COLOUR TELEVISION DISPLAY TUBE WITH COMA CORRECTION

Abstract: Colour television display tube comprising an electron gun system (5) of the "in-line" type and an electromagnetic deflection unit. The extremity of the electron gun system is provided with field shapers comprising, for example, annular elements (34,34′) of a material having a high magnetic per­ meability which are positioned around the two outer beams and are adapted to compensate coma. These elements are located in a more advanced position towards the screen than is usual in positions where the outer beams have undergone a pre-deflection of at least 0.5 mm so as to reduce the "green droop" (= anisotropic Y-coma) and to reduce the need of a negative field 6-pole. It is, for example, a characteristic feature that the elements (34,34′) are present "above" the bottom of the centring bush (28), or the fact that the distance between focusing gap of the electron gun and elements (34,34′) is more than 10 mm, or the fact that the axial position of the elements (34,34′) coincides with the axial beginning of the turns on the line deflection coil, or with respect to the axial beginning is located closer to the display screen.

Description: The invention relates to a colour television display tube comprising an electron gun system of the "in-line" type in an evacuated envelope for generating three electron beams whose axes are in a plane and which converge on a display screen provided on a wall of the envelope and are deflected in the operative display tube across said display screen in two mutually perpendicular directions by means of a deflection unit comprising deflection coils producing a first and a second deflection field, the direction of the first deflection field being parallel to the said plane, said electron gun system comprising correction elements of a magnetically permeable material positioned around the two outer beams at the extremity of the gun facing the display screen. A colour television display tube of this type is known from United States Patent US-A- 4,196,370 and from European Patent application EP-A 0 170 319. A frequent problem in colour television display tubes incorporating an electron gun system of the "in-line" type is what is commonly referred to as the line and field coma error. This error becomes manifest in that the dimensions of the rasters scanned by the three electron beams on the display screen are different. This is due to the excentric location of the outer electron beams relative to the fields for horizontal and vertical deflection, respectively. The Patent Specification cited above sums up a large number of Patents giving partial solutions. These solutions consist of the use of field shapers. These are magnetic field conducting and/or protective annular and plate shaped elements mounted on the extremity of the gun and locally strengthening or weakening the deflection field or the deflection fields along part of the electron beam paths. In colour television display tubes various types of deflection units may be used for the deflection of the electron beams. These deflection units in tubes having an "in-line" electron gun system are mostly self convergent. One of the frequently used deflection unit types is what is commonly referred to as the hybrid deflection unit. It comprises a saddle line deflection coil and a toroidal field deflection coil. Due to the winding technique used for manufacturing the field deflection coil it is not possible to make the coil completely self convergent. Usually such a winding distribution is chosen that a certain convergence error remains, which is referred to as coma. This coma error becomes manifest, for example, in a larger raster (horizontal and vertical) for the outer beams relative to the central beam. The horizontal and vertical deflection of the central beam is smaller than that of the outer beams. As has been described, inter alia, in the United States Patent US-A-4,196,370 and the European Patent application EP-A 0 170 319 cited above, this may be corrected by providing elements of a material having a high permeability (for example, mu-metal) around the outer beams. The peripheral field is slightly shielded by these elements at the area of the outer electron beams so that these beams are slightly less deflected and the coma error is reduced. Two problems then present themselves. The first problem is that the shielding of the outer electron beams also results in these beams being deflected to a lesser extent at the area where the field astigmatism is corrected in the field deflection coil. Since the (barrel-shaped = negative 6-pole) vertical deflection field can only perform an astigmatism correction by the grace of pre-deflection, the astigmatism correction of the field deflection coil becomes less. This can be corrected by positioning the electron gun as a whole further away from the screen and hence away from the coil, but this results in a display tube with a greater build-in depth. Another solution may be to provide an extra barrel-shaped component in the deflection field of the field deflection coil, but this causes the need for coma correction to be increased again. A second problem which presents itself is that the correction of the field coma (Y-coma) is anisotropic. In other words, the correction in the corners is less than the correction at the end of the vertical axis. This is caused by the positive "lensing" action of the line deflection coil (approximately quadratic with the line deflection) for