Planar light source, display device and method for manufacturing the same

Light-emitting elements are divided into chromaticity groups on the basis of their chromaticity. In a planar light source, light-emitting elements selected from two of the chromaticity groups are alternately provided in a longitudinal direction and in a transverse direction, respectively. The two chromaticity groups are substantially equally away from a target chromaticity in directions reverse to each other. Further, pairs of two of the light-emitting elements belonging to the two chromaticity groups which are separated from each other, are provided so that distances d2 at which two of the light-emitting elements in each of the pairs are provided are less than distances d1 at which the pairs are provided.

This Nonprovisional application claims priority under U.S.C. §119(a) on Patent Application No. 338331/2007 filed in Japan on Dec. 27, 2007, the entire contents of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to (i) a planar light source using an LED which planar light source can be for use in, for example indicator, illumination or displaying, (ii) a display device such as a liquid crystal display device using the planar light source and (iii) a method for manufacturing the planar light source and the display device.

BACKGROUND OF THE INVENTION

With a rapid progress of LED techniques, especially, techniques for a blue LED (light-emitting diode) which is a III nitride semiconductor light-emitting element, a white LED (hereinafter referred to as “light-emitting element”) which is a combination of a blue LED and a fluorescent substance (e.g. yellow fluorescent substance) has been put into practical use. The white LED has a luminous efficiency (efficiency of conversion of electricity to light) higher than an incandescent lamp, and is expected to become, in the future, an energy saving light source having a luminous efficiency higher than a fluorescent lamp.

It is possible to realize a planar light source by mounting a plurality of light-emitting elements on a substrate. The planar light source can be used as an illumination light source or a backlight for a non-light-emitting display device such as a liquid crystal display device. Japanese Patent Application Publication Tokukai No. 2007-227389 (published on Sep. 6, 2007, and hereinafter referred to as Patent Document 1) discloses a liquid crystal display device including a backlight in which blue, green and red LEDs are provided on a back side of a liquid crystal display panel. A method thus using three-color LEDs has the advantage of widening a color reproducibility region of a liquid crystal display device, whereas has the disadvantage of a necessity of increase in the distance between the backlight and the liquid crystal display panel so that a good mixture of the three colors is realized in the liquid crystal display panel. In contrast, when a white LED (light-emitting element) composed of a blue LED and a fluorescent substance is used as a backlight, it is possible to realize a good color mixture even if the backlight is provided to be close to the liquid crystal display panel. Such a flat display device can be suitably used as a wall-hung display and a portable display. However, when the backlight is provided to be close to the liquid crystal display panel, the influence due to production tolerance of the light-emitting elements themselves comes out. This causes in-plane distribution of chromaticity to become visible.

Japanese Patent Application Publication Tokukai No. 2007-80530 (published on Mar. 29, 2007, and hereinafter referred to as Patent Document 2) discloses an LED illumination device using a white LED, in which a color mixture is substantially the same as a target chromaticity coordinate e of white. For example, according to the LED illumination device disclosed in Patent Document 2, (i) a group of white LEDs in which x and y of chromaticity coordinates (x, y) are larger than the target chromaticity coordinates e and (ii) a group of white LEDs in which x and y of the chromaticity coordinates (x, y) are smaller than the target chromaticity coordinates e are combined so that a mixed color has the chromaticity close to the target chromaticity coordinates e.

However, the LED illumination device disclosed in Patent Document 2 still has the following problems to be solved.

First, in a planar light source in which light-emitting elements are provided two-dimensionally, unevenness of color and unevenness of luminous intensity must be fully restrained.

Second, when the planar light source in which the light-emitting elements are provided two-dimensionally is used as a backlight of a liquid crystal display device or the like, unevenness of color and unevenness of luminous intensity of the light-emitting elements must be fully restrained in a display panel provided near a backlight. In other words, a flat display device must be realized in which the backlight can be provided close to the display panel.

Third, a method which is directed to a combination of light-emitting elements and which is suitable for reducing in-plane distribution must be provided.

Fourth, a displaying method suitable for a display device using a planar light source whose in-plane distribution is reduced must be provided.

Fifth, the utilization ratio (yield) of light-emitting elements used in a planar light source must be improved.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a planar light source which can effectively restrain unevenness of color and unevenness of luminous intensity, and to provide a flat display device in which unevenness of color and unevenness of luminous intensity are restrained even if the distance between a backlight and a display panel is reduced.

In order to attain the above object, the planar light source of the present invention is a planar light source including a plurality of light-emitting elements, wherein: each of the plurality of light-emitting elements is comprised of a combination of a semiconductor light-emitting element for emitting primary light and fluorescent substances for absorbing the primary light and emitting secondary light having a longer wavelength than the primary light, each of the plurality of light-emitting elements emitting combination light of the primary light and the secondary light, said each of the plurality of light-emitting elements, provided in at least a partial area of the planar light source, belongs to a first chromaticity group or a second chromaticity group on a basis of chromaticity of the combination light, (i) a difference between chromaticity of light-emitting elements belonging to the first chromaticity group and a target chromaticity and (ii) a difference between chromaticity of light-emitting elements belonging to the second chromaticity group and the target chromaticity are substantially equal and are away from the target chromaticity in directions reverse to each other, respectively, and the light-emitting elements belonging to the first chromaticity group and the light-emitting elements belonging to the second chromaticity group are alternately provided, in said at least a partial area, in a longitudinal direction and a transverse direction, respectively.

In order to attain the above object, another planar light source of the present invention is a planar light source comprising a plurality of light-emitting elements, wherein: each of the plurality of light-emitting elements is comprised of a combination of a semiconductor light-emitting element for emitting primary light and fluorescent substances for absorbing the primary light and emitting secondary light having a longer wavelength than the primary light, each of the plurality of light-emitting elements emitting combination light of the primary light and the secondary light, said each of the plurality of light-emitting elements, provided in at least a partial area of the planar light source, belongs to a first chromaticity group or a second chromaticity group on a basis of chromaticity of the combination light, (i) a difference between chromaticity of light-emitting elements belonging to the first chromaticity group and a target chromaticity and (ii) a difference between chromaticity of light-emitting elements belonging to the second chromaticity group and the target chromaticity are substantially equal and are away from the target chromaticity in directions reverse to each other, respectively, and pairs of two of the light-emitting elements belonging to the first and second chromaticity groups, respectively are provided so that intervals at which two of the light-emitting elements in each of the pairs are provided are less than 0.5 times as long as intervals at which the pairs are provided.

