Patent Publication Number: US-2022229222-A1

Title: Color Liquid Crystal Displays and Display Backlights

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. Utility application Ser. No. 15/985,150, filed May 21, 2018, which claims the benefit of priority to U.S. Provisional Application No. 62/510,119, filed May 23, 2017, each of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to color liquid crystal displays (LCDs) and in particular backlight arrangements for operating high color gamut color LCDs. 
     BACKGROUND OF THE INVENTION 
     Color LCDs find application in a variety of electronics devices including televisions, computer monitors, laptops, tablet computers and smart phones. As is known, most color LCDs comprise a liquid crystal (LC) display panel and a white light emitting backlight for operating the display panel. 
     The present invention intends to improve the color gamut of LCD backlights and color LCDs, where color gamut refers to the entire range of colors that the display can produce. The invention further intends to improve the luminous efficacy of LCD backlights and Color LCDs. 
     SUMMARY OF THE INVENTION 
     Embodiments of the invention concern color LCDs and display backlights that include red and green photoluminescence materials (e.g. phosphors, quantum dots, organic dyes or combinations thereof), which when excited by excitation light (typically blue) generate white light for operating the display. 
     In accordance with one or more embodiments, there is provided a backlight comprising a europium activated sulfide phosphor. The europium activated sulfide phosphor can have a general composition based on MA2S4:Eu, where M is at least one of Mg, Ca, Sr and Ba, A is at least one of Ga, Al, In, La and Y. In some embodiments, the europium activated sulfide phosphor comprises strontium, gallium and sulfur and has a general composition and crystal structure of SrGa2S4:Eu. In some embodiments, the europium activated sulfide phosphor has a general composition (M)(A)2S4:Eu, M′, A′ where M is at least one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; A is at least one of Ga, Al, and In; and A′ is at least one of Si, Ge, La, Y and Ti. In this formula, the dopants Eu, M′ and A′ may be present in substitutional sites or interstitial sites. In some embodiments, the europium activated sulfide phosphor may have a composition (M,M′)(A,A′)2S4:Eu, wherein: M is at least one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; A is at least one of Ga, Al, In, La and Y; and A′ is at least one of Si, Ge and Ti; wherein M′ substitutes for M, and A′ substitutes for A in the MA2S4 crystalline lattice. The red photoluminescence material and/or europium activated sulfide phosphor can comprise a wavelength converting layer that is located remotely to one or more light emitting devices that comprise an excitation source, typically a blue LED, for generating excitation light. In other embodiments, one or both of the red photoluminescence material and/or europium activated sulfide phosphor can be located in the one or more light emitting devices. Typically, the wavelength converting layer comprises a part of the backlight, though it may be considered to comprise a part of the display. The wavelength converting layer, which typically comprises a film, is of a size corresponding to size of the display. The wavelength converting layer can be incorporated with or used to replace the diffuser layer of known displays/backlights. The red photoluminescence material can comprise a phosphor material, quantum dots, organic dies and combination thereof. 
     According to one or more embodiments a display backlight comprises: an excitation source for generating blue excitation light with a dominant emission wavelength in a range 445 nm to 465 nm; a red photoluminescence material with a peak emission wavelength in a range 610 nm to 650 nm; a europium activated sulfide phosphor having a peak emission wavelength in a range 525 nm to 545 nm; and a wavelength converting layer located remotely to the excitation source, wherein the wavelength converting layer comprises at least one of the red photoluminescence material and the europium activated sulfide phosphor. In some embodiments, the europium activated sulfide phosphor generates light having a peak emission wavelength in a range 535 nm to 540 nm. A particular benefit of using a europium activated sulfide phosphor is that this can improve luminous efficacy of the backlight/display. The europium activated sulfide phosphor advantageously comprises strontium (Sr), gallium (Ga), and sulfur (S). In some embodiments, the europium activated sulfide phosphor has a general composition and crystal structure of SrGa2S4:Eu. The sulfide phosphor can comprise further elements such as alkaline earth metals or a halogen and can be coated to improve its reliability. 
     In some embodiments, the europium activated sulfide phosphor is located in the wavelength converting layer remote to the one or more light emitting devices including the excitation sources. Since some europium activated sulfide phosphors, more particularly SrGa2S4:Eu, may have problems with thermal quenching, locating this phosphor material in the wavelength converting layer remotely to the light emitting device (LED chip), provides a lower operating temperature environment for the phosphor can ameliorate the problems of thermal quenching. A further benefit of locating the europium activated sulfide phosphor in a separate wavelength converting layer and the red photoluminescence material in the one or more light emitting devices (that is, both the red and green are not located in the same physical location) is that this can improve luminous efficacy of the backlight. The increase in luminous efficacy results in part from the relative size difference between the light emitting device(s) (small area) and the wavelength converting layer (large area) and this difference in areas can minimize absorption of green light by the red photoluminescence material in the light emitting device(s). Additionally, by locating the green europium activated sulfide phosphor in the wavelength converting layer downstream of the light emitting device(s), longer wavelength (lower energy) red light which is incapable of exciting the europium activated sulfide phosphor will be able to pass through the wavelength converting layer with little or no absorption thereby improving luminous efficacy. 
     In embodiments, the red photoluminescence material can comprise a manganese-activated fluoride phosphor. In some embodiments, the manganese-activated fluoride phosphor comprises a manganese-activated potassium hexafluorosilicate phosphor of composition K2SiF6:Mn4+ (KSF) or a manganese-activated potassium hexafluorogermanate phosphor of composition K2GeF6:Mn4+ (KGF). The manganese-activated fluoride phosphor can comprise a phosphor of composition: K2TiF6:Mn4+, K2SnF6:Mn4+, Na2TiF6:Mn4+, Na2ZrF6:Mn4+, Cs2SiF6:Mn4+, Cs2TiF6:Mn4+, Rb2SiF6:Mn4+, Rb2TiF6:Mn4+, K3ZrF7:Mn4+, K3NbF7:Mn4+, K3TaF7:Mn4+, K3GdF6:Mn4+, K3LaF6:Mn4+or K3YF6:Mn4+. When using a manganese-activated fluoride phosphor, more particularly though not exclusively KSF and/or KGF, it is preferably included with the one or more light emitting devices including the excitation source. A particular benefit of including KSF or KGF phosphor in the light emitting device(s) is a substantial reduction in phosphor usage compared with including it in the wavelength converting layer. KSF and KGF have a low blue (excitation) absorption efficiency requiring high material solid loadings in use. In large color LCDs such as televisions, computers and tablet computers, use of this material in the large area wavelength converting layer could be prohibitively expensive. In an embodiment of the invention the europium activated sulfide phosphor and red photoluminescence material are located in the wavelength converting layer. 
     In various embodiments of the invention, backlights comprising a red photoluminescence material and a green europium activated sulfide phosphor, such backlight can have an emission spectrum with a color gamut of at least 95% of NTSC (National Television System Committee) and/or at least 100% of DCI-P3 (Digital Cinema Initiatives) RGB color space standards. Such a color gamut is comparable to backlights based on QDs (non-cadmium containing) and exceeds known backlights composed of KSF and β-SiAlON. In this patent specification, a high color gamut backlight and/or color display refers to a backlight/display capable of producing light of colors that are at least 95% of NTSC and/or at least 100% of DCI-P3 RGB color space standards. 
     In various embodiments, the backlight can have an emission spectrum comprising red, green and blue emission peaks, wherein said red peak has chromaticity coordinates CIE x=0.6700 to 0.6950, CIE y=0.3300 to 0.2950; the green peak has chromaticity coordinates CIE x=0.1950 to 0.2950, CIE y=0.7250 to 0.6250; and the blue peak has chromaticity coordinates CIE x=0.1600 to 0.1400, CIE y=0.0180 to 0.0600. 
     In various embodiments, comprising a wavelength converting layer, the wavelength converting layer comprises a separate film that is fabricated separate to other components of the backlight. In other embodiments, the photoluminescence wavelength conversion layer can be fabricated as a part of another component of the backlight or display, for example it can be deposited directly onto a component of the backlight or display, that is, in direct contact with the component. 
     In various embodiments, backlights of the invention can comprise edge-lit or direct-lit arrangements. 
     In edge-lit arrangements, the backlight further comprises a light guide and the light emitting device is configured to couple light into at least one edge of the light guide and the wavelength converting layer is disposed adjacent to the light guide. In some embodiments, the wavelength converting layer is in direct contact with the light guide. To increase emission brightness, the backlight can further comprise a Brightness Enhancement Film (BEF) and the wavelength converting layer is disposed between the light guide and the brightness enhancement film. The wavelength converting layer can be in direct contact with the BEF. 
     In some edge-lit arrangements, the backlight can further comprise a light reflective surface and the wavelength converting layer be disposed between the light reflective surface and the light guide. The wavelength converting layer can be in direct contact with the light guide or in direct contact with the light reflective surface. 
     In direct-lit arrangements, the backlight can further comprise a Brightness Enhancement Film (BEF) and the wavelength converting layer is disposed adjacent to the brightness enhancement film. The wavelength converting layer can be in direct contact with the BEF. 
