Patent Publication Number: US-2013242264-A1

Title: Lighting optical system and projection display device including the same

Description:
TECHNICAL FIELD 
     The present invention relates to a lighting optical system and a projection display device including the same. 
     BACKGROUND ART 
     Recently, in the projection display device (i.e., projector) that uses a liquid crystal panel or a digital micromirror device (DMD) as a display element, technology that uses a light-emitting diode (LED) as the light source has been a focus of attention (e.g., see Patent Literature 1). 
     The projector using the LED as the light source (i.e., LED projector) has an advantage of a long life and high reliability which is due to the long life/high reliability of the LED. 
     CITATION LIST 
     Patent Literature 
     
         
         Patent Literature 1: JP 2003-186110 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, as described below, the LED projector has a problem that it is difficult to achieve a high-luminance image display due to limitations of etendue. 
     In the lighting optical system that projects light to the display element, the limitations of etendue determined by the light-emitting area and the radiation angle of the light source must be taken into consideration. In other words, to effectively use light from the light source as projected light, the product value of the light-emitting area and the radiation angle of the light source must be set equal to or lower than that of the area of the display element and the capturing angle determined by the F-number of the lighting optical system. In the LED, however, the amount of light is smaller than that of the other light sources. Therefore, even if the amount of light can be increased by increasing the size of the light-emitting area, this leads to the increase of etendue. Consequently, since light use efficiency is lowered, it becomes impossible to achieve the high-luminance image display. 
     Thus, there is a demand for increasing the amount of light without increasing the size of the light-emitting area in the light source of the projector. However, it is difficult to achieve this only by using the LED. 
     From the standpoint of increasing the amount of light, a light source other than the LED may be used for each color light. However, this is not desirable because it will increase the number of components, thus increasing the size of the entire projector. 
     It is therefore an object of the present invention to provide a lighting optical system capable of increasing brightness without increasing etendue or device size. It is another object of the invention to provide a projection display device that includes the lighting optical system. 
     Solution to Problem 
     To achieve the above object, a lighting optical system according to the present invention includes a first light source for emitting first color light and second color light, and a second light source for emitting third color light. The first light source includes a semiconductor laser element that emits a linearly polarized laser beam, excitation light generation means for spatially and temporally separating the laser beam emitted from the semiconductor laser element to generate first excitation light and second excitation light, a first phosphor that is excited by the first excitation light to emit first color light, and a second phosphor that is excited by the second excitation light to emit second color light. The excitation light generation means includes a liquid crystal element that converts the incident laser beam into one of two lights orthogonal to each other in polarization direction, and light space separation means for spatially separating the two lights converted by the liquid crystal element into the first excitation light and the second excitation light according to a difference between the two lights in polarization direction. 
     A projection display device according to the present invention includes: the lighting optical system described above; an optical modulation device that modulates light that is output from the lighting optical system according to an image signal; and a projection optical system that projects the light modulated by the optical modulation device. 
     Advantageous Effects of Invention 
     Thus, the present invention can provide a lighting optical system capable of increasing brightness without increasing etendue or device size, and a projection display device that includes the same. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  shows a schematic diagram showing the configuration of a liquid crystal projector that includes a lighting optical system according to a first embodiment of the present invention; 
         FIG. 2  shows characteristics of wavelength-transmittance of a dichroic prism of the liquid crystal projector shown in  FIG. 1 ; 
