Abstract:
A projector with reduced size and higher contrast includes a prism assembly, a light system, and a Digital Micro-mirror Device (DMD). Only In the “ON” state of DMD, the light from the light system reflects to a projection screen through the prism assembly and the DMD. The prism assembly includes two prisms and a medium layer. The prism assembly is appropriately designed so as to disable the light in the “OFF” state from reflecting to the projection screen by two-time total internal reflection in the prism assembly, and also to reduce the size of the projector.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a projector; more specifically, relates to an innovatively designed prism assembly for a projector for reducing size of the projector and providing higher contrast. 
     2. Description of the Prior Art 
     Please refer to  FIG. 1 .  FIG. 1  is the schematic view of a prior art projector  100  with smaller size. As shown in  FIG. 1 , projector  100  has a light system  110 , a lens assembly  120 , a digital micro-mirror device (DMD)  130  and a prism assembly  140 . 
     Light system  110  generates lights, which emit to prism assembly  140 ; after that, the lights are reflected to DMD  130 , which will again reflect the lights. DMD  130  comprises a dust-proof cover and a plurality of micro mirrors M. Micro mirrors M of DMD  130  are used to reflect the lights from the light system  110  reflected through prism assembly. Each micro mirror M rotates along a rotating axis to the ON state S ON  (the solid lines of DMD  130  in  FIG. 1 ) or to the OFF state S OFF  (the broken lines of DMD  130  in  FIG. 1 ), respectively, according to a control signal. More specifically, each micro mirror M is in the FLAT state S FLAT  before receiving the control signal, and is paralleled to the dust-proof cover of DMD  130 . As receiving the control signal for enabling, micro mirrors M rotate clockwise to an angle θ S ; as receiving the control signal for disabling, micro mirrors M rotate counterclockwise to an angle θ S . Therefore, the included angle of micro mirrors M between the ON state S ON  and the OFF state S OFF  is 2θ S . In the ON state S ON , micro mirrors M will reflect the incident light through prism assembly  140 , then into lens assembly  120  so as to project the light onto the projection screen. In the OFF state S OFF , micro mirrors M will rotate to an included angle 2θ S  to reflect the incident light through the prism assembly  140  so that after the light passes through prism assembly  140 , it will carry on in the direction away from the optical axis A 2  of the lens assembly  120  instead of entering into lens assembly  120 . 
     Prism assembly  140  comprises two prisms T A  and T B , and a medium layer X. Prisms T A  and T B  are usually glass pillars; prism T A  comprises three planes P 1 , P 2 , and P 3 ; prism T B  comprises three planes P 4 , P 5 , and P 6 . Medium layer X is usually air layer locating between the plane P 2  of prism T A  and the plane P 4  of prism T B . Prisms T A  and T B  have a refractive index N 1 , medium layer X has a refractive index N 2 ; N 2  is smaller N 1 , which means compare to prisms T A  and T B , medium layer X is a less dense medium. When the light emits into plane P 2  of prism T A  from the light system  110  and the incident angle is smaller than the total reflection angle of prism T A , the total reflection will occur on plane P 2 . Additionally, plane P 3  is paralleled with DMD  130 , plane P 5  is paralleled with lens assembly  120  (i.e. P 5  is perpendicular to the optical axis A 2  of lens assembly  120 ). The included angles between plane P 1  and plane P 2  and between plane P 2  and plane P 3  respectively are β and α. The included angle between planes P 6  and P 5  is γ, which is an acute or right angle, i.e. the angle γ is smaller or equal to a right angle. 
     Light system  110  is usually a gas discharge lamp using elliptic lampshade to gather lights, which emit along an optical axis A 1 . In other words, light system  110  is a light source with focal length f/# (f−number), in which the optical axis A 1  is about perpendicular to plane P 1 . 
     Please still refer to  FIG. 1 . The lights from the light system  110  move along optical axis A 1  and pass through plane P 1 ; after emitting to prism T A , the lights are totally reflected from plane P 2  to the plane of DMD  130  (i.e. the dust-proof cover of DMD  130 ) though plane P 3 , and an included angle between the normal to the plane of DMD  130  and the incident light is θ AOI . Next, micro mirrors M will again reflect the incident lights. When in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 1 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  to lens assembly  120 . When in the OFF state S OFF , the lights reflected by micro mirrors M (the broken lines in  FIG. 1  ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  in the direction away from the optical axis A 2  of lens assembly  120  instead of entering into lens assembly  120 . 
