Abstract:
An illuminating apparatus ( 100 ) has: a light source unit ( 140 ), which includes a light emitting element ( 110 ), and a luminous flux control member ( 120 ); a columnar light guide rod ( 160 ), which is composed of a transparent material; and a cover ( 170 ), which is disposed to cover the light source unit ( 140 ) and the light guide rod ( 160 ), said cover being disposed with respect to the light source unit ( 140 ) and the light guide rod ( 160 ) with an air layer therebetween. The light source unit ( 140 ) is disposed such that an output surface ( 126 ) of the luminous flux control member ( 120 ) faces an end surface ( 162 ) of the light guide rod ( 160 ).

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an illumination device which uses a light emitting element as a light source, and can be used in place of fluorescent tubes and the like. 
       BACKGROUND ART 
       [0002]    In recent years, in view of energy saving and environmental conservation, illumination devices (such as LED bulbs and LED fluorescent tubes) using a light-emitting diode (hereinafter also referred to as “LED”) as a light source have been increasingly used as an illumination device in place of electric light bulbs and fluorescent tubes. In generally used LED fluorescent tubes, a plurality of LEDs are linearly disposed on a substrate at a predetermined interval, and a cover is disposed so as to cover the LEDs. However, the conventional LED fluorescent tubes have a problem that bright spots corresponding to LEDs are seen through the cover, and the luminance unevenness is significant. It is conceivable to increase the number of LEDs or reduce the light transmittance of the cover in order to make the bright spots less noticeable; however, such solutions to the problem are not preferable in view of energy saving. In addition, the conventional LED fluorescent tubes have a problem that the light distribution angle is narrow (for example, 150°). 
         [0003]    In order to solve the problems of the conventional LED fluorescent tubes, there has been proposed an illumination device in which an LED is disposed at an end portion of a light emission region (see, for example, PTL 1).  FIG. 1  is a side view illustrating a configuration of the illumination device disclosed in PTL 1. 
         [0004]    As illustrated in  FIG. 1 , illumination device  10  disclosed in PTL 1 includes LED  12  serving as a light source, round-columnar optical member  14  made of a transparent material, cylindrical cover  16  made of a light transmissive material, and handle part  18  made of a non-light transmissive material. LED  12  is disposed in such a manner that it faces an end surface of round columnar optical member  14 . The outer peripheral surface of optical member  14  is roughened in order to provide a light expansion function. In handle part  18 , LED  12 , a driving section, a power source, and the like are housed. 
         [0005]    In illumination device  10  disclosed in PTL 1, light emitted from LED  12  enters round columnar optical member  14  from an end surface of optical member  14 . Part of the light having entered optical member  14  is output to the outside while being diffused at the outer peripheral surface of optical member  14 . The light output from the outer peripheral surface of optical member  14  is transmitted through cover  16  and then output to the outside (see  FIG. 1 ). In the case of illumination device  10  disclosed in PTL 1, LED  12  is disposed at the end portion of the light emission region, and therefore the bright spot corresponding to LED  12  is not seen through cover  16 . In addition, since the light is output in all directions from outer peripheral surface of optical member  14 , the light distribution angle is wide. 
       CITATION LIST 
     Patent Literature 
       [0006]    PTL 1 
         [0007]    Japanese Patent Application Laid-Open No. 2009-169157 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0008]    However, illumination device  10  disclosed in PTL 1 has a problem that the region around LED  12  cannot be used as effective light emission region  20 . As illustrated in  FIG. 1 , in illumination device  10  disclosed in PTL 1, the region corresponding to cover  16  serves as effective light emission region  20 , and the region corresponding to handle part  18  serves as non-light emission region  22 . Thus the region around LED  12  cannot be used as effective light emission region  20 . If LED  12  is covered by light transmissive cover  16 , not by handle part  18 , the luminance in the region around LED  12  becomes significantly high in comparison with the other regions, and thus luminance unevenness is caused. For this reason, in illumination device  10  disclosed in PTL 1, covering LED  12  by cover  16  is not realistic. 
         [0009]    In addition, illumination device  10  disclosed in PTL 1 has another problem that luminance unevenness is caused in effective light emission region  20 . As illustrated in  FIG. 1 , in illumination device  10  disclosed in PTL 1, the distribution of the light emitted from LED  12  is not controlled, and the light enters optical member  14  as it is. As a result, in illumination device  10  disclosed in PTL 1, the amount of light that reaches the opposite end portion of optical member  14  is insufficient, and the luminance is significantly different between the both ends of effective light emission region  20 . 
         [0010]    An object of the present invention is to provide an illumination device which includes a light emitting element, and can achieve both expansion of the effective light emission region and uniformization of the luminance in the effective light emission region. 
