Abstract:
This invention realizes an illumination optical system with a small etendue that has a longer lifetime and a high degree of brightness. The invention includes: a laser light source that generates excitation light; a phosphor that generates a fluorescent light by means of the excitation light; a light tunnel that projects the excitation light that is incident at one end towards the phosphor from another end, and projects a fluorescent light generated with the phosphor from the one end; and a dichroic mirror that is provided between the laser light source and the light tunnel, and that allows the fluorescent light or the excitation light to pass through, and reflects the remaining excitation or fluorescent light.

Description:
TECHNICAL FIELD 
     The present invention relates to an illumination optical system that generates illumination lights of a plurality of colors for forming image lights of a plurality of colors, and a projector that projects the image lights produced by the illumination optical system. 
     BACKGROUND ART 
     Technology that uses an LED (Light Emitting Diode) as a light source of a projector that projects an image onto a screen such as a liquid crystal projector or a DMD (Digital Micromirror Device) projector has been receiving attention (see Patent Literature 1). 
     Because an LED has a long lifetime and offers high reliability, projectors that employ an LED as a light source have the advantages of long lifetime and high reliability. 
     However, because the brightness of the light of an LED is low for use as a projector, it is not easy to obtain a projected image that has sufficient brightness with a projector employing an LED as a light source. The extent to which a display panel can utilize light from a light source as projection light is limited by the etendue. More specifically, unless the value of the product of a light-emission area of a light source and the angle of radiation is made less than or equal to a value of the product of the area of the plane of incidence of the display panel and the capturing angle that is determined by an f-number of the illumination optical system, the light from the light source can not be effectively utilized as projection light. 
     Although the light quantity of a light source that employs an LED can be increased by increasing the light-emission area, if the light-emission area increases, the etendue of the light source will also increase. As a light source for a projector, it is desirable in terms of the limitation produced by the etendue to increase the light quantity without increasing the light-emission area. However, it is difficult for a light source that employs an LED to increase the light quantity without increasing the light-emission area. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP2003-186110A 
     SUMMARY OF INVENTION 
     Technical Problem 
     The etendue of a light source that using only an LED is increases. The present invention realizes an illumination optical system with a small etendue, a longer lifetime, and a high level of brightness. 
     Solution to Problem 
     An illumination optical system of the present invention comprises:
     a laser light source that generates an excitation light;   a phosphor that generates fluorescent light by means of the excitation light;   

     a light tunnel that projects the excitation light that is incident at one end towards the phosphor from another end, and projects fluorescent light generated at the phosphor from the one end; 
     a dichroic mirror that is disposed between the laser light source and the light tunnel, and that allows the fluorescent light or the excitation light to pass through, and reflects the remaining excitation or fluorescent light. Further, a projector according to the present invention comprises the above described illumination optical system. 
     Advantageous Effects of Invention 
     According to the present invention, since a laser with a high energy density converges on a phosphor as excitation light, and since fluorescent light emitted from the place at which the laser converges is used, an illumination optical system can be realized that has a small etendue, a longer lifetime and a higher level of brightness. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram that illustrates the configuration of a first embodiment of an illumination optical system according to the present invention. 
         FIG. 2(   a ) is a plan view of phosphor wheel  104  as viewed from the plane of incidence of laser light generated by laser light source  101  (from the left side towards the right side in  FIG. 1) , and  FIGS. 2(   b ), ( c ), and ( d ) are sectional views that illustrate the structure of blue fluorescent light region  401 , green fluorescent light region  402 , and red fluorescent light region  403 . 
         FIG. 3  is a block diagram illustrating the configuration of another exemplary embodiment of the illumination optical system according to the present invention. 
         FIG. 4(   a ) is a block diagram illustrating the configuration of another exemplary embodiment of the illumination optical system according to the present invention, and  FIG. 4(   b ) is a sectional view illustrating the structure of phosphor  303 . 
         FIG. 5  is a block diagram illustrating the circuit configuration of a projector that uses the illumination optical system of the exemplary embodiment shown in  FIG. 1 . 
