Abstract:
A backlight unit ( 49 ) for a display device ( 69 ) provided with a liquid crystal display panel ( 59 ) comprises a chassis ( 41 ), a diffusion plate ( 43 ) supported by the chassis, and point-like light sources supported by mounting substrates ( 21 ) provided on the chassis. The point-like light sources comprise light emitting modules (MJ). The mounting substrates are laid in a rectangular region ( 41   a ) adapted for arranging the mounting substrates therein and set on the chassis. The gaps at the boundaries between the mounting substrates do not continue in either the direction along the long sides and/or the direction along the short sides of the rectangular region so as to enable the rectangular region to be seen from end to end.

Description:
TECHNICAL FIELD 
       [0001]    The present invention is related to an illumination device, a display device including the illumination device, and a television receiver incorporating the display device. 
       BACKGROUND ART 
       [0002]    Display devices using a non-self-luminous display panel such as, for example, a liquid crystal display panel typically incorporates an illumination device for illuminating the display panel from behind. Various types of light sources such as cold cathode tubes and light emitting elements are used as light sources of this type of illumination device. Examples of the light emitting elements include a light emitting diode (hereinafter referred to as “LED”), an organic electroluminescence element, an inorganic electroluminescence element, etc., among which the LED is most commonly used today. An illumination device described in Patent Literature 1 also uses LEDs as light sources. 
         [0003]    In the illumination device described in Patent Literature 1, as shown in  FIG. 12 , an LED  122  is mounted on a mounting substrate  121 , and further, a lens  124  is attached to the mounting substrate  121  to cover the LED  122 . The mounting substrate  121 , the LED  122 , and the lens  124  together form a light emitting module mj. A large number of light emitting modules mj are arranged in a matrix form to form a planar light source. 
         [0004]    In the illumination device described in Patent Literature 1, a large number of point light sources are arranged. On the other hand, a large number of linear light sources such as cold cathode tubes are arranged in an illumination device described in Patent Literature 2. In cases where, as in these two examples, an illumination device formed by arranging a plurality of light sources is incorporated in a display device, if light from the light sources directly enters the illumination device, it results in uneven brightness of the display surface. To prevent such uneven brightness, a diffusion plate is disposed between the light sources and the display device for diffusing light. A diffusion plate, as described in Patent Literature 2, is typically built as part of the illumination device. 
         [0005]    In some cases where a large area needs to be illuminated with a planar light source formed by arranging a large number of point light sources, it may be necessary to arrange a plurality of mounting substrates, each supporting a plurality of point light sources. An example of such a case is disclosed in Patent Literature 3. 
       CITATION LIST 
     Patent Literature 
       [0006]    Patent Literature 1: JP-A-2008-41546 
         [0007]    Patent Literature 2: JP-A-2005-19065 
         [0008]    Patent Literature 3: JP-A-2006-301209 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    Many of mounting substrates that support point light sources have a reflection sheet bonded to a surface thereof for higher light reflectance. If a reflection sheet of the size of a mounting substrate is bonded to the mounting substrate, a large difference in light reflectance arises between the mounting substrate and the other parts, and this may invite the following problem. 
         [0010]    That is, boarders between mounting substrates, in other words, gaps formed at boundaries between the mounting substrates are, though depending on their widths, perceived as shadows when seen from outside the diffusion plate. This problem will be described with reference to  FIGS. 13 and 14 . 
         [0011]      FIG. 13  shows mounting substrates  101 , each of which supports a plurality of point light sources  102  such as LEDs. The mounting substrates  101  are rectangular elongated sideways, and the point light sources  102  are arranged into a matrix form of 4 rows and 11 columns on each of the mounting substrates  101 . The mounting substrates  101  are arranged in 4 rows and 2 columns, to together form a rectangular planar light source that includes the point light sources  102  arranged in 16 rows and 22 columns in total. 
         [0012]      FIG. 14  shows a diffusion plate  103  illuminated by the planar light source. The figure illustrates a state where gaps at the boundaries between the mounting substrates  101  appear as shadows S. The gaps at the boundaries between the mounting substrates  101  align straight, forming a pattern like a chess board, and thus the shadows S tend to be long and undesirably noticeable. 