         FIG. 3  shows a schematic diagram showing the configuration of a DMD projector that includes a lighting optical system according to a second embodiment of the present invention. 
         FIG. 4  shows a schematic front view showing the configuration of a DMD in the DMD projector shown in  FIG. 3 ; and 
         FIG. 5  shows a schematic sectional view showing the inclined state of a micromirror in the DMD shown in  FIG. 4 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be described with reference to the drawings. 
     First Embodiment 
     First, a lighting optical system of a projection display device that uses a liquid crystal panel as a display element (i.e., liquid crystal projector) according to a first embodiment of the present invention will be described. 
       FIG. 1  shows a schematic diagram showing the configuration of an optical system of the liquid crystal projector according to this embodiment. 
     Liquid crystal projector  1  includes lighting optical system  2  that includes a first light source for emitting first color light and second color light, and a second light source for emitting third color light. Hereinafter, an example where the first color light and the second color light are respectively red light and green light and the third color light is blue light will be described. However, the present invention is not limited to this example. For example, the first color light can be green light, and the second color light can be red light, or the third color light can be red or green light. As described above, while the major feature of the present invention is the configuration of the first light source for emitting two color lights, an arbitrary light source can be used for the second light source. Thus, the combination of the two color lights in the first light source can be selected by taking the configuration of the second light source into consideration. 
     First light source  10  includes laser light source unit (laser light source part)  11  that emits a linearly polarized laser beam, red phosphor (first phosphor)  12  that emits red light (first color light) R, and green phosphor (second phosphor)  13  that emits green light (second color light) G. Specifically, in this embodiment, red phosphor  12  and green phosphor  13  are excited by laser beams to emit red light R and green light G. Further, first light source  10  includes excitation light generation means  18  for spatially and temporally separating the laser beam emitted from laser light source unit  11  to generate first excitation light E 1  and second excitation light E 2 . First excitation light E 1  is used for exciting the red phosphor, and second excitation light E 2  is used for exciting the green phosphor. In this embodiment, excitation light generation means  18  enables use of the common laser light source (laser light source unit  11 ) without using any different laser light sources respectively for two independently arranged phosphors  12  and  13 . Thus, an increase in the number of components and an accompanying increase in the size of the device can be prevented. 
     Laser light source unit  11  includes a plurality of blue laser diodes  11   a  as semiconductor laser elements for emitting laser beams. In other words, in this embodiment, blue laser beams are used as excitation light for exciting red phosphor  12  and green phosphor  13 . Laser light source unit  11  further includes collimator lens  11   b  for converting the laser beams emitted from blue laser diodes  11   a  into collimated light beams, mechanism component  11   c  for holding blue laser diodes  11   a  and collimator lenses  11   b , and a cooling unit (not shown) for cooling blue laser diodes  11   a . In this embodiment, each blue laser diode  11   a  is disposed in laser light source unit  11  so that the polarization direction of the laser beam can be parallel to the paper surface shown in  FIG. 1 . 
     Excitation light generation means  18  includes liquid crystal element  14  that temporally separates the laser beam from laser light source unit  11 , and dichroic prism  15  for spatially separating the two lights temporally separated by liquid crystal element  14 . 
     Liquid crystal element  14  functions to change the polarization direction of the incident laser beam according to an applied voltage. In other words, liquid crystal element  14  can change the polarization direction of the laser beam that is transmitted through liquid crystal element  14  by switching between a state in which voltage is not applied (i.e., OFF state) and a state in which voltage is applied (i.e., ON state). Specifically, liquid crystal element  14  can directly transmit the laser beam in the OFF state, while liquid crystal element  14  can rotate the polarization direction of the laser beam by 90° to transmit it in the ON state. The OFF state and the ON state can be switched in time division. Accordingly, liquid crystal element  14  can time-divisionally output the two lights orthogonal to each other in a polarization direction. 
