Abstract:
A light-beam control member according to the present invention includes: an entrance face and an exit face. The exit face includes: a first exit face constituting an inner face of a second concave portion located so as to intersect the central axis; and a second exit face constituting a convex curved face located so as to surround the first exit face. The second exit face has multiple, circular ring-shaped convex portions disposed concentrically about the central axis and projecting in the direction along the central axis. The pitch of the convex portions in the direction perpendicular to the central axis in a cross section including the central axis is constant.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a light flux controlling member, and a light-emitting device, a surface light source device and a display device including the light flux controlling member. 
       BACKGROUND ART 
       [0002]    Some transmission type image display apparatuses such as liquid crystal display apparatuses and sign boards use a direct surface light source device as a backlight. In recent years, direct surface light source devices having a plurality of light emitting elements as the light source are used. In such surface light source devices, a light flux controlling member for controlling the distribution of the light emitted from the light emitting element is disposed over the light emitting element (see, for example, PTL 1). 
         [0003]      FIG. 1  is a sectional view of surface light source device  10  disclosed in PTL 1. As illustrated in  FIG. 1 , surface light source device  10  includes light-emitting device  20 , and light diffusion member  30  disposed on light-emitting device  20  with an air layer therebetween. In addition, light-emitting device  20  includes light emitting element  40 , and a lens (light flux controlling member)  50  disposed on light emitting element  40 . Lens  50  is a condenser lens including incidence surface  51  disposed on light emitting element  40  side, reflecting surface  52  disposed to surround incidence surface  51  and configured to reflect light incident on incidence surface  51 , and emission surface  53  configured to emit light incident on incidence surface  51  and light incident on incidence surface  51  which is reflected by reflecting surface  52 . In addition, in surface light source device  10  disclosed in PTL 1, a light diffusion treatment is performed on incidence surface  51 , reflecting surface  52  or emission surface  53 . A part of light emitted from light emitting element  40  which is incident on incidence surface  51  is reflected by reflecting surface  52  and then emitted from emission surface  53  to the outside. In addition, the other part of light emitted from light emitting element  40  which is incident on incidence surface  51  is emitted from emission surface  53  to the outside without being reflected by reflecting surface  52 . In lens  50  of surface light source device  10  disclosed in PTL 1, incidence surface  51 , reflecting surface  52  or emission surface  53  is subjected to a light diffusion treatment to prevent color unevenness in light diffusion member  30 . 
       CITATION LIST 
     Patent Literature 
     PTL 1 
     Japanese Patent Application Laid-Open No. 2007-005218 
     SUMMARY OF INVENTION 
     Technical Problem 
       [0004]    In surface light source device  10  disclosed in PTL 1, incidence surface  51 , reflecting surface  52  or emission surface  53  of lens  50  is subjected to a light diffusion treatment. Here, in the case where, in lens  50  which has a desired light distribution property in the state where it is subjected to no light diffusion treatment, incidence surface  51 , reflecting surface  52  or emission surface  53  is subjected to a light diffusion treatment, light is scattered on the surface subjected to the light diffusion treatment, and consequently a desired light distribution property cannot be achieved. While lens  50  disclosed in PTL 1 is a so-called condenser lens, the same applies to a so-called diffusion lens which smoothly spreads light emitted from light emitting element  40 . 
         [0005]    In view of this, an object of the present invention is to provide a light flux controlling member which can suppress color unevenness of emission light without performing a light diffusion treatment on an optical surface. In addition, another object of the present invention is to provide a light emitting element, a surface light source device and a display device including the light flux controlling member. 
       Solution to Problem 
       [0006]    A light flux controlling member according to an embodiment of the present invention is disposed such that an optical axis of light emitted from a light emitting element and a central axis of the light flux controlling member coincide with each other, the light flux controlling member being configured to control a distribution of the light emitted from the light emitting element, and including: an incidence surface composed of an internal surface of a first recess and configured to allow incidence of the light emitted from light emitting element, the first recess being disposed on the light emitting element side to intersect the central axis; and an emission surface disposed on a side opposite to the incidence surface to intersect the central axis, and configured to emit light incident on the incidence surface to outside of the light flux controlling member. The emission surface includes a first emission surface composed of an internal surface of a second recess disposed to intersect the central axis, and a second emission surface composed of a protruding curved surface disposed to surround the first emission surface, the second emission surface includes a plurality of annular protrusions concentrically disposed around the central axis and protruded in a direction along the central axis, and a pitch of the protrusions in a direction perpendicular to the central axis is constant in a cross section including the central axis. 
         [0007]    A light-emitting device according to the embodiment of the present invention includes: a light emitting element; and the light flux controlling member. 
         [0008]    A surface light source device according to the embodiment of the present invention includes: the light-emitting device; and a light diffusion member configured to allow light from the light-emitting device to pass therethrough while diffusing the light. 
         [0009]    A display device according to the embodiment of the present invention includes: the surface light source device; and a display member to which light emitted from the surface light source device is applied. 
