ratio k between the front and after electron lenses, namely, the smaller the aperture D, of the front electron lens 1 relative to 40 the aperture D2 of the after electron lens 2, the greater is the improvement in the aberration.

Thus, a lens system causing low spherical aberration may be formed by forming an indepen- dent front lens region and an independent after lens region, and forming the front electron lens and the after electron lens so that the aperture ratio k therebetween is small. The coefficient Cs of the total spherical aberration of the composite lens system consisting of the lenses 1 and 2 45 formed by the front and after lens regions is expressed by Cs=1k.W+11M14. M/V2)312.CS2 (2) where Cs' and W are the coefficients of spherical aberration of the lenses 1 and 2, respec- 50 tively.

Therefore, the amount of aberration Ar is - Ar=MI."M2.1k:.Csl(o,)+ 1 /M 14.(01)312.CS2 (02)l X (j/0 0)312.CI03 (3) 55 where k is the aperture ratio of the aperture D, of the front electron lens to the aperture D2 Of the after electron lens, M, and M2 are the respective magnifications of the front and after electron lenses, 00=V1/V1, 01=V2/V1, 02=V2/Y,, and V,, V2 andV3 are voltages applied to the first, second and third electrodes, respectively.

It is apparent from Equation (3) that reducing the aperture ratio k is effective for-reducing the 60 total aberration.

Thus, the aberration characteristics of the main electron lens consisting of the two indepen- dent lenses 1 and 2 can be improved by designing the lenses 1 and 2 so that the aperture ratio k is small; however, such a main electron lens is unsatisfactory with regard to astigmatism and curvature of field. Accordingly, even if such a composite lens system were to be employed as a 65

4 GB2193595A 4 common main electron lens for a plurality of electron beams, for example three electron beams as previously explained with reference to Fig. 6, and is designed so that the three beams BR, BG and BB Will intersect each other at a position to make coma aberration zero to meet the Fraunhofer conditions, the respective spots of the side beams B, and B, are liable to bloom.

Japanese Patent Application Provisional Publication No. 55-19755 discloses an electron gun 5 arrangement intended to improve the condition of the spots of the side beams. In this known electron gun arrangement, a main electron lens comprises front electron lenses, namely a front electron lens region, and an after electron lens separate from the front electron lenses, namely an after electron lens region. The front electron lenses, in particular, are individual electron lenses corresponding to the electron beams, respectively, while the after electron lens is a 10 common electron lens for all the electron beams, having low characteristics of astigmatism and curvature of field. The aperture of each front electron lens is smaller than that of the after electron lens. In this electron gun arrangement, the electrodes forming the front electron lenses of the main electron lens may, for example, also serve as the electrodes of the cathode prefocusing electron lens, and hence the same focusing voltage is applied to these electrodes, 15 whereby the cathode current is caused to vary by variation of the dynamic focusing voltage, and the brightness of the fluorescent screen is caused to vary from position to position.

