Abstract:
An optical module is composed of three light-guide units and a holder for the units. Each of the light-guide units is made of a mirror plane used for reflecting and transmitting light installed between two prisms via the use of circularly applied glue. The holder includes three installation areas one installation area for each light-guide unit. Additionally, the holder has two frames that are arranged perpendicularly to each other and each frame is installed perpendicularly to the joining sides of two installation areas i.e. one frame is installed perpendicular to the side where installation area  1  and installation area  2  meet and the other frame is installed perpendicular to the side where installation area  2  and installation area  3  meet.

Description:
BACKGROUND OF INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical module and its assembling method, and more specifically, to an optical module and its assembling method of a projecting apparatus. 
     2. Description of the Prior Art 
     A projecting apparatus for generating an image is disclosed in U.S. Pat. No. 6,089,719 “Projecting Apparatus For Displaying Electrical Images”. Please refer to FIG. 1 showing the projecting apparatus  10  according to U.S. Pat. No. 6,089,719. To display an image on a screen  34 , a projecting apparatus  10  comprised of a light source device  22 , three modulating units  24 ,  26 ,  28 , a dichroic-polarization beam splitter prism  30  composed of four triangular prisms  36  of equal proportion, and a projecting lens  32  is used. The light source device  22  is used to generate three different-colored rays red, green, and blue with uniform illumination but different polarities. Each of the modulating units  24 ,  26 ,  28  modulates and changes the polarity of one of the single-colored polarized raysby means of reflection. The beam splitter prism  30  is used to first receive the polarized rays of red, green and blue, then guide the rays to their respective modulating units  24 ,  26 ,  28  for modulation, and finally converge the three modulated rays into an output beam all of which are accomplished via mirror planes, which transmit or reflect light based its polarity, plated onto the triangular prisms  36  of the beam splitter prism  30 . The projecting lens  32  is installed in front of the output face of the beam splitter prism  30  for projecting the output beam to a screen  34 . 
     Other projecting apparatus arealso disclosed in U.S. Pat. No. 6,247,814 “Projecting Apparatus For Displaying Electronic Images” and U.S. Pat. No. 6,364,488 “Projection Display Device For Displaying Electrically Encoded Images” thatincorporate the use of an L-shaped optical module to create an optical path of approximately the same length for the three monochrome rays red, green, and blue in order to reduce the optical design of the projecting apparatus. Please refer to FIG. 2 showing the projecting apparatus  40  according to U.S. Pat. No. 6,247,814. The projecting apparatus  40  includes a light source  42 , three modulating units  44 ,  46 ,  48 , an L-shaped optical module  50 , an input lens set  52  and a projecting lens  54 . 
     The light source  42  is for generating monochrome rays in red, green and blue in the same polarity. The three modulating units  44 ,  46 ,  48  are for modulating a single-colored polarized ray and changing its polarity by manner of reflection. The L-shaped optical module  50  is for controlling the path of each single-colored polarized ray. The input lens set  52  is installed between the light source  42  and the inner side of the L-shaped optical module  50 . The projecting lens  54  is for projecting the beam output from the L-shaped optical module  50  to a screen  56 . 
     To elaborate further upon the L-shaped optical  50 , its makeup consists of three rectangular, transparent light-guide units, which are named respectively as a first, second, and third light-guide unit  60 ,  62 ,  64 . Each light-guide unit is composed of a mirror sandwiched between the diagonals of two triangular prisms  66 . The first and third light-guide units  60 , 64  have a polarization light beam splitter mirror  70 , 74  respectively while the second light-guide unit has a dichroic mirror  72 . 
     The arrangement of the light-guide units has the second light-guide unit  62  restingat the apexof the first and the third light-guide units  60 ,  64 . This arrangement ideally causes the first and the third polarization beam splitter mirror  70 ,  74  to be aligned along the same plane, and the second dichroic mirror  72  to be perpendicular to both the first and the third polarization beam splitter mirrors  70 ,  74 . Light is input through the right angle that is located on the inside of the L-shaped optical module  50  and formed by of perpendicular sides  61 ,  65  of the first and the third light-guide units  60 ,  64 . 
