Abstract:
A reflective display device ( 2 ) comprises a plurality of controllable light absorption layers ( 8 ) arranged in a stack. Each of the layers ( 8 ) is capable of absorbing incident light in a specified wavelength band. A selective reflector ( 10 ) is immediately behind at least one of the layers ( 8 ) and is adapted to reflect at least some wavelengths of light within the wavelength band and substantially to transmit light of other wavelengths.

Description:
The present invention relates to a reflective display, notably to a reflective full colour display. 
     A reflective display is a non-emissive device in which ambient light for viewing the displayed information is reflected from the display back to the viewer rather than light from behind the display being transmitted through the display. A problem with reflective displays is that light must pass through a number of layers twice, and unwanted absorption by those layers can reduce efficiency. This problem is increased if a stack of controllable layers is used to allow independent control of different colours. 
     SUMMARY OF THE INVENTION 
     Aspects of the invention are specified in the independent claims. Preferred features are specified in the dependent claims. 
     By putting a spectrally selective reflection layer immediately beneath a controllable absorption layer, any light in that spectral band not absorbed by the layer can be returned to the viewer without having to traverse any lower layers. This results in improved performance. 
     For example, if the top switchable layer controls absorption in the blue channel, a blue reflector can be placed immediately underneath. Red and green light will then continue to the lower layers, perhaps with red being modulated next. A red reflector could then be placed immediately underneath this layer, so that only green light would continue to the third layer, behind which either a broadband or a green reflector would be located. 
     With this arrangement, the lower controllable absorption layers need not be as spectrally selective; in the example given above, the second and third controllable absorption layers could also absorb blue light without affecting the performance of the device, as they are underneath the blue reflector. This might allow better colour performance by hiding the unwanted absorption in the other bands of the spectrum. 
     The order of the colour layers can be chosen for best optical effect—either to minimise the losses in the most sensitive part of the spectrum (if for example the electrodes are particularly absorbing in the blue, placing the blue layer at the top would be preferable), or to improve the colour performance of the display as described above. 
     The colour-selective layers can be made by a number of well-known methods, such as layers of curable cholesteric liquid crystal materials (two layers might be wanted to reflect both left and right hand polarizations of light) or Bragg reflective stacks. The reflector can be put immediately under the controllable absorption layer, before the incoming light has hit the second electrode for that layer, which gives the least loss for that part of the spectrum, at the cost of increasing the distance of the electrode from the controllable layer. Alternatively the reflector may be located anywhere between the layer whose spectrum it matches and the next controllable layer. 
     If a metal layer is part of the selective reflector layer, it may be possible to use that metal layer as the electrode for the neighbouring controllable layer, further simplifying the device and reducing unwanted losses. 
     It is not necessary to have three selectively reflecting layers—there may be benefit from just one under the top controllable layer and e.g. a broadband reflector at the bottom of the stack. 
     As it is desirable in many reflective displays to give a diffuse background, rather than for the display to have a specular appearance, it may be useful to perturb the reflective layers so that the light is slightly diffused, for example by spatially modulating the orientation of the reflector so that it is not quite coplanar with the display plane in a way that changes from region to region. The divergence from coplanarity with the display substrates may be quite small, for example ±2°, notably ±1°. 
     The invention provides a display which is brighter than one without a selective reflection layer. 
     Reflective layers can be made easily, for example by use of curable cholesteric materials such as those from Merck. 
     A further benefit of this invention arises with the use of dichroic dyed layers as the controllable absorption layers. To absorb both polarizations in their absorbing orientation, prior art has suggested using a quarter-wave plate behind the layer, in order to convert the unabsorbed polarization into one which on reflection will be absorbed during the return path through the layer. See for example, Wu &amp; Yang,  Reflective Liquid Crystal Displays , Wiley-SID 2001, 4.2 Cole-Kashnow Cell, 136-137. Normally it is difficult to make a waveplate that gives the correct phase shift across the whole visible spectrum. In the case of the present invention, individual waveplates can be placed in front of each Bragg mirror, and can be more easily made to have the correct phase shift over the narrower spectrum for that corresponding layer. 
