# Patent

## Title
Projection optical system

## Abstract

A 1-2nd lens group includes two aspherical resin lenses, and thereby it is possible to perform effective correction in order to suppress aberration fluctuation to be reduced. At this time, a glass lens is disposed between the two lenses, and thereby it is possible to control an incident light angle to the resin lens and to reduce power of the resin lenses, and it is possible to prevent variations of shapes of the resin lens. Even in a case where the second optical group is configured of one mirror, it is possible for a primary image to have appropriate aberration and to hereby reduce aberration for a good image which is finally projected onto a screen through the second optical group.

## Background
BACKGROUND
1. Technical Field
The present invention relates to a projection optical system suitable for being incorporated in a projector which performs enlargement projection of an image of an image display element.
2. Related Art
A refraction optical system configured to include a plurality of lenses as a projection optical system for a projector which can perform projection from a short distance and can obtain a large picture plane by having a wide angle of view substantially equal to a half angle of view of 60 degrees, is proposed (see JP-A-2007-147970). However, in a case where a significantly wide angle of view is obtained by the refraction optical system configured only of the lenses, there are drawbacks in that, particularly, a lens disposed on the enlargement side is likely to be enormously increased in side. In addition, when the refraction optical system performs projection at a wide angle of view, it is considered that a large number of lenses are needed in order to correct chromatic aberration of magnification occurring by a negative meniscus lens which is positioned on the enlargement side and has great power.
As a method to solve the drawbacks of the refraction optical system, a refraction optical system formed of a plurality of lenses and a refraction/reflection complex optical system which uses at least one curved reflective mirror have been proposed (for example, see JP-A-2006-235516 and JP-A-2007-079524). In the refraction/reflection complex optical system, since a reflective mirror is used as a final unit which obtains a wide angle of view, the chromatic aberration of magnification is unlikely to occur, compared to the refraction optical system using only the lenses described above.
However, for example, in JP-A-2006-235516, a significantly wide angle of view is obtained using the refraction optical system and a concave mirror; however, the curved mirror needs to be enormously increased in size and the entire length thereof needs to be enormously increased. In addition, in JP-A-2007-079524, for example, the angle of view is about 60 degrees in the eighth example, a mirror size is decreased by combining a concave mirror and a convex mirror. However, similar to JP-A-2006-235516, the entire length needs to be enormously increased. In addition, the F-number is about 3 and it is dark and an optical system using a transmissive liquid crystal is defective in terms of brightness. Moreover, two mirrors have an aspherical surface, which causes a high degree of difficulty in terms of achieving accuracy and assembly.
As above, in the refraction/reflection complex optical system, an ultra-wide angle of view is obtained but it is difficult to decrease the entire length, and thereby there are drawbacks in that the mirror is increased in size. The system is suitable for the usage in which the optical system is bent and disposed in a large housing such as one in a rear projector. However, the system is not suitable for equipment such as a front projector in which portability is important.
In comparison, a system in which a reflective mirror is used as a front projector has been known (JP-A-2008-250296, JP-A-2012-203139, or the like). For example, in JP-A-2008-250296, one or two aspherical lens is disposed before an aspherical mirror, and thereby a compact configuration is achieved; however, in a system having brightness with the F-number of about 1.7, a range of magnification change is narrowed to about 1.2 times. Conversely, in a system having a range of magnification change of about 2 times, it is dark with the F-number of about 1.85. In addition, in JP-A-2012-203139, a positive lens is disposed closest to the mirror side in the refraction optical system, and thereby it is possible to miniaturize the mirror, which enables the entire optical system to be miniaturized. However, since the system is applied only to the F-number of about 1.8, sufficient brightness is not obtained.
Incidentally, in the related art, a projector for the proximity application is usually used by being fixed during installment such as ceiling installment or wall installment, with respect to a fixed screen. However, recently, there is a high demand that not only the projector is disposed upright, and performs projection onto a table surface with a relatively small projection size, but also the projector moves to a relatively large room and can be applied to a large picture plane projection. In a case of the large picture plane projection, in order to obtain sufficient contrast even in a relatively bright place, there is a need to use a bright optical system even to a small extent.
SUMMARY
An advantage of some aspects of the invention is to provide a projection optical system which can cover a wide range of magnification change in an application to a short throw type projector and can be applied to an image display element having high resolution.
A projection optical system according to an aspect of the invention includes: in order from a reduction side, a first optical group which is formed of a plurality of lenses and has positive power; and a second optical group which has one reflective surface having a concave aspherical shape. The first optical group is formed to have a 1-1st lens group having positive power, on the reduction side, and a 1-2nd lens group having weaker positive or negative power, compared to the power of the 1-1st lens group, on the enlargement side, with the widest air interval as a boundary. The 1-2nd lens group includes at least two aspherical resin lenses and at least one glass lens disposed between the two aspherical resin lenses. Here, in comparison between power of the lenses and the lens groups, the relatively weak power means a less value in a case where absolute values of power are compared. In other words, for the power which the 1-2nd lens group has, the above description means that the absolute value thereof is less than an absolute value of power which the 1-1st lens group has.
First, in this case of the configuration described above, the first optical group plays a role of causing an image of an object (that is, a panel section) to be formed as an image in front of a mirror of the second optical group and forming a primary image in order to form an image again on a screen by the mirror of the second optical group. At this time, since the second optical group is configured only of one mirror, it is difficult to individually correct aberration. Accordingly, in order to obtain a final image having small aberration on the screen by the second optical group, there is a need to form the primary image containing aberration in the first optical group.
Further, in the ultra-wide angle projection optical system having the configuration described above, when the projection magnification is changed, the aberration fluctuation is likely to increase because an angle of view is abnormally wide. Accordingly, the first optical group needs to form the primary image which contains aberration corresponding to the change of image forming magnification even when the image forming magnification is changed.
In comparison, in the projection optical system according to the aspect of the invention, as described above, the 1-2nd lens group includes at least two aspherical resin lenses, and thereby it is possible to form a primary image needed to obtain a good image on a screen. In application to a short throw type projector, a wide range of magnification change is covered and it is possible to be also applied to an image display element having high resolution.
Particularly, in order to obtain an image having high contrast with respect to a high-definition panel and having low distortion, a plurality of focusing lens groups configuring the 1-2nd lens group and a mirror configuring the second optical group need to be appropriately disposed, and, especially, it is preferable that the 1-2nd lens group includes at least two aspherical lenses molded using a resin.
