Field of the Invention

The present invention relates to cathode ray tubes having tension masks, and more particularly to a tube having a tension mask with a mask structure and support frame of dissimilar thermal expansion materials.

Background of the Invention

A color cathode ray tube, or CRT, includes an electron gun for forming and directing three electron beams to a screen of the tube. The screen is located on the inner surface of the faceplate panel of the tube and is made up of an array of elements of three different color- emitting phosphors. A shadow mask, which may be either a formed mask or a tension mask, is located between the electron gun and the screen. The electron beams emitted from the electron gun pass through apertures in the shadow mask and strike the screen causing the phosphors on the screen to emit light so that an image is displayed on the viewing surface of the faceplate panel.

One type of shadow mask structure may comprise a tension mask having an array of apertures. The aperture array terminates near the edges of the tension mask with a solid etched border that is welded to a support frame. The solid border of the mask serves as an optical edge for forming the black surround of the matrix which in turn defines the borders of the screen array of the tube screen.

Low expansion material is desirable for the shadow mask, because it reduces electron beam mislandings on the phosphor stripes as the mask warms from electron bombardment during tube operation. It has been found desirable to make the mask structure and support

frame from different materials to reduce the cost of the mask-frame assembly. The consequence of having mask borders welded to the frame when the mask and frame have different thermal expansion coefficients is that inelastic deformation of the solid borders can occur along the mask-to-frame weld during thermal processing of the CRT in production.

One way to address these problems would be to form both the mask structure and frame of materials having low coefficients of thermal expansion (CTE). However, low CTE materials are expensive and frame structures are relatively massive, so such an approach results in increased cost.

It is therefore desirable to provide an improved tension mask border and a solution to the problem that exists when there is a mismatch in coefficient of thermal expansion between the tension mask structure and support frame.

Summary of the Invention

The invention provides a cathode ray tube (CRT) having a tension mask and support frame assembly mounted within the tube between an electron gun and a luminescent screen. A frame is located in the CRT and has at least two opposed sides, the frame being of a material having a first coefficient of thermal expansion. A tension mask formed of a material having a second coefficient of thermal expansion is supported by the frame. The tension mask has an active array area formed of a plurality of elongated apertured portions located between mask borders and a plurality of tails extending outward from the mask borders to the opposed sides of the frame. A strip border is attached to each mask border near the outer edges of the active array along the opposed sides wherein the strip border is formed of a material having a coefficient of thermal expansion similar to the second coefficient of thermal expansion.

Brief Description of the Drawings

The invention will now be described by way of example with reference to the accompanying figures of which:

Figure 1 is a cross sectional view of a CRT containing the improved tension mask of the present invention.

Figure 2 is a perspective view of a tension mask assembly including a support assembly according to the present invention.

Figure 3 is an exploded perspective view of an area of the tension mask assembly as shown in Figure 2.

Figure 4 is a plan view of an area of an alternate tension mask assembly.

Detailed Description of the Invention

Figure 1 shows a cathode ray tube (CRT) 1 having a glass envelope 2 comprising a rectangular faceplate panel 3 and a tubular neck 4 connected by a funnel 5. The funnel 5 has an internal conductive coating (not shown) that extends from an anode button 6 toward the faceplate panel 3 and to the neck 4. The faceplate panel 3 comprises a viewing faceplate 8 and a peripheral flange or sidewall 9, which is sealed to the funnel 5 by a glass frit 7. A three- color phosphor screen 12 is carried by the inner surface of the faceplate panel 3. The screen 12 can be a line screen with the phosphor lines arranged in triads, each of the triads including a phosphor line of each of the three colors. A tension mask support frame assembly 10 is removably mounted in predetermined spaced relation to the screen 12. An electron gun 13, shown schematically by dashed lines in Figure 1 , is centrally mounted within the neck 4 to

generate and direct three inline electron beams, a center beam and two side or outer beams, along convergent paths through the tension mask frame assembly 10 to the screen 12.

The CRT 1 is designed to be used with an external magnetic deflection yoke 14 shown in the neighborhood of the funnel-to-neck junction. When activated, the yoke 14 subjects the three beams to magnetic fields which cause the beams to scan horizontally and vertically in a rectangular raster over the screen 12.

The tension mask support frame assembly 10, as shown in Figure 2, has a frame 20 which includes two long sides 22 and 24, and two short sides 26 and 28. The two long sides 22, 24 of the frame 20 are parallel to a central major axis, X, of the tube; and the two short sides 26, 28 are parallel to a central minor axis, Y, of the tube. The sides 22, 24, 26, 28 are preferably formed of rectangular tubular material. It should be understood however that other geometry tubular materials or other solid materials could be utilized to form these sides. The two long sides 22, 24 and two short sides 26, 28 preferably form a continuous mask support frame 20 and lie in a common plane generally parallel to a tension mask 30. A pair of opposed support blade members 40 are mounted on the two long sides 22, 24. The support blade members 40 could alternatively be mounted on the two short sides 26, 28. The mask 30 is supported by edges 42 of each support blade member 40 as will be described below. Although the tension mask assembly 10 may be formed of many suitable materials, steel is suitable because of its low cost, high strength and ability to maintain the mask 30 in tension.

