DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0074] The image display apparatus having a three-dimensionally decentered optical path according to the present invention will be described below by way of Examples.

[0075] A coordinate system used in the following Examples is defined as follows. The direction of the observer's visual axis (forward direction) is defined as a Z-axis. The horizontal direction is defined as an X-axis, and the vertical direction is defined as a Y-axis.

[0076] Numerical data (shown later) in each Example is data obtained by backward ray tracing from a pupil 1 to an image display device (image plane) 3 . The following description of the arrangement will also be made in the order of backward ray tracing.

[0077] In Examples 1 to 4 shown below, an ocular prism 10 is a decentered prism that is two-dimensionally decentered in the YZ-plane. An optical path distributing prism 20 is positioned closer to the image display device 3 than the ocular prism 10 . The optical path distributing prism 20 is a three-dimensionally decentered prism in which all the segments of the axial principal ray are not present in the same plane. Examples 1 to 4 will be described below with reference to the accompanying drawings.

EXAMPLE 1

[0078] FIG. 1 is a perspective view of the optical system of an image display apparatus according to Example 1. FIG. 2 is a perspective view showing the axial principal ray 2 in the optical system according to Example 1. FIG. 3 is a right-hand side view of the optical system in Example 1. FIG. 4 is a left-hand side view of the optical system in Example 1. FIG. 5 is a three-plane view showing optical surfaces and principal rays in the optical system according to Example 1. It should be noted that in FIGS. 1 to 4 , surfaces are expressed in the form of meshes.

[0079] The illustrated optical system is an optical system for a right eye when it is arranged for use in a head-mounted image display apparatus designed to be fitted for both eyes. An optical system for a left eye is constructed in plane symmetry with respect to the YZ-plane. The same is the case with the following Examples.

[0080] In the optical system, the axial principal ray (optical axis) 2 in the backward ray tracing emerges from the pupil 1 where the pupil of an observer's right eye should be positioned in a case where the optical system is arranged for use in a head-mounted image display apparatus. The axial principal ray 2 enters the ocular prism 10 while being refracted through a surface 11 of the ocular prism 10 . The surface 11 also serves as a totally reflecting surface 13 . Then, the axial principal ray 2 is internally reflected by a surface 12 and incident on the surface 13 at an angle exceeding the critical angle. Consequently, the axial principal ray 2 is totally reflected by the surface 13 and exits the ocular prism 10 while being refracted through a surface 14 . Then, the axial principal ray 2 enters the optical path distributing prism 20 while being refracted through a surface 21 . The axial principal ray 2 is internally reflected by a surface 22 and further internally reflected by a surface 23 . Then, the axial principal ray 2 is internally reflected by a surface 24 and exits the optical path distributing prism 20 while being refracted through a surface 25 . Then, the axial principal ray 2 reaches the image display device 3 .

[0081] The surfaces 11 to 14 of the ocular prism 10 are in plane symmetry with respect to the YZ-plane and each two-dimensionally decentered in the YZ-plane.

[0082] The axial principal ray 2 exiting the ocular prism 10 enters the optical path distributing prism 20 through the surface 21 and is then reflected by the reflecting surface 22 in a direction not in the same plane as the decentration direction (Y-direction) of the ocular prism 10 . In this Example, a plane H 1 defined by points of intersection between the axial principal ray 2 and the surfaces 12 , 13 and 14 and a plane H 2 defined by points of intersection between the axial principal ray 2 and the surfaces 21 , 22 and 23 are not identical with each other but decentered in an orthogonal relation to each other. Thereafter, the axial principal ray 2 is reflected by the surface 23 away from the pupil 1 and then reflected by the surface 24 toward the pupil 1 . The surfaces 23 and 24 are decentered in a direction perpendicularly intersecting the decentration plane (YZ-plane) of the ocular prism 10 . The axial principal ray 2 reflected from the surface 24 passes through the surface 25 to reach the image display device 3 placed on the observer's side of the optical path distributing prism 20 .

[0083] At least one reflecting surface of each of the ocular prism 10 and the optical path distributing prism 20 has a positive power so that both the ocular prism 10 and the optical path distributing prism 20 have a positive power. Thus, an intermediate image 4 of the image display device 3 is formed in the vicinity of the surface 21 (the same is the case with the other Examples).