       Advantageous Effects of Invention 
       [0010]    With the light flux controlling member of the embodiment of the present invention, a desired light distribution can be achieved while suppressing color unevenness. Therefore, according to the present invention, it is possible to provide a surface light source device and a display device which can suppress luminance unevenness and color unevenness. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0011]      FIG. 1  is a sectional view of a surface light source device disclosed in PTL 1; 
           [0012]      FIG. 2A  and  FIG. 2B  illustrate a configuration of a surface light source device according to an embodiment; 
           [0013]      FIG. 3A  and  FIG. 3B  are sectional views illustrating a configuration of the surface light source device according to the embodiment; 
           [0014]      FIG. 4  is a partially enlarged sectional view of  FIG. 3B ; 
           [0015]      FIG. 5A  to  FIG. 5E  illustrate a configuration of a light flux controlling member according to the embodiment; 
           [0016]      FIG. 6A  is a graph showing a cross-sectional shape of the light flux controlling member according to the embodiment, and  FIG. 6B  is a graph obtained by subtracting a measurement result of a cross-sectional shape of a light flux controlling member of a comparative example from a measurement result of a cross-sectional shape of light flux controlling member shown in  FIG. 6A ; 
           [0017]      FIG. 7A  and  FIG. 7B  are partially enlarged views of  FIG. 6A ; 
           [0018]      FIG. 8A  and  FIG. 8B  are partially enlarged views of  FIG. 6A ; 
           [0019]      FIG. 9A  to  FIG. 9F  are photographs of emission surfaces of light flux controlling members A to F; 
           [0020]      FIG. 10A  is a graph showing a cross-sectional shape of light flux controlling member A according to the embodiment, and  FIG. 10B  is a graph obtained by subtracting a measurement result of a cross-sectional shape of light flux controlling member E of the comparative example from a measurement result of the cross-sectional shape of light flux controlling member A; 
           [0021]      FIG. 11A  is a graph showing a cross-sectional shape of light flux controlling member B according to the embodiment, and  FIG. 11B  is a graph obtained by subtracting a measurement result of the cross-sectional shape of light flux controlling member E of the comparative example from a measurement result of the cross-sectional shape of light flux controlling member B; 
           [0022]      FIG. 12A  is a graph showing a cross-sectional shape of light flux controlling member C according to the embodiment, and  FIG. 12B  is a graph obtained by subtracting a measurement result of the cross-sectional shape of light flux controlling member E of the comparative example from a measurement result of the cross-sectional shape of light flux controlling member C; 
           [0023]      FIG. 13A  is a graph showing a cross-sectional shape of light flux controlling member D according to the embodiment, and  FIG. 13B  is a graph obtained by subtracting a measurement result of the cross-sectional shape of light flux controlling member E of the comparative example from a measurement result of the cross-sectional shape of light flux controlling member D; 
           [0024]      FIG. 14A  is a graph showing a luminance on a light diffusion member in the case where light flux controlling member A is used,  FIG. 14B  is a graph showing a chromaticity on the light diffusion member in the case where light flux controlling member A is used, and  FIG. 14C  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member A is used; 
           [0025]      FIG. 15A  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member B is used,  FIG. 15B  is a graph showing a chromaticity on the light diffusion member in the case where light flux controlling member B is used, and  FIG. 15C  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member B is used; 
           [0026]      FIG. 16A  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member C is used,  FIG. 16B  is a graph showing a chromaticity on the light diffusion member in the case where light flux controlling member C is used, and  FIG. 16C  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member C is used; 
           [0027]      FIG. 17A  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member D is used,  FIG. 17B  is a graph showing a chromaticity on the light diffusion member in the case where light flux controlling member D is used, and  FIG. 17C  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member D is used; 
           [0028]      FIG. 18A  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member E is used,  FIG. 18B  is a graph showing a chromaticity on the light diffusion member in the case where light flux controlling member E is used, and  FIG. 18C  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member E is used; 
           [0029]      FIG. 19A  is a graph showing a cross-sectional shape of a light flux controlling member according to a modification of the embodiment, and  FIG. 19B  is a graph obtained by subtracting a designed value of a cross-sectional shape of light flux controlling member E of the comparative example from a designed value of a cross-sectional shape of the light flux controlling member shown in  FIG. 19A ; and 
           [0030]      FIG. 20A  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member F is used,  FIG. 20B  is a graph showing a chromaticity on the light diffusion member in the case where light flux controlling member F is used, and  FIG. 20C  is a graph showing a luminance on the light diffusion member in the case where light flux controlling member F is used. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0031]    In the following, an embodiment of the present invention is described in detail with reference to the accompanying drawings. Here, as a typical example of the surface light source device according to the embodiment of the present invention, a surface light source device suitable for a backlight of a liquid crystal display apparatus will be described. When used with a display member (for example, a liquid crystal panel) to which light from surface light source device is applied, the surface light source device can be used as a display apparatus. 
       [Configuration of Surface Light Source Device and Light-Emitting Device] 
       [0032]      FIG. 2  to  FIG. 4  illustrate a configuration of surface light source device  100  according to the embodiment of the present invention.  FIG. 2A  is a plan view of surface light source device  100 , and  FIG. 2B  is a side view of surface light source device  100 .  FIG. 3A  is a sectional view taken along line A-A of  FIG. 2B , and  FIG. 3B  is a sectional view taken along line B-B of  FIG. 2A .  FIG. 4  is an enlarged sectional view illustrating a part of  FIG. 3B . 
