Abstract:
Transflective display device provided with an optical waveguide which splits light coming in from aside into two light beams having a mutually opposite circular polarization. Polarization splitting is achieved at the interface of areas in the optical waveguide having a chiral nematic structure.

Description:
FIELD OF THE INVENTION 
     The invention relates to a display device comprising an image display panel having a first substrate which is provided with light-reflecting electrodes at the area of pixels, an illumination system comprising an optical waveguide of an optically transparent material having an exit face facing the image display panel and a plurality of end faces, at least one of said end faces being an entrance face for light, while light can be coupled into said end face of the optical waveguide. 
     The image display panel may comprise an electro-optical medium (between two substrates) such as liquid crystalline material or an electrochromic material. It may also be based on electrostatic forces (deformable mirrors). 
     The invention also relates to an illumination unit (or front light) for such a display device and to methods of manufacturing such illumination units. 
     Such reflective display devices are used in, for example, portable apparatus such as laptop computers, mobile telephones, personal organizers, etc. With a view to saving energy, it is desirable that the light source can be switched off in the case of sufficient ambient light. 
     BACKGROUND OF THE INVENTION 
     A display device of the type mentioned above is described in WO 99/22268 (PHN 16.374). In the optical waveguide described in this document, an unpolarized beam from the light source is split up into two mutually perpendicularly polarized beam components. Polarization separation is obtained by causing the unpolarized beam to be incident on an interface between an area of isotropic material having a refractive index n p  and an area of anisotropic material having refractive indices n o  and n e , in which one of the two indices n o  or n e  is equal or substantially equal to n p . When an unpolarized beam is incident on such an interface, the beam component which does not experience any refractive index difference at the transition between isotropic and anisotropic material is passed in an undeflected form, whereas the other beam component is deflected or reflected. One of the two beam components is subsequently passed by a polarizer to a reflective liquid crystal panel. The optical waveguide shown exhibits much less image distortion than a known optical waveguide with a groove structure (microprisms) on the viewing side of the optical waveguide. The image distortion is produced because the groove structure has different slopes, which results in multiple image formation. Generally, this multiple image formation is prevented by providing an optical compensator having a complementary groove structure. 
     However, in the display device described in WO 99/22268 (PHN 16.374), stray light is generated in the viewing direction on the interface between the areas with isotropic and anisotropic material. 
     Moreover, the light which is deflected in the direction of the image display panel sometimes undergoes partial reflections in the image display panel and in the optical waveguide before the light reaches the reflecting pixels. 
     These drawbacks apply to the same or an even greater extent to optical waveguides which are based on a groove structure. 
     SUMMARY OF THE INVENTION 
     It is, inter alia, an object of the present invention to provide a solution to the above-mentioned problem. 
     To this end, a display device according to the invention is characterized in that the optical waveguide is present between the image display panel and a circular polarizer, and the optical waveguide comprises polarizing means for substantially circularly polarizing the entering light. In this application, the word circular is also understood to be “elliptical”. In certain circumstances (when less contrast is sufficient) it is also possible to work with elliptically polarized light. 
     The polarizer may be integrated in the display device. 
     The polarizing means have a similar function as in the known device, namely polarizing light rays from the light source, in which light of one kind of polarization (for example, levorotatory polarization) is deflected in the direction of the image display panel. In the relevant case, light exiting on the viewing side (dextrorotatory polarized light in the same example) is not passed by the polarizer. 
     Due to polarization, an unpolarized beam from the light source is split up into two mutually oppositely polarized beam components (levorotatory and dextrorotatory). Such a polarization separation is obtained, for example, by causing the unpolarized beam to be incident on an interface between an area of isotropic material and an area of chiral nematic material, for example, a chiral nematic liquid crystal material provided in, for example, a groove structure, or a (patterned) chiral nematic network. When an unpolarized beam is incident on such an interface, a beam component of one handedness is passed undeflected on the transition between isotropic and chiral nematic material, while the beam component having the other, opposite handedness is deflected or reflected. 
     A suitable embodiment is characterized in that the pitch of the chiral nematic liquid crystal material or the chiral nematic polymer network within a groove varies. A larger bandwidth of the reflected light can thereby be obtained. 
     A first method of manufacturing such an illumination unit (or front light) provided with polarizing means for circularly (or elliptically) polarizing the entering light is characterized in that a surface of a transparent body is provided with grooves, and the transparent body within the grooves is provided with a chiral nematic liquid crystal material or the chiral nematic polymer network. 