         FIG. 6  is a block diagram illustrating the circuit configuration of a projector that uses the illumination optical system of the exemplary embodiment shown in  FIG. 3 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Next, exemplary embodiments are described with reference to the drawings. 
       FIG. 1  is a block diagram that illustrates the configuration of one exemplary embodiment of an illumination optical system according to the present invention. 
     The present exemplary embodiment includes laser light source  101 , dichroic mirror  102 , light tunnel  103 , phosphor wheel  104 , and reflecting prism  105 . 
     Laser light source  101  generates laser light that provides excitation light having a wavelength λ 1 . The laser light generated by laser light source  101  is incident on phosphor wheel  104  through dichroic mirror  102  and light tunnel  103 . Phosphor wheel  104  includes a plurality of fluorescent light generation regions that generate light of respectively different wavelengths by means of laser light generated by laser light source  101 . 
       FIG. 2(   a ) is a plan view of phosphor wheel  104  as viewed from the plane of incidence of laser light generated by laser light source  101  (from the left side towards the right side in  FIG. 1) . 
     The circular phosphor wheel  104  includes blue fluorescent light region  401 , green fluorescent light region  402 , and red fluorescent light region  403 . The respective regions are defined by the angle from the center. When laser light generated by laser light source  101  is incident on blue fluorescent light region  401 , green fluorescent light region  402 , or red fluorescent light region  403 , blue fluorescent light, green fluorescent light, or red fluorescent light having wavelengths λ 2 , λ 3 , λ 4  (λ 2 &lt;λ 3 &lt;λ 4 ) that are longer than wavelength λ 1  are generated, respectively. 
       FIGS. 2(   b ), ( c ), and ( d ) are sectional views that illustrate the structure of blue fluorescent light region  401 , green fluorescent light region  402 , and red fluorescent light region  403 . 
     In blue fluorescent light region  401  shown in  FIG. 2(   b ), reflective layer  405  that reflects light having wavelengths λ 2  to λ 4  and blue phosphor layer  406  are formed in a layered manner on substrate  404 . When excitation laser light having wavelength λ 1  is incident on blue phosphor layer  406 , blue phosphor layer  406  generates blue fluorescent light of wavelength λ 2 . 
     In green fluorescent light region  402  shown in  FIG. 2(   c ), green phosphor layer  407  is formed on reflective layer  405 . When excitation laser light having wavelength λ 1  is incident on green phosphor layer  407 , green phosphor layer  407  generates green fluorescent light having wavelength λ 3 . 
     In green fluorescent light region  403  shown in  FIG. 2(   d ), red phosphor layer  408  is formed on reflective layer  405 . When excitation laser light having wavelength λ 1  is incident on red phosphor layer  408 , red phosphor layer  408  generates red fluorescent light having wavelength λ 4 . 
     The phosphor wheel having the above described structure rotates around the center thereof, and the incidence position of laser light generated by laser light source  101  is in the vicinity of the outer circumferential portion thereof. Therefore, blue fluorescent light, green fluorescent light, and red fluorescent light are generated in sequence in a state in which laser light generated by laser light source  101  is incident thereon, and the generated light is reflected by reflective layer  405  and is incident again on light tunnel  103 . 
     According to the present exemplary embodiment, lights having four wavelengths, λ 1  to λ 4 , are used, and the relationship between the wavelengths is λ 1 &lt;λ 2 &lt;λ 3 &lt;λ 4 . Dichroic mirror  102  reflects light of λ 2 , λ 3 , and λ 4 , and allows light of λ 1  to pass therethrough. Light tunnel  103  is formed in a tapered shape in which the sizes of two end faces that serve as light incidence/projection surfaces are different. Because light tunnel  103  has a tapered shape, the angle distribution of fluorescent light generated at each phosphor and diffused can be changed and homogenized. In this case, the term “light tunnel” includes a light tunnel in which a hollow internal surface is formed with a mirror, and a light tunnel which comprises a solid, transparent polygonal column and utilizes total reflection. The latter is also referred to as a “rod lens”. 