     Please refer to FIG. 3 now to follow how a projecting apparatus  40  operates. Generally, image signals wish to be displayedare input into the projecting apparatus  40  where images corresponding to the input signals are generated. For instance, the signal from the output port of a computers video card can be connected to the projecting apparatus  40  in order to display the operational mode of the computer. The three modulating units  44 ,  46 ,  48  of the projecting apparatus  40  each modulate their respective monochromatic beam according to received image signals. Then an image from each monochromatic beam (a red image  12 , a green image  14  and a blue image  16 ) is outputted and brought together to create one image for users to see. 
     Continuing with the example of displaying a computer operational mode, the three images (red image  12 , the green image  14  and the blue image  16 ) have equal resolutions (e.g. 800*600 or 1024*768) composed from a plurality of pixels  18 . This means that pixels  18  from each of the three images with the same coordinates all correspond to one another. Under ideal conditions, the angles at which the red image  12 , the green image  14  and the blue image  16  are projected onto the screen  56  are less than the maximum tolerance level, resulting in the overlap of pixels from the three images with the same coordinates at the same position. 
     For instance, if the projection angles of the red image  12 , the green image  14  and the blue image  16  on the screen  56  are each less than the maximum tolerance level, a pixel  20 R on the upper left corner of the red image  12 , a pixel  20 G on the upper left corner of the green image  14 , and a pixel  20 B on the upper left corner of the red image  16  will overlap one another and form a single pixel for users to see. However, if any of the projection angles of the red image  12 , the green image  14  and the blue image  16  onto the screen  56  is larger than the maximum tolerance level, the pixels of the images with larger than maximum tolerance levels will not be in-line thereby decreasing projection quality. Therefore, it is imperative that the projection angles of the red image  12 , the green image  14  and the blue image  16  on the screen  56  each be made less than the maximum tolerance level. 
     The part that has the biggest effect on whether the projection angles fall within tolerance levels is the L-shaped optical  50  more specifically the three light-guide units  60 ,  62 ,  64 . Reason being if the three light-guide units  60 ,  62 ,  64  are not in proper position with respect to one another, the light-guide units  60 ,  62 ,  64  will project their respective single-colored polarized rays at different angles resulting in image quality degradation. Therefore, the design of a conventional L-shaped optical module  50  usually incorporates a holder  80  (FIG. 4) to align and maintain the positions of the three light-guide units  60 ,  62 ,  64  so as to guarantee the paths of the single-colored polarized rays. 
     However, the current-conventional method for assembling the L-shaped optical module  50  is not ideal because it easily leads to misalignment of parts. As can be deduced from the above-given information, any small misalignment can cause any, two, or all of the three beams of red, blue and green to be projected at angles above the maximum tolerance. Pixels therefore do not overlap but lie on different positions on the screen, resultingin lower than expected image quality. 
     Under conventional methods the manufacturing of light-guide units  60 ,  62 ,  64  involves gluing a mirror between two prisms, the mirror of choice either a dichroic mirror  72  or a polarization beam splitter mirror  70 , 74  depending on the type of light-guide unit is being produced. The assembled light-guide units  60 ,  62 ,  64  in FIG. 2 are then attached to the holder  80  as shown in FIG.  4 . Each light-guide unit  60 ,  62 ,  64  is glued toa different set of four points located on the holder  80 . For instance, the second light-guide unit  62  is glued onto the set of points  82 ,  84 ,  86 ,  88  of holder  80 , wherein one prism  66  is glued to points  82 ,  84  and the other prism  66  is glued to points  86 ,  88 . 
     The problem lies in tolerated errors that occur during the manufacturing process. More specifically, when the two prisms  66  are being glued to one of the two mirrors, the bottoms of the three parts are not always perfectly level when glued together. This means that the assembled light-guide  60 ,  62 ,  64  will not lie flush with surface of the holder  80 . The effect of such an error is shown in FIG.  5 . 