The display device of the present invention is a display device including any one of the planar light sources described in the above arrangements and a display panel for displaying an image by driving a plurality of passive pixels, the planar light source illuminating the display panel.

A method for manufacturing a planar light source of the present invention includes the steps of: preparing a plurality of light-emitting elements, measuring light emitted from the light-emitting elements via red, green and blue filters, dividing the light-emitting elements into red, green and blue luminescence intensity groups, for each luminescence intensity group, based on differences between their luminescence intensity and target luminescence intensity, respectively, and alternately providing light-emitting elements selected from two of the plurality of luminescence intensity groups in a longitudinal direction and in a transverse direction, respectively, the light-emitting elements being selected in a manner such that differences between luminescence intensity of the light-emitting elements and the target luminescence intensity are substantially equal and are away from the target luminescence intensity in directions reverse to each other, respectively.

DESCRIPTION OF THE EMBODIMENTS

Explained in the present embodiment is (i) a planar light source in which light-emitting elements are provided in accordance with a predetermined planar arrangement rule so that unevenness of chromaticity and unevenness of luminous intensity are restrained and (ii) a display device using the planar light source.

FIG. 1is a cross-sectional view of a light-emitting element10of the present embodiment. In the light-emitting element10, a semiconductor light-emitting element1is die-bonded to a frame4including a concavity having a reflection surface7and is sealed by a resin3. Fluorescent substances2are, in advance, dispersed in the resin3. The light-emitting element10is designed so that (i) when primary light emitted from the semiconductor light-emitting element1passes through the resin3, the fluorescent substances2are partially excited by the primary light and are converted into secondary light, and (ii) the primary light and the secondary light are combined so as to be emitted as substantially white light.

The semiconductor light-emitting element1is a GaN semiconductor light-emitting element including a conductive substrate. A bottom surface electrode is provided on a bottom surface of the conductive substrate, and a top surface electrode is provided on a surface which is on the opposite side of the bottom surface. The light emitted from the semiconductor light-emitting element1has a peak wavelength of 457 nm which is a wavelength of the blue light.

The frame4is formed by inserting a pair of lead frames5into a nylon material. Ends of the lead frames5are exposed in the bottom surface of the concavity of the frame4, and are provided so as to be separated from each other. Further, the other ends of the lead frames5, serving as external terminals, are cut off to a predetermined length, and are folded along an external wall of the frame4.

On the bottom surface of the concavity of the frame4, the semiconductor light-emitting element1is die-bonded to the end of one of the lead frames5with the use of a brazing filler material, so as to be fixed to a predetermined position. Further, the top surface electrode of the semiconductor light-emitting element1and the end of the other lead frame5are wire-bonded by a wire6so that the semiconductor light-emitting element1and the lead frame5are electrically connected.

In order for the light to efficiently be emitted, a reflection film7is preferably a metal film including silver or aluminum.

Used as the fluorescent substances2are a combination of green fluorescent substances2a(e.g. Eu-activated β-sialon) for emitting green (emission peak wavelength of not less than 500 nm and not more than 550 nm) secondary light and red fluorescent substances2b(e.g. CaAlSiN3: Eu) for emitting red (emission peak wavelength of not less than 600 nm and not more than 780 nm) secondary lights. This makes it possible to obtain a three-wavelength type light-emitting elements having high color rendering properties. Instead, it is possible to use, as the fluorescent substances2, yellow fluorescent substances (e.g. Ce: YAG, BOSE (Ba, Sr, O, Eu), Eu-activated α-sialon) for emitting yellow (emission peak wavelength of approximately 560 nm) secondary light. In this case, blue primary light and yellow secondary light are combined, and therefore a so-called quasi-white light-emitting element can be obtained. It should be noted that even the yellow fluorescent substances have green components and red components in addition to yellow components.

The resin3is required to have high durability to the primary light having a short wavelength. In view of this, silicon resin is preferably used.

In order to prevent sedimentation of the fluorescent substances2, a sedimentation suppression member8such as silica fine particles can be mixed in the resin3. This makes it possible to suppress the influence of the sedimentation of the fluorescent substances2even if time required to harden the resin3is not uniform. It is preferable that the fluorescent substances2aand2bare kept to be evenly dispersed in the resin3. By mixing the sedimentation suppression member8, it is possible to prevent production tolerance and unevenness of color of the emitted light due to unevenness of the fluorescent substances2aand2b.

FIG. 2(a) is a graph showing a spectral distribution of a three-wavelength type light-emitting element. The distribution has three peaks at wavelength of blue, green and red light.FIG. 2(b) is a graph showing a spectral distribution of a quasi-white light-emitting element. The distribution has two peaks at wavelength of blue and yellow light.

FIG. 3is a chromaticity diagram showing a chromaticity distribution of a population of light-emitting elements belonging to a single mold lot. As shown inFIG. 3, in a single mold lot, the chromaticity has a one-dimensional distribution in the chromaticity diagram. One of the contributing factors causing this unevenness in chromaticity is unevenness of density of the fluorescent substances2aand2b. If the densities of the fluorescent substances2aand2bare high, the chromaticity (x, y) approaches the chromaticity of the fluorescent substances2aand2b. Whereas, if the densities of the fluorescent substances2aand2bare low, the chromaticity (x, y) approaches the chromaticity of the semiconductor light-emitting element1. Unevenness of sedimentation of the fluorescent substances can cause a phenomenon similar to unevenness of chromaticity due to the density dependency of the fluorescent substances. The sedimentation suppression member8has an effect of restraining the unevenness of chromaticity caused by the sedimentation of the fluorescent substances.

Although two fluorescent substances, that is, the fluorescent substances2aand2bare used, the chromaticity has a one-dimensional distribution (seeFIG. 3). It can be presumed that this is because the densities of the two fluorescent substances change in a similar manner.

A plurality of light-emitting elements are subjected to measurements of their chromaticity and their axial luminous intensity (brightness), and ones of the light-emitting elements having luminous intensity falling within a certain range are selected as the light-emitting element10.FIG. 5is a graph showing an example of relationship between chromaticity and luminous intensity of a light-emitting element. As shown inFIG. 5, there is a tendency that the larger the chromaticity x becomes, the higher the luminous intensity becomes. Therefore, it is more preferable that the allowable luminous intensity range shifts toward a higher range as the chromaticity (or color temperature) becomes higher, as compared with a case in which an allowable luminous intensity range stays constant over the whole area of the chromaticity x. That is, it is preferable to set the allowable luminous intensity range as shown by the parallelogram ofFIG. 5. Each chromaticity of the light-emitting elements is divided into seven chromaticity groups of “2”, “3”, “4”, “5”, “6”, “7” and “8” shown inFIG. 3.

