DISPLAY DEVICE

Abstract: In a display device (LCD) (1) driven in the transmission mode and based on the combination of (LCD) switching elements and a phosphor pattern (8) which is irradiated, for example by an UV source (10) the light output is considerably increased by arranging an interference filter (14) between the radiation source (10) and the phosphor layer (8).

Description: The invention relates to a display device comprising an electro-optical medium between two parallel transparent substrates having drive means to influence the transmission state of the electro-optical medium, one of the substrates having a pattern of at least two luminescent materials, said display device further comprising a radiation source suitable for emitting sufficiently short-wave radiation to excite the luminescent materials and an interference filter being arranged between the radiation source and the luminescent materials, which filter substantially completely passes the short-wave radiation and substantially completely reflects radiation generated in the luminescent materials. The said display devices are used, for example, in colour television or in colour monitors for the purpose of data display in computer systems, and in, for example, display devices in dashboards, etc. A display device of the type mentioned in the opening paragraph is described in the non-prepublished application EP-A-0 267 686. In the device shown in this application a liquid crystal display device consisting of a liquid crystalline material between two glass plates is driven in the transmission mode. The device is exposed with UV radiation on one side, whilst the device on the side of the UV source comprises a pattern of phosphors. Areas of the display device associated with this pattern can be separately switched by means of switching electrodes. The ultraviolet radiation emitted by the UV source produces conversion in the luminescent layers into a colour in the visible part of the spectrum (for example, into the primary colours red, green and blue, so that visible light passes to the other side of the devide in defendence on the state of the electro-optical medium to form a (colour) picture. To prevent during this conversion loss of light due to emission and dispersion in the direction of the radiation source an interference filter is arranged between the radiation source and the luminescent materials, which filter substantially completely passes the short-wave radiation and substantially completely reflects radiation generated in the luminescent materials. Without this filter a large part of the quantity of light generated in the phosphors is lost during conversion. In fact, the conversion is effected within a very thin layer (approximately 2 to 3 microns) on the side of the incident ultraviolet radiation. Since the generated visible light is emitted in all directions and is also dispersed by the phosphors, a large part thereof (aproximately 60 to 70%) leaves the phosphor layer on the side of the UV source. This leads to a lower brightness, but moreover a part of the light generated in the phosphors may be dispersed back via reflection from various surfaces (for example a glass-air surface when the luminescent material between the UV source and the electro-optical medium is provided on a glass substrate), and then at an unacceptably large spatial angle or in undesired areas. All this leads to a loss of resolution and a decreased contrast. By using an interference filter the radiation required for excitation, for example a characteristic wavelength of 254 nm (Hg resonance line) or a wavelength range of 360-380 nm is passed either for all angles of incidence or for a selected angle range. Furthermore, the presence of the filter causes light which is generated in the phosphor layer and is emitted in the direction of the radiation source to be substantially completely reflected by the filter for all angles of incidence. A light output gain which may be a factor of 2 to 3 is then obtained with respect to a device without an interference filter. An additional advantage is that there are substantially no reflections against surfaces located closer to the radiation source because substantially all radiation in the direction of the source is reflected by the interference filter. Consequently, resolution and contrast are improved considerably. In the embodiment shown in EP-A-0 267 686 the interference filter and the luminescent materials are arranged between the radiation source and the elctro-optical medium, whilst the electro-optical medium comprises a light switch for the radiation generated in the luminescent materials. The invention provides a device with a different arrangement of the interference filter with respect to the electro-optical medium and is characterized in that the interference filter is arranged between the electro-optical medium and the luminescent materials on the side of the electro-optical medium remote from the radiation source, whilst the electro-optical medium comprises a switch for the radiation exciting the luminescent materials. The phosphors preferably emit visible light, for example the three primary colours red, green and blue, whilst the electro-optical medium will generally be a liquid crystalline medium, although electroscopic displays may be used alternatively. There