Abstract:
A projector includes: a light source that emit light; an illumination optical system that includes a superimposing optical element capable of performing superimposition illumination and uniformizes the light emitted from the light source to form illumination light; a color separating optical system that includes a dichroic mirror which reflects a predetermined color light component of the illumination light and transmits other light components, thereby separating the light components into a first optical path and a second optical path, a first mirror which bends the first optical path, and a second mirror which bends the second optical path, and adjusts the reflection angle of the predetermined color light component by the dichroic mirror and the bent angles of the optical paths by the first and second mirrors to provide a predetermined difference between the length of the first optical path and the length of the second optical path, the predetermined difference corresponding to a difference between the focal, distances of the first and second optical paths; light modulating devices that are illuminated by the color light components emitted from the color separating optical system and form color optical images; a combining optical system that combines the color optical images; and a projection optical system that projects an image combined by the combining optical system.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to projector that includes a color separating optical system for separating illumination light into color light components having corresponding wavelength ranges and combines and projects color optical images emitted from liquid crystal panels illuminated by the separated light components. 
         [0003]    2. Related Art 
         [0004]    In general, in projectors, a dichroic mirror separates light emitted from a light source device into a plurality of color light components, but an axial chromatic aberration occurs due to the difference between the wavelengths of color light components. A technique for adjusting the length of an optical path related to a specific color light beam separated from light emitted from a light source has been proposed in order to compensate the axial chromatic aberration (see JP-A-205-181240). 
         [0005]    However, the compensation of the axial chromatic aberration is performed on the overall structure of the light source device, and the light source device includes a large number of components. Therefore, it is not easy to adjust the amount of compensation, and the compensation needs to be repeatedly performed. 
       SUMMARY 
       [0006]    An advantage of some aspects of the invention is that it provides a projector capable of easily compensating chromatic aberration to prevent the irregularity or blur of an image, thereby improving the usage efficiency of light. 
         [0007]    According to an aspect of the invention, a projector includes: a light source that emit light; an illumination optical system that includes a superimposing optical element capable of performing superimposition illumination and uniformizes the light emitted from the light source to form illumination light; a color separating optical system that includes a dichroic mirror which reflects a predetermined color light component of the illumination light and transmits other light components, thereby separating the light components into a first optical path and a second optical path, a first mirror which bends the first optical path, and a second mirror which bends the second optical path, and adjusts the reflection angle of the predetermined color light component by the dichroic mirror and the bent angles of the optical paths by the first and second mirrors to provide a predetermined difference between the length of the first optical path and the length of the second optical path, the predetermined difference corresponding to a difference between the focal distances of the first and second optical paths of the superimposing optical element; light modulating devices that are illuminated by the color light components emitted from the color separating optical system and form color optical images; a combining optical system that combines the color optical images; and a projection optical system that projects an image combined by the combining optical system. 
         [0008]    In the projector according to this aspect, the color separating optical system adjusts the reflection angle of the predetermined color light component by the dichroic mirror and the bent angles of the optical paths by the first and second mirrors to provide a predetermined difference between the length of the first optical path and the length of the second optical path that corresponds to a difference between the focal distances of the first and second optical paths of the superimposing optical element. In this way, it is possible to accurately compensate for the chromatic aberration caused by the superimposing optical element before the light modulating devices form images and thus to prevent the irregularity or blur of an image to be projected, which results in an improvement in the usage efficiency of light. In many cases, the superimposing optical system has the greatest effect on the chromatic aberration when the light modulating devices corresponding to, for example, red, green, and blue light are illuminated. Therefore, it is know that the compensation of the chromatic aberration caused by the superimposing optical element makes it possible to effectively prevent the irregularity of a projected image and thus improve the quality of the projected image. 
         [0009]    In the projector according to the above-mentioned aspect, preferably, the superimposing optical element is composed of a single lens. In addition, preferably, when a main wavelength of the predetermined color light component passing through the first optical path and main wavelengths of the other color light components passing through the second optical path are used as reference wavelengths, the length of the first optical path is L a , the length of the second optical path is L b , the refractive index of the single lens with respect to the main wavelength of the predetermined color light component is n λa  the refractive index of the single lens with respect to the main wavelengths of other color light components is n λb , a curvature radius of an incident surface of the single lens is r 1 , a curvature radius of an emission surface of the single lens is r 2 , and the thickness of the single lens is d, the color separating optical system satisfies the following Expression related to a difference L a −L b : 
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         [0010]    In this case, it is possible to accurately calculate the chromatic aberration caused by the single lens of the superimposing optical element, and adjust the reflection angle of a predetermined color light component by the dichroic mirror and the bent angles of the optical paths by the first and second mirrors, on the basis of the calculated chromatic aberration, to provide the predetermined difference required to compensate for the chromatic aberration. 
         [0011]    In the projector according to the above-mentioned aspect, preferably, the illumination optical system includes a pair of fly-eye lenses. In this case, the pair of fly-eye lenses serve as light beam separating optical elements for separating a light beam into a plurality of partial light beams and uniformizing illumination light. 