     Please refer to  FIG. 2 .  FIG. 2  is the schematic view of prior art projector  100  with lower contrast when in the OFF state. The lights from the edge of light system  110  pass through plane P 1 ; after emitting to prism T A , the lights are totally reflected from plane P 2  through plane P 3 , to DMD  130 . Since the light system  110  has focal length F, the direction of lights from the edge of light system  110  is different than that from the center. As in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 2 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  to lens assembly  120 . As in the OFF state S OFF , the lights reflected by micro mirrors M (the broken lines in  FIG. 2 ) will pass through plane P 3  and be refracted between planes P 2  and P 3 , then emit out to plane P 6 . After being totally reflected from plane P 6 , the lights will again emit out from plane P 5  to lens assembly  120 , as shown in  FIG. 2 . Thus the contrast of projector  100  will be reduced. 
     Please refer to  FIG. 3 .  FIG. 3  is the schematic view of prior art projector  200  with high contrast. In  FIG. 3 , except prism assembly  240 , the remaining elements are identical to those of projector  100 ; the description related to such functions thus will not be stated herein. 
     Similarly, prism assembly  240  also comprises two prisms T A  and T B  and a medium layer X. The lights from light system  110  move along the optical axis A 1  and pass through plane P 1 ; after emitting to prism T A , the lights are totally reflected from plane P 2  through plane P 3  to DMD  130 , and an included angle between the normal to the plane of DMD  130  and the incident light is θ AOI . Next, the micro mirrors M will again reflect the incident lights. As in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 3 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  to lens assembly  120 . As in the OFF state S OFF , the lights reflected by micro mirrors M (the broken lines in  FIG. 3 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  in the direction away from the optical axis A 2  of lens assembly  120  instead of entering into lens assembly  120 . 
     Please refer to  FIG. 4 .  FIG. 4  is the schematic view of the prior art projector  200  with increased contrast when in the OFF state. Lights from the edge of the light system  110  pass through plane P 1 ; after being emitted to prism T A , the lights are totally reflected from plane P 2  through plane P 3  to DMD  130 . Since the light system  110  has focal length F, the direction of lights from the edge of light system  110  is different than that from the center. As in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 4 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 3  to lens assembly  120 . As in the OFF state S OFF , the lights reflected by micro mirrors (the broken lines in  FIG. 4 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  instead of entering into lens assembly  120 , as shown in  FIG. 4 . Thus the contrast of projector  200  may be increased. Nonetheless, compare to prism assembly  140 , prism assembly  240  has greater size hence the size of projector  200  becomes larger, that made it inconvenient for users. 
     Therefore in the OFF state, prior art projector  100  is not able to keep all lights away from lens assembly  120  (i.e. there are still stray lights entering into lens assembly  120 ), which would result in low contrast or even light leakage in projector  100 . It is necessary to improve an image quality as bad as it is. In the prior art projector  200 , however, size of prism T B  in prism assembly  240  is increased to enhance contrast; thus the size of projector  200  is increased and that has made it inconvenient for users. 
     SUMMARY OF THE INVENTION 
     The present invention provides a projector with reduced size and higher contrast. The projector comprises a light system with a first optical axis for emitting a first light, a digital micro-mirror device (DMD), a prism assembly, and a lens assembly. The DMD comprises a plurality of micro mirrors rotatable to a first angle or a second angle. The prism assembly comprises a medium layer having a reference refractive index, a first prism, and a second prism. The first prism locates at a first side of the medium and has a prism refractive index which is larger than the reference refractive index. The first prism is used to totally reflect the first light to form a second light to the DMD. The second prism locates at a second side of the medium layer and has the prism refractive index. The lens assembly is with a second optical axis. As the micro mirrors of the DMD rotate to the first angle, the micro mirrors of the DMD reflect the second light into the lens assembly through the first prism, the medium layer, and the second prism. As the micro mirrors of the DMD rotate to the second angle, the second light is reflected from the micro mirrors of the DMD to the prism assembly and emits out of the prism assembly in a direction away from the second optical axis after two-time internally total reflection in the prism assembly. 