are also various possibilities for the nature of the interference filter used. For example, a so-called all-dielectric multi-layer filter comprising at least 20 but preferably 30 to 40 layers alternately having a high and a low refractive index may be chosen. An advantage thereof is that no absorption occurs in the filter, provided that the materials chosen are transparent both to the exciting and the generated radiation. The selected number of layers (at least 20) also ensures a very broad reflection band for the entire visible spectrum. On the other hand an advantageous choice is a metal-dielectric filter comprising only 3 to 5 layers of alternately a metal layer and a layer of dielectric material. Various choices are also possible for the phosphors. When using a radiation source based on the 254 nm Hg resonance line the following combination is very satisfactory: BaMg₂Al₁₆O₂₇:Eu as a blue phosphor (maximum emission at 450 nm); CeMgAl₁₁O₁₉:Tb as a green phosphor (maximum emission at 545 nm); Y₂O₃:Eu as a red phosphor (maximum emission at 612 nm). The associated emission wavelengths are satisfactorily suitable fo the maximum sensitivity of each of the three colour receptors of the eye; this renders a eminent colour rendition possible. When using a radiation source mainly with long-wave UV radiation (for example a high-pressure mercury lamp), for example ZnS:Ag (blue), (Zn,Cd)S:Cu,Al (green) and Y₂O₂S:Eu (red) are very suitable. The invention will now be described in greater detail and by way of example with reference to the accompnaying drawing in which Figure 1 is a diagrammatic cross-section of an embodiment of a display device according to the invention, Figure 2 is a diagrammatic cross-section of an interference filter for use in such display devices, and Figure 3 is a diagrammatic cross-section of a second interference filter for use in such display devices. Figure 1 shows diagrammatically a display device 1, in this embodiment a liquid crystal display device having a liquid crystal 2 as an electro-optical medium which is present between two parallel transparent substrates 3, 4 which are transparent for UV radiation. Transparent strip-shaped electrodes 5, 6 are provided on these substrates, in this embodiment electrodes of tin oxide, defining a matrix of switching elements. The electrodes are coated in this embodiment with a layer of orienting material 7, of, for example, silicon oxide. Furthermore, the device comprises a pattern 8 of luminescent materials (phosphors) which are provided in accordance with a given arrangement and which radiate in this embodiment red, green and blue light if they are irradiated by, for example, UV light 9 from a radiation source 10. The red, green and blue phosphor dots coincide with the switching points defined by the electrodes 5 and 6, whilst the liquid cyrstal 2 serves as a light switch for the light generated in the phosphors. The liquid crystal material 2 functions as a UV radiation switch and the phosphor pattern 8 is present on the viewing side of the device. The liquid crystal material and UV wavelength used can be optimized in such a way that a maximum contrast is achieved. In the embodiment shown the liquid ROTN 3010 from Messrs. Hoffmann-La Roche is chosen for the liquid crystal material and the wavelength of the exciting radiation 9 is approximately 370 nm. the cell structure is such that a twist of the director of approximately 270° occurs (SBE cell) between the surfaces of the two orientation layers 7. The polariser 11 and the analyserr 12 are suitable for UV radiation. If desired, an extra glass plate (transparent to UV light) may be arranged between the analyser 12 and the interference filter 14. The analyser 12 and the filter 14 may also change places. The device further comprises an extra substrate layer 13. There are various possibilities for the structure of the interference filter 14. For example, use can be made of a so-called "all-dielectric" multilayer filter comprising a minimum number of 20 layers alternately having a high and a low refractive index. Figure 2 shows such a filter 14 comprising 30 to 40 layers 21, 22, the layers 21 consisting of a material having a low refractive index and the layers 22 consisting of a material having a high refractive index. For example, magnesium fluoride (MgF₂) or silicon oxide (SiO₂) is chosen for the layers 21, which materials are suitable for all wavelengths. Materials which may be chosen for the layers 22 are, for example, hafnium oxide (HfO₂) which is suitable for all wavelengths or zirconium oxide (ZrO₂), suitable for λ > 350 nm, or one of the materials titanium oxide (TiO₂), tantalum oxide (Ta₂O₅) or niobium oxide (Nb₂O₅) which are suitable for λ > 370 nm. Their choice also depends on the choice of the radiation source 10. Alternatively, the filter may be in the form of a "metal-dielectric" filter (see Figure 3) of 3-5 layers, in this embodiment 5 layers alternately having a metal layer 23 and a dielectric layer 24. Both filters may be manufactured by means of electron-beam vapour-deposition techniques, whilst the filter of Figure 4 may be alternatively obtained by means of sputtering. A radiation source 10 is chosen which emits radiation in the wavelength range of 350-390 nm, for example, a high-pressure mercury lamp. As an alternative