Abstract:
In a device for generating a quadrangular illuminating field, having a light mixing rod, which has a quadrangular cross-section as well as a quadrangular inlet area and a quadrangular outlet area and guides light coupled in via the inlet area to the outlet area, in order to generate therein the quadrangular illuminating field, the outlet area is limited by four rectilinear sides, of which two each meet in one of the corners of the outlet area, and, in one or two corners, the sides meeting therein converge at right angles while, in the other corners, the sides meeting therein converge at an angle, which is not equal to 90°.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority of German patent application no. 101 03 098.3, filed Jan. 24, 2001, the complete disclosure of which is hereby incorporated by reference. 
     FIELD OF THE INVENTION 
     The invention relates to a device for generating a quadrangular illuminating field, having a light mixing rod, which has a quadrangular cross-section as well as a quadrangular inlet area and a quadrangular outlet area and guides light coupled in via the inlet area to the outlet area, in order to generate therein the quadrangular illuminating field, said outlet area being limited by four rectilinear sides, of which two each converge in one of the corners of the outlet area. The invention further relates to the use of such device for generating a quadrangular illuminating field in an optical device comprising a surface to be illuminated, which has a predetermined shape, and illumination optics, which form an image of the outlet area on the surface to be illuminated. 
     BACKGROUND OF THE INVENTION 
     Such device for generating a quadrangular illuminating field is employed, for example, in the illumination optics of a digital projector, as it is called, which also comprises, in addition to the illumination optics, a generally rectangular imaging element, such as a tilting mirror matrix or an LCD matrix, and projection optics for projecting the image generated by the imaging element. In such a digital projector, it is required for high-quality images that the imaging element be illuminated as uniformly as possible by the illumination optics. Therefore, the illumination optics contain the light mixing rod, whose outlet area conventionally has a rectangular shape, if the imaging element is rectangular. It has been found, however, that a complicated optical system is necessary for forming an image of the illuminating field in the outlet area of the light mixing rod on the imaging element, so as to ensure that the image is formed in a manner as free from distortion as possible. In many cases, this also raises the additional problem that the outlet area and the surface of the imaging element are not situated in planes which are parallel to each other, but are inclined relative to each other, which also causes distortions leading to decreased uniformity of the illumination of the imaging element. 
     OBJECT AND SUMMARY OF THE INVENTION 
     Therefore , it is an object of the invention to improve the above-described type of device for generating a quadrangular illuminating field such that it allows illuminating, as uniformly as possible, a surface which is to be illuminated and has a predetermined shape. 
     According to the invention, this object is achieved, in connection with the above-described type of device for generating a quadrangular illuminating field, in that the sides meeting in one or two corners converge at a right angle and the sides meeting in the remaining corners converge at an angle which is not equal to 90°. Thus, the shape of the outlet area of the light mixing rod may be selected such that the distortion caused by forming an image of the outlet area on the surface to be illuminated is compensated and the surface to be illuminated is uniformly illuminated. The light mixing rod may preferably extend rectilinearly or at an angle. 
     Since the outlet area, and preferably also the remaining cross-sectional areas of the light mixing rod, comprises one or two right angles, the production of the light mixing rod is simplified. In particular, if the four sides of each cross-sectional area of the light mixing rod converge at the same angles as the sides of the outlet area, the light mixing rod may easily be produced with the required accuracy. If it is a solid mixing rod, it may be formed by grinding and polishing a blank. If it is a hollow mixing rod, it may be produced from four reflectively coated plates. 
     Moreover, the device according to the invention may be further embodied such that the cross-sectional area of the light mixing rod decreases from the inlet area to the outlet area. Since, in a light mixing rod, the product of the inlet area with the aperture of the inlet area equals the product of the outlet area with the aperture of the outlet area, a larger aperture is possible at the outlet area. At the same time, the inlet area may be large, which facilitates the coupling-in of the light in the light mixing rod. 
