Cathode ray tube having a curved display window and colour display device

Abstract: A cathode ray tube comprising an electron gun, a display screen on an inner surface of an at least substantially rectangular curved display window and a colour selection electrode arranged in front of the display screen, characterized in that the inner surface exhibits a deviation from an arc shape along the long axis such that, in operation, the effect of doming is reduced. Preferably, the deviation decreases according as the distance to the long axis increases. In an embodiment, the inner surface also exhibits a deviation from an arc shape along lines in the y-direction.

Description: The invention relates to a cathode ray tube comprising an electron gun, a display screen provided on an inner surface of an at least substantially rectangular curved display window having a long (x) and short (y) curved axis and a colour selection electrode arranged in front of the display screen, the shape of the colour selection electrode corresponding approximately to the shape of inner surface. The invention also relates to a colour display device comprising a cathode ray tube. Such cathode ray tubes are known. In operation, the electrons of an electron beam emitted by the electron gun impinge on the colour selection electrode, thereby heating it. It is noted that approximately 80% of all electrons are captured by the colour selection electrode. The heating of the colour selection electrode causes said electrode to expand. As a result thereof the apertures in the colour selection electrode are displaced relative to the display screen. This phenomenon is called "doming". One type of doming is the so-called local doming. Local doming occurs as a result of differences in the intensity of the image displayed. As a result thereof, certain parts of the colour selection electrode are heated more than others, thereby causing the colour selection electrode to bulge locally, which brings about colour errors. One of the objects of the invention is to provide a colour display device in which a measure to reduce the effect of doming, in particular of local doming, of the colour selection electrode is applied. The cathode ray tube according to the invention is characterized in that the z-coordinate of points on the long axis of the inner surface of the display window, z being the distance between a plane tangential to the display screen through the centre of the display screen and a plane parallel thereto, through points of the long axis, is approximately represented by: z=A₁ - (A₁²-x²) 1/2 + f(x) where x is the x-coordinate of the points on the long axis, x being the distance between the centre of the display window and the points on the long axis, where f(x) is nil for x=0 and for the end of the long axis, f(x) is negative at least substantially everywhere between these points, is decreasing from nil for x=0 to a minimum in the range between 0.5L and 0.9L where L is the x-coordinate for the end of the long axis and then increasing to nil for x=L, and where for the value f(x) for the minimum it holds that: 0.0005L<| f(x) | The inner surface of the display window exhibits a deviation from an arc shape along the long axis, which deviation reduces the effect of doming, in particular local doming, of the colour selection electrode. It is noted that the shape of the colour selection electrode approximately corresponds to the shape of the inner surface. By superposing an outwardly directed deviation f(x), hereinafter also referred to as "bulge", on the arc shape of the long axis, represented by the function A₁-(A₁²-x²) 1/2 , the radius of cuvature in the x-direction of the inner surface and the radius of curvature in the x-direction of the colour selection electrode, whose shape is adapted to the inner surface, decrease along the long axis according as the value of x increases. As a result thereof, the effect of local doming is reduced. Preferably, f(x) has an extremum for 0,65 L < x < 0.80 L. In a further embodiment the z-coordinate of points on lines parallel to the long axis of the inner surface of the display window, z being the distance between a plane tangential to the display screen through the centre of the display screen and a plane parallel thereto, through points of lines parallel to the long axis, is approximately represented by: z=z₀+ A₁′ - (A₁′²-x²) 1/2 + f′(x) where z₀ is a constant for the given line, x is the x-coordinate of the points on the said line, where f′(x) is nil for x=0 and x=L, f′ (x) is negative at least substantially everywhere between these points, is decreasing from nil for x=0 to a minimum in the range between 0.5L and 0.9L and then increasing to nil for x=L, and with the value of the minimum decreasing (in absolute value) as the value of y increases and where the value of the minimum of f′(x) at the edges at both ends of the short axis of the substantially rectangular display screen is less than 1/5 th of the value of the minimum of f′(x) on the long (x)-axis. In the above-mentioned embodiment, along lines perpendicular to the short axis and parallel to the long axis, the deviation (= the "bulge") is a function of the distance to the long axis. The deviation from an arc shape in the inner surface varies over the inner surface. As a result thereof, a further reduction of the effect of