Wavelength converted semiconductor light emitting device

BACKGROUND

FIELD OF INVENTION

0001This invention relates to wavelength-converted semiconductor light-emitting devices.

DESCRIPTION OF RELATED ART

0002Semiconductor light-emitting devices including light emitting diodes (LEDs) are among the most efficient light sources currently available. Materials systems currently of interest in the manufacture of high-brightness light emitting devices capable of operation across the visible spectrum include Group III-V semiconductors, particularly binary, ternary, and quaternary alloys of gallium, aluminum, indium, and nitrogen, also referred to as III-nitride materials. Typically, III-nitride light emitting devices are fabricated by epitaxially growing a stack of semiconductor layers of different compositions and dopant concentrations on a sapphire, silicon carbide, III-nitride, or other suitable substrate by metal-organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), or other epitaxial techniques. Sapphire is often used as the growth substrate due to its wide commercial availability and relative ease of use. The stack grown on the growth substrate typically includes one or more n-type layers doped with, for example, Si, formed over the substrate, a light emitting or active region formed over the n-type layer or layers, and one or more p-type layers doped with, for example, Mg, formed over the active region. III-nitride light emitting devices efficiently emit UV through green light.

0003Illumination systems have been proposed which convert the color of light emitted by light emitting diodes by means of a fluorescent material such as a phosphor.

0004A dichromatic illumination system, which mixes the primary emission of a blue LED with light emitted by a yellow phosphor is described in <patcit id="pcit0001" dnum="US5998925A"><text>U.S. Patent 5,998,925</text></patcit>. A Y₃Al₅O₁₂:Ce³⁺ phosphor is coated on a III-nitride LED, and a portion of the blue light emitted from the LED is converted to yellow light by the phosphor. Another portion of the blue light from the LED is transmitted through the phosphor. Thus, this system emits both blue light emitted from the LED, and yellow light emitted from the phosphor. The mixture of blue and yellow emission bands are perceived as white light by an observer with a CRI between about 75 and about 80 and a color temperature, T_c, that ranges from about 6000 K to about 8000 K.

0005However, white light LEDs based on the dichromatic approach can only be used to a limited extent for general-purpose illumination, on account of poor color rendering caused by the absence of red color components.

0006A red-deficiency-compensating illumination system is illustrated in <figref idref="f0001">Fig. 1</figref> and described in more detail in <patcit id="pcit0002" dnum="US6351069B"><text>U.S. Patent 6,351,069</text></patcit>. LED 34 of <figref idref="f0001">Fig. 1</figref> is designed to produce white output light that is well-balanced with respect to color to provide illumination for good color rendition. "The LED 34 includes the Gallium Nitride (GaN) die 12 that is positioned on the reflector cup lead frame 14 and is electrically coupled to the leads 16 and 18. The leads 16 and 18 provide excitation power to the GaN die 12. The GaN die 12 may generally be in a shape of a square. In the preferred embodiment, the GaN die 12 is configured to emit primary light having a peak wavelength of 470 nm, which lies within the blue region of the light spectrum, i.e., blue light. The GaN die 12 is covered by a spacing layer 36 made of a transparent material. The transparent material may be clear epoxy or glass.

0007"Adjacent to the spacing layer 36 is a fluorescent layer 38. The fluorescent layer 38 includes the fluorescent material 22 and a second fluorescent material 40. The fluorescent material 22 has a property to absorb the primary light and emit secondary light having a first peak wavelength, while the fluorescent material 40 has a property to absorb the primary light and emit secondary light having a second peak wavelength. Preferably, the secondary light emitted by the fluorescent material 22 has a broadband spectral distribution centered in the yellow region of the visible spectrum. However, the secondary light emitted by the fluorescent material 40 has a ... spectral distribution that is intense in the red region of the visible spectrum. Thus, when the primary light and the secondary lights emitted by the fluorescent materials 22 and 40 are combined, white light is created that is rich in red color, in addition to other colors. The peak wavelengths of the secondary lights depend on the composition of the fluorescent materials 22 and 40, in addition to the peak wavelength of the primary light.