     In various embodiments of the invention, the wavelength converting layer can further comprise particles of a light scattering material. The inclusion of particles of a light scattering material can increase uniformity of light emission from the wavelength converting layer and can eliminate the need for a separate light diffusive layer as are commonly used in known displays. Additionally, incorporating particles of a light scattering material with the red or green photoluminescence materials of the wavelength converting layer can result in an increase in light generation by the photoluminescence wavelength conversion layer as well as a substantial, up to 40%, reduction in the quantity of photoluminescence material required to generate a given color of light. Given the relatively high cost of photoluminescence materials, the inclusion of a light scattering material can result in a significant reduction in manufacturing cost for larger displays such a tablet computers, laptops, TVs and monitors. Additionally, the light emitting device can further comprise particles of a light scattering material. 
     The light scattering material can comprise, for example, particles of zinc oxide (ZnO), silicon dioxide (SiO2), titanium dioxide (TiO2), magnesium oxide (MgO), barium sulfate (BaSO4), aluminum oxide (Al2O3), or combinations thereof. The light scattering material particles can have an average diameter such that they scatter excitation light more than photoluminescence generated red or green light. In some embodiments, the light diffusive material particles have an average diameter (D50) of 200 nm of less, typically 100 nm to 150 nm. 
     In accordance with one or more embodiments, there is provided a backlight in which the red and green photoluminescence materials are located at different physical locations along the light path of the backlight/display. For example, one of the red and green photoluminescence materials can be located within the one or more light emitting devices including the excitation source and the other photoluminescence material located in a photoluminescence wavelength converting layer that is located remotely to the one or more light emitting devices. In preferred embodiments, the red photoluminescence material is located within the one or more light emitting devices and the green photoluminescence material is located within the photoluminescence wavelength converting layer. In other embodiments, the red and green photoluminescence materials can be located in respective wavelength converting layers or within respective light emitting devices. The red and green photoluminescence materials can comprise a phosphor material, quantum dots, organic dies and combination thereof. 
     According to one or more embodiments a display backlight comprises: one or more light emitting devices comprising an excitation source for generating blue excitation light with a dominant emission wavelength in a range 445 nm to 465 nm and a red photoluminescence material with a peak emission wavelength in a range 610 nm to 650 nm; and a wavelength converting layer located remotely to the light emitting device; said wavelength converting layer comprising a green photoluminescence material with a peak emission wavelength in a range 525 nm to 545 nm. A particular benefit of locating the red photoluminescence material in the one or more light emitting device(s) and the green photoluminescence material in a separate wavelength converting layer (that is, both the red and green are not located in the same physical location) is that this can improve luminous efficacy of the backlight. As discussed above, the increase in luminous efficacy results in part from the size difference between the light emitting device(s) (small area) and the wavelength converting layer (large area) and this difference in areas can minimize absorption of green light by the red photoluminescence material in the light emitting device(s). Additionally, locating the green europium activated sulfide phosphor in the wavelength converting layer downstream of the light emitting device(s), enables longer wavelength red light to pass through the wavelength converting layer with little or no absorption thereby improving luminous efficacy. In one such arrangement, the green photoluminescence can comprise a β-SiAlON phosphor and the red photoluminescence material can comprise a Group IIA/IIB selenide sulfide-based phosphor, for example having a composition MSe1-xSx: Eu, wherein M is at least one of Mg, Ca, Sr, Ba and Zn and 0&lt;x&lt;1.0. 
     The green photoluminescence material advantageously comprises a europium activated sulfide phosphor. The europium activated sulfide phosphor can have a general composition based on MA2S4:Eu, where M is at least one of Mg, Ca, Sr and Ba, A is at least one of Ga, Al, In, La and Y. In some embodiments, the europium activated sulfide phosphor comprises strontium, gallium, and sulfur and can have a general composition and crystal structure SrGa2S4:Eu. In some embodiments, the europium activated sulfide phosphor has a general composition (M)(A)2S4:Eu, M′, A′ where M is at least one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; A is at least one of Ga, Al, and In; and A′ is at least one of Si, Ge, La, Y and Ti. In this formula, the dopants Eu, M′ and A′ may be present in substitutional sites or interstitial sites. In some embodiments, the europium activated sulfide phosphor may have a composition (M,M′)(A,A′)2S4:Eu, wherein: M is at least one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; A is at least one of Ga, Al, In, La and Y; and A′ is at least one of Si, Ge and Ti; wherein M′ substitutes for M, and A′ substitutes for A in the MA2S4 crystalline lattice. The sulfide phosphor can comprise further elements such as alkaline earth metals or a halogen. A particular benefit of using a europium activated sulfide phosphor is that this can improve luminous efficacy. Since SrGa2S4:Eu can have problems with thermal quenching, locating this phosphor material in the wavelength converting layer remotely to the light emitting device (LED chip), provides a lower operating temperature environment for the phosphor can ameliorate the problems of thermal quenching. 
     Additionally or alternatively, the green photoluminescence can comprise a quantum dot material. 
     In embodiments, the red photoluminescence material can comprise a manganese-activated fluoride phosphor. In some embodiments, the manganese-activated fluoride phosphor comprises a manganese-activated potassium hexafluorosilicate phosphor of composition K2SiF6:Mn4+ (KSF) or a manganese-activated potassium hexafluorogermanate phosphor of composition K2GeF6:Mn4+ (KGF). A particular benefit of using KSF or KGF phosphor in the light emitting device(s) is a substantial reduction in red phosphor usage compared with including it in the wavelength converting layer. KSF and KGF have a low blue (excitation) absorption efficiency requiring high material solid loadings in use. In large color LCDs such as televisions, computers and tablet computers, use of this material in the large area wavelength converting layer would be prohibitively expensive. A further advantage when using SrGa2S4:Eu phosphor in in the wavelength converting layer is to avoid it chemically reacting with KSF or KGF phosphors. In other embodiments, the manganese-activated fluoride phosphor can comprise a phosphor of composition selected from the group consisting of: K2TiF6:Mn4+, K2SnF6:Mn4+, Na2TiF6:Mn4+, Na2ZrF6:Mn4+, Cs2SiF6:Mn4+, Cs2TiF6:Mn4+, Rb2SiF6:Mn4+, Rb2TiF6:Mn4+, K3ZrF7:Mn4+, K3NbF7:Mn4+, K3 TaF7:Mn4+, K3 GdF6:Mn4+, K3LaF6:Mn4+ and K3YF6:Mn4+. 
     In various embodiments of the invention, backlights comprising a manganese-activated fluoride red phosphor and a green sulfide phosphor, such backlight can have an emission spectrum with a color gamut of at least 95% of NTSC (National Television System Committee) and/or at least 100% of DCI-P3 (Digital Cinema Initiatives) RGB color space standards. Such a color gamut is comparable to backlights based on QDs (non-cadmium containing) and exceeds that of known backlights composed of KSF and β-SiAlON. In this patent specification, a high color gamut backlight and/or color display refers to a backlight/display capable of producing light of colors that are at least 95% of NTSC and/or at least 100% of DCI-P3 RGB color space standards. 
     In various embodiments, the backlight can have an emission spectrum comprising red, green and blue emission peaks, wherein said red peak has chromaticity coordinates CIE x=0.6700 to 0.6950, CIE y=0.3300 to 0.2950; the green peak has chromaticity coordinates CIE x=0.1950 to 0.2950, CIE y=0.7250 to 0.6250; and the blue peak has chromaticity coordinates CIE x=0.1600 to 0.1400, CIE y=0.0180 to 0.0600. 
     In various embodiments, the wavelength converting layer comprises a separate film that is fabricated separate to other components of the backlight. In other embodiments, the photoluminescence wavelength conversion layer can be fabricated as a part of another component of the backlight or display, for example it can be deposited directly onto a component of the backlight or display, that is, in direct contact with the component. 
     In various embodiments, backlights of the invention can comprise edge-lit or direct-lit arrangements. 
     In edge-lit arrangements, the backlight further comprises a light guide and the light emitting device is configured to couple light into at least one edge of the light guide and the wavelength converting layer is disposed adjacent to the light guide. In some embodiments, the wavelength converting layer is in direct contact with the light guide. To increase emission brightness, the backlight can further comprise a Brightness Enhancement Film (BEF) and the wavelength converting layer is disposed between the light guide and the brightness enhancement film. The wavelength converting layer can be in direct contact with the BEF. 
     In some edge-lit arrangements the backlight can further comprise a light reflective surface and the wavelength converting layer be disposed between the light reflective surface and the light guide. The wavelength converting layer can be in direct contact with the light guide or in direct contact with the light reflective surface. 
     In direct-lit arrangements, the backlight can further comprise a Brightness Enhancement Film (BEF) and the wavelength converting layer is disposed adjacent to the brightness enhancement film. The wavelength converting layer can be in direct contact with the BEF. 
     In various embodiments of the invention, the wavelength converting layer can further comprise particles of a light scattering material. The inclusion of particles of a light scattering material can increase uniformity of light emission from the wavelength converting layer and can eliminate the need for a separate light diffusive layer as are commonly used in known displays. Additionally, incorporating particles of a light scattering material with the red or green photoluminescence materials of the wavelength converting layer can result in an increase in light generation by the photoluminescence wavelength conversion layer as well as a substantial, up to 40%, reduction in the quantity of photoluminescence material required to generate a given color of light. Given the relatively high cost of photoluminescence materials, the inclusion of a light scattering material can result in a significant reduction in manufacturing cost for larger displays such a tablet computers, laptops, TVs and monitors. Additionally, the light emitting device can further comprise particles of a light scattering material. 
