Projection optical system and image projection device

A projection optical system for projecting an image on a surface is provided. The image is an enlarged image of an image which is formed on an image forming element. The projection optical system includes a coaxial optical system having an optical axis; and a non-coaxial optical system including a rotationally asymmetric curved-surface mirror. The non-coaxial optical system does not share the optical axis with the coaxial optical system. The coaxial optical system includes a first lens having a positive refractive power and being an aspheric plastic lens; and a second lens having a negative refractive power and being an aspheric plastic lens. The first lens has a first refractive index distribution, and the second lens has a second refractive index distribution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

An aspect of this disclosure relates to a projection optical system and an image projection device including the projection optical system.

2. Description of the Related Art

Recently, for a liquid-crystal projector that is widely known as an image projection device, a resolution improvement of a liquid-crystal panel, an improvement on brightness based on an efficiency improvement of a light-source lamp, and a price reduction are progressing. Further, small and light weight image projection devices that utilize DMD (Digital Micro-mirror Device) and the like become common, and the image projection devices are widely used not only in offices or schools, but also in households. Especially, as portability of front projectors are improved, front projectors are used for small conferences for several people. As a projection optical system to be mounted in such an image projection device, a projection optical system including a curved-surface mirror has been proposed (for example, cf. Patent Document 1 (Japanese Published Unexamined Application No. 2006-235516), Patent Document 2 (Japanese Registered Patent No. 4210314), and Patent Document 3 (Japanese Published Unexamined Application No. 2009-157223)).

SUMMARY OF THE INVENTION

In one aspect, there is provided a projection optical system for projecting an image on a surface to be projected, the image being an enlarged image which is formed on an image forming element. The projection optical system includes a coaxial optical system having an optical axis; and a non-coaxial optical system including a rotationally asymmetric curved-surface mirror. The non-coaxial optical system does not share the optical axis with the coaxial optical system. The coaxial optical system and the non-coaxial optical system are arranged in this order on a light path from the image forming element to the surface. The coaxial optical system includes at least a first lens having a positive refractive power and being an aspheric plastic lens; and a second lens having a negative refractive power and being an aspheric plastic lens. The first lens has a first refractive index distribution along a first direction from the center of the first lens to a peripheral portion of the first lens, and the second lens has a second refractive index distribution along a direction from the center of the second lens to a peripheral portion of the second lens. One of the first lens and the second lens is arranged at a position closest to the curved-surface mirror among the optical elements included in the coaxial optical system.

In another aspect, there is provided a projection optical system for projecting an image on a surface to be projected, the image being an enlarged image which is formed on an image forming element. The projection optical system includes a coaxial optical system having an optical axis; and a non-coaxial optical system including a rotationally asymmetric curved-surface mirror. The non-coaxial optical system does not share the optical axis with the coaxial optical system. The coaxial optical system and the non-coaxial optical system are arranged in this order on a light path from the image forming element to the surface. The coaxial optical system includes at least a group of lenses including a first lens and a second lens. The first lens has a first refractive power, a surface of the first lens close to the image forming element being an aspheric surface, and a surface of the first lens close to the curved-surface mirror being a spherical surface. The second lens has a second refractive power, a surface of the second lens close to the image forming element being a spherical surface, and a surface of the second lens close to the curved-surface mirror being an aspheric surface. The spherical surface of the first lens and the spherical surface of the second lens are joined. The first lens has a first refractive index distribution along a first direction from the center of the first lens to a peripheral portion of the first lens, and the second lens has a second refractive index distribution along a second direction from the center of the second lens to a peripheral portion of the second lens. The second lens is arranged at a position closest to the curved-surface mirror among the optical elements included in the coaxial optical system.

In another aspect, there is provided an image projection device including an image forming element that forms an image thereon in accordance with a modulated signal; and a projection optical system that irradiates light from a light source to the image forming element and projects an image on a surface to be projected, the image being an enlarged image which is formed on the image forming element. The projection optical system includes a coaxial optical system having an optical axis; and a non-coaxial optical system including a rotationally asymmetric curved-surface mirror. The non-coaxial optical system does not share the optical axis with the coaxial optical system. The coaxial optical system and the non-coaxial optical system are arranged in this order on a light path from the image forming element to the surface. The coaxial optical system includes, at least, a first lens having a positive refractive power and being an aspheric plastic lens; and a second lens having a negative refractive power and being an aspheric plastic lens. The first lens has a first refractive index distribution along a first direction from the center of the first lens to a peripheral portion of the first lens, and the second lens has a second refractive index distribution along a second direction from the center of the second lens to a peripheral portion of the second lens. One of the first lens and the second lens is arranged at a position closest to the curved-surface mirror among the optical elements included in the coaxial optical system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, for example, a projection optical system in which a free-form surface mirror is adopted as a curved-surface mirror and in which a plastic lens is arranged in the vicinity of the free-form surface mirror is considered. In such a projection optical system, there is a problem that, if an error occurs in the production process of the projection optical system, an uncorrectable deformation occurs in an image projected onto a surface, such as a screen, when a focus adjustment is performed. Among the errors that can occur in the production process, especially, the refractive index distribution that occurs during the shaping process of a plastic lens, which is to be arranged in the vicinity of the free-form surface mirror, can be a cause of field curvature.

