Abstract:
A phosphor-based multi-colored display is provided which includes a spatial light modulator comprising an addressable array of apertures each corresponding to a respective sub-pixel in the display; an array of phosphor regions each indexed to a corresponding aperture of the spatial light modulator; and a backlight which provides quasi-monochromatic light to phosphor within each of the phosphor regions. The phosphor within each of the phosphor regions, upon being excited by the quasi-monochromatic light of the backlight, emits light having a color different from a color of the quasi-monochromatic light, through the corresponding aperture of the spatial light modulator.

Description:
TECHNICAL FIELD 
     The present invention relates to a colour display that includes an electrically addressable spatial light modulator. Furthermore, the present invention relates to a spatially extended illumination panel that may be used for general lighting. In particular, the invention pertains to a colour display that has high efficiency and can be used in devices such as mobile phones, PDAs, computer monitors and liquid crystal display (LCD) TVs. 
     BACKGROUND ART 
     JP 2008096547 (O. Ito et al.; published Apr. 24, 2008) describes a display which uses an arrangement of localized phosphors to act as local light sources for a display. The phosphors are placed between two lens sheets. The lower lens sheet acts to focus light from a collimated backlight into the phosphors. The second lens sheet acts to focus light emitted by a phosphor though a selected TFT aperture of an LCD panel. The phosphor and dual lens-array assembly is separated from the main part of the display. 
     JP 2010066437 (O. Ito et al.; published Mar. 25, 2010) describes a display in which an arrangement of localized phosphors is placed above an LC panel and a collimated backlight. Light from the collimated backlight is focused through the TFT apertures of the display panel and into the correctly indexed phosphor region. This arrangement is subject to unwanted phosphor excitation by ambient light. Some ambient light also reflects in the chambers that house the phosphors. These aspects can lower the contrast of the display unless viewed in a darkened environment. 
     WO 2004/099664 (T. Jüstel; published November 2004) describes a phosphor on a lightguide which may be excited from rays leaking from the lightguide. The publication does not cover pixilated arrangements appropriate for making a display. 
     U.S. Pat. No. 7,339,638 (K. Kitamura; published Apr. 13, 2006) uses a dual lens layer, each layer containing lenses near every LC pixel. Colours are separated by angle and pass through the LC. This system is more suited to a projector than a display. 
     U.S. Pat. No. 5,146,355 (J. Prince et al.; published Sep. 8, 1992) describes an arrangement of phosphors situated below an LC panel. A transflective configuration is described. 
     U.S. Pat. No. 6,864,626 B1 (S. Weiss et al.; published Mar. 8, 2005) describes display configurations involving optically pumped luminescent nanocrystals. 
     U.S. Pat. No. 5,608,554 (Y. Do et al.; published Mar. 4, 1997) describes an LCD which contains a phosphor layer and a backlight source with a main emitting peak in the region 380-420 nm. The phosphor distribution resides on the opposite side of the LC layer from the backlight. 
     U.S. Pat. No. 6,473,144 (N. Ichikawa et al., published Oct. 29, 2002) and US 2007/0047221 (S. Park; published Mar. 1, 2007) use a holographic array or diffractive gratings to separate colours and focus them onto separate pixels. This approach is technically demanding and expensive to implement. 
     SUMMARY OF INVENTION 
     According to an aspect of the invention, a phosphor-based multi-coloured display is provided which includes a spatial light modulator comprising an addressable array of apertures each corresponding to a respective sub-pixel in the display; an array of phosphor regions each indexed to a corresponding aperture of the spatial light modulator; and a backlight which provides quasi-monochromatic light to phosphor within each of the phosphor regions. The phosphor within each of the phosphor regions, upon being excited by the quasi-monochromatic light of the backlight, emits light having a colour different from a colour of the quasi-monochromatic light, through the corresponding aperture of the spatial light modulator. 
     According to another aspect, the colour of the quasi-monochromatic light is one of red, green, blue and yellow, and the colour of the light emitted by the phosphor regions comprises one or more of the others of red, green, blue and yellow. 
