Lens system and projector utilizing the same

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2006-334018, filed on Dec. 12, 2006, the entire contents of which are incorporated herein by reference.

1. Field of the Invention

The present invention relates to a lens system including lens elements of small diameters for projecting on to a screen in an enlarged fashion images from light valves such as mainly DMDs (Digital Micromirror Devices) for forming images by changing reflecting directions of light.

2. Description of the Related Art

Projectors utilizing DMDs as light valves are considered as being more advantageous in miniaturization than those utilizing other image forming methods. Currently, compact portable projectors have been well accepted by many users, and especially so are data projectors which are convenient for presentations.

In projectors which are developed for use as portable ones, reducing the thickness dimension is of importance, and it can be said that the reduction in thickness is one of the most important factors for projectors which are in many cases carried together with note-type personal computers or the like for combined use. As an example of a means for solving the problem, for example, the Japanese Unexamined Patent Publication No. 2003-121736 describes a design method for reducing the effective diameter of a projection lens.

The present invention has been made in view of the situations and an objective thereof is to realize a lens system made up of lens elements of small diameters, so as to provide a projector incorporating the lens system so realized which is thus thin enough for convenient carriage and which can project an enlarged image with high image quality even in a limited space.

According to a preferred aspect of the invention, there is provided a lens system including, sequentially in that order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole.

The first lens group is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 1 a lens group having a negative refractive power as a whole and a second sub-lens group or 1 b lens group having a positive refractive power as a whole, and the second lens group is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 2 a lens group having a negative refractive power as a whole and a second sub-lens group or 2 b lens group having a positive refractive power as a whole.

In addition, this lens system is such that the following conditional expression (1) is satisfied with respect to a power set for the first lens group, the following conditional expression (2) is satisfied with respect to an afocal magnification set for the first lens group, the following conditional expression (3) is satisfied with respect to a power set for the 1 a lens group, the following conditional expression (4) is satisfied with respect to a power set for the 2 a lens group, the following conditional expression (5) is satisfied with respect to a power set for the 2 b lens group, and the following conditional expression (6) is satisfied with respect to a thickness dimension at an optical axis of the first lens group;

−0.5 ≦f/f I ≦−0.1  (1)

0.3 ≦h IE /h IX ≦0.5  (2)

−1.5 ≦f/f Ia ≦−0.5  (3)

−0.3 ≦f/f IIa ≦0.3  (4)

0.15 ≦f/f IIb ≦0.7  (5)

1.6 ≦T I /f≦ 2.6  (6)

where,

f: Composite focal length of the whole lens system; f I : Composite focal length of the first lens group; h IE : Height of paraxial ray incident on to a magnifying-side surface of a lens element which is disposed outermost on a magnifying side of the first lens group; h IX : Height of paraxial ray emerging from a contracting-side surface of a lens element which is disposed outermost on a contracting side of the first lens group; f Ia : Composite focal length of the 1 a lens group which makes up the first lens group; f IIa : Composite focal length of the 2 a lens group which makes up the second lens group; f I : Composite focal length of the 2 b lens group which makes up the second lens group; and T I : Distance on the optical axis between the magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the first lens group to the contracting-side surface of the lens element which is disposed outermost on the contracting side of the first lens group.

FIG. 1 is a drawing showing the configuration of lens elements of a lens system according to a first embodiment of the invention,

FIG. 2 is a drawing showing aberrations produced by the lens system according to the first embodiment;

FIG. 3 is a drawing showing the configuration of lens elements of a lens system according to a second embodiment of the invention,

FIG. 4 is a drawing showing aberrations produced by the lens system according to the second embodiment;

FIG. 5 is a drawing showing the configuration of lens elements of a lens system according to a third embodiment of the invention,

FIG. 6 is a drawing showing aberrations produced by the lens system according to the third embodiment;

FIG. 7 is a drawing showing the configuration of lens elements of a lens system according to a fourth embodiment of the invention,