However, in conventional projection optical systems including the projection optical systems described in Patent Documents 1-3, errors that occur in actual production processes of the projection optical systems have not been considered. Especially, the refractive index distribution that occurs during the shaping process of the plastic lens has not been considered at all.

Embodiments have been developed in view of the above problem. An objective of the embodiments is to provide a projection optical system for which the refractive index distribution that occurs during the shaping process of the plastic lens is considered and an image projection device including the projection optical system.

Hereinafter, the embodiments are explained with reference to the figures. In the embodiments, same reference numerals may be used for corresponding parts that are common to the embodiments, in order to avoid overlapping explanations. Incidentally, in each embodiment, a long axis direction of a screen (a horizontal direction) is defined to be an X-axis direction, a short axis direction of the screen (a vertical direction) is defined to be a Y-axis direction, and a normal direction of the screen is defined to be a Z-axis direction.

First Embodiment

FIG. 1is a schematic diagram illustrating an image projection device10according to a first embodiment. Schematically, the image projection device10shown inFIG. 1irradiates light emitted from a light source11onto an image forming element17and projects a magnified image of the image forming element17onto a screen90using a projection optical system18. As the light source11, for example, a halogen lamp, a xenon lamp, a metal halide lamp, an extra high pressure mercury lamp, and an LED can be used. As the image forming element17, for example, a DMD (Digital Micro-mirror Device) or a liquid crystal panel can be used.

Here, the image projection device10is specifically explained. The light emitted from the light source11is condensed at an entrance of an integrator rod13by a reflector12. The integrator rod13is, for example, a light pipe which is shaped like a tunnel by combining four mirrors. The light condensed at the entrance of the integrator rod13repeats reflecting on mirror surfaces on the interior of the integrator rod13. Thus, the amount of light distribution is uniform at the exit of the integrator rod13. The exit of the integrator rod13may be deemed as a surface light source which emits illuminating light whose light intensity distribution is uniform. An image of the surface light source is formed, for example, on the image forming element17through the lenses for illumination14, a first mirror15, and a second mirror16. Since the image forming element17is irradiated by the light whose light intensity distribution is uniform, the light intensity distribution over the image projected onto the screen90, which is the magnified image of the image forming element17, is also uniform.

When the image forming element17is the DMD, the image forming element17includes many infinitesimal mirrors. An angle of each infinitesimal mirror can be varied, for example, within a range between minus 12 degrees and 12 degrees inclusive. An angle of the illuminating light toward the DMD may be adjusted, so that, for example, when the angle of the infinitesimal mirror is minus 12 degrees, the illuminating light reflected on the infinitesimal mirror enters the projection optical system18, and when the angle of the infinitesimal mirror is 12 degrees, the illuminating light reflected on the infinitesimal mirror does not enter the projection optical system18. In this manner, a digital image can be formed on the screen90by controlling the inclination angles of the infinitesimal mirrors included in the DMD.

Incidentally, plural image forming elements17corresponding to, for example, red, green, and blue may be used. Further, each image forming element17may be irradiated with light which has passed through a corresponding color filter. Then a color image can be projected onto the screen90, when light fluxes reflected from the plural image forming elements17are combined with a light combining unit and the combined light fluxes enter the projection optical system18.

FIG. 2is a ray diagram schematically illustrating the projection optical system18according to the first embodiment. With reference toFIG. 2, the projection optical system18includes a coaxial optical system19including lenses having a common optical axis or a group of lenses having a common optical axis, and a curved-surface mirror20which is a non-coaxial optical system which does not have a common optical axis with the coaxial optical system19.

In the projection optical system18, the coaxial optical system19and the curved-surface mirror20are arranged in this order on a light path from the image forming element17to the screen90, on which an image is to be projected. The curved-surface mirror20is a single rotationally asymmetric curved-surface mirror. Here, the non-coaxial optical system may include an optical element other than the curved-surface mirror20. Further, the symbol “A” shown in the figures indicates the optical axis of the coaxial optical system19.