         [0012]    In the projector according to the above-mentioned aspect, preferably, the first optical path and the second optical path are arranged perpendicular to each other with respect to the combining optical system. In this case, the combining optical system combines color optical images without deviation among the color optical images. 
         [0013]    In the projector according to the above-mentioned aspect, preferably, in the illumination optical system, an optical axis up to the second mirror is perpendicular to an optical axis from the second mirror to the combining optical system. In addition, preferably, in the color separating optical system, the dichroic mirror and the first mirror are arranged substantially in parallel to each other. In this case, when the dichroic mirror is inclined such that optical axes before and after the dichroic mirror are not perpendicular to each other, a component, such as the first mirror, of the color separating optical system is appropriately arranged to correspond to the arrangement of the optical axes, which allows to adjust the optical paths to provide the predetermined difference between the lengths of the optical paths. 
         [0014]    In the projector according to the above-mentioned aspect, preferably, in the illumination optical system, an optical axis up to the second mirror is not perpendicular to an optical axis from the second mirror to the combining optical system, and in the color separating optical system, the dichroic mirror and the second mirror are arranged substantially in parallel to each other. In this case, when the second mirror and the dichroic mirror are inclined such that optical axes before and after the second mirror and the dichroic mirror are not perpendicular to each other, the light source and other components of the color separating optical system are appropriately arranged to correspond to the arrangement of the optical axes, which allows to adjust the optical paths to provide the predetermined difference between the lengths of the optical paths. 
         [0015]    In the projector according to the above-mentioned aspect, preferably, the predetermined color light component is a red light component, and the length of the first optical path is larger than the length of the second optical path. In this case, since the length of the optical path of a red light component that has weak refractive power and is within a relatively long wavelength range becomes long, the difference between the length of the first optical path and the second optical path occurs, which allows to compensate the chromatic aberration. 
         [0016]    In the projector according to the above-mentioned aspects, preferably, the predetermined color light component is a blue light component, and the length of the first optical path is shorter than the length of the second optical path. In this case, since the length of the optical path of a blue light component that has strong refractive power and is within a relatively short wavelength range becomes short, the difference between the length of the first optical path and the second optical path occurs, which allows to compensate the chromatic aberration. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0018]      FIG. 1  is a conceptual diagram illustrating a projector according to a first embodiment of the invention. 
           [0019]      FIG. 2  is a plan view illustrating a color separating optical system of the projector according to the first embodiment. 
           [0020]      FIG. 3  is a diagram schematically illustrating optical paths of the projector according to the first embodiment, 
           [0021]      FIG. 4  is a plan view illustrating a color separating optical system of a projector according to a modification of the first embodiment. 
           [0022]      FIG. 5  is a plan view illustrating a color separating optical system of a projector according to a second embodiment of the invention. 
           [0023]      FIG. 6  is a plan view illustrating a color separating optical system of a projector according to a modification of the second embodiment. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Embodiment 
       [0024]      FIG. 1  is a diagram illustrating a projector according to a first embodiment of the invention. A projector  100  according to the first embodiment includes a light source device  10 , an illumination optical system  20 , a color separating optical system  30 , a light modulating device  40 , a cross dichroic prism  50 , serving as a combining optical system, and a projection lens  60 , serving as a projection optical system. 
         [0025]    The light source device  10  includes a light source  11  that emits light within a visible light wavelength range, a reflector  12  that reflects the light emitted from the light source, and a collimating lens  13 , which is a collimating unit for collimating light beams. 
         [0026]    In the light source device  10 , the light source  11  is, for example, a high-pressure mercury lamp, and emits substantially white light having the amount of light required to form an optical image. The reflector  12  reflects the light to converge on a predetermined focus. The collimating lens  13  converts the traveling directions of light beams to be parallel to each other. However, the curved surface of the reflector  12  is generally an ellipsoid, but the invention is not limited thereto. For example, the curved surface of the reflector  12  may be a paraboloid. When the reflector  12  having a paraboloid as a reflecting surface is used, the collimating lens  13  may not be needed. 
         [0027]    The illumination optical system  20  is an optical system for dividing a light beam emitted from the light source device  10  into a plurality of partial light beams and making the plurality of light beams incident on an illumination region such that the light beams are superposed to uniformize the in-plane illuminance of the illumination region. The illumination optical system  20  serves as an illuminating device for forming uniform illumination light from the light emitted from the light source. The illumination optical system  20  includes first and second fly-eye lenses  21   a  and  21   b , a polarizing element  22 , and a superimposing lens  23 , which is a single lens of a superimposing optical element. 
