Patent Publication Number: US-2022221729-A1

Title: Beam splitting and combining device and electronic device

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The invention relates to an optical field, and more particularly to a beam splitting and combining device and an electronic device. 
     Description of the Related Art 
       FIG. 1  is a schematic diagram of a conventional liquid crystal on silicon (LCOS) projector  1 . In operation, light emitted from a light source  11  of the projector  1  and passing through two color lenses  12  and  13  is split into red light, green light and blue light. The red light, green light and blue light respectively pass through the corresponding polarized beam splitters  14 ,  15  and  16  and are reflected to the corresponding LCOS panels  17 ,  18  and  19 , by which the polarizations of the red light, green light and blue light are changed. Then, the red light, green light and blue light are reflected back to the corresponding polarized beam splitters  14 ,  15  and  16 , are combined by a beam-combining prism  10 , and are projected from a projection lens of the projector  1 . 
       FIG. 2  is a schematic diagram of the beam-combining prism  10  of  FIG. 1 . As shown in  FIG. 2 , the beam-combining prism  10  is formed by connecting four prisms  10   a ,  10   b ,  10   c  and  10   d . The four prisms  10   a ,  10   b ,  10   c  and  10   d  have a gap A therebetween. When combined by the beam-combining prism  10 , the red light, green light and blue light propagating therein may reaches the gap A. That causes imaging error and affects the image quality. 
       FIG. 3  is a schematic diagram of a known digital light processing (DLP) projector  20 . In operation, light emitted from a light source  21  of the projector  20  and passing through a total reflection prism  22  is split into red light, green light and blue light by a beam splitting and combining prism  23 . The red light, green light and blue light respectively pass through the corresponding digital micromirror devices (DMDs), are reflected back to the beam splitting and combining prism, are combined, and are projected from a projection lens  24  of the projector  20 . 
     The beam splitting and combining prism  23  is formed by connecting three prisms which are placed in a staggered arrangement. Therefore, the incident light does not propagate through the gap formed between the three prisms. However, the beam splitting and combining prism  23  is large and heavy and miniaturization of the beam splitting and combining prism  23  is difficult. Also, a projector provided with such a beam splitting and combining prism is too large in volume. 
     BRIEF SUMMARY OF THE INVENTION 
     As described, a known beam splitting and combining device has gaps between prisms affecting the image quality and generally is large in volume. The invention therefore provides a beam splitting and combining device (BSC device) to address the described problems. 
     A BSC device in accordance with an embodiment of the invention includes a first prism, a second prism and a first optical film. The first prism includes a first surface, a second surface and a third surface. The second prism includes a fourth surface, a fifth surface and a sixth surface. The fifth surface and the second surface are attached to each other. The first optical film is formed between the second surface and the fifth surface by coating. The first surface allows a beam in a first range of wavelengths to pass through. The fourth surface allows a beam in a second range of wavelengths to pass through. The first optical film allows the beam in the first range of wavelengths to pass through, and reflects the beam in the second range of wavelengths. The sixth surface allows the beams in the first and second ranges of wavelengths to pass through and reflects a beam in a third range of wavelengths. The beam in the first range of wavelengths is configured to pass through the first surface, the second surface, the first optical film, the fifth surface and the sixth surface in order or in reverse order, or is configured to pass through the first surface and the third surface in order or in reverse order. 
     In another embodiment, the BSC device includes a first prism and a second prism attached to each other. The first prism includes a first surface, a second surface and a third surface. The first surface allows a beam in a first range of wavelengths to pass through. 
     The second prism includes a fourth surface, a fifth surface and a sixth surface. The fifth surface and the second surface are attached to each other. A first optical film is formed between the fifth surface and the second surface by coating. The first optical film allows the beam in the first range of wavelengths to pass through, and reflects a beam in a second range of wavelengths. The first surface and the second surface have an included angle of 45 degrees therebetween. The fourth surface is perpendicular to the first surface. The fifth surface and the fourth surface have an included angle of 45 degrees therebetween. The sixth surface and the first surface have an included angle of 45 degrees therebetween. 
     In yet another embodiment, the BSC device includes a prism. The prism includes a first surface, a second surface and a third surface. The first surface allows a beam in a first range of wavelengths to pass through, and reflects a beam in a second range of wavelengths. The beam in the second range of wavelengths is incident on the second surface. The third surface allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. The beam in the first range of wavelengths is configured to pass through the first surface and the third surface in order or in reverse order. The beam in the second range of wavelengths is configured to pass through the second surface, to reach the first surface and to be reflected on the first surface, or is configured to be reflected on the first surface, to reach the second surface and to pass through the second surface. The beam in the third range of wavelengths is reflected on the third surface. 
     In another embodiment, the third surface and the sixth surface are in parallel or coplanar. The third surface allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. A part of the beam in the first range of wavelengths is able to pass through the first surface and the third surface in order or in reverse order. Another part of the beam in the third range of wavelengths is reflected on the third surface. 
     In yet another embodiment, the BSC device further includes a third prism. The third prism includes a seventh surface, an eighth surface, a ninth surface, a tenth surface and an eleventh surface. The seventh surface allows the beam in the third range of wavelengths to pass through. The eighth surface and the sixth surface are attached to each other. A second optical film is sandwiched between the sixth surface and the eighth surface. The second optical film allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. The ninth surface allows the beams in the first, second and third ranges of wavelengths to pass through. The third surface and the sixth surface are disposed in parallel or are coplanar. The third surface allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. The tenth surface allows the beam in the second range of wavelengths to pass through. The eleventh surface allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. 
     In another embodiment, the beam in the first range of wavelengths passes through the first surface, the third surface, the eleventh surface, and the ninth surface in order or in reverse order, and/or passes through the first surface, the second surface, the first optical film, the fifth surface, the sixth surface, the second optical film, the eighth surface and the ninth surface in order or in reverse order. The beam in the second range of wavelengths passes through the tenth surface, is reflected on the eleventh surface, and passes through the ninth surface in order or in reverse order, and/or passes through the fourth surface, reaches the fifth surface, is reflected on the first optical film, passes through the sixth surface, passes through the second optical film, passes through the eighth surface and passes through the ninth surface in order or in reverse order. The beam in the third range of wavelengths is reflected on the third surface, passes through the eleventh surface, and passes through the ninth surface in order or in reverse order, or passes through the seventh surface, reaches the eighth surface, is reflected on the second optical film and passes through the ninth surface in order or in reverse order. 
     In yet another embodiment, the BSC device further includes a third prism. The third prism includes a seventh surface, an eighth surface and a ninth surface. The seventh surface allows the beam in the third range of wavelengths to pass through. The eighth surface and the third surface are attached to each other. A second optical film is formed between the third surface and the eighth surface by coating. The second optical film allows the beam in the first range of wavelengths to pass through, and reflects the beam in the third range of wavelengths. The ninth surface allows the beam in the first range of wavelengths to pass through, and reflects the beam in the second range of wavelengths. The third surface and the sixth surface are disposed in parallel or are coplanar. The third surface allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. The beam in the first range of wavelengths passes through the first surface, the second surface, the first optical film, the fifth surface, and the sixth surface in order or in reverse order, or passes through the first surface, the third surface, the second optical film, the eighth surface and the ninth surface in order or in reverse order. The beam in the second range of wavelengths passes through the fourth surface, is reflected on the first optical film formed between the fifth surface and the second surface, and passes through the sixth surface in order or in reverse order, or is reflected on the ninth surface. The beam in the third range of wavelengths passes through the seventh surface, is reflected on the second optical film formed between the eighth surface and the third surface, and passes through the ninth surface in order or in reverse order, or is reflected on the sixth surface. 
     In another embodiment, the first prism is a square pyramidal prism, the first surface is a bottom surface of the first prism, the second prism is a triangular pyramidal prism, the third prism is a triangular pyramidal prism, and the ninth surface and the fifth surface are disposed in parallel or are coplanar. 
     In yet another embodiment, the first prism is a square pyramidal prism and further includes a seventh surface allowing the beam in the second range of wavelengths to pass through, and an eighth surface allowing the beam in the third range of wavelengths to pass through. The second prism is a triangular pyramidal prism and further includes a ninth surface allowing the beam in the third range of wavelengths to pass through. A part of the beam in the first range of wavelengths passes through the first surface, the second surface, the first optical film, the fifth surface, and the sixth surface in order or in reverse order. Another part of the beam in the first range of wavelengths passes through the first surface and the third surface in order or in reverse order. The beam in the second range of wavelengths passes through the first surface, is reflected on the first optical film formed between the second surface and the fifth surface, and passes through the seventh surface in order or in reverse order. A part of the beam in the third range of wavelengths passes through the first surface, is reflected on the third surface, and passes through the eighth surface in order or in reverse order. 
     In another embodiment, the BSC device further includes a fourth prism. The fourth prism includes a twelfth surface, a thirteenth surface, a fourteenth surface, and a fifteenth surface. The twelfth surface allows the beam in the third range of wavelengths to pass through. The thirteenth surface and the eleventh surface are attached to each other. A third optical film is provided between the thirteenth surface and the eleventh surface by coating. The third optical film allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. The fourteenth surface allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. The fifteenth surface and the third surface are attached to each other. A fourth optical film is provided between the fifteenth surface and the third surface by coating. The fourth optical film allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. The beam in the first range of wavelengths passes through the first surface, the second surface, the first optical film, the fifth surface, the sixth surface, the second optical film, the eighth surface and the ninth surface in order or in reverse order, and/or passes through the first surface, the third surface, the fourth optical film, the fifteenth surface, the thirteenth surface, the third optical film, the eleventh surface and the ninth surface in order or in reverse order. The beam in the second range of wavelengths passes through the fourth surface, reaches the fifth surface, is reflected on the first optical film, passes through the sixth surface, passes through the second optical film, passes through the eighth surface and passes through the ninth surface in order or in reverse order, or passes through the tenth surface, is reflected on the third optical film formed between the thirteenth surface and the eleventh surface, and passes through the ninth surface in order or in reverse order. The beam in the third range of wavelengths passes through the seventh surface, reaches the eighth surface, is reflected on the second optical film, and passes through the ninth surface in order or in reverse order, or passes through the twelfth surface, is reflected on the fourth optical film formed between the fifteenth surface and the third surface, passes through the eleventh surface and passes through the ninth surface in order or in reverse order. 
     In yet another embodiment, the first prism is a square pyramidal prism, the first surface is a bottom surface of the first prism, the second prism is a triangular pyramidal prism, the third prism is a square pyramidal prism, the ninth surface is a bottom surface of the third prism, and the fourth prism is a triangular pyramidal prism. 
     In another embodiment, any two of the first, second and third ranges do not overlap. The BSC device satisfies at least one of the following conditions (1) and (2). (1) any wavelength of the beam in the first range and that in the second range have a difference greater than 20 nm, any wavelength of the beam in the second range and that in the third range have a difference greater than 20 nm, and any wavelength of the beam in the third range and that in the first range have a difference greater than 20 nm; (2) the third range is not between the first range and the second range. 
     A projector in accordance with an embodiment of the invention includes a light source, the above-mentioned BSC device, a spatial light modulator, and a projection lens. 
     A head-mounted display in accordance with an embodiment of the invention includes a light source, the above-mentioned BSC device and a projection lens. 
