Abstract:
A projection apparatus and a light guide module for use in the projection apparatus are provided. The projector apparatus comprises an illumination mechanism, at least one digital micromirror device (DMD), and a light guide module, wherein the light guide module includes a plurality of interfaces. The illumination mechanism provides a light beam which travels through the plurality of interfaces of the light guide module along a first axis without any rotation. The light beam orderly performs total internal reflections on each of the interfaces of the light guide module. Accordingly, the light beam travels to the at least one DMD, and then the light beam is reflected along a second axis.

Description:
[0001]    This application claims priority to Taiwan Patent Application No. 097134678 filed on Sep. 10, 2008, the content of which is incorporated herein by reference in its entirety. 
       CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0002]    Not applicable. 
       BACKGROUND OF THE INVENTION 
       [0003]    1. Field of the Invention 
         [0004]    The present invention provides an optical structure, in particular, to a light guide module for use in a projection apparatus and the projection apparatus. 
         [0005]    2. Descriptions of the Related Art 
         [0006]    DLP (Digital Light Processing) projectors, which are developed by Texas Instruments, are projection displays that utilize a particular light source modulation scheme. The most prominent feature of DLP projectors is that, as a fully digitalized reflection projector, they can not only present finer projection images, but also allow for effective reduction in both volume and weight of the projectors, thereby making the projectors lighter, thinner, shorter and smaller. DLP projectors are classified into either single-chip or three-chip projectors, and are mainly used as super-lightweight portable and high-luminance projectors. 
         [0007]    A DLP projector comprises a light source, a color wheel, a digital micromirror device (DMD) chip and a projection lens. Light from the light source is collected by a light collecting cover, focused via a lens and then sent through filters of three colors (i.e., red, green and blue) on a color wheel to the DMD chip. A memory associate with each pixel of the DMD records the value of a digital signal corresponding to the pixel, while the digital signal is transmitted to a drive electrode to induce positive or negative deflections of the micromirrors and control the deflection time. By controlling the rotational speed of the color wheel, alternation of the three primary colors (i.e., red, green and blue) can be accomplished to obtain a full color effect. 
         [0008]      FIG. 1  is a schematic view of a projection apparatus  1  that adopts the conventional DLP technology. The projection apparatus  1  comprises a light source  101 , a color wheel  103 , a light integration rod  105 , a relay mirror assembly  107 , a TIR (Total Internal Reflection) prism  109  (including a first prism  109   a  and a second prism  109   b ), a DMD  111  and a projection lens  113 . It should be noted that the color wheel  103  has three primary colors formed on individual portions adjacent to each other, with each color being formed on one portion respectively. Between a surface  1091  of the first prism  109   a  and a surface  1092  of the second prism  109   b,  there is an air gap  110 . The two surfaces  1091 ,  1092  are substantially parallel to each other. 
         [0009]    The light source  101  emits a white light beam (as indicated by the arrow), which is adapted to pass through the rotatable color wheel  103  to be split into different color lights. Then, the color lights travel through the light integration rod  105  to the relay mirror assembly  107 . It should be particularly noted that the relay mirror assembly  107  comprises a first reflecting mirror  107   a  and a second reflecting mirror  107   b,  while the color lights exiting from the light integration rod  105  are reflected sequentially by the first reflecting mirror  107   a  and the second reflecting mirror  107   b  to the first prism  109   a.  The first reflecting mirror  107   a  and the second reflecting mirror  107   b  are not in the same plane, so there is a three-dimensional rotation of the light beam reflected by these surfaces. 
         [0010]    Upon entering the first prism  109   a,  the color lights undergo a total internal reflection and then travel to the DMD  111 . Then, the color lights are selectively reflected by the DMD  111  back to the first prism  109   a,  passes through the first prism  109   a,  the air gap  110  and the second prism  109   b  in turn, and finally enters the projection lens  113  to form an image to be projected. 
