Optical system and rear projector

An optical system that projects projection light from an image generating device, onto a screen is provided. The optical system comprises: a lens system on which the projection light from the image generating device is incident; and an aspherical mirror with negative power as a whole that is disposed on an optical path between the lens system and the screen, wherein a curvature of the aspherical mirror in the radial direction changes positive to negative on a way from a center to a periphery thereof.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-360017, filed Dec. 13, 2004, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to an optical system that magnifies and projects projection light that has been modulated by an image generating device such as a liquid crystal device, DMD, based on image information onto a screen, and to a rear projector that uses the same.

Japanese Laid-Open Patent Publication No. 2001-264627 discloses an optical system for a rear projector that includes an image forming optical system with a plurality of lenses and a positive power, and a projection system with a curved mirror of a negative power.

The Publication also discloses that the optical system having an image generating device that emits projection light, the lens system, and the curved mirror is disposed below a screen so as to images are displayed with the projection light incident on the screen diagonally from below. In this system, a screen (Fresnel screen) where prism-type Fresnel lenses are disposed at a narrow pitch is used. A normal Fresnel screen cannot be used when the incident angle is large due to the amount of light being insufficient. Contrary, with a Fresnel screen that uses prism-type Fresnel lenses, when the incident angle of the projection light is sufficiently large, the projection light is refracted in the desired direction and therefore bright images can be displayed. Therefore, when the incident angle is too low, the images displayed by a prism-type Fresnel screen become dark. Accordingly, combined-type screens where prism-type Fresnel lenses are disposed in a top part of the screen where the incident angle for the projected light is large are now used, but such combined-type screens are difficult to manufacture and are expensive. There is a further problem that stray light is produced due to the mixture of different types of Fresnel in a screen.

When the light is incident on the screen diagonally from below, the incident angle is smallest in a lower central region of the screen and becomes larger toward an upper part and edges of the screen. For this reason, to produce a sufficiently large incident angle in the lower central part of the screen, it is preferable to use an optical system that is a combination of a wide-angle lens system and a curved lens with negative power. However, there is the problem that when a sufficiently high incident angle is produced in the lower central part of the screen, rays are scattered in the upper part and at the edges of the screen, resulting in it becoming difficult to achieve a sufficiently high resolution. For this reason, for a slim rear projector, it is difficult to display bright, clear images across the entire screen.

SUMMARY

It is an object of the present invention to provide an optical system that achieves a sufficient incident angle for the entire screen and can also suppress the scattering of rays in the upper part and edge parts of the screen. It is a further object of the present invention to provide a slim rear projector that uses the above optical system and can therefore display clear, bright images across the entire screen.

One of aspects of the present invention is an optical system that projects projection light from an image generating device, which outputs the projection light that has been modulated according to image information, onto a screen. The optical system includes: a lens system on which the projection light from the image generating device is incident; and an aspherical mirror with negative power as a whole that is disposed on an optical path between the lens system and the screen, wherein a curvature of the aspherical mirror in the radial direction changes positive to negative on a way from a center to a periphery thereof.

It is preferable that the curvature of the aspherical mirror decreases from the center to the periphery. That is, it is preferable that the curvature falls from the center toward the periphery with the sign of the curvature becoming inverted on the way.

The aspherical mirror has a negative power as a whole, but has an element of a positive power in the periphery part that is an outer circumferential part of the mirror. Therefore suppresses the scattering of rays of projection light that are projected in an upper part and edge parts of the screen. Accordingly, by combining the aspherical mirror with a wide-angle lens system, for example, a lens system where a lens closest to the screen, that is the lens located at an end of a projection side of the lens system is a meniscus lens that is convex on the projection side, it is possible to project the projection light with a sufficiently large incident angle in a lower central part of the screen and it is also possible to suppress scattering of rays of projected light in an upper part and at edge parts of the screen. The screen side described above means the side to which the projection light is emitted. Therefore, the screen side is referred to as the projection side in this specification.

Accordingly, it is possible to provide an optical system that can project the projection light with an incident angle of at least 45°, for example, on the entire area of the screen. For this reason, it becomes possible to provide a projection system including a screen whose entire area is a prism-type Fresnel screen. Therefore, by constructing a rear projector of an image generating device, a screen, and the above projection optical system and projecting the projected light from a rear side of the screen, it is possible to provide a rear projector that is slim and can display clear, bright images across the entire area of the screen.