       Solution to Problem 
       [0011]    An illumination device of embodiments of the present invention includes: a light guiding member; a light source unit including a light emitting element and a light flux controlling member that controls a distribution of light emitted from the light emitting element, the light source unit being disposed in such a manner that light passed through the light flux controlling member is incident on an end surface of the light guiding member; and a cover disposed in such a manner as to cover at least part of the light source unit and the light guiding member, with an air layer interposed between the cover and at least part of the light source unit and between the cover and the light guiding member, wherein an exterior surface of the cover corresponding to at least part of the light source unit and the light guiding member serves as a light emission region. 
       Advantageous Effects of Invention 
       [0012]    According to the present invention, it is possible to provide an illumination device which can achieve both expansion of the effective light emission region and uniformization of the luminance in the effective light emission region. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0013]      FIG. 1  is a front view illustrating a configuration of an illumination device disclosed in PTL 1; 
           [0014]      FIG. 2  is a perspective view of an illumination device of embodiment 1; 
           [0015]      FIG. 3A  is a plan view of the illumination device of embodiment 1, and  FIG. 3B  is a side view of the illumination device of embodiment 1; 
           [0016]      FIG. 4  is a sectional view taken along line A-A and line B-B of  FIG. 3B . 
           [0017]      FIG. 5  is a partially enlarged sectional view of the part shown by the broken line in  FIG. 4 ; 
           [0018]      FIGS. 6A and 6B  are schematic views illustrating light paths in the part shown by the broken line in  FIG. 4 ; 
           [0019]      FIG. 7  is a schematic view for describing a method for determining light distribution characteristics; 
           [0020]      FIG. 8  is a graph of the light distribution characteristics of the illumination device of embodiment 1; 
           [0021]      FIG. 9  is a sectional view of an illumination device of embodiment 2; 
           [0022]      FIG. 10  is a schematic view illustrating light paths in the part shown by the broken line in  FIG. 9 ; 
           [0023]      FIG. 11  is a graph of the light distribution characteristics of the illumination device of embodiment 2; 
           [0024]      FIG. 12A  is a sectional view of an illumination device of embodiment 3, and  FIG. 12B  is a partially enlarged sectional view of the part shown by the broken line in  FIG. 12A ; 
           [0025]      FIG. 13A  is a plan view of an illumination device of embodiment 4, and  FIG. 13B  is a side view of the illumination device of embodiment 4; and 
           [0026]      FIGS. 14A to 14D  are sectional views of the illumination device of embodiment 4. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0027]    Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following description explains an illumination device which can be used in place of fluorescent tubes, as a typical example of the illumination device of the embodiments of the present invention. 
       Embodiment 1 
     Configuration of Illumination Device 
       [0028]      FIG. 2  is a perspective view of illumination device  100  of embodiment 1 of the present invention.  FIG. 3A  is a plan view of illumination device  100 , and  FIG. 3B  is a side view of illumination device  100 .  FIG. 4  is a sectional view taken along line A-A and line B-B of  FIG. 3B .  FIG. 5  is a partially enlarged sectional view of the region shown by the broken line in  FIG. 4 . 
         [0029]    As illustrated in  FIG. 2  to  FIG. 5 , illumination device  100  includes light emitting element  110 , light flux controlling member  120 , holder  130 , heat sink  150 , light guiding rod  160  and cover  170 . Light emitting element  110 , light flux controlling member  120  and holder  130  function as light source unit  140 . 
         [0030]    Light emitting element  110  is a light source of illumination device  100  and is disposed on a substrate mounted on heat sink  150  (see  FIG. 5 ). Light emitting element  110  is composed of, for example, a light-emitting diode (LED) such as a white light-emitting diode. The substrate is made of, for example, a metal having a high thermal conductivity such as aluminum and copper. 
         [0031]    Light flux controlling member  120  controls the travelling direction of light emitted from light emitting element  110 . That is, light flux controlling member  120  controls the distribution (diffusion) of the light emitted from light emitting element  110 . Light flux controlling member  120  is disposed in such a manner that its central axis matches the optical axis of light emitting element  110  (see  FIG. 5 ). As illustrated in  FIG. 5 , light flux controlling member  120  includes incidence surface  122  on which light emitted from emitting element  110  is incident, total reflection surface  124  that totally reflects part of the light incident on incidence surface  122 , and emission surface  126  that emits part of the light incident on incidence surface  122  and the light reflected by total reflection surface  124 . Therefore, most of the light emitted from emitting element  110  enters light flux controlling member  120  from incidence surface  122 , and is emitted out of light flux controlling member  120  from emission surface  126 . 
