Projection optical system and projector

FIG. 2 is a light ray diagram of a projection optical system according to Example 1 including an optical path deflector. A projection optical system 3 A according to the present example is formed of the following components sequentially arranged from the enlargement side: a first lens group LG 1 having negative power; a second lens group LG 2 having positive power; an aperture stop O; and a third lens group LG 3 having positive power. The projection optical system 3 A includes 11 lenses in total.

The projection optical system 3 A includes an optical path deflector that is disposed between the first lens group LG 1 and the second lens group LG 2 and deflects the optical path, as shown in FIG. 2 . In the present example, the optical path deflector is a mirror R 1 . In the projection optical system 3 A, the mirror R 1 bends an optical axis L by 90° in a position between the first lens group LG 1 and the second lens group LG 2 . When the projection distance of the projection optical system 3 A is changed, the space between the lens group LG 1 and the lens group LG 2 is changed for focusing.

The light modulator 18 is located in the reduction-side image plane of the projection optical system 3 A. The screen S is located in the enlargement-side image plane of the projection optical system 3 A. The projection optical system 3 A includes the cross dichroic prism 19 between the light modulator 18 and a reduction-side first lens located in a position closest to the reduction side. The reduction-side first lens is the twelfth lens L 12 .

The first lens group LG 1 is formed of three negative lenses and one positive lens. The three negative lenses are each a single lens. Specifically, the first lens group LG 1 is formed of a first lens L 1 having a convex surface facing the enlargement side and having negative power, a second lens L 2 having a convex surface facing the enlargement side and having negative power, a third lens L 3 having a convex surface facing the enlargement side and having negative power, and a fourth lens L 4 having a convex surface facing the enlargement side in the vicinity of the optical axis and having positive power. The first lens L 1 , the second lens L 2 , the third lens L 3 , and the fourth lens L 4 each have a meniscus shape. The first lens L 1 has aspheric surfaces on opposite sides. The fourth lens L 4 has aspheric surfaces on opposite sides.

The second lens group LG 2 is formed of a fifth lens L 5 , a sixth lens L 6 , a seventh lens L 7 , and an eighth lens L 8 . The second lens group LG 2 includes at least one positive lens. In the present example, the sixth lens L 6 and the eighth lens L 8 are each a positive lens. The fifth lens L 5 , the sixth lens L 6 , and the seventh lens L 7 are bonded to each other to form a first triplet jointed lens SL 1 . The eighth lens L 8 is a single lens. The fifth lens L 5 has a meniscus shape having a convex surface facing the enlargement side and has negative power. The sixth lens L 6 has a convex shape both on the enlargement and reduction sides. The seventh lens L 7 has a meniscus shape having a convex surface facing the reduction side and has negative power. The eighth lens L 8 has a convex shape both on the enlargement and reduction sides.

The third lens group LG 3 includes a plurality of positive lenses. The third lens group LG 3 includes a jointed lens including at least one of the plurality of positive lenses. Further, the twelfth lens L 12 , which is located in a position closest to the reduction side in the third lens group LG 3 , is a positive lens having a convex surface facing the reduction side. In other words, the reduction-side first lens is a positive lens having a convex surface facing the reduction side. That is, the third lens group LG 3 is formed of four lenses, the ninth lens L 9 , the tenth lens L 10 , the eleventh lens L 11 , and the twelfth lens L 12 . The ninth lens L 9 and the eleventh lens L 11 are each a positive lens. The eleventh lens L 11 along with the tenth lens L 10 forms a second jointed lens SL 2 .

More specifically, the ninth lens L 9 has a convex shape both on the enlargement and reduction sides. The ninth lens L 9 has aspheric surfaces on opposite sides. The tenth lens L 10 has a meniscus shape having a convex surface facing the enlargement side and has negative power. The eleventh lens L 11 has a meniscus shape having a convex surface facing the enlargement side. The twelfth lens L 12 has a convex shape both on the enlargement and reduction sides.

Data on the projection optical system 3 A are as follows:

f: 5.18;

FNo: 2.00; and

ω: 67.70,

where f denotes the focal length of the entire optical system, FNo denotes the f number thereof, and ω denotes the half angle of view thereof.

Data on the lenses of the projection optical system 3 A are as follows: The number in the column labeled with “Lens” denotes the reference character with which the lenses of the projection optical system 3 A are each labeled. STO denotes the aperture stop O. The number 19 represents the cross dichroic prism. A surface having a surface number with * is an aspheric surface. R denotes the radius of curvature. D denotes the on-axis inter-surface space (mm) and represents the lens thickness or the inter-lens space. nd denotes the refractive index at the d line. νd denotes the Abbe number at the d line. The variable space 1 represents the distance between the screen Sand the first lens L 1 . The variable space 2 represents the distance between the first lens group LG 1 and the second lens group LG 2 . The variable space 1 changes in accordance with the projection distance, and the variable space 2 changes in accordance with the focusing performed when the projection distance is changed.