     The light scattering material can comprise, for example, particles of zinc oxide (ZnO), silicon dioxide (SiO2), titanium dioxide (TiO2), magnesium oxide (MgO), barium sulfate (BaSO4), aluminum oxide (Al2O3), or combinations thereof. The light scattering material particles can have an average diameter such that they scatter excitation light more than photoluminescence generated red or green light. In some embodiments, the light diffusive material particles have an average diameter (D50) of 200 nm of less, typically 100 nm to 150 nm. 
     According to one or more embodiments, there is contemplated a display backlight comprising: a light emitting device comprising an excitation source for generating blue excitation light with a dominant emission wavelength in a range 445 nm to 465 nm and a manganese-activated potassium hexafluorosilicate phosphor of composition K2SiF6:Mn4+; and a wavelength converting layer located remotely to the light emitting device, and comprising a green photoluminescence material having a peak emission wavelength in a range 525 nm to 545 nm, said green photoluminescence material comprising strontium, gallium, and sulfur having general composition and crystal structure of SrGa2S4:Eu. 
     The green photoluminescence material in the wavelength converting layer can comprise a europium activated sulfide phosphor comprising strontium, gallium, and sulfur. 
     The europium activated sulfide phosphor can have a general composition and crystal structure of SrGa2S4:Eu. 
     According to one or more embodiments, there is contemplated a display backlight comprises a light emitting device comprising an excitation source for generating blue excitation light with a dominant emission wavelength in a range 445 nm to 465 nm and a red photoluminescence material with a peak emission wavelength in a range 610 nm to 650 nm; and a wavelength converting layer located remotely to the light emitting device; said wavelength converting layer comprising a green photoluminescence material with a peak emission wavelength in a range 525 nm to 545 nm, said green photoluminescence material comprising a quantum dot material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the present invention is better understood, embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic cross-sectional representation of a color LCD in accordance with an embodiment of the present invention; 
         FIG. 2  is a schematic cross-sectional representation of a front plate of the color LCD of  FIG. 1 ; 
         FIG. 3  is a schematic diagram of a unit pixel of a color filter plate of the color LCD of  FIG. 1 ; 
         FIG. 4  shows the filtering characteristics, light transmission versus wavelength, for red, green and blue filter elements of a color filter plate of a color LCD display according to an embodiment of the invention; 
         FIG. 5  is a schematic cross-sectional representation of a back plate of the color LCD of  FIG. 1 ; 
         FIG. 6  is a cross-sectional side view of a light emitting device in accordance with an embodiment of the invention; 
         FIG. 7  is a schematic cross-sectional representation of an edge-lit backlight of the color LCD of  FIG. 1  in which a photoluminescence layer is located between a light guide and a BEF (Brightness Enhancement Film); 
         FIG. 8  is a schematic cross-sectional representation of an edge-lit backlight in accordance with an embodiment of the invention in which a photoluminescence layer is located between a light guide and a light reflective layer; 
         FIG. 9  is a schematic exploded cross-sectional representation of a direct-lit backlight in accordance with an embodiment of the invention; 
         FIG. 10  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for a light emitting device in accordance with an embodiment of the invention; 
         FIG. 11  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for a photoluminescence wavelength converting layer in accordance with an embodiment of the invention; 
         FIG. 12  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for a backlight in accordance with an embodiment of the invention before and after the BEF; 
         FIG. 13  shows the 1931 CIE color coordinates of the NTSC standard and RGB color coordinates of a backlight according to some embodiments; 
         FIG. 14  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for backlight BL.2 (tuned to DCI-P3 white point) before and after the AUO color filter; and 
         FIG. 15  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for backlight BL.3 (tuned to DCI-P3 white point) before and after the AUO color filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the invention concern color LCD backlights that include red-emitting and green-emitting photoluminescence materials (e.g. phosphors, quantum dots and/or organic dyes), which when excited by excitation light (typically blue light) generate a combined white light output for operating the display. 
     In accordance with some embodiments of the invention a backlight comprises a europium activated sulfide phosphor such as for example a europium activated sulfide phosphor of general composition based on MA2S4:Eu, where M is at least one of Mg, Ca, Sr and Ba, A is at least one of Ga, Al, In, La and Y. In some embodiments, the europium activated sulfide phosphor has a general composition and crystal structure of SrGa2S4:Eu. The sulfide phosphor can comprise further elements such as a halogen and can be coated to improve its reliability. The red photoluminescence material and/or europium activated sulfide phosphor can comprise a wavelength converting layer that is located remotely to one or more light emitting devices that comprise an excitation source, typically a blue LED, for generating excitation light. In other embodiments one or both of the red photoluminescence material and/or europium activated sulfide phosphor can be located in the one or more light emitting devices. Typically, the wavelength converting layer comprises a part of the backlight, though it may be considered to comprise a part of the display. The wavelength converting layer, which typically comprises a film, is of a size corresponding to size of the display. The wavelength converting layer can be incorporated with or used to replace the diffuser layer of the known displays/backlights. The red photoluminescence material can comprise a phosphor material, quantum dots, organic dies and combination thereof. 
     In accordance with other embodiments of the invention a backlight comprises locating the red and green photoluminescence materials at different physical locations along the light path of the backlight/display. For example, the red and green photoluminescence materials can be located within separate components of the backlight, i.e. at separate physical locations, with one photoluminescence material being located in a light emitting package containing an excitation source, typically a blue LED and the other photoluminescence material being located in a photoluminescence wavelength converting layer that is located remotely to the light emitting package. “Remotely” in this specification means two components which are spatially separated such as to reduce transfer of heat between components. The components can be separated by air or a light transmissive medium. In preferred embodiments, the red photoluminescence material is located within the one or more light emitting devices and the green photoluminescence material is located within the photoluminescence wavelength converting layer. In other embodiments, the red and green photoluminescence materials can be located in respective wavelength converting layers or within respective light emitting devices. A particular benefit of locating the red photoluminescence material in the one or more light emitting device(s) and the green photoluminescence material in a separate wavelength converting layer is that this can improve luminous efficacy of the backlight. 
     Embodiments of the present invention will now be described in detail with reference to the drawings, which are provided as illustrative examples of the invention so as to enable those skilled in the art to practice the invention. Notably, the figures and examples below are not meant to limit the scope of the present invention to a single embodiment, but other embodiments are possible by way of interchange of some or all of the described or illustrated elements. Moreover, where certain elements of the present invention can be partially or fully implemented using known components, only those portions of such known components that are necessary for an understanding of the present invention will be described, and detailed descriptions of other portions of such known components will be omitted so as not to obscure the invention. In the present specification, an embodiment showing a singular component should not be considered limiting; rather, the invention is intended to encompass other embodiments including a plurality of the same component, and vice-versa, unless explicitly stated otherwise herein. Moreover, applicants do not intend for any term in the specification or claims to be ascribed an uncommon or special meaning unless explicitly set forth as such. Further, the present invention encompasses present and future known equivalents to the known components referred to herein by way of illustration. Throughout this specification like reference numerals are used to denote like features. 
     Referring to  FIG. 1  there is shown a schematic cross-sectional representation of a light transmissive Color LCD (Liquid Crystal Display)  100  formed in accordance with an embodiment of the invention. The Color LCD  100  comprises a LC (Liquid Crystal) Display Panel  102  and a Display Backlight  104 . The Backlight  104  ( FIGS. 7-9 ) is operable to generate white light  140  for operating the LC Display Panel  102 . 
     LC Display Panel 
     As shown in  FIG. 1 , the LC display panel  102  comprises a transparent (light transmissive) Front (light/image emitting) Plate  106 , a transparent Back Plate  108  and a Liquid Crystal (LC)  110  filling the volume between the Front and Back Plates  106 ,  108 . 