     A head-up display in accordance with an embodiment of the invention includes a light source, a projection lens, a reflective mirror, a spatial light modulator, and the above-mentioned BSC device. 
     The BSC device has the following merits: when the prisms are attached to each other, gaps can be merely formed at the corners of the prisms. It can be avoided that gaps are formed between prisms to affect the propagation of beams when the beams are incident on the surfaces of the prisms. Such arrangement can effectively avoid the influence on the formed images due to arrival of the incident beam at the gaps. Further, the volume and weight of the BSC device can be reduced that benefits miniaturization of the whole structure. 
     In the invention, the above-mentioned projection lens has a reduced volume, an enlarged field of view and a reduced F-number and is still capable of good optical performance. 
     The projection lens in accordance with an embodiment of the invention includes a first lens, a second lens, a third lens, a fourth lens and a fifth lens. The first lens is with refractive power. The second lens is with refractive power. The third lens is with refractive power. The fourth lens is with refractive power. The fifth lens is with negative refractive power and includes a concave surface facing an image source side. The first, second, third, fourth and fifth lenses are sequentially arranged from a projection side to the image source side along an optical axis. The second lens and the third lens are cemented together, to form a cemented lens which is with positive refractive power. 
     In another embodiment, the projection lens further includes a stop disposed between the projection side and the first lens. The first lens is with positive refractive power and includes a convex surface facing the image source side. The second lens is with negative refractive power and includes a concave surface facing the projection side and another concave surface facing the image source side. The third lens is with positive refractive power and includes a convex surface facing the projection side and another convex surface facing the image source side. The fourth lens is with positive refractive power and includes a convex surface facing the projection side and another convex surface facing the image source side. The fifth lens further includes a concave surface facing the projection side. 
     In yet another embodiment, the first lens further includes a concave surface or a convex surface facing the projection side. The fourth lens and the fifth lens are cemented together. 
     In another embodiment, the projection lens satisfies any one of the following conditions: 1.9≤f 1 /f≤3.7; −1.1≤f 2 /f≤−0.6; 0.8≤f 3 /f≤1.2; 0.8≤f 4 /f≤1.1, where f 1  is an effective focal length of the first lens, f 2  is an effective focal length of the second lens, f 3  is an effective focal length of the third lens, f 4  is an effective focal length of the fourth lens, and f is an effective focal length of the projection lens. 
     In yet another embodiment, the projection lens satisfies any one of the following conditions: −17≤f 23 /f 5 ≤30; −1.15≤f 4 /f 5 ≤−0.75; −33≤f 23 /f≤16 where f 4  is an effective focal length of the fourth lens, f 5  is an effective focal length of the fifth lens, f 23  is an effective focal length of combination of the second lens and the third lens, and f is an effective focal length of the projection lens. 
     In another embodiment, the projection lens satisfies any one of the following conditions: 0.3≤f/TTL≤0.45; 0.09≤BFL/TTL≤0.22; 0.5≤IH/f≤0.65 where f is an effective focal length of the projection lens, TTL is an interval from the stop to the image source along the optical axis, BFL is an interval from the image source side surface to the image source along the optical axis, and IH is a half image height of the projection lens. 
     In yet another embodiment, the projection lens satisfies the following conditions: −1.2≤f 5 /f≤−0.9 where f 5  is an effective focal length of the fifth lens, and f is an effective focal length of the projection lens. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of a conventional liquid crystal on silicon (LCOS) projector. 
         FIG. 2  is a schematic diagram of the beam-combining prism of  FIG. 1 . 
         FIG. 3  is a schematic diagram of a known digital light processing (DLP) projector. 
         FIGS. 4A, 4B and 4C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a beam splitting and combining device (BSC device) in accordance with the first embodiment of the invention. 
         FIGS. 5A, 5B and 5C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the second embodiment of the invention. 
         FIG. 6A  is a schematic diagram of a BSC device in accordance with the third embodiment of the invention. 
         FIG. 6B  is an exploded diagram of the BSC device in accordance with the third embodiment of the invention. 
         FIG. 6C  depicts the light paths of the beams in the first and second ranges of wavelengths for the BSC device in accordance with the third embodiment of the invention. 
         FIG. 6D  depicts the light path of the beam in the third range of wavelengths for the BSC device in accordance with the third embodiment of the invention. 
         FIG. 6E  depicts the light paths for the BSC device in accordance with the third embodiment of the invention. 
         FIGS. 7A, 7B and 7C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the fourth embodiment of the invention. 
         FIG. 8A  is a schematic diagram of a BSC device in accordance with the fifth embodiment of the invention. 
         FIG. 8B  is an exploded diagram of the BSC device in accordance with the fifth embodiment of the invention. 
         FIG. 8C  depicts the light paths of the beams in the first and second ranges of wavelengths for the BSC device in accordance with the fifth embodiment of the invention. 
         FIG. 8D  depicts the light path of the beam in the third range of wavelengths for the BSC device in accordance with the fifth embodiment of the invention. 
         FIG. 8E  depicts the light paths for the BSC device in accordance with the fifth embodiment of the invention. 
         FIGS. 9A, 9B and 9C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the sixth embodiment of the invention. 
         FIGS. 10A, 10B and 10C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the seventh embodiment of the invention. 
         FIGS. 11A, 11B and 11C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the eighth embodiment of the invention. 
         FIG. 12A  is a schematic diagram of a BSC device in accordance with the ninth embodiment of the invention. 
         FIG. 12B  is an exploded diagram of the BSC device in accordance with the ninth embodiment of the invention. 
         FIG. 12C  depicts the light paths of the beams in the first and second ranges of wavelengths for the BSC device in accordance with the ninth embodiment of the invention. 
         FIG. 12D  depicts the light path of the beam in the third range of wavelengths for the BSC device in accordance with the ninth embodiment of the invention. 
         FIG. 12E  depicts the light paths for the BSC device in accordance with the ninth embodiment of the invention. 
         FIGS. 13A, 13B and 13C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the tenth embodiment of the invention. 
         FIGS. 14A and 14B  are respectively a schematic diagram and a light path diagram of a BSC device in accordance with the eleventh embodiment of the invention. 
         FIG. 15  is a schematic diagram of a projector including a BSC device of the invention. 
         FIG. 16  is a schematic diagram of a head-mounted display including a BSC device of the invention. 
         FIG. 17  depicts arrangement of the lenses and the light paths for a projection lens which is included in an electronic device of the invention. 
         FIG. 18A  depicts the field curvature of the projection lens of  FIG. 17 . 
         FIG. 18B  depicts the distortion of the projection lens of  FIG. 17 . 
         FIG. 18C  depicts the modulation transfer function of the projection lens of  FIG. 17 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The purpose, technical scheme and merits of the invention can be fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings. It is understood that this description is made for the purpose of illustrating the invention and should not be taken in a limiting sense. 
       FIGS. 4A, 4B and 4C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a beam splitting and combining device (hereinafter BSC device) in accordance with the first embodiment of the invention. As shown in  FIGS. 4A-4C , in the first embodiment, the BSC device  100  includes a first prism  110  and a second prism  120  connected to each other. The first prism  110  includes a first surface  1101  allowing a beam in a first range of wavelengths to pass through along a first direction X, a second surface  1102  allowing the beam in the first range of wavelengths to pass through and reflecting a beam in a second range of wavelengths, and a third surface  1103  allowing the beam in the first range of wavelengths to pass through and reflecting a beam in a third range of wavelengths. The second surface  1102  and the third surface  1103  are adjacent to each other. When observed in the first direction X, the second surface  1102  and the third surface  1103  are shaded from view by the first surface  1101 . 
     The second prism  120  includes a fourth surface  1201  allowing the beam in the second range of wavelengths to pass through, a fifth surfaced  1202  contacting the second surface  1102 , allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, and a sixth surface  1203  allowing the beams in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths. The fourth, fifth and sixth surfaces  1201 ,  1202  and  1203  are adjacent to each other. The third surface  1103  and the sixth surface  1203  may be disposed in parallel or be coplanar. Further, the third surface  1103  and the sixth surface  1203  may be parallel to the second direction Y. 
     The second surface  1102  and the fifth surface  1202  may be glued together with a first optical film  1104  sandwiched therebetween. The first optical film  1104  may be coated on the second surface  1102  or on the fifth surface  1202 . By either arrangement, the beam in the second range of wavelengths can be reflected thereon. The first optical film  1104  is provided for allowing the beam in the first range of wavelengths to pass through and for reflecting the beam in the second range of wavelengths. An optical film is coated on the sixth surface  1203  for allowing the beams in the first and second range of wavelengths to pass through and for reflecting the beam in the third range of wavelengths. The first, second and third ranges of wavelengths may respectively be one of the following ranges: 380 nm-450 nm (purple), 450 nm-475 nm (blue), 476 nm-495 nm (cyan), 495 nm-570 nm (green), 570 nm-590 nm (yellow), 590 nm-620 nm (orange), 620 nm-750 nm (red), all of which are wavelengths of visible light. However, the invention is not limited thereto. Preferably, any two of the first, second and third ranges of wavelengths do not overlap. Further, at least one of the following conditions is satisfied: a difference between any two wavelengths in the first, second and third ranges is greater than 20 nm, and the third range is not between the first range and the second range. 
     The light path of the first embodiment is described as follows; As shown, the beam in the first range of wavelengths which is incident in the first direction X sequentially passes through the first surface  1101 , the second surface  1102 , the first optical film  1104 , the fifth surface  1202  and the sixth surface  1203 , and is emitted in the first direction X. The beam in the second range of wavelengths which is incident in the second direction Y passes through the fourth surface  1201 , is reflected on the first optical film  1104  provided between the second surface  1102  and the fifth  1202 , passes through the sixth surface  1203 , and is emitted in the first direction X. The beam in the third range of wavelengths which is incident in the third direction Z reaches the sixth surface  1203 , is reflected on the sixth surface  1203 , and is emitted in the first direction X. Thus, the beam in the first, second and third ranges of wavelengths are combined. 
     The above-mentioned propagation of the beams can be reversed, described as follows: A combined beam is incident on the sixth surface  1203  in the first direction X. The beam in the first range of wavelengths passes through the sixth surface  1203 , the fifth surface  1202 , the first optical film  1104 , the second surface  1102 , and the first surface  1101  and is emitted in the first direction X. The beam in the second range of wavelengths passes through the sixth surface  1203 , reaches the fifth surface  1202 , is reflected on the first optical film  1104 , and is emitted from the fourth surface  1201  in the second direction Y. The beam in the third range of wavelengths is reflected on the sixth surface  1203  and is emitted in the third direction Z. Thus, the combined beam is split into three beams in the first, second and third ranges of wavelengths. 
     When the area onto which the beam is incident is relatively large or the size of the BSC device is relatively small, the beams in the first and third ranges of wavelengths may simultaneously reaches plural surfaces. Under such circumstance, the beam in the first range of wavelengths which is incident in the first direction passes through the first surface  1101 . A part of the beam in the first range of wavelengths sequentially passes through the second surface  1102 , the first optical film  1104 , the fifth surface  1202  and the sixth surface  1203  and is emitted in the first direction X. Another part of the beam in the first range of wavelengths passes through the third surface  1103  and is emitted in the first direction X. The beam in the second range of wavelengths which is incident in the second direction Y passes through the fourth surface  1201 , is reflected on the first optical film  1104  provided between the second surface  1102  and the fifth surface  1202  and passes through the sixth surface  1203 , and is emitted in the first direction X. A part of the beam in the third range of wavelengths which is incident in the third direction Z is reflected on the sixth surface  1203  and is emitted in the first direction X, while another part of the beam in the third range of wavelengths which is incident in the third direction Z is reflected on the third surface  1103  and is emitted in the first direction X. The emitted beams in the first and second ranges of wavelengths and the reflected beam in the third range of wavelengths are combined and emitted. Thus, combination of the beams in the first, second and third ranges of wavelengths is performed. The above embodiment can be modified in which the beam is arranged without passing through the third surface  1103 . 