[0084] By three-dimensionally decentering the optical system as stated above, the optical system can be folded in a desired direction. Accordingly, the whole image display apparatus can be constructed in a compact form. Thus, it is possible to provide an image display apparatus having a minimized dead space in consideration of other members. In addition, the degree of design freedom increases favorably.

[0085] It should be noted that in this Example decentration aberrations can be favorably corrected at a horizontal field angle of 35° , as will be clear from the aberrational diagram (described later).

[0086] Further, the axial principal ray 2 from the two-dimensionally decentered ocular prism 10 is three-dimensionally decentered by the reflecting surface 22 of the optical path distributing prism 20 so that the direction of decentration thereafter is perpendicular to the decentration direction of the ocular prism 10 . Accordingly, the apparatus can be prevented from increasing in size.

[0087] In addition, it becomes unnecessary to place the image display device 3 horizontally by rotating the reflecting surface 22 of the optical path distributing prism 20 about an axis parallel to the vertical direction (Y-axis). If the reflecting surface 22 is rotated through 45° as in this Example, the image display device 3 can be placed vertically. Accordingly, it is possible to prevent the image display apparatus from increasing in size in the horizontal direction. Further, even if the aspect ratio of the image display device 3 is set at 9:16, the overall size of the apparatus will not become very large.

EXAMPLE 2

[0088] FIG. 6 is a three-plane view showing optical surfaces and principal rays in the optical system of an image display apparatus according to Example 2. This Example is approximately the same as Example 1. Example 2 differs from Example 1 as follows. In the backward ray tracing, the axial principal ray 2 is reflected toward the pupil 1 by the reflecting surface 23 of the optical path distributing prism 20 and then reflected away from the pupil 1 by the surface 24 . Then, the axial principal ray 2 passes through the surface 25 and reaches the image display device 3 placed on the side of the optical path distributing prism 20 remote from the observer.

[0089] In this Example also, the optical system is decentered three dimensionally, thereby allowing the optical system to be folded in a desired direction. Accordingly, the whole image display apparatus can be constructed in a compact form. Thus, it is possible to provide an image display apparatus having a minimized dead space in consideration of other members. In addition, the degree of design freedom increases favorably.

[0090] In this Example also, decentration aberrations can be favorably corrected at a horizontal field angle of 35°.

[0091] Further, the axial principal ray 2 from the two-dimensionally decentered ocular prism 10 is three-dimensionally decentered by the reflecting surface 22 of the optical path distributing prism 20 so that the direction of decentration thereafter is perpendicular to the decentration direction of the ocular prism 10 . Accordingly, the apparatus can be prevented from increasing in size. In addition, it becomes unnecessary to place the image display device 3 horizontally by rotating the reflecting surface 22 of the optical path distributing prism 20 about an axis parallel to the vertical direction (Y-axis). If the reflecting surface 22 is rotated through 45° as in this Example, the image display device 3 can be placed vertically. Accordingly, it is possible to prevent the image display apparatus from increasing in size in the horizontal direction. Further, even if the aspect ratio of the image display device 3 is set at 9:16, the overall size of the apparatus will not become very large.

EXAMPLE 3

[0092] FIG. 7 is a three-plane view showing optical surfaces and principal rays in the optical system of an image display apparatus according to Example 3. This Example is approximately the same as Example 2. Example 3 differs from Example 2 as follows. In the backward ray tracing, the axial principal ray 2 is reflected acutely toward the pupil 1 by the reflecting surface 23 of the optical path distributing prism 20 . Thus, the axial principal ray 2 crosses itself once in the optical path distributing prism 20 in the course of traveling from the surface 22 to the surface 25 . The axial principal ray 2 passes through the surface 25 and reaches the image display device 3 placed on the side of the optical path distributing prism 20 remote from the observer.

[0093] In this Example also, the optical system is decentered three dimensionally, thereby allowing the optical system to be folded in a desired direction. Accordingly, the whole image display apparatus can be constructed in a compact form. Thus, it is possible to provide an image display apparatus having a minimized dead space in consideration of other members. In addition, the degree of design freedom increases favorably.

[0094] In this Example also, decentration aberrations can be favorably corrected at a horizontal field angle of 35°.

