The invention relates to a color display tube comprising a display window, a color selection electrode having a shadow mask and a frame, the display window is provided with supporting elements and the frame with suspension elements for coupling the color selection electrode to the supporting elements, the shadow mask is having an apertl£ed central area, a skirt and a circumferential blind edge located between the apertured central area and the skirt.

The invention further relates to a color selection electrode for use in such a color display tube, and to a shadow mask of said color selection electrode.

A color display tube as described in the opening paragraph is disclosed in the United States Patent US 3, 809, 945. In this patent specification a shadow mask is disclosed in which the region between the apertured central area and the peripheral portion is provided with a transition structure. This structure has a yield strength which is intermediate between the yield strength of the apertured central area and that of the peripheral portion. This intermediate yield strength is realized by providing said region with partially etched blind holes.

However, the color display tube of the prior art has the disadvantage that the shadow mask is rather sensitive to microphony and drop test, due to the abrupt changes of the material properties at the edges of the shadow mask. These abrupt changes lead to a large difference in yield strength and this makes the mask drawing process more difficult. As a consequence the apertures at the edges of the mask may be deformed and this will deteriorate the front of screen performance of the color display tube, and in particular the color purity of the picture being displayed on the screen of the color display tube.

It is an object of the invention to provide a color display tube having a color selection electrode with an improved shadow mask over the type described in the opening

paragraph, which strongly diminishes the registration errors on the display window, so improving the color purity of the color display tube.

According to the present invention, this object is realized with a color display tube which is characterized in that the amount of material per unit area in the circumferential blind edge increases from the apertured central area in the direction of the skirt.

The invention is based on the insight that the shadow mask will be less susceptible for microphony and drop test when the abrupt changes in material properties can be prevented, which can be realized by slowly and continuously increasing the amount of material between the apertured central area and the skirt.

First of all, a definition for the amount of material per unit area. In a blank sheet of mask material-that is before etching the holes in it-the amount of material per unit area equals the physical density of the material used. After the etching process of the shadow mask the apertured central area will contain a large number of apertures and this will lower the amount of material per unit area proportional to the amount of material that has been removed by the etching process. So, for example, say that the shadow mask has obtained a transmission of about 20%, by etching 20% of the shadow mask material, the amount of material per unit area is then only 80% of the blank sheet. This amount of material per unit area can also be expressed as an effective density or an effective thickness of the shadow mask.

So, when the amount of material in the circumferential blind edge of the shadow mask gradually increases between the apertured central area and the skirt, then the abrupt changes in material properties will be strongly diminished or will even be absent. This will result in a shadow mask having a yield strength which varies smoothly between the apertured central area and the skirt.

A preferred embodiment is characterized in that, the amount of material per unit area in the circumferential blind edge at the side of the apertured central area equals the amount of material per unit area of said apertured central area.

This embodiment has the advantage that in the transition between the apertured central area and the circumferential blind edge the amount of material per unit area shows a continuous behavior. In this case the change in material properties will be absent.

In a further preferred embodiment the amount of material per unit area of the circumferential blind edge at the side of the skirt equals the amount of material per unit area of said skirt.

This embodiment renders a continuous transition of material properties between the circumferential blind edge and the skirt.

A further embodiment is characterized in that the circumferential blind edge is provided with a pattern of blind holes.

In the manufacturing process of present day shadow masks, the pattern of apertures is realized by an photo-chemical etching process. Normally the apertures are etched from both sides of the shadow mask. Blind holes-that are holes which are not etched so far to become an aperture, but that remain a pit-can amongst others be realized by one-sided etching as is disclosed in US 3, 809, 945.

However for the present invention it is required that the amount of material per unit area varies smoothly between the apertured central area and the skirt. This can be realized by continuously varying the amount of etched material. This can be done by either varying the number of blind holes or by changing the size of the blind holes.

These last two embodiments are characterized in that the number of blind holes decreases from the apertured central area in the direction of the skirt, or in that the depth of the blind holes decreases from the apertured central area in the direction of the skirt, respectively.

In a still further embodiment, the circumferential blind edge is provided with a circumferential groove of which the depth decreases from the apertured central area in the direction of the skirt.