         [0013]    In an illumination device disclosed in Patent Literature 3, as shown in  FIG. 15 , a strip-shaped mounting substrate  101  has point light sources  102  arranged thereon in a row, and three mounting substrates  101  are serially arranged in a row, and a plurality of rows of mounting substrates  101  are arranged to form a planar light source. The rows each composed of three mounting substrates  101  are displaced from each other, as a result of which the gaps at the boundaries between the mounting substrates  101  form a zigzag or a staggered pattern, and thus, even if the shadows S as shown in  FIG. 14  appear in the diffusion plate  103 , they would not be so long as to be undesirably noticeable. 
         [0014]    However, in the illumination device of Patent Literature 3, ends of the mounting substrates  101  are not aligned, and thus the point light sources  102  are less dense at edges of the planar light source. With this configuration, as shown in  FIG. 16 , shadows  51  resulting from insufficient light appear at edges of a diffusion plate  103 . An attempt to hide the shadows S 1  by covering them with a frame-shaped case in an electronic apparatus incorporating the illumination device would require the frame shape to be wide, preventing a narrower frame desirable in the design of the electronic apparatus. 
         [0015]    The present invention has been made in view of the foregoing, and an object of the present invention is, in an illumination device including a diffusion plate, a chassis which supports the diffusion plate, and a light source which is formed of a plurality of mounting substrates each of which supports a plurality of point light sources, to prevent gaps at boundaries between the mounting substrates from appearing in an undesirably noticeable manner. 
       Solution to Problem 
       [0016]    According to a preferable embodiment of the present invention, an illumination device includes: a diffusion plate; a chassis which supports the diffusion plate; and a light source which is formed of a plurality of mounting substrates each of which supports a plurality of point light sources. Here, the mounting substrates are laid out in a rectangular mounting-substrate-layout region set on the chassis; and gaps at boundaries between the mounting substrates do not align straight in the rectangular mounting-substrate-layout region end to end at least in one of a short-side direction and a long-side direction thereof. 
         [0017]    With this configuration, since the mounting substrates are laid out in the rectangular mounting-substrate-layout region, it is possible to gain the amount of light that a planar light source is required to cover all over the rectangular mounting-substrate-layout region. Further, since the gaps at the boundaries between the mounting substrates do not align straight in the rectangular mounting-substrate-layout region end to end at least in one of a short-side direction and a long-side direction thereof, no shadow appears to be so long as to be undesirably noticeable in the diffusion plate. 
         [0018]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, a plurality of kinds of rectangular mounting substrates are laid out in the rectangular mounting-substrate-layout region; and gaps at boundaries between the mounting substrates do not align straight in the rectangular mounting-substrate-layout region end to end either in the short-side direction or in the long-side direction thereof. 
         [0019]    With this configuration, no shadow appears to be so long as to be undesirably noticeable in the diffusion plate either in the long-side direction or in the short-side direction of the diffusion plate. 
         [0020]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, a plurality of mounting-substrate rows, each of which is formed by serially arranging strip-shaped mounting substrates of different lengths in a longitudinal direction, are laid out in the rectangular mounting-substrate-layout region; and gaps at boundaries between the strip-shaped mounting substrates are displaced from each other between adjacent ones of the mounting-substrate rows such that the gaps at the boundaries between the mounting substrates do not align straight in the rectangular mounting-substrate-layout region end to end at least in one of the short-side direction and the long-side direction thereof. 
         [0021]    This configuration facilitates the designing of the layout of the mounting substrates. 
         [0022]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the point light sources are light emitting elements which are mounted on the mounting substrates, and each of the light emitting elements is covered with a lens. 
         [0023]    With this configuration, it is possible to adjust the directivity of light emitted from the light emitting elements by using the lens. 
         [0024]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the lens has a light diffusing function. 
         [0025]    With this configuration, the light diffusing function of the lens helps achieve satisfactory diffusion of light. This widens the range of directions in which light is emitted from the light emitting elements, and this makes it possible to cover a wide area with comparatively a small number of light emitting elements. 
         [0026]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the light diffusing function is given to the lens by applying surface-roughing treatment to a mountain-board-side surface of the lens. 