FIG. 4(a) is a top view showing how a planar light source60is arranged in the present embodiment. According to the planar light source60, a plurality of light-emitting elements10A and10B are provided on a surface of a mounting substrate61. The light-emitting elements10A and the light-emitting elements10B are explained in detail in the following description of planar arrangement rules. The light-emitting elements10A and the light-emitting elements10B are alternately provided at intervals of C1in a transverse direction and at intervals of C0in a longitudinal direction, respectively. A reflection sheet62from which parts where the light-emitting elements10A and the light-emitting elements10B are provided are removed is attached onto the mounting substrate61. Further, posts63for supporting a diffusion plate71and a liquid crystal display panel72are provided in the four corners of the mounting substrate61. The diffusion plate71and the liquid crystal display panel72are later described.

FIG. 4(b) is a cross-sectional view of a liquid crystal display device70using the planar light source60. The liquid crystal display device70includes the diffusion plate71provided so as to keep a distance t from the planar light source60, and the liquid crystal display panel72provided above the diffusion board71.

The planar light source60used in the present embodiment can be used as a backlight of a passive display device (a display device, including a display panel for displaying an image by driving a plurality of passive pixels modulating the light from a backlight device, which is not self-luminous) other than a liquid crystal display device.

In order to reduce unevenness of color of a planar light source, the inventors of the present invention studied the following planar arrangement rules and evaluated the in-plane distribution of unevenness of color, by the use of light-emitting elements which are divided into a plurality of chromaticity groups as shown inFIG. 3.

FIG. 6(a) is an explanatory view showing how the chromaticity at the point P can be expressed when a light-emitting element A having the chromaticity (y+Δy) and a light-emitting element B having the chromaticity (y−Δy) are combined (the chromaticity at the point P can be expressed in the same manner when the chromaticity y is replaced by the chromaticity x or color temperature T). Note that it is assumed that whole luminous flux of the light-emitting element A is the same as that of the light-emitting element B, a difference in visibility between the chromaticity (y+Δy) and the chromaticity (y−Δy) is not considered, and transfer functions of luminous intensity of the light-emitting element A and the light-emitting element B are fAand fB, respectively. As shown inFIG. 6(a), unevenness of chromaticity at the point P can be expressed as F·Δy, where F=(fA−fB)/(fA+fB). It follows that the unevenness of chromaticity becomes F times smaller. The same is true in a case of a plurality of light-emitting elements. Hereinafter, F is referred to as a “chromaticity unevenness parameters”.

FIG. 6(b) is a view showing another calculation method, where a transfer function f of the light-emitting element A is 0.5, and each transfer function f of four light-emitting elements N, S, E and W which adjoin the light-emitting element A is 0.125.FIG. 6(c) is a view showing a further calculation method where a transfer function f of the light-emitting element A is 0.6, and each transfer function f of four light-emitting elements N, S, E and W which adjoin the light-emitting element A is 0.1. each of the light-emitting elements is nearer to the point P inFIG. 6(c) than inFIG. 6(b).

An example of a planar arrangement rule1is shown inFIG. 7(a).FIG. 7(a) is a schematic view of a planar light source in which light-emitting elements10A belonging to a first chromaticity group “7” (chromaticity y+Δy) and light-emitting elements10B belonging to a second chromaticity group “3” (chromaticity y−Δy) are alternately provided at regular intervals in the longitudinal and transverse directions, respectively.

FIG. 7(b) shows an in-plane distribution (%) of the chromaticity unevenness parameters calculated by using the method shown inFIG. 6(b), where a numeral value 1 is assigned to the chromaticity group “7”, and a numeral value −1 is assigned to the chromaticity group “3”. As is clear fromFIG. 7(b), the chromaticity unevenness parameters become 0% on the light-emitting elements except the light-emitting elements on the periphery of the planar light source. That is, almost no unevenness of color is caused. Similarly, the result calculated based on the method ofFIG. 6(c) is shown inFIG. 7(c). In this case, the chromaticity unevenness parameters are 20% or −20% on the light-emitting elements except the light-emitting elements on the periphery of the planar light source. That is, the unevenness of chromaticity is ((x−0.2Δx, y−0.2Δy) through (x+0.2Δx, y+0.2Δy)) which is twenty percent of the original unevenness of chromaticity ((x−Δx, y−Δy) through (x+Δx, y+Δy)).

An example of a planar arrangement rule2is shown inFIG. 8(a).FIG. 8(a) is a schematic view of a planar light source in which light-emitting elements belonging to the first chromaticity group “7” (numerical value 1 is assigned), the second chromaticity group “3” (numerical value −1 is assigned), the third chromaticity group “6” (numerical value 0.5 is assigned) and the fourth chromaticity group “4” (numerical value −0.5 is assigned) are provided at regular intervals in the longitudinal and transverse directions, respectively. The result obtained by evaluating in-plane unevenness of this case by using the method ofFIG. 6(b) is shown inFIG. 8(b). In this case, the chromaticity unevenness parameters are 6.25% or −6.25% on the light-emitting elements except the light-emitting elements on the periphery of the planar light source. Further, the evaluation result by using the method ofFIG. 6(c) is shown inFIG. 8(c). In this case, the chromaticity unevenness parameters of 25% (maximum value) or −25% (minimum value) can be obtained on the respective light-emitting elements.

It should be noted that the chromaticity unevenness parameters of the planar arrangement rule2are slightly inferior to those of the planar arrangement rule1although the chromaticity groups “4” and “6” used in the planar arrangement rule2have smaller unevenness of chromaticity than the chromaticity groups “3” and “7” used in the planar arrangement rule1.