Here, a plurality of naspherical lenses are used, and thereby high definition and low distortion are relatively easily achieved. However, in a case of using the plurality of aspherical surfaces, in general, high surface accuracy of the respective aspherical surfaces is needed, and eccentricity between the aspherical surfaces of the respective lenses is likely to result in the performance deterioration. Further, as the general characteristics of the resin-molded lens, since the shrinkage factor of the material is great, it is difficult to secure accuracy of the surface, compared to the glass-molded lens or the like. In addition, when the power becomes excessively strong, an uneven thickness ratio representing a ratio of the lens thickness in the vicinity of the optical axis and the lens thickness at the outer circumferential portion is increased, and thus it is known that internal distortion occurs in a gate portion which is formed during molding or in an outer circumferential portion, which influences on the performance. A linear expansion coefficient, a refractive index, or a temperature coefficient of the resin is increased by one digit, compared to the case of the glass lens, which results in a focal position shift due to a focal distance change according to a surrounding temperature or a temperature change during an operation. Accordingly, for the resin lens, it is not preferable that a single body has strong power. For this reason, it is preferable that the resin lens has weak power to the smallest extent, and has a low uneven thickness ratio.
In comparison, in the aspect of the invention, at least one glass lens is disposed between a plurality of resin lenses, and thereby the power of the resin lens is lowered. An angle of the light flux incident to the resin lens is controlled by the glass lens. In this manner, the optimal incident angle to the resin lens is obtained, and thereby it is possible to prevent performance deterioration due to variations of shapes or internal distortion.
In a specific aspect of the invention, an interval between the two aspherical resin lenses of the 1-2nd lens group is changed when focusing is performed in response to the magnification change. In this case, the interval between the aspherical lenses is changed, and thereby it is possible to achieve high definition and low distortion of the image over the wide range of the magnification change.
In a case where the plurality of aspherical lenses are arranged in parallel, sensitivity to the interval of the lenses or sensitivity to eccentricity between the lenses is excessively increased and excessively high accuracy such as frame accuracy of the moving group is needed. In comparison, in the above aspect, the glass lens (spherical lens) is disposed between the aspherical lenses, and thereby it is possible to decrease the sensitivity between the aspherical lenses. Further, the aspherical lenses move as the moving group, and as a result, it is possible to achieve high definition and low distortion.
Further, for the resin-molded lens, a lens having a large aperture can be relatively low in cost, and it is advantageous that the resin-molded lens can be applied to a lens having a shape other than the circular shape, relatively simply.
In another aspect of the invention, the 1-1st lens group is configured to have an aperture therein and two lenses of a positive lens with a convex surface facing the enlargement side and a negative lens with the concave surface facing the enlargement side, in this order from the reduction side, on the enlargement side from the aperture. In this case, the two lenses as described above are disposed on the enlargement side from the aperture, and thereby it is possible to maintain good performance even in a wide range of the magnification change.
The 1-1st lens group plays a role of efficiently receiving the light flux from the object (that is, the panel) side and sending the light flux to the 1-2nd lens group as the focusing lens group. The 1-2nd lens group, as the focusing lens group, needs to play a role of forming an appropriate intermediate image even in a wide range of the magnification change. As described above, the lens group disposed on the enlargement side from the aperture of the 1-1st lens group is two positive and negative lenses, particularly, the positive lens with the convex surface facing the enlargement side and the negative lens with the concave surface facing the enlargement side are disposed in this order from the reduction side, and thereby it is possible to define the surface of the 1-1st lens group on the outermost enlargement side as an emission surface. The 1-1st lens group and the 1-2nd lens group as the focusing lens group are hereby combined, then, it is possible to correct the field curvature and astigmatism characteristics in the wide range of the magnification change and it is possible to obtain stable performance in order to form an appropriate intermediate image.
In still another aspect of the invention, the 1-1st lens group has an aperture therein, includes at least two sets of cemented lenses of positive lenses and negative lenses disposed on the reduction side from the aperture, and has at least one concave naspherical surface facing the enlargement side. In this case, an occurrence of chromatic aberration is prevented even in a configuration in which a small number of lenses are used, assembly variations is decreased, and it is possible to increase numerical aperture (brighten).
The plurality of lenses disposed on the reduction side from the aperture of the 1-1st lens group play a role of efficiently receiving the light flux from the object (that is, the panel) side. In a case where the plurality of lenses are configured of only the spherical lenses, there is a possibility that the number of lenses needs to be increased. When the number of lenses is increased, transmittance is reduced, the entire length of the lens is increased due to the increase of the lenses, and the configurational number of lenses needs to be set to the minimum extent.
For example, in order to cover the brightness having F-number of about 1.6, at least surface having the concave aspherical shape on the enlargement side is included on the reduction side from the aperture of the 1-1st lens group, thereby the brightness is secured, an occurrence of flare is suppressed, and it is possible to provide an image having high contrast. In addition, at least two sets of cemented lenses are configured to be disposed on the reduction side from the aperture of the 1-1st lens group, thereby an occurrence of chromatic aberration is suppressed to the smallest extent, and assembling properties are enhanced by cementing.
In still another aspect of the invention, the numerical aperture on the object side is equal to or more than 0.3. In this case, it is possible to form a sufficiently bright projection image.
In still another aspect of the invention, the reduction side is substantially telecentric.
In still another aspect of the invention, elements configuring the first optical group and the second optical group all have a rotationally symmetric system.
In still another aspect of the invention, a range of magnification change is equal to or greater than 1.5 times.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements.
FIG. 1 is a diagram showing a schematic configuration of a projector in which a projection optical system of an embodiment is incorporated.
FIG. 2 is a diagram showing light fluxes and a configuration from an object surface to a projection surface in the projection optical system of the embodiment or Example 1.
FIG. 3 is an enlarged diagram showing a part from the object surface to a concave reflective mirror in FIG. 2.
FIG. 4 is a diagram showing a configuration of the projection optical system of Example 1.
FIGS. 5A to 5C are diagrams showing aberration on a reduction side of the projection optical system of Example 1.
FIGS. 6A to 6E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 5A.
FIGS. 7A to 7E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 5B.
FIGS. 8A to 8E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 5C.
FIG. 9 is a diagram showing a configuration of a projection optical system of Example 2.
FIGS. 10A to 10C are diagrams showing aberration on a reduction side of the projection optical system of Example 2.
FIGS. 11A to 11E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 10A.
FIGS. 12A to 12E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 10B.
FIGS. 13A to 13E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 10C.
FIG. 14 is a diagram showing a configuration of a projection optical system of Example 3.
FIGS. 15A to 15C are diagrams showing aberration on a reduction side of the projection optical system of Example 3.
FIGS. 16A to 16E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 15A.
FIGS. 17A to 17E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 15B.
FIGS. 18A to 18E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 15C.