[0095] Further, the axial principal ray 2 from the two-dimensionally decentered ocular prism 10 is three-dimensionally decentered by the reflecting surface 22 of the optical path distributing prism 20 so that the direction of decentration thereafter is perpendicular to the decentration direction of the ocular prism 10 . Accordingly, the apparatus can be prevented from increasing in size. In addition, it becomes unnecessary to place the image display device 3 horizontally by rotating the reflecting surface 22 of the optical path distributing prism 20 about an axis parallel to the vertical direction (Y-axis). If the reflecting surface 22 is rotated through 45° as in this Example, the image display device 3 can be placed vertically. Accordingly, it is possible to prevent the image display apparatus from increasing in size in the horizontal direction. Further, even if the aspect ratio of the image display device 3 is set at 9:16, the overall size of the apparatus will not become very large.

EXAMPLE 4

[0096] FIG. 8 is a three-plane view showing optical surfaces and principal rays in the optical system of an image display apparatus according to Example 4. This Example is approximately the same as Example 3. Example 4 differs from Example 3 in that the axial principal ray 2 is made incident on the image display device 3 approximately at right angles without considering the use of the image display device 3 in common to left and right eyes.

[0097] In this Example also, the optical system is decentered three dimensionally, thereby allowing the optical system to be folded in a desired direction. Accordingly, the whole image display apparatus can be constructed in a compact form. Thus, it is possible to provide an image display apparatus having a minimized dead space in consideration of other members. In addition, the degree of design freedom increases favorably.

[0098] In this Example also, decentration aberrations can be favorably corrected at a horizontal field angle of 35°.

[0099] Further, the axial principal ray 2 from the two-dimensionally decentered ocular prism 10 is three-dimensionally decentered by the reflecting surface 22 of the optical path distributing prism 20 so that the direction of decentration thereafter is perpendicular to the decentration direction of the ocular prism 10 . Accordingly, the apparatus can be prevented from increasing in size. In addition, it becomes unnecessary to place the image display device 3 horizontally by rotating the reflecting surface 22 of the optical path distributing prism 20 about an axis parallel to the vertical direction (Y-axis). If the reflecting surface 22 is rotated through 45° as in this Example, the image display device 3 can be placed vertically. Accordingly, it is possible to prevent the image display apparatus from increasing in size in the horizontal direction. Further, even if the aspect ratio of the image display device 3 is set at 9:16, the overall size of the apparatus will not become very large.

[0100] Next, constituent parameters in the above-described Examples 1 to 4 will be shown. In the constituent parameters in each Example, the axial principal ray 2 is defined by a light ray passing perpendicularly through the center of the exit pupil 1 of the optical system and reaching the center of the image display device 3 in the backward ray tracing. Further, in the backward ray tracing, the center of the pupil 1 is defined as the origin of each decentered optical surface constituting a decentered optical system. The direction along which the axial principal ray 2 travels until it intersects the surface 11 of the ocular prism 10 is defined as a Z-axis. The direction in which the Z-axis extends from the pupil 1 toward the surface 11 is defined as a positive direction of the Z-axis. A plane in which the optical axis is folded in the ocular prism 10 is defined as a YZ-plane. An axis passing through the origin and perpendicularly intersecting the YZ-plane is defined as an X-axis. The direction in which the X-axis extends toward the image display device 3 is defined as a positive direction of the X-axis. An axis that constitutes a right-handed orthogonal coordinate system in combination with the X- and Z-axes is defined as a Y-axis. This coordinate system is coincident with those shown in FIGS. 5 to 8 .

[0101] Regarding decentered surfaces, each surface is given displacements in the X-, Y- and Z-axis directions (X, Y and Z, respectively) of the vertex position of the surface from the origin of the optical system, and tilt angles (degrees) of the center axis of the surface [the Z-axis of the above equation (a) in regard to free-form surfaces; the Z-axis of the following equation (d) in the case of aspherical surfaces] with respect to the X-, Y- and Z-axes (α, β and γ, respectively). In this case, the positive α and β mean counterclockwise rotation relative to the positive directions of the corresponding axes, and the positive γ means clockwise rotation relative to the positive direction of the Z-axis. It should be noted that the way of rotating the center axis of each surface through α, β and γ is as follows. First, the center axis of the surface and the XYZ orthogonal coordinate system are rotated through α counterclockwise about the X-axis. Then, the rotated center axis of the surface is rotated through β counterclockwise about the Y-axis of the new coordinate system, and the coordinate system once rotated is also rotated through β counterclockwise about the Y-axis. Then, the center axis of the surface, which has been rotated twice, is rotated through γ clockwise about the Z-axis of the new coordinate system.