As illustrated in Fig. 17 of the accompanying drawings, this known electron gun arrangement has, for example, a main electron lens comprising front electron lenses of a decelerating bipoten tial electron lens system and an after electron lens of an accelerating bipotential electron lens 20 system. Cathodes K,, K, and K, for emitting, for example, electron beams BR, B, and B, for the colours red, green and blue, respectively, are provided and first grids G1R, G1G and G,,, second grids G21, G2G and G2., and third grids G,,, GIG and G,, for the electron beams BR, 13G and BB, respectively, are arranged sequentially. Fourth and fifth grids G, and G,, namely, commonly grids, are arranged sequentially after the third grids. One end of the fourth grid G, facing the third grids 25 G3R, G3, and G3B is trifurcated. in three cylindrical electrodes G,,, G4G and G41, respectively, corresponding to the third grids G3R, GIG and G,,. Voltages according to an inequality V2<V1<V3 are applied to the respective electrodes, where V, is a voltage applied to the third grids, V2 is a voltage applied to the fourth grid and V3 is a voltage applied to the fifth grid. For example, the voltages V, and V3 may be equal to an anode voltage V,. The electrodes G,, , G4, and G41 of the 30 fourth grid G, and the third grids G, constitute the decelerating bipotential front electron Lens,,, Lens,, and Lens,, individually for the beams BR, 13G and B,, respectively, of a main electron lens, while the fourth grid G4 and the fifth grid G, constitute an accelerating bipotential after electron lens 2 commonly for the beams BR, B, and B,. The aperture ratio k of the aperture D, of the electrodes G41, G,, and G,, of the fourth grid G, to the aperture D2 of the fourth grid G, at one 35 end thereof facing the fifth grid G, is smaller than unity, namely, k=D, /D2<1. The length of the fourth grid G, is greater than D,+D2 to separate the lens region of the after electron lens 2 from the lens region of the front Lens1R, Lens,, and Lens,,. The electron beams BR, B, and B, are caused to intersect each other substantially at the centre of the after electron lens 2 so as to meet the Fraunhofer conditions. 40 Referring to Fig. 18 of the accompanying drawings showing another previously-proposed electron gun arrangpment, the after electron lens 2_ of this electron gun arrangement is an extension type (extended field- type) bipotential electron lens. The after electron lens 2 comprises fourth, fifth and sixth grids G4, G, and G6, and voltages V, to V, applied respectively to the third grids G,, the fourth grid G, the fifth grid G, and the sixth grid G, may meet, for example, the 45 following conditions: V,=;74=anode voltage, V,/V,=0.25 to 0.40, and V, /V40.4 to 0.6.

Accordingly, forming the main electron lens of the front electron lenses respectively for the electron beams, each with a small aperture, and the after electron lens commonly for all the electron beams, with a large aperture, solves the problems of astigmatism of the front electron lenses and of curvature of field, and enables the employment of an electron lens having small 50 astigmatism and curvature of field as the after electron lens. Consequently, such a main electron lens of a multibeam single electron gun type is able to solve the problem of blooming of the spots of the side beams attributable to astigmatism and curvature of field.

Fig. 19 of the accompanying drawings illustrates the electrode configuration of the foregoing electron gun arrangement. First grids G,,, G,G and G,, are provided for cathodes KR, K, and K,, 55 respectively, while second to sixth grids G, to G, are common grids. Thus, front lenses Lens,,, Lens,,.and Lens,, of a main electron lens are provided for electron beams emitted from the cathodes K,, KG and K,, respectively. The lenses Lens,,, LensIG and Lens,, are formed of electron beam transmission apertures h3R, h3G and h3B formed in the front end plate of the common third grid G,, and electron beam transmission apertures h4R, h4, and h,, formed in the 60 front end plate of the common fourth grid G,, respectively. In similar manner to the previously mentioned front electron lenses, the front electron lenses Lens1R, LenS,G and Lens,, are formed to meet the required relationship described above. In forming the front electron lenses, the electron beam transmission apertures h3R, h,, and h3B formed in the front end plate of the common third grid G, and the electron beam transmission apertures h4R, h4G and h40 formed in 65 GB2193595A 5 the front end plate of the common fourth grid G, are formed with a press to form cylindrical walls Ws around the respective apertures, so as to prevent mutual disturbance in the respective electric fields. Fig. 20 of the accompanying drawings shows this part of the electron gun arrangement in enlarged detail. Referring again to Fig. 19, electron beam transmission apertures are formed in the respective front end plates of the first grids G1R, (31G and G113, the second grid 5 G2 and the third grid G. to form cathode prefocusing lenses respectively for the electron beams.