FIG. 8 is a drawing showing aberrations produced by the lens system according to the fourth embodiment;

FIG. 9 is a drawing showing the configuration of lens elements of a lens system according to a fifth embodiment of the invention,

FIG. 10 is a drawing showing aberrations produced by the lens system according to the fifth embodiment;

FIG. 11 is a drawing showing the configuration of lens elements of a lens system according to a sixth embodiment of the invention,

FIG. 12 is a drawing showing aberrations produced by the lens system according to the sixth embodiment;

FIG. 13 is a drawing showing the configuration of lens elements of a lens system according to a seventh embodiment of the invention,

FIG. 14 is a drawing showing aberrations produced by the lens system according to the seventh embodiment;

FIG. 15 is a drawing showing the configuration of lens elements of a lens system according to an eighth embodiment of the invention,

FIG. 16 is a drawing showing aberrations produced by the lens system according to the eighth embodiment;

FIG. 17 is a drawing showing an external appearance of a projector which utilizes the lens system of the invention.

Hereinafter, embodiments of the invention will be described by reference to specific numerical examples. In Embodiments 1 to 8 below, a lens system is configured to include, sequentially in that order from a magnifying side, a first lens group LG 1 which makes up a substantially afocal optical system as a whole and a second lens group LG 2 having a positive refractive power as a whole.

The first lens group LG 1 is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 1 a lens group LG 1 a having a negative refractive power as a whole and a second sub-lens group or 1 b lens group LG 1 b having a positive refractive power as a whole.

The 1 a lens group LG 1 a is configured to include three lens elements which are, sequentially in that order from a magnifying side, a lens element (lens element designated as L 111 , a magnifying-side surface as 111 , a contracting-side surface as 112 ) which has a positive refractive power (hereinafter, referred to as a positive lens element), a lens element (lens element designated as L 112 , a magnifying-side surface as 113 , a contracting-side surface as 114 ) which is formed into a meniscus shape which is convex on a magnifying side thereof and which has a negative refractive power (hereinafter, referred to as a negative lens element), and a negative lens element (lens element designated as L 113 , a magnifying-side surface as 115 , a contracting-side surface as 116 ).

The 1 b lens group LG 1 b , which follows the 1 a lens group LG 1 a , is configured to include four lens elements in total, that is, a negative lens element (lens element designated as L 121 , a magnifying-side surface as 121 , a contracting-side surface as 122 ), a positive lens element (lens element designated as L 122 , a magnifying-side surface as 123 , a contracting-side surface as 124 , however, in the event that the lens element constitutes a cemented lens element together with the preceding negative lens element, the lens surface 122 and the lens surface 123 constitute the same lens surface), a negative lens element (lens element designated as L 123 , a magnifying-side surface as 125 , a contracting-side surface as 126 ), and a positive lens element (lens element designated as L 124 , a magnifying-side surface as 127 , a contracting-side surface as 128 ) which has a meniscus shape which is convex on a contracting side thereof, or is configured to include five lens elements in total, that is, a negative lens element (lens element designated as L 121 , a magnifying-side surface as 121 , a contracting-side surface as 122 ), a positive lens element (lens element designated as L 122 , a magnifying-side surface as 123 , a contracting-side surface as 124 , however, in the event that the lens element constitutes a cemented lens element together with the preceding negative lens element, the lens surface 122 and the lens surface 123 constitute the same lens surface), a negative lens element (lens element designated as L 123 , a magnifying-side surface as 125 , a contracting-side surface as 126 ), a positive lens element (designated as L 124 , a magnifying-side surface as 127 , a contracting-side surface as 128 ), and a positive lens element which has a meniscus shape which is convex on a contracting side thereof or a negative lens element which has a meniscus shape which is convex on a contracting side thereof (lens element designated as L 125 , a magnifying-side surface as 129 , a contracting-side surface as 1210 ).