The projection optical system18forms an intermediate image (real image) of the image forming element17once in between the coaxial optical system19and the curved-surface mirror20, which is the non-coaxial optical system. The projection optical system18is an intermediate image system that projects the intermediate image onto the screen90by lifting up the intermediate image with the curved-surface mirror20. A concrete configuration of the coaxial optical system19is described later. Hereinafter, an example case is explained in which a free-form surface mirror is used as the curved-surface mirror20.

The curved-surface mirror20is further explained in detail below. In order to project the image onto the screen90at close range, it may be required to form the image above the image projection device10, such as a projector, so that the screen can be easily seen. Therefore, as shown inFIG. 3, for example, a center of the image forming element17is not placed on the optical axis A of the coaxial optical system19, but the center of the image forming element17is eccentrically placed with respect to the optical axis A. The quality of the image is maintained by taking a wide performance guarantee range of the coaxial optical system19(namely, by setting the coaxial optical system19to be a wide-angle lens). However, there is a limit for the width, when the coaxial optical system19is a wide-angle lens. Thus, in order to project the image from a position closer to the screen90using the coaxial optical system19, it may be required to lengthen the light path using a mirror. This method of lengthening the light path using a mirror has been used for rear-projection televisions. However, it is difficult to use a mirror in a portable image projection device, which is usually used in a meeting room. If such a mirror were used in a portable image projection device, a large mirror might be required as well as a large space and cost. Therefore, the method shown inFIG. 3is not preferable.

As an example which is different from that ofFIG. 3, there is a method in which an image is projected obliquely using a curved mirror. The oblique projection is, for example, as shown inFIG. 4, to project the image at close range by obliquely arranging the image forming element17or the coaxial optical system19with respect to the screen90. With this method, the image can be projected at close range. However, there is a disadvantage that the screen is deformed in a trapezoidal shape. Therefore, the method shown inFIG. 4is not preferable.

In view of the problems on the methods corresponding toFIGS. 3 and 4, in the embodiment, the optical systems are arranged as shown inFIG. 2. The deformation of the screen in the trapezoidal shape is efficiently corrected by using the free-form surface mirror as the curved-surface mirror20. Here, the free-form surface mirror is, for example, as shown inFIG. 5, a mirror such that a curvature in the X-axis direction of its surface varies along the Y-axis direction. Specifically, when the vertical direction of the screen90, on which the image is to be projected, is set to be the X-axis direction and the horizontal direction of the screen90is set to be the Y-axis direction, the curvature of the curved-surface mirror20in the X-axis direction becomes greater as a coordinate value of the Y-axis varies from a coordinate value corresponding to an end portion of the curved-surface mirror20which is closer to the optical axis A of the coaxial optical system19to the coordinate value corresponding to another end portion of the curved-surface mirror20which is separated from the optical axis A of the coaxial optical system19.

The projection optical system18functions to form the real image of the image forming element17on the screen90. A size of the image to be displayed on the screen90and a distance between the image projection device10and the screen90may be adjusted by a user of the image projection device10. In order to form the real image of the image forming element17on the screen90, a focus adjustment process is performed. In a projection optical system for a usual projector (namely, in a coaxial optical system which is rotationally symmetric), a whole focusing method or a focus adjusting method, in which one of lenses (or one of groups of lenses, where each group of lenses includes plural lenses) is moved, has been used. Here, in the whole focusing method, the entire projection optical system is moved and the focus is adjusted.

For the projection optical system18according to the embodiment, it is preferable to adopt a focus adjusting method in which a lens or a group of lenses that is closest to the image forming element17is fixed, while either two or more lenses or two or more groups of lenses are moved in the optical axis direction. Namely, it is preferable that a distance between the lens or the group of lenses that are the closest to the image forming element17not be varied. The reason is as follows. Here, the image is projected onto the screen90at close range, and the deformation of the image is mainly corrected using the curved-surface mirror20, which is the non-coaxial optical system. Therefore, if the whole focusing method or the focus adjusting method, in which one lens or one group of lenses are moved, were used, a correction of the amount of deformation would be insufficient.

Additionally, when the lens or the group of lenses which are closest to the image forming element17is fixed, brightness does not change depending on the size of the screen, compared to the case when the whole focusing method or the focus adjusting method, in which one lens or one group of lenses are moved, is used. In other words, the reflected light from the image forming element17needs to reach the screen90(in order to improve the light use efficiency). When the lens or the group of lenses which are the closest to the image forming element17are not fixed and moved in the optical axis direction, a portion of the reflected light is scattered by an outer frame of the lens or an outer frame of the group of the lenses. Thus it is not ensured that all the reflected light reaches the screen90. Here, there would be no problem, if an outer diameter of the lens or the group of lenses which are closest to the image forming element17were sufficiently large compared to the diameter of the flux of the reflected light from the image forming element17. However, it is not preferable to enlarge the diameter of the lens or the group of lenses from the viewpoints of resource saving and downsizing of the products.

