This invention relates to a display device comprising a display screen, said display screen being provided with a selective light-absorbing outer coating on an outer side of said screen.

As the use of different kind of displays, such as CRTs (cathode ray tubes), LCDs (liquid crystal displays), and PDPs (plasma display panels) is becoming more and more wide spread, the demands on these displays have increased. For example, there is a demand that undue reflections in a display screen surface, due to stray external light, shall be avoided. Both specularly and diffusely reflected external light, for example, results in that the displayed picture will have a reduced daylight contrast. This has led to the development of optically functional layers which are arranged on the outer surface of the display screen surface.

Many solutions to the above problems have been suggested. For example, anti- reflection (AR) films are very effective in reducing the specularly reflected ambient light.

The present invention primarily relates to the problem of diffusely reflected ambient light, which originates from daylight, which is reflected, for example, at the phosphor layer of a CRT. The application of spectrally selective coating structures attached to the outer surface of the display screen will firstly reduce the amount of ambient light reaching the phosphor layer, and will secondly reduce the amount of this light after it has been diffusely reflected at the phosphor layer. Thus, the spectrally selective coating will reduce reflected ambient light twice, whereas the luminance L of the light generated by the phosphor layer under electron bombardment (i. e. the useful picture content) is reduced only once. The so called luminance contrast performance (LCP) of the display is enhanced in this manner. LCP is defined as L/4Zdiff, where Rdiff represents the diffuse reflection coefficient of the display screen. The higher the value of LCP, the better the display performs in terms of daylight contrast and light output, and hence picture quality. The use of such films and coatings is well known and wide spread, and as an example, such a coating has been developed and manufactured by the applicant of the present invention under the name CAS

coating (color and anti-static coating). Further examples of such spectrally selective coatings are disclosed in the patent documents EP-0 426 037 and EP-0 335 680. However, a disadvantage of the above known structures is that they tend to have a purple or violet appearance, so that an image displayed on said display gets a purplish tone, especially in darker parts of a displayed picture, which is undesirable for most applications.

One way of overcoming the above problem has been proposed in the patent document EP-0 603 941. Here a display screen is provided with an outer filtering layer, comprising a black dye which contains a metal oxide. A black or grey screen appearance is achieved thereby. However, the inclusion of a black dye in the outer filtering layer results in a deterioration of the amount of light exiting the display without any improvement of the LCP.

Consequently, an object of the present invention is to provide a display device which has a non-colored appearance and which exhibits an improved luminance contrast performance (LCP), at the same time overcoming the drawbacks of the prior art.

The above and other objects are achieved by a display device as described in the opening paragraph, said display device being characterized in that said display screen is further provided with a wavelength-selective absorbing inner layer on an inner side of said screen. Light within a first wavelength interval will thus be absorbed by said absorbing inner layer, and light within a second wavelength interval will be absorbed by the selective light- absorbing outer coating. This renders it possible to neutralize the color reflected from said display device. Further, said wavelength-selective absorbing inner layer is suitably arranged so as to absorb wavelengths outside a wavelength interval of about 500 to 560 nm, i. e. essentially green light is transmitted. The violet appearance of prior art absorbing coatings on the outside of the display may be neutralized thereby.

In a first embodiment of the invention, said wavelength-selective absorbing inner layer is provided as a color filter, such as a green color filter. A construction is achieved thereby that is usable with most display types, such as CRTs, PDPs, and LCDs. Preferably, said display device comprises a fluorescent phosphor pattern, and while said wavelength- selective absorbing inner layer is arranged between said fluorescent phosphor pattern and the inner side of said screen. Moreover, said display device comprises a fluorescent phosphor pattern comprising separated fluorescent areas for the colors red, green, and blue, respectively, and said wavelength-selective absorbing inner layer is arranged between the

green fluorescent phosphor areas and the inner side of said screen. A construction is achieved thereby, that may be used with prior art devices having a common spectrally selective outer coating with a violet appearance. In a second embodiment of the invention, said display device comprises a fluorescent phosphor pattern, and said wavelength-selective absorbing inner layer is arranged as a pigment coating around phosphor particles of said phosphor pattern. The invention may then be realized in a CRT or the like, without adding extra components, apart from the pigments. Preferably, said display device comprises a fluorescent phosphor pattern comprising separated fluorescent areas for the colors red, green, and blue, respectively, while said wavelength-selective absorbing inner layer is arranged as a pigment coating around the green fluorescent phosphor particles and the inner side of said screen. A construction is achieved thereby, that may be used with prior art devices having a common spectrally selective outer coating with a violet appearance.

