Patent Publication Number: US-2021191092-A1

Title: Lens unit, projection optical system, and projector

Description:
The present application is based on, and claims priority from JP Application Serial Number 2019-228979, filed Dec. 19, 2019, the disclosure of which is hereby incorporated by reference herein in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a lens unit, a projection optical system, and a projector. 
     2. Related Art 
     A jointed lens which can be adopted as a lens which constitutes a projection optical system is described in JP-A-2003-140037 (Document 1), JP-A-2010-266496 (Document 2), JP-A-2011-053332 (document 3), and JP-A-06-347612 (Document 4). Document 1 discloses a jointed lens obtained by jointing two or three lenses to each other with an adhesive. In Document 1, the curvature radius of the joint surface of the two lenses adjacent to each other is specified to thereby prevent the two lenses from being separated from each other. Document 2 discloses a jointed lens obtained by jointing two lenses to each other with resin. In Document 2, the thermal expansion coefficients of the lenses and the resin are specified to thereby prevent the stress from occurring. Document 3 discloses a jointed lens obtained by jointing two lenses to each other with resin. In Document 3, a tilted surface part is provided outside the effective diameter of each lens. In Document 3, the two lenses are made to have contact with each other via the tilted surface part to thereby make the optical axes of the two lenses coincide with each other. Document 4 discloses a jointed lens obtained by jointing two lenses to each other with an adhesive. In Document 4, a planar section in a direction perpendicular to a surface axis is disposed outside the effective diameter of each of the lenses. In Document 4, the two lenses are bonded to each other with reference to the planar sections to thereby prevent each of the lenses from tilting. 
     There has been proposed no configuration of accurately keeping the thickness of the jointed lens having been bonded in a desired value. 
     SUMMARY 
     In view of the problems described above, a lens unit according to the present disclosure includes a jointed lens having a first optical member, a second optical member disposed on an optical axis of the first optical member, and a jointing member having a light transmissive property and disposed between the first optical member and the second optical member, and a holding mechanism configured to hold the first optical member and the second optical member. The holding mechanism holds the first optical member and the second optical member so that a distance along an optical axis direction of the optical axis between a first lateral surface at an opposite side to a second optical member side in the first optical member and a second lateral surface at an opposite side to a first optical member side in the second optical member becomes a preset distance. The jointing member adheres to the first optical member and the second optical member so that the distance becomes the preset distance. 
     A projection optical system according to the present disclosure includes the lens unit described above disposed between a reduction-side imaging plane and an elargement-side imaging plane. 
     A projector according to the present disclosure includes a light source, a light modulator configured to modulate light emitted from the light source to provide an image to the reduction-side imaging plane, and the projection optical system described above configured to project the image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a lens unit according to Practical Example 1. 
         FIG. 2  is a perspective view of the lens unit shown in  FIG. 1  when being cut along the line A-A. 
         FIG. 3  is a cross-sectional view of the lens unit shown in  FIG. 1 . 
         FIG. 4  is a perspective view of a lens unit according to Practical Example 2. 
         FIG. 5  is a perspective view of the lens unit shown in  FIG. 4  when being cut along the line B-B. 
         FIG. 6  is a perspective view of the lens unit shown in  FIG. 4  when a holding ring is removed from the lens unit. 
         FIG. 7  is an explanatory diagram of a first fixation member fixed to a first lens. 
         FIG. 8  is a perspective view of a lens unit according to Practical Example 3. 
         FIG. 9  is a cross-sectional view of the lens unit shown in  FIG. 8  when being cut along the line C-C. 
         FIG. 10  is a partial perspective view of the lens unit shown in  FIG. 8  when being cut along the line D-D. 
         FIG. 11  is a perspective view of the lens unit in the state in which a holding ring shown in  FIG. 8  is detached. 
         FIG. 12  is a schematic configuration diagram of a projector equipped with a projection optical system. 
         FIG. 13  is a ray chart schematically showing a whole of a projection optical system according to Practical Example 4. 
         FIG. 14  is a ray chart of the projection optical system according to Practical Example 4. 
         FIG. 15  is a ray chart of a second optical system in Practical Example 4. 
         FIG. 16  is a graph of a relationship between the thickness of the lens unit and a TV distortion of a final image. 
         FIG. 17  is an explanatory diagram of a distortion aberration of the final image due to the thickness of the lens unit. 
         FIG. 18  is an explanatory diagram of a distortion aberration of the final image due to the thickness of the lens unit. 
         FIG. 19  is a diagram showing MTF by the elargement-side of the projection optical system according to Practical Example 4. 
         FIG. 20  is a ray chart schematically showing a whole of a projection optical system according to Practical Example 5. 
         FIG. 21  is a ray chart of the projection optical system according to Practical Example 5. 
         FIG. 22  is a ray chart of a second optical system in Practical Example 5. 
         FIG. 23  is a diagram showing MTF by the elargement-side of the projection optical system according to Practical Example 5. 
         FIG. 24  is a ray chart schematically showing a whole of a projection optical system according to Practical Example 6. 
         FIG. 25  is a ray chart of the projection optical system according to Practical Example 6. 
         FIG. 26  is a ray chart of a second optical system in Practical Example 6. 
         FIG. 27  is a diagram showing MTF by the elargement-side of the projection optical system according to Practical Example 6. 
     
    
    