         [0032]    Incidence surface  122  is an internal surface of a recess formed on the bottom side of light flux controlling member  120 . Incidence surface  122  is so formed as to face light emitting element  110  and to intersect with the central axis of light flux controlling member  120 . Incidence surface  122  is a rotationally symmetrical surface symmetrical about the central axis of light flux controlling member  120 . 
         [0033]    Total reflection surface  124  is a surface which extends from the outer edge of the bottom of light flux controlling member  120  to the outer edge of emission surface  126 , and reflects the light incident on incidence surface  122  to emission surface  126 . Total reflection surface  124  is a rotationally symmetrical surface that is so formed as to surround the central axis of light flux controlling member  120 . The diameter of total reflection surface  124  gradually increases from incidence surface  122  side (bottom side) toward emission surface  126  side. The generatrix of total reflection surface  124  is an arc-like curve protruding outward (away from the central axis) (see  FIG. 5 ). 
         [0034]    Emission surface  126  is located on the side opposite to incidence surface  122  (bottom) in light flux controlling member  120 , and is so formed as to intersect with the central axis of light flux controlling member  120 . Emission surface  126  is a circular plane centered at the central axis of light flux controlling member  120 . Emission surface  126  has a diameter substantially the same as that of end surface  162  of light guiding rod  160 . 
         [0035]    As described above, while the light having entered light flux controlling member  120  is basically output from emission surface  126 , part of the light having entered light flux controlling member  120  may be output from the side surface of light flux controlling member  120  (see  FIG. 6A ). Such a configuration can actively make up the shortage of the light around light source unit  140 . Part of the light having entered light flux controlling member  120  can be output from the side surface of light flux controlling member  120  by, for example, providing a cutout to a side part of light flux controlling member  120 , roughening part of a side surface of light flux controlling member  120 , or forming part of the side surface of light flux controlling member  120  as a refracting surface instead of the total reflection surface. 
         [0036]    Light flux controlling member  120  is formed by integral molding. The material of light flux controlling member  120  is not particularly limited as long as light having the desired wavelengths can be transmitted through light flux controlling member  120 . Examples of the material of light flux controlling member  120  include light transmissive resins such as polymethylmethacrylate (PMMA), polycarbonate (PC), and epoxy resin (EP); and glass. 
         [0037]    Holder  130  surrounds light emitting element  110  and light flux controlling member  120 , and sets the position of light flux controlling member  120  in such a manner that the central axis of light flux controlling member  120  matches the optical axis of light emitting element  110 . The material of holder  130  is not particularly limited. Examples of the material of holder  130  include: resins such as polymethylmethacrylate (PMMA), polycarbonate (PC), and epoxy resin (EP); glass; and a metal such as aluminum. As described later, holder  130  may have light transmissivity, or light reflectivity. Holder  130  having light transmissivity can be manufactured by using a light transmissive material (a transparent resin or glass). In addition, a light diffusing property can be provided to light transmissive holder  130  by adding a diffusing member such as beads in the light transmissive material, or by performing a light expansion treatment (for example, roughening treatment) on the internal surface or external surface of manufactured holder  130  manufactured by using light transmissive materials. On the other hand, holder  130  having light reflectivity can be manufactured by using a light reflective material (white resin or glass, or metal). It is also possible to paint the surface of holder  130  manufactured by using various kinds of materials, with a light reflective coating material (for example, white paint). 
         [0038]    Light source unit  140  is disposed in such a manner that light having passed through light flux controlling member  120  is incident on end surface  162  of light guiding rod  160  described later. To be more specific, light source unit  140  is disposed in such a manner that emission surface  126  of light flux controlling member  120  faces end surface  162  of light guiding rod  160  (see  FIG. 5 ). Thus, most of the light emitted from emission surface  126  of light emitting element  110  enters light guiding rod  160  from end surface  162 . In view of efficiently leading light to enter light guiding rod  160 , emission surface  126  and end surface  162  are preferably in contact with each other. Holder  130  also has a function to set the position of light guiding rod  160  in such a manner that end surface  162  of light guiding rod  160  faces emission surface  126  of light flux controlling member  120 . 
         [0039]    Heat sinks  150  are disposed on the both ends of illumination device  100 , and have a function to cool light emitting element  110 . In addition, in heat sink  150 , a circuit that connects light emitting element  110  with an external power source circuit is formed. Heat sink  150  is manufactured by using, for example, a metal having high thermal conductivity such as aluminum and copper. 
         [0040]    Light guiding rod  160  is a light transmissive light guiding member having a columnar shape. Light guiding rod  160  allows the light whose light distribution has been controlled by light flux controlling member  120  to enter thereto from end surface  162 . That is, end surface  162  of light guiding rod  160  functions as an incidence surface. The light having entered light guiding rod  160  travels in light guiding rod  160  by a predetermined distance, and is then output from outer peripheral surface (side surface)  164  of light guiding rod  160 . That is, outer peripheral surface  164  of light guiding rod  160  functions as an emission surface. 
