COLOUR DISPLAY TUBE INCLUDING A COLOUR SELECTION ELECTRODE WITH EDGE

Abstract: A colour display tube (1) comprises an envelope having a picture display window (2), an electron gun (9) for generating electron beams (10,11,12), a deflection system (13) for deflecting electron beams along electron beam paths, a colour selection electrode (7) having a large plurality of apertures, the colour selection electrode having an edge (7b) having an interior portion which extends towards the display window and an exterior portion extending away from the display window, and suspension means (8) for suspending the colour selection electrode in the display window envelope, characterized in that the edge is of such a shape that substantially every point of the edge of the colour selection electrode is an element of an electron beam path.

Description: The invention relates to a colour display tube comprising an envelope having a display window, an electron gun for generating electron beams, a deflection system for deflecting electron beams along electron beam paths, a colour selection-electrode having a large plurality of apertures, the colour selection electrode having an edge including an interior portion which extends towards the display window and an exterior portion extending away from the display window and suspension means for suspending the colour selection-electrode in the envelope. Such a colour display tube is disclosed in United States Patent 4,551,651. Said patent describes a colour display tube comprising an envelope including a substantially rectangular display widow having an upright edge and a substantially rectangular colour selection-electrode which is suspended in the corners of the display window with the aid of suspension means which each include a flat spring element which is connected to the colour selection-electrode and is perpendicular to the electron beams which are deflected towards the relevant corner. The prior art colour-electrode has an edge comprising a ridge extending towards the display window and a collar extending away from the display window, the centre of a cross-section through the ridge with collar being locted substantially in the plane of the shadow mask. A phenomenon occuring in a colour display tube of the type defined in the opening paragraph, important for the picture quality, is what is commonly referred to as the "overall doming" of the colour selection electrode. During usage, in response to impingement of the colour selection electrode by electrons, the curvature of the colour selection electrode changes. This bulging of the electrode results in the landing spot of the electron beam on the picture screen to be displaced, a so-called landing displacement. The significance of landing displacements will be described in further detail in the description of the Figure. In an embodiment of the prior art display tube the collar is at such an angle to the longitudinal axis of the picture tube that electrons directly reflected by the collar fall on the colour selection electrode outside the pattern of apertures in the colour electron electrode, whereby the overall surface area of the colour selection electrode is impinged by electrons, which reduces distortion of the colour selection electrode and consequently overall doming effects due to thermal effects. Experiments have however shown that in this prior art construction overall doming does still negatively affect the picture quality. It is therefore an object of the invention to reduce overall doming and thus to improve the picture quality of a colour display tube. To that end, a colour display tube of the type defined in the opening paragraph, is characterized in that the edge is of such a shape that substantially every point of the edge of the colour selection electrode constitutes an element of an electron beam path, so that during operation of the cathode ray tube the electron beams impinge on the entire edge. As has been found during experiments, this reduces overall doming of the colour selection electrode. A preferred embodiment of the colour display tube according to the invention, is characterized in that for substantially every point of the edge the angle between the plane tangent to the edge in said point and the electron beam path of which said point is an element, exceeds 5 degrees. If for a portion of the edge said angle is less than 5° a significant portion of the electrons impinging on said portion is not absorbed but reflected and said portion is irradiated in a less efficient manner. A preferred embodiment of the colour display tube according to the invention, is characterized in that the average mass per unit of surface of the colour selection electrode over substantially the entire colour selection electrode is substantially uniform. The colour selection electrode has a perforated portion and an unperforated edge. In the prior art display tube the average mass per unit of surface exceeds the average mass per unit of surface of the perforated portion. Consequently, the perforated portion warms-up at a higher rate than the edge. An substantially uniform average mass per unit of surface further