The electron beam transmission apertures forming the cathode prefocusing lenses and the front electron lenses are coaxial with axes OR, OG and OB, which are in alignment with the respective electron beams. The axes 0, and OB corresponding to the side beams are each inclined at a predetermined angle 0 to the axis 0. corresponding to the centre electron beam. Accordingly, 10 the cylindrical walls Ws formed around the electron beam transmission apertures h3R, h3G and h3,3 formed in the front end plate of the third grid G3 and the electron beam transmission apertures h4R, h4G and h4B formed in the rear end plate of the fourth grid G4 should be formed coaxially with the axes 0, 0. and 0, respectively, which requires complicated manufacturing processes and tends to cause problems during manufacture such as the problem of maintaining the axes at 15 accurate positions.

According to the present invention there is provided an electron gun arrangement comprising:

a plurality of cathodes; and a main electron lens having front electron lenses each provided in the path of an electron beam emitted from each cathode and each forming a front electron lens region, and a common 20 back electron lens forming a common electron lens region separate from said front electron lens regions, and provided in the respective paths of the electron beams emitted from said cathodes; wherein:

the aperture of each of said front electron lenses respectively forming said front electron lens regions is smaller than the aperture of said back electron lens forming said back electron lens 25 region; the respective centre axes of the electron beam transmission apertures of electrodes forming said front electron lens regions on the paths of the respective electron beams are parallel to each other; and each front electron lens forming said front electron lens region is formed so as to meet 30 Fraunhofer conditions.

Embodiments of the invention, to be described in greater detail hereinafter, overcome or at least reduce the foregoing problems to provide an electron gun arrangement for emitting a plurality of electron beams of the type which comprises a main electron lens having a plurality of front electron lens regions respectively formed for the electron beams and provided with electron, 35 beam transmission apertures formed with the respective centre axes thereof parallel to each other, and an after electron tens region common to the plurality of electron beams, which electron gun arrangement can be accurately manufactured using simple manufacturing processes.

Preferably, the electron gun arrangement includes a cathode prefocusing lens and a main electron lens having grids serving also as the grids of the cathode prefocusing lens, and being 40 capable of preventing variations of cathode current caused by dynamic focusing currents applied to the grids.

The preferred electron gun arrangement has a centre cathode for emitting a centre electron beam which is retracted away from a main electron lens relative to side cathodes for emitting side electron beams, and which is capable of preventing distortion of an. electric field which 45

I- exerts a particular lens action on the centre electron beam.

In embodiments of the present invention, although the front electron lenses are provided individually for the rpspective electron beams, the respective centre axes of the electron beam transmission apertures of the front electron lenses are parallel to each other. Therefore, the electron beam transmission holes can be formed easily and with high accuracy, and even when 50 circumferential walls are formed around the electron beam transmission apertures of the front electron lenses, the electron beam transmission apertures can be formed accurately relative to the intervals between the centre axes thereof and the axial alignment. Furthermore, although the side electron beams travel at an angle to the respective optical axes of the corresponding front electron lenses, because the centre axes of the front electron lenses are parallel to each other, 55 aberration attributable to the differences between the paths of the electron beams and the axes of the corresponding front electron lenses is not a significant problem because the _front electron lenses are formed so as to meet Fraunhofer conditions.