0008Layer 38 including the two fluorescent materials is a phosphor-resin mixture that "includes two fluorescent materials that are combined with a resin paste." The phosphor-resin mixture "is deposited over the encapsulation layer to form a fluorescent layer that uniformly covers the encapsulation layer. The deposited phosphor-resin mixture may then be gelled, i.e., partially cured." Thus, in the red-deficiency compensating system of <patcit id="pcit0003" dnum="US6351069B"><text>U.S. Patent 6,351,069</text></patcit>, two fluorescent materials are mixed together, then suspended in a resin layer.

0009<patcit id="pcit0004" dnum="WO0124283A1"><text>WO 01/24283 A1</text></patcit> discloses a phosphor-converted LED device with one or more phosphor thin films disposed on a side of a substrate opposite to the side of the substrate on which a light emitting structure is disposed.

0010<patcit id="pcit0005" dnum="JP2000031531A"><text>JP 2000 031531</text></patcit> discloses a wavelength converted LED having two layers comprising phosphor embedded in a resin matrix.

SUMMARY

0011In accordance with embodiments of the invention, a wavelength converted semiconductor light emitting device includes a first wavelength converting material and a second wavelength converting material. The first wavelength converting material emits light having a shorter wavelength than light emitted by the second wavelength converting material. The second fluorescent material layer is disposed on a plurality of discrete regions on the semiconductor light emitting device and the first fluorescent material layer overlies the second fluorescent material layer. In some embodiments, the first and second wavelength converting materials are arranged to maximize one or more of the luminous equivalent, the color rendering index, or the gamut of the combined visible light emitted by the device.

BRIEF DESCRIPTION OF THE DRAWINGS

0012<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> illustrates a prior art red-deficiency-compensating illumination system.</li><li><figref idref="f0002">Fig. 2</figref> illustrates the excitation spectrum of Sr₂Si₅N₈Eu²⁺ and the emission spectra of several phosphors.</li><li><figref idref="f0003">Figs. 3, 4, 5, and 6</figref> illustrate light-emitting devices employing discrete phosphor layers to minimize phosphor interaction, <figref idref="f0003">figure 6</figref> illustrating an embodiment of the current invention.</li><li><figref idref="f0004">Fig. 7</figref> illustrates a simulated spectrum and two experimental spectra of a system including a blue light emitting diode and Y₃Al₅O₁₂:Ce³⁺ and CaS:Eu²⁺ phosphors.</li><li><figref idref="f0005">Fig. 8</figref> illustrates a display according to embodiments of the present invention.</li><li><figref idref="f0006">Figs. 9</figref>, <figref idref="f0007">10</figref>, <figref idref="f0008">11</figref>, <figref idref="f0009">12</figref>, and <figref idref="f0010">13</figref> illustrate five examples of LED and phosphor combinations implemented in the device of <figref idref="f0005">Fig. 8</figref>.</li><li><figref idref="f0011">Fig. 14</figref> illustrates the excitation and emission spectra of Y₃Al₅O₁₂:Ce³⁺ and CaS:Eu²⁺ phosphors.</li><li><figref idref="f0012">Fig. 15</figref> illustrates two experimental spectra of a system including a blue light emitting diode and Y₃Al₅O₁₂:Ce³⁺ and Sr₂Si₅N₈:Eu²⁺ phosphors.</li></ul>

DETAILED DESCRIPTION

0013In accordance with embodiments of the invention, systems including multiple wavelength converting materials such as phosphors combined with semiconductor light emitting devices are disclosed. In the below description, "mixed" or "composite" light refers to the combination of light emitted by the semiconductor light emitting device and light emitted by all the phosphors in the system.