Here, when the distance between the projection optical system18, which is assumed as a product, and the screen90is almost constant (for example, when the usage of the projection optical system18is limited to a case in which the distance between the projection optical system18and the screen90is 500 mm plus/minus 5 mm), the whole focusing method or the focus adjusting method, in which one lens or one group of lenses are moved, can be adopted without any problem.

In the embodiment, the coaxial optical system19of the projection optical system18has a configuration as shown inFIG. 6A.FIG. 6Ais a ray diagram exemplifying the projection optical system according to the first embodiment. The coaxial optical system19inFIG. 2is more specifically shown inFIG. 6A. The coaxial optical system19shown inFIG. 6Aincludes, from a side of the image forming element17, a lens19a, a lens19b, a lens19cand a lens19d, in this order. In the coaxial optical system19, the lens19ais a lens having a positive refractive power. The lens19bis a lens having a negative refractive power. The lens19cis a lens having a positive refractive power. The lens19dis a lens having a negative refractive power. Here, each of the lenses19a-19dmay be one of a group of lenses.

The lens19ais fixed. The lenses19b,19c, and19dare independently reciprocable in the Z-axis direction (the direction of the optical axis A).

Namely, for the coaxial optical system19a floating focusing method is adopted. In the floating focusing method, the plural lenses (the lenses19b,19c, and19d) in the coaxial optical system19move corresponding distances, which are different from each other, in the Z-axis direction (in the direction of the optical axis A), and the focus is adjusted. Here, in the embodiment, the curved-surface mirror20is fixed and does not move when the focus is adjusted. The reason is that, when a component such as the curved-surface mirror20, which is large and serves as the most important function for the correction of the deformation, is moved, a positional error with respect to the coaxial optical system19becomes greater, and this leads to an increase of the deformation.

When the size of the screen90is reduced by moving the screen90close to the curved-surface mirror20from the state indicated inFIG. 6Aand the focus is adjusted, the lenses19b,19c, and19dare moved away from the lens19ain the Z-axis direction, which is the direction of the optical axis A, as shown inFIG. 7. Here, the displacements of the lenses19b,19c, and19dmay be different from each other. Further, when the size of the screen90is enlarged by separating the screen90from the curved-surface mirror20and the focus is adjusted, the lenses19b,19c, and19dare moved close to the lens19ain the Z-axis direction, which is the direction of the optical axis A, contrary to the case ofFIG. 7. Here, the displacement of the lenses19b,19c, and19dmay be different from each other.

In this manner, an irregularly-shaped deformation can be corrected by adjusting the focus through moving the plural lenses. Here, the irregularly-shaped deformation is specific to the optical system including the curved-surface mirror20, which is the free-formed surface mirror. Further, the displacement of the plural lenses may be different from each other. Note that, when the focus is adjusted by moving the plural lenses while fixing the curved-surface mirror20, which is the free-formed surface mirror, the spacing of three or more lenses is changed.

InFIG. 6A, the lenses19cand19dare aspheric plastic lenses. The deformation of the screen, which is specific to a free surface optical system, can be suppressed by using the free-form surface mirror as the curved-surface mirror20and arranging the lenses19cand19d, which are the aspheric plastic lenses, in the vicinity of the curved-surface mirror20. Hereinafter, this is explained.

InFIG. 6A, when an aspheric lens is placed in the vicinity of the image forming element17, the aspheric lens only gives the same aspheric effects to light fluxes corresponding to different angles of view. However, when the aspheric lens is placed closer to the curved-surface mirror20, the light fluxes corresponding to the different angles of view are divided. Thus, the aspheric lens can provide different aspheric effects to the light fluxes corresponding to the different angles of view. This is the reason why the aspheric lenses are placed in the vicinity of the curved-surface mirror20, which is the free-form surface mirror.

As shown inFIG. 6A, the effect of correcting the deformation with the aspheric lens is greater, when the aspheric lens is close to the curved-surface mirror20, which is the free-form surface mirror. However, the outer diameter of the aspheric lens may also be greater, when the aspheric lens is close to the curved-surface mirror20. Thus, a lens made of glass is undesirable from a viewpoint of cost and weight reduction. As in the case of the embodiment, it is preferable to use a plastic lens as the aspheric lens. Here, the aspheric plastic lens can be produced at low cost, can be produced to be lightweight, and can be shaped using a metallic mold.