A distance D between an aspherical mirror end of the lens system and the aspherical mirror and a combined focal length f of the lens system should preferably satisfy a condition below.
1.0|f|<D<3.0|f|(A)

If the distance D falls below the lower limit of the above condition, there is the possibility of rays reflected by the aspherical mirror being interfered with by the lenses system. On the other hand, if the distance D exceeds the upper limit of the above condition, the distance to the aspherical mirror is too great, resulting in the problems of the aspherical mirror becoming unnecessarily large, which prevents the rear projector from being made compact, and of alignment becoming difficult.

The lens system should preferably include, from the projection side, a front group with negative refractive power, a rear group with positive refractive power, and one of a flat mirror and a prism that bends an optical path between the front group and the rear group. A retrofocus-type combination of a front group with negative refractive power and a rear group with positive refractive power is suited to being telecentric on the incident side, having a long back focus, and projecting projection light generated or modulated by an image generating element such as a liquid crystal device or a DMD.

By providing a sufficient distance between the front group and the rear group but inserting a flat mirror or a prism for bending the optical path between the front group and the rear group, the entire length of the optical system in the direction in which the projection light is reflected from the aspherical mirror can be shortened, and therefore a projection optical system suited to a slim rear projector can be provided.

The front group should preferably include, from the projection side, a first lens group, which has a negative refractive power and includes at least a meniscus lens that is convex on the projection side, and a second lens group which has a positive refractive power. The lens system becomes a retrofocus-type that is negative, positive, and positive from the projection side and also has a power distribution suited to the lens system being made telecentric on the incident side. In addition, by using a construction where the first lens group includes at least a meniscus lens that is convex on the projection side, it is possible to provide a wide-angle lens system.

For example, the first lens group may include, in order from the projection side, a meniscus lens that is convex on the projection side, a meniscus plastic aspherical lens that is convex on the projection side, and a negative meniscus lens that is convex on the projection side. Also, the rear group may include, in order from an image generating device side (the opposite side to the projection side) thereof, a meniscus plastic aspherical lens that is convex on the image generating device side, a positive lens, and a plastic aspherical lens.

DESCRIPTION

FIG. 1shows the overall arrangement of a rear projector according to the one of the aspect of the present invention. The rear projector1includes, inside a housing2, a large screen9, an image generating device5including a light source3and a light modulator (light valve)4that modulates emitted light from the light source3based on an image signal (data or information) to generate images, a projection optical system10that projects projection light6from the image generating device5onto the screen9from the rear surface thereof, and a large flat mirror8that reflects the projection light6and guides the projection light6to the screen9. As the image generating device5, it is possible to use a CRT or other kind of a light-emitting optical device where the functions of the light source3and the functions of the light valve4are integrated. In rear projectors produced in recent years, a liquid crystal display (LCD) or a DMD is often used as the light valve4. The DMD includes micromirror elements that are driven to modulate the emitted light. In the rear projector1, a DMD panel is used as the light valve4, and since the light valve4is a reflective-type, the light source3is disposed on the same side of the light valve4as the optical system10.

FIG. 2is an enlarged view of the projector part provided in the lower portion of the rear projector1. It should be noted that although a lens system11is housed inside the housing2so as to be bent at an intermediate part as shown inFIG. 1, the lens system11is shown inFIG. 2in a state where the optical axis has been extended in a straight line to show the general arrangement of the lens system11. In the rear projector1, the optical path is bent by a mirror surface or the like. In the specification, the expressions “the screen9side” or “the image generating device5side” refer to positional relationships along the optical axis, and do not indicate positional relationships inside the housing2. So, instead of “the screen9side”, the expression “projection side” is sometimes used hereafter.

FIG. 3shows an enlargement of the optical system10. The optical system10includes the lens system11on which the projection light6that has been modulated by the light valve4of the image generating device5is incident and an aspherical mirror12with negative power as a whole. The aspherical mirror12is provided at a position facing the large flat mirror8on the optical paths between the lens system11and the screen9. The lens system11is composed of a front group GF with a negative refractive power that is disposed on the projection side (the screen9side), that is, the aspherical mirror12side and a rear group GR with a positive refractive power that is disposed on the image generating device5side. A certain distance is provided between the front group GF and the rear group GR, and the lens system11further includes a flat mirror13for bending the optical path, the mirror13is disposed between the front group GF and the rear group GR. The flat mirror13can be replaced with a prism.

