Patent Publication Number: US-9429299-B2

Title: Optical member with prisms

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a Continuation of application Ser. No. 14/454,042 filed Aug. 7, 2014, which claims the benefit of Japanese Application No. 2013-184074 filed Sep. 5, 2013. The disclosures of the prior applications are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an illuminating apparatus in which the light distribution can be controlled. 
     2. Description of the Related Art 
     In illuminating apparatuses to be installed outside, various light distribution characteristics are generally required depending on the installation environment. For example, with regard to an illuminating apparatus such as a tunnel lamp or a roadway lamp, different light distribution characteristics may be required for a travelling direction and a width direction of the roadway, and light distribution characteristics that are asymmetrical relative to the reference axis of light distribution may also be required within a specific plane (e.g., a vertical plane parallel to the width direction). 
     As a typical example in which these kind of light distribution characteristics are required, there is a case in which tunnel lamps on an expressway are installed on a wall surface of only one side of the tunnel (e.g., refer to Japanese Patent Application Laid-Open (JP-A) No. 2004-311259 (refer to  FIGS. 3 to 5 )). In general, tunnel lamps on an expressway are required to illuminate the road surface as well as a predetermined range (e.g., within a range of a certain height from the road surface) of the wall surface on both sides within the tunnel in order to reduce driver&#39;s anxiety and the like. In order to satisfy this requirement, tunnel lamps on an expressway are normally installed facing each other on both wall surfaces of the tunnel such that the wall surface on one side (as well as the road surface) is illuminated by the tunnel lamps installed on the other wall surface. However, it is preferable to install tunnel lamps on a wall surface of only one side of the tunnel in terms of the cost of the tunnel lamps and their wiring fixtures, the ease of maintenance, and the like. In response to such problems, in the invention disclosed in JP-A No. 2004-311259, the light distribution characteristics of the tunnel lamps in a vertical plane (cross-section of the tunnel) parallel to the road width direction are configured to be asymmetrical relative to the reference axis of light distribution, thereby illuminating the road surface as well as both wall surfaces within the tunnel so as to satisfy a predetermined illumination standard with tunnel lamps installed on the wall surface of one side. 
       FIG. 12  illustrates the light distribution characteristics of the illuminating apparatus disclosed in JP-A No. 2004-311259. In  FIG. 12 , light distributions a and b within two mutually orthogonal planes including a light distribution reference axis of the illuminating apparatus (hereinafter also referred to as “optical axis of the illuminating apparatus” or simply “optical axis”) C 0  are indicated by a solid line and a dashed line, respectively. Herein, in  FIG. 12 , with regard to the angle around a photometric center O, the angle of the optical axis C 0  is regarded as 0° and the counterclockwise direction is regarded as the positive direction. 
     As illustrated in  FIG. 12 , the light distribution a has peaks in both the positive and negative angular directions, and exhibits an asymmetrical distribution relative to the optical axis C 0 . Thus, in this light distribution, the distribution profile of a distribution A having a peak in the negative angular direction is different from that of a distribution B having a peak in the positive angular direction. In particular, the absolute value of the angle at which the peak occurs and the light intensity at the peak are different in each of the distributions A and B. 
     When using an illuminating apparatus having such light distribution characteristics as tunnel lamps installed on the wall surface of one side within a tunnel, a plane including the light distribution a illustrated in  FIG. 12  corresponds to a vertical plane that is parallel to the width direction of the roadway. Further, the light distribution characteristics of the illuminating apparatus are adjusted according to the predetermined installation position, installation angle, and the like of the illuminating apparatus such that a predetermined range of the side wall on the side on which the illuminating apparatus is installed is illuminated by illumination light corresponding to the distribution A of the light distribution a, and a predetermined range of the side wall on the opposite side with the roadway therebetween is illuminated by illumination light corresponding to the distribution B of the light distribution a which is brighter than the distribution A. Thereby, the road surface as well as both side walls within the tunnel can be illuminated so as to satisfy a predetermined illumination standard. 
     JP-A No. 2004-311259 discloses the following as an illuminating apparatus having the above-described light distribution characteristics: an illuminating apparatus  100  including a straight tube-shaped fluorescent lamp  110  and a reflecting member  112  disposed on the rear side of the fluorescent lamp  110  (refer to  FIG. 13 ). The reflecting member  112  is formed to have an inverse U-shaped cross-section by connecting a first and a second reflecting panel  113  and  114 , whose reflecting surfaces are constituted by a single curved surface, to each other in a continuously integral manner at one end side thereof. Illumination light corresponding to the distribution A is generated when the first reflecting panel  113  reflects light from the fluorescent lamp  110 , whereas illumination light corresponding to the distribution B which is brighter than the distribution A is generated when the second reflecting panel  114 , whose reflecting surface is larger than the reflecting surface of the first reflecting panel  113 , reflects light from the fluorescent lamp  110 . 
     SUMMARY OF THE INVENTION 
     However, the following problems exist in the illuminating apparatus  100  which uses the reflecting member  112  consisting of the reflecting panels  113  and  114  for control of the light distribution of illumination light as disclosed in JP-A No. 2004-311259. First, since the reflecting member  112  is formed to have an inverse U-shaped cross-section, it is difficult to make the illuminating apparatus thin. Further, since the reflecting panels  113  and  114  are normally made of metal panels, the reflectivity of the reflecting panels  113  and  114  is low, and it is difficult to improve the utilization efficiency of light from the light source due to loss of light. In addition, since the reflecting panels  113  and  114  are molded by sheet-metal processing, it is difficult to achieve fine adjustment of the light distribution. 
     The present invention was created in consideration of the above-described problems, and an object thereof is to provide an illuminating apparatus that can easily control the light distribution while remaining thin and highly efficient. 
     The embodiments of the invention described below are examples of the structure of the present invention. In order to facilitate the understanding of the various structures of the present invention, the explanations below are divided into aspects. Each aspect does not limit the technical scope of the present invention, and the technical scope of the present invention can also include structures in which a portion of the components in the aspects below is substituted or deleted, or another component is added upon referring to the best modes for carrying out the invention. 
     According to a first aspect of the present invention, there is provided an illuminating apparatus including: a light source, and an optical member that controls light distribution of light emitted from the light source in a forward direction, wherein a plurality of prisms extending in one direction are provided on at least one among two principal surfaces of the optical member in regions on both sides when divided at a virtual plane that includes a reference axis of the optical member, the plurality of prisms include reflecting prisms that reflect light from a light source that is disposed virtually so as to include an optical axis on the virtual plane that includes the reference axis and emit the light from the optical member, and the light source is disposed such that its optical axis is shifted relative to the reference axis to a region on one side when divided at the virtual plane that includes the reference axis. 
     With this structure, light distribution that is asymmetrical relative to the optical axis of the light source can be realized within a plane that is orthogonal to the direction in which the plurality of reflecting prisms extend. Further, with this structure, this kind of light distribution control is carried out using an optical member in which a plurality of prisms are provided on at least one principal surface thereof, and the plurality of prisms include reflecting prisms. Thus, an illuminating apparatus that is thin and exhibits high efficiency with low loss of light can be realized. In addition, with this structure, the light distribution characteristics of the illuminating apparatus can be finely and easily adjusted based on the arrangement of the optical member and the light source, the optical design of the plurality of prisms provided to the optical member, and the like. 
     According to the first aspect of the present invention, a plurality of prisms disposed near the virtual plane that includes the reference axis are configured as refracting prisms that refract the light from the light source that is disposed virtually so as to include an optical axis on the virtual plane that includes the reference axis and emit the light from the optical member. 
     With this structure, compared to a case in which the plurality of prisms are constituted by only reflecting prisms, the occurrence of stray light caused by reflecting prisms can be suppressed, and in turn the light emitting efficiency can be improved and the controllability of the asymmetrical light distribution portion can be improved. 
     Also, with this structure, among the light distribution that is asymmetrical relative to the optical axis of the light source within a plane that is orthogonal to the direction in which the plurality of reflecting prisms extend, the balance between the amount of light of the primary (larger amount of light) distribution and the amount of light of the secondary (smaller amount of light) distribution can be easily adjusted by the action of the refracting prisms. 
     According to the first aspect of the present invention, a boundary between the reflecting prisms and the refracting prisms provided on a side on which the light source is disposed among the regions on both sides when divided at the virtual plane that includes the reference axis is located more toward the reference axis than the optical axis of the light source. 