reduces temperature differences in the colour selection electrode. The edge may, for example, be provided with pits or be thinner than the apertured portion of the colour selection electrode. Preferably, the edge is provided with pits and is at least substantially equally thin as the apertured portion of the colour selection electrode. The mechanical strength of the colour selection electrode is than at least substantially uniform, which reduces mechanical strain. An embodiment of the colour display tube according to the invention in which the colour selection electrode is coated with a layer having a high electron reflection coefficient, is characterized in that this layer is substantially uniformly applied across the colour selection-electrode inclusive of the edge of the colour selection electrode. Such a layer provided on the colour selection electrode increases the electron reflection of the colour selection electrode, which results in a reduction of the temperature of this colour selection electrode. A uniform distribution of this layer over the overall colour selection electrode, inclusive of the edge of the colour selection electrode, result in a higher degree of uniformity of the temperature over the colour selection electrode, inclusive of the edge of the colour selection electrode, which reduces temperature differences across the colour selection electrode and, consequently, overall doming. A further embodiment of the colour display tube according to the invention, in which the display window is provided with a layer having a high thermal absorption coefficient, is characterized in that this layer is applied substantially uniformly over the display window inclusive of the edge of the display window. Such a layer provided on the display window has for its object to absorb the heat given off by the colour selection electrode. In the colour display tube according to the invention also the edge of the colour selection electrode radiates heat. A uniform distribution of above said layer over the entire display screen, inclusive of the edges, that is to say inclusive of those portions of the display window to which the edge of the colour selection electrode radiates heat predominantly, reduces temperature differences over the colour selection. A still further embodiment of the colour display tube according to the invention is characterized in that the edge of the colour selection electrode is substantially entirely provided between the display window and the plane of the colour selection electrode. Experiments have proved that in the prior art construction a portion of the electrons reflected by the collar still pass through the apertures of the colour selection electrode and impinge on the display screen, which detrimentally affects the picture quality. In a colour display tube according to the invention the reflected electrons do not pass through the apertures of the colour selection electrode. Some embodiments of the invention will now be described in greater detail with reference to the accompanying drawings. Herein Fig. 1 is a schematic, cross-sectional view of a colour display tube according to the invention; Fig. 2a is a perspective schematic drawing of a colour selection electrode known from the prior art; Fig. 2b is a sectional view taken on a portion of the line AA′ of Fig. 2a; Fig. 3a is a perspective schematic view of a colour selection electrode suitable for a colour display tube of the invention; Fig. 3b is a sectional view taken on a portion of the line BB′ of Fig. 3a; Fig. 4 is a sectional view taken on a portion of the line AA′ of Fig. 2a; Fig. 5 shows a sectional view of a detail of a display tube, which illustrates the landing displacement caused by a uniform heating of the colour selection electrode; Fig. 6 shows a sectional view of a detail of a display tube, which illustrates the landing displacement caused by a non-uniform heating of the colour selection electrode; Fig. 7a shows in the form of a graph the landing displacement along a line OCW on the display screen; Fig. 7b is a view of a display window, illustrating the line OCW; Fig. 8a shows in the form of a graph the landing displacement in two points A and B as a function of the time; Fig. 8b is a view of a display window in which the points A and B are denoted; Fig. 9 shows a sectional view of a colour selection electrode suitable for a colour display tube according to the invention. The Figures are shown schematically, not to scale, corresponding components in the different embodiments usually having been given the same reference numerals. Fig. 1 is a sectional view of a colour display tube according to the invention. The colour display tube includes an envelope 1 which, in this example, comprises a substantially rectangular display window 2 which has an upright edge 3. In addition, the colour display tube has a cone 4 and a neck 5. A pattern of phosphors 6 luminescing in the colour red, green and blue is provided on the display window 2. At a short distance from the display window 2 a substantially rectangular colour selection electrode 7 having a large number of apertures and provided with an edge 7b is