The variable spaces 1 and 2 are set as follows in the focusing performed when the projection distance is changed. The projection distance when the variable space 1 , which is the distance between the first lens L 1 and the screen S, is 500 mm is defined as a reference distance. The projection distance when the variable space 1 is 1000 mm is defined as a far-projection distance. The projection distance when the variable space 1 is 300 mm is defined as a near-projection distance.

Data on the aspheric surfaces of the first lens L 1 labeled with the surface numbers 1 and 2, data on the aspheric surfaces of the fourth lens L 4 labeled with the surface numbers 7 and 8, and data on the aspheric surfaces of the ninth lens L 9 labeled with the surface numbers 16 and 17 are as follows. The amount of displacement z of an aspheric surface at a height h from the optical axis is expressed by Expression 1 below.

z = c · h 2 1 + 1 - ( k + 1 ) ⁢ c 2 · h 2 + A ⁢ ⁢ 04 · h 4 + A ⁢ ⁢ 06 · h 6 + A ⁢ ⁢ 08 · h 8 + A ⁢ ⁢ 10 · h 10 + A ⁢ ⁢ 12 · h 12

where c denotes the curvature, k denotes a conic coefficient, and A04 to A12 denote higher-order aspheric coefficients.

Surface number 1 2 7 8 16 17 k 0.0000 0.0000 0.0000 0.0000 −0.5950 7.8691 A04 1.1753E−05 −6.8775E−06  −4.8632E−05  −4.5036E−05 4.2946E−06 2.8853E−05 A06 −3.3422E−09  4.4761E−08 3.2042E−07  2.6232E−07 1.9810E−07 2.0773E−07 A08 6.1662E−13 −5.9937E−11  −1.2811E−09  −1.5029E−09 −7.6998E−10  −8.1695E−10  A10 0.0000E+00 0.0000E+00 0.0000E+00  1.3924E−12 0.0000E+00 0.0000E+00 A12 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00

Projection Image and Enlarged Image

The projection image formed by the light modulator 18 and the enlarged image generated when the projection optical system 3 A projects the projection image will next be described. FIG. 3 describes the projection image. FIG. 3 shows a case where one long edge of the light modulator 18 so sized that the width and height thereof are 12.8 mm and 8 mm is located on an optical axis L of the projection optical system 3 A. The thick lines in FIG. 3 conceptually represent the projection image. The thin line in FIG. 3 represents the image display area of the light modulator 18 . FIG. 4 describes the enlarged image generated when the projection image is projected on the screen S via the projection optical system 3 A. The thick lines in FIG. 4 conceptually represent the enlarged image generated when the projection image in FIG. 3 is projected on the screen S. The thin line in FIG. 4 conceptually represents the image display area of the light modulator 18 projected on the screen S. The chain line in FIG. 4 conceptually represents the image display area of the light modulator 18 formed when a projection optical system that produces no distortion aberration projects the image display area on the screen S.

The light modulator 18 forms a projection image having distortion that corrects the distortion aberration produced by the projection optical system 3 A, as shown in FIG. 3 . When the projection optical system 3 A projects the projection image on the screen S, an oblong enlarged image having no distortion is formed, as shown in FIG. 4 .

The projection image has a shape that allows reduction in trapezoidal distortion of the enlarged image with respect to the ideal oblong image formed on the screen S. That is, the projection image has distortion opposite the trapezoidal distortion of the enlarged image. The projection image is therefore so formed that the shortest edge thereof is the edge having the largest image height on the screen S. That is, the projection image is compressed at the maximum image height in a lower portion of the image display area of the light modulator 18 . The projection image has no field curvature. When the projection optical system that produces no distortion is used to project the projection image on the screen S, the image is undesirably smaller than an image formed when the projection optical system 3 A according to the present example is used to project the projection image, as indicated by the chain line in FIG. 4 . The projection optical system 3 A according to the present example can therefore generate a larger enlarged image on the screen S than the enlarged image formed by the projection optical system that has the same focal length but produces no distortion aberration.

The projection optical system 3 A according to the present example satisfies Conditional Expressions (1) and (2) below:

0.4< d/f< 12.0  (1)

−45%< Dst<− 10%  (2)

where f denotes the focal length of the entire optical system, d denotes the space between the first lens group LG 1 and the second lens group LG 2 , and Dst denotes the distortion aberration at the reduction-side maximum image height.

The space d between the first lens group LG 1 and the second lens group LG 2 is the air space between the fourth lens L 4 and the fifth lens L 5 . Further, the space d between the first lens group LG 1 and the second lens group LG 2 is the value when the projection distance of the projection optical system 3 A is the reference distance. In the present example, d=31.25 and f=5.18 are satisfied. d/f=6.03 is therefore satisfied, so that Conditional Expression (1) is satisfied. In the present example, since the air space between the first lens group LG 1 and the second lens group LG 2 satisfies Conditional Expression (1), the optical path deflector, such as the mirror R 1 , is readily disposed in the projection optical system 3 A to deflect the optical path, as shown in FIG. 2 . Further, in the present example, since the Conditional Expression (1) is satisfied, an increase in the size of the projection optical system 3 A can be suppressed even when the mirror R 1 is disposed in the projection optical system 3 A.