         [0033]    As illustrated in  FIG. 2A  to  FIG. 4 , surface light source device  100  according to the present embodiment includes casing  110 , substrate  120 , a plurality of light-emitting devices  130  and light diffusion member  160 . Substrate  120  is disposed on the bottom plate of casing  110 , and light-emitting devices  130  are disposed on substrate  120  at a constant interval. The top plate of casing  110  is provided with an opening. Light diffusion member  160  is disposed over light-emitting devices  130  to close the opening such that light diffusion member  160  is substantially parallel to substrate  120  and functions as a light emitting surface. The size of light emitting surface is, but not limited to, about 400 mm×about 700 mm, for example. 
         [0034]    Each light-emitting device  130  is fixed on substrate  120 . Each light-emitting device  130  includes light emitting element  131  and light flux controlling member  141 . 
         [0035]    Light emitting element  131  is a light source of surface light source device  100 . Light emitting element  131  is a light-emitting diode (LED) such as a white light-emitting diode for example. 
         [0036]    Light flux controlling member  141  controls the distribution of light emitted from light emitting element  131 . Light flux controlling member  141  is disposed over light emitting element  131  such that the central axis CA of light flux controlling member  141  coincides with optical axis OA of light emitting element  131 . Here, the “the optical axis of the light emitting element” means a central light beam of a stereoscopic light flux from light emitting element  131 . The optical axis of light-emitting device  130  coincides with optical axis OA of light emitting element  131  and central axis CA of light flux controlling member  141  (see  FIG. 4 ). A gap for dissipating the heat emitted from light emitting element  131  to the outside is formed between substrate  120  and light flux controlling member  141 . 
         [0037]    Light flux controlling member  141  is formed by integral shaping. The material of light flux controlling member  141  is not limited as long as light of a desired wavelength can pass therethrough. Examples of the material of light flux controlling member  141  include: light transmissive resins such as polymethylmethacrylate (PMMA), polycarbonate (PC), and epoxy resin (EP); or glass. The shape of controlling member  141  will be separately described in detail. 
         [0038]    Light diffusion member  160  is a plate-shaped member having a light diffusing property, and allows the light emitted from light-emitting device  130  to pass therethrough while diffusing the light. Normally, the size of light diffusion member  160  is substantially the same as that of the member to be irradiated, such as a liquid crystal panel. For example, light diffusion member  160  is formed of a light transmissive resin such as polymethylmethacrylate (PMMA), polycarbonate (PC), polystyrene (PS), and styrene methyl methacrylate copolymerization resin (MS). For the purpose of providing a light diffusing property, minute irregularities are formed on the surface of light diffusion member  160 , or diffusing members such as beads are dispersed in light diffusion member  160 . 
         [0039]    The light emitted from light emitting element  131  is spread in the radial direction (the direction away from central axis CA) by light flux controlling member  141 . This effect is significant in light having a small angle to central axis CA in the light flux emitted from light emitting element  131 . The light emitted from light-emitting device  130  reaches light diffusion member  160 . The light reaching light diffusion member  160  passes through light diffusion member  160  while being diffused. 
       [Configuration of Light Flux Controlling Member] 
       [0040]      FIG. 5A  to  FIG. 5E  illustrate a configuration of light flux controlling member  141  according to the present embodiment.  FIG. 5A  is a plan view of light flux controlling member  141 ,  FIG. 5B  is a front view of light flux controlling member  141 ,  FIG. 5C  is a rear view of light flux controlling member  141 ,  FIG. 5D  is a bottom view of light flux controlling member  141 , and  FIG. 5E  is a sectional view taken along line A-A of  FIG. 5A . 
         [0041]    As illustrated in  FIG. 5A  to  FIG. 5E , light flux controlling member  141  includes incidence surface  142 , rear surface  143 , emission surface  144 , flange part  145  and a plurality of leg parts  146 . 
         [0042]    Incidence surface  142  is the internal surface of first recess  147  which is disposed on light emitting element  131  side at a center portion on the lower side of light flux controlling member  141  so as to intersect central axis CA. Incidence surface  142  allows light emitted from light emitting element  131  to enter light flux controlling member  141  while controlling the travelling direction thereof. The shape of incidence surface  142  is not limited. In the present embodiment, incidence surface  142  is an aspherical surface having a semi-ellipse shape in cross section. In addition, incidence surface  142  is rotationally symmetrical about central axis CA. 
         [0043]    Rear surface  143  is a plane which is located on the lower side (light emitting element  131  side) of light flux controlling member  141  and extends from the opening edge of first recess  147  in the radial direction (a direction perpendicular to central axis CA). On rear surface  143 , a plurality of leg parts  146  are disposed at even intervals. 
         [0044]    Emission surface  144  emits, to the outside, the light having entered light flux controlling member  141  from incidence surface  142  while controlling the travelling direction of the light. Emission surface  144  is disposed on light emitting element  131  side to intersect central axis CA, and protrudes upward (light diffusion member  160  side) relative to flange part  145 . 
