Patent Publication Number: US-9429832-B2

Title: Oblique projection optical system and projection type display apparatus using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of U.S. application Ser. No. 13/860,381, filed on Apr. 10, 2013, which is a continuation of U.S. application Ser. No. 12/636,951, filed Dec. 14, 2009, now U.S. Pat. No. 8,425,049, which claims benefit of priority to Japanese Patent Application Nos. 2009-002124 filed on Jan. 8, 2009 and 2009-197587 filed on Aug. 28, 2009, the entire contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a projection type display apparatus for enlarging an image displayed on a display screen of a display device and projecting the enlarged image on a projection screen or a projection board, and a projection optical system thereof, or in particular to a projection type display apparatus for projecting obliquely on the projection screen an image displayed on the display screen of a display device, an optical system thereof, and a method of machining a molded plastic lens constituting the optical system. 
     In the projection type display apparatus for displaying in an enlarged form, on a projection screen or a projection board, an image on a display screen of a liquid crystal panel of reflection type or transmission type or a displayed image of a display device with an array of a plurality of micro mirrors arranged thereon, it is important not only to produce a sufficiently enlarged image on the projection screen but also to prevent the shadow of a presenter from entering the projection screen and the enlarged image light from intruding directly into the eyes of the presenter. To meet this requirement, a projection optical system of what is called the short projection type in which the distance between the projection type display apparatus and the projection screen is decreased has begun to be placed on the market. This projection optical system is so configured that the enlarged image light enters the projection screen from an oblique direction (for example, JP-A-2008-250296). 
     Means for optically adjusting the oblique projection optical system using a curved mirror for oblique projection is also known (for example, JP-A-2002-35077). 
     On the other hand, a projection type display apparatus of rear projection type is known in which a mirror for returning optical path is interposed between the projection type display apparatus and the projection screen to decrease the apparent projection distance (for example, JP-A-2006-259252). 
     SUMMARY OF THE INVENTION 
     In the conventional art, or especially in an oblique projection optical system disclosed in JP-A-2008-250296 in which enlarged image light enters a projection screen from an oblique direction, a constitution is adopted in which a curved mirror is arranged between the projection optical system and the projection screen, and an intermediate image formed between the curved mirror and a coaxial projection optical system is enlarged by the curved mirror and projected as an enlarged image on the projection screen. 
     According to the technique disclosed in JP-A-2008-250296, therefore, the curved mirror is required to be moved in parallel along the optical axis of the coaxial projection optical system to change the magnification of the enlarged image on the screen, and for this purpose, a high-accuracy movement adjusting mechanism to prevent the curved mirror from tilting with respect to the optical axis. Such an adjusting mechanism, however, is not disclosed in JP-A-2008-250296. 
     JP-A-2002-350774, on the other hand, discloses an adjusting method in which a free-form curved mirror is simply moved, and fails to take into consideration a specific method of correcting phenomena unique to the oblique projection optical system such as trapezoidal distortion of the projected image due to the oblique projection on the projection screen and the aberration due to a difference in projection distances in the vertical direction on the projection screen. Also, JP-A-2002-350774 describes nothing about a method of fabricating the free-form curved mirror having negative power arranged between the projection optical system and the projection screen. 
     Further, the projector disclosed in JP-A-2006-259252 includes a mirror mechanism to rotate a mirror for reflecting light projected from a projection lens of the projector body, in which a fixing unit of the mechanism is fixed on the projector body to maintain the projection light from the projector body at a predetermined projection angle to the mirror. Thus, the rear projection is possible without shifting the projection lens in the projector body on the one hand, and the projector body is accommodated in the mirror mechanism for front projection (direct projection on the screen) on the other hand. In this way, this projector is designed to have a compact form in the case where the projector body is accommodated in the mirror. 
     In the projector disclosed in JP-A-2006-259252, however, the relative positions of the mirror and the projector body for rear projection are fixed, and there is not consideration on a method of changing the magnification of the enlarged image on the screen and a technical means for adjusting the position of the enlarged image for rear projection. 
     The present invention has been achieved in view of the aforementioned problems, and an object thereof is to provide an oblique projection optical system for projecting an image obliquely on the projection screen and a projection type display apparatus using such a system, wherein the trapezoidal distortion and the aberration due to the oblique projection are corrected with a compact configuration and in the case where the projection light is returned by a flat mirror and projected on the projection screen, limitation on the setting of the projection type display apparatus is remarkably improved. 
     Another object of the invention is to provide a projection type display apparatus including an oblique projection optical system configured of a plurality of lenses including at least one aspheric plastic lens, wherein the plastic lens is formed in an aspheric shape and thereby the machining time of the molding die is reduced as compared with the lens having a free-form curved surface, and the employment of a flat mirror for returning the optical path remarkably reduces the development cost. 
     In order to achieve the objective described above, according to one aspect of the invention, there is provided what is called an oblique projection optical system including a plurality of lenses for enlarging the image displayed on a display screen and projecting the enlarged image obliquely on a projection screen, wherein the lens nearest to the projection screen has a vertical effective area of the image through which an image light flux is passed and which is arranged at a position not containing the optical axis shared by a largest number of lenses among the plurality of lenses, and wherein the shape of the lens is asymmetrical about the optical axis thereof, thereby making it possible to correct the aberration caused by the ultra-wide angle and the distortion caused by the oblique projection. 
     For the reason described above, in the oblique projection optical system according to the invention, the height (shift amount) of the position of the displayed enlarged image with respect to the optical axis in the vertical direction of the screen can be increased, and therefore, a small flat mirror for returning the optical path can be arranged between the lens located at a nearest position to the projection screen and the projection screen. 
     Also, in order to reduce the size of the projection type display apparatus having the oblique projection optical system, an upper end of the effective area of the lens nearest to the projection screen in the vertical direction is arranged above an lower end of the effective area of the flat mirror in the vertical direction of the screen, and wherein this flat mirror has a predetermined angle of elevation with respect to the optical axis shared by the largest number of the lenses of the oblique projection optical system as well as this angle of elevation is changeable by a rotation adjusting mechanism. 
     Further, the oblique projection optical system according to the invention can maintain the image quality even at an ultra-wide angle. Accordingly, in the projection type display apparatus having this oblique projection optical system, a high magnification image can be produced even in the case where the distance is short between the projection screen and the projection type display apparatus, and the flat mirror can be moved by a flat mirror moving mechanism along the optical axis shared by the largest number of lenses. 
     Furthermore, the lens constituting the oblique projection optical system according to the invention and arranged nearest to the projection screen has the vertical effective area of the image, through which the light flux is passed, which is arranged at a position not containing the optical axis shared by the largest number of lenses among the plurality of lenses, and which has such a shape asymmetric about the center axis of the effective area thereof and a portion of the aspheric shape symmetric about the optical axis shared by the largest number of lenses is truncated. 
     According to another aspect of the invention, there is provided a projection type display apparatus including an oblique projection optical system configured of a plurality of lenses for enlarging the image displayed on the display screen and projecting the enlarged image obliquely on the projection screen, wherein the lens nearest to the projection screen has a vertical effective area of the image, through which the light flux passes, which is arranged at a position not including the optical axis shared by the largest number of lenses among the plurality of lenses, and which is accommodated in the range of maximum vertical width of the surface of the housing of the projection type display apparatus facing the projection screen. 
     According to still another aspect of the invention for achieving the objective described above, there is provided a projection type display apparatus including an oblique projection optical system configured of a plurality of lenses for enlarging the image on the display screen and projecting the enlarged image obliquely on the projection screen, wherein the lens nearest to the projection screen has a vertical effective area of the image through which the image light flux passes and which is arranged at a position not including the optical axis shared by the largest number lenses among the plurality of the lenses, wherein a flat mirror for returning the optical path is arranged between the lens nearest to the projection screen and the projection screen and has a rotation adjusting mechanism for changing the angle of the flat mirror with respect to the optical axis shared by the largest number of lenses, wherein in a first state where the flat mirror is arranged at a predetermined angle with respect to the optical axis shared by the largest number of lenses, an image enlarged by the image light flux returned by the flat mirror is obtained toward the display screen, while in a second state where the flat mirror is accommodated in the projection type display apparatus, an enlarged image is obtained in the direction along the extended optical axis shared by the largest number of lenses among the plurality of lenses. 
     According to still another aspect of the invention, there is provided a projection type display apparatus including a unit for detecting the rotation angle of the flat mirror and an image correction function for automatically correcting the distortion of the image projected on the screen in accordance with the detected rotation angle. 
     According to yet another aspect of the invention, there is provided a projection type display apparatus wherein in the case where the flat mirror for returning the optical path is arranged at a predetermined angle θ1 with respect to the axis perpendicular to the optical axis shared by the largest number of lenses, an enlarged image is obtained from the image light flux returned by the flat mirror as an image enlarged in the direction toward the display screen, the projection type display apparatus being tilted by θ2 with respect to a reference flat surface substantially perpendicular to the enlarged image, where θ1 and θ2 satisfy the following relationship:
 