DESCRIPTION OF EXEMPLARY EMBODIMENTS
Hereinafter, a projection optical system according to an embodiment of the invention will be described in detail with reference to the drawings.
As illustrated in FIG. 1, a projector 2, in which the projection optical system according to an embodiment of the invention is incorporated, includes an optical system section 50 which projects an image light, and a circuit device 80 which controls an operation of the optical system section 50.
In the optical system section 50, a light source 10 is, for example, an extra-high pressure mercury lamp, and emits light fluxes including an R light flux, a G light flux, and a B light flux. The light source 10 may be a discharge light source, in addition to an extra-high pressure mercury lamp, or may be a solid-state light source, such as an LED or a laser. A first integrator lens 11 and a second integrator lens 12 have a plurality of lens elements arranged in an array. The first integrator lens 11 divides a light flux from the light source 10 into a plurality of light fluxes. Each lens element of the first integrator lens 11 condenses the light flux from the light source 10 in the vicinity of the lens elements of the second integrator lens 12. The lens elements of the second integrator lens 12 form images of the lens elements of the first integrator lens 11 on the liquid crystal panels 18R, 18G, and 18B in cooperation with a superimposing lens 14. In this configuration, the entire display regions of the liquid crystal panels 18R, 18G, and 18B are illuminated with a light flux from the light source 10, in substantially uniform brightness.
A polarization conversion element 13 converts a light flux from the second integrator lens 12 to a predetermined linearly polarized light. The superimposing lens 14 superimposes the image of each lens element of the first integrator lens 11 on the display regions of the liquid crystal panels 18R, 18G, and 18B through the second integrator lens 12.
A first dichroic mirror 15 reflects the R light flux incident from the superimposing lens 14 and transmits the G light flux and the B light flux. The R light flux reflected from the first dichroic mirror 15 is incident to the liquid crystal panel 18R serving as an optical modulator through a reflective mirror 16 and a field lens 17R. The liquid crystal panel 18R modulates the R beam in response to an image signal so as to form an R-color image.
A second dichroic mirror 21 reflects the G beam from the first dichroic mirror 15 and transmits the B beam. The G beam reflected from the second dichroic mirror 21 is incident to the liquid crystal panel 18G serving as an optical modulator through a field lens 17G. The liquid crystal panel 18G modulates the G beam in response to an image signal to form a G-color image. The B beam transmitted through the second dichroic mirror 21 is incident to the liquid crystal panel 18B serving as an optical modulator through relay lenses 22 and 24, reflective mirrors 23 and 25, and a field lens 17B. The liquid crystal panel 18B modulates the B beam in response to an image signal to form a B-color image.
A cross dichroic prism 19 is a prism for beam composition, combines beams modulated by the liquid crystal panels 18R, 18G, and 18B to form an image beam, and causes the image beam to travel to a projection optical system 40.
The projection optical system 40 is a zoom lens for projection, which projects an image beam modulated by the liquid crystal panels 18G, 18R, and 18B and combined by the cross dichroic prism 19 onto a screen (not shown) on an enlarged scale.
The circuit device 80 includes an image processing unit 81 to which an external image signal, such as a video signal, is input, a display driving unit 82 which drives the liquid crystal panels 18G, 18R, and 18B provided in the optical system section 50 on the basis of an output of the image processing unit 81, a lens driving unit 83 which operates a driving mechanism (not shown) provided in the projection optical system 40 to adjust a state of the projection optical system 40, and a central control unit 88 which performs overall control of the operations of the circuit portions 81, 82, and 83, and the like.
The image processing unit 81 converts the input external image signal to an image signal including the tone of each color or the like. The image processing unit 81 may perform various image processes, such as distortion correction or color correction, on the external image signal.
The display driving unit 82 can operate the liquid crystal panels 18G, 18R, and 18B on the basis of an image signal output from the image processing unit 81, and can form an image corresponding to the image signal or an image corresponding to an image signal subjected to the image process, on the liquid crystal panels 18G, 18R, and 18B.
The lens driving unit 83 operates under the control of the central control unit 88, and appropriately moves some optical components configuring the projection optical system 40 along an optical axis OA through an actuator AC, thereby it is possible to perform focusing in response to magnification change (focusing during magnification change) in projection of an image on a screen by the projection optical system 40. Further, the lens driving unit 83 can change a vertical position of the image projected on the screen, through adjustment of a tilt at which the entire projection optical system 40 moves vertically perpendicular to the optical axis OA.
Hereinafter, the projection optical system 40 of the embodiment will be specifically described with reference to FIG. 2 and FIG. 3. The projection optical system 40 illustrated in FIG. 2 or the like has the same configuration as the projection optical system 40 of Example 1 to be described below.
The projection optical system 40 of the embodiment projects an image formed on a projection-performed surface of the liquid crystal panel 18G (18R or 18B), onto a screen (not shown). A prism PR corresponding to the cross dichroic prism 19 in FIG. 1 is disposed between the projection optical system 40 and the liquid crystal panel 18G (18R or 18B).
The projection optical system 40 includes a first optical group 40a which is formed of a plurality of lenses and has positive power and a second optical group 40b which is configured of one mirror MR with a reflective surface having a concave aspherical shape. The first optical group 40a is formed to have a 1-1st lens group 41 having positive power, on the reduction side, and a 1-2nd lens group 42 having weaker positive or negative power, compared to the power of the 1-1st lens group 41, on the enlargement side, with the widest air interval BD, as a boundary, of a space formed between included lenses.
The 1-1st lens group 41 has an aperture ST inside thereof and is formed to have a lens group E1 on a reduction side from the aperture ST and a lens group E2 on an enlargement side from the aperture ST.
The 1-2nd lens group 42 has, in order from the reduction side, three moving lens groups of an F1 lens group (hereinafter, lens group F1), an F2 lens group (hereinafter, lens group F2), and an F3 lens group (hereinafter, lens group F3), which individually move in an optical axis direction when focusing is performed in response to the magnification change, and a fixed lens group H (enlargement-side fixed lens group) which is fixed when focusing is performed in response to the magnification change. As shown in FIG. 2, the fixed lens group H is configured of two negative lenses L15 and L16 and the lens L16 disposed on the outermost enlargement side is the aspherical lens molded using a resin. The lens L15 disposed on the reduction side is the glass lens. In addition, of the three lens groups F1 to F3 (moving lens groups), the lens L14 disposed on the outermost enlargement side, that is, at a position in the vicinity of the fixed lens group H (enlargement-side fixed lens group) is an aspherical lens molded using a resin. In other words, the 1-2nd lens group 42 is configured to have at least two aspherical resin lens and at least one glass lens disposed between the two aspherical resin lenses. In addition, the three lens groups F1 to F3 (moving lens group) are caused to individually move by the actuator AC in a direction A1 along the optical axis OA when focusing is performed in response to magnification change. The lenses L14 and L16 as the two aspherical resin lenses of the 1-2nd lens group 42 have an interval therebetween, which is changed when focusing is performed in response to the magnification change. Further, the actuator AC can cause the lens groups F1 to F3 to move in various modes by performing of the focusing during magnification change and, for example, the three groups may completely individually move, but may be linked to each other using a cam mechanism or the like.
Hereinafter, the lenses configuring each lens group will be described in order from the reduction side. Of the first optical group 40a, the lens group E1 of the 1-1st lens group 41 has eight lenses L1 to L8 and the lens group E2 has two lenses L9 and L10. The lens group F1 as the moving lens group of the 1-2nd lens group 42 has one lens L11, the lens group F2 has one lens L12, the lens group F3 has two lenses L13 and L14, and the fixed lens group H positioned on the outermost enlargement side has two lenses L15 and L16 described above. In other words, the first optical group 40a is configured to have 16 lenses L1 to L16 as a whole.
Of the lenses L1 to L8 configuring the lens group E1, the lens L3 as a positive lens and the lens L4 as a negative lens form a cemented lens and the lens L7 and the lens L8 form a cemented lens. Particularly, the lens L6 is a negative glass lens with both surfaces of aspherical shapes and is a negative aspherical meniscus lens with a concave naspherical shape on the enlargement side. In other words, the 1-1st lens group 41 has at least two sets of cemented lenses of the positive lenses and the negative lenses on the reduction side from the aperture ST and has at least one surface of the concave aspherical surface facing the enlargement side. Further, of the lenses configuring the lens group E1, the lenses other than the lens L6 are spherical glass lenses. In addition, the lenses L1 to L8 have a circular shape which is symmetric about the optical axis OA.
For the two lenses L9 and L10 configuring the lens group E2, the lenses L9 is the positive lens and the lens L10 is the negative lens. Particularly, the lens L10 is a negative meniscus lens with the concave surface facing the enlargement side. In other words, the 1-1st lens group 41 is configured to have two lenses of a positive lens with the convex surface facing the enlargement side and a negative lens with the concave surface facing the enlargement side, in this order from the reduction side on the enlargement side from the aperture ST. Further, lenses L9 and L10 are spherical glass lenses having a circular shape which is symmetric about the optical axis OA.