     According to the present exemplary embodiment, laser light generated by laser light source  101  passes through dichroic mirror  102  and is incident on phosphor wheel  104  through light tunnel  103 . Blue fluorescent light, green fluorescent light, and red fluorescent light that are sequentially generated at phosphor wheel  104  are incident again on light tunnel  103 . The lights are reflected at dichroic mirror  102  and reflecting prism  105  and are emitted as illumination light. 
     As described above, according to the illumination optical system of the present exemplary embodiment, uniformized red fluorescent light, green fluorescent light, and blue fluorescent light appear in sequence and are used as illumination light. 
       FIG. 3  is a block diagram illustrating the configuration of another exemplary embodiment of the illumination optical system according to the present invention. 
     According to the exemplary embodiment illustrated in  FIG. 1 , fluorescent light of three colors is generated by a single excitation light source by using a phosphor wheel that includes three fluorescent light regions. In contrast, the present exemplary embodiment is provided with individual excitation light sources for the respective phosphors of each color. 
     The present exemplary embodiment comprises laser light sources  201 ,  205 ,  209 , dichroic mirrors  202 ,  206 ,  210 , light tunnels  203 ,  207 ,  211 , blue phosphor  204 , green phosphor  208 , red phosphor  212 , and cross dichroic prism  212 . 
     Laser light sources  201 ,  205 , and  209  generate laser light that is used as excitation light of wavelength λ 1 . Blue phosphor  204 , green phosphor  208 , and red phosphor  212  generate blue fluorescent light, green fluorescent light, and red fluorescent light having wavelengths λ 2 , λ 3 , and λ 4  (λ 2 &lt;λ 3 &lt;λ 4 ) that are longer than wavelength λ 1 , respectively, when laser light generated by laser light source  201  is incident thereon. 
     The structure of blue phosphor  204 , green phosphor  208 , and red phosphor  212  is the same as the structure of blue fluorescent light region  401 , green fluorescent light region  402 , and red fluorescent light region  403  shown in  FIG. 2(   b ), ( c ), and ( d ), in which blue phosphor, green phosphor, and red phosphor are formed on a reflective layer formed on a substrate, respectively. 
     Dichroic mirror  202  allows light of wavelength λ 1  to pass therethrough and reflects light of λ 2 . Dichroic mirror  206  reflects light of wavelength λ 1 , and allows light of λ 3  to pass therethrough. Dichroic mirror  210  reflects light of wavelength λ 1 , and allows light of λ 4  to pass therethrough. 
     Light tunnels  203 ,  207 , and  211  are formed in a tapered shape in which sizes of the two ends are different, similarly to light tunnel  103  shown in  FIG. 1 , and thus the angle distribution of fluorescent light that is generated at each phosphor and that is diffused can be changed and homogenized. The term “light tunnel” includes a light tunnel in which a hollow internal surface is formed with a mirror, and a light tunnel which comprises a solid, transparent polygonal column and which utilizes total internal reflection. 
     Laser light generated by laser light source  201  is incident on blue phosphor  204  through dichroic mirror  202  and light tunnel  203 . Blue fluorescent light that is generated at blue phosphor  204  passes through light tunnel  203 , is reflected by dichroic mirror  202 , and is incident on cross dichroic prism  213 . 
     Laser light generated by laser light source  205  is reflected by dichroic mirror  206  and is incident on green phosphor  208  through light tunnel  207 . Green fluorescent light generated at green phosphor  208  passes through light tunnel  207  and dichroic mirror  202  and is incident on cross dichroic prism  213 . 
     Laser light generated by laser light source  209  is reflected by dichroic mirror  210  and is incident on red phosphor  221  through light tunnel  211 . Red fluorescent light generated at red phosphor  212  is incident on cross dichroic prism  213  through light tunnel  211  and dichroic mirror  210 . 