     A particularly advantageous further embodiment of the device according to the invention consists in that the light mixing rod is made of a light-transparent material and is, consequently, a solid mixing rod, in which the light which does not travel directly from the inlet area to the outlet area is guided by total reflexion. Since no losses occur in said total reflexion and, consequently, only the losses caused by material absorption are present, nearly all of the coupled-in light is guided to the outlet area, so that a very high efficiency in light transmission is realized. Moreover, it is also an advantage that excessive heating of the light mixing rod is prevented because total reflexion involves no losses which might contribute to such heating. 
     A particular further embodiment of the device according to the invention consists in that the light mixing rod has a hollow cross-section, which is formed by four reflective surfaces extending from the inlet area to the outlet area. This configuration of the light mixing rod allows simple and accurate production. A further particular advantage of this embodiment is that the outlet area is not formed by an end surface of the material, from which the light mixing rod is produced, but is the region at the end of the light mixing rod between the four reflective surfaces, thus usually an air portion, since the light mixing rod is conventionally present in a surrounding atmosphere and thus in air. This leads to the advantage that on the outlet area no soiling or misting can form, which might adversely affect the uniformity of the generated quadrangular illuminating field. 
     In particular, the device according to the invention may be further embodied such that the light mixing rod comprises a solid mixing rod portion made of a light-transparent material and an end portion optically coupled therewith, which is provided with reflective surfaces and has a hollow cross-section and whose end averted from the solid mixing rod portion forms the outlet or inlet area. As the outlet or inlet area is formed by the end of the end portion averted from the solid mixing rod portion, this prevents that soiling or misting may accumulate in this area, so that the illuminating field generated in the outlet area is not adversely affected. 
     In a further preferred embodiment of the device according to the invention, the reflective surfaces partially extend across the solid mixing rod portion. This allows particularly simple production of the light mixing rod. Thus, the reflective surfaces may, for example, be joined with the solid mixing rod portion using an optical cement which is transparent to light. Advantageously, this type of joint does not require a further holding device or socket for the end portion. 
     Moreover, the device according to the invention may be further embodied such that the cross-sectional area of the end portion is greater than that of the solid mixing rod portion. Thus, a device is provided for generating a quadrangular illuminating field having a light mixing rod, which device allows the cross-sectional area to be easily changed. If the reflective surfaces, according to this further embodiment, also partially overlap the solid mixing rod portion, the end portion may be arranged such that it does not contact the solid mixing rod portion and that a gap is present between them. This has the advantageous effect that the overlapping part of the end portion does not eliminate the total reflexion of the solid mixing rod portion in this region. 
     A particularly advantageous embodiment of the device according to the invention consists in that the light mixing rod comprises a starting portion provided with reflective surfaces, which has a hollow cross-section and whose end averted from the solid mixing rod portion forms the inlet area. This light rod allows to effectively prevent soiling and misting from forming in both the inlet and the outlet area and thus also to prevent any deterioration of the optical properties of the light mixing rod. 
     The device according to the invention may also be further embodied such that the light mixing rod comprises a first and second light guiding region for separately guiding components of the coupled-in light due to a partition extending from the inlet area to the outlet area. Thus, it is possible to create an intentional non-uniformity of the luminance in the illuminating field. This non-uniformity may be adapted to uniformly illuminate the surface to be illuminated. 
     Moreover, the device according to the invention may be used to generate a quadrangular illuminating field, in particular in an optical device comprising a surface to be illuminated, which has a predetermined shape, and illumination optics, which form an image of the outlet area on the surface to be illuminated. Preferably, projection optics for forming an image of the surface to be illuminated on a projection surface are also provided. Thus, an optical device is provided wherein a more uniform illumination of the surface to be illuminated is possible due to the device according to the invention, because the distortions caused by the illumination optics may be compensated by suitably selecting the shape of the quadrangular outlet area of the light mixing rod. 