         [0041]    While light guiding rod  160  has a round columnar shape in the present embodiment, the shape of light guiding rod  160  is not particularly limited as long as light guiding rod  160  has a columnar shape which includes end surface  162  and outer peripheral surface  164 . For example, light guiding rod  160  may have a rectangular columnar shape. In addition, the length and thickness of light guiding rod  160  is appropriately set in accordance with the usage, the strength of light emitted from light emitting element  110 , and the like. 
         [0042]    Light guiding rod  160  is formed by, for example, injection molding, extrusion molding, casting molding, and the like. The material of light guiding rod  160  is not particularly limited as long as light having the desired wavelengths can pass through light guiding rod  160 . Examples of the material of light guiding rod  160  include light transmissive resins such as polymethylmethacrylate (PMMA), polycarbonate (PC), and epoxy resin (EP); and glass. In addition, a diffusing member such as beads may be dispersed in the above-mentioned light transmissive resins or glass. By dispersing a diffusing member in light guiding rod  160  at a suitable density, a forward diffusing property can be provided to light guiding rod  160  (see  FIG. 6A  and  FIG. 6B ). In addition, a light expansion treatment (for example, roughening treatment) may be performed on outer peripheral surface  164  of light guiding rod  160 . 
         [0043]    By cover  170 , the light emitted from outer peripheral surface  164  of light guiding rod  160  is transmitted to the outside while being diffused. In addition, by cover  170 , the light which has not entered light guiding rod  160  but has reached cover  170  is transmitted to the outside while being diffused (see  FIG. 6A ). Cover  170  is disposed in such a manner as to cover at least part of light source unit  140  and light guiding rod  160 , or more correctly, to cover at least part of a side surface of light source unit  140  and outer peripheral surface  164  of light guiding rod  160 , with an air layer interposed between cover  170  and at least part of light source unit  140  and between cover  170  and light guiding rod  160 . Thus, the air layer exists not only between light guiding rod  160  and cover  170 , but also between light source unit  140  and cover  170  (see  FIG. 5 ). The thickness of the air layer is not particularly limited as long as the light emitted from outer peripheral surface  164  of light guiding rod  160  can travel between light source unit  140  and cover  170 . The exterior surface of cover  170  corresponding to at least part of light source unit  140  and light guiding rod  160  serves as an effective light emission region. The term “effective light emission region” refers to a light emission region that satisfies the quality required for illumination device  100 . 
         [0044]    The shape of cover  170  is not particularly limited as long as it can cover light source unit  140  and light guiding rod  160  with the air layer therebetween. For example, while cover  170  has a cylindrical form in the present embodiment, cover  170  may has a substantially cylindrical form as described in embodiment 4. 
         [0045]    The material of cover  170  is not particularly limited as long as the material has light transmissivity. Examples of the material of cover  170  include light transmissive resins such as polymethylmethacrylate (PMMA), polycarbonate (PC), polystyrene (PS), and styrene methyl methacrylate copolymerization resin (MS). In addition, the method for providing cover  170  with a light expansion function is not particularly limited. For example, a light expansion treatment (for example, roughening treatment) may be performed on the internal surface or external surface of cover  170 , or a diffusing member such as beads may be dispersed in the above-mentioned light transmissive resins. 
         [0046]      FIGS. 6A and 6B  are schematic views illustrating light paths during use, in the region shown by the broken line in  FIG. 4 .  FIG. 6A  is a schematic view of illumination device  100  in a mode where light is emitted also from the side surface of light flux controlling member  120 . In this mode, holder  130  has light transmissivity. On the other hand,  FIG. 6B  is a schematic view of illumination device  100  in a mode where light is emitted only from emission surface  126  of light flux controlling member  120 . In this mode, holder  130  has light reflectivity. 
         [0047]    As illustrated in  FIG. 6A  and  FIG. 6B , the light emitted from light emitting element  110  enters light flux controlling member  120 . The distribution of the light having entered light flux controlling member  120  is controlled such that the light reaches light guiding rod  160  with a good balance in the range from the near end to the other end of light guiding rod  160 , and then the light is emitted from emission surface  126  toward end surface  162  of light guiding rod  160 . The light emitted from emission surface  126  enters light guiding rod  160 . The light having entered light guiding rod  160  travels in light guiding rod  160 , and then emitted from outer peripheral surface  164 . As illustrated in  FIG. 6A  and  FIG. 6B , the light emitted from outer peripheral surface  164  is transmitted through the air layer, and then reaches the internal surface of cover  170 . At this time, since the air layer is provided also between light source unit  140  (holder  130 ) and cover  170 , the light emitted from outer peripheral surface  164 , the light reflected on the internal surface of cover  170  and the like reach the internal surface of cover  170  also in a region near light source unit  140  (light emitting element  110 ). The light having reached the internal surface of cover  170  is transmitted through cover  170  while being diffused. As a result, the light is substantially uniformly emitted from the entirety of the exterior surface of cover  170 . 