[0102] Among optical surfaces constituting the optical system in each Example, a specific surface and a surface subsequent thereto are given a surface separation when these surfaces form a coaxial optical system. In addition, the refractive index and Abbe's number of each medium are given according to the conventional method.

[0103] The configuration of each free-form surface used in the present invention is defined by the above equation (a). The Z-axis of the defining equation is the axis of the free-form surface.

[0104] Aspherical surfaces used in the present invention are rotationally symmetric aspherical surfaces given by the following equation: 5 $\begin{array}{cc}\begin{array}{c}Z=\left(y^2/R\right)/\left[1+{\left\{1-\left(1+K\right)y^2/R^2\right\}}^{1/2}\right]+\\ {\mathrm{Ay}}^4+{\mathrm{By}}^6+{\mathrm{Cy}}^8+{\mathrm{Dy}}^{10}+\dots \end{array}& \left(d\right)\end{array}$

[0105] In the above equation, Z is an optical axis (axial principal ray) for which the direction of travel of light is defined as a positive direction, and y is taken in a direction perpendicular to the optical axis. R is a paraxial curvature radius, and K is a conic constant. A, B, C, D . . . are 4th-, 6th-, 8th- and 10th-order aspherical coefficients, respectively. The Z-axis of this defining equation is the axis of the rotationally symmetric aspherical surface.

[0106] In the constituent parameters (shown later), those terms concerning free-form surfaces and aspherical surfaces for which no data is shown are zero. The refractive index is expressed by the refractive index for the spectral d-line (wavelength: 587.56 nanometers). Lengths are given in millimeters.

[0107] It should be noted that when Examples 1 to 4 are arranged in the form of a viewing optical system, the viewing field angles are as follows. The horizontal half field angle is 17.5°, and the vertical half field angle is 13.3°. The size of the image display device 3 is 8.9×6.7 millimeters. The pupil diameter is 4 millimeters.

[0108] It should be noted that in the tables below, “FFS” denotes a free-form surface, and “ASS” denotes an aspherical surface, and further “RE” denotes a reflecting surface. 1

[0109] 2

[0110] 3

[0111] 4

[0112] FIGS. 9 and 10 are aberrational diagrams showing lateral aberrations in the above-described Example 1. In the diagrams showing lateral aberrations, the numerals in the parentheses denote (horizontal field angle, vertical field angle), and lateral aberrations at the field angles are shown.

[0113] In the foregoing Examples 1 to 3, it is possible to construct an inexpensive head-mounted image display apparatus allowing observation of a single image display device 3 with both left and right eyes EL and ER by arranging left and right optical systems in bilateral symmetry with respect to a plane P, as shown in FIGS. 11 ( a ) and 11 ( b ), which passes through the center of the image surface of the image display device 3 at right angles thereto and extends in the longitudinal direction of the image surface. It should be noted that FIGS. 11 ( a ) and 11 ( b ) show the optical system of Example 1 as arranged in the form of a head-mounted image display apparatus allowing observation with both eyes. FIG. 11 ( a ) is a plan view showing only the optical surfaces and the principal rays as viewed from above an observer fitted with the image display apparatus. FIG. 11 ( b ) is a front view showing the optical surfaces and the principal rays as seen from the observer's side. In the figures, L and R are suffixed to reference symbols denoting the surfaces and the pupils to distinguish the left and right optical systems from each other.

[0114] It should be noted that in these Examples the images of the image display device 3 that are displayed in the left and right eyes EL and ER are in 180° rotated relation to each other. Therefore, it is necessary to arrange the system so that a display image for the right eye and a display image for the left eye are alternately displayed on the image display device 3 , and when the image for the right eye is displayed, the displayed image reaches only the right eye ER, whereas when the image for the left eye is displayed, the displayed image reaches only the left eye EL. That is, when the image display device 3 is a reflection type image display device such as a reflection type liquid crystal display device, as shown in FIG. 11 ( a ) by way of example, a small hole is provided in the reflecting surface 24 R of the optical system for the right eye, and an illuminating light source 5 L for the left eye is positioned to face the hole. Similarly, a small hole is provided in the reflecting surface 24 L of the optical system for the left eye, and an illuminating light source 5 R for the right eye is positioned to face the hole. When a display image for the right eye is displayed on the image display device 3 , the illuminating light source 5 R for the right eye is turned on, whereas when a display image for the left eye is displayed on the image display device 3 , the illuminating light source 5 L for the left eye is turned on, whereby the images displayed on the image display device 3 are alternately distributed to the left and right eyes EL and ER. Alternatively, optical shutters 6 L and 6 R may be placed between the left and right optical systems on the one hand and the left and right pupils 1 L and 1 R on the other. In this case, when a display image for the right eye is displayed on the image display device 3 , the optical shutter 6 R for the right eye is opened, whereas when a display image for the left eye is displayed on the image display device 3 , the optical shutter 6 L for the left eye is opened, whereby the images displayed on the image display device 3 are alternately distributed to the left and right eyes EL and ER.

