Patent Publication Number: US-2023140825-A1

Title: Beam splitter/combiner and projection apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefits of Chinese application serial no. 202111286082.7, filed on Nov. 2, 2021, and Chinese application serial no. 202210214752.2, filed on Mar. 7, 2022. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification. 
     BACKGROUND 
     Technical Field 
     The disclosure relates to an optical device and a display apparatus, in particular to a beam splitter/combiner and a projection apparatus. 
     Description of Related Art 
     Generally speaking, when making the illumination module of a projection apparatus (projector), the light sources of various colors (such as red, green, and blue) are placed on the same substrate and integrated into the same module with the lens array. The optical path is designed in such a way that the centers of different color beams coincide with each other before entering the projector module of the projection apparatus to improve color uniformity, reduce the size of the projection apparatus, and lower the manufacturing cost of the projection apparatus. 
     However, as the size of the illumination module gradually shrinks, the light sources become closer to each other, and the color beams tend to be incident on the side or the edge of the light-transmitting substrate during the merging process, resulting in a loss of combining efficiency. In addition, it is difficult to make a beam splitter/combiner by zoning the coating on the same light-transmitting substrate. At the same time, when the light source and the beam splitter/combiner are integrated into the illumination module, it is necessary to align the optical path to the boundary of the coated optical film, which makes it more difficult to manufacture the illumination module. 
     On the other hand, in the process of light source packaging, due to the uncertainty in the process, the light output angle may be deviated, for example, the blue/green image may show an asymmetry in the uniformity due to some factors when the light output from the light source, for example, the projected image shows the phenomenon of left blue and right green. 
     The information disclosed in this Background section is only for enhancement of understanding of the background of the described technology and therefore it may contain information that does not form the prior art that is already known to a person of ordinary skill in the art. Further, the information disclosed in the Background section does not mean that one or more problems to be resolved by one or more embodiments of the invention was acknowledged by a person of ordinary skill in the art. 
     SUMMARY 
     The disclosure provides a beam splitter/combiner and a projection apparatus, capable of combining different color beams or splitting color beams, while reducing a size of a projection apparatus and the manufacturing cost of the projection apparatus. 
     Other purposes and advantages of the disclosure can be further understood from the technical features disclosed in the disclosure. 
     In order to achieve one or part or all of the above objectives or other objectives, an embodiment of the disclosure provides a beam splitter/combiner. The beam splitter/combiner includes a first light-transmitting substrate, a first light transmission element, and a second light transmission element. The first light-transmitting substrate has a first optical surface facing incident light and a second optical surface opposite to the first optical surface. The first light transmission element is disposed on the first optical surface. The second light transmission element is disposed on the second optical surface. A first color beam incident on the first light-transmitting substrate is reflected by the first light transmission element and leaves the first light-transmitting substrate. A second color beam incident on the first light-transmitting substrate passes through the first light transmission element, is reflected by the second light transmission element, then passes through the first light transmission element, and leaves the first light-transmitting substrate. Paths of the first color beam and the second color beam incident on or leaving the first light-transmitting substrate coincide. 
     In order to achieve one or part or all of the above objectives or other objectives, the disclosure provides a projection apparatus. The projection apparatus includes a light source module, a beam splitter/combiner, a light valve, and a projection lens. The light source module is configured to generate a first color beam and a second color beam. The beam splitter/combiner is disposed on a transmission path of the first color beam and the second color beam. The beam splitter/combiner is configured to form an illumination beam from the first color beam and the second color beam. The beam splitter/combiner includes a first light-transmitting substrate, a first light transmission element, and a second light transmission element. The first light-transmitting substrate has a first optical surface facing incident light and a second optical surface opposite to the first optical surface. The first light transmission element is disposed on the first optical surface. The second light transmission element is disposed on the second optical surface. The first color beam incident on the first light-transmitting substrate is reflected by the first light transmission element and leaves the first light-transmitting substrate, and the second color beam incident on the first light-transmitting substrate passes through the first light transmission element, is reflected by the second light transmission element, then passes through the first light transmission element and leaves the first light-transmitting substrate. Paths of the first color beam and the second color beam incident on or leaving the first light-transmitting substrate coincide. The light valve is disposed on a transmission path of the illumination beam, and the light valve is configured to convert the illumination beam into an image beam. The projection lens is disposed on a transmission path of the image beam for projecting the image beam outside the projection apparatus. 
     Based on the above, by changing the thickness of the first light-transmitting substrate, it is possible to control positions of emitted light of different color beams to achieve a combined light effect with center axes of beams aligned with each other. In addition, the disclosure may prevent a loss of light leakage caused by the beam incident to a side or an edge of the first light-transmitting substrate. The light transmission element on the first light-transmitting substrate of the disclosure is a complete optical film, which may effectively reduce the manufacturing cost of the beam splitter/combiner. At the same time, when the light source and the beam splitter/combiner are integrated into an illumination module, it is much easier to align light paths than the first light-transmitting substrate with zoned coating, which may reduce the difficulty of manufacturing the illumination module. 
     Other objectives, features and advantages of the disclosure will be further understood from the further technological features disclosed by the embodiments of the disclosure wherein there are shown and described preferred embodiments of this disclosure, simply by way of illustration of modes best suited to carry out the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the disclosure and, together with the description, serve to explain the principles of the disclosure. 
         FIG.  1    is a schematic diagram of a projection apparatus according to an embodiment of the disclosure. 
         FIG.  2    is a schematic diagram of an illumination module according to an embodiment of the disclosure. 
         FIG.  3    is a schematic diagram of an illumination module according to another embodiment of the disclosure. 
         FIG.  4    is a schematic diagram of a beam splitter/combiner according to an embodiment of the disclosure. 
         FIG.  5    is a schematic diagram of an illumination module according to another embodiment of the disclosure. 
         FIG.  6    is a schematic diagram of an illumination module according to another embodiment of the disclosure. 
         FIG.  7    is a schematic diagram of an illumination module according to another embodiment of the disclosure. 
         FIG.  8    is a schematic diagram of an illumination module according to another embodiment of the disclosure. 
         FIG.  9    is a schematic diagram of an illumination module according to another embodiment of the disclosure. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” etc., is used with reference to the orientation of the Figure(s) being described. The components of the disclosure can be positioned in a number of different orientations. As such, the directional terminology is used for purposes of illustration and is in no way limiting. On the other hand, the drawings are only schematic and the sizes of components may be exaggerated for clarity. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the disclosure. Also, it is to be understood that the phraseology and terminology used herein are for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. Unless limited otherwise, the terms “connected,” “coupled,” and “mounted” and variations thereof herein are used broadly and encompass direct and indirect connections, couplings, and mountings. Similarly, the terms “facing,” “faces” and variations thereof herein are used broadly and encompass direct and indirect facing, and “adjacent to” and variations thereof herein are used broadly and encompass directly and indirectly “adjacent to”. Therefore, the description of “A” component facing “B” component herein may contain the situations that “A” component directly faces “B” component or one or more additional components are between “A” component and “B” component. Also, the description of “A” component “adjacent to” “B” component herein may contain the situations that “A” component is directly “adjacent to” “B” component or one or more additional components are between “A” component and “B” component. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive. 
     The foregoing and other technical contents, features and effects of the disclosure will be clearly presented in the following detailed description of a preferred embodiment with reference to the drawings. The directional terms (e.g., up, down, left, right, forward or backward, etc.) referred to in the following embodiments are intended to refer only to the direction of the additional views. Accordingly, the directional terms used are for illustrative purposes and are not intended to limit the disclosure. 
       FIG.  1    is a schematic diagram of a projection apparatus according to an embodiment of the disclosure. A projection apparatus  200  includes an illumination module  150   a , a light valve  220 , and a projection lens  230 . The illumination module  150   a  includes a beam splitter/combiner  100   a  and a light source module  210 . 