     In accordance with another aspect, the array of phosphor regions further including scattering regions in between the phosphor regions with the scattering regions also being indexed to corresponding apertures of the spatial light modulator, wherein the backlight provides the quasi-monochromatic light to scattering material within each of the scattering regions and the scattering material scatters the quasi-monochromatic light though the corresponding aperture of the spatial light modulator. 
     According to another aspect, the backlight comprises a lightguide and a quasi-monochromatic light source which introduces the light into the lightguide from a side of the lightguide, and the array of phosphor regions is located directly above the lightguide such that light is extracted directly from the lightguide into the phosphor regions. 
     According to yet another aspect, a distance of separation between the phosphor regions and the corresponding apertures is less than a size of the corresponding apertures. 
     According to another aspect, the display further includes a colour filter associated with each aperture. 
     In accordance with another aspect, the plurality of phosphor regions are positioned within corresponding chambers which act to redirect light emitted by the phosphor towards an outward normal of the display. 
     In still another aspect, walls of the chambers are formed by mirrored sections and/or low index regions. 
     In yet another aspect, the shape of the chambers approximate compound parabolic reflectors. 
     According to another aspect, the display further includes an interference filter between the array of phosphor regions and the backlight, the interference filter being designed to allow the quasi-monochromatic light to pass through and excite the phosphor within the phosphor regions over a predefined angular acceptance range, and to reflect the light emitted by the phosphor away from the backlight. 
     According to another aspect, the backlight includes a blue light source. 
     In accordance with still another aspect, the backlight includes a UV light source. 
     According to another aspect, the array of phosphor regions includes phosphor regions respectively including red, green and blue phosphors. 
     In accordance with another aspect, light from the backlight passes through the array of phosphor regions by way of top apertures within the array of phosphor regions, and the display further includes reflectors positioned between the array of phosphor regions and the spatial light modulator which reflect the light passing through the array of phosphor regions back towards the phosphor regions to excite the phosphor therein. 
     According to another aspect, the spatial light modulator includes at least one of a liquid crystal cell and a polymer dispersion liquid crystal cell. 
     According to another aspect, the backlight is a quasi-collimated backlight. 
     In accordance with still another aspect, the display further includes a lens array for focusing the collimated light into the phosphor regions. 
     In yet another aspect, the backlight includes a lightguide. 
     According to another aspect, the backlight is a direct view backlight. 
     The invention relates to an optical arrangement that can be used to improve the efficiency of a liquid crystal display (LCD). As illustrated in cross-section in  FIG. 2 , this arrangement includes a quasi-monochromatic backlight  20 , a layer  21  containing a patterned array of phosphor and scattering material, and a spatial light modulator such as an LCD display panel  1   n . By “quasi-monochromatic” or single colour light source, it is meant that the backlight  20  emits light over a narrow spectral range in comparison to the spectral range of visible light. The backlight spectral range is narrow compared to the absorption bandwidth of chosen phosphors. Excitation of the phosphors by the quasi-monochromatic light results in the phosphors emitting light of generally another, different, colour component or components. Each individual phosphor region in the layer  21  acts as a local light source for a colour sub-pixel of the display. In a preferred embodiment, red and green light emitting phosphors are excited by a blue backlight (although it will be appreciated that other colour arrangements may be utilized without departing from the scope of the invention). Light from the red and green phosphors, after passing through a liquid crystal cell, polarisers and correctly indexed TFT aperture  38 , form the red and green sub-pixels of the display. The blue sub-pixels are formed by scattering elements or regions  26  within the array layer  21  which elastically scatter the LED light. 
     The efficiency of the device is increased primarily due the much reduced absorption in colour filters compared to current state of the art displays. In some embodiments of the invention, the colour filters are removed altogether. 
     In some embodiments of the invention, the phosphors are located directly above a lightguide. Light of appropriate wavelength from one or more light emitting diodes enters the lightguide from its sides. Light is extracted directly from the lightguide into the phosphor or scattering regions, but is prevented from leaving elsewhere due to a refractive index step maintaining total internal reflection. This display geometry is highly compact and is an integrated form in the sense that air gaps are eliminated from the main part of the display. Aside from enabling an ultra-thin display, this aspect reduces unwanted Fresnel reflection losses, increases the robustness of the display and reduces the possibility of contamination from the environment. 