The lens L11 configuring the lens group F1 is a positive biconvex lens. Further, the lens L11 is the spherical glass lens having a circular shape which is symmetric about the optical axis OA.
The lens L12 configuring the lens group F2 is a positive biconvex lens. Further, the lens L12 is the spherical glass lens having a circular shape which is symmetric about the optical axis OA.
Of the lenses L13 and L14 configuring the lens group F3, the lens L13 is a negative meniscus lens. Further, the lens L13 is the spherical glass lens having a circular shape which is symmetric about the optical axis OA. In comparison, the lens L14 is the aspherical resin lens as described above. More specifically, the lens L14 is a lens (aspherical lens) which has negative power and has both surface subjected to an aspherical surface process, and is a lens (resin lens) molded using a resin. Further, the lens L14 has the circular shape which is symmetric about the optical axis OA.
Of the lenses L15 and L16 configuring the fixed lens group H disposed on the outermost enlargement side of the lens groups, the lens L15 is a negative biconcave lens. Further, the lens L15 is the spherical glass lens having a circular shape which is symmetric about the optical axis OA. The lens L16 is a lens (aspherical lens) which has negative power with both surface subjected to an aspherical surface process, and is a lens (resin lens) molded using a resin. Further, the lens L16 is not formed to have a circular shape, but to have a partially notched shape which is notched on the upper side (side on which an image beam is projected) from a state of a circular shape which is symmetric about the optical axis OA.
The second optical group 40b is configured of one mirror MR having a concave aspherical shape and the mirror MR reflects the image beam emitted from the first optical group 40a, and thereby projects the image beam to a screen.
Further, as described above, in the projection optical system 40, of all of the lenses L1 to L16 configuring the first optical group 40a, lenses L1 to L15 have a circular shape which is symmetric about the optical axis OA and lens L16 has a partially notched shape from a circular shape which is symmetric about the optical axis OA. In addition, the mirror MR configuring the second optical group 40b also has a partially notched shape from a circular shape which is symmetric about the optical axis OA. In other words, elements configuring the first optical group 40a and the second optical group 40b all belong to a rotationally symmetric system. In addition, as shown in FIG. 1, the reduction side in the projection optical system 40 is substantially telecentric. For example, as described above, in a case where beams modulated by the respective liquid crystal panels 18R, 18G, and 18B in the cross dichroic prism 19 are combined into an image beam, it is possible to hereby easily absorb variations due to assembly.
In general, a short throw projection optical system including the projection optical system 40 as above has an abnormally short distance to a screen. In the projection optical system 40 described above, an object positioned on a panel surface PI of the liquid crystal panel 18G (18R or 18B) (that is, an image on a panel) in the first optical group 40a, is temporarily formed as an image in front of a mirror of the second optical group 40b, is again formed as an image on a screen by one mirror MR configuring the second optical group 40b, and thereby short throw projection is performed in the first optical group 40a. In other words, in this case, the first optical group 40a plays a role of forming a primary image (intermediate image) in front of the mirror MR. In the projection as described above, aberration fluctuation due to focusing in response to the magnification change is greater than a case of general projection, and thus it is common not to have a significantly large range of magnification change. Accordingly, when an angle of view is significantly wide and thereby projection magnification is changed, the aberration fluctuation is increased. Even in this case, the primary image formed by the first optical group 40a needs to be appropriately used. In addition, in the common short throw projection optical system, it is easy to increase contrast reduction due to field curvature and astigmatism fluctuation which directly influence on image performance, and distortion due to movement of a focus group is highly likely to be also increased more than in a normal lens system.
In comparison, in the present embodiment, as described above, the 1-2nd lens group 42 of the first optical group 40a has the aspherical resin lens in addition to the one disposed on the outermost enlargement side, for example. In other words, the 1-2nd lens group 42 includes at least two aspherical lens molded using the resin. Further, at least one glass lens is disposed between the plurality of resin lenses. Specifically, the lens L15 as the spherical glass lens is disposed between the lens L14 and the lens L16 as aspherical resin lenses. The angle of the beam incident to the resin lens L16 from the glass lens L15 is hereby controlled, the resin lenses L14 and L16 have weak power, and thus it is possible to prevent variations in the shapes of the resin lens L16. Further, a plurality of resin lenses are provided, and thereby it is possible to hereby perform effective correction in order to suppress aberration fluctuation to be decreased. Even in a case where the second optical group 40b is configured of one mirror MR, it is possible to hereby achieve a good image having small aberration, compared to an image of which the primary image has moderate aberration, and which is finally projected on the screen through the second optical group 40b. In other words, in the projector 2 as a short throw type projector, a wide range of magnification change is covered and it is possible to be also applied to an image display element having high resolution. In addition to the above description, when focusing is performed in response to the magnification change, the fixed lens group H as the fixed group is disposed on the enlargement side, the moving lens group, which moves when focusing is performed in response to the magnification change, is disposed on the reduction side from the fixed lens group H, and thereby it is possible to perform effective correction in order to suppress the aberration fluctuation to be reduced.
In the first optical group 40a, when one aspherical lens is included in the lenses configuring the 1-2nd lens group 42 as a focusing group disposed on the enlargement side, there is a concern that a sufficient range of magnification change will not be secured by a design. When two aspherical lenses are included in the 1-2nd lens group 42, it is possible to widen the range of the magnification change; however, in this case, when the spherical glass lens is not inserted therebetween as described above, the shapes of the respective aspherical lenses have a strong tendency to be changed into severe aspherical shapes by separating large surface shape from the spherical shape in order to obtain sufficiently wide range of magnification change, surface sensitivity or refractive index sensitivity is enhanced, further, eccentricity sensitivity between the surfaces is enhanced, and then it is possible for significant variation in the final lens performance to occur.
In addition, for the lens disposed on the outermost enlargement side, such as the aspherical lens (lens L16) of the 1-2nd lens group 42, in order to prevent interference with a beam reflected from the mirror of the second optical group 40b, not only there is a need to have an atypical shape such as a partially notched circular shape, but also, an aspherical resin-molded surface is normally used because the diameter is relatively great such that, in the present embodiment, the lens L16 is also an aspherical resin lens having an atypical shape. However, in general, the aspherical resin-molded surface also has lower accuracy than the aspherical glass-molded surface, and thus there is a need to sufficiently reduce sensitivity at the time of design because the system is likely to be influenced by variations due to surface accuracy or refractive index as described above.
In comparison, in the present embodiment, even when the 1-2nd lens group 42 as the focusing group includes two aspherical lenses (lens L14, L16), a negative glass lens (lens L15) is disposed therebetween. In this manner, the negative power of the aspherical lens is appropriately distributed, it is possible to reduce relative sensitivity between the aspherical surface, and it is possible to reduce the aberration fluctuation even in a wide range of the magnification change.
Further, in the 1-2nd lens group 42 as the focusing group, it is also possible to dispose the fixed lens group on the reduction side. In this case, for example, the number of moving lens groups of the focusing groups is decreased (in Example 2 to be described below, instead of three moving lens groups, configured of two moving lens groups), a frame structure or the like which fixes the lenses is simplified, and it is possible to obtain the entire apparatus at a low cost.
Further, in the 1-1st lens group 41, if a configuration on the reduction side from the aperture ST has only the spherical lenses, it is considered that it is possible to be applied only to brightness having the F-number of about 1.8, when applied to a wide range of the magnification change. In comparison, in the present embodiment, the aspherical glass surface (lens L6) is appropriately disposed on the reduction side from the aperture ST, and thereby it is possible to achieve an image having less flare and high contrast even at the F-number of about 1.6. Even when the number of (in the present embodiment, 16) lenses is substantially the same as an example in the related art, the numerical aperture on the object side is equal to or more than 0.3, that is, with brightness having the F-number of about 1.6, a range of magnification change of high magnification of 1.5 (further, equal to or more than 1.6) is secured, and performance of sufficient application to the image display element having high resolution is achieved.
EXAMPLE
Hereinafter, a specific example of the projection optical system 40 will be described. Meanings of specifications common in Examples 1 to 3 in the following description are defined as follows.
f Focal Length of Entire system
ω Half Angle of View
Na Numerical Aperture
R Curvature Radius
D Axial Top Surface Interval (Lens Thickness Or Lens Interval)
Nd Refractive Index of d Line
Vd Abbe Number of d Line
The aspherical surface is defined by the following polynomial equation (aspherical surface equation).