         [0028]    Each of the first and second fly-eye lenses  21   a  and  21   b  is composed of a plurality of element lenses arranged a matrix, and each of the element Lenses divides light passing through the collimating lens  13  of the light source device  10  and condenses and diffuses the divided light component-s. More specifically, the first fly-eye lens  21   a  serves as a light beam dividing optical element that divides a light beam passing through the collimating lens  13  into a plurality of partial light beams, and includes a plurality of element lenses in the plane orthogonal to an optical axis OA of illumination light. The outline of each of the element lenses is similar to the shape of an illuminated region (an effective pixel region) of each of liquid crystal light valves  40   a ,  40   b , and  40   c , which will be described later. The second fly-eye lens  21   b  is an optical element that condenses the plurality of partial light beams divided by the first fly-eye lens  21   a , and includes a plurality of element lenses n the plane orthogonal to the optical axis OA of illumination light, similar to the first fly-eye lens  21   a . However, since the second fly-eye lens  21   b  is provided in order to condense light beams, it is unnecessary that the outline of each of the element lenses correspond to the shape of the illuminated region of each of the liquid crystal light valves  40   a ,  40   b , and  40   c.    
         [0029]    The polarizing element  22  comprises a PBS array, and has a function of linearly polarizing the partial light beams divided by the first fly-eye lens  21   a  in one direction. Although not shown in  FIG. 1 , the polarizing element  22  has a structure in which polarizing films and reflecting mirrors inclined with respect to the optical axis OA of illumination light are alternately arranged. The polarizing film transmits one of a P polarized light beam and an S polarized light beam included in the partial light beams, but reflects the other light beam. The reflected polarized light, beam is reflected by the reflecting mirror and is then emitted in the direction in which the transmitted polarized light beam is emitted, that is, along the optical axis OA of illumination light. All the emitted polarized light beams are polarized by a retardation plate provided in a strip shape on the light emission surface of the polarizing element  22 , so that all the polarized light beams are polarized in the same direction. The use of the polarizing element  22  allows to polarize the light beams emitted from the light source device  10  in the same direction, and thus it is possible to improve the usage efficiency of light used for the liquid crystal light valves  40   a ,  40   b , and  40   c.    
         [0030]    The superimposing lens  23  is a superimposing optical system that condenses the plurality of partial light beams passing through the first fly-eye lens  21   a , the second fly-eye lens  21   b , and the polarizing element  22  and superimposes the light beams on an image forming region (effective region) of each of the liquid crystal light valves  40   a ,  40   b , and  40   c  forming the light modulating device  40 . That is, the superimposing lens  23  can superimpose the partial light beams divided by the first fly-eye lens  21   a  on the liquid crystal light valves  40   a ,  40   b , and  40   c , which allows to illuminate the liquid crystal light valves  40   a ,  40   b , and  40   c  with light having uniform illuminance. 
         [0031]    The illumination light formed by the illumination optical system  20  is within a visible light wavelength range, and the superimposing lens  23  of the illumination optical system  20  has a unique refractive index for each wavelength. These factors cause chromatic aberration to occur in the illumination optical system  20 , and color irregularity or blur occurs in the light modulating device  40  in the subsequent state due to the chromatic aberration, which may cause the usage efficient of light in the projection  100  to be reduced. In particular, the chromatic aberration generated by the superimposing lens  23  of the illumination optical system  20  has a great effect on the operation of the projector  100 . Therefore, it is necessary to compensate the chromatic aberration (which will be described later). 
         [0032]    The color separating optical system  30  includes a first dichroic mirror  31 , a second dichroic mirror  32 , a first reflecting mirror  33 , a second reflecting mirror  34   a , a third reflecting mirror  34   b , and three field lenses  35   a ,  35   b , and  35   c . The color separating optical system  30  separates the illumination light formed by the illumination optical system  20  into red (R), green (G), and blue (B) light components, and guides the R, G, and B light components to the liquid crystal light valves  40   a ,  40   b , and  40   c  in the subsequent state, respectively. More specifically, firstly, the first and second dichroic mirrors  31  and  32  separates the illumination light by reflecting and transmitting light components in a predetermined wavelength range of the visible light wavelength range included in the illumination light. In particular, in this embodiment, the first dichroic mirror  31  reflects the R light component, but transmits the G and B light components. The second dichroic mirror  32  reflects the G light component, but transmits the B light component. That is, the first dichroic mirror  31  separates light emitted from the light source into R, G, and B Might components. The second dichroic mirror  32  separates the light passing through the first dichroic mirror  31  into G and B light components. In this way, as shown in  FIG. 1 , the R light component is separated by the first dichroic mirror  31  and passes through a first optical path OP 1 . The G light component is separated by the first and second dichroic mirrors  31  and  32  and passes through a second optical path OP 2 . The B light component is separated by the second dichroic mirror  32  and sequentially passes through a portion of the second optical path OP 2  and a third optical path OP 3 . As described above; in this embodiment, the R light component, the G light component, and the B light component correspond to the first optical path OP 1 , the second optical path OP 2 , and the third optical path OP 3 , respectively. 
