Patent Publication Number: US-2011051092-A1

Title: Optical element, optical unit, and projection display apparatus for switching polarization direction

Description:
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2009-195470, filed Aug. 26, 2009, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical element, an optical unit, and a projection display apparatus for switching the polarization direction. 
     2. Description of the Related Art 
     There have been developed projection display apparatuses (hereinafter, also referred to as projectors, as needed) having both the function to project normal images (hereinafter, referred to as two-dimensional images) on a projection surface and the function to project parallax images on a projection surface so as to provide stereoscopic images (hereinafter, referred to as three-dimensional images) to a viewer. A polarized glasses method is one of methods for showing three-dimensional images to viewers. Polarized glasses provide filtering on incident light so that polarization components perpendicular to each other (such as P-polarized light and S-polarized light) or circularly polarized light components of which the directions of rotation are opposite to each other (such as right-handed circularly polarized light and left-handed circularly polarized light) enter the right eye and left eye of a viewer, respectively. 
     There have been proposed various methods for projector systems employing such a polarized glasses method. For example, there is proposed a method (first method) in which two projectors, a first projector for projecting images for the right eye and a second projector for projecting images for the left eye, are prepared, and a polarizing plate for transmitting a polarization component for the right eye is placed posterior to the projection lens of the first projector (more specifically, in the path of light emitted from the projection lens), and a polarizing plate for transmitting a polarization component for the left eye is placed posterior to the projection lens of the second projector. 
     There is also proposed a method (second method) in which one projector, which projects images for the right eye and left eye by switching the images in a time-division manner, is used and a polarization switcher is placed posterior to the projection lens of the projector. The polarization switcher includes a polarizing plate and a liquid crystal element for switching the polarization direction according to a voltage applied to the element. 
     Further, there is proposed a method (third method) for switching the color component or polarization direction in a time-division manner by rotating a color wheel onto which polarizing elements are attached. 
     The first method described above requires two projectors. Also, in the first and second methods above, although an existing projector for two-dimensional images can be easily diverted to a projector for three-dimensional images, since a polarizing plate or polarization switcher is provided posterior to the projection lens, a larger size of the polarizing plate or polarization switcher is required. 
     In the third method described above, polarizing elements can be miniaturized. However, in a projector according to the third method, since non-polarized light emitted from a light source passes through the polarizing elements attached onto the color wheel even when a two-dimensional image is projected, the amount of light for projecting a two-dimensional image is reduced. More specifically, the light amount is reduced to half the intrinsic amount of light for projecting a two-dimensional image. The light amount will be reduced in the same way also in the first and second methods if the polarizing plate or polarization switcher posterior to the projection lens is not removed, but such reduction of light amount can be prevented by removing the polarizing plate or polarization switcher. 
     There can be considered a method in which polarizing elements are not integrated with the color wheel as described in the third method but are independently provided within the projector. In a projector according to such a method, however, the polarizing elements need be removed from the light path in order to prevent the reduction of the amount of light for projecting a two-dimensional image, and a mechanism therefor is also necessary. 
     SUMMARY OF THE INVENTION 
     An optical unit of one embodiment of the present invention comprises: a first polarization separation element and a second polarization separation element which each are configured to reflect one of two polarization components perpendicular to each other included in incident light and transmit the other polarization component; and a switching element placed between the first polarization separation element and the second polarization separation element and configured to be switchable between a first state of transmitting an incident polarization component as it is and a second state of converting an incident polarization component to the other polarization component perpendicular thereto and transmitting the converted component. 
     Another embodiment of the present invention is a projection display apparatus. The apparatus comprises: the optical unit state above; and a control unit configured to switch between a two-dimensional mode for displaying a two-dimensional image and a three-dimensional mode for displaying a three-dimensional image. The control unit provides control such that the switching element is placed in the first state or the second state in accordance with switching between the two-dimensional mode and the three-dimensional mode. 
     Yet another embodiment of the present invention is an optical element. The optical element is of wire grid type and includes metal wires and half-wave plates inserted between the metal wires. 