     DESCRIPTION OF AN EXEMPLARY EMBODIMENT 
     A lens unit, a projection optical system, and a projector according to an embodiment of the present disclosure will hereinafter be described with reference to the drawings. First, an example of a lens unit adoptable to the projection optical system will hereinafter be described. Then, an overall configuration of the projector will be described. Subsequently, a projection optical system equipped with the lens unit will be described as an example of the projection optical system which can be installed in the projector. 
     Lens Unit According to Practical Example 1 
       FIG. 1  is a perspective view of a lens unit according to Practical Example 1.  FIG. 2  is a perspective view of the lens unit shown in  FIG. 1  when being cut along the line A-A.  FIG. 3  is a cross-sectional view of the lens unit shown in  FIG. 1 . As shown in  FIG. 1 , the lens unit  50 A according to Practical Example 1 has a jointed lens  40  and a holding mechanism  55 . 
     As shown in  FIG. 2 , as optical members, the jointed lens  40  is provided with a first lens  51 , a second lens  52  arranged on the optical axis L of the first lens  51 , and a third lens  53  disposed between the first lens  51  and the second lens  52 . Further, the jointed lens  40  is provided with a jointing member  54  located between the first lens  51  and the second lens  52 . The jointing member  54  has a light transmissive property. The jointing member  54  is elastically deformable, and adheres to the two lenses located before and after the jointing member  54  in the optical axis L direction. As the jointing member  54 , the jointed lens  40  is provided with a first jointing member  54   a  and a second jointing member  54   b , wherein the first jointing member  54   a  is located between the first lens  51  and the third lens  53 , and adheres to the first lens  51  and the third lens  53 , and the second jointing member  54   b  is located between the third lens  53  and the second lens  52 , and adheres to the third lens  53  and the second lens  52 . In the present example, the jointing member  54  is an adhesive. As the jointing member  54 , a gel resin member can be adopted. 
     The holding mechanism  55  holds the first lens  51  and the second lens  52 . As shown in  FIG. 3 , the holding mechanism  55  keeps the distance between a first-lens lateral surface  51   a  at the opposite side to the second lens  52  in the first lens  51  and a second-lens lateral surface  52   a  at the opposite side to the first lens  51  in the second lens  52  in a preset distance T set in advance. The holding mechanism  55  is provided with a first recessed part  56  provided to a first ring-like outer circumferential surface  51   b  along the optical axis L of the first lens  51 , a second recessed part  57  provided to a second ring-like outer circumferential surface  52   b  around the optical axis L of the second lens  52 , and inter-surface distance keeping members  58  each disposed at an outer side in a radial direction of the first lens  51  and the second lens  52 . The first recessed part  56  and the second recessed part  57  correspond respectively to a first latched part and a second latched part to be engaged with the inter-surface distance keeping members  58 . 
     The first recessed part  56  of the first lens  51  is disposed outside an effective ray range of the first lens  51 . The second recessed part  57  of the second lens  52  is disposed outside an effective ray range of the second lens  52 . The first recessed part  56  of the first lens  51  and the second recessed part  57  of the second lens  52  are each disposed in the entire circumference around the optical axis L. The holding mechanism  55  is provided with three inter-surface distance keeping members  58  as the inter-surface distance keeping members  58 . As shown in  FIG. 1 , the three inter-surface distance keeping members  58  are disposed at regular angular intervals. 
     As shown in  FIG. 3 , the inter-surface distance keeping members  58  are each provided with a main body part  60  extending in the optical axis L direction, a first protrusion  61  protruding toward the inner side in the radial direction from the main body part  60 , and a second protrusion  62  protruding toward the inner side in the radial direction at a position distant as much as a predetermined distance U from the first protrusion  61  in the optical axis L direction. The first protrusion  61  is fitted into the first recessed part  56  of the first lens  51  from the outer side in the radial direction. The second protrusion  62  is fitted into the second recessed part  57  of the second lens  52  from the outer side in the radial direction. In other words, the first protrusion  61  is a first latch part to be engaged with the first recessed part  56 , and the second protrusion  62  is a second latch part to be fitted into the second recessed part  57 . In the present example, the inter-surface distance keeping members  58  are made of metal. 
     Further, the lens unit  50 A is provided with a holding ring  65  for holding the three inter-surface distance keeping members  58  from the outer circumferential side. The holding ring  65  makes contact with each of the inter-surface distance keeping members  58  from the outer circumferential side. 
     According to the present example, the first protrusion  61  of the inter-surface distance keeping member  58  is fitted into the first recessed part  56  of the first lens  51  from the outer side in the radial direction, and the second protrusion  62  distant as much as the predetermined distance U from the first protrusion  61  in the optical axis L direction is fitted into the second recessed part  57  of the second lens  52  from the outer side in the radial direction. Thus, since the positions of the first lens  51  and the second lens  52  in the optical axis L direction can be defined, the distance between the first-lens lateral surface  51   a  of the first lens  51  and the second-lens lateral surface  52   a  of the second lens  52  can be kept in the preset distance T set in advance. 
     Here, the first jointing member  54   a  and the second jointing member  54   b  are elastically deformable. Therefore, when the distance between the first lens  51  and the second lens  52  changes for setting the distance between the first lens  51  and the second lens  52  to the preset distance T, the first jointing member  54   a  changes in the thickness in the optical axis L direction to keep the state of adhering to the first lens  51  and the third lens  53 . Further, the second jointing member  54   b  also changes in the thickness in the optical axis L direction to keep the state of adhering to the third lens  53  and the second lens  52 . 
     Lens Unit According to Practical Example 2 
       FIG. 4  is a perspective view of a lens unit according to Practical Example 2.  FIG. 5  is a perspective view of the lens unit shown in  FIG. 4  when being cut along the line B-B.  FIG. 6  is a perspective view of the lens unit shown in  FIG. 4  when the holding ring is removed from the lens unit.  FIG. 7  is an explanatory diagram of a first fixation member  71  fixed to the first lens. The lens unit  50 B according to the present example is provided with constituents corresponding to those of the lens unit  50 A according to Practical Example 1. Therefore, the description will be presented denoting the corresponding constituents by the same reference numerals. 
     As shown in  FIG. 4 , the lens unit  50 B according to Practical Example 2 has the jointed lens  40  and the holding mechanism  55 . As shown in  FIG. 5 , as optical members, the jointed lens  40  is provided with a first lens  51 , a second lens  52  arranged on the optical axis L of the first lens  51 , and a third lens  53  disposed between the first lens  51  and the second lens  52 . Further, the jointed lens  40  is provided with the first jointing member  54   a  and the second jointing member  54   b , wherein the first jointing member  54   a  is located between the first lens  51  and the third lens  53 , and adheres to the first lens  51  and the third lens  53 , and the second jointing member  54   b  is located between the third lens  53  and the second lens  52 , and adheres to the third lens  53  and the second lens  52 . The first jointing member  54   a  and the second jointing member  54   b  have a light transmissive property. The first jointing member  54   a  and the second jointing member  54   b  are elastically deformable. In the present example, the jointing member  54  is an adhesive. 
     The holding mechanism  55  holds the first lens  51  and the second lens  52 . The holding mechanism  55  keeps the distance between the first-lens lateral surface  51   a  at the opposite side to the second lens  52  in the first lens  51  and the second-lens lateral surface  52   a  at the opposite side to the first lens  51  in the second lens  52  in the preset distance T set in advance. As shown in  FIG. 5  and  FIG. 6 , the holding mechanism  55  is provided with first fixation members  71  to be fixed to the outer circumferential edge of the first lens  51 , second fixation members  72  to be fixed to the outer circumferential edge of the second lens  52 , and the inter-surface distance keeping members  58  each disposed at the outer side in the radial direction of the first fixation member  71  and the second fixation member  72 . The first fixation members  71  and the second fixation members  72  are each disposed at three places around the optical axis L at regular angular intervals. The first fixation members  71  and the second fixation members  72  respectively overlap each other when viewed from the optical axis L direction. Further, the holding mechanism  55  is provided with three inter-surface distance keeping members  58  as the inter-surface distance keeping members  58 . The three inter-surface distance keeping members  58  are each disposed at the outer side in the radial direction of the first fixation member  71  and the second fixation member  72 . In the present example, the first fixation members  71  and the second fixation members  72  are made of metal. 
     As shown in  FIG. 6 , the first fixation member  71  is provided with a first facing surface  71   a  opposed to an outer circumferential edge part of the first-lens lateral surface  51   a  in the optical axis L direction. The second fixation member  72  is provided with a second facing surface  72   a  opposed to an outer circumferential edge part of the second-lens lateral surface  52   a  in the optical axis L direction. More specifically, as shown in  FIG. 7 , the first fixation member  71  is provided with a first contact part  73  making contact with the first-lens lateral surface  51   a  from the optical axis L direction, a second contact part  74  making contact with the outer circumferential edge part of a first-lens medial surface  51   c  at the second lens  52  side in the first lens  51  from the optical axis L direction, and a first coupling part  75  and a second coupling part  76  for coupling the first contact part  73  and the second contact part  74  to each other at two places in the circumferential direction. The first facing surface  71   a  is provided to the first contact part  73 . The first facing surface  71   a  is a taper surface tilted in a direction of getting away from the outer circumferential edge part of the outer circumferential surface of the first lens  51  from the inner circumferential side toward the outer circumferential side. Between the first facing surface  71   a  and the outer circumferential edge part of the first-lens lateral surface  51   a  in the optical axis L direction, there is disposed the first recessed part  56  recessed toward the inner side in the radial direction. The first recessed part  56  is disposed outside the effective ray range of the first lens  51 . The first recessed part  56  is the latched part to be engaged with the inter-surface distance keeping member  58 . 
     The first fixation member  71  and the second fixation member  72  are members the same as each other. The second fixation member  72  is fixed to the second lens  52  with the posture flipped in the optical axis L direction from that of the first fixation member  71 . Therefore, as shown in  FIG. 6 , the second fixation member  72  is provided with the first contact part  73  making contact with the second-lens lateral surface  52   a  from the optical axis L direction, the second contact part  74  making contact with the outer circumferential edge part of a second-lens medial surface  52   c  at the first lens  51  side in the second lens  52  from the optical axis L direction, and the first coupling part  75  and the second coupling part  76  for coupling the first contact part  73  and the second contact part  74  to each other at two places in the circumferential direction. The second facing surface  72   a  is provided to the second contact part  74 . The second facing surface  72   a  is a taper surface tilted in a direction of getting away from the outer circumferential edge part of the second-lens lateral surface  52   a  from the inner circumferential side toward the outer circumferential side. Between the second facing surface  72   a  and the outer circumferential edge part of the second-lens lateral surface  52   a  in the optical axis L direction, there is disposed the second recessed part  57  recessed toward the inner side in the radial direction. The second recessed part  57  is disposed outside the effective ray range of the second lens  52 . The second recessed part  57  is the latched part to be engaged with the inter-surface distance keeping member  58 . 