     Other embodiments of the invention rely on light from a quasi-collimated backlight being focused into the phosphor regions by a lens sheet. An example is shown in  FIG. 8 . Several designs are available for the collimated backlight. Some examples are shown in  FIGS. 9(   a )- 9 ( d ). The designs in  FIGS. 9(   a )- 9 ( c ) rely upon extraction features situated on a planar lightguide. The geometry of these features is optimized to give good collimation in the extracted light about the normal to the lightguide. The extraction features may be sub-wavelength, as in the example shown in  FIG. 9(   a ), and hence diffractive in nature. To increase extraction efficiency and control uniformity, the lightguide may be wedge shaped with a small wedge angle that can vary along the lightguide. Alternatively, a number of separate shallow wedge features can be applied to one or both interfaces of the lightguide.  FIG. 9(   d ) shows an example of collimated direct view backlight. Light from a tiled arrangement of single reflection light-emitting diodes (SRLEDs)  75  forms the backlight. Each SRLED  75  comprises an emitting LED chip  76  and a curved mirror  77 . Light from the LED chip  76  is reflected in the mirror  77  towards the normal direction. 
     Many lightguide-based collimated backlight schemes give much better collimation in one direction than in the other. If such backlights are used, it is appropriate to use a lenticular lens and phosphor arrangement. If a backlight showing good collimation in both directions is used, then a pixilated lens and printed phosphor arrangement is preferred. Even if a lenticular phosphor arrangement is adopted, the phosphor lines need to be periodically interrupted in order aid the extraction of downconverted light from the phosphors. 
     The phosphors are deposited inside chambers which act to focus the light emitted from the phosphors towards the display normal. The phosphors are preferentially localized to regions considerably smaller than a sub-pixel. This localization increases the collimation of the downconverted light that can be attained so that the majority of the light can pass through the correctly addressed TFT aperture  38 . In this way, loss from colour filters and the black-mask associated with the TFT array is greatly reduced leading to further efficiency enhancement. 
     Referring again to  FIG. 2 , phosphor and scattering regions are formed within chambers defined at least in part by mirrored sections  28  made of reflective metal or the like, and/or low refractive index regions  27 . The chambers serve to re-direct light that is emitted sideways into the upwards direction. An interference filter  42  ( FIG. 5 ) can be placed below the phosphor layer to reflect downward propagating light emitted by the phosphor. The interference filter  42  is designed to allow light at the LED wavelength to pass through over a defined angular acceptance range. 
     Use of a single colour LED and the remote phosphor arrangement removes a component of loss that manifests in displays which rely on white LEDs. A white LED contains phosphors within its packaging. Due to the diffuse nature of light emitted by a phosphor and scattering cause by the phosphor, LED package loss is larger for white LEDs than for single colour ones. 
     The efficiency enhancements described above allow for a brighter display and/or a lower power usage. The latter aspect leads to a lower environmental impact. At a given brightness, the lower power required leads to a longer battery life for a mobile device. 
     The remote phosphor backlight described here can be applied to standard display panels, even if colour filters are present. 
     Instead of using a blue backlight and diffusive material to form a blue sub-pixel, a UV backlight can be used in which the blue sub-pixels are also phosphor based. 
     Additional phosphors can be used to provide more than three sub-pixel types. For example a yellow phosphor can be used with red and green ones and a blue backlight to form a RGBY display. 
     The LCD panel acts as a natural shield for ambient light. This implies that little UV or blue ambient light will excite the phosphors which would compromise contrast. The configuration also minimizes ambient light reflections within the phosphor chambers that could otherwise degrade the viewed images. In a display operating with a UV backlight, little of this light can escape from the device so as to present a health risk. 
     For small area display applications, the phosphors will be nanophosphors of known types. Here, the term nanophopsphor encompasses dye-molecule based and semiconductor nano-crystal based wavelength converters. These can be deposited within a resin by ink-jet printing or other known techniques. For larger area displays it may be possible to use standard phosphors. 
     To the accomplishment of the foregoing and related ends, the invention, then, comprises the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       In the annexed drawings, like references indicate like parts or features: 
         FIG. 1  illustrates a typical display of conventional type. 