     The light source module  210  includes a first color light source  210 G and a second color light source  210 B for generating a first color beam and a second color beam, respectively. The first color light source  210 G includes first color light units  210 G 1  and  210 G 2  for generating first color beams LG 1  and LG 2 , respectively. The second color light source  210 B includes second color light units  210 B 1  and  210 B 2  for generating second color beams LB 1  and LB 2 , respectively. Further, wavelengths of the first color beams LG 1  and LG 2  are different from wavelengths of the second color beams LB 1  and LB 2 . According to this embodiment, the first color light source  210 G is, for example, a green light source to generate a green light beam (the first color beam), and the second color light source  210 B is, for example, a blue light source to generate a blue light beam (the second color beam). According to some embodiments, a number of the first color light unit of the first color light source  210 G and the second color light unit of the second color light source  210 B may be one or more than one, and the disclosure is not limited thereto. The light source module  210  used in this embodiment is, for example, a laser diode (LD) or a laser diode bank. Specifically, light source modules that meet the volume requirements can be implemented according to the actual design, and the disclosure is not limited thereto. As shown in  FIG.  1   , the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  are incident on the beam splitter/combiner  100   a  respectively. According to some embodiments, the first color beams LG 1  and LG 2  generated by the first color light units  210 G 1  and  210 G 2  are parallel to the second color beams LB 1  and LB 2  generated by the second color light units  210 B 1  and  210 B 2 . 
     The beam splitter/combiner  100   a  is disposed on a transmission path of incident light emitted by the light source module  210 . The incident light includes the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2 . The beam splitter/combiner  100   a  is configured to combine the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  to form an illumination beam L. 
     The beam splitter/combiner  100   a  includes a light-transmitting substrate  110 . The light-transmitting substrate  110  has a first optical surface  112  facing the incident light and a second optical surface  114  opposite to the first optical surface  112 . According to some embodiments, a material of the light-transmitting substrate  110  may be glass or other light-transmitting materials, and the disclosure is not limited thereto. According to some embodiments, the first optical surface  112  and the second optical surface  114  may or may not be parallel to each other, and the disclosure is not limited thereto. 
     The beam splitter/combiner  100   a  further includes a first light transmission element  116  disposed on the first optical surface  112 , and a second light transmission element  118  disposed on the second optical surface  114 . According to some embodiments, a material of the first light transmission element  116  and the second light transmission element  118  may be titanium dioxide (TiO 2 ) or silicon dioxide (SiO 2 ), or other suitable materials. According to some embodiments, the material of the first light transmission element  116  and the second light transmission element  118  are different. According to some embodiments, the material of the first light transmission element  116  and the second light transmission element  118  may be the same, but a thickness of the first light transmission element  116  and the second light transmission element  118  are different. The term “light transmission element” is defined as an element that provides a light transmission effect of reflecting a beam or allowing light to pass through. 
     According to some embodiments, the first light transmission element  116  is designed to reflect green light and allow blue light to pass through, and the second light transmission element  118  is designed to reflect blue light. Further, according to other embodiments, the first light transmission element  116  may reflect the first color beams LG 1  and LG 2  (green light beam), the second color beams LB 1  and LB 2  (blue light beam) may pass through the first light transmission element  116 , and the second light transmission element  118  may reflect the second color beams LB 1  and LB 2  (blue light beam). In addition, according to other embodiments, the first light transmission element  116  may be designed to reflect blue light and allow green light to pass through, and the second light transmission element  118  may be designed to reflect green light. Further, according to other embodiments, the first color beams LG 1  and LG 2  may be, for example, blue light beams, and the second color beams LB 1  and LB 2  may be, for example, green light beams. The first light transmission element  116  may reflect the first color beams LG 1  and LG 2  (blue light beams), the second color beam LB 1  and LB 2  (green light beams) may pass through the first light transmission element  116 , and the second light transmission element  118  may reflect the second color beams LB 1  and LB 2  (green light beams). According to this embodiments, the first light transmission element  116  is, for example, a dielectric coating layer with a beam splitting function attached to the light-transmitting substrate  110 , so that different light transmission effects (reflection or passing through) may be provided for the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  of different wavelengths, respectively, to obtain a beam splitting effect. The second light transmission element  118  is, for example, a dielectric coating layer with a beam splitting function attached to the light-transmitting substrate  110 , or a reflective mirror layer formed by a metallic coating attached to the light-transmitting substrate  110  to provide a reflective effect on the second color beams LB 1  and LB 2 . Further, when the first light transmission element  116  or the second light transmission element  118  is a dielectric coating layer, the beam splitting effect may be obtained by designing the first light transmission element  116  and the second light transmission element  118  to have different materials or thicknesses. The following is an example to illustrate the function of the beam splitter/combiner  100   a  with the first color light unit  210 G 1  and the second color light unit  210 B 1 . 
     As shown in  FIG.  1   , the first light transmission element  116  of the disclosure is configured to reflect the green light beams and allow the blue light beams to pass through. The second light transmission element  118  of the disclosure is configured to reflect the blue light beams. 
     When the first color beam LG 1  emitted by the first color light unit  210 G 1  is incident on the beam splitter/combiner  100   a , the first color beam LG 1  is incident at an incidence angle θ on the light-transmitting substrate  110  of the beam splitter/combiner  100   a . The first color beam LG 1  is reflected by the first light transmission element  116  on the first optical surface  112  of the light-transmitting substrate  110  and leaves the light-transmitting substrate  110 . The incidence angle of the first color beam LG 1  onto the light-transmitting substrate  110  is defined as an angle between the first color beam LG 1  and a normal line of the first optical surface  112 . 
     On the other hand, when the second color beam LB 1  emitted by the second color light unit  210 B 1  is incident on the beam splitter/combiner  100   a , since the second color beam LB 1  and the first color beam LG 1  are parallel to each other, the second color beam LB 1  is also incident at the same incidence angle θ as the first color beam LG 1  on the light-transmitting substrate  110  of the beam splitter/combiner  100   a . After the second color beam LB 1  passes through the first light transmission element  116  on the first optical surface  112  of the light-transmitting substrate  110 , the second color beam LB 1  is reflected by the second light transmission element  118  on the second optical surface  114  of the light-transmitting substrate  110 , and then passes through the first light transmission element  116  and leaves the light-transmitting substrate  110 . The incidence angle of the second color beam LB 1  onto the light-transmitting substrate  110  is defined as an angle between the second color beam LB 1  and the normal line of the first optical surface  112 . 
     Since an optical path is affected by a thickness and refractive index of the light-transmitting substrate  110 , when the second color beam LB 1  is reflected by the second light transmission element  118  on the second optical surface  114  of the light-transmitting substrate  110 , passes through the first light transmission element  116  and leaves the light-transmitting substrate  110 , a path of the second color beam LB 1  leaving the light-transmitting substrate  110  is not always the same as a path of the first color beam LG 1  leaving the light-transmitting substrate  110 . In other words, when the second color beam LB 1  leaves the light-transmitting substrate  110 , a center axis of the second color beam LB 1  and a center axis of the first color beam LG 1  are not always aligned with each other. If the first color beam LG 1  and the second color beam LB 1  do not coincide in their paths when leaving the light-transmitting substrate  110 , uniformity of color output of entire projection system is affected. 
     In order to make the path of the second color beam LB 1  leaving the light-transmitting substrate  110  the same as the path of the first color beam LG 1  leaving the light-transmitting substrate  110 , that is, the center axes of the second color beam LB 1  and the first color beam LG 1  are aligned with each other and combined, a path of the second color beam LB 1  inside the light-transmitting substrate  110  may be changed by adjusting a thickness t between the first optical surface  112  and the second optical surface  114 , so that the path of the second color beam LB 1  leaving the light-transmitting substrate  110  is the same as the path of the first color beam LG 1  leaving the light-transmitting substrate  110 . 
     Displacement of the path of the first color beam LG 1  leaving the light-transmitting substrate  110  and the path of the second color beam LB 1  leaving the light-transmitting substrate  110  is related to a distance d between the first color beam LG 1  and the second color beam LB 1  before they are incident on the light-transmitting substrate  110 , refractive index n of the light-transmitting substrate  110 , and the incidence angle θ of the first color beam LG 1  and the second color beam LB 1  incident on the first optical surface  112  of the light-transmitting substrate  110 . 
     According to some embodiments, when the first optical surface  112  of the light-transmitting substrate  110  is parallel to the second optical surface  114  of the light-transmitting substrate  110 , the thickness of the light-transmitting substrate  110  is t, 
     