vertical beam displacements. (The field deflection coil has a corresponding lensing action, but it does not contribute to the relevant anisotropic effect). The elimination of such an anisotropic Y-coma error by adapting the winding distribution of the coils is a complicated matter and often introduces an anisotropic X-coma. It is an object of the invention to provide a colour television display tube in which it is possible to correct the field coma errors on the vertical axis and in the corners to a more equal extent and in which the coma correction per mm of corrected field coma has a reduced influence on the field astigmatism without requiring the winding distribution of the coils to be notably adapted. In an embodiment of the invention a colour display tube of the type described in the opening paragraph is characterized in that the correction elements are placed in positions in which the outer beams have undergone a predeflection of at least 1 mm in a deflection direction perpendicular to the said plane when in operation the electron beams are deflected in said direction to an extremity of the display screen. In practice very good results were obtained when the correction were placed in positions in which the beams had undergone a 1 to 2 mm pre-deflection in the vertical direction. As a result the (field shielding) elements are closer to or even in the magnetic deflection field. In an embodiment of the invention a colour display tube of the type described in the opening paragraph is characterized in that the axial position of the correction elements is not further away from the display screen than the axial position of the gun-sided extremity of the deflection coil for the second deflection coil, at which positions the outer beams have undergone a substantial predeflection in a direction perpendicular to the said plane when in operation the electron beams are deflected in said deflection direction to an extremity of the display screen. The invention is based on the recognition that the field-astigmatic effect of the negative 6-pole component in the vertical deflection field only operates by the grace of pre-deflection at the area of this 6-pole and that it is less affected as the correction elements are closer to the screen. The invention is also based on the recognition that the problem of the anisotropic Y-coma can be reduced by suitabley utilising the Z dependence of the anisotropic Y-coma. This dependence implies that as the coma correction is effected at a larger distance (in the Z direction) from the "lens" constituted by the line deflection coil its "lensing" action becomes more effective so that the coma correction acquires a stronger anisotropic character. In order to place the correction element in a position which is closer to the display screen than is usual, the (conventional) electron gun system can be positioned closer to the display screen. An alternative presented by the invention is to elongate the electron gun system towards the display screen and this in such a manner that the distance between the correction elements and the focusing gap is increased. In conventional systems this distance is less than 10 mm. An embodiment of the invention is characterized in that the correction elements are located at a distance of at least 10 mm and preferably still further away from the focusing gap of the electron gun system. Within the scope of the invention the electron gun system can be elongated in different manners. A practical manner is characterized in that the side of the electron gun system facing the display screen is provided with a centring bush having a bottom remote from the display screen and a lid facing said bottom, each having apertures for passing the electron beams, and in that the correction elements are mounted on the lid. If the correction elements give rise to an overcompensation of the field coma, the invention provides a further correction possibility. It is characterized in that a further correction element is placed around the position of the central beam, which further correction element is located at a greater distance from the display screen than the correction elements around the outer beams. The display tube according to the invention is very suitable for use in combination with a deflection unit of the hybrid type, particularly when a combination is concerned which should be free from raster correction. It is remarked that EP-A-0 160 970 discloses a colour television display tube in which between an electron gun system and a deflection yoke correction elements are positioned some distance apart from the gun sided-end of the deflection yoke. The invention will now be further described with reference to a drawing in which Figure 1 is a longitudinal section through a display tube according to the invention; Figure 2 is a perspective elevational view of an electron gun system for a tube as shown in Figure 1; Figure 3a shows the beam path on deflection towards a vertical axis extremity in a conventional display system; Figure 3b shows the beam path upon deflection towards a screen corner in a conventional display system; Figure 4a shows the beam path upon deflection towards a vertical axis extremity in a display system according to the invention; Figure 4b shows the beam path upon deflection towards a screen corner in a display system according to the invention; Figure 5a is a longitudinal section through part of a conventional electron gun; Figures 5b, 5c, 5d show three examples of