         [0048]    In the exemplary case illustrated in  FIG. 6A , part of the light having entered light flux controlling member  120  is emitted from the side surfaces, not from emission surface  126 , and is then transmitted through holder  130  before reaching the internal surface of cover  170 . Also, part of the light emitted from emission surface  126  leaks from the location between emission surface  126  and end surface  162  toward cover  170 . As illustrated in  FIG. 6A , by utilizing such pieces of light, the shortage of light around light source unit  140  (light emitting element  110 ) can be actively made up. On the other hand, in the exemplary case illustrated in  FIG. 6B , the outside surface of holder  130  has light reflectivity. Thus, light having reached the region around light source unit  140  does not enter holder  130 . With this configuration, the loss of light around light source unit  140  can be prevented. 
         [0049]    In the present embodiment, in the case where light guiding rod  160  in which a diffusing member is dispersed is used, when the density of the diffusing member is increased, the amount of forward diffusing light decreases whereas the amount of rearward diffusing light increases. In addition, in the case where light guiding rod  160  in which the surface of outer peripheral surface  164  is roughened is used, when the roughness of the surface increased, the amount of the rearward diffusing light increases. Since the rearward diffusing light tends to reach the air layer between light source unit  140  and cover  170 , the shortage of light around light source unit  140  (light emitting element  110 ) can be further made up by increasing the amount of the rearward diffusing light. It should be noted that, since the amount of the light which reaches the opposite end portion of light guiding rod  160  (the end portion at which light source unit  140  is not disposed) decreases as the amount of the rearward diffusing light increases, the increase of the amount of the rearward diffusing light is not suitable for light guiding rod  160  in which the light guiding distance is long. 
       (Evaluation Test) 
       [0050]    The light distribution characteristics of illumination device  100  of embodiment 1 were evaluated.  FIG. 7  is a schematic view illustrating a method for determining the light distribution characteristics. As illustrated in  FIG. 7 , an illuminometer was disposed at a location (reference position 0°) just above (in the direction orthogonal to the longitudinal direction) the center of illumination device  100  at a distance of 1 m. The illuminance was measured in such a manner that the position of the illuminometer is changed at intervals of 2 degrees in the right-handed rotation (+θ direction) from the reference position to the point of 90 degrees, about the center of illumination device  100  as the rotational center. Likewise, the illuminance was measured in such a manner that the position of the illuminometer is changed at the intervals of 2 degrees in the left-handed rotation (−θ direction) from the reference position to the point of −90 degrees. The points of 90 degrees and −90 degrees are located on the extended line of light guiding rod  160  in the longitudinal direction. As illustrated in  FIG. 7 , of the two end portions of illumination device  100 , light source unit  140  is disposed on the end portion on the −90 degrees side. 
         [0051]    In this test, the light distribution characteristics of illumination device  100  including the following light transmissive holder  130 , light guiding rod  160  and cover  170  were determined. 
       &lt;Holder&gt; 
       [0052]    Material: polycarbonate 
       &lt;Light Guiding Rod&gt; 
       [0053]    Length: 300 mm 
         [0054]    Thickness: 4 mm, 8 mm or 16 mm 
         [0055]    Material: acrylic resin (added with 1 wt % of silicone particles having a mean particle diameter of 6.8 μm) 
       &lt;Cover&gt; 
       [0056]    Outer diameter: 26 mm 
         [0057]    Thickness: 1 mm 
         [0058]    Material: acrylic resin 
         [0059]      FIG. 8  is a graph of the light distribution characteristics of illumination device  100  of embodiment 1. The lines of white marks represent the light distribution characteristics of the case where cover  170  is not mounted, and the lines of black marks represent the light distribution characteristics of the case where cover  170  is mounted. Round marks (∘, •) represent the illuminance value of illumination device  100  including light guiding rod  160  having a diameter of 4 mm, quadrangular marks (⋄, ♦) represent the illuminance value of illumination device  100  including light guiding rod  160  having a diameter of 8 mm, and triangular marks (Δ, ▴) represent the illuminance value of illumination device  100  including light guiding rod  160  having a diameter of 16 mm. 
         [0060]    In  FIG. 8 , the measurement results (white marks) of the state where cover  170  is not mounted show that light guiding rod  160  used in the test has a forward diffusing property (the peaks of the illuminance are at about 50 to 60 degrees). On the other hand, the measurement results (black marks) of the state where cover  170  is mounted show that well-balanced light distribution characteristics can be achieved with cover  170  (the peaks of the illuminance are at about 0 degree). 