The invention will now be described by way of example with reference to the accompanying drawings, throughout which like parts are referred to by like references, and in which: 60 Figure 1 is a longitudinal sectional view showing an electrode arrangement of an electron gun arrangement, in a preferred embodiment according to the present invention; Figure 2 is a fragmentary sectional view of an important portion of the electron gun arrange- ment of Fig. 1; - 65 Figure 3 is a fragmentary sectional view ofa modified portion of the electron gun arrangement 6 GB2193595A 6 of Fig. 1; Figure 4 is a graph showing variation of spot size with cathode current in the electron gun arrangement of Fig. 1; Figure 5 is a graph showing variation of spot size with cathode current in a previously- proposed electron gun arrangement; 5 Figures 6 and 7 are respectively a longitudinal sectional view and a fragmentary longitudinal sectional view of the electrode arrangement of a previously-proposed electron gun arrangement; Figures 8, 9, 11 and 12 are schematic sectional views of electron gun arrangements used for explanining the construction and performance thereof; Figure 10 is a graph showing variation of spherical aberration with the ratio of focal length to 10 lens aperture for the ratio of length to diameter of the electron lens; Figure 13 is a diagrammatic illustration used for explaining the characteristics of the electron lens; Figure 14 is a graph showing the respective variations of the distance between a front electron lens and an after electron lens, and the object distance with the aperture ratio of the 15 front electron lens to the after electron lens; Figures 15 and 16 are graphs showing the variations of the coefficient of spherical aberration with the ratio of focusing voltage to anode voltage; Figures 17 to 20 are sectional views showing the respective electrode arrangements of previously-proposed electron gun arrangements; 20 Figures 21 and 22 are sectional views showing respective electrode arrangements of electron gun arrangements, in a second embodiment according to the present invention; Figures 23 to 26 are empirical graphs showing the variations of cathode current with focusing voltage; and Figure 27 is a longitudinal sectional view showing an electrode arrangement of an electron gun 25 arrangement, in a third embodiment according to the present invention.

A first embodiment of an electron gun arrangement is shown in Figs. 1 and 2. The electron gun arrangement employs a main electron lens of a unipotential type. Cathodes K,, K, and K, for emitting electron beams, for example the colours red, green and blue, respectively, are mounted as shown in Fig. 1. The centre cathode KG is-mounted with its centre axis in alignment with the 30 centre axis 0, of the electron gun arrangement, and the cathodes KR and K, are mounted on opposite sides of the cathode K, with their centre axes inclined at an angle to the centre axis OG of the electron gun arrangment. First grids G1R, G,, and G,, are associated with the cathodes KR, K, and K,, respectively. Second to sixth grids G, to G,, which are common to all three of the electron beams B,, B, and B,, are arranged sequentially after the first grids G,,, G,, and G,,. 35 The third grid G, has a front end plate 31 which faces the second grid G2 and has a rear end plate 32 which faces the fourth grid G,. The fourth grid G, has a front end plate 41 which faces the third grid G3. - The first grids G,R, GIG and G,, are coaxial with the cathodes K,, K, and K,, respectively, and electron beam transmission apertures h,,, hIG and h1B are formed coaxially with the cathodes K,, 40 KG and KB in the first grids G,,, GIG and G,,, respectively. Electron beam transmission apertures h2R, h2, and hM, and electron beam transmission apertures h31R, h31G and h31B are formed in the second grid G2 and in the front end plate 31 of the third grid G3 coaxially with the correspond ing electron beam transmission apertures h,R, h1G and h,,, respectively. The electron beam transmission apertures of the first grids G,,, G,,, G,,, the second grid G2 and the front end plate 45 31 of the third grid G3 are formed so that the common centre axis 0, of the electron beam transmission apertures h1R, h2, and h31R for the side electron beam BR, and the common centre axis 0, of the electron bearn transmission apertures h1B, h2,, and h31B for the side electron beam B, are each inclined at a predetermined angle 0 to the common centre axis 0, of the electron transmission, apertures h1G, h2G and h31G for the centre electron beam B,, and intersect each other 50 at a predetermined point on the centre axis PG.

Electron beam transmission apertures h3R, h3, and hW, and corresponding electron beam transmission apertures h4R, h,,-and h-,,, for transmitting the respective electron beams BR, 13G and B, are formed in the rear end plate 32 of the third grid G. and in the front end plate 41 of the fourth grid G4, respectively. 55 Individual front electron lenses LensIR, Lens,, and Lens,, for the respective electron beams BR, 13G and B, of a main electron lens are formed between the corresponding electron beam transmission apertures of the third grid G, and.the fourth grid G,. The centre axis of the electron beam transmission apertures h3G and h4G for the electron beam BG is aligned with the centre axis 0,; the respective centre axes 0,' and 0,' of the electron beam transmission apertures h3, and 60 h,, for the side electron beam B,, and the electron beam transmission apertures h3B and h4B for the side electron beam B, are parallel to the centre axis 0, and are symmetrical with respect to the centre axis OG as shown in Fig. 2. That is, the centre axes of the cathode prefocusing lenses for the side electron beams B. and B, are inclined at the predetermined angle 0 to the centre axis of the cathode pref. ocusing lens for the centre electron beam B,, and the axes of the 65 7 GB2193595A 7 front lenses Lens1G and Lens,, of the main electron lens are parallel to each other.