     Dichroic prism  15  is disposed on the output side of liquid crystal element  14 . Dichroic prism  15  is configured to spatially separate the two lights (linearly polarized lights) that are output from liquid crystal element  14  and are orthogonal to each other in a polarization direction into first excitation light E 1  and second excitation light E 2  according to the difference between the two lights in polarization direction. Specifically, dichroic prism  15  has a polarized light separation mechanism of transmitting linearly polarized light that enters dichroic prism  15  as P-polarized light and of reflecting linearly polarized light that enters dichroic prism  15  as S-polarized light. Accordingly, when liquid crystal element  14  is in the OFF state, dichroic prism  15  can directly transmit the laser beam transmitted through liquid crystal element  14  to output it as first excitation light E 1  On the other hand, when liquid crystal element  14  is in the ON state, dichroic prism  15  can reflect the laser beam whose polarization direction has been changed by liquid crystal element  14  to output it as second excitation light E 2 . 
     Further, dichroic prism  15  is configured to reflect red light R emitted from red phosphor  12  and to transmit green light G emitted from green phosphor  13 . Dichroic prism  15  of this embodiment accordingly has a function of combining red light R and green light G in addition to the polarized light separation function. Thus, the device can be further miniaturized. 
     Liquid crystal element  14  is desirably configured to change the time ratio of the ON state to the OFF state per unit time. Accordingly, by changing the generation ratio of first excitation light E 1  to second excitation light E 2  from dichroic prism  15 , the ratio of the amount of red light R to the amount of green light G per unit time can be adjusted. Further, the laser output of the laser light source unit can also be desirably adjusted to synchronize with the time ratio. With this configuration, the time ratio of the ON state to the OFF state of active diffraction element  14  can be adjusted according to an image signal to be displayed, and the laser output can be adjusted to synchronize with the time ratio. As a result, contrast can be improved and power consumption can be reduced. 
     In this description, the P-polarized light that is transmitted through dichroic prism  15  is defined as first excitation light E 1 , and the S-polarized light that is reflected by dichroic prism  15  is defined as second excitation light E 2 . Needless to say, however, the reverse can be defined. 
     Referring to  FIG. 2 , the principle of transmitting the P-polarized light and reflecting the S-polarized light by dichroic prism  15  will be described. 
       FIG. 2  shows characteristics of wavelength-transmittance of dichroic prism  15 .  FIG. 2  shows the transmittance characteristic curves of dichroic prism  15  for the P-polarized light and the S-polarized light. 
     As can be understood from  FIG. 2 , the transmittance characteristic curve of dichroic prism  15  for the P-polarized light has a tendency of widening to the shorter wavelength side and the longer wavelength side with respect to the S-polarized light. This enables, even when the P-polarized light and the S-polarized of equal wavelengths enter dichroic prism  15 , transmission of one polarized light while reflecting the other polarized light. Thus, by selecting the wavelength of the laser beam emitted from laser light source  11  to, for example, λ EX , dichroic prism  15  can transmit the P-polarized light and reflect the S-polarized light. 
     As shown in  FIG. 1 , condenser lens groups  16  and  17  are respectively arranged on the front sides of red phosphor  12  and green phosphor  13 . 
     In this embodiment, red light R and green light G are emitted from light source  10  on the same optical path. However, lights must enter liquid crystal units  40   r ,  40   g , and  40   b  described below through different optical paths. Accordingly, lighting optical system  2  includes, on the optical path of color light RG emitted from first light source  10 , first dichroic mirror  37  that is disposed to reflect red light R and to transmit green light G. Between first dichroic mirror  37  and first light source unit  10 , lens arrays  33  and  34  that make the illumination distribution of the incident light uniform and PS converter (polarization conversion element)  35  that aligns the polarization direction of light with a predetermined direction are arranged via reflection mirror  15  and condenser lens  36 . In this embodiment, PS converter  35  is designed so that the light that is output from PS converter  35  can be converted into S-polarized light for first dichroic mirror  37 . 