     As shown in  FIG. 5 , the inter-surface distance keeping members  58  are each provided with the main body part  60  extending in the optical axis L direction, the first protrusion  61  protruding toward the inner side in the radial direction from the main body part  60 , and the second protrusion  62  protruding toward the inner side in the radial direction at a position distant as much as the predetermined distance U from the first protrusion  61  in the optical axis L direction. The main body part  60  is disposed between the first coupling part  75  and the second coupling part  76  in the first fixation member  71 , and between the first coupling part  75  and the second coupling part  76  in the second fixation member  72 . The first protrusion  61  is fitted into the first recessed part  56  of the first lens  51  from the outer side in the radial direction. The second protrusion  62  is fitted into the second recessed part  57  of the second lens  52  from the outer side in the radial direction. In other words, the first protrusion  61  is a first latch part to be engaged with the first recessed part  56 , and the second protrusion  62  is a second latch part to be fitted into the second recessed part  57 . In the present example, the inter-surface distance keeping members  58  are made of metal. 
     Further, the lens unit  50 B is provided with the holding ring  65  for holding the three inter-surface distance keeping members  58  from the outer circumferential side. The holding ring  65  makes contact with each of the inter-surface distance keeping members  58  from the outer circumferential side. 
     According to the present example, by fixing the first fixation member  71  to the first lens  51 , the first recessed part  56  recessed toward the inner side in the radial direction is disposed between the first fixation member  71  and the first lens  51 . Further, by fixing the second fixation member  72  to the second lens  52 , the second recessed part  57  recessed toward the inner side in the radial direction is disposed between the second fixation member  72  and the second lens  52 . Further, the first protrusion  61  of the inter-surface distance keeping member  58  is fitted into the first recessed part  56  from the outer side in the radial direction, and the second protrusion  62  distant as much as the predetermined distance U from the first protrusion  61  in the optical axis L direction is fitted into the second recessed part  57  from the outer side in the radial direction. Thus, since the positions of the first lens  51  and the second lens  52  in the optical axis L direction can be defined, the distance between the first-lens lateral surface  51   a  of the first lens  51  and the second-lens lateral surface  52   a  of the second lens  52  can be kept in the preset distance T set in advance. 
     Further, the first jointing member  54   a  and the second jointing member  54   b  are elastically deformable. Therefore, when the distance between the first lens  51  and the second lens  52  changes for setting the distance between the first lens  51  and the second lens  52  to the preset distance T, the first jointing member  54   a  changes in the thickness along the optical axis L to keep the state of adhering to the first lens  51  and the third lens  53 . Further, the second jointing member  54   b  also changes in the thickness along the optical axis L to keep the state of adhering to the third lens  53  and the second lens  52 . 
     Lens Unit According to Practical Example 3 
       FIG. 8  is a perspective view of a lens unit according to Practical Example 3.  FIG. 9  is a cross-sectional view of the lens unit shown in  FIG. 8  when being cut along the line C-C.  FIG. 10  is a cross-sectional view of the lens unit shown in  FIG. 8  when being cut along the line D-D.  FIG. 11  is a perspective view of the lens unit in the state in which a holding ring is detached. It should be noted that since the lens unit  50 C according to Practical Example 3 is provided with constituents corresponding to those of the lens unit  50 A according to Practical Example 1, the corresponding constituents are denoted by the same reference symbols. 
     As shown in  FIG. 8 , the lens unit  50 C according to Practical Example 3 has the jointed lens  40  and the holding mechanism  55 . As shown in  FIG. 9 , as optical members, the jointed lens  40  is provided with the first lens  51 , the second lens  52  arranged on the optical axis L of the first lens  51 , and the third lens  53  disposed between the first lens  51  and the second lens  52 . Further, the jointed lens  40  is provided with the first jointing member  54   a  and the second jointing member  54   b , wherein the first jointing member  54   a  is located between the first lens  51  and the third lens  53 , and adheres to the first lens  51  and the third lens  53 , and the second jointing member  54   b  is located between the third lens  53  and the second lens  52 , and adheres to the third lens  53  and the second lens  52 . The first jointing member  54   a  and the second jointing member  54   b  have a light transmissive property. Further, the first jointing member  54   a  and the second jointing member  54   b  are elastically deformable. In the present example, the jointing member  54  is an adhesive. 
     The holding mechanism  55  holds the first lens  51  and the second lens  52 . The holding mechanism  55  keeps the distance between the first-lens lateral surface  51   a  at the opposite side to the second lens  52  in the first lens  51  and the second-lens lateral surface  52   a  at the opposite side to the first lens  51  in the second lens  52  in the preset distance T set in advance. The adjustment mechanism  55  is provided with first reference surfaces  81  disposed by the outer circumferential side of the effective ray range of the first-lens lateral surface  51   a , and second reference surfaces  82  disposed by the outer circumferential side of the effective ray range of the second-lens lateral surface  52   a . Further, the holding mechanism  55  is provided with the inter-surface distance keeping members  58  disposed at the outer side in the radial direction of the first lens  51  and the second lens  52 , first pressing members  83  for pressing the first lens  51 , and second pressing members  84  for pressing the second lens  52 . 
     The first reference surfaces  81  are disposed at three places around the optical axis L in the first lens  51 . The second reference surfaces  82  are disposed at three places around the optical axis L in the second lens  52 . In the present example, the three first reference surfaces  81  and the three second reference surfaces  82  are each disposed at regular angular intervals. The first reference surfaces  81  and the second reference surfaces  82  are located at the same angular positions around the optical axis L, respectively. More specifically, as shown in  FIG. 11 , the first lens  51  is provided with first protruding parts  85  protruding outward in the radial direction at three places at regular angular intervals around the optical axis L. The surface at an opposite side to the second lens  52  of each of the first protruding parts  85  corresponds to the first reference surface  81  perpendicular to the optical axis L. The second lens  52  is provided with second protruding parts  86  protruding outward in the radial direction at three places at regular angular intervals around the optical axis L. When viewed from the optical axis L direction, the first protruding parts  85  and the second protruding parts  86  overlap each other. The surface at an opposite side to the first lens  51  of each of the second protruding parts  86  corresponds to the second reference surface  82  perpendicular to the optical axis L. 
     Here, the first protruding parts  85  and the first reference surfaces  81  can be formed using a metal mold when molding the first lens  51 . It should be noted that it is possible to form the first reference surfaces  81  without providing the first protruding parts  85  to the first lens  51 . In this case, the first reference surfaces  81  can be formed by cutting work outside the effective ray range of the first lens  51 . Similarly, the second protruding parts  86  and the second reference surfaces  82  can be formed using a metal mold when molding the second lens  52 . It should be noted that it is possible to form the second reference surfaces  82  without providing the second protruding parts  86  to the second lens  52 . In this case, the second reference surfaces  82  can be formed by cutting work outside the effective ray range of the second lens  52 . 
     The inter-surface distance keeping members  58  are each a ring-like member. As shown in  FIG. 9  and  FIG. 10 , the inter-surface distance keeping members  58  are each provided with the main body part  60  surrounding the first lens  51  and the second lens  52  from the outer side in the radial direction, the first protrusion  61  which has a ring-like shape and protrudes toward the inner side in the radial direction from an end portion at one side in the optical axis L direction of the main body part  60 , and the second protrusion  62  which has a ring-like shape and protrudes toward the inner side in the radial direction from an end portion at the other side in the optical axis L direction of the main body part  60 . The first protrusion  61  is provided with a first positioning surface  61   a  which can make contact with the first reference surface  81  of the first lens  51  from the optical axis L direction. The second protrusion  62  is provided with a second positioning surface  62   a  which can make contact with the second reference surface  82  of the second lens  52  from the optical axis L direction. The second positioning surface  62   a  is distant as much as a predetermined distance V from the first positioning surface  61   a  in the optical axis L direction. The inter-surface distance keeping members  58  can be formed of metal. 
     Further, as shown in  FIG. 10  and  FIG. 11 , the holding mechanism  55  is provided with the first pressing members  83  for pressing the first lens  51  toward the optical axis L direction, and the second pressing members  84  for pressing the second lens  52  toward the optical axis L direction. The first pressing members  83  and the second pressing members  84  are each disposed at three places around the optical axis L. The first pressing members  83  are each a coil spring, and are each disposed between the second protrusion  62  of the inter-surface distance keeping member  58  and the first protruding part  85  of the first lens  51  to bias the first lens  51  toward the first positioning surface  61   a . Thus, the first reference surface  81  makes contact with the first positioning surface  61   a . The second pressing members  84  are each a coil spring, and are each disposed between the first protrusion  61  of the inter-surface distance keeping member  58  and the second protruding part  86  of the second lens  52  to bias the second lens  52  toward the second positioning surface  62   a . Thus, the second reference surface  82  makes contact with the second positioning surface  62   a . It should be noted that as the first pressing members  83  and the second pressing members  84 , it is possible to use an elastic member such as rubber. 
     As shown in  FIG. 11 , each of the first protruding parts  85  of the first lens  51  is provided with a first groove  85   a  for disposing the second pressing member  84  between the first protrusion  61  and the second protruding part  86  of the second lens  52 . The first groove  85   a  penetrates the first protruding part  85  in the optical axis L direction. Further, each of the second protruding parts  86  of the second lens  52  is provided with a second groove  86   a  for disposing the first pressing member  83  between the second protrusion  62  and the first protruding part  85  of the first lens  51 . The second groove  86   a  penetrates the second protruding part  86  in the optical axis L direction. 
     The inter-surface distance keeping members  58  are each provided with the first positioning surface  61   a  which can make contact with the first reference surface  81  of the first lens  51  from the optical axis L direction. Further, the inter-surface distance keeping members  58  are each provided with the second positioning surface  62   a  which can make contact with the second reference surface  82  of the second lens  52  from the optical axis L direction at a position distant as much as the predetermined distance V from the first positioning surface  61   a  in the optical axis L direction. Further, the holding mechanism  55  is provided with the first pressing members  83  for pressing the first lens  51  toward the first positioning surfaces  61   a  to make the first reference surfaces  81  make contact with the first positioning surfaces  61   a , and the second pressing members  84  for pressing the second lens  52  toward the second positioning surfaces  62   a  to make the second reference surfaces  82  make contact with the second positioning surfaces  62   a . Thus, since the positions of the first lens  51  and the second lens  52  in the optical axis L direction can be defined, the distance between the first-lens lateral surface  51   a  of the first lens  51  and the second-lens lateral surface  52   a  of the second lens  52  can be kept in the preset distance T set in advance. 
     Further, the first jointing member  54   a  and the second jointing member  54   b  are elastically deformable. Therefore, when the distance between the first lens  51  and the second lens  52  changes for setting the distance between the first lens  51  and the second lens  52  to the preset distance T, the first jointing member  54   a  changes in the thickness along the optical axis L to keep the state of adhering to the first lens  51  and the third lens  53 . Further, the second jointing member  54   b  also changes in the thickness along the optical axis L to keep the state of adhering to the third lens  53  and the second lens  52 . 
     Modified Example of Lens Unit 