         FIG. 2  illustrates preferred embodiment to the present invention. 
         FIG. 3  illustrates an embodiment of the invention in which the lower polarizer is an in-cell type. 
         FIG. 4  illustrates an embodiment in which colour filters are included to reduce cross-talk. 
         FIG. 5  illustrates an embodiment in which a planar wavelength selective filter is included to reflect downward propagating light at downconverted wavelengths. The filter is such that it allows backlight light to pass through. 
         FIG. 6  illustrates an embodiment in which separate wavelength selective filters reside under each phosphor and scattering region. 
         FIGS. 7(   a ) and  7 ( b ) Illustrate respective means of achieving strong elastic scattering within blue sub-pixel chambers. 
         FIG. 8  shows an embodiment that incorporates a collimated backlight. Light from the collimated backlight is focused into the phosphor and scattering regions. 
         FIGS. 9(   a ),  9 ( b ),  9 ( c ) and  9 ( d ) show respective examples of collimated backlight designs. 
         FIG. 10  shows an embodiment of the invention that incorporates a UV primary light source and red, green and blue phosphors. 
         FIG. 11  shows an embodiment of the invention that includes extraction features placed to top of the liquid crystal panel. 
         FIG. 12  shows an embodiment of the invention that includes a polymer dispersed liquid crystal cell. 
         FIG. 13  shows an embodiment that employs curved phosphor and scattering region chamber mirrors to improve the collimation of light leaving the chambers. 
         FIG. 14  shows an embodiment that incorporates angled mirror sections to reflect focused backlight rays into modified phosphor and scattering region chambers. 
     
    
    
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
         
           
               1  refers to a conventional LCD panel comprising two polarizers, a liquid crystal cell and a TFT layer. 
               1   n  refers to an LCD panel used in most embodiments of the invention. 
               2  refers to a conventional lightguide-based backlight unit. 
               3  refers to a light source such as an LED. 
               3   n  refers to a blue light source such as an LED used in most embodiments of the invention. 
               3   u  refers to a UV light source such as an LED used in one embodiment of the invention. 
               4  refers to a planar lightguide used in a conventional display. 
               4   n  refers to a lightguide used in most embodiments of the invention. 
               4   p  refers to a tapered lightguide. 
               5  refers to a mirror placed beneath the lightguide. 
               6  refers to features on the lightguide which induce extraction of light from the lightguide. 
               7  refers to a strong diffuser. 
               8  refers to a prism sheet. 
               9  refers to a prism sheet that is oriented in an orthogonal direction to prism sheet  8 . 
               10  refers to a weak diffuser. 
               11  refers to an additional layer which can be an interference film. 
               12  refers to the rear polarizer of an LCD panel. 
               13  Shows the trajectory of a typical path through the system. 
               20  refers to a quasi-monochromatic backlight assembly used in several embodiments of the invention. 
               20   p  refers to a collimated backlight. 
               21  refers to a layer containing the phosphor and scattering material that act as secondary light sources for the displays in the invention. 
               25   r  refers to a red phosphor region. 
               25   g  refers to a green phosphor region. 
               25   b  refers to a blue phosphor region. 
               26  refers to scattering regions made up of scattering material such as elastic scattering material. 
               27  refers to low refractive index regions placed above and to the sides of the phosphor and scattering materials. Preferably this layer is formed of a material having a low refractive index. 
               28  refers to mirrored sections of the chambers that contain phosphors or scattering material. 
               29  refers to low refractive index material. This material may be air. 
               30  refers to the lower polarizer of the display. 
               30   i  refers to an in-cell polarizer used as the lower polarizer of a display. 
               31  refers to a glass substrate layer. 
               32  refers to a liquid crystal cell. 
               33  refers to the TFT layer. 
               34  refers to a glass substrate layer. 
               35  refers to the top polarizer of the display. 
               36  refers to planarizing material such as a resin. 
               37  refers to corrugated features added to the bottom of the lightguide to control out-coupling of light from the lightguide into the phosphors and scattering material. 
               38  refers to apertures in the TFT and black-mask array through which light may pass. 
               41   r  refers to a red colour filter. 