         [0011]    However, the design of the projection apparatus  1  has the following disadvantages: 
         [0012]    Firstly, in the conventional projection apparatus, the light integration rod  105  has a profile (e.g., a rectangle with an aspect ratio of 16:9, with a long direction thereof lying in the vertical direction) that is rotated by 90° relative to that of the DMD  111  (e.g., a rectangle with an aspect ratio of 9:16, with a long direction thereof lying in the horizontal direction). In more detail, after passing through the light integration rod  105 , the light beam will also present a rectangular profile whose long direction lies in the vertical direction and whose aspect ratio is 16:9. When this incident light beam is guided by the rotation of the first reflecting mirror  107   a,  the reflected light beam thereof will exhibit a profile whose normal direction is rotated by a certain angle relative to the incident direction and then enters the second reflecting mirror  107   b.  Afterwards, the light beam is guided by the rotation of the second reflecting mirror  107   b  again, causing the normal direction of the reflected light beam&#39;s profile to be rotated by a further angle before the light beam enters the TIR prism  109 . As a result, the light beam entering the TIR prism  109  has a rectangular profile whose long direction lies in the horizontal direction with an aspect ratio of 9:16. Then, through a TIR in the TIR prism  109 , the light beam is reflected to the DMD  111  to be imaged and then is projected the projection lens  113 . As the reflected light beam presents attenuated luminance, this may degrade the utilization efficiency of the light source within the projection apparatus to an extent that the image finally obtained has inadequate luminance. 
         [0013]    Secondly, to collect light effectively in the aforesaid projection apparatus between the light integration rod and multiple light sources, the optical structure is designed in such a way that two light sources, namely a left and a right light sources, are arranged at an upper and a lower position respectively (i.e., arranged asymmetrically with one at an upper left position and the other at a lower right position; or alternatively, with one at a lower left position and the other at an upper right position). Three light integration rods are used (i.e., two of them are arranged at an upper and a lower position respectively near the light sources, and the third one is arranged far away from the light sources to integrate light beams from the aforesaid two light integration rods). Consequently, the conventional projection apparatus with multiple light sources is difficult to design and exhibits a poor space utilization. 
         [0014]    Thirdly, in the projection apparatus  1 , a fixing structure (not shown) is needed to position the reflecting mirrors  107   a,    107   b.  However, because orientations of the mirrors have a direct influence on the light path, they must be positioned accurately in order to ensure a correct light path. Consequently, the procedure of positioning the mirror assembly is very complex and tedious, which is unfavorable for mass production of projection apparatuses. 
         [0015]    In view of this, it is important to provide a projection apparatus with improved imaging quality, miniaturized profile and light weight by improving the complex light path design of the conventional projection apparatus and reducing loss of luminance of the light source. 
       SUMMARY OF THE INVENTION 
       [0016]    One objective of the present invention is to provide a light guide module for use in a projection apparatus. By using an improved TIR prism design instead of the reflecting mirrors as used in conventional projection apparatuses, this light guide module overcomes the problem of luminance loss in the conventional light path and simplifies the layout of components in the projection apparatus. 
         [0017]    The present invention provides a light guide module for use in a projection apparatus. The projection apparatus comprises an illumination mechanism and a DMD (Digital Micromirror Device). The illumination mechanism is adapted to provide a light beam, which travels along a first axis to the DMD via the light guide module and is then reflected along a second axis. The light guide module comprises a plurality of interfaces adapted to enable the light beam to undergo a plurality of total internal reflections on each of the interfaces in turn. The total internal reflections have a traveling direction parallel to a reference plane, while the first and second axes are parallel to each other. 
         [0018]    Another objective of the present invention is to provide a projection apparatus that utilizes an improved TIR prism design. This, on one hand, reduces the back focal length of the projection lens in the projection apparatus to allow for reduction in the volume of the projection apparatus and facilitate the development towards miniaturized and lightweight projection apparatuses. On the other hand, the design makes it easy to integrate multiple light sources into the projection apparatus and enhance the ability of the projection lens to collect light beams, thereby improving the quality of images projected. 
         [0019]    To this end, the present invention provides a projection apparatus, which comprises an illumination mechanism, at least one DMD (Digital Micromirror Device) and a light guide module. The illumination mechanism is adapted to provide a light beam, and the light guide module is adapted to guide a traveling direction of the light beam. The light beam travels along a first axis to the DMD via the light guide module, and is then reflected along a second axis. The light guide module comprises a plurality of interfaces adapted to enable the light beam to undergo a plurality of total internal reflections on each of the interfaces in turn. The total internal reflections have a traveling direction parallel to a reference plane, and the first axis and the second axis are parallel to each other. 
         [0020]    The detailed technology and preferred embodiments implemented for the subject invention are described in the following paragraphs accompanying the appended drawings for people skilled in this field to well appreciate the features of the claimed invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIG. 1  is a schematic view of a conventional projection apparatus utilizing DLP technology; 
           [0022]      FIG. 2  is a schematic view of a projection apparatus according to an embodiment of the present invention; 
           [0023]      FIG. 3  is a schematic view of a light guide module according to an embodiment of the present invention; 
           [0024]      FIG. 4  is a schematic view illustrating propagation of a light beam within the light guide module according to an embodiment of the present invention; and 
           [0025]      FIG. 5  is a schematic view illustrating a beam light incident on a light integration rod according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    In the following descriptions, the present invention will be explained with reference to embodiments thereof. However, these embodiments are only for purposes of illustration, but not to limit the present invention to any specific environment, applications or particular implementations described in these embodiments. It should be appreciated that in the following embodiments and the attached drawings, elements not directly related to this invention are omitted from depiction. 