doming is possible. The deviation decreases in a direction transversely to the long axis. In yet another embodiment, viewed from the long axis, the deviation, i.e. the value of the extremum of f(x), at the extreme edges is less than 1/5 th of the deviation on the long axis. Preferably, the deviation at the extreme edges is approximately nil. In embodiments, the maximum deviation on the long axis is less than 2% of the length of the long axis. By virtue of the bulge the effect of local doming in the x-direction is reduced. However, still other image errors may occur, inter alia , the so-called raster errors. Disturbing raster errors occur when the maximum deviation- is more than 2% of the length of the long axis. The maximum deviation on the long axis is more than 0.05% of the length of the long axis. In the case of deviations smaller than 0.05%. the positive effect on local doming is small. In a further embodiment the z-coordinate of points on lines parallel to the short (y) axis of the inner surface of the display window, z being the distance between a plane tangential to the display screen through the centre of the display screen and a plane parallel thereto, through points of lines parallel to the short axis, is approximately represented by: z=z₀′ + A₁″ - (A₁″²-y²) 1/2 + f″(y) where z₀′ is a constant for the given line, y is the y-coordinate of the points on the said line, where f″(y) is nil for y=0 and for the end of the short axis (y=L₁), f″(y) is negative at least substantially everywhere between these points, is decreasing from nil for y=0 to a minimum in the range between 0.5L₁ and 0.9L₁ where L¹ is the y-coordinate for the end of the short axis and then increasing to nil to y=L₁, and the value of the minimum is dependent on the x-coordinate of the said line and increases (in absolute value) according as the value of x increases. Preferably, the maximum value of the extremum of f″(y) is smaller than 2% of the length of the short axis. A larger maximum value may lead to disturbing raster errors. The invention is of great importance to cathode ray tubes having a curvature of the display window along the short axis which is larger, i.e, the radius of curvature R y is smaller, than the curvature along the long axis. In an embodiment, the ratio between the radius of curvature along the long axis R x and the radius of curvature along the short axis R y (R y : R x ) is less than 3 : 4. The invention is of great importance to cathode ray tubes in which the ratio between the lengths of the short axis and the long axis is less than 3 : 4. In an example the said ratio is approximately 9 : 16. By way of example, a few embodiments of the cathode ray tube and the colour display device according to the invention will be described and explained in more detail with reference to the accompanying drawing, in which: Fig. 1 is a sectional view of a colour display device according to the invention; Fig. 2 is a partly perspective top view of a part of the inner surface of a display window suitable for a cathode ray tube according to the invention; Fig. 3 is a graphic representation of the distance Z for the long axis and for a number of lines located at a distance from the long axis; Fig. 4a shows the deviations from an arc shape for the lines shown in Fig. 3; Figs. 4b and 4c are perspective elevational views of two examples of "bulges" in the inner surface of the display window; Fig. 4d is a graphic representation of the radius of curvature in the x-direction R x along the long axis; Fig. 5 is a sectional view of details of a cathode ray tube, by means of which several aspects of local doming are explained; Figs. 6a and 6b give a few values of beam displacements caused by local doming; Figs. 6c and 6d give a few values of beam displacements caused by overall doming; Fig. 7 shows the distance in the z-direction between the centre of the inner surface of the display window and points on the inner surface of the display window along lines parallel to the short axis or y-axis; Fig. 8 shows the deviations from an arc shape for the lines shown in Fig. 7; Figs. 9a and 9b are illustrations of the effect of the deviations from a perfect arc shape shown in Figs. 7 and 8. The Figures are diagrammatic representations and are not drawn to scale, corresponding parts in the various embodiments generally bearing the same reference numerals. Fig. 1 is a sectional view of a colour display device according to the invention. Said colour display device comprises a cathode ray tube 1 having an envelope with a substantially rectangular curved display window 2. Said envelope further comprises a cone 3 and a neck 4. A pattern of phosphors 5 luminescing in the colours blue, red and green is provided on the display window 2. A substantially rectangular colour selection electrode 6 having a large number of apertures is suspended at a short distance from the display window 2 by means of suspension means 7 near the corners of the colour selection electrode. An electron gun 8 for generating three electron beams 9, 10 and 11 is arranged in the neck 4 of the cathode ray tube 1. Said beams are deflected by a deflection system 12 and intersect each other substantially at the location of the colour selection electrode 6, after which each electron beam impinges on one of the three phosphors provided on the screen. Fig. 2 