     With this structure, when adjusting the balance between the amount of light of the primary (larger amount of light) distribution and the amount of light of the secondary (smaller amount of light) distribution among the light distribution that is asymmetrical relative to the optical axis of the light source within a plane that is orthogonal to the direction in which the plurality of reflecting prisms extend, the illuminating apparatus is particularly advantageous with respect to increasing the ratio of the amount of light of the secondary distribution relative to the amount of light of the primary distribution. 
     According to the first aspect of the present invention, the light source is disposed more toward the optical member than a focal point of the plurality of reflecting prisms. 
     With this structure, the light distribution of light emitted from the light source can be precisely adjusted in a broader range by adjusting the relative distance between the optical member and the light source relative to the focal length of the plurality of prisms. 
     According to the first aspect of the present invention, the plurality of prisms are divided into a plurality of small regions at at least one virtual plane parallel to the virtual plane that includes the reference axis, and one or more prisms disposed in each of the plurality of small regions are configured to have each different focal length than the focal length of the one or more prisms disposed in adjacent small regions. 
     With this structure, the light distribution of illumination light can be more precisely adjusted by adjusting the focal lengths of the one or more prisms disposed in each small region and the distance between the optical member and the light source relative to such focal lengths. 
     According to the first aspect of the present invention, one or more of the reflecting prisms disposed in each of the plurality of small regions included on a side on which the light source is disposed among the regions on both sides when divided at the virtual plane that includes the reference axis are configured such that the focal length thereof decreases as the small regions are distanced from the reference axis. 
     With this structure, the occurrence of stray light caused by the reflecting prisms can be suppressed, and decreases in the emitting efficiency can be reduced. 
     According to the first aspect of the present invention, the plurality of prisms is provided on a principal surface of the optical member that faces the light source, and each of the reflecting prisms includes a first surface that faces the reference axis and a second surface that reflects at least a portion of light that enters from the first surface to the side of the principal surface of the optical member on which the plurality of prisms are not provided. 
     With the structures described above, an illuminating apparatus that can easily control the light distribution while remaining thin and highly efficient can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a side surface view illustrating the essential parts of an illuminating apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a graph illustrating the light distribution characteristics of the illuminating apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a side surface view illustrating the essential parts of an illuminating apparatus according to a second embodiment of the present invention; 
         FIG. 4  is a graph illustrating the light distribution characteristics of the illuminating apparatus illustrated in  FIG. 3 ; 
         FIG. 5  is a side surface view illustrating the essential parts of an illuminating apparatus according to a third embodiment of the present invention; 
         FIG. 6  is a graph illustrating the light distribution characteristics of the illuminating apparatus illustrated in  FIG. 5 ; 
         FIG. 7  is a side surface view illustrating the essential parts of another example of the illuminating apparatus according to the third embodiment of the present invention; 
         FIG. 8  is a graph illustrating the light distribution characteristics of the illuminating apparatus illustrated in  FIG. 7 ; 
         FIG. 9  is a side surface view illustrating the essential parts of an illuminating apparatus according to a fourth embodiment of the present invention; 
         FIG. 10  is a graph illustrating the light distribution characteristics of the illuminating apparatus illustrated in  FIG. 7  together with measurement results using an actual device; 
         FIG. 11  is a side surface view illustrating the essential parts of an alternative embodiment of the illuminating apparatus according to the present invention; 
         FIG. 12  is a graph illustrating the light distribution characteristics of a conventional illuminating apparatus; and 
         FIG. 13  is a side surface view illustrating the essential parts of a conventional illuminating apparatus having the light distribution characteristics illustrated in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the present invention will be explained below referring to the attached drawings. All of the drawings that illustrate the structure of an illuminating apparatus of the present invention ( FIGS. 1, 3, 5, 7, 9, and 11 ) are schematic views that illustrate only the essential parts. Therefore, the illuminating apparatuses according to the embodiments of the present invention can include other constituent elements omitted from the drawings such as an enclosure that retains the illustrated constituent elements therewithin. Also, the relative dimensions of each illustrated portion are intended to exaggerate the features for the purpose of explanation, and do not necessarily reflect an actual reduced scale. 
     An illuminating apparatus  10  according to a first embodiment of the present invention includes a light source  12  and an optical member  14  arranged opposing the light source  12 . In this embodiment, the optical member  14  is a sheet-shaped (thin panel-shaped) member including two principal surfaces  14   a  and  14   b . One principal surface  14   b  is arranged facing the light source  12 . Also, in this embodiment, the optical member  14  is formed in an approximately rectangular shape in a plan view. However, in the present invention, the outer shape of the optical member  14  is not particularly limited as long as it includes a plurality of prisms  15  to be explained later. 
     With regard to the term “sheet-shaped” mentioned above, for example, compared to the similar terms “panel-shaped” and “film-shaped”, it has generally been suggested that a panel, a sheet (thin panel), and a film exhibit decreasing thickness in that order. However, “sheet-shaped” is not always differentiated from terms such as “panel-shaped” and “film-shaped” based on a clear technical meaning with respect to, for example, a thickness in the presence or absence of flexibility. Thus, in the present invention, the term “sheet-shaped” is used as a term that can be appropriately substituted with terms such as “panel-shaped” and “film-shaped” including “thin panel-shaped” in order to merely specifically indicate a shape that has two principal surfaces  14   a  and  14   b.    
     Herein, in the illuminating apparatus  10 , the direction from the light source  12  toward the optical member  14  is referred to as the “forward direction”. In other words, the optical member  14  controls the light distribution of light emitted in the forward direction from the light source  12 . Further, in the illuminating apparatus  10 , the light source  12  is configured to emit light mainly in the forward direction. In addition, the light source  12  preferably emits light such that it spreads radially in the forward direction in at least a plane parallel to the paper surface in  FIG. 1 . 
     With regard to the light source  12 , the axis indicated by reference numeral C 2  in  FIG. 1  is a reference axis of light distribution of the light source  12 . This axis is normally established as a virtual axis that is perpendicular to a light-emitting surface of the light source  12  and passes through the photometric center (a point estimated as an origin point of light that disperses from the light source  12 ) (hereinafter, the axis C 2  will also be referred to as the optical axis C 2  of the light source  12 ). In the illustrated example, for the sake of explanation, the light-emitting surface of the light source  12  corresponds to a front surface  12   a  in terms of the outer shape of the light source  12 , and the photometric center thereof is positioned at the geometric center of the light-emitting surface  12   a . However, in the illuminating apparatus  10 , the light source  12  includes cases in which the light-emitting surface is unclear as a surface in the outer shape of the light source  12  or is a curved surface. In such cases, the light-emitting surface and the photometric center used in the definition of the optical axis C 2  are respectively determined as an appropriate virtual surface and position considering the shape of the light source  12  and the like. In the following explanation, the light-emitting surface of the light source  12  is referred to using reference numeral  12   a  including the above-described cases. If the light source  12  has symmetrical light distribution around an axis perpendicular to the light-emitting surface  12   a , the optical axis C 2  is normally the axis of symmetry of this light distribution, and typically corresponds to the geometric center axis of the light-emitting surface  12   a.    
     As will be explained below, the illuminating apparatus  10  controls the light distribution of light emitted from the light source  12  to a desired light distribution by the optical member  14 , and emits light whose light distribution is controlled in this way as illumination light. However, the illuminating apparatus  10  is configured such that the reference axis of the light distribution of the illumination light (the optical axis of the illuminating apparatus  10 ) coincides with the optical axis C 2  of the light source  12 . 
     The optical member  14  includes a reference axis C 1  that is a virtual axis that serves as a reference for the light distribution control effect of the optical member  14  (or a reference for arranging the plurality of prisms). A plurality of prisms  15  are provided on the principal surface  14   b  of the optical member  14  that faces the light source  12  based on the reference axis C 1  as explained below. 
     The plurality of prisms  15  that extend in one direction (the direction orthogonal to the paper surface in  FIG. 1 ) are provided on the principal surface  14   b  of the optical member  14  in regions on both sides when divided at a virtual plane (not illustrated; hereinafter also referred to as a “reference plane”) including the reference axis C 1 . In  FIG. 1 , the reference plane is a virtual plane including the reference axis C 1  that is orthogonal to the paper surface, and the plurality of prisms  15  extending parallel to the reference plane are aligned on the principal surface  14   b  of the optical member  14  in a direction that is orthogonal to the direction in which the prisms  15  extend and are provided in regions to the left side and the right side of the reference axis C 1  in  FIG. 1 . 