     Cross dichroic prism  213  allows light having wavelength λ 2  to pass and reflects light having wavelength λ 3  and λ 4 . Consequently, the respective fluorescent lights that are generated at the respective phosphors are projected in the same direction. 
     According to the present exemplary embodiment arranged as described above, since units that generate fluorescent light are provided for each color, a plurality of fluorescent lights can be generated simultaneously. Further, by driving laser light sources  201 ,  205 , and  209  in sequence, each fluorescent light can also be output sequentially, similarly to the illumination optical system illustrated in  FIG. 1 . 
     The present exemplary embodiment provides a unit that generates fluorescent light with respect to each color. Two kinds of units are provided: a unit for blue light in which laser light of laser light source  201  passes through dichroic mirror  202  and is incident on light tunnel  203 , and units for green light and red light in which laser lights of laser light sources  205  and  209  are reflected by dichroic mirrors  206  and  210  and are incident on light tunnels  207  and  211 . These may be mixed, or naturally only one of these may be used. Since the same fluorescent lights can be obtained with different optical systems, the design freedom can be improved. 
       FIG. 4(   a ) is a block diagram that illustrates the arrangement of another exemplary embodiment of an illumination optical system according to the present invention. 
     Relative to the exemplary embodiment shown in  FIG. 3 , the present exemplary embodiment is a modification example of a unit in which, among units provided for each color, excitation light generated by a laser light source is reflected by a dichroic mirror and is incident on a light tunnel, and which increases the light output. 
     The present exemplary embodiment includes laser light sources  301  and  302 , phosphor  303 , light tunnel  304 , and dichroic mirror  305 . Laser light sources  301  and  302  generate laser light having the same wavelength as excitation light. 
       FIG. 4(   b ) is a sectional view that illustrates the structure of phosphor  303 . As shown in the figure, reflective layer  307  and phosphor layer  308  are formed in a layered manner on substrate  306 . By means of the laser light of laser light sources  301  and  302 , phosphor layer  308  generates fluorescent light having a longer wavelength that that of the laser light in question. Reflective layer  307  allows laser light generated by laser light sources  301  and  302  to pass therethrough, and reflects fluorescent light generated by phosphor layer  308 . 
     Dichroic mirror  305  reflects laser light generated by laser light sources  301  and  302 , and allows fluorescent light generated by phosphor layer  308  to pass therethrough. 
     Laser light generated by laser light source  301  passes through reflective layer  307  and is incident on phosphor layer  308 . Laser light generated by laser light source  302  is reflected by dichroic mirror  305  and is incident on phosphor layer  308 . At phosphor layer  308 , fluorescent light is generated by means of laser light from laser light sources  301  and  302  that is incident thereon. The fluorescent light generated at phosphor layer  308  is output to the outside through light tunnel  304  and dichroic mirror  305 , and is utilized as illumination light. Although laser light that does not contribute to the generation of fluorescent light also exists among the laser light of laser light sources  301  and  302 , since the laser light in question is reflected by dichroic mirror  305 , the laser light is not projected to the outside. 
     Phosphor  303  of the present exemplary embodiment may also comprise an illumination optical system that outputs each color in sequence as the phosphor wheel illustrated in  FIG. 1 . Further, the unit of the present exemplary embodiment may also comprise the illumination optical system illustrated in  FIG. 3  as three units that generate respectively different fluorescent lights. 
       FIG. 5  is a block diagram illustrating the circuit configuration of a projector that uses the illumination optical system of the exemplary embodiment shown in  FIG. 1 . 
     The projector shown in  FIG. 5  includes user interface  501 , controller  502 , storage portion  503 , video signal processor  504 , synchronization signal processor  505 , LD driver  506 , phosphor wheel driver  508 , display element driver  509 , rotational state detector  510 , and display element  511 , as well as laser light source  101  and phosphor wheel  104  shown in  FIG. 1 . 
     User interface  501  accepts instructions input from a user, and outputs the instructions to controller  502 . User interface  501  also displays the current operating state of the projector on a display apparatus (not shown) such as an indicator or a display panel. 