Cylindrically shaped walls Ws are formed around the electron beam transmission apertures h,,, h3G and h,,, formed in the end plate 32 of the third grid G3, and the electron beam transmission apertures h4R, h4G and h4B formed in the end plate 41 of the fourth grid G4, respectively, to isolate the lenses LerisjR, Lens1G and Lens,, from each other. The cylindrically shaped walls Ws 5 can be formed in the end plates 32 and 41, for example by forming the electron beam transmission apertures h3R, h3G, h3B h4R h4G and h4B in the end plates 32 and 41 which are each formed of a thick plate so that the thickness of the thick plate corresponds to the height of the cylindrical shaped walls. Ws, as shown in Fig. 2, If it is difficult to form the electron beam transmission apertures in the thick end plates 32 and 41 with high accuracy, the end plates 32 10 and 41 each may be a laminated plate formed by laminating a plurality of thin plates previously provided with holes that match each other, for example by using a laser laminating process.

It is also possible to form the cylindrical circumferential walls Ws by pressing the electron beam transmission apertures h3R, h3, and h,,,, and the electron beam transmission apertures h,,, h,, and h4B in the end plates 32 and 41, which are in that case formed of a comparatively thin 15 plate, as shown in Fig. 3.

In the electron gun arrangement constructed in this manner, since the respective optical axes of the lenses Lens1R, Lens1G and Lens1,3 are parallel to each other, the side electron beams BR and B, travel at a predetermined angle to the respective optical axes of the corresponding lenses Lens,, and Lens113. However, since the lenses Lens,, and Lens,, are formed at positions meeting 20 Fraunhofer conditions, namely at positions where the coma aberration of the lenses is zero, the blooming of spots, for example, on the fluorescent screen of a colour cathode-ray tube due to the aberration of the electron beams B, and BB 'S prevented. Fig. 4 is a graph showing the measured results of the variation of spot size with cathode current lk in the electron gun arrangement according to the construction shown in Fig. 1 and 2, and Fig. 5 is a graph similar 25 to that of Fig. 4 for the previously-proposed electron gun arrangement of the construction shown in. Fig. 19. It is apparent from comparing Figs. 4 and 5 that the present electron gun arrangement having lenses Lens1R, Lens1G and Lens1B arranged with their optical axes parallel to each other is superior to the previously-proposed electron gun arrangement relative to spot size variation', because the present electron gun arrangement meets Fraunhofer conditions. 30 Fig. 21 shows a second embodiment of an electron gun arrangement. Since the second embodiment is substantially the same as the first embodiment in its electrode arrangement, only those portions which are different from the first embodiment will be described in detail.

In the second embodiment shown in Fig. 21, a high anode voltage V,, for example 27 kV, is applied to the fourth grid G4 and the sixth grid' G,. The anode voltage is applied to the fourth 35 grid G, and -the sixth grid G. through a lead pin electrically interconnecting the fourth grid G, and the sixth grid G, which is connected to an internal conductive film formed over the inner surface of the cathode-ray tube and which is connected to an anode button (not shown) provided in the funnel of the cathode-ray tube. Voltages V, and VF+d are applied individually to the third grid G3 and the fifth grid G, through leads 1 and 2 connected to terminal pins (not shown)-which 40 penetrate a stem (not shown) provided at the rear end of the cathode-ray tube. The prefocusing voltage VF applied to the third grid Q3 is a fixed voltage equal to, for example, 25 to 30% of the i voltage V,, for example 8 M A dynamic focusing voltage of a parabolic waveform varying, for example, in'the range of 0 to +60OV, in synchronism with the horizontal and vertical deflection of the electron bearns, is applied to the fifth grid G, in addition to the prefocusing voltage VF, 45 Voltages applied to the first grids G, (G1R, G1G, Gj and the second grid G2 are, for example, in the order of 0 to several tens of volts and in the order of 600 to 70OV, respectively.