The above study demonstrates that light-emitting elements are preferably selected from two chromaticity groups having similar chromaticity differences when light-emitting elements selected from chromaticity groups having chromaticity differences are provided in longitudinal and transverse directions so that the unevenness of chromaticity is reduced. Therefore, when a backlight device (planar light source) for use in a liquid crystal display device is realized by combining light-emitting elements belonging to various chromaticity groups, it is preferable that the light-emitting elements are provided in such a backlight device (planar light source) as shown inFIG. 9.FIG. 9is a top view of the backlight device which is divided into nine vertical areas and sixteen horizontal areas. A plurality of light-emitting elements (1, 4, 9, 16 or 25 light-emitting elements) are used in each of the areas. The numeral values written in each of the areas ofFIG. 9indicate the chromaticity groups of light-emitting elements used in each of the areas. For example, “8-2” ofFIG. 9indicates the area in which the light-emitting elements belonging to the chromaticity groups “8” and “2” having large chromaticity differences are used, and “5-5” indicates the area in which the light-emitting elements belonging to the chromaticity group “5” having the same chromaticity are used. It is possible to obtain a good planar light source and a liquid crystal display device using the same, in which (i) the light-emitting elements having unevenness of chromaticity can be effectively used and (ii) the unevenness of color is hardly perceptible, when first through third areas are arranged as follows. The combinations “5-5” of the light-emitting elements belonging to the chromaticity group “5” having no chromaticity difference are used in the second area which is an area near the center (the area does not include corners) on which area a viewer fixes its eyes in a liquid crystal display device. The light-emitting elements belonging to the chromaticity groups “8-2” having large chromaticity differences are used in the first area which is an area near the periphery (the area includes corners), and the light-emitting elements belonging to the chromaticity groups “6-4” and “7-3” having smaller chromaticity differences than “8-2” are used in the third area which is an area between the area near the center and the area near the periphery.

FIG. 10is a view showing a result of two-dimensional simulation of the chromaticity unevenness parameters obtained when the light-emitting element10A having an unevenness of chromaticity of numeral value 1 and the light-emitting element10B having an unevenness of chromaticity of numeral value −1 are alternately provided in the longitudinal and transverse directions in accordance with the planar arrangement rule1. Note that the chromaticity unevenness parameters were obtained at the central area of 16 light-emitting elements provided in a matrix manner (4 rows and 4 columns), under the assumptions that (i) angle dependency of light emission distribution is lambertian (angle dependency of luminescence intensity is indicated by cos θ at an angle θ with a normal direction) and (ii) equations of c0=c1=0.7 and t=0.84(t/c0=1.2) are satisfied

The chromaticity unevenness parameters were −7.8% above the light-emitting element10B and 7.8% above the light-emitting element10A. It is clear that the unevenness of color is reduced drastically.

FIG. 11is a view showing a result of two-dimensional simulation of chromaticity unevenness parameters in the planar arrangement rule3. In the planar arrangement rule3, the chromaticity unevenness parameters were obtained at the central area of 32 light-emitting elements provided in a matrix manner (4 rows and 8 columns, andFIG. 11show a part of the 32 light-emitting elements), under the assumptions that equations of c1(intervals in a transverse direction)=0.5, c0(intervals in a transverse direction)=1 and t=0.84 are satisfied Thus, the area density of the light-emitting elements of the planar arrangement rule3is the same as that of the planar arrangement rule1.

The range in which the maximum value and the minimum value of the chromaticity unevenness parameters fall can be summarized as follows.

This shows that the unevenness of color can be reduced more in the planar arrangement rule3(the light-emitting elements are alternately provided in a rectangular shape where c1<c0is met) than in the planar arrangement rule1(the light-emitting elements are alternately provided in a square shape) which has the same area density of light-emitting elements as that of the planar arrangement rule3.

The Embodiment 1 deals with a planar light source in which light-emitting elements belonging to respective chromaticity groups are provided at regular intervals in a transverse direction in accordance with the planar arrangement rule1,2or3. However, the present embodiment deals with a case where pairs of two light-emitting elements belonging to different chromaticity groups are provided so as to be adjacent to each other in accordance with a planar arrangement rule4or5so that in-plane unevenness of chromaticity is reduced. Further, the present embodiment deals with a liquid crystal display device90including a planar light source80having the above arrangement.

FIG. 12(a) andFIG. 12(b) are a top view and a cross-sectional view, respectively, showing an arrangement of a display device. The display device90includes the planar light source80, a diffusion plate91provided so as to keep a distance t from the planar light source80, and a liquid crystal display panel92provided above the diffusion plate91. The planar light source80backlights the liquid crystal display panel92. Posts83(later described) for supporting the diffusion plate91and the liquid crystal display panel92are provided in four corners of a mounting substrate81. The diffusion plate91and the liquid crystal display panel92are described later.

FIG. 13(a) andFIG. 13(b) are a top view and a cross-sectional view, respectively, showing an arrangement of a light-emitting element20used in the present embodiment. The light-emitting element20includes a substrate21on which surface an electrode pattern is formed and the semiconductor light-emitting element1. The semiconductor light-emitting element1is die-bonded on the electrode pattern and is sealed with a resin23in which the green fluorescent substances2aand the red fluorescent substances2bare dispersed. A package of the light-emitting element20is a rectangular parallelepiped.

The light-emitting element20has emission angle dependency of a spectral distribution. One of possible causes of this is the shape of the package. When the fluorescent substances2are evenly dispersed in the resin23sealing the semiconductor light-emitting element1, short light path length along which primary light travels causes an increase in ratio of the primary light, whereas long light path length causes an increase in ratio of the secondary light. The light path length becomes long, when primary light is emitted by the semiconductor light-emitting element1in an oblique direction of the drawing, and is then emitted from a surface of the resin3. This causes an increase in percentage of the secondary light to the primary light. Therefore, the present embodiment adopts a light-emitting element20, in which the emission angle dependency of a spectral distribution is reduced by causing the light emitted by the semiconductor light-emitting element1to be emitted from a side surface of the resin23so that a change in light path length to the emission angle is reduced.

A light-emitting element30is arranged so that the change in spectral distribution due to the emission angle is further reduced. A top view and a cross-sectional view of this light-emitting element30are shown inFIGS. 14(a) and14(b), respectively. The light-emitting element30includes a substrate31on which surface an electrode pattern is provided, and the semiconductor light-emitting element1. The semiconductor light-emitting element1is die-bonded on the electrode pattern, and is sealed with resin33in which the green fluorescent substances2aand the red fluorescent substances2bare dispersed. A package of the light-emitting element30is a truncated pyramid. An inclination angle of an inclined surface33aof the resin33is preferably 60°. With this shape, differences in light path lengths to the emission angle is reduced because the primary light, obliquely emitted by the semiconductor light-emitting element1, partially passes through the inclined surface33a. Therefore, it is possible to restrain the unevenness of color of the emitted light. Note that it is possible to form the shape of the package by using a method such as a molding method or a dicing method.