     Still yet another embodiment of the present invention is an optical unit. The optical unit comprises the optical element state above, a quarter-wave plate provided posterior to the optical element, and a reflecting plate provided posterior to the quarter-wave plate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments will now be described, by way of example only, with reference to the accompanying drawings which are meant to be exemplary, not limiting, and wherein like elements are numbered alike in several Figures, in which: 
         FIG. 1  is a schematic diagram that shows a configuration of a projection display apparatus according to an embodiment 1 of the present invention; 
         FIG. 2  are diagrams that show a configuration of a reflecting optical unit according to an embodiment 2, in which  FIG. 2A  shows a state in a three-dimensional mode while  FIG. 2B  shows a state in a two-dimensional mode; 
         FIG. 3  is a schematic diagram that shows a configuration of a projection display apparatus according to a modification  1  of the embodiment 1 of the present invention; 
         FIG. 4  are diagrams that show a configuration of a reflecting optical unit according to a modification  1  of the embodiment 2, in which  FIG. 4A  shows a state  1  in the three-dimensional mode while  FIG. 4B  shows a state  2  in the three-dimensional mode; 
         FIG. 5  are diagrams that show a configuration of a reflecting optical unit according to a modification  2  of the embodiment 2, in which  FIG. 5A  shows a state  1  in the three-dimensional mode while  FIG. 5B  shows a state  2  in the three-dimensional mode; 
         FIG. 6  are diagrams that show a configuration of a reflecting optical unit according to a modification  3  of the embodiment 2, in which  FIG. 6A  shows a state  1  in the three-dimensional mode while  FIG. 6B  shows a state  2  in the three-dimensional mode; 
         FIG. 7  is a diagram that shows a configuration of a reflecting optical unit according to a modification  4  of the embodiment 2; 
         FIG. 8  are diagrams that show configurations of reflecting optical units according to a modification  5  of the embodiment 2, in which  FIG. 8A  shows an example in which the second polarization separation element  73  is configured to be a convex while  FIG. 8B  shows the configuration as shown in  FIG. 2 ; 
         FIG. 9  is a diagram that shows a configuration of an optical element according to an embodiment 3; 
         FIG. 10  is a diagram that shows a configuration of an optical unit according to an embodiment 4; and 
         FIG. 11  is a diagram that shows a configuration of an optical unit according to an embodiment 5. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will now be described by reference to the preferred embodiments. This does not intend to limit the scope of the present invention, but to exemplify the invention. 
       FIG. 1  is a schematic diagram that shows a configuration of a projection display apparatus  100  according to an embodiment 1 of the present invention. The projection display apparatus  100  is capable of projecting a two-dimensional image on a projection surface, such as a screen, to display the image (hereinafter, such usage is referred to as a two-dimensional mode) and also capable of projecting a parallax image on a projection surface to display a three-dimensional image (or to allow a viewer to sense a three-dimensional image, more strictly) (hereinafter, such usage is referred to as a three-dimensional mode). 
     A light source  1  provides light to an optical unit  50  under the control of a lump driving unit  12 . For the light source  1 , a halogen lamp having the electrode configuration with a filament, a lamp having the electrode configuration of generating arc discharge, such as a metal halide lamp, xenon short arc lamp, and high-pressure mercury lamp, or an LED lamp may be used. Light emitted from a luminous tube  1   a  provided in the center part of the light source  1  is collected by a reflector  1   b  having an elliptical surface or a hyperboloid, so as to be provided to the optical unit  50 . 
     The optical unit  50  includes a color wheel  3 , a rod integrator  5 , and condenser lenses  6   a  and  6   b . Light emitted from the light source  1  passes through the color wheel  3 , rod integrator  5 , and condenser lenses  6   a  and  6   b  in this order. The color wheel  3  is discoid in shape and provided perpendicular to the optical axis of light emitted from the light source  1 . The color wheel  3  rotates about a rotation axis parallel to the optical axis under the control of a wheel driving unit  11 . 
     More specifically, the color wheel  3  has a surface facing incident light, on which are formed an R region for transmitting the red component of incident light, a G region for transmitting the green component of incident light, and a B region for transmitting the blue component of incident light. As it rotates, the color wheel  3  sequentially transmits red light, green light, and blue light in a time-division manner. There may be further formed on the surface facing incident light a W region for transmitting all the color components of incident light in addition to the R, G, and B regions. Also, there may be formed a Cy region, a Ye region, and an Mg region for transmitting complimentary colors of cyan, yellow, and magenta, for example. 
     The rod integrator  5  is provided on the optical axis mentioned above posterior to the color wheel  3 . The rod integrator  5  equalizes the illuminance of light incident from the entrance face  5   a  and allows the light to outgo from the exit face  5   b.    
     The condenser lenses  6   a  and  6   b  are arranged on the aforementioned optical axis so as to face the exit face  5   b  of the rod integrator  5 . The condenser lenses  6   a  and  6   b  collect light from the exit face  5   b  of the rod integrator  5  and provide the light to a reflecting optical unit  7 . 
     The reflecting optical unit  7  is arranged on the aforementioned optical axis so as to have a certain inclination relative to the optical axis. The reflecting optical unit  7  reflects light from the condenser lenses  6   a  and  6   b  to provide the light to a imager  8 . The reflecting optical unit  7  has a function to switch the mode between the two-dimensional mode and three-dimensional mode according to an instruction from a control unit  10 . The reflecting optical unit  7  will be detailed later. 
     The imager  8  modulates light reflected by the reflecting optical unit  7  according to an image signal specified by the control unit  10  and provides the light thus modulated to a time-division polarization switching element  13 . There is shown an example in which a DMD (Digital Micromirror Device) is used. A DMD includes multiple micromirrors relative to the number of pixels and generates a desired image by having the direction of each micromirror controlled according to each pixel signal. 