     The present invention further provides a projector with reduced size and higher contrast. The projector comprises a light system with a first optical axis for emitting a first light, a DMD, a prism assembly, and a lens assembly. The DMD comprises a plurality of micro mirrors rotatable to a first angle or a second angle. The prism assembly comprises a medium layer having a reference refractive index, a first prism, and a second prism. The first prism locates at a first side of the medium layer and has a prism refractive index which is greater than the reference refractive index. The first prism comprises a first plane passed through by the first light, a second plane coupled to the first plane of the first prism and locating at the first side of the medium layer for totally reflecting the first light to form a second light, and a third plane coupled to the first plane of the first prism and the second plane of the first prism, and being paralleled with the DMD. The second prism locates at a second side of the medium layer and has the prism refractive index. The second prism comprises a fourth plane locating at the second side of the medium layer, a fifth plane coupled to the fourth plane of the second prism, and a sixth plane coupled to the fourth plane of the second prism and the fifth plane of the second prism. An obtuse angle is included between the fifth plane of the second prism and the sixth plane of the second prism. The lens assembly is opposite to the fifth plane of the second prism and is with a second optical axis. As the micro mirrors of the DMD rotate to the first angle, the micro mirrors of the DMD reflect the second light into the lens assembly through the first prism, the medium layer, and the second prism. As the micro mirrors of the DMD rotate to the second angle, the micro mirrors reflect the second light to the sixth plane of the second prism, and the second light is totally reflected for the first time from the sixth plane of the second prism to the fifth plane of the second prism, and after being totally reflected for the second time from the fifth plane of the second prism, the second light emits out of the prism assembly in a direction away from the second optical axis. 
     The present invention further provides a projector with reduced size and higher contrast. The projector comprises a light system with a first optical axis for emitting a first light, a DMD, a prism assembly, and a lens assembly. The DMD comprises a plurality of micro mirrors rotatable to a first angle or a second angle. The prism assembly comprises a medium layer having a reference refractive index, a first prism, and a second prism. The first prism locates at a first side of the medium layer and has a prism refractive index which is greater than the reference refractive index. The first prism comprises a first plane passed through by the first light, a second plane coupled to the first plane of the first prism and locating at the first side of the medium layer for reflecting the first light to form a second light, a third plane coupled to the first plane of the first prism and being paralleled with the DMD; and a cross section coupled to the second plane of the first prism and the third plane of the first prism. The second prism locates at a second side of the medium layer and has the prism refractive index. The second prism comprises a fourth plane locating at the second side of said medium layer, a fifth plane coupled to the fourth plane of the second prism, and a sixth plane coupled to the fourth plane of the second prism and the fifth plane of the second prism. The lens assembly is opposite to the fifth plane of the second prism and is with a second optical axis. As micro mirrors of the DMD rotate to the first angle, the micro mirrors of the DMD reflect the second light into the lens assembly through the first prism, the medium layer, and the second prism. As the micro mirrors of the DMD rotate to the second angle, the second light is reflected from the micro mirrors of the DMD to the cross section of the first prism, is totally reflected for the first time from the cross section to the second plane of the first prism whereon the second light is totally reflected for the second time, and emits out of the prism assembly in a direction away from the second optical axis. 
     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 THE DRAWINGS 
         FIG. 1  is the schematic view of a prior art projector with smaller size. 
         FIG. 2  is the schematic view of the smaller-sized prior art projector with reduced contrast when in the OFF state. 
         FIG. 3  is the schematic view of a prior art projector with higher contrast. 
         FIG. 4  is the schematic view of the prior art projector with increased contrast when in the OFF state. 
         FIG. 5  is the schematic view of a projector of a first embodiment in the present invention. 
         FIG. 6  is the schematic view illustrating enhancing contrast and further reducing light leakage using the prism assembly of the first embodiment in the present invention. 
         FIG. 7  is the schematic view illustrating size comparison between the prism assembly of the first embodiment in the present invention and the prism assembly of the prior art projector with higher contrast. 
         FIG. 8  is the schematic view of a projector of a second embodiment in the present invention. 
         FIG. 9  is the schematic view illustrating enhancing contrast and further reducing light leakage using the prism assembly of the second embodiment in the present invention. 