     As shown in  FIG. 2 , the Front Plate  106  can comprise a glass plate  112  having on its upper surface, that is the face of the plate comprising the viewing face  114  of the display, a first polarizing filter layer  116 . Optionally, the outermost viewing surface of the front plate can further comprise an anti-reflective layer  118 . On its underside, that is the face of the front plate  106  facing the liquid crystal (LC)  110 , the glass plate  112  can further comprise a color filter plate  120  and a light transmissive common electrode plane  122  (for example transparent Indium Tin Oxide, ITO). 
     The color filter plate  120  comprises an array of different color sub-pixels filter elements  124 ,  126 ,  128  which respectively allow transmission of red (R), green (G), and blue (B) light. Each unit pixel  130  of the display comprises a group of three sub-pixels filter elements  124 ,  126 ,  128 .  FIG. 3  is a schematic diagram of a unit pixel  130  of the color filter plate  132 . As shown, each RGB sub-pixel  124 ,  126 ,  128  comprises a respective color filter pigment, typically an organic dye, which allows passage of light corresponding to the color of the sub-pixel only. The RGB sub-pixel elements  124 ,  126 ,  128  can be deposited on the glass plate  112  with opaque dividers/walls (black matrix)  132  between each of the sub-pixels  124 ,  126 ,  128 . The black matrix  132  can be formed as a grid mask of metal, such as for example chromium, defining the sub-pixels  124 ,  126 ,  128  and providing an opaque gap between the sub-pixels and unit pixels  130 . To minimize reflection from the black matrix, a double layer of Cr and CrOx may be used, but of course, the layers may comprise materials other than Cr and CrOx. The black matrix film which can be sputter-deposited underlying or overlying the photoluminescence material may be patterned using methods that include photolithography.  FIG. 4  shows the filtering characteristics, light transmission versus wavelength, for red (R), green (G) and blue (B) filter elements of a Hisense filter plate optimized for TV applications. 
     Referring to  FIG. 5 , the back plate  108  can comprise a glass plate  134  having on its upper surface (the surface facing the LC) a TFT (Thin Film Transistor) layer  136 . The TFT layer  136  comprises an array of TFTs, in which there is a transistor corresponding to each individual color sub-pixel  124 ,  126 ,  128  of each unit pixel  130 . Each TFT is operable to selectively control passage of the light to its corresponding sub-pixel. On a lower surface of the glass plate  134  there is provided a second polarizing filter layer  138 . The directions of polarization of the two polarizing filters  116  and  138  are aligned perpendicular to one another. 
     Backlight 
     The Backlight  104  is operable to generate and emit white light  140  from a front light emitting face  142  (upper face that faces the rear of the Display Panel,  FIG. 7 ) for operating the LC Display Panel  102 . 
     Backlight: Light Emitting Device 
       FIG. 6  is a schematic cross-sectional representation of a light emitting device  146  according to some embodiments. The light emitting device  146  is operable to generate composite light comprising a combination of blue excitation light and one of red (peak emission wavelength 610 nm -650 nm) or green (peak emission wavelength 530 nm -545 nm) photoluminescence light. 
     As shown in  FIG. 6 , the device  146  can comprise a blue light emitting GaN LED chip  42  (dominant emission wavelength 445 nm -465 nm), preferably 445 nm -455 nm, housed within a package. The package, which can for example comprise a low temperature co-fired ceramic (LTCC) or high temperature polymer, comprises upper and lower body parts  44 ,  46 . The upper body part  44  defines a recess  48 , often circular in shape, which is configured to receive one or more LED chip(s)  42 . The package further comprises electrical connectors  50  and  52  that also define corresponding electrode contact pads  54  and  56  on the floor of the recess  48 . Using for example adhesive or solder, the LED chip  42  can be mounted to a thermally conductive pad  58  located on the floor of the recess  48 . The LED chip&#39;s electrode pads are electrically connected to corresponding electrode contact pads  54  and  56  on the floor of the package using bond wires  60  and  62  and the recess  48  is completely filled with a light transmissive (transparent) polymer material  64 , typically a silicone, which is loaded with a photoluminescence material, such as a phosphor, such that the exposed surfaces of the LED chip  42  are covered by the phosphor/polymer material mixture. To enhance the emission brightness of the device the walls of the recess  48  can be inclined and have a light reflective surface. In accordance with the invention the photoluminescence material comprises either a green- or a red-emitting photoluminescence material. In preferred embodiments, the red or green photoluminescence materials comprise narrow-band phosphors. In operation the light emitting device  146  generates composite light  148  comprising a combination of blue excitation light from the LED chip  42  and photoluminescence light generated by the photoluminescence material in response to excitation by the blue excitation light. Depending on the photoluminescence material present in the light emitting device, the photoluminescence light can be green or red. 
     As shown in  FIG. 7 , the backlight  104  can comprise an edge-lit arrangement comprising a light guide (waveguide)  144  with one or more light emitting devices  146  located along one or more edges of the light guide  144 . As indicated, the light guide  144  can be planar; though, in some embodiments, it can be tapered (wedge-shaped) for promoting a more uniform emission of composite-light from a front light emitting face (upper face as shown in  FIG. 7  that faces the Display Panel) of the light guide. The light emitting devices  146  are configured such that in operation, they generate composite light  148  which is coupled into one or more edges of the light guide  144  and then guided, by total internal reflection, throughout the volume of the light guide  144  and finally emitted from the front face (upper face that faces the Display Panel  102 ) of the light guide  144 . As shown in  FIG. 7 , and to prevent the escape of light from the backlight  104 , the rear face (lower face as shown) of the light guide  144  can comprise a light reflective layer (surface)  150  such as Vikuiti™ ESR (Enhanced Spectral Reflector) film from 3M. 
     On a front light emitting face (upper face as shown) of the light guide  144  there is provided a photoluminescence wavelength converting layer  152  and a Brightness Enhancement Film (BEF)  154 . In the embodiment illustrated in  FIG. 7  the photoluminescence wavelength converting layer  152  is disposed between the light guide  144  and BEF  154 . 
     Backlight: Brightness Enhancement Film (BEF) 
     The Brightness Enhancement Film (BEF), also known as a Prism Sheet, comprises a precision micro-structured optical film and controls the emission of light  140  from the backlight within a fixed angle (typically 70 degrees), thereby increasing luminous efficacy of the backlight. Typically, the BEF comprises an array of micro-prisms on a light emitting face of the film and can increase brightness by 40 -60%. The BEF  154  can comprise a single BEF or a combination of multiple BEFs and in the case of the latter even greater increases in brightness can be achieved. Examples of suitable BEFs include Vikuiti™ BEF II from 3M or prism sheets from MNTech. In some embodiments, the BEF  154  can comprise a Multi-Functional Prism Sheet (MFPS) that integrates a prism sheet with a diffusion film and can have a better luminous efficiency than a normal prism sheet. In some embodiments, the BEF  154  can comprise a Micro-Lens Film Prism Sheet (MLFPS) such as those available from MNTech. 
     Backlight: Photoluminescence Wavelength Converting Layer 
     For the sake of brevity, in the following description the photoluminescence wavelength converting layer will be referred to as the “photoluminescence layer”. 
     The photoluminescence layer  152  contains either a red- or green-emitting photoluminescence material and in operation converts at least a portion of the blue excitation light of the composite light  148  generated by the device  146  to produce a white light emission product  140  for operating the LC display panel  104 . More specifically, the photoluminescence layer  152  contains either a blue light excitable red-emitting (Peak emission wavelength λpe=600 nm -650 nm) photoluminescence material or a green-emitting (Peak emission wavelength λpe=530 nm -545 nm) photoluminescence material. The combination of photoluminescence generated light  158  and composite light  148  results in a white light emission product  140 . To optimize the efficacy and color gamut of the display, the red- and green-emitting photoluminescence materials are selected to match their peak emission (PE) wavelength λpe with the transmission characteristic of their corresponding color filter elements. Preferably, the green-emitting photoluminescence material has a peak emission wavelength λpe≈535 nm. In order to maximize display color gamut and efficacy, the red-emitting and/or green-emitting photoluminescence materials present in the light emitting device  146  and photoluminescence layer  152  preferably comprise narrow-band photoluminescence materials having an emission peak with a FWHM (Full Width Half Maximum) of about 50 nm of less. 
     The red- and green-emitting photoluminescence materials can comprise phosphor materials, quantum dots (QDs), organic dyes or combinations thereof. For the purposes of illustration only, the current description specifically refers to photoluminescence materials embodied as phosphor materials. The phosphor materials can comprise inorganic and organic phosphor materials. Inorganic phosphors can comprise aluminate, silicate, phosphate, borate, sulfate, chloride, fluoride or nitride phosphor materials. As is known phosphor materials are doped with a rare-earth element called an activator. The activator typically comprises divalent europium, cerium or tetravalent manganese. Dopants such as halogens can be substitutionally or interstitially incorporated into the crystal lattice and can for example reside on lattice sites of the host material and/or interstitially within the host material. 
     Red-Emitting Phosphor Materials 
     In this patent specification, a red-emitting phosphor refers to a phosphor material which generates light having a peak emission wavelength in a range 610 nm -650 nm, that is in the orange to red region of the visible spectrum. Preferably, the red-emitting phosphor is a narrow-band phosphor material and has a full width at half maximum emission intensity of less than about 50 nm. Examples of suitable red-emitting phosphor materials for use in the light emitting device  146  and photoluminescence layer  152  are given in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Red-emitting phosphors 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 λpe  
                 FWHM  
               