  z
  =
  
    
      
        ch
        2
      
      
        1
        +
        
          
            1
            -
            
              
                (
                
                  k
                  +
                  1
                
                )
              
              ⁢
              
                c
                2
              
              ⁢
              
                h
                2
              
            
          
        
      
    
    +
    
      
        A
        4
      
      ⁢
      
        h
        4
      
    
    +
    
      
        A
        6
      
      ⁢
      
        h
        6
      
    
    +
    
      
        A
        8
      
      ⁢
      
        h
        8
      
    
    +
    
      
        A
        10
      
      ⁢
      
        h
        10
      
    
    +
    
      
        A
        12
      
      ⁢
      
        h
        12
      
    
    +
    
      
        A
        14
      
      ⁢
      
        h
        14
      
    
  




Here,

c: Curvature (1/R)
h: Height From Optical Axis
k: Coefficient Of The Conic Of Aspherical Surface
Ai: Coefficient Of Aspherical Surface In High Order Equation
Example 1
Data of lens surfaces of Example 1 is shown in Table 1. Further, OBJ means the panel surface PI and STO means the aperture ST. In addition, a surface having “*” before a surface reference number means a surface having aspherical shape.







TABLE 1







f 4.150


ω 72.9


NA 0.313












R
D
Nd
Vd















OBJ
Infinity
9.400




1
Infinity
27.908 
1.51633
64.14


2
Infinity
0.000


3
40.014
7.200
1.49700
81.54


4
−255.649
0.200


5
36.347
7.800
1.49700
81.54


6
−152.931
0.200


7
29.838
9.200
1.48749
70.24


8
−37.003
1.200
1.90366
31.31


9
26.317
0.100


10
16.951
7.500
1.48749
70.24


11
−838.804
0.100


*12
113.564
1.600
1.74320
49.29


*13
30.000
0.100


14
18.311
8.000
1.53172
48.84


15
−16.680
1.200
1.79952
42.22


16
34.631
4.000


STO
Infinity
3.280


18
172.340
4.200
1.76182
26.52


19
−28.820
5.137


20
65.966
1.400
1.79952
42.22


21
38.454
variable




interval


22
65.125
8.500
1.58913
61.13


23
−573.281
variable




interval


24
94.051
9.000
1.69680
55.53


25
−132.718
variable




interval


26
−78.738
2.500
1.80518
25.42


27
−416.408
6.327


*28
−96.291
3.500
1.53116
56.04


*29
187.907
variable




interval


30
−81.006
2.800
1.80518
25.42


31
−345.990
2.500


*32
−78.832
3.500
1.53116
56.04


*33
110.871
120.000 


*34
−63.642
variable
reflective




interval
surface


35
Infinity









In Table 1 and the following Tables, an exponent of 10 (for example, 1.00×10+18) is described using E (for example, 1.00E+18).
Table 2 shows an aspherical surface coefficient of a lens surface of Example 1.







TABLE 2







Aspherical Surface Coefficient














A04
A06





K
A12
A14
A08
A10
















12
41.7552
−2.7588E−05
4.4936E−07
−2.7734E−09
1.1565E−11




−2.0673E−14
0.0000E+00


13
4.8011
−1.1587E−05
4.5419E−07
−3.0749E−09
1.2463E−11




−2.7707E−14
0.0000E+00


28
0.0000
−1.7926E−05
2.4112E−08
−8.4914E−12
−2.1642E−15




0.0000E+00
0.0000E+00


29
0.0000
−2.1140E−05
2.3908E−08
−9.6403E−12
0.0000E+00




0.0000E+00
0.0000E+00


32
3.2860
2.8102E−06
4.2059E−09
4.2001E−12
−1.3317E−14




7.4306E−18
0.0000E+00


33
−1.0000
−1.0542E−05
1.5971E−08
−1.7320E−11
1.4535E−14




−1.0344E−17
3.7595E−21


34
−1.0000
2.4351E−08
−2.8251E−11
2.8195E−15
−4.9422E−19




3.9175E−23
−2.1047E−27









Table 3 shows values of variable intervals 21, 23, 25, 29, and 34 in Table 2 at the projection magnification of 135 times, the projection magnification of 106 times, and the projection magnification of 179 times.