     The terms “behind” and “beneath” are used herein with reference to the relationship between a controllable absorption layer and its corresponding selective reflector, to denote that the reflector is disposed to receive incident light after the light has passed through the absorption layer. The term “immediately” in this context means that light which passes through a controllable absorption layer impinges on the corresponding reflection layer without passing through any intermediate controllable absorption layer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be further described, by way of example only, with reference to the following drawings in which: 
         FIG. 1  is a schematic sectional view through a prior art reflective display device; 
         FIGS. 2-5  are schematic sectional views through embodiments of reflective display devices in accordance with an aspect of the present invention; 
         FIG. 6  is a graph of reflectivity v wavelength for model cholesteric mirrors suitable for use in embodiments of the invention; 
         FIG. 7  is a graph of reflectivity v wavelength for the prior art reflective display device of  FIG. 1 ; 
         FIGS. 8-10  are graphs of reflectivity v wavelength for the model embodiments of  FIGS. 3-5 ; and 
         FIG. 11  is a schematic sectional view through a reflective display device in accordance with a further embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     The prior art reflective display device  2  shown in  FIG. 1  comprises a stack of selective absorption layers  8 B,  8 R and  8 G, in this example liquid crystal layers  8 , which can be made to absorb, respectively, blue, red and green light. The blue-absorbing layer  8 B is at the top of the stack and the green-absorbing layer  8 G is at the bottom of the stack. 
     Each absorption layer  8  is sandwiched between transparent substrates  4  and transparent conductors  6  and can be wholly or partly actuated by the application of suitable electric signals via the conductors  6 . Thus selected pixel regions of each absorption layer  8  may be made either to absorb light in a particular wavelength band or substantially to transmit all incident light. A silver mirror  10 W functions as a broadband reflector which reflects light of all wavelengths. The silver mirror  10 W, is disposed at the bottom of the device  2  and reflects light back through the layers  4 , 6 , 8  to the viewer. 
     The light passes 12 times through conductor layers  6 . Ignoring aperture issues, the best reflectivity will be determined by the loss in the conductors  6  and the reflectivity of the silver mirror  10 W. Various translucent conductors  6  are known to those skilled in the art of display manufacture, for example indium tin oxide (ITO) or poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (Pedot-PSS). 
     Given: 
     conductor transmission ˜97.5%; and 
     silver reflectivity ˜92%; 
     the expected peak reflectivity of 0.92×(0.975) 12  is about 68%. 
     Turning now to  FIG. 2 , a reflective display device  2  in accordance with an aspect of the invention includes a wavelength selective mirror  10 B between the conductor  6  below the blue-absorbing LC  8 B and the substrate  4  immediately below the conductor  6 . The device  2  shown in  FIG. 3  includes a corresponding additional wavelength selective mirror  10 R below the red-absorbing LC  8 R, and the device  2  of  FIG. 4  includes a further corresponding additional wavelength selective mirror  10 G below the green-absorbing LC  8 G. The mirrors  10 R and  10 G reflect red light and green light respectively. The device  2  of  FIG. 5  is similar to the device of  FIG. 4  but does not include the silver mirror  10 W since in principle the three selective mirrors  10 B,  10 R and  10 G should, between them, reflect substantially all of the non-absorbed incident light. 
     The wavelength selective mirrors could be made from reactive mesogen cholesteric films, for example Merck materials RMS03-008 (blue reflective), RMS03-010 (green) and RMS03-009 (red). 
     Model 
     Modelling of devices in accordance with aspects of the invention was carried out using the method described by D W Berreman:  Optics in stratified media:  4×4  matrix formulation ; Optical Society of America, 62(4):502-10, 1972. 
     In order to model the device  2  we need to have representative values for each layer. 
     We start by leaving out the LC  8  and setting n o  &amp; n e  to 1.52. We use the same value for the substrate  4  and the conductor  6 , and we assume that the incident medium has the same index rather than 1.0 This removes any reflective losses from the first interface. In practice we would add an anti-reflection coating to achieve almost the same. 
     We assume that the substrates  4  are 50 μm, and that the LC  8  is 3 μm thick. 
     We need values for the imaginary part of the refractive index for the conductor  6 , and we need to design the mirrors  10 . 
     We assume that the thickness of the conductor is 100 nm and model a layer surrounded by the same index (1.52) media. We then calculate the transmission and find the value of the imaginary part of the refractive index for the conductor that gives a transmission of 97.5% for a wavelength of ˜550 nm. Modelling gives the value as −0.011. 
     Silver Mirror 
     We use data sheet values for silver:
     n=0.27   k=−4.18
 
and assume a thickness of 200 nm.
   

