Projection device and associated display device

Abstract: Radiation from a radiation source (2) is con­ verted into light of a desired spectral character by caus­ ing the radiation from tne source (2) to excite luminescent material (18) which is present between two interference filters (22, 23). In combination with opto-electronic elements monochrome pictures are obtained which are suitable for use in projection television. The sandwich of luminescent material (18) and interference filters (22, 23) may form part of a display device (3), for example an LCD, but it may alternatively be integrated, together with the radiat­ ion source, to form a separate illumination source.

Description: The invention relates to a projection device comprising at least one radiation source and at least one monochrome display device for operation in the transmission mode, with an electro-optical means between two parallel substrates provided with drive means to cause the electro-optical means to change its transmission state at least locally and to define a pattern in the electro-optical means. The electro-optical means may be one of the many liquid crystalline materials, but alternatively the display device may be an electroscopic display. The invention also relates to a display device and an illumination source which can be used in such a projection device, but which are alternatively usable for other applications. It has been proposed quite frequently to replace the cathode ray tube in projection television devices by other display elements such as, for example LCD's. An example of such a device is given in the Article "LCD Fullcolor video projector" by S. Morozumi et al. in SID '86, Digest, pages 375 - 378. The device shown in this Article has a white light source, two dichroic mirrors, three LCD-display elements (light valves), one dichroic prism and a projection lens with an associated screen. The light from the source is split into three sub-beams (red, green, blue) and each beam passes an associated liquid crystal cell which serves as a light switch. After traversing the light switches, the three sub-pictures thus obtained are combined and imaged on the projection screen. A problem in such a projection device is the use of the white light source which has a wide spectrum as regards the emitted radiation. In the optical set-up of the system this gives rise to an incomplete split-up into the three primary colours (red, green, blue) and to chromatic aberration in the image on the projection screen. It is an object of the invention to obviate these drawbacks. To this end a projection device according to the invention is characterized in that it is provided with a layer of material luminescing in one colour, which material can be excited by radiation from the radiation source and in that a first interference filter is arranged between the radiation source and the layer of luminescent material, which filter passes radiation suitable for excitation and reflects radiation generated in the luminescent material, whilst a second interference filter is arranged on the other side of the layer of luminescent material, which filter has a high transmission for light of the desired spectral characteristic and in which the reflection considerably increases for light rays of the desired spectral characteristic extending at an angle having a value larger than 20° to 35° to the normal on the filter. Such a device has a high light output (> 85 %) and passes light of the desired spectral lines at small angles, whereas light of unwanted spectral lines cannot emanate at all due to the action of the second filter. The first interference filter is used to prevent loss of light due to emission and dispersion in the luminescent layer in the direction of the radiation source, as has been further described in the Application EP-A-0 272 760 filed simultaneously and published on 29.06.1988. As described in this Application the first interference filter and the luminescent layer, viewed from the radiation source, may be arranged both in front of and behind the liquid crystalline material; in the latter case the liquid crystalline material serves as a switch for the radiation emitted by the source. The use of luminescent material, in combination with the second interference filter, has the advantage that (sub)pictures are obtained which are substantially entirely monochromatic so that there is little chromatic aberration and no lens corrections are required. By providing the second interference filter a limited transmission angle is obtained for the desired spectral lines or band, while in addition the reflection in the direction of the luminescent layer considerably increases for light rays extending at an angle having a value larger than 20° to 35 to the normal on the filter. At larger angles up to 90° there is substantially no