1.5≦θ2/θ1≦2.0
 
     According to a further aspect of the invention, there is provided an oblique projection optical system, wherein the lens nearest to the projection screen among the plurality of lenses is formed of plastics and has a vertical effective area of the image, through which the light flux passes, which is arranged at a position not including the optical axis shared by the largest number of lenses among the plurality of lenses, and which has such a shape that a portion of the aspheric shape symmetric about the optical axis shared by the largest number of the lenses constituting the projection optical system is truncated, thereby making it possible to machine a plurality of molding dies arranged symmetrically about the optical axis shared by the largest number of lenses. 
     According to a still further aspect of the invention, there is provided an oblique projection optical system for projecting an image obliquely to the projection screen and a projection type display apparatus using the oblique projection optical system, wherein the trapezoidal distortion and the aberration due to the oblique projection are corrected with a compact structure that a small flat mirror can be accommodated in the projection type display apparatus, wherein in the case where the projection light is returned by the flat mirror and projected on the projection screen, the position of the projection image can be changed by adjusting the angle of the flat mirror, and wherein the limitation on the setting of the projection type display apparatus can be remarkably improved by moving the flat mirror in the direction along the optical axis and thus changing the magnification of the projection image. 
     According to a yet further aspect of the invention, there is provided an oblique projection optical system configured of a plurality of lenses, wherein the lens nearest to the projection screen has a vertical effective area of the image, through which the image light flux passes, and which is arranged at a position not including the optical axis shared by the largest number of lenses among the plurality of lenses, and 
     wherein the particular lens has such a shape that a portion of the aspheric shape symmetric about the particular optical axis is truncated to remarkably reduced the time for machining the molding dies, and a flat mirror is used for the mirror for returning the optical path for a greatly reduced development cost. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view of a projection type display apparatus including an oblique projection optical system and an optical path return mirror according to an embodiment of the invention. 
         FIG. 2  is a side view of a projection type display apparatus including an oblique projection optical system and an optical path return mirror according to another embodiment of the invention. 
         FIG. 3  is a side view of a projection type display apparatus including an oblique projection optical system and an optical path return mirror according to still another embodiment of the invention. 
         FIG. 4  is a side view of a projection type display apparatus including an oblique projection optical system and an optical path return mirror according to yet another embodiment of the invention. 
         FIG. 5  is a side view of a projection type display apparatus including an oblique projection optical system and an optical path return mirror according to a further embodiment of the invention. 
         FIG. 6  is a front view of a projection type display apparatus including an oblique projection optical system and an optical path return mirror according to an embodiment of the invention. 
         FIG. 7  is a side view of a projection type display apparatus including an oblique projection optical system according to an embodiment of the invention. 
         FIG. 8  is a front view of a projection type display apparatus including an oblique projection optical system according to an embodiment of the invention. 
         FIG. 9  is a sectional view of the projection lens showing the lens configuration of the oblique projection optical system according to this invention. 
         FIG. 10  is a sectional view showing the lens configuration of the oblique projection optical system and the light ray tracking result according to the invention. 
         FIG. 11  is a sectional view showing the lens configuration of the oblique projection optical system and the light ray tracking result according to the invention. 
         FIG. 12  is a sectional view of the projection lens having the lens configuration of the oblique projection optical system according to an embodiment (the lens data shown in  FIGS. 25A, 25B ) of the invention. 
         FIG. 13  is a sectional view of the projection lens having the lens configuration of the oblique projection optical system according to another embodiment (the lens data shown in  FIGS. 26A, 26B ) of the invention. 
         FIG. 14  is a sectional view of the projection lens having the lens configuration of the oblique projection optical system according to still another embodiment (the lens data shown in  FIGS. 27A, 27B ) of the invention. 
         FIG. 15  is a sectional view illustrating the lens configuration of the oblique projection optical system and the light ray tracking result according to an embodiment (the lens data shown in  FIGS. 25A, 25B ) of the invention. 
         FIG. 16  is a diagram representing the spot shape of the projection image of the oblique projection optical system according to an embodiment (the lens data shown in  FIGS. 25A, 25B ) of the invention. 
         FIG. 17  is a diagram representing the spot shape of the projection image of the oblique projection optical system according to another embodiment (the lens data shown in  FIGS. 26A, 26B ) of the invention. 
         FIG. 18  is a diagram representing the spot shape of the projection image of the oblique projection optical system according to still another embodiment (the lens data shown in  FIGS. 27A, 27B ) of the invention. 
         FIG. 19  is a diagram illustrating a configuration of the illumination optical system of the projection type display apparatus according to an embodiment. 
         FIG. 20  is a diagram schematically illustrating the configuration of a lens surface shaping machine for the die to mold the plastic lens. 
         FIG. 21  is a diagram schematically illustrating a method of the machining operation on the lens surface shaping machine for the die to mold the plastic lens. 
         FIG. 22  is a diagram schematically illustrating the method of shaping the lens surface for the die to mold the plastic lens according to an embodiment of the invention. 
         FIG. 23  is a diagram schematically illustrating the method of shaping the lens surface for the die to mold the plastic lens according to another embodiment of the invention. 
         FIG. 24  is a diagram illustrating the contour of the plastic lens according to an embodiment of the invention. 
         FIG. 25A  represents the data on the spherical system included in the lens data adapted for the projection lens to realize the oblique projection optical system according to a first embodiment of the invention. 
         FIG. 25B  represents the data on the aspheric system included in the lens data adapted for the projection lens to realize the oblique projection optical system according to the first embodiment of the invention. 
         FIG. 26A  represents the data on the spherical system included in the lens data adapted for the projection lens to realize the oblique projection optical system according to a second embodiment of the invention. 
         FIG. 26B  represents the data on the aspheric system included in the lens data adapted for the projection lens to realize the oblique projection optical system according to the second embodiment of the invention. 
         FIG. 27A  represents the data on the spherical system included in the lens data adapted for the projection lens to realize the oblique projection optical system according to a third embodiment of the invention. 
         FIG. 27B  represents the data on the aspheric system included in the lens data adapted for the projection lens to realize the oblique projection optical system according to the third embodiment of the invention. 
         FIG. 28  is a sectional view of the projection lens illustrating the lens configuration of the oblique projection optical system according to an embodiment of the invention. 
         FIG. 29  is a diagram illustrating the configuration of a part of the projection type display apparatus according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The best aspect for implementing the invention is explained in detail below with reference to the accompanying drawings. Incidentally, in each of the drawings, elements having a same function are designated by same reference numerals, respectively, and explanations thereof are omitted. 