That is, when d/f in Conditional Expression (1) is smaller than the lower limit, it is difficult to ensure a space large enough to accommodate the optical path deflector. On the other hand, when d/f in Conditional Expression (1) is greater than the upper limit, a redundant space is formed in the projection optical system 3 A, and the entire optical system tends to be longer. Further, when the air space between the first lens group LG 1 and the second lens group LG 2 increases in such away that d/f in Conditional Expression (1) is greater than the upper limit, it is necessary to increase the diameter of the first lens group LG 1 , resulting in an increase in the size of the first lens group LG 1 .

Further, in the present example, Dst=−30.0% is satisfied, so that Conditional Expression (2) is satisfied. In the present example, since the distortion aberration at the reduction-side maximum image height satisfies Conditional Expression (2), deterioration of the quality of the enlarged image can be avoided or suppressed. Further, the diameter of the first lens L 1 can be reduced, and the number of lenses that form the projection optical system 3 A can be reduced.

That is, when Dst in Conditional Expression (2) is smaller than the lower limit so that the amount of negative distortion aberration on the reduction side increases, the amount of distortion of the projection image is too large. The quality of the enlarged image therefore deteriorates. On the other hand, when Dst in Conditional Expression (2) is greater than the upper limit so that the amount of negative distortion aberration on the reduction side decreases, it is necessary to increase the diameter of the first lens L 1 and increase the number of lenses that form the projection optical system 3 A. As a result, the size of the projection optical system 3 A increases. Further, the manufacturing cost of the projection optical system 3 A increases.

In the present example, the projection image is compressed at the maximum image height in a lower portion of the image display area of the light modulator 18 . The amount of information on the image in the lower portion of the image display area of the light modulator 18 is therefore smaller than the amount of information on the original image. As a result, the quality of the enlarged image deteriorates on the screen S. To solve the problem described above, when the distortion aberration at the reduction-side maximum image height satisfies Conditional Expression (2), an excessive decrease in the amount of information on the projection image from the amount of information on the original image can be avoided. The deterioration of the enlarged image on the screen S can therefore be suppressed. When the projector 1 projects, for example, a movie, and captions or any other information is displayed in a portion close to the optical axis on the screen S, that is, in a lower portion of the enlarged image, a decrease in the resolution is small. The captions are therefore legible.

In the present example, the first lens group LG 1 includes three negative lenses. The negative lenses are each a single lens. The projection optical system 3 A can therefore have a sufficient back focal length and a flat enlargement-side image plane while producing a distortion aberration that falls within the range expressed by Conditional Expression (2).

The projection optical system 3 A according to the present example next satisfies Conditional Expressions (3) and (4) below:

1.5<| f 1/ f|< 2.5  (3)

0.2<| f 1/ f 23|<0.6  (4)

where f denotes the focal length of the entire optical system, f1 denotes the focal length of the first lens group LG 1 , and f23 denotes the combined focal length of the combination of the second lens group LG 2 and the third lens group LG 3 .

In the present example, f1=−10.191 is satisfied. |f1/f|=1.97 is therefore satisfied, so that Conditional Expression (3) is satisfied. Further, in the present example, f23=33.349 is satisfied. |f1/f23|=0.31 is therefore satisfied, so that Conditional Expression (4) is satisfied.

In the present example, the combined focal length of the combination of the second lens group LG 2 and the third lens group LG 3 satisfies Conditional Expression (4), and the focal length of the entire optical system and the focal length of the first lens group LG 1 satisfy Conditional Expression (3). The projection optical system 3 A can therefore correct comma and field curvature with the distortion aberration maintained at a desired value. Further, the projection optical system 3 A can ensure a sufficient back focal length while having a wide angle of view. The cross dichroic prism 19 is therefore readily disposed.

That is, when |f1/f| in Conditional Expression (3) is smaller than the lower limit, the focal length of the first lens group LG 1 is shorter than the focal length of the entire optical system. As a result, the power of each of the negative lenses of the first lens group LG 1 increases, and it is therefore difficult to correct comma and field curvature in a well-balanced manner with the distortion aberration maintained at a desired value. On the other hand, when |f1/f| in Conditional Expression (3) is greater than the upper limit, the focal length of the first lens group LG 1 is longer than the focal length of the entire optical system. As a result, the negative power of the first lens group LG 1 is too small, and it is therefore difficult to increase the angle of view of the projection optical system 3 A and ensure a sufficient back focal length.

The projection optical system 3 A further satisfies Conditional Expression (5) below:

4.0< f 2/ f< 12.0  (5)

where f2 denotes the focal length of the second lens group LG 2 .

In the present example, f2=45.983 is satisfied. f2/f=8.88 is therefore satisfied, so that Conditional Expression (5) is satisfied. In the present example, Conditional Expression (5) is satisfied, whereby comma and astigmatism can be corrected in a well-balanced manner. That is, when f2/f in Conditional Expression (5) is smaller than the lower limit, the positive power of the second lens group LG 2 is too large, and it is therefore difficult to correct comma and astigmatism in a well-balanced manner. When f2/f in Conditional Expression (5) is greater than the upper limit, the positive power of the second lens group LG 2 is too small. As a result, the burden on the positive power of the third lens group LG 3 undesirably increases, so that a variety of aberrations are likely to occur.