0014In some embodiments, a blue light emitting diode is combined with a yellow- or green-emitting phosphor and a red-emitting phosphor. Examples of suitable yellow- or green-emitting phosphors include (Lu_1-x-y-a-bY_xGd_y)₃(Al_1-zGa_z)₅O₁₂:Ce_a³⁺Pr_b³⁺ wherein 0 &lt; x &lt; 1, 0 &lt; y &lt; 1, 0 &lt; z ≤ 0.1, 0 &lt; a ≤ 0.2 and 0 &lt; b ≤ 0.1 including, for example, Lu₃Al₅O₁₂:Ce³⁺ and Y₃Al₅O₁₂:Ce³⁺; (Sr_1-a-bCa_bBa_c)Si_xN_yO_z:Eu_a²⁺ (a = 0.002 - 0.2, b = 0.0 - 0.25, c = 0.0 - 0.25, x = 1.5 - 2.5, y = 1.5 - 2.5, z = 1.5 - 2.5) including, for example, SrSi₂N₂O₂:Eu²⁺; (Sr_1-u-v-xMg_uCa_vBa_x)(Ga_2-y-zAl_yIn_zS₄):Eu²⁺ including, for example, SrGa₂S₄:Eu²⁺; and Sr_1-xBa_xSiO₄:Eu²⁺. Examples of suitable red-emitting phosphors include (Ca_1-XSr_x)S:Eu²⁺ wherein 0 &lt; x ≤ 1 including, for example, CaS:Eu²⁺ and SrS:Eu²⁺; and (Sr_1-x-yBa_xCa_y)_2-zSi_5-aAl_aN_8-aO_a:Eu_z²⁺ wherein 0 ≤ a &lt; 5, 0 &lt; x ≤ 1, 0 ≤ y ≤ 1, and 0 &lt; z ≤ 1 including, for example, Sr₂Si₅N₈:Eu²⁺.

0015In some embodiments, a UV light emitting diode is combined with a blue-emitting phosphor, a yellow- or green-emitting phosphor and a red-emitting phosphor. Examples of suitable yellow- or green-emitting phosphors and suitable red-emitting phosphors are listed above. Examples of suitable blue-emitting phosphors include, for example, MgSrSiO₄.

0016Though the embodiments described below refer to a blue LED combined with two phosphors and a UV LED combined with three phosphors, it is to be understood that more or fewer phosphors and LEDs emitting other colors may be used.

0017The excitation and emission spectra of some of the above-listed phosphors are illustrated in <figref idref="f0002">Fig. 2</figref>. In <figref idref="f0002">Fig. 2</figref>, spectrum a is the excitation spectrum of Sr₂Si₅N₈:Eu²⁺; spectrum b is the emission spectrum of Sr₂Si₅N₈:Eu²⁺, spectrum c is the emission spectrum of MgSrSiO₄; spectrum d is the emission spectrum of Sr_1-xBa_xSiO₄:Eu²⁺; spectrum e is the emission spectrum of SrGa₂S₄:Eu²⁺; and spectrum f is the emission spectrum of SrSi₂N₂O₂:Eu²⁺.

0018The inventors have observed that when some phosphors are mixed together, interaction between the mixed phosphors may adversely affect the efficiency and spectrum of the device. Accordingly, depending on the phosphors in the combination, depositing the phosphors as separate, discrete layers, as described below in <figref idref="f0003">Figs. 3-6</figref>, may improve the performance of the device. Whether the preferred phosphor arrangement is a mix of phosphors as in <figref idref="f0001">Fig. 1</figref> or discrete layers as in <figref idref="f0003">Figs. 3-6</figref> may depend on the excitation and emission spectra of the phosphors and on the application. Three applications are described below: an illumination device with the phosphors deposited directly on an LED, a display with the phosphors spaced apart from the LEDs, and an illumination device with the phosphors spaced apart from the LEDs. For an illumination device, the phosphor arrangement may be chosen to maximize the color rendering index, given as CRI or Ra. For a display device, the phosphor arrangement may be chosen to maximize color gamut for the filters used in the device. In either an illumination device or a display device, it is desirable to maximize the luminous equivalent. The luminous equivalent is the highest efficiency possible for a given spectrum and is expressed in lumens/W.