         [0045]    Emission surface  144  includes first emission surface  148  located in a predetermined range around central axis CA of light flux controlling member  141 , and second emission surface  149  continuously formed at the periphery of first emission surface  148  (see  FIG. 5E ). 
         [0046]    First emission surface  148  is the internal surface of a second recess protruding downward (to light emitting element  131  side) which is disposed at a position to intersect central axis CA of light flux controlling member  141  (optical axis OA of light emitting element  131 ). In other words, the second recess (first emission surface  148 ) is disposed at a position to intersect central axis CA (optical axis OA of light emitting element  131 ) of emission surface  144  of light flux controlling member  141  formed in a protruding curved shape as a whole. It is to be noted that first emission surface  148  may be formed such that the generatrix from the central axis to the external edge of first emission surface  148  (the shape of first emission surface  148  in the cross section including central axis CA) is a curve which is recessed with respect to the lower side (the light emitting element  131  side) (or protruded upward (to the light diffusion member  160  side)). 
         [0047]    Second emission surface  149  is a curved surface (protruding curved surface) protruding upward (to light diffusion member  160  side) as a whole which is disposed to surround first emission surface  148 . Second emission surface  149  has a protruding shape obtained by cutting out a part of an annular surface as a whole. As described in detail later, a plurality of minute annular protrusions  150  are formed on emission surface  144  (see  FIG. 7A  to  FIG. 8B ). It is to be noted that, although not illustrated in the drawings, emission surface  144  may have a third emission surface around second emission surface  149 . In the cross section including central axis CA, the third emission surface may have a linear shape, or a curved shape. 
         [0048]    In the case of first emission surface  148  whose generatrix from the central axis to the external edge is a curve which is recessed with respect to the lower side (the light emitting element  131  side) (or protruded upward (to the light diffusion member  160  side)), the boundary between first emission surface  148  and second emission surface  149  may be the middle point between the apex of emission surface  144  and the intersection of first emission surface  148  with central axis CA in a direction along central axis CA, and the like. 
         [0049]    Flange part  145  is located between the outer periphery part of emission surface  144  and the outer periphery part of rear surface  143 , and is radially outwardly protruded. Flange part  145  has a substantially annular shape. Flange part  145  may be omitted; however, with flange part  145 , ease of handling and alignment of light flux controlling member  141  is increased. The thickness of flange part  145  is not limited, and is determined in consideration of the desired planar dimension of emission surface  144 , workability of flange part  145  and the like. 
         [0050]    Leg parts  146  are columnar-shaped members protruding downward (to light emitting element  131  side) from rear surface  143  at the outer periphery part of rear surface  143 . Leg parts  146  support light flux controlling member  141  at an appropriate position with respect to light emitting element  131 . 
         [0051]    The light emitted from light emitting element  131  enters light flux controlling member  141  from incidence surface  142 . At this time, with the shape of incidence surface  142 , the light from light emitting element  131  (in particular, the light in a region around optical axis OA) is spread in a radial direction (a direction away from optical axis OA of light emitting element  131 ). The light having entered light flux controlling member  141  is emitted to the outside of emission surface  144 . Also at this time, with the shape of emission surface  144 , the light from light emitting element  131  is further spread in a radial direction (a direction away from optical axis OA of light emitting element  131 ). As a result, light-emitting device  130  emits light smoothly spread in a wide angle range. 
         [0052]    Now emission surface  144  is described in detail.  FIG. 6A  to  FIG. 8B  are graphs for describing emission surface  144 .  FIG. 6A  shows a measurement result of a cross-sectional shape including central axis CA of light flux controlling member  141  according to the present embodiment.  FIG. 6B  shows a result obtained by subtracting a measurement result of a cross-sectional shape including central axis CA of a light flux controlling member of the comparative example provided with no protrusion  150  from a measurement result of a cross-sectional shape including central axis CA of light flux controlling member  141  according to the present embodiment provided with protrusion  150  ( FIG. 6A ).  FIG. 7A  is a partially enlarged view of region a of  FIG. 6A , and  FIG. 7B  is a partially enlarged view of region b of  FIG. 6A .  FIG. 8A  is a partially enlarged view of region c of  FIG. 6A , and  FIG. 8B  is a partially enlarged view of region d of  FIG. 6A . In  FIG. 6A  to  FIG. 8B , the abscissa indicates a distance from central axis CA of light flux controlling member  141  (d 1 ; mm). In  FIG. 6A ,  FIG. 7A ,  FIG. 7B ,  FIG. 8A  and  FIG. 8B , the ordinate indicates a height with respect to the center of first emission surface  148  (h 1 ; mm). In addition, in  FIG. 6B , the ordinate indicates a result (Δh 1 ; mm) obtained by subtracting a measurement result of a cross-sectional shape including central axis CA of the light flux controlling member of the comparative example provided with no protrusion  150  from a measurement result of a cross-sectional shape including central axis CA of light flux controlling member  141  according to the present embodiment ( FIG. 6A ). It is to be noted that, since emission surface  144  is formed to be rotationally symmetrical about central axis CA (optical axis OA),  FIG. 6A  to  FIG. 8B  show a result of only the right half of the cross section including central axis CA. 