         [0061]    In addition, the external appearance of illumination device  100  was observed with the naked eye during the determination of the light distribution characteristics, and the entirety of cover  170  was bright, and almost no luminance unevenness was observed. 
       (Effect) 
       [0062]    Since illumination device  100  of embodiment 1 has a configuration in which the light from light emitting element  110  disposed at the end portion is guided by light guiding rod  160 , the bright spot corresponding to light emitting element  110  is not seen through cover  170 . In addition, since light is emitted from outer peripheral surface  164  of light guiding rod  160  in all directions, the light distribution angle is wide. 
         [0063]    In addition, since the air layer is provided between light source unit  140  and cover  170  so that the light reaches the internal surface of cover  170  also in a region near light source unit  140  in illumination device  100  of embodiment 1, the region near light source unit  140  can be used as the effective light emission region. Further, since the distribution of the light emitted from light emitting element  110  is adjusted by light flux controlling member  120  in illumination device  100  of embodiment 1, it is possible to prevent the luminance in the region near light source unit  140  from being excessively increased, and to prevent the luminance in the region distanced from light source unit  140  from being significantly decreased. 
         [0064]    As described above, illumination device  100  of embodiment 1 can achieve both expansion of effective light emission region and uniformization of the luminance in the effective light emission region. 
       Embodiment 2 
     Configuration of Illumination Device 
       [0065]      FIG. 9  is a sectional view illustrating a configuration of illumination device  200  of embodiment 2 of the present invention. Illumination device  200  of embodiment 2 is different from illumination device  100  of embodiment 1 in that light source unit  140  (first light source unit  140   a  and second light source unit  140   b ) is disposed at the both ends of light guiding rod  160 . Here, the same components as those of illumination device  100  of embodiment 1 are denoted by the same reference numerals, and the descriptions thereof are omitted. 
         [0066]    First light source unit  140   a  includes first light emitting element  110   a , first light flux controlling member  120   a  and first holder  130   a . Likewise, second light source unit  140   b  includes second light emitting element  110   b , second light flux controlling member  120   b  and second holder  130   b . First light source unit  140   a  is disposed in such a manner that it faces first end surface  162   a  of light guiding rod  160 , and second light source unit  140   b  is disposed in such a manner that it faces second end surface  162   b  of light guiding rod  160 . To be more specific, first light source unit  140   a  is disposed in such a manner that the emission surface of first light flux controlling member  120   a  faces first end surface  162   a  of light guiding rod  160 . In addition, second light source unit  140   b  is disposed in such a manner that the emission surface of second light flux controlling member  120   b  faces second end surface  162   b  of light guiding rod  160 . 
         [0067]    Light guiding rod  160  is the same as light guiding rod  160  of illumination device  100  of embodiment 1. In view of effectively using a pair of light source units  140  (first light source unit  140   a  and second light source unit  140   b ) disposed at the both ends of light guiding rod  160 , light guiding rod  160  preferably has a forward diffusing property. 
         [0068]      FIG. 10  is a schematic view illustrating light paths during use, in the region shown by the broken line in  FIG. 9 . In the mode illustrated in  FIG. 10 , holder  130  has light transmissivity. In addition, light guiding rod  160  has a forward diffusing property. 
         [0069]    As illustrated in  FIG. 10 , in illumination device  200  of embodiment 2, not only the light emitted from first light source unit  140   a , but also the light emitted from second light source unit  140   b  is emitted from outer peripheral surface  164  of light guiding rod  160 . At this time, part of the forward diffusing light derived from the light emitted from first light source unit  140   a  travels between second light source unit  140   b  (holder  130   b ) and cover  170 . In addition, part of the forward diffusing light derived from the light emitted from second light source unit  140   b  travels between first light source unit  140   a  (holder  130   a ) and cover  170 . As a result, in comparison with the case of illumination device  100  of embodiment 1, the amount of light which reaches the internal surface of cover  170  in a region near light source unit  140  is large in illumination device  200  of embodiment 2. 
       (Evaluation Test) 
       [0070]    In the above-described procedure, the light distribution characteristics of illumination device  200  of embodiment 2 were evaluated (see  FIG. 7 ). In this test, the light distribution characteristics of illumination device  200  including the following light transmissive first holder  130   a , second holder  130   b , light guiding rod  160  and cover  170  were determined. 