     FIG. 5 shows a cross-sectional view of the optical module  50  along the line  5 — 5  in FIG. 4 as viewed from the upper right corner. While FIG. 5 assumes that only one light-guide unit has been assembled with error, one, two, or all three may have the error illustrated by the figure. Because the two prisms  66  of the second light-guide unit  62  were not glued to the dichroic mirror  72  at an even level, the prism  66  glued to points  86 ,  88  rests higher than the prism  66  glued to points  82 ,  84 . As a result,the dichroic mirror  72  lies at a slant causing the paths of the single-colored polarized rays in the L-shaped optical module  50  to deviate from the intended path. For instance, a green polarized ray G* is reflected by a modulating unit  44  and then passes through the first light-guide unit  60  to the second dichroic mirror  72 . Because the dichroic mirror  72  lies at aslant, the polarized ray G* will deviate from the intended path. Therefore, due to the slight error in assembly of the conventional optical module  50 , the projection angles of the red image  12 , the green image  14  and the blue image  16  may possibly be larger than the maximum tolerance level, which causes low image quality in the optical module  50 . 
     As stated before every manufacturing process has a tolerance level for errors, meaning that every part has slight imperfections. One imperfect part such as the example in FIG. 5 may or may not affect the projection angle enough to cause the angle to be above the maximum tolerance level. However, it is more likely that more than one part is imperfect. The culmination of errors from all parts with imperfections that fall within assembly tolerance increases the likelihood of the projection angles of the monochrome rays to be greater than the maximum tolerance level. In other words, even though the assembly error for every part falls within the manufacturing tolerance for error, there is no guarantee that the angles at which the monochrome rays are projected will fall under the maximum tolerance level. 
     In addition to the manufacturing problem, there is the problem of the effect of temperature. The temperature difference between the off/onstates of the projecting apparatus  40  can be up to a few tens of degrees (e.g. room temperature is 20° C. while the projecting apparatus  40  is up to 50° C. in operation). However, since the prisms  66  and the holder  80  are made of different materials (in this case glass and metal respectively), the two have different expansion coefficients meaning that the prisms  66  will contract or expand at a rate different from the rate the holder  80  contracts or expands when the projecting apparatus  40  is switched off or on. This effect leads to two things one being the two prisms  66  will push each other away and the other being the holder will push each prism individually up. The ultimate effect is the light-guide unit  60 ,  62 ,  64  will become misaligned. 
     Please refer to FIG. 6 diagramming the forces caused by expansion when temperature rises after the projecting apparatus  40  is switched on. As mentioned above, the prisms  66  are made of glass, and the holder  80  is made of metal. When the temperature rises, the holder  80  will exert forces of F 1 , F 2 , F 3 , F 4  on the points  82 ,  84 ,  86 ,  88  to which the prisms  66  are glued because the holders  80  expansion coefficient is larger than that of the prisms  66 . In addition, the two prisms  66  will exert forces F 5 , F 6  pushing each other away. With the holder only applying the forces F 1 , F 2 , F 3 , F 4  to only the points  82 ,  84 ,  86 ,  88  instead of to the whole surface of the prisms  66  coupled with the prisms  66  pushing each other away, the light-guide unit  62  will rotate because it as a whole experiences an unbalanced moment of force. The rotation of light-guide unit  62  causes the path of the single-colored polarized rays to be changed. 
     SUMMARY OF INVENTION 
     It is therefore a primary objective of the present invention to provide an optical module and a method of assembly for a projecting apparatus to solve the problems mentioned above. 
     Briefly summarized, an optical module includes three light-guide units and a holder. Each of the light-guide units is composed of a mirror plane used to reflect and transmit light sandwiched between two prisms. The holder is composed of three installation areas, one for each light-guide unit, and two frames, each located between two of the installation areas. That is to say, the first frame is located between the first and second installation area while the second frame is located between the second installation area and the third installation area. 