               
                 Phosphor family 
                   
                 Composition 
                 (nm) 
                 (nm) 
               
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Hexafluorosilicate 
                 KSF 
                 K 2 SiF 6 :Mn 4+   
                 ≈632 
                 ≈10 
               
               
                 Hexafluorotitanate 
                 KTF 
                 K 2 TiF 6 :Mn 4+   
                 ≈632 
                 ≈10 
               
               
                 Hexafluorogermanate 
                 KGF 
                 K 2 GeF 6 :Mn 4+   
                 ≈632 
                 ≈10 
               
               
                 Selenide sulfide 
                 CSS 
                 MSe 1−x S x : Eu 
                 600-635 
                 50-55 
               
               
                   
                   
                 M = Mg, Ca, Sr  
                   
                   
               
               
                   
                   
                 and/or Ba 
                   
                   
               
               
                 Selenide sulfide 
                 CSS 
                 CaSeS:Eu 
                 610-635 
                 50-55 
               
               
                 Silicon-nitride 1:1:1:3 
                 CASN 
                 CaAlSiN 3 :Eu 
                 625-650 
                 ≈75 
               
               
                   
                   
                 (Ca 1−x Sr x )AlSiN 3 :Eu 
               
               
                   
               
            
           
         
       
     
     Narrow-Band Red Phosphors: Manganese-Activated Fluoride Phosphors 
     An example of a manganese-activated fluoride phosphor is manganese-activated potassium hexafluorosilicate phosphor (KSF)—K2SiF6:Mn4+. An example of such a phosphor is NR6931 KSF phosphor from Intematix Corporation, Fremont Calif., USA which has a peak emission wavelength of about 632 nm. KSF phosphor is excitable by blue excitation light and generates red light with a peak emission wavelength (λpe) of between about 631 nm and about 632 nm with a FWHM of ·5 nm to ·10 nm (depending on the way it is measured: i.e. whether the width takes account of a single or double peaks). Other manganese-activated phosphors can include: K2GeF6:Mn4+, K2TiF6:Mn4+, K2SnF6:Mn4+, Na2TiF6:Mn4+, Na2ZrF6:Mn4+, Cs2SiF6:Mn4+, Cs2TiF6:Mn4+, Rb2SiF6:Mn4+, Rb2TiF6:Mn4+, K3ZrF7:Mn4+, K3NbF7:Mn4+, K3TaF7:Mn4+, K3GdF6:Mn4+, K3LaF6:Mn4+ and K3YF6:Mn4+. 
     Narrow-Band Red Phosphors: Group IIA/IIB Selenide Sulfide-Based Phosphors 
     An example of a Group IIA/BB selenide sulfide-based phosphor material has a composition MSe1-xSx:Eu, wherein M is at least one of Mg, Ca, Sr, Ba and Zn and 0&lt;x&lt;1.0. A particular example of this phosphor material is CSS phosphor (CaSe1-xSx:Eu). Details of CSS phosphors are provided in co-pending U.S. patent application Ser. No. 15/282,551 filed 30 Sep. 2016, which is hereby incorporated by reference in its entirety. The CSS narrow-band red phosphors described in U.S. patent application Ser. No. 15/282,551 can be used in the present invention. The peak emission wavelength of CSS phosphors can be tuned from 600 nm to 650 nm by changing the ratio of S/Se in the composition and exhibits a narrow-band red emission spectrum with FWHM in the range ·48 nm to ·60 nm (longer wavelength typically has a larger FWHM value). 
     Green-Emitting Phosphor Materials 
     In this patent specification, a green-emitting phosphor refers to a phosphor material which generates light having a peak emission wavelength in a range 525 nm to 545 nm, that is in the green red region of the visible spectrum. In some embodiments, the green-emitting phosphor generates light having a peak emission wavelength in a range 535 nm to 540 nm. Preferably, the green-emitting phosphor is a narrow-band phosphor material and has a full width at half maximum emission intensity of less than about 50 nm. Examples of suitable green-emitting phosphor materials for use in the light emitting device  146  and photoluminescence layer  152  are given in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Green-emitting phosphor materials 
               
            
           
           
               
               
               
               
               
            
               
                 Phosphor  
                   
                   
                 λpe  
                 FWHM  
               
               
                 family 
                   
                 Composition 
                 (nm) 
                 (nm) 
               
               
                   
               
               
                 Sulfide 
                   
                 MA 2 S 4 :Eu 
                 525-545 
                 48-50 
               
               
                   
                   
                 M = Mg, Ca, Sr and/or Ba 
                   
                   
               
               
                   
                   
                 A = Ga, Al, In, La and/or Y 
                   
                   
               
               
                 Sulfide 
                   
                 SrGa 2 S 4 :Eu 
                 525-545 
                 48-50 
               
               
                 Sulfide 
                   
                 (M)(A) 2 S 4 :Eu, M′, A′ 
                 525-545 
                 48-50 
               
               
                   
                   
                 M = Mg, Ca, Sr and/or Ba 
                   
                   
               
               
                   
                   
                 A = Ga, Al and/or In 
                   
                   
               
               
                   
                   
                 M′ = Li, Na and/or K 
                   
                   
               
               
                   
                   
                 A′ = Si, Ge, La, Y and/or Ti 
                   
                   
               
               
                 Sulfide 
                   
                 (M,M′)(A,A′) 2 S 4 :Eu 
                 525-545 
                 48-50 
               
               
                   
                   
                 M = Mg, Ca, Sr and/or Ba 
                   
                   
               
               
                   
                   
                 A = Ga, Al, In, La and/or Y 
                   
                   
               
               
                   
                   
                 M′ = Li, Na and/or K 
                   
                   
               
               
                   
                   
                 A′ = Si, Ge and/or Ti 
                   
                   
               
               
                 β-SiAlON 
                   
                 M x Si 12−(m+n) Al m+n O n N 16−n :Eu 
                 525-545 
                 50-52 
               
               
                   
                   
                 M = Mg, Ca and/or Sr 
                   
                   
               
               
                 Aluminate 
                 YAG 
                 Y 3 (Al 1−x Ga x ) 5 O 12 : Ce 
                 500-550 
                 ≈110 
               
               
                 Aluminate 
                 LuAG 
                 Lu 3 (Al 1−x M x ) 5 O 12 : Ce 
                 500-550 
                 ≈110 
               
               
                 Silicate 
                   
                 A 2 SiO 4 :Eu 
                 500-550 
                  ≈70 
               
               
                   
                   
                 A = Mg, Ca, Sr and/or Ba 
                   
                   
               
               
                 Silicate 
                   
                 (Sr 1−x Ba x ) 2 SiO 4 : Eu 
                 500-550 
                  ≈70 
               
               
                   
               
            
           
         
       
     
     Green-Emitting Phosphor Materials: Green Sulfide Phosphors 
     An example of a green sulfide phosphor material has a general composition based on MA2S4:Eu, where M is at least one of Mg, Ca, Sr and Ba, A is at least one of Ga, Al, In, La and Y. To improve reliability, the phosphor particles can be coated with one or more oxides chosen from the group of materials consisting of aluminum oxide, silicon oxide, titanium oxide, zinc oxide, magnesium oxide, zirconium oxide and chromium oxide. An example of such a phosphor is NBG phosphor from Intematix Corporation, Fremont California, USA which has a peak emission wavelength of between about 535 nm -540 nm. Details of green sulfide phosphors are provided in co-pending PCT patent publication No. WO2018/080936 published 3 May 2018, which is hereby incorporated by reference in its entirety. The green sulfide phosphors described in PCT patent publication WO2018/080936 can be used in the present invention. For example, a narrow band green phosphor may have a composition (M)(A) 2 S 4 : Eu, M′, A′ where M is at least one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; A is at least one of Ga, Al, and In; and A′ is at least one of Si, Ge, La, Y and Ti. In the latter formula the dopants Eu, M′ and A′ may be present in substitutional sites, although other options for incorporation are envisaged, such as interstitial sites. Furthermore, the green sulfide phosphor may have a composition (M,M)(A,A) 2 S 4 :Eu, wherein: M is at least one of Mg, Ca, Sr and Ba; M′ is at least one of Li, Na and K; A is at least one of Ga, Al, In, La and Y; and A′ is at least one of Si, Ge and Ti; wherein M′ substitutes for M, and A′ substitutes for A in the MA 2 S 4  crystalline lattice. In the latter formula the specific substitution sites are identified, although it is envisaged that alternative substitutional sites may exist; for example, it is envisaged that for doping with Li and Si the following structure may provide an alternative substitutional site for the Li: Sr(Ga 1-2x Si x Li 2x ) 2 S 4 :Eu, wherein: M is at least one of Mg, Ca, Sr and Ba; and A is at least one of Ga, Al, In, La and Y; wherein 0&lt;x&lt;0.1. 
     Quantum Dot (QD) Materials 
     A quantum dot (QD) is a portion of matter (e.g. semiconductor) whose excitons are confined in all three spatial dimensions that may be excited by radiation energy to emit light of a particular wavelength or range of wavelengths. QDs can comprise different materials, for example cadmium selenide (CdSe). The color of light generated by a QD is enabled by the quantum confinement effect associated with the nano-crystal structure of the QD. The energy level of each QD relates directly to the physical size of the QD. For example, the larger QDs, such as red QDs, can absorb and emit photons having a relatively lower energy (i.e. a relatively longer wavelength). On the other hand, green QDs, which are smaller in size can absorb and emit photons of a relatively higher energy (shorter wavelength). Examples of suitable QDs can include: CdZnSeS (cadmium zinc selenium sulfide), CdxZn1-x Se (cadmium zinc selenide), CdSexS1-x (cadmium selenium sulfide), CdTe (cadmium telluride), CdTexS1-x (cadmium tellurium sulfide), InP (indium phosphide), InxGa1-x P (indium gallium phosphide), InAs (indium arsenide), CuInS2 (copper indium sulfide), CuInSe2 (copper indium selenide), CuInSxSe2-x (copper indium sulfur selenide), CuInxGa1-x S2 (copper indium gallium sulfide), CuInxGal-xSe2 (copper indium gallium selenide), CuInxAl1-x Se2 (copper indium aluminum selenide), CuGaS2 (copper gallium sulfide) and CuInS2xZnS1-x (copper indium selenium zinc selenide). The QD materials can comprise core/shell nano-crystals containing different materials in an onion-like structure. For example, the above described exemplary materials can be used as the core materials for the core/shell nano-crystals. The optical properties of the core nano-crystals in one material can be altered by growing an epitaxial-type shell of another material. Depending on the requirements, the core/shell nano-crystals can have a single shell or multiple shells. The shell materials can be chosen based on the band gap engineering. For example, the shell materials can have a band gap larger than the core materials so that the shell of the nano-crystals can separate the surface of the optically active core from its surrounding medium. In the case of the cadmium-based QDs, e.g. CdSe QDs, the core/shell quantum dots can be synthesized using the formula of CdSe/ZnS, CdSe/CdS, CdSe/ZnSe, CdSe/CdS/ZnS, or CdSe/ZnSe/ZnS. Similarly, for CuInS2 quantum dots, the core/shell nanocrystals can be synthesized using the formula of CuInS2/ZnS, CuInS2/CdS, CuInS2/CuGaS2, CuInS2/CuGaS2/ZnS and so on. 
     Examples of suitable quantum dots composition are given in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Quantum dot composition 
               