               41   g  refers to a green colour filter 
               41   b  refers to a blue colour filter. 
               42  refers to a reflecting band pass filter layer. 
               42   b  refers to reflecting band pass filters placed under the phosphor and scattering materials. 
               51  refers to dielectric Mie scatterers. 
               52  refers to the dielectric medium such as a resin 
               53  refers to a dielectric buffer layer above the scatterer-containing medium. 
               53   a  refers to a dielectric buffer layer. 
               53   b  refers to a dielectric layer such as glass of polymer onto which microlenses or roughness may be added. 
               54  refers to an arrangement of microlenses. 
               62  refers to a substrate layer between microlenses and the phosphor containing layer. 
               63  refers to a microlens layer. 
               64  refers to an air gap between a collimated backlight and a focusing lens array. 
               71  refers to a layer placed above a planar lightguide into which a grating may be embossed or etched. Preferentially this layer has a low refractive index. 
               72  refers to a diffractive grating layer. 
               73  refers to non-diffractive extraction features placed on a lightguide. 
               74  refers to a light turning layer placed above a wedge lightguide. 
               75  refers to a single reflection LED (SRLED). 
               76  refers to the emitting chip of the SRLED. 
               77  refers to the curved mirror of the SRLED. 
               81  refers to extraction features placed on top of the display to aid extraction of light from the display. 
               91  refers to a polymer dispersed liquid crystal cell. 
               101  refers to angled reflectors used to reflect light from the quasi-monochromatic backlight into the phosphor and scattering region chambers. 
             D refers to the separation between the phosphors and the TFT layer. 
           
         
       
    
     DETAILED DESCRIPTION OF INVENTION 
     The present invention will now be described in detail with reference to the drawings, in which like reference numerals are used to refer to like elements throughout. 
       FIG. 1  illustrates a conventional small area liquid crystal display that is well known. The display consists of a liquid crystal display panel  1  and a backlight unit  2 . The backlight unit  2  involves a white light source  3  that can be a fluorescent tube, an LED with phosphor, an RGB LED group, a laser or other light source. The chosen source is positioned at the edge of a thin lightguide  4 . Light  13  entering the lightguide  4  from the light source  3  is guided by total internal reflection (TIR) unless interrupted by outcoupling features  6  positioned on one or both of the large faces of the lightguide  4 . Any light that leaves a bottom interface of the lightguide  4  is reflected back up through it by a mirror  5 . Light which leaves the top of the lightguide  4  generally has unfavourable angular characteristics and may suffer from small-scale spatial inhomogeneity. To rectify this, a strong diffuser  7 , two crossed prism sheets  8  and  9  in orthogonal directions and a weak diffuser  10  are used to produce the correct angular distribution and ensure local homogeneity. Often the weak diffuser  10  is incorporated into the top prism layer  9 . A further layer  11  can be added to the assembly. The layer  11  is typically an interference film that preferentially reflects one polarization and transmits the other. The reflected light is recycled through the diffusers  10  and  7  to become unpolarised and is them reflected by the mirror  5  toward the interference film  11  again. The result is that the light is polarized so that it is better transmitted by a rear polarizer  12  of the liquid crystal spatial light modulator (SLM)  1 . 
     The optical efficiency of this conventional display is limited by many factors, including absorption loss in the colour filters, absorption in the black mask of the LCD pixels, absorption loss in the phosphor within the LED and loss in the LED packaging itself. 
     A schematic representation of the preferred embodiment of this invention is shown in  FIG. 2 . Included in this embodiment is a liquid crystal display panel  1   n , a lightguide  4   n  into which blue light is injected from one or more light sources  3   n , and the patterned phosphor and scattering material layer  21  that acts as a local light source for illuminating the sub-pixels within the panel  1   n . The layer  21  contains an arrangement of red and green phosphor regions  25   r , 25   g  containing red and green phosphor, respectively, and scattering regions  26  containing scattering material. The phosphor regions  25   r  and  25   g  act to absorb blue light and re-emit red and green light, respectively. The periodic patterning of the phosphor regions  25   r , 25   g  and scattering regions  26  coincides with that of the TFT sub-pixel array. The LCD panel  1   n  can be identical to one described in the conventional art except that the colour filter layer may be removed as will be appreciated. 