     When the propagation of the beams described above is reverse, operation of beam splitting is performed. In detail, a combined beam which is incident in the first direction X reaches the sixth surface  1203  and the third surface  1103 . A part of the beam in the first range of wavelengths sequentially passes through the sixth surface  1203 , the fifth surface  1202 , the first optical film  1104 , the second surface  1102  and the first surface  1101 , and is emitted in the first direction X, while another part of the beam in the first range of wavelengths passes through the third surface  1103  and is emitted from the first surface  1101  in the first direction X. The beam in the second range of wavelengths passes through the sixth surface  1203 , reaches the fifth surface  1202 , is reflected on the first optical film  1104  and is emitted from the fourth surface  1201  in the second direction Y. A part of the beam in the third range of wavelengths is reflected on the sixth surface  1203  and is emitted in the third direction Z, while another part of the beam in the third range of wavelengths is reflected on the third surface  1103  and is emitted in the third direction Z. Thus, splitting a combined beam into three beams in the first, second and third ranges of wavelengths is performed. 
     In the above embodiment shown in  FIGS. 4A-4C , the first prism  110  is a square pyramidal prism and the first surface  1101  is the bottom surface thereof. The second prism  120  is a triangular pyramidal prism. The first, second and third directions are perpendicular to each other. The first surface  1101  and the second surface  1102  have an included angle of 45 degrees therebetween. The first surface  1101  and the third surface  1103  have an included angle of 45 degrees therebetween. The fourth surface  1201  is perpendicular to the first surface  1101 . The fifth surface  1202  and the fourth surface  1201  have an included angle of 45 degrees therebetween. The sixth surface  1203  and the first surface  1101  have an included angle of 45 degrees therebetween. However, the invention is not limited to the above embodiment. The angles between the surfaces may be changed and the incident direction of the beam may be changed, to perform the operation of splitting and combining the beams in the first, second and third ranges of wavelengths. In addition, the forms of the first prism  110  and the second prism  120  may be changed. 
     In the above embodiment, the BSC device  100  merely has two prisms, namely the first prism  110  and the second prism  120 . The first prism  110  and the second prism  120  are connected, with the second surface  1102  and the fifth surface  1202  contacting each other. Therefore, where gaps can be formed is only at the corners of the first prism  110  and the second prism  120 . Such arrangement can effectively avoid the negative influence on the formed images due to arrival of the incident beam at the gaps. Further, the volume and weight of the BSC device can be reduced, which benefits miniaturization of the projector. 
       FIGS. 5A, 5B and 5C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the second embodiment of the invention. As shown in  FIG. 5A-5C , the second embodiment is a modification of the first embodiment. In the second embodiment, the BSC device  200  includes a first prism  210  and a second prism  220 . The BSC device  200  of the second embodiment and the BSC device  100  of the first embodiment are structurally symmetrical with respect to the first direction X. Therefore, description of the part of the second embodiment same as or similar to that of the second embodiment is omitted. 
       FIGS. 6A and 6B  are respectively a schematic diagram and an exploded diagram of a BSC device in accordance with the third embodiment of the invention,  FIG. 6C  depicts the light paths of the beams in the first and second ranges of wavelengths for the BSC device in accordance with the third embodiment of the invention,  FIG. 6D  depicts the light path of the beam in the third range of wavelengths for the BSC device in accordance with the third embodiment of the invention, and  FIG. 6E  depicts the light paths for the BSC device in accordance with the third embodiment of the invention. The third embodiment is a modification of the first embodiment. Therefore, description of the parts of the third embodiment same as that of the first embodiment is omitted. As shown in  FIGS. 6A-6E , in the third embodiment, the BSC device  300  includes a first prism  310 , a second prism  320  and a third prism  330  connected to each other. The first prism  310  includes a first surface  3101  allowing a beam in a first range of wavelengths to pass through, a second surface  3102  allowing the beam in the first range of wavelengths to pass through and reflecting a beam in a second range of wavelengths, and a third surface  3103  allowing the beam in the first range of wavelengths to pass through and reflecting a beam in a third range of wavelengths. The second surface  3102  and the third surface  3103  are adjacent to each other. When observed in the first direction X, the second surface  3102  and the third surface  3103  are shaded from view by the first surface  3101 . 
     The second prism  320  includes a fourth surface  3201  allowing the beam in the second range of wavelengths to pass through, a fifth surfaced  3202  contacting the second surface  3102 , allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, and a sixth surface  3203  allowing the beams in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths. The fourth, fifth and sixth surfaces  3201 ,  3202  and  3203  are adjacent to each other. 
     The third prism  330  is a square pyramidal prism and includes a seventh surface  3301  allowing the beam in the third range of wavelengths to pass through, an eighth surface  3302  contacting the sixth surface  3301 , opposing the seventh surface  3301 , allowing the beams in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths, a ninth surface  3303  being the bottom surface of the third prism and allowing the beams in the first, second and third ranges of wavelengths to pass through, a tenth surface  3304  allowing the beam in the second range of wavelengths to pass through, and an eleventh surface opposing the tenth surface  3304 , allowing the beams in the first and third ranges of wavelengths to pass through and reflecting the beam in the second range of wavelengths. When observed in the first direction X, the seventh surface  3301 , the eighth surface  3302 , the tenth surface  3304  and the eleventh  3305  are shaded from view by the ninth surface  3303 . 
     The second surface  3102  and the fifth surface  3202  can be connected by gluing. A first optical film  3104  is provided between the second surface  3102  and the fifth surface  3202  by coating. The first optical film  3104  may be coated on the second surface  3102  or the fifth surface  3202 . The first optical film  3104  allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. 
     The sixth surface  3203  and the eighth surface  3302  can be connected by gluing. A second optical film  3306  is provided between the sixth surface  3203  and the eighth surface  3302 . The second optical film  3306  may be coated on the sixth surface  3203  or on the eighth surface  3302 , to reflect the beam in the third range of wavelengths thereon. The second optical film  3306  allows the beams in the first and second ranges of wavelengths to pass through and reflects the beam in the third range of wavelengths. 
     The fourth surface  3201  and the tenth surface  3304  may be disposed in parallel or be coplanar. The eleventh surface  3305  and the fifth surface  3202  may be disposed in parallel or be coplanar. The sixth surface  3203  and the third surface  3103  may be disposed in parallel or be coplanar. Further, the sixth surface  3203  and the third surface  3103  are parallel to the direction in which the beam in the second range of wavelengths passes through the fourth surface  3201 . In  FIG. 6C , the eleventh surface  3305  and the fifth surface  3202  (or the second surface  3102 ) are represented by bold lines, both of which are the reflective surfaces reflecting the beam in the second range of wavelengths and are parallel to the third direction Z. In  FIG. 6D , the sixth surface  3203  (or the eighth surface  3302 ) and the third surface  3103  are represented by bold lines, both of which are reflective surfaces reflecting the beam in the third range of wavelengths and are parallel to the second direction Y. 
     The light paths in the third embodiment are described as follows: As shown, the beam in the first range of wavelengths which is incident in the first direction X sequentially passes through the first surface  3101 , the second surface  3102 , the first optical film  3104 , the fifth surface  3202 , the sixth surface  3203 , the second optical film  3306  and the eighth surface  3302 , and is emitted from the ninth surface  3303  in the first direction X. The beam in the second range of wavelengths which is incident in the second direction Y passes through the fourth surface  3201 , is reflected on the first optical film  3104 , passes through the sixth surface  3203 , the second optical film  3306  and the eighth surface  3302 , and is emitted from the ninth surface  3303  in the first direction X. The beam in the third range of wavelengths which is incident in the third direction Z passes through the seventh surface  3301 , reaches the eighth surface  3302 , is reflected on the second optical film  3306 , and is emitted from the ninth surface  3303  in the first direction X. The above-mentioned propagation of the beams can be reversed: The beam in the first range of wavelengths sequentially passes through the ninth surface  3303 , the eighth surface  3302 , the second optical film  3306 , the sixth surface  3203 , the fifth surface  3202 , the first optical film  3104  and the second surface  3102 , and is emitted from the first surface  3303  in the first direction X. The beam in the second range of wavelengths sequentially passes through the ninth surface  3303 , the eighth surface  3302 , the second optical film  3306  and the sixth surface  3203 , is reflected on the first optical film  3104  provided between the second surface  3102  and the fifth surface  3202 , and is emitted from the fourth surface  3201  in the second direction Y. The beam in the third range of wavelengths passes through the ninth surface  3303  and the eighth surface  3302 , is reflected on the second optical film  3306  to the seventh surface  3301 , and is emitted from the seventh surface  3301  in the third direction Z. 
     When the area onto which the beam is incident is relatively large or the size of the BSC device is relatively small, the beams in the first and third ranges of wavelengths may simultaneously reaches plurality surfaces. Under such circumstance, a part of the beam in the first range of wavelengths which is incident in the first direction X sequentially passes through the first surface  3101 , the second surface  3102 , the first optical film  3104 , the fifth surface  3202 , the sixth surface  3203 , the second optical film  3306  and the eighth surface  3302 , and is emitted from the ninth surface  3303  in the first direction X. Another part of the beam in the first range of wavelengths which is incident in the first direction X passes through the third surface  3103  and the eleventh surface  3305  and is emitted from the ninth surface  3303  in the first direction X. 
     A part of the beam in the second range of wavelengths which is incident in the second direction Y passes through the fourth surface  3201 , is reflected on the first optical film  3104 , passes through the sixth surface  3203 , the second optical film  3306  and the eighth surface  3302 , and is emitted from the ninth surface  3303  in the first direction X. Another part of the beam in the second range of wavelengths which is incident in the second direction Y passes through the tenth surface  3304 , is reflected on the eleventh surface  3305 , and is emitted from the ninth surface  3303  in the first direction X. 
     A part of the beam in the third range of wavelengths which is incident in the third direction Z passes through the seventh surface  3301 , reaches the eighth surface  3302 , is reflected on the second optical film  3306  to the ninth surface  3303  and is emitted from the ninth surface  3303  in the first direction X. Another part of the beam in the third range of wavelengths which is incident in the third direction Z is reflected on the third surface  3103  to the eleventh surface  3305 , passes through the eleventh surface  3305 , and is emitted from the ninth surface  3303  in the first direction X. Thus, combination of the beams in the first, second and third ranges of wavelengths is performed. 
     Reverse propagation of the beams is described as follows: A combined beam is incident on the ninth surface  3303 . A part of the beam in the first range of wavelengths sequentially passes through the eighth surface  3302 , the second optical film  3306 , the sixth surface  3203 , the fifth surface  3202 , the first optical film  3104 , the second surface  3102 , and is emitted from the first surface  3303  in the first direction X. Another part of the beam in the first range of wavelengths sequentially passes through the ninth surface  3303 , the eleventh surface  3305 , the third surface  3103  and is emitted from the first surface  3101  in the first direction X. A part of the beam in the second range of wavelengths sequentially passes through the ninth surface  3303 , the eighth surface  3302 , the second optical film  3306 , the sixth surface  3203 , is reflected on the first optical film  3104  provided between the second surface  3102  and the fifth surface  3202 , and is emitted from the fourth surface  3201  in the second direction Y. Another part of the beam in the second range of wavelengths passes through the ninth surface  3303 , is reflected on the eleventh surface  3305 , and is emitted from the tenth surface  3304  in the second direction Y. A part of the beam in the third range of wavelengths passes through the ninth surface  3303  and the eleventh surface  3305 , reaches the third surface  3103 , and is reflected on the third surface  3103  in the third direction Z. Another part of the beam in the third range of wavelengths passes through the ninth surface  3303 , reaches the eighth surface  3302 , is reflected on the second optical film  3306 , and is emitted from the seventh surface  3301  in the third direction Z. Thus, splitting a combined beam into three beams in the first, second and third ranges of wavelengths is performed. In the third embodiment, the beam may be arranged without passing through the third surface  3103 . Alternatively, the beam may be arranged without passing through the second prism  320 . 
     In the third embodiment shown in  FIGS. 6A-6E , the first prism  310  is a square pyramidal prism and the first surface  3101  is the bottom surface of the first prism  310 . The second prism  320  is a triangular pyramidal prism. The third prism  330  is a square pyramidal prism and the ninth surface  3303  is the bottom surface of the third prism  330 . The first direction X, the second direction Y and the third direction Z are perpendicular to each other. The first surface  3101  and the second surface  3102  have an included angle of 45 degrees therebetween. The first surface  3101  and the third surface  3103  also have an included angle of 45 degrees therebetween. The fourth surface  3201  is perpendicular to the first surface  3101 . The sixth surface  3203  and the first surface  3101  have an included angle of 45 degrees therebetween. The seventh surface  3301  and the first surface  3101  have an included angle of 90 degrees therebetween. The eighth surface  3302  and the first surface  3101  have an included angle of 45 degrees therebetween. The ninth surface  3303  and the first surface  3101  are disposed in parallel. The tenth surface  3304  is perpendicular to the first surface  3101 . 