     A second method is characterized in that a surface of a transparent body is provided with a layer of a chiral nematic material which is locally converted into isotropic material. 
     These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is a cross-section of an embodiment of a reflective display device according to the invention. 
     FIG. 2 is a cross-section of an optical waveguide, while 
     FIGS. 3 and 4 are variants of FIG. 2, and 
     FIGS. 5 to  10  are cross-sections of an optical waveguide during one stage of a plurality of possible manufacturing methods. 
    
    
     The Figures are diagrammatic and not to scale. Corresponding components generally have the same reference numerals. 
     DETAILED DESCRIPTION OF THE INVENTION 
     The display device  1  shown diagrammatically in FIG. 1 comprises an image display panel  2  and an illumination system (or front light)  8 . 
     The image display panel  2  comprises a liquid crystalline material  5  between two substrates  3 ,  4 , based on the twisted nematic (TN), the supertwisted nematic (STN) or the ferroelectric effect so as to modulate the direction of polarization of incident light. The image display panel comprises, for example, a matrix of pixels for which light-reflecting picture electrodes  6  are provided on the substrate  3 . The substrate  4  is light-transmissive and has one or more light-transmissive electrodes  7  of, for example, ITO (indium tin oxide). The picture electrodes are provided with electric voltages via connection wires  6 ′,  7 ′ which are provided with drive voltages by means of a drive unit  9 . The substrates and electrodes are coated with orientation layers  15  in known manner. 
     The illumination system  8  comprises an optical waveguide  18  which is made of an optically transparent material and has four end faces  10 ,  10 ′. A light source  12  whose light is coupled into the optical waveguide  18  via one of the end faces, for example  10 , is situated opposite this end face. The light source  12  may be, for example, a rod-shaped fluorescence lamp. The light source may alternatively be constituted by one or more light-emitting diodes (LED), notably in flat panel display devices having small image display panels such as, for example, portable telephones. Moreover, the light source  12  may be detachable. 
     The exit face  16  of the optical waveguide  8  faces the image display panel  2 . Each end face  10 ′ of the transparent plate in which light is not coupled in may be provided with a reflector. In this way, light which is not coupled out on the exit face  16  and consequently propagates through the optical waveguide and arrives at an end face is thus prevented from leaving the optical waveguide  11  via this end face  10 ′. 
     To prevent light from leaving the optical waveguide  18  without contributing to the light output of the illumination system, light of the lamp  12  is preferably coupled into the optical waveguide  18  via coupling means  13 , for example, by means of a wedge-shaped optical waveguide which limits the angle of the entering beam with respect to the exit faces  16 ,  17  to, for example, 15 degrees. Moreover, the contrast is enhanced because there is no stray light. 
     A light beam  20  from the lamp  12  is converted in a manner to be described below into circularly polarized light so that mainly light of one handedness is deflected towards the reflective image display panel  2  (beams  21 ) and, dependent on the state of a pixel, reflected (beam  22 ) with the same or the opposite handedness. After reflection on the pixel, the circularly polarized light of the opposite handedness is converted in a phase plate or retarder  24  into linearly polarized light and reaches a polarizer  25  with such a direction of the transmission axis in this embodiment that the reflected light is absorbed. Similarly, circularly polarized light of the same handedness is passed by the polarizer  25 . 
     Stray light, which is reflected on internal surfaces (for example, the surface  16 ), has a handedness which is opposed to that of the beam  22  and is also converted by the retarder  24  into linearly polarized light which is absorbed by the polarizer  25  (beams  26 ). Also parasitic light generated in the optical waveguide  18  due to internal reflection is absorbed by the polarizer  25  (beam  27 ). 
     FIG. 2 is a cross-section of a first embodiment of an optical waveguide with which the above-mentioned effect can be achieved. On an exit face  19 , the optical waveguide  18  has a plurality of grooves  30  which are filled with a chiral nematic liquid crystalline mixture and are covered with a 20-50 μm thick plate  31  of, for example, acryl or glass. On the side facing the light source, the grooves  30  preferably extend at an angle of 45 degrees to the surface  19  so that light  21  coupled out by the grooves leaves the optical waveguide substantially perpendicularly to the surface  16  in the direction of the display device  2 . Consequently, a very efficient illumination of the reflective display device  2  is achieved. Since the grooves are filled with chiral nematic liquid crystalline material, levorotatory or dextrorotatory circularly polarized light is reflected (beams  21 ), dependent on the material used and on the surface treatment. In this embodiment, levorotatory light is reflected in a spectral range determined by the pitch p of the chiral nematic liquid crystalline material and the refractive indices n e , n o  (n e : extraordinary refractive index and n o : ordinary refractive index); light having a wavelength in the range between λ e =n e .p and λ o =n o .p is reflected. Dextrorotatory polarized light  20 ′ remains within the optical waveguide  19  due to reflection on the surfaces  16 ,  19  and due to a favorably chosen angle of incidence of the beam  20 , and, after internal reflections can again be reflected on a groove  30 . 