transmission. The light reflected at these larger angles is dispersed by the luminescent layer. A part thereof returns at smaller angles and can still contribute to the light output; by arranging such a second interference filter a light output gain by a factor of 1.8 to 2.4 is achieved in this application.. Interference filters having similar spectral properties are known per se from EP-A-0 170 320 for use in projection cathode ray tubes. In projection television devices with dichroic mirrors and interference filters having such a considerably limited transmission angle, there is not only a gain in brightness by a factor of 1.8 to 2.4 but also a gain in contrast by a factor of more than 2 and also a considerable colour improvement because the second interference filter also ensures a spectral filtering, particularly filtering out long-wave spectral components. If there are no interference filters, these long-wave spectral components are not correctly reflected by the dichroic mirrors (for the blue component) or passed (for the green component), which leads to a colour variation across the projection screen and also to a loss of contrast via multiple reflections. When using interference filters in combination with the dichroic prisms or mirrors, this problem is prevented or is reduced to a considerable extent. The second interference filter preferably has a number of layers (at least 9, but preferably 14) consisting alternately of a material having a high refractive index and of a material having a low refractive index, each with an optical thickness n . d in which n is the refractive index of the material of the layer and d is the thickness, which optical thickness n . d is between 0.2 λ f and 0.3 λ f in which λ f is equal to p x λ , in which λ is the central wavelength of the relevant display device and p is a number between 1.16 and 1.30. The average optical thickness is 0.25 λ f and λ f is the central wavelength of the filter. The filter thus comprises exclusively or substantially exclusively layers having an optical thickness of approximately 0.25 λ f . This gives the filter the special property of the very broad reflection band (no transmission) for light rays which extend at an angle having a value larger than 20° to 35° and 90° to the normal on the filter. The layer thicknesses in the filter according to the invention are thus chosen to be such that at a desired wavelength the reflection starts to increase considerably for light rays from the luminescent material which extend at an angle having a value larger than 20° to 35° to the normal on the filter. The broad reflection band ensures that light rays extending at angles having a value larger than 20° to 35° are reflected as much as possible and after dispersion in the luminescent material again get a chance of emanating in the forward direction, that is to say within an angle of at most 20° to 35° to the normal on the filter. Consequently, a maximum light output in the forward direction is produced, which is at least 60% larger than without use of the filter. In addition the broad reflection band ensures that the halo effect is greatly reduced and that light emanates to a smaller extent at large angles. The optical thickness n.d of the layers of the filter is preferably not equal for all layers but varies between 0.2 λ f and 0.27 λ f . Due to this variation in thickness a flatter response characteristic of the filter is obtained . Since the response characteristic of the filter is wavelength-dependent, a combination of the relevant phosphor and an adapted filter provides the possibility of improving the colour point of the light rays which traverse the filter within an angle of 20° to 35° to the normal........... There are various possibilities for the first interference filter to be used. For example, it is possible to choose a so-called "all-dielectric" multilayer filter comprising at least 6 but preferably 9 to 30 layers alternately having a high and a low refractive index. A sufficiently broad reflection bandwidth for one colour can be obtained with this number of layers. An advantage is that there is no absorption in the filter, provided that the materials chosen are transparent both to the exciting and to the generated radiation. On the other hand a "metal-dielectric" filter may alternatively be chosen 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 predominantly long-wave UV-radiation, for example 360-380 nanometers, for example a ZnS: Ag - blue phosphor; (Zn, Cd)S:Cu, Ag - green phosphor; Y2DtS: Eu - red phosphor are suitable. When using a radiation source based on the 254 nm Hg-resonance line the following combination is very satisfactory: BaMg₂Al₁₆0₂₇:Eu as a blue phosphor (maximum emission at 450 nm); CeMgAl₁₁0₁₉: Tb as a green phosphor (maximum emission at 545 nm); Y₂0₃: Eu as a red phosphor (maximum emission at 612 nm). The