     The time-division polarization switching element  13  is placed between the imager  8  and a projection lens  9 . In the present embodiment, the time-division polarization switching element  13  is provided directly on the entrance of the projection lens  9  or with a certain space in between. In the three-dimensional mode, the time-division polarization switching element  13  switches the polarization direction of light modulated by the imager  8  between two polarization directions perpendicular to each other in a time-division manner, according to an instruction from the control unit  10 . The time-division polarization switching element  13  will be detailed later. 
     The projection lens  9  projects light modulated by the imager  8  onto a screen or another projection surface, which is not illustrated. Since the projection lens  9  forms an image from the light modulated by the imager  8 , the image generated by the imager  8  is enlarged and displayed on the projection surface. In the three-dimensional mode, the polarization direction of light incident on the projection lens  9  is switched by the time-division polarization switching element  13  in a time-division manner. 
     The control unit  10  controls the lump driving unit  12 , wheel driving unit  11 , reflecting optical unit  7 , imager  8 , and time-division polarization switching element  13 . More specifically, the control unit  10  provides an on/off signal to the lump driving unit  12  so as to allow the lump driving unit  12  to turn on or off the power of the light source  1 . The control unit  10  also provides a rotation control signal to the wheel driving unit  11  so as to allow the wheel driving unit  11  to rotate the color wheel  3 . Further, the control unit  10  provides an image signal to the imager  8  so as to allow the imager  8  to generate a desirable image. 
     The control unit  10  also provides a mode switching signal to the reflecting optical unit  7  so as to allow the reflecting optical unit  7  to switch the mode between the two-dimensional mode and three-dimensional mode. This mode switching process will be detailed later. The control unit  10  provides a polarization switching signal to the time-division polarization switching element  13  in the three-dimensional mode to allow the time-division polarization switching element  13  to switch the polarization direction in a time-division manner. In the two-dimensional mode, since the polarization direction need not be fixed, there is no need to allow the time-division polarization switching element  13  to switch the polarization direction in a time-division manner. 
       FIG. 2  are diagrams that show a configuration of a reflecting optical unit according to the embodiment 2:  FIG. 2A  shows a state in the three-dimensional mode while  FIG. 2B  shows a state in the two-dimensional mode. The reflecting optical unit according to the embodiment 2 is an optical element suitable for the reflecting optical unit  7  in the projection display apparatus  100  according to the embodiment 1. In the following, description is given on the premise that the reflecting optical unit  7  according to the embodiment 2 is provided in the projection display apparatus  100  according to the embodiment 1. 
     The reflecting optical unit  7  according to the embodiment 2 comprises a first polarization separation element  71 , a switching element  72 , and a second polarization separation element  73 . Each of the first polarization separation element  71  and second polarization separation element  73  reflects one of two polarization components perpendicular to each other (such as P-polarized light and S-polarized light) included in incident light and transmits the other polarization component. The switching element  72  is placed between the first polarization separation element  71  and second polarization separation element  73  and is switchable between a first state of transmitting an incident polarization component as it is and a second state of converting an incident polarization component to the other polarization component perpendicular thereto and transmitting the converted component. 
     In the reflecting optical unit  7 , the second polarization separation element  73 , switching element  72 , and first polarization separation element  71  may be laminated in this order and integrally formed. The second polarization separation element  73  and switching element  72  may be in contact with each other, or there may be provided a space in between. The relationship between the switching element  72  and first polarization separation element  71  is also the same. 
     The first polarization separation element  71  is a layer for receiving non-polarized incident light. The first polarization separation element  71  reflects a first polarized light, which is either S-polarized light perpendicular to the plane of incidence or P-polarized light parallel with the plane of incidence included in non-polarized incident light, and transmits a second polarized light, which is the other polarized light. 
     For the first polarization separation element  71 , a wire grid can be used. A wire grid is a non-absorbing polarizing plate prepared by evaporating metal material (such as aluminum) onto a glass substrate and forming wire-like grid by fine etching at the nanometric level. Generally, a wire grid reflects S-polarized light and transmits P-polarized light in incident light. By changing the direction of ribs forming a grid, another type of wire grid, which reflects P-polarized light and transmits S-polarized light in incident light, can be also obtained. Besides a wire grid, a dielectric multilayer film with a polarization separation coating applied thereto may be also used for the first polarization separation element  71 . 
     In the following, there will be described an example using the first polarization separation element  71  that reflects S-polarized light and transmits P-polarized light in incident light. With regard to the case of using the first polarization separation element  71  that reflects P-polarized light and transmits S-polarized light in incident light, the following description is also applicable by interchanging P-polarized light and S-polarized light therein. 
     The switching element  72  is switchable between a first state of transmitting P-polarized light provided from the first polarization separation element  71  as it is and a second state of converting P-polarized light provided from the first polarization separation element  71  to S-polarized light and transmitting the converted light. 