FIG. 15(a) andFIG. 15(b) are a top view and a cross-sectional view, respectively, showing an arrangement of a light-emitting element40. In the light-emitting element40, a package has a dome shape. With this shape, when the semiconductor light-emitting element1is considered as a point light source, the light path length along which the primary light travels in the resin43does not change because the semiconductor light-emitting element1is sealed with the resin43having a uniform thickness. Therefore, it is possible to restrain the unevenness of color of the emitted light. Note that it is possible to form the shape of the package by using a method such as a molding method.

In the present embodiment, it is possible to use any one of the light-emitting elements10,20,30and40. Alternatively, it is possible that the semiconductor light-emitting element1is replaced with a semiconductor light-emitting element including two electrodes on its surface, and the electrodes are wire-bonded by two wires. The semiconductor light-emitting element can be subjected to flip chip mounting in which a pair of electrodes faces the frame4or the substrates21,31and41and is then die-bonded by a conductive brazing filler material.

In the present embodiment, a planar light source is arranged such that (i) pairs of two light-emitting elements20A and20B belonging to different chromaticity groups are provided so as to keep a distance d2from each other and so as to be adjacent to each other, and (ii) adjacent two pairs are provided so as to keep a distance d1in the transverse direction and so as to keep a distance d0in the longitudinal direction (hereinafter referred to as “adjacently paired arrangement”). The effect of averaging the chromaticity distribution which is brought about by this arrangement is simulated as follows.

The result of two-dimensional simulation of chromaticity unevenness parameters in the planar arrangement rule4is shown inFIGS. 16 through 19. In the planar arrangement rule4, pairs of the light-emitting elements20A and20B are provided so as to be adjacent to each other in the transverse direction, and adjacent two pairs in the longitudinal direction are provided so that an alignment of the light-emitting elements20A and20B in one of the adjacent two pairs is reverse to that in the other of the adjacent two pairs. In the simulation, an in-plane distribution was obtained in a case of 16 light-emitting elements provided in a matrix manner (4 rows and 4 columns) (second through fourth rows and second through fourth columns are shown in the drawings), under the assumptions that (i) angle dependency of luminescence intensity of the light-emitting elements20A and20B is lambertian, and (ii) equations of d0=d1=1 and t=0.84 are satisfied. In this case, two light-emitting elements exist, on average, in a 1×1 square. Therefore, the area density of the light-emitting elements is the same as that of the simulation shown inFIGS. 10 and 11.

The distance d2between the light-emitting elements20A and20B was used as a parameter d2was set to 0.4d1inFIG. 16, d2was set to 0.3d1inFIG. 17, d2was set to 0.2d1inFIG. 18, d2was set to 0.1d1inFIG. 19. In the simulation, the chromaticity unevenness parameters were found, assuming that the light-emitting elements20A have a numeral value 1 and the light-emitting elements20B have a value −1.

The range in which the maximum value and the minimum value of the chromaticity unevenness parameters fall can be summarized as follows.

Thus, when d2is 0.4d1or less, there was an effect of improvement in the unevenness of chromaticity, as compared with the planar arrangement rule3in which the light-emitting elements20A and20B are not adjacently provided. When d2is 0.3d1or less, it is possible to obtain an effect of the maximum point of the chromaticity distribution being not above the light-emitting elements20A and20B. The smaller d2/d1is, the greater effect on the unevenness of color is brought about. On the other hand, under the same in-plane mounting density of the light-emitting elements and the same distance t, distribution of the luminous intensity (light intensity) itself tends to become slightly larger. Therefore, it is preferable that d2/d1be set so that the unevenness is least perceivable for a person, by taking into consideration the unevenness of color and the unevenness of luminous intensity comprehensively. The distance d2can be decreased to such a degree that the light-emitting elements do not overlap each other, that is, the distance d2can be as small as the width of the light-emitting element.

As shown inFIGS. 16 through 19, the above simulation deals with an arrangement in which an alignment of the light-emitting elements20A and20B in one of adjacent two rows is reversed to that in the other of the adjacent two rows. Instead, an alignment of the light-emitting elements20A and20B in one of adjacent two rows can be the same as that in the other of the adjacent two rows. Especially, when d2/d1is small, it is possible to obtain a substantially the same excellent outcome as this simulation.

A result of two-dimensional simulation of parameters of unevenness of chromaticity in the planar arrangement rule5is shown inFIG. 20. In the planar arrangement rule5, in the light-emitting elements of the four rows used in the calculation of d2=0.1d1, the first and third horizontal rows are shifted so that the pairs form an isosceles triangle. In each of the pairs of the first and third rows, the light-emitting element20A is provided on the left side and the light-emitting element20B is provided on the right side in the same manner as the second and fourth rows.

In this case, the range, in which the maximum value and the minimum value of the chromaticity unevenness parameters fall is as follows.

This result is almost the same as the planar arrangement rule shown inFIG. 19. It should be noted that the pairs can be provided so as to form an equilateral triangle, instead of an isosceles triangle.

Therefore, a planar light source using such planar arrangement rules can be suitably used as a backlight of the liquid crystal display device90. In this case, the unevenness of color can be reduced.

It is preferable that (i) an actual value of the interval t is not less than 0.5 cm and not more than 3 cm, for example, 1.85 cm, and (ii) the distance d1between the pairs is 0.5 through 2 times the distance t. When a light emission distribution property of light-emitting elements is changed from lambertian so that most of the light is directed in the transverse direction, the distance t can be further reduced. This is suitably applicable to a flat display device.

Embodiment 3 deals with a displaying method for correcting chromaticity of a planar light source when the chromaticity differs from target white chromaticity.

(Offset Correction of Chromaticity by a Liquid Crystal Display Device)

FIG. 21is a chromaticity diagram for explaining adjustment of the chromaticity of a liquid crystal display device. Plotted inFIG. 21are target chromaticity W, and chromaticity WA and WB obtained when white colors are displayed by liquid crystal display device A and B, respectively, which are two samples of the liquid crystal display device90.

At least because of the reason that mold lots of light-emitting elements constituting planar light sources which the liquid crystal display devices A and B include, respectively, are different from each other, and other reasons, the light-emitting elements to be used have different chromaticity distributions. Therefore, the chromaticity WA and WB do not coincide with the target chromaticity W.

However, when the liquid crystal display device90includes chromaticity correction means93a, it is possible for the respective chromaticity WA and WB to be coincident with the target chromaticity W. For example, it is possible for the chromaticity WA to be coincident with the target chromaticity W, by carrying out the following offset correction so that transmittance of green light is reduced in the liquid crystal display device A. It is possible for the chromaticity WB to be coincident with the target chromaticity W, by carrying out the following offset correction so that transmittance of green light is increased in the liquid crystal display device B. Thus, it is possible to cause the unevenness of chromaticity between the different liquid crystal display devices to fall within a certain range.