       FIGS. 1 to 3  are side views each schematically illustrating a projection type display apparatus according to an embodiment of the invention. Especially, a projection type display apparatus is provided with an oblique projection optical system (not shown) in which a flat mirror (not shown) is fixed on a fixing frame  2  and an image light flux  3  is returned and projected obliquely on a projection screen directed toward a display screen (not shown). The main parts including the oblique projection optical system described above, an illumination optical system and circuit parts (not shown) are accommodated in a lower housing  9  and an upper housing  1   a.    
     In  FIGS. 1 to 3 , reference numeral  4  designates a flat mirror rotation/fixing mechanism, and numeral  5  a rotation mechanism capable of adjusting the angle of the mirror fixing frame and, if required, includes a detection unit capable of detecting the angle of elevation of the flat mirror is provided. Numerals  6  and  7  designate fixing units for fixing a flat mirror moving mechanism, numeral  8  the flat mirror moving mechanism, R 1  an upper limit of the enlarged image light flux in the vertical direction of the screen, and R 2  an lower limit of the enlarged image light flux in the vertical direction of the screen. 
     Incidentally, a specific example of the lens configuration for realizing the oblique projection optical system of the projection type display apparatus according to this invention is explained in detail later. 
     In the projection type display apparatus according to this invention, as shown in  FIG. 2 , the flat mirror (not shown) is fixed on the fixing frame  2 , and the image light flux  3  can be returned and projected obliquely on the projection screen in the direction toward the display screen (not shown). Accordingly, not only the apparent projection distance from the projection type display apparatus to the projection screen can be greatly reduced, but also the flat mirror can be moved by the moving mechanism  8  relatively to the housing of the projection type display apparatus and fixed at a predetermined position by the fixing units  6 ,  7 , thereby making it possible to change the projection distance to the projection screen without moving the housing. As a result, not only the magnification of the image on the projection screen can be easily changed, but also, as shown in  FIGS. 3 and 4 , the angle of the flat mirror can be adjusted from a predetermined angle of elevation by the flat mirror rotation/fixing mechanism  4  or the mirror fixing frame rotation mechanism  5 , thereby making it possible to move the image display position on the projection screen arbitrarily. 
     At the same time, one or both of the flat mirror rotation/fixing mechanism  4  and the mirror fixing frame rotation mechanism  6  is provided with a rotation angle detection unit (for example, a rotary encoder) capable of detecting the rotation angle of the flat mirror with respect to the predetermined angle of elevation, so that the automatic Keystone correction of the video circuit can be made in the vertical direction of the screen in accordance with the rotation angle obtained thereby to further improve the facility in operation. 
     Also, the projection type display apparatus according to this invention, as shown in  FIG. 2 , is designed in such a manner that in the case where the flat mirror (not shown) is fixed on the fixing frame  2  and the image light flux  3  is returned and projected obliquely on the projection screen in the direction toward the display screen (not shown), in order to reduce the effective area of the flat mirror, the interval between the projection type display apparatus and the flat mirror is shortened, and the amount of shift (defined as 10:0 in the case where the optical axis of the projection lens is superposed on the lower vertical end of the enlarged image, and as a negative value in the case where the lower vertical end of the enlarged image is above the optical axis assuming that the enlarged image width in the vertical direction is 10) of the oblique projection optical system assumes a negative value (i.e. so that the lower vertical end of the enlarged image is located above the optical axis). 
     In the projection type display apparatus according to this invention, in order to further shorten the interval between the projection type display apparatus and the flat mirror, the flat mirror is tilted, as shown in  FIG. 4 , by θ1 from a predetermined angle of elevation by the rotation/fixing mechanism  4  or the mirror fixing frame rotation mechanism  5 , and the projection type display apparatus is tilted correspondingly by θ2 from the plane perpendicular to the projection screen. In this way, an enlarged image having little distortion of the projection screen can be obtained. In connection with this, the projection type display apparatus according to this invention was actually manufactured by way of trial and the relationship between the vertical keystone distortion correction and the image quality reduction due to the image processing was studied. 
     As a result, it was found that in the case where the ratio between θ2 and θ1 (θ2/θ1) is not larger than 1.5, the magnification of the image in the upper part of the projection screen in the vertical direction was enlarged than that of the image in the lower part thereof to such an excessive degree that the image quality is deteriorated even after the vertical Keystone correction was executed in the video circuit. On the contrary, in the case where the ratio between θ2 and θ1 (θ2/θ1) is not less than 2.5, the magnification of the image in the lower part of the projection screen in the vertical direction was enlarged as compared with that of the image in the upper part thereof to such an excessive degree that the image is deteriorated considerably even after making the vertical Keystone correction was executed by the video circuit. It has thus been found that in the case where the ratio between θ2 and θ1 (θ2/θ1) is set to near 2.0, the image quality deterioration can be minimized. 
       FIGS. 5 and 6  are a side view and a front view, respectively, schematically illustrating the projection type display apparatus according to this invention in which the flat mirror (not shown) for returning the optical path is accommodated in the upper part of the set housing. 
     In the projection type display apparatus according to this invention, as illustrated in  FIG. 5 , the flat mirror (not shown) for returning the optical path fixed to the fixing frame  2  can be accommodated at a predetermined position in the upper part of the set housing by the flat mirror rotation/fixing mechanism  4 , the mirror fixing frame rotation mechanism  5 , the moving mechanism  8  and the fixing units  6 ,  7 , and when the flat mirror is accommodated at such a position, the enlarged projection is possible in the direction along the optical axis shared by the largest number of lenses constituting the oblique projection optical system without returning the optical path. 
       FIG. 6  is a front view of the set schematically illustrating the projection type display apparatus according to this invention when the flat mirror (not shown) for returning the optical path is accommodated in the upper part of the set housing. Among the plurality of lenses constituting the oblique projection optical system, the lens (designated by L 17  in  FIG. 6 ) arranged at a position nearest to the projection screen is made to be a rectangle or a trapezoid shape having substantially the same aspect ratio as the effective area of the display screen. The vertical effective image area of this lens, which shuts off unrequired light and through which the light flux passes, is arranged at a position not including the optical axis shared by the largest number of lenses among the plurality of lenses, thereby the vertical effective image area of the lens is accommodated within the maximum vertical screen width of the surface of the housing of the projection type display apparatus facing the projection screen, resulting in a remarkably improved design quality. 
     The structure of the projection type display apparatus according to an embodiment of this invention has been described above in which an enlarged image is obtained regardless of whether the flat mirror for returning the optical path is accommodated or set open as shown in  FIGS. 5 and 6 . Nevertheless, a projection type display apparatus which is not usable when the optical path returning flat mirror is accommodated in the housing is of course within the scope of the present invention, as long as being provided with the oblique projection optical system according to this invention. 