     Controller  502  controls each component comprising the projector in accordance with a program stored in storage portion  503 . 
     Storage portion  503  stores a control program of controller  503 , or temporarily stores video data. 
     Video signal processor  504  converts a video signal input from the outside into a video signal to be used inside the projector. Since video signals of the present exemplary embodiment are formed by illumination lights of respective colors being output sequentially by an illumination optical system as described above, video signals according to each color are generated sequentially. 
     Synchronization signal processor  505  converts synchronization signals that are synchronized with video signals input from the outside into video signals to be used inside the projector. More specifically, synchronization signal processor  505  generates and outputs synchronization signals that show the output timing of video signals of each color. 
     LD driver  506  controls a lighting state of laser light source  101  according to synchronization signals output from synchronization signal processor  505 . 
     Rotational state detector  510  detects a rotational state of phosphor wheel  104 , and outputs the detected result to phosphor wheel driver  508 . 
     Phosphor wheel driver  508  controls the rotational state of phosphor wheel  104  so that a color of a video signal indicated by a synchronization signal output by synchronization signal processor  505  and a color output by the illumination optical system that indicates a rotational state of phosphor wheel  104  detected by rotational state detector  510  match. 
     Display device driver  509  drives display element  511  in accordance with video signals output by the video signal processor. In this case, a display element that sequentially displays images of each color such as a reflective image forming element in which a plurality of micromirrors are arranged in a matrix and which forms an image according to a reflection state of each micromirror, or a transmission-type liquid crystal display element or a reflective liquid crystal display element is used as the display element  511 . 
     According to the projector configured as described above, display element  511  that displays images corresponding to each color by means of illumination light of each color sequentially output from the illumination optical system is illuminated, and reflection images or transmission images of display element  511  are sequentially projected through a projection optical system (unshown). 
       FIG. 6  is a block diagram illustrating the circuit configuration of a projector that uses the illumination optical system of the exemplary embodiment shown in  FIG. 3 . 
     The projector shown in  FIG. 6  includes user interface  501 , controller  502 , storage portion  503 , video signal processor  504 , synchronization signal processor  505 , LD driver  506 ′, display element driver  509 ′, and display element  511 , as well as laser light sources  201 ,  205 , and  209  shown in  FIG. 1 . 
     Since the configuration and operation of user interface  501 , controller  502 , storage portion  503 , video signal processor  504 , and synchronization signal processor  505  are the same as in the exemplary embodiment illustrated in  FIG. 5 , these components are assigned the same reference numbers as in  FIG. 5 , and a description thereof is omitted below. 
     LD driver  506 ′ controls a lighting state of laser light sources  201 ,  205 , and  209  in accordance with synchronization signals output by synchronization signal processor  505 . 
     Display element driver  509 ′ drives display element  511 ′ in accordance with video signals output by the video signal processor. In this case, similarly to the display element  511  shown in  FIG. 5 , a display element that sequentially displays images of each color such as a reflective image forming element in which a plurality of micromirrors are arranged in a matrix and which forms an image according to the reflection state of each micromirror, or a transmission-type liquid crystal display element or a reflective liquid crystal display element is used as the display element. Hence, LD driver  506 ′ turns on laser light sources  201 ,  205 , and  209  in accordance with image colors displayed by display element  511 ′. 
     In this connection, some transmission-type liquid crystal display elements and reflective liquid crystal display elements display color images. When a display element that performs a color display is used as display element  511 ′, LD driver  506 ′ turns on laser light sources  201 ,  205 , and  209  at the same time. 
     According to the projector configured as described above, display element  511 ′ that displays images corresponding to each color by means of illumination light of each color sequentially output from the illumination optical system is illuminated, and reflection images or transmission images of display element  511 ′ are sequentially projected through a projection optical system (not shown). 
     DESCRIPTION OF SYMBOLS 
     
         
           101  Laser light source 
           102  Dichroic mirror 
           103  Light tunnel 
           104  Phosphor wheel 
           105  Reflecting prism