       
         
           
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                   ⁢ 
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     In this condition, the second color beam LB 1  and the first color beam LG 1  leave the light-transmitting substrate  110  on the same path, that is, the center axes of the second color beam LB 1  and the first color beam LG 1  are aligned with each other and combined. 
     Similarly, when the second color beam LB 2  emitted by the second color light unit  210 B 2  passes through the first light transmission element  116  and leaves the light-transmitting substrate  110 , the path of the second color beam LB 2  leaving the light-transmitting substrate  110  is the same as the first color beam LG 2  emitted by the first color light unit  210 G 2  leaving the light-transmitting substrate  110 , that is, the center axes of the second color beam LB 2  and the first color beam LG 2  are aligned with each other and combined. The first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  form an illumination beam L leaving the beam splitter/combiner  100   a  and incident on the light valve  220 . 
     According to some embodiments, the incidence angle θ of the first color beam LG 1  and the second color beam LB 1  incident on the first optical surface  112  of the light-transmitting substrate  110  (i.e., the angle between the first color beam LG 1  and the second color beam LB 1  and the normal line of the first optical surface  112 ) falls roughly in a range of 30 degrees to 75 degrees. Considering the thickness of the light-transmitting substrate and the size of the beam splitter/combiner, the incidence angle θ falls roughly in a range of 30 degrees to 60 degrees. According to some embodiments, the incidence angle θ is roughly 45 degrees. 
     According to some embodiments, the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  form same size light spots when incident on the beam splitter/combiner  100   a . Geometric centers of the light spots formed by the first color beam LG 1  and the second color beam LB 1  leaving the light-transmitting substrate  110  coincide, and the geometric centers of the light spots formed by the first color beam LG 2  and the second color beam LB 2  leaving the light-transmitting substrate  110  coincide. 
     According to some embodiments, the illumination module  150   a  of the projection apparatus  200  further includes a third color light source, e.g., a red color light source (not shown in  FIG.  1   ). The third color light source may be disposed at any position of the projection apparatus  200 , and the third color light source is configured to emit a third color beam, e.g., a red light beam. The third color beam, the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  form the illumination beam L, incident on the light valve  220 . 
     According to some embodiments, if emission positions of the first color beam LG 1  and the second color beam LB 1 , the first color beam LG 2  and the second color beam LB 2  are changed to incident positions, the paths of the first color beam LG 1  and the second color beam LB 1  incident on the light-transmitting substrate  110  coincide, the paths of the first color beam LG 2  and the second color beam LB 2  incident on the light-transmitting substrate  110  coincide, the geometric centers of the light spots formed by the first color beam LG 1  and the second color beam LB 1  incident on the light-transmitting substrate  110  coincide, and the geometric centers of the light spots formed by the first color beam LG 2  and the second color beam LB 2  incident on the light-transmitting substrate  110  coincide. The beam splitter/combiner  100   a  may split the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  of the illumination beam L. The emission positions of the first color beams LG 1 , LG 2  and the second color beams LB 1 , LB 2  after being split may be controlled by changing the thickness t of the light-transmitting substrate  110 . 
     The light valve  220  is disposed on a transmission path of the illumination beam L, and the light valve  220  is configured to convert the illumination beam L into an image beam LI. According to some embodiments, the light valve  220  is, for example, a digital micro-mirror device (DMD) or a liquid-crystal-on-silicon panel (LCOS panel). However, according to other embodiments, the light valve  220  may also be a transparent liquid crystal panel or other spatial light modulator, and the disclosure is not limited thereto. 
     The projection lens  230  is disposed on a transmission path of the image beam LI, and is configured to project the image beam LI outside the projection apparatus  200 , such as on a screen (not shown) outside the projection apparatus  200 , to form an image. The projection lens  230  includes, for example, a combination of one or more optical lenses with refractive power. The optical lens includes, for example, a non-planar lens such as a biconcave lens, a biconvex lens, a concave-convex lens, a convex-concave lens, a plano-convex lens, a plano-concave lens, etc., or various combinations thereof. According to an embodiment, the projection lens  230  may also include a flat optical lens. Here, the embodiment does not limit the form and the type of the projection lens  230 . 
       FIG.  2    is a schematic diagram of an illumination module according to an embodiment of the disclosure. An illumination module  150   b  includes a beam splitter/combiner  100   b  and a light source module  210 . The illumination module  150   b  shown in  FIG.  2    is similar to the illumination module  150   a  in  FIG.  1   . The difference between  FIG.  2    and  FIG.  1    is that in  FIG.  2   , a first color light source  210 G of the light source module  210  has three first color light units  210 G 1 ,  210 G 2 , and  210 G 3 . That is, a number of the first color light source units in the embodiment of  FIG.  2    is greater than a number of second color light source units. According to some embodiments, the number of the first color light units may be equal to or less than the number of the second color light units, and the disclosure is not limited thereto. Since the first color light source  210 G has three first color light units  210 G 1 ,  210 G 2 , and  210 G 3 , a light spot formed when a first color beam LG emitted by the first color light source  210 G is incident on the beam splitter/combiner  100   b  is larger than a light spot formed when a second color beam LB emitted by the second color light source  210 B with two second color light units  210 B 1  and  210 B 2  is incident on the beam splitter/combiner  100   b . Therefore, when the first color beam LG and the second color beam LB are incident at an incidence angle θ on a light-transmitting substrate  110  of the beam splitter/combiner  100   b , the light spot projected by the first color beam LG on a first optical surface  112  of the beam splitter/combiner  100   b  will be larger than the light spot projected by the second color beam LB on the first optical surface  112  of the beam splitter/combiner  100   b.    
     Although the first color beam LG and the second color beam LB have different light spot sizes projected on the first optical surface  112  of the beam splitter/combiner  100   b , paths of the first color beam LG and the second color beam LB leaving the light-transmitting substrate  110  may be the same by changing a thickness of the light-transmitting substrate  110 . The geometric centers of the light spots formed by the first color beam LG and the second color beam LB coincide with each other. 
     According to some embodiments, as shown in  FIG.  2   , the first color light units  210 G 1 ,  210 G 2 , and  210 G 3  of the first color light source  210 G are combined to emit the first color beam LG, and the second color light units  210 B 1  and  210 B 2  of the second color light source  210 B are combined to emit the second color beam LB. An optical axis of the first color beam LG is in the middle of the first color light units  210 G 1 ,  210 G 2 , and  210 G 3 . An optical axis of the second color beam LB is in the middle of the second color light units  210 B 1  and  210 B 2 . 
     When the first color beam LG emitted by the first color light source  210 G is incident on the light-transmitting substrate  110 , the first color beam LG incident on the light-transmitting substrate  110  is reflected by a first light transmission element  116  and leaves the light-transmitting substrate  110 . 
     When the second color beam LB emitted by the second color light source  210 B is incident on the light-transmitting substrate  110 , the second color beam LB passes through the first light transmission element  116 , then the second color beam LB is reflected by the second light transmission element  118 , then passes through the first light transmission element  116 , and leaves the light-transmitting substrate  110 . 
     In order to coincide the paths of the first color beam LG and the second color beam LB leaving the light-transmitting substrate  110 , the beam splitter/combiner  100   b  satisfies the following conditions. 
     The first optical surface  112  and a second optical surface  114  are parallel to each other. When a distance between the optical axis of the first color beam LG and the optical axis of the second color beam LB before they are incident on the first optical surface  112  is d, refractive index of the light-transmitting substrate  110  is n, and an angle (incidence angle) between the incident first color beam LG (the second color beam LB) and the normal line of the first optical surface  112  is θ, a thickness of the light-transmitting substrate  110  is t, 
     