In the present example, the twelfth lens L 12 , which is located in a position closest to the reduction side in the third lens group LG 3 , is a positive lens having a convex surface facing the reduction side. In the present example, Conditional Expression (6) below is satisfied:

3.0< f 3/ f< 8.0  (6)

where f3 denotes the focal length of the third lens group LG 3 .

In the present example, f3=29.010 is satisfied. f3/f=5.60 is therefore satisfied, so that Conditional Expression (6) is satisfied. The projection optical system 3 A satisfies Conditional Expression (6) and therefore readily ensures a sufficient back focal length and a reduction-side numerical aperture for capturing the light flux from the light modulator 18 .

FIG. 5 is an aberration diagram when the projection distance of the projection optical system 3 A is the reference distance. FIG. 6 is an aberration diagram when the projection distance of the projection optical system 3 A is the far-projection distance. FIG. 7 is an aberration diagram when the projection distance of the projection optical system 3 A is the near-projection distance. In each of the aberration diagrams of FIGS. 5 to 7 , the right figure is a distortion aberration diagram, the middle figure is an astigmatism diagram, and the left figure is a spherical aberration diagram. The spherical aberration, astigmatism, and distortion aberration are satisfactorily corrected in the projection optical system 3 A, as shown in the aberration diagrams of FIGS. 5 to 7 .

The projection optical system 3 A according to the present example forms no intermediate image therein and permits occurrence of the distortion aberration. The size of the projection optical system 3 A can therefore be reduced. On the other hand, the light modulator 18 forms a projection image having distortion that corrects the distortion aberration produced by the projection optical system 3 A. The projector 1 can therefore project an enlarged image having a suppressed distortion aberration.

In the projection optical system 3 A according to the present example, the mirror R 1 or any other component may be disposed as a first optical path deflector in a position between the second lens group LG 2 and the third lens group LG 3 instead of a position between the first lens group LG 1 and the third lens group LG 2 . The air space d between the second lens group LG 2 and the third lens group LG 3 is 26.0. d/f=5.02 is therefore satisfied, so that Conditional Expression (1) is satisfied. In the present example, the air space between the second lens group LG 2 and the third lens group LG 3 satisfies Conditional Expression (1).

In the projection optical system 3 A according to the present example, not only may the first optical path deflector be disposed between the first lens group LG 1 and the second lens group LG 2 , but a second optical path deflector may be disposed between the second lens group LG 2 and the third lens group LG 3 . FIG. 8 is a light ray diagram when the first and second optical path deflectors are disposed in the projection optical system 3 A. In the example shown in FIG. 8 , the mirror R 1 is provided as the first optical path deflector, and a second mirror R 2 is provided as the second optical path deflector. The second mirror R 2 is disposed between the second lens group LG 2 and the third lens group LG 3 .

That is, in the projection optical system 3 A, the air space between the second lens group LG 2 and the third lens group LG 3 is 24 mm, which is a long air space. The second mirror R 2 is therefore readily disposed between the second lens group LG 2 and the third lens group LG 3 . Since the projection optical system 3 A includes the mirror R 1 and the second mirror R 2 , the optical path can be bent by 180° in the projection optical system 3 A. The direction of the light reflected off the mirror R 1 may be so changed that the first lens group LG 1 is opposite the third lens group LG 3 with respect to the second lens group LG 2 sandwiched therebetween.

FIG. 9 is a configuration diagram of the projection optical system according to Example 2. A projection optical system 3 B according to the present example is formed of the following components sequentially arranged from the enlargement side: a first lens group LG 1 having negative power; a second lens group LG 2 having positive power; an aperture stop O; and a third lens group LG 3 having positive power. The projection optical system 3 B includes 9 lenses in total.

The projection optical system 3 B includes an optical path deflector that is disposed between the first lens group LG 1 and the second lens group LG 2 and deflects the optical path, as shown in FIG. 9 . The optical path deflector is a prism block P. The light modulator 18 is located in the reduction-side image plane of the projection optical system 3 B. The screen S is located in the enlargement-side image plane of the projection optical system 3 B. The projection optical system 3 B includes the cross dichroic prism 19 between the light modulator 18 and a reduction-side first lens located in a position closest to the reduction side. The reduction-side first lens is the ninth lens L 9 . When the projection distance of the projection optical system 3 B is changed, the space between the lens group LG 1 and the lens group LG 2 is changed for focusing.

The first lens group LG 1 is formed of three negative lenses. The three negative lenses are each a single lens. That is, the first lens group LG 1 is formed of a first lens L 1 having a convex surface facing the enlargement side and having negative power, a second lens L 2 having a convex surface facing the enlargement side and having negative power, and a third lens L 3 having a convex surface facing the reduction side and having negative power. The first lens L 1 , the second lens L 2 , and the third lens L 3 each have a meniscus shape. In the present example, the first lens L 1 has aspheric surfaces on opposite sides.