The second lens group LG 2 is configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 2 a lens group LG 2 a having a negative refractive power as a whole and a second sub-lens group or 2 b lens group LG 2 b having a positive refractive power as a whole.

The 2 a lens group LG 2 a is configured to include three lens elements, that is, a positive lens element (lens element designated as L 211 , a magnifying-side surface as 211 , a contracting-side surface as 212 ), a positive lens element (designated as L 212 , a magnifying-side surface as 213 , a contracting-side surface as 214 ), and a negative lens element (lens element designated as L 213 , a magnifying-side surface as 215 , a contracting-side surface as 216 , however, in the event that the lens element constitutes a cemented lens element together with the preceding positive lens element, the lens surface 214 and the lens surface 215 constitute the same lens surface).

The 2 b lens group LG 2 b is configured to include three lens elements in total, that is, a positive lens element (lens element designated as L 221 , a magnifying-side surface as 221 , a contracting-side surface as 222 ), a negative lens element (lens element designated as L 222 , a magnifying-side surface as 223 , a contracting-side surface as 224 ), and a positive lens element (lens element designated as L 223 , a magnifying-side surface as 225 , a contracting-side surface as 226 , however, in the event that the lens element constitutes a cemented lens element together with the preceding negative lens element, the lens surface 224 and the lens surface 225 constitute the same lens surface).

Additionally, the 1 a lens group LG 1 a and the 1 b lens group LG 1 b , which make up the first lens group LG 1 , are fixed to a first lens barrel, and the 2 a lens group LG 2 a and the 2 b lens group LG 2 b , which make up the second lens group LG 2 , are fixed to a second lens barrel.

In addition, depending upon applications, the lens system is configured to include a third lens group LG 3 which is made up of a positive lens element (lens element designated as L 301 , a magnifying-side surface designated as 301 , a contracting-side surface designated as 302 ) which is disposed in the vicinity of a light valve such as a DMD on the contracting side of the second lens group LG 2 . A cover glass CG (a magnifying-side surface as C 01 , a contracting-side surface as C 02 ), which is a constituent component of the light valve such as the DMD, is provided between a contracting side of the third lens group LG 3 and a surface of the light valve with a slight airspace provided therebetween.

In this way, according to the embodiment, there is provided a lens system comprising, sequentially in that order from a magnifying side, a first lens group which makes up a substantially afocal optical system as a whole and a second lens group having a positive refractive power as a whole, the first lens group being configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 1 a lens group having a negative refractive power as a whole and a second sub-lens group or 1 b lens group having a positive refractive power as a whole, the second lens group being configured to include, sequentially in that order from a magnifying side, a first sub-lens group or 2 a lens group having a negative refractive power as a whole and a second sub-lens group or 2 b lens group having a positive refractive power as a whole, wherein, the following conditional expression (1) is satisfied with respect to a power set for the first lens group, the following conditional expression (2) is satisfied with respect to an afocal magnification set for the first lens group, the following conditional expression (3) is satisfied with respect to a power set for the 1 a lens group, the following conditional expression (4) is satisfied with respect to a power set for the 2 a lens group, the following conditional expression (5) is satisfied with respect to a power set for the 2 b lens group, and the following conditional expression (6) is satisfied with respect to a thickness dimension at an optical axis of the first lens group;

−0.5 ≦f/f I ≦−0.1  (1)

0.3 ≦h IE /h IX ≦0.5  (2)

−1.5 ≦f/f Ia ≦−0.5  (3)

−0.3 ≦f/f IIa ≦0.3  (4)

0.15 ≦f/f IIb ≦0.7  (5)

1.6 ≦T I /f≦ 2.6  (6)

where,

f: Composite focal length of the whole lens system; f I : Composite focal length of the first lens group; h IE : Height of paraxial ray incident on to a magnifying-side surface of a lens element which is disposed outermost on a magnifying side of the first lens group; h IX : Height of paraxial ray emerging from a contracting-side surface of a lens element which is disposed outermost on a contracting side of the first lens group; f Ia : Composite focal length of the 1 a lens group which makes up the first lens group; f IIa : Composite focal length of the 2 a lens group which makes up the second lens group; f IIb : Composite focal length of the 2 b lens group which makes up the second lens group; and T I : Distance on the optical axis between the magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the first lens group to the contracting-side surface of the lens element which is disposed outermost on the contracting side of the first lens group.