     The projection type display apparatus according to another embodiment of this invention, as shown in  FIG. 7 , having no flat mirror for returning the optical path, is configured to project the image in enlarged form in the direction along the optical axis shared by the largest number of lenses constituting the oblique projection optical system. 
       FIG. 8  is a front view of the set schematically illustrating a form of the projection type display apparatus according to another embodiment of this invention shown in  FIG. 7 , in which the lens (designated as L 17  in  FIG. 8 ) nearest to the projection screen among the plurality of lenses constituting the oblique projection optical system is of a rectangle or a trapezoid shape having substantially the same aspect ratio as the effective display screen area thereof, and the vertical effective image area of the lens to shut off the unrequired light and permit light flux to pass therethrough is arranged at a position not containing the optical axis shared by the largest number of lenses among the plurality of lenses. The vertical effective image area of the particular lens is thus accommodated within the maximum vertical screen width of the surface of the housing of the projection type display apparatus facing to the projection screen. Further, the center of the contour of the lens L 17  mentioned above is located above the center line of the surface of the housing facing to the projection screen, thereby to exhibit a satisfactory balance of the appearance and a remarkably improved design quality. 
     Next, with reference to  FIGS. 9 to 18 , an oblique projection optical system employed in the projection type display apparatus is specifically explained. In order to minimize the projection distance of the projection type display apparatus, the lens nearest to the projection screen among the plurality of lenses constituting the oblique projection optical system is of a rectangle or a trapezoid shape having substantially the same aspect ratio as the effective display screen area thereof, and the vertical effective image area of the lens to shut off the unrequired light and permit the light flux to pass therethrough is arranged at a position not containing the optical axis shared by the largest number of lenses among the plurality of lenses. In this way, a short projection distance for oblique projection in enlarged form is secured. 
     &lt;Oblique Projection Optical System for Short Distance Projection&gt; 
     First,  FIG. 9  is a sectional view illustrating the basic configuration of the projection optical system illustrated by the YZ section of the XYZ orthogonal coordinate system. For the convenience of explanation of the projection optical system, the liquid crystal panel  122  providing the display screen and the cross prism  111  are shown on the right side and the projection screen on the left side. This embodiment corresponds to the lens data represented in  FIGS. 27A, 27B , and the lens L 17  arranged at a position nearest to the projection screen is an aspheric plastic lens, of which the effective area for passing the image light flux is arranged at a position not including the optical axis  11  shared by a plurality of lenses constituting the oblique projection optical system. Thus, the light flux focused around the projection screen can be controlled simply by the shape of the lens L 17 , thereby making it possible to correct the trapezoidal distortion due to the oblique projection or the aberration due to the ultra-wide angle lens (especially, a high-order coma aberration and astigmatism). Also, the shape of the lens L 17  is a rectangle having the aspect ratio substantially equal to that of the effective area of the display screen or a trapezoid corresponding to the area where the image light flux is passed, resulting in an advantage of shutting off the unrequired light which otherwise might deteriorate the focusing performance. Further, the contour of the lens L 17  which is not circular symmetric about the optical axis  11  makes it possible to reduce the size. As a result, even in the case where the oblique projection optical system according to this embodiment is accommodated in the housing of the projection type display apparatus, the contour of the lens L 17  can be accommodated within the maximum vertical screen width of the surface of the housing facing to the projection screen. At the same time, the center of the contour of the lens L 17  is located above the center line of the surface of the housing facing the projection screen. Thus, a satisfactory balance of the external appearance can be secured and at the same time the design quality can be remarkably improved. 
     In  FIG. 9 , only the lens L 17  is formed in the shape of a rectangle having substantially the same aspect ratio as the effective area of the display screen or a trapezoid (in the figure, the sectional shape is shown) corresponding to the area through which the image light flux passes. In the lens configuration for realizing the oblique projection optical system according to this invention, however, as illustrated in  FIG. 15 , the contours of the lenses L 14  and L 15 , which also have an area not permitting the image light flux to pass therethrough, may be determined excepting such an area. In this way, the contour size can be reduced as compared with the contour of the conventional lens symmetric with respect of the optical axis, and the projection type display apparatus having this optical system can be effectively reduced in size and weight. 
     Although the lenses L 3  and L 11  are also aspheric plastic lenses, the effective area of each lens, through which the image light flux passes, is arranged at a position including the optical axis  11  shared by a plurality of lenses constituting the oblique projection optical system, and therefore, has an aspheric shape symmetric about the optical axis  11 . The projection lens for realizing the oblique projection optical system according to this embodiment is composed of totally 17 lenses including 14 glass lenses and 3 plastic lenses and fixedly held by four lens barrels (B 1 , B 2 , B 3 , B 4 ). Incidentally, in the case where the magnification is changed by changing the projection distance, the focus can be adjusted by changing the position of the lens barrel B 4  relative to the position of the lens barrel B 3 . 
     In the embodiment shown in  FIGS. 10 to 15 , the origin of the XYZ orthogonal coordinate system is set at the center of the display screen of the liquid crystal panel  122  for displaying an image by modulating the illumination light flux with video signals, and the Z axis is assumed to be parallel to the normal to the liquid crystal panel  122  (not shown) for displaying the image. The Y axis is parallel to a short side of the display screen of the liquid crystal panel  122  for displaying the image, and assumed to be equal to the vertical direction of the image display liquid crystal panel  122 . The X axis is parallel to a long side of the display screen of the liquid crystal panel  122  for displaying the image, and assumed to be equal to the horizontal (lateral) direction of the liquid crystal panel  122  for displaying the image. Also,  FIG. 10  is a sectional view of the projection lens of the oblique projection optical system of the projection type display apparatus according to an embodiment, and  FIG. 10  is a sectional view of the projection lens including but not showing the flat mirror for returning the optical path. 
     A sectional view of the projection lens for realizing the oblique projection optical system according to a first embodiment of this invention is shown in  FIG. 12 , data on a spherical system included in lens data obtainable in the process is represented in  FIG. 25A , and data on an aspheric system similarly obtainable is represented in  FIG. 25B . Also, a sectional view of the projection lens for realizing the oblique projection optical system according to a second embodiment is shown in  FIG. 13 , data on a spherical system included in lens data obtainable in the process is represented in  FIG. 26A , and data on an aspheric system obtainable is represented in  FIG. 26B . Similarly, a sectional view of the projection lens for realizing the oblique projection optical system according to a third embodiment is shown in  FIG. 14 , data on a spherical system included lens data obtainable in the process is represented in  FIG. 27A , and the data on an aspheric system similarly obtainable is represented in  FIG. 27B . 
     In each lens data, projection distances L 0 , L 1 , an amount of shift S 1  from the optical axis at the image display position and a vertical image size Dv represented in  FIGS. 10 and 11  are summarized in Table 1 below. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 L0 (mm) 
                 Dv (mm) 
                 S1 (mm) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                  60″ Projection Embodiment 1 
                 650.1 
                 747.1 
                 186.8 
               