The second lens group LG 2 is formed of three lenses, a fourth lens L 4 , a fifth lens L 5 , and a sixth lens L 6 . The second lens group LG 2 includes at least one positive lens. In the present example, the fourth lens L 4 and the fifth lens L 5 are each a positive lens. The fourth lens L 4 has a convex shape both on the enlargement and reduction sides. The fifth lens L 5 and the sixth lens L 6 are bonded to each other to form a first jointed lens SL 1 . The fifth lens L 5 has a convex shape both on the enlargement and reduction sides. The sixth lens L 6 has a concave shape both on the enlargement and reduction sides and has negative power.

The third lens group LG 3 includes a plurality of positive lenses. The third lens group LG 3 includes a jointed lens including at least one of the plurality of positive lenses. Further, the ninth lens L 9 , which is located in a position closest to the reduction side in the third lens group LG 3 , is a positive lens having a convex surface facing the reduction side. In other words, the reduction-side first lens is a positive lens having a convex surface facing the reduction side.

More specifically, the third lens group LG 3 is formed of three lenses, the seventh lens L 7 , the eighth lens L 8 , and the ninth lens L 9 . The seventh lens L 7 and the ninth lens L 9 are each a positive lens. The seventh lens L 7 along with the eighth lens L 8 forms a second jointed lens SL 2 . The seventh lens L 7 has a convex shape both on the enlargement and reduction sides. The eighth lens L 8 has a concave surface facing the enlargement side and has negative power. The ninth lens L 9 has a convex shape both on the enlargement and reduction sides. The ninth lens L 9 has aspheric surfaces on opposite sides.

Data on the projection optical system 3 B are as follows:

f: 5.18;

FNo: 2.00; and

ω: 67.40,

where f denotes the focal length of the entire optical system, FNo denotes the f number thereof, and ω denotes the half angle of view thereof.

Data on the lenses of the projection optical system 3 B are as follows.

The variable spaces 1 and 2 are set as follows in the focusing performed when the projection distance is changed.

Data on the aspheric surfaces of the first lens L 1 labeled with the surface numbers 1 and 2 and data on the aspheric surfaces of the ninth lens L 9 labeled with the surface numbers 18 and 19 are as follows.

Surface number 1 2 18 19 k 0.0000 0.0000 −5.7661 −0.5864 A04 1.3857E−05 −8.5468E−06  −1.8813E−06  3.3312E−05 A06 −7.4869E−09  4.2432E−08 4.8186E−09 −3.6921E−08  A08 3.7381E−12 −6.1728E−11  −7.3622E−12  9.3453E−11 A10 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00 A12 0.0000E+00 0.0000E+00 0.0000E+00 0.0000E+00

Projection Image and Enlarged Image

FIG. 10 describes the projection image. The thick lines in FIG. 9 conceptually represent the projection image. The thin line in FIG. 10 represents the image display area of the light modulator 18 . FIG. 11 describes the enlarged image generated when the projection image is projected on the screen S via the projection optical system 3 B. The thick lines in FIG. 11 conceptually represent the enlarged image generated when the projection image in FIG. 10 is projected on the screen S. The thin line in FIG. 11 conceptually represents the image display area of the light modulator 18 projected on the screen S. The chain line in FIG. 11 conceptually represents the image display area of the light modulator 18 formed when a projection optical system that produces no distortion aberration projects the image display area on the screen S.

Also in the present example, the light modulator 18 forms a projection image having distortion that corrects the distortion aberration produced by the projection optical system 3 B, as shown in FIG. 10 . When the projection optical system 3 B projects the projection image on the screen S, an oblong enlarged image having no distortion is formed, as shown in FIG. 11 . The projection image is compressed at the maximum image height in a lower portion of the image display area of the light modulator 18 . The projection image has no field curvature. The projection optical system 3 B according to the present example can generate a larger enlarged image on the screen S than the enlarged image formed by the projection optical system that has the same focal length but produces no distortion aberration, as indicated by the chain line in FIG. 11 .

The projection optical system 3 B according to the present example satisfies Conditional Expressions (1) and (2) below:

0.4< d/f< 12.0  (1)

−45%< Dst<− 10%  (2)

where f denotes the focal length of the entire optical system, d denotes the space between the first lens group LG 1 and the second lens group LG 2 , and Dst denotes the distortion aberration at the reduction-side maximum image height.

The space between the first lens group LG 1 and the second lens group LG 2 is the sum of the air space between the first lens L 1 and the prism block P, the thickness of the prism block P, and the air space between the prism block P and the second lens group L 2 . Further, the space between the first lens group LG 1 and the second lens group LG 2 is the value when the projection distance of the projection optical system 3 B is the reference distance. In the present example, d=50.240 and f=5.18 are satisfied. d/f=9.70 is therefore satisfied, so that Conditional Expression (1) is satisfied. Therefore, in the projection optical system 3 B, the prism block P is readily disposed in the projection optical system 3 B to deflect the optical path. Further, an increase in the size of the projection optical system 3 B can be suppressed even when the optical path deflector is disposed in the projection optical system 3 B.