         [0027]    This configuration helps achieve more satisfactory diffusion of light. 
         [0028]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the light emitting elements are LEDs. 
         [0029]    This configuration makes it possible to obtain a bright illumination device by using LEDs brightness of which has been significantly increased these days. It also makes it possible to achieve a light source with longer life and less power consumption. 
         [0030]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the LEDs are formed to emit white light by applying a fluorescent substance having an emission peak in a yellow range to blue light emitting chips. 
         [0031]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the LEDs are formed to emit white light by applying fluorescent substances, one of which having an emission peak in a green range, the other having an emission peak in a red range, to blue light emitting chips. 
         [0032]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, wherein the LEDs are formed to emit white light by combining red light emitting chips with blue light emitting chips to which a fluorescent substance having an emission peak in a green range is applied. 
         [0033]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the LEDs are formed to emit white light by combining blue, green, and red light emitting chips. 
         [0034]    In the white light emitted from a white-light-emitting LED, even color tone may be a hard target due to, for example, stronger blue. However, by assembling the LEDs to emit white light as in the present invention, it is possible to obtain illumination of leveled, substantially even color tone. 
         [0035]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the LEDs are formed to emit white light by combining ultraviolet light emitting chips with a fluorescent substance. 
         [0036]    According to a preferred embodiment of the present invention, in the illumination device configured as described above, the LEDs are formed to emit white light by applying fluorescent substances, one of which having an emission peak in a blue range, another having an emission peak in a red range, and the other having an emission peak in a green range, to ultraviolet light emitting chips 
         [0037]    In cases where an ultraviolet light emitting chip is used as a light source, light emitted from the light source tends to have uneven color tone, but with the configuration of the present invention, it is possible to obtain illumination of leveled, substantially even color tone. 
         [0038]    According to a preferred embodiment of the present invention, a display device is configured to include any one of the illumination devices configured as described above and a display panel which receives light from the illumination device. 
         [0039]    With this configuration, it is possible to obtain a display device that suffers less from uneven brightness. 
         [0040]    According to a preferred embodiment of the present invention, in the display device configured as described above, the display panel is a liquid crystal display panel. 
         [0041]    With this configuration, it is possible to obtain a liquid crystal display device that suffers less from uneven brightness. 
         [0042]    According to a preferred embodiment of the present invention, a television receiver includes the display device configured as described above. 
         [0043]    With this configuration, it is possible to obtain a television receiver that suffers less from uneven brightness. 
       ADVANTAGEOUS EFFECTS OF INVENTION 
       [0044]    According to the present invention, it is possible to obtain a satisfactory amount of light as a planar light source all across the rectangular mounting-substrate-layout region, and to reduce occurrence of shadows that appear to be so long as to be undesirably noticeable in the diffusion plate, which would otherwise result in degraded quality of illumination. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0045]      FIG. 1  An exploded perspective view of a display device including an illumination device according to a preferred embodiment of the present invention; 
           [0046]      FIG. 2  A sectional view showing part of an illumination device; 
           [0047]      FIG. 3  A plan view showing a mounting-substrate layout according to a first embodiment; 
           [0048]      FIG. 4  A plan view of a diffusion plate illuminated by mounting substrates laid out according to the first embodiment; 
           [0049]      FIG. 5  A plan view of a mounting-substrate layout according to a second embodiment; 
           [0050]      FIG. 6  A plan view of a mounting-substrate layout according to a third embodiment; 
           [0051]      FIG. 7  A plan view of a mounting-substrate layout according to a fourth embodiment; 
           [0052]      FIG. 8  A plan view of a mounting-substrate layout according to a fifth embodiment; 
           [0053]      FIG. 9  A plan view of a mounting-substrate layout according to a sixth embodiment; 
           [0054]      FIG. 10  A plan view of a mounting-substrate layout according to a seventh embodiment; 
           [0055]      FIG. 11  An exploded perspective view of a television receiver; 
           [0056]      FIG. 12  An exploded perspective view of a conventional illumination device; 
           [0057]      FIG. 13  A plan view showing an example of mounting-substrate layout; 
           [0058]      FIG. 14  A plan view of a diffusion plate illuminated by the mounting substrates laid out in a manner shown in  FIG. 13 ; 
           [0059]      FIG. 15  A plan view showing another example of mounting-substrate layout; 
           [0060]      FIG. 16  A plan view of a diffusion plate illuminated by mounting substrates laid out in a manner shown in  FIG. 15 ; 
           [0061]      FIG. 17  A graph showing how illuminance differs in different directions of irradiation from an LED; and 
           [0062]      FIG. 18  A conceptual diagram showing a collective brightness of a plurality of LEDs. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0063]    A description will be given, based on  FIGS. 1 to 4 , of the configuration of an embodiment of a display device incorporating an illumination device according to a preferred embodiment of the present invention. In  FIG. 1 , a display device  69  is illustrated as being horizontally placed with a display surface thereof facing upward. 