associated emission wavelengths are readily suitable for the maximum sensitivity of each of the three colour receptors of the eye; this renders an eminent colour display possible. The invention also relates to a display device which is particularly but not exclusively suitable for use in the projection device described. Such a device may also be used, for example in monochrome display devices (data graphic displays). Such a display device comprising electro-optical means between two parallel substrates provided with drive means to cause the electro-optical means to change its state at least locally is characterized in that one of the substrates is provided with a layer of monochrome luminescent material between a first interference filter which passes radiation suitable for excitation of the luminescent material and reflects radiation generated in the luminescent material, and a second interference filter having a high transmission for light of the desired spectral characteristic which is emitted by the luminescent layer at small angles to the normal on the filter and in which the reflection considerably increases for light rays of the said spectral characteristic which extend at an angle having a value larger than 20° to 35° to the normal on the filter. The invention also relates to an illumination source for a display device. This illumination source comprising a space accomodating a radiation source and bounded by at least a flat wall is characterized in that the wall is provided with at least one layer of material luminescing in one colour, which can be excited by radiation source and in that a first interference filter is arranged between the radiation source and the layer of luminescent material, which filter passes the radiation suitable for excitation and reflects radiation generated in the luminescent material, whilst a second interference filter is arranged on the other side of the layer of luminescent material, which filter has a high transmission for light of the desired spectral characteristic which is emitted by the luminescent layer at small angles to the normal on the filter and in which the reflection considerably increases for light rays of said spectral characteristic which extend at an angle having a value larger than 20° to 35° to the normal on the filter. Consequently, a light source can be obtained which emits light of a desired colour (determined by spectral lines or band) at a limited angle. Besides its use as a separate light source for the projection LCD-TV described, such a light source may also be used in, for example direct-vision display devices in which a small viewing angle is sufficient. The invention will now be described in greater detail with reference to the accompanying drawings in which Figure 1 shows diagrammatically a first embodiment of a projection device according to the invention; Figure 2 is a diagrammatic cross-section of a first embodiment of a display device for use in a projection device according to the invention; Figure 3 is a diagrammatic cross-section of a modification of the device of Figure 2; Figure 4 is a diagrammatic cross-section of a second embodiment of such a display device; Figure 5 is a diagrammatic cross-section of an interference filter; Figure 6 is a modification thereof, whilst Figure 7 shows diagrammatically a drive circuit of such a device, and Figure 8 shows diagrammatically an illumination source according to the invention. Figure 1 shows diagrammatically a projection device 1 having three radiation sources 2, mirrors 43 and three monochrome display devices 3 in which a red (3R), a green (3G) and a blue (3B) picture are generated. The three pictures are imaged via dichroic prisms 4 and a lens 5 on a screen 6 on which they register in such a manner that an overall colour picture is obtained. The axes of the display devices 3B and 3R coincide and are perpendicular to the axis 11 of the display device 3G wnich also constitutes the principle axis of the projection device. Tne pictures from the display devices 3B and 3R may not only be imaged on the screen 6 by means of prisms 4 but also by means of two plane dichroic mirrors which intersect each other and each of which extends at an angle of 45° to the axis 11 and which extend perpendicularly to the plane of the axes of the display device through the point of intersection of the axes. According to the invention the device 1 comprises a radiation sources 2 which emits radiation 10 (in this embodiment UV radiation). This radiation which is shown as a single line is actually incident through a wide angle range and has such a wavelength that it can excite phosphors present in the display devices 3 according to the invention. The radiation sources 2, for example high-pressure mercury lamps, need not be entirely identical. The gas filling of the lamps may be adapted in such a