         [0033]    Next, in the color separating optical system  30 , the R light component reflected from the first dichroic mirror  31  is incident on the field lens  35   a  for adjusting the incident angle of light through the first reflecting mirror  33 . In addition, the G light component having passed through the first dichroic mirror  31  and then reflected from the second dichroic mirror  32  is incident on the field lens  35   b  for adjusting the incident angle of light. Further, the B light component having passed through the first and second dichroic mirrors  31  and  32  is incident on the field lens  35   c  for adjusting the incident angle of light through relay lenses LL 1  and LL 2  and the second and third reflecting mirrors  34   a  and  34   b.    
         [0034]    In the color separating optical system  30  according to this embodiment, in order to compensate for chromatic aberration occurring in the superimposing lens  23  of the illumination optical system  20 , the angle of the first dichroic mirror  31  deviates from a reference angle of 45° for reflecting the R light component by a predetermined angle, which will be described later. Therefore, the first reflecting mirror  33  also deviates from the reference angle of 45° for reflecting the R light component by a predetermined angle. 
         [0035]    The light modulating device  40  is composed of the liquid crystal light valves  40   a ,  40   b , and  40   c . The liquid crystal light valves  40   a ,  40   b , and  40   c  are light modulating devices of a non-emission tree for modulating the spatial intensity distribution of incident illumination light. The liquid crystal light valves  40   a ,  40   b , and  40   c  include liquid crystal panels  41   a ,  41   b , and  41   c  that are illuminated by the R, G, and B light components emitted from the color separating optical system  30 , first polarizing filters  42   a ,  42   b , and  42   c  arranged on the incident sides of the liquid crystal panels  41   a  to  41   c , and second polarizing filters  43   a  to  43   c  arranged on the emission sides of the liquid crystal panels  41   a  to  41   c , respectively. The R light component reflected from the first dichroic mirror  31  is incident on the liquid crystal panel  41   a  of the liquid crystal light valve  40   a  through, for example, the field lens  35   a . The G light component having passed through the first dichroic mirror  31  and then reflected from the second dichroic mirror  32  is incident on the liquid crystal panel  41   b  of the liquid crystal light valve  40   b  through, for example, the field lens  35   b . The B light component having passed through the first and second dichroic mirrors  31  and  32  is incident on the liquid crystal panel  41   c  of the liquid crystal light valve  40   c  through, for example, the field lens  35   c . The liquid crystal panels  41   a  to  41   c  modulate the spatial intensity distribution of the incident light components, and the three color components incident on the corresponding liquid crystal panels  41   a  to  41   c  are modulated according to driving signals or image signals incident on the liquid crystal panels  41   a  to  41   c  as electric signals. In this case, the polarizing directions of the light components incident on the liquid crystal panels  41   a  to  41   c  are adjusted by the first polarizing filters  42   a  to  42   c , respectively. Light components emitted from the liquid crystal panels  41   a  to  41   c  are polarized in predetermined polarizing directions by the second polarizing films  43   a  to  43   c , respectively. In this way, the liquid crystal light valves  40   a ,  40   b , and  40   c  form R, G and B optical images. 
         [0036]    The cross dichroic prism  50  combines the R, G, and B optical images emitted from the liquid crystal light valves  40   a ,  40   b , and  40   c . More specifically, the cross dichroic prism  50  are formed by bonding four right-angled prisms and has a substantially square shape in plan view. A pair of dielectric multi-layer films  11   a  and  51   b  is formed in an X shape at interfaces among the right-angled prisms. The first dielectric multi-layer film  51   a  reflects an R right component, and the second dielectric multi-layer film  51   b  reflects a B light component. In the cross dichroic prism  50 , the dielectric multi-layer film  51   a  reflects the R light component emitted from the liquid crystal light valve  40   a  on the right side, and the dielectric multi-layer films  51   a  and  51   b  transmit the G light component emitted from the liquid crystal light valve  40   b . In addition, the dielectric multi-layer film  51   b  reflects the B light component emitted from the liquid crystal light valve  40   c  on the left side. In this way, the cross dichroic prism  50  combines the R, G, and B light components to form combined light, which is color image light. 
         [0037]    The projection lens  60  enlarges the image light, which is the combined light formed by the cross dichroic prism  50  at a predetermined enlargement ratio and projects a color image on a screen (not shown). 
         [0038]      FIG. 2  is a plan view illustrating the detailed structure of the color separating optical system  30  of the projector  100 . In  FIG. 9 , the same components as those in  FIG. 1  have the same reference numerals. 