suspended with the aid of suspension means 8 near the corners of said upright edge 3. An electron gun 9 to generate three electron beams 10, 11 and 12 is mounted in the neck 5 of the colour display tube. These beams are deflected by means of a system of coils 13 and intersect each other substantially in the region of the colour selection electrode 7, whereafter each of the electron beams impinges on one of the three phosphors provided on the window. Each suspension means 8 includes a first element attached to the colour selection electrode, this element in this example being a plate-shaped spring element and being arranged transversely of the electron beams 10, 11 and 12 deflected towards the relevant corner and a second element, this element in this example being provided near the corner of the upright edge 3 of the display window 2. Fig. 2a is a perspective view of a colour selection electrode of the prior art. In colour selection electrode 14 is constituted by a thin metal plate whose central portion 15 is provided with a large number of apertures 16. The colour selection electrode 14 is bulged in accordance with the shape of the display window as is shown in Fig. 2a in a plan view. The edge 17 of the colour selection electrode 14 is provided with ridges 18 extending towards the display window and collars 19 extending away from the display window. In this prior art embodiment the collars are folded outwardly and enclose an angle of 25.5 degrees with the longitudinal axis of the tube. Fig. 2b is a cross-sectional view along a portion of the line AA′ of Fig. 2a. In this example the ridge 18 has a height H of 5 mm, a width B of 3.5 mm and the collar 19 has a length L of 8.6 mm. Fig. 2b also shows electron beams 20a and 20c which are deflected towards the edge 17 of the colour selection electrode 14. A portion of the edge 17, the portion 21, is not directly irradiated by the electron beam 20 as will be obvious from the Figure. This portion is irradiated indirectly by reflected electrons. In Fig. 2b, electron beam 20d obtained by reflection from electron beam 20c irradiates this portion. However, it has been found that in spite of this indirect radiation, significant thermal distortions nevertheless occur. Fig. 3a is a perspective view of a colour selection electrode suitable for a colour display tube of the invention. The colour selection electrode 22 has an edge 23 which includes an interior portion 24 extending towards the display window, and a collar 25. Fig. 3b is a cross-sectional view taken on a portion of the line BB′ of Fig. 3b. The portions 24 and 25 are of such a shape that they are directly irradiated by the electron beams 26a and 26b which are deflected to the edge 23. This causes a uniform supply of heat by the electron beam(s) to the colour selection electrode, which reduces the thermal distortion of the colour selection electrode. The angle α between the interior portion 24 and the electron beam 26a deflected towards this interior portion and the angle α between the exterior portion 25 and the electron beam 26 deflected towards this exterior portion are both much greater than 5° in this example. All portions of the edge are then directly irradiated, with an appropriate efficiency. In this example, neither of the portions 24 and 25 are beneath the plane of the colour selection electrode which plane is indicated in the Figure by broken line 27. The reason therefor is as follows. Electrons directly reflected by the collar 19 are reflected perpendicularly to the longitudinal axis of the tube along beam 20b and cannot pass through apertures 16 to impinge on the display window (Fig. 2b). It has however been found that for a construction of a prior art colour selection electrode of the type shown in Fig. 2, a portion of the reflected electrons still pass through the apertures in the colour selection electrode. The most probable explanation for this phenomenon is that a portion of the electrons are not directly reflected by the colour selection electrode, but, after having impinged on the colour selection electrode, remain absorbed for a certain period of time at the surface of the colour selection electrode whereafter (re-)emission occurs. For such a process it does not hold for all the electrons that the angle of incidence of the electrons is equal to the angle of emission, but each point of the surface of the colour emits as a punctilinear source. Fig. 4 which shows the result of this effect, is substantially identical to Fig. 2b. Electron beam 20a impinges on the collar 19 in point A; the directly reflected electron beam 20b skims along the colour selection electrode and does not pass into the apertures 16 in this colour selection electrode. Due to the above-described effect, electron emission occurs however also in other directions, these directions are denoted in Fig. 4 by means of broken lines. Electrons emitted in accordance with line 20e do indeed enter the apertures 16 and, after having been reflected once again, impinge on the display screen. This results in a haze being produced at the edge of the display screen. In order to prevent electrons from passing through the