The conditional expression (1) specifies an appropriate power distribution to the first lens group which makes up the substantially afocal optical system as a whole. In the event that an upper limit is surpassed, the positive power of the first lens group becomes too large, and the back focus becomes short. In the event that a lower limit is surpassed, the negative power of the first lens group becomes too large, and the load borne by the rear group increases, whereby the aberrations are deteriorated.

In addition, the conditional expression (2) specifies the magnification of the first lens group which functions as a wide converter, and in the event that an upper limit thereof is surpassed, the magnification becomes too high, whereby the load in terms of aberrations borne by the front group becomes too large, resulting in a deteriorated performance, whereas a lower limit is surpassed, the magnification decreases, which is disadvantageous in wide-angle setting.

The conditional expression (3) is associated with the negative power set for the 1 a lens group which corresponds to a dispersion system in the first lens group which makes up the wide converter. In the event that an upper limit is surpassed, the negative power becomes weak, whereby the focal length of the whole converter system is increased, and the system has to be enlarged. In the event that a lower limit is surpassed, the negative power becomes too large, which is advantageous in miniaturization but is disadvantageous in correcting the aberrations.

The following conditional expressions (4), (5) are associated with the power configuration of the second lens group. Although the second lens group, which makes up the rear group, has the positive power as the whole system, the second lens group is characterized in that since the 2 a lens group, which is situated on the magnifying side, constitutes the dispersion system, the 2 a lens group is configured to have the negative power as a whole, and since the 2 b lens group, which is situated on the contracting side, constitutes a condenser system, the 2 b lens group is configured to have the positive power as a whole.

The conditional expression (4) specifies the negative power of the 2 a lens group, and in the event that an upper limit is surpassed, the negative power becomes small, whereby the back focus is shortened. On the contrary, in the event that a lower limit is surpassed, the negative power becomes too large, giving rise to enlargement of the system, and the aberrations are increased by the negative power which now becomes excessive.

The conditional expression (5), which follows the expression (4), specifies the positive power of the 2 b lens group so as to obtain an appropriate size and performance for the whole lens system together with the conditional expression (4). In the event that an upper limit is surpassed, the positive power becomes immoderate, and the performance is decreased and the back focus is shortened. In the event that a lower limit is surpassed, the positive power becomes small, giving rise to enlargement of the system.

The conditional expression (6) specifies the thickness of the first lens group in the optical axis direction. Functioning as the afocal converter, the first lens group needs to keep a certain axial thickness for correction of the aberrations, although it depends upon the afocal magnification thereof. In the event that an upper limit is surpassed, although it is advantageous in terms of performance, the system has to be enlarged, whereas in the event that a lower limit is surpassed, although it is advantageous in terms of miniaturization, the powers of the 1 a lens group and the 1 b lens group become too large, whereby the aberrations are increased.

In addition, the 1 a lens group, which makes up the first lens group, is configured to include three lens elements, sequentially in that order from a magnifying side, a positive lens element, a negative lens element which has a meniscus shape which is convex on a magnifying side thereof, and a negative lens element, and it is preferable that the following conditional expression (7) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1 a lens group, and the following conditional expression (8) is satisfied with respect to the refractive index of a glass material used for each of the lens elements which make up the 1 a lens group;

0.6 ≦R Ia4 /f≦ 1.1  (7)

1.65≦N Ia   (8)

where,

R Ia4 : Radius of curvature of the contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1 a lens group; and N Ia : Mean value of refractive indices of the lens elements which make up the 1 a lens group relative to the d line.