     The phosphor regions  25   r , 25   g  and the scattering regions  26  are contained within chambers that may be constructed from low refractive index regions  27  and/or mirrored sections  28 . In the preferred embodiment, the phosphor and scattering material layer  21  resides directly above the lightguide  4   n . Regions of low index material  29  between the phosphor regions  25   r , 25   g  and scattering regions  26  prevent light leakage from the lightguide  4   n  except into the phosphor regions  25   r , 25   g  or scattering regions  26 . The low index material  29  may be air, for example, although other materials may be utilized as will be appreciated. A planarizing material  36  such as resin completes the layer  21 . 
     The operation of the preferred embodiment proceeds as follows. Continuing to refer to  FIG. 2 , light from the light source  3   n  enters the lightguide  4   n  and subsequently enters into the phosphor regions  25   r , 25   g  and scattering regions  26 . Ideally, all the light entering the phosphor regions  25   r , 25   g  is fully absorbed by the phosphors therein. Some of the light subsequently emitted by the phosphor regions  25   r , 25   g  or scattered in the scattering regions  26  may be reflected by the chamber walls as formed by the mirrored sections  28  and/or low index regions  27  so that it is redirected towards the outward normal to the display. Light that ultimately escapes upwards from one of the chambers acts as a local light source of appropriate colour for a sub-pixel of the display. This light enters the LC panel  1   n  comprising a bottom polarizer  30 , a glass substrate  31 , a liquid crystal cell  32 , a TFT layer  33 , a second glass substrate  34  and a second polarizer  35 . Light from a phosphor or scattering chamber substantially passes through the correctly indexed TFT aperture  38  if the sub-pixel is in the “on” state and then diverges from the display where it may be viewed. To increase the fraction of light which can pass through the correct TFT aperture  38 , the separation D between the phosphor and scattering materiallayer  21  and the TFT layer  33  should be small. Preferentially, D is smaller than the sub-pixel size (e.g., size of apertures  38 ). It is noted that the figures are not drawn to scale in such regard. 
     The bottom polarizer  30  and substrate  31  may be thinned from industry standard values by applying etching techniques. The bottom polarizer  30  may be replaced by an in-cell polarizer  30   i  as shown in  FIG. 3 . This revised design constitutes the second embodiment of the invention. The substrate layer  31  may be omitted so as to further reduce the distance between the phosphor and scattering regions and the TFT apertures  38 . 
     Colour filters  41   r ,  41   g ,  41   b  may reside within or close to the TFT apertures  38 . An example of this modification to embodiment one is shown in  FIG. 4 . The colour filters  41   r ,  41   g ,  41   b  are respectively aligned with the phosphor regions  25   r , 25   g  and scattering region  26  and act to reduce cross talk involving light from a local source of one colour passing through an incorrectly addressed sub-pixel with a different colour. For example, colour filter  41   g  prevents red light from phosphor region  41   r  from passing through the green sub-pixel. Since light rays emerging from a phosphor or scattering region chamber mostly head towards the correct colour filter, loss in the colour filters is much lower than in a standard panel. The filters can also act to adjust the colour balance and colour gamut range of the display at some expense to efficiency. 
     Since colour filters can be retained, it will be clear that the invention can be used in conjunction with standard, unaltered, LCD panels whilst still showing efficiency improvement. 
     A bandpass filter layer  42  may be placed between the lightguide  4   n  and the phosphor and scattering material layer  21  as shown in  FIG. 5 . The filter  42  is designed to transmit light at the primary light source  3   n  wavelength over the range of incident angles appropriate for light extraction from the lightguide. The filter  42  may be an interference or a thin metal layer based filter. The filter  42  may be patterned into individual filters  42   b  so as to exist only beneath the phosphor regions  25   r , 25   g  and the scattering regions  26 , as shown in  FIG. 6 . 
     The phosphor materials as used herein may be dye or nano-crystal based nanophosphors. 