     For the switching element  72 , a liquid crystal element that changes its state according to whether or not a voltage is applied thereto can be used. The liquid crystal element changes the orientation of liquid crystal molecules according to the voltage. When no voltage is applied, the liquid crystal element behaves as a half-wave plate (also called a λ/2 phase plate) (this state corresponds to the second state set forth above). A half-wave plate gives the phase difference of 180 degrees to perpendicular components of incident light, thereby rotating linearly polarized light. The rotation angle is adjustable with the angle between the incident polarized light and the slow axis, and, when linearly polarized light makes a 45-degree angle with the slow axis, the plane of the linearly polarized light rotates by 90 degrees. Namely, S-polarized light can be converted to P-polarized light, and vice versa. 
     When voltage is applied, the liquid crystal element behaves as a simple transmission plate that gives no phase difference to perpendicular components of incident light (this state corresponds to the first state set forth above). Accordingly, incident light passes through the plate as it is, i.e., when S-polarized light comes in, the S-polarized light passes through the plate as it is, and, when P-polarized light comes in, the P-polarized light passes through the plate as it is. 
     The second polarization separation element  73  reflects S-polarized light and transmits P-polarized light provided from the switching element  72 . The second polarization separation element  73  can be configured in the same way as the first polarization separation element  71 . 
     The control unit  10  applies a voltage to the switching element  72  in the three-dimensional mode so as to provide control such that the switching element  72  is placed in the first state (see  FIG. 2A ), while the control unit  10  applies no voltage to the switching element  72  in the two-dimensional mode so as to provide control such that the switching element  72  is placed in the second state (see  FIG. 2B ). 
     As shown in  FIG. 2A , the switching element  72  behaves as a simple transmission plate in the three-dimensional mode, so that the switching element  72  transmits P-polarized light provided from the first polarization separation element  71  as it is. The second polarization separation element  73  then also transmits the P-polarized light provided from the switching element  72 . Thus, in the three-dimensional mode, the whole reflecting optical unit  7  functions as a polarization separation element that reflects S-polarized light and transmits P-polarized light. 
     On the other hand, as shown in  FIG. 2B , the switching element  72  behaves as a half-wave plate in the two-dimensional mode, so that the switching element  72  convert P-polarized light provided from the first polarization separation element  71  to S-polarized light and transmits the converted light. The second polarization separation element  73  then reflects the S-polarized light provided from the switching element  72 . The switching element  72  then converts the S-polarized light provided from the second polarization separation element  73  to P-polarized light and transmits the converted light. Thereafter, the first polarization separation element  71  transmits the P-polarized light provided from the switching element  72 . Thus, in the two-dimensional mode, the whole reflecting optical unit  7  functions as a mirror that reflects both S-polarized light and P-polarized light. 
     For the time-division polarization switching element  13  shown in  FIG. 1 , the aforementioned liquid crystal element can be used similarly to the switching element  72 . In the three-dimensional mode, the control unit  10  switches, in a time-division manner, between the state in which a voltage is applied to the time-division polarization switching element  13  and the state in which no voltage is applied to the element  13 . In the two-dimensional mode, since the polarization direction need not be fixed, such switching in a time-division manner is not required and the state is fixed to either state. In terms of power consumption, it is preferable to fix the state to the no-voltage applied state. 
     The time-division polarization switching element  13  behaves as a half-wave plate when no voltage is applied thereto, so that the element  13  converts S-polarized light provided from the reflecting optical unit  7  via the imager  8  to P-polarized light and transmits the converted light. On the other hand, the time-division polarization switching element  13  behaves as a simple transmission plate when a voltage is applied thereto, so that the element  13  transmits S-polarized light provided from the reflecting optical unit  7  via the imager  8  as it is. Accordingly, parallax image light to be provided to each of the right eye and left eye of a viewer wearing polarized glasses can be provided as an appropriate polarization component. 
     The time-division polarization switching element  13  may include a quarter-wave plate (also called a λ/4 phase plate) in addition to the aforementioned liquid crystal element. A quarter-wave plate gives the phase difference of 90 degrees to perpendicular components of incident light, thereby converting linearly polarized light to circularly polarized light, and vice versa. When linearly polarized light makes a 45-degree angle with the slow axis, the linearly polarized light is converted to circularly polarized light. For example, S-polarized light is converted to right-handed circularly polarized light, while P-polarized light is converted to left-handed circularly polarized light. Conversely, S-polarized light may be converted to left-handed circularly polarized light, while P-polarized light is converted to right-handed circularly polarized light. 
     In this way, by adding a quarter-wave plate to the time-division polarization switching element  13 , the element  13  can provide right-handed circularly polarized light and left-handed circularly polarized light in a time-division manner, thereby also handling the case of circularly polarized glasses. In comparison with linearly polarized glasses, circularly polarized glasses have a feature of keeping crosstalk between left and right images low even when the viewer wearing the glasses faces another direction. 