Further, in the present example, Dst=−29.0% is satisfied, so that Conditional Expression (2) is satisfied. In the present example, since the distortion aberration at the reduction-side maximum image height satisfies Conditional Expression (2), deterioration of the quality of the enlarged image can be avoided or suppressed. Further, the diameter of the first lens L 1 can be reduced, and the number of lenses that form the projection optical system 3 B can be reduced.

Further, in the present example, the first lens group LG 1 includes three negative lenses. The negative lenses are each a single lens. The projection optical system 3 B can therefore have a sufficient back focal length and a flat enlargement-side image plane while producing a distortion aberration that approximately falls within the range expressed by Conditional Expression (2).

The projection optical system 3 B according to the present example next satisfies Conditional Expressions (3) and (4) below:

1.5<| f 1/ f|< 2.5  (3)

0.2<| f 1/ f 23|<0.6  (4)

where f denotes the focal length of the entire optical system, f1 denotes the focal length of the first lens group LG 1 , and f23 denotes the combined focal length of the combination of the second lens group LG 2 and the third lens group LG 3 .

In the present example, f1=−10.117 is satisfied. |f1/f|=1.95 is therefore satisfied, so that Conditional Expression (3) is satisfied. Further, in the present example, f23=34.927 is satisfied. |f1/f23|=0.29 is therefore satisfied, so that Conditional Expression (4) is satisfied. The projection optical system 3 B can therefore correct comma and field curvature with the distortion aberration maintained at a desired value. Further, the projection optical system 3 B can ensure a sufficient back focal length while having a wide angle of view. The cross dichroic prism 19 is therefore readily disposed.

The projection optical system 3 B further satisfies Conditional Expression (5) below:

4.0< f 2/ f< 12.0  (5)

where f2 denotes the focal length of the second lens group LG 2 .

In the present example, f2=44.502 is satisfied. f2/f=8.59 is therefore satisfied, so that Conditional Expression (5) is satisfied. The projection optical system 3 B can therefore correct comma and astigmatism in a well-balanced manner.

In the present example, the ninth lens L 9 , which is located in a position closest to the reduction side in the third lens group LG 3 , is a positive lens having a convex surface facing the reduction side. In the present example, Conditional Expression (6) below is satisfied:

3.0< f 3/ f< 8.0  (6)

where f3 denotes the focal length of the third lens group LG 3 .

In the present example, f3=25.500 is satisfied. f3/f=4.92 is therefore satisfied, so that Conditional Expression (6) is satisfied. The projection optical system 3 B therefore readily ensures a sufficient back focal length and a reduction-side numerical aperture for capturing the light flux from the light modulator.

The light rays that enter the prism block P are refracted at the light incident surface of the prism block P. The projection optical system 3 B therefore readily suppresses an increase in the diameter of the first lens group LG 1 as compared with a case where no prism block P is provided between the first lens group LG 1 and the second lens group LG 2 .

FIG. 12 is an aberration diagram when the projection distance of the projection optical system 3 B is the reference distance. FIG. 13 is an aberration diagram when the projection distance of the projection optical system 3 B is the far-projection distance. FIG. 14 is an aberration diagram when the projection distance of the projection optical system 3 B is the near-projection distance. In each of the aberration diagrams of FIGS. 12 to 14 , the right figure is a distortion aberration diagram, the middle figure is an astigmatism diagram, and the left figure is a spherical aberration diagram. The spherical aberration, astigmatism, and distortion aberration are satisfactorily corrected in the projection optical system 3 B, as shown in the aberration diagrams of FIGS. 12 to 14 .

The projection optical system 3 B according to the present example forms no intermediate image therein and permits occurrence of the distortion aberration. The size of the projection optical system 3 B can therefore be reduced. On the other hand, the light modulator 18 forms a projection image having distortion that corrects the distortion aberration produced by the projection optical system 3 B. The projector 1 can therefore project an enlarged image having a suppressed distortion aberration.

A description will next be made of a case where a triangular prism is used as the optical path deflector. FIG. 15 is a light ray diagram when a triangular prism P 1 is disposed between the first lens group LG 1 and the second lens group LG 2 . The triangular prism P 1 has a first reflection surface M 1 and a second reflection surface M 2 . Therefore, in the present example, the thus disposed triangular prism P 1 reverses the optical path by 180° in the projection optical system 3 B.

When the triangular prism P 1 is disposed between the first lens group LG 1 and the second lens group LG 2 , the first lens group LG 1 , which is located on the enlargement side of the triangular prism P 1 , and the second lens group LG 2 and the third lens group LG 32 , which are located on the reduction side of the triangular prism P 1 , are allowed to approach to each other. The size of the projection optical system 3 B can therefore be reduced.

FIG. 16 is a light ray diagram of a projection optical system according to Example 3 including an optical path deflector. A projection optical system 3 C according to the present example is formed of the following components sequentially arranged from the enlargement side: a first lens group LG 1 having negative power; a second lens group LG 2 having positive power; an aperture stop O; and a third lens group LG 3 having positive power. The projection optical system 3 C includes 9 lenses in total.