0019<figref idref="f0001">Figs. 1</figref> and <figref idref="f0003">3-6</figref> illustrate a first application, illumination devices with phosphors deposited directly on an LED, either as a mix in <figref idref="f0001">Fig. 1</figref> or as discrete layers in <figref idref="f0003">Figs. 3-6</figref>. In an illumination device, the phosphor arrangement is selected to maximize luminous equivalent and color rendering index. Determination of which of the different phosphor arrangements of <figref idref="f0001">Figs. 1</figref> and <figref idref="f0003">3-6</figref> are appropriate for a particular combination of phosphors is described in the context of two examples, a device with Y₃Al₅O₁₂:Ce³⁺ and Sr₂Si₅N₈:Eu²⁺ phosphors and a device with Y₃Al₅O₁₂:Ce³⁺ and CaS:Eu²⁺ phosphors.

0020In a first example, a blue light emitting diode is combined with Y₃Al₅O₁₂:Ce³⁺ and Sr₂Si₅N₈:Eu²⁺ phosphors. <figref idref="f0002">Fig. 2</figref> illustrates the excitation and emission spectra of Y₃Al₅O₁₂:Ce³⁺ and Sr₂Si₅N₈:Eu²⁺. In a first approximation of the combined light emitted by the system, the spectra of the blue light emitting diode and the two phosphors are added. The emission spectra of Y₃Al₅O₁₂:Ce³⁺ and Sr₂Si₅N₈:Eu²⁺ illustrate that both phosphors have strong emission at orange wavelengths, for example at approximately 600 nm. The overlap of the emission spectra at orange wavelengths shifts the appearance of the mixed light away from red toward shorter wavelengths. This shift may negatively impact the color rendering of the mixed light.

0021In addition, <figref idref="f0002">Fig. 2</figref> illustrates that the emission spectrum of Y₃Al₅O₁₂:Ce³⁺ overlaps with the excitation spectrum of Sr₂Si₅N₈:Eu²⁺. As a result, a portion of the emission from the Y₃Al₅O₁₂:Ce³⁺ phosphor may be consumed by the Sr₂Si₅N₈:Eu²⁺ phosphor, reducing the amount of green/yellow light in the mixed light. In addition, absorption by Sr₂Si₅N₈:Eu²⁺ of light emitted by Y₃Al₅O₁₂:Ce³⁺ exacerbates the shift of the mixed light toward shorter, orange wavelengths. Both effects may negatively impact the color rendering of the mixed light.

0022Absorption by the red-emitting phosphor of light emitted by the green/yellow emitting phosphor may be reduced by separating the green/yellow and red phosphors into discrete regions. <figref idref="f0003">Figs. 3-6</figref> illustrate light-emitting devices where the red-emitting phosphor and green/yellow-emitting phosphor are deposited such that absorption by the red-emitting phosphor of light emitted by the green/yellow-emitting phosphor is reduced, <figref idref="f0003">figure 6</figref> illustrating an embodiment of the current invention.

0023In the device illustrated in <figref idref="f0003">Fig. 3</figref>, a semiconductor light emitting device 1 is disposed in a reflector cup 2. The green/yellow phosphor 5 is mixed with a resin, silicone, or other transparent material and disposed on one side of a reflector cup 2, while any other phosphors 4, including a red-emitting phosphor, are mixed separately with a resin, silicone, or other transparent material and disposed on the other side of reflector cup 2, such that slurry 5 does not appreciably mix with slurry 4. In some embodiments, the viscosity of the transparent material forming the slurry is selected to avoid mixing phosphor 4 with phosphor 5. Since green/yellow-emitting phosphor 5 and any other phosphors 4 are adjacent to each other, rather than mixed in the same slurry, light emitted by green/yellow phosphor 5 is less likely to be absorbed by any red-emitting phosphors in slurry 4.