     Furthermore, when the light source  12  used in the illuminating apparatus  10  is disposed virtually such that its optical axis C 2  coincides with the reference axis C 1 , the plurality of prisms  15  include reflecting prisms that reflect light from the light source disposed in this way (a light source  13  indicated by dashed lines in  FIG. 1 ) so that it is emitted from the optical member  14 . In the illuminating apparatus  10 , the plurality of prisms  15  are configured as these reflecting prisms  15  across an entire range A straddling the reference plane of the optical member  14 . 
     Herein, each of the plurality of reflecting prisms  15  is a so-called TIR (Total Internal Reflection) prism. Specifically, each reflecting prism  15  includes a pair of prism surfaces  15   a  and  15   b  consisting of a first surface  15   a  that faces the reference axis C 1  and a second surface  15   b  that faces the opposite side of the reference axis C 1 . Light emitted from the light source  13  enters each prism  15  from the first surface  15   a , and at least a portion of the light that has entered proceeds toward the principal surface  14   a  (hereinafter also referred to as an “emitting surface  14   a ”) of the optical member  14  on which the reflecting prisms  15  are not provided by total internal reflection at the second surface  15   b  and is emitted from the emitting surface  14   a  (refer to the light tracks indicated by the dashed line arrows in  FIG. 1 ). 
     In addition, in the illustrated example, the plurality of reflecting prisms  15  are configured such that the focal point is located on the reference axis C 1  with regard to the lens effect thereof. A light-emitting surface  13   a  of the light source  13  is located at this focal point, and light emitted radially from the light source  13  in at least a plane that is orthogonal to the direction in which the reflecting prisms  15  extend (within a plane parallel to the paper surface in  FIG. 1 ) is converted to light that is substantially parallel to the optical axis C 2  direction in this plane as schematically illustrated by the light tracks indicated by the dashed line arrows in  FIG. 1 . 
     In the illuminating apparatus  10 , under the above-described structure of the optical member  14 , the actual light source  12  is configured such that its optical axis C 2  is disposed at a position that is shifted relative to the reference axis C 1  to a region on one side when divided at the reference plane (the right side of the reference axis C 1  in the example illustrated in  FIG. 1 ). In more detail, in the illuminating apparatus  10 , the light source  12  is disposed at a position that is shifted from the position at which the light source  13  is disposed to a region on one side when divided at the reference plane along a direction orthogonal to the reference plane in an orientation in which the optical axis C 2  is maintained parallel to the reference axis C 1 . Thus, the distance between the optical member  14  and the light-emitting surface  12   a  of the light source  12  is the same as a focal length F of the plurality of reflecting prisms  15 . 
     Herein, the optical member  14  normally has uniform optical characteristics in the direction in which the reflecting prisms  15  extend (the direction orthogonal to the paper surface in  FIG. 1 ; hereinafter also referred to as a “vertical direction”). In this case, the position of the reference axis C 1  in the vertical direction can be set to any appropriate position in accordance with the specific structure of the illuminating apparatus  10 . For example, the position of the reference axis C 1  in the vertical direction can be the center position in the vertical direction of the outer shape of the optical member  14 . 
     In the above explanation, the light source  13  of the illuminating apparatus  10  was disposed such that its light-emitting surface  13   a  is positioned at the focal point on the reference axis C 1 . However, if the optical member  14  has uniform optical characteristics in the direction in which the reflecting prisms  15  extend, the focal points of the plurality of reflecting prisms  15  are distributed continuously linearly on the reference plane (in a direction orthogonal to the paper surface in  FIG. 1 ). In this case, the position at which the light source  13  is disposed can be a position at which the optical axis thereof (omitted from the illustration associated with the light source  13 , but will be referred to using reference numeral C 2  similar to the optical axis C 2  of the light source  12 ) and the reference axis C 1  do not coincide as long as the optical axis C 2  is included in the reference plane (in other words, coincides with any one of the virtual axes within the reference plane that are parallel to the reference axis C 1 ) in accordance with the structure of the illuminating apparatus  10  and the like. 
     Also, in the above explanation, the position at which the light source  12  is disposed was set to a position shifted from the position at which the light source  13  is disposed along a direction orthogonal to the reference plane such that the position in the vertical direction of the optical axis C 2  of the light source  12  coincides with the position in the vertical direction of the reference axis C 1  (the optical axis C 2  of the light source  13 ). However, in the illuminating apparatus  10 , if the optical member  14  has uniform optical characteristics in the direction in which the reflecting prisms  15  extend, the position in the vertical direction at which the light source  12  is disposed is not necessarily limited to the above-described position, and can be set to any appropriate position in accordance with the structure of the illuminating apparatus  10  and the like. 
     Herein, in the illuminating apparatus  10 , the light source  12  is preferably made of a point light source including a light-emitting diode. However, in the illuminating apparatus  10 , the light source  12  can also be a linear light source. In this case, the light source  12  used in the illuminating apparatus  10  and the light source  13  in which the light source  12  is virtually disposed are arranged in the above-described predetermined position and orientation with regard to the light-emitting surfaces  12   a  and  13   a  and the optical axes C 2 , and are arranged such that the direction in which the linear light sources  12  and  13  extend coincides with the direction in which the plurality of reflecting prisms  15  extend. In the illuminating apparatus  10 , such a linear light source can include, for example, a straight tube-shaped fluorescent tube, or a plurality of point light sources that are arranged linearly. 
     The operational effects of the illuminating apparatus  10  configured as described above are as follows. 
     In the following, a cross-section of the illuminating apparatus  10  that is orthogonal to the vertical direction (the direction in which the plurality of reflecting prisms  15  extend) is referred to as a “transverse cross-section”. Further, the transverse cross-section including the optical axis C 2  of the light source  12  typically includes the reference axis C 1 . However, in a typical optical member  14  having uniform optical characteristics in the vertical direction, light distribution control as described below is also achieved in the case that the reference axis C 1  is not included in the transverse cross-section including the optical axis C 2  of the light source  12 . In this case, the term “reference axis C 1 ” in the following explanation can be replaced with the phrase “an axis established upon projecting the reference axis C 1  in the vertical direction on a transverse cross-section including the optical axis C 2  of the light source  12 ”. 
     In the illuminating apparatus  10 , by configuring the light source  12  and the optical member  14  as described above, in the transverse cross-section including the optical axis C 2 , a portion of the emitted light from the light source  12  is emitted from the emitting surface  14   a  of the optical member  14  so as to be tilted relative to the optical axis C 2  direction in a direction (the right direction in  FIG. 1 ) in which the optical axis C 2  is shifted from the reference axis C 1  as illustrated by the light tracks schematically illustrated by dot-dot-dash line arrows R 1  to R 3  in  FIG. 1 . Further, another portion of the emitted light from the light source  12  is emitted from the emitting surface  14   a  of the optical member  14  so as to be tilted relative to the optical axis C 2  direction in a direction (the left direction in  FIG. 1 ) opposite to the direction in which the optical axis C 2  is shifted from the reference axis C 1  as illustrated by the light tracks schematically illustrated by a dot-dot-dash line arrow L 1  in  FIG. 1 . 
     The above point will be explained in more detail below. 
     In the transverse cross-section including the optical axis C 2 , in the reflecting prisms  15  disposed on the side (the left side of the reference axis C 1  in  FIG. 1 ) on which the light source  12  is not disposed among the regions on both sides when divided at the reference plane, emitted light from the light source  12  enters into the reflecting prisms  15  from the first surface  15   a  of the reflecting prisms  15  similar to emitted light from the light source  13  that is virtually disposed, and then at least a portion of this light is reflected at the second surface  15   b  and emitted from the emitting surface  14   a  of the optical member  14  as illustrated by the dot-dot-dash line arrows R 1  in  FIG. 1 . However, since the light source  12  is disposed at a position that is shifted in one direction (the right direction in  FIG. 1 ) relative to the reference axis C 1 , this emitted light is emitted so as to be tilted in this shifted direction (the right direction in  FIG. 1 ) relative to the optical axis C 2  direction. 