     The invention will be explained below by way of example and with reference to the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other objects of the present invention and various features and details of the operation and construction thereof are hereinafter more fully set forth with reference to the accompanying drawings, wherein: 
     FIG. 1 is a perspective view of a light mixing rod according to the invention; 
     FIG. 2 is a front view of the light mixing rod according to the invention as shown in FIG. 1; 
     FIG. 3 is a schematic representation of an optical device according to the invention, which comprises the light mixing rod according to the invention as shown in FIGS. 1 and 2; 
     FIG. 4 is a front view of a light mixing rod of the invention according to another embodiment; 
     FIG. 5 is a front view of a further embodiment of the light mixing rod of FIG. 4; 
     FIG. 6 is a side view of a light mixing rod of the invention according to another embodiment; 
     FIG. 7 is a front view of a light mixing rod shown in FIG. 6; 
     FIG. 8 is a side view of a light mixing rod of the invention according to a further embodiment; 
     FIG. 9 is a front view of a light mixing rod of the invention according to a further embodiment; 
     FIG. 10 is a side view of a light mixing rod of the invention according to a further embodiment; 
     FIG. 11 is a front view of a light mixing rod of the invention according to a further embodiment, and 
     FIG. 12 is a front view of a light mixing rod of the invention according to a further embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a schematic top view of a projection device according to the invention, wherein a ray path is indicated by way of example. The projection device contains a light source  1 , a light mixing rod  2 , into which light from the light source  1  may be coupled, and, subsequent to the light mixing rod  2 , illumination optics  3  which can illuminate a surface  4  to be illuminated by the light exiting from the light mixing rod  2 . The surface  4  to be illuminated is preferably an imaging element and may, for example, be a tilting mirror matrix or an LCD matrix having a quadrangular, in particular a rectangular or square, shape. The projection device further comprises projection optics  5 , by which the surface  4  to be illuminated may be projected onto a projection surface  6 . 
     The light mixing rod  2  comprises an inlet area  7  facing the light source  1  and an outlet area  8  facing the illumination optics  3 . It is arranged such that the outlet area  8  is not parallel to the surface  4  to be illuminated, but encloses an angle therewith, both in the top view shown in FIG.  3  and in a side view of the projection device. 
     The illumination optics  3  are designed such that an image of the outlet area  8  is formed on the surface  4  to be illuminated, and, to this end, they are provided with lens units  9 ,  10  and a deflecting prism  11 . In the deflecting prism  11 , the light exiting from the light mixing rod  2  is deflected only by refraction, and not by reflexion. 
     The projection optics  5  comprise the lens unit  10  and a further lens unit  12 . By the use of the deflecting prism  11  and by employing the lens unit  10  for both the illumination optics  3  and the projection optics  5 , a very compact projection device may be formed. 
     As best shown in FIGS. 1 and 2, the light mixing rod  2  consists of material transparent to light from the light source  1 , has a solid cross-section and, by total reflexion at the interfaces between the side surfaces  13 ,  14 ,  15  and  16  of the light mixing rod  2  and the surrounding area, guides those light rays from the inlet area  7  to the outlet area  8  which do not travel directly from the inlet area  7  to the outlet area  8 . Such a light mixing rod  2  is referred to as a “solid mixing rod”. 
     The shape of the light mixing rod  2  is selected to have a quadrangular cross-section, wherein, in the case of the quadrangular outlet area  8 , two interior angles α, δ circumferentially adjacent to the outlet area  8  have a value of 90°, while a further interior angle γ is greater than 90°, and the last interior angle is less than 90°. The interior angles α, β, γ, δ are selected such that the distortion in the image of the outlet area  8 , which distortion is caused by the position of the outlet area  8  of the light mixing rod  2  relative to the surface  4  to be illuminated and by the illumination optics  3 , is compensated so as to uniformly illuminate the surface  4  to be illuminated, which is a rectangular surface in this case. Preferably, the illumination optics  3  and the light mixing rod  2  are designed such that the surface  4  to be illuminated is slightly over-illuminated, i.e. the image of the outlet area  8  is somewhat larger than the surface  4  to be illuminated and protrudes on all four sides of the surface  4  to be illuminated. Thus, for example, the illumination of the surface  4  to be illuminated is ensured if the image of the outlet area still differs slightly from the rectangular shape or if the orientation of the surface  4  to be illuminated is not optimal. 