Finally, a yet further preferred embodiment of this invention, said display device further comprises a coating which is one from among the following: an anti-reflection coating, an anti-static coating, and an electrostatically shielding coating, or any combination thereof, on an outer side of said selective light-absorbing outer coating. It is possible thereby to combine good anti-reflection properties of the display with an improved LCP. Suitably, said selective light-absorbing outer coating is integrally formed with said second outer coating in order to minimize the number of coatings that is to be applied.

Currently preferred embodiments of the present invention will now be described in closer detail, with reference to the accompanying drawings.

Fig. 1 is a schematic drawing showing a cross-section of a cathode ray tube in which the present invention is applied.

Fig. 2 is a schematic drawing showing a detail of a display screen of a cathode ray tube, as shown in Fig. 1.

Fig. 3 is a graph showing the reflection relative to that of bare glass as a function of the wavelength of incident light, using a modified IRIS coating, i. e. an AR coating with a spectrally selective absorbing dye.

For LCP improvement, it is desired that a selective light-absorbing outer coating 2, such as an absorbing dye transmission coating, on the outer side of a display screen

1, should absorb light (daylight) in between the green and red peaks of the radiation spectrum. This is approximately 560 nm (orange), which results in the use of a dye having a purple color. For the reasons stated above, this purple appearance is undesired for most applications. Consequently, there is a need to reduce such a purple appearance of a display screen, and it is to such a reduction that this invention relates.

The main inventive idea behind this invention is that the color appearance of the selective light-absorbing coating may be influenced by adding a color to the inside of the screen. For this purpose, a compensation material is at least partly provided on the inside of the display screen, so that external light reflected at, for example, a phosphor layer, is given a color complementary to purple, i. e. green light. The light reflected in said display screen will appear to be less purple to a viewer as a result.

In Fig. 1, a cross-section of a cathode ray tube display (CRT), to which the invention may be applied, is shown. The display comprises a display screen 1 having an inner side and an outer side. The central part of the display screen 1 is manufactured from a transparent material such as glass, and is not of major importance to this invention. However, on the outer side of the display screen, facing a potential viewer, a selective light-absorbing outer coating 2 is arranged. Preferably, said selective light-absorbing outer coating is a chrominance coating comprising a transparent matrix with a dye or pigment incorporated therein. On the opposite side of the display screen 1, i. e. the inner side of the screen, a wavelength-selective absorbing inner layer 3 is arranged, said layer partly covering said inner surface of the display screen 1.

A screen as shown in Fig. 1 may be used for generating a full-color CRT display. Traditionally, the inner surface of the display is provided with a layer of luminescent material, such as phosphor, and an electron beam is arranged to hit the phosphor, thereby generating light. In a full-color display, such as an RGB display, separate phosphor pixels or stripes are arranged for generating the colors red, green, and blue, respectively. In such a display, the invention may be realized in two ways, as will be described below.

A first embodiment of this invention is shown in greater detail in Fig. 2, showing a portion of the display screen, for example for a CRT, in cross-section. On the inner side of the display screen, a patterned fluorescent layer 4 is arranged, here shown as stripes 4R, 4G, 4B, for generating red, green, and blue light, respectively. A filtering layer 3' for absorbing the complementary color of green is arranged for every third stripe 4G generating the color green, between said fluorescent phosphor layer 4G and said inner surface of the display screen 1. As is shown in Fig. 2, the external light reflected by the phosphor of

the phosphor stripe 4G will only comprise green light, thereby neutralizing the violet light transmitted by the outer coating, which will result in a more neutral color appearance of the display screen while retaining the LCP improvement generated by the outer coating, as described above. Light will be reflected as in prior art displays from the remaining phosphor stripes 4R, 4B.

In a second embodiment (not explicitly shown), pigmented green phosphor may be used, for example through the use of greenish pigments. Here a patterned fluorescent layer 4 is arranged on the inner side of the display screen, shown as stripes 4R, 4G, 4B for generating red, green, and blue light, respectively. Every third stripe 4G generating the color green is constituted by a heavily pigmented green phosphor. An example of such a phosphor pigmentation is given, for example, in the patent document EP-0 836 215 and will therefore not be described in more detail herein. The green pigment layer will function as a filter (3) for each covered phosphor particle as a result of this. Otherwise, the function of this embodiment is as described above with reference to the color filter application.

