WO2003075304A1

WO2003075304A1 - Cathode ray tube having a reduced difference in light transmittance between a central region and a peripheral region of a panel face thereof

Info

Publication number: WO2003075304A1
Application number: PCT/IB2003/000702
Authority: WO
Inventors: Andre Van Der Voort; Evert-Jan Van Donkelaar; Tjerk G. Spanjer; Sipke J. Bijlsma; Frank Gersmann; Joseph W. J. M. Van Der Heijden
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2002-03-04
Filing date: 2003-02-21
Publication date: 2003-09-12
Also published as: TW200305182A; AU2003206058A1

Abstract

A display tube having a glass front panel, an inner surface of the front panel carrying a phosphor pattern embedded in a black matrix, wherein a glass front panel has an outer surface with a transparent plastic sheet adhered thereto, the transparent plastic sheet being adhered by means of a pressure-sensitive adhesive, the assembly of plastic sheet and adhesive having a first transmittance T1 in an area corresponding to a central region of said panel and a second, higher transmittance T2 in an area corresponding to a peripheral region of said panel.

Description

CATHODE RAY TUBE HAVING A REDUCED DIFFERENCE IN LIGHT TRANSMITTANCE BETWEEN A CENTRAL REGION AND A PERIPHERAL REGION OF A PANEL FACE THEREOF

The invention relates to a display tube having a glass front panel, an inner surface of the front panel carrying a phosphor pattern embedded in a black matrix.

Such display tubes are, for example, Cathode Ray Tubes, Field Emission Displays, and Plasma Display Panels. In considering the performance of a display tube, the "dead" tube (power switch-off) is often not taken into account. However, from a set-design point of view this is also an important item. That is why it was considered by the inventors in a recent perception test. Looking at the current WSRF tubes, it is always found that the center area in the dead tube is much brighter than the areas towards the edges. This "brightness" can be expressed as the diffuse reflectivity of the tube. The diffuse reflectivity is determined by the glass transmittance, the amount of Black Matrix (i.e. the graphite reflection), and the amount of phosphor (i.e. the phosphor reflection). In tube design, Luminance and Luminance Decrease requirements define the ratio between these parameters. Clear glass is used in combination with a foil having an uniform transmittance in the current RF types. Advantage of this is the fact that the glass has a given transmittance distribution value or gradient.

Taking the above-mentioned situation into consideration, it is an object of this invention to provide a cathode ray tube which has a reduced difference in brightness between a central region and a peripheral region of a front panel.

According to the invention, there is provided a cathode ray tube of the type described in the preamble using a glass front panel having an outer surface with a transparent plastic sheet adhered thereto, the transparent plastic sheet being adhered by means of a pressure-sensitive adhesive, the assembly of plastic sheet and adhesive having a first transmittance Tj in an area corresponding to a central region of said panel and a second, higher transmittance T₂ in an area corresponding to a peripheral region of said panel. Providing a transparent sheet-adhesive assembly having a gradient in its transmittance creates a new design parameter to optimize a cathode ray tube.

According to a first embodiment, the plastic sheet has a uniform transmittance (and is preferably uncolored or clear), and the adhesive is a colored adhesive, said colored adhesive being thick in an area corresponding to a central region of said panel and thinner in an area corresponding to a peripheral region of said panel.

The use of a colored adhesive with a graded thickness not only improves the distribution of the diffuse reflectivity, but, if the adhesive was provided previously on the transparent sheet, very much simplifies the production process. In particular, a transmittance gradient either in the East-West or in the North-South direction of the front panel can be made in this manner.

According to a second embodiment, the adhesive has a uniform thickness, (and preferably is uncolored or clear), and the plastic sheet is colored, the colored sheet having a dark color tone in an area corresponding to a central region of said panel and a lighter color tone in an area corresponding to a peripheral region of said panel.

The coloring of the plastic sheet may advantageously be carried out by means of deep-dyeing. The use of a deep-dyeing process makes it possible even to produce a transmittance gradient from the center towards the corners. The invention also relates to a method of manufacturing a display tube comprising a glass front panel having an outer surface with a plastic sheet thereon. Said method is characterized in that a plastic sheet having a surface with a pressure-sensitive adhesive thereon is provided and is applied to the outer surface of the glass front panel with its pressure adhesive layer facing the glass front panel, said assembly of plastic sheet and pressure-sensitive adhesive having a first transmittance Ti in a central region and a second, higher, transmittance T₂ in a peripheral region.

The inventive method is useful in particular if an assembly of a clear plastic sheet and a pressure-sensitive adhesive is used which produces the desired transmittance variation by means of a thickness variation of the adhesive, which in that case is colored. The colored adhesive may be produced by adding pigments, in particular soot.

An advantageous method of providing an adhesive layer with a varying thickness on a plastic sheet is the use of wet chemical deposition techniques. In particular, the use of gravure roller coating is very suitable for this object, the more as the inventive object demands layer thicknesses between 10 and 50 microns and thickness variations of from 5 to 20 microns.