to the radiation source 10 a UV source may be chosen which directly emits the 254 nm Hg resonance line (for example, a low pressure mercury lamp with a quartz envelope). Although at this wavelength quartz must also be chosen for the substrates 3 and 13 phosphors which are very efficient at this wavelength are known for the display of blue, green and red such as: BaMg₂Al₁₆O₂₇:Eu as a blue phosphor, λ max = 450 nm; CeMgAl₁₁O₁₉:Tb as a green phosphor, λ max = 545 nm; Y₂O₃:Eu as a red phosphor, λ max = 612 nm. A long-wave UV source such as a high pressure mercury lamp may alternatively by chosen as a radiation source. The invention is of course not limited to the embodiments referred to, but several variations within the scope of the inventin are possible to those skilled in the art. For example, when choosing a radiation source having a longer wavelength (350-390 nm), normal glass instead of quartz glass can be chosen and ZnS:Ag (blue phosphor), (Zn,Cd)S:Cu,Al (green phosphor) and Y₂O₂S:Eu (red phosphor) are suitable as phosphors. The phosphor layer and the interference filter may be entirely provided on the outside (viewing side). To prevent parallax, the UV light 9 must then be substantially parallel. As already stated, a wide choice is also possible with respect to the electro-optical element (various types of LCD, such as TH-LCD, SBE, HBE, FELCD, electroscopic displays etc.). Finally a pattern-shaped second interference filter registering with the phosphor dots may be arranged on the other side (viewing side of the phosphor layer in which the filter parts only pass light at a small spatial angle and in which extra brightness is obtained at a small angle because the reflection greatly increases for light rays extending at a angle of more than 20 to 35° to the normal on the filter such as has been described for monochrome display devices in EP-A-0 275 601 filed simultaneously. Such a measure, in which the previously mentioned spatial broadening is compensated, may be useful in data display devices (data graphic displays).

Claims: A display device (1) comprising an electro-optical medium (2) between two parallel transparent substrates (3, 4) having drive means (17) to influence the transmission state of the electro-optical medium, one of the substrates having a pattern (8) of at least two luminescent materials, said display device further comprising a radiation source (10) suitable for emitting sufficiently short-wave radiation (9) to excite the luminescent materials, and an interference filter (14) being arranged between the radiation source (10) and the luminescent materials, which filter substantially completely passes the short-wave radiation and substantially completely reflects radiation generated in the luminescent materials, whereby the interference filter (14) is arranged between the electro-optical medium (2) and the luminescent materials on the side of the electro-optical medium remote from the radiation source (12), whilst the electro-optical medium (2) comprises a switch for the radiation exciting the luminescent materials. A display device as claimed in Claim 1, characterized in that the electro-optical medium (2) comprises a liquid crystalline material. A display device as claimed in Claim 1 or 2, characterized in that the interference filter (14) is a dielectric filter comprising at least 20 layers (21, 22) alternately having a high and a low refractive index, said layers substantially completely passing the exciting radiation and substantially completely reflecting the radiation generated in the luminescent materials. A display device as claimed in Claim 3, characterized in that the layers (22) having a high refractive index comprise one or more of the materials hafnium oxide (HfO₂), zirconium oxide (ZrO₂), titanium oxide (TiO₂), tantalum oxide (TaO₅) or niobium oxide (Nb₂O₅ and the layers (21) having a low refractive index comprise one or more of the materials magnesium fluoride (MgF₂) or silicon oxide (SiO₂). A display device as claimed in Claim 1 or 2, characterized in that the interference filter comprises 3-5 layers (28, 24) of alternately a metal layer(23) and a layer of dielectric material (24). A display device as claimed in any one of the preceding Claims, characterized in that the radiation source (10) emits radiation at a central wavelength of 254 nm and in that one or more of the following materials are chosen for the pattern of luminescent material. BaMg₂Al₁₆O₂₇:Eu as a blue phosphor; CeMgAl₁₁O₁₉:Tb as a green phosphor; Y₂O₃:Eu as a red phosphor. A display device as claimed in any one of Claims 1 to 5, characterized in that the radiation source emits radiation at a wavelength in the range of 360-380 nm. A display device as claimed in any one of the preceding Claims, characterized in that a pattern-shaped second interference filter is arranged on the other side of the pattern of luminescent material viewed from the radiation source, the pattern of said second interference filter registering with the pattern of luminescent material whilst the reflection of the second interference filters greatly increases for light rays which extend at an angle of more than 20 to 35° to the normal on the filter.

Inventors: VRIENS, LEENDERT
Applicants: N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: G02F1
IPC codes: G02B5, G02F1, G09F9