         [0064]    The display device  69  incorporates a liquid crystal display panel  59  as a display panel. The liquid crystal display panel  59  and a backlight unit  49  which illuminates the liquid crystal display panel  59  from behind are accommodated in a housing. The housing is made by combining a front housing member HG 1  and a rear housing member HG 2 . 
         [0065]    The liquid crystal display panel  59  is made by bonding together an active matrix substrate  51 , which includes switching elements such as thin film transistors (TFTs), and a counter substrate  52 , which faces the active matrix substrate  51 , with an unillustrated sealing material in between, and then filling the space between the active matrix substrate  51  and the counter substrate  52  with liquid crystal. 
         [0066]    A polarization film  53  is fixed to a light receiving side of the active matrix substrate  51  and to a light emission side of the counter substrate  52 . The liquid crystal display panel  59  forms an image by making use of variation in light transmittance resulting from different tilts of liquid crystal molecules. 
         [0067]    The backlight unit  49 , which embodies the illumination device of the present invention, has the following configuration. That is, the backlight unit  49  includes light emitting modules MJ, a chassis  41 , a diffusion plate  43 , a prism sheet  44 , and a microlens sheet  45 . 
         [0068]    The chassis  41  has a tray-like shape where walls rise from edges of a rectangular main flat surface. 
         [0069]    The light emitting modules MJ each include a mounting substrate  21 , a point light source disposed on the mounting substrate  21 , a lens  24  which covers the point light source, and a reflection sheet  11  which is bonded to a surface of the mounting substrate  21 . The size of the reflection sheet  11  as a whole is the same as the size of the mounting substrate  21 . The point light source is a light emitting element mounted on the mounting substrate  21 . The light emitting element of this embodiment is an LED  22 . 
         [0070]    As the reflection sheet  11 , there may be adopted a resin foam sheet containing a large number of fine air bubbles and exploiting the interface reflection in the air bubbles to the full to reflect light. This type of reflection sheet has a high optical reflectance; polyethylene-terephthalate (PET) foam sheets having a reflectance of 98% or more are available, and thus, it is desirable to adopt such a resin foam sheet. 
         [0071]    The lens  24  is provided with a light diffusing function. A description will be given of the significance of the light diffusing function that the lens  24  is provided with. Take, for example, the illumination device disclosed in Patent Literature 1. Although the illumination device shown in  FIG. 12  is combined with lenses  124 , since light from each of the LEDs  122  is emitted in a small range of directions, a large number of light emitting modules mj need to be arranged densely in order to avoid uneven brightness. This increases the cost for components and for mounting the components, making the illumination device expensive as a whole. 
         [0072]    Recently, the brightness of LEDs has been significantly increased, so that it is now possible to obtain a sufficient amount of light to cover the entire screen with a comparatively small number of LEDs. However, if a small number of high-brightness LEDs are sparsely arranged, it is impossible to avoid uneven brightness, and thus, it is preferable to use a lens that is highly capable of diffusing light in combination with each LED. The lens provided with the light diffusing function will herein be referred to as “diffusion lens”. 
         [0073]      FIG. 17  is a graph showing how illuminance (unit Lux) differs in different irradiation directions in a case of a bare LED and in a case of an LED combined with a diffusion lens. In the case of the bare LED, the illuminance is highest at an angle of 90°, which is the angle of the optical axis, and sharply decreases farther away from there. In contrast, in the case of the LED combined with a diffusion lens, illuminance of a certain level or higher can be obtained in a wider area, and the peak of illuminance can be set at an angle that is different from the angle of the optical axis. Needless to say, the pattern of illuminance shown in the figure can be changed as desired by accordingly designing the diffusion lens. 