            
           
           
               
               
               
            
               
                   
                 Green (525 nm-545 nm) 
                 Red (610 nm-650 nm) 
               
               
                   
                   
               
               
                   
                 CdSe ~2.9 nm 
                 CdSe ~4.2 nm 
               
               
                   
                 Cd x Zn 1−x  Se 
                 Cd x Zn 1−x  Se 
               
               
                   
                 CdZnSeS 
                 CdZnSeS 
               
               
                   
                 CdSe x S 1−x   
                 CdSe x S 1−x   
               
               
                   
                 CdTe 
                 CdTe 
               
               
                   
                 CdTe x S 1−x   
                 CdTe x S 1−x   
               
               
                   
                 CdS 
                 — 
               
               
                   
                 InP 
                 InP 
               
               
                   
                 In x Ga 1−x  P 
                 In x Ga 1−x  P 
               
               
                   
                 — 
                 InAs 
               
               
                   
                 CuInS 2   
                 CuInS 2   
               
               
                   
                 CuInSe 2   
                 CuInSe 2   
               
               
                   
                 CuInS x Se 2−x   
                 CuInS x Se 2−x   
               
               
                   
                 Cu In x Ga 1−x  S 2   
                 Cu In x Ga 1−x  S 2   
               
               
                   
                 Cu In x Gal, Se 2   
                 Cu In x Gal, Se 2   
               
               
                   
                 CuGaS2 
                 CuGaS2 
               
               
                   
                 Cu In x Al 1−x  Se 2   
                 Cu In x Al 1−x  Se 2   
               
               
                   
                 Cu Ga x Al 1−x  Se 2   
                 — 
               
               
                   
                 CuInS 2x ZnS 1−x   
                 CuInS 2x ZnS 1−x   
               
               
                   
                 CuInSe 2x ZnSe 1−x   
                 CuInSe 2x ZnSe 1−x   
               
               
                   
                   
               
            
           
         
       
     
     There are a variety of ways of implementing backlights in accordance with the invention. For example, as described above, in some embodiments the red-emitting photoluminescence material can be located in the light emitting device  146  and the green-emitting photoluminescence material located in the photoluminescence layer  152 . In other embodiments the green-emitting photoluminescence material can be located in the light emitting device  146  and the red-emitting photoluminescence material located in the photoluminescence layer  152 . It is contemplated, in other embodiments, to locate both the red-emitting and the green-emitting photoluminescence material in the photoluminescence layer  152 . It will be appreciated that in such arrangements the light emitting device  146  need not include red-emitting and green-emitting photoluminescence materials and that the light emitting device  146  may generate only blue excitation light. In some arrangements, the red-emitting and the green-emitting photoluminescence materials can be incorporated in the photoluminescence layer  152  as a mixture. In other arrangements, the red-emitting and the green-emitting photoluminescence materials can be incorporated in separate respective photoluminescence layers. In the context of this specification, “photoluminescence layer” contemplates both a single layer and multiple layers; that is “photoluminescence layer” includes “photoluminescence layers”. Regardless of the location of the red and green photoluminescence materials, the photoluminescence layer can be implemented in a number of ways. 
     In some embodiments, the photoluminescence layer  152  is disposed adjacent to the BEF  154 . When using inorganic phosphor materials, the red-emitting or green-emitting phosphors, which are in the form of particles, can be incorporated as a mixture in a curable light transmissive liquid binder material and the mixture deposited as a uniform layer on a light transmissive substrate using for example screen printing or slot die coating. In some embodiments, the BEF  154  can comprise the light transmissive substrate and the photoluminescence layer  152  can be deposited directly onto the BEF  154 . In this patent specification, depositing directly means in direct contact with, in that is there is no intervening layer or air gap between the layers. When depositing the photoluminescence layer using screen printing, the light transmissive binder material can comprise for example a light transmissive UV-curable acrylic adhesive such as UVA4103 clear base from STAR Technology of Waterloo, Ind. USA. An advantage of depositing the photoluminescence layer directly onto the BEF is that this can increase light emission from the backlight by eliminating an air interface between the photoluminescence layer and BEF. Such an air interface could otherwise lead to a greater probability of internal reflection of light within the photoluminescence layer and reduce light coupling into the BEF. 
     In other embodiments, the photoluminescence layer  152  can be fabricated as a separate film and the resulting film disposed between the lightguide  144  and BEF  154 . Fabricating the photoluminescence layer separately can be advantageous when the lower face of the BEF  154  includes a pattern of features or surface texturing. 
     For example, in one arrangement, the red- or green-emitting phosphors and light transmissive material are deposited, for example, by screen printing as a uniform layer onto a light transmissive film, such as for example mylar™. In other embodiments, the red- or green-emitting phosphors can be incorporated in and homogeneously distributed throughout a film which can then be applied to the BEF  154 . 
     In other embodiments, the photoluminescence layer  152  can be disposed adjacent to the light guide  144 . For example in  FIG. 7  the photoluminescence layer  152  is disposed between the light guide  144  and the BEF  154  adjacent to the front light emitting face (upper face as shown that faces the Display Panel) of the light guide  144 . In some embodiments, the photoluminescence layer  152  can be deposited directly onto the front light emitting face of the light guide  144 . An advantage of depositing the photoluminescence layer directly onto the front face of the light guide is that this can increase overall light emission from the backlight through the elimination an air interface between the light guide and photoluminescence layer. Such an air interface, if present, could reduce light coupling from the light guide into the photoluminescence layer and reduce overall light emission from the backlight. 
     In other embodiments, the photoluminescence layer  152  can be fabricated as a separate film and the resulting film then applied to the front light emitting face of the light guide  144 . Such an arrangement can be advantageous when the front light emitting face of the light guide  144  includes a pattern of features or texturing that is used to aid in a uniform light extraction of light from the light guide. 
     In other embodiments, and as indicated in  FIG. 8  the photoluminescence layer  152  is disposed between the rear face (lower face as shown) of the light guide  144  and the light reflective layer  150 . In some embodiments, the photoluminescence layer  152  can be deposited directly onto the rear face of the light guide  144 . An advantage of depositing the photoluminescence layer directly onto the rear face of the light guide is that this can increase overall light emission from the backlight through the elimination an air interface between the light guide and photoluminescence layer. Such an air interface, if present, could reduce light coupling from the light guide into photoluminescence layer and reduce overall light emission from the backlight. 
     In other embodiments, the photoluminescence layer  152  can be deposited directly onto the light reflective layer  150 . An advantage of depositing the photoluminescence layer directly onto the light reflective layer  150  is that this can increase overall light emission from the backlight through the elimination an air interface between the photoluminescence layer and light reflective layer. Such an air interface if present, could reduce backward directed light being reflected back in a direction towards the light emitting face  142  of the backlight. 
     In yet other embodiments the photoluminescence layer  152  can be fabricated as a separate film and the resulting film then applied to the rear face of the light guide  144 . Such an arrangement can be advantageous when the rear emitting face of the light guide  144  includes a pattern of features of texturing to aid in a uniform light extraction of light from the light guide. 
     An advantage of having a photoluminescence layer as compared with known displays that utilize white LEDs, is that due to the light diffusive nature of phosphor materials this can eliminate the need for a separate light diffusive layer and the associated interface losses and thereby increase display efficacy as well as reducing production costs. 
     However, due to the isotropic nature of photoluminescence light generation, photoluminescence light  158  by the red- or green-emitting phosphors in the photoluminescence layer will be emitted in all directions including directions towards the light guide  144 . To reduce the likelihood of such light reaching the light guide  144 , the backlight can further comprise a light diffusive layer disposed between the photoluminescence layer  152  and the light guide  144 . 
     While in the foregoing embodiments the backlight has been an edge-lit arrangement utilizing a light guide, the invention finds utility in direct-lit backlights that comprise an array of light emitting devices configured over the surface of the LC display panel.  FIG. 9  illustrates such an embodiment in which an array of light emitting devices  146  containing one of the red- or green-emitting phosphors are provided on the floor  158  of a light reflective enclosure  160  and a separate photoluminescence layer  152  provided overlaying the enclosure. 
     In any of the embodiments described ( FIGS. 6-8 ) the photoluminescence layer  152  can further incorporate particles of a light scattering (diffusive) material, preferably zinc oxide (ZnO). The light diffusive material can comprise silicon dioxide (SiO2), titanium dioxide (TiO2), magnesium oxide (MgO), barium sulfate (BaSO4), aluminum oxide (Al2O3) or combinations thereof. Inclusion of a light scattering material can increase uniformity of light emission from the photoluminescence layer and can eliminate the need for a separate light diffusive layer. Additionally, incorporating particles of a light scattering material with the red- or green-emitting phosphor can result in an increase in light generation by the photoluminescence layer and a substantial, up to 40%, reduction in the quantity of phosphor materials required to generate a given color of light. Given the relatively high cost of phosphor materials, inclusion of an inexpensive light scattering material can result in a significant reduction in manufacturing cost for larger displays such a tablet computers, laptops, TVs and monitors. Further details of an exemplary approach to implement scattering particles are described in U.S. Pat. No. 8,610,340 issued Dec. 17, 2013, which is hereby incorporated by reference in its entirety. The size of the light scattering particles can be selected to scatter excitation light relatively more than light generated by the phosphor. In some embodiments, the light scattering material particles have an average diameter (D50) of 200 nm of less, typically 100 nm to 150 nm. 
     As described above, due to the isotropic nature of photoluminescence light generation, photoluminescence light  158   160  will be emitted in all directions including emission in directions towards the light guide  144 . To reduce the likelihood of such light reaching the light guide  144 , the backlight can in some embodiments further comprise a light diffusive layer disposed between the photoluminescence layer  152  and the light guide  144  even when the photoluminescence layer  152  already includes light scattering material. In other embodiments the photoluminescence layer  152  and light diffusive layer can be fabricated as separate films and the films then applied to one another. 
     Example Color Display Backlights 
     Table 4 tabulates details of preferred example backlights in accordance with the invention for use in high color gamut LCD television. The example backlights preferably comprise the edge-lit configuration illustrated in  FIG. 7 . 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Example backlights 
               