     As described above, the laser beam and the phosphors are used for generating red light R and green light G. On the other hand, the LED that is a semiconductor light-emitting element is used for generating blue light B. In other words, liquid crystal projector  1  includes blue LED  20  as a second light source. 
     As in the case of first light source  10 , several optical elements are arranged on the optical path of blue light B emitted from blue LED  20 . On the light-emitting side of blue LED  20 , two condenser lenses  21  and  23  are arranged via reflection mirror  22  to condense blue light B emitted from blue LED  20 . Lens arrays  24  and  25 , PS converter (polarization conversion element)  26 , and condenser lens  27  are similarly arranged. 
     Liquid crystal projector  1  according to this embodiment includes liquid crystal units (optical modulation devices)  40   r ,  40   g , and  40   b  that modulate color lights R, G, and B output from lighting optical system  2  according to an image signal. Liquid crystal units  40   r ,  40   g , and  40   b  respectively include liquid crystal panels  41   r ,  41   g , and  41   b  for modulating color lights R, G, and B, incident-side polarization plates  42   r ,  42   g , and  42   r  arranged on the incident sides of liquid crystal panels  41   r ,  41   g , and  41   b , and output-side polarization plates  43   r ,  43   g , and  43   r  arranged on the output sides of liquid crystal panels  41   r ,  41   g , and  41   b.    
     Between lighting optical system  10  and liquid crystal units  40   r ,  40   g , and  40   b , reflection mirrors  44   r ,  44   g , and  44  for changing the optical paths of color lights R, G, and B, and condenser lenses  45   r ,  45   g , and  45   b  for adjusting incident angles to liquid crystal units  40   r ,  40   g , and  40   b  are arranged. PS converter  26  is designed so that the S-polarized light can enter reflection mirrors  44   r ,  44   g , and  44   b.    
     Further, liquid crystal projector  1  includes cross dichroic prism (light-combining optical system)  51  for combining color lights R, G, and B modulated by liquid crystal units  40   r ,  40   g , and  40   b  to output combined light, and projection lens (projection optical system)  52  for projecting and displaying the combined light on a screen or the like. 
     Next, referring again to  FIG. 1 , the operation of projecting an image in liquid crystal projector  1  of this embodiment will be described. 
     The laser beam emitted from laser light source  11  enters liquid crystal element  14 . The linearly polarized laser beam is temporally separated into light that is directly transmitted through liquid crystal element  14  and light that is transmitted through liquid crystal element  14 , and whose polarization direction is changed. The two linearly polarized lights that are transmitted through liquid crystal element  14  enter dichroic prism  15 . 
     The linearly polarized light that enters dichroic prism  15  as P-polarized light is transmitted through dichroic prism  15  to be output as first excitation light E 1 . Then, first excitation light E 1  is condensed by condenser lens group  16  to enter red phosphor  12  disposed on the optical axis of laser light source unit  11 . Red phosphor  12  is excited by first excitation light E 1  to emit randomly polarized red light R. Condenser lens group  16  concentrates red light R that is emitted from red phosphor  12  so that it will enter dichroic prism  15 . 
     On the other hand, the linearly polarized light that enter dichroic prism  15  as S-polarized light is reflected by dichroic prism  15  to be output as second excitation light E 2 . Then, second excitation light E 2  is condensed by condenser lens group  17  to enter green phosphor  13 . Green phosphor  13  is excited by second excitation light E 2  to emit randomly polarized green light G. Condenser lens group  17  concentrates green light G that is emitted from green phosphor  13  so that it will enter dichroic prism  15 . 
     Red light R is reflected by dichroic prism  15  while green light G is transmitted through dichroic prism  15 . Accordingly, red light R and green light G are combined by dichroic prism  15 . Combined color light RG is reflected by reflection mirror  31 . Then, lens arrays  33  and  34  make the irradiation distribution of combined color light RG uniform, and PS converter converts color light RG to be S-polarized light for first dichroic mirror  37 . Thus, color light RG, whose illumination distribution has been made uniform and whose polarization direction has been aligned, is condensed by condenser lens  36  to enter first dichroic mirror  37 . 
     Color light RG, which has entered first dichroic mirror  37 , is separated into red light R and green light G. These lights are respectively transmitted to liquid crystal units  40   r  and  40   g  via reflection mirrors  44   r  and  44   g  and condenser lenses  45   r  and  45   g.    