               
                  60″ Projection Embodiment 2 
                 651.3 
                 747.1 
                 186.8 
               
               
                  60″ Projection Embodiment 3 
                 650.0 
                 747.1 
                 186.8 
               
               
                  80″ Projection Embodiment 1 
                 885.6 
                 996.1 
                 249.0 
               
               
                  80″ Projection Embodiment 2 
                 885.6 
                 996.1 
                 249.0 
               
               
                  80″ Projection Embodiment 3 
                 882.4 
                 996.1 
                 249.0 
               
               
                 100″ Projection Embodiment 1 
                 1011.7 
                 1245.0 
                 331.3 
               
               
                 100″ Projection Embodiment 2 
                 1011.9 
                 1245.0 
                 331.3 
               
               
                 100″ Projection Embodiment 3 
                 1011.8 
                 1245.0 
                 331.3 
               
               
                   
               
            
           
         
       
     
     In the projection lens for realizing the oblique projection optical system according to the invention, the amount of shift S 1  for the size Dv in the vertical direction of the image can be set to 20% or more. Also, the ratio D/L 0  between the projection screen size D (mm) and the projection distance L 0  (mm) for the general the projection type display apparatus of 60 inches has a screen size of 1524 (mm) and a projection distance of 1800 (mm), and in this case the ratio D/L 0  is 0.85. A shorter projection distance is about 1000 (mm), and in this case the ratio D/L 0  is 1.52. In the oblique projection optical system according to this invention, as described above, even a D/L of 2.0 or more is realizable, and according to an embodiment, a ratio of 2.34 is actually obtained. 
     According to this invention, the projection type display apparatus having a flat mirror for returning the optical path can also be realized as shown in  FIG. 11 . The result of manufactured by way of trial indicates that the interval between the lens arranged at a position nearest to the projection screen and the flat mirror is about 150 mm, and therefore, L 1  represented in  FIG. 11  is shortened by about 150 mm compared to L 0 . Also, in the case where the depth of the set is assumed to be 350 mm, the distance from the projection type display apparatus according to the invention to the projection screen with the light returned by the flat mirror is shortened by 500 mm compared to L 0 . This leads to great advantages that a large image can be obtained with a small installation space and the presenter does not directly view the image light of the projection type display apparatus. 
     Next, a specific method of reading the lens data and the mechanism for correcting the aberration of the projection lens to realize the oblique projection optical system according to an embodiment of the invention are explained with reference to the lens data represented in  FIGS. 25A, 25B  for the projection lens according to the first embodiment having a configuration in  FIG. 12 . In general, the lenses are configured in three groups, and those L 16  to L 12  from the projection screen side form the third group. All of L 16  to L 13  are concave lenses having a telocentric configuration. At the same time, in order to reduce the chromatic aberration of magnification, a convex lens formed of a lens material small in Abbe number is arranged as L 12 . Further, as represented in  FIG. 15 , L 16  is arranged at a position not including the optical axis  11  shared by a plurality of lenses constituting the oblique projection optical system. In this way, a strong aspheric shape in which the light flux focused around the screen can be controlled by the lens shape of L 16  alone used to correct the trapezoidal distortion caused by the oblique projection and the aberration due to the ultra-wide angle lens (especially, the high-order coma aberration and the astigmatism). In the process, the contour of the lens L 16  is not required to be circular symmetric about the optical axis for the aforementioned reason, but may be a rectangle shape having an aspect ratio substantially equal to that of the effective area of the displays screen or a trapezoid corresponding to the area through which the image light flux is passed. As a result, unrequired light which otherwise might deteriorate the focusing performance can be advantageously shut off. Further, the fact that the contour of the lens L 16  is not circular symmetric about the optical axis can reduce the size, with the result that even in the case where the oblique projection optical system according to this embodiment is accommodated in the housing of the projection type display apparatus, the contour of L 16  can be set within the vertical maximum screen width of the surface of the housing facing to the projection screen. At the same time, the contour center of L 16  is located above the center line of the surface of the housing facing the projection screen, resulting in a satisfactory balance of external appearance for a remarkably improved design property. 
     Further, L 11  to L 9  make up a second group, in which L 11  is a lens having a strong aspheric shape with a weak negative refractive power, and corrects the spherical aberration and the low-order coma aberration caused by a light flux substantially parallel to the optical axis and passing through a place distant from the optical axis. The lenses L 10  and L 9 , on the other hand, take charge of a part of the refractive power of the glass projection lens having a positive refractive power, and at the same time the lens L 9  suppresses the occurrence of the coma aberration and the astigmatism in the shape of a meniscus convex toward the projection screen. 
     Finally, L 8  to L 1  constitute a first group, in which the doublet lens of L 8  and L 7  and the triplet lens of L 6  to L 4  are rendered to have a negative refractive power and thus a strong telecentric property. Further, the lens L 3  having a strong aspheric shape corrects zonal coma aberration caused by a light flux oblique to the optical axis and passing through a place distant from the optical axis. As a result, the distortion was suppressed and a satisfactory focusing performance of the projection lens as a whole was realized in oblique projection. The projection lenses according to other embodiments of the invention illustrated in  FIGS. 13, 14  are equivalent to a configuration in which L 15  illustrated in  FIG. 12  is divided into L 15  and L 16  for an improved aberration correction ability and in which the aspheric lens L 16  illustrated in  FIG. 12  is simply replaced by the lens L 17 , while the result that the effects and the configuration of the projection optical system remain the same. 
     Next, the oblique projection optical system described above is further explained using specific numerical values with reference to  FIGS. 25A, 25B ,  FIGS. 26A, 26B  and  FIGS. 27A, 27B . 