       
         
           
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                       2 
                     
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                       θ 
                     
                   
                 
                 
                   sin 
                   ⁢ 
                   2 
                   ⁢ 
                   θ 
                 
               
               × 
               
                 d 
                 . 
               
             
           
         
       
     
     In these conditions, the path of the first color beam LG leaving the light-transmitting substrate  110  is the same as the path of the second color beam LB leaving the light-transmitting substrate  110 , that is, when the first color beam LG and the second color beam LB leave the light-transmitting substrate  110 , center axes of the first color beam LG and the second color beam LB are aligned with each other and combined. In addition, the geometric centers of the respective light spots formed by the first color beam LG and the second color beam LB when leaving the light-transmitting substrate  110  coincide. 
     Although the light spot sizes of the first color beam LG and the second color beam LB are different, by designing the thickness t of the light-transmitting substrate  110  of the beam splitter/combiner  100   b , emission positions (the positions when the first color beam LG and the second color beam LB leaving the light-transmitting substrate  110 ) of the first color beam LG and the second color beam LB may be adjusted so that the first color beam LG and the second color beam LB are combined by aligning the center axes of the beams. 
     According to some embodiments, if the emission positions of the first color beam LG and the second color beam LB are changed to incident positions, paths of the first color beam LG and the second color beam LB incident on the light-transmitting substrate  110  coincide. The beam splitter/combiner  100   b  may split the first color beam LG and the second color beam LB of an illumination beam L. The emission positions of the first color beam LG and the second color beam LB after being split may be controlled by adjusting the thickness t of the light-transmitting substrate  110 . 
       FIG.  3    is a schematic diagram of an illumination module according to another embodiment of the disclosure. An illumination module  150   c  includes a beam splitter/combiner  100   c  and a light source module  210 . The illumination module  150   c  shown in  FIG.  3    is similar to the illumination module  150   b  shown in  FIG.  2   . The difference is that the light source module  210  in  FIG.  3    includes not only a first color light source  210 G and a second color light source  210 B, but also a third color light source  210 R. The third color light source  210 R includes third color light source units  210 R 1 ,  210 R 2 , and  210 R 3 . According to some embodiments, a number of the third color light source units of the third color light source  210 R may be one or more than one, and the disclosure is not limited thereto. The third color light source units  210 R 1 ,  210 R 2 , and  210 R 3  are combined to emit a third color beam LR. The third color beam LR is parallel to a first color beam LG and a second color beam LB. Further, a wavelength of the first color beam LG, a wavelength of the second color beam LB, and a wavelength of the third color beam LR are not the same as each other. According to this embodiment, the first color light source  210 G is, for example, a green light source to generate a green light beam (the first color beam), and the second color light source  210 B is, for example, a blue light source to generate a blue light beam (the second color beam), and the third color light source  210 R is, for example, a red light source to generate a red light beam (the third color beam). Since the third color light source  210 R has three third color light source units  210 R 1 ,  210 R 2 , and  210 R 3 , a light spot formed when the third color beam LR emitted by the third color light source  210 R is incident on the beam splitter/combiner  100   c  is larger than a light spot formed when the second color beam LB emitted by the second color light source  210 B with two second color light units  210 B 1  and  210 B 2  is incident on the beam splitter/combiner  100   c . Therefore, when the third color beam LR and the second color beam LB are incident at an incidence angle θ on a light-transmitting substrate  110  of the beam splitter/combiner  100   c , the light spot projected by the third color beam LR on a first optical surface  112  of the beam splitter/combiner  100   c  will be larger than the light spot projected by the second color beam LB on the first optical surface  112  of the beam splitter/combiner  100   c . In addition, in  FIG.  3   , the beam splitter/combiner  100   c  further includes a third light transmission element  130 . According to some embodiments, the third light transmission element  130  is a flat mirror. A gas layer  140  is located between the third light transmission element  130  and the second light transmission element  118 . According to some embodiments, a material of the gas layer  140  is air, but the disclosure is not limited thereto. 
     Although light spot sizes projected by the first color beam LG, the second color beam LB, and the third color beam LR on the first optical surface  112  of the beam splitter/combiner  100   c  are different, by designing a thickness t1 of the light-transmitting substrate  110  and a thickness t2 of the gas layer  140 , paths of the first color beam LG, the second color beam LB, and the third color beam LR leaving the light-transmitting substrate  110  may coincide. 
     According to some embodiments, as shown in  FIG.  3   , the first color light units  210 G 1 ,  210 G 2 , and  210 G 3  of the first color light source  210 G are combined to emit the first color beam LG. The second color light units  210 B 1  and  210 B 2  of the second color light source  210 B are combined to emit the second color beam LB. The third color light source units  210 R 1 ,  210 R 2 , and  210 R 3  of the third color light source  210 R are combined to emit the third color beam LR. An optical axis of the first color beam LG is in the middle of the first color light units  210 G 1 ,  210 G 2 , and  210 G 3 . An optical axis of the second color beam LB is in the middle of the second color light units  210 B 1  and  210 B 2 . An optical axis of the third color beam LR is in the middle of the third color light source unit  210 R 1 ,  210 R 2 , and  210 R 3 . 
     When the first color beam LG emitted by the first color light source  210 G is incident on the light-transmitting substrate  110 , the first color beam LG incident on the light-transmitting substrate  110  is reflected by a first light transmission element  116  and leaves the light-transmitting substrate  110 . 
     When the second color beam LB emitted by the second color light source  210 B is incident on the light-transmitting substrate  110 , the second color beam LB incident on the light-transmitting substrate  110  passes through the first light transmission element  116 , then the second color beam LB is reflected by the second light transmission element  118 , then passes through the first light transmission element  116  and leave the light-transmitting substrate  110 . 
     When the third color beam LR emitted by the third color light source  210 R is incident on the light-transmitting substrate  110 , the third color beam LR incident on the light-transmitting substrate  110  passes through the first light transmission element  116 , the second light transmission element  118 , and the gas layer  140 , and then is reflected by the third light transmission element  130 , passes through the gas layer  140 , the second light transmission element  118 , and the first light transmission element  116  sequentially, and leaves the light-transmitting substrate  110 . 
     In order to coincide the paths of the first color beam LG, the second color beam LB, and the third color beam LR leaving the light-transmitting substrate  110 , the beam splitter/combiner  100   c  satisfies the following conditions. 
     The second color beam LB is located between the first color beam LG and the third color beam LR. The first optical surface  112 , a second optical surface  114 , and the third light transmission element  130  are parallel to each other. When a distance between the optical axis of the first color beam LG and the optical axis of the second color beam LB is d1, and a distance between the optical axis of the first color beam LG and the optical axis of the third color beam LR is d2, refractive index of the light-transmitting substrate  110  is n, and an angle (incidence angle) between the incident first color beam LG, the second color beam LB, the third color beam LR, and the normal line of the first optical surface  112  is θ, a thickness of the light-transmitting substrate  110  is t1, 
     