     However, the invention is not limited to the above embodiment. The angles between the surfaces may be changed and the incident direction of the beam may be changed, to perform the operation of splitting and combining the beams in the first, second and third ranges of wavelengths. In addition, the forms of the first prism  310 , the second prism  320  and the third prism  330  may be changed. 
       FIGS. 7A, 7B and 7C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the fourth embodiment of the invention. As shown in  FIG. 7A-7C , the fourth embodiment is a modification of the third embodiment. In the fourth embodiment, the BSC device  400  includes a first prism  410 , a second prism  420  and a third prism  430 . The BSC device  400  of the fourth embodiment and the BSC device  300  of the third embodiment are structurally symmetrical with respect to the first direction X. Therefore, description of the parts of the fourth embodiment same as or similar to that of the third embodiment is omitted. 
       FIGS. 8A and 8B  are respectively a schematic diagram and an exploded diagram of a BSC device in accordance with the fifth embodiment of the invention,  FIG. 8C  depicts the light paths of the beams in the first and second ranges of wavelengths for the BSC device in accordance with the fifth embodiment of the invention,  FIG. 8D  depicts the light path of the beam in the third range of wavelengths for the BSC device in accordance with the fifth embodiment of the invention, and  FIG. 8E  depicts the light paths for the BSC device in accordance with the fifth embodiment of the invention. The fifth embodiment is a modification of the first embodiment. Therefore, description of the parts of the fifth embodiment same as that of the first embodiment is omitted. As shown in  FIGS. 8A-8E , in the fifth embodiment, the BSC device  500  includes a first prism  510 , a second prism  520  and a third prism  530 . The first prism  510  includes a first surface  5101  allowing a beam in a first range of wavelengths to pass through, a second surface  5102  allowing the beam in the first range of wavelengths to pass through and reflecting a beam in a second range of wavelengths, and a third surface  5103  allowing the beam in the first range of wavelengths to pass through and reflecting a beam in a third range of wavelengths. The second surface  5102  and the third surface  5103  are adjacent to each other. When observed in the first direction X, the second surface  5102  and the third surface  5103  are shaded from view by the first surface  5101 . 
     The second prism  520  includes a fourth surface  5201  allowing the beam in the second range of wavelengths to pass through, a fifth surfaced  5202  contacting the second surface  5102 , allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, and a sixth surface  5203  allowing the beams in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths. The fourth, fifth and sixth surfaces  5201 ,  5202  and  5203  are adjacent to each other. 
     The third prism  530  includes a seventh surface  5301  allowing the beam in the third range of wavelengths to pass through, an eighth surface  5302  contacting the third surface  5103 , allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the third range of wavelengths, a ninth surface  5303  allowing the beams in the first and third ranges of wavelengths to pass through and reflecting the beam in the second range of wavelengths. 
     The second surface  5102  and the fifth surface  5202  can be connected by gluing. A first optical film  5104  is provided between the second surface  5102  and the fifth surface  5202  by coating. The first optical film  5104  may be coated on the second surface  5102  or the fifth surface  5202 . The first optical film  5104  allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. The sixth surface  5203  allows the beams in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths. To achieve this purpose, the sixth surface  5203  may be coated with an optical film. 
     The third surface  5103  and the eighth surface  5302  can be connected by gluing. A second optical film  5305  is provided between the third surface  5103  and the eighth surface  5302 . The second optical film  5305  may be coated on the third surface  5103  or on the eighth surface  5302 , to reflect the beam in the third range of wavelengths thereon. The second optical film  5305  allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. 
     The ninth surface  5303  and the fifth surface  5202  may be disposed in parallel or be coplanar. The sixth surface  5203  and the third surface  5103  may be disposed in parallel or be coplanar. In  FIG. 8C , the ninth surface  5303  and the fifth surface  5202  (or the second surface  5102 ) are represented by bold lines, both of which are the reflective surfaces reflecting the beam in the second range of wavelengths and are parallel to the third direction Z. In  FIG. 8D , the sixth surface  5203  and the third surface  5103  (or the eighth surface  5302 ) are represented by bold lines, both of which are reflective surfaces reflecting the beam in the third range of wavelengths and are parallel to the second direction Y. 
     As shown, a part of the beam in the first range of wavelengths which is incident in the first direction X sequentially passes through the first surface  5101 , the second surface  5102 , the first optical film  5104  and the fifth surface  5202 , and is emitted from the sixth surface  5203  in the first direction X. Another part of the beam in the first range of wavelengths which is incident in the first direction X passes through the third surface  5103 , the second optical film  5305  and the eighth surface  5302  and is emitted from the ninth surface  5303  in the first direction X. 
     The beam in the second range of wavelengths is incident in the second direction Y. A part of the beam in the second range of wavelengths passes through the fourth surface  5201 , is reflected on the first optical film  5104  provided between the second surface  5102  and the fifth surface  5202 , passes through the sixth surface  5203 , and is emitted in the first direction X. Another part of the beam in the second range of wavelengths is reflected on the ninth surface  5303  and is emitted in the first direction X. 
     The beam in the third range of wavelengths is incident in the third direction Z. A part of the beam in the third range of wavelengths passes through the seventh surface  5301 , reaches the eighth surface  5302 , is reflected on the second optical film  5305  to the ninth surface  5303 , and is emitted from the ninth surface  5303  in the first direction X. Another part of the beam in the third range of wavelengths is reflected on the sixth surface  5203  and is emitted in the first direction X. Accordingly, combination of the beams in the first, second and third ranges of wavelengths are performed. 
     Reversed propagation of the beams is described as follows: A part of the combined beam reaches the ninth surface  5303 . The beam in the first range of wavelengths sequentially passes through the ninth surface  5303 , the eighth surface  5302 , the second optical film  5305  and the third surface  5103 , and is emitted from the first surface  5101 . The beam in the second range of wavelengths is reflected on the ninth surface  5303  and is emitted in the second direction Y. The beam in the third range of wavelengths passes through the ninth surface  5303  and the eighth surface  5302 , is reflected on the second optical film  5305 , and is emitted from the seventh surface  5301  from the seventh surface  530 . Accordingly, splitting the combined beam into the beams in the first, second and third ranges of wavelengths is performed. Another part of the combined beam reaches the sixth surface  5203 . The beam in the first range of wavelengths sequentially passes through the sixth surface  5203 , the fifth surface  5202 , the first optical film  5104 , the second surface  5102  and the first surface  5101  and is emitted in the first direction X. The beam in the second range of wavelengths passes through the sixth surface  5203  and the fifth surface  5202 , is reflected on the first optical film  5104 , and is emitted from the fourth surface  5201  in the second direction Y. The beam in the third range of wavelengths is reflected on the sixth surface  5203  and is emitted in the third direction Z. However, the invention is not limited thereto. The light paths of the fifth embodiment may be modified to be the same as those of the first embodiment or may be arranged without passing through the second prism  520 . 
     In the fifth embodiment shown in  FIGS. 8A-8C , the first prism  510  is a square pyramidal prism and the first surface  5101  is the bottom surface of the first prism  510 . The second prism  520  is a triangular pyramidal prism. The third prism  530  is a triangular pyramidal prism. The first direction X, the second direction Y and the third direction Z are perpendicular to each other. The first surface  5101  and the second surface  5102  have an included angle of 45 degrees therebetween. The first surface  5101  and the third surface  5103  also have an included angle of 45 degrees therebetween. The fourth surface  5201  is perpendicular to the first surface  5101 . The sixth surface  5203  and the first surface  5101  have an included angle of 45 degrees therebetween. The seventh surface  5301  is perpendicular to the first surface  5101  and the fourth surface  5201 . The eighth surface  5302  and the seventh surface  5301  have an included angle of 45 degrees therebetween. 
     However, the invention is not limited to the above embodiment. The angles between the surfaces may be changed and the incident direction of the beam may be changed, to perform the operation of splitting and combining the beams in the first, second and third ranges of wavelengths. In addition, the forms of the first prism  510 , the second prism  520  and the third prism  530  may be changed. 
       FIGS. 9A, 9B and 9C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the sixth embodiment of the invention. As shown in  FIG. 9A-9C , the sixth embodiment is a modification of the fifth embodiment. In the sixth embodiment, the BSC device  600  includes a first prism  610 , a second prism  620  and a third prism  630 . The BSC device  600  of the sixth embodiment and the BSC device  500  of the fifth embodiment are structurally symmetrical with respect to the first direction X. Therefore, description of the part of the sixth embodiment same as or similar to that of the fifth embodiment is omitted. 
       FIGS. 10A, 10B and 10C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the seventh embodiment of the invention. The seventh embodiment is a modification of the first embodiment. Therefore, description of the parts of the seventh embodiment same as that of the first embodiment is omitted. The BSC device  700  of the seventh embodiment, having the same structure as that of the first embodiment, includes a first prism  710  and a second prism  720  connected to each other. The first prism  710  includes a first surface  7101  allowing beams in first, second and third ranges of wavelengths to pass through, a second surface  7102  allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, a third surface  7103  allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the third range of wavelengths, a seventh surface  7105  allowing the beam in the second range of wavelengths to pass through, and an eighth surface  7106  allowing the beam in the third range of wavelengths to pass through. The second surface  7102  and the third surface  7103  are adjacent to each other. When observed in the first direction X, the second surface  7102  and the third surface  7103  are shaded from view by the first surface  7101 . The second prism  720  includes a fourth surface  7201 , a fifth surfaced  7202  contacting the second surface  7102 , allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, a sixth surface  7203  allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the third range of wavelengths, and a ninth surface  7204  allowing the beam in the third range of wavelengths to pass through. The fourth, fifth and sixth surfaces  7201 ,  7202  and  7203  are adjacent to each other. 
     The second surface  7102  and the fifth surface  7202  can be connected by gluing. A first optical film  7104  is provided between the second surface  7102  and the fifth surface  7202  by coating. The first optical film  7104  may be coated on the second surface  7102  or the fifth surface  7202 . The first optical film  7104  allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. The sixth surface  7203  allows the beams in the first and second ranges of wavelengths to pass through and reflects the beam in the third range of wavelengths. To achieve the purpose, the sixth surface  7203  may be coated with an optical film. 
     The third surface  7103  and the sixth surface  7203  may be disposed in parallel or be coplanar. Further, the third surface  7103  and the sixth surface  7203  are parallel to the direction in which the beam in the second range of wavelengths passes through the seventh surface  7105 . The third surface  7103  allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. To achieve this purpose, the third surface  7103  may be coated with an optical film. 
     In the seventh embodiment, the propagation of the beams in the first, second and third ranges of wavelengths and the incident direction and the emitting direction of the combined beam are reverse to those in the first embodiment. 
     The beam in the first range of wavelengths which is incident in the first direction X sequentially passes through the sixth surface  7203 , the fifth surface  7202 , the first optical film  7104  and the second surface  7102 , and is emitted from the first surface  7101  in the first direction X. The beam in the second range of wavelengths which is incident in the second direction Y passes through the seventh surface  7105 , is reflected on the first optical film  7104  provided between the second surface  7102  and the fifth surface  7202 , and is emitted from the first surface  7101  in the first direction X. The beam in the third range of wavelengths which is incident in the third direction Z passes through the ninth surface  7204 , reaches the sixth surface  7203 , is reflected on the sixth surface  7203 , passes through the fifth surface  7202 , the first optical film  7104  and the second surface  7102 , and is emitted from the first surface  7101  in the first direction X. 
     The above-mentioned propagation of the beams can be reversed: A combined beam is incident in the first direction X. A beam in the first range of wavelengths sequentially passes through the first surface  7101 , the second surface  7102 , the first optical film  7104  and the fifth surface  7202 , and is emitted from the sixth surface  7203  in the first direction X. A beam in the second range of wavelengths passes through the first surface  7101 , is reflected on the first optical film  7104  provided between the second surface  7102  and the fifth surface  7202 , and is emitted from the seventh surface  7105  in the second direction Y. A beam in the third range of wavelengths passes through the first surface  7101 , the second surface  7102 , the first optical film  7104  and the fifth surface  7202 , is reflected on the sixth surface  7203 , and is emitted from the ninth surface  7204  in the third direction Z. 