0024In the device illustrated in <figref idref="f0003">Fig. 4</figref>, the green/yellow-emitting phosphor 5 and other phosphors 4 are deposited over LED 1 as discrete layers. Phosphor layer 4, including any red-emitting phosphors, is deposited closest to LED 1. Green/yellow-emitting phosphor 5 is then deposited over phosphor layer 4. Phosphor layers 4 and 5 may be separated by an optional transparent layer 6. Phosphor layers 4 and 5 may be deposited as slurries in a resin or other transparent material; deposited as thin films by, for example, electron beam evaporation, thermal evaporation, rf-sputtering, chemical vapor deposition, or atomic layer epitaxy; or deposited as conformal layers over LED 1 by, for example, screen printing, stenciling as described in <patcit id="pcit0006" dnum="US6650044B"><text>U.S. Patent 6,650,044</text></patcit>, or by electrophoretic deposition as described in <patcit id="pcit0007" dnum="US6576488B"><text>U.S. Patent 6,576,488</text></patcit>. Thin films are described in more detail in <patcit id="pcit0008" dnum="US6696703B"><text>U.S. Patent 6,696,703</text></patcit>. Each of <patcit id="pcit0009" dnum="US6696703B"><text>U.S. Patent 6,696,703</text></patcit>, <patcit id="pcit0010" dnum="US6650044B"><text>U.S. Patent 6,650,044</text></patcit> and <patcit id="pcit0011" dnum="US6576488B"><text>U.S. Patent 6,576,488</text></patcit> are incorporated herein by reference. In contrast to a thin film, which typically behaves as a single, large phosphor particle, the phosphor in a conformal layer generally behaves as multiple phosphor particles. In addition a thin film typically contains no materials other than phosphor. A conformal layer often includes materials other than phosphor, such as, for example, silica.

0025In some embodiments, one or more dichroic filters are included in the device. A dichroic filter designed to transmit light emitted by LED 1 but to reflect light emitted by phosphors 4 and 5 may be included between LED 1 and phosphor layer 4. Layer 6 between green/yellow-emitting phosphor 5 and red-emitting phosphor 4 may be a dichroic filter designed to transmit light emitted by red-emitting phosphor 4 and LED 1, and reflect light emitted by green/yellow-emitting phosphor 5. Dichroic filters may reduce the amount of radiation back-scattered by phosphor layers 4 and 5 into LED 1, where it can be absorbed. In the device illustrated in <figref idref="f0003">Fig. 5</figref>, the green/yellow-emitting phosphor 5 and other phosphors 4 are deposited on LED 1 in a plurality of small regions. The different regions may form a pattern, such as a checkerboard pattern. If light from LED 1 is to escape unconverted, as in the case where blue light emitted by the LED mixes with green and red light emitted by phosphors to make white light, the amount of unconverted light may be controlled by controlling the thickness of phosphor regions 4 and 5, or by leaving regions of LED 1 uncovered, or covered by an optional transparent material 7 that does not convert the light emitted by LED 1. Patterns of different phosphor layers as illustrated in <figref idref="f0003">Fig. 5</figref> may be formed by depositing a first layer of phosphor by electrophoretic deposition, patterning that layer using conventional lithography and etching techniques, then depositing a second phosphor layer by electrophoretic deposition. Alternatively, patterns of phosphor layers may be deposited by screen printing or ink jet printing. In some embodiments, a pattern of phosphor layers may be formed by pipetting the individual phosphor mixes 4 and 5 into wells in a clear plastic microplate used for microbiology. The phosphor-filled microplate is then placed on LED 1. Phosphor-filled microplates may be formed separately from LED 1.

0026In the embodiment illustrated in <figref idref="f0003">Fig. 6</figref>, a plurality of small regions of phosphor 4, which includes any red-emitting phosphors, is formed on the surface of LED 1. A layer of green/yellow-emitting phosphor 5 is deposited over the plurality of regions of phosphor 4.

0027Each of the devices illustrated in <figref idref="f0003">Figs. 3-6</figref> may reduce the above-described absorption problem. In each case, light emitted by LED 1 is incident on the red-emitting phosphor first, or is incident on the red-emitting phosphor and the green/yellow-emitting phosphor in separate regions. The arrangements illustrated in <figref idref="f0003">Figs. 3-6</figref> thus reduce the probability that light emitted from the green/yellow-emitting phosphor will be absorbed by a red-emitting phosphor.