The cathodes K (kR, Kg, KJ, the-first grids G, (G,R, G,G, GJ, the second grid G2 and the third grid G3 constitute cathode prefocusing lenses, respectively, for the electron beams BR, 8G and BB.

The third grid G3 and the fourth grid G, constitute the front electron lenses Lens,,, Lens,, and 50 Lens,, each having a small aperture of a main electron lens, while the fourth grid G,,'the fifth grid G. and the sixth grid G.6 constitute an after electron lens Lens 2 of a unipotentia[ type having a large aperture of the main-electron lens. That is, the main electron lens comprises the front electron lenses LensIR, Lensle, Lens,, and the after electron lens Lens 2. A fixed focusing voltage is applied to the prefocusing electron lens of the main electron lens, that is the third grid 55 G3, while the dynamic focusing voltage d for correcting for variations of the distance between scanning positions on the fluorescent screen and the main electron lens is applied, in addition to the fixed prefocusing voltage, to the focusing electrode of the after electron lens, namely the fifth grid G,.

Although the second embodiment of the present invention comprises an electron _gun arrange- 60 ment comprising a main - electron lens having front electron lenses each having an aperture smaller than that of the after electron lens, of the main electron lens, the present invention is not limited thereto in its application, and it is also-applicable to various electron gun arrangements in which some of a plurality of electrodes of a main electron lens to which a focusing voltage is applied are associated with a cathode prefocusing electron lens. 65 8 GB2193595A 8 Fig. 22 shows an example of such an electron gun arrangement, which comprises three cathodes K,, KG and K,, and first to sixth grids G, to G, which are arranged sequentially and which are common to all the cathodes. The cathodes KR, KG and KB, and the first to third grids G, to G, form a cathode prefocusing electron lens, the third to fifth grids G, to G, form unipotential electron lenses LenSAR, LenSAG and LenSAB, and the fifth and sixth grids G,, and G. 5 form bipotential electron lenses Lens,,, Lens,, and Lens,,. The unipotential electron lenses and the bipotential electron lenses constitute a main electron lens. Ordinarily, a fixe low voltage VG2 is applied to the second grid G, and the fourth grid G4, and a focusing voltage is applied to the third grid G, and the fifth. grid G,. According to this embodiment of the present invention, different voitages are applied to the third grid G, and the fifth grid G,. That is, a fixed focusing 10 voltage V, is applied to the third grid G3, and a dynamic focusing voltage is applied to the fifth grid G3 in addition to the focusing voltage V,.

The first embodiment shown in Fig. 1 shows the present invention embodied in an electron gun arrangement of a unipotenfial type corresponding to the electron gun arrangement shown in Fig. 18. The present invention is also applicable to electron gun arrangements of various types 15 such as, for example, an electron gun arrangement of a bipotential type such as shown in Fig.

17 and an electron gun arrangement in which some of the electrodes of the main electron grids to which a focusing voltage is applied function also as the grids of a cathode prefocusing electron lens.

Fig. 27 shows a third embodiment of the present invention. In Fig. 27, those parts similar to 20 those previously described with reference to Fig. 1 are marked with the same reference charac ters and description thereof is not repeated. In the third embodiment, a centre cathode K, is positioned at a distance greater than the distance between the after electron lens Lens 2 of a main electron lens and cathodes K, and K, from the after electron lens Lens 2, namely, the centre cathode KG is placed further away from the after electron lens Lens 2 relative to the 25 cathodes K, and K,, and a first grid G,, corresponding to the cathode KG is Moved away so that the respective distances between the cathodes K,, KG and K, and the corresponding first grids G1R, G,G and G,, are substantially the same.