            
           
           
               
               
            
               
                   
                 Red and green photoluminescence materials 
               
            
           
           
               
               
               
            
               
                 Back- 
                 Device 146 
                 Photoluminescence layer 152 
               
            
           
           
               
               
               
               
               
            
               
                 light 
                 Material 
                 λ pe  (nm) 
                 Material 
                 λ pe  (nm) 
               
               
                   
               
               
                 BL.1 
                 K 2 SiF 6 :Mn 4+  (KSF) 
                 632 
                 SrGa2S4:Eu 
                 536 
               
               
                 BL.2 
                 K 2 SiF 6 :Mn 4+  (KSF) 
                 632 
                 ZnS coated InP (QD) 
                 538 
               
               
                 BL.3 
                 K 2 SiF 6 :Mn 4+  (KSF) 
                 632 
                 ZnS coated CdSe (QD) 
                 533 
               
               
                 BL.4 
                 K 2 GeF 6 :Mn 4+  (KGF) 
                 632 
                 SrGa 2 S 4 :Eu 
                 525-545 
               
               
                 BL.5 
                 K 2 TiF 6 :Mn 4+  (KTF) 
                 632 
                 SrGa 2 S 4 :Eu 
                 525-545 
               
               
                 BL.6 
                 CASN 
                 630-650 
                 SrGa 2 S 4 :Eu 
                 525-545 
               
               
                 BL.7 
                 CASN 
                 630-650 
                 β-SiAlON 
                 525-545 
               
               
                 BL.8 
                 β-SiAlON 
                 525-545 
                 CaSeS:Eu (CSS) 
                 630 
               
               
                   
               
            
           
         
       
     
     In the example denoted BL.1, the red-emitting phosphor comprises a narrow-band red-emitting manganese-activated potassium hexafluorosilicate phosphor of composition K2SiF6:Mn4+ (KSF), peak emission wavelength λpe=632 nm, and is located in the light emitting device  146 . The light emitting device  146  comprises a 7020 cavity package containing two 300 mW GaN LED chips with a dominant emission wavelength of 453 nm. The KSF phosphor is incorporated in, and homogeneously distributed throughout, a UV curable light transmissive silicone encapsulant (e.g. Dow Corning OE-6370 HF optical encapsulant) and the mixture deposited in the cavity recess such as to cover the LED chip. 
     In BL.1 the green-emitting phosphor comprises a narrow-band green-emitting strontium gallium sulfide phosphor of composition SrGa2S4:Eu, peak emission wavelength λpe=536 nm and is located in photoluminescence layer  152 . The green-emitting phosphor is incorporated in, and homogeneously distributed throughout a UV curable light transmissive acrylic binder (UVA4103 from STAR Technology) and the mixture screen printed as a ≈50 μm thickness layer on a ≈140 μm light transmissive PET (Polyethylene terephthalate) film. 
       FIG. 10  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for the light emitting device  146  of BL.1. 
       FIG. 11  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for the photoluminescence wavelength converting layer  152  of BL.1. 
       FIG. 12  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for backlight BL.1 before and after the BEF  154 . 
       FIG. 13  shows the 1931 CIE color coordinates of the NTSC (National Television System Committee) colorimetry 1953 (CIE 1931) standard and RGB color coordinates of the backlight BL.1. 
     Table 5 tabulates the optical characteristics of the backlight BL.1. An AUO (AU Optronics Corp.) high color gamut color filter characteristic was used to calculate the Red, Green and Blue emission spectra of an LCD display incorporating the backlight BL.1. As can be seen from Table 5 backlight BL.1 in accordance with the invention can produce light with color gamut of 100.7% (area) of the NTSC and 104.7% of DCI-P3 RGB color space standards. For comparison known high color gamut LCD display utilizing phosphors have a DCI-P3 of 99% to 100%. More specifically test have shown that backlights in accordance with the invention have an emission spectrum comprising red, green and blue emission peaks in which the red peak has chromaticity coordinates CIE x=0.6700 to 0.6950, CIE y=0.3300 to 0.2950; the green peak has chromaticity coordinates CIE x=0.1950 to 0.2950, CIE y=0.7250 to 0.6250; and the blue peak has chromaticity coordinates CIE x=0.1600 to 0.1400, CIE y=0.0180 to 0.0600. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Optical characteristics of backlight BL.1 
               
               
                 Drive condition: I f  = 20 mA, V f  = 5.2 V: Values for  
               
               
                 backlight tuned to NTSC white point 
               
            
           
           
               
               
            
               
                 Parameter 
                 Value 
               
               
                   
               
            
           
           
               
               
            
               
                 Backlight CIE x after BEF &amp; before color filter 
                 0.2783 
               
               
                 Backlight CIE y after BEF &amp; before color filter 
                 0.2482 
               
               
                 Backlight brightness (lm) after BEF &amp; before color filter 
                 6.18 
               
               
                 LCD white CIE x 
                 0.3260 
               
               
                 LCD white CIE y 
                 0.3213 
               
               
                 LCD Brightness (lm) 
                 5.07 
               
               
                 Brightness LCD/backlight (%) 
                 82.1 
               
               
                 Red CIE x after LCD red filter 
                 0.6923 
               
               
                 Red CIE y after LCD red filter 
                 0.3075 
               
               
                 Green CIE x after LCD green filter 
                 0.2361 
               
               
                 Green CIE y after LCD green filter 
                 0.6723 
               
               
                 Blue CIE x after LCD blue filter 
                 0.1495 
               
               
                 Blue CIE y after LCD blue filter 
                 0.0429 
               
               
                 NTSC (%) 
                 100.7 
               
               
                 DCI-P3 (%) 
                 104.9 
               
               
                   