     The blue light scattering material used in the scattering regions  26  may contain dielectric Mie scatterers  51  within a dielectric medium  52  such as resin. A dielectric buffer layer  53  is formed above the dielectric medium  52 . The Mie scatterers  51  show a resonance at or near the primary light source  3   n  wavelength. The situation is shown schematically in  FIG. 7(   a ). Instead of distributed scatterers  51 , the necessary scattering may be produced by a random microlens array  54  or a roughened interface between media of different refractive index. The former case is illustrated in  FIG. 7(   b ). For example, the microlens array  54  is formed on a dielectric layer  53   b  such as glass or polymer formed atop a different dielectric buffer layer  53   a , which in turn is formed on the dielectric medium  52 . 
     The lightguide  4   n  is illuminated by a light source  3   n  that emits over a narrow wavelength range, with a sub 50 nm bandwidth preferred. The light source  3   n  can be a packaged blue LED or laser. 
     In a third embodiment of the current invention, the phosphor and scattering material layer  21  and LCD panel  1   n  are located above a collimated backlight  20   p  as shown in  FIG. 8 . A bottom glass substrate  62  and a lens array layer  63  are placed beneath the phosphor and scattering material layer  21 . An air gap  64  exists between the backlight  20   p  and the rest of the assembly. The lens array layer  63  acts to focus light from the collimated backlight  20   p  into the phosphor regions  25   r  and  25   g  where the light is absorbed, and into the scattering regions  26  where it is elastically scattered. The action of the display after the phosphor excitation proceeds identically with the first embodiment described herein with respect to  FIG. 2 . A filter (not shown) may be placed beneath phosphor and scattering material layer  21  to reflect downwards propagating light emitted from the phosphors. This filter is designed to allow the narrow band collimated backlight light to pass though. The filter is easier to design and fabricate than its counterpart  42  in the embodiment of  FIG. 5 . It will also be less lossy. 
     There are several known methods to realize the collimated backlight  20   p . One example is shown in  FIG. 9(   a ). The collimated backlight  20   p  includes the light source  3   n , back reflector  5  and lightguide  4   n . A planar layer  71  resides on top of the lightguide  4   n . The layer  71  preferentially has a lower refractive index that the lightguide  4   n , Diffractive features  72  are imprinted or etched on the top layer  71 . These features extract light from the lightguide  42  only into angles close to the lightguide normal. To improve extraction efficiency and uniformity, wedge shaped features  37  are added to the lightguide  4   n . Another example of a lightguide based collimated backlight is shown in  FIG. 9(   b ). Extraction features  73 , which may be produced by imprinting or etching techniques, are added to the top of the lightguide  42 . These features  73  are substantially non-diffractive, being several times the wavelength in scale, and are designed to extract light into directions close to the backlight normal. In  FIG. 9(   c ), a wedge-shaped lightguide  4   p  is used to control the light extraction rate. A turning film  74  is used to re-steer the light rays escaping from the lightguide  4   n  so that they are centered about the normal direction. In  FIG. 9(   d ), an example of a collimated direct view backlight is given. It comprises a tiled array of single reflection light emitting diodes (SRLEDs)  75 . Each SRLED gives collimated output by reflecting light from a small LED chip  76  in a curved mirror  77 . 
     Many collimated backlight schemes, particularly those based on lightguides and LED light injection, give good collimation in one direction only. If such a backlight is used, it is sufficient to use a lenticular lens arrangement. If the chosen backlight gives collimation in two directions, the lenses should ideally be pixilated in two dimensions to maximize the benefit from focusing into the phosphors. 
     A fourth embodiment of the invention is shown in  FIG. 10 . The blue light source has been replaced by a UV light source  3   u . This light source  3   u  excites the red and green phosphors in regions  25   r , 25   g  as before. A blue phosphor in a corresponding phosphor region  25   b  replaces the scattering material and region  26  used in the preferred embodiment. Use of a UV source can lead to increased absorption and fluorescent loss in the lightguide and glass substrates. UV resistant materials should preferably be used in the device construction. The introduction of a wavelength shift in the blue sub-pixel, and the increased wavelength shifts in the red and green phosphors, implies the Stokes shift related efficiency loss inherent to the downconversion process has been increased. Nevertheless, this embodiment naturally leads to the angular properties of all colour pixel types being closely similar so that colour balance can be more easily maintained over a range of viewer angles. 