       
         
           
             
               
                 t 
                 ⁢ 
                 1 
               
               = 
               
                 
                   
                     
                       
                         n 
                         2 
                       
                       - 
                       
                         
                           sin 
                           2 
                         
                         ⁢ 
                         θ 
                       
                     
                   
                   
                     sin 
                     ⁢ 
                     2 
                     ⁢ 
                     θ 
                   
                 
                 × 
                 d 
                 ⁢ 
                 1 
               
             
             , 
           
         
       
     
     a thickness of the gas layer  140  is t2, 
     
       
         
           
             
               t 
               ⁢ 
               2 
             
             = 
             
               
                 
                   
                     d 
                     ⁢ 
                     2 
                   
                   - 
                   
                     d 
                     ⁢ 
                     1 
                   
                 
                 
                   2 
                   × 
                   sin 
                   ⁢ 
                   θ 
                 
               
               . 
             
           
         
       
     
     At these conditions, the paths of the first color beam LG, the second color beam LB, and the third color beam LR leaving the light-transmitting substrate  110  are the same, that is, center axes of the first color beam LG, the second color beam LB, and the third color beam LR are aligned with each other and combined. In addition, geometric centers of the respective light spots formed by the first color beam LG, the second color beam LB, and the third color beam LR when leaving the light-transmitting substrate  110  coincide. 
     According to some embodiments, the incidence angle θ of the first color beam LG, the second color beam LB, and the third color beam LR incident on the first optical surface  112  of the light-transmitting substrate  110  (i.e., the angle between the first color beam LG, the second color beam LB, the third color beam LR, and the normal line of the first optical surface  112 ) falls roughly in a range of 30 degrees to 75 degrees. Considering the thickness of the light-transmitting substrate and the size of the beam splitter/combiner, the incidence angle θ falls roughly in a range of 30 degrees to 60 degrees. According to some embodiments, the incidence angle θ is roughly 45 degrees. 
     Although light spot sizes of the first color beam LG, the second color beam LB, and the third color beam LR are different, by designing the thickness t1 of the light-transmitting substrate  110  and the thickness t2 of the of the gas layer  140  of the beam splitter/combiner  100   c , emission positions of the first color beam LG, the second color beam LB, and the third color beam LR leaving the light-transmitting substrate  110  may be adjusted so that the first color beam LG, the second color beam LB, and the third color beam LR are combined by aligning the center axes of the beams. 
     It should be noted that the illumination module  150   c  in  FIG.  3    includes a beam splitter/combiner  100   c  and a light source module  210 , which may be provided with the light valve  220  and the projection lens  230  shown in  FIG.  1    to form a projection apparatus according to another embodiment of the disclosure. The structure of the light valve  220  and the projection lens  230  are as described above, and therefore will not be repeated in the following. 
       FIG.  4    is a schematic diagram of a beam splitter/combiner according to an embodiment of the disclosure. The difference between  FIG.  4    and  FIG.  3    lies in a direction of the optical path, where emission positions of the first color beam LG, the second color beam LB, and the third color beam LR in  FIG.  3    are changed to incident positions, paths of the first color beam LG, the second color beam LB, and the third color beam LR incident on the light-transmitting substrate  110  coincide, and geometric centers of light spots formed by the first color beam LG, the second color beam LB, and the third color beam LR incident on the light-transmitting substrate  110  coincide. As shown in  FIG.  4   , a beam splitter/combiner  100   c  may split a first color beam LG, a second color beam LB, and a third color beam LR of an illumination beam L. Emission positions of the first color beam LG, the second color beam LB, and the third color beam LR after being split may be controlled by adjusting a thickness t1 of the light-transmitting substrate  110  and a thickness t2 of a gas layer  140 . 
       FIG.  5    and  FIG.  6    are schematic diagrams of an illumination module according to another embodiment of the disclosure. Referring to  FIG.  5    and  FIG.  6    at the same time, an illumination module  150   d  includes a beam splitter/combiner  100   d  and a light source module  210 . The first color beam LG emitted by the light source module  210  is parallel to the second color beam LB. According to some embodiments, the first color beam LG and the second color beam LB do not form the same light spot size on the light-transmitting substrate  110 , and the geometric centers of the light spots formed by the first color beam LG and the second color beam LB incident on or leaving the light-transmitting substrate  110  coincide. According to other embodiments, the first color beam LG and the second color beam LB form the same size light spots on the light-transmitting substrate  110 . 
     The illumination module  150   d  shown in  FIG.  5    is similar to the illumination module  150   b  shown in  FIG.  2   . The difference is that in  FIG.  5   , there is a gap, i.e., a gas layer  170 , between the second light transmission element  118  and the light-transmitting substrate  110 . According to some embodiments, a thickness tg1 of the gas layer  170  is about 0 to 2 mm, but is not limited thereto. In the embodiment of  FIG.  5   , the first light transmission element  116  is a coating, such as a dielectric coating layer with a light splitting function. According to the embodiment of  FIG.  5   , the second light transmission element  118  is an adjustable flat mirror to provide a reflection effect to the second color beam LB. According to some embodiments, the second light transmission element  118  is substantially parallel to the second optical surface  114 . According to some embodiments, the material of the first light transmission element  116  is not the same as the material of the second light transmission element  118 . According to some embodiments, the thickness of the first light transmission element  116  is not the same as the thickness of the second light transmission element  118 . 
     When the first color beam LG emitted by the first color light source  210 G is incident on the light-transmitting substrate  110 , the first color beam LG incident on the light-transmitting substrate  110  is reflected by the first light transmission element  116  and leaves the light-transmitting substrate  110 . 
     When the second color beam LB emitted by the second color light source  210 B is incident on the light-transmitting substrate  110 , after the second color beam LB passes through the first light transmission element  116 , the second color beam LB sequentially passes through the second optical surface  114  and the gas layer  170 , is reflected by a reflective surface  118 S of the second light transmission element  118 , passes through the gas layer  170 , the second optical surface  114  sequentially, and then passes through the first light transmission element  116  and leaves the light-transmitting substrate  110 . 
     The displacement of the path of the first color beam LG leaving the light-transmitting substrate  110  and the path of the second color beam LB leaving the light-transmitting substrate  110  is related to the distance d between the first color beam LG and the second color beam LB before they are incident on the light-transmitting substrate  110 , the refractive index n of the light-transmitting substrate  110 , and the incidence angle θ of the first color beam LG (the second color beam LB) incident on the first optical surface  112  of the light-transmitting substrate  110 . 
     According to some embodiments, when the first optical surface  112  is parallel to the second optical surface  114 , and when the distance between the first color beam LG and the second color beam LB is d, the refractive index of the light-transmitting substrate  110  is n, the angle between the first color beam LG and the normal line of the first optical surface  112  is “0”, then the thickness of the light-transmitting substrate  110  is t, 
     
