FIELD OF THE INVENTION

The present invention relates to a display panel comprising a light valve layer, such as a liquid crystal display panel.

BACKGROUND OF THE INVENTION

Such a display panel is described e.g. in US 4,927,240. The display panel has two liquid crystal layers. In between the two layers a polarizer is present. Further external polarizers may be present on the outer surfaces of the display panel. Although such a construction improves the contrast ratio, the brightness of the display panel may not be sufficient to provide a good image quality.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a display panel, which is able to provide a high brightness image. This object is achieved by means of a display device according to claim 1. The invention is defined by the independent claims. The dependent claims define advantageous embodiments.

The arrangement is able to substantially eliminate the light leakage that, in prior art displays, occurs in dark areas and causes black not to be true black. Therefore a high contrast ratio may be achieved even at high brightness levels, which is substantially the product of the contrast ratios of the light valve layer and the auxiliary light valve layer, when neglecting the influence of reflective polarizers.

The auxiliary layer is sandwiched between a first and a second polarizer, the first and/or the second polarizer being a reflective polarizer. This allows light, that would otherwise be absorbed in parts of the display panel which have to reproduce a dark image, to be reflected. The reflected light may then be directed to other parts of the display, thereby increasing the achievable peak-brightness in these other parts. Consequently, such an arrangement results in a behavior similar to a cathode ray tube by allowing a relatively high peak brightness in parts of the display area.

Preferably, the primary light valve layer comprises an active matrix Liquid Crystal Display, also called LCD, layer, and the auxiliary layer comprises a passive matrix LCD layer. This allows a low-cost display to be produced.

Preferably a diffuser layer is sandwiched between the primary light valve layer and the auxiliary light valve layer. This reduces artifacts, resulting from the parallax between the patterns of the layers.

The display device may be applied in a direct view LCD or in a projection LCD device, wherein the image of the light valve layer is projected onto a projection surface. The display device may be used in a display apparatus, such as a television set, a projector, a computer monitor, a portable computer device, a presentation display or an electronic cinema. The apparatus further comprises signal processing circuitry for converting an input signal into a format suitable for driving the display device.

A display device comprising the display panel and a back-lighting arrangement with a reflector has the advantage that a relatively large part of the light reflected back by the reflective polarizer(s) is again reflected by this reflector, so that this light may be used in areas of the display panel, which have to reproduce a high brightness.

Preferably, the back-lighting arrangement comprises a reflector, which is placed to sandwich the light source between the reflector and one of the reflective polarizers. This provides a more energy efficient display. Preferably, a control signal is further provided to the light source in such a way that the brightness of light areas in the image is limited to a maximum level. Alternatively, or in combination, the control signal may limit an average brightness of the display device to a maximum average value over a predefined time interval. This lowers the energy consumption of the display. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 illustrates schematically in cross section a display device according to prior art,

Fig. 2 illustrates the display device of Fig. 1, modified according to an embodiment of the invention,

Fig. 3 illustrates more detailed a display device according to a preferred embodiment of the invention, in cross section,

Fig. 4 illustrates image data to be displayed by a display device according to an embodiment of the invention,

Fig. 5 illustrates image data, extracted from the image data in Fig. 4 and being intended to be supplied to an auxiliary light valve layer, Fig. 6 illustrates a control arrangement in a display device according to an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 illustrates schematically in cross section a display device according to prior art. The device comprises a display panel with a light valve layer 1, for instance an active matrix LCD with a plurality of individually controllable picture elements (pixels). The light valve layer 1 comprises a number of sub- layers, such as polarizers, color filters and a layer comprising the LC material, which is controlled by applying voltages to electrodes, as is known in the art. The device further comprises a back-lighting arrangement 2, e.g. comprising fluorescent tubes or light emitting diodes (LEDs). The back-lighting arrangement 2 projects preferably white light 3 towards the light valve layer 1 , which receives driving signals so as to modulate incoming light in different ways in different areas of the display (indicated by different hatchings in the light valve layer 1). Thus, the light valve layer 1 may emit e.g. red light 4 in one area. In a specific area 5, the display is intended to be black. However some leakage 6 of light normally occurs, which may cause that black will not be true black. This affects the contrast of the display negatively. The leakage 6 can of course be reduced by decreasing the light generation in the back-lighting arrangement 2, but this reduces also the brightness of the white parts. in the display device.