As described above, there is an advantage for placing the aspheric lens in the vicinity of the curved-surface mirror20, which is the free-form surface mirror. However, there is a disadvantage for this arrangement. The disadvantage is that, when field curvature occurs, namely, when a focal position depends on a position on the screen90due to an error in a production process, and when a position is focused on the screen90at which a depth of field is shallow, the image is deformed.

Hereinafter, this is explained usingFIGS. 8 and 9. Here, configurations shown inFIGS. 8 and 9are the same as that ofFIGS. 2 and 3, respectively. For example, inFIG. 8, the depth of field at a position C on the screen90is shallower than the depth of field at a position B on the screen90. Here, the depth of field is a range in the direction perpendicular to the screen90(the Z-axis direction) within which an image is visually in focus. For example, at the position B, the image is visually in focus within a range L. However, at the position C, the image is visually in focus only within a range M. In the direction perpendicular to the screen90(the Z-axis direction), the depth of field is shallower at the position C than the depth of field at the position B. At the position C, an incident angle with respect to the screen90is greater.

Here, it is assumed that, at the position C, the image is visually in focus within a range N, instead of the range M that is the range within which the image is originally supposed to be in focus, because of some errors in the production process. In this case, the depth of field corresponding to the range L is relatively longer. Thus, in order for the image to be focused over the whole screen90, for example, the image may be focused by moving the screen90to a predetermined position within the range N, as shown with the dashed line inFIG. 8. As another example, the image may be focused by moving the ranges L and N toward the screen90.

In this manner, when the image is focused within the depth of field which is the shallower of the two, a deformation specific to the free surface is generated on the image. Here, note that the image is focused at a position shifted from the position at which the image was originally supposed to be in focus. On the other hand, as shown inFIG. 9, when the optical system does not include the curved-surface mirror20, which is the free-form surface mirror, and when the image is focused at a predetermined position within the range N, instead of a predetermined position within the range M at which the image is originally supposed to be in focus, the deformation on the screen does not change significantly.

As described above, for the projection optical system including the plastic lens that is placed at or in the vicinity of the free-form surface mirror, there is a problem that, when errors occur in the production process, field curvature occurs. Further, when the focus is adjusted, the uncorrectable field curvature occurs in the image projected onto the projection surface, such as the screen. Among the errors in the production process, particularly, a refractive index distribution which is generated during the molding process of the plastic lens can be a cause of the occurrence of the field curvature. Here, the plastic lens is to be placed in the vicinity of the free-form surface mirror. Hereinafter, this is explained in detail.

FIG. 10Ashows examples of refractive index distributions of a convex lens50and a concave lens60, which are aspheric plastic lenses. Recently, large aspheric lenses used in laser printers or projectors are manufactured mainly through the molding of plastic materials. This is because large aspheric lenses can be manufactured at low cost. Further, large aspheric lenses can be manufactured to be lightweight. Furthermore, the aspheric shapes can be easily formed. During a plastic molding process of an optical element, a thermally molten plastic material is shaped in metal molds. Then the shaped plastic material is cooled in the metal molds. During the cooling process, a peripheral portion is cooled relatively quickly compared to a central portion. Density at the portion which is cooled quickly becomes relatively denser, compared to density at a portion which is cooled slowly. Thus density distribution inside the shaped plastic material becomes inhomogeneous, or degradation occurs in the shaped plastic material. Therefore, the refractive index becomes inhomogeneous inside the shaped lens, and the refractive index distribution is generated.

The refractive index inside the plastic lens at the peripheral portion is higher than that of the central portion. As described above, this is because, during the shaping of the lens, the peripheral portion is cooled more quickly than the central portion, and the density at the peripheral portion becomes relatively denser compared to the density at the central portion. Therefore, the refractive index distributions become, for example, the distributions shown inFIG. 10A. When the concave lens60having such a refractive index distribution is placed, for example, at the closest position to the curved-surface mirror20inFIG. 6A, refractive power to a light flux passing through the position at which the refractive index is high (the light which travels toward the upper portion on the screen) is insufficient, and the range within which the image is in focus is displaced to the left side of the screen (the direction which is opposite to the direction from the screen90to N inFIG. 8). Thus, field curvature occurs. In this manner, when the aspheric lens is placed in the vicinity of the curved-surface mirror20, which is the free-form surface mirror, the field curvature occurs during focus adjustment.

However, in the embodiment, the lenses19cand19d, which are aspheric plastic lenses, are placed in the vicinity of the curved-surface mirror20, which is the free-form surface mirror, instead of placing only one aspheric lens in the vicinity of the curved-surface mirror20. Therefore, the field curvature caused by the refractive index distribution of the aspheric plastic lens can be reduced.