manner that the emission spectrum satisfactorily suits the associated phosphor layer. Different wavelengths for the different phosphors may be desired, for example UV radiation for the blue phosphor,, blue light for the green phospnor and blue or blue-green light for the red phosphor. All sources 2 may also emit long-wave UV radiation with the following phosphors being satisfactory: ZnS:Ag (Bleu). (Zn,Cd)S: Cu, Ag (green) and Y₂0₂S: Eu (red). Figure 2 shows diagrammatically one of the display devices 3, in this embodiment a liquid crystal display device having a liquid crystal 12 as an electro-optical medium which is present between two parallel transparent substrates 13, 14 of, for example, glass. Transparent strip-shaped electrodes 15, 16,in this embodiment of tin oxide, defining a matrix of switching points are present on these substrates. In this embodiment the electrodes are coated with a layer of orienting material 17 of, for example, silicon oxide. In the relevant embodiment the display device also comprises a polariser 19 and an analyser 20 while the assembly is secured to a support 21. This support must be transparent to radiation from the radiation source 2 and will be made of, for example quartz when using short-wave UV radiation. According to the invention the display device 3 has a phosphor layer 18 which, dependent on the type of phosphor used, emits, for example red, green or blue light if the layer 18 is excited by radiation 10 suitable for this purpose, and a first interference filter 22 is arranged in the radiation path in front of the phosphor layer 18, which filter substantially completely passes the radiation 10 and substantially completely reflects the light generated in the phosphor layer. It is achieved thereby that light generated in the phosphor layer 18 which would otherwise leave the phosphor layer 18 on the side of the filter 22 is reflected by this layer 22 and thus contributes to the overall light output. In this way a greater brightness (2 to 3 times) is obtained than if the interference filter 22 were not provided. According to the invention the device comprises a second interference filter 23 having such a composition that the colour generated in the phosphor layer 18 is passed at a limited transmission angle. As already stated , the use of such a filter not only leads to a gain in light output but also to an improvement of colour purity and contrast. Various materials may be chosen for the liquid crys t alline material 12. For example, use can be made of the so-called twisted nematic materials, but materials whose effect is based on birefringence such as, for example described in EP-A-0,131,216 (270°-twist) or in DE-A-34,31,871 (180°-twist) or ferro-electric liquid crystals are alternatively suitable. The invention may be alternatively used for other electro-optical display devices used in the transmission mode such as , for example (dry) electroscopic displays. Figure 3 shows diagrammatically a modification of the device of Figure 2 in which the display device is divided into a part 3 A accommodating the liquid crystal 12 and the electrodes 15, 16 with the associated intermediate layers and a part 3 B in which, for example UV radiation 10 which is incident at a wide angle range is converted into visible substantially monochromatic radiation 10′ with a small angle range in the direction of propagation. The part 3 B accommodates the fluorescent layer 18 and the interference filters 22, 23 between supports 21 and 28. The polarisers 19 and 19′ may optionally be provided on the part 3 A or on the part 3 B . Tne structure shown has the advantage that heat generated in the fluorescent layer 18 is less likely to produce a higher temperature in the liquid crystal. This effect can be enhanced by utilizing the gap 29 between the parts 3 A and 3 B for cooling purposes, for example by means of a suitably transparent coolant. The reference numerals for the remaining components have the same significance as those in Figure 2. Figure 4 shows a further embodiment of a device according to the invention. The liquid crystal material 12 now functions as a switch for the exciting radiation, for example UV radiation, and the phosphor pattern 18 is on the other side. The liquid crystal material used and the wavelength may be optimized in such a way that a maximum contrast is achieved. In the relevant embodiment the liquid crystal ROTN3010 from Messrs. Hoffmann-Laroche is chosen and the wavelength of the exciting radiation 10 is approximately 370 nanometers. The cell structure is such that a twist of the director of approximately 270° (SBE-cell) occurs between the surfaces of the two orientation layers 17. Otherwise the reference numerals in Figure 3 have the same significance as those in Figure 2. In this respect it is to be noted that the polariser 19 and the analyser 20 are now