     When the area onto which the beam is incident is relatively large or the size of the BSC device is relatively small, the incident beams may simultaneously reaches plurality surfaces. Under such circumstance, the beam in the first range of wavelengths is incident in the first direction. A part of the beam in the first range of wavelengths sequentially passes through the sixth surface  7203 , the fifth surface  7202 , the first optical film  7104  and the second surface  7102 , and is emitted from the first surface  7101  in the first direction X. Another part of the beam in the first range of wavelengths which is incident in the first direction X passes through the third surface  7103  and is emitted from the first surface  7101  in the first direction X. The beam in the second range of wavelengths which is incident in the second direction Y passes through the seventh surface  7105 , is reflected on the first optical film  7104  provided between the second surface  7102  and the fifth surface  7202 , and is emitted from the first surface  7101  in the first direction X. A part of the beam in the third range of wavelengths which is incident in the third direction Z passes through the eighth surface  7106 , reaches the third surface  7103 , is reflected on the third surface  7103 , and is emitted from the first surface  7101  in the first direction X. Another part of the beam in the third range of wavelengths which is incident in the third direction Z passes through the ninth surface  7204 , reaches the sixth surface  7203 , is reflected on the sixth surface  7203 , passes through the fifth surface  7202 , the first optical film  7104  and the second surface  7102 , and is emitted from the first surface  7101  in the first direction X. 
     The reverse propagation of the beams is described as follows: A combined beam is incident on the first surface  7101  in the first direction X. A part of the beam in the first range of wavelengths sequentially passes through the first surface  7101 , the second surface  7102 , the first optical film  7104  and the fifth surface  7202 , and is emitted from the sixth surface  7203  in the first direction X. Another part of the beam in the first range of wavelengths passes through the first surface  7101  and is emitted from the third surface  7103  in the first direction X. The beam in the second range of wavelengths passes through the first surface  7101 , is reflected on the first optical film  7104  provided between the second surface  7102  and the fifth surface  7202 , and is emitted from the seventh surface  7105  in the second direction Y. A part of the beam in the third range of wavelengths passes through the first surface  7101 , is reflected on the third surface  7103 , and is emitted on the eighth surface  7106  in the third direction Z. Another part of the beam in the third range of wavelengths passes through the first surface  7101 , the second surface  7102 , the first optical film  7104  and the fifth surface  7202 , is reflected on the sixth surface  7203 , and is emitted from the ninth surface  7204  in the third direction Z. 
     In the seventh embodiment, the first prism  710  is a square pyramidal prism and the first surface  7101  is the bottom surface of the first prism  710 . The second prism  720  is a triangular pyramidal prism. 
       FIGS. 11A, 11B and 11C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the eighth embodiment of the invention. As shown in  FIG. 11A-11C , the eighth embodiment is a modification of the seventh embodiment. In the eighth embodiment, the BSC device  800  includes a first prism  810  and a second prism  820 . The BSC device  800  of the eighth embodiment and the BSC device  700  of the seventh embodiment are structurally symmetrical with respect to the first direction X. Therefore, description of the part of the eighth embodiment same as or similar to that of the seventh embodiment is omitted. 
       FIGS. 12A and 12B  are respectively a schematic diagram and an exploded diagram of a BSC device in accordance with the ninth embodiment of the invention,  FIG. 12C  depicts the light paths of the beams in the first and second ranges of wavelengths for the BSC device in accordance with the ninth embodiment of the invention,  FIG. 12D  depicts the light path of the beam in the third range of wavelengths for the BSC device in accordance with the ninth embodiment of the invention, and  FIG. 12E  depicts the light paths for the BSC device in accordance with the ninth embodiment of the invention. The ninth embodiment is a modification of the fourth and fifth embodiments. Therefore, description of the parts of the ninth embodiment same as or similar to that of the fifth embodiment is omitted. As shown in  FIGS. 12A-12C , in the ninth embodiment, the BSC device  900  includes a first prism  910 , a second prism  920 , a third prism  930  and a fourth prism  940 . The first prism  910  includes a first surface  9101  allowing a beam in a first range of wavelengths to pass through, a second surface  9102  allowing the beam in the first range of wavelengths to pass through, and a third surface  9103  being adjacent to the second surface  9102 , allowing the beam in the first range of wavelengths to pass through and reflecting a beam in a third range of wavelengths. When observed in a direction perpendicular to the first surface  9101  (i.e. the first direction X), the second surface  9102  and the third surface  9103  are shaded from view by the first surface  9101 . 
     The second prism  920  includes a fourth surface  9201  allowing the beam in the second range of wavelengths to pass through, a fifth surfaced  9202  contacting the second surface  9102 , allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, and a sixth surface  9203  allowing the beams in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths. The fourth, fifth and sixth surfaces  9201 ,  9202  and  9203  are adjacent to each other. 
     The third prism  930  is a square pyramidal prism and includes a seventh surface  9301  allowing the beam in the third range of wavelengths to pass through, an eighth surface  9302  contacting the sixth surface  9203 , opposing the seventh surface  9301 , allowing the beams in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths, a ninth surface  9303  being the bottom surface of the third prism  930  and allowing the beams in the first, second and third ranges of wavelengths to pass through, a tenth surface  9304  allowing the beam in the second range of wavelengths to pass through, and an eleventh surface  9305  allowing the beams in the first and third ranges of wavelengths to pass through and reflecting the beam in the second range of wavelengths. When observed in the first direction X, the seventh surface  9301 , the eighth surface  9302 , the tenth surface  9304  and the eleventh  9305  are shaded from view by the ninth surface  9303 . 
     The fourth prism  940  includes a twelfth surface  9401  allowing the beam in the third range of wavelengths to pass through, a thirteenth surface  9402  contacting the eleventh surface  9305 , allowing the beams in the first and third ranges of wavelengths to pass through and reflecting the beam in the second range of wavelengths, a fourteenth surface  9403  allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, and a fifteenth surface  9404  contacting the third surface  9103 , allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the third range of wavelengths. 
     The second surface  9102  and the fifth surface  9202  can be connected by gluing. A first optical film  9104  is provided between the second surface  9102  and the fifth surface  9202  by coating. The first optical film  9104  may be coated on the second surface  9102  or the fifth surface  9202 . The first optical film  9104  allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. 
     The sixth surface  9203  and the eighth surface  9302  can be connected by gluing. A second optical film  9306  is provided between the sixth surface  9203  and the eighth surface  9302 . The second optical film  9306  may coated on the sixth surface  9203  or on the eighth surface  9302 . The second optical film  9306  allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. 
     The thirteenth surface  9402  and the eleventh surface  9305  can be connected by gluing. A third optical film  9405  is provided between the thirteenth surface  9402  and the eleventh surface  9305 . The third optical film  9405  may coated on the thirteenth surface  9402  or on the eleventh surface  9305 , to perform reflection of the second range of wavelengths thereon. The third optical film  9405  allows the beam in the first range of wavelengths to pass through and reflects the beam in the second range of wavelengths. 
     The fifteenth surface  9404  and the third surface  9103  can be connected by gluing. A fourth optical film  9406  is provided between the fifteenth surface  9404  and the third surface  9103 . The fourth optical film  9406  may be coated on the fifteenth surface  9404  or on the third surface  9103  to perform reflection of the beam in the third range of wavelengths thereon. The fourth optical film  9406  allows the beam in the first range of wavelengths to pass through and reflects the beam in the third range of wavelengths. 
     The fourth surface  9201  and the tenth surface  9304  may be disposed in parallel or be coplanar. The eleventh surface  9305  and the fifth surface  9202  may be disposed in parallel or be coplanar. The sixth surface  9203  and the third surface  9103  may be disposed in parallel or be coplanar. Further, the sixth surface  9203  and the third surface  9103  are parallel to the direction in which the beam in the second range of wavelengths passes through the fourth surface  9201 . In  FIG. 12C , the eleventh surface  9305  (or the thirteenth surface  9402 ) and the fifth surface  9202  (or the second surface  9102 ) are represented by bold lines, both of which are the reflective surfaces reflecting the beam in the second range of wavelengths and are parallel to the third direction Z. In  FIG. 12D , the sixth surface  9203  (or the eighth surface  9302 ) and the third surface  9103  (or the fifteenth surface  9404 ) are represented by bold lines, both of which are reflective surfaces reflecting the beam in the third range of wavelengths and are parallel to the second direction Y. 
     The light paths in the ninth embodiment are described as follows: The beam in the first range of wavelengths which is incident in the first direction X sequentially passes through the first surface  9101 , the second surface  9102 , the first optical film  9104 , the fifth surface  9202 , the sixth surface  9203 , the second optical film  9306  and the eighth surface  9302 , and is emitted from the ninth surface  9303 . The beam in the second range of wavelengths which is incident in the second direction Y passes through the fourth surface  9201 , is reflected on the first optical film  9104  provided between the second surface  9102  and the fifth surface  9202 , passes through the sixth surface  9203 , the second optical film  9306  and the eighth surface  9302 , and is emitted from the ninth surface  9303  in the first direction X. The beam in the third range of wavelengths which is incident in the third direction Z passes through the seventh surface  9301 , reaches the eighth surface  9302 , is reflected on the second optical film  9306 , and is emitted from the ninth surface  9303  in the first direction X. 
     The above-mentioned propagation of the beams can be reversed: A combined beam is incident on the ninth surface  9303  in the first direction X. The beam in the first range of wavelengths sequentially passes through the eighth surface  9302 , the second optical film  9306 , the sixth surface  9203 , the fifth surface  9202 , the first optical film  9104  and the second surface  9102 , and is emitted from the first surface  9101  in the first direction X. The beam in the second range of wavelengths sequentially is incident on the ninth surface  9303 , sequentially passes through the eighth surface  9302 , the second optical film  9306  and the sixth surface  9203 , is reflected on the first optical film  9104  provided between the second surface  9102  and the fifth surface  9202 , and is emitted from the fourth surface  9201  in the second direction Y. The beam in the third range of wavelengths is incident on the ninth surface  9303 , is reflected on the second optical film  9306  provided between the eighth surface  9302  and the sixth surface  9203 , and is emitted from the sixth surface  9203  in the third direction Z. Accordingly, the combined beam is split into the beams in the first, second and third ranges of wavelengths. 
     When the area onto which the beam is incident is relatively large or the size of the BSC device is relatively small, the beams in the first and third ranges of wavelengths may simultaneously reaches plurality surfaces. Under such circumstance, the beam in the first range of wavelengths is incident on the first surface  9101  in the first direction X. A part of the beam in the first range of wavelengths sequentially passes through the first surface  9101 , the second surface  9102 , the first optical film  9104 , the fifth surface  9202 , the sixth surface  9203 , the second optical film  9306  and the eighth surface  9302 , and is emitted from the ninth surface  9303 . Another part of the beam in the first range of wavelengths sequentially passes through the third surface  9103 , the fourth optical film  9406 , the fifteenth surface  9404 , the thirteenth surface  9402 , the third optical film  9405  and the eleventh surface  9305  and is emitted from the ninth surface  9303 . 
     The beam in the second range of wavelengths is incident in the second direction Y. A part of the beam in the second range of wavelengths passes through the fourth surface  9201 , is reflected on the first optical film  9104  provided between the second surface  9102  and the fifth surface  9202 , passes through the sixth surface  9203 , the second optical film  9306  and the eighth surface  9302 , and is emitted from the ninth surface  9303  in the first direction X. Another part of the beam in the second range of wavelengths which is incident in the second direction Y passes through the tenth surface  9304 , is reflected on the third optical film  9405  provided between the thirteenth surface  9402  and the eleventh surface  9305 , and is emitted from the ninth surface  9303  in the first direction X. In  FIG. 12C , the bold lines represent the reflective surfaces reflecting the beam in the second range of wavelengths. 
     The beam in the third range of wavelengths is incident in the third direction Z. A part of the beam in the third range of wavelengths passes through the seventh surface  9301 , reaches the eighth surface  9302 , is reflected on the second optical film  9306  to the ninth surface  9303  and is emitted from the ninth surface  9303  in the first direction X. Another part of the beam in the third range of wavelengths passes through the twelfth surface  9401 , reaches the fifteenth surface  9404 , is reflected on the fourth optical film  9406  provided between the fifteenth surface  9404  and the third surface  9103 , passes through the eleventh surface  9305 , and is emitted from the ninth surface  9303  in the first direction X. Thus, combination of the beams in the first, second and third ranges of wavelengths is performed. 