     It should be noted that in the lens units  50 A through  50 C, it is possible to assume that the jointed lens  40  has two lenses, namely the first lens  51  and the second lens  52 . In this case, the first lens  51  and the second lens  52  are bonded to each other with the jointing member  54  provided with a light transmissive property. The jointing member  54  is elastically deformable. The jointing member  54  adheres to the first lens  51  and the second lens  52  located before and after the jointing member  54  in the optical axis L direction. 
     Also in such a manner, when the lens unit is provided with the holding mechanism  55 , it is possible to define the positions of the first lens  51  and the second lens  52  in the optical axis L direction. Therefore, the distance between the first-lens lateral surface  51   a  at the opposite side to the second lens  52  in the first lens  51  and the second-lens lateral surface  52   a  at the opposite side to the first lens  51  in the second lens  52  can be kept in the preset distance T set in advance. The jointing member  54  is elastically deformable. Therefore, when the distance between the first lens  51  and the second lens  52  changes for setting the distance between the first lens  51  and the second lens  52  to the preset distance T, the jointing member  54  changes in the thickness along the optical axis L to keep the state of adhering to the first lens  51  and the second lens  52 . 
     Projector 
       FIG. 12  is a schematic configuration diagram of a projector equipped with the projection optical system  3  according to the present disclosure. As shown in  FIG. 12 , the projector  1  is provided with an image formation section  2  for generating a projection image to be projected on a screen S, the projection optical system  3  for enlarging the projection image to project the enlarged image on the screen S, and a control section  4  for controlling an operation of the image formation section  2 . 
     Image Generation Optical System and Control Section 
     The image formation section  2  is provided with a light source  10 , a first integrator lens  11 , a second integrator lens  12 , a polarization conversion element  13 , and a superimposing lens  14 . The light source  10  is formed of, for example, a super-high pressure mercury lamp or a solid-state light source. The first integrator lens  11  and the second integrator lens  12  each have a plurality of lens elements arranged in an array. The first integrator lens  11  divides a light beam from the light source  10  into a plurality of light beams. Each of the lens elements of the first integrator lens  11  converges the light beam from the light source  10  in the vicinity of the corresponding one of the lens elements of the second integrator lens  12 . 
     The polarization conversion element  13  converts the light from the second integrator lens  12  into predetermined linearly polarized light. The superimposing lens  14  superimposes the images of the respective lens elements of the first integrator lens  11  on the display area of each of a liquid crystal panel  18 R, a liquid crystal panel  18 G, and a liquid crystal panel  18 B described later via the second integrator lens  12 . 
     Further, the image formation section  2  is provided with a first dichroic mirror  15 , a reflecting mirror  16  and a field lens  17 R, and the liquid crystal panel  18 R. The first dichroic mirror  15  reflects R light as a part of the light beam having entered the first dichroic mirror  15  from the superimposing lens  14 , and transmits G light and B light each of which is a part of the light beam having entered the first dichroic mirror  15  from the superimposing lens  14 . The R light having been reflected by the first dichroic mirror  15  enters the liquid crystal panel  18 R via the reflecting mirror  16  and the field lens  17 R. The liquid crystal panel  18 R is a light modulator. The liquid crystal panel  18 R modulates the R light in accordance with an image signal to thereby form a red projection image. 
     Further, the image formation section  2  is provided with a second dichroic mirror  21 , a field lens  17 G, and the liquid crystal panel  18 G. The second dichroic mirror  21  reflects the G light as a part of the light beam from the first dichroic mirror  15 , and transmits the B light as a part of the light beam from the first dichroic mirror  15 . The G light having been reflected by the second dichroic mirror  21  enters the liquid crystal panel  18 G via the field lens  17 G. The liquid crystal panel  18 G is a light modulator. The liquid crystal panel  18 G modulates the G light in accordance with the image signal to thereby form a green projection image. 
     Further, the image formation section  2  is provided with a relay lens  22 , a reflecting mirror  23 , a relay lens  24 , a reflecting mirror  25  and a field lens  17 B, and the liquid crystal panel  18 B. The B light having been transmitted through the second dichroic mirror  21  enters the liquid crystal panel  18 B via the relay lens  22 , the reflecting mirror  23 , the relay lens  24 , the reflecting mirror  25 , and the field lens  17 B. The liquid crystal panel  18 B is a light modulator. The liquid crystal panel  18 B modulates the B light in accordance with the image signal to thereby form a blue projection image. 
     The liquid crystal panel  18 R, the liquid crystal panel  18 G, and the liquid crystal panel  18 B surround a cross dichroic prism  19  from three directions. The cross dichroic prism  19  is a light combining prism, and generates the projection image obtained by combining the light beams modulated by the respective liquid crystal panels  18 R,  18 G, and  18 B with each other. 
     Here, the cross dichroic prism  19  constitutes apart of the projection optical system  3 . The projection optical system  3  projects the projection image (the images formed by the respective liquid crystal panels  18 R,  18 G, and  18 B) combined by the cross dichroic prism  19  on the screen S in an enlarged manner. The screen S is an elargement-side imaging plane of the projection optical system  3 . 
     The control section  4  is provided with an image processing section  6  to which an external image signal such as a video signal is input, and a display drive section  7  for driving the liquid crystal panel  18 R, the liquid crystal panel  18 G, and the liquid crystal panel  18 B based on the image signals output from the image processing section  6 . 
     The image processing section  6  converts the image signal input from external equipment into image signals including the tones and so on of the respective colors. The display drive section  7  operates the liquid crystal panel  18 R, the liquid crystal panel  18 G, and the liquid crystal panel  18 B based on the projection image signals of the respective colors output from the image processing section  6 . Thus, the image processing section  6  displays the projection images corresponding to the image signals on the liquid crystal panel  18 R, the liquid crystal panel  18 G, and the liquid crystal panel  18 B, respectively. 
     Projection Optical System 
     Then, the projection optical system  3  will be described. Practical Example 1 through Practical Example 3 will hereinafter be described as configuration examples of the projection optical system  3  to be implemented in the projector  1 . It should be noted that in each of Practical Example 1 through Practical Example 3, the liquid crystal panel  18 R, the liquid crystal panel  18 G, and the liquid crystal panel  18 B are described as the liquid crystal panel  18  in the ray chart of the projection optical system. Further, each of Practical Example 1 through Practical Example 3 is provided with the lens unit  50 . The lens unit  50  is what is obtained by jointing the two or three lenses to each other with the jointing member  54  elastically deformable. In the lens unit  50 , the jointing member  54  is an adhesive. Further, the lens unit  50  is provided with any one of the holding mechanisms  55  described in the lens units  50 A through  50 C described above. 
     Projection Optical System According to Practical Example 4 
       FIG. 13  is a ray chart schematically showing a whole of a projection optical system according to Practical Example 4.  FIG. 13  schematically shows the light beams which reach the screen S from the projection optical system  3 A according to the present example with the light beams F 1  through F 3 . The light beam F 1  is a light beam which reaches the position where the image height is the lowest. The light beam F 3  is a light beam which reaches the position where the image height is the highest. The light beam F 2  is a light beam which reaches a position between the positions of the light beam F 1  and the light beam F 3 .  FIG. 14  is a ray chart of the projection optical system  3 A according to Practical Example 4.  FIG. 15  is a ray chart of a second optical system in the projection optical system  3 A. 
     As shown in  FIG. 14 , the projection optical system  3 A according to the present example is constituted by a first optical system  31  and a second optical system  32  arranged in sequence from the reduction-side toward the elargement-side. The first optical system  31  is a refracting optical system provided with a plurality of lenses. The second optical system  32  is the lens unit  50  having three lenses bonded to each other. The second optical system  32  has a mirror M provided with a concavely curved surface. The projection optical system  3 A forms a first intermediate image  351  conjugate with a reduction-side imaging plane in the middle of the first optical system  31 . Further, the projection optical system  3 A forms a second intermediate image  352  conjugate with the reduction-side imaging plane in the middle of the second optical system  32 . Further, the projection optical system  3 A forms a final image conjugate with the second intermediate image  352  on the elargement-side imaging plane. 
     By the reduction-side imaging plane, there is disposed the liquid crystal panel  18  of the image formation section  2 . The liquid crystal panel  18  forms the projection image at the other side of the optical axis N of the first optical system  31 . The first intermediate image  351  is formed at one side of the optical axis N of the first optical system  31 . The second intermediate image  352  is formed at the other side of the optical axis N of the first optical system  31 . The elargement-side imaging plane is disposed at one side of the optical axis N of the first optical system  31 . In the elargement-side imaging plane, there is disposed the screen S. 
     In the following description, three axes perpendicular to each other are defined as an X axis, a Y axis, and a Z axis for the sake of convenience. Further, it is assumed that the optical axis direction along the optical axis N of the first optical system  31  is the Z-axis direction, one side of the optical axis N is an upper side Y 1  in the Y-axis direction, and the other side is a lower side Y 2  in the Y-axis direction. It is assumed that a plane which is perpendicular to the X axis and includes the Y axis and the Z axis is a Y-Z plane. Therefore, the liquid crystal panel  18  forms the projection image in the lower side Y 2  of the optical axis N. The first intermediate image  351  is formed in the upper side Y 1  of the optical axis N. The second intermediate image  352  is formed in the lower side Y 2  of the optical axis N. The screen S is located in the upper side Y 1  of the optical axis N. The lateral direction of the screen S corresponds to the X-axis direction. The second intermediate image  352  is an image vertically flipped in the Y-axis direction with respect to the enlarged image to be formed in the screen S.  FIG. 13 ,  FIG. 14 , and  FIG. 15  are each a ray chart in the Y-Z plane. 
     As shown in  FIG. 14 , the first optical system  31  has the cross dichroic prism  19 , and sixteen lenses L 1  through L 16 . The first lens L 1  through the 16-th lens L 16  are arranged in this order from the reduction side toward the elargement side. In the present example, the third lens L 3  and the fourth lens L 4  are bonded to each other to form a first jointed lens L 21 . The 11-th lens L 11  and the 12-th lens L 12  are bonded to each other to form a second jointed lens L 22 . The 14-th lens L 14  and the 15-th lens L 15  are bonded to each other to form a third jointed lens L 23 . 
     As shown in  FIG. 15 , the second optical system  32  is constituted by a 17-th lens L 17 , an 18-th lens L 18 , and a 19-th lens L 19 . The first jointing member  54   a  intervenes between the 17-th lens and the 18-th lens, and the second jointing member  54   b  intervenes between the 18-th lens and the 19-th lens. By an outer side in the redial direction of the 17-th lens L 17 , the 18-th lens L 18 , and the 19-th lens L 19 , there is disposed the holding mechanism  55  for keeping the distance between a 17-th-lens lateral surface Ll 7   a  at an opposite side to the 19-th lens L 19  in the 17-th lens L 17  and a 19-th-lens lateral surface Ll 9   a  at an opposite side to the 17-th lens L 17  in the 19-th lens L 19  in the preset distance T set in advance. Further, the second optical system  32  has the mirror M provided with the concavely curved surface. The mirror M is a reflective coating layer provided to the 19-th-lens lateral surface Ll 9   a . The mirror M reflects the ray from the first optical system  31  toward the upper side Y 1 . 
     Lens Data 
     The lens data of the projection optical system  3 A is as follows. The surface numbers are provided in sequence from the reduction side toward the elargement side. The reference symbols are the reference symbols of the lenses and the mirrors. The data of the surface numbers not corresponding to any lenses or any mirrors are dummy data. The reference symbol R represents a curvature radius. The reference symbol D represents an axial surface distance. The reference symbol A represents an effective diameter. The units of R, D, and A are millimeter. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 REFERENCE 
                 SURFACE 
                   