       &lt;Holder&gt; 
       [0071]    Material: polycarbonate 
       &lt;Light Guiding Rod&gt; 
       [0072]    Length: 300 mm 
         [0073]    Thickness: 4 mm, 8 mm or 16 mm 
         [0074]    Material: acrylic resin (added with 1 wt % of silicone particles having a mean particle diameter of 6.8 μm) 
       &lt;Cover&gt; 
       [0075]    Outer diameter: 26 mm 
         [0076]    Thickness: 1 mm 
         [0077]    Material: acrylic resin 
         [0078]      FIG. 11  is a graph of the light distribution characteristics of illumination device  200  of embodiment 2. The lines of white marks represent the light distribution characteristics of the case where cover  170  is not mounted, and the lines of black marks represent the light distribution characteristics of the case where cover  170  is mounted. Round marks (∘, •) represent the illuminance value of illumination device  100  including light guiding rod  160  having a diameter of 4 mm, quadrangular marks (⋄, ♦) represent the illuminance value of illumination device  200  including light guiding rod  160  having a diameter of 8 mm, and triangular marks (Δ, ▴) represent the illuminance value of illumination device  200  including light guiding rod  160  having a diameter of 16 mm. 
         [0079]    As illustrated in the graph, since light source unit  140  (first light source unit  140   a  and second light source unit  140   b ) is disposed at the both ends of light guiding rod  160  in illumination device  200  of embodiment 2, the peaks of the illuminance were at two points, at about 50 to 60 degrees and about −50 to −60 degrees when cover  170  is not mounted (see white marks). On the other hand, the peaks of the illuminance were at about 0 degree when cover  170  is mounted (see black marks). Comparing the graphs of  FIG. 8  and  FIG. 11 , in the case where illumination device  200  of embodiment 2 is used, the illuminance is more than double in comparison with the case where illumination device  100  of embodiment 1 is used. 
       (Effect) 
       [0080]    Since the amount of the light which reaches a region around light source unit  140  is large, illumination device  200  of embodiment 2 can achieve an effect that the luminance unevenness in the effective light emission region can be further reduced, in addition to the effect of illumination device  100  of embodiment 1. 
       Embodiment 3 
     Configuration of Illumination Device 
       [0081]      FIG. 12A  is a sectional view illustrating a configuration of illumination device  300  of embodiment 3 of the present invention.  FIG. 12B  is a partially enlarged sectional view of the region shown by the broken line in  FIG. 12A . Illumination device  300  of embodiment 3 is different from illumination device  200  of embodiment 2 in that a plurality of prisms  372  are formed on the internal surface of cover  370 . Here, the same components as those of illumination device  200  of embodiment 2 are denoted by the same reference numerals, and the descriptions thereof are omitted. 
         [0082]    Cover  370  has a form which is obtained by rounding a prism sheet having a plurality of prisms  372  formed in parallel to one another on one surface thereof, with prisms  372  facing the inside. The form in cross-section orthogonal to the ridgeline of each prism  372  is a substantially triangular shape. In addition, the circle formed by the ridgeline of each prism  372  is orthogonal to the center line of light guiding rod  160 . Prisms  372  change the travelling directions of the forward diffusing light derived from the light from first light source unit  140   a  and the forward diffusing light derived from the light from second light source unit  140   b , thereby achieving an efficient output of the forward diffusing light. 
       (Effect) 
       [0083]    Since the forward diffusing light emitted from light guiding rod  160  can be efficiently output, illumination device  300  of embodiment 3 can achieve an effect that the luminance can be further increased, in addition to the effect of illumination device  200  of embodiment 2. 
       Embodiment 4 
     Configuration of Illumination Device 
       [0084]      FIG. 13A  is a plan view of illumination device  400  of embodiment 4 of the present invention,  FIG. 13B  is a side view of illumination device  400 .  FIG. 14A  is a sectional view taken along line C-C illustrated in  FIG. 13B ,  FIG. 14B  is a sectional view taken along line D-D illustrated in  FIG. 13B ,  FIG. 14C  is a sectional view taken along line E-E illustrated in  FIG. 13A , and  FIG. 14D  is a sectional view taken along line F-F illustrated in  FIG. 13A . 
         [0085]    Illumination device  400  of embodiment 4 is different from illumination device  200  of embodiment 2 in that illumination device  400  includes two light guiding rods  160  (first light guiding rod  160   a  and second light guiding rod  160   b ), and that light source units  140  (first light source unit  140   a  and second light source unit  140   b , or third light source unit  140   c  and fourth light source unit  140   d ) are disposed at the both ends of each of light guiding rods  160   a  and  160   b . Here, the same component as those of illumination device  200  of embodiment 2 are denoted by the same reference numerals, and the descriptions thereof are omitted. 