suitable for UV radiation while also the substrate 13 is transparent to UV radiation. In addition the radiation 10 must now be substantially parallel. If desired an extra glass plate (transparent to UV radiation) can be arranged between the analyser 20 and the interference filter 22. The analyser 20 and the filter 22 may also change places. Figure 5 shows by way of example a 20-layer filter 23 and the adjacent layer of phosphor 18 (Ph). The filter consists of layers 24 (Si0₂) denoted by a letter L and L′ (refractive index n = 1.47) and layers 25 (Ti0₂ ( n = 2.35)) denoted by a letter H. The layers have a thickness of approximately 0.25 λ f . The last layer 25 which is approximately 0.25 λ f thick is coated with a 0.125 λ f thick terminal layer 24 (L′). The phosphor 18 for, for example green is either a Tb-pnosphor with λ = 545 nm or willemite (Zn₂Si0₄: Mn) with λ = 535 nm. At p = 1.20, λ f will be equal to 654 nm for Tb and 642 nm for willemite. The composition of the filter 23 is shown in the following Table: Generally the filters 22 have at least 6 but preferably 9 to 30 layers alternately having a low refractive index and a high refractive index. For the layers 24 having a low refractive index, for example magnesium fluoride (MgF₂) or silicon oxide (Si0₂) is chosen, which materials are suitable for all wavelengths. For the layers 25, for example hafnium oxide (Hf0₂) is chosen which is suitable for all wavelengths, or zirconium oxide (Zr0₂) which is suitable for λ > 350 nm, or one of the materials titanium oxide (Ti0₂), tantalum oxide (Ta₂0₅) or niobium oxide (Nb₂0₅) which are suitable for λ > 370 nm. The choice also depends on the choice of the radiation source. The filter 22 may also be a "metal-dielectric" filter (see Figure 6) of 3-5 layers, in this embodiment 5 layers alternately with a metal layer 26 and a dielectric layer 27. Both filters 22 and 23 may be manufactured by means of electron beam vapour-deposition techniques, whilst the filter of Figure 6 may also be obtained by means of sputtering. Instead of a long-wave UV source, an UV source which directly emits the 254 nm Hg resonance line may be alternatively chosen as a radiation source (for example a low-pressure mercury lamp with a quartz envelope). Although at this wavelength quartz must be chosen for the substrate 13 in the device of Figures 2, 3, very efficient phosphors for the display of blue, green and red at this wavelength are known, such as: BaMg₂Al₁₆0₂₇: Eu as a blue phosphor, λ max = 450 nm; CeMgAl₁₁0₁₉ Tb as a green phosphor, λ max = 545 nm; Y₂0₃: Eu as a red phosphor, λ max = 612 nm. Figure 7 shows diagrammatically a drive unit 31 for processing signals for a projection device as described above. The drive unit 31 which is fed via one or more terminals 33 receives, for example a video signal via an input 32. Dependent on the state of the drive unit, which can be adjusted by means of control signals 34, sub-signals are applied for the three sub-colours to the sub-drive units 35 (35 B , 35 G and 35 R , respectively) via signal lines 38 (38 B , 38 G and 38 R ). The sub-drive units 35 drive the display devices 3 by means of signal lines 39. For the purpose of mutual synchronisation, synchronising and clock signals are generated in the drive unit 31, which are applied via synchronising lines 36 and clock lines 37 to the sub-drive units 35 and the display devices 3, respectively. Finally, Figure 8 shows in a cross-section an illumination source 40 according to the invention which can be used, for example in the device of Figure 3 but also in other devices in which light having given properties (spectral properties, limited angle range, polarisation) is desired. The radiation source 2 is present in a space having a structure 41 constituting a flat wall on one side, whilst the radiation 10 emitted by the source 2 is incident on the flat wall structure 41 at different angles via a second, reflective, wall 42 which is, for example convex or cylindrical. The flat wall structure 41 has a phosphor layer 18 between two interference filters 22, 23 and supports 21, 28 having similar properties as described with reference to the previous embodiments. If necessary, a polariser 19 may be provided. Radiation emitted bh the source 2 (for example UV radiation) is converted by this combination into long-wave light 10 having a small angle range, whilst the interference filter 23 provides a possibility of a very satisfactory colour selection with a light output increase by a factor of 1.8 to 2.4. Since the use of the interference filter 22 increases the light output by a factor of 2 to 3,the gain in light output with respect to a similar device without interference filters is a factor of 4 to 7. This also applies if the combination of phosphor and interference filters is directly provided on a liquid crystal or another electro-optical device.