The projection optical system 3 C includes an optical path deflector that is disposed between the first lens group LG 1 and the second lens group LG 2 and deflects the optical path, as shown in FIG. 16 . In the present example, the optical path deflector is a mirror R 1 . In the projection optical system 3 C, the mirror R 1 bends an optical axis L by 90° in a position between the first lens group LG 1 and the second lens group LG 2 .

The light modulator 18 is located in the reduction-side image plane of the projection optical system 3 C. The screen S is located in the enlargement-side image plane of the projection optical system 3 C. The projection optical system 3 C includes the cross dichroic prism 19 between the light modulator 18 and a reduction-side first lens located in a position closest to the reduction side. The reduction-side first lens is the ninth lens L 9 . When the projection distance of the projection optical system 3 C is changed, the space between the lens group LG 1 and the lens group LG 2 is changed for focusing.

The first lens group LG 1 includes three negative lenses. The three negative lenses are each a single lens. That is, the first lens group LG 1 is formed of a first lens L 1 having a convex surface facing the enlargement side and having negative power, a second lens L 2 having a convex surface facing the enlargement side and having negative power, and a third lens L 3 having a convex surface facing the enlargement side and having negative power. The first lens L 1 , the second lens L 2 , and the third lens L 3 each have a meniscus shape. In the present example, the first lens L 1 has aspheric surfaces on opposite sides.

The second lens group LG 2 is formed of a fourth lens L 4 . The fourth lens L 4 has a convex shape both on the enlargement and reduction sides and has positive power.

The third lens group LG 3 includes a plurality of positive lenses. The third lens group LG 3 includes a jointed lens including at least one of the plurality of positive lenses. Further, the ninth lens L 9 , which is located in a position closest to the reduction side in the third lens group LG 3 , is a positive lens having a convex surface facing the reduction side. In other words, the reduction-side first lens is a positive lens having a convex surface facing the reduction side.

That is, the third lens group LG 3 is formed of five lenses, the fifth lens L 5 , the sixth lens L 6 , the seventh lens L 7 , the eighth lens L 8 , and the ninth lens L 9 . The fifth lens L 5 , the seventh lens L 7 , and the ninth lens L 9 are each a positive lens. The fifth lens L 5 and the sixth lens L 6 form a first jointed lens SL 1 . The seventh lens L 7 is a single lens. The eighth lens L 8 and the ninth lens L 9 form a second jointed lens SL 2 .

More specifically, the fifth lens L 5 has a convex shape both on the enlargement and reduction sides. The sixth lens L 6 has a concave shape both on the enlargement and reduction sides and has negative power. The seventh lens L 7 has a convex shape both on the enlargement and reduction sides. The eighth lens L 8 has a concave shape on the enlargement side and has negative power. The eighth lens L 8 has a meniscus shape. The ninth lens L 9 has a convex shape both on the enlargement and reduction sides. The seventh lens L 7 has aspheric surfaces on opposite sides.

Data on the projection optical system 3 C are as follows:

f: 5.18;

FNo: 2.00; and

ω: 67.80,

where f denotes the focal length of the entire optical system, FNo denotes the f number thereof, and ω denotes the half angle of view thereof.

Data on the lenses of the projection optical system 3 C are as follows.

The variable spaces 1 and 2 are set as follows in the focusing performed when the projection distance is changed.

Data on the aspheric surfaces of the first lens L 1 labeled with the surface numbers 1 and 2 and data on the aspheric surfaces of the seventh lens L 7 labeled with the surface numbers 13 and 14 are as follows.

Surface number 1 2 18 19 k −1.0000 0.0000 −3.4078 −1.7863 A04 1.9172E−05 3.9061E−06 −3.2265E−05  −2.7256E−05  A06 −1.7116E−08  2.9215E−08 1.3933E−07 −2.6737E−08  A08 1.2240E−11 −5.7492E−11  −3.1067E−10  1.1708E−10 A10 −3.9460E−15  8.1749E−15 0.0000E+00 0.0000E+00 A12 1.3529E−19 1.6156E−17 0.0000E+00 0.0000E+00

Projection Image and Enlarged Image

FIG. 17 describes the projection image. The thick lines in FIG. 17 conceptually represent the projection image. The thin line in FIG. 17 represents the image display area of the light modulator 18 . FIG. 18 describes the enlarged image generated when the projection image is projected on the screen S via the projection optical system 3 C. The thick lines in FIG. 18 conceptually represent the enlarged image generated when the projection image in FIG. 17 is projected on the screen S. The thin line in FIG. 18 conceptually represents the image display area of the light modulator 18 projected on the screen S. The chain line in FIG. 18 conceptually represents the image display area of the light modulator 18 formed when a projection optical system that produces no distortion aberration projects the image display area on the screen S.

