Abstract:
A rod integrator comprises a solid rod-shaped first member and a hollow second member. The first member guides a luminous flux from a light source incident on one end to the other end while totally reflecting the luminous flux on side faces. The second member guides the luminous flux from the first member incident on one end to the other end while specularly reflecting the luminous flux, and then emits the luminous flux from the other end. The first and second members are combined such that substantially all of the luminous flux emitted from the other end of the first member is made incident on one end of the second member.

Description:
RELATED APPLICATIONS  
         [0001]    This application claims the priority of Japanese Patent Application No. 2001-243198 filed on Aug. 10, 2001, which is incorporated herein by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a rod integrator which can achieve a uniform luminous flux density within a plane perpendicular to an optical axis for a luminous flux from a light source, and an illumination optical system using the same.  
           [0004]    2. Description of the Prior Art  
           [0005]    Conventionally, video projectors and the like, for example, have often been configured so as to dispose luminous flux density homogenizing means between an illumination light source and an image display device such as a liquid crystal panel in order for the image display device to be illuminated uniformly.  
           [0006]    Known as typical luminous flux density homogenizing means are rod integrators. As conventional rod integrators, those disclosed in Japanese Unexamined Patent Publication No. HEI 9-33881 and No. HEI 11-326727, for example, have been known, which are formed as a solid rod prism  100  made of glass as shown in FIG. 6, for example. As shown in FIG. 7, for example, the rod prism  100  guides light beams, incident thereon directly or after being reflected by a reflector  101 B from a light source section  101 A, toward its light exit end while totally reflecting them a plurality of times on its inner wall faces, thereby yielding a substantially uniform density distribution within a plane orthogonal to the optical axis at the light exit end thereof. Thereafter, illumination light with thus homogenized luminous flux density is turned into a parallel luminous flux by a convex lens  102 . As backlight, for example, the resulting parallel luminous flux irradiates an image display device  103  such as a liquid crystal panel, whereby the luminous flux carrying image information is projected onto a screen, which is not depicted, by a projection lens  104 .  
           [0007]    Meanwhile, such an illumination optical system is configured such that the light exit end face of the rod prism  100  and the device surface of the image display device  103  have an optically conjugate relationship with each other. Therefore, flaws and dust particles on the light exit end face of the rod prism  100  may form images on the device surface of the image display device  103 . The images of flaws and dust particles on the device surface may be superposed on the image displayed by the image display device  103 , so as to be projected onto the screen as being enlarged by about 50× or greater, for example. Since the flaws and dust particles on the light exit end face of the rod prism  10  are hard to eliminate completely, image qualities have been likely to deteriorate on the screen when such a rod prism  100  is used.  
           [0008]    For overcoming such a problem, a hollow prism  105  having mirror inner faces such as the one shown in FIG. 8 has been known. This hollow prism  105  is formed by providing one surface of each of four rectangular glass sheet materials with a reflective coating, and bonding and assembling them so as to form a box having a rectangular cross section such that their reflective coating surfaces are arranged inside. In this hollow prism  105 , luminous fluxes incident thereon from the light source side are reflected by the mirror surfaces of the inner walls of the prism a plurality of times so as to be guided to the light exit end. Since the light exit end has no end face which may cause flaws or dust particles attached thereto, no images of flaws and dust particles will be formed on the device surface of an image display device having a conjugate relationship with the light exit end position.  
           [0009]    However, unlike the total reflection by the side faces of the rod prism  100  mentioned above, the hollow prism  105  depends on the reflection by the inner wall faces turned into mirror surfaces by the reflective coating. Since the reflective coating on such an inner wall face is hard to attain a reflectance of 100%, the quantity of light reaching the light exit end face may greatly decrease if the reflection is repeated on the inner wall faces by a plurality of times.  
         SUMMARY OF THE INVENTION  
         [0010]    In view of such circumstances, it is an object of the present invention to provide a rod integrator which can prevent flaws and dust particles on the light exit end face of a prism from forming images on the device surface of an image display device and keep the quantity of light from decaying while passing therethrough, and an illumination optical system using the same.  
           [0011]    The present invention provides a rod integrator for receiving a luminous flux from a light source, homogenizing a density of the luminous flux, and then emitting thus homogenized luminous flux, the rod integrator comprising:  
           [0012]    a solid rod-shaped first member, positioned on the luminous flux entrance side, for guiding the luminous flux from the light source incident on one end to the other end while totally reflecting the luminous flux on a side face thereof; and  
           [0013]    a hollow second member, positioned on the luminous flux exit side, for guiding the luminous flux from the first member incident on one end to the other end while specularly reflecting the luminous flux and then emitting the luminous flux from the other end;  
           [0014]    wherein the first and second members are combined such that substantially all of the luminous flux emitted from the other end of the first member is made incident on one end of the second member.  