         [0074]      FIG. 18  conceptually shows a collective brightness of a plurality of LEDs. In the figure, the solid-line waveforms indicate the brightness of LEDs each combined with a diffusion lens, while the broken-line waveforms indicate the brightness of bare LEDs. The horizontal lines among the waveforms indicate widths (full width at half maximum) of the waveforms at brightness of levels half the peak levels. In the case of LEDs each combined with a diffusion lens, each waveform can have a large width, and thus it is easy to generate integrated collective brightness as flat brightness as shown in the upper part of the figure. In contrast, in the case of bare LEDs, the waveforms each have a high peak but have a narrow width, and thus it is impossible to avoid generation of waves in the brightness made by gathering the waveforms. Unevenly bright images are not desirable, so it is almost indispensably necessary to adopt the LED combined with a diffusion lens. 
         [0075]    In view of the above, the light emitting module MJ is provided with a diffusion lens  24 . 
         [0076]    Surface-roughing treatment such as surface texturing (grain finishing) may be applied to a surface of the diffusion lens  24  that faces the mounting substrate  21 , to thereby give the surface a light diffusing function. This allows still more effective diffusion of light. 
         [0077]    The mounting substrate  21  is rectangular, and on its upper surface which is formed as a mounting surface  21 U, a plurality of electrodes (not shown) are formed to be arranged in a matrix form, and LEDs  22  are mounted on the electrodes. The mounting substrate  21  functions as a common mounting substrate for the LEDs  22 . A plurality of pairs of an LED  22  and a diffusion lens  24  are arranged in a matrix form in the X arrow direction and the Y arrow direction shown in  FIG. 1 . The mounting substrate  21  is fixed to the chassis  41  by an appropriate method such as swaging, bonding, screwing, or riveting. 
         [0078]    The diffusion lens  24  is circular in plan, and has a plurality of legs  24   a  on its lower surface. The tips of the legs  24   a  are bonded to the mounting surface  21 U of the mounting substrate  21  with an adhesive, and thereby the diffusion lens  24  is attached to the mounting substrate  21 . The reflection sheet  11  has formed therein through holes in which the legs  24   a  of the diffusion lens  24  are inserted. The presence of the legs  24   a  generates a gap between the mounting substrate  21  and the diffusion lens  24 . An air flow passes through the gap, and the LED  22  is cooled by the air flow. Incidentally, on the condition that heat dissipation is ensured, it is possible to use an integrally molded light emitting module in which an LED is embedded in a diffusion lens. 
         [0079]    Various types of LEDs can be used as the LED  22 . For example, it is possible to use an LED that is formed to emit white light by applying, to a blue light emitting chip, a fluorescent substance having an emission peak in the yellow range. It is also possible to use an LED that is formed to emit white light by applying, to a blue light emitting chip, fluorescent substances, one of which having an emission peak in the green range and the other having an emission peak in the red range. It is also possible to use an LED that is formed to emit white light by combining a red light emitting chip with a blue light emitting chip to which a fluorescent substance having an emission peak in the green range is applied. It is also possible to use an LED which is formed to emit white light by combining blue, green, and red light emitting chips. 
         [0080]    LEDs that are formed to emit white light tend to emit white light in which the blue component appears stronger than the other components, and this may cause uneven color tone. By using LEDs emitting white light in the above-described manners, it is possible to obtain illumination of leveled, substantially even color tone. 
         [0081]    In addition to the LEDs of the types described above, it is possible to use an LED that is formed to emit white light by combining an ultraviolet light emitting chip with a fluorescent substance, specifically, by applying fluorescent substances, one of which having an emission peak in a blue range, another having an emission peak in a green range, and the other having an emission peak in a red range, to an ultraviolet light emitting chip. 
         [0082]    Use of an ultraviolet light emitting chip as the light source tends to result in uneven color tone, but with the above configuration, it is possible to obtain illumination of leveled, substantially even color tone. 