The conditional expression (7) is associated with the shape of the contracting-side surface of the lens element which is disposed second outermost from the magnifying side of the 1 a lens group, and this lens element plays an important role due to the lens element being disposed outermost on the magnifying side as a negative lens in the lens group having the negative power. Namely, the contracting-side surface of the relevant lens element is formed substantially into a concentric shape relative to a bundle of rays on the magnifying side while being made to have a strong power, and basically, the concentric shape so formed is made to suppress the generation of aberrations. Consequently, in the event that a lower limit is surpassed, spherical aberration and comatic aberration are overcorrected, whereas in the event that an upper limit is surpassed, they become undercorrected, on the contrary.

The conditional expression (8) is associated with the characteristics of the refractive index of the 1 a lens group which has the particularly strong negative power. In order to mitigate the intensity of curvature of acquiring the strong negative power, it is inevitable that the refractive index is as high as possible, and in the event that a lower limit of the conditional expression is surpassed, the curvature becomes excessive, whereby the spherical aberration and the comatic aberration become excessive, and the Petzval sum becomes too small, thereby making it impossible to obtain a good performance.

In addition, the 1 b lens group is configured to include four lens elements in total, that is, a negative lens element, a positive lens element, a negative lens element, and a positive lens element which has a meniscus shape which is convex on a contracting side thereof, or is configured to include five lens elements in total, that is, a negative lens element, a positive lens element, a negative lens element, a positive lens element, and a positive lens element which has a meniscus shape which is convex on a contracting side thereof or a negative lens element which has a meniscus shape which is convex on a contracting side thereof, and it is preferable that the following conditional expression (9) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed outermost on the contracting side of the 1 b lens group, the following conditional expression (10) is satisfied with respect to the dispersion properties of a glass material used for each lens element which makes up the 1 b lens group, and the following conditional expression (11) is satisfied with respect to the refractive index of a glass material used for each of the negative lens elements which make up the 1 b lens group;

−2.8 ≦R IbL /f≦− 1.0  (9)

V IbP −V IbN ≦−5  (10)

1.58≦N IbN   (11)

where,

R IbL : Radius of curvature of the contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 1 b lens group; V Ibp : Mean value of Abbe numbers of the positive lens elements which make up the 1 b lens group; V IbN : Mean value of Abbe numbers of the negative lens elements which make up the 1 b lens group; and N IbN : Mean value of the refractive indices of the negative lens elements which make up the 1 b lens group relative the d line.

The conditional expression (9) is associated with the shape of the lens element which is disposed outermost on the contracting side of the 1 b lens group, and the shape has a role of transmitting a bundle of dispersed rays from the 1 a lens group to the rear while maintaining the generation of aberrations to a low level. In the event that an upper limit is surpassed, the correction of a large negative distortion produced by the 1 b lens group becomes insufficient, whereas in the event that a lower limit is surpassed, although it is advantageous in correcting the distortion, the meniscus shape becomes too intensive, and high-order spherical aberration and comatic aberration are generated.

The conditional expression (10) is associated with the achromatism of the 1 b lens group and specifies an appropriate combination of Abbe numbers in the glass material. In the event that a lower limit is surpassed, it becomes difficult to correct chromatic aberration.

The conditional expression (11) specifies a restricting condition for correction of field curvature, spherical aberration and comatic aberration. Since the Petzval sum tends to become negative easily, it is good to correct it by increasing the refractive indices of the negative lens elements, and together with this, the correction of spherical aberration and comatic aberration is intended by increasing the radius of curvature by the high refractive indices. Due to this, in the event that a lower limit is surpassed, it becomes difficult to correct field curvature.