FIG. 22is a block diagram showing how the liquid crystal display device of the present embodiment operates. A liquid crystal display panel92includes a liquid crystal cell92aand a filter92b. The liquid crystal display panel92is backlit by the planar light source80. The liquid crystal cell92ais arranged so that its light transmittance can be controlled in response to a signal supplied from a driving circuit93. The liquid crystal cell92aincludes a plurality of electrodes. Pixels are comprised of parts of the liquid crystal cell92acorresponding to the plurality of electrodes and parts of the filter92b, respectively. When an electric field is applied across desired electrodes, the liquid crystal cell92aserves as a shutter causing a change in light transmittance so that a corresponding pixel is opened or closed. The driving circuit93including the chromaticity correction means93ais connected to the liquid crystal cell92a. An image is formed in response to a signal supplied from the driving circuit93. Further, it is possible to correct the chromaticity by changing time-average light transmittance (a ratio of opening time and closing time) of a pixel corresponding to any color.

The present embodiment deals with how light-emitting elements having different chromaticity are combined in a case where a chromaticity distribution of a population of light-emitting elements varies two-dimensionally.

FIG. 23is a chromaticity diagram showing a chromaticity distribution of a population of light-emitting elements. As shown inFIG. 23, two groups of chromaticity distributions each showing an upward-sloping linear distribution appear. This is partially because, for example, light-emitting elements are manufactured by using semiconductor light-emitting elements1and fluorescent substances2aand2beach belonging to a plurality of lots.

When colors of a plurality of light-emitting elements selected from the population having such chromaticity distributions are compared, for example, when colors of M1and M2shown inFIG. 23(b) are compared, the color of M1looks different from the color of M2. This is because coordinates of M1and M2on the chromaticity diagram are different from each other, even though color temperature of M1is the same as that of M2. Therefore, it is necessary to combine light-emitting elements so as to compliment each other's chromaticity.

The following description deals with how to combine light-emitting elements. In the population of light-emitting elements constituting groups1and2of chromaticity distributions, light-emitting elements each having luminescence intensity falling in a certain range are selected. The selected light-emitting elements are sorted into a plurality of color temperature groups. Specifically, the selected light-emitting elements are sorted into L1, M1, H1for the group1, and L2, M2, and H2for the group2, respectively, in the order of increasing color temperature. Color temperature 6200K along the trajectory of a black-body radiation is a threshold temperature for discriminating between L1and M1, and between L2and M2. Color temperature 7200K along the trajectory of a black-body radiation is a threshold temperature for discriminating between M1and H1, and between M2and H2. It should be noted that the color temperature groups are chromaticity groups divided along the color temperature on the trajectory of the black-body radiation.

A planar light source is prepared with the use of (i) any one of combinations of L1H2, M1M2and H1L2and (ii) any one of the planar arrangement rules1through5described in the Embodiments 1 and 2. This causes the planar light source to have chromaticity which falls in a range of the target chromaticity.

It should be noted that the number of groups of chromaticity distributions is not limited to two. Even if the number of the groups is more than two, it is possible to realize a planar light source in which the chromaticity distributions are restrained by a method similar to the method as described above. Further, the number of the color temperature groups can be four, five, six or seven, instead of three.

In the Embodiments 1 though 4, the light-emitting elements are divided into chromaticity groups on the basis of the chromaticity (x, y) of each of the light-emitting elements. However, when light-emitting elements are used as a backlight of a liquid crystal display device, an observer sees light traveled via a filter which is provided in a liquid crystal panel. In this case, even if chromaticity is uniform as a whole, it would appear that different chromaticity of the light reaches the observer via the filter. In view of the circumstances, the present embodiment luminescence intensity of each color of each light-emitting element is divided into a plurality of groups (one kind of chromaticity groups).

Each luminescence intensity of the light emitted from each light-emitting element via a filter R, a filter G and a filter B is measured. It should be noted that sensitivity of a measuring device is set to the human visibility (a visibility correction filter is attached to a Si photo diode). The filter R, filter G and filter B have the same characteristics as red, green and blue color filters provided in a liquid crystal panel, respectively.

The light-emitting elements are divided into luminescence intensity groups R3, R4, R5, R6, R7and others with no rank on the basis of the luminescence intensity of the light-emitting elements which passed through the filter R. The light-emitting elements are divided into the luminescence intensity groups G3, G4, G5, G6, G7and others with no rank on the basis of a data measured via the filter G. The light-emitting elements are divided into the luminescence intensity groups of B3, B4, B5, B6, B7and others with no rank on the basis of a data measured via the filter B. It should be noted that average light-emitting elements belong to the luminescence intensity groups of R5, G5and B5. The luminescence intensity groups for the respective colors are shown inFIG. 24. Thus, light-emitting elements with some ranks belong to any one of 53=125 luminescence intensity groups for the respective colors.

FIG. 25is an explanatory view showing combinations of light-emitting elements,FIG. 25(a) shows an example in which R7G7B3and R3G3B7are combined, andFIG. 25(b) shows an example in which R7G7B4and R3G3B6are combined. When the density of fluorescent substances is high, there is a correlation in which (i) R and G are in conjunction with each other so that their respective intensity become high and (ii) the intensity of B becomes low. Therefore, there are a lot of light-emitting elements belonging to such luminescence intensity groups. Two RGB combinations should be carried out so that an average of the numeral values of luminescence intensity groups (7of “R7”) of two RGB combinations becomes 5.

FIG. 25(c) shows an example of a combination in which green fluorescent substances and red fluorescent substances are slightly unbalanced. Also in this case, the two RGB combinations has an average of the numeral values of luminescence intensity groups of the two RGB combinations becomes 5.

The description has dealt with the example in which light-emitting elements are divided into a plurality of groups in accordance with their luminescence intensity. However, the present embodiment is not limited to this. Namely, instead of the division into the luminescence intensity groups, it is possible to select and combine (i) light-emitting elements having first numeral values of luminescence intensity measured for R, G and B and (ii) light-emitting elements having second numeral values of luminescence intensity measured for R, G and B. The differences between the first numeral values and target values are close to the differences between the second numerals values and the target values, respectively. The first numeral values are opposite to the second numeral values with respect to the target values, respectively.