     First,  FIG. 12  shows the configuration of the projection optical system according to this embodiment based on an example of the numerical values listed in  FIGS. 25A, 25B . In other words,  FIG. 12  shows the configuration of the YZ section in the XYZ orthogonal coordinate described above. The image projection apparatus according to the invention, as illustrated in  FIGS. 1 to 6 , is realizable also in a configuration having a mirror for returning the optical path (not shown). For convenience&#39; sake, however, an explanation is given on the assumption that the mirror for returning the optical path is not present as illustrated in  FIGS. 7, 8 . The configuration of the projection optical system of  FIG. 12  is shown as a development in the direction along Z axis. This is also the case with  FIGS. 13 to 15 . 
     Light emitted from the display screen P 0  (a liquid crystal panel according to the embodiment) indicated under the optical axis  11  in  FIG. 12  passes first through the first and second groups configured of only the projection lenses having a rotationally symmetric surface among the plurality of lenses. Then, the image light passes through the third group including the aspheric lens L 16  rotationally asymmetric about the center of the lens contour and projected in enlarged form on the projection screen. 
     Here, the first and second groups of the projection lenses are configured of a plurality of lenses all having rotationally symmetric refraction surfaces, and four of the refraction surfaces are rotationally symmetric aspheric surfaces, while the other surfaces are spherical. The rotationally symmetric surfaces here are expressed by Equation (1) represented in  FIG. 25B  using the local cylindrical coordinate system for each surface. 
     In Equation (1), h is the distance from the optical axis, Z is the sag amount of the lens surface shape, c is the radius of curvature at the vertex, K is the conic constant and A to J are the coefficients of the terms of the powers of h. 
     Also, the radius of curvature of each surface is listed in  FIG. 25A . In  FIG. 25A , the radius of curvature having its center on a left side of the surface is expressed by a positive value, and otherwise, by a negative value. In  FIG. 25A , the inter-surface distance indicates the distance from the vertex of a lens surface to the vertex of a next lens surface. The inter-surface distance is expressed as a positive value in the case where the next lens surface is located on the left side of a given lens surface in  FIG. 25A , and as a negative value in the case where the next lens surface is located on the right side. Further, in  FIG. 25A , the surface numbers ( 9 ), ( 10 ), ( 23 ), ( 24 ), ( 33 ) and ( 34 ) indicate aspheric surfaces rotationally symmetric about the optical axis, “aspheric” being written beside the surface number in the table of  FIG. 25A  to be easily understood. 
     The coefficients of these six aspheric surfaces are listed in  FIG. 25B . 
     From the table represented in  FIG. 25B , it is understood that the conic constant K is 0 according to this embodiment. The trapezoidal distortion caused by the oblique incidence is very large in the direction of oblique incidence, and small in the direction perpendicular to the direction. Therefore, considerably different functions are required in the direction of oblique incidence and in the direction perpendicular thereto. Thus, the asymmetric aberration can be satisfactorily corrected by not using the conic constant K functioning in all the directions in the rotational symmetry. 
     Incidentally, the numerical values described in the tables of  FIGS. 25A, 25B  assume a case in which an optically modulated optical image (modulated image) in a range of 0.59 inch diagonal with an aspect ratio of 16×9 on the display screen of the liquid crystal panel is projected in a form enlarged to sizes of 60 inches, 80 inches and 100 inches diagonal on the projection screen. In order to secure the optimum focusing performance for the enlarged image of each size, the lenses L 15  and L 16  are moved in parallel to the optical axis in such a manner that the lens intervals ( 30 ) and ( 34 ) assume the inter-surface intervals listed in the lower table in  FIG. 25B . 
     The lens data listed in  FIGS. 26A, 26B and 27A, 27B  are also written in a similar format. 
     Spot shapes of the image projected in the form enlarged to 80 inches using the lens data numerically listed in  FIGS. 25A, 25B  according to the first embodiment are represented in  FIG. 16 , and spot shapes of the image projected in the form enlarged to 80 inches using the lens data numerically listed in  FIGS. 26A, 26B  according to the second embodiment are represented in  FIG. 17 . Further, spot shapes of the image projected in the form enlarged to 80 inches using the lens data numerically listed in  FIGS. 27A, 27B  according to the third embodiment are represented in  FIG. 18 . 
     In  FIG. 16 , the spots of light fluxes emitted from ten points including the X-Y locating at (0, 3.67), (−6.53, 3.67), (−3.92, 2.20), (0.0, 0.0), (0, 0), (−6.53, 0.0), (−3.92, −2.20), (0, −3.67), (−5.53, −3.67), (−5.22, −3.67), (−5.22, 3.67) on the display screen of the liquid crystal panel are represented in the order from the bottom to up. Its scale unit is 5 mm. The horizontal direction in the spot diagram corresponds to the X direction on the display screen of the liquid crystal panel, and the vertical direction corresponds to the Y direction on the display screen of the liquid crystal panel. The spot diagrams represented in  FIGS. 17, 18  are also obtained by the light flux from the points of similar values on the X-Y coordinate on the display screen of the liquid crystal panel. From these diagrams, it is understood that a satisfactory performance is maintained. 
     An oblique projection optical system for realizing the projection type display apparatus according to an embodiment of the invention have been described in detail above. The example described above is so configured that the light emitted from the projection lens is returned by the optical path returning flat mirror and proceeds toward the display screen of the liquid crystal panel. Nevertheless, the invention is not limited to this configuration, and depending on the position where the projection lens is arranged, the returning flat mirror may of course be omitted. 
     Next, an illumination optical system used in the projection type display apparatus according to an embodiment of the invention is explained with reference to  FIG. 19 . In  FIG. 19 , a light source  101  includes a lamp  98  and a reflector  99 . The lamp  98  is a high-pressure white mercury lamp. Also, the reflector  99 , arranged in such a position as to cover the lamp  98  from behind, has a reflection surface in a shape of a paraboloid of revolution and a circular or polygonal exit opening. Light emitted from this lamp  98  is reflected by the reflector  99  having a reflection surface in the shape of the paraboloid of revolution, proceeds in a direction substantially parallel to an optical axis  115 , and is emitted from the light source  101  as a substantially parallel light flux. The light emitted from the light source  101  enters an integrator of multi-lens type  103 . 