Two experiments with a display screen structure as described above will now be described below: 1) LCP and Rdiff measurements on a CRT with a green filter only, said filter being placed between the green light emitting phosphor stripe and the inner screen surface.

2) LCP and Rdiff measurements on a CRT with red, green, and blue color filters, respectively, i. e. every stripe is provided with a suitably colored color filter.

EXPERIMENT 1 A first display screen provided on its inner surface with a green color filter was treated with a first dye solution during the coating process, said dye solution having a first die concentration of 0.5%. Correspondingly, a second display screen, provided on its inner surface with a green color filter was treated with a second dye solution during the coating process, said dye solution having a second die concentration of 1.0%.

The resulting body color impression was as follows. The uncoated display screen (before the application of the coating as above) had a green appearance. The display screen coated with the 0.5 % solution had a neutral appearance. Finally, the display screen coated with the 1.0% solution had a clear violet appearance. The following measurements where made: Uncoated 0.5% violet 1.0% violet Difference LCP 271. 0 284. 8 +5. 1% (White-D65) 272. 9 298. 9 +9. 5% Rdiff 10. 96% 9.24%-15. 7% 10.63% 7.09%-33. 3%

EXPERIMENT 2 A first and a second display screen provided on its inner surface with a red, green, and blue color filter, respectively, was treated as above with a 0.5% solution and a 1.0% solution, respectively.

The resulting body color impression was as follows. The uncoated display screen (before the application of the coating as above) had a greenish appearance. The display screen coated with the 0.5 % solution had a light violet appearance. Finally, the display screen coated with the 1.0% solution had a violet appearance. The following measurements where made: Uncoated 0.5% violet 1.0% violet Difference LCP 338. 8 363. 8 +7. 4% (White-D65) 340. 8 390. 3 +14. 5% Rdiff 5. 69% 4. 51%-20. 7% 5. 48%. 3. 44%-37.2% A first indication is that a violet dye in combination with a greenish screen appearance, i. e. provided with a green color filter or strong green pigmented phosphor, can give an LCP gain of approximately 5%. For CRT displays used as computer monitors, this may further be provided at no extra cost or only a low extra cost, since these display screens are already being provided with a coating. Furthermore, as seen from the tables above, a higher LCP gain can be obtained for applications for which a violet appearance may be accepted.

Consequently, a display screen having a less purple or even neutral color appearance is achieved, and furthermore, since the chrominance layer also absorbs orange light, as discussed above, the inventive display screen will display a more saturated red color.

As was seen above, the LCP improvement is about 5 to 10% and the screen has a more neutral color. It may also be concluded from the above and from experiment 1 with only green color filters that a neutral display appearance is achieved.

In an embodiment of the invention, the inventive inner layer and outer coating may be used together with an additional outer coating, such as an AR coating, an antistatic coating or an electrostatically shielding coating, or a combination thereof. For example, an additional coating may be formed integrally with said outer coating, thus limiting the number of separate coatings for manufacturing purposes. An example of the above is shown in Fig. 3, showing the AR behavior of a modified AR coating (a LCP enhancing purple dye had been <BR> <BR> added to a standard AR coating. ) The use of such an AR outer coating on the outside of the display screen in combination with the inner layer and outer coating of the present invention is advantageous, for example for the picture quality. The AR coating used in Fig. 3 is an IRIS coating. The IRIS coating is a 2-or 3-layer AR coating that is commercially available on, for example, monitor tubes, manufactured by the applicant of the present invention, and that in fact incorporates a CAS coating, as described above, in one or two of its constituting layers.

The graph of Fig. 3 shows that even with the dye, the AR properties of the IRIS coating remain intact (more or less the same performance as for the standard IRIS coating), and moreover the dye will increase the LCP. In other words, it is both theoretically and practically possible to combine the present invention with the benefits of AR coatings.

It should be noted that, although the invention was described above with reference to a cathode ray tube, the invention is equally applicable to other types of displays having separated emission areas for different colors of a displayed picture, such as PDPs and LCDs. Furthermore, it is also possible to use the invention in displays with more or fewer colors, such as a monochrome display. Furthermore, the invention may be used in combination with other coating structures, such as an AR coating, as described above, or an ES (electrostatically in shielding) coating.