                   
                   
                 GLASS 
                 REFRACTION/ 
                   
               
               
                 SYMBOL 
                 NUMBER 
                 SHAPE 
                 R 
                 D 
                 MATERIAL 
                 REFLECTION 
                 A 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 0 
                 SPHERE 
                 INFINITE 
                 5.3900 
                   
                 REFRACTION 
                 0.0000 
               
               
                   
                 1 
                 SPHERE 
                 INFINITE 
                 25.9100 
                 BSC7 
                 REFRACTION 
                 11.9263 
               
               
                 19 
                 2 
                 SPHERE 
                 INFINITE 
                 4.0000 
                   
                 REFRACTION 
                 13.9142 
               
               
                   
                 3 
                 SPHERE 
                 INFINITE 
                 0.3397 
                   
                 REFRACTION 
                 14.8696 
               
               
                 L1 
                 4 
                 SPHERE 
                 −328.1403 
                 8.9875 
                 TAFD5F 
                 REFRACTION 
                 14.8702 
               
               
                   
                 5 
                 SPHERE 
                 −32.9236 
                 0.5000 
                   
                 REFRACTION 
                 15.6675 
               
               
                 L2 
                 6 
                 SPHERE 
                 35.2778 
                 10.6178 
                 TAF1 
                 REFRACTION 
                 14.2863 
               
               
                   
                 7 
                 SPHERE 
                 2158.7790 
                 0.5956 
                   
                 REFRACTION 
                 12.0454 
               
               
                 L3 
                 8 
                 SPHERE 
                 16.4198 
                 5.7747 
                 FCD515 
                 REFRACTION 
                 9.8503 
               
               
                 L4 
                 9 
                 SPHERE 
                 158.3827 
                 1.9851 
                 FD225 
                 REFRACTION 
                 8.1367 
               
               
                   
                 10 
                 SPHERE 
                 12.3305 
                 2.8995 
                   
                 REFRACTION 
                 6.2473 
               
               
                   
                 11 
                 SPHERE 
                 INFINITE 
                 1.6564 
                   
                 REFRACTION 
                 5.6369 
               
               
                 L5 
                 12 
                 SPHERE 
                 −14.2698 
                 13.1316 
                 SFSL5 
                 REFRACTION 
                 5.6271 
               
               
                   
                 13 
                 SPHERE 
                 −20.3638 
                 30.0346 
                   
                 REFRACTION 
                 7.5000 
               
               
                 L6 
                 14 
                 SPHERE 
                 54.1315 
                 17.4918 
                 SLAH89 
                 REFRACTION 
                 29.0000 
               
               
                   
                 15 
                 SPHERE 
                 −167.1383 
                 21.0941 
                   
                 REFRACTION 
                 26.1025 
               
               
                 L7 
                 16 
                 SPHERE 
                 −37.2827 
                 15.0000 
                 EC3 
                 REFRACTION 
                 22.4382 
               
               
                   
                 17 
                 SPHERE 
                 54.5260 
                 12.1913 
                   
                 REFRACTION 
                 25.8190 
               
               
                 L8 
                 18 
                 SPHERE 
                 −76.5126 
                 14.7917 
                 TAFD33 
                 REFRACTION 
                 26.4354 
               
               
                   
                 19 
                 SPHERE 
                 −36.2470 
                 38.4506 
                   
                 REFRACTION 
                 29.0033 
               
               
                   
                 20 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 31.6561 
               
               
                 L9 
                 21 
                 SPHERE 
                 133.8789 
                 15.8392 
                 SFSL5 
                 REFRACTION 
                 31.8487 
               
               
                   
                 22 
                 SPHERE 
                 −68.9741 
                 2.5994 
                   
                 REFRACTION 
                 31.6470 
               
               
                 L10 
                 23 
                 SPHERE 
                 31.3378 
                 11.0553 
                 TAF1 
                 REFRACTION 
                 23.6500 
               
               
                   
                 24 
                 SPHERE 
                 64.5697 
                 0.5000 
                   
                 REFRACTION 
                 21.2712 
               
               
                 L11 
                 25 
                 SPHERE 
                 52.2125 
                 20.0000 
                 FCD100 
                 REFRACTION 
                 20.3631 
               
               
                 L12 
                 26 
                 SPHERE 
                 −25.0000 
                 10.7786 
                 TAFD32 
                 REFRACTION 
                 11.6726 
               
               
                   
                 27 
                 SPHERE 
                 32.9539 
                 6.4395 
                   
                 REFRACTION 
                 7.8949 
               
               
                 L13 
                 28 
                 SPHERE 
                 143.1217 
                 7.4044 
                 FCD515 
                 REFRACTION 
                 10.7085 
               
               
                   
                 29 
                 SPHERE 
                 −20.0723 
                 0.5000 
                   
                 REFRACTION 
                 12.1527 
               
               
                 L14 
                 30 
                 SPHERE 
                 −58.8944 
                 2.6061 
                 FDS90SG 
                 REFRACTION 
                 13.2638 
               
               
                 L15 
                 31 
                 SPHERE 
                 26.6089 
                 14.8509 
                 SLAH97 
                 REFRACTION 
                 16.1290 
               
               
                   
                 32 
                 SPHERE 
                 −52.1867 
                 25.0657 
                   
                 REFRACTION 
                 18.4714 
               
               
                 L16 
                 33 
                 SPHERE 
                 78.7767 
                 13.4658 
                 SNBH57 
                 REFRACTION 
                 32.5354 
               
               
                   
                 34 
                 SPHERE 
                 −208.7929 
                 5.0000 
                   
                 REFRACTION 
                 32.4274 
               
               
                   
                 35 
                 SPHERE 
                 INFINITE 
                 19.5666 
                   
                 REFRACTION 
                 30.9799 
               
               
                 L17 
                 36 
                 ASPHERIC 
                 −369.8594 
                 15.0386 
                 E48R_ZEON 
                 REFRACTION 
                 26.5882 
               
               
                   
                   
                 SURFACE 
               
               
                 L18 
                 37 
                 SPHERE 
                 150.0000 
                 15.0000 
                 SBAL12 
                 REFRACTION 
                 23.1486 
               
               
                   
                 38 
                 SPHERE 
                 −103.0000 
                 10.0779 
                 E48R_ZEON 
                 REFRACTION 
                 20.1205 
               
               
                 L19, M 
                 39 
                 ASPHERIC 
                 −18.5869 
                 −10.0779 
                 E48R_ZEON 
                 REFLECTION 
                 20.2000 
               
               
                   
                   
                 SURFACE 
               
               
                 L18 
                 40 
                 SPHERE 
                 −103.0000 
                 −15.0000 
                 SBAL12 
                 REFRACTION 
                 14.3544 
               
               
                   
                 41 
                 SPHERE 
                 150.0000 
                 −15.0386 
                 E48R_ZEON 
                 REFRACTION 
                 32.9333 
               
               
                 L17 
                 42 
                 ASPHERIC 
                 74.397 
                 −240.9501 
                   
                 REFRACTION 
                 37.4796 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 43 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 1450.6157 
               
               
                   
               
            
           
         
       
     
     The aspherical coefficient of each of the aspheric surfaces is as follows. 
     
       
         
           
               
               
               
               
             
               
                   
               
               
                 SURFACE NUMBER 
                 36 
                 39 
                 42 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Y CURVATURE 
                 −369.8593556 
                 −18.58690118 
                 74.39788526 
               
               
                 RADIUS 
               
               
                 CONIC 
                 0 
                 −4.147826268 
                 2.563567409 
               
               
                 CONSTANT (K) 
               
               
                 4-th-ORDER 
                 2.37906E−05 
                 −1.85425E−05  
                 1.00621E−06 
               
               
                 COEFFICIENT (A) 
               
               
                 6-th-ORDER 
                 −2.90682E−08  
                 5.74483E−08 
                 7.74934E−10 
               
               
                 COEFFICIENT (B) 
               
               
                 8-th-ORDER 
                 2.72699E−11 
                 −7.17873E−11  
                 −7.32277E−13  
               
               
                 COEFFICIENT (C) 
               
               
                 10-th-ORDER 
                 −1.36402E−14  
                 4.39201E−14 
                 2.98841E−16 
               
               
                 COEFFICIENT (D) 
               
               
                   
               
            
           
         
       
     