In addition, the 2 a lens group is configured to include three lens elements, that is, a positive lens element, a positive lens element and a negative lens element, and it is preferable that the following conditional expressions (12), (13) are satisfied, respectively, with respect to the refractive index and dispersion properties of a glass material used for each of the lens elements which make up the 2 a lens group, the following conditional expression (14) is satisfied with respect to a power set for the negative lens element which is disposed outermost on the contracting side of the 2 a lens group, and the following conditional expression (15) is satisfied with respect to the shape of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2 a lens group and the shape of a magnifying-side surface of the lens element which is disposed second outermost from the magnifying side of the same lens group;

0.15 ≦N IIaN −N IIaP   (12)

10 ≦V IIaP −V IIaN   (13)

−1.5 ≦f/f IIaN ≦−0.4  (14)

−0.75 ≦R IIa2 /R IIa3 ≦−0.1  (15)

where,

N IIaN : Mean value of refractive indices of the negative lens element which makes up the 2 a lens group relative to the d line; N IIaP : Mean value of refractive indices of the positive lens elements which make up the 2 a lens group relative to the d line; V IIaP : Mean value of Abbe numbers of the positive lens elements which make up the 2 a lens group; V IIaN : Mean value of Abbe numbers of the negative lens elements which make up the 2 a lens group; f IIaN : Focal length of the lens element which is disposed outermost on the contracting side of the lens elements which make up the 2 a lens group; R IIa2 : Radius of curvature of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the lens elements which make up the 2 a lens group; and R IIa3 : Radius of curvature of a magnifying-side surface of the lens element which is disposed second outermost from the magnifying side of the lens elements which make up the 2 a lens group.

The conditional expression (12) specifies a condition for field curvature correction, which corrects a field curvature produced by the strong negative power of the 2 a lens group. Namely, it is good to use a material having a high refractive index for the negative lens element and a material having a low refractive index for the positive lens element, so as to prevent the Petzval sum from becoming too small. In the event that a lower limit is surpassed, the Petzval sum becomes too small, whereby the field curvature is worsened.

The conditional expression (13) is a condition for correction of chromatic aberration of the 2 a lens group. In the event that an upper limit is surpassed, the Abbe numbers come nearer to each other, and the chromatic aberration effect becomes insufficient.

The conditional expression (14) is, as with the conditional expression (13), a condition for correction of chromatic aberration of the 2 a lens group, as well. As is known, since chromatic aberration can be corrected by an appropriate combination of Abbe number and power, the conditional expression (14) specifies the power. In the event that an upper limit and a lower limit are surpassed, in both cases, the achromatic power of the 2 a lens group becomes inappropriate, whereby the chromatic aberration are worsened.

The conditional expression (15) is associated with correction of spherical aberration and comatic aberration within the 2 a lens group. The correction of those aberrations can be enabled by designing the shape of the convex lens element within the 2 a lens group in such a manner as to fall within a range restricted by the conditional expression (15). In the event that a lower limit is surpassed, the spherical aberration becomes over, and the comatic aberration relative to a bundle of off-axis rays is also worsened. In the event that an upper limit is surpassed, the spherical aberration becomes under excessively.

In addition, the 2 b lens group is configured to include three lens elements in total, that is, a positive lens element, a negative lens element, and a positive lens element, and it is preferable that the following conditional expressions (16), (17) are satisfied with respect to refractive index and dispersion properties of a glass material used for each of the lens elements which make up the 2 b lens group, respectively, the following conditional expression (18) is satisfied with respect to a power set for the lens element which is disposed second outermost from the magnifying side of the 2 b lens group, and the following conditional expression (19) is satisfied with respect to the shape of a magnifying-side surface and the shape of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2 b lens group;

0.15 ≦N IIbN −N IIbP   (16)

0 ≦V IIbP −V IIbN   (17)

−1.0 ≦f/f IIb2 ≦−0.2  (18)

−2.0 ≦R IIb1 /R IIb2 ≦−0.3  (19)

where,

N IIbN : Mean value of refractive indices of the negative lens element which makes up the 2 b lens group relative to the d line; N IIbP : Mean value of refractive indices of the positive lens elements which make up the 2 b lens group relative to the d line; V IIbP : Mean value of Abbe numbers of the positive lens elements which make up the 2 b lens group; V IIbN : Mean value of Abbe numbers of the negative lens element which makes up the 2 b lens group; f IIb2 : Focal length of the lens element which lies second outermost from the magnifying side of the lens elements which make up the 2 b lens group; R IIb1 : Radius of curvature of a magnifying-side surface of the lens element which is disposed outermost on the magnifying side of the 2 b lens group; and R IIb2 : Radius of curvature of a contracting-side surface of the lens element which is disposed outermost on the magnifying side of the 2 b lens group.