TABLE 3







Variable Interval











135x
106x
179x
















21
23.996
24.133
23.834



23
2.753
1.500
3.999



25
6.845
6.831
6.866



29
12.105
13.234
11.000



34
−600.000
−477.350
−783.927










FIG. 4 is a sectional diagram showing the projection optical system of Example 1. The projection optical system in FIG. 4 corresponds to the projection optical system 40 of Embodiment 1. Further, the lens L16 or the mirror MR having a partially notched shape from a circle in FIG. 3 or the like is depicted intact without a notch in FIG. 4. In FIG. 4, the projection optical system performs enlargement projection of an image on the panel surface PI to the screen at a magnification depending on a distance. In other words, the projection optical system has 16 lenses L1 to L16 of the lenses L1 to L8 configuring the lens group E1 of the 1-1st lens group 41, the lenses L9 and L10 configuring the lens group E2 thereof, the lens L11 configuring lens group F1 of the 1-2nd lens group 42, the lens L12 configuring the lens group F2 thereof, the lenses L13 and L14 configuring the lens group F3 thereof, and the lenses L15 and L16 configuring the fixed lens group H, in this order from the reduction side. For example, as in a case where projection onto a wall surface is changed to projection onto a floor surface, the magnification change occurs due to the change of a projection position (change of projection distance), and the lens group H configuring the 1-1st lens group 41 and the 1-2nd lens group 42 are fixed as is when focusing is performed during the magnification change, whereas the lens groups F1 to F3 configuring the 1-2nd lens group 42 individually move. The interval between the lenses L14 and L16 as the two aspherical resin lens of the 1-2nd lens group 42 is hereby changed when focusing is performed in response to the magnification change.
Further, the respective lenses L1 to L16 will be described in detail. In the 1-1st lens group 41, the lens L1 as a first lens is a positive lens, the lens L2 as a second lens is a positive lens, the lens L3 as a third lens is a positive lens, the lens L4 as a fourth lens is a negative lens, the lens L3 and the lens L4 form a cemented lens, the lens L5 as a fifth lens is a positive lens, the lens L6 as a sixth lens is a negative meniscus lens having both surfaces subjected to the aspherical surface process with a concave surface facing the enlargement side, the lens L7 as a seventh lens is a positive biconvex lens, the lens L8 as an eighth lens is a negative biconcave lens, the lens L7 and the lens L8 form a cemented lens, the lens L9 as a ninth lens is a positive biconvex lens, and the lens L10 as the tenth lens is a negative meniscus lens with the concave surface facing the enlargement side. In addition, in the 1-2nd lens group 42, the lens L11 as an eleventh lens is a positive lens, the lens L12 as a twelfth lens is a positive lens, the lens L13 as a thirteenth lens is a negative lens, the lens L14 as a fourteenth lens is a negative lens having both surfaces subjected to the aspherical surface process, the lens L15 as a fifteenth lens is a negative lens, and the lens L16 as a sixteenth lens is a negative lens having both surfaces subjected to the aspherical surface process. In addition, the second optical group 40b is configured of one mirror having a concave aspherical surface.
FIG. 5A is a diagram showing aberration (spherical aberration, astigmatism, and distortion) on the reduction side of the projection optical system when the projection magnification of 135 times is performed. FIG. 5B is a diagram showing aberration on the reduction side of the projection optical system when the projection magnification of 106 times is performed. FIG. 5C is a diagram showing aberration on the reduction side of the projection optical system when the projection magnification of 179 times is performed. In addition, FIGS. 6A to 6E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 5A. FIG. 6A is a diagram showing the lateral aberration in a case of the maximum angle of view and FIGS. 6A to 6E are diagrams showing lateral aberration at five angles of view. Similarly, FIGS. 7A to 7E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 5B. FIGS. 8A to 8E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 5C.
Example 2
Data of lens surfaces of Example 2 is shown in Table 4. Further, OBJ means the panel surface PI and STO means the aperture ST. In addition, a surface having “*” before a surface reference number means a surface having aspherical shape.







TABLE 4







f 4.171


ω 72.9


NA 0.313












R
D
Nd
Vd















OBJ
Infinity
9.400




1
Infinity
27.908 
1.51633
64.14


2
Infinity
0.000


3
35.294
9.800
1.49700
81.54


4
−85.567
0.200


5
33.124
7.200
1.49700
81.54


6
−301.905
0.200


7
64.998
8.000
1.51633
64.14


8
−27.374
1.200
1.90366
31.31


9
61.009
0.200


10
21.894
6.700
1.51633
64.14


11
−30.792
1.600
1.83220
40.10


*12
48.420
1.000


13
26.724
6.400
1.54814
45.78


14
−26.724
0.615


15
−59.344
1.200
1.78590
44.20


16
29.247
5.419


STO
Infinity
6.100


18
158.188
4.600
1.76182
26.52


19
−35.477
4.300


20
67.005
1.400
1.79952
42.22


21
37.995
24.589 


22
48.661
9.000
1.60311
60.64


23
473.356
variable




interval


24
−600.814
7.000
1.62299
58.16


25
−68.918
2.500
1.80518
25.42


26
−119.549
variable




interval


*27
−245.455
3.500
1.53116
56.04


*28
117.002
12.147 


29
−49.204
2.800
1.80518
25.42


30
−155.067
variable




interval


*31
−74.300
3.500
1.53116
56.04


*32
148.695
114.289 


*33
−62.079
variable
reflective




interval
surface


34
Infinity









Table 5 shows an aspherical surface coefficient of a lens surface of Example 2.







TABLE 5







Aspherical Surface Coefficient














A04
A06





K
A12
A14
A08
A10
















12
10.7205
2.2599E−05
−3.6917E−08
5.2139E−11
−1.5830E−12




0.0000E+00
0.0000E+00


27
0.0000
−3.0294E−05
2.4795E−08
3.5116E−12
−5.4910E−15




0.0000E+00
0.0000E+00


28
0.0000
−3.4688E−05
3.3814E−08
−1.4840E−11
5.8670E−15




0.0000E+00
0.0000E+00


31
3.2860
9.4048E−06
1.0286E−08
−2.0983E−11
1.5102E−14




−3.9051E−18
0.0000E+00


32
−1.0000
−4.2315E−06
1.2030E−08
−7.5394E−12
−5.8590E−15




9.2843E−18
−3.4672E−21


33
−1.0000
−3.5735E−08
8.1044E−12
−1.1988E−14
2.6318E−18




−3.3105E−22
1.6230E−26









Table 6 shows values of variable intervals 23, 26, 30, and 33 in Table 5 at the projection magnification of 134 times, the projection magnification of 105 times, and the projection magnification of 178 times.