         [0053]    As described above, emission surface  144  includes a plurality of minute annular protrusions  150 . That is, an annular recess is formed between two protrusions  150 . Protrusion  150  and the annular recess are smoothly connected with each other, and protrusions  150  and the annular recess are visually recognized as waviness of emission surface  144  in external appearance. Protrusions  150  are formed at at least second emission surface  149 . In the present embodiment, protrusions  150  are formed in the entirety of first emission surface  148  and second emission surface  149 . In comparison with an emission surface on which no protrusion  150  is formed, the direction of light emission is slightly changed with emission surface  144  on which minute protrusions  150  which are visually recognized as waviness are formed. With this configuration, light subjected to color separation at emission surface  144  is moderately mixed together on the illuminated surface. On the other hand, cyclically formed minute protrusions  150  cyclically change the emission direction of light, which is different from scattering (a state where light reaching the minute range is dispersed all directions), and therefore, the light distribution characteristics of the entire light flux controlling member  141  is not substantially changed. As a result, color unevenness can be suppressed while achieving desired light distribution characteristics. 
         [0054]    Minute annular protrusions  150  are concentrically disposed around central axis CA in first emission surface  148  and second emission surface  149 . In addition, protrusions  150  disposed in first emission surface  148  and second emission surface  149  protrude in a direction along central axis CA. As illustrated in  FIG. 6B  and  FIG. 7A  to  FIG. 8B , in plan view of first emission surface  148  and second emission surface  149 , the pitch of protrusions  150  (the distance between vertices (ridgelines) of protrusions  150 ) is constant from the center portion of first emission surface  148  to the outer periphery portion of second emission surface  149 . That is, in the cross section including central axis CA, the pitch of protrusions  150  in a direction perpendicular to central axis CA is constant. The pitch of protrusions  150  in a direction perpendicular to central axis CA is not limited, and is preferably 0.1 to 0.5 mm When the pitch of protrusions  150  is smaller than 0.1 mm, the angle variation of the emission surface in one protrusion  150  is excessively large, and consequently there is a risk that a desired light distribution cannot be achieved. On the other hand, when the pitch of protrusions  150  is greater than 0.5 mm, the angle variation of the emission surface in one protrusion  150  is excessively small, and consequently there is a risk that color unevenness cannot be sufficiently suppressed. In this manner, with protrusions  150  formed at a constant interval in a direction perpendicular to central axis CA in emission surface  144  (first emission surface  148  and second emission surface  149 ), the distribution of light emitted from emission surface  144  can be continuously (successively) changed, and color unevenness on light diffusion member  160  can be suppressed. In addition, with protrusion  150  protruding in a direction along central axis CA, an undercut portion is not formed, and manufacturing of a metal mold is facilitated. 
         [0055]    The height of protrusion  150  is not limited, and is preferably 0.05 mm or smaller. When the height of protrusion  150  is greater than 0.05 mm, the angle variation of the emission surface in one protrusion  150  is excessively large, and consequently there is a risk that a desired light distribution cannot be achieved. It is to be noted that at a position in the radial direction in emission surface  144 , the height of protrusion  150  may gradually decrease as the distance to central axis CA decreases (see the modification described later). That is, it suffices that protrusion  150  is formed only at a position where color unevenness is effectively suppressed. In addition, in the case where protrusion  150  is formed in first emission surface  148 , it is preferable that the apex of protrusion  150  and central axis CA do not intersect each other so that the effect of spreading light in a region around optical axis OA is not impaired. Here, the “height of the protrusion” is the amplitude of waviness formed by the waveform, and the length corresponding to half the distance (interval in the direction parallel to central axis CA passing through the vertex of the protrusion) between the straight line connecting the vertices of the adjacent two protrusions  150 , and the straight line connecting the recess between the two protrusions  150  and the valley bottoms of two recesses on the both sides of the recess, in the cross section including central axis CA. 
       [Experiment 1] 
       [0056]    In Experiment 1, four light flux controlling members  141  which are different from each other in designed pitch of protrusion  150  and/or height of protrusion  150  in the cross section including central axis CA were observed, and the Y-chromaticity value and the luminance distribution on light diffusion member  160  were measured in surface light source devices  100  using respective light flux controlling members A to E. Experiment 1 used a light flux controlling member having a pitch of 0.288 mm and a height of 0.015 mm (hereinafter referred to also as “light flux controlling member A”), a light flux controlling member having a pitch of 0.288 mm and a height of 0.030 mm (hereinafter referred to also as “light flux controlling member B”), a light flux controlling member having a pitch of 0.192 mm and a height of 0.015 mm (hereinafter referred to also as “light flux controlling member C”), and a light flux controlling member having a pitch of 0.192 mm and a height of 0.030 mm (hereinafter referred to also as “light flux controlling member D”) (the above-mentioned numerical values are designed values). Further, for comparison, a light flux controlling member provided with no protrusion  150  (pitch 0 mm and height 0 mm) (hereinafter referred to also as “light flux controlling member E”) was also used in the measurement. 