         FIG. 10  is the schematic view illustrating size comparison between the prism assembly of the second embodiment in the present invention and the prism assembly of the prior art projector with higher contrast. 
         FIG. 11  is the schematic view of the actual value of the defined included angles in the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Therefore, the present invention provides an improved prism assembly for both better size and contrast of the projector. 
     Please refer to  FIG. 5 .  FIG. 5  is the schematic view of projector  300  of a first embodiment in the present invention. In  FIG. 5 , except prism assembly  340 , the remaining elements are identical to those of projector  100 ; the description related to such functions thus will not be stated herein. 
     Prism assembly  340  is the internally shrank partial block of the prism assembly  140  to form the shape as shown in  FIG. 5 . Similarly, prism assembly  340  comprises two prisms T A  and T B , and a medium layer X. Prisms T A  and T B  are glass pillars for example; prism T A  has three planes P 1 , P 2 , and P 3 ; prism T B  has three planes P 4 , P 5 , and P 6 . Medium layer X is an air layer for example; locating between plane P 2  of prism T A  and plane P 4  of prism T B . Prisms T A  and T B  have a refractive index N 1 , medium X has a refractive index N 2 ; and N 2  is less than N 1 . In addition, plane P 3  is paralleled with DMD  130 , plane P 5  is paralleled with lens assembly  120  (i.e. plane P 5  is perpendicular to the optical axis A 2  of lens assembly  120 ). The included angles between planes P 1  and P 2  and between plane P 2  and plane P 3  respectively are β and α. The included angle between planes P 5  and P 6  is γ; and γ is an obtuse angle, which is a feature of this embodiment of the present invention. 
     Please still refer to  FIG. 5 . The lights from the light system  110  move along optical axis A 1  and pass through plane P 1 ; after emitting to prism T A , the lights are totally reflected from plane P 2  to prism assembly  130  through plane P 3 , and an included angle between the normal to the plane of DMD  130  and the light is θ AOI . Micro mirrors M will again reflect the incident lights. As in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 5 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  to the lens assembly  120 . As in the OFF state S OFF , the lights reflected by the micro mirrors M (the broken lines in  FIG. 5 ) will pass through plane P 3  and be refracted between the planes P 2  and P 4 , then emit out to plane P 6 ; the lights are totally reflected from plane P 6  for the first time then emit to plane P 5 ; and are totally reflected from plane P 5  for the second time then emit out from plane P 1  in the direction away from the optical axis A 2  of lens assembly  120  instead of entering into lens assembly  120 . Furthermore, the dotted lines part in  FIG. 5  is to illustrate the emitted lights from the prior art prism assembly  140  in the OFF state. 
     Please refer to  FIG. 6 .  FIG. 6  is the schematic view illustrating enhancing contrast and further reducing light leakage using the prism assembly of the first embodiment in the present invention. After the lights from the edge of light system  110  emit into prism T A  through plane P 1 , they are totally reflected from plane P 2  to DMD  130  through plane P 3 . Because light system  110  has focal length F, the direction of lights from the edge of light system  110  is different than that from the center. As in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 6 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  to lens assembly  120 . As in the OFF state S OFF , the lights reflected by micro mirrors M (the broken lines in  FIG. 6 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit to plane P 6 . The lights are totally reflected for the first time at plane P 6  then are reflected to plane P 5 ; and totally reflected for the second time at plane P 5  then emit out from plane P 1  in the direction away from the optical axis A 2  of lens assembly  120  instead of entering into lens assembly  120 . In contrast to the lights emitted out from the prior art projector  100  in the OFF state S OFF  (the dotted lines in  FIG. 6 ), the lights emitted out under such condition in the present invention will not enter into lens assembly  120  and that would result in bad contrast. Therefore, the contrast will be enhanced and the light leakage will be reduced in the OFF state by using the size-limited prism assembly  340  of the first embodiment in the present invention. 
     Please refer to  FIG. 7 .  FIG. 7  is the schematic view illustrating the size comparison between the prism assembly  340  of the first embodiment in the present invention and the prism assembly  240  of the prior art projector. As shown in  FIG. 7 , the prism assembly  340  of the first embodiment in the present invention still has smaller size compare to the prism assembly  240  of prior art, and it can also enhance the contrast of projector. 