In particular, portions of the end plate of the second grid G2 which are provided with electron beam transmitting apertures h,,, h1G and h,, are formed in different flat planes, respectively, so 30 that the portion are positioned parallel to and substantially at the same distance from the corresponding first grids G,,, G,, and G,,, respectively. So as to form the second grid G, in such a shape, the -central portion of the second grid G, which- is provided with the electron beam transmitting apertures h2G is extended towards the first grid G1G by pressing to form a cylindrical portion-SG2 having a diameter greater than that of the first grid G,, and a bottom plane 35 extending parallel to the first grid G1G. The central portion of the front end plate 31 of the third grid G3 which is provided with the electron beam transmitting aperture h31G is extended towards the second grid G2 by pressing to form a cylindrical portion SG, having a diameter smaller than that of the cylindrical portion SG,. The third grid G, is disposed so that the bottom wall of the cylindrical portion S,, extends parallel to the bottom-portion %, of the second grid G.. 40 Thus, the respective distances of the first grids G, from the corresponding cathodes K are the same, the respective distances of the second grids G2 from the corresponding first grids G, are the same, and the respective distances of the third grids G, from the corresponding second grids G2 are the same, so that all the electron beams are subjected to the same effect from the cathode prefocusing lens and the equipotential surface between the second grid G2G and the 45 third grid G3G for the centre electron beam B, is formed in a flat plane and is not distorted into a curved surface as shown in Fig. 7 so as to exert an undesirable lens effect on the centre electron beam B,. 1 Since the central portion of the front end plate 31 of the third grid G3 extends backwardly to form the cylindrical portion SG3, the distance between the cylindrical portion SG3 and the rear end 50 plate 32 is greater than those between the portions of the front end plate 31 for the side electron beams and the rear end plate 32. However, no irregular electric field which would exert undesirable lens action on the electron beams iis formed, because the end plates 31 and 32 are integral parts of the,third grid G,.

The after electron lens of unipotential type of the main electron lens employed, in the foregoing 55 embodiments 'may be substituted by an electron lens of an extended field unipotential type.

As will'be apparent from the foregoing description, according to embodiments of the present invention, the main electron lenses and an after electron lens which are formed separately, and the front electron lenses are each formed with an aperture smaller than that of the after electron lens to reduce aberration. The front electron lenses are formed with the respective optical axes 60 thereof parallel to each other without causing an increase in aberration. Therefore, the electron gun arrangement can be easily -manufactured, the axes of the electron lenses can be accurately maintained during machining, and the electron gun arrangement Can be manufactured precisely in conformity with design conditions.

Furthermore, the centre cathode is extended backwardly relative to the other cathodes so as 65 9 GB2193595A 9 to subject all of the electron beams to the same effect of the focusing voltage and, particularly, a low voltage is applied to the third grid of the electron gun arrangement so that the grids forming the cathode prefocusing electron lens can be mounted close to each other, whereby all of the electron beams are subjected to the same effect of the electric field, and thus undesirable lens effects are avoided. 5 Also, as mentioned above with reference to the embodiments of the present invention, a fixed focusing voltage is applied to the electrodes of the main electron lens, also serving as the components of the prefocusing electron lens, while a dynamic focusing voltage for correcting the variation of the focus attributable to the variation of the distance between the main electron lens and a scanning position on the fluorescent screen is. applied, in addition to the fixed focusing 10 voltage, to the focusing electrodes associated only with the main electron lens. As a result of this, brightness irregularity on the fluroescent screen of the cathode- ray tube attributable to the effect of the dynamic focusing voltage on the cathode focusing electron lens is prevented.