         [0057]    First, the external shapes of light flux controlling members A to E were observed. In addition, a light flux controlling member in which protrusion  150  is formed only in second emission surface  149  was produced, and the emission surface was observed.  FIG. 9A  is a photograph of the emission surface of light flux controlling member A,  FIG. 9B  is a photograph of the emission surface of light flux controlling member B,  FIG. 9C  is a photograph of the emission surface of light flux controlling member C,  FIG. 9D  is a photograph of the emission surface of light flux controlling member D,  FIG. 9E  is a photograph of the emission surface of light flux controlling member E, and  FIG. 9F  is a photograph of the emission surface of the light flux controlling member in which protrusion  150  is formed only in second emission surface  149 . In the photographs, protrusions  150  can be visually recognized at a portion where illuminating light is reflected (lower left portion). 
         [0058]    As illustrated in  FIG. 9A  to  FIG. 9E , in the produced light flux controlling members A to E, protrusions  150  formed in emission surface  144  were observed as thin lines. In addition, as illustrated in  FIG. 9F , with the broken line as the boundary, presence/absence of protrusion  150  was observed. 
         [0059]    Next, in the cross section including central axis CA, the shapes of emission surfaces  144  of light flux controlling members A to E were measured.  FIG. 10A  shows a measurement result of a cross-sectional shape including central axis CA of the emission surface of light flux controlling member A, and  FIG. 10B  shows a difference in shape of the emission surfaces of light flux controlling member A and light flux controlling member E.  FIG. 11A  shows a measurement result of a cross-sectional shape including central axis CA of the emission surface of light flux controlling member B, and  FIG. 11B  shows a difference in shape of the emission surfaces of light flux controlling member B and light flux controlling member E.  FIG. 12A  shows a measurement result of a cross-sectional shape including central axis CA of the emission surface of light flux controlling member C, and  FIG. 12B  shows a difference in shape of the emission surfaces of light flux controlling member C and light flux controlling member E.  FIG. 13A  shows a measurement result of a cross-sectional shape including central axis CA of the emission surface of light flux controlling member D, and  FIG. 13B  shows a difference in shape of the emission surfaces of light flux controlling member D and light flux controlling member E. In  FIG. 10A to 13B , the abscissa indicates a distance from central axis CA of light flux controlling member  141  (d 2 ; mm). In  FIG. 10A ,  FIG. 11A ,  FIG. 12A  and  FIG. 13A , the ordinate indicates the height with respect to the center of first emission surface  148  (h 2 ; mm). In  FIG. 10B ,  FIG. 11B ,  FIG. 12B  and  FIG. 13B , the ordinate indicates the difference in shape of the emission surfaces of each of light flux controlling members A to D, and light flux controlling member E (Δh 2 ; mm). As illustrated in  FIG. 10A  to  FIG. 13B  (light flux controlling member E is not illustrated), five light flux controlling members A to E which are different from each other in pitch and height in the cross section including central axis CA were prepared. The measurement results show the fact that the heights corresponding to the designed values were not obtained due to working problems. 
         [0060]      FIG. 14A  to  FIG. 18C  are graphs showing the distance from the center of light flux controlling member, and a measurement result of the luminance distribution or the Y-chromaticity value. The measurement of the Y-chromaticity value and the luminance distribution was performed using surface light source device  100  provided with only one light-emitting device  130 . It is to be noted that, in surface light source device  100  used in the measurement, the distance between substrate  120  and light diffusion member  160  was set to 24 mm. 
         [0061]      FIG. 14A  shows a luminance distribution on light diffusion member  160  in the case where light flux controlling member A was used,  FIG. 14B  is a graph showing a relationship between a distance from central axis CA of light flux controlling member A (mm), and a Y-chromaticity value on light diffusion member  160 , and  FIG. 14C  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member A, and a luminance (cd/m 2 ) on light diffusion member  160 .  FIG. 15A  shows a luminance distribution on light diffusion member  160  in the case where light flux controlling member B was used,  FIG. 15B  shows a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member B, and a Y-chromaticity value on light diffusion member  160 , and  FIG. 15C  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member B, and a luminance (cd/m 2 ) on light diffusion member  160 .  FIG. 16A  shows a luminance distribution on light diffusion member  160  in the case where light flux controlling member C was used,  FIG. 16B  a graph showing a relationship between is a distance (mm) from central axis CA of light flux controlling member C, and a Y-chromaticity value on light diffusion member  160 , and  FIG. 16C  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member C, and a luminance (cd/m 2 ) on light diffusion member  160 .  FIG. 17A  shows a luminance distribution on light diffusion member  160  in the case where light flux controlling member D was used,  FIG. 17B  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member D, and a Y-chromaticity value on light diffusion member  160 , and  FIG. 17C  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member D, and a luminance (cd/m 2 ) on light diffusion member  160 .  FIG. 18A  shows a luminance distribution on light diffusion member  160  in the case where light flux controlling member E was used,  FIG. 18B  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member E, and a Y-chromaticity value on light diffusion member  160 , and  FIG. 18C  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member E, and a luminance (cd/m 2 ) on light diffusion member  160 . In  FIGS. 13B and 13C  to  FIGS. 17B and 17C , the abscissa indicates a distance (d 3 ; mm) from central axis CA of light flux controlling member  141  on light diffusion member  160 . In addition, in  FIG. 14B ,  FIG. 15B ,  FIG. 16B ,  FIG. 17B  and  FIG. 18B , the ordinate indicates a Y-chromaticity value on light diffusion member  160  (c). In  FIG. 14C ,  FIG. 15C ,  FIG. 16C ,  FIG. 17C  and  FIG. 18C , the ordinate indicates a luminance (L 1 ; cd/m 2 ) on light diffusion member  160 . 