is a partly perspective top view of a part, in this Figure a quarter, of the inner surface of a display window suitable for use in a cathode ray tube according to the invention. Point A denotes the centre of the inner surface of the display window. The long axis is referred to as the x-axis, the short axis is referred to as the y-axis, for simplicity, the ends of the x-axis and the y-axis have been given values for x and y, respectively, of 1. In fact, the length of the long axis is, for example, 332 mm and the length of the short axis is, for example, 188 mm, which corresponds to a length-width ratio of approximately 16 : 9. Point B is the corner of the inner surface of the display window. The direction perpendicular to the x-axis and the y-axis is the z-direction. Fig. 3 shows the z-value for four lines. The x-value is plotted on the horizontal axis, the z-value in mm is plotted on the vertical axis. Line A₁ is the intersecting line of the inner surface of the display window with the plane y = 0. Line A₂ is the intersecting line of the inner surface with the plane y = 0.3. Line A₃ is the intersecting line of the inner surface with the plane y = 0.7. Finally, line A₄ is the intersecting line of the inner surface with the plane y = 1.0. In this case, z is defined as having a positive value. When z is plotted as a function of x, there is a deviation from an arc-shaped relation between z and x. An arc-shaped relation is to be understood to mean that z can be expressed by z = z₀ + A₁′ - (A₁′²-x²) ½ . Fig. 4a shows the deviation f′(x) from an arc shape for the lines A₁ up to and including A₄ through the beginning and the end of said lines. In this Figure, the line f′(x) = 0 corresponds to perfectly arc-shaped lines (spherical sections) through the beginning and the end of the lines A₁ up to and including A₄. The deviation f′(x) (in mm) of the lines A₁ up to and including A₄ from the arc shape is plotted on the vertical axis. This deviation is negative, i.e. viewed from the cathode ray tube the deviation is outwardly directed. The deviation is nil for x = 0 and x = 1. This can be attributed to the fact that the arc-shaped lines are selected such that they pass through the beginning and the end of the lines A i . The deviations exhibit an extremum for x approximately equal to 0.7. The value of the extremum decreases according as the lines are further removed from the x-axis, i.e. according as the value of y increases. Along the x-axis (the long axis) z is represented by z = A₁ - (A₁²-x²) ½ + f(x), A₁ and f(x) being of opposite sign; for the entire system of lines A₁ up to and including A₄, z as a function of x can be expressed by z = z₀ + A₁′ - (A₁′²-x²) ½ + f′(x), where z₀, A₁′ and f′(x) may be, and in this example are, dependent on y. Figs. 4b and 4c are examples of two "bulges" in the inner surface of the display window. The analytical shapes of the superposed "bulges", i.e. f′(x), are shown in Figs. 4b and 4c. Fig. 4d is a graphic representation of the radius of curvature in the x-direction (R x ) along the longitudinal axis as a function of the x-value. Line 41 shows a perfect arc shape, i.e. a constant R x ; line 42 shows R x for a colour display device according to the invention. The colour selection electrode expands as a result of the heating of the electrode by exposing it to electron beams. This brings about a landing displacement Æ, as shown in Fig. 5. Fig. 5 is a sectional view of a detail of a colour display tube. This Figure illustrates the effect of the local heating of the colour selection electrode 6, which effect is termed "local doming". In the "cold state", the electron beam 10 is incident on the display screen 5 on the inside of the display window 2 at point 13. A local heating of the colour selection electrode 6, which may occur, for example, when the image displayed exhibits large differences in intensity, i.e. dark and light areas, causes the colour selection electrode to bulge locally, as shown by bulge 6a in Fig. 5. As a result thereof, the apertures through which the electron beam 10 passes are displaced relative to the display screen 5. The electron beam 10 then impinges on the display screen 5 at point 14. The distance between the points 13 and 14 is the beam displacement Δ. Figs. 6a, 6b give the local doming values for a few positions on the display screen of a 86 FS colour display tube having a length : width ratio of 16 : 9, the values being measured for a known colour display device (Fig. 6b) and for a colour display device according to the invention (Fig. 6a). The colour selection electrode was manufactured from a iron-nickel alloy having a low coefficient of thermal expansion. In these tests, areas measuring 10 cm by 10 cm were exposed to an electron beam having a power of 33 Watts. A marked reduction, namely with 10 to 20%, in beam displacements caused by local doming is obtained. Figs. 6c and 6d give the overall doming for the same tubes. "Overall doming" is the effect which occurs when the colour selection electrode heats up integrally. Fig. 6d gives the landing displacement as a result of overall doming for a known display device and Fig. 6c for a display device according to the invention. Overall doming also has been reduced by a few percent. Also in the case of colour display devices comprising an iron colour selection electrode it appears