0028Separating the phosphors as illustrated in <figref idref="f0003">Figs. 3-6</figref> significantly improves the color rendering of an illumination system including a blue LED, Y₃Al₅O₁₂:Ce³⁺, and Sr₂Si₅N₈:Eu²⁺ phosphors. <figref idref="f0012">Fig. 15</figref> illustrates spectra of mixed light from a blue LED, Y₃Al₅O₁₂:Ce³⁺, and Sr₂Si₅N₈:Eu²⁺ phosphors in a mixed phosphor arrangement as illustrated in <figref idref="f0001">Fig. 1</figref> (curve a), and in a layered phosphor arrangement as illustrated in <figref idref="f0003">Fig. 4</figref> (curve b). Both phosphor arrangements have a high luminous equivalent, 296 for the layered arrangement and 343 for the mixed arrangement, but the layered arrangement shows a significantly higher color rendering index: 87, compared to 75 for the mixed arrangement.

0029In some light-emitting devices, separating the green/yellow- and red-emitting phosphors does not improve the performance of the device, as illustrated in a second example of an illumination system, where a blue LED is combined with Y₃Al₅O₁₂:Ce³⁺ and CaS:Eu²⁺ phosphors. <figref idref="f0011">Fig. 14</figref> illustrates the excitation and emission spectra of Y₃Al₅O₁₂:Ce³⁺ and CaS:Eu²⁺. The left most solid curve is the excitation spectrum of Y₃Al₅O₁₂:Ce³⁺. The center solid curve is the emission spectrum of Y₃Al₅O12:Ce³⁺. The dashed curve is the excitation spectrum of CaS:Eu²⁺. The right most solid curve is the emission spectrum of CaS:Eu²⁺. <figref idref="f0004">Fig. 7</figref> illustrates three spectra of composite light from a system including a blue LED and Y₃Al₅O₁₂:Ce³⁺ and CaS:Eu²⁺ phosphors. Curve a is a simulated spectrum calculated by the superposition of the emission spectrum of the light emitting diode and the two phosphors. Curve b is an observed spectrum from a device formed by mixing the two phosphors in a single layer, as illustrated in <figref idref="f0001">Fig. 1</figref>. Curve c is an observed spectrum from a device including two discrete phosphor layers, as illustrated in <figref idref="f0003">Fig. 4</figref>. Layering the phosphors decreases the luminous equivalent of the device. The layered device has a color rendering index of 96 and a luminous equivalent of 265. The mixed device has a Ra of 91 and a luminous equivalent of 300. Accordingly, layering the phosphors reduces the luminous equivalent of the device, without significantly improving the color rendering. As such, mixing the phosphors is preferred.

0030Many factors may influence whether multiple phosphors are best mixed or formed as discrete layers. The difference between the refractive indices of the two phosphors and the particle size of the two phosphors influences the likelihood of absorption by the red phosphor of emission from the green/yellow phosphor. As the step in index of refraction between two materials increases, the likelihood that light incident on the interface between the two materials will be reflected rather than absorbed also increases. Thus, if the refractive indices of the two phosphors are quite different, light emitted by the green/yellow phosphor is likely to be scattered, rather than absorbed, when incident on the red phosphor. In addition, the location of the red-emitting phosphor emission spectrum may influence whether two phosphors should be mixed. As described above, interaction between a green/yellow-emitting phosphor and a red-emitting phosphor may shift the red component of the composite light from the system toward shorter, more orange wavelengths, resulting in reduced color rendering. The longer the peak wavelength of the red-emitting phosphor, the more the system can tolerate shifting of the red toward shorter wavelengths without impacting color rendering. Also, the overlap between the emission spectrum of the green/yellow emitting phosphor and the excitation spectrum of the red-emitting phosphor may influence whether two phosphors should be mixed. The more significant the overlap, the more likely a significant amount of emission from the green/yellow-emitting phosphor will be absorbed by the red-emitting phosphor. Accordingly, the more significant the overlap, the more likely the performance of the system will be changed by separating the phosphors.