Fig. 2 illustrates the display device of Fig. 1, modified according to an embodiment of the invention. The display panel now comprises next to a primary light valve layer 1 an auxiliary light valve layer 7. The auxiliary light valve layer is arranged to reduce the light flow from the light source of the backlighting arrangement 2 to picture elements in the primary light valve layer 1, which has to display dark or black areas. This substantially eliminates the leakage 6 illustrated in Fig. 1 , without reducing the brightness in the white areas at all. Moreover, as will be shown later, light blocked by the auxiliary light valve layer 7 may even be re-used in other areas of the display, thereby increasing the brightness in the white areas.

Fig. 3 illustrates, in cross section and more detailed, a display device according to a preferred embodiment of the invention. The primary light valve layer 1

comprises an active matrix LCD arrangement 10 sandwiched between first 11 and second 12 absorption polarizers. Such a construction is known in the art and need not be discussed further.

The backlighting arrangement 2 in this embodiment comprises a light source with three fluorescent tubes 13, 14, 15 and a backlighting reflector 16, directing emitted light towards the primary light valve layer 1.

The auxiliary light valve layer 7 comprises a passive matrix TN (twisted nematic) display arrangement 18, which is relatively inexpensive. The display arrangement 18 is sandwiched between first 19 and second 20 reflective polarizers. Reflective polarizers provide the advantage that light that is blocked by the auxiliary layer 7 from reaching the primary light valve layer 1, is not absorbed, but reflected, for example, towards the back¬ lighting reflector 16. The backlighting reflector 16 is usually a white diffusor, which depolarizes the reflected light, whereafter it is reflected again towards the primary light valve layer 1. This light may therefore to a great extent be reused in areas of an image with a relatively high brightness.

The reflection at the second reflective polarizer 20 is straight forward: the polarized light that is not transmitted through the second reflective polarizer 20 is reflected to the reflector 16, which reflects back depolarized light again towards the second reflective polarizer 20. After a number of reflection cycles the majority of the light emitted by the tubes 13, 14, 15 is thus provided as polarized light to the display arrangement 18.

The reflection at the first reflective polarizer 19 is more complicated: If a high voltage is applied across LC cells of the auxiliary light value layer 7, then there is substantially no change of polarization direction of light passing through these LC cells. This light passes through the first reflective polarizer 19 to provide light to the primary light value layer 1.

If a low voltage is applied across LC cells of the auxiliary light value layer 7, then the polarization direction is changed 90°. In this case the first reflective polarizer reflects this light back through these LC cells. As a result, the polarization direction is changed back 90° and therefore the second reflective polarizer 20 passes this light back to the reflector 16, which reflects back depolarized light, which may be used again for providing light to LC cells which have to pass light to the primary light value layer 1. So, after a number of reflection cycles the majority of the light is transferred via the LC cells, which are receiving a high voltage, to the primary light value layer 1.

If an intermediate voltage is applied to LC-cells of the auxiliary light value layer 7, meaning a voltage between the low voltage resulting in a substantially 90° change of polarization and the high voltage resulting in substantially no change of polarization, than also "reflection cycles" take place, be it in a more complicated manner. The LC-cells of the auxiliary layer 7 which receive this intermediate voltage rotate the polarization direction of the transmitted light between 0° and 90°. As a result, part of this light is reflected by the first reflective polarizer 19. The polarization of this reflected polarized light, is partly rotated back by the concerned LC-cells. Some of this light will be reflected back by the second reflective layer 20 and is then reuseable. The rest of this light is passed through the 2 ^nd reflective layer 20 and then depolarized and reflected back via the reflector 16. Again after a number of reflection cycles, the majority of the light generated by the tubes 13, 14, 15 is used to pass light through the auxiliary light value layer 7 at the desired locations.