embodiments of electron guns for a colour television display tube according to the invention; Figures 6a, 6b show two modifications of coma correction elements which may be used within the scope of the invention. Figure 1 shows in a longitudinal section a display tube according to the invention. It is a colour television display tube of the "in-line" type. In a glass envelope 1, which is composed of a display window 2, a cone 3 and a neck 4, this neck accommodates an integrated electron gun system 5 generating three electron beams 6, 7, and 8 whose axes are co-planar prior to deflection. The axis of the central electron beam 7 coincides with the tube axis 9. The inside of the display window 2 is provided with a large number of phosphor element triplets. The elements may consist of lines or dots. Each triplet comprises an element consisting of a blue-luminescing phosphor, an element consisting of a green-luminescing phosphor and an element consisting of a red-luminescing phosphor. All triplets combined constitute the display screen 10. The phosphor lines are substantially perpendicular to the said plane through the beam axes. Positioned in front of the display screen is a shadow mask 11 having a very large number of elongated apertures 12 which allow the electron beams 6, 7 and 8 to pass, each beam inpinging only on phosphor elements of one colour. The three co-planar electron beams are deflected by a system of deflection coils 13 comprising a line deflection coil 14, a yoke ring 15 and a field deflection coil 16. Figure 2 is a perspective elevational view of an embodiment of an electron gun system as used in the colour television diplay tube of Figure 1. The electron gun system has a common cup shaped control electrode 20 in which three cathodes (not visible in the Figure) are secured, and a common plate shaped first anode 21. The three electron beams whose axes are co-planar are focused with the aid of the second anode 22 and third anode 23 which are common for the three electron beams. Anode 22 consists of three cup shaped parts 24, 25 and 26. The open ends of parts 25 and 26 are connected together. Part 25 is coaxially positioned relative to part 24. Anode 23 has one cup shaped part 27 whose bottom, likewise as the bottoms of the other cup shaped parts, is apertured. Anode 23 also includes a centring bush 28 used for centring the electron gun system in the neck of the tube. This centring bush is provided for that purpose with centring springs not shown. The electrodes of the electron gun system are connected together in a conventional manner with the aid of brackets 29 and glass rods 30. The bottom of the centring bush 28 has three apertures 31, 32 and 33. A mirrored centring bush 42 having a lid with three apertures 43, 44, 45 faces centring bush 28. Substantially annular correction elements 34, 34' are provided around the apertures 43 and 45 for the outer electron beams. The centring bushes 28, 42 are, for example, 6.5 mm deep and have an external diameter of 22.1 mm and an internal diameter of 21.6 mm in a tube having a neck diameter of 29.1 mm. The distance between the centres of two adjoining apertures in the bottom of centring bush 28 is 6.5 mm. Figure 3a is a side elevation of the three beams upon deflection towards a vertical axis extremity in a conventional display system in which the correction elements are placed on the bottom of centring bush 28. At the rear in the tube red and blue are deflected to a lesser extent than green, so that the beams coincide again on the screen. Figure 3b is a side elevation upon deflection towards a screen corner. Due to the increasing focusing effect of lens L as a result of the line deflection and the distance between G on the one hand and R and B on the other hand, the green beam on the screen is less far deflected in the corner than are the red and blue beams. This effect with respect to the vertical axis situation is referred to as the "green droop". Figure 4 shows analogously to Figure 3a the side elevation of the three beams upon deflection towards a vertical axis extremity in the case of the display system of Figures 1 and 2. As compared with the conventional system the correction elements are placed 13 mm to the front. Now again the red and blue beams are deflected to a lesser extent than green, but this in an axial position which is closer to the display screen. The total discrimination as is visible on the screen is equal to that of the original case (Figure 3a), but the discrimination is less at the area of lens L and also at the area of the negative 6-pole component of the field deflection field (sometimes generated by means of a soft magnetic "astigmatism correction"member). As can be seen in Figure 4a the red and blue beams at the area of the coil (= approximately the position of lens L) is more deflected than in the conventional situation (Figure 3a). This extra deflection is of great importance because the field astigmatic effect of a field 6-pole is proportional to the deflection of the beams at the area of this 6-pole. A greater deflection means that there is less vertical 6-pole field required to procedure a similar astigmatism effect. Figure 4b shows analogously to Figure 3b the side elevation upon deflection towards the corner of the display screen in the case of the display system of Figures 1 and 2. Since at the area of lens L the vertical distance between the green beam on the one hand and the red and blue beams on the other hand has become less with respect to the