     The liquid crystal element and the quarter-wave plate may be placed in this order or may be placed in the reverse order. 
     Thus, according to the embodiments 1 and 2, the reflecting optical unit  7  is configured to have the switching element  72 , which is capable of behaving as both a half-wave plate and a simple transmission plate, between the first polarization separation element  71  and second polarization separation element  73 , thereby switching between non-polarization state and polarization state with such a simple configuration while preventing the reduction of light amount. 
     More specifically, it is only necessary to place, within an existing projection display apparatus  100 , the reflecting optical unit  7  according to the embodiment 2 instead of a reflecting mirror at the position where the reflecting mirror has been placed and add the time-division polarization switching element  13 . The configurations of the components to be added and provided as a replacement are simple because these are all small parts and electrically controlled. Since all polarization components can be reflected in the two-dimensional mode, an almost normal amount of light can be maintained also in the two-dimensional mode. If the reflecting optical unit  7  is configured only with the first polarization separation element  71 , the light amount will be reduced by half because P-polarized light is not reflected. 
       FIG. 3  is a schematic diagram that shows a configuration of the projection display apparatus  100  according to a modification  1  of the embodiment 1 of the present invention. In comparison with the projection display apparatus  100  shown in  FIG. 1 , in the projection display apparatus  100  according to the modification  1 , the time-division polarization switching element  13  is provided as a constituting element of the reflecting optical unit  7  and integrally configured therein, instead of being placed at the entrance of the projection lens  9 . Since the operation of each constituting element of the projection display apparatus  100  according to the modification  1  is identical with that of the projection display apparatus  100  shown in  FIG. 1 , the description thereof is omitted here. 
       FIG. 4  are diagrams that show a configuration of a reflecting optical unit according to a modification  1  of the embodiment 2:  FIG. 4A  shows a state  1  in the three-dimensional mode while  FIG. 4B  shows a state  2  in the three-dimensional mode. The reflecting optical unit according to the modification  1  of the embodiment 2 is an optical element suitable for the reflecting optical unit  7  in the projection display apparatus  100  according to the modification  1  of the embodiment 1. In the following, description is given on the premise that the reflecting optical unit  7  according to the modification  1  of the embodiment 2 is provided in the projection display apparatus  100  according to the modification  1  of the embodiment 1. The same premise will be given also in the case of the reflecting optical units  7  according to modifications  2  and  3  of the embodiment 2 described later. 
     The reflecting optical unit  7  according to the modification  1  of the embodiment 2 comprises the first polarization separation element  71 , switching element  72  (given as a mode switching element  72  in the modifications  1 - 3  of the embodiment 2), second polarization separation element  73 , and a time-division polarization switching element  74 . In the reflecting optical unit  7 , the second polarization separation element  73 , mode switching element  72 , first polarization separation element  71 , and time-division polarization switching element  74  are laminated in this order and integrally formed. 
     As with the time-division polarization switching element  13  discussed previously, the time-division polarization switching element  74  behaves as a simple transmission plate when a voltage is applied thereto (see  FIG. 4A ), so that the element  74  transmits S-polarized light provided from the first polarization separation element  71  as it is. On the other hand, the time-division polarization switching element  74  behaves as a half-wave plate when no voltage is applied thereto (see  FIG. 4B ), so that the element  74  converts S-polarized light provided from the first polarization separation element  71  to P-polarized light and transmits the converted light. 
     Since it does not act on non-polarized light, the time-division polarization switching element  74  has no influence on light provided from the condenser lenses  6   a  and  6   b  or light provided from the first polarization separation element  71  in the two-dimensional mode. It is because such light is not polarized. 
       FIG. 5  are diagrams that show a configuration of a reflecting optical unit according to a modification  2  of the embodiment 2:  FIG. 5A  shows a state  1  in the three-dimensional mode while  FIG. 5B  shows a state  2  in the three-dimensional mode. 
     The reflecting optical unit  7  according to the modification  2  of the embodiment 2 comprises the first polarization separation element  71 , mode switching element  72 , second polarization separation element  73 , time-division polarization switching element  74 , and a quarter-wave plate  75 . In the reflecting optical unit  7 , the second polarization separation element  73 , mode switching element  72 , first polarization separation element  71 , quarter-wave plate  75 , and time-division polarization switching element  74  are laminated in this order and integrally formed. Namely, the reflecting optical unit  7  according to the modification  2  has a configuration in which the quarter-wave plate  75  is added between the first polarization separation element  71  and time-division polarization switching element  74  in the reflecting optical unit  7  according to the modification  1  described previously. 
     In the three-dimensional mode, the quarter-wave plate  75  converts S-polarized light reflected by the first polarization separation element  71  to right-handed circularly polarized light. Since it does not act on non-polarized light, the quarter-wave plate  75  does not act in the two-dimensional mode. 