0031In some embodiments, though the phosphors are separated into discrete layers, a small amount of, for example, a red-emitting phosphor may be included in a green/yellow emitting phosphor layer. The presence of a small amount of red-emitting phosphor in the green/yellow emitting phosphor layer may improve the color rendering of the composite light.

0032<figref idref="f0005">Fig. 8</figref> illustrates a second application, a display device with phosphors spaced apart from one or more LEDs. The device illustrated in <figref idref="f0005">Fig. 8</figref> is described in more detail in Application Serial No. <patcit id="pcit0012" dnum="US10678541B"><text>10/678,541, filed October 3, 2003</text></patcit>, titled "LCD Backlight Using Two-Dimensional Array LEDs,"

0033<figref idref="f0005">Fig. 8</figref> is a side view of an LCD display. An array of LEDs 24 is placed on the rear panel of the backlight 26. The backlight 26 is covered with a diffusing cover plate 40. The diffuser 40 is for example made of acrylic or glass, with a roughened surface for diffusing light. Alternatively, the diffuser 40 may have light scattering particles with the acrylic or glass sheet. Many types of diffusers are known and may be used with the backlight 26. A transparent plate may be used instead of the diffuser 40 if the light output of the backlight 26 is sufficiently diffuse without a diffuser. Additional films (not shown) for increasing the brightness or efficiency might be used on top of the diffuser, just before the LCD, as for example Brightness Enhancement Film and Dual Brightness Enhancement Film, as for example produced by 3M.

0034The back plane and the sidewalls of the backlight 26 are covered with highly reflective materials. Good results have been obtained with a white diffuse reflective film on the back (e.g., E60L, produced by Toray, Japan), and a specular reflecting material on the sidewalls (e.g., Miro material, as produced by Alanod, Germany), but other configurations work as well. The materials used should have a high coefficient of reflection, preferably >90%. By using these high reflective materials, a high recycling efficiency is achieved. This is in particular important when Brightness Enhancement Films are used, as mentioned above, as these films reflect the light which can not be used in the first pass, and which needs to be recycled in order to contribute to the output of the LCD during a second or third pass.

0035LCD panel 14 is placed in front of the backlight 26. The LCD panel 14 may be a conventional LCD, having a first polarizing filter, a thin film transistor array for developing an electric field across selected areas of the liquid crystal layer, a liquid crystal layer, an RGB color filter array, and a second polarizing filter. The color filter array has red, green and blue subpixels. Between the LCD panel 14 and the backlight 26, additional films can be used, such as a brightness enhancement film (BEF) or polarization recovery film (DBEF).

0036LEDs 26 are generally blue or UV-emitting LEDS. A phosphor layer 39, which may include multiple phosphors mixed as in <figref idref="f0001">Fig. 1</figref>, or layered as in <figref idref="f0003">Figs. 3-6</figref>, is formed on the cover plate 40, rather than directly on LEDs 26. In some embodiments, different phosphor layers are formed on different surfaces of cover plate 40. The cover plate 40 may or may not be a diffuser, depending on the amount of diffusing performed by the phosphor. Spacing phosphor layer 39 apart from LEDs 26 is attractive because light emitted from the phosphor to the rear of the backlight 26 has a larger recycling efficiency than into the LED chips, due to the high reflectivity of the films used in the backlight 26. In addition to the recycling efficiency, the phosphors are not required to withstand the high temperatures near the LEDs and are not required to be chemically compatible with the LEDs, increasing he number of possible suitable phosphors and potentially improving the efficiency and lifetime of the device. From a logistics point of view, this solution is attractive as well, as the blue backlight can be used for a large range of different displays, with different types of color filters, and only the phosphor layer thickness and phosphor concentration has to be optimized to fit a particular LCD.

0037In the display device illustrated in <figref idref="f0005">Fig. 8</figref>, the phosphor arrangement is selected to maximize luminous equivalent and the color gamut.