     Blue light B emitted from blue LED  20  enters lens arrays  24  and  25  via condenser lenses  21  and  23  and reflection mirror  22 . Lens arrays  24  and  25  make the illumination distribution of blue light B uniform, and PS converter  26  converts blue light B to be S-polarized light for reflection mirror  44   b . Then, blue light B enters condenser lens  27 . Blue light B condensed by condenser lens  27  is transmitted to liquid crystal unit  40   b  via reflection mirror  44   b  and condenser lens  45   b.    
     Color lights R, G, and B are modulated by liquid crystal units  40   r ,  40   g , and  40   b  according to the image signal. Modulated color lights R, G, and B are output to cross dichroic prism  51 , and combined by cross dichroic prism  51 . The combined light enters projection lens  52 , and is projected to the screen or the like by projection lens  52  to be displayed as an image. 
     As mentioned above, the lighting optical system according to this embodiment uses the combination of the semiconductor laser element and the phosphors as the light sources of the red light and the green light. In contrast to a case in which the LED is used as a light source, this enables an increase in the amount of light without causing the size of the light-emitting area to increase. Thus, by preventing an increase of etendue, light use efficiency can be increased, and brightness of the lighting optical system can be improved. Therefore, according to the embodiment, by using the excitation light generation means that includes the liquid crystal element and the dichroic prism and is capable of spatially and temporally separating the laser beam, the common laser light source can be used for the two independently arranged phosphors. As a result, the above-mentioned improvement of brightness can also be achieved without causing an increase in the number of components and an accompanying increase in the size of the device. 
     In this embodiment, the LED is used as the second light source for emitting the third color light. However, as described above, the light source is not limited to an LED, accordingly, a light source other than the LED can be used. For example, the second light source can be configured to emit blue light by exciting the phosphor with the laser beam as in the case of the first light source. 
     Second Embodiment 
     Next, the lighting optical system of a projection display device that uses a digital micromirror device (DMD) as a display element (i.e., DMD projector) according to a second embodiment of the present invention will be described. 
       FIG. 4  shows a schematic diagram showing the configuration of an optical system of the DMD projector according to this embodiment. 
     This embodiment is a modification of the first embodiment where the configuration of the display element (optical modulation device) is changed. In the embodiment, a DMD is used in place of the liquid crystal unit of the first embodiment. The arrangement configuration of the optical system of this embodiment is accordingly changed from that of the first embodiment. However, the configuration of each of light sources  10  and  20  is similar to that of the first embodiment. Hereinafter, members similar to those of the first embodiment will be denoted by similar reference numerals shown, and description thereof will be omitted. 
     In lighting optical system  4  according to this embodiment, in contrast to that of the first embodiment, second dichroic mirror  38  for transmitting red light R and green light G and reflecting blue light B is added. Second dichroic mirror  38  is disposed between first light source  10  and reflection mirror  31 . Blue LED  20  is arranged so as to cause blue light B to enter second dichroic mirror  38  via condenser lens group  29 . This enables second dichroic mirror  38  to output combined light RGB including three color lights R, G, and B. In this embodiment, first dichroic mirror  37  in the first embodiment is not provided, and optical elements other than the condenser lenses associated with second light source (blue LED)  20  in the first embodiment are not provided. Since output light need not be converted into light of a specific polarization component, the polarization conversion element (PS converter  35 ) of the first embodiment is also not provided. 
     In DMD projector  3  according to this embodiment, as described below, a color image is projected by using a single plate method. Accordingly, lighting optical system  4  must output red light R, green light G, and blue light B not only, as combined light RGB on the same optical path, but also in time division. Thus, in this embodiment, laser light source unit  11  and blue LED  20  are configured to be time-divisionally switched on and off according to the time ratio of the OFF state to the ON state of liquid crystal element  14 . Table 1 shows an example of time-division operation patterns for the respective color components of the color image. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Color component 
               