[0115] FIG. 12 is a perspective view showing the left and right optical systems of the head-mounted image display apparatus allowing observation with both eyes shown in FIGS. 11 ( a ) and 11 ( b ). In the figure, the ocular prisms of the left and right optical systems are denoted by reference symbols 10 L and 10 R, respectively, and the left and right optical path distributing prisms are denoted by reference symbols 20 L and 20 R, respectively. As will be clear from the figure, the left and right optical path distributing prisms 20 L and 20 R are integrated into a single prism member.

[0116] In Example 4, the angle of inclination of the principal rays incident on the image display device 3 is very small. Therefore, even an image display device having a small viewing angle, such as a liquid crystal display device, is usable. However, Example 4 does not take into consideration the scheme of allowing observation of a single image display device 3 with both eyes as in Examples 1 to 3.

[0117] Incidentally, an image display apparatus for a single eye can be formed by preparing one set of optical system components arranged as stated above. It is also possible to form an image display apparatus for two eyes by supporting two sets of such optical system components in bilateral symmetry with each other as stated above. In this way, it is possible to form a stationary or portable image display apparatus that allows observation with a single eye or two eyes.

[0118] FIG. 13 shows the arrangement of an image display apparatus designed to be fitted for a single eye (in this case, the apparatus is fitted for the left eye). FIG. 14 shows the arrangement of an image display apparatus designed to be fitted for two eyes. In FIGS. 13 and 14 , reference numeral 131 denotes a display apparatus body unit. In the case of FIG. 13 , the display apparatus body unit 131 is fixed by a support member through the observer's head such that the display apparatus body unit 131 is held in front of the observer's left eye. In the case of FIG. 14 , the display apparatus body unit 131 is fixed by a support member through the observer's head such that the display apparatus body unit 31 is held in front of both the observer's eyes. The support member for the display apparatus body unit 131 has a pair of front frames 132 (left and right) each joined at one end thereof to the display apparatus body unit 131 . The left and right front frames 132 extend from the observer's temples to the upper portions of his/her ears, respectively. A pair of rear frames 133 (left and right) are joined to the other ends of the left and right front frames 132 , respectively, and extend over the left and right side portions of the observer's head. In the case of FIG. 14 , the support member further has a top frame 134 joined at both ends thereof to the other ends of the left and right rear frames 133 , respectively, such that the top frame 134 lies over the top of the observer's head.

[0119] A rear plate 135 is joined to one front frame 132 near the joint to the rear frame 133 . The rear plate 135 is formed from an elastic member, e.g. a metal leaf spring. In the case of FIG. 14, a rear cover 136 , which forms a part of the support member, is joined to the rear plate 135 such that the rear cover 136 can support the apparatus at a position behind the observer's ear in a region extending from the back part of the head to the base of the neck. A speaker 139 is mounted inside the rear plate 135 or the rear cover 136 at a position corresponding to the observer's ear.

[0120] A cable 141 for transmitting external image and sound signals is led out from the display apparatus body unit 131 . In the case of FIG. 14 , the cable 141 extends through the top frame 134 , the rear frames 133 , the front frames 132 and the rear plate 135 and projects to the outside from the rear end of the rear cover 136 . In the case of FIG. 13 , the cable 141 projects from the rear end of the rear plate 135 . The cable 141 is connected to a video-replaying unit 140 . It should be noted that reference numeral 140 a in the figures denotes a switch and volume control part of the video-replaying unit 140 .