apertures, the entire edge is consequently contained in the invention between the display window and the plane of the colour selection electrode. Due to warm-up of the colour selection electrode by irradiation of this electrode by the electrode beams, the colour selection electrode expands. This produces a landing displacement as is shown in Fig. 5. Fig. 5 is a detail of the display tube of Fig. 1 and illustrates the effect of uniform colour selection electrode expansion. In the "cold" state electron beams 27 and 28 passing through the respective apertures 29 and 30 of the colour selection electrode 7 impinge on the screen 2 in the points 31 and 32 respectively. The colour selection electrode expands when warmed-up, which causes the positions of the points 29 and 30 to move to the points 29′ and 30′, which results in a displacement of the points 31 and 32 to 31′ and 32′ respectively. The distance between the points 31 and 31′ and 32 and 32′ will be denoted the landing displacement. Taking in a direction from the centre of the display window points 31 and 32 move outwardly. For clarity, in the drawing the position of the colour selection electrode in the warm state is shifted relative to the colour selection electrode in the cold state. Generally, the landing displacement due to uniform warm-up of the colour selection electrode is approximately linear, that is to say the landing displacement is approximately directly proportional to the distance of a point to the centre of the window. The linear expansion of the shadow mask can be compensated for by suspending the colour selection electrode such that when heated it moves towards the display window, more specifically in such a manner that the landing displacement is compensated. An example of such a suspension is described in the above-cited Unites States Patent 4,551,651 in which, due to the spring action of the flat spring-loaded elements at this position of the flat springloaded elements in the colour selection electrode moves towards the display window when the temperature increases. Actually, the temperature across the colour selection electrode is not uniform. This has for its effect that an additional bulging of the shadow mask occurs. Fig. 6 shows the effect of this bulging on the landing displacement. In this Figure, colour selection electrode 7 is represented in the "cold" state by a solid line, after warm-up by a dotted line. In the cold state the electron beams 35 and 36 impinge on the display window 2 in the respective points 37 and 38. Due to the additional bulging of the colour selection electrode these points shift to the respective points 37′ and 38′, Fig. 7a shows in the form of a graph an example of a landing displacement resulting therefrom along the line OCW across the display window shown in Fig. 7b. The landing displacement Δ is shown in µm. The landing displacements are negative as they are directed towards the centre of the display window. These landing displacements do not only depend on the position on the window but also on time. In Fig. 7a lines 39 and 40 represent the landing displacement at instants t and t′ respectively. This landing displacement cannot or only partly be compensated for by the suspension means. The total landing displacement resulting after the compensation co-determines the picture quality. A landing displacement results in the electron beams not or not appropriately impinging on the phosphors which negatively affects the picture quality. It will be obvious that the landing displacement in Fig. 7a as a result of the additional bulging of the colour selection electrode must be minimized as much as possible. This is the object of the invention. The effect of the invention is illustrated in Fig. 8a which shows, in the form of a graph, landing displacements 4 as a function of time in two points on the display window. the curve 41 represents the landing displacement for the prior art construction in the point B on the display window, curve 42 represents the landing displacements for a picture tube according to the invention in the same point. Fig. 8b is a view of a display window and shows this point B and the centre C. It will be obvious that a reduction in the landing displacement occurs. A further embodiment of the picture display tube according to the invention, in which the colour selection electrode is provided with a layer having a high electron reflection coefficient, is characterized in that this layer is uniformly applied across the colour selection electrode, inclusive of the edge. Such a layer provided on the colour selection electrode increases the electron reflection of the colour selection electrode, which results in a decrease in the temperature of this colour selection electrode. The use of layers of this type is known per se, they may consist of, for example, bismuth oxide or a lead borate glass. A uniform distribution of this layer across the overall colour selection electrode inclusive of the edge reduces temperature differences across the colour selection electrode, and, consequently, overall doming. Curve 42 in Fig. 8a represents the landing displacement of a display tube according