When light-emitting elements having the two RGB combinations thus prepared are provided in accordance with any one of the planar arrangement rules described in the present specification, it is possible to realize light-emitting elements in which their in-plane distribution of chromaticity is restrained.

The present embodiment relates to an improvement in yield of a planar light source for lighting.

It is necessary to use light-emitting elements having color temperature of (6500K±200K), when a planar light source having a certain color temperature, for example, a planar light source having a color temperature of (6500K±200K) is prepared by arranging a plurality of light-emitting elements. However, in the production field, even if the producer tries to prepare light-emitting elements having a color temperature of 6500K, there is unevenness of color temperature among produced plural light-emitting elements. Therefore, it is necessary that the unevenness of color temperature among produced light-emitting elements falls within a range (centered at 6500K) of (6500K±200K). Especially, when a white lighting module (planar light source for lighting) is prepared, hundreds of light-emitting elements are used. As such, the preparation of such a white lighting module is susceptible to the unevenness of production.

The inventors of the present invention studied in detail and found out that it was very difficult to cause the unevenness of color temperature among the produced light-emitting elements to fall within such a range of (6500K±200K). This is because in the production process, there were unevenness of emission wavelength among semiconductor light-emitting elements, unevenness of emission wavelength among fluorescent substances, unevenness of amount among resins, unevenness of compounding ratio among fluorescent substances and unevenness of preparation time. For such reasons, light-emitting elements whose color temperature do not fall within the range of (6500K±200K) are judged as defective products. This gives rise to the reality that planar light sources can not be prepared with good process yield. Further, the yield becomes lower, as the target color temperature of a planar light source becomes narrower, for example, as X of (6500K±XK) becomes smaller.

Used as a light-emitting element302was a light-emitting element10, provided that yellow fluorescent substances2, which is dispersed in the resin3, includes 2(Sr0.93Ba0.05Eu0.02)O.SiO2.

FIG. 26shows a top schematic view of a planar light source300. 22×15 light-emitting elements302are provided, in accordance with the planar arrangement rule1, on a glass-epoxy substrate301having a rectangular shape. Further, the planar light source300is arranged to include pairs of arbitrary two light-emitting elements (e.g.303and304shown inFIG. 26).

It was assumed that a target color temperature of the planar light source300was 6500K, and an allowable range was from 6200K to 7200K. An amount of the fluorescent substances, dispersed into each of the light-emitting elements302used in the planar light source300, was adjusted so that the color temperature of emission color was 6500K. However, the unevenness of color temperature among the light-emitting elements was caused during the production. In view of the circumstances, the light-emitting elements302provided in accordance with the planar arrangement rule1were divided into three color temperature groups1through3(chromaticity groups divided along on the color temperature on the trajectory of the black-body radiation) on the basis of color temperature Tc of emission color. In the color temperature group1, light-emitting elements have their emission color whose color temperatures fall a range of not less than 6200K and less than 7200K, in the color temperature group2, light-emitting elements have their emission color whose color temperatures fall a range of not less than 7200K, and in the color temperature group3, light-emitting elements have their emission color whose color temperatures fall a range of less than 6200K. Each of the pairs of two light-emitting elements is definitely comprised of (i) two light-emitting elements belonging to the color temperature group1or (ii) one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3.

Specifically, the pair303was comprised of one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3.FIG. 27shows emission spectra of the light-emitting elements belonging to the respective color temperature groups. InFIG. 27, a spectrum1is a spectrum of the light-emitting elements belonging to the color temperature group3, and their color temperatures were 5525K. InFIG. 27, a spectrum2is a spectrum of the light-emitting elements belonging to the color temperature group2, and their color temperatures were 8383K. When the emission spectrum was measured while only the two light-emitting elements of the pair303were being turned on, a spectrum3inFIG. 27was obtained. Their color temperatures were 6452K. It is clear that the spectrum3has emission spectrum and color temperature which are an average of those of the two light-emitting elements.

The pair304was comprised of two light-emitting elements belonging to the color temperature group1.FIG. 28shows emission spectra of the light-emitting elements. InFIG. 28, spectra4and5are, respectively, spectra of the two light-emitting elements, and their color temperatures were 6251K and 6880K, respectively. When the emission spectrum was measured while only the two light-emitting elements of the pair304were being turned on, a spectrum6inFIG. 28was obtained. Their color temperatures were 6524K.

FIG. 29shows an emission spectrum measured while the whole light-emitting elements of the planar light source300thus prepared were being turned on. The color temperature was 6392K. This slightly deviates from the target color temperature of the planar light source, but fell within an allowable color temperature range.

As described above, even though light-emitting elements whose color temperatures have deviated from each other during the production process, it is possible to prepare a planar light source having a target color temperature, when such light-emitting elements are provided in accordance with an ingenious arrangement method of the present invention. This is because the ingenious arrangement method allows the color temperature of such light-emitting elements to be averaged. According to Example 1, it was possible to use light-emitting elements whose color temperatures fall in a range from 5000K to 9000K, and the yield of the light-emitting elements was 99%.

COMPARATIVE EXAMPLE 1

Light-emitting elements used in Comparative Example 1 all belong to the color temperature group1of the Example 1. The light-emitting elements were not divided and were simply provided in an array shape. There is no difference from the Example 1 except this.

FIG. 30shows an emission spectrum measured while whole of the light-emitting elements of the planar light source300, thus prepared, of the Comparative Example 1 is being turned on. The color temperature was 6414K. This slightly deviates from the target color temperature of the planar light source, but falls within the allowable color temperature range. In the Comparative Example 1, only light-emitting elements whose color temperatures fall in a range from 6200K to 7200K were used. Therefore, the yield of the light-emitting elements was 85%.

Used as a light-emitting element502was a light-emitting element10, provided that green fluorescent substances2aand red fluorescent substances2b, which are dispersed in the resin3, include (Ca0.9Mg0.1)3(Sc0.90Ce0.10)2(SiO4)3and (Ca0.98Eu0.02)AlSiN3, respectively. A wavelength conversion section was prepared by dispersing a mixture of the green fluorescent substances2aand the red fluorescent substances2binto the silicon resin3. Thus, a planar light source500of Example 2 was prepared.

FIG. 31shows a top schematic view of a planar light source500. 75×4 light-emitting elements502are provided, in accordance with the planar arrangement rule1, on a glass-epoxy substrate501having a linear shape. Further, the planar light source500is arranged to include pairs of arbitrary two light-emitting elements (e.g.503and504shown inFIG. 31).