The conditional expression (16) is associated with the refractive index of a glass material for each of the lens elements which make up the 2 b lens group which makes up the second lens group and has a large positive power. By giving a difference in refractive index between the positive and negative lens elements, a correcting capability of spherical aberration is made use of on a cemented surface while maintaining the generation of chromatic aberration to a minimum level, and the effect of field curvature correction is also expected. In the event that a lower limit is surpassed in the conditional expression (16), it results in overcorrection of field curvature and results in undercorrection of spherical aberration.

The conditional expression (17) specifies a condition for color correction in the 2 b lens group, and in order to correct single-color aberrations at the same time, it is necessary that the powers of the respective lens elements do not become excessive, and to make this happen, it is a necessary condition that Abbe numbers of glass materials used for the positive lens elements and the negative lens element satisfy the conditional expression (17). In the event that a lower limit is surpassed, it becomes difficult to correct chromatic aberration.

The conditional expression (18) similarly specifies a condition for chromatic aberration correction or achromatism of the 2 b lens group. In the event that both an upper limit and a lower limit are surpassed, in both cases, the achromatic power in the 2 b lens group becomes inappropriate, and the chromatic aberration is worsened.

The conditional expression (19) specifies a restricting condition for the shape of the lens element which is disposed outermost on the magnifying side of the 2 b lens group and is associated with spherical aberration and comatic aberration. In the event that an upper limit is surpassed, the spherical aberration becomes under largely, whereas in the event that a lower limit is surpassed, the generation of comatic flare in a bundle of marginal rays becomes remarkable.

In addition, as a method for adjusting the focal point of the whole lens system, a method for shifting the whole projection lens in the optical axis direction cannot be said to be a good method in a case where there is imposed a limitation on the size of a structure to be shifted. Then, in adopting a so-called inner focusing method in which the focal point is adjusted by shifting part of an optical system of a lens system, when taking it into consideration that the first lens group is substantially afocal, it is considered to make use of the lens group which is disposed closer to the contracting side than the first lens group, and it is preferable to adjust the focal point by shifting the second lens group in the optical axis direction because a spatial amount for shifting is sufficient and the change in aberrations due to distance can be suppressed to a least level.

Furthermore, in order to focus bundles of rays from the DMDs on to the surface of a screen with good efficiency, the angle of a principal ray of a bundle of rays emerging from the DMD must be set according to the properties of the illuminating optical system. Due to this, the lens system as being a projection lens is set substantially telecentric in many cases. However, when attempting to design such that all the rays are captured, the effective diameter of the second lens group on the contracting side thereof becomes gigantic, which in many cases causes a problem with the arrangement of the second lens group relative to the illuminating optical system. As this occurs, although there is imposed a limitation on the effective diameter in the vicinity of the emerging side of the second lens group depending upon the arrangement of the second lens group with the illuminating optical system, the objective is preferably attained by providing a third lens group which is made up of a single positive lens in the vicinity of the light valve in the airspace between the second lens group and the light valve.