         [0039]    The illumination light emitted from the superimposing lens  23  positioned in the last stage of the illumination optical system  20  (see  FIG. 1 ) is incident on the first dichroic mirror  31 . In this embodiment, an intersection between the optical axis OA of illumination light, which is a reference optical path of light emitted from the illumination optical system  20  and an incident surface of the first dichroic mirror  31  is referred to as a separation point SP. Similarly, an intersection between the previous state of a first optical path OP 1 , which is a reference optical path of the R light component, and a reflecting surface of the first dichroic mirror  33  is referred to as a reflection point RP 1 , and an intersection between the previous state of a second optical path OP 2 , which is a reference optical path of the G light component, and a reflecting surface of the second dichroic mirror  32  is referred to as a reflection point RP 2 . In this case, first, the R light component, which is a predetermined color light component, and the C light component, which is another color light component, of the illumination light are separated into the first optical path OP 1  and the second optical path OP 2  respectively, using the separation point SP of the first dichroic mirror  31  as a reference point. In this case, the B light component is separated into the second optical path OP 2  together with the G light component. Then, the first optical path OP 1  related to the R light component is reflected at a predetermined angle, that is, about a right angle, at the reflection point RP 1  by the first reflecting mirror  33 , which is a first mirror (more specifically, the first optical path OP 1  is bent at an angle corresponding to a bent angle α). In addition, the second optical path OP 2  related to the G light component is reflected at a predetermined angle, that is, about a right angle, at the reflection point RP 2  by the second reflecting mirror  32 , which is a second mirror. 
         [0040]    In this embodiment, the optical axis OA of illumination light, which is a system optical axis up to the second dichroic mirror  32  and is also an optical axis in the next stage of the superimposing lens  23  provided in the illumination optical system  20 , and an optical axis OB of emission light from the dichroic mirror  32  to the cross dichroic prism  50  are orthogonal to each other. In addition, the first dichroic mirror  31  is arranged substantially parallel to the first reflecting mirror  33 . In this case, the reflection angle of the R light component by the first dichroic mirror  31  is maintained at a predetermined angle of, for example, about 45°, and there is a predetermined difference between the length of the first optical path OP 1  and the length of the second optical path OP 2 . More specifically, the incident surface of the first dichroic mirror  31  is arranged at an angle slightly larger than 45° with respect to the optical axis OA of illumination light such that the reflection angle of the R light component by the first dichroic mirror  31  is slightly smaller than 45° by a minute angle, that is, such that the first optical path OP 1  is bent at a predetermined angle α smaller than 90°. In this way, it is possible to make the length of the first optical path OP 1  slightly larger than the length of the second optical path OP 2  and thus perform compensation to correspond to chromatic aberration, as compared to the related art. However, in this case, the first reflecting mirror  33  is arranged substantially parallel to the first dichroic mirror  31  so as to correspond to the arrangement, that is, inclination of the first dichroic mirror  31 , so that the first optical path OP 1  is orthogonal to the incident surface of the liquid crystal light valve  40   a . As a result, light is guided to an appropriate direction. 
         [0041]    Next, the comparison between the general structure of the projector according to the related art and the structure of the projector  100  according to this embodiment of the invention will be described with reference to  FIGS. 2 and 3 .  FIG. 3  is a diagram illustrating an optical path according to this embodiment and an optical path according to the related art. 
         [0042]    In the projection according to the related art, as shown in  FIG. 2 , generally, a dichroic mirror VD corresponding to the first dichroic mirror  31  is inclined at an angle of 45° with respect to the optical axis OA of illumination light. Therefore, in this case, an optical path VOP of an R light component, which is reflected light, is bent on the right side at an angle of 90° by the reflection of light by the dichroic mirror VD. In addition, a dichroic mirror VM corresponding to the first dichroic mirror  33  is inclined at an angle of 45′ with respect to the optical axis VOP. Therefore, the optical path VOP is bent on the left side at an angle of 90°. A reflection point VRP is an intersection between the optical axis of the optical path VOP and the reflecting surface of the reflecting mirror VM. 
         [0043]    As described above, the R light component emitted from the superimposing lens  23  is incident on the liquid crystal light valve  40   a  through the field lens  35   a  along the optical path VOP bent in a crank shape. However, G and B light components having passed through the dichroic mirror VD travel along the second and third optical paths OP 2  and OP 3 , respectively, similar to this embodiment of the invention. 
         [0044]    As described above, according to the related art, the length of the optical path VOP, which is an optical path of the R light component, is equal to the length of the second optical path OP 2 , which is an optical path of the C light component. In particular, in this case, the optical path VOP and the second optical path OP 2  are symmetric with respect to a plane AX (which is represented by a dotted line) including an Intersecting line CS between a separation point SP and a pair of dielectric multi-layer films  51   a ,  51   b  that intersect each other in an X shape in the cross dichroic prism  50 . Here, the term ‘symmetry’ includes rotational symmetry using the plane AX as a reference surface as well as line symmetry using the plane AX as a reference line. That is, the symmetry may also include point symmetry using a middle point between a point indicating the intersecting point CS and the separation point SP as a central point of rotation on the plane shown in  FIG. 2 . 
         [0045]    In contrast, in this embodiment of the invention, first dichroic mirror  31  and the first dichroic mirror  33  are inclined to make the length of the first optical path OP 1  larger than the length of the second optical path OP 2 , which causes the first and second optical paths to be asymmetric with respect to the plane AX. In this embodiment, since the arrangement, that is, inclination of the first reflecting mirror  33  is set appropriately, the first optical path OP 1  of light reflected by the first reflecting mirror  33  is adjusted so as to be aligned with the optical path VOP of light reflected by the reflecting mirror VM. Therefore, in this case, light components are incident on the cross dichroic prism  50  shoe in  FIG. 1  along the first optical path OP 1  and the second optical path OP 2  orthogonal to each other. 