     In the embodiment of FIG. 3, the grooves  30  are provided with chiral nematic polymer networks. The pitch of the chiral nematic material in each groove  30 R,  30 G,  30 B is adapted in such a way that red (beam  21 R), blue (beam  21 B) and green (beam  21 G) light is reflected and leaves the optical waveguide substantially perpendicularly to the surface  16  in the direction of the display device  2 . The reference numerals again denote the same components as those in FIG.  2 . In this way, different grooves couple different parts of the spectrum, with a very good adaptation being possible to the wavelength of the light source(s)  12 , notably when LEDs having a narrow emission spectrum are used for this purpose. When the choice of the liquid crystal material and the pitch limits the reflection band to a very narrow band (at most equal to that of the spectrum emitted by the LED) the light to be reflected and the reflected light are minimally disturbed during use in reflection (when the light source  12  is switched off). 
     The pitch of the chiral nematic material within a groove  30  may also vary to such an extent that a wide spectrum is reflected so that each groove  30  reflects beams  21 R,  21 G and  21 B (FIG.  4 ). 
     The mutual parts (display device, optical waveguide and retarder-polarizer combination) are preferably mutually secured by means of a transparent adhesive having a low refractive index. The choice of a low refractive index also prevents the above-mentioned parasitic reflections. 
     FIGS. 5 to  7  show the method of manufacturing an optical waveguide which does not have microgrooves but generates circularly polarized light which is deflected towards the reflective image display panel  2 . A thin layer  33  of a chiral nematic liquid crystal polymer material is provided on a basic substrate  32  of, for example, glass and is coated, if necessary, with an isotropic transparent protective coating  34 . The coating  34  is subsequently made locally isotropic, in this case by means of laser beams  35  which are incident at an angle of 45 degrees. The chiral nematic liquid crystal polymer material remains anisotropic in the areas which are not irradiated. By suitable choice of the ratio between isotropic and anisotropic areas, an optical waveguide  18  is obtained with areas  30  which convert an incident light beam into light beams  21  leaving the optical waveguide in the direction of the display device  2 . 
     In the method shown in FIG. 6, the substrate is coated with a mixture of chiral nematic monomers, which mixture is subsequently exposed via the mask  36  (by means of, for example UV radiation  37  again incident at an angle of 45 degrees) up to a temperature below the isotropic transition temperature. The chiral nematic ordering is thereby locally frozen (areas  30 ). Subsequently, the assembly is heated to a temperature above the isotropic transition temperature (by means of, for example thermal radiation) so that the unexposed parts  39  become isotropic and are fixed to a polymer network by means of local illumination  38  (flood exposure). 
     Finally, the method shown in FIG. 7 makes use of “photo-isomerizable” chiral nematic polymers, a layer of which is provided again between a basic substrate and a coating. The material is chosen to be such (pitch, refractive indices) that it reflects the desired wavelength. By local UV illumination via the mask  36 , the value of the reflected wavelength shifts to higher values, for example, to infrared. The unexposed parts  30  continue reflecting the desired wavelength, while the other reflection is not visible to the human eye. 
     The protective scope of the invention is not limited to the embodiments described. It has already been noted that elliptically polarized light may be used alternatively, although this is at the expense of the suppression of stray light. Also, other electro-optical effects may be used, for example, electrochromic effects. As mentioned in the opening paragraph a display comprising deformable mirrors may be used as well. Circularly polarized light may also be obtained in the optical waveguide by providing a (pattern of) ¼λ plate(s), combined with (a) linear reflector and/or mirror(s). The invention resides in each and every novel characteristic feature and each and every combination of characteristic features. Reference numerals in the claims do not limit their protective scope. Use of the verb “comprise” and its conjugations does not exclude the presence of elements other than those mentioned in the claims. Use of the article “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.