     As described above, the multi-lens integrator  103  is configured of a first multi-lens element  103   a  and a second multi-lens element  103   b . The first multi-lens element  103   a  is configured of a matrix array of a plurality of lens cells each in a shape of a rectangle substantially similar to the liquid crystal panels  122   a ,  122   b ,  122   c  as viewed from the direction along the optical axis  115 . The light incident from the light source is divided into a plurality of light rays through the plurality of the lens cells and thereby efficiently led through the second multi-lens element  103   b  and a polarization conversion element  104 . Specifically, the first multi-lens element  103   a  is designed so that the lamp  98  and each lens cell of the second multi-lens element  103   b  are in optically conjugate with each other. 
     The plurality of lens cells of the second multi-lens element  103   b , like those of the first multi-lens element  103   a , are each in a shape of a rectangle as viewed from the direction along the optical axis  115  and arranged in matrix. Each of the lens cells constituting the second multi-lens element  103   b  projects (maps) the shape of the corresponding lens cell of the first multi lens element  103   a  on the liquid crystal panels  122   a ,  122   b  and  122   c  in cooperation with the superposition lenses  108   a ,  108   b  and  108   c . In the process, the polarization conversion element  104  functions to set the light from the second multi-lens element  103   b  in a predetermined direction of polarization. At the same time, the image projected through each lens cell of the first multi-lens element  103   a  is superposed by the superposition lenses  108   a ,  108   b  and  108   c , thereby the light amount is uniformly distributed on the corresponding liquid crystal panels  112   a ,  112   b  and  112   c.    
     Among the plurality of lenses constituting the oblique projection optical system according to the invention, the lens arranged nearest to the projection screen is made of plastics, and the effective area thereof in the vertical direction of the image through which the light flux passes is arranged at a position not containing the optical axis shared by the largest number of lenses among the plurality of lenses. This plastic lens is in such a shape that a portion of the aspheric shape symmetric about the optical axis shared by the largest number of lenses constituting the projection optical system is truncated. 
     The molding die is machined by a method shown in  FIG. 21(A)  in which a die being a work is rotated and cut with a cutting tool on a multi-spindle machine to produce a die in the shape corresponding to the desired lens shape, or by a method shown in  FIG. 21(B)  in which a work is fixed while rotating a cutting tool thereby to machine the die to the required shape. To obtain a mirror surface, the method (B) requires the cutting time about 10 to 20 times longer than the method (A). The method (B) is advantageously used to machine a rotationally asymmetric free-form curved surface, while the method (A) is suitable to machining a rotationally symmetric aspheric work in a shorter time. 
     Among the plurality of the lenses constituting the oblique projection optical system according to the invention, the lens arranged nearest to the projection screen is made of plastics, and the vertical effective image area thereof through which the light flux passes is arranged at a position not containing the optical axis shared by the largest number of lenses among the plurality of lenses. This plastic lens is in such a shape that a portion of the aspheric shape symmetric about the optical axis shared by the largest number of lenses constituting the projection optical system is truncated. Thus, the method of machining the molding die described above can be employed in which as shown in  FIG. 21(A) , the die being a work is rotated and cut with the cutting tool on the multi-spindle machine tool thereby to produce a die in the shape corresponding to the desired lens shape. As a result, the die machining operation is made possible in a short machining time and the development cost can be reduced. 
     In the process, according to this invention, a plurality of dies can be machined at the same time with the C axis as a rotation axis of the multi-spindle machine tool shown in  FIG. 21(A) . In the case where two dies are machined at the same time, for example, as shown in  FIG. 22 , two dies (work pieces) are arranged symmetrically about the rotation axis. Then, the desired die shape can be obtained with a high accuracy in satisfactory cutting balance in the machining operation. Further,  FIG. 23  illustrates an optimum arrangement for machining four dies at the same time. Also in the case where an odd number of dies are machined at the same time, a similar effect can of course be obtained by arranging the adjacent dies at positions apart from each other by the rotation angle (360 degrees) divided by the number of the dies machined at the same time. 
     In the oblique projection optical system according to the invention described above, the contour shape of the lens arranged at a position nearest to the projection screen is, for example, a rectangle as shown in  FIG. 24 . In that case, the shape of the lens surface is asymmetric about the center axis based on the contour shape of the effective lens surface, but an aspheric shape symmetric about the optical axis shared by the largest number of lenses constituting the oblique projection optical system. 
     Incidentally, in  FIG. 9 , only the lens L 17  nearest to the projection screen is formed in a shape lacking the area where the image light flux is not passed, and arranged at a position not including the optical axis  11  shared by the largest number of lenses of the oblique projection optical system. Nevertheless, not only the lens L 17  arranged nearest to the projection screen but also a plurality of lenses next nearest to the projection screen may be formed in the shape lacking the area where the image light flux is not passed. In this way, the oblique projection optical system can be reduced in size and weight. 
     Specifically, in the configuration illustrated in  FIG. 28 , for example, not only the lens L 17  nearest to the projection screen but also the lens L 16  next nearest to the projection screen is arranged at a position not containing the optical axis  11  shared by the largest number of lenses of the oblique projection optical system. Further, the lens L 15  arranged at the second next nearest position to the projection screen has a contour in a shape lacking a lower end area through which the image light flux is not passed. In the configuration shown in  FIG. 28 , therefore, as compared with the shape having the area through which the image light flux is not passed, the structure of the oblique projection optical system can be reduced in size and weight by an amount equivalent to a space  12  defined by a dotted line. 