         [0062]    As indicated with the broken lines in  FIG. 14B ,  FIG. 15B ,  FIG. 16B ,  FIG. 17B , and  FIG. 18B , in the case where light flux controlling members A to D according to the present embodiment were used, color contrast was reduced, and color unevenness was eliminated. In particular, color contrast was significantly reduced, and color unevenness was significantly eliminated in the configuration in which the pitch of protrusions  150  is set to a large value (light flux controlling member A and B) in comparison with the configuration in which the pitch of protrusions  150  is set to a small value. From a study in light of the measurement results shown in  FIG. 10B ,  FIG. 11B ,  FIG. 12B  and  FIG. 13B , the effect of the pitch cannot be confirmed by comparison between configurations having the same actual height of protrusion  150 . The reason that light flux controlling member B achieves the effect of reducing color unevenness in comparison with light flux controlling member D may possibly be not by the difference in pitch, but by the difference in height of protrusion  150  in the actual product. However, with the actual pitch and height of protrusion  150  in light flux controlling members A to D according to the present embodiment, the effect of reducing color unevenness was confirmed. It is to be noted that, although not illustrated in the drawings, even with a light flux controlling member in which protrusions  150  having a low height are formed at a small pitch only in second emission surface  149 , color contrast was reduced, and color unevenness was eliminated. Further, as can be understood from comparison among  FIGS. 14A  to  FIG. 17C  and  FIGS. 18A to 18C , even with light flux controlling members A to D according to the present embodiment, a luminance distribution similar to that of light flux controlling member E of the comparative example was obtained. 
       [Effect] 
       [0063]    Light flux controlling member  141  according to the present embodiment is provided with a plurality of minute annular protrusions  150  formed in emission surface  144  at a constant pitch in a direction perpendicular to central axis CA and protruded in a direction along central axis CA, and therefore can eliminate color unevenness by slightly changing the emission direction of the light emitted from light emitting element  131  while spreading the light as with a light flux controlling member provided with no protrusion  150 . In addition, since light flux controlling member  141  according to the present embodiment is provided with no undercut portion, a metal mold for manufacturing light flux controlling member  141  can be readily produced. 
         [0064]    On the other hand, when a light diffusion treatment is performed on emission surface  53  in lens  50  (light flux controlling member) disclosed in PTL 1, light emitted from emission surface  53  is scattered in all directions. At this time, regarding light travelling in a lateral direction from emission surface  53 , the distance to light diffusion member  30  is long, and has only a small influence on color unevenness in light diffusion member  30  of light-emitting device  20 . On the other hand, regarding light travelling directly upward from emission surface  53 , the distance to light diffusion member  30  is short, and has a large influence on color unevenness in light diffusion member  30  of light-emitting device  20 . That is, even when a light diffusion treatment is performed on emission surface  53 , lens  50  disclosed in PTL 1 causes color unevenness on light diffusion member  30  of light-emitting device  20 . In addition, when it is assumed that emission surface  53  has a spherical surface or an aspherical surface as in the present invention, a light diffusion treatment cannot be uniformly performed even when a blast process is performed in a direction along central axis CA. 
         [0065]    In addition, when incidence surface  51  or reflecting surface  52  is subjected to a light diffusion treatment in lens  50  disclosed in PTL 1, an undercut portion is formed, and the structure of lens  50  for manufacturing a metal mold is complicated. 
       [Modification] 
       [0066]    Next, a light flux controlling member according to a modification of the present embodiment is described. The light flux controlling member according to the modification is different from light flux controlling member  141  according to the embodiment in that the height of the protrusion decreases as the distance to central axis CA decreases. In view of this, the components same as those of light flux controlling member  141  according to the present embodiment are denoted with the same reference numerals, and description thereof is omitted, and only components different from those of light flux controlling member  141  according to the present embodiment are described. 
         [0067]      FIG. 19A  and  FIG. 19B  are graphs for describing a shape of an emission surface of the light flux controlling member according to the modification.  FIG. 19A  shows a designed value of a cross-sectional shape including central axis CA of the light flux controlling member according to the modification of the present embodiment.  FIG. 19B  shows a result obtained by subtracting a designed value of a cross-sectional shape including central axis CA of light flux controlling member E of the comparative example provided with no protrusion  150  from a designed value of a cross-sectional shape including central axis CA of the light flux controlling member according to the modification of the present embodiment provided with protrusion  150  ( FIG. 19A ). In  FIG. 19A  and  FIG. 19B , the abscissa indicates a distance from central axis CA of the light flux controlling member (d 4 ; mm). In  FIG. 19A , the ordinate indicates a height with respect to the center of the first emission surface (h 4 ; mm). In addition, in  FIG. 19B , the ordinate indicates a difference between a designed value of the emission surface of the light flux controlling member according to the modification of the present embodiment and a designed value of the emission surface of light flux controlling member E of the comparative example (Δh 4 ; mm). It is to be noted that, since the emission surface is formed to be rotationally symmetrical about central axis CA (optical axis OA),  FIG. 19A  shows a result of only the right half of the cross section including central axis CA. 