to the invention, in which a bismuth oxide is not uniformly applied, that is to say it is only applied on that portion of the colour selection electrode that is provided with apertures. Curve 43 in Fig. 8a represents the landing displacement for a picture tube according to the invention, in which a bismuth oxide layer is uniformly applied. It will be obvious that this results in a reduced landing displacement. A further embodiment of the colour display tube according to the invention in which the display window is provide with a layer having a high thermal absorption coefficient, is characterized in that this layer is applied uniformly over the display window, inclusive of the display window edge. Such a layer applied on the display window has for its object to absorb the heat radiated by the colour selection electrode. The use of such a layer is known per se and the layer consists, for example, of a coating containing a mixture of soot and graphite. In the colour display tube according to the invention also the edge of the colour selection electrode gives off heat. A uniform distribution of the above-mentioned layer over the entire display window inclusive of the edge, that is to say including those portions of the display window to which the edge of the colour selection electrode radiates heat predominantly, reduces temperature differences across the colour selection electrode. Fig. 9 shows a sectional view of a colour selection electrode for an embodiment of a colour display tube according to the invention. The colour selection electrode 22 includes a perforated portion with apertures 16 and an edge 23. The edge 23 has a height a of 0.5 cm and a width b of 0.5 cm. The edge 23 is further provided with pits 44. The volume of the pits 44 per unit of surface is approximately equal to the volume of the apertures 16 per unit of surface and the thickness of the colour selection electrode is uniform, so that the average mass per unit of surface across at least substantially the entire colour selection electrode 22 is at least substantially uniform. The supply heat being approximately equal per unit of surface, the edge and the perforated portion warm-up approximately equally fast, so that temperature differences across the colour selection electrode decrease. The colour selection electrode is framed in an edge 45 and is provided with a layer 46 which has a high electron reflection coefficient. It will be obvious that many variations are possible for a person skilled in the art, within the scope of the claims.

Claims: 1. A colour dispaly tube (1) comprising an envelope having a display window (2), and electron gun (9) for generating electron beams (10, 11, 12), a deflection system (13) for deflecting electron beams along electron beam paths, a colour selection electrode (7) having a large plurality of apertures, the colour selection electrode having an edge (7b) including an interior portion which extends towards the display window and an exterior portion extending away from the display window, and suspension means (8) for suspending the colour selection electrode in the envelope, characterized in that the edge (7b) is of such a shape that substantially every point of the edge of the colour selection electrode constitutes an element of an electron beam path, so that during operation of the cathode ray tube the electron beams impinge on the entire edge. 2. A colour display tube as claimed in Claim 1, characterized in that for substantially every point of the edge of the colour selection electrode the angle between the plane tangent to the edge in said point and the electron beam path of which said point is an element exceeds 5°. 3. A colour display tube as claimed in Claim 1 or 2, characterized in that the average mass per unit of surface of the colour selection electrode across substantially the entire colour selection electrode is substantially uniform. 4. A colour display tube as claimed in Claim 3, characterized in that the edge is provided with pits (44). 5. A colour display tube as claimed in Claim 3, characterized in that the edge is thinner than the apertured portion of the colour selection electrode. 6. A colour display tube as claimed in any one of the preceding Claims, in which the colour selection electrode is provided with a layer (46) having a high electron reflection coefficient, characterized in that this layer is uniformly applied over the colour selection electrode inclusive of the edge of the colour selection electrode. 7. A colour display tube as claimed in any one of the preceding Claims, in which the display window is provided with a layer having a high thermal absorption coefficient, characterized in that this layer is applied substantially uniformly over the display window, inclusive of the edge of the display window. 8. A colour display tube as claimed in any one of the preceding Claims, characterized in that the edge is located substantially entirely between the display window and the plane of the colour selection electrode.

Inventors: HENS, THEODOOR CHRISTIAAN ANNA
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1988
Main classification: H01J29
IPC codes: H01J29