     The model gives a reflectivity of 92% (for light incident from a medium with n=1.52). The reflectivity does not vary much with wavelength. 
     Cholesteric Mirrors 
     The reflectivity of these mirrors  10 B, 10 R, 10 G is determined by the refractive indices and the pitch. They only reflect one handedness of circularly polarised light. To reflect unpolarised light two layers with opposite twist are needed. We have assumed that each layer is 5 μm thick. 
     We set n o =1.49, n e =1.66—fairly typical parameters. 
     One can tune the reflection band by varying the pitch. We have tuned the three mirrors as shown in  FIG. 6 , where reflectivity peaks  12 B,  12 G and  12 R correspond respectively to reflection from the blue reflector  10 B, green reflector  10 G and red reflector  10 R. The cholesteric pitch controls the central wavelength. We have used: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 Red 
                 0.39 μm 
               
               
                   
                 Green 
                 0.33 μm 
               
               
                   
                 Blue 
                  0.28 μm. 
               
               
                   
                   
               
             
          
         
       
     
     Modelling of % reflectivity v wavelength for the prior art device of  FIG. 1  gives the graph shown in  FIG. 7 . The reflectivity of about 68% is in line with expectation. (The oscillations are due to interference between the layers. The model assumes full coherence which amplifies these effects.) 
     Modelling of % reflectivity v wavelength for the device of  FIG. 3  (using blue and red mirrors  10 B and  10 R) is shown in  FIG. 8 . The model shows substantial enhancement of reflectivity where the cholesteric mirrors  10 B,  10 R have an effect. The enhancement is strongest for the first layer (in this example, blue). The order of the layers could of course be changed. 
     As shown in  FIG. 9 , adding a third selective reflector (in this example a green reflector  10 G) has relatively little effect. 
     Modelled reflectivity for the device of  FIG. 5  is shown in  FIG. 10 . Here, three selective reflectors are used without the silver mirror  10 W. As expected, reflectivity is better than for the prior art device, although there may be some colour shifts with viewing angle. 
     Turning now to  FIG. 11 , an embodiment is illustrated which is similar to  FIG. 4  but in which a retarder  14  is disposed between each controllable absorption layer  8  and its corresponding Bragg selective reflector  10 . The retarder  14  exhibits a quarter wave retardation. Where the controllable light absorption layer  8  is a dyed LC material with an untwisted configuration, the layer  8  will typically absorb light of one polarisation while transmitting light of opposite polarity. By passing the plane polarised light through a retarder  14  the light may be circularly polarised. On reflection from the Bragg reflector  10 , the polarisation of the light is inverted and after passing back through the retarder  14  it has a polarisation opposite to its original state, i.e. of a polarity to be absorbed by the dyed LC material  8 . This arrangement improves absorption efficiency. 
     The retarder  14  may comprise a single quarter-wave plate or it may comprise a combination of two, three or more waveplates, for example a quarter-wave plate and a half-wave plate in combination. Such combinations may broaden the wavelength range and are known per se. For example, U.S. Pat. No. 7,169,447 describes a combination of half-wave and quarter-wave plates made from polymerised liquid crystals. P Harihan, in “Broad-band superchromatic retarders”, Meas. Sci. Technol. Vol. 9 (1998) 1678-1682 describes a combination of four plates. 
     Despite technological advances, it is difficult to produce a retarder which works to the same efficiency across the whole visible spectrum. Accordingly, by providing one or more selective reflectors  10 , an associated retarder  14  need work only over the limited waveband range of the reflector  10 . Each retarder  14  may be selected or tuned for optimal performance with its associated reflector  10 , thereby improving performance of the device. 
     It will be understood that, for purposes of illustration, the various layers shown in  FIGS. 1-5  and  11  have been drawn not necessarily to scale. 
     The articles, ‘a’ and ‘an’ are used herein to denote ‘at least one’ unless the context otherwise dictates. 
     It is appreciated that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable combination. 
     It is to be recognized that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the ambit of the present invention as specified in the claims.