     The solid mixing rod  2  may be produced, for example, from a parallelepiped blank by grinding and polishing. 
     Alternatively, the light mixing rod  2  may be provided as a hollow mixing rod, as shown in FIG.  4 . In this embodiment, the mixing rod is formed by four side plates  17 ,  18 ,  19  and  20 , whose internal surfaces  21 ,  22 ,  23  and  24  are reflectively coated. 
     The four side plates  17  to  20  each form a substantially rectangular cross-section, with the side plates  17  and  19  being provided, at their inner surfaces  21 ,  23 , at both ends, with recesses  25 ,  26 ,  27  and  28  which extend from the inlet area  7  to the outlet area  8 . These recesses  25 ,  26 ,  27 ,  28 , into which the side plates  18  and  20  are inserted, are designed such that the desired interior angles α, β, γ, δ are present in the assembled state shown in FIG.  4 . Preferably, the side plates  17  to  20  are held together by fitting a piece of shrink tubing (not shown) over the side plates in their assembled state shown in FIG. 4, which tubing is then heated and contracts so that the side plates  17  to  20  are urged together by elastic pretension. This shrink tubing may be disposed, for example, in a central portion of the light mixing rod  2 . There may also be provided two pieces of shrink tubing, one in a region adjacent to the inlet area  7  and one in a region adjacent to the outlet area  8 . 
     FIG. 5 shows a further embodiment of the hollow mixing rod shown in FIG.  4 . In this further embodiment, a partition is provided which is formed by a plate  29  reflectively coated on both sides and extends from the inlet area  7  toward the outlet area  8  and, as shown in FIG. 5, is disposed diagonally, in cross-section, in the light mixing rod  2 . This plate  29 , which is reflectively coated on both sides, preferably extends a predetermined distance into the light mixing rod  2  from the inlet area  7 , which predetermined distance may one half to one third of the length of the light mixing rod  2 . By this partition, first and second light guiding regions  30  and  31  are formed, each of which has a triangular shape in cross-section. Due to this cross-sectional shape, the luminance distribution at the end of the light guiding regions  30  and  31  is different than it is in the same place in the light mixing rod shown in FIG. 4, so that the luminance distribution in the outlet area  8  also differs from that of the light mixing rod  2  shown in FIG.  4 . Thus, said partition leads to an intentional inhomogenization of the luminance distribution in the outlet area  8 , which effect is even stronger as the length of the plate  29  increases. This may be employed, e.g. in the case of the projection device shown in FIG. 3, to illuminate the surface  4  to be illuminated even more uniformly. The plate  29  is preferably disposed in the hollow mixing rod such that its front side  47  facing the outlet area  8  is not situated in the outlet area  8 , but inside the hollow mixing rod. The front side  47  and the corresponding opposite front side at the other end of the plate may be polished and plane, if the plate  29  is made of a light-transparent material, so that the plate  29  serves as a light guide for the light incident on the opposite front side. Then, only the minor losses of reflection and absorption occur, so that the plate  29  advantageously causes only very minor losses. Alternatively, the opposite front side may also be blackened, so that the light incident thereon is not passed on, but is blocked. 
     In a further preferred embodiment of the invention, such partition, in accordance with the embodiment of FIG. 5, may also be provided in a solid mixing rod. To this end, the starting portion of the light mixing rod  2 , which is to be provided with the partition, is formed by two prisms separated from each other by a gap. At the interfaces between the prism surfaces and the gap, total reflexion of light beams having predetermined angles occurs, so that the gap between the prisms also provides a partition leading to the inhomogenization of the luminance distribution in the outlet area  8 . 
     FIGS. 6 and 7 show a further embodiment of the light mixing rod  2  according to the invention. In this embodiment, the light mixing rod  2  comprises a solid mixing rod portion  32 , made of a light-transparent material, such as glass, and having a solid cross-section, and an end portion  33 , which is optically coupled with the solid mixing rod portion  32  at the outlet end thereof. 