Also in the present example, the light modulator 18 forms a projection image having distortion that corrects the distortion aberration produced by the projection optical system 3 C, as shown in FIG. 17 . When the projection optical system 3 C projects the projection image on the screen S, an oblong enlarged image having no distortion is formed, as shown in FIG. 18 . The projection image is compressed at the maximum image height in a lower portion of the image display area of the light modulator 18 . The projection image has no field curvature. The projection optical system 3 C according to the present example can generate a larger enlarged image on the screen S than the enlarged image formed by the projection optical system that has the same focal length but produces no distortion aberration, as indicated by the chain line in FIG. 18 .

The projection optical system 3 C according to the present example satisfies Conditional Expressions (1) and (2) below:

0.4< d/f< 12.0  (1)

−45%< Dst<− 10%  (2)

where f denotes the focal length of the entire optical system, d denotes the space between the first lens group LG 1 and the second lens group LG 2 , and Dst denotes the distortion aberration at the reduction-side maximum image height.

The space d between the first lens group LG 1 and the second lens group LG 2 is the air space between the third lens L 3 and the fourth lens L 4 . Further, the space d between the first lens group LG 1 and the second lens group LG 2 is the value when the projection distance of the projection optical system 3 C is the reference distance. In the present example, d=36.066 and f=5.18 are satisfied. d/f=6.96 is therefore satisfied, so that Conditional Expression (1) is satisfied. An optical path deflector, such as the mirror R 1 , is therefore readily disposed in the projection optical system 3 C to deflect the optical path. Further, since Conditional Expression (1) is satisfied in the present example, an increase in the size of the projection optical system 3 C can be suppressed even when the mirror R 1 is disposed in the projection optical system 3 C.

Further, in the present example, Dst=−30.0% is satisfied, so that Conditional Expression (2) is satisfied. Therefore, in the projection optical system 3 C, deterioration of the quality of the enlarged image can be avoided or suppressed. Further, the diameter of the first lens L 1 can be reduced, and the number of lenses that form the projection optical system 3 C can be reduced.

Further, in the present example, the first lens group LG 1 includes three negative lenses. The negative lenses are each a single lens. The projection optical system 3 C can therefore have a sufficient back focal length and a flat enlargement-side image plane while producing a distortion aberration that approximately falls within the range expressed by Conditional Expression (2).

The projection optical system 3 C according to the present example next satisfies Conditional Expressions (3) and (4) below:

1.5<| f 1/ f|< 2.5  (3)

0.2<| f 1/ f 23|<0.6  (4)

where f denotes the focal length of the entire optical system, f1 denotes the focal length of the first lens group LG 1 , and f23 denotes the combined focal length of the combination of the second lens group LG 2 and the third lens group LG 3 .

In the present example, f1=−9.571 is satisfied. |f1/f|=1.85 is therefore satisfied, so that Conditional Expression (3) is satisfied. Further, in the present example, f23=33.859 is satisfied. |f1/f23|=0.28 is therefore satisfied, so that Conditional Expression (4) is satisfied. The projection optical system 3 C can therefore correct comma and field curvature with the distortion aberration maintained at a desired value. Further, the projection optical system 3 C can ensure a sufficient back focal length while having a wide angle of view. The cross dichroic prism 19 is therefore readily disposed.

The projection optical system 3 C further satisfies Conditional Expression (5) below:

4.0< f 2/ f< 12.0  (5)

where f2 denotes the focal length of the second lens group LG 2 .

In the present example, f2=35.010 is satisfied. f2/f=6.76 is therefore satisfied, so that Conditional Expression (5) is satisfied. The projection optical system 3 C can therefore correct comma and astigmatism in a well-balanced manner.

In the present example, the ninth lens L 9 , which is located in a position closest to the reduction side in the third lens group LG 3 , is a positive lens having a convex surface facing the reduction side. The projection optical system 3 C satisfies Conditional Expression (6) below:

3.0< f 3/ f< 8.0  (6)

where f3 denotes the focal length of the third lens group LG 3 .

In the present example, f3=25.429 is satisfied. f3/f=4.91 is therefore satisfied, so that Conditional Expression (6) is satisfied. The projection optical system 3 C therefore readily ensures a sufficient back focal length and a reduction-side numerical aperture for capturing the light flux from the light modulator.

FIG. 19 is an aberration diagram when the projection distance of the projection optical system 3 C is the reference distance. FIG. 20 is an aberration diagram when the projection distance of the projection optical system 3 C is the far-projection distance. FIG. 21 is an aberration diagram when the projection distance of the projection optical system 3 C is the near-projection distance. In each of the aberration diagrams of FIGS. 19 to 21 , the right figure is a distortion aberration diagram, the middle figure is an astigmatism diagram, and the left figure is a spherical aberration diagram. The spherical aberration, astigmatism, and distortion aberration are satisfactorily corrected in the projection optical system 3 C, as shown in the aberration diagrams of FIGS. 19 to 21 .

The projection optical system 3 C according to the present example forms no intermediate image therein and permits occurrence of the distortion aberration. The size of the projection optical system. 3 C can therefore be reduced. On the other hand, the light modulator 18 forms a projection image having distortion that corrects the distortion aberration produced by the projection optical system 3 C. The projector 1 can therefore project an enlarged image having a suppressed distortion aberration.