       
         
           
             t 
             = 
             
               
                 
                   
                     
                       n 
                       2 
                     
                     - 
                     
                       
                         sin 
                         2 
                       
                       ⁢ 
                       θ 
                     
                   
                 
                 
                   sin 
                   ⁢ 
                   2 
                   ⁢ 
                   θ 
                 
               
               × 
               
                 d 
                 . 
               
             
           
         
       
     
     At these conditions, the paths of the second color beam LB and the first color beam LG leaving the light-transmitting substrate  110  are the same, that is, the center axes of the second color beam LB and the first color beam LG are aligned with each other and combined. 
     According to some embodiments, the incidence angle θ of the first color beam LG and the second color beam LB incident on the first optical surface  112  of the light-transmitting substrate  110  (i.e., the angle between the first color beam LG 1  and the second color beam LB 1  and the normal line of the first optical surface  112 ) falls roughly in the range of 30 degrees to 75 degrees. Considering the thickness of the light-transmitting substrate and the size of the beam splitter/combiner, the incidence angle θ is roughly in the range of 30 degrees to 60 degrees. According to some embodiments, the incidence angle θ is roughly 45 degrees. 
     During the manufacturing process of the illumination module  150   d , due to various manufacturing tolerances in the process, it is possible that the paths of the first color beam LG and the second color beam LB cannot be coincident when they leave the light-transmitting substrate  110 , resulting in an asymmetry in the image uniformity of the first color beam and the second color beam (e.g., green light and blue light) due to the light source emission factor. Thus, by adjusting the relative angle between the second light transmission element  118  and the second optical surface  114 , the path of the second color beam LB leaving the light-transmitting substrate  110  after being reflected by the reflective surface  1185  of the second light transmission element  118  coincides with the path of the first color beam LG leaving the light transmitting substrate  110 . 
     According to some embodiments, a relative angle θ2 between the second light transmission element  118  and the second optical surface  114  may be regulated, and the relative angle θ2 ranges from −1 degree to 1 degree. According to other embodiments, the relative angle θ2 ranges from −0.5 degrees to 0.5 degrees. As shown in  FIG.  6   , by adjusting the relative angle θ2 between the second light transmission element  118  and the second optical surface  114 , the path of the second color beam LB reflected by the reflective surface  1185  of the second light transmission element  118  may coincide with the path of the first color beam LG when leaving the light-transmitting substrate  110 . 
     Thus, due to a gas layer  170  between the second light transmission element  118  and the second optical surface  114 , so that the second light transmission element  118  is adjustable in angle, the path of the second color beam LB and the first color beam LG leaving the light-transmitting substrate  110  may coincide by adjusting the relative angle between the second light transmission element  118  and the second optical surface  114 , so as to improve the color uniformity when imaging. 
       FIG.  7    and  FIG.  8    are schematic diagrams of an illumination module according to another embodiment of the disclosure. Referring to  FIG.  7    and  FIG.  8    at the same time, an illumination module  150   e  includes a beam splitter/combiner  100   e  and a light source module  210 . The illumination module  150   e  shown in  FIG.  7    is similar to the illumination module  150   c  in  FIG.  3   . The difference between  FIG.  7    and  FIG.  3    is that in  FIG.  7   , there is a gap, i.e., a gas layer  170 , between the second light transmission element  118  and the light-transmitting substrate  110 . The illumination module  150   e  includes a third light transmission element  130 . There is a gas layer  140  between the third light transmission element  130  and the second light transmission element  118 . According to some embodiments, the second light transmission element  118  may reflect the second color beam LB and allow the third color beam LR to pass through. According to the embodiment of  FIG.  7   , the first light transmission element  116  is a coating, such as a dielectric coating layer with a light splitting function. According to the embodiment of  FIG.  7   , the second light transmission element  118  is a coating film attached to an adjustable thin substrate, such as a dielectric coating layer with a light splitting function attached to the adjustable thin substrate. The thin substrate is only configured as a carrier for the coating film, so it is necessary to minimize the influence of the thickness of the thin substrate on the refraction of the light beam. As a result, a thickness of the thin substrate is much smaller than the thickness of the light-transmitting substrate  110 . According to the embodiment of  FIG.  7   , the third light transmission element  130  is an adjustable flat mirror, or the third light transmission element  130  may also be a coating film attached to an adjustable thin substrate, such as a dielectric coating layer with a light splitting function attached to the adjustable thin substrate to provide reflection effect to the third color beam LR. In addition, the thin substrate is only configured as a carrier for the coating, so it is necessary to minimize the influence of the thickness of the thin substrate on the refraction of the light beam. As a result, a thickness of the thin substrate is much smaller than the thickness of the light-transmitting substrate  110 . According to some embodiments, the third light transmission element  130  is substantially parallel to the second optical surface  114 . According to some embodiments, the material of the first light transmission element  116  is not the same as the material of third light transmission element  130 . According to some embodiments, the thickness of the first light transmission element  116  is not the same as the thickness of third light transmission element  130 . 
     When the first color beam LG emitted by the first color light source  210 G is incident on the light-transmitting substrate  110 , the first color beam LG incident on the light-transmitting substrate  110  is reflected by the first light transmission element  116  and leaves the light-transmitting substrate  110 . 
     When the second color beam LB emitted by the second color light source  210 B is incident on the light-transmitting substrate  110 , after the second color beam LB passes through the first light transmission element  116 , the second color beam LB sequentially passes through the second optical surface  114  and the gas layer  170 , and then is reflected by the reflective surface  118 S of the second light transmission element  118 , passes through the gas layer  170 , the second optical surface  114  sequentially, and then passes through the first light transmission element  116  and leaves the light-transmitting substrate  110 . 
     When the third color beam LR emitted by the third color light source  210 R is incident on the light-transmitting substrate  110 , the third color beam LR incident on the light-transmitting substrate  110  passes through the first light transmission element  116 , the second light transmission element  118 , and the gas layer  140 , and then the third color beam LR is reflected by the third light transmission element  130 , and then passes through the gas layer  140 , the second light transmission element  118 , and the first light transmission element  116  sequentially, and leaves the light-transmitting substrate  110 . 
     The displacement of the path of the first color beam LG leaving the light-transmitting substrate  110  and the path of the second color beam LB leaving the light-transmitting substrate  110  relative to the path of the third color beam LR leaving the light-transmitting substrate  110  is related to the distance d1 between the first color beam LG and the second color beam LB before they are incident on the light-transmitting substrate  110 , the distance d2 between the first color beam LG and the third color beam LR before they are incident on the light-transmitting substrate  110 , the refractive index n of the light-transmitting substrate  110 , and the incidence angle θ of the first color beam LG and the second color beam LB incident on the first optical surface  112  of the light-transmitting substrate  110 . 
     According to some embodiments, when the distance between the first color beam LG and the second color beam LB is d1, the distance between the first color beam LG and the third color beam LR is d2, the refractive index of the light-transmitting substrate  110  is n, the angle between the first color beam LG and the normal line of the first optical surface  112  is “0”, then the thickness of the light-transmitting substrate  110  is t1, 
     