         [0046]    Next, the comparison between this embodiment of the invention and the related art will be described with reference to  FIG. 3 . More specifically, the difference between the length of the first optical path OP 1  and the second optical path OP 1  according to) this embodiment of the invention will be described below. 
         [0047]    First, in this embodiment, the length of the first optical path OP 1  is referred to as L R , and the length of the second optical path OP 2  is referred to as L G . That is, the difference between the length of the first optical path OP 1  and the second optical path OP 2  is represented by L R −L G . In addition, an inclination angle θ indicates an angle formed between the first optical path OP 1  of this embodiment of the invention and the optical path VOP of the related art. That is, θ=90°−α. 
         [0048]    Hereinafter, the value of the difference L R −L G  between the length of the first optical path OP 1  and the optical path VOP will be described below. 
         [0049]    As can be seen from  FIG. 3 , while the first optical path OPD reaches the reflection point VRP from the separation point SP via the reflection point RP 1 , the optical path VOP directly reaches the reflection point VRP from the separation point SP, which results in the difference between the length of the first optical path OP 1  and the length of the optical path VOP. More specifically, when the distance from the separation point SP to the reflection point VRP is ‘x’, the distance from the separation point SP to the reflection point VRP in the first optical path OP is represented by x·(1/cos θ+tan θ) since the distance from the separation point SP to the reflection point RP 1  is x/cos θ and the distance from the reflection point PR, to the reflection point VRP is x·tan θ. Therefore, the difference between the lengths is expressed by x·(1/cos θ+tan θ−1). In this case, as described above, since the length of the optical path VOP is equal to the length of the second optical path OP 2 , the difference between the lengths is the difference between the length of the first optical path OP 1  and the length of the second optical path OP 2 . That is, the difference L R −L G  can be ex-pressed by x·(1/cos θ+tan θ−1). However, because θ=90°−α, the value of the difference L R −L G  can be determined directly and exclusively by adjusting the inclination angle α. Since the value of α is sufficiently small, x·(1/cos θ+tan θ−1) is approximately x·θ. 
         [0050]    Meanwhile, as described above, it is necessary to compensate optical axis chromatic aberration occurring in the superimposing lens  23 . Therefore, it is possible to accurately compensate the chromatic aberration of illumination light by determining the difference L R −L G  on the basis of the main wavelength in the wavelength range of the K light component traveling along the first optical path OP 1  and the main wavelength in the wavelength range of the G light component traveling along the second optical path OP 2  in the characteristics of the superimposing lens  23 . 
         [0051]    The refractive indexes of the superimposing lens  23  with respect to the main wavelength of the R light component traveling along the first optical path OP 1  and the main wavelength of the G light component traveling along the second optical path OP 2  are referred to as n R  and n G , respectively. As characteristics of the superimposing lens  23 , the curvature radius of an incident surface of the superimposing lens  23  is referred to as r 1 , the curvature radius of an emission surface of the superimposing lens  23  is referred to as r 2 , and the thickness of the superimposing lens  23  is referred to as d. In this case, the difference between the length of the first optical path OP 1  and the length of the second optical path OP 2  suitable for axial compensating chromatic aberration is represented by Expression 1 given below. 
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         [0052]    The above-mentioned Expression 1 makes it possible to appropriately compensate chromatic aberration in the wavelength ranges of R, C, and B light components on the basis of characteristics of the superimposing lens  23  used in this embodiment, such as a material forming the superimposing lens  23  and the shape thereof. Therefore, the difference L R −L G  may be determined so as to satisfy Expression 1. That is, the inclination angle θ of the first dichroic mirror  31  shown in  FIG. 1  or  2  is set to a value satisfying x·(1/cos θ+tan θ−1)=L R −L G =f R −f G  or a value approximate thereto, and the inclination of the first reflecting mirror  33  is adjusted on the basis of the value, thus obtaining the desired value of the difference L R −L G . In this way, it is possible to accurately compensate chromatic aberration. As a result, the projector  100  according to this embodiment can prevent the color irregularity or blur of an image and thus improve the usage efficiency of light. In addition, it is possible to compensate the chromatic aberration of the B light component incident on the cross dichroic prism  50  through the third optical path OP 3  by appropriately adjusting the length of the third optical path OP 3  or relay optical systems LL 1  and LL 2 . 
         [0053]    In this embodiment, the R light component is reflected by the first dichroic mirror  31  to travel through the first optical path OP 1 , but the invention is not limited thereto. For example, a combination of light components passing through the first and second optical paths OP 1  and OP 2  can be appropriately changed under the following conditions. 