According to an alternative embodiment of the inventive manufacturing method, a plastic sheet with pressure-sensitive adhesive assembly is used, the plastic sheet of which is colored, said colored sheet having a color variation for producing the first and second transmittances Ti and T₂. In this case the adhesive can be clear. The invention also relates to a plastic sheet having a surface with a pressure- sensitive adhesive thereon, the adhesive being colored and being thick in an area corresponding to a central region of the sheet and thinner in an area corresponding to a peripheral region of the sheet.

These and further aspects of the invention will be explained in greater detail by way of example and with reference to the accompanying drawings, in which Fig. 1 is display device; Figs. 2 A, 2B illustrate plastic sheet/adhesive assemblies in accordance with the invention;

Fig. 3 shows a luminance distribution over a CRT window surface. Fig. 4 is a graph showing the thickness distribution of an adhesive layer on a plastic sheet and the resulting transmittance distribution; Fig. 5 schematically shows a gravure roller coater, and

Fig. 6 shows gravure roller surface patterns.

The Figs, are not drawn to scale. In general, like reference numerals refer to like parts.

A color display device 1 (Fig. 1) comprises an evacuated envelope 2 with a display window 3, a cone portion 4, and a neck 5. Said neck 5 accommodates an electron gun 6 for generating three electron beams 7, 8, and 9. A display screen 10 is present on the inside of the display window. Said display screen 10 comprises a phosphor pattern of phosphor elements luminescing in red, green, and blue. On their way to the display screen the electron beams 7, 8, and 9 are deflected across the display screen 10 by means of a deflection unit 11 and pass through a shadow mask 12 which is arranged in front of the display window 3 and which comprises a thin plate having apertures 13. The shadow mask is suspended in the display window by suspension means 14. The three electron beams converge and pass through the apertures of the shadow mask at a small angle with respect to each other and, consequently, each electron beam impinges on phosphor elements of only one color. In Fig. 1, the axis (z-axis) of the envelope is also indicated. A plastic sheet 15 is present in front of the display window 3. The plastic sheet 15 is adhered to the display window 3 via a pressure- sensitive adhesive 16. The sheet 15 may be provided for various reasons, such as to reduce reflection or transmittance, to provide for a touch-screen, or to enhance implosion protection. In this embodiment, the display device is or comprises a cathode ray tube (CRT). Although the invention is of particular importance for such devices, within the scope of the present invention the term display device covers other display devices as well, such as Plasma Display Panel display devices and Field Emission Display Devices.

Figs. 2 A and 2B present two alternative embodiments of the sheet (15)/ adhesive (16) assembly, arranged on the outer surface of the CRT glass window (3). In both embodiments the plastic sheet 15 is made of Poly Ethylene Terephtalate (PET), the thickness being 188 μm. Properties of this PET sheet are:

Structure crystalline Density 1.37 Elongation 70 % Tensile strength 6600 psi Melting point 248° C

Dielectric str. 20 KV/mm UN resistance Good Transition temperature 74° C

The invention is not restricted to the use of PET. Other suitable synthetic materials may also be used, for example, TAC.

One or both surfaces of the sheet 15 may be provided with a primer layer. The surface facing the CRT glass window (3) has a colored Pressure Sensitive Adhesive (PSA) (16) thereon. A preferred adhesive material is, for example, polyacrylic-acid ester in which a (black) pigment, for example, soot, is dispersed. The opposite surface of the sheet (15) may have a hard coating (18) thereon, optionally in combination with anti-reflection and/or antistatic layers.

The adhesive film 16 has a thickness variation such that it is thick in an area corresponding to a central region of the CRT glass window (3) and thin in an area corresponding to a peripheral region of the CRT glass window (3) so as to reduce the difference in diffuse reflection between the central region and the peripheral region of said CRT glass window (improved luminance distribution). An example of such an improved luminance distribution, obtained by a thickness variation of the adhesive film 16, is presented in Fig. 3. The bottom part of the Fig. 4 graph shows the variation in thickness t (in μm) of an adhesive film provided on a sheet substrate from which three plastic sheets are to be cut. It can be seen that the thickness t ranges from 22.5 to 30 microns. The thickness variation in this case is parabolic. The resulting transmittance T is shown in the upper part of the Fig. 4 graph. The transmittance is approximately 57 % in the central region and 64 % in the peripheral regions of each future plastic sheet. This is a useful result, in particular for "32WSRT tubes". In this case the transmittance variation or gradient is chosen so as to extend in the vertical direction (along the North-South axis of the CRT window). However, if desired, the transmittance variation may alternatively be set in the horizontal (East- West) direction.

The tables in Annex A give an overview of some important parameters when a graded T plastic sheet/PSA assembly is applied compared with the design requirements for 32WSRF tubes.

It follows from the tables in Annex A that the maximum difference in diffuse reflectivity decreases from 9.2 - 4.9 in the standard design to 8.6 -> 5.4 % in the graded T design. The plastic sheet/PSA assembly has to be made with a gradient of 57 % in the center and 64 % towards the North and South edges. The tube thus becomes darker in the center and lighter in the corners compared with the standard tube, which gives a more uniform dead tube. Further it follows that a small improvement in LCP (Luminance Contrast Performance) can be achieved.