Namely, the lenses19cand19dhave the refractive index distributions similar to the refractive index distributions of the convex lens50and the concave lens60shown inFIG. 10A(cf.FIGS. 10B and 10C). Suppose that the lenses19cand19dhave similar refractive index distributions. For example, each refractive index distribution is such that the refractive index becomes higher as the position moves from the center of the lens to the peripheral portion, as shown inFIGS. 10B and 10C. As the refractive index of the lens19cbecomes higher at the end portion of the lens19c, the point of focus at the position C on the screen is displaced to the front side of the screen, as shown inFIG. 8.

However, as shown inFIG. 10C, as the refractive index of the lens19dbecomes higher at the end portion of the lens19d, similarly to the case of the lens19c, the point of the focus at the position C on the screen is displaced to the opposite side (the rear side of the screen). Namely, the field curvature can be reduced by using both the lens19cand the lens19d. Because of this effect, it is preferable to arrange a set of lenses including a lens with a positive refractive power and a lens with a negative refractive power, when aspheric lenses having large deformation correcting effects are used in the optical system including the curved-surface mirror20, which is the free-form surface mirror.

However, when the refractive index at the center of the lens19cis defined to be N1iand the refractive index at the peripheral portion of the lens19cis defined to be N1oas shown inFIG. 10B, and when the refractive index at the center of the lens19dis defined to be N2iand the refractive index at the peripheral portion of the lens19dis defined to be N2oas shown inFIG. 100, it may not be necessary that characteristics of the refractive index distributions of the lenses19cand19dbe such that the refractive index becomes higher as the position moves from the center of the lens to the peripheral portion of the lens, as shown inFIG. 10A, provided that a sign of a difference of the refractive indices ΔN1=N1o−N1ifor the lens19c, which is the difference between the refractive index at the center of the lens19cand the refractive index at the peripheral portion of the lens19c, is equal to a sign of a difference of the refractive indices ΔN2=N2o−N2ifor the lens19d, which is the difference between the refractive index at the center of the lens19dand the refractive index at the peripheral portion of the lens19d. The reason is that, when the sign of the difference of the refractive indices ΔN1=N1o−N1ifor the lens19cand the sign of the difference of the refractive indices ΔN2=N2o−N2ifor the lens19dare the same, the effects of the refractive index distributions can be canceled out.

Further, when the difference between the refractive index at the center of the lens19cand the refractive index at the peripheral portion of the lens19cis defined to be ΔN1, a distance from a point, at which the outermost light beam among light beams passing through the lens19cexits the lens19c, to the optical axis is defined to be W1(cf.FIG. 6B), the difference between the refractive index at the center of the lens19dand the refractive index at the peripheral portion of the lens19dis defined to be ΔN2, and a distance from a point, at which the outermost light beam among light beams passing through the lens19dexits the lens19d, to the optical axis is defined to be W2(cf.FIG. 6B), it is preferable that (ΔN1/W1) and (ΔN2/W2) be substantially equal. Further, it is preferable that a focal distance of the lens19cand a focal distance of the lens19dbe substantially equal. That is because the effects of the refractive index distributions can be effectively canceled out.

Further, as described above, when the light fluxes reflected on the image forming element17approaches the curved-surface mirror20, the light fluxes are divided into the plural light fluxes corresponding to the angles of view. Since the intensity of the light used in the image projection device10, such as the projector, is strong and the lens closer to the image forming element17is irradiated by the light fluxes not being separated, the temperature of the lens closer to the image forming element17tends to become higher compared to an outside air temperature. On the other hand, the temperature of the lens closer to the curved-surface mirror20does not become as high in comparison to the lens closer to the image forming element17. Thus an expansion or a change in the refractive index caused by heat does not tend to occur in the lens closer to the curved-surface mirror20in comparison to the lens closer to the image forming element17. Since the aspheric plastic lens is sensitive to the expansion or the change in the refractive index caused by heat, it is preferable that the aspheric plastic lens be arranged at a position closer to the curved-surface mirror20. Especially, it is preferable that one of the lenses19cand19dbe arranged at the closest position to the curved-surface mirror20among the optical elements included in the coaxial optical system19. Further, when the lenses19cand19dare arranged adjacent to each other, temperatures of the lenses19cand19dare nearly equal. Thus the effect of heat can be reduced.