     Further, in the transverse cross-section including the optical axis C 2 , in the reflecting prisms  15  which are on the opposite side of the reference axis C 1  relative to the optical axis C 2  of the light source  12  and are disposed at a position separated from the optical axis C 2  among the reflecting prisms  15  disposed on the side (the right side of the reference axis C 1  in  FIG. 1 ) on which the light source  12  is disposed among the regions on both sides when divided at the reference plane, emitted light from the light source  12  enters into the reflecting prisms  15  from the first surface  15   a  of the reflecting prisms  15  similar to emitted light from the light source  13  that is virtually disposed, and then at least a portion of this light is reflected at the second surface  15   b  and emitted from the emitting surface  14   a  of the optical member  14  as illustrated by the dot-dot-dash line arrows R 3  in  FIG. 1 . However, since the light source  12  is disposed at a position that is shifted in one direction (the right direction in  FIG. 1 ) relative to the reference axis C 1 , this emitted light is emitted so as to be tilted in this shifted direction (the right direction in  FIG. 1 ) relative to the optical axis C 2  direction. 
     On the other hand, in the transverse cross-section including the optical axis C 2 , in the reflecting prisms  15  which are disposed near the optical axis C 2  of the light source  12  (including the reflecting prisms  15  that are disposed on the opposite side of the reference axis C 1  relative to the optical axis C 2  and the reflecting prisms  15  disposed between the optical axis C 2  of the light source  12  and the reference axis C 1 ) and the reflecting prisms  15  which are disposed between the optical axis C 2  of the light source  12  and the reference axis C 1  (in a range that is not necessarily limited to near the optical axis C 2  of the light source  12 ), emitted light from the light source  12  enters into the reflecting prisms  15  from the second surface  15   b  of the reflecting prisms  15  unlike the emitted light from the light source  13  that is virtually disposed, as illustrated by the dot-dot-dash line arrows L 1  and R 2  in  FIG. 1 . Most of this light that has entered from the second surface  15   b  of the reflecting prisms  15  is emitted from the emitting surface  14   a  of the optical member  14  without entering the first surface  15   a  as illustrated by the dot-dot-dash line arrows L 1  in  FIG. 1 . As a result, this light is emitted so as to be tilted in a direction (the left direction in  FIG. 1 ) that is opposite to the shifted direction of the light source  12  relative to the optical axis C 2  direction. 
     Also, at least a portion of the light that has entered from the second surface  15   b  is reflected at the first surface  15   a  and emitted from the emitting surface  14   a  of the optical member  14  as illustrated by the dot-dot-dash line arrow R 2  in  FIG. 1 . As a result, this light is emitted so as to be tilted in the shifted direction of the light source  12  (the right direction in  FIG. 1 ) relative to the optical axis C 2  direction. 
     Herein, in the illuminating apparatus  10 , the light source  12  and the optical member  14  are configured and disposed such that most of the light that is emitted from the light source  12  and enters the optical member  14  is emitted so as to be tilted in the right direction relative to the optical axis C 2  direction as illustrated by the light tracks indicated by the dot-dot-dash line arrows R 1  to R 3  in  FIG. 1 . 
     Hereinafter, in the transverse cross-section including the optical axis C 2 , when the emitting direction of light emitted from the emitting surface  14   a  of the optical member  14  is divided into two different directions, emitted light on the side at which the amount of emitted light is greater will be referred to as primary light, and emitted light on the side at which the amount of emitted light is smaller will be referred to as secondary light. 
     In the case of the illuminating apparatus  10 , light that is emitted from the emitting surface  14   a  of the optical member  14  so as to be tilted relative to the optical axis C 2  direction in a direction (the right direction in  FIG. 1 ) in which the optical axis C 2  is shifted from the reference axis C 1  as illustrated by the light tracks schematically illustrated by the dot-dot-dash line arrows R 1  to R 3  in  FIG. 1  is primary light R 1  to R 3 , and light that is emitted from the emitting surface  14   a  of the optical member  14  so as to be tilted relative to the optical axis C 2  direction in a direction (the left direction in  FIG. 1 ) opposite to the direction in which the optical axis C 2  is shifted from the reference axis C 1  as illustrated by the light tracks schematically illustrated by the dot-dot-dash line arrows L 1  in  FIG. 1  is secondary light L 1 . 
     In this arrangement configuration of the light source  12  and the optical member  14 , the average emission angle of the primary light R 1  to R 3  (tilt angle toward the right direction relative to the optical axis C 2  direction) that is emitted from the emitting surface  14   a  of the optical member  14  is normally different from the average emission angle of the secondary light L 1  (tilt angle toward the left direction relative to the optical axis C 2  direction). Thereby, in the illuminating apparatus  10 , illumination light emitted from the optical member  14  can realize asymmetrical light distribution for both the amount of light and the emission angle relative to the optical axis C 2  of the light source  12  (in other words, the optical axis of the illuminating apparatus  10 ) in the transverse cross-section including the optical axis C 2 . 
     Further, in the illuminating apparatus  10 , in the transverse cross-section including the optical axis C 2 , the average emission angle (tilt angle relative to the optical axis C 2  direction) of the primary light R 1  to R 3  that is emitted from the emitting surface  14   a  of the optical member  14  increases as the distance in the transverse cross-section over which the optical axis C 2  of the light source  12  is shifted relative to the reference axis C 1  increases. Therefore, in this arrangement configuration of the light source  12  and the optical member  14 , the emitting direction of the primary light R 1  to R 3  can be controlled by adjusting the distance in the transverse cross-section including the optical axis C 2  between the reference axis C 1  and the optical axis C 2  of the light source  12 . 
     In addition, in the transverse cross-section including the optical axis C 2 , as the distance in the transverse cross-section between the reference axis C 1  and the optical axis C 2  of the light source  12  increases, the number of reflecting prisms  15  that exists between the reference axis C 1  and the optical axis C 2  of the light source  12  increases, and thus the ratio of the amount of the secondary light L 1  relative to the amount of the primary light R 1  to R 3  also increases. Therefore, in this arrangement configuration of the light source  12  and the optical member  14 , the ratio of the amount of the secondary light L 1  relative to the amount of the primary light R 1  to R 3  can be controlled by adjusting the distance in the transverse cross-section between the reference axis C 1  and the optical axis C 2  of the light source  12 . 
     Also, in the illuminating apparatus  10 , this kind of light distribution control is carried out using the optical member  14  which has a plurality of the reflecting prisms  15  on one principal surface  14   b  thereof. Thus, an illuminating apparatus  10  that is thin and exhibits high efficiency with low loss of light can be realized. Further, in the illuminating apparatus  10 , the light distribution characteristics of the illuminating apparatus  10  can be finely and easily adjusted based on the arrangement configuration of the optical member  14  and the light source  12  and the optical design of the plurality of prisms  15  of the optical member  14 . 
     Moreover, for example, if the light source  12  is constituted by a point light source with a relatively wide light-emitting surface area such as a so-called COB (Chip On Board) LED, light that enters the plurality of prisms  15  from a position separated from the optical axis C 2  among the emitted light from the light source  12  has a narrower incident angle range than that of light that enters the plurality of prisms  15  from near the optical axis C 2 , and thereby it is easier to control the light distribution of this light. Thus, in the illuminating apparatus according to the present invention, in order to ensure efficiency and controllability of light distribution, it is preferable to dispose the reflecting prisms  15  which have excellent efficiency in regions separated from the optical axis C 2  and to configure the plurality of the prisms  15  such that the majority thereof are reflecting prisms  15  so as to exhibit a light distribution control function. In the illuminating apparatus  10  according to the present embodiment, the plurality of prisms  15  are configured as these reflecting prisms  15  across the entire range A straddling the reference plane of the optical member  14 , and thereby a configuration of the plurality of prisms  15  that is preferable from the above-described perspective is realized. 
     As described above, in the optical member  14 , the plurality of reflecting prisms  15  normally have uniform optical characteristics in the direction in which the plurality of reflecting prisms  15  extend. Thus, the light distribution of the illumination light of the illuminating apparatus  10  in a plane that is orthogonal to the transverse cross-section including the optical axis C 2  (hereinafter, this plane is also referred to as a “vertical cross-section”) is a direct reflection of the light distribution within this plane of the light source  12 . In particular, when the light distribution within the vertical cross-section including the optical axis C 2  of the light source  12  is symmetrical relative to the optical axis C 2 , the light distribution within this plane of the illumination light is also symmetrical relative to the optical axis C 2 . 
       FIG. 2  is a graph illustrating the results upon analyzing (simulation by ray tracing) the light distribution of illumination light in a model corresponding to the illuminating apparatus  10 . 