     The reverse propagation of the beams is described as follows: A combined beam is incident on the ninth surface  9303  in the first direction X. A part of the beam in the first range of wavelengths sequentially passes through the eleventh surface  9305 , the third optical film  9405 , the thirteenth surface  9402 , the fifteenth surface  9404 , the fourth optical film  9406 , the third surface  9103 , and is emitted from the first surface  9101  in the first direction X. Another part of the beam in the first range of wavelengths sequentially passes through the eighth surface  9302 , the second optical film  9306 , the sixth surface  9203 , the fifth surface  9202 , the first optical film  9104 , the second surface  9102  and is emitted from the first surface  9101  in the first direction X. 
     A part of the beam in the second range of wavelengths is incident on the ninth surface  9303 , sequentially passes through the eighth surface  9302 , the second optical film  9306 , the sixth surface  9203 , is reflected on the first optical film  9104  provided between the second surface  9102  and the fifth surface  9202 , and is emitted from the fourth surface  9201  in the second direction Y. Another part of the beam in the second range of wavelengths is incident on the ninth surface  9303 , is reflected on the third optical film  9405  provided between the thirteenth surface  9402  and the eleventh surface  9305 , and is emitted from the tenth surface  9304  in the second direction Y. 
     A part of the beam in the third range of wavelengths is incident on the ninth surface  9303 , is reflected on the second optical film  9306  provided between the eighth surface  9302  and the sixth surface  9203 , and is emitted from the seventh surface  9301  in the third direction Z. Another part of the beam in the third range of wavelengths is incident on the ninth surface  9303 , passes through the eleventh surface  9305 , reaches the fifteenth surface  9404 , is reflected on the fourth optical film  9406  provided between the fifteenth surface  9404  and the third surface  9103 , and is emitted from the twelfth surface  9401  in the third direction Z. Thus, splitting a combined beam into three beams in the first, second and third ranges of wavelengths is performed. 
     The light paths described above are only an exemplary embodiment of the invention. After incident on the ninth surface  9303  in the first direction X, the combined beam may merely reach the eleventh surface  9305  without reaching the eighth surface  9302 . The paths of the spilt beams and the paths of the corresponding combined beam can be deduced by reference to the above-mentioned light paths, and therefore the description thereof is omitted. 
     In the ninth embodiment shown in  FIGS. 12A-12E , the first direction X, the second direction Y and the third direction Z are perpendicular to each other. The first surface  9101  and the second surface  9102  have an included angle of 45 degrees therebetween. The fourth surface  9201  is perpendicular to the first surface  9101 . The fifth surface  9202  and the fourth surface  9201  have an included angle of 45 degrees therebetween. The seventh surface  9301  is perpendicular to the first surface  9101  and the fourth surface  9201 . The eighth surface  9302  and the seventh surface  9301  have an included angle of 45 degrees therebetween. 
     However, the invention is not limited to the above embodiment. The angles between the surfaces may be changed and the incident direction of the beam may be changed, to perform the operation of splitting and combining the beams in the first, second and third ranges of wavelengths. 
     In the ninth embodiment, the first prism  910  is a square pyramidal prism and the first surface  9101  is the bottom surface of the first prism  910 . Further, except for the second surface  9102  and the third surface  9103 , the other side surfaces are perpendicular to the first surface  9101 . The second prism  920  is a triangular pyramidal prism. The third prism  930  is a square pyramidal prism and the ninth surface  9303  is the bottom surface of the third prism  930 . The seventh surface  9301  and the tenth surface  9304  are perpendicular to each other. However, the invention is not limited thereto. The forms of the first, second, third and fourth prisms  910 ,  920 ,  930  and  940  can be changed. 
       FIGS. 13A, 13B and 13C  are respectively a schematic diagram, an exploded diagram and a light path diagram of a BSC device in accordance with the tenth embodiment of the invention. As shown in  FIG. 13A-13C , the tenth embodiment is a modification of the ninth embodiment. In the tenth embodiment, the BSC device  1000  includes a first prism  1010 , a second prism  1020 , a third prism  1030  and a fourth prism  1040 . The BSC  1000  of the tenth embodiment and the BSC device  900  of the ninth embodiment are structurally symmetrical with respect to the first direction X. Therefore, description of the part of the tenth embodiment same as or similar to that of the ninth embodiment is omitted. 
       FIGS. 14A and 14B  are respectively a schematic diagram and a light path diagram of a BSC device in accordance with the eleventh embodiment of the invention. The BSC device  1000  of the eleventh embodiment includes a prism  1100  having the same structure as that of the second embodiment. The prism  1100  may be a triangular prism, including a first surface  11001  allowing the beam in the first range of wavelengths to pass through and reflecting the beam in the second range of wavelengths, a second surface  11002  allowing the beam in the second range of wavelengths to pass through, and a third surface  11003  allowing the beam in the first and second ranges of wavelengths to pass through and reflecting the beam in the third range of wavelengths. 
     In the eleventh embodiment as shown in  FIGS. 14A and 14B , the beam in the first range of wavelengths which is incident on the first surface  11001  in the first direction X sequentially passes through the first surface  11001 , and is emitted from the third surface  11003  in the first direction X. The beam in the second range of wavelengths is incident on the second surface  11002  in the second direction Y, is reflected on the first surface  11001 , and is emitted from the third surface  11003  in the first direction X. The beam in the third range of wavelengths is incident on the third surface  11003  in the third direction Z, and is reflected in the first direction X. 
     Reversed propagation of the above beams is described as follows: A combined beam is incident on the third surface  11003  in the first direction X. A beam in the first range of wavelengths passes through the third surface  11003  and is emitted from the first surface  11001  in the first direction X. A beam in the second range of wavelengths passes through the third surface  11003 , is reflected on the first surface  11001 , and is emitted from the second surface  11002  in the second direction Y. A beam in the third range of wavelengths is incident on the third surface  11003  and is reflected to propagate in the third direction Z. Accordingly, only the prism  1100  is able to perform the beam splitting and combining. 
     In the BSC device as described above, where gaps can be formed is only at the corners of the prisms. It can be avoided that gaps are formed between prisms to affect the propagation of beams when the beams are incident on the surfaces of the prisms. The arrangement of these embodiments can effectively avoid the influence on the formed images due to arrival of the incident beam at the gaps. Further, the volume and weight of the BSC device can be reduced that benefits miniaturization of the projector, improves the quality of projection images, and increases the light energy utilization efficiency. Various embodiments are described above. However, people skilled in the art can still make modifications. For example, the structure of the BSC device  700  of the seventh embodiment is the same as that of the first embodiment. However, the propagation of the beams in the first, second and third ranges of wavelengths of the seventh embodiment is reverse to that of the first embodiment. It is understood that other embodiments can be modified in a similar way. Square pyramidal prisms are described in the above embodiments. However, the invention is not limited thereto. For example, a square pyramidal prism can be replaced with two triangular pyramidal prisms which are connected to each other. An optical film is sandwiched therebetween, allowing beams in the first, second and third ranges of wavelengths to pass through. Any embodiments can have the merits of the invention as long as the beams are incident on the surfaces of the prisms without reaching the gaps. In the embodiments described above, the arrangements of the reflective surfaces enable the beams to be combined in the BSC devices before emitted therefrom. Accordingly, the beams can be combined more efficiently and more uniformly, the light energy utilization efficiency can be increased, the quality of formed images can be improved, and the beams can be spilt more uniformly in later operation. Further, the optical film is disposed between the prisms to be protected, whereby the quality of the picture images can be stabilized and the light path and the quality of the picture images can be balanced. In the above embodiments, providing an optical film between two surfaces is described, while providing an optical film on a single surface is not particularly described. However, people skilled in the art can understand that any surface reflecting a beam in a specific range of wavelengths has an optical film coated thereon. 
     The invention provides an electronic device, such as a projecting related device, to include the BSC device described above. For example, a projector  1200  as shown in  FIG. 15  includes a light source  1201 , a projection lens  1202 , a spatial light modulator (SLM)  1203 , a plurality of beam splitters  1205 ,  1205 ′,  1205 ″, a plurality of relay lenses  1207 , an assembly of integration lens and P-S converter  1206 , and the BSC device  1204  described in the above embodiments. In operation, light emitted from the light source  1201  sequentially passes through the beam splitter  1205 ″, the assembly of integration lens and P-S converter  1206 , the beam splitter  1205 ′, the beam splitters  1205 , the relay lenses  1207 , the SLM  1203 , the BSC device  1204 , and the projection lens  1202  to form image on a screen. It is understood that any of the beam splitters  1205  can be replaced with a BSC devices described in the above embodiments to function the same. Also, it is understood that another BSC device (not shown) can be additionally provided between the beam splitter  1205 ′ and the assembly of integration lens and P-S converter  1206  in the invention. Also, it is worth noting that the assembly of integration lens and P-S converter  1206  can be deemed a relay lens. Under such circumstance, the relay lenses  1207  and the assembly of integration lens and P-S converter  1206  constitute a relay lens assembly. 
     The projecting related device of the invention may be a head-mounted display  1300  as shown in  FIG. 16 , which includes a light source  1301 , a relay lens assembly  1307 , a projection lens  1302 , and the BSC device  1303  described above. In operation, light emitted from the light source  1301  sequentially passes through the relay lens assembly  1307 , the BSC device  1303  and the projection lens  1302 . 
     The electronic device of the invention may be a head-up display. The head-up display includes a light source, a projection lens, a reflective mirror, a spatial light modulator, a displaying element, and the BSC device described above. In operation, light emitted from the light source sequentially passes through the BSC device, the spatial light modulator, the projection lens, the reflective mirror and the displaying element, wherein the displaying element includes an imaging forming surface (like a screen). 
     The electronic device of the invention may be a laser range finder. The laser range finder includes an optical sensor, a light emitter and the BSC device described above. 
     The electronic device of the invention may be a colorimeter. The colorimeter includes an optical sensor, a digital signal processor and the BSC device described above. 
     The electronic device of the invention may be a panel. The panel includes a light source, a projection lens, a spatial light modulator and the BSC device described above. In operation, the light emitted from the light source sequentially passes through the BSC device, the spatial light modulator, and the projection lens. 
       FIG. 17  depicts arrangement of the lenses and the light paths for a projection lens or a relay lens assembly which is included in an electronic device of the invention. As shown in  FIG. 17 , the projection lens  5  includes a stop ST 5 , a first lens L 51 , a second lens L 52 , a third lens L 53 , a fourth lens L 54  and a fifth lens L 55 , which are sequentially arranged along an optical axis OAS from a projection side to an image source side. In projection, light coming from the image source IS 5  is projected towards the projection side. The first lens L 51  is made of plastic, is with positive refractive power, and is able to converge the beam with a large angle. Further, the first lens L 51  has a convex surface S 53  facing the image source side and a convex surface S 52  facing the projection side. The surfaces S 52  and S 53  are both aspheric surfaces. The second lens L 52  is with negative refractive power and is made of glass. The second lens L 52  has a concave surface S 55  facing the image source side and a concave surface S 54  facing the projection side. The surfaces S 54  and S 55  are both spherical surfaces. The third lens L 53  is made of glass, is with positive refractive power, and is provided with the function of correcting spherical aberration and chromatic aberration. Further, the third lens L 53  has a convex surface S 56  facing the image source side and a convex surface S 55  facing the projection side. The surface S 55  is a spherical surface while the surface S 56  is an aspheric surface. The fourth lens L 54  is with positive refractive power and is made of glass. The second lens L 58  has a convex surface S 58  facing the image source side and a convex surface S 57  facing the projection side. The surfaces S 57  and S 58  are both aspheric surfaces. The fifth lens L 55  is made of plastic and is with negative refractive power. The fifth lens L 55  has a concave surface S 510  facing the image source side and a concave surface S 59  facing the projection side. The surfaces S 59  and S 510  are both aspheric surfaces. The second lens L 52  and the third lens L 53  are cemented together without an air gap formed therebetween, to form a cemented lens which is with positive refractive power. The cemented lens functions as a relay lens for the front and rear lens groups so as to reduce the volume of the entire lenses and promote the field of view. By the design of the above lenses L 51 -L 55 , the stop ST 5  and at least one of the following conditions (1)-(11) satisfied, the projection lens  5  is able to have the volume effectively reduced, F-number effectively reduced, the field of view effectively promoted, the optical aberration effectively corrected, and the chromatic aberration effectively corrected. 
       −1.2≤ f   5   /f≤− 0.9  (1)
 