     As best shown in FIG. 7, the end portion  33  has a hollow cross-section, which is limited by four plates  34 ,  35 ,  36 ,  37 . The inner sides  38 ,  39 ,  40 ,  41  of the plates  34  to  37  are reflectively coated, and the end of the end portion  33  averted from the inlet area  7  forms the outlet area  8 . On the side where the interior angles are not equal to 90°, the opposed plates  34  and  36  comprise recesses  43 ,  44  which extend in the longitudinal direction of the light mixing rod  2  and in which the plate  35  is guided. The recesses  43  and  44  are formed such that the desired interior angles β, γ are present in the outlet area  8  of the light mixing rod  2 . The right angle between the plates  34  and  37  may be realized in that the side surface of the plate  34  supported on the inner side  41  of the plate  37  extends perpendicularly to the inner side  38  of the plate  34 . The right angle between the plates  37  and  36  may be realized in the same manner, as shown in FIG.  7 . 
     The plates  34  to  37  are arranged such that the inner sides  38  to  41  extend perpendicularly to an end surface  42  of the solid mixing rod portion  32  opposed to the inlet area  7 , so that the light mixing rod  2  also extends rectilinearly. Of course, the plates  34  to  37  may also be arranged such that the inner sides  38  to  41  do not extend at a right angle, but at a different angle to the end surface  42 . In this case, the light mixing rod  2  will be bent. 
     As best shown in FIG. 6, the plates  34  to  37  extend, partially across the solid mixing rod portion  32 , in the longitudinal direction of the light mixing rod  2 , so that the end portion  33  partially overlaps the solid mixing rod portion  32 . The plates  34  to  37  are fixed to the solid mixing rod portion  32  using an optical cement. Alternatively, instead of using the optical cement, a piece of shrink tubing (not shown) may be fitted over the plates  34  to  37 , preferably in the region of the solid mixing rod portion  32 , and then heated so as to contract such that the plates  34  to  37  are urged against the solid mixing rod portion  32  by elastic pretension and thus fixed thereto. In this light mixing rod  2 , the plates  34  to  37  eliminate total reflexion in their region of contact with the solid mixing rod portion  32  and, instead, cause reflexions at the reflectively coated inner sides  38  to  41 . 
     The light mixing rod  2  is preferably designed such that, in the condition shown in FIG. 6, the length of the part of the end portion  33  (i.e. the protruding hollow portion) protruding from the solid mixing rod portion  32  in the longitudinal direction of the light mixing rod  2  is greater than the depth of focus of the illumination optics  3 . Preferably, the length of the protruding part is at least one order of magnitude greater than the depth of focus. This ensures that any soiling or misting on the end surface  42  of the solid mixing rod portion  32  will not lead to a substantial deterioriation in the illumination of the surface to be illuminated. 
     The light mixing rod  2  shown in FIGS. 6 and 7 extends rectilinearly. However, for example, it may be bent somewhere along the solid mixing rod portion  32 . It is essential that the light mixture of the coupled-in light be effected substantially in the solid mixing rod portion  32  so as to benefit from the advantage of the very small transmission losses of a solid mixing rod, and that the outlet area  8  be defined in such a manner by the end portion  33  having the hollow cross-section that the outlet area  8  is always free from misting and soiling. The contribution of the end portion  31  to the light mixture depends on its length and increases as its length increases. 
     In this embodiment, the light mixing rod may be supported by a holding device, which only engages the end portion  33 . This holding device is preferably connected with the external surfaces of the four plates  34  to  37  and does not have an adverse effect on the light mixture, since it does not affect the inner sides  38  to  41  of the plates  34  to  37 . If the size of the overlapping part of the end portion  33  does not suffice to hold the light rod by a holding device which only engages the end portion  33 , a further holding device may be provided which engages the solid mixing rod portion  32 . However, since such a holding device eliminates total reflexion in the region of contact with the solid mixing rod portion  32 , this will always lead to undesired losses. The latter may be reduced by reflectively coating the part of the holding device contacting the solid mixing portion  32 . However, also in this case, there is the advantage that the losses caused by the holding device are smaller than in a pure solid mixing rod, since the latter is usually supported by a holding device engaging at least two locations of the solid mixing rod spaced apart from each other in the longitudinal direction, thus causing losses at these two locations. Consequently, the losses caused by the holding device may be advantageously reduced in the light mixing rod shown in FIG.  6 . 