TABLE 6







variable interval











134x
105x
178x
















23
6.678
4.000
9.332



26
8.961
10.793
7.163



30
3.403
4.248
2.546



33
−600.000
−478.094
−782.728










FIG. 9 is a sectional diagram showing the projection optical system of Example 2. Further, the lens L16 or the mirror MR having a partially notched shape from a circle in an actual optical system is depicted intact without a notch in FIG. 9. In FIG. 9, the projection optical system performs enlargement projection of an image on the panel surface PI to the screen at a magnification depending on a distance. In other words, the projection optical system has 16 lenses L1 to L16 of the lenses L1 to L8 configuring the lens group E1 of the 1-1st lens group 41, the lenses L9 and L10 configuring the lens group E2 thereof, the lens L11 configuring a first fixed lens group H1 of the 1-2nd lens group 42, the lenses L12 and L13 configuring the lens group F1 thereof, the lenses L14 and L15 configuring the lens group F2 thereof, and the lens L16 configuring a second fixed lens group H2, in this order from the reduction side. For example, as in a case where projection onto a wall surface is changed to projection onto a floor surface, the magnification change occurs due to the change of a projection position (change of projection distance), and the first and second fixed lens groups H1 and H2 configuring the 1-1st lens group 41 and the 1-2nd lens group 42 are fixed as is when focusing is performed during the magnification change, whereas the lens groups F1 and F2 configuring the 1-2nd lens group 42 individually move. The interval between the lenses L14 and L16 as the two aspherical resin lens of the 1-2nd lens group 42 is hereby changed when focusing is performed in response to the magnification change.
As above, in Example 2, the first optical group 40a is configured to have 16 lenses from the lens L1 (first lens) to the lens L16 (sixteenth lens) numbered from the reduction side, and the first optical group 40a can be divided into the 1-1st lens group 41 having positive power, on the reduction side, and the 1-2nd lens group 42 having weaker positive or negative power, compared to the power of the 1-1st lens group 41, on the enlargement side, with the widest air interval BD as a boundary.
More specifically, the 1-1st lens group 41 is configured to include the lens group E1 having the positive lens L1, the positive lens L2, the cemented lens of the positive lens L3 and the negative lens L4, the cemented lens of the positive lens L5 and the negative lens L6 subjected to the aspherical surface process with the concave surface facing the enlargement side, the positive lens L7, and the negative lens L8, the aperture ST, and the lens group E2 having the positive biconvex lens L9, and the negative meniscus lens L10 with the concave surface facing the enlargement side, in this order from the reduction side. In other words, a total of ten lenses in lens groups E1 and E2 are sequentially arranged.
The 1-2nd lens group 42 is configured to include the first fixed lens group H1 having the positive lens L11, the lens group F1 (F1 lens group) having the cemented lens of the positive lens L12 and the negative lens L13, the lens group F2 (F2 lens group) having the negative lens L14 having both surfaces subjected to the aspherical surface process and the negative lens L15, and the second fixed lens group H2 having the negative lens L16 having both surfaces subjected to a aspherical surface process, in this order from the reduction side. In other words, a total of six lenses in the fixed lens groups H1 and H2 and the lens group F1 and F2 are sequentially disposed. The lens L14 and the lens L16 are lenses molded using a resin and the lens L15 disposed between the lens L14 and the lens L16 is the glass lens. In addition, the 1-2nd lens group 42 performs focusing by causing the two lens groups F1 and F2 to individually move, when the projection distance is changed during the magnification change.
The second optical group 40b is configured of one mirror having a concave aspherical surface.
FIG. 10A is a diagram showing aberration (spherical aberration, astigmatism, and distortion) on the reduction side of the projection optical system when the projection magnification of 134 times is performed. FIG. 10B is a diagram showing aberration on the reduction side of the projection optical system when the projection magnification of 105 times is performed. FIG. 10C is a diagram showing aberration on the reduction side of the projection optical system when the projection magnification of 178 times is performed. In addition, FIGS. 11A to 11E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 10A. FIG. 11A is a diagram showing the lateral aberration in a case of the maximum angle of view and FIGS. 11A to 11E are diagrams showing lateral 1 aberration at five angles of view. Similarly, FIGS. 12A to 12E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 10B. FIGS. 13A to 13E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 10C.
Example 3
Data of lens surfaces of Example 3 is shown in Table 7. Further, OBJ means the panel surface PI and STO means the aperture ST. In addition, a surface having “*” before a surface reference number means a surface having aspherical shape.







TABLE 7







f 4.170


ω 73.0


NA 0.313












R
D
Nd
Vd















OBJ
Infinity
9.400




1
Infinity
27.908 
1.51633
64.14


2
Infinity
0.000


3
32.799
9.800
1.49700
81.54


4
−125.875
0.200


5
33.199
7.200
1.49700
81.54


6
−279.426
0.200


7
64.435
8.000
1.51633
64.14


8
−28.940
1.200
1.90366
31.31


9
73.268
0.200


10
21.610
6.700
1.51633
64.14


11
−26.005
1.600
1.83220
40.10


*12
48.530
1.000


13
26.267
6.400
1.54814
45.78


14
−26.267
0.100


15
−54.604
1.200
1.78590
44.20


16
29.080
6.251


STO
Infinity
6.100


18
182.921
4.600
1.76182
26.52


19
−36.035
2.435


20
71.170
1.400
1.79952
42.22


21
40.859
26.025 


22
48.777
9.000
1.58913
61.13


23
628.170
variable




interval


24
161.530
7.000
1.74400
44.79


25
−137.069
2.500


26
−123.922
2.500
1.80518
25.42


27
1354.514
variable




interval


*28
−130.621
3.500
1.53116
56.04


*29
198.616
variable




interval


30
−63.407
2.800
1.80518
25.42


31
1650.129
12.324 


*32
−73.950
3.500
1.53116
56.04


*33
193.169
variable




interval


*34
−61.972
variable
reflective




interval
surface


35
Infinity









Table 8 shows an aspherical surface coefficient of a lens surface of Example 3.







TABLE 8







Aspherical Surface Coefficient














A04
A06





K
A12
A14
A08
A10
















12
10.8601
2.3660E−05
−4.2016E−08
4.0629E−11
−1.5830E−12




0.0000E+00
0.0000E+00


28
0.0000
−2.8133E−05
2.8535E−08
−1.4118E−12
−5.4910E−15




0.0000E+00
0.0000E+00


29
0.0000
−3.3294E−05
3.8307E−08
−1.9340E−11
5.8670E−15




0.0000E+00
0.0000E+00


32
3.2860
9.2013E−06
1.2095E−08
−2.1197E−11
1.4603E−14




−4.0310E−18
0.0000E+00


33
−1.0000
−1.7505E−06
8.4160E−09
−3.3971E−12
−5.7594E−15




6.8674E−18
−2.7020E−21


34
−1.0000
−2.4860E−08
1.1410E−11
−1.3441E−14
2.9180E−18




−3.4938E−22
1.6230E−26









Table 9 shows values of variable intervals 23, 27, 29, 33, and 34 in Table 8 at the projection magnification of 135 times, the projection magnification of 105 times, and the projection magnification of 222 times.