     It will be understood that the UV light source  3   u  and blue phosphor regions  25   b  can also be used to replace the blue light source  3   n  and scattering regions  26  in other embodiments described herein and remain within the scope of the invention. 
     In all embodiments described herein, a UV filter (not shown) may be incorporated into the display above the phosphor, or phosphor and scattering material, layer  21 . This acts to prevent incursion of UV ambient light into the display, a small fraction of which could reach and excite the phosphors. If a UV primary source is used, the UV filter prevents ultraviolet exposure of the viewers from any residual un-converted UV light escaping from the device. 
     A fifth embodiment of the invention is shown in  FIG. 11 . Features  81  are placed on the top interface of the embodiment of  FIG. 2  to aid light extraction into air. These features  81  may be, but are not restricted to, microlens or pyramidal geometries (the latter example is shown schematically in  FIG. 11 ). The features  81  can also be diffractive. Such extraction features  81  are only necessary if insufficient collimation of the light emerging from the phosphor and scattering chambers has been achieved. It is also only necessary if the separation D (e.g.,  FIG. 2 ) is small enough that rays close to or beyond the TIR condition at the top of the display can pass through the correctly addressed sub-pixel TFT aperture  38 . It will be understood that the extraction features  81  can also be added to other embodiments detailed herein. 
     A sixth embodiment of the invention is shown in  FIG. 12 . The lower polarizer assembly of the embodiment of  FIG. 2  has been removed and the liquid crystal cell replaced by polymer dispersion liquid crystal cell (PDLC)  91 . The PDLC acts as both a polarizer and polarization rotator when an appropriate voltage is applied. 
     A seventh embodiment of the invention is shown in  FIG. 13 . The chambers holding the phosphor regions  25   r , 25   g  and scattering regions  26  possess curved sides (e.g., via curved mirror sections  28   p ) to improve the collimation of the light leaving these chambers. The phosphor samples within the phosphor regions are localized to a scale significantly smaller than the sub-pixel size. Preferentially, the area of the base of the phosphor is less than 1/10 th  of the sub-pixel area. Preferentially, the shapes of the chambers approximate to compound parabolic reflectors. 
       FIG. 14  shows an eighth embodiment that incorporates metal reflectors  101  to reflect backlight light down into the phosphor regions  25   r , 25   g  and scattering regions  26 . The mirrored sections  28   q  defining the corresponding chambers are modified so that metal mirrors exist on all sides except the top aperture located between chambers from the perspective of the backlight. The metal reflectors are aligned with the top apertures, and the backlight light is focused between the chambers so that the majority of this light strikes the reflectors  101  and is reflected towards the open apertures of the chambers defined by the mirrored sections  28   q . The extraction of light subsequently emitted by the phosphor regions  25   r , 25   g  or scattered in the scattering regions  26  is improved by the chamber design. This embodiment circumvents the need for incorporating wavelength selective filters in the design to improve the extraction efficiency. 
     Although the invention has been shown and described with respect to a certain embodiment or embodiments, equivalent alterations and modifications may occur to others skilled in the art upon the reading and understanding of this specification and the annexed drawings. In particular regard to the various functions performed by the above described elements (components, assemblies, devices, compositions, etc.), the terms (including a reference to a “means”) used to describe such elements are intended to correspond, unless otherwise indicated, to any element which performs the specified function of the described element (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein exemplary embodiment or embodiments of the invention. In addition, while a particular feature of the invention may have been described above with respect to only one or more of several embodiments, such feature may be combined with one or more other features of the other embodiments, as may be desired and advantageous for any given or particular application. 
     INDUSTRIAL APPLICABILITY 
     The invention pertains to a high efficiency colour display that can be used in devices such as mobile phones, PDAs, computer monitors and liquid crystal display TVs. In some embodiments, an entirely standard matrix addressable LCD panel is used. In essence the invention relates to a particular form of phosphor-based backlight in which the requisite coloured light is produced locally to each sub-pixel. This phosphor backlight is relatively cheap to produce, and its use in conjunction with a standard LCD panel leads to a competitive unit price. Efficiency and image quality can be further improved if a modified panel is used.