       1.9≤ f   1   /f≤ 3.7  (2)
 
       −1.1≤ f   2   /f≤− 0.6  (3)
 
       0.8≤ f   3   /f≤ 1.2  (4)
 
       0.8≤ f   4   /f≤ 1.1  (5)
 
       0.3≤ f/TTL≤ 0.45  (6)
 
       0.09≤ BFL/TTL≤ 0.22  (7)
 
       0.5≤ IH/f≤ 0.65  (8)
 
       −17≤ f   23   /f   5 ≤30  (9)
 
       −1.15≤ f   4   /f   5 ≤−0.75  (10)
 
       −33≤ f   23   /f≤ 16  (11)
 
     where f is an effective focal length of the projection lens  5 , f 1  is an effective focal length of the first lens L 51 , f 2  is an effective focal length of the second lens L 52 , f 3  is an effective focal length of the third lens L 53 , f 4  is an effective focal length of the fourth lens L 54 , f 5  is an effective focal length of the fifth lens L 55 , f 23  is an effective focal length of combination of the second lens L 52  and the third lens L 53 , TTL is an interval from the stop ST 5  to the image source IS 5  along the optical axis OAS, BFL is an interval from the image source side surface  5511  to the image source IS 5  along the optical axis OAS, and IH is a half image height of the projection lens  5 . 
     The chief ray angle can be effectively reduced and the field of view can be enlarged when the condition (1): −1.2≤f 5 /f≤−0.9 is satisfied. The quality of the formed image can be effectively improved when the condition (2): 1.9≤f 1 /f≤3.7 is satisfied. The field of view can be effectively enlarged when the condition (3): −1.1≤f 2 /f≤−0.6 is satisfied. The chromatic aberration and the field curvature can be effectively corrected when the condition (4): 0.8≤f 3 /f≤1.2 is satisfied. The quality of the formed image can be effectively improved when the condition (5): 0.8≤f 4 /f≤1.1 is satisfied. The total lens length can be effectively reduced when the condition (6): 0.3≤f/TTL≤0.45 is satisfied. The back focal length can be effectively controlled when the condition (7): 0.09≤BFL/TTL≤0.22 is satisfied. The size of the formed image can be effectively controlled when the condition (8): 0.5≤IH/f≤0.65 is satisfied. The chromatic aberration and the field curvature can be effectively corrected when the condition (10): −1.15≤f 4 /f 5 ≤−0.75 is satisfied. 
     Table 1 shows the optical specification of the projection lens  5  in  FIG. 17 . 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Effective Focal Length = 5.2 mm F-number = 1.406 
               
               
                 Total Lens Length = 14.943 mm Field of View = 61.400 degrees 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Refractive 
                 Abbe 
                 Effective 
                   
               
               
                 Surface 
                 Radius of 
                 Thickness 
                 Index 
                 number 
                 Focal Length 
               
               
                 Number 
                 Curvature (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 (mm) 
                 Remark 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 S51 
                 ∞ 
                 0.4 
                   
                   
                   
                 Stop ST5 
               
               
                 S52 
                 18.2146 
                 0.845 
                 1.661 
                 20.412 
                 18.875 
                 First Lens L51 
               
               
                 S53 
                 −41.1835 
                 1.758 
               
               
                 S54 
                 −3.1212 
                 0.400 
                 1.684 
                 26.671 
                 −4.594 
                 Second Lens L52 
               
               
                 S55 
                 8.5649 
                 3.099 
                 1.883 
                 40.765 
                 6.117 
                 Third Lens L53 
               
               
                 S56 
                 −5.9414 
                 0.100 
               
               
                 S57 
                 6.2765 
                 4.202 
                 1.883 
                 40.765 
                 4.567 
                 Fourth Lens L54 
               
               
                 S58 
                 −7.9273 
                 0.151 
               
               
                 S59 
                 −10.4971 
                 1.754 
                 1.661 
                 20.412 
                 −6.014 
                 Fifth Lens L55 
               
               
                 S510 
                 7.0283 
                 2.234 
               
               
                   
               
            
           
         
       
     
     The aspheric surface sag z of each lens in table 1 can be calculated by the following formula: 
         z=ch   2 /{1−[1−( k+ 1) c   2   h   2 ] 1/2   }±Ah   4   +Bh   6   +Ch   8   +Dh   10   +Eh   12   ±Fh   14  
 
     where c is curvature, h is the vertical distance from the lens surface to the optical axis, k is conic constant and A, B, C, D, E and F are aspheric coefficients. 
     The conic constant k and the aspheric coefficients A, B, C, D, E and F of each surface are shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 A 
                 B 
                 C 
                 D 
               
               
                   
                 k 
                 E 
                 F 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 S52 
                 0.00 
                 0.000 
                 −0.00357253 
                 −0.000176302 
                 −0.000114911 
               
               
                   
                   
                 2.68637E−05 
                 −5.19113E−06 
                   
                   
               
               
                 S53 
                 0.00 
                 0.000 
                 −0.005560758 
                 −9.49865E−05 
                 −0.000170705 
               
               
                   
                   
                 3.26231E−05 
                 −4.17519E−06 
                   
                   
               
               
                 S56 
                 0.00 
                 0.000 
                 0.000169508 
                 2.36741E−05 
                 5.1921E−07 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S57 
                 0.00 
                 0.000 
                 −0.000490452 
                 1.17107E−05 
                 −1.52217E−06 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S58 
                 0.00 
                 0.000 
                 0.001376433 
                 −1.18118E−05 
                 −2.45508E−07 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S59 
                 0.00 
                 0.000 
                 −0.000439748 
                 0.000169894 
                 −4.79224E−06 
               
               
                   
                   
                 6.59737E−08 
                 2.68869E−10 
                   
                   
               
               
                 S510 
                 −4.5561 
                 0.000 
                 0.001168273 
                 9.51808E−05 
                 5.09445E−05 
               
               
                   
                   
                 −4.59297E−06 
                 4.00223E−07 
               
               
                   
               
            
           
         
       
     
     Table 3 shows the parameters and condition values for conditions (1)-(11) in accordance with the embodiment of  FIG. 17 . It can be seen from Table 3 that the projection lens  5  of the embodiment of  FIG. 17  satisfies the conditions (1)-(11). 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
             