that when beam displacements caused by local doming are measured the effect of local doming is reduced in the colour display device according to the invention. A measurement carried out on a known colour display device yielded a beam displacement of 150 µm was measured, while for the colour display device according to the invention a displacement of 120 µm was obtained. Fig. 7 shows the distance in the z-direction between the centre of the inner surface of the display window and points on the inner surface of the display window along lines which extend parallel to the short axis or y-axis. Fig. 7 shows the z-value for five lines. The y-value is plotted on the horizontal axis, the z-value in mm is plotted on the vertical axis. Line B₁ is the intersecting line of the inner surface with the plane x = 0. Line B₂ is the intersecting line of the inner surface with the plane x = 0.3. Line B₃ is the intersecting line of the inner surface with the plane x = 0.6. Line B₄ is the intersecting line of the inner surface with the plane x = 0.8. Finally, line B₅ is the intersecting line of the inner surface with the plane x = 1.0. In this case, z has been defined as a positive value. When z is plotted as a function of y, there is a marked deviation from an arc-shaped relation between z and y. An arc-shaped relation is to be understood to mean that z can be expressed by z = z₀ + A₁″ - (A₁″²-x²) ½ . In this example, the radius of curvature in the y-direction is approximately 900 mm and, hence, smaller than the radius of curvature R x along the long axis which is approximately 1400 mm (see Fig. 4D). Fig. 8 shows the deviation f″(y) from an arc shape through the beginning and the end of the lines for the lines B₁ up to and including B₅. In this Figure, the line z = 0 corresponds to perfectly arc-shaped lines (spherical sections) through the beginning and the end of the lines B₁ up to and including B₅. The deviation f″(y) from the arc shape of the lines B₁ up to and including B₅ (in mm) is plotted on the vertical axis. Said deviation is negative, i.e. viewed from the cathode ray tube the deviation is directed outwards. Said deviation is nil for y = 0 and y = 1. This is caused by the fact that the arc shapes are selected such that they pass through the beginning and the end of the lines B i . The deviations exhibit an extremum for a value of y approximately equal to 0.7. The value of the extremum increases according as the lines are further removed from the y-axis. Along the y-axis (the short axis) z is expressed by z = A₁″ - (A₁″²-y²) ½ ; for the entire system of lines B₁ up to and including B₅, z as a function of y can be expressed by z = z₀′ + A₁″ - (A₁″²-y²) ½ + f″(y) , where z₀′, A₁″ and f″(y) may be, and in this example are, dependent on x, and where the absolute value of f″(y) increases according as the x-value increases. Figs. 9a and 9b show the effect of the deviations from perfect spherical lines in the y-direction shown in Figs. 7 and 8. Fig. 9a shows, in the form of lines with equal landing displacements, the effect of local doming as a function of x and y for a colour display device the inner surface of the display window of which has a "bulge" on the long axis, the height of said bulge decreasing according as y increases and the inner surface along lines in the y-direction extending as perfectly spherical lines; Fig. 9b shows, in the form of lines of equal landing displacement, the effect of local doming in a colour display device in which also the inner surface of the display window exhibits a deviation from a perfect sphere along lines in the y-direction, as shown in Figs. 7 and 8. In both Figures, standardized beam displacements in the x-direction are shown, the beam displacement at the point x = 2/3, y = 0 of Fig. 9b being set at 100. The effect of local doming exhibits a marked decrease by providing a "bulge" in the y-direction, the reduction increasing according as the x-value increases. For x = 0.7 and y = 0.9, the landing displacement caused by local doming is approximately 30% higher in Fig. 9a than in Fig. 9b. Further, it is noted that in Fig. 9b lines of equal landing displacements extend approximately parallel to the y-axis, whereas lines of equal landing displacement in Fig. 9a clearly describe a curved path. In particular for an in-line colour display device, i.e. a colour display device having an in-line electron gun, it is advantageous when lines of equal landing displacement extend approximately parallel to the y-axis, i.e. parallel to the axis transverse to the in-line plane. In an in-line colour display device the width of the phosphor lines is approximately constant for a line which extends parallel to the y-axis, and the spot width, i.e. the width of the electron spot is approximately constant. consequently, a line extending parallel to the y-axis has an approximately constant spatial guard band which is determined by the difference between the above-mentioned width dimensions. Preferably, lines of equal landing displacement extend in the same manner as lines having an equal spatial guard band, i.e. parallel to the y-axis, as shown in Fig. 9b. As has been noted, the colour selection electrode has a shape which is adapted to that of the screen. It will be obvious that within the scope of the claims many variations are possible to those skilled in the art.