As described above, in the first embodiment, the projection optical system18includes the coaxial optical system19including the lens19chaving the positive refractive power and the lens19dhaving the negative refractive power; and the curved-surface mirror20which is the non-coaxial optical system that does not share the optical axis with the coaxial optical system19. Further, the lenses19cand19dmay be aspheric plastic lenses. The refractive index distribution of the lens19cbetween the center of the lens19cand the peripheral portion of the lens19c(for example, the refractive index distribution such that the refractive index becomes higher as the position moves from the center of the lens19cto the peripheral portion of the lens19c) may be similar to the refractive index distribution of the lens19dbetween the center of the lens19dand the peripheral portion of the lens19d. One of the lenses19cand19dmay be arranged at the closest position to the curved-surface mirror20among the elements included in the coaxial optical system19. As a result, the effect of the refractive index distributions are cancelled out by the lens19chaving the positive refractive power and the lens19dhaving the negative refractive power. Thus the field curvature can be reduced. Further, by arranging the lenses19cand19d, which are the aspheric plastic lenses, at the positions closer to the curved-surface mirror20, the effect of temperature rise in the image projection device10can be reduced.

Second Embodiment

In a second embodiment, the projection optical system18and a coaxial optical system29are arranged as shown inFIG. 11. Namely, the coaxial optical system29has a common configuration with the coaxial optical system19except that the lenses19cand19dare replaced by lenses29cand29d. In the coaxial optical system29, the lens29chas a common configuration with the lens19cexcept that a portion of the lens19c(substantially the upper half portion) is removed. The lens29chas a positive refractive power. The lens29cis an aspheric plastic lens. Further, the lens29dhas a common configuration with the lens19dexcept that a portion of the lens19d(substantially the upper half portion) is removed. The lens29dhas a negative refractive power. The lens29dis an aspheric plastic lens. Here, each of the lenses19a,19b,29c, and29dmay be one lens in a group of lenses.

The lens19ais fixed. The lenses19b,29c, and29dare independently reciprocable in the Z-axis direction (the direction of the optical axis A). Namely, the coaxial optical system29adopts the floating focusing method in which the plural lenses (the lenses19b,29c, and29d) in the coaxial optical system29move corresponding distances, which are different from each other, in the Z-axis direction (in the direction of the optical axis A) and the focus is adjusted.

In the coaxial optical system19shown inFIG. 6A, even if the lens placed closer to the curved-surface mirror20is shaped rotationally symmetric, the light fluxes only pass through a portion of the lens corresponding to a half of the diameter. Further, the outer diameter of the lens placed closer to the curved-surface mirror20is larger than that of the lens placed closer to the image forming element17. This can be a cause of an increase in size of the image projection device10. Therefore, it is preferable that the lens29cbe shaped such that a portion (substantially the upper half portion) of the rotationally symmetric shape (the shape of the lens19c), through which the light fluxes do no pass, is removed from the rotationally symmetric shape and that the lens29dbe shaped such that a portion (substantially the upper half portion) of the rotationally symmetric shape (the shape of the lens19d), through which the light fluxes do no pass, is removed from the rotationally symmetric shape. This facilitates the downsizing of the image projection device10.

Since the lenses29cand29dare aspheric plastic lenses, the lenses29cand29dcan be formed by metal molding. Therefore, as shown inFIG. 11, the lens29ccan be easily formed to have the shape such that the portion (substantially the upper half portion), through which the light fluxes do not pass, is removed from the rotationally symmetric shape. Similarly, the lens29dcan be easily formed to have the shape such that the portion (substantially the upper half portion), through which the light fluxes do not pass, is removed from the rotationally symmetric shape. The lenses29cand29dcan be produced with less amounts of plastic material compared to the lenses19cand19d. Therefore, using the lenses29cand29dfacilitates resource saving and cost reduction.

As described above, the second embodiment provides the effect described below, in addition to the effect similar to the effect of the first embodiment. Namely, in the second embodiment, the lenses29cand29d, which are placed closer to the curved-surface mirror20, are formed, so that each of the lenses29cand29dhas the shape such that the portion (substantially the upper half portion), through which the light fluxes do not pass, is removed from the rotationally symmetric shape. Thus the second embodiment can facilitate downsizing, resource saving, and cost reduction of the image projection device10.

Third Embodiment

In the third embodiment, the projection optical system18and a coaxial optical system39are arranged as shown inFIG. 12. Namely, the coaxial optical system39has a common configuration with the coaxial optical system19except that the lenses19cand19dare replaced by lenses39cand39d. In the coaxial optical system39, the lens39cis a lens having a negative refractive power and is an aspheric plastic lens. Further, the lens39dis a lens having a positive refractive power and is an aspheric plastic lens. Here, each of the lenses19a,19b,39c, and39dmay be one lens in a group of lenses.

The lens19ais fixed. The lenses19b,39c, and39dare independently reciprocable in the Z-axis direction (the direction of the optical axis A). Namely, the coaxial optical system39adopts the floating focusing method in which the plural lenses (the lenses19b,39c, and39d) in the coaxial optical system39move corresponding distances, which are different from each other, in the Z-axis direction (in the direction of the optical axis A) and the focus is adjusted.