               
            
           
         
       
     
     Table 6 tabulates the optical characteristics of the backlight BL.2. An AUO (AU Optronics Corp.) high color gamut color filter characteristic was used to calculate the Red, Green and Blue emission spectra of an LCD display incorporating the backlight BL.2. As can be seen from Table 6 backlight BL.1 in accordance with the invention can produce light with color gamut of 102.2% (area) of the NTSC and 106.6% of DCI-P3 RGB color space standards.  FIG. 14  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for backlight BL.2 (tuned to DCI-P3 white point) before and after the AUO color filter  120 . 
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Optical characteristics of backlight BL.2 tuned to  
               
               
                 DCI-P3 and NTSC white points 
               
               
                 Drive condition: I f  = 20 mA, V f  = 5.2 V 
               
            
           
           
               
               
            
               
                   
                 Value 
               
            
           
           
               
               
               
            
               
                   
                 Tuned to  
                 Tuned to  
               
               
                 Parameter 
                 DCI-P3 
                 NTSC 
               
               
                   
               
            
           
           
               
               
               
            
               
                 Backlight CIE x after BEF &amp; before color filter 
                 0.2677 
                 0.2637 
               
               
                 Backlight CIE y after BEF &amp; before color filter 
                 0.2456 
                 0.2336 
               
               
                 Backlight brightness (lm) after BEF &amp;  
                 14.44 
                 14.61 
               
               
                 before color filter 
                   
                   
               
               
                 LCD white CIE x 
                 0.3141 
                 0.3107 
               
               
                 LCD white CIE y 
                 0.3268 
                 0.3149 
               
               
                 LCD Brightness (lm) 
                 11.78 
                 11.91 
               
               
                 Brightness LCD/backlight (%) 
                 81.5 
                 81.5 
               
               
                 Red CIE x after LCD red filter 
                 0.6901 
                 0.6902 
               
               
                 Red CIE y after LCD red filter 
                 0.3098 
                 0.3096 
               
               
                 Green CIE x after LCD green filter 
                 0.2481 
                 0.2477 
               
               
                 Green CIE y after LCD green filter 
                 0.6873 
                 0.6848 
               
               
                 Blue CIE x after LCD blue filter 
                 0.1544 
                 0.1545 
               
               
                 Blue CIE y after LCD blue filter 
                 0.0345 
                 0.0332 
               
               
                 NTSC (%) 
                 — 
                 102.2 
               
               
                 DCI-P3 (%) 
                 106.6 
                 — 
               
               
                   
               
            
           
         
       
     
     Table 7 tabulates the optical characteristics of the backlight BL.3. An AUO (AU Optronics Corp.) high color gamut color filter characteristic was used to calculate the Red, Green and Blue emission spectra of an LCD display incorporating the backlight BL.3. As can be seen from Table 7 backlight BL.1 in accordance with the invention can produce light with color gamut of 112.3% (area) of the NTSC and 117.2% of DCI-P3 RGB color space standards.  FIG. 15  shows emission spectrum, intensity (a.u.) versus wavelength (nm), for backlight BL.3 (tuned to DCI-P3 white point) before and after the AUO color filter  120 . 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Optical characteristics of backlight BL.3 tuned to  
               
               
                 DCI-P3 and NTSC white points 
               
               
                 Drive condition: I f  = 20 mA, V f  = 5.2 V 
               
            
           
           
               
               
            
               
                   
                 Value 
               
            
           
           
               
               
               
            
               
                   
                 Tuned to  
                 Tuned to  
               
               
                 Parameter 
                 DCI-P3 
                 NTSC 
               
               
                   
               
            
           
           
               
               
               
            
               
                 CIE x after BEF &amp; before color filter 
                 0.2633 
                 0.2601 
               
               
                 CIE y after BEF &amp; before color filter 
                 0.2392 
                 0.2271 
               
               
                 Brightness (lm) after BEF &amp; before color filter 
                 15.97 
                 15.85 
               
               
                 LCD white CIE x 
                 0.3135 
                 0.3110 
               
               
                 LCD white CIE y 
                 0.3284 
                 0.3157 
               
               
                 LCD Brightness (lm) 
                 13.86 
                 13.75 
               
               
                 Brightness LCD/backlight (%) 
                 86.8 
                 86.6 
               
               
                 Red CIE x after LCD red filter 
                 0.6934 
                 0.6934 
               
               
                 Red CIE y after LCD red filter 
                 0.3065 
                 0.3064 
               
               
                 Green CIE x after LCD green filter 
                 0.1962 
                 0.1962 
               
               
                 Green CIE y after LCD green filter 
                 0.7211 
                 0.7180 
               
               
                 Blue CIE x after LCD blue filter 
                 0.1537 
                 0.1539 
               
               
                 Blue CIE y after LCD blue filter 
                 0.0402 
                 0.0383 
               
               
                 NTSC (%) 
                 — 
                 112.3 
               
               
                 DCI-P3 (%) 
                 117.2 
                 — 
               
               
                   
               
            
           
         
       
     
     More specifically test have shown that backlights in accordance with the invention have an emission spectrum comprising red, green and blue emission peaks in which the red peak has chromaticity coordinates CIE x=0.6700 to 0.6950, CIE y=0.3300 to 0.2950; the green peak has chromaticity coordinates CIE x=0.1950 to 0.2950, CIE y=0.7250 to 0.6250; and the blue peak has chromaticity coordinates CIE x=0.1600 to 0.1400, CIE y=0.0180 to 0.0600. 
     Table 8 tabulates RGB color space values for NTSC (National Television System Committee) colorimetry 1953 (CIE 1931) and DCI-P3 (Digital Cinema Initiatives) RGB color space standards. 
     
       
         
           
               
             
               
                 TABLE 8 
               
             
            
               
                   
               
               
                 NTSC (National Television System Committee) and DCI (Digital 
               
               
                 Cinema Initiatives) RGB color space (color gamut) standards 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Red 
                 Green 
                 Blue 
                 White point 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Standard 
                 CIE x 
                 CIE y 
                 CIE x 
                 CIE y 
                 CIE x 
                 CIE y 
                 CIE x 
                 CIE y 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 NTSC 
                 0.6700 
                 0.3300 
                 0.2100 
                 0.7100 
                 0.1400 
                 0.0800 
                 0.3101 
                 0.3162 
               
               
                 DCI-P3 
                 0.6800 
                 0.3200 
                 0.2650 
                 0.6900 
                 0.1500 
                 0.0600 
                 0.3127 
                 0.3290 
               
               
                   
               
            
           
         
       
     
     It will be appreciated that the present invention is not restricted to the specific embodiments described and that variations can be made that are within the scope of the invention. 
     For example while in the foregoing embodiments one or both of the red-emitting and/or green-emitting photoluminescence materials is located in the photoluminescence layer, it is envisaged in further embodiments to locate the red-emitting and/or green-emitting photoluminescence materials in the one or more light emitting devices thereby eliminating the need for a photoluminescence layer. Such an arrangement is found to be particularly advantageous where the green-emitting photoluminescence material comprises a europium activated sulfide phosphor. Further, it is also advantageous where the red-emitting photoluminescence material comprises a manganese-activated fluoride phosphor. In some arrangements, the red-emitting and the green-emitting photoluminescence materials can be incorporated in the same light emitting device in the form of a mixture or in separate locations/layers in the same light emitting device. In other arrangements, the red-emitting and the green-emitting photoluminescence materials can be located in separate respective light emitting devices. The inventors have discovered that such an arrangement can increase luminous efficacy, and offers the advantages of reduced complexity, ease of manufacture and reduced manufacturing costs. 
     REFERENCE NUMERALS 
       42  LED chip 
       44  Upper body part 
       46  Lower body part 
       48  Recess 
       50  Electrical connector 
       52  Electrical connector 
       54  Contact pad 
       56  Contact pad 
       58  Thermally conductive pad 
       60  Bond wire 
       62  Bond wire 
       100  Color LCD 
       102  LC Display Panel 
       104  Edge-lit backlight 
       106  Front plate 
       108  Back plate 
       110  Liquid Crystal (LC) 
       112  Glass plate 
       114  Viewing face 
       116  First polarizing filter layer 
       118  Anti-reflective layer 
       120  Color filter plate 
       122  Light transmissive common electrode plane 
       124  Red sub-pixel filter element 
       126  Green sub-pixel filter element 
       128  Blue sub-pixel filter element 
       130  Unit pixel 
       132  Opaque divider/black matrix 
       134  Glass plate 
       136  TFT 
       138  Second polarizing filter layer 
       140  White Light 
       142  Light emitting face of Backlight 
       144  Light guide 
       146  Light emitting device 
       148  Composite light 
       150  Light reflective layer 
       152  Photoluminescence wavelength converting layer (photoluminescence layer) 
       154  Brightness Enhancement Film (BEF) 
       156  Light diffusive layer 
       158  Photoluminescence light 
       160  Floor of light reflective enclosure 
       162  Light reflective enclosure