Referring now to Figure 3, an exploded section of a first embodiment of the tension mask frame assembly 10 is shown. The tension mask 30 is formed from a thin sheet of low expansion material, such as INVAR or other iron-nickel alloys, which are etched or otherwise processed to form a plurality of elongated apertured portions 32 extending parallel to the minor axis. The apertured portions 32 extend between top and bottom mask borders 36 to

define an active array area 37. The mask borders are formed as a solid section between the aperrured portions 32 and a plurality of tails 33. The tails 33 extend outward from each mask border 36 and are attached to each of the support blade members 40 at an edge 42, for example, by a wheel-type resistance welder or by other welding methods, such as laser welding.

Strip borders 46 are located either on the screen facing side as shown in Figure 3 or alternatively on the gun facing side of the tension mask 30. As a further alternative, strip borders 46 could be located on both the screen facing side and gun facing side of the tension mask 30. They are fixedly attached along the mask borders 36 near the upper and lower outer edges of the active array area 37, such as by welding. Although the strip borders 46 are shown in Figure 3 as being attached to the tails 33, they could alternatively be attached to mask borders 36 or may have attachment points on both the borders 36 and tails 33. The strip borders 46 extend generally parallel to the support blade member edge 42 and can be formed of a low thermal expansion material, such as INVAR or other iron-nickel alloy, similar to the low expansion material of the tension mask 30. In a preferred embodiment, the strip border 46 and the tension mask 30 are made of materials having the same coefficient of thermal expansion. Also, having the strip borders 46 and tension mask 30 constructed of similar materials (i.e., having relative expansion characteristics being less than 50% of one another) have been shown to minimize the impact of thermal cycling on the active aperture array area 37. Experiments have even shown that strip borders 46 being materials having coefficient of thermal expansion values of a factor of 2.5 times greater than those of the tension mask material are effective. Additionally, the tails 33 allow for some degree of relative movement between the support blade members 40 and the tension mask 30 while the mask borders 36 and strip borders 46 reduce or prevent inelastic deformation in the active aperture array area

37. The strip borders 46 also provide the tension mask 30 with increased buckling stiffness to prevent any remaining inelastic deformation from causing mask wrinkles. For example, a tension mask constructed of Invar without the strip borders 46, attached to a steel frame has been found to excessively stretch in the major axis direction during thermal cycling in tube processing, thereby causing wrinkling in the tension mask 30.

Figure 4 illustrates an alternate embodiment wherein the mask border 136, strip border 146, and tails 133 have been modified. In this embodiment, the active aperture array area 37 remains unchanged with a plurality of elongated aperture portions 32. The mask borders 136 can have curvature. The curvature is formed such that the mask borders 136 extend closer to the support blade member edge 42 at the center 145 and progressively farther from the support blade member edge 42 approaching the edges 149. The strip border 146 is tapered and formed to have a first radius of curvature along an active array edge 147 and a different radius of curvature along a support member edge 148. The active array edges 147 and the support member edge 148 are arranged such that the strip border 146 is wider at the center 145 than at the edges 149. This can be seen in Figure 4 wherein the center dimension is labeled A and is greater than the edge dimensions labeled B. The strip border 146 is similarly secured to the mask border 136 as described above in the first embodiment. Tails 133 vary in length with longer tails 133 being located at the edges 149 as shown by dimension D and shorter tails located in the center 145 as shown by dimension C. The tails 133 are similarly attached to the support blade member edge 42 as described above in the first embodiment. This tapered design of the strip border 146 having varying width provides greater stiffness where the sum of axial thermal forces are higher near the center 145 and less stiff where the sum of axial thermal forces are lower near the edges 149. Each tail 133 acts as a beam in bending with its ends differently displaced by an amount equal to the difference in expansion between the

tension mask 30 and the support blade member edge 42 at the location of the tail 133, minus the deformation due to mechanical stress in the same direction. Since the difference in expansion between the tension mask 30 and the support blade member edge 42 is greatest at the edges 149 of the tension mask 30 and smallest at the center 145, greater isolation is provided by increasing tail lengths toward the edges 149 of the tension mask 30. Since the sum of axial forces from the tails on the borders is greatest at the center, it is desirable to have a wider border and strip border at the mask center to resist the axial forces; thus preventing over-stressing the border and strip border, and protecting the mask array. Since the sum of axial forces is smaller near the mask edge, it is desirable to increase tail lengths near the edge (as well as narrowing the adjacent mask border and strip border) to reduce the edge tails' contribution to the sum of axial forces on the mask border at all inboard locations.

The embodiments as shown in Figures 3 and 4 according to the invention allow thermal processing of a CRT in production while reducing the possibility of wrinkling or otherwise deforming the tension mask 30 when materials for the support blade member 40 and the tension mask 30 have dissimilar coefficients of thermal expansion.

Upon conjunction of the faceplate panel 8 with the tension mask 30 during final CRT assembly, the tension mask 30 is mounted on studs (not shown) extending from the faceplate panel 8. The electron gun 13 produces an electron beam whose center of deflection is substantially coincident, in effect, with the pathway followed by the light source used in producing and locating the phosphor stripes on the screen 12. With the use of matrix and screening processes known in the art, the border 46 can be used to define the periphery in the matrix process and also define where the phosphor stripes are terminated.

The foregoing illustrates some of the possibilities for practicing the invention. Many other embodiments are possible within the scope and spirit of the invention. For example,

while both embodiments have been described with support blade members and associated borders along a major axis of the CRT, the invention may be utilized with similar frame to mask arrangements along the minor axis of the CRT. It is, therefore, intended that the foregoing description be regarded as illustrative rather than limiting, and that the scope of the invention is given by the appended claims together with their full range of equivalents.