            
           
           
               
               
               
               
            
               
                   
                 Green 
                 Red 
                 Blue 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Liquid crystal element 14 
                 ON 
                 OFF 
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Laser light source unit 11 
                 ON 
                 OFF 
               
               
                   
                 Blue LED 20 
                 OFF 
                 ON 
               
               
                   
                   
               
            
           
         
       
     
     Further, DMD projector  3  according to this embodiment includes DMD  61  that is a display element, and total reflection (TIR) prism  62  disposed on the front side of DMD  61 , i.e., between DMD  61  and projection lens  52 . Between lighting optical system  4  and total internal reflection (TIR) prism  62 , reflection mirror  63  for changing the optical path of combined light RGB and condenser lens  64  are arranged. 
     Now, the configuration of DMD  61  used in DMD projector  3  according to this embodiment will be described. 
       FIG. 4(   a ) shows a schematic front view showing the configuration of DMD  61 , and  FIG. 5(   b ) is an enlarged schematic front view showing the vicinity of a region surrounded with dotted lines shown in  FIG. 4(   a ). 
     DMD  61  includes many micromirrors (pixels)  61   a  arrayed in a matrix, and is disposed in DMD projector  3  so that light can enter from the arrow direction shown in  FIG. 4(   a ). Each micromirror  61   a  is configured to incline by ±12° with axis  61   a  orthogonal to incident light set as the rotational axis. Rotational axis  61   a  of micromirror  61   a  is the diagonal direction of each micromirror  61  whose shape is square, and inclines by 45° with respect to the arraying direction of micromirrors  61   a.    
       FIG. 5  shows a schematic sectional view taken along line A-A′ shown in  FIG. 4(   b ).  FIGS. 5(   a ) and  5 ( b ) show micromirrors  61   a  respectively inclined by +12° and −12°. In  FIGS. 5(   a ) and  5 ( b ), the arrangement of projection lens  52  with respect to micromirror  61   a  is also schematically shown. 
     Micromirror  61   a  is set in the ON state when it inclines by +12°. Specifically, as shown in  FIG. 5(   a ), in the ON state, light that enters micromirror  61  (see arrow L 1 ) is reflected in a direction (refer to arrow L 2 ) that allows it to enter projection lens  52 . On the other hand, micromirror  61   a  is set in the OFF state when it inclines by −12°. Specifically, as shown in  FIG. 5(   b ), light that enters micromirror  61   a  (see arrow L 1 ) is reflected in a direction (see arrow L 3 ) that prevents it from entering projection lens  52 . 
     Thus, DMD  61  can project the color image through projection lens  52  by switching between the ON state and the OFF state of each micromirror  61   a  in synchronization with color lights R, G, and B entered in time division. 
     Lastly, referring again to  FIG. 3 , the operation of projecting an image by DMD projector  3  of this embodiment will be described. 
     Red light R and green light G are, as in the case of the first embodiment, emitted from first light source  10  on the same optical path to enter second dichroic mirror  38 . Blue light B emitted from blue LED  20  also enters second dichroic mirror  38  via condenser lens group  29 . 
     Red light R and green light G are transmitted through second dichroic mirror  38  while blue light B is reflected by second dichroic mirror  38 . Accordingly, three color lights are combined by second dichroic mirror  38 . Combined color light RGB is reflected by reflection mirror  31 . Lens arrays  33  and  34  make the illumination distribution of combined color light RGB uniform. Then, combined light RGB is condensed by condense lens  36  so that it exits from lighting optical system  4 . 
     Color light RGB output from lighting optical system  4  enters TIR prism  62  via reflection mirror  63  and condenser lens  64 . Color light RGB that enters TIR prism  62  is reflected on an air gap surface in TIR prism  62  so that it enters DMD  61 , and is modulated by DMD  61  according to an image signal. The modulated light is transmitted through TIR prism  62  so that it enters projection lens  52 , and is projected to the screen or the like by projection lens  52  to be displayed as an image. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1  Liquid crystal projector 
               2 ,  4  Lighting optical system 
               3  DMD projector 
               10  First light source 
               11  Laser light source unit 
               11   a  Blue laser diode 
               11   b  Collimator lens 
               11   c  Mechanism component 
               12  Red phosphor 
               13  Green phosphor 
               14  Liquid crystal element 
               15  Dichroic prism 
               16 ,  17 ,  29  Condenser lens group 
               18  Excitation light generation means 
               20  Blue LED 
               21 ,  23 ,  27 ,  36 ,  45   r ,  45   g ,  45   b ,  64  Condenser lens 
               22 ,  31 ,  44   r ,  44   g ,  44   b ,  63  Reflection mirror 
               24 ,  25 ,  33 ,  34  Lens array 
               26 ,  35  PS converter 
               37  First dichroic mirror 
               38  Second dichroic mirror 
               40   r ,  40   g ,  40   b  Liquid crystal unit 
               41   r ,  41   g ,  41   b  Liquid crystal panel 
               42   r ,  42   g ,  42   b  Incident-side polarization plate 
               40   r ,  40   g ,  40   b  Output-side polarization plate 
               51  Cross dichroic prism 
               52  Projection lens 
               61  DMD 
               62  TIR prism