     As with the time-division polarization switching element  13  discussed previously, the time-division polarization switching element  74  behaves as a simple transmission plate when a voltage is applied thereto (see  FIG. 5A ), so that the element  74  transmits the right-handed circularly polarized light provided from the quarter-wave plate  75  as it is. On the other hand, the time-division polarization switching element  74  behaves as a half-wave plate when no voltage is applied thereto (see  FIG. 5B ), so that the element  74  converts the right-handed circularly polarized light provided from the quarter-wave plate  75  to left-handed circularly polarized light and transmits the converted light. 
       FIG. 6  are diagrams that show a configuration of a reflecting optical unit according to a modification  3  of the embodiment 2:  FIG. 6A  shows a state  1  in the three-dimensional mode while  FIG. 6B  shows a state  2  in the three-dimensional mode. 
     The reflecting optical unit  7  according to the modification  3  of the embodiment 2 comprises the first polarization separation element  71 , mode switching element  72 , second polarization separation element  73 , time-division polarization switching element  74 , and quarter-wave plate  75 . In the reflecting optical unit  7 , the second polarization separation element  73 , mode switching element  72 , first polarization separation element  71 , time-division polarization switching element  74 , and quarter-wave plate  75  are laminated in this order and integrally formed. Namely, the reflecting optical unit  7  according to the modification  3  has a configuration in which the time-division polarization switching element  74  and quarter-wave plate  75  are interchanged in the reflecting optical unit  7  according to the modification  2  described previously. 
     As with the time-division polarization switching element  13  discussed previously, the time-division polarization switching element  74  behaves as a simple transmission plate when a voltage is applied thereto (see  FIG. 6A ), so that the element  74  transmits S-polarized light provided from the first polarization separation element  71  as it is. The quarter-wave plate  75  then converts the S-polarized light provided from the time-division polarization switching element  74  to right-handed circularly polarized light. On the other hand, the time-division polarization switching element  74  behaves as a half-wave plate when no voltage is applied thereto (see  FIG. 6B ), so that the element  74  converts S-polarized light provided from the first polarization separation element  71  to P-polarized light and transmits the converted light. In this case, the quarter-wave plate  75  converts the P-polarized light provided from the time-division polarization switching element  74  to left-handed circularly polarized light. 
     Thus, the modification  1  of the embodiment 1 and the modifications  1 - 3  of the embodiment 2 provide effects similar to those provided by the basic example of the embodiment 1 and 2 shown in  FIGS. 1 and 2 . A designer may employ any of the configuration shown in  FIG. 2  (based on  FIG. 1 ) and the configurations shown in  FIGS. 4-6  (based on  FIG. 3 ) in consideration of cost or restriction caused by the configurations other than the reflecting optical unit  7  and time-division polarization switching element  13  (or time-division polarization switching element  74 ). 
       FIG. 7  is a diagram that shows a configuration of a reflecting optical unit according to a modification  4  of the embodiment 2. The reflecting optical unit according to the modification  4  of the embodiment 2 has a configuration in which an absorbing plate  76  is added to the reflecting optical unit  7  according to the basic example of the embodiment 2 shown in  FIG. 2 . In the reflecting optical unit  7  according to the modification  4 , the absorbing plate  76 , second polarization separation element  73 , switching element  72 , and first polarization separation element  71  are laminated in this order and integrally formed. 
     For the absorbing plate  76 , a metal plate with black paint applied thereto can be used. In the three-dimensional mode, the second polarization separation element  73  transmits P-polarized light. The absorbing plate  76  then absorbs the P-polarized light. 
     In the modification  4 , there may be provided a cooling means for cooling the absorbing plate  76  within the projection display apparatus  100 . For example, a fan  14  may be provided. The control unit  10  allows the fan  14  to rotate so as to cool the absorbing plate  76  in the three-dimensional mode. In the two-dimensional mode, since the second polarization separation element  73  does not transmit P-polarized light (or S-polarized light, obviously) and no light is provided from the second polarization separation element  73  to the absorbing plate  76 , the necessity to cool the absorbing plate  76  is low. Accordingly, the control unit  10  may stop the fan  14  in the two-dimensional mode. 
     The absorbing plate  76  may not necessarily be integrally configured in the reflecting optical unit  7  and may be placed at any position in the path of light provided from the second polarization separation element  73 . Also, a polarizing plate that absorbs P-polarized light may be used instead of the absorbing plate  76 . Further, a cooling means of contact type, such as a Peltier device, may be used instead of the fan  14 . The absorbing plate  76  and fan  14  according to the modification  4  are also applicable to the reflecting optical unit  7  according to each of the modifications  1 - 3  of the embodiment 2. 
     Thus, according to the modification  4  of the embodiment 2, the absorbing plate  76  prevents P-polarized light that has passed through the second polarization separation element  73  from being diffusely reflected within the projection display apparatus  100  and entering the primary light path, and also prevents heat generation in each component within the projection display apparatus  100 . Also, a cooling means including the fan  14  prevents temperature rise in the absorbing plate  76 . 