         [0083]    As the mounting substrates  21 , a plurality of types of mounting substrates  21  are provided; the mounting substrates  21  are all rectangular shaped, but they have different rectangular shapes and different sizes. In a mounting-substrate layout according to a first embodiment shown in  FIGS. 1 and 3 , a total of 11 mounting substrates  21  are laid out all across a rectangular mounting-substrate-layout region  41   a  (see  FIG. 3 ) which is set on the chassis  41 . The light emitting modules MJ supported by the mounting substrates  21  together form a matrix uniformly spread out all over the rectangular mounting-substrate-layout region  41   a.    
         [0084]    When the LEDs  22  of the light emitting modules MJ are lit, light emitted from the LEDs  22  illuminates the diffusion plate  43  from behind. Fractional light from the LEDs  22  does not directly travel toward the diffusion plate  43 ; it is reflected by the reflection sheet  11  toward the diffusion plate  43 . Light is diffused in the diffusion plate  43 , and thus, as seen from outside, the diffusion plate  43  appears to be a plane having comparatively uniform brightness. 
         [0085]    Gaps at the boundaries between the mounting substrates  21  do not align straight in the rectangular mounting-substrate-layout region  41   a  end to end either in the long-side direction or in the short-side direction thereof. As a result, as shown in  FIG. 4 , even if there should be any shadows S, they are not so long as to be undesirably noticeable either in the long-side direction or in the short-side direction of the diffusion plate  43 . On the other hand, since the mounting substrates  21  are laid out all across the rectangular mounting-substrate-layout region  41   a , the amount of light that a planar light source is required to cover can be obtained all over the rectangular mounting-substrate-layout region  41   a.    
         [0086]      FIGS. 5 to 10  show mounting-substrate layouts according to other preferred embodiments. 
         [0087]    In a mounting-substrate layout according to a second embodiment shown in  FIG. 5 , a total of eight mounting substrates  21  are laid out in the rectangular mounting-substrate-layout region  41   a . On each of the mounting substrates  21 , four light emitting modules MJ are arranged in the column direction, that is, in the Y arrow direction shown in  FIG. 1 . As to the row direction, that is, the X arrow direction shown in  FIG. 1 , the mounting substrate  21  on the left in the first row from the top in  FIG. 5  has a width sufficient to hold five columns of light emitting modules MJ, while the mounting substrate  21  on the right has a width sufficient to hold 17 columns of light emitting modules MJ. This also applies to the mounting substrates  21  in the third row from the top. Of the two mounting substrates  21  in the second row from the top, the one on the left has a width sufficient to hold 10 columns of light emitting modules MJ, while the one on the right has a width sufficient to hold 12 columns of light emitting modules MJ. This also applies to the mounting substrates  21  in the fourth row from the top. 
         [0088]    Gaps at the boundaries between the mounting substrates  21  do not align straight in the rectangular mounting-substrate-layout region  41   a  end to end in the short-side direction thereof, and thus, at least in this direction, a shadow that is so long as to be undesirably noticeable does not appear in the diffusion plate  43 . On the other hand, since the mounting substrates  21  are laid out in the rectangular mounting-substrate-layout region  41   a , the amount of light that a planar light source is required to cover can be obtained all over the rectangular mounting-substrate-layout region  41   a.    
         [0089]    In a mounting-substrate layout according to a third embodiment shown in  FIG. 6 , a total of eight mounting substrates  21  are laid out in a rectangular mounting-substrate-layout region  41   a . On each of the mounting substrates  21 , four light emitting modules MJ are arranged in the column direction, that is, in the Y arrow direction shown in  FIG. 1 . As to the row direction, that is, the X arrow direction shown in  FIG. 1 , the mounting substrate  21  on the left in the first row from the top in  FIG. 6  has a width sufficient to hold five columns of light emitting modules MJ, while the mounting substrate  21  on the right has a width sufficient to hold 17 columns of light emitting modules MJ. This also applies to the mounting substrates  21  in the third row from the top. Of the two mounting substrates  21  in the second row from the top, the one on the left has a width sufficient to hold 17 columns of light emitting modules MJ, while the one on the right has a width sufficient to hold five columns of light emitting modules MJ. This also applies to the mounting substrates  21  in the fourth row from the top. 