         [0054]    The chromatic aberration occurs due to the difference among the unique refractive indexes with respect to the wavelengths of R, G, and B light components. In general, light having a short wavelength is refracted at a large angle, but light having a long wavelength is refracted at a small angle. Therefore, as the wavelength of light becomes longer, a longer optical path is needed to compensate the chromatic aberration. For the reason, in this embodiment, it is preferable that the wavelength of light passing through the first optical path OP 1  be longer than the wavelength of light passing through the second optical path OP 2  as selection conditions of R, G, and B light components passing through the first and second optical paths OP 1  and OP 2 . Therefore, in this embodiment, for example, it is also preferable that the R light component travel through the first optical path OP 1  and the B light component travel through the second optical path OP 2 . In addition, for example, it is also preferable that the G light component travel through the first optical path OP 1  and the B light component travel through the second optical path OP 2 . 
         [0055]      FIG. 4  is a plan view illustrating a color separating optical system of a projector according to a modification of this embodiment of the invention. In  FIG. 4 , the same components as those in this embodiment are denoted by the same reference numerals, and a description of components having the same functions as those in  FIG. 1  will be omitted. 
         [0056]    In the above-mention embodiment, the length of the first optical path OP 1  is larger than that of the second optical path OP 2 , but in the modification, the length of the first optical path OP 1  is smaller than that of the second optical path OP 2 . That is, in a color separating optical system  130 , a first dichroic mirror  131  is inclined in a direction opposite to the direction in the above-mentioned embodiment. In this way, the length of the first optical path OP 1  is smaller than the length of the second optical path OP 2 . In this case, light having a relatively short wavelength travels through the first optical path OP 1 . Hereinafter, the modification will be described in detail below. 
         [0057]    First, the first dichroic mirror  131  reflects a B light component in a short wavelength range among R, G, and B light components, but transmits the G and B light components. A second dichroic mirror  139  reflects the G light component, but transmits the R light component. That is, the first dichroic mirror  131  separates the B light, which is a predetermined color light component, to travel through the first optical path OP 1 , and separates the G and F light components to travel through the second optical path OP 2 . The second dichroic mirror  132  separates the G light, which is another color light component, to travel through the second optical path OP 2 , and separates the R light component to sequentially travel through a portion of the second optical path OP 2  and the third optical path OP 3 . As described above, in this modification, the B light component corresponds to the first optical path OP 1 , the G light component corresponds to the second optical path OP 2 , and the R light component corresponds to the third optical path OP 3 . 
         [0058]    In this modification, the first dichroic mirror  131  is arranged such that the bent angle α of the first optical path OP 1  is larger than 90°. In this way, it is possible to make the length of the first optical path OP 1  shorter than the length of the second optical path OPT. In this case, the first reflecting mirror  33  is also arranged such that the first optical path OP 1  is formed in an appropriate direction according to the arrangement, that is, the inclination of the first dichroic mirror  131 . 
         [0059]    In the above-mentioned structure, the difference between the length of the first optical path OP 1  and the length of the second optical path OP 2  is calculated in the same manner as that used in the above-mentioned embodiment. In addition, similar to the above-mentioned embodiment, it is possible to calculate the difference between the lengths of the optical paths suitable for compensating axial chromatic aberration by finding the main wavelengths of the B and G light components satisfying a conditional expression related to characteristics of the superimposing lens  23 . 
         [0060]    In this modification, patterns other than the above-mentioned example will be considered. That is, as the selection conditions of light components traveling through the first and second optical paths OP 1  and OP 2 , the wavelength of light traveling through the first optical path OP 1  is preferably shorter than the wavelength of light traveling through the second optical path OP 2 , considering the cause of the chromatic aberration. Therefore, alternatively, for example, the B light component may pass through the first optical path OP 1 , and the R light component may pass through the second optical path OP 2 . In addition, for example, the G light component may pass through the first optical path OP 1 , and the R light component may pass through the second optical path OP 2 . 
       Second Embodiment 
       [0061]    In the first embodiment, the dichroic mirror is inclined at a predetermined angle with respect to the optical axis OA of illumination light in the illumination optical system. However, in a second embodiment, the optical axis OA of illumination light is inclined, which will be described below. That is, will the first embodiment, as shown in  FIG. 1 , the optical axis OA of illumination light, which is an optical axis of the illumination optical system  20  and is also an optical axis of a system up to the second dichroic mirror  32 , is perpendicular to an optical axis OB of light emitted from the second dichroic mirror  32  to the cross dichroic prism  50 . However, in the second embodiment, the optical axis OA is not perpendicular to the optical axis OB. In addition, in the first embodiment, as shown in  FIG. 1 , the first dichroic mirror  31  is arranged so as to be substantially parallel to the first reflecting mirror  33 . However, in the second embodiment, a first dichroic mirror  231  is arranged so as to be substantially parallel to a second dichroic mirror  232  (see  FIG. 5 ). 
         [0062]      FIG. 5  is a plan view illustrating a color separating optical system  230  of a projector according to the second embodiment. The overall structure of the projector according to the second embodiment is the same as that of the projector  100  according to the first embodiment shown in  FIG. 1 , and thus a description thereof will be omitted. In  FIG. 5 , the same components as those in the first embodiment are denoted by the same reference numerals, and a description of components having the same functions as those in  FIG. 1  will be omitted. 