       FIG. 8  are diagrams that show configurations of reflecting optical units according to a modification  5  of the embodiment 2:  FIG. 8A  shows an example in which the second polarization separation element  73  is configured to be a convex while  FIG. 8B  shows the configuration as shown in  FIG. 2 . In the above, there have been described configurations in which the first polarization separation element  71 , switching element  72 , and second polarization separation element  73  are formed as plane plates, and air gaps are provided between the first polarization separation element  71  and switching element  72  and between the switching element  72  and second polarization separation element  73 . These configurations are suitable to discharge heat accumulated in each of the first polarization separation element  71 , switching element  72 , and second polarization separation element  73 . 
     However, in the two-dimensional mode, there is a problem that a difference of the traveling direction of the light beam occurs between S-polarized light reflected by the first polarization separation element  71  toward the imager  8  and P-polarized light reflected by the second polarization separation element  73  toward the imager  8  (see  FIG. 8B ). In other words, there occurs a difference of the position of illumination area on the imager  8  between both the light beams. Since the part where the illumination areas of both the light beams do not overlap is too dark to be used, only the part where the two illumination areas overlap each other will be used, resulting in the reduction of use efficiency of light. 
     On the other hand, by configuring the second polarization separation element  73  as a convex as shown in  FIG. 8A , the position of the illumination area on the imager  8  of S-polarized light reflected by the first polarization separation element  71  and that of P-polarized light reflected by the second polarization separation element  73  can be substantially matched. 
     Even when the second polarization separation element  73  is configured as a plane, the focus positions of both the light beams can be brought close to each other to some extent by tilting the second polarization separation element  73  with respect to the first polarization separation element  71 . However, since there occurs an optical path difference between S-polarized light reflected by the first polarization separation element  71  and P-polarized light reflected by the second polarization separation element  73 , if the optical system is designed so that the S-polarized light is focused on the imager  8 , the P-polarized light will not be focused on the imager  8 . 
     In such a case, by placing the imager  8  between the focal position of the S-polarized light and that of the P-polarized light, the light provided on the imager  8  can be homogenized. 
     As described above, the modification  5  of the embodiment 2 enables both the discharge of heat accumulated in the first polarization separation element  71 , switching element  72 , and second polarization separation element  73 , and the prevention of occurrence of a difference between the focus position of S-polarized light and that of P-polarized light in the two-dimensional mode. More specifically, providing air gaps between the first polarization separation element  71  and switching element  72  and between the switching element  72  and second polarization separation element  73  facilitates the discharge of heat. Also, configuring the second polarization separation element  73  as a convex or tilting the second polarization separation element  73  prevents the occurrence of a difference of the focus position between S-polarized light and P-polarized light in the two-dimensional mode. Even in the case where air gaps are not provided between the first polarization separation element  71  and switching element  72  and between the switching element  72  and second polarization separation element  73 , since there occurs an optical path difference of the thickness of the substrate forming each of the elements, the measures stated above are also effective. 
       FIG. 9  is a diagram that shows a configuration of an optical element according to an embodiment 3. The optical element according to the embodiment 3 is made based on a conventional wire grid. Multiple metal wires  22  are disposed on a glass substrate  21  with certain gaps between the wires. Each metal wire may be configured with an aluminum rib. 
     In the embodiment 3, half-wave plates  23  are inserted between the multiple metal wires. Namely, the metal wires  22  and half-wave plates  23  are aligned in stripes. Although a conventional wire grid reflects S-polarized light and transmits P-polarized light included in incident light, the wire grid according to the embodiment 3 (hereinafter, referred to as a λ/2 wire grid  20 ) reflects S-polarized light and converts P-polarized light in incident light to S-polarized light to transmit the converted light. 
       FIG. 10  is a diagram that shows a configuration of an optical unit  30  according to an embodiment 4. The optical unit  30  according to the embodiment 4 includes the λ/2 wire grid  20  according to the embodiment 3. The optical unit  30  according to the embodiment 4 comprises the λ/2 wire grid  20 , a quarter-wave plate  31 , and a reflecting plate  32 . In the optical unit  30 , the λ/2 wire grid  20 , quarter-wave plate  31 , and reflecting plate  32  are placed in this order as viewed from the incident light side. For example, the reflecting plate  32  (configured with a conventional mirror, for example), quarter-wave plate  31 , and λ/2 wire grid  20  may be laminated in this order and integrally formed. 
     The λ/2 wire grid  20  reflects S-polarized light and converts P-polarized light in incident light to S-polarized light to transmit the converted light. The quarter-wave plate  31  converts the S-polarized light, which has been converted and transmitted by the λ/2 wire grid  20 , to right-handed circularly polarized light and transmits the converted light. The reflecting plate  32  converts the right-handed circularly polarized light provided from the quarter-wave plate to left-handed circularly polarized light and reflects the converted light. The quarter-wave plate  31  then converts the left-handed circularly polarized light provided from the reflecting plate  32  to P-polarized light and transmits the converted light. Thereafter, the λ/2 wire grid  20  converts the P-polarized light provided from the quarter-wave plate  31  to S-polarized light and transmits the converted light. Thus, the optical unit  30  according to the embodiment 4 can convert every component of non-polarized light incident thereon to S-polarized light. 