     In the configuration shown in  FIG. 28 , the lens L 17  is formed of plastics and arranged at a position not containing the optical axis shared by the largest number of lenses constituting the projection optical system. This plastic lens has such a shape that a portion of the aspheric shape symmetric about the optical axis is truncated. Thus, the molding die explained with reference to  FIG. 20  can be machined in a shorter time at a lower initial cost, and resultingly, the lens can be fabricated at a low cost. Further, the lens L 16 , though formed of glass, is arranged at a position not containing the optical axis shared by the largest number of lenses constituting the projection optical system, and has such a shape that a portion of the spherical shape symmetric about the optical axis is truncated. Two lenses L 16  can be fabricated by cutting one spherical glass into two parts, thereby making it possible to fabricate even glass lens at a low cost. Specifically, the greater the number of lenses which are arranged at a position not containing the optical axis  11  shared by the largest number of lenses constituting the oblique projection optical system and which have such a shape that a portion of the aspheric or spherical shape symmetric about the optical axis is truncated, the less expensively the oblique projection optical system can be fabricated. In the configuration shown in  FIG. 28 , only the lenses L 17  and L 16  correspond to such lenses. Nevertheless, a structure is desirable in which not only the single lens nearest to the projection screen, but as many lenses as possible not less than two, arranged nearest to the projection screen and having area through which the image light flux is not passed, are arranged at a position not containing the optical axis  11  shared by the largest number of lenses of the oblique projection optical system, and formed in such a shape that a portion of the aspheric or spherical shape symmetric about the optical axis is truncated. 
     As a result, like in the configuration of the oblique projection optical system illustrated in FIG.  28 , not only the lens nearest to the projection screen but also a predetermined number of lenses successively arranged from the position nearest to the projection screen are formed in the shape lacking the area through which the image light flux is not passed. In this way, the oblique projection optical system can be reduced in both size and weight and can be fabricated at a lower cost. 
     Thus, the use of this oblique projection optical system more preferably makes it possible to fabricate a compact, lightweight and low cost projection type display apparatus. 
     With the configuration of the oblique projection optical system illustrated in  FIG. 28 , the size, weight and cost can be reduced for both the projection type display apparatus having the flat mirror as shown in  FIGS. 1 to 6  and the projection type display apparatus having no flat mirror as illustrated in  FIGS. 7 and 8 . 
     With reference to  FIG. 29 , an explanation is given about an example in which the size of the projection type display apparatus having a flat mirror is suitably reduced by utilizing the space  12  of the oblique projection optical system thereof illustrated in  FIG. 28 . 
       FIG. 29  illustrates a portion of the oblique projection optical system of the projection type display apparatus having a flat mirror, a flat mirror  14 , a mirror holding unit  15  for holding the flat mirror, and a mirror rotation shaft  13  providing a support to rotate the mirror holding unit  15 . In the example shown in  FIG. 29 , the mirror rotation shaft  13  is arranged in a space corresponding to the space  12  shown in  FIG. 28 . In  FIG. 29 , the flat mirror is shown at a position for projecting an image, and when the flat mirror is accommodated in the upper part of the set housing, it can be accommodated by rotating the mirror holding unit  15  around the mirror rotation shaft  13  in the illustrated direction. 
     In the projection type display apparatus having the flat mirror illustrated in  FIGS. 1 to 6 , the flat mirror is accommodated in the upper part of the set housing by the flat mirror rotation/fixing mechanism  4 , the rotation mechanism  5  and the moving mechanism  8  for the mirror fixing frame, the fixing unit  6  and the fixing unit  7 . Namely, the position of the flat mirror is changed by the two types of rotational motions and the two types of slide motions. 
     On the contrary, in the configuration illustrated in  FIG. 29 , only by the rotational motion around the mirror rotation shaft  13 , the angle that the flat mirror forms to the optical axis  11  shared by the largest number of lenses of the oblique projection optical system can be changed thereby to change the operating and accommodation of the positions of the flat mirror. Specifically, in the configuration illustrated in  FIG. 29 , both the operating and the accommodation positions of the mirror can be changed by a simpler rotation adjusting mechanism including the mirror rotation shaft  13  and the mirror holding unit  15 . 
     Further, in the example illustrated in  FIG. 29 , the mirror rotation shaft  13  is formed of a single metal shaft to improve the holding accuracy of the mirror rotation shaft  13 . If it is supposed that a single mirror rotation shaft formed of metal is attempt to be used in a display apparatus not having the compact structure of the oblique projection optical system as illustrated in  FIG. 28 , the position of the mirror rotation shaft would interfere with the lenses of the oblique projection optical system unless the position of the mirror rotation shaft is located ahead in the direction of projection or lower than the position corresponding to the space  12  illustrated in  FIG. 28 . In the example illustrated in  FIG. 29 , in contrast, the mirror rotation shaft  13 , which is arranged in a space corresponding to the space  12  illustrated in  FIG. 28 , is not required to be arranged at a position ahead in the direction of projection or lower than the position corresponding to the space  12 , even if it is formed of a single metal shaft, thereby preventing the set housing from becoming bulky. 
     Specifically, the configuration illustrated in  FIG. 29  can further reduce the size of the projection type display apparatus in a suitable way. 
     In  FIG. 29 , the mirror rotation shaft  13  is arranged at a position corresponding to the space  12 .  FIG. 28  illustrates that the space  12  includes the space formed continuously from the position nearest to the projection screen, at a position not including the optical axis  11  shared by the largest number of lenses of the oblique projection optical system and being inverted symmetrically about the optical axis  11  with respect to the space which is occupied by the plurality of lenses which have such a shape that a portion of the aspheric or spherical shape symmetric about the optical axis is truncated. 
     Accordingly, if the mirror rotation shaft  13  is arranged at a position superposed on at least a portion of the space inverted symmetrically about the optical axis  11  with respect to the space which is occupied by the plurality of lenses formed continuously from the position nearest to the projection screen at a position not including the optical axis  11  shared by the largest number of lenses of the oblique projection optical system and which have such a shape that a portion of the aspheric or spherical shape symmetric about the optical axis is truncated, then at least the effect similar to that of the configuration shown in  FIG. 29  can be obtained. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.