         [0063]    As described above, in the second embodiment, the optical axis OA of illumination light is not perpendicular to the optical axis OB of emission light. In particular, in  FIG. 5 , an angle formed between the optical axis OA of illumination light and the optical axis OB of emission light is smaller than 90°. In this case, the second dichroic mirror  232 , which is a second mirror for bending the second optical path OP 2 , is arranged at a predetermined angle so that the second optical path OP 2  is aligned with the optical axis OB of emission light. In addition, the first dichroic mirror  231  is arranged so as to be substantially parallel to the second dichroic mirror  232 , and the first optical path OP 1  is substantially parallel to the second optical path OP 2 . The first reflecting mirror  233  is inclined at an angle of 45° with respect to the first optical path OP 1 , which causes the first optical path OP 1  to be bent at a right angle. In this way, the first optical path OP 1  and the second optical path OP 2  are perpendicular to each other such that light components passing through the first and second optical paths are incident on the cross dichroic prism  50  at right angles to each other. 
         [0064]    In the above-mentioned structure, the length of the first optical path OP 1  is shorter than the length of the second optical path OP 2 . Therefore, it is possible to pass light in a relatively short wavelength range (for example, a B light component) through the first optical path OP 1  and thus to obtain a difference between the length of the first optical path OP 1  and the length of the second optical path OP 2  suitable for compensating axial chromatic aberration, similar to the first embodiment. 
         [0065]      FIG. 6  is a plan view illustrating a color separating optical system  330  of a projector according to a modification of the second embodiment. In this modification, the projector is similar to the projector shown in  FIG. 5  except for the arrangement of components of the color separating optical system, and thus a description of the overall structure of the projector will be omitted. 
         [0066]    In this modification, contrary to the structure shown in  FIG. 5 , an angle formed between the optical axis OA of illumination light and the optical axis OB of emission light is larger than 90°. In this case, similar to the second embodiment, a second dichroic mirror  332  for bending the second optical path OP 2  is inclined at a predetermined angle to align the second optical path OP 2  with the optical axis OB of emission light. In addition, a first dichroic mirror  331  is arranged substantially in parallel to the second dichroic mirror  332 , and the first optical path OP 1  is substantially parallel to the second optical path OP 2 . A first reflecting mirror  333  is arranged so as to be inclined at an angle of  455  with respect to the first optical path OP 1 , which causes the first optical path OP 1  to be bent at a right angle. In this way, the first optical path OP 1  and the second optical path OP 2  are perpendicular to each other such that light components passing through the first optical path OP 1  and second optical path OP 2  are incident on the cross dichroic prism  50  at right angles to each other. 
         [0067]    In the above-mentioned structure, the length of the first optical path OP 1  is larger than the length of the second optical path OP 2 . Therefore, it is possible to pass light in a relatively long wavelength range (for example, an R light component) through the first optical path OP 1  and thus to obtain a difference between the length of the first optical path OP 1  and the length of the second optical path OP 2  suitable for compensating axial chromatic aberration, similar to the first embodiment. 
         [0068]    In the above-mentioned embodiments, characteristics of a dichroic film attached to the first dichroic mirror  31  may vary according to an angle at which the first dichroic mirror  31  is arranged. That is, in general, the dichroic mirror films are designed so as to have the optimum film characteristics at an angle of 45°, which is a basic incident angle of light; however, in this embodiment, the dichroic mirror films may be designed according to the incident angle of light that varies in accordance with the arrangement angles of the dichroic mirrors. 
         [0069]    In the above-mentioned embodiments, the third optical path OP 3  among the first to third optical paths OP 1  to OP 3  is relayed, but the invention is not limited thereto. For example, the invention may be applied to a projector that separates light into light components passing through optical paths having the same length. 
         [0070]    The invention is not limited to the above-mentioned embodiments, but it can be modified in various ways without departing from the scope and spirit of the invention. For example, the following modifications can be made. 
         [0071]    In the above-mentioned embodiments, a high-pressure mercury lamp is used as the light source  11 . However, instead of the high-pressure mercury lamp, other lamps such as a metal halide lamp may be used as the light source. 
         [0072]    In the above-mentioned embodiments, the projector  10  includes three liquid crystal light valves  41   a  to  41   c . However, the invention may be applied to a projector using one liquid crystal panel, a projector using two liquid crystal panels, or a projector using four or more liquid crystal panels. 
         [0073]    In the above-mentioned embodiments, a front protector that projects an image on the screen in the viewing direction. However, the invention may be applied to a rear projector that projects an image in a direction opposite to the viewing direction. 
         [0074]    The priority applications Numbers JP2006-127413 upon which this patent application is based is hereby incorporated by reference. While this invention has been described in conjunction with the specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, preferred embodiments of the invention as set forth herein are intended to be illustrative, not limiting. There are changes that may be made without departing from the spirit and scope of the invention.