     In a further embodiment of the light mixing rod  2  according to the invention, there is no overlap of the solid mixing rod portion  32  by the end portion  33 , as shown in FIG.  8 . In this embodiment, the end surface  42  of the solid mixing rod portion  32  lies in the same plane as the inlet area of the end portions  33 , so that there are no losses of light at the transition between the solid mixing rod portion  32  and the end portion  33 . Further, total reflexion is not eliminated in any region of the solid mixing rod portion  32 . 
     FIG. 9 shows a further embodiment of the light mixing rod  2  according to the invention. In this embodiment, the cross-sectional area of the end portion  33  is greater than the cross-sectional area of the solid mixing rod portion  32 . The solid mixing rod portion  32  and the end portion  33  are arranged such that the centers of their cross-sectional areas are situated on the central longitudinal axis of the light mixing rod  2 . The end portion  33  is formed, in a manner similar to that of the embodiment shown in FIG. 7, by four plates  34  to  37 . If the end portion  33  overlaps the solid mixing rod portion  32 , this will result in a circumferentially extending gap  45  being present in the overlapping region between the solid mixing rod portion  32  and the end portion  33 . Consequently, the light in the light mixing rod  2  will be guided to the end surface  42  of the solid mixing rod portion  32  by total reflexion and will be guided by reflexion at the plates  34  to  37  only in the protruding hollow portion of the end portion  33 . Thus, the losses caused by the reflexion at the plates  34  to  37  will be reduced in comparison to the light mixing rod  2  shown in FIGS. 6 and 7. 
     Alternatively, the light mixing rod  2  according to the invention may also be used such that the end portion  33  faces the light source  1  and that its end forms the inlet area  7  (thus, the inlet and outlet areas have been changed around as compared with the embodiment shown in FIG.  3 ). This will reduce the thermal stresses on the material of the light mixing rod  2 , since the secondary focus of the light source  1 , in the optical device shown in FIG. 3, is situated in the plane of the inlet area  7 , which, due to the hollow cross-section of the end portion  33 , is not a material end surface of the light mixing rod  2 . 
     In a further embodiment of the light mixing rod  2  according to the invention, the solid mixing rod portion  32  is provided with an end portion  33  and a starting portion  46  at its opposite ends in the longitudinal direction, as shown in FIG.  10 . As shown by way of example in FIG. 10, the starting and end portions  46 ,  33  may both be designed as in the embodiment shown in FIGS. 6 and 7. However, they may also be provided as in the other embodiments described herein, and may, in particular, be different from each other. In such a light mixing rod  2 , both the inlet area  7  and the outlet area  8  are formed in air, thus combining the above-described advantages of such inlet and outlet areas. 
     If, in this embodiment, both the end portion  33  and the starting portion  46  contact the solid mixing rod portion  32 , the light mixing rod  2  may be held by a holding device, which engages the end portion  33 , on the one hand, and the starting portion  46 , on the other hand, so that no losses are caused by the holding device, because the holding device may be connected with the external plate surfaces of the end portions  33  and of the starting portion  46 , and these external surfaces are not involved in the light mixture. 
     The cross-sectional shape of the quadrangular light mixing rod  2  is not restricted to the shape shown in FIG.  2 . Thus, for example, the two opposite interior angles α and γ may also be right angles as shown in FIG.  11 . In the example represented therein, the angle β will then be greater than 90° and the angle δ will be less than 90°. 
     Alternatively, the light mixing rod  2  according to the invention may also have a cross-sectional shape in which only one right angle is present in the outlet area  8 , as shown in FIG.  12 . In the embodiment shown in said FIG., only the angle α is a right angle, while the other angles β, γ and δ are not equal to 90°.