     One of the features of the present invention is that, a first plane of the first light-guide unit is attached to and glued to a first side of the first frame, a second plane of the second light-guide unit is attached to and glued to a second side of the first frame, and a third plane of the third light-guide unit is attached to and glued to a third side of the second frame. 
     These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 illustrates a conventional projecting apparatus. 
     FIG. 2 illustrates another conventional projecting apparatus. 
     FIG. 3 illustrates how the projecting apparatus in FIG. 1 operates. 
     FIG. 4 illustrates how to install the three light-guide units in FIG. 1 onto the holder. 
     FIG. 5 illustrates a cross section of the optical module along line  5 - 5 ″ in FIG.  4 . 
     FIG. 6 illustrates the forces exerted on the two prisms after the projecting apparatus is switched on. 
     FIG. 7 illustrates an optical module installed on a projecting apparatus according to the present invention. 
     FIG. 8 illustrates adiagram of the first light-guide device, the second light-guide device, and the third light-guide device in FIG. 7 each separated into two prisms. 
     FIG. 9 illustrates a holder of the optical module in FIG.  7 . 
     FIG. 10 illustrates an assembled optical module in FIG.  7 . 
     FIG. 11 illustrates a cross section of the optical module along line  11 - 11 ″ in FIG.  10 . 
     FIG. 12 illustrates a cross section of the optical module along line  12 - 12 ″ in FIG.  12 . 
     FIG. 13 illustrates how the forces are exerted on the two prisms of the first light-guide unit after the projecting apparatus is switched on. 
     FIG. 14 illustrates another holder according to the present invention. 
     FIG. 15 illustrates the dichroic-polarization beam splitter prism in FIG. 1 installed onto the holder in FIG.  14 . 
    
    
     DETAILED DESCRIPTION 
     Please refer to FIG. 7 showing an optical module  100  installed on a projecting apparatus  90  according to the present invention. The projecting apparatus  90  includes a light source  92 , three modulating units  94 ,  96 ,  98 , an optical module  100 , an input lens set  102  and a projecting lens  104 . The light source  92  is for generating polarized rays in red, green and blue in the same polarity. The three modulating units  94 ,  96 ,  98  are for modulating a single-colored polarized ray and changing its polarity by manner of reflection. The optical module  100  is for controlling the path of each single-colored polarized ray, which includes three rectangular transparent light-guide devices referred to as a first light-guide device  110 , a secondlight-guide device  112 , and a thirdlight-guide device  114 , wherein the second light-guide device  112  is positioned at the apex of the first light-guide device  110  and the thirdlight-guide device  114 . The input lens set  112  is installed between the light source  92  and the inner side of the optical module  100 . The projecting lens  104  is for projecting the beam output from the optical module  100  to a screen  106 . 
     Please refer to FIG. 8 showing a diagram of the three light-guide devices  110 ,  112 ,  114  in FIG. 7 each one separated into two prisms. Each light-guide device is composed of a mirror plane, used to reflect and transmit light, installed between two prisms by gluing the mirror to the prisms. For light-guide device  110 , the prisms are prisms  121 ,  122  and the mirror is mirror plane  130 . For light-guide device  112 , the prisms are prisms  123 ,  124  and the mirror is mirror plane  132 . For light-guide device  114 , the prisms are prisms  121 ,  122  and the mirror is mirror plane  134 . 
     In this embodiment, mirror planes  130 ,  134  of the optical module  100  are polarization beam splitter mirrors while mirror plane  132  is a dichroic mirror. The mirrors are arranged in such a manner that the mirror planes  130 ,  134  lie in the same plane and themirror plane  132  lies perpendicular to both of them. This arrangement provides the optical module  100  with the same optical characteristic as that of the optical module  50  disclosed in U.S. Pat. No. 6,247,814. 