The luminance distribution shown in Fig. 3 is linear (in one direction).

In the above embodiment, the PSA (16) is colored and the plastic sheet (15) is clear.

In an alternative embodiment, the PSA (16) is clear and the plastic sheet (15) is colored, preferably by means of deep dyeing. The latter situation is illustrated in Fig. 2B: a sub-layer (19) has a dark color tone in an area corresponding to a central region of the CRT window (3) and a light color tone in an area corresponding to a peripheral region of the CRT window (3), so as to reduce the difference in diffuse reflection between the central region and a peripheral region. A preferred method of providing a plastic sheet with a color tone variation such as the above is to apply a dye in varying thickness onto an outer surface of a plastic sheet and having the dye impregnate the plastic sheet. A preferred technique for applying the dye is, for example, gravure roller coating, which renders it possible to apply very thin coatings of low-viscosity liquids at high speed. An important feature of a gravure roller coater (Fig. 5) is the patterned chrome gravure roller. A pattern of, for example, cells or grooves (Fig. 6) is engraved into the surface of the gravure roller, either by mechanical engraving (knurling), by chemical etching, or by electromechanical engraving. By adapting the pattern of the gravure roller it is possible to transfer a dye layer having a desired local thickness variation to a plastic sheet. After the impregnation step, the plastic sheet will have a dyed sub-layer, the dyed sub-layer having a desired pattern of lighter and darker color tones. This makes it possible not only to produce a plastic sheet with a "linear" transmittance gradient, (gradient along a single direction), but also a "circularly symmetrical" gradient (a gradient, for example, from the central region towards each of the comers).

The invention does not only relate to obtaining a transmittance distribution for reducing differences in diffuse reflection , but it is also suitable for obtaining other transmittance distributions for other objects. For example, if tubes are made with wider guardbands in the comers, in the framework of better feasibility, which involves that the phosphor stripes in the comers must be smaller, the relative luminance in the comer regions would be smaller. This disadvantageous effect can be compensated for by using an inventive plastic sheet/PSA assembly, the transmittance of which increases from the central region towards the comer regions. Luminance values in the comers which are 70 to 80 % of the luminance in the central CRT window area would be possible in this manner. Summarizing, the invention relates to a display tube having a glass front panel, an inner surface of the front panel carrying a phosphor pattern embedded in a black matrix, characterized by the use of a glass front panel having an outer surface with a transparent plastic sheet adhered thereto, the transparent plastic sheet being adhered by means of a pressure-sensitive adhesive, the assembly of plastic sheet and adhesive having a first transmittance Ti in an area corresponding to a central region of said panel and a second, higher transmittance T₂ in an area corresponding to a peripheral region of said panel. ANNEX A

Claims

CLAIMS:

1. A display tube having a glass front panel, an inner surface of the front panel carrying a phosphor pattern embedded in a black matrix, characterized by the use of a glass front panel having an outer surface with a transparent plastic sheet adhered thereto, the transparent plastic sheet being adhered by means of a pressure-sensitive adhesive, the assembly of plastic sheet and adhesive having a first transmittance Ti in an area corresponding to a central region of said panel and a second, higher transmittance T₂ in an area corresponding to a peripheral region of said panel.

2. A display tube as claimed in claim 1 , characterized in that the plastic sheet has a uniform transmittance and preferably is uncolored, i.e. clear, and the adhesive is a colored adhesive, said colored adhesive being thick in an area corresponding to a central region of said panel and thinner in an area corresponding to a peripheral region of said panel.

3. A display tube as claimed in claim 1 , characterized in that the adhesive has a uniform thickness and preferably is uncolored, i.e. clear, and the plastic sheet is colored, the colored sheet having a dark color tone in an area corresponding to a central region of said panel and a lighter color tone in an area corresponding to a peripheral region of said panel.

4. A method of manufacturing a display tube comprising a glass front panel having an outer surface with a plastic sheet thereon, characterized in that a plastic sheet having a surface with a pressure-sensitive adhesive thereon is provided and is applied to the outer surface of the glass front panel with its pressure-adhesive layer down, said assembly of plastic sheet and pressure-sensitive adhesive having a first transmittance Ti in a central area and a second, higher, transmittance T₂ in a peripheral area.

5. A method as claimed in claim 4, characterized in that an assembly of a plastic sheet and a pressure-sensitive adhesive is used, the adhesive of which is colored and has a thickness variation for producing the first and second transmittances Ti and T₂.

6. A method as claimed in claim 5, characterized in that the adhesive having a thickness variation is provided on the plastic sheet by means of a wet chemical deposition technique.

7. A method as claimed in claim 4, characterized in that an assembly of a plastic sheet and a pressure-sensitive adhesive is used, the plastic sheet of which is colored, said colored sheet having a color variation for producing the first and second transmittances Ti and T₂.

8. A plastic sheet having a surface with a pressure-sensitive adhesive thereon, the adhesive being colored and being thick in an area corresponding to a central region of the sheet and thinner in an area corresponding to a peripheral region of the sheet.