            
               
                 BFL 
                 2.234 mm 
                 IH 
                 2.780 mm 
                 f 23   
                 −167.027 mm 
               
               
                 f 1 /f 
                 3.630 
                 f 2 /f 
                 −0.884 
                 f 3 /f 
                 1.176 
               
               
                 f 4 /f 
                 0.878 
                 f 5 /f 
                 −1.156 
                 f/TTL 
                 0.348 
               
               
                 BFL/TTL 
                 0.150 
                 f 23 /f 5   
                 27.775 
                 IH/f 
                 0.535 
               
               
                 f 4 /f 5   
                 −0.759 
                 f 23 /f 
                 −32.121 
               
               
                   
               
            
           
         
       
     
     Further, the optical performance of the projection lens  5  of  FIG. 17  can meet the requirements. It can be seen from  FIG. 18A  that the field curvature of the projection lens  5  ranges from ˜20 μm to 16 μm. It can be seen from  FIG. 18B  that the distortion of the projection lens  5  ranges from ˜10% to 0%. It can be seen from  FIG. 18C  that the modulation transfer function of the projection lens  5  ranges from 0.32 to 1.0. It is obvious that the field curvature and the distortion of the projection lens  5  can be corrected effectively, and the resolution of the projection lens  5  can meet the requirements. Therefore, the projection lens  5  is capable of good optical performance. 
     In the embodiment of  FIG. 17 , the fourth lens L 54  and the fifth lens L 55  are spaced without being cemented together. That is, the fourth lens L 54  and the fifth lens L 55  have an air gap therebetween. However, it is understood that the fourth lens L 54  and the fifth lens L 55  may be cemented as a cemented lens. Such arrangement also belongs to the category of the invention. 
     The projection lens of  FIG. 17  can be modified as follows. In a modified embodiment, the projection lens includes a stop, a first lens, a second lens, a third lens, a fourth lens and a fifth lens, which are sequentially arranged along an optical axis from a projection side to an image source side. In projection, light coming from the image source is projected towards the projection side. The first lens is made of plastic and has a convex surface facing the projection side. The fifth lens is made of glass. By the design of the above lenses, the stop and at least one of the above-mentioned conditions (1)-(11) satisfied, the projection lens is able to have the volume effectively reduced, F-number effectively reduced, the field of view effectively promoted, the optical aberration effectively corrected, and the chromatic aberration effectively corrected. 
     Table 4 shows the optical specification of the projection lens of the modified embodiment. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Effective Focal Length = 5.2 mm F-number = 1.734 
               
               
                 Total Lens Length = 12.829 mm Field of View = 61.400 degrees 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Refractive 
                 Abbe 
                 Effective 
                   
               
               
                 Surface 
                 Radius of 
                 Thickness 
                 Index 
                 number 
                 Focal Length 
               
               
                 Number 
                 Curvature (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 (mm) 
                 Remark 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 S41 
                 ∞ 
                 0.4 
                   
                   
                   
                 Stop 
               
               
                 S42 
                 21.4292 
                 0.613 
                 1.661 
                 20.412 
                 10.159 
                 First Lens 
               
               
                 S43 
                 −9.9232 
                 1.708 
               
               
                 S44 
                 −2.9698 
                 0.300 
                 1.714 
                 25.000 
                 −3.427 
                 Second Lens 
               
               
                 S45 
                 12.6770 
                 2.766 
                 1.883 
                 40.765 
                 4.649 
                 Third Lens 
               
               
                 S46 
                 −4.1645 
                 0.100 
               
               
                 S47 
                 4.8827 
                 4.820 
                 1.535 
                 56.115 
                 5.496 
                 Fourth Lens 
               
               
                 S48 
                 −4.9247 
                 0.617 
               
               
                 S49 
                 −4.1024 
                 0.300 
                 1.535 
                 56.115 
                 −4.833 
                 Fifth Lens 
               
               
                 S410 
                 7.3143 
                 1.205 
               
               
                   
               
            
           
         
       
     
     The definition of aspheric surface sag z of each lens in table 4 is the same as that in table 1, and therefore the descriptions thereof are omitted. The conic constant k and the aspheric coefficients A, B, C, D, E and F of each surface are shown in Table 5. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                   
                   
                 A 
                 B 
                 C 
                 D 
               
               
                   
                 k 
                 E 
                 F 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 S42 
                 0.00 
                 0.000 
                 −0.007679523 
                 −0.000750565 
                 −0.00075766 
               
               
                   
                   
                 0.000233471 
                 −6.15785E−05 
                   
                   
               
               
                 S43 
                 0.00 
                 0.000 
                 −0.008182363 
                 −0.001224555 
                 −0.000328041 
               
               
                   
                   
                 9.36368E−05 
                 −3.02009E−05 
                   
                   
               
               
                 S46 
                 0.00 
                 0.000 
                 0.000613733 
                 7.92385E−06 
                 5.78579E−06 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S47 
                 0.00 
                 0.000 
                 −0.001024131 
                 2.09729E−05 
                 −9.97465E−07 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S48 
                 0.00 
                 0.000 
                 0.002831994 
                 −3.8779E−05 
                 5.39114E−06 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S49 
                 0.00 
                 0.000 
                 0.001075986 
                 0.000446506 
                 −2.98748E−05 
               
               
                   
                   
                 1.31161E−06 
                 9.39233E−09 
                   
                   
               
               
                 S410 
                 4.2335 
                 0.000 
                 −0.003262972 
                 −0.000927107 
                 0.000161951 
               
               
                   
                   
                 −1.30166E−05 
                 4.03241E−07 
               
               
                   
               
            
           
         
       
     
     Table 6 shows the parameters and condition values for conditions (1)-(11) in accordance with the modified embodiment. It can be seen from Table 6 that the projection lens of the modified embodiment satisfies the conditions (1)-(11). 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
             
            
               
                 BFL 
                 1.205 mm 
                 IH 
                 2.780 mm 
                 f 23   
                 40.355 mm 
               
               
                 f 1 /f 
                 1.954 
                 f 2 /f 
                 −0.659 
                 f 3 /f 
                 0.894 
               
               
                 f 4 /f 
                 1.057 
                 f 5 /f 
                 −0.929 
                 f/TTL 
                 0.405 
               
               
                 BFL/TTL 
                 0.094 
                 f 23 /f 5   
                 −8.350 
                 IH/f 
                 0.535 
               
               
                 f 4 /f 5   
                 −1.137 
                 f 23 /f 
                 7.761 
               
               
                   
               
            
           
         
       
     
     Also, the optical performance of the projection lens of the modified embodiment can meet the requirements. That is, the field curvature and the distortion of the projection lens can be corrected effectively, and the resolution of the projection lens can meet the requirements. Therefore, the projection lens of the modified embodiment is capable of good optical performance. 
     Table 7 shows the optical specification of the projection lens of another modified embodiment. 
     
       
         
           
               
             
               
                 TABLE 7 
               
             
            
               
                   
               
               
                 Effective Focal Length = 5.3 mm F-number = 1.432 
               
               
                 Total Lens Length = 14.152 mm Field of View = 59.260 degrees 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                 Refractive 
                 Abbe 
                 Effective 
                   
               
               
                 Surface 
                 Radius of 
                 Thickness 
                 Index 
                 number 
                 Focal Length 
               
               
                 Number 
                 Curvature (mm) 
                 (mm) 
                 Nd 
                 Vd 
                 (mm) 
                 Remark 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 S21 
                 ∞ 
                 0.4 
                   
                   
                   
                 Stop 
               
               
                 S22 
                 −285.3517 
                 1.163 
                 1.799 
                 44.125 
                 15.989 
                 First Lens 
               
               
                 S23 
                 −12.2128 
                 1.737 
               
               
                 S24 
                 −3.2799 
                 0.400 
                 1.667 
                 27.781 
                 −5.732 
                 Second Lens 
               
               
                 S25 
                 7.1063 
                 3.457 
                 1.883 
                 40.765 
                 5.815 
                 Third Lens 
               
               
                 S26 
                 −5.9410 
                 0.100 
               
               
                 S27 
                 5.8686 
                 3.401 
                 1.883 
                 40.765 
                 4.569 
                 Fourth Lens 
               
               
                 S28 
                 −9.6686 
                 0.100 
               
               
                 S29 
                 −23.7112 
                 0.300 
                 1.661 
                 20.412 
                 −5.464 
                 Fifth Lens 
               
               
                 S210 
                 4.3744 
                 3.094 
               
               
                   
               
            
           
         
       
     
     The definition of aspheric surface sag z of each lens in table 7 is the same as that in table 1, and therefore the descriptions thereof are omitted. The conic constant k and the aspheric coefficients A, B, C, D, E and F of each surface are shown in Table 8. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                   
                   
                 A 
                 B 
                 C 
                 D 
               
               
                   
                 k 
                 E 
                 F 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 S22 
                 0.00 
                 0.000 
                 −0.003607331 
                 −0.000349028 
                 5.71539E−05 
               
               
                   
                   
                 −1.80322E−05 
                 6.17019E−07 
                   
                   
               
               
                 S23 
                 0.00 
                 0.000 
                 −0.004566779 
                 −0.000175655 
                 −2.16784E−05 
               
               
                   
                   
                 2.5321E−06 
                 −6.57088E−07 
                   
                   
               
               
                 S26 
                 0.00 
                 0.000 
                 0.000895517 
                 −1.72671E−05 
                 1.61556E−06 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S27 
                 0.00 
                 0.000 
                 −0.000120795 
                 −1.98143E−05 
                 −1.52334E−06 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S28 
                 0.00 
                 0.000 
                 0.001683345 
                 −4.93966E−05 
                 2.91484E−07 
               
               
                   
                   
                 0 
                 0 
                   
                   
               
               
                 S29 
                 0.00 
                 0.000 
                 −0.001240842 
                 0.000397266 
                 −2.44664E−05 
               
               
                   
                   
                 6.76513E−07 
                 −6.78939E−09 
                   
                   
               
               
                 S210 
                 −0.197 
                 0.000 
                 −0.001738521 
                 0.000573864 
                 −2.52463E−05 
               
               
                   
                   
                 4.20356E−06 
                 −2.90402E−07 
               
               
                   
               
            
           
         
       
     
     Table 9 shows the parameters and condition values for conditions (1)-(11) in accordance with this modified embodiment. It can be seen from Table 9 that the projection lens of this modified embodiment satisfies the conditions (1)-(11). 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
             
            
               
                 BFL 
                 3.094 mm 
                 IH 
                 2.780 mm 
                 f 23   
                 32.502 mm 
               
               
                 f 1 /f 
                 3.017 
                 f 2 /f 
                 −1.082 
                 f 3 /f 
                 1.097 
               
               
                 f 4 /f 
                 0.862 
                 f 5 /f 
                 −1.031 
                 f/TTL 
                 0.374 
               
               
                 BFL/TTL 
                 0.219 
                 f 23 /f 5   
                 −5.948 
                 IH/f 
                 0.525 
               
               
                 f 4 /f 5   
                 −0.836 
                 f 23 /f 
                 6.132 
               
               
                   
               
            
           
         
       
     
     Also, the optical performance of the projection lens of this modified embodiment can meet the requirements. That is, the field curvature and the distortion of the projection lens can be corrected effectively, and the resolution of the projection lens can meet the requirements. Therefore, the projection lens of the modified embodiment is capable of good optical performance. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.