     Functions and Advantages 
     Here, the lens unit  50  constituting the second optical system  22  is provided with the refracting surface and the reflecting surface, and the ray is folded at these surfaces. Therefore, when the thickness of the lens unit  50  changes from the design value, there is a problem that the direction of the ray passing through the lens unit  50  varies to increase the TV distortion in the final image. 
       FIG. 16  is a graph showing a simulation of a relationship between the change in thickness of each of the members constituting the lens unit and the TV distortion in the final image to be projected to the screen S in the projection optical system constituted by the first optical system formed of the refracting optical system and the second optical system formed of the lens unit. The lens unit targeted by the simulation is one corresponding to the lens unit  50 . Specifically, the lens unit is constituted by an incident-side lens which the ray from the first optical system enters, a reflection-side lens provided with a mirror formed of a coating layer, and an intermediate lens located between the incident-side lens and the reflection-side lens. In terms of the projection optical system  3 A according to the present example, the incident-side lens corresponds to the 17-th lens L 17 . The reflection-side lens corresponds to the 19-th lens L 19 . The intermediate lens corresponds to the 18-th lens L 18 . Between the incident-side lens and the intermediate lens, there intervenes the first jointing member  54   a  adhering to these lenses. Between the intermediate lens and the reflection-side lens, there intervenes the second jointing member  54   b  adhering to these lenses. 
     In  FIG. 16 , the values in the vertical axis in the fields of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens each represent the variation of the TV distortion when the thickness increases or decreases as much as 30 μm from the design value. These values represent when the thickness of one of the members changes alone, and the thicknesses of the other of the members are kept in the design values. Further, the value in the vertical axis in the field of the total represents the variation of the TV distortion when all of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens each increase as much as 30 μm, or each decrease as much as 30 μm. It should be noted that when the vertical trapezoid of the TV distortion positively varies means the state in which an upper side of the screen becomes longer than the lower side, and when the vertical trapezoid thereof negatively varies means the reverse. When the vertical pin-cushion of the TV distortion positively varies means the state in which the height at right and left sides becomes larger than the height at the center of the screen, and when the vertical pin-cushion thereof negatively varies means the reverse. When the horizontal pin-cushion of the TV distortion positively varies means the state in which the width at right and left sides becomes larger than the width at the center of the screen, and when the horizontal pin-cushion thereof negatively varies means the reverse. 
       FIG. 17  is an explanatory diagram of the variation of the vertical trapezoid when the thickness of the lens unit takes the design value, and when all of the members of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens each become thicker as much as 30 μm with respect to the points set at twenty places in the final image to be shown in the screen S.  FIG. 18  is an explanatory diagram of the variation of the vertical trapezoid when the thickness of the lens unit takes the design value, and when all of the members of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens each become thinner as much as 30 μm with respect to the points in the final image to be shown in the screen S. In  FIG. 17  and  FIG. 18 , the white circles represent the positions of the points when the thickness of the lens unit takes the design value, and the gray circles represent the positions of the points when the thickness of the lens unit increases or decreases from the design value. 
     According to  FIG. 16 , it is understood that the TV distortion varies when the thickness of each of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens increases or decreases. Further, it is also understood that when all of the thicknesses of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens increase or decrease, the TV distortion significantly varies compared to the value of the TV distortion at the design value. Also in  FIG. 17  and  FIG. 18 , it is understood that the TV distortion significantly varies compared to the value of the TV distortion at the design value when all of the thicknesses of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens increase or decrease. 
     In contrast, from  FIG. 16 , it is understood that the TV distortion varies at a constant rate when the thickness of any one of the incident-side lens, the first adhesive, the intermediate lens, the second adhesive, and the reflection-side lens increases or decreases irrespective of what member increases or decreases in thickness. Therefore, it can be determined that the TV distortion can be suppressed by approximating the distance between the plane of incidence of the incident-side lens and the reflecting surface of the reflection-side lens as the total thickness of the jointed lens to the design value even when changing the thicknesses of the two members, namely the first adhesive and the second adhesive, which deform elastically. 
     From such a point of view, in the projection optical system  3 A according to the present example, the second optical system  32  is formed of the lens unit  50  provided with the holding mechanism  55 . Therefore, the distance between the 17-th-lens lateral surface Ll 7   a  at the opposite side to the 19-th lens L 19  in the 17-th lens L 17  and the 19-th-lens lateral surface Ll 9   a  at the opposite side to the 17-th lens L 17  in the 19-th lens L 19  can be kept in the preset distance T set in advance. Therefore, it is possible to prevent the TV distortion from occurring, and thus, it is possible to prevent the distortion aberration from occurring in the final image. 
       FIG. 19  is a diagram showing MTF by the elargement side of the projection optical system  3 A. The horizontal axis of MTF represents the spatial frequency. The vertical axis of MTF represents a contrast reproduction ratio. In the drawing, the black graph represents a tangential ray (T), and the gray graph represents a radial ray (R). Further, in each of the tangential ray (T) and the radial ray (R), the solid line represents the light beam F 1 , the dashed line represents the light beam F 2 , and the dotted line represents the light beam F 3 . As shown in  FIG. 16 , the projection optical system  3 A according to the present example is high in resolution. 
     Projection Optical System According to Practical Example 5 
       FIG. 20  is a ray chart schematically showing a whole of a projection optical system according to Practical Example 5.  FIG. 20  schematically shows the light beams which reach the screen S from the projection optical system  3 B according to the present example with the light beams F 1  through F 3 .  FIG. 21  is a ray chart of the projection optical system according to Practical Example 5.  FIG. 22  is a ray chart of a second optical system in Practical Example 5. It should be noted that since the projection optical system  3 B according to Practical Example 5 is provided with constituents corresponding to those of the projection optical system  3 A described above, the description will be presented denoting the corresponding constituents by the same reference symbols. 
     As shown in  FIG. 20 , the projection optical system  3 B according to the present example is constituted by the first optical system  31  and the second optical system  32  arranged in sequence from the reduction-side toward the elargement-side. The first optical system  31  is a refracting optical system provided with a plurality of lenses. The second optical system  32  is the lens unit  50  having two lenses bonded to each other. As shown in  FIG. 22 , the second optical system  32  has the mirror M provided with the concavely curved surface. The projection optical system  3 B forms an intermediate image  35  conjugate with a reduction-side imaging plane in the middle of the second optical system  32 . Further, the projection optical system  3 B forms a final image conjugate with the intermediate image  35  in an elargement-side imaging plane. 
     By the reduction-side imaging plane, there is disposed the liquid crystal panel  18  of the image formation section  2 . The liquid crystal panel  18  forms the projection image in the upper side Y 1  of the optical axis N. The intermediate image  35  is formed in the lower side Y 2  of the optical axis N. The screen S is located in the upper side Y 1  of the optical axis N. The intermediate image  35  is an image vertically flipped in the Y-axis direction with respect to the enlarged image to be formed in the screen S. 
     The first optical system  31  has the cross dichroic prism  19 , and the fourteen lenses L 1  through L 14 . The first lens L 1  through the 14-th lens L 14  are arranged in this order from the reduction side toward the elargement side. In the present example, the second lens L 2  and the third lens L 3  are bonded to each other to form the first jointed lens L 21 . The fourth lens L 4  and the 5-th lens L 5  are bonded to each other to form the second jointed lens L 22 . The 9-th lens L 9  and the 10-th lens L 10  are bonded to each other to form the third jointed lens L 23 . 
     As shown in  FIG. 22 , the second optical system  32  is constituted by a 15-th lens L 15  and a 16-th lens L 16 . The jointing member  54  intervenes between the 15-th lens L 15  and the 16-th lens L 16 . By an outer side in the redial direction of the 15-th lens L 15  and the 16-th lens L 16 , there is disposed the holding mechanism  55  for keeping the distance between a 15-th-lens lateral surface Ll 5   a  at an opposite side to the 16-th lens L 16  in the 15-th lens L 15  and a 16-th-lens lateral surface Ll 6   a  at an opposite side to the 15-th lens L 15  in the 16-th lens L 16  in the preset distance T set in advance. Further, the second optical system  32  has the mirror M provided with the concavely curved surface. The mirror M is a reflective coating layer provided to the 16-th-lens lateral surface Ll 6   a . The mirror M reflects the ray from the first optical system  31  toward the upper side Y 1 . 
     Lens Data 
     The lens data of the projection optical system  3 B is as follows. The surface numbers are provided in sequence from the reduction side toward the elargement side. The reference symbols are the reference symbols of the lenses and the mirrors. The data of the surface numbers not corresponding to any lenses or any mirrors are dummy data. The reference symbol R represents a curvature radius. The reference symbol D represents an axial surface distance. The reference symbol A represents an effective diameter. The units of R, D, and A are millimeter. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 REFERENCE 
                 SURFACE 
                   
                   
                   
                 GLASS 
                 REFRACTION/ 
                   
               
               
                 SYMBOL 
                 NUMBER 
                 SHAPE 
                 R 
                 D 
                 MATERIAL 
                 REFLECTION 
                 A 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 0 
                 SPHERE 
                 INFINITE 
                 5.0771 
                   
                 REFRACTION 
                 0.0000 
               
               
                   
                 1 
                 SPHERE 
                 INFINITE 
                 21.5089 
                 SBSL7 
                 REFRACTION 
                 7.6965 
               
               
                   
                 2 
                 SPHERE 
                 INFINITE 
                 3.0000 
                   
                 REFRACTION 
                 9.6301 
               
               
                   
                 3 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 10.0417 
               
               
                 19 
                 4 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 10.0417 
               
               
                   
                 5 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 10.0417 
               
               
                 L1 
                 6 
                 SPHERE 
                 54.8940 
                 5.0419 
                 451969.7922 
                 REFRACTION 
                 10.1721 
               
               
                   
                 7 
                 SPHERE 
                 −21.7949 
                 0.1000 
                   
                 REFRACTION 
                 10.2241 
               
               
                 L2 
                 8 
                 SPHERE 
                 38.0215 
                 5.3428 
                 483914.7992 
                 REFRACTION 
                 9.4314 
               
               
                 L3 
                 9 
                 SPHERE 
                 −17.1657 
                 1.0000 
                 838742.337  
                 REFRACTION 
                 9.0855 
               
               
                   
                 10 
                 SPHERE 
                 −73.3538 
                 0.1000 
                   
                 REFRACTION 
                 8.9564 
               
               
                 L4 
                 11 
                 SPHERE 
                 43.7198 
                 4.6777 
                 458568.7954 
                 REFRACTION 
                 8.6729 
               
               
                 L5 
                 12 
                 SPHERE 
                 −16.6916 
                 1.0000 
                 836961.3727 
                 REFRACTION 
                 8.3746 
               
               
                   
                 13 
                 SPHERE 
                 582.5473 
                 0.1000 
                   
                 REFRACTION 
                 8.3056 
               
               
                 L6 
                 14 
                 SPHERE 
                 60.5198 
                 2.4189 
                 846663.2378 
                 REFRACTION 
                 8.2894 
               
               
                   
                 15 
                 SPHERE 
                 −101.1944 
                 0.1004 
                   
                 REFRACTION 
                 8.1713 
               
               
                 L7 
                 16 
                 SPHERE 
                 −78.4977 
                 2.5160 
                 529212.7106 
                 REFRACTION 
                 8.1703 
               
               
                   
                 17 
                 SPHERE 
                 −23.6259 
                 11.0686 
                   
                 REFRACTION 
                 8.1000 
               
               
                 O 
                 18 
                 SPHERE 
                 INFINITE 
                 10.8450 
                   
                 REFRACTION 
                 5.9273 
               
               
                 L8 
                 19 
                 SPHERE 
                 −11.1826 
                 1.0000 
                 457192.8527 
                 REFRACTION 
                 5.3000 
               
               
                   
                 20 
                 SPHERE 
                 −14.2102 
                 4.4064 
                   
                 REFRACTION 
                 5.6852 
               
               
                 L9 
                 21 
                 SPHERE 
                 21.2833 
                 5.6421 
                 621528.3349 
                 REFRACTION 
                 8.1133 
               
               
                 L10 
                 22 
                 SPHERE 
                 −14.5583 
                 1.0000 
                 838219.3417 
                 REFRACTION 
                 8.1514 
               