     In the model used for this analysis, the refractive index of the optical member  14  was 1.58 (assuming a polycarbonate is used as the molding material), and the width of the plurality of reflecting prisms  15  in the arrangement direction (the left-right direction on the paper surface in  FIG. 1 ) was 95 mm. The focal length F of the plurality of reflecting prisms  15  was set to 15 mm by setting the apex angle of the reflecting prisms  15  to 40° and the arrangement pitch to 50 μm and by adjusting the tilt angle of the prism surfaces  15   a  and  15   b  of the plurality of reflecting prisms  15  relative to the principal surface of the optical member  14  (e.g., the emitting surface  14   a ). Also, the light distribution of the light source  12  was modeled based on a light-emitting diode which is a point light source (a COB-type LED having a light-emitting diameter of 20 mm), and the distance that the optical axis C 2  of the light source  12  is shifted from the reference axis C 1  in the transverse cross-section including the optical axis C 2  was 15 mm. 
     In  FIG. 2 , the coordinates in the circumferential direction indicate an angle of beam spread [°] when the optical axis C 2  direction (forward direction) is 0°, and a negative angle corresponds to a tilt angle toward the right direction relative to the optical axis C 2  direction in  FIG. 1 , whereas a positive angle corresponds to a tilt angle toward the left direction relative to the optical axis C 2  direction in  FIG. 1 . The coordinates in the radial direction indicate a light intensity [cd]. In  FIG. 2 , a light distribution curve in the transverse cross-section including the optical axis C 2  of illumination light of the illuminating apparatus  10  is illustrated with a solid line, and a light distribution curve in the vertical cross-section including the optical axis C 2  is illustrated with a dashed line. 
     From the light distribution curve illustrated with a solid line in  FIG. 2 , it can be understood that in the illuminating apparatus  10 , light distribution that is asymmetrical relative to the optical axis C 2  is realized in the transverse cross-section including the optical axis C 2  of illumination light. In this light distribution curve, the angle of beam spread at the distribution center of a light distribution C corresponding to the primary light is approximately −15°, and the angle of beam spread at the distribution center of a light distribution D corresponding to the secondary light is approximately 35°. Further, the ratio of the peak light intensity in the light distribution D corresponding to the secondary light relative to the peak light intensity in the light distribution C corresponding to the primary light is approximately 25%. 
     From the light distribution curve illustrated with a dashed line in  FIG. 2 , it can be understood that the light distribution in the vertical cross-section including the optical axis C 2  of illumination light is substantially symmetrical relative to the optical axis C 2 . 
     Further, although not illustrated, similar analyses were also conducted using similar models in which the distance that the optical axis C 2  of the light source  12  is shifted from the reference axis C 1  in the transverse cross-section including the optical axis C 2  was set to 0 mm, 5 mm, and 10 mm. From the above results as well as the results of these similar analyses, it was confirmed that the emission angle of the primary light as well as the ratio of the amount of secondary light relative to the amount of primary light are dependent on the distance that the optical axis C 2  of the light source  12  is shifted from the reference axis C 1  as described above. 
     The illuminating apparatus  10  having the above-described light distribution characteristics can be suitably used as, for example, a tunnel lamp that is installed on a wall surface of one side of a tunnel and illuminates the wall surfaces on both sides and the road surface within the tunnel. In this case, a plane having the light distribution illustrated with a solid line in  FIG. 2  (the transverse cross-section including the optical axis C 2 ) is set to coincide with a vertical plane that is parallel to the width direction of the roadway, and the light distribution characteristics of the illuminating apparatus  10  are adjusted in accordance with the predetermined installation position, installation angle, and the like of the illuminating apparatus  10  such that a predetermined range of the wall surface on the side on which the illuminating apparatus  10  is installed is illuminated by illumination light corresponding to the secondary light D and a predetermined range of the wall surface on the opposite side with the roadway therebetween is illuminated by illumination light corresponding to the primary light C which is brighter than the secondary light D. Thereby, the road surface as well as the wall surfaces on both sides within the tunnel can be illuminated so as to satisfy a predetermined illumination standard. 
     Also, the illuminating apparatus  10  can also be suitably used as a roadway lamp that is erected toward the road shoulder on one side in the width direction of a roadway on which sidewalks are provided on the outside of the road shoulder on both sides in the width direction to illuminate the sidewalks on both sides of the roadway as well as the road surface. In this case, a plane having the light distribution illustrated with a solid line in  FIG. 2  (the transverse cross-section including the optical axis C 2 ) is set to coincide with a vertical plane that is parallel to the width direction of the roadway, and the light distribution characteristics of the illuminating apparatus  10  are adjusted in accordance with the predetermined installation position, installation angle, and the like of the illuminating apparatus  10  such that the sidewalk on the side on which the illuminating apparatus  10  is installed is illuminated by illumination light corresponding to the secondary light D and the sidewalk on the opposite side with the roadway therebetween is illuminated by illumination light corresponding to the primary light C which is brighter than the secondary light D. Thereby, the road surface as well as the sidewalks on both sides can be illuminated so as to satisfy a predetermined illumination standard. 
     Next, referring to  FIGS. 3 to 9 , further embodiments of the illuminating apparatus of the present invention will be explained. In the explanation of the following embodiments, explanations of features that are the same as those in the previously explained embodiment(s) will be appropriately omitted, and the explanations will focus mainly on the unique features of each embodiment. 
     The basic structure of an illuminating apparatus  20  of a second embodiment of the present invention illustrated in  FIG. 3  is similar to that of the illuminating apparatus  10  illustrated in  FIG. 1 . However, the illuminating apparatus  20  differs from the illuminating apparatus  10  in that among the plurality of prisms  15  and  22  provided in regions on both sides when divided at the reference plane on a principal surface  24   b  of an optical member  24  facing the light source  12 , the plurality of prisms  22  disposed near the reference plane (in the range indicated by B in  FIG. 3 ) are configured as refracting prisms  22 . 
     In the optical member  24 , a plurality of reflecting prisms  15  similar to the reflecting prisms  15  of the illuminating apparatus  10  illustrated in  FIG. 1  are provided on the outsides (in the ranges indicated by A in  FIG. 3 ) of the range B near the reference plane among the regions on both sides when divided at the reference plane. Therein, the boundary between the reflecting prisms  15  and the refracting prisms  22  provided on the side on which the light source  12  is disposed (the right side of the reference axis C 1  in  FIG. 3 ) among the regions on both sides when divided at the reference plane is located more toward the reference axis C 1  than the optical axis C 2  of the light source  12 . 
     Similar to the illuminating apparatus  10  illustrated in  FIG. 1 , the plurality of reflecting prisms  15  are configured such that the focal point is located on the reference axis C 1  with regard to the lens effect thereof, and the light source  13  is disposed virtually such that the light-emitting surface  13   a  is located at this focal point. 
     When the light source  12  used in the illuminating apparatus  10  is disposed virtually such that its optical axis C 2  coincides with the reference axis C 1 , the plurality of refracting prisms  22  refract light from the light source disposed in this way (the light source  13  indicated by dashed tracks in  FIG. 3 ) so that it is emitted from the optical member  24 . 
     Specifically, each refracting prism  22  includes a first surface  22   a  that is arranged tilted relative to a principal surface (e.g., an emitting surface  24   a ) of the optical member  24 , and each of the plurality of prisms  22  refracts light that enters from the first surface  22   a . Thereby, the refracting prisms  22  are configured to function as linear Fresnel lenses (corresponding to a cylindrical lens that protrudes toward a rear direction). Further, each refracting prism  22  (excluding the refracting prisms  22  whose first surfaces  22   a  are directly connected to each other from both sides of the reference axis C 1  in the example illustrated in  FIG. 3 ) also includes a second surface  22   b  that is approximately orthogonal to the principal surface of the optical member  24  and is connected to the first surface  22   a  of an adjacent refracting prism  22 . 
     The illuminating apparatus  20  configured as described above achieves the same operational effects as the illuminating apparatus  10  described above. Therein, in the illuminating apparatus  20 , by providing the plurality of reflecting prisms  15  on the optical member  24  on the outsides (the ranges indicated by A in  FIG. 3 ) of the range B near the reference plane, a preferable configuration of the plurality of prisms  22  and  15  is realized for the case in which the light source  12  is constituted by a point light source with a relatively wide light-emitting surface area as described above. 
     In addition, the illuminating apparatus  20  also achieves the following unique operational effects compared to the illuminating apparatus  10 . 
     First, in the illuminating apparatus  20 , in the transverse cross-section including the optical axis C 2  of the light source  12 , light that is emitted from the light source  12  and enters into the refracting prisms  22  is emitted from the emitting surface  24   a  of the optical member  24  so as to be tilted relative to the optical axis C 2  direction in a direction (the left direction in  FIG. 1 ) opposite to the direction in which the light source  12  is shifted from the light source  13  as illustrated by the light line illustrated by a dot-dot-dash line arrow L 2  in  FIG. 3 . 