Any one of the convex lens and the concave lens, which are the aspheric plastic lenses, can be placed closer to the curved mirror20. In the coaxial optical system19shown inFIG. 6A, the concave lens (the lens19d) is placed closer to the curved-surface mirror20. However, in the coaxial optical system39shown inFIG. 12, the convex lens (the lens39d) is placed closer to the curved-surface mirror20. Namely, placing the convex lens closer to the curved-surface mirror20and placing the concave lens closer to the curved-surface mirror20provide similar effects, provided that the convex lens and the concave lens are arranged at corresponding positions where the light fluxes are separated. Here, a glass lens may be placed between the convex lens and the concave lens. Placing the glass lens between the convex lens and the concave lens still provides the effect to cancel out the field curvature, similarly to the case in which the convex lens and the concave lens are arranged adjacent to each other.

As described above, in the third embodiment, the convex lens (the lens39d) is placed closer to the curved-surface mirror20. Placing the convex lens closer to the curved-surface mirror20provides the effect similar to the effect of the first embodiment, in which the concave lens (the lens19d) is placed closer to the curved-surface mirror20.

Fourth Embodiment

In the fourth embodiment, the projection optical system18and a coaxial optical system49are arranged as shown inFIG. 12. Namely, the coaxial optical system49has a common configuration with the coaxial optical system19except that the lenses19cand19dare replaced by a group of lenses49c. In the coaxial optical system49, the group of lenses49chas a configuration such that the group of lenses49cincludes a lens49c1and a lens49c2that are joined and integrated. Here, each of the lenses19aand19bmay be one of many lenses included in a group. Further, the group of lenses49cmay include a lens other than the lenses49c1and49c2.

The lens19ais fixed. The lens19band the group of lenses49care independently reciprocable in the Z-axis direction (the direction of the optical axis A). Namely, the coaxial optical system49adopts the floating focusing method in which the lens and the group of lenses (the lens19band the group of lenses49c) in the coaxial optical system49move corresponding distances, which are different from each other, in the Z-axis direction (in the direction of the optical axis A) and the focus is adjusted.

The lens49c1is a lens having a negative refractive power. The lens49c1is a plastic lens such that the surface closer to the image forming element17is an aspheric surface and the surface closer to the curved-surface mirror20is a spherical surface. Further, the lens49c2is a lens having a positive refractive power. The lens49c2is a plastic lens such that the surface closer to the image forming element17is a spherical surface and the surface closer to the curved-surface mirror20is an aspheric surface. The spherical surface of the lens49c1and the spherical surface of the lens49c2are joined. In this case, the lenses49c1and49c2receive similar effects of temperature change which is caused by high-intensity light fluxes from the image projection device10. Thus the effects of the temperature change can be efficiently cancelled out. Further, it is especially preferable that absolute values of the refractive powers of the lenses49c1and49c2be set to be close. When the absolute values of the refractive powers of the lenses49c1and49c2are close, the effect of cancelling out the thermal effect is improved.

As described above, the fourth embodiment provides the effect described below, in addition to the effect similar to the effect of the first embodiment. Namely, in the coaxial optical system49, the group of lenses49cis arranged. The lens49cincludes the lens49c1having a negative refractive power and being a plastic lens such that the surface closer to the image forming element17is the aspheric surface and the surface closer to the curved-surface mirror20is the spherical surface; and the lens49c2having a positive refractive power and being a plastic lens such that the surface closer to the image forming element17is the spherical surface and the surface closer to the curved-surface mirror20is the aspheric surface. Here, the spherical surfaces of the lenses49c1and49c2are joined. As a result, the effect of the temperature change on the group of lenses49ccan be cancelled out. Here, the temperature change is caused by the high-intensity light fluxes from the image projection device10.

Incidentally, inFIG. 13, the group of lenses49carranged in the coaxial optical system49may have the following configuration. Namely, the group of lenses49cincludes the lens49c1and the lens49c2. Here, the lens49c1may have a positive refractive power, and the lens49c1may be a plastic lens such that the surface closer to the image forming element17is an aspheric surface and the surface closer to the curved-surface mirror20is a spherical surface. Further, the lens49c2may have a negative refractive power, and the lens49c2may be a plastic lens such that the surface closer to the image forming element17is a spherical surface and the surface closer to the curved-surface mirror20is an aspheric surface. Here, the spherical surfaces of the lenses49c1and49c2are joined.

The preferred embodiments are described above. However, the present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope of the present invention.

The present application is based on Japanese Priority Application No. 2010-290068 filed on Dec. 27, 2010, the entire contents of which are hereby incorporated herein by reference.