original situation (Figure 3b), the green droop effect is also reduced. This means that the difference in Y-coma between screen corners and vertical axis has become less. Figure 5a shows the plan view of a conventional coma-correction system. The coma-correcting elements 34, 34' are positioned on the bottom of the centring bush 28. In Figure 5b centring bush 28 has a lid 29 on which the coma-correction elements 34, 34' are placed. The dimensions of the elements 34 are to be adapted so as to obtain, measured on the screen, an approximately equal coma-correction level as in the case of Figure 5a. In Figure 5c an inverted cup 42 on which the coma-correcting elements 34, 34' are placed is positioned on the centring bush 28. Also in this case the size of the (annular) elements 34, 34' is adapted to obtain the desired coma-correction level. Figure 5d shows a third modification in which the component 27' is elongated and which is no longer equal to component 26. Components 26 and 27 are usually identical in order to cause main lenses which are formed by the gap between 26 and 27 to be symmetrical. The component 27 is considered to be elongated within the scope of coma correction improvement when the distance between centring bush and the focusing gap formed between components 26 and 27 and 26 and 27', respectively, is more than 10 mm. In conventional systems this distance is always less than 10 mm. A common value is approximately 8 mm. In this respect it is to be noted that the gun length is more or less equal for all conventional types of guns, provided that they are operated at the same high voltage both in the case of mini neck tubes and narrow neck tubes. Figures 6a and 6b show that the elements 34, 34' do not necessarily require a purely annular shape. The shapes as shown in 6a and 6b are intended to be able to correct line coma effects.

Claims: A colour television display tube comprising an electron gun system (5) in an evacuated envelope (1) for generating three electron beams (6, 7, 8) whose axes are in a plane and which converge on a display screen (10) provided on a wall of the envelope and are deflected in the operative display tube across said display screen in two mutually perpendicular directions by means of a deflection unit (13) comprising deflection coils (14, 16) producing a first and a second deflection field, the direction of the first deflection field being parallel to the said plane, said electron gun system comprising correction elements (34, 34') of a magnetically permeable material positioned around the two outer beams (6, 8) at the extremity of the gun facing the display screen, characterized in that the correction elements are placed in positions in which the outer beams having undergone a predeflection of at least 1 mm in a deflection direction perpendicular to the said plane when in operation the electron beams are deflected in said deflection direction to an extremity of the display screen. A colour television display tube as claimed in Claim 1, characterized in that the axial position of the correction elements is not further away from the display screen than the axial position of the gun-sided extremity of the deflection coil (14) for the second deflection field. A colour television display tube as claimed in Claim 1 or 2, characterized in that the correction elements are located at a distance of at least 10 mm from the focusing gap of the electron gun system. A colour television display tube as claimed in Claim 3, characterized in that the side of the electron gun system facing the display screen is provided with a centring bush (28) having a bottom remote from the display screen and a lid (29) facing said bottom, each having apertures for passing the electron beams, and in that the correction elements are mounted on the lid. A colour television display tube comprising an electron gun system (5) in an evacuate envelope (1) for generating three electron beams (6, 7, 8) whose axes are co-planar and which converge on a display screen (10) provided on a wall of the envelope and are deflected in the operative display tube across said display screen in two mutually perpendicular directions by means of a deflection unit (13) comprising deflection coils (14, 16) producing a first and a second deflection field, the direction of the first deflection field being parallel to the said plane, said electron gun system comprising correction elements (34, 34') of a magnetically permeable material position around the two outer beams (6, 8) at the extremity facing the display screen, characterized in that the axial position of the correction elements is not further away from the display screen than the axial position of the gun-sided extremity of the deflection coil (14) for the second deflection field, at which positions the outer beams have undergone a substantial predeflection in a deflection direction perpendicular to the said plane when in operation the electron beams are deflected in said deflection direction to an extremity of the display screen. A colour television display tube as claimed in Claim 1 or 5, characterized in that a further correction element (38) is placed around the position of the central beam (7), which further correction element is located at a greater distance from the display screen than the correction elements around the outer beams.

Inventors: SLUYTERMAN, ALBERTUS AEMILIUS SEYNO C/O INT.OCTR.
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1986
Main classification: H01J29
IPC codes: H01J29