           [0015]    The present invention provides an illumination optical system comprising the rod integrator and an illumination light source for emitting the luminous flux incident on the rod integrator.  
           [0016]    Preferably, in this case, the rod integrator is positioned and secured by a holding mechanism, whereas the holding mechanism comprises a substrate and a pressing member having an elasticity.  
           [0017]    Preferably, the rod integrator is in contact with the pressing member by a plurality of protrusions provided with the pressing member.  
           [0018]    Preferably, each of the first and second members has a substantially rectangular cross section, whereas both of them are shaped in conformity to an image display device irradiated with the luminous flux emitted from the second member.  
           [0019]    Preferably, the second member has a total length shorter than that of the first member. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1 is a perspective view showing the rod integrator in accordance with an embodiment of the present invention;  
         [0021]    [0021]FIG. 2 is a view showing operations of the rod integrator shown in FIG. 1;  
         [0022]    [0022]FIG. 3 is a perspective view showing a rod holding mechanism for holding the rod integrator in accordance with the above-mentioned embodiment of the present invention;  
         [0023]    [0023]FIGS. 4A and 4B are respective perspective views showing a pressing plate and a pressing member which are constituent members of the rod holding mechanism shown in FIG. 3;  
         [0024]    [0024]FIGS. 5A to  5 C are schematic views showing respective rod integrators whose modes of prism combination are different from that of the rod integrator shown in FIG. 1;  
         [0025]    [0025]FIG. 6 is a perspective view showing a rod integrator constituted by a rod prism alone;  
         [0026]    [0026]FIG. 7 is a schematic view showing an example of illumination optical system using a rod prism; and  
         [0027]    [0027]FIG. 8 is a schematic view showing an example of hollow prism. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0028]    In the following, rod integrators and illumination optical systems in accordance with embodiments of the present invention will be explained with reference to the drawings.  
         [0029]    [0029]FIG. 1 is a schematic perspective view showing the rod integrator in accordance with an embodiment. This rod integrator  11  is constituted by a combination of a solid rod prism  12  made of glass and a hollow prism  13  having inner faces provided with a reflective coating. Namely, the rod prism  12  is made of a rod-shaped glass rod having a rectangular cross section, whereas the hollow prism  13  is formed by four glass sheets each having one surface provided with the reflective coating which are bonded so as to form a box having a rectangular cross section such that the reflective coating surfaces are arranged inside, one end part of the rod prism  12  being slightly inserted into the hollow prism  13 .  
         [0030]    For example, this rod integrator  11  is disposed in place of the rod prism  100  in the above-mentioned illumination optical system of FIG. 7, such that the rod prism  12  and the hollow prism  13  are positioned on the light source side and image display device side, respectively. Here, the hollow prism  13  has a length shorter than that of the rod prism  12 .  
         [0031]    In thus arranged rod integrator  11 , as shown in FIG. 2, a luminous flux outputted from the light source  101  is incident on the light entrance end  14  of the rod prism  12  disposed on the light source side. Since this luminous flux is incident on the light entrance end  14  of the rod prism  12  at various angles, its angles of incidence and reflection on inner wall faces  15  of the rod prism  12  vary (though the angle of incidence is such that total reflection occurs at the interface between glass and air). The luminous flux emitted from the rod prism  12  is incident on the hollow prism  13 , and is reflected by reflective faces  17  of the inner walls of the hollow prism  13  as in the reflection on the side faces of the rod prism  12 , so as to be emitted from the light exit end  16  of the hollow prism  13 . Since the mode of reflection varies on the inner wall faces of the rod integrator  11  as in the above-mentioned rod prism  100  and the like, the luminous flux density is homogenized in a cross section perpendicular to the optical axis when the luminous flux is emitted from the light exit end  16 .  
         [0032]    As a result of such a configuration, since the light exit end  16  is hollow in the rod integrator  11  in accordance with this embodiment, there is no fear of flaws and dust particles forming images on the device surface of the image display device  103  having a conjugate positional relationship with the light exit end  16 , unlike the prior art. Also, since the length of the rod prism  12  attaining a luminous flux reflection efficiency of nearly  100 % on the side wall face  15  occupies a large proportion of the whole length of the rod integrator  11 , the decay of light quantity can be kept lower than that in the prior art using the hollow prism  13  alone.  
         [0033]    Though the length of the hollow prism  13  is preferably as short as possible, it is necessary to secure such a length that influences of flaws, dust particles, and the like on the light exit end face of the rod prism  12  do not extend over the device surface of the image display device  103 .  
         [0034]    [0034]FIG. 3 is a schematic perspective view showing a rod holding mechanism  20  for holding the rod integrator  11  at a predetermined position of an optical system.  
         [0035]    This rod holding mechanism  20  comprises a plate member  21  for positioning the rod integrator  11  while in a state where the rod integrator  11  is mounted thereon, and a pressing member  31  for pressing thus positioned rod integrator  11  from thereabove so as to secure it at this position in the vertical direction.  