TABLE 9







Variable Interval











135x
105x
222x
















23
2.148
0.541
4.637



27
10.547
11.204
9.495



29
4.445
5.114
3.500



33
113.625
113.905
113.133



34
−600.000
−476.336
−969.054










FIG. 14 is a sectional diagram showing the projection optical system of Example 3. Further, the lens L16 or the mirror MR having a partially notched shape from a circle in an actual optical system is depicted intact without a notch in FIG. 14. In FIG. 14, the projection optical system performs enlargement projection of an image on the panel surface PI to the screen at a magnification depending on a distance. In other words, the projection optical system has 16 lenses L1 to L16 of the lenses L1 to L8 configuring the lens group E1 of the 1-1st lens group 41, the lenses L9 and L10 configuring the lens group E2 thereof, the lens L11 configuring the fixed lens group H of the 1-2nd lens group 42, the lenses L12 and L13 configuring the lens group F1 thereof, the lens L14 configuring the lens group F2 thereof, and the lenses L15 and L16 configuring the lens group F3, in this order from the reduction side. Further in the present embodiment, in the 1-2nd lens group 42, the fixed lens group does not exist on the enlargement side. For example, as in a case where projection onto a wall surface is changed to projection onto a floor surface, the magnification change occurs due to the change of a projection position (change of projection distance), and the fixed lens group H configuring the 1-1st lens group 41 and the 1-2nd lens group 42 are fixed as is when focusing is performed during the magnification change, whereas the lens groups F1 to F3 configuring the 1-2nd lens group 42 moves, respectively. The movement causes the interval between the lenses L14 and L16 as the two aspherical resin lens of the 1-2nd lens group 42 to be changed when focusing is performed in response to the magnification change.
As above, in Example 3, the first optical group 40a is configured to have 16 lenses from the lens L1 (first lens) to the lens L16 (sixteenth lens) numbered from the reduction side, and the first optical group 40a can be divided into the 1-1st lens group 41 having positive power, on the reduction side, and the 1-2nd lens group 42 having weaker positive or negative power, compared to the power of the 1-1st lens group 41, on the enlargement side, with the widest air interval BD as a boundary.
More specifically, the 1-1st lens group 41 is configured to include the lens group E1 having the positive lens L1, the positive lens L2, the cemented lens of the positive lens L3 and the negative lens L4, the cemented lens of the positive lens L5 and the negative lens L6 subjected to the aspherical surface process with the concave surface facing the enlargement side, the positive biconvex lens L7, and the negative biconcave lens L8, the aperture ST, and the lens group E2 having the positive biconvex lens L9, and the negative meniscus lens L10 with the concave surface facing the enlargement side, in this order from the reduction side. In other words, a total of ten lenses in lens groups E1 and E2 are sequentially arranged.
The 1-2nd lens group 42 is configured to include the fixed lens group H having the positive lens L11, the lens group F1 (F1 lens group) having the positive lens L12 and the negative lens L13, the lens group F2 (F2 lens group) having the negative lens L14 having both surfaces subjected to the aspherical surface process, and the lens group F3 (F3 lens group) having the negative lens L15 and the negative lens L16 having both surfaces subjected to an aspherical surface process, in this order from the reduction side. In other words, a total of six lenses in the fixed lens groups H and the lens groups F1 to F3 are sequentially disposed. The lens L14 and the lens L16 are lenses molded using a resin and the lens L15 disposed between the lens L14 and the lens L16 is the glass lens. In addition, the 1-2nd lens group 42 performs focusing by causing the three lens groups F1, F2, and F3 to move individually for the focus when focusing is performed in response to the magnification change.
The second optical group 40b is configured of one mirror having a concave aspherical surface.
FIG. 15A is a diagram showing aberration (spherical aberration, astigmatism, and distortion) on the reduction side of the projection optical system when the projection magnification of 135 times is performed. FIG. 15B is a diagram showing aberration on the reduction side of the projection optical system when the projection magnification of 105 times is performed. FIG. 15C is a diagram showing aberration on the reduction side of the projection optical system when the projection magnification of 222 times is performed. In addition, FIGS. 16A to 16E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 15A. FIG. 16A is a diagram showing the lateral aberration in a case of the maximum angle of view and FIGS. 16A to 16E are diagrams showing horizontal aberration at five angles of view. Similarly, FIGS. 17A to 17E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 15B. FIGS. 18A to 18E are diagrams showing lateral aberration of the projection optical system, which correspond to FIG. 15C.
Summary of Examples
In any one of Examples, a simple configuration, in which the lens on the outermost enlargement side is one aspherical resin lens, is employed while a wide angle of view is equal to or greater than a half angle of view of 70° at a wide angle end.
The invention is not limited to the embodiments or examples described above and can be performed in various aspects within a range without departing from the gist thereof.
In addition, in the respective Examples, one or more lenses having substantially no power may be added before and after or between the lenses configuring each lens group.
In addition, a target of enlargement projection by the projection optical system 40 is not limited to the liquid crystal panels 18G, 18R, and 18B, but it is possible for the projection optical system 40 to perform enlargement projection of an image formed by various light modulating elements such as a digital micro mirror device, in which a micro mirror functions as a pixel.
The entire disclosure of Japanese Patent Application No. 2015-029306, filed Feb. 18, 2015 is expressly incorporated by reference herein.

## Claims

1. A projection optical system comprising:
in order from a reduction side,
a first optical group which is formed of a plurality of lenses and has positive power; and
a second optical group which has one reflective surface having a concave aspherical shape,
wherein the first optical group is formed to include a 1-1st lens group having positive power, on the reduction side, and a 1-2nd lens group having weaker positive or negative power, compared to the power of the 1-1st lens group, on the enlargement side, with the widest air interval as a boundary, and
wherein the 1-2nd lens group includes at least two aspherical resin lenses and at least one glass lens disposed between the two aspherical resin lenses.

2. The projection optical system according to claim 1,
wherein an interval between the two aspherical resin lenses of the 1-2nd lens group is changed when focusing is performed in response to the magnification change.

3. The projection optical system according to claim 1,
wherein the 1-1st lens group is configured to have an aperture therein and two lenses of a positive lens with a convex surface facing the enlargement side and a negative lens with the concave surface facing the enlargement side, in this order from the reduction side, on the enlargement side from the aperture.

4. The projection optical system according to claim 1,
wherein the 1-1st lens group has an aperture therein, includes at least two sets of cemented lenses of positive lenses and negative lenses disposed on the reduction side from the aperture, and has at least one concave aspherical surface facing the enlargement side.

5. The projection optical system according to claim 1,
wherein the numerical aperture on the object side is equal to or more than 0.3.

6. The projection optical system according to claim 1,
wherein the reduction side is substantially telecentric.

7. The projection optical system according to claim 1,
wherein elements configuring the first optical group and the second optical group all have a rotationally symmetric system.

8. The projection optical system according to claim 1,
wherein a range of magnification change is equal to or greater than 1.5 times.