         [0027]      FIG. 2  illustrates a schematic view of an embodiment of a projection apparatus  4  according to the present invention. The projection apparatus  4  comprises an illumination mechanism  401 , a color wheel  403 , a light integration rod  405 , a relay mirror assembly  406 , a DMD  211 , a light guide module  2  and a projection lens  413 . It should be appreciated that although this embodiment shown in  FIG. 2  is described with reference to a single DMD chip, the present invention is not limited thereto, and those of ordinary skill in the art may use a Philips prism (not shown) with a projection apparatus that has three DMD chips upon reviewing the present disclosure. Therefore, applications where the projection apparatus has three DMD chips are omitted from description herein. Furthermore, the projection apparatus of the present invention is particularly suited to an illumination mechanism with multiple light sources integrated therein, although the present invention is not limited thereto. Hereinafter, an example in which the illumination mechanism  401  comprises two light emitting devices  401   a,    401   b  will be described to explain features of the present invention. 
         [0028]      FIG. 3  illustrates a schematic view of a light guide module  2  in the projection apparatus  4 . The light guide module  2  of the present invention comprises at least one prism, each of which has a plurality of surfaces adapted to define a plurality of interfaces. In this embodiment, the light guide module  2  comprises a first prism  209   a  and a second prism  209   b,  both of which are triangular prisms. Specifically, the first prism  209   a  is approximately an isosceles triangular prism with a first surface  2091 , a second surface  2092  and a third surface  2093 . The second prism  209   b  is approximately a right-angle triangular prism with a first surface  2094 , a second surface  2095  and a third surface  2096 . The third surface  2093  of the first prism  209   a  is substantially parallel to the first surface  2094  of the second prism  209   b.    
         [0029]    Furthermore, the first prism  209   a  and the second prism  209   b  have a high refractive index. For example, the first prism  209   a  and the second prism  209   b  are made of a glass material with a refractive index of 1.6 to 2.0. For example, an SF57 glass material with a refractive index of 1.8, although the present invention is not limited thereto. A medium  210  is interposed between the adjacent surfaces of the first prism  209   a  and the second prism  209   b.  In particular, the medium  210  is interposed between the third surface  2093  of the first prism  209   a  and the first surface  2094  of the second prism  209   b,  and has a refractive index smaller than those of the first prism  209   a  and the second prism  209   b  to reduce the divergence angle of the light beam and consequently facilitate the occurrence of total internal reflections when the light beam propagates from the first prism  209   a  (a dense medium) to the interface with the medium  210  (a spare medium). In this embodiment, the medium  210  is air with a refractive index of substantially 1.0. 
         [0030]    The projection apparatus  4  of the present invention is characterized in that the light beam from the illumination mechanism  401  undergoes a plurality of total internal reflections within the light guide module  2 , so the reflecting mirrors (e.g., the reflecting mirrors  107   a,    107   b  in  FIG. 1 ) of the relay mirror assembly in the conventional optical mechanism can be simplified. In particular, the light beam supplied by the illumination mechanism  401  of this embodiment passes through the light integration rod  405  and the color wheel  403  in turn, and then enters the first prism  209   a  of the light guide module  2  along a first axis p 1 . Because the refractive index of the first prism  209   a  is much higher than that of the medium  210 , a total internal reflection is more likely to take place each time when a light beam travels to the interface between the first prism  209   a  and the medium  210 . Moreover, the larger the difference in refractive indexes, the smaller the critical angle at which total internal reflection takes place and therefore, the more likely total internal reflection will take place. 
         [0031]    In particular, in this embodiment, the light beam undergoes three total internal reflections within the first prism  209   a  and then enters the DMD  211  via the first prism  209   a,  where it is imaged and reflected by micromirrors (not shown) distributed on the DMD  211  along a second axis p 2  and is projected through the projection lens  413 . Here, the first axis p 1  and the second axis p 2  are parallel to each other, and the three total internal reflections take place on the first surface  2091 , the second surface  2092  and the third surface  2093  of the first prism  209   a  respectively. 