As an example for an alternative solution, the circumferential blind edge can also be provided with a groove parallel to the circumference and with a varying depth in such a way that the amount of material per unit area increases from the apertured central area to the skirt.

The invention further relates to a color selection electrode for use in such a color display tube, and to a shadow mask of said color selection electrode.

These and other aspects of the invention are apparent from and will be elucidated by way of non-limitative examples with reference to the drawings and the embodiments described hereinafter.

In the drawings: Fig. 1 is a sectional view of a color display tube according to the invention; Fig. 2 is a schematic view of a color selection electrode;

Figs. 3A and 3B are cross sections of a prior art shadow mask; Figs. 4A, 4B, 4C and 4D are cross sections of a shadow mask according to the invention.

The color display tube 1 shown in Fig. 1 comprises an evacuated glass envelope 2 with a display window 3, a funnel shaped part 4 and a neck 5. On the inner side of the display window 3 a screen 6 having a pattern of for example lines or dots of phosphors luminescing in different colors (e. g. red, green and blue) may be arranged. the phosphor pattern is excited by the three electron beams 7,8 and 9 that are generated by the electron gun 10. On their way to the screen the electron beams 7, 8 and 9 are deflected by the deflection unit 11 ensuring that the electron beams 7,8 and 9 systematically scan the screen 6. Before the electrons hit the screen 6 they pass through a color selection electrode 12. This color selection electrode 12 comprises a shadow mask 13, which is the real color selective part: it intersects the electron beams so that the electrons only hit the phosphor of the appropriate color. The shadow mask 13 may be an apertured mask having circular or elongate apertures, or a wire mask. Further, the color selection electrode 12 comprises a frame 14 for supporting the shadow mask 13.

In this example, the color selection'electrode 12 is suspended from the display window 3 by using supporting elements 17, which are secured in the upright edge of the corner areas 18 of the display window 3.

Fig. 2 gives a schematic view of the color selection electrode 12, comprising a shadow mask 13 and a frame 14. In this example the frame is provided with corner sections 16 for suspending the color selection electrode 12 from the display window 3. It is emphasized that this suspension system merely serves as an example; the invention is not limited to a color selection electrode 12 with corner suspension. The invention is also applicable to other suspension systems, like for instance three or four point axial systems and also to frames which are formed as a ring or frames which are assembled from different frame parts like corner sections 16 and diaphragm parts interconnecting the corner sections 16.

Also in Fig. 2, the different regions of the shadow mask 13 are indicated: a central apertured area 20, a circumferential blind edge 21 and a skirt 22.

The central apertured area 20-indicated by the regions with apertures 25- extends to the dashed line 23. The circumferential blind edge is located between the dashed

line 23 and the bending line 24, while the skirt 22 is the upright part of the shadow mask 13, which is coupled to the frame 14.

Figs. 3A and 3B show a cross section over the line II-II of the shadow mask 13 from Fig. 2 according to the prior art. In Fig. 3A the circumferential blind edge 21 does not show any pattern of blind holes, which means that there is a discontinuity, at the transition area 23, in the yield strength between the apertured central area 20 and the circumferential blind edge 21. This discontinuity will hamper the stretching of the shadow mask 13 in the drawing process and this will lead to a deformation of the apertures 30 at the edges of the apertured central area 20 of the shadow mask 13. Due to this deformation the landing performance will diminish.

The prior art as disclosed in US 3,809, 945 is shown in Fig. 3B and describes a circumferential blind edge 21 with blind holes 31. These blind holes 31 all have the same dimensions, which are designed to give the circumferential blind edge a yield strength that is in between the relative high yield strength of the skirt 22 and the relative low yield strength of the apertured central area 20. This makes that the shadow mask will have two discontinuities in yield strength: one between the apertured central area 20 and the circumferential blind edge 21 and the other between the circumferential blind edge 21 and the skirt 22. This embodiment will give an improvement with respect to material properties over the embodiment of Fig. 3A.

However, it is still not good enough to meet the increasingly stronger demands of new designs. Especially in real flat color picture tubes the mask-forming process should fulfil very high demands in order to meet the required visual performance-e. g. color purity - of the color picture tube 1.