     Please refer to  FIG. 8 .  FIG. 8  is the schematic view of projector  400  of a second embodiment in the present invention. In  FIG. 8 , except prism assembly  440 , the remaining elements are identical to those of projector  100 ; the related functions hence will not be stated herein. 
     Prism assembly  440  is the internally shrank partial block of prism assembly  140  and a cross section P 7  is placed between planes P 2  and P 3  to form the shape as shown in  FIG. 8 . Similarly, prism assembly  440  comprises two prisms T A  and T B , and a medium layer X. Prism T A  and T B  are glass pillars for example; prism T A  has four planes P 1 , P 2 , P 3 , and the cross section P 7 ; prism T B  has three planes P 4 , P 5 , and P 6 . In other words, prism assembly  440  is the planes P 2  and P 3  of prism T A  in prism assembly  340  that has been partially cut off to form the cross section P 7 . Medium layer X is an air layer for example, locating between plane P 2  of prism T A  and plane P 4  of prism T B . Prisms T A  and T B  has a refractive index N 1 , medium layer X has a refractive index N 2 ; N 2  is less than N 1 . In addition, plane P 3  is paralleled with DMD  130 , plane P 5  is paralleled with lens assembly  120  (i.e. plane P 5  is perpendicular to the optical axis A 2  of lens assembly  120 ). The included angles between planes P 1  and P 2  and between plane P 2  and plane P 3  respectively are β and α. 
     Please still refer to  FIG. 8 . After the lights from the light system  110  move along optical axis A 1  to the prism T A  and pass through plane P 1 , they are totally reflected from plane P 2  to DMD  130  through plane P 3 , and an included angle between the normal to the plane of DMD  130  and the light is θ AOI . After that, the micro mirrors M will again reflect the incident lights. As in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 8 ) will pass through plane P 3  and be refracted between P 2  and P 4 , then emit out from plane P 5  to lens assembly  120 . As in the OFF state S OFF , the lights reflected by micro mirrors M (the broken lines in  FIG. 8 ) will pass through plane P 3  and they will be totally reflected for the first time at the cross section P 7  then be reflected to plane P 2 ; and totally reflected for the second time at plane P 2  then emit out from plane P 1  in the direction away from the optical axis A 2  of lens assembly  120  instead of entering into lens assembly  120 . In addition, the dotted lines appeared in  FIG. 8  is to illustrate the emitted lights from the prior art prism assembly  140  in the OFF state. 
     Please refer to  FIG. 9 .  FIG. 9  is the schematic view illustrating enhancing contrast and further reducing light leakage by using the prism assembly  440  of the second embodiment in the present invention. After the lights from the edge of light system  110  enter into prism T A  through plane P 1 , they are totally reflected from plane P 2  to DMD  130  through plane P 3 . Because the light system  110  has focal length F, the direction of the lights from the edge of the light system  110  is different than that from the center. When in the ON state S ON , the lights reflected by micro mirrors M (the solid lines in  FIG. 9 ) will pass through plane P 3  and be refracted between planes P 2  and P 4 , then emit out from plane P 5  to the lens assembly  120 . When in the OFF state S OFF , the lights reflected by micro mirrors M (the broken lines in  FIG. 9 ) will pass through plane P 3  to plane P 2 ; the lights will be totally reflected for the first time at the cross section P 7  then be reflected to plane P 2 , the lights will then be totally reflected for the second time at plane P 2  and emit out from plane P 1  in the direction away from the optical axis A 2  of lens assembly  120  instead of entering into lens assembly  120 . In contrast to the lights emitted out from the prior art projector  100  in the OFF state S OFF  (the dotted lines in  FIG. 9 ), the lights emitted out under this condition in the present invention will not enter into lens assembly  120  and as a result, the contrast is not good. By using the size-limited prism assembly  440  of the second embodiment of the present invention, the contrast of the projector will be enhanced and the light leakage will be reduced. 
     Please refer to  FIG. 10 .  FIG. 10  is the schematic view illustrating size comparison between the prism assembly  440  of the second embodiment of the present invention and the prism assembly  240  of prior art. As shown in  FIG. 10 , the prism assembly  440  of the second embodiment in the present invention has smaller size compare to the prism assembly  240  of prior art, and it is still able to enhance the contrast of projector. 