         [0032]    It should be noted that the aforesaid total internal reflections have a traveling direction parallel to a reference plane. In more detail, when traveling along the first axis p 1 , the light beam propagates in form of a plane wave w 1  as shown in  FIG. 4 . The plane wave w 1  of the light beam is distributed in the xz plane of the three-dimensional space (taking the coordinate system (x, y, z) as an example), and the direction in which the light beam travels (i.e., a normal direction n 1  of the plane wave w 1 ) is parallel to the y axis of the three-dimensional coordinate system (x, y, z). When the light beam undergoes a total internal reflection within the first prism  209   a,  all variations of the normal direction n 1  of the plane wave w 1  are parallel to the xy plane; i.e., the light beam entering the light guide module  2  of the present invention travels along a direction parallel to the xy plane at all times, so no three-dimensional rotation of the light beam occur with. 
         [0033]    As described above, the light guide module  2  of the present invention is adapted to have the light beam undergo a plurality of total internal reflections parallel to a reference plane without incurring three-dimensional rotations, so the optical mechanism disclosed in the projection apparatus of the present invention is particularly applicable to the integration of multiple light sources. In particular, because the light beam only undergoes total internal reflections without incurring three-dimensional rotations when traveling in the light guide module  2 , the aspect ratio of the profile of the light integration rod  405  may be identical to that of the profile of the DMD  211  in the present invention (e.g., the two profiles are both rectangles with an aspect ratio of 9:16 and the long direction of each rectangle lying in the horizontal direction). On the other hand, because the profile of the light integration rod  405  is a rectangle whose long direction lies in the horizontal direction, the cross section of the light integration rod  405  may be divided, in terms of the light path arrangement, into a left and a right regions  405   a,    405   b  adapted to receive light beams from the left and the right light source of the illumination structure  401  respectively, as shown in  FIG. 5 . In other words, in the illumination structure  401  of the projection apparatus  4  of the present invention, two light emitting devices  401   a,    401   b  may be easily disposed horizontally and symmetrically so that light beams from the light emitting devices  401   a,    401   b  are reflected by two side edges of the prism  4011  respectively and then collected by the regions  405   a,    405   b  of the light integration rod  405  along the first axis p 1 . This facilitates effective utilization and arrangement of the internal space of the projection apparatus  4 , and as compared to prior art solutions where the left and the right light source must be arranged asymmetrically at an upper and a lower position respectively, to coordinate with the light integration rod, (i.e., arranged asymmetrically with one at an upper left position and the other at a lower right position; or alternatively, with one at a lower left position and the other at an upper right position), the projection apparatus  4  of the present invention allows for effective utilization of the limited space within the projection apparatus, thereby resulting in a smaller volume. 
         [0034]    It should be noted that the light guide module  2  of the present invention guides the light beams to undergo total internal reflections therein parallel to a reference plane, so the relay mirror assembly  406  of the projection apparatus  4  of the present invention can eliminate use of two reflecting mirrors (e.g., the reflecting mirrors  107   a,    107   b  in  FIG. 1 ) that would otherwise be needed in the prior art optical mechanism, thereby making the projection apparatus lightweight and eliminating the complex procedure of positioning the reflecting mirrors and difficulty in controlling the positioning accuracy. Moreover, the problem that the multiple reflections of conventional light paths may result in luminance loss of light beams can also be solved. 
         [0035]    Furthermore, the projection apparatus  4  of the present invention is characterized in that a substantial decrease in the thickness of the first prism  209   a  of the light guide module  2  is made possible. As shown in  FIG. 3 , when the micromirrors on the DMD  211  are tilted by 12° relative to the second axis p 2 , a negative correlation relationship will be obtained between the angle θ of the first prism  209   a  and the refractive index N of the first prism  209   a,  i.e., θ=sin −1 (1/N)−sin −1 (0.21/N). For example, when the refractive index N is 1.8, the angle θ of the first prism  209   a  is 27.11°; and when the refractive index N is 2.0, the angle θ of the first prism  209   a  is 24°. Therefore, the larger the refractive index of the prism  209   a  is, the smaller the angle θ of the first prism  209   a  and, accordingly, the thinner the prism. It is particularly important that as the prism decreases in thickness, a back focal length h between the inner end of the projection lens  413  and the DMD  211  decreases correspondingly, which helps to further shrink the volume of the projection apparatus  4  and remarkably enhance the projection lens&#39;s ability to collect light beams, thereby improving the quality of the output images. 
         [0036]    The above disclosure is related to the detailed technical contents and inventive features thereof. People skilled in this field may proceed with a variety of modifications and replacements based on the disclosures and suggestions of the invention as described without departing from the characteristics thereof. Nevertheless, although such modifications and replacements are not fully disclosed in the above descriptions, they have substantially been covered in the following claims as appended.