In this way a light recycling mechanism is provided. This entails a higher brightness in white areas of the display, or a lower power consumption at a given brightness level. The resolution of the auxiliary layer 7 may be as high as the resolution of the primary layer 1 , but embodiments where a lower resolution is used are also conceivable. Using a resolution in the auxiliary layer higher than the resolution of the primary layer is of course also possible, but considered uneconomical. A diffuser layer 21 may be sandwiched between the primary light valve layer 1 and the auxiliary light valve layer 7. The diffuser layer 21 is provided in order to avoid visual artifacts, caused by the parallax between the pixel patterns of the primary layer 1 and of the auxiliary layer 7, such as e.g. a visible moire pattern. This diffuser layer 21 should preferably be of a type which does not depolarize the transmitted light. The second absorption polarizer 12 and the first reflective polarizer 19 may be combined into one polarizer. The auxiliary layer may in principle also be color dependent, if desired. This may be realized by using an ECB (Electrically Controlled Birefringent) display panel in the auxiliary layer. Light passing through such a display panel may be spectrally controlled. As an alternative, three auxiliary layers in a stack may be used, each having reflective polarizers, reflecting light of an individual color, such as red, green and blue. By controlling the three auxiliary layers individually, the amount of red, green and blue light that reaches the primary panel may be individually controlled.

The above-described embodiment relates to a direct view LCD device. The invention may however also be realized in other ways, e.g. as an LCD projection device. The auxiliary layer may then be placed from a few millimeters to a few centimeters from the

primary layer. In many cases, no diffusing layer may be needed, due to the beam aperture of the illuminating light beam.

Fig. 4 illustrates schematically image data 22 to be displayed by a display device according to an embodiment of the invention. The data 22 comprise an image with two black areas 23, 24, and a plurality of areas, having other colors 25, 26, 27. In accordance with an embodiment of the invention, image data relating to black or dark areas of an image are extracted to provide auxiliary image data, to be fed to the auxiliary light valve layer 7. Fig. 5 illustrates such auxiliary image data 22', extracted from the image data in Fig. 4 and being intended to be supplied to an auxiliary light valve layer 7. Only dark or black areas, 23', 24' are visible in the auxiliary image. In general, the auxiliary layer 7 may be used to block or reduce light flow, i.e. may have both gray and black areas. A gray area in the auxiliary layer 7 may be combined with e.g. a red area in the primary layer to provide a dark red output.

Fig. 6 illustrates schematically a control arrangement in a display device according to an embodiment of the invention. The control arrangement comprises a control unit 30, which receives image data I, corresponding to an image to be displayed by the display device, e.g. corresponding to the image 22 in Fig. 4. The control unit 30 generates, based on these image data I, a primary control signal S _p , which is fed to the primary light valve layer 1 , and an auxiliary control signal S ₃ , which is fed to the auxiliary light valve layer 7. The auxiliary control signal S ₃ corresponds to the dark parts of the image, e.g. corresponding to the auxiliary image data 22' illustrated in Fig. 5. The primary control signal Sp may be adapted in dependence of the auxiliary control signal S _a , so as to ensure a correct relative brightness of all picture elements of the primary light valve layer 1.

The control unit 30 thus realizes a method of controlling a display device wherein a control signal is provided to an auxiliary light valve layer in such a way that the auxiliary light valve layer reduces light flow from the light source to picture elements in the primary light valve layer, which have to display dark parts of an image.

Preferably, the control signals S _n and S _a may be adjusted such that the primary light valve layer absorbs as little light as possible, i.e. has as high brightness as possible with regard to the displayed image information.

Preferably, the control unit 30 may also control the output of the light sources 13, 14, 15. For instance if only relatively small light or white areas appear in an image, the use of reflective polarizers concentrates the light flow to these areas, where the brightness may become too high. Therefore the control unit 30 preferably controls the output from the

light sources in such a way that the brightness of these areas is limited to a maximum level. This lowers the energy consumption. The output of the light sources may also be controlled based on an average brightness level of the auxiliary layer or of the image to be displayed.

In summary, the present invention relates to a display device, such as an active matrix LCD display with a primary LCD arrangement and an auxiliary light valve layer, such as a passive matrix LCD panel. This auxiliary layer may prevent light of the backlighting arrangement from reaching the primary arrangement at those areas where the display device has to display black or dark areas of an image, thus substantially eliminating light leakage in these areas. This provides improved contrast. Preferably the blocked light is re-used in other areas of the display by using reflective polarizers in the auxiliary layer and a reflector. The invention also relates to a method of controlling such a display panel.

It should be mentioned that the above-described embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. For instance, the primary light valve layer may be a high quality passive matrix display, such as a Ferro-Elcctric LCD or an STN (super twisted nematic) LCD display. Even if such displays by themselves do not reach the picture performance level achieved by the active matrix LCD, the use of an auxiliary passive LCD layer, as described above, may improve the performance of such a device to an acceptable level at a low cost. One or more of the layers of the display panel may be incorporated in the backlighting arrangement.