       
         
           
             
               
                 t 
                 ⁢ 
                 1 
               
               = 
               
                 
                   
                     
                       
                         n 
                         2 
                       
                       - 
                       
                         
                           sin 
                           2 
                         
                         ⁢ 
                         θ 
                       
                     
                   
                   
                     sin 
                     ⁢ 
                     2 
                     ⁢ 
                     θ 
                   
                 
                 × 
                 d 
                 ⁢ 
                 1 
               
             
             , 
           
         
       
     
     the thickness of the gas layer  140  is tg2, 
     
       
         
           
             
               tg 
               ⁢ 
               2 
             
             = 
             
               
                 
                   
                     d 
                     ⁢ 
                     2 
                   
                   - 
                   
                     d 
                     ⁢ 
                     1 
                   
                 
                 
                   2 
                   × 
                   sin 
                   ⁢ 
                   θ 
                 
               
               . 
             
           
         
       
     
     According to some embodiments, the incidence angle θ of the first color beam LG, the second color beam LB, and the third color beam LR incident on the first optical surface  112  of the light-transmitting substrate  110  (i.e., the angle between the first color beam LG 1 , the second color beam LB 1 , and the third color beam LR and the normal line of the first optical surface  112 ) falls roughly in the range of 30 degrees to 75 degrees. Considering the thickness of the light-transmitting substrate and the size of the beam splitter/combiner, the incidence angle θ is roughly in the range of 30 degrees to 60 degrees. According to some embodiments, the incidence angle θ is roughly 45 degrees. 
     During the manufacturing process of the illumination module  150   e , due to various manufacturing tolerances in the process, it is possible that the paths of the first color beam LG, the second color beam LB, and the third color beam LB cannot be coincident when they leave the light-transmitting substrate  110 , resulting in an asymmetry in the image uniformity of the first color beam, the second color beam, and the third color beam (e.g., green light, blue light, and red light) due to the light source emission factor. Thus, by adjusting the relative angle between the second light transmission element  118  and the second optical surface  114 , the path of the second color beam LB leaving the light-transmitting substrate  110  after being reflected by the reflective surface  1185  of the second light transmission element  118  coincides with the path of the first color beam LG leaving the light transmitting substrate  110 . By adjusting the relative angle between the third light transmission element  130  and the second optical surface  114 , the path of the third color beam LR passing through the second light transmission element  118  and leaving the light-transmitting substrate  110  after being reflected by a reflective surface  130 S of the third light transmission element  130  coincides with the path of the first color beam LG leaving the light-transmitting substrate  110 . 
     According to some embodiments, a relative angle θ3 between the third light transmission element  130  and the second optical surface  114  may be regulated, and the relative angle θ3 ranges from −1 degree to 1 degree. According to other embodiments, the relative angle θ3 ranges from −0.5 degrees to 0.5 degrees. As shown in  FIG.  8   , by adjusting the relative angle θ2 between the second light transmission element  118  and the second optical surface  114 , the path of the second color beam LB reflected by the reflective surface  1185  of the second light transmission element  118  leaving the light-transmitting substrate  110  may coincide with the path of the first color beam LG leaving the light-transmitting substrate  110 . After determining the relative angle θ2 between the second light transmission element  118  and the second optical surface  114 , the relative angle θ3 between the third light transmission element  130  and the second optical surface  114  is adjusted so that the path of the third color beam LR reflected by the reflective surface  130 S of the third light transmission element  130  leaving the light-transmitting substrate  110  may coincide with the path of the first color beam LG leaving the light-transmitting substrate  110 . 
     Thus, by adjusting the relative angle θ2 between the second light transmission element  118  and the second optical surface  114  and adjusting the relative angle θ3 between the third light transmission element  130  and the second optical surface  114 , the paths of the second color beam LB and the third color beam LR and the first color beam LG leaving the light-transmitting substrate  110  may coincide, so as to improve the color uniformity when imaging. 
       FIG.  9    is a schematic diagram of an illumination module according to another embodiment of the disclosure. An illumination module  150   f  includes a beam splitter/combiner  100   f  and the light source module  210 . The illumination module  150   f  shown in  FIG.  9    is similar to the illumination module  150   c  in  FIG.  3   . The difference between  FIG.  9    and  FIG.  3    is that the beam splitter/combiner  100   f  also includes a light-transmitting substrate  160 . The light-transmitting substrate  160  is located between the second light transmission element  118  and the third light transmission element  130 , and the third light transmission element  130  is located on the light-transmitting substrate  160 . Specifically, the light-transmitting substrate  160  has a third optical surface  162  facing the light-transmitting substrate  110  and a fourth optical surface  164  opposite to the third optical surface  162 , and the light transmission element  130  is located on the fourth optical surface  164  of the light-transmitting substrate  160 . According to some embodiments, a material of the light-transmitting substrate  160  may be glass or other light-transmitting materials, but the disclosure is not limited thereto. According to some embodiments, there is a gas layer  140  between the light-transmitting substrate  160  and the second light transmission element  118 . According to some embodiments, the gas layer  140  may also be omitted, so the light-transmitting substrate  160  is located on the second light transmission element  118  and is in direct contact with the second light transmission element  118 . According to the embodiment of  FIG.  9   , the first light transmission element  116  is a coating film attached to the light-transmitting substrate  110 , such as a dielectric coating layer with a light-splitting function attached to the light-transmitting substrate  110 . According to the embodiment of  FIG.  9   , the second light transmission element  118  is a coating film attached to the light-transmitting substrate  110 , such as a dielectric coating layer with a light-splitting function attached to the light-transmitting substrate  110 . According to the embodiment of  FIG.  9   , the third light transmission element  130  is a coating film attached to the light-transmitting substrate  160 , such as a dielectric coating layer with a light-splitting function attached to the light-transmitting substrate  160 , or a reflective mirror layer formed by a metallic coating attached to the light-transmitting substrate  160  to provide a reflective effect on the third color light beam LR. 