     Basically, at least a portion of the light that is emitted from the light source  12  and becomes the primary light R 2  due to the action of the reflecting prisms  15  disposed between the optical axis C 2  and the reference axis C 1  and the light that is emitted from the light source  12  and becomes the primary light R 1  due to the action of the reflecting prisms  15  disposed near the reference axis C 1  among the reflecting prisms  15  disposed on the side (the left side of the reference axis C 1  in  FIG. 1 ) on which the light source  12  is not disposed among the regions on both sides when divided at the reference plane in the illuminating apparatus  10  is emitted as secondary light L 2  in the illuminating apparatus  20  and thus contributes to the overall light distribution. 
     Therefore, in the illuminating apparatus  20 , replacing a portion of the reflecting prisms  15  with the refracting prisms  22  functions as a means for adjusting the balance between the amount of secondary light L 1  and L 2  and the amount of primary light R 1  to R 3  in accordance with the illumination standard of the environment in which the illuminating apparatus is installed and the like. In particular, the structure of the illuminating apparatus  20  is more advantageous than the illuminating apparatus  10  in terms of increasing the ratio of the amount of secondary light L 1  and L 2  relative to the amount of primary light R 1  to R 3 . 
     Further, compared to the illuminating apparatus  10 , the illuminating apparatus  20  is advantageous in terms of improving the light controllability and emitting efficiency as described below. In the reflecting prisms  15 , the tilt angles of the pair of first and second surfaces  15   a  and  15   b  relative to the principal surface (e.g., the emitting surface  24   a ) of the optical member  24  are relatively large. Thus, for example, some light may leak to the outside upon passing through the first surface  15   a  after entering into the reflecting prism  15  from the second surface  15   b , and this light is referred to as so-called stray light, which may inhibit the improvement of light controllability and emitting efficiency in the illuminating apparatus  20 . 
     In contrast, the majority of light that is emitted from the light source  12  and enters into the plurality of refracting prisms  22  is more likely to enter into the refracting prisms  22  from the first surface  22   a , whose tilt angle relative to the principal surface (e.g., the emitting surface  24   a ) of the optical member  24  is less than that of the first and second surfaces  15   a  and  15   b  of the reflecting prisms  15 , and then be emitted from the emitting surface  24   a  of the optical member  24 . Therefore, compared to the illuminating apparatus  10  in which all of the plurality of prisms are configured as reflecting prisms  15 , the occurrence of stray light as described above can be suppressed, and in turn the light emitting efficiency can be improved and the controllability of the asymmetrical light distribution portion can be improved. 
       FIG. 4  is a graph similar to  FIG. 2  illustrating the results upon analyzing (simulation by ray tracing) the light distribution of illumination light in a model corresponding to the illuminating apparatus  20 . 
     The conditions of the model used in this analysis are similar to those of the model corresponding to the illuminating apparatus  10  as described above in relation to  FIG. 2 , except that the plurality of refracting prisms  22  are set in a range of ±6 mm on both sides of the reference plane. 
     From the light distribution curve illustrated with a solid line in  FIG. 4 , it can be understood that in the illuminating apparatus  20  as well, light distribution that is asymmetrical relative to the optical axis C 2  is realized in the transverse cross-section including the optical axis C 2  of illumination light. 
     Further, in this light distribution curve, the ratio of the peak light intensity in the light distribution D corresponding to the secondary light relative to the peak light intensity in the light distribution C corresponding to the primary light is approximately 46%. Thus, it can be understood that the ratio of the amount of secondary light relative to the amount of primary light can be increased by replacing the plurality of reflecting prisms  15  disposed near the reference plane with the plurality of refracting prisms  22 . 
     Herein, in the example illustrated in  FIG. 3 , the plurality of refracting prisms  22  are configured such that the plurality of refracting prisms  22  whose first surface  22   a  faces the opposite side of the reference axis C 1  are disposed in the regions on both sides when divided at the reference plane, and the plurality of refracting prisms  22  in one region and the plurality of refracting prisms  22  in the other region are disposed symmetrically relative to the reference plane. However, in this case, the focal point of the plurality of refracting prisms  22  is not necessarily the same as the focal point of the plurality of reflecting prisms  15 . Further, even if the plurality of refracting prisms  22  are provided in the regions on both sides when divided at the reference plane, the configuration of the plurality of refracting prisms  22  does not have to be symmetrical relative to the reference plane. For example, all of the refracting prisms  22  can be configured such that their first surfaces  22   a  face the side of the optical axis C 2  of the light source  12  similar to the refracting prisms  22  disposed on the right side of the reference plane in  FIG. 3 . Also, in the illuminating apparatus  20 , the plurality of refracting prisms  22  can be provided in the region on only one side (e.g., the side on which the light source  12  is disposed) when divided at the reference plane. 
     The basic structure of an illuminating apparatus  30  of a third embodiment of the present invention illustrated in  FIG. 5  is similar to that of the illuminating apparatus  10  illustrated in  FIG. 1 . However, the illuminating apparatus  30  differs from the illuminating apparatus  10  in that a distance G between the optical member  14  and the emitting surface  12   a  of the light source  12  is shorter than the focal length F of the plurality of reflecting prisms  15  and the light source  12  is disposed more toward the optical member  14  than the focal point of the plurality of reflecting prisms  15 . 
     In addition to operational effects similar to those of the illuminating apparatus  10  described above, the illuminating apparatus  30  also achieves the following unique operational effects compared to the illuminating apparatus  10 . 
     In the illuminating apparatus  30 , as in the illuminating apparatus  10  illustrated in  FIG. 1 , in the transverse cross-section including the optical axis C 2 , in the reflecting prisms  15  disposed on the side (the left side of the reference axis C 1  in  FIG. 5 ) on which the light source  12  is not disposed among the regions on both sides when divided at the reference plane, emitted light from the light source  12  enters into the reflecting prisms  15  from the first surface  15   a  of the reflecting prisms  15  and then at least a portion of this light is reflected at the second surface  15   b  and emitted from the emitting surface  14   a  of the optical member  14  as illustrated by the dot-dot-dash line arrows R 1  in  FIG. 5 . 
     However, in the illuminating apparatus  30 , the emission angle (tilt angle relative to the optical axis C 2  direction) of this emitted light R 1  is larger than the emission angle of the emitted light R 1  that is emitted by the action of the reflecting prisms  15  disposed in the same position in the illuminating apparatus  10  (in other words, the emission angle of this emitted light R 1  approaches a direction parallel to the emitting surface  14   a  of the optical member  14 ). This emission angle increases as the distance G between the optical member  14  and the emitting surface  12   a  of the light source  12  decreases relative to the focal length F of the plurality of the prisms  15  by one or both of adjusting the focal length F of the plurality of prisms  15  and adjusting the distance G between the optical member  14  and the emitting surface  12   a  of the light source  12 . 
     Meanwhile, in the transverse cross-section including the optical axis C 2 , in the reflecting prisms  15  which are on the opposite side of the reference axis C 1  relative to the optical axis C 2  of the light source  12  and are disposed at a position separated from the optical axis C 2  among the reflecting prisms  15  disposed on the side (the right side of the reference axis C 1  in  FIG. 5 ) on which the light source  12  is disposed among the regions on both sides when divided at the reference plane, as illustrated in a dot-dot-dash line arrows L 3  in  FIG. 5 , emitted light from the light source  12  enters into the reflecting prisms  15  from the first surface  15   a  of the reflecting prisms  15  and then at least a portion of this light is reflected at the second surface  15   b  as in the illuminating apparatus  10  illustrated in  FIG. 1  (refer to the dot-dot-dash line arrows R 3  in  FIG. 1 ). However, unlike in the illuminating apparatus  10  illustrated in  FIG. 1 , when this reflected light is emitted from the emitting surface  14   a  of the optical member  14 , it is emitted so as to be tilted relative to the optical axis C 2  direction in a direction (the left direction in  FIG. 5 ) opposite to the direction in which the light source  12  is shifted from the light source  13 . 
     In the illuminating apparatus  30 , the amount of this emitted light L 3  increases as the distance G between the optical member  14  and the emitting surface  12   a  of the light source  12  decreases relative to the focal length F of the plurality of prisms  15  by one or both of adjusting the focal length F of the plurality of prisms  15  and adjusting the distance G between the optical member  14  and the emitting surface  12   a  of the light source  12 . 