         [0036]    The plate member  21  has a groove  22  extending longitudinally at the center part thereof, and is configured such that, while one longitudinal edge of the rod integrator  11  is arranged at the bottom, two side faces meeting at this edge fit into the groove  22 . Also, while in the state placed in the groove  22 , the rod integrator  11  is restrained from moving longitudinally. Namely, a part of the wall face of the light exit end  16  of the hollow prism  13 , which is one end part of the rod integrator  11 , abuts against an abutment  23  raised from the groove  22 . In this state, the light entrance end face  14  of the rod prism  12  is urged toward the other end by a pressing plate  25  (see FIG. 4A) attached to the plate member  21 .  
         [0037]    As shown in FIG. 4A, the pressing plate  25  has an L-shaped cross section. In its raised wall part  26 , a rectangular opening  27  smaller than the light entrance end face  14  of the rod prism  12  is formed at substantially the center part thereof. Its bottom wall part  28 , on the other hand, is formed with a plurality of screw holes  29  for fastening the pressing plate  25  to the plate member  21  with screws.  
         [0038]    When the pressing plate  25  is secured to the plate member  21  by screws, an edge part of the light exit end face  14  of the rod integrator  11  positioned at a predetermined position is supported by an edge part of the opening  27  of the pressing plate  25 .  
         [0039]    The pressing member  31  comprises a pressing part  32  for pressing the rod integrator  11  from thereabove at four points of the upper face thereof by an elastic force; and a securing part  33 , joined to the pressing part  32 , having a rectangular U-shaped cross section for securing the pressing part  32  to the bottom face of the groove  22  of the plate member  21  (see FIG. 4B).  
         [0040]    The pressing part  32  is configured such that both longitudinal end parts thereof descend from the center part thereof. Each end part is bifurcated, whereas a protrusion  34  projecting from the lower face is formed at the center of each of the bifurcated leading parts.  
         [0041]    When the rod integrator  11  is set at a predetermined position so that the securing part  33  of the pressing member  31  is secured to the bottom face of the plate member  21 , both end parts of the pressing part  32  of the pressing member  31  urge the rod integrator  11  downward by their own elastic forces. This restrains the rod integrator  11  from moving vertically.  
         [0042]    At that time, the rod integrator  11  and the pressing part  32  of the pressing member  31  are in contact with each other by the four protrusions  34 , whereby they are in a point-contact state at the four protrusions  34 . The point-contact state is attained as such in order to minimize the contact area of members with outer wall faces, which may change the state of reflection when light is reflected by side wall faces of the rod integrator  11 . In particular, since the light is totally reflected by the interface between glass and air in the inner wall faces of the rod prism  12 , the point contact is preferred as the contact with the pressing member  31 .  
         [0043]    The rod holding mechanism is disposed so as to place the rod integrator  11  at a predetermined position between the light source  101  and the convex lens  102 .  
         [0044]    The combination of the rod prism  12  and hollow prism  13  is not limited to the mode shown in FIG. 2. For example, as shown in FIGS. 5A and 5B, the hollow prism  13  may have an inner diameter greater than the outer diameter of the rod prism  12 . Also, as shown in FIG. 5C, the rod prism  12  may have an outer diameter greater than the inner diameter of the hollow prism  13 . When the light exit end of the rod prism  12  and the light entrance end of the hollow prism  13  are separate from each other, it is preferred that an appropriate mask be disposed at a predetermined position between the two prisms  12 ,  13  so as to prevent external light from entering the hollow prism  13 .  
         [0045]    The hollow prism  13  may be formed not only from glass but also from a metal or the like. In the latter case, surfaces to become inner wall faces may be formed beforehand by specular processing.  
         [0046]    Though the hollow prism  13  is constituted by four sheet materials in the above-mentioned embodiment, it may also be constructed by two members each having an L-shaped cross section as a matter of course. The original members may have any form as long as inner wall faces can be formed as light reflecting surfaces.  
         [0047]    Though the rod prism  12  and hollow prism  13  have a rectangular cross section in conformity to the form of the image display device, they may have other forms such as circular cross-sectional forms, for example, as a matter of course. Operations and effects of the present invention can be attained in the latter case as well.  
         [0048]    In the rod integrator and illumination optical system of the present invention, a hollow prism is disposed on the image display device side, so that the light exit end face is hollow, whereby there is no fear of flaws and dust particles forming images on the device surface of the image display device having a conjugate positional relationship with the light exit end, which have been problematic in the prior art. Also, since the length of the rod prism yielding a luminous flux reflection efficiency of nearly 100% at its side wall faces occupies a large proportion of the total length of the rod integrator, the decay of light quantity can be kept lower than that in the prior art using the hollow prism alone.