[0121] The cable 141 may have a jack and plug arrangement attached to the distal end thereof so that the cable 141 can be detachably connected to an existing video deck or the like. The cable 141 may also be connected to a TV signal receiving tuner so as to enable the user to enjoy watching TV. Alternatively, the cable 141 may be connected to a computer to receive computer graphic images or message images or the like from the computer. To eliminate the bothersome cord, the head-mounted image display apparatus may be arranged to receive external radio signals through an antenna connected thereto.

[0122] The optical system of the image display apparatus according to the present invention is also usable as an image-forming optical system by introducing light from a subject from the pupil 1 side and placing an image pickup device at the position of the image display device 3 . FIG. 15 is a schematic view showing an arrangement in which the optical system according to the present invention is incorporated into an objective optical system 42 of a photography part of an electronic camera 40 . In this example, the objective optical system 42 for photography placed in an optical path 41 for photography uses an optical system similar to Example 2 except that the optical path is reverse to that in Example 2. A stop 1 ′ is placed at the position of the pupil 1 . An object image produced by the objective optical system 42 for photography is formed on an image pickup surface 46 of a CCD 45 through a filter 44 , e.g. a low-pass filter or an infrared cutoff filter. The object image received by the CCD 45 is processed in a processor 51 and displayed in the form of an electronic image on a liquid crystal display device (LCD) 52 . The processor 51 also controls a recording device 54 for recording the object image detected by the CCD 45 in the form of electronic information. The image displayed on the LCD 52 is led to an observer's eyeball E through an ocular optical system 53 . The ocular optical system 53 is formed from a decentered prism similar to the ocular prism 10 . As the ocular optical system 53 , it is also possible to use a three-dimensionally decentered optical system including the ocular prism 10 and the optical path distributing prism 20 according to the present invention. It should be noted that the objective optical system 42 for photography may include another lens (a positive lens or a negative lens) as a constituent element on the object side of the prism 10 or on the image side of the prism 20 .

[0123] In the camera 40 arranged as stated above, the objective optical system 42 for photography can be constructed with a minimal number of optical members. Accordingly, the whole apparatus can be constructed in a compact form. Further, it is possible to provide a camera having a minimized dead space in consideration of other associated members at reduced costs. In addition, the degree of design freedom increases favorably.

[0124] Although in this example a plane-parallel plate is placed as a cover member 43 of the objective optical system 42 for photography, it is also possible to use a lens having a power as the cover member 43 .

[0125] The optical system of the image display apparatus according to the present invention can also be used as a projection optical system by placing an image plane for projection at the position of the image display device 3 and providing a screen in front of the pupil 1 . FIG. 16 is a schematic view showing an arrangement in which a decentered prism optical system according to the present invention is used in a projection optical system 96 of a presentation system formed by combining together a personal computer 90 and a liquid crystal projector 91 . In this example, an optical system similar to Example 2 is used in the projection optical system 96 . In the figure, image and manuscript data prepared on the personal computer 90 is branched from a monitor output and delivered to a processing control unit 98 in the liquid crystal projector 91 . In the processing control unit 98 of the liquid crystal projector 91 , the input data is processed and output to a liquid crystal panel (LCP) 93 . The liquid crystal panel 93 displays an image corresponding to the input image data. Light from a light source 92 is applied to the liquid crystal panel 93 . The amount of light transmitted by the liquid crystal panel 93 is determined by the gradation of the image displayed on the liquid crystal panel 93 . Light from the liquid crystal panel 93 is projected on a screen 97 through a projection optical system 96 comprising a field lens 95 placed immediately in front of the liquid crystal panel 93 and prisms 20 and 10 constituting the optical system according to the present invention, together with a cover lens 94 which is a positive lens.

[0126] The projector arranged as stated above can be constructed with a minimal number of optical members. Accordingly, a low-cost projector can be realized. In addition, the projector can be constructed in a compact form.

[0127] As will be clear from the foregoing description, the present invention provides an image display apparatus capable of attaining a reduction in overall size of the apparatus by using a three-dimensionally decentered optical system in which all the segments of an axial principal ray extending from a pupil to an image display device are not within the same plane. The image display apparatus further allows various constituent members to be effectively placed in a dead space, thereby permitting the image display apparatus to be designed in a variety of ways. In addition, the present invention provides an image display apparatus favorably corrected for decentration aberrations despite the use of an optical system decentered in a three-dimensional space.