     As stated above, according to the embodiment 4, every component of non-polarized light can be converted to S-polarized light, thereby fixing polarization to a certain direction while preventing the reduction of light amount. 
     The optical unit  30  according to the embodiment 4 may be employed as the optical unit  7  according to the embodiment 1 or 2. More specifically, the λ/2 wire grid  20 , quarter-wave plate  31 , and reflecting plate  32  may be used instead of the first polarization separation element  71 , switching element  72 , and second polarization separation element  73 . 
     With the optical unit  30  according to the embodiment 4, since the light amount does not decrease even in the three-dimensional mode, there is no need to switch the state between the two-dimensional mode and three-dimensional mode. It is because there may be both the non-polarization state and the state in which polarization is fixed to a certain direction in the two-dimensional mode. Since there is no need to switch the state between the two-dimensional mode and three-dimensional mode, electrical control is also unnecessary. 
     Thus, in addition to the effects of the optical unit according to the embodiment 2, the optical unit  30  according to the embodiment 4 further provides advantageous effects, such that the light amount for each image does not decrease even in the three-dimensional mode and that electrical control is unnecessary and the configuration can be more simplified. 
       FIG. 11  is a diagram that shows a configuration of an optical unit  40  according to an embodiment 5. The optical unit  40  according to the embodiment 5 also includes the λ/2 wire grid  20  according to the embodiment 3. The optical unit  40  according to the embodiment 5 comprises a first reflecting plate  41 , a normal wire grid  42 , the λ/2 wire grid  20 , and a second reflecting plate  43 . In the optical unit  40 , the normal wire grid  42 , the first reflecting plate  41  and second reflecting plate  43  (these two are in the same ordinal position), and the λ/2 wire grid  20  are placed in this order as viewed from the incident light side. 
     The normal wire grid  42  reflects S-polarized light and transmits P-polarized light included in incident light. The first reflecting plate  41  then reflects the S-polarized light reflected by the normal wire grid  42 . Also, the second reflecting plate  43  reflects the P-polarized light transmitted by the normal wire grid  42 . The λ/2 wire grid  20  reflects the S-polarized light provided from the first reflecting plate  41  and also converts the P-polarized light provided form the second reflecting plate  43  to S-polarized light to transmit the converted light. Thus, the optical unit  40  according to the embodiment 5 also can convert every component of non-polarized light incident thereon to S-polarized light. 
     As described above, the optical unit  40  according to the embodiment 5 provides the same effect as the optical unit  30  according to the embodiment 4. 
     The present invention has been described with reference to some embodiments. The embodiments are intended to be illustrative only, and it will be obvious to those skilled in the art that various modifications to constituting elements or processes could be developed and that such modifications also fall within the scope of the present invention. 
     There have been described examples in which the optical unit according to the embodiment 2, 4, or 5 is applied to the projection display apparatus  100 . However, such an optical unit is also applicable to a display apparatus of non-projection type, such as a liquid crystal display and an organic EL display. 
     The optical unit and time-division polarization switching element according to each embodiment exert the effects as described above, as long as they are placed in any positions between a light source and a screen. Also, although the embodiments describe examples using a liquid crystal element that behaves as a half-wave plate using birefringence of liquid crystal, the switching of the polarization direction is also enabled using optical rotation of liquid crystal. 
     The embodiment 2 describes an example in which the first polarization separation element  71  reflects S-polarized light and the second polarization separation element  73  also reflects S-polarized light. However, the first polarization separation element  71  may be set to reflect P-polarized light while the second polarization separation element  73  is also set to reflect P-polarized light, the first polarization separation element  71  may be set to reflect S-polarized light while the second polarization separation element  73  is set to reflect P-polarized light, or the first polarization separation element  71  may be set to reflect P-polarized light while the second polarization separation element  73  is set to reflect S-polarized light. 
     In the first setting of the three setting examples cited above, the control unit  10  applies a voltage to the switching element  72  in the three-dimensional mode so as to provide control such that the switching element  72  is placed in the first state set forth above (a simple transmission plate), while the control unit  10  applies no voltage to the switching element  72  in the two-dimensional mode so as to provide control such that the switching element  72  is placed in the second state set forth above (a half-wave plate). 
     In the second and third settings of the three setting examples cited above, on the other hand, the control unit  10  applies a voltage to the switching element  72  in the two-dimensional mode so as to provide control such that the switching element  72  is placed in the first state set forth above (a simple transmission plate), while the control unit  10  applies no voltage to the switching element  72  in the three-dimensional mode so as to provide control such that the switching element  72  is placed in the second state set forth above (a half-wave plate).