0038<figref idref="f0006">Figs. 9</figref>, <figref idref="f0007">10</figref>, <figref idref="f0008">11</figref>, <figref idref="f0009">12</figref>, and <figref idref="f0010">13</figref> illustrate the performance of several phosphor compositions combined with blue light emitting diodes. In each of <figref idref="f0006 f0007 f0008 f0009 f0010">Figs. 9-13</figref>, curve a represents the Planckian locus, which is considered true white light, and curve b represents the CIE chart. Curve c illustrates the gamut of colors possible with the filters in RGB pixel filter of the device of <figref idref="f0005">Fig. 8</figref>. Point d represents the color of mixed light emitted by the LED and phosphors. Curve e represents the gamut of colors demanded by the NTSC standard. Point f represents the color of light after being filtered by each of the red, blue, and green filters. Points g illustrate the spectrum of composite light before filtering. Curves h, i, and j illustrate the performance of the blue, green, and red filters of the RGB pixel filter of <figref idref="f0005">Fig. 8</figref>. Curve k illustrates the composite light emitted by the combination of the LEDs and phosphors. Curve 1 illustrates the composite light after passing through the blue filter represented in curve h. Curve m illustrates the composite light after passing through the green filter represented in curve i. Curve n illustrates the composite light after passing through the red filter represented in curve j.

0039In the device illustrated in <figref idref="f0006">Fig. 9</figref>, a blue LED emitting light at, for example, 455 nm, is combined with a single phosphor, Y₃Al₅O₁₂:Ce³⁺. The device illustrated in <figref idref="f0006">Fig. 9</figref> operates at a luminous equivalent of 299 and has a color gamut that is 62% of the area of the color gamut of the NTSC standard.

0040In the device illustrated in <figref idref="f0007">Fig. 10</figref>, a red-emitting phosphor, SrS:Eu²⁺, is added to the device of <figref idref="f0006">Fig. 9</figref>. The addition of SrS:Eu²⁺ does not significantly impact the performance of the device. The device illustrated in <figref idref="f0007">Fig. 10</figref> operates at a luminous equivalent of 291 and has a color gamut that is 62% of the area of the color gamut of the NTSC standard.

0041The device illustrated in <figref idref="f0008">Fig. 11</figref> combines a blue light emitting diode with SrGa₂Sa:Eu²⁺ and CaS:Eu²⁺. This device demonstrates an excellent color gamut (86% of NTSC) but a poor luminous equivalent of 200. In this case, it is preferable to form discrete phosphor regions as illustrated in <figref idref="f0003">Figs. 3-6</figref>, rather than mix the phosphors as illustrated in <figref idref="f0001">Fig. 1</figref>.

0042In the device illustrated in <figref idref="f0009">Fig. 12</figref>, the CaS:Eu²⁺ of the device of <figref idref="f0008">Fig. 11</figref> is replaced by SrS:Eu²⁺. This device demonstrates both excellent color gamut (73% of NTSC) and excellent luminous equivalent (298). In this case, it is preferable to form discrete phosphor regions as illustrated in <figref idref="f0003">Figs. 3-6</figref>, rather than mix the phosphors as illustrated in <figref idref="f0001">Fig. 1</figref>.

0043The device illustrated in <figref idref="f0010">Fig. 13</figref> combines a blue light emitting diode with Sr_1-xBa_xSiO₄:Eu²⁺ and Sr₂Si₅N₈:Eu²⁺. The device has 72% the color gamut of NTSC and a luminous equivalent of 241.

0044In a third application, an illumination device includes phosphors spaced apart from one or more LEDs. An example of such a device is the device illustrated in <figref idref="f0005">Fig. 8</figref> minus LCD 14. In such a device, the phosphor arrangement is selected to maximize luminous equivalent and color rendering index. Phosphor combinations such as those described above in the first application may be appropriate.

0045Having described the invention in detail, those skilled in the art will appreciate that, given the present disclosure, modifications may be made to the invention without departing from the scope of the invention as defined by the claims.