         [0090]    In the same manner as in the second embodiment, in the mounting-substrate layout according to the third embodiment, gaps at the boundaries between the mounting substrates  21  do not align straight in the rectangular mounting-substrate-layout region  41   a  end to end in the short-side direction thereof, and thus, at least in this direction, a shadow that is so long as to be undesirably noticeable does not appear in the diffusion plate  43 . On the other hand, since the mounting substrates  21  are laid out in the rectangular mounting-substrate-layout region  41   a , the amount of light that a planar light source is required to cover can be obtained all over the rectangular mounting-substrate-layout region  41   a.    
         [0091]    In a mounting-substrate layout according to a fourth embodiment shown in  FIG. 7 , a total of eight mounting substrates  21  are laid out in a rectangular mounting-substrate-layout region  41   a . On each of the mounting substrates  21 , four light emitting modules MJ are arranged in the column direction, that is, in the Y arrow direction shown in  FIG. 1 . As to the row direction, that is, the X arrow direction shown in  FIG. 1 , the mounting substrate  21  on the left in the first row from the top in  FIG. 7  has a width sufficient to hold five columns of light emitting modules MJ, while the mounting substrate  21  on the right has a width sufficient to hold 17 columns of light emitting modules MJ. This also applies to the mounting substrates  21  in the second row from the top. Of the two mounting substrates  31  in the third row from the top, the one on the left has a width sufficient to hold 17 columns of light emitting modules MJ, while the one on the right has a width sufficient to hold five columns of light emitting modules MJ. This also applies to the mounting substrates  31  in the fourth row from the top. 
         [0092]    In the same manner as in the second embodiment, in the mounting-substrate layout according to the fourth embodiment, gaps at the boundaries between the mounting substrates  21  do not align straight in the rectangular mounting-substrate-layout region  41   a  end to end in the short-side direction thereof, and thus, at least in this direction, a shadow that is so long as to be undesirably noticeable does not appear in the diffusion plate  43 . On the other hand, since the mounting substrates  21  are laid out in the rectangular mounting-substrate-layout region  41   a , the amount of light that a planar light source is required to cover can be obtained all over the rectangular mounting-substrate-layout region  41   a.    
         [0093]    In a mounting-substrate layout according to a fifth embodiment shown in  FIG. 8 , a total of eight mounting substrates  21  are laid out in a rectangular mounting-substrate-layout region  41   a . On each of the mounting substrates  21 , four light emitting modules MJ are arranged in the column direction, that is, in the Y arrow direction shown in  FIG. 1 . As to the row direction, that is, the X arrow direction shown in  FIG. 1 , the mounting substrate  21  on the left in the first row from the top in  FIG. 8  has a width sufficient to hold five columns of light emitting modules MJ, while the mounting substrate  21  on the right has a width sufficient to hold 17 columns of light emitting modules MJ. This also applies to the mounting substrates  21  in the second and third rows from the top. Of the two mounting substrates  21  in the fourth row from the top, the one on the left has a width sufficient to hold 17 columns of light emitting modules MJ, while the one on the right has a width sufficient to hold five columns of light emitting modules MJ. 
         [0094]    In the same manner as in the second embodiment, in the mounting-substrate layout according to the fifth embodiment, gaps at the boundaries between the mounting substrates  21  do not align straight in the rectangular mounting-substrate-layout region  41   a  end to end in the short-side direction thereof, and thus, at least in this direction, a shadow that is so long as to be undesirably noticeable does not appear in the diffusion plate  43 . On the other hand, since the mounting substrates  21  are laid out across the rectangular mounting-substrate-layout region  41   a , the amount of light that a planar light source is required to cover can be obtained all over the rectangular mounting-substrate-layout region  41   a.    