Claims: A projection device comprising at least one radiation source (2) and at least one monochrome display device (3) for operation in the transmission mode, with an electro-optical means (12) between two parallel substrates (13, 14) provided with drive means to cause the electro-optical means to change its transmission state at least locally and to define a pattern in the electro-optical means characterized in that the display device is provided with a layer of material (18) luminescing in one colour, which material can be excited by radiation (10) from the radiation source (2) and in that a first interference filter (22) is arranged between the radiation source (2) and the layer of luminescent material (18), which filter passes radiation suitable for excitation and reflects radiation generated in the luminescent material, whilst a second interference filter (23) is arranged on the other side of the layer of luminescent material (18), which filter has a high transmission for light of the desired spectral characteristic which is emitted by the luminescent layer at small angles to the normal on the filter and in which the reflection considerably increases for light rays of the said spectral characteristic which extend at an angle having a value larger than 20° to 35° to the normal on the filter. A display device (2) comprising electro-optical means (12) between two parallel substrates (13, 14) provided with drive means to cause the electro-optical means to change its state at least locally, characterized in that one of the substrates (13, 14) is provided with a layer of monochrome luminescent material between a first interference filter (22) which passes the radiation suitable for excitation of the luminescent material and reflects radiation generated in the luminescent material, and a second interference filter (23) having a high transmission for light of the desired spectral characteristic which is emitted by the luminescent layer at small angles to the normal on the filter and in which the reflection considerably increases for light rays of the spectral characteristic which extend at an angle having a value larger than 20° to 35° to the normal on the filter. An illumination source (40) for a display device comprising a space accommodating a radiation source (2) and bounded by at least a flat wall (41), characterized in that the wall is provided with at least one layer of material (18) luminescing in one colour, which can be excited by the radiation source and in that a first interference filter (22) is arranged between the radiation source and the layer of luminescent material, which filter passes the radiation suitable for excitation and reflects radiation generated in the luminescent material, whilst a second interference filter (23) is arranged on the other side of the layer of luminescent material, which filter has a high transmission for light of the desired spectral characteristic which is emitted by the luminescent layer at small angles to the normal on the filter and in which the reflection considerably increases for light rays of the said spectral characteristic which extend at an angle having a value larger than 20° to 35° to the normal on the filter. A device as claimed in Claim 1 or Claim 2, characterized in that the electro-optical means comprises a liquid crystalline material. A device as claimed in any one of the preceding Claims, characterized in that the first interference filter is a dielectric filter comprising at least six layers alternately having a high and a low refractive index. A device as claimed in Claim 5, characterized in that the layers having a high refractive index comprise one or more of the materials hafnium oxide (HfO₂), zirconium oxide (ZrO₂), titanium oxide (TiO₂), tantalum oxide (Ta₂O₅) or niobium oxide (Nb₂O₅) and the layers having a low refractive index comprise one or more of the materials magnesuim fluoride (MgF₂) or silicon oxide (SiO₂). A device as claimed in any one of Claims 1 to 6, characterized in that the first interference filter has 3 to 5 layers of alternately a metal layer (26) and a layer of dielectric material (27). A device as claimed in any one of the preceding Claims, characterized in that the second interference filter comprises a plurality of layers (24, 25) manufactured alternately from a material having a high refractive index and from a material having a low refractive index and is composed of at least nine layers each having an optical thickness n . d ., in which n is the refractive index of the material of the layer and d is the thickness, said optical thickness n . d . being between 0.2 λ f and 0.3 λ f in which λ f is equal to p x λ, in which λ is the central wavelength of the relevant display device, in which p is a number between 1.16 and 1.30 and in which the average optical thickness is 0.25 λ f . A device as claimed in Claim 8, characterized in that the second interference filter is composed of 9 to 30 layers. A projection device as claimed in Claim 9, characterized in that the optical thickness n . d . is between 0.2 λ f and 0.27 λ f .

Inventors: VRIENS LEENDERT, VRIENS, LEENDERT
Applicants: PHILIPS NV, N.V. PHILIPS' GLOEILAMPENFABRIEKEN
Application year: 1987
Main classification: H04N9
IPC codes: H04N9, G02F1, H04N3, G09F9