It was assumed that a target color temperature of the planar light source500was 5000K, and an allowable range was from 4700K to 5300K. The light-emitting elements502provided in accordance with the planar arrangement rule1were divided into three color temperature groups1through3(chromaticity groups divided along on the color temperature on the trajectory of the black-body radiation) on the basis of color temperature Tc of emission color. In the color temperature group1, light-emitting elements have their emission color whose color temperatures fall a range of not less than 4700K and less than 5300K, in the color temperature group2, light-emitting elements have their emission color whose color temperatures fall a range of not less than 5300K, and in the color temperature group3, light-emitting elements have their emission color whose color temperatures fall a range of less than 4700K. Each of the pairs of two light-emitting elements is definitely comprised of (i) two light-emitting elements belonging to the color temperature group1or (ii) one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3.

Specifically, the pair503was comprised of one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3. The pair504was comprised of two light-emitting elements belonging to the color temperature group1.

COMPARATIVE EXAMPLE 2

Light-emitting elements and a planar light source were manufactured in the same way as the Example 2. Like the Example 2, the 75×4 light-emitting elements were simply provided in an array shape on the glass-epoxy substrate501having a linear shape. Because the target color temperature of the planar light source was 5000K, and the allowable color temperature was from 4700K to 5300K, light-emitting elements used in Comparative Example 2 all belong to the color temperature group1of the Example 2.

Used as a light-emitting element602was a light-emitting element10, provided that green fluorescent substances2aand red fluorescent substances2b, which are dispersed in the resin3, include 2(Ba0.65Sr0.33Eu0.02)O.SiO2and (Ca0.98Eu0.02)AlSiN3, respectively. A wavelength conversion section was prepared by dispersing a mixture of the green fluorescent substances2aand the red fluorescent substances2binto the silicon resin3. Thus, a planar light source600of Example 3 was prepared.

FIG. 32shows a top schematic view of the planar light source600. 280 light-emitting elements602are provided, in accordance with the planar arrangement rule1, on a glass-epoxy substrate601having a circular shape. Further, the planar light source600is arranged to include pairs of arbitrary two light-emitting elements (e.g.603and604shown inFIG. 32).

It was assumed that a target color temperature of the planar light source600was 2500K, and an allowable range was from 2300K to 2700K. The light-emitting elements602provided in accordance with the planar arrangement rule1were divided into three color temperature groups1through3(chromaticity groups divided along on the color temperature on the trajectory of the black-body radiation) on the basis of color temperature Tc of emission color. In the color temperature group1, light-emitting elements have their emission color whose color temperatures fall a range of not less than 2300K and less than 2700K, in the color temperature group2, light-emitting elements have their emission color whose color temperatures fall a range of not less than 2700K, and in the color temperature group3, light-emitting elements have their emission color whose color temperatures fall a range of less than 2300K. Each of the pairs of two light-emitting elements is definitely comprised of (i) two light-emitting elements belonging to the color temperature group1or (ii) one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3.

Specifically, the pair603was comprised of one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3. The pair604was comprised of two light-emitting elements belonging to the color temperature group1.

COMPARATIVE EXAMPLE 3

Light-emitting elements and a planar light source were manufactured in the same way as the Example 3. Like the Example 3, the 280 light-emitting elements were simply provided in an array shape on the glass-epoxy substrate601having a circular shape. Because the target color temperature of the planar light source was 2500K, and the allowable color temperature was from 2300K to 2700K, light-emitting elements used in Comparative Example 3 all belong to the color temperature group1of the Example 3.

Used as a light-emitting element302was a light-emitting element10, provided that green fluorescent substances2aand red fluorescent substances2b, which are dispersed in the resin3, include (Ca0.9Mg0.1)3(Sc0.90Ce0.10)2(SiO4)3and (Ca0.98Eu0.02)AlSiN3, respectively. A wavelength conversion section was prepared by dispersing a mixture of the green fluorescent substances2aand the red fluorescent substances2binto the silicon resin3. Thus, a planar light source of Example 4 was prepared.

Like the example 1, 22×15 light-emitting elements302are provided, in accordance with the planar arrangement rule1, on a glass-epoxy substrate301having a rectangular shape (seeFIG. 26). Further, the planar light source is arranged to include pairs of arbitrary two light-emitting elements (e.g.303and304shown inFIG. 26).

It was assumed that a target color temperature of the planar light source was 3000K, and an allowable range was from 2500K to 3600K. The light-emitting elements302provided in accordance with the planar arrangement rule1were divided into three color temperature groups1through3(chromaticity groups divided along on the color temperature on the trajectory of the black-body radiation) on the basis of color temperature Tc of emission color. In the color temperature group1, light-emitting elements have their emission color whose color temperatures fall a range of not less than 2800K and less than 3200K, in the color temperature group2, light-emitting elements have their emission color whose color temperatures fall a range of not less than 3200K, and in the color temperature group3, light-emitting elements have their emission color whose color temperatures fall a range of less than 2800K. Each of the pairs of two light-emitting elements is definitely comprised of (i) two light-emitting elements belonging to the color temperature group1or (ii) one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3.

Specifically, the pair303was comprised of one light-emitting element belonging to the color temperature group2and one light-emitting element belonging to the color temperature group3. The pair304was comprised of two light-emitting elements belonging to the color temperature group1.

COMPARATIVE EXAMPLE 4

Light-emitting elements and a planar light source were manufactured in the same way as the Example 4. Like the Example 4, the 22×15 light-emitting elements302were simply provided in an array shape on the glass-epoxy substrate301having a rectangular shape. Because the target color temperature of the planar light source was 3000K, and the allowable color temperature was from 2800K to 3200K, light-emitting elements used in Comparative Example 4 all belong to the color temperature group1of the Example 4.

The evaluation result of characteristics of the planar light sources of Examples 1 through 4 and Comparative Examples 1 through 4 is shown in Table 1.

It should be noted that brightness was obtained by turning on a planar light source under the condition that a forward current (IF) is 20 mA and by converting white lights from the planar light source into a photocurrent. Further, Tc and the general color rendering index (Ra) was obtained by turning on a planar light source under the condition that a forward current (IF) is 20 mA and by measuring white light from the planar light source with MCPD-2000 made by Otsuka Electronics Co., Ltd.

As shown in Table 1, as compared with the Comparative Examples, it is possible for the planar light sources of the Examples to widen the chromaticity range of light-emitting elements without causing a deviation in color temperature of the planar light sources and without greatly declining the light flux and the color rendering. Therefore, it is possible to remarkably increase the yield of products.