     When the first color beam LG emitted by the first color light source  210 G is incident on the light-transmitting substrate  110 , the first color beam LG incident on the light-transmitting substrate  110  is reflected by the first light transmission element  116  and leaves the light-transmitting substrate  110 . 
     When the second color beam LB emitted by the second color light source  210 B is incident on the light-transmitting substrate  110 , after the second color beam LB passes through the first light transmitting element  116 , the second color beam LB passes through the second optical surface  114 , is reflected by the second light transmitting element  118 , and then passes through the first light transmitting element  116  and leaves the light-transmitting substrate  110 . 
     When the third color beam LR emitted by the third color light source  210 R is incident on the light-transmitting substrate  110 , the third color beam LR incident on the light-transmitting substrate  110  passes through the first light transmission element  116 , the second light transmission element  118 , and the gas layer  140 , the third color beam LR continues to pass through the light-transmitting substrate  160 , is reflected by the third light transmission element  130 , passes through the light-transmitting substrate  160 , the gas layer  140 , the second light transmission element  118 , and the first light transmission element  116  sequentially, and leaves the light-transmitting substrate  110 . 
     By adjusting the thickness t1 of the light-transmitting substrate  110 , the paths of the first color beam LG and the second color beam LB may coincide when leaving the light-transmitting substrate  110 . By adjusting the thickness t1 of the light-transmitting substrate  110  and the thickness t3 of the light-transmitting substrate  160 , the paths of the first color beam LG and the third color beam LR may coincide when leaving the light-transmitting substrate  110 . In this case, the paths of the first color beam LG, the second color beam LB, and the third color beam LR may coincide when leaving the light-transmitting substrate  110  because only the thickness of the light-transmitting substrate  110  and the light-transmitting substrate  160  need to be adjusted. Since only one adjustment is needed to adjust the incident angle θ of the first color beam LG, it is simple to manufacture and may effectively reduce the production cost. 
     According to some embodiments, when the distance between the first color beam LG and the second color beam LB before they are incident on the light-transmitting substrate  110  is d1, the distance between the first color beam LG and the third color beam LR before they are incident on the light-transmitting substrate  110  is d2, the refractive index of the light-transmitting substrate  110  is n1, the refractive index of the light-transmitting substrate  160  is n2, the angle between the first color beam LG and the normal line of the first optical surface  112  is “θ”, then the thickness of the light-transmitting substrate  110  is t1, 
     
       
         
           
             
               
                 t 
                 ⁢ 
                 1 
               
               = 
               
                 d 
                 ⁢ 
                 1 
                 × 
                 
                   
                     
                       
                         n 
                         ⁢ 
                         
                           1 
                           2 
                         
                       
                       - 
                       
                         
                           sin 
                           2 
                         
                         ⁢ 
                         θ 
                       
                     
                   
                   
                     sin 
                     ⁢ 
                     2 
                     ⁢ 
                     θ 
                   
                 
               
             
             , 
           
         
       
     
     the thickness of the light-transmitting substrate  160  is t3, 
     
       
         
           
             
               t 
               ⁢ 
               3 
             
             = 
             
               
                 ( 
                 
                   
                     d 
                     ⁢ 
                     2 
                   
                   - 
                   
                     d 
                     ⁢ 
                     1 
                   
                 
                 ) 
               
               × 
               
                 
                   
                     
                       
                         n 
                         ⁢ 
                         
                           2 
                           2 
                         
                       
                       - 
                       
                         
                           sin 
                           2 
                         
                         ⁢ 
                         θ 
                       
                     
                   
                   
                     sin 
                     ⁢ 
                     2 
                     ⁢ 
                     θ 
                   
                 
                 . 
               
             
           
         
       
     
     In summary, the disclosure may be utilized to adjust the thickness of the light-transmitting substrate of the beam splitter/combiner, so that beams of different colors may be incident on the beam splitter/combiner to obtain a combined light effect with center axes of beams aligned with each other. On the other hand, when a color beam formed by different colors of light is incident on the beam splitter/combiner, emission positions of the different colors of light may also be changed by adjusting a thickness of the light-transmitting substrate of the beam splitter/combiner. In addition, by adjusting the angle of the light transmission element, the error in the manufacturing process may be corrected, thus enabling different colors to obtain a better combined light effect. The disclosure may further reduce the size and manufacturing difficulty of the beam splitter/combiner, and reduce the manufacturing cost of the projection apparatus. 
     The foregoing description of the preferred embodiments of the disclosure has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to best explain the principles of the disclosure and its best mode practical application, thereby to enable persons skilled in the art to understand the disclosure for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the disclosure be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. Therefore, the term “the disclosure”, “the present disclosure” or the like does not necessarily limit the claim scope to a specific embodiment, and the reference to particularly preferred exemplary embodiments of the disclosure does not imply a limitation on the disclosure, and no such limitation is to be inferred. The disclosure is limited only by the spirit and scope of the appended claims. Moreover, these claims may refer to use “first”, “second”, etc. following with noun or element. Such terms should be understood as a nomenclature and should not be construed as giving the limitation on the number of the elements modified by such nomenclature unless specific number has been given. The abstract of the disclosure is provided to comply with the rules requiring an abstract, which will allow a searcher to quickly ascertain the subject matter of the technical disclosure of any patent issued from this disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Any advantages and benefits described may not apply to all embodiments of the disclosure. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the disclosure as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.