Claims: A cathode ray tube (1) comprising an electron gun (8), a display screen (5) provided on an inner surface of an at least substantially rectangular curved display window (2) having a long (x) and short (y) curved axis and a colour selection electrode (6) arranged in front of the display screen, the shape of the colour selection electrode corresponding approximately to the shape of inner surface, characterized in that the z-coordinate of points on the long axis of the inner surface of the display window, z being the distance between a plane tangential to the display screen through the centre of the display screen and a plane parallel thereto, through points of the long axis, is approximately represented by: z=A₁ - (A₁²-x²) 1/2 + f(x) where x is the x-coordinate of the points on the long axis, x being the distance between the centre of the display window and the points on the long axis, where f(x) is nil for x=0 and for the end of the long axis, f(x) is negative at least substantially everywhere between these points, is decreasing from nil for x=0 to a minimum in the range between 0.5L and 0.9L where L is the x-coordinate for the end of the long axis and then increasing to nil for x=L, and where for the value f(x) for the minimum it holds that: 0.0005L < | f(x) | A cathode ray tube as claimed in claim 1, characterized in that the minimum lies in the range between 0.65L and 0.8L. A cathode ray tube as claimed in claim 1 or 2, characterized in that the z-coordinate of points on lines parallel to the long axis of the inner surface of the display window, z being the distance between a plane tangential to the display screen through the centre of the display screen and a plane parallel thereto, through points of lines parallel to the long axis, is approximately represented by: z=z₀+ A₁′ - (A₁′²-x²) 1/2 + f′(x) where z₀ is a constant for the given line, x is the x-coordinate of the points on the said line, where f′(x) is nil for x=0 and x=L, f′(x) is negative at least substantially everywhere between these points, is decreasing from nil for x=0 to a minimum in the range between 0.5L and 0.9L and then increasing to nil for x=L, and with the value of the minimum decreasing (in absolute value) as the value of y increases and where the value of the minimum of f′(x) at the edges at both ends of the short axis of the substantially rectangular display screen is less than 1/5 th of the value of the minimum of f′(x) on the long (x)-axis. A cathode ray tube as claimed in one of the claims 1, 2 or 3, characterized in that for the value f(x) for the minimum on the long (x)-axis it holds that: | f(x) |<0.02L A cathode ray tube as claimed in any one of the preceding claims characterized in that the z-coordinate of points on lines parallel to the short (y) axis of the inner surface of the display window, z being the distance between a plane tangential to the display screen through the centre of the display screen and a plane parallel thereto, through points of lines parallel to the short axis, is approximately represented by: z=z₀′ + A₁″ - (A₁″²-y²) 1/2 + f″(y) where z₀′ is a constant for the given line, y is the y-coordinate of the points on the said line, where f″(y) is nil for y=0 and for the end of the short axis (y=L₁), f″(y) is negative at least substantially everywhere between these points, is decreasing from nil for y=0 to a minimum in the range between 0.5L₁ and 0.9L₁ where L₁ is the y-coordinate for the end of the short axis and then increasing to nil to y=L₁, and the value of the minimum is dependent on the x-coordinate of the said line and increases (in absolute value) according as the value of x increases. A cathode ray tube as claimed in claim 5 characterized in that for the maximum value of f″(y) (in absolute value) it holds: | f″(y) |<0.02L₁. A cathode ray tube as claimed in any one of the preceding claims, characterized in that the radius of curvature of the inner surface of the display window is smaller along the short (y)-axis than along the long (x)-axis. A cathode ray tube as claimed in any of the preceding claims characterized in that the ratio between the lengths of the short (y)-axis and the long (x)-axis is less than 3:4. A colour display device comprising a cathode ray tube as claimed in one of the preceding claims.

Inventors: CROOYMANS MARCUS THEODORUS MAR, PENNINGA JOHANNES, VAN NES JOHANNES CORNELIS ADRI, CROOYMANS, MARCUS THEODORUS MARIA, PENNINGA, JOHANNES, VAN NES, JOHANNES CORNELIS ADRIANUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS ELECTRONICS N.V.
Application year: 1991
Main classification: H01J29
IPC codes: H01J29, H01J31