               
                   
                 23 
                 SPHERE 
                 19.4601 
                 0.7648 
                   
                 REFRACTION 
                 8.7013 
               
               
                 L11 
                 24 
                 SPHERE 
                 29.6398 
                 7.0827 
                 608267.3465 
                 REFRACTION 
                 8.7391 
               
               
                   
                 25 
                 SPHERE 
                 −12.2091 
                 0.1000 
                   
                 REFRACTION 
                 9.2494 
               
               
                 L12 
                 26 
                 SPHERE 
                 −12.8956 
                 1.0000 
                 755000.5232 
                 REFRACTION 
                 9.1962 
               
               
                   
                 27 
                 SPHERE 
                 −110.0697 
                 0.9959 
                   
                 REFRACTION 
                 10.6854 
               
               
                 L13 
                 28 
                 ASPHERIC 
                 16.8297 
                 3.5898 
                 E48R_ZEON 
                 REFRACTION 
                 12.9970 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 29 
                 ASPHERIC 
                 21.1795 
                 10.1838 
                   
                 REFRACTION 
                 13.5215 
               
               
                   
                   
                 SURFACE 
               
               
                 L14 
                 30 
                 ASPHERIC 
                 −164.8780 
                 3.5898 
                 E48R_ZEON 
                 REFRACTION 
                 15.6318 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 31 
                 ASPHERIC 
                 38.8134 
                 1.0000 
                   
                 REFRACTION 
                 16.5107 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 32 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 16.1444 
               
               
                 L15 
                 33 
                 ASPHERIC 
                 33.1742 
                 12.0355 
                 Z330R_ZEON 
                 REFRACTION 
                 15.9142 
               
               
                   
                   
                 SURFACE 
               
               
                 L16 
                 34 
                 SPHERE 
                 INFINITE 
                 14.7119 
                 EFEL1 
                 REFRACTION 
                 13.9140 
               
               
                 M 
                 35 
                 ASPHERIC 
                 −13.8214 
                 0.0000 
                 EFEL1 
                 REFLECTION 
                 12.8228 
               
               
                   
                   
                 SURFACE 
               
               
                 L16 
                 36 
                 SPHERE 
                 INFINITE 
                 −14.7119 
                 EFEL1 
                 REFRACTION 
                 21.8507 
               
               
                 L15 
                 37 
                 SPHERE 
                 INFINITE 
                 −12.0355 
                 Z330R_ZEON 
                 REFRACTION 
                 21.4485 
               
               
                   
                 38 
                 ASPHERIC 
                 33.1742 
                 −65.8130 
                   
                 REFRACTION 
                 21.1755 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 39 
                 SPHERE 
                 INFINITE 
                 −56.8385 
                   
                 REFRACTION 
                 294.3897 
               
               
                   
                 40 
                 SPHERE 
                 INFINITE 
                 −180.0883 
                   
                 REFRACTION 
                 493.6966 
               
               
                   
                 41 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 1125.1844 
               
               
                   
               
            
           
         
       
     
     The aspherical coefficient of each of the aspheric surfaces is as follows. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
             
            
               
                 SURFACE NUMBER 
                 28 
                 29 
                 30 
                 31 
               
               
                   
               
               
                 Y CURVATURE 
                 16.8296875 
                 21.17946978 
                 −164.8779938 
                 38.81338024 
               
               
                 RADIUS 
               
               
                 CONIC 
                 −0.164206666 
                 −7.634008001 
                 90 
                 0 
               
               
                 CONSTANT (K) 
               
               
                 4-th-ORDER 
                 −1.15543E−04 
                 −6.14677E−05 
                 −9.39007E−06 
                 −1.74055E−04 
               
               
                 COEFFICIENT (A) 
               
               
                 6-th-ORDER 
                  1.00751E−07 
                 −2.60513E−07 
                 −4.62232E−07 
                  3.89048E−07 
               
               
                 COEFFICIENT (B) 
               
               
                 8-th-ORDER 
                 −6.25176E−10 
                  1.05249E−09 
                  1.18170E−09 
                 −8.39080E−10 
               
               
                 COEFFICIENT (C) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 SURFACE NUMBER 
                 33 
                 35 
                 38 
               
               
                   
               
               
                 Y CURVATURE 
                 33.17421232 
                 −13.82144938 
                 33.17421232 
               
               
                 RADIUS 
               
               
                 CONIC 
                 1.424812957 
                 −3.41862312 
                 1.424812957 
               
               
                 CONSTANT (K) 
               
               
                 4-th-ORDER 
                 −8.30393E−06  
                 −3.65499E−05  
                 −8.30393E−06  
               
               
                 COEFFICIENT (A) 
               
               
                 6-th-ORDER 
                 8.68876E−09 
                 3.27225E−07 
                 8.68876E−09 
               
               
                 COEFFICIENT (B) 
               
               
                 8-th-ORDER 
                 1.32570E−10 
                 −1.53172E−09  
                 1.32570E−10 
               
               
                 COEFFICIENT (C) 
               
               
                 10-th-ORDER 
                 −4.27302E−13  
                 3.26740E−12 
                 −4.27302E−13  
               
               
                 COEFFICIENT (D) 
               
               
                 12-th-ORDER 
                 3.93005E−16 
                   
                 3.93005E−16 
               
               
                 COEFFICIENT (E) 
               
               
                   
               
            
           
         
       
     
     Functions and Advantages 
     In the projection optical system  3 B according to the present example, the second optical system  32  is formed of the lens unit  50  provided with the holding mechanism  55 . Therefore, in the second optical system  32 , the distance between the 15-th-lens lateral surface Ll 5   a  of the 15-th lens L 15  and the 16-th-lens lateral surface Ll 6   a  of the 16-th lens L 16  is kept in the preset distance T set in advance. Thus, it is possible to prevent the distortion aberration from occurring.  FIG. 23  is a diagram showing MTF by the elargement side of the projection optical system  3 B. As shown in  FIG. 23 , the projection optical system  3 B according to the present example is high in resolution. 
     Projection Optical System According to Practical Example 6 
       FIG. 24  is a ray chart schematically showing a whole of a projection optical system according to Practical Example 6.  FIG. 24  schematically shows the light beams which reach the screen S from the projection optical system  3 C according to the present example with the light beams F 1  through F 3 .  FIG. 25  is a ray chart of the projection optical system according to Practical Example 6.  FIG. 26  is a ray chart of a second optical system. It should be noted that since the projection optical system  3 C according to Practical Example 6 is provided with constituents corresponding to those of the projection optical system  3 C described above, the description will be presented denoting the corresponding constituents by the same reference symbols. 
     As shown in  FIG. 24 , the projection optical system  3 C according to the present example is constituted by the first optical system  31  and the second optical system  32  arranged in sequence from the reduction-side toward the elargement-side. The first optical system  31  is a refracting optical system provided with a plurality of lenses. The second optical system  32  is the lens unit  50  having three lenses bonded to each other. As shown in  FIG. 25 , the second optical system  32  has the mirror M provided with the concavely curved surface. The projection optical system  3 C forms the intermediate image  35  conjugate with the reduction-side imaging plane in the middle of the second optical system  32 . Further, the projection optical system  3 C forms the final image conjugate with the intermediate image  35  in the elargement-side imaging plane. 
     By the reduction-side imaging plane, there is disposed the liquid crystal panel  18  of the image formation section  2 . The liquid crystal panel  18  forms the projection image in the upper side Y 1  of the optical axis N. The intermediate image  35  is formed in the lower side Y 2  of the optical axis N. The screen S is located in the upper side Y 1  of the optical axis N. The intermediate image  35  is an image vertically flipped in the Y-axis direction with respect to the enlarged image to be formed in the screen S. 
     The first optical system  31  has the cross dichroic prism  19 , and the fourteen lenses L 1  through L 14 . The first lens L 1  through the 14-th lens L 14  are arranged in this order from the reduction side toward the elargement side. In the present example, the second lens L 2  and the third lens L 3  are bonded to each other to form the first jointed lens L 21 . The fourth lens L 4  and the 5-th lens L 5  are bonded to each other to form the second jointed lens L 22 . The 9-th lens L 9  and the 10-th lens L 10  are bonded to each other to form the third jointed lens L 23 . 
     As shown in  FIG. 26 , the second optical system  32  is constituted by a 15-th lens L 15 , a 16-th lens L 16 , and a 17-th lens L 17 . The first jointing member  54   a  intervenes between the 15-th lens and the 16-th lens, and the second jointing member  54   b  intervenes between the 16-th lens and the 17-th lens. By an outer side in the redial direction of the 15-th lens L 15 , the 16-th lens L 16 , and the 17-th lens L 17 , there is disposed the holding mechanism  55  for keeping the distance between a 15-th-lens lateral surface Ll 5   a  at an opposite side to the 17-th lens L 17  in the 15-th lens L 15  and a 17-th-lens lateral surface Ll 7   a  at an opposite side to the 15-th lens L 15  in the 17-th lens L 17  in the preset distance T set in advance. Further, the second optical system  32  has the mirror M provided with the concavely curved surface. The mirror M is a reflective coating layer provided to the 17-th-lens lateral surface Ll 7   a . The mirror M reflects the ray from the first optical system  31  toward the upper side Y 1 . 
     Lens Data 
     The lens data of the projection optical system  3 C is as follows. The surface numbers are provided in sequence from the reduction side toward the elargement side. The reference symbols are the reference symbols of the lenses and the mirrors. The data of the surface numbers not corresponding to any lenses or any mirrors are dummy data. The reference symbol R represents a curvature radius. The reference symbol D represents an axial surface distance. The reference symbol A represents an effective diameter. The units of R, D, and A are millimeter. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 REFERENCE 
                 SURFACE 
                   
                   
                   
                 GLASS 
                 REFRACTION/ 
                   
               
               