     In particular, in the illuminating apparatus  30 , by decreasing the distance G between the optical member  14  and the emitting surface  12   a  of the light source  12  relative to the focal length F of the plurality of prisms  15  as described above, the amount of light that is emitted from the emitting surface  14   a  of the optical member  14  so as to be tilted relative to the optical axis C 2  direction in a direction (the left direction in  FIG. 5 ) opposite to the direction in which the optical axis C 2  is shifted from the reference axis C 1  as illustrated by the light tracks schematically illustrated by the dot-dot-dash line arrows L 1  and L 3  in  FIG. 5  can be increased compared to the amount of light emitted from the emitting surface  14   a  of the optical member  14  so as to be tilted relative to the optical axis C 2  direction in a direction (the right direction in  FIG. 5 ) in which the optical axis C 2  is shifted from the reference axis C 1  as illustrated by the light tracks schematically illustrated by the dot-dot-dash line arrows R 1  and R 4  in  FIG. 5 . In this case, the emitted light L 1  and L 3  becomes primary light and the emitted light R 1  and R 4  becomes secondary light. 
     In this way, in the illuminating apparatus  30 , by adjusting the distance G between the optical member  14  and the emitting surface  12   a  of the light source  12  relative to the focal length F of the plurality of prisms  15 , the light distribution of light emitted from the light source  12  can be precisely adjusted in a broader range. Further, the light L 1  and L 3  that is emitted so as to be tilted in a direction (the left direction in  FIG. 5 ) opposite to the direction in which the optical axis C 2  is shifted from the reference axis C 1  generally has a relatively broad distribution at the tilt angle relative to the optical axis C 2  direction. Thus, the illuminating apparatus  30  is advantageous in that, by configuring the emitted light L 1  and L 3  as primary light, it can achieve relatively broad light distribution with respect to the primary light L 1  and L 3  in accordance with the illumination standard of the environment in which the illuminating apparatus is installed and the like. 
     Furthermore, in the illuminating apparatus  30 , among the light emitted from the light source  12 , light that enters into the reflecting prisms  15  disposed near the optical axis C 2  of the light source  12  from a portion of the first surface  15   a  near the emitting surface  14   a  of the optical member  14  is emitted from the emitting surface  14   a  of the optical member  14  without entering and being reflected at the second surface  15   b  as illustrated by the dot-dot-dash line arrow R 4  in  FIG. 5 . As a result, this light is emitted so as to be tilted in a direction (the right direction in  FIG. 5 ) in which the light source  12  is shifted relative to the optical axis C 2  direction. 
     This emitted light R 4  is emitted from the light source  12  near the optical axis C 2 , and thus the amount thereof is normally large. Therefore, this light greatly contributes to increasing the amount of light R 1  and R 4  emitted from the emitting surface  14   a  of the optical member  14  so as to be tilted relative to the optical axis C 2  direction in a direction (the right direction in  FIG. 5 ) in which the optical axis C 2  is shifted from the reference axis C 1  among the overall emitted light distribution. Further, the emission angle (the tilt angle relative to the optical axis C 2  direction) of the emitted light R 4  tends to be large. 
     Thereby, light distribution control by broadening the angle between the average emitting direction of the primary light L 1  and L 3  and the average emitting direction of the secondary light R 1  and R 4  can be easily carried out. Thus, for example, when using the illuminating apparatus  30  as a tunnel lamp or a roadway lamp, light distribution suitable for a tunnel lamp or roadway lamp can be easily achieved in accordance with the illumination standard of the installation environment and the like. 
       FIG. 6  is a graph similar to  FIG. 2  illustrating the results upon analyzing (simulation by ray tracing) the light distribution of illumination light in a model corresponding to the illuminating apparatus  30 . 
     The conditions of the model used in this analysis are similar to those of the model corresponding to the illuminating apparatus  10  as described above in relation to  FIG. 2 , except that the distance G between the emitting surface  12   a  of the light source  12  and the optical member  14  was set to 15 mm relative to the focal length F (80 mm) of the plurality of reflecting prisms  15 . 
     From the light distribution curve illustrated with a solid line in  FIG. 6 , it can be understood that in the illuminating apparatus  30  as well, light distribution that is asymmetrical relative to the optical axis C 2  is realized in the transverse cross-section including the optical axis C 2  of illumination light. However, in this light distribution curve illustrated with a solid line in  FIG. 6 , the distribution center of the light distribution C corresponding to the primary light occurs in the positive direction of the angle of beam spread (tilt angle toward the left direction relative to the optical axis C 2  direction in  FIG. 5 ), and the value thereof is about 15°. Also, from this light distribution curve, it can be understood that the light distribution C of the primary light exhibits a half-value width that is prominently wider than that of the light distribution C of the primary light in the light distribution curve illustrated with a solid line in  FIG. 2  with regard to the illuminating apparatus  10 . 
     Further, in the light distribution curve illustrated with a solid line in  FIG. 6 , the angle of beam spread of the distribution center of the light distribution D corresponding to the secondary light occurs in the negative direction of the angle of beam spread (tilt angle toward the right direction relative to the optical axis C 2  direction in  FIG. 5 ), and the value thereof is about −50°. From this, it can be understood that the absolute value of this angle of beam spread is greater than the angle of beam spread (about 35°) at the distribution center of the light distribution D of the secondary light in the light distribution curve illustrated with a solid line in  FIG. 2  with regard to the illuminating apparatus  10 . 
       FIG. 7  is a side surface view illustrating the essential parts of another example of the illuminating apparatus according to the third embodiment of the present invention. The basic structure of an illuminating apparatus  40  illustrated in  FIG. 7  is similar to that of the illuminating apparatus  20  illustrated in  FIG. 3 . However, the illuminating apparatus  40  differs from the illuminating apparatus  20  in that the distance G between the optical member  14  and the emitting surface  12   a  of the light source  12  is smaller than the focal length F of the plurality of reflecting prisms  15  and the light source  12  is disposed more toward the optical member  14  than the focal point of the plurality of reflecting prisms  15 . 
     In addition to operational effects similar to those of the illuminating apparatus  30  illustrated in  FIG. 5 , the illuminating apparatus  40  configured as described above also achieves operational effects similar to those of the illuminating apparatus  20  illustrated in  FIG. 3  with respect to including the plurality of refracting prisms  22 . 
       FIG. 8  is a graph similar to  FIG. 2  illustrating the results upon analyzing (simulation by ray tracing) the light distribution of illumination light in a model corresponding to the illuminating apparatus  40 . 
     The conditions of the model used in this analysis are similar to those of the model corresponding to the illuminating apparatus  20  as described above in relation to  FIG. 3 , except that the distance G between the emitting surface  12   a  of the light source  12  and the optical member  14  was set to 15 mm relative to the focal length F (80 mm) of the plurality of refracting prisms  22 . The focal length of the plurality of refracting prisms  22  does not necessarily have to be the same as the focal length F of the reflecting prisms  15 , and in the present embodiment, the focal length of the plurality of refracting prisms  22  is set to 15 mm, which is the distance between the emitting surface  12   a  of the light source  12  and the optical member  14 . 
     From the light distribution curve illustrated with a solid line in  FIG. 8 , it can be understood that the illuminating apparatus  40  has features relative to the illuminating apparatus  20  similar to the features described above of the illuminating apparatus  30  relative to the illuminating apparatus  10 . 
     Next, an illuminating apparatus  50  according to a fourth embodiment of the present invention will be explained referring to  FIG. 9 . The illuminating apparatus  50  illustrated in  FIG. 9  includes the light source  12  and an optical member  54  opposing the light source  12 . A plurality of prisms  55 ,  22 ,  56 , and  57  that extend in one direction (the direction orthogonal to the paper surface in  FIG. 9 ) are provided on a principal surface  54   b  of the optical member  54  in regions on both sides when divided at a reference plane (a virtual plane including the reference axis C 1 ). In  FIG. 9 , the reference plane is a virtual plane that includes the reference axis C 1  and is orthogonal to the paper surface. The plurality of prisms  55 ,  22 ,  56 , and  57  which extend parallel to the reference plane are arranged on the principal surface  54   b  of the optical member  54  in a direction that is orthogonal to the direction in which the prisms  55 ,  22 ,  56 , and  57  extend, and are provided in regions to the left side and the right side of the reference axis C 1  in  FIG. 9 . 
