Abstract:
A transflective color display having apertures in reflective electrodes through which light from a backlight ( 9 ) passes in the transmissive mode. The switching behavior for both the reflective and the transmissive mode is made identical by introducing a retardation plate ( 11 ). The transmission efficiency is further increased by using only a monochrome green mode in transmission.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to a display device comprising a display panel with a first light-transmitting substrate provided with reflective material, a second light-transmitting substrate and an electro-optical material between said two substrates. 
     Such display devices are used, for example, in (portable) display screens in hand-held telephones, organizers but also, for example, in automotive applications. 
     A (transfective) display device of the type mentioned in the opening paragraph is described in IBM TDB Vol. 15, No. 8, pp. 2435-6. In the reflective state, ambient light is reflected by the reflective material, in this case a reflective electrode (a partly covered mirror) of, for example, chromium or aluminum. In the transmissive state, these electrodes pass light, and in the reflective state they reflect incident light. The actual picture elements (characters) are provided on the electrodes by means of etching. 
     To ensure that sufficient light can be passed in the transmissive state, the mirror must not be thick (in the case of aluminum, for example, thinner than 15 nm). It is very difficult to provide such mirrors with sufficient accuracy. Variations in thickness cause large variations in light transmission and, as a result, lead to non-uniform behavior in both the reflective state and the transmissive state. In the case of relatively large panels, the small thickness additionally influences the drive behavior because the square resistance becomes too high. 
     Another problem arises if birefringent material, for example twisted nematic (liquid-crystal) material is used in such a display device, because said material causes the transmission-voltage curve to be different in the transmissive mode and in the reflective mode. 
     SUMMARY OF THE INVENTION 
     The present invention aims, inter alia, at obviating one or more of the above-mentioned drawbacks. 
     To achieve this, a display device in accordance with the invention is characterized in that at the location of picture elements the reflective material is provided with at least one aperture. 
     By providing the layer of reflective material with (an) aperture(s) (occupying for example up to 30% of the surface area), sufficient light from a light source (backlight) is passed, while, on the other hand, the layer of reflective material (for example of aluminum) has such a thickness now (for example approximately 250 nm) that thickness variations of a few nanometers caused by process variations do not influence the uniformity of the display panel. Also the square resistance decreases considerably. 
     The above-mentioned apertures can be provided in individual picture electrodes in accordance with a pattern. In another embodiment, the aperture defines the individual picture elements. 
     A preferred embodiment of a display device in accordance with the invention is characterized in that the electro-optical material is switchable between two states having a different birefringence, the display panel is provided with polarizers and with a retardation foil between the first substrate and a first (back)polarizer. Particularly in the case of panels based on (super)twisted nematic effect (S)TN, the voltage dependence for the transmissive mode differs substantially from that for the reflective mode. For use in the reflective mode, a display panel is generally embodied so that, after passing a front polarizer, light of a(n) (average) wavelength λ is subject to a change in polarization in the liquid crystal material, such that, dependent upon the voltage, elliptically to circularly polarized light impinges on the reflecting electrode (retardation ¼λ). Dependent upon the drive voltage, after reflection a smaller or larger degree of extinction occurs at the location of the front polarizer. By providing a retardation filter between the first substrate and the first (back)polarizer, said apertures (in the transmissive mode) pass light at the location of the reflector, which light is elliptically polarized and, in particular, circularly polarized. As a result, the black-state is optimally corrected. Consequently, the voltage-dependence of the transmissive mode is practically identical to that of the reflective mode, so that the use of a single voltage region is sufficient, thus saving costs. 
     Depending on the circumstances, it may be sufficient to use a green light-emitting light source as the backlight. Generally, the transmissive mode is used during less than 5% of the above-mentioned applications, so that it is hardly disturbing that in the transmissive mode not the entire color palette is used. This means that very efficient green backlights can be employed. 
     Preferably, the apertures are situated at the location of green picture elements. The wavelength of the light source is preferably adapted to the transmission peak of the green part of a color filter present in the display cell. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the drawings: 
     FIG. 1 is a plan view of a part of a display device in accordance with the invention, 
     FIG. 2 is a variant of FIG. 1, 
     FIG. 3 is a cross-sectional view taken on the line III—III in FIG. 1, 
     FIG. 4 is a cross-sectional view taken on the line IV—IV in FIG. 2, 
     FIG. 5 shows the variation of the transmission and reflection as a function of the voltage for various types of devices, 
     FIG. 6 is a plan view of a part of another display device in accordance with the invention, and 
     FIG. 7 is a cross-sectional view taken on the line VII—VII in FIG. 6, and 
     FIG. 8 is a variant of FIG.  7 . 
    