         [0086]    As illustrated in  FIG. 13A  to  FIG. 14D , illumination device  400  includes four light source units  140  (first light source unit  140   a , second light source unit  140   b , third light source unit  140   c  and fourth light source unit  140   d ), heat sink  450 , two light guiding rod  160  (first light guiding rod  160   a  and second light guiding rod  160   b ) and cover  470 . Each light source unit  140  includes light emitting element  110 , light flux controlling member  120  and holder  130 . 
         [0087]    Each light source unit  140  is disposed in such a manner that it faces end surface  162  of light guiding rod  160 . To be more specific, first light source unit  140   a  is disposed in such a manner that it faces first end surface  162   a  of first light guiding rod  160   a , and second light source unit  140   b  is disposed in such a manner that it faces first end surface  162   b  of first light guiding rod  160   a . First light source unit  140   a , second light source unit  140   b  and first light guiding rod  160   a  compose a first illumination unit. Likewise, third light source unit  140   c  is disposed in such a manner that it faces first end surface  162   c  of second light guiding rod  160   b , and fourth light source unit  140   d  is disposed in such a manner that it faces second end surface  162   d  of second light guiding rod  160   b . Third light source unit  140   c , fourth light source unit  140   d  and second light guiding rod  160   b  compose a second illumination unit. 
         [0088]    Heat sink  450  is disposed at the both end portions, center portion and bottom portion of illumination device  400 , and has a function to cool light emitting element  110  of each light source unit  140 . In addition, heat sink  450  also has a function to arrange, in series, the first illumination unit (first light source unit  140   a , second light source unit  140   b  and first light guiding rod  160   a ) and the second illumination unit (third light source unit  140   c , fourth light source unit  140   d  and second light guiding rod  160   b ). 
         [0089]    By cover  470 , the light emitted from the outer peripheral surfaces of first light guiding rod  160   a  and second light guiding rod  160   b  is transmitted to the outside while being diffused. Cover  470  is disposed in such a manner as to cover the first illumination unit (first light source unit  140   a , second light source unit  140   b  and first light guiding rod  160   a ) and the second illumination unit (third light source unit  140   c , fourth light source unit  140   d  and second light guiding rod  160   b ), with an air layer interposed between cover  470  and the first illumination unit and between cover  470  and the second illumination unit. In the present embodiment, cover  470  has a substantially cylindrical form (the form which is obtained by cutting out part of a cylindrical form). 
       (Effect) 
       [0090]    By additionally providing the illumination unit (light guiding rod  160  and a pair of light source units  140  disposed at the both ends of light guiding rod  160 ), illumination device  400  of embodiment 4 can achieve an effect that the length can be increased while maintaining the luminance, in addition to the effect of illumination device  200  of embodiment 2. 
       [Modification] 
       [0091]    It is to be noted that, while light source unit  140  has holder  130  in the above-mentioned embodiments, holder  130  is not an essential component of light source unit  140 . For example, the position of light flux controlling member  120  may be set by utilizing a leg part formed in light flux controlling member  120 . 
         [0092]    In addition, while a condenser lens is used as light flux controlling member  120  in the above-mentioned embodiments, light flux controlling member  120  may not be the condenser lens. For example, light flux controlling member  120  may be a reflector that guides the light from light emitting element  110  to end surface  162  of light guiding rod  160 . 
         [0093]    In addition, while light flux controlling member  120 , holder  130  and light guiding rod  160  are separately formed the above-mentioned embodiments, these components may be integrally formed. For example, light flux controlling member  120  and holder  130  may be integrated, or holder  130  and light guiding rod  160  may be integrated. Alternatively, light flux controlling member  120 , holder  130  and light guiding rod  160  may be integrated. 
         [0094]    In addition, the light guiding member included in the illumination device of the embodiments of the present invention is not limited to the round columnar light guiding rod  160  described in the above-mentioned embodiments. For example, the light guiding member may have a substantially round columnar shape, a plate-shape, an annular shape, or the like. 
       INDUSTRIAL APPLICABILITY 
       [0095]    The illumination device of the embodiments of the present invention can be used in place of fluorescent tubes, and is therefore widely applicable to various kinds of illumination devices. 
         [0096]    This application is entitled to and claims the benefit of Japanese Patent Application No. 2012-008902 filed on Jan. 19, 2012, the disclosure of which including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  Illumination device 
           12  LED 
           14  Optical member 
           16  Cover 
           18  Handle part 
           20  Effective light emission region 
           22  Non-light emission region 
           100 ,  200 ,  300 ,  400  Illumination device 
           110  Light emitting element 
           120  Light flux controlling member 
           122  Incidence surface 
           124  Total reflection surface 
           126  Emission surface 
           130  Holder 
           140  Light source unit 
           150 ,  450  Heat sink 
           160  Light guiding rod 
           162  End surface 
           164  Outer peripheral surface 
           170 ,  370 ,  470  Cover 
           372  Prism