         [0068]    The emission surface of the light flux controlling member according to the modification includes a first emission surface and a second emission surface. The first emission surface and the second emission surface include a plurality of minute annular protrusions. That is, an annular recess is formed between two annular protrusions. The protrusions and recesses are smoothly connected with each other, and are visually recognized as waviness of the emission surface in external appearance. In the cross section including central axis CA, the protrusions of the first emission surface and the protrusions of the second emission surface have a wavy shape. 
         [0069]    The height of each protrusion decreases toward central axis CA from the external edge of the second emission surface. The degree of the reduction in height of the protrusion is not limited. The height of the protrusion may be reduced in a uniform manner, or may be reduced such that the reduction length is gradually increased, or, may be reduced such that the reduction length is gradually reduced. In addition, the reduction length of the height of the protrusion may be reduced after it is increased. In the present embodiment, the reduction length of the height of the protrusion is small on the external edge side of the second emission surface, and is reduced after being increased toward the central axis. Here, the reduction length is a difference between the height of a certain protrusion and the height of another protrusion internally adjacent to the certain protrusion. It is to be noted that, preferably, at a position where the first emission surface intersects central axis CA, the height of the protrusion is 0. That is, preferably, the first emission surface and central axis CA perpendicularly intersect each other. 
       [Experiment 2] 
       [0070]    In experiment 2, in a surface light source device using light flux controlling member F according to the modification in which the height of the protrusions decreases from the external edge toward the center portion of the second emission surface, a Y-chromaticity value and a luminance distribution on light diffusion member  160  were measured. 
         [0071]      FIG. 20A  to  FIG. 20C  are graphs showing a distance from the center of light flux controlling member F, and a measurement result of a luminance distribution or a Y-chromaticity value. The measurement of the Y-chromaticity value and the luminance distribution was performed with use of a surface light source device including only one light-emitting device. It is to be noted that, in the surface light source device used for the measurement, the distance between substrate  120  and light diffusion member  160  was set to 24 mm. It is to be noted that the reduction length of the height of the protrusions is set to be reduced after being increased from the external edge of the second emission surface side toward the central axis (see  FIG. 19B ). 
         [0072]      FIG. 20A  shows a luminance distribution on light diffusion member  160  in the case where light flux controlling member F is used,  FIG. 20B  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member F and a Y-chromaticity value on light diffusion member  160 , and  FIG. 20C  is a graph showing a relationship between a distance (mm) from central axis CA of light flux controlling member F and a luminance (cd/m 2 ) on light diffusion member  160 . In  FIG. 20B  and  FIG. 20C , the abscissa indicates the distance from central axis CA of the light flux controlling member (d 5 ; mm). In  FIG. 20B , the ordinate indicates a Y-chromaticity value (c) on light diffusion member  160 . In addition, in  FIG. 20C , the ordinate indicates a luminance (L 2 ; cd/m 2 ) on light diffusion member  160 . 
         [0073]    As indicated with the broken line in  FIG. 20B , in the case where light flux controlling member F according to the modification of the present embodiment is used, color contrast was reduced, and color unevenness was further eliminated in comparison with the case where the light flux controlling member  141  according to the embodiment is used. In addition, as can be understood from comparison between  FIGS. 18A to 18C  and  FIGS. 20A to 20C , also with light flux controlling member F according to the modification of the present embodiment, a luminance distribution similar to that of light flux controlling member E of the comparative example was obtained. 
         [0074]    This application is entitled to and claims the benefit of Japanese Patent Application No. 2014-175671 filed on Aug. 29, 2014, and Japanese Patent Application No. 2015-059483 filed on Mar. 23, 2015 the disclosure each of which including the specification, drawings and abstract is incorporated herein by reference in its entirety. 
       INDUSTRIAL APPLICABILITY 
       [0075]    A surface light source device including the light flux controlling member according to the embodiment of the present invention is applicable to a backlight of a liquid crystal display, a sign board, a generally-used illumination apparatus and the like, for example. 
       REFERENCE SIGNS LIST 
       [0000]    
       
           10  Surface light source device 
           20  Light-emitting device 
           30  Light diffusion member 
           40  Light emitting element 
           50  Lens 
           51  Incidence surface 
           52  Reflecting surface 
           53  Emission surface 
           100  Surface light source device 
           110  Casing 
           120  Substrate 
           130  Light-emitting device 
           131  Light emitting element 
           141  Light flux controlling member 
           142  Incidence surface 
           143  Rear surface 
           144  Emission surface 
           145  Flange part 
           146  Leg part 
           147  First recess 
           148  First emission surface 
           149  Second emission surface 
           150  Protrusion 
           160  Light diffusion member 
         CA Central axis of light flux controlling member 
         OA Optical axis of light emitting element