The present invention gives a solution of the problem of the discontinuities in yield strength. The general idea is that the amount of material per unit area in the circumferential blind edge 21 should be increased continuously when going form the apertured central area 20 to the skirt 22. The best solution is obtained when the amount of material per unit area in the circumferential blind edge 21 at the side of the apertured central area 20 equals the amount of material per unit area in the apertured central area 20, and when the amount of material per unit area in the circumferential blind edge 21 at the side of the skirt 22 equals the amount of material per unit area in the skirt 22. In that case the yield strength will not have any discontinuities in the transition areas 23,24. In order to meet this objective, it is necessary to have a grading of the amount of material per unit area in the

circumferential blind edge. There are several ways to realize this, as is illustrated in Figs. 4A, 4B and 4C.

The embodiment of Fig. 4A is provided with a pattern of blind holes 32a, 32b, 32c, 32d, having the same size, but the distance between the blind holes increases in the direction of the skirt 22. This means that the density of blind holes in the direction of the skirt 22 decrease so the amount of material per unit area increases. In this way a smooth transition in yield strength can be realized between the apertured central area 20 and the skirt 22.

An alternative embodiment is shown in Fig. 4B, here the dimensions of the blind holes 33a, 33b, 33c are varied. This can be realized by either changing the depth of the blind holes or by changing the size of the blind hole. Also a combination of these two measures is a possibility. This should be done in such a way that the amount of material which is etched away decreases in the direction of the skirt 22. So, blind hole 33a should be larger than blind hole 33b, which is larger than blind hole 33c. This embodiment renders a smooth transition in yield strength as well.

Evidently, it will also be possible to manufacture a shadow mask 13 which has a blind edge 21, combining the measures of the embodiments of Figs. 4A and 4B.

For the shape of the blind holes 32,33 the person skilled in the art can still choose from a large variety, for instance circular, elliptical, oblong or rectangular.

In a third embodiment, as shown in Fig. 4C, the variation of the amount of material per unit area has been realized by providing the circumferential blind edge 21 with a circumferential groove 34. This depth 35 of this circumferential groove 34 is largest at the side of the apertured central area 20 and is chosen such that the amount of material per unit area continuously fits to the apertured central area 20, equalizing the yield strength at both sides of the transition area 23. The depth 35 diminishes to zero at the side of the skirt, also giving an equal yield strength at the transition area 24.

The circumferential groove 34 extends along the entire circumferential blind edge 21. Instead of one large groove over the entire width of the circumferential blind edge 21, alternatively, a number of circumferential grooves 36a, 36b, 36c-as shown in Fig. 4D- can be used, provided the continuity conditions with respect to the amount of material per unit area are met at the transition areas 23,24. In this embodiment the depth of the grooves 36a, 36b, 36c as well as the width and their mutual distance can be varied.

The most commonly used manufacturing process for shadow mask 13 is a photo-chemical etching process. The shadow mask-as a flat sheet of metal-is provided with a photosensitive material and subsequently both sides are exposed using a photo

negative with the required pattern, in order to transfer the desired pattern of apertures on both sides of the metal sheet. The chemical etching process only etches the exposed regions of the metal sheet, so realizing the pattern of apertures. This process is very well suited for making the blind holes in the circumferential blind edge 21. Etching only has to be done at one side of the shadow mask-either screen side or gun side-and the size of the dots on photo negative determines the size of the blind holes.

Summarizing, in present day color display tubes 1 the shadow mask 13 has an apertured central area 20, a skirt 22 and a circumferential blind edge 21 located between the apertured central area 20 and the skirt 22. In the prior art shadow mask 1'3 the amount of material per unit area shows a discontinuity at the transition areas 23,24 between the apertured central are 20 and the circumferential blind edge 21 and between the circumferential blind edge 21 and the skirt 22. This leads to a different yield strength in these areas, which makes the drawing process of the shadow mask 13 more difficult.

According to the present invention this problem is solved by a circumferential blind edge 21 which has a gradual increasing amount of material per unit area when going from the apertured central area 20 in the direction of the skirt 2). This enables a continuous behavior of the yield strength over the transition areas 23,24. It will be easier to draw the flat mask into its final shape, the deformations will be less and consequently, the performance of the color display tube 1 will be better with respect to color purity, microphony and droptest.