    
     The Figures are diagrammatic and not drawn to scale. In general, like reference numerals refer to like parts. 
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG.  1  and FIG. 2 are schematic plan views, and FIG.  3  and FIG. 4 are cross-sectional views of a part of a display device comprising an electro-optical display cell, in this example a liquid crystal cell  1  with a twisted nematic liquid-crystal material  2  which is sandwiched between two transparent substrates  3 ,  4  of, for example, glass, provided with electrodes  5 ,  6 . The electrode  5  is made of a light-transmitting material, while the electrode  6  is made of a reflective or diffusely reflective material, such as aluminium or silver. The thickness of the reflective material is chosen to be such (150-400 nm) that no light is passed. To allow light originating from an illumination source (backlight) ( 9 ) to pass nevertheless in the transmissive state, the reflective electrode material is provided with at least one aperture. For example, the electrodes  6  are surrounded by apertures  10  (FIGS. 1,  3 ) or provided with apertures  10 ′ (FIGS. 2,  4 ) which occupy, for example, maximally 25% of the electrode surface. During operation in the reflective mode, incident light is now fully reflected by the electrodes  6  and absorbed at the location of apertures  10 ,  10 ′, which leads to an increase of the contrast, while during operation of the illumination source  9 , the apertures  10 ,  10 ′ allow sufficient light to pass. As shown in FIG. 4, the electrodes  6  may be provided, if necessary, on transparent (ITO) electrodes  12 . 
     Different electro-optical effects may be applied, in particular liquid crystal effects, such as (S)TN, guest-host, PDLC, ECB, ferro-electrics etc. In this example, the device comprises two polarizers  7 ,  8  whose directions of polarization are mutually perpendicular in this example. The device further includes orientation layers (not shown) which orient the nematic liquid crystal material at the inner walls of the substrates, in this example, in such a way that the liquid crystal layer has a twist angle of approximately 60 degrees. In this case, the liquid crystal material has a positive optical anisotropy and a positive dielectric anisotropy. Consequently, if the electrodes  5 ,  6  are energized by an electric voltage, the molecules and hence the directors orient themselves towards the field. In FIG. 5, the curve indicated by dashed lines shows the reflection-voltage characteristic of such a device. Incident light  13  is transformed at a voltage V 2   40   to elliptically (preferably circularly) polarized light which is reflected at the location of the reflecting electrode  6  and reaches the polarizer  7  as practically linearly (at right angles to the direction of polarization of the polarizer  7 ) polarized light and is absorbed (complete extinction). At a decreasing voltage across the liquid crystal cell, the birefringence increases until, at a voltage V 1 ′, the retardation of the liquid crystal layer is such that practically maximum reflection occurs. When the display cell is used in the transmissive mode, the transmission-voltage characteristic corresponds approximately to the continuous line in FIG. 5, if no special measures are taken. 
     In accordance with a further aspect of the invention, in this example, a retardation foil  11  is situated between the polarizer  8  and the liquid crystalline material  2 , which retardation foil converts linearly polarized light passed by the polarizer  8  into elliptically polarized light, preferably, of the same ellipticity as the light which, in the reflective mode, is incident on the reflective electrode  6  at a voltage V 2 ′. In the present example, in which the polarizers cross each other at right angles, a ¼λ plate is used as the retardation foil  11 , so that the light originating from the source  9  reaches the liquid crystal layer as circularly polarized light and the switching behavior (transmission-voltage curve) becomes practically identical to the dashed line shown in FIG.  5 . In particular, V 2  becomes practically equal to V 2 ′, so that the curves in the region near complete extinction coincide. 
     To reproduce color images, the device of FIG. 3,  4  is provided with a color filter  14 . As mentioned in the opening paragraph, the transmissive mode is generally used during less than 5% of the life time. A white light source (backlight)  9 , which emits all colors of the spectrum is generally less efficient (in lumens per watt) in the green portion of the spectrum where the eye is most sensitive. The color filter  14  absorbs light in a large portion of the spectrum, so that absorption of light from a white light source (backlight) increases further. For this reason, a green light source, for example an electroluminescent source or an LED, is often used in specific applications (particularly telephones, organizers) where the light source  9  is used comparatively rarely. In this case, the wavelength of the source  9  is adapted, for example, to the transmission peak of the green (part of the) color filter. If necessary, the green picture elements may also be embodied, so as to be larger (1.3-2 times) than the red or blue picture elements to further increase the transmission. If necessary, the green picture elements are embodied so as to form separate rows of picture elements between rows comprising both red and blue picture elements. 
     In the Table below, the light output in the transmissive mode is compared for 6,5″ reflective display panels with 640 (×3)×240 picture elements, having respectively, a green and a white light source. 
     
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                   
               
               
                   
                 green 
                 white 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 backlight efficiency (lm/W) 
                 6 
                 4 
               
               
                   
                 light output at 100 mW (cd/m 2 ) 
                 23.6 
                 15.7 
               
               
                   
                 Colour filter transmission (%) 
                 95% 
                 50% 
               
               
                   
                 light output display (cd/m 2 ) 
                 1.62 
                 0.56 
               
               
                   
                 Power at 2 cd/m 2  (W) 
                 124 mW 
                 357 mW 
               
               
                   
                   
               
             
          
         
       
     
     In both cases, the apertures  10  occupy approximately 20% of the overall surface area. As shown in the Table, the use of a green light source leads, under equal conditions, to a higher light output in the transmissive mode. Therefore, in applications in which the transmissive function is less important, it is more favorable to choose a green light source. 
     FIG. 6 is a plan view of a part of a display cell with a green picture element whose surface area is 1.4 times the surface area of a blue or red picture element (whose surface areas are identical). Since, as mentioned above, the wavelength of the light source  9  is adapted to the transmission peak of the green portion of the color filter, the apertures in the reflective (metal) layer  15  are situated only at the location of the green picture element. The overall surface area of the apertures  10  is approximately 28.5% of the overall surface area of the green picture element, so that for each of the three types of picture elements (red, green, blue) the reflective surface area is the same. 
     In this example, the reflector is embodied so as to be a separate metal layer  15  on which the (now light-transmitting) ITO picture electrode  16  is provided. A passivation layer  15  and the picture electrode  16 . The as in the previous examples. FIG. 8 shows a the metal layer  15 . The other elements, if any, etc., are not shown in FIG. 7,  8 .