         [0095]    In a mounting-substrate layout according to a sixth embodiment shown in  FIG. 9 , a total of 48 mounting substrates  21  are laid out in a rectangular mounting-substrate-layout region  41   a . The mounting substrates  21  are formed as laterally-long strips, and on each of them, a plurality of light emitting modules MJ are arranged in the row direction, that is, in the X arrow direction shown in  FIG. 1 . The mounting substrates  21  are not necessarily of the same length. As to the three mounting substrates  21  arranged in the first row from the top in  FIG. 9 , the one on the left has a width sufficient to hold five light emitting modules MJ, the one at the center has a width sufficient to hold 12 light emitting modules MJ, and the one on the right has a width sufficient to hold five light emitting modules MJ. As to the three mounting substrates  21  arranged in the second row from the top, the one on the left has a width sufficient to hold seven light emitting modules MJ, the one at the center has a width sufficient to hold eight light emitting modules MJ, and the one on the right has a width sufficient to hold seven light emitting modules MJ. The configurations of the first and second rows are alternately repeated in the rest of the rows. 
         [0096]    Consequently, the mounting-substrate rows, each formed of three mounting substrates  21  arranged laterally side by side, are positioned such that boundaries between the mounting substrates  21  are displaced between any adjacent ones of the mounting-substrate rows, for example, the first and second mounting-substrate rows from the top. As a result, gaps at the boundaries between the mounting substrates  21  do not align straight in the rectangular mounting-substrate-layout region  41   a  end to end in the short-side direction thereof, and thus, at least in this direction, a shadow that is so long as to be undesirably noticeable does not appear in the diffusion plate  43 . On the other hand, since the mounting substrates  21  are laid out in the rectangular mounting-substrate-layout region  41   a , the amount of light that a planar light source is required to cover can be obtained all over the rectangular mounting-substrate-layout region  41   a.    
         [0097]    In a mounting-substrate layout according to a seventh embodiment shown in  FIG. 10 , a total of 32 mounting substrates  21  are laid out in a rectangular mounting-substrate-layout region  41   a . Of the two mounting substrates  21  in the first row from the top, the one on the left has a width sufficient to hold eight light emitting modules MJ, while the one on the right has a width sufficient to hold 14 light emitting modules MJ. Of the two mounting substrates  21  in the second row from the top, the one on the left has a width sufficient to hold 14 light emitting modules MJ, while the one on the right has a width sufficient to hold eight light emitting modules MJ. The configurations of the first and second rows are alternately repeated in the rest of the rows. 
         [0098]    Consequently, mounting-substrate rows, each including two mounting substrates  21  arranged laterally side by side, are positioned such that boundaries between the mounting substrates  21  are displaced between any adjacent ones of the mounting-substrate rows, for example, the first and second mounting-substrate rows from the top. As a result, gaps at the boundaries between the mounting substrates  21  do not align straight in the rectangular mounting-substrate-layout region  41   a  end to end in the short-side direction thereof, and thus, at least in this direction, a shadow that is so long as to be undesirably noticeable does not appear in the diffusion plate  43 . On the other hand, since the mounting substrates  21  are laid out in the rectangular mounting-substrate-layout region  41   a , the amount of light that a planar light source is required to cover can be obtained all over the rectangular mounting-substrate-layout region  41   a.    
         [0099]    The mounting-substrate layouts of the first to seventh embodiments are not meant to limit the scope of the present invention. The total number of mounting substrates  21 , the number of light emitting modules MJ supported by each mounting substrate  21 , the matrix pattern of the light emitting modules MJ, etc. may be set freely. 
         [0100]      FIG. 11  shows an example of the configuration of a television receiver in which the display device  69  is incorporated. A television receiver  89  is arranged such that the display device  69  and a group of control boards  92  are housed in a cabinet composed of a front cabinet  90  and a rear cabinet  91  which are attached to each other, the cabinet being supported by a stand  93 . 
         [0101]    It should be understood that the embodiments specifically described above are not meant to limit the present invention, and that many variations and modifications can be made within the spirit of the present invention. 
       INDUSTRIAL APPLICABILITY 
       [0102]    The present invention is widely applicable to illumination devices incorporating a diffusion plate which is irradiated with light from a light source. The present invention is also widely applicable to display devices incorporating the illumination device, and television receivers provided with the display device. 
       LIST OF REFERENCE SYMBOLS 
       [0000]    
       
         
           
               49  backlight unit 
               41  chassis 
               43  diffusion plate 
             MJ light emitting module 
               21  mounting substrate 
               22  LED 
               24  diffusion lens 
               11  reflection sheet 
               59  liquid crystal display panel 
               69  display device 
               89  television receiver