                 SYMBOL 
                 NUMBER 
                 SHAPE 
                 R 
                 D 
                 MATERIAL 
                 REFLECTION 
                 A 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                   
                 0 
                 SPHERE 
                 INFINITE 
                 5.0771 
                   
                 REFRACTION 
                 0.0000 
               
               
                   
                 1 
                 SPHERE 
                 INFINITE 
                 21.5089 
                 SBSL7 
                 REFRACTION 
                 7.7525 
               
               
                   
                 2 
                 SPHERE 
                 INFINITE 
                 3.0000 
                   
                 REFRACTION 
                 9.8396 
               
               
                   
                 3 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 10.2842 
               
               
                 19 
                 4 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 10.2842 
               
               
                   
                 5 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 10.2842 
               
               
                 L1 
                 6 
                 SPHERE 
                 65.2535 
                 5.0547 
                 459231.8119 
                 REFRACTION 
                 10.4081 
               
               
                   
                 7 
                 SPHERE 
                 −21.5572 
                 0.1000 
                   
                 REFRACTION 
                 10.4808 
               
               
                 L2 
                 8 
                 SPHERE 
                 33.7121 
                 5.5287 
                 466801.7961 
                 REFRACTION 
                 9.6397 
               
               
                 L3 
                 9 
                 SPHERE 
                 −17.7888 
                 1.0000 
                 838592.34  
                 REFRACTION 
                 9.2943 
               
               
                   
                 10 
                 SPHERE 
                 −76.9653 
                 0.1000 
                   
                 REFRACTION 
                 9.1655 
               
               
                 L4 
                 11 
                 SPHERE 
                 39.5109 
                 4.6487 
                 449763.8088 
                 REFRACTION 
                 8.8624 
               
               
                 L5 
                 12 
                 SPHERE 
                 −18.1989 
                 1.0000 
                 836854.375  
                 REFRACTION 
                 8.5659 
               
               
                   
                 13 
                 SPHERE 
                 2344.7694 
                 0.1000 
                   
                 REFRACTION 
                 8.4650 
               
               
                 L6 
                 14 
                 SPHERE 
                 60.9938 
                 2.3391 
                 846663.2378 
                 REFRACTION 
                 8.4276 
               
               
                   
                 15 
                 SPHERE 
                 −135.2249 
                 0.1027 
                   
                 REFRACTION 
                 8.2862 
               
               
                 L7 
                 16 
                 SPHERE 
                 −97.0588 
                 2.5154 
                 503784.5221 
                 REFRACTION 
                 8.2856 
               
               
                   
                 17 
                 SPHERE 
                 −24.9159 
                 11.0686 
                   
                 REFRACTION 
                 8.1000 
               
               
                 O 
                 18 
                 SPHERE 
                 INFINITE 
                 12.9279 
                   
                 REFRACTION 
                 6.0487 
               
               
                 L8 
                 19 
                 SPHERE 
                 −10.7625 
                 1.0000 
                 737044.5312 
                 REFRACTION 
                 5.3000 
               
               
                   
                 20 
                 SPHERE 
                 −13.7917 
                 2.9113 
                   
                 REFRACTION 
                 5.7749 
               
               
                 L9 
                 21 
                 SPHERE 
                 18.6067 
                 5.2604 
                 587629.3674 
                 REFRACTION 
                 8.1488 
               
               
                 L10 
                 22 
                 SPHERE 
                 −18.7692 
                 1.0000 
                 843594.2683 
                 REFRACTION 
                 8.1900 
               
               
                   
                 23 
                 SPHERE 
                 19.9365 
                 1.0936 
                   
                 REFRACTION 
                 8.6264 
               
               
                 L11 
                 24 
                 SPHERE 
                 43.1388 
                 6.3428 
                 708049.282  
                 REFRACTION 
                 8.6290 
               
               
                   
                 25 
                 SPHERE 
                 −12.4385 
                 0.1000 
                   
                 REFRACTION 
                 9.1114 
               
               
                 L12 
                 26 
                 SPHERE 
                 −12.7727 
                 1.0000 
                 755000.5232 
                 REFRACTION 
                 9.0592 
               
               
                   
                 27 
                 SPHERE 
                 −175.0400 
                 0.1000 
                   
                 REFRACTION 
                 10.4570 
               
               
                 L13 
                 28 
                 ASPHERIC 
                 17.2789 
                 3.5898 
                 E48R_ZEON 
                 REFRACTION 
                 12.1337 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 29 
                 ASPHERIC 
                 24.8934 
                 10.9770 
                   
                 REFRACTION 
                 12.4998 
               
               
                   
                   
                 SURFACE 
               
               
                 L14 
                 30 
                 ASPHERIC 
                 −162.0425 
                 3.5898 
                 E48R_ZEON 
                 REFRACTION 
                 14.2609 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 31 
                 ASPHERIC 
                 32.6131 
                 1.0000 
                   
                 REFRACTION 
                 15.4862 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 32 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 15.4078 
               
               
                 L15 
                 33 
                 ASPHERIC 
                 33.8937 
                 11.9019 
                 Z330R_ZEON 
                 REFRACTION 
                 15.3575 
               
               
                   
                   
                 SURFACE 
               
               
                 L16 
                 34 
                 SPHERE 
                 INFINITE 
                 7.0000 
                 EFEL1 
                 REFRACTION 
                 13.7122 
               
               
                   
                 35 
                 SPHERE 
                 INFINITE 
                 8.0616 
                 Z330R_ZEON 
                 REFRACTION 
                 13.0735 
               
               
                 L17, M 
                 36 
                 ASPHERIC 
                 −13.5088 
                 0.0000 
                 Z330R_ZEON 
                 REFLECTION 
                 12.8981 
               
               
                   
                   
                 SURFACE 
               
               
                 L16 
                 37 
                 SPHERE 
                 INFINITE 
                 −8.0616 
                 Z330R_ZEON 
                 REFRACTION 
                 22.5140 
               
               
                   
                 38 
                 SPHERE 
                 INFINITE 
                 −7.0000 
                 EFEL1 
                 REFRACTION 
                 6.6445 
               
               
                 L15 
                 39 
                 SPHERE 
                 INFINITE 
                 −11.9019 
                 Z330R_ZEON 
                 REFRACTION 
                 16.2844 
               
               
                   
                 40 
                 ASPHERIC 
                 33.8937 
                 −65.8130 
                   
                 REFRACTION 
                 21.4543 
               
               
                   
                   
                 SURFACE 
               
               
                   
                 41 
                 SPHERE 
                 INFINITE 
                 −56.8385 
                   
                 REFRACTION 
                 299.7208 
               
               
                   
                 42 
                 SPHERE 
                 INFINITE 
                 −180.0883 
                   
                 REFRACTION 
                 508.1933 
               
               
                   
                 43 
                 SPHERE 
                 INFINITE 
                 0.0000 
                   
                 REFRACTION 
                 1169.1953 
               
               
                   
               
            
           
         
       
     
     The aspherical coefficient of each of the aspheric surfaces is as follows. 
     
       
         
           
               
               
               
               
               
             
               
                   
               
             
            
               
                 SURFACE NUMBER 
                 28 
                 29 
                 30 
                 31 
               
               
                   
               
               
                 Y CURVATURE 
                 17.27891973 
                 24.89343087 
                 −162.042457 
                 32.6131135 
               
               
                 RADIUS 
               
               
                 CONIC 
                 0.154036661 
                 −9.791011994 
                 90 
                 0 
               
               
                 CONSTANT (K) 
               
               
                 4-th-ORDER 
                 −1.14296E−04 
                 −6.32387E−05 
                 −3.30805E−05 
                 −2.08195E−04 
               
               
                 COEFFICIENT (A) 
               
               
                 6-th-ORDER 
                  1.22744E−07 
                 −1.68865E−07 
                 −4.85441E−07 
                  4.55554E−07 
               
               
                 COEFFICIENT (B) 
               
               
                 8-th-ORDER 
                 −8.35882E−10 
                  8.66939E−10 
                  1.36489E−09 
                 −9.60761E−10 
               
               
                 COEFFICIENT (C) 
               
               
                   
               
            
           
           
               
               
               
               
            
               
                 SURFACE NUMBER 
                 33 
                 36 
                 40 
               
               
                   
               
               
                 Y CURVATURE 
                 33.89369796 
                 −13.50883191 
                 33.89369796 
               
               
                 RADIUS 
               
               
                 CONIC 
                 1.07073038 
                 −3.463988043 
                 1.07073038 
               
               
                 CONSTANT (K) 
               
               
                 4-th-ORDER 
                 −7.60572E−06  
                 −3.68891E−05 
                 −7.60572E−06  
               
               
                 COEFFICIENT (A) 
               
               
                 6-th-ORDER 
                 8.47827E−09 
                  2.94214E−07 
                 8.47827E−09 
               
               
                 COEFFICIENT (B) 
               
               
                 8-th-ORDER 
                 1.32167E−10 
                 −1.38970E−09 
                 1.32167E−10 
               
               
                 COEFFICIENT (C) 
               
               
                 10-th-ORDER 
                 −4.27006E−13  
                  3.15769E−12 
                 −4.27006E−13  
               
               
                 COEFFICIENT (D) 
               
               
                 12-th-ORDER 
                 4.32766E−16 
                   
                 4.32766E−16 
               
               
                 COEFFICIENT (E) 
               
               
                   
               
            
           
         
       
     
     Functions and Advantages 
     In the projection optical system  3 C according to the present example, the second optical system  32  is formed of the lens unit  50  provided with the holding mechanism  55 . Therefore, in the second optical system  32 , the distance between the 15-th-lens lateral surface L 15   a  of the 15-th lens L 15  and the 17-th-lens lateral surface Ll 7   a  of the 17-th lens L 17  is kept in the preset distance T set in advance. Thus, it is possible to prevent the distortion aberration from occurring.  FIG. 27  is a diagram showing MTF by the elargement side of the projection optical system  3 C. As shown in  FIG. 27 , the projection optical system  3 C according to the present example is high in resolution.