     Alternatively, one could use polarization beam splitter mirrors for all three mirror planes  130 ,  132 ,  134  in the optical module  100 . This arrangement provides the optical module  100  with the same optical characteristic as that of the L-shaped optical module  50  disclosed in U.S. Pat. No. 6,364,488. Under this arrangement though, the projecting lens  104  of the projecting apparatus  90  must be moved from the topside of the second light-guide device  112  to the left side of the second light-guide device  112 . 
     Please refer to FIG. 9 showing a holder  150  of the optical module  100  in FIG.  7 . The holder  150  is made of metal and has three installation areas first installation area  152 , second installation area  154 , third installation area  156  for installing their respective light-guides first light-guide device  110 , second light-guide device  112 , third light-guide device  114 . The holder  150  also has two frames first frame  160  and a second frame  162 . The first frame  160  is installed perpendicularly between the first installation area  152  and the second installation area  154 , and the second frame  162  is installed perpendicularly between the second installation area  154  and the third installation area  156  and perpendicular to the first frame  160 . Moreover, the first frame  160  and second frame  162  respectively form two light paths  170 ,  172  for light from the three light-guide devices  110 ,  112 ,  114  to travel along. 
     Please refer to FIG.  8 -FIG.  10 . FIG. 10 shows an assembled optical module  100  in FIG.  7 . As shown in FIG. 9 to assemble the optical module  100 , first apply some glue  180  on the two sides  164 ,  166  of the first frame  160  and one side  168  of thesecond frame. Then circularly apply some glue  180  in each of the installation areas  152 ,  154 ,  156 . Finally attach each of the light-guides first light-guide device  110 , second light-guide device  112 , third-light-guide device  114  to its respective installation areas first installation area  152 , second installation area  154 , third installation area  156 . The orientation of the light-guide devices  110 ,  112 ,  114  should be the same as that shown in FIG. 7 in which the first mirror plane  130  and the third mirror plane  134  lie in the same plane, and the second mirror plane  132  lies perpendicular to them both. 
     All three light-guide units  110 ,  112 ,  114  are all attached to the holder  150  in the same manner. The light-guide unit has only the side of one of its prisms attached to one side of one of the frames and both of its prisms attached to the installation area via the previously applied glue  180 . In this embodiment, first light-guide unit  110  has the side  140  of prism  121  attached to side  164  of the first frame  160  and both of its prisms  121 ,  122  attached to installation area  152 . Second light-guide unit  112  has the side  142  of prism  123  attached to side  166  of the first frame  160  and both of its prisms  123 ,  124  attached to installation area  152 . Finally, the third light-guide unit  114  has the side  144  of prism  125  attached to side  168  of the second frame  162  and both of its prisms  125 ,  126  attached to installation area  152 . 
     In order to describe the advantages of the present invention, please refer to FIG.  7  and FIG.  10 -FIG.  12 . FIG. 11 shows a cross section of the optical module  100  along the line  11 - 11 ″ in FIG. 10 while FIG. 12 shows a cross section of the optical module  100  along the line  12 - 12 ″ in FIG.  10 . 
     Please refer to FIG. 11 for the first description and FIG. 12 for the second. As described above, there are three light-guide units  110 ,  112 ,  114  attached to the holder  150 . One case involves light-guide units  110 ,  112  where the side  140  of prism  121  from light-guide unit  110  is attached to side  164  of the first frame  160  and the side  142  of prism  123  from light-guide unit  112  is attached to side  166  of the first frame  160 . Because the two sides of  164 ,  166  of the first frame  160  are inherently parallel to each other,the first mirror plane  130  is perpendicular to the second mirror plane  132  (as shown in FIG. 7) instead of being oblique as the mirror plane  72  is to mirror plane  70  (as shown in FIG.  5 ). 