In addition, FIG. 17 shows an external appearance of a projector which utilizes the lens system of the invention. As is shown in FIG. 17 , a projector 10 is formed substantially into a rectangular parallelepiped shape, and the projector 10 has a lens cover 19 which covers a projection opening provided at an end of a front plate 12 which makes up a main body housing, and a plurality of vent holes 18 are provided in the front plate 12 . In addition, although the illustration thereof is omitted in FIG. 17 , a key/indicator section is provided on an upper plate 11 which also makes up the main body housing, and this key/indicator section includes keys and indicators such as a power supply switch, a power indicator lamp which informs that the power supply is on or off, a lamp switch key for turning on a lamp of a light source unit, an indicator lamp which indicates that the lamp is turned on and an overheat indicator lamp which informs that the light source unit is under overheat when it actually happens. Furthermore, provided on a back plate, which is not shown, of the main body housing are an input/output connector section where USB terminals, D-SUB terminals for image signal input, S terminals, RCA terminals and the like are provided and an Ir receiving section where control signals from a power supply adapter plug and a remote controller are received. In addition, a plurality of vent holes 18 are provided in a right-hand side plate, not shown, which is a side plate of the main body housing, and a left-hand side plate 15 , which is a side plate shown in FIG. 17 , and a projection lens which adopts the lens system that has been described heretofore is installed inside the left-hand side plate 15 therealong.

A numerical example of a first embodiment of a lens system of the invention is shown in Table 1. In addition, FIG. 1 is a drawing showing the configuration of the lens system, and FIG. 2 is a drawing showing aberrations thereof. In the following tables and drawings, f denotes a focal length of the whole lens system, F no an F number, and 2ω total angle of view of the lens system. In addition, r denotes a radius of curvature, d a lens element thickness or spacing between lens elements, n d a refractive index relative to the d line, and ν d Abbe number at the d line. In the drawings showing spherical aberration of the drawings showing the aberrations, CA 1 , CA 2 , CA 3 denote aberration curves at wavelengths of CA 1 =550.0 nm, CA 2 =435.8 nm and CA 3 =640.0 nm, and S.C denotes a sine condition. In the drawing showing astigmatism, S denotes sagital, and M denotes meridional. In addition, throughout the tables, unless otherwise described, the wavelength used in calculation of numerical values is CA 1 =550.0 nm, and the drawings showing the aberrations show aberrations which result with an object distance of 1700 mm which is used in many cases for evaluation of a projection lens.

A numerical example of a second embodiment of a lens system of the invention is shown in Table 2. In addition, FIG. 3 is a drawing showing the configuration of the lens system, and FIG. 4 is a drawing showing aberrations thereof.

A numerical example of a third embodiment of a lens system of the invention is shown in Table 3. In addition, FIG. 5 is a drawing showing the configuration of the lens system, and FIG. 6 is a drawing showing aberrations thereof.

A numerical example of a fourth embodiment of a lens system of the invention is shown in Table 4. In addition, FIG. 7 is a drawing showing the configuration of the lens system, and FIG. 8 is a drawing showing aberrations thereof.

A numerical example of a fifth embodiment of a lens system of the invention is shown in Table 5. In addition, FIG. 9 is a drawing showing the configuration of the lens system, and FIG. 10 is a drawing showing aberrations thereof.

A numerical example of a sixth embodiment of a lens system of the invention is shown in Table 6. In addition, FIG. 11 is a drawing showing the configuration of the lens system, and FIG. 12 is a drawing showing aberrations thereof.

A numerical example of a seventh embodiment of a lens system of the invention is shown in Table 7. In addition, FIG. 13 is a drawing showing the configuration of the lens system, and FIG. 14 is a drawing showing aberrations thereof.

A numerical example of an eighth embodiment of a lens system of the invention is shown in Table 8. In addition, FIG. 15 is a drawing showing the configuration of the lens system, and FIG. 16 is a drawing showing aberrations thereof.

Next, values corresponding to the conditional expression (1) to conditional expression (19) in the first embodiment to the eight embodiment are shown in Table 9 altogether.

As is obvious from Table 9, the numerical values related to the respective examples of the first to eighth embodiments satisfy the conditional expressions (1) to (19), and as is obvious from the aberration drawings of the respective embodiments, the respective aberrations were corrected properly.