     Additionally, what is worth attention is, in the first and the second embodiments of the present invention, the incident angle θ AOI  is approximately greater than the rotatable angle 2θ S  of micro mirrors M for enhancing the penetration rate of the lights reflected by DMD  130  at prism T A  and T B . In contrast to this invention, when the rotatable angle 2θ S  of micro mirrors M is 24°, the incident angle θ AOI  is also configured as 24° in prior art so that in the ON state, the lights from the light system can be emitted out and paralleled to the optical axis A 2  of lens assembly  120 . Such method, however, when the lights are entering from prisms T A  to T B , the incident angle is larger so that the penetration rate of lights is lower. In the present invention, when the rotatable angle 2θ S  of DMD  130  is 24°, the incident angle θ AOI  can be designed to 25°. Such that in the ON state, when the lights from the light system enter into prism T B , the incident angle is smaller so that the penetration rate of lights is higher, and can be emitted out in the direction approximately away from the optical axis A 2  of lens assembly  120  after being refracted and reflected. The way the incident angle θ AOI  is adjusted in the present invention may be done by rotating the angle of the light system  110 , which is adjusting the angle of the optical axis A 1 , so that the optical axis A 1  may still be about perpendicular to plane P 1  but the incident angle θ AOI  may thus increase to 25°. 
     Moreover, the present invention further defines the included angles α, β, and γ so as to enhance the contrast of projector. These angles are defined as follow:
 
α=(α IN +α OUT )/ 2    (1)
 
β=α+sin −1 [ sin(θ AOI )/ N   1 ]  (2)
 
γ=(180−θ CRI −α)   (3)
 
wherein:
 
α IN =θ CRI −sin −1 [ sin(2θ S −θ CONE )/ N   1 ]
 
α OUT =θ CRI −sin −1 [ sin(θ CONE +θ AOI −2θ S )/ N   1 ]
 
θ CRI =sin −1 (1 /N   1 )
 
θ CONE =sin−1(NA)
 
NA=1/(2 F )
 
     wherein NA is the numerical aperture (NA) of light system  110 ; θ CRI  is the total reflection threshold angle with which the lights enter into the prism assembly of the present invention through the air; θ CONE  is the included angle between the emitting light beam and the optical axis A 1  of the light system  110 . Since the energy of the light source is in Gaussian distribution, as the angle α equals to α IN  or α OUT , the light energy projected by the projector in the ON state is the lowest. Thus the present invention defines angle α as the average of α IN  and α OUT , so that the light energy projected by the projector in the ON state is the highest. 
     Please refer to  FIG. 11 .  FIG. 11  is the schematic view of the actual value of the included angles α and β defined above in the present invention. As shown in  FIG. 11 , in type  1  the focal length F is 2.4; the incident angle θ AOI  is 24 degrees; αand β are 33.37 degrees and 48.92 degrees, respectively. In type  3 , the focal length F is 2.4; the incident angle θ AOI  is 26 degrees; αand β are 32.07 degrees and 48.87 degrees, respectively. In type  2 , the focal length F is 2.4; the incident angle θ AOI  is 25 degrees; α and β are 32.72 and 48.89, respectively, wherein Type  2  is the design with the highest performance of the projector in the present invention. 
     To conclude, the present invention provides an improved prism assembly that when the projector is in the OFF state, the light can be emitted out after two-time total reflection in the prism assembly, so that the projector contrast may be enhanced and the light leakage in the OFF state can be further reduced; at the same time the size of the prism assembly also may be decreased and this makes it more convenient for the users. In contrast to the first conventional prism assembly, the prism assembly of a first embodiment provided according to the spirit of the present invention has a smaller size, therefore the projector that uses the prism assembly of the present invention has a reduced size and higher contrast. In contrast to the second conventional one, the prism assembly of a second embodiment provided according to the spirit of the present invention also has a smaller size, and since one of the prisms in the prism assembly has an obtuse angle, it may also enhance the contrast and decrease light leakage so that the projector that uses the prism assembly of the present invention has a smaller size and higher contrast. Thus, users may use the prism assembly and the projector provided in the present invention to reduce the space taken by a projector and to enhance the projector contrast in order to obtain a greater convenience. 
     The descriptions above are merely the preferred embodiments of the present invention. Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.