     The other case involves light-guides  112 ,  114 , where light-guide  112  is attached in the manner described above and the side  144  of prism  125  from light-guide unit  114  is attached to side  168  of the second frame  162 . Because the first frame  160  is perpendicular to the second frame  162 , the same result is achieved. Namely, the second mirror plane  132  is perpendicular to the third mirror plane  134  (as shown in FIG. 7) instead of being oblique as the mirror plane  72  is to mirror plane  70  (as shown in FIG. 5) 
     Therefore, when the three light-guide devices  110 ,  112 ,  114  are glued onto the holder  150  in the manner described above, the three mirror planes  130 ,  132 ,  134  will be held fixed at the correct position to the holder  150 . Each single-colored polarized ray can then pass along the intended path without deviation. No deviation means that the quality of projection does not suffer. 
     In addition, compared with the optical module  50 , the path of single-colored polarized rays in the optical module  100  is less influenced by temperature. Please refer to FIG. 13 showing how forces are exerted on the two prisms  121 ,  122  of the first light-guide unit  110  after the projecting apparatus  90  is switched on. Since the holder  150  and the prisms  121 ,  122  are respectively made of metal and glass, the expansion coefficient of the holder  150  is larger than that of the prisms  121 ,  122 . The result is that when the projecting apparatus  100  is switched on and the temperature rises, the expansion per unit of length of the holder is larger than that of either of the prisms  121 ,  122 . 
     In the design of this invention, the direction of force exerted by the holder  150  via the glue  180  onto the prism 121  is from the center of the prism  121  towards the three sides of the prism  121 . Because the glue is applied circularly instead of just at four points as shown in FIG. 6 of the Prior Art, the vector sum of the exerted forceson prism  121  is zero. The result is that even though the prism  121  is under condition of force and moment balance, the position of the first mirror plane  130  on the holder  150  will not deviate due to temperature. All the other prisms  122 - 126  experience the same result as prism  121  meaning that; the position of the second mirror plane  132  and the third mirror plane  134  will not deviate.As a result,the paths of each single-colored polarized ray will not be changed due to temperature. Moreover, the glue  180  for used on the prisms  121 - 126  is flexible so that even if the holder  150  and the six prisms  121 - 126  pull or push each other due to the surrounding temperature, the prisms  121 - 126  will not be damaged. 
     The holder and the assembling method according to the present invention not only can be applied in the L-shaped optical module  50 , but also can be used for fixing the conventional dichroic-polarization beam splitter prism  30  in FIG.  1 . Please refer to FIG. 14 showing another holder  200  according to the present invention, and FIG. 15 showing the dichroic-polarization beam splitter prism  30  in FIG. 1 installed on the holder  200  in FIG.  14 . The holder  200  includes two frames  202 ,  204  integrated into each other and an installation area  206  wherein the first frame  202  and the second frame  204  respectivelyform a first light path  208  and a second light path  210  for light to pass along. 
     When installing the dichroic-polarization beam splitter prism  30  on the holder  200 , as shown in FIG. 15, apply glue along the inner sides of the first frame  202 , second frame  204 , and the installation area  206 . The attach the dichroic-polarization beam splitter  30  to the two frames  202 ,  204  and the installation area  206  with the two inners sides of the dichroic-polarization beam splitter  30  facing the two frames  202 ,  204 . In this manner, the dichroic-polarization beam splitter prism  30  can be fixed to the holder  200 . 
     In contrast to the prior art, the present invention calls for the holder of the optical module to have two frames installed perpendicularly on the top. Furthermore, the side of one prism from each of the light-guide units of the optical module is attached to the side of one of the frames. In addition, when the glue used to attach the prisms to the holder is to be applied in a circular fashion so as to reach the condition of force and moment balance without deviation more easily. The sum of these changes result is that when the optical module is assembled, the influence on the paths of single-colored rays due to manufacturing tolerance of the light-guide devices can be reduced to the minimum. 
     Those skilled in the art will readily observe that numerous modifications and alterations of the device may be made while retaining the teachings of the invention. Accordingly, the above disclosure should be construed as limited only by the metes and bounds of the appended claims.