     In the illuminating apparatus  50 , the plurality of prisms  55 ,  22 ,  56 , and  57  are divided into a plurality (four in  FIG. 9 ) of small regions A 1 , B, A 2 , and A 3  at one or more (three in  FIG. 9 ) virtual planes (not illustrated) that are parallel to the reference plane, and one or more prisms  55 ,  22 ,  56 , and  57  are disposed in each of the plurality of small regions A 1 , B, A 2 , and A 3 . The small regions A 1 , B, A 2 , and A 3  include a small region that includes the reference axis C 1  (the small region B in  FIG. 9 ) and small regions A 1 , A 2 , and A 3  that are provided outside of the small region B including the reference axis C 1 . The optical axis C 2  of the light source  12  is disposed so as to be included in one of the small regions A 1 , A 2 , and A 3  provided outside of the reference axis C 1  (the small region A 2  adjacent on the right to the small region B in  FIG. 9 ). 
     In  FIG. 9 , the plurality of the prisms  55 ,  22 ,  56 , and  57  are disposed in all of the small regions A 1 , B, A 2 , and A 3 . However, in the illuminating apparatus  50 , it is sufficient as long as at least one prism  55 ,  22 ,  56 , and  57  is disposed in each of the small regions A 1 , B, A 2 , and A 3 . 
     In the illuminating apparatus  50 , the one or more prisms  55 ,  22 ,  56 , and  57  disposed in adjacent small regions A 1 , B, A 2 , and A 3  are configured to have mutually different focal lengths. 
     In other words, in the example illustrated in  FIG. 9 , at the very least, the focal length of the plurality of prisms  55  included in the small region A 1  is different from the focal length of the plurality of prisms  22  included in the small region B, and the focal length of the plurality of prisms  22  included in the small region B is different from the focal length of the plurality of prisms  56  included in the small region A 2 , and the focal length of the plurality of prisms  56  included in the small region A 2  is different from the focal length of the plurality of prisms  57  included in the small region A 3 . However, for example, the focal lengths of the plurality of prisms  55  and  57  included in the small regions A 1  and A 3 , which are not adjacent, can be the same. 
     In the example illustrated in  FIG. 9 , among the plurality of small regions A 1 , B, A 2 , and A 3 , the plurality of prisms  22  included in the small region B near the reference axis C 1  are configured as refracting prisms  22 , and the plurality of prisms  55 ,  56 , and  57  included in the small regions A 1 , A 2 , and A 3  outside of the small region B are configured as reflecting prisms  55 ,  56 , and  57 . 
     Therein, the one or more reflecting prisms  56  and  57  disposed in each of the plurality of small regions A 2  and A 3  included on the side (the right side of the reference axis C 1  in  FIG. 9 ) on which the light source  12  is disposed among the regions on both sides when divided at the reference plane are configured such that the focal length thereof decreases the farther away the small regions A 2  and A 3  are from the reference axis C 1 . In other words, in the example illustrated in  FIG. 9 , the focal length of the plurality of reflecting prisms  57  disposed in the small region A 3  is shorter than the focal length of the plurality of reflecting prisms  56  disposed in the small region A 2 . 
     In the illuminating apparatus  50 , a distance H between the optical member  54  and the emitting surface  12   a  of the light source  12  is appropriately set in accordance with the desired light distribution control by the optical member  54 . 
     In the illuminating apparatus  50  configured as described above, asymmetrical light distribution for both the amount of light and the emission angle is realized relative to the optical axis C 2  in the transverse cross-section including the optical axis C 2 , similar to the illuminating apparatuses  10 ,  20 ,  30 , and  40  according to the first to third embodiments described above. 
     In addition, in the illuminating apparatus  50 , by adjusting the focal length of each small region A 1 , B, A 2 , and A 3  as well as the distance H between the optical member  54  and the emitting surface  12   a  of the light source  12  relative to these focal lengths, the light distribution of the illumination light can be more precisely adjusted. 
     Further, in the illuminating apparatus  50 , by configuring the one or more reflecting prisms  56  and  57  disposed in each of the plurality of small regions A 2  and A 3  included on the side (the right side of the reference axis C 1  in  FIG. 9 ) on which the light source  12  is disposed among the regions on both sides when divided at the reference plane such that the focal length thereof decreases the farther away the small regions A 2  and A 3  are from the reference axis C 1 , the occurrence of stray light (e.g., light that enters the reflecting prisms  57  from a first surface  57   a , is reflected at a second surface  57   b , and then reenters the first surface  57   a  and leaks to the outside upon passing through the first surface  57   a ) can be suppressed, and decreases in the emitting efficiency can be reduced. 
     In the illuminating apparatus  50  illustrated in  FIG. 9 , the distance H between the optical member  54  and the emitting surface  12   a  of the light source  12  is set to be shorter than the focal lengths of the plurality of reflecting prisms  55 ,  56 , and  57  disposed in the small regions A 1 , A 2 , and A 3 , and the plurality of refracting prisms  22  are disposed in the small region B that is near the reference axis C 1 . Therefore, the illuminating apparatus  50  basically achieves operational effects similar to those of the illuminating apparatus  40  illustrated in  FIG. 7  as well as the operational effects unique to the illuminating apparatus  50  described above. 
     In the example illustrated in  FIG. 9 , one small region A 1  is provided on the side (the left side of the reference axis C 1  in  FIG. 9 ) on which the light source  12  is not disposed among the regions on both sides when divided at the reference plane, and one small region B is provided near the reference axis C 1  straddling the regions on both sides when divided at the reference plane. However, in the illuminating apparatus  50 , these small regions A 1  and B can be further divided into multiple small regions. 
       FIG. 10  is a graph similar to  FIG. 2  illustrating the results upon fabricating an actual device corresponding to the illuminating apparatus  40  illustrated in  FIG. 7  and measuring the light distribution of illumination light thereof, together with the results upon analysis of the light distribution of illuminating light in a model corresponding to the illuminating apparatus  40 . In  FIG. 10 , the light distribution curve illustrated with a solid line is the measurement results of the actual device, and the light distribution curve b illustrated with a dashed line is the analysis results of the model. Comparing these light distribution curves a and b, the light distribution of the actual device and the light distribution upon analyzing the model match well, and from these results the effectiveness of the illuminating apparatus according to the present invention was confirmed. 
     The present invention was explained above based on preferred embodiments thereof. However, the illuminating apparatus according to the present invention is not limited to the above embodiments. 
     For example, the illuminating apparatus according to the present invention can be configured like an illuminating apparatus  60  illustrated in  FIG. 11 , in which a plurality of prisms  16  and  23  provided on an optical member  64  are disposed on a principal surface (emitting surface)  64   a  of the optical member  64  on the opposite side of a principal surface  64   b  on the side that faces the light source  12 . The illuminating apparatus  60  illustrated in  FIG. 11  has a structure in which a plurality of refracting prisms  23  are provided near the reference plane and reflecting prisms  16  are provided on the outsides of the refracting prisms  23 . 
     Each reflecting prism  16  includes a pair of prism surfaces  16   a  and  16   b  consisting of a first surface  16   a  that faces the reference axis C 1  and a second surface  16   b  that reflects at least a portion of light that enters into the reflecting prism  16 . However, in this case, emitted light from the light source  12  enters into each reflecting prism  16  from the principal surface  64   b  side of the optical member  64  that faces the light source  12 , is reflected at the second surface  16   b , passes through the first surface  16   a , and then is emitted as illumination light. 
     Further, in the example illustrated in  FIG. 11 , the plurality of refracting prisms  23  all include a first surface  23   a  (refracting surface) that faces the opposite side of the optical axis C 2 . Thus, by configuring the plurality of refracting prisms  23  to include the refracting surfaces  23   a  whose tilt directions relative to a principal surface (e.g., the principal surface  64   b ) of the optical member  64  are aligned on one side, the occurrence of stray light can be effectively suppressed. 
     Moreover, in the illuminating apparatus according to the present invention, the plurality of prisms can be provided on both principal surfaces of the optical member. Also, in the illuminating apparatus according to the present invention, a plurality of light scattering elements formed in, for example, a dome shape can be provided on a principal surface of the optical member on the side on which the plurality of prisms is not disposed, or in a region of the principle surface of the optical member in which the plurality of prisms are not disposed. 
     Further, the illuminating apparatus according to the present invention can be suitably applied to not only a tunnel lamp or roadway lamp as described above, but also, for example, an indoor light such as a base light or a desk lamp and the like.