Figs. 23 to 26 are graphs showing the experimental results of cathode current variations with the focusing voltage VF when the same focusing voltage V, is applied to the third grid G, and 15 the fifth grid G, of the electron gun arrangement shown in Fig. 21. In the experiments, the voltage applied to the second grid G, was regulated so that the cathode cutoff voltage E KCO was + 1 OOV when the focusing voltage V, was 7.7 kV, which caused the electron beams to be focused precisely at the centre of the fluoescent screen, the cathode voltage was adjusted to make the cathode current lk equal 50, 100, 20 and 400 MA, and the focusing voltage was 20 varied in the range of 6.7 to 8.7 W. It is apparent from Figs. 23 to 26 that the variation of the cathode current lk is dependent on the focusing voltage and, as best shown in Fig. 23, the smaller the cathode current lk, the greater is the variation of the cathode current lk.

Claims

CLAIMS 25

1. An electron gun arrangement comprising:

a plurality of cathodes; and a main electron lens having front electron lenses each provided in the path of an electron beam emitted from each cathode and-each forming a front electron lens region, and a common back electron lens forming -a common electron lens region separate from said front electron lens 30 regions, and provided in the respective paths of the electron beams emitted from said cathodes; wherein:

the aperture of each of said front electron lenses respectively forming said front electron lens regions is smaller than the aperture of said back electron lens forming said back electron lens region; 35- the respective centre axes of the electron beam transmission apertures of electrodes forming said front electron lens regions on the paths of the respective electron beams are parallel to each other; and each front electron lens forming said front electron lens region is formed so as to meet Fraunhofer conditions. 40

2. An electron gun arrangement according to claim 1, wherein:

some of the electrodes forming a cathode prefocusing electron lens serve aisq as some of the electrodes forming the front electron lenses of said main electron lens, means being provided for applying a focusing volage to the electrodes forming said main electron lens and to those electrodes serving as the electrodes of said cathode prefocusing electron lens and also to the 45 front electron lenses of said main electron lens; the electrodes of said cathode prefocusing electron lens which serve also as the electrodes of said main electron lens are electrically isolated, from the rest of the electrodes of said main electron lens to which the focusing voltage is applied; and means for applying a fixed focusing voltage to the electrodes of said cathode prefocusing 50 electron lens is provided, serving also as the electrodes of said main electron lens, and a dynamic focusing voltage is applied thereto in addition to said fixed focusing voltage, and to the rest of the electrodes of said main electron lens.

3. An electron gun arrangement according to claim 1 or claim 2, wherein:

the distance of the centre cathode from the plurality of cathodes from said back electron lens 55 is greater than the distance of the side cathodes from the plurality of cathodes from said back electron lens; and portions of one of the grids of the cathode prefocusing electron lens, disposed adjacent to said cathodes, are formed in respective flat planes, and portions of the grid among those forming said cathode prefocusing electron lens, disposed adjacent and corresponding to the 60 respective portions of the former grid, are formed in respective flat planes which extend parallel to the respective portions of the former grid.

4. An electron gun arrangement according to claim 1, claim 2 or claim 3, wherein a grid adjacent one of said cathodes is formed with a centre planar portion having a beam opening, and two outer planar portions having beam openings which form obtuse angles with said centre 65 GB2193595A 10 planar portion.

5. An electron gun arrangement according to claim 1, claim 2 or claim 3, wherein a grid adjacent one of said cathodes is formed with a Centre planar portion having a beam opening, and two outer planar portions having beam openings which are offset in the beam direction from said Centre planar portion. 5

6. An electron gun arrangement according to claim 5, wherein said two outer portions each make an obtuse angle with said Centre portion.

7. An electron gun arrangement substantially as hereinbefore described with reference to any of Figs. 1 to 4 and 21 to 27 of the accompanying drawings.

Published 1988 at The Patent Office, State House, 66/71 High Holborn, London WC 1 R 4TP. Further copies may be obtained from The Patent Office, Sales Branch, St Mary Cray, Orpington, Kent BR5 3RD. Printed by Burgess & Son (Abingdon) Ltd. Con, 1/87.