The front group GF includes, in order from the projection side, that is, the aspherical mirror12side, a first lens group G1with negative refractive power and a second lens group G2with positive refractive power, and therefore the lens system11is a retrofocus-type composed of three groups that are negative, positive, and positive in order from the projection side. The retrofocus lens system11is telecentric on the input side, has a long back focus, and is constructed so as to be suited to the projection optical system10that projects and magnifies the projected light6with a liquid crystal device or a DMD as the light valve4.

The optical system10has rotational symmetry around the optical axis19and on the optical axis includes the aspherical mirror12and the lens system11from the screen9side (the projection side). The lens system11includes, from the aspherical mirror12side (the projection side), the first lens group G1and the second lens group G2that construct the front group GF, the rear group GR, and the flat mirror13that is disposed between the front group GF and the rear group GR for bending the optical path. Parallel glasses FG1and FG2disposed between the lens system11and the light valve4are optical low-pass filters.

The first lens group G1positioned furthest on the aspherical mirror12side has an overall negative refractive power and is constructed of three lenses that in order from the aspherical mirror12side are a positive meniscus lens L11that is convex on the aspherical mirror12side, a negative meniscus plastic aspherical lens L12that is convex on the aspherical mirror12side, and a negative meniscus lens L13that is convex on the aspherical mirror12side.

The second lens group G2is also composed of three lenses that in order from the aspherical mirror12side are a negative lens L21, a positive lens L22, and a negative meniscus lens L23that is convex on the light valve4side, the lenses L22and L23composing a cemented lens.

The rear group GR is composed of a total of eleven lenses that in order from the light valve4(the image generating device5) side that is the opposite side to the projection side are a positive meniscus plastic aspherical lens L41that is convex on the light valve4side, a positive lens L40, a negative plastic aspherical lens L39, and four cemented lenses respectively composed of a positive lens L38and a negative lens L37, a positive meniscus lens L36that is convex on the aspherical mirror12side and a negative meniscus lens L35, a positive lens L34and a negative lens L33, and a positive meniscus lens L32that is convex on the aspherical mirror12side and a negative meniscus lens L31. A stop S is also disposed on the aspherical mirror12side of the lens L33.

The design data for the optical system shown inFIG. 3is as follows. In the data, “r” represents the radius of curvature (expressed in mm) of the respective lenses and mirrors in order from the light valve4side, “d” represents the distance between the surfaces of the respective lenses and mirrors in order from the light valve4side, “nd” represents the refractive index (d line) of the respective lenses, and “vd” represents the Abbe number (d line) of the respective lenses. The abbreviation “inf” indicates a flat surface. The “Type” column shows whether a lens surface is spherical (SPH) or aspherical (ASP), while “AASP” shows that a lens surface is an aspherical surface expressed by an odd-order polynomial.

Both surfaces of the lens L41(the 5th and 6th surfaces when counting from the light valve4), both surfaces of the lens L39(the 9th and 10th surfaces), both surfaces of the lens L12(the 31st and 32nd surfaces) and the aspherical mirror12(the 35th surface) are aspherical. In addition, out of such surfaces, the 5th, the 32nd, and 35th surface (i.e., the aspherical mirror12) are aspherical surfaces expressed by odd-order polynomials. An aspherical surface is expressed by Expression (1) below. It should be noted that c has been substituted for 1/r. The value “h” is the distance from the optical axis.
z=ch2/(1+√(1+K)c2h2)+A4h4+A6h6+A8h8+A10h10(Expression 1)

Various parameters for the optical system10are as follows.

Combined focal length f (mm) of the lens system:4.82F number of the lens system:2.5Half-field angle (degrees) of the lens system:74.2Back focus length (mm) of the lens system:36.90Distance D (mm) between the aspherical mirror7.3012 side end of the lens system and the asphericalmirror 12:Condition (A):4.82 < D < 14.46

In the lens system11, the lens closest to the screen, that is, the lens L11that is located at an end of the projection side and next to the aspherical mirror12is a positive meniscus lens that is convex on the projection side, thereby making the lens system a wide-angle lens system with a half-field angle of 74.2°. Accordingly, by combining this lens system with the aspherical mirror12that has a negative power, it is possible to project the projection light6onto the screen9with a large incident angle. For a prism-type Fresnel screen, the incident angle should preferably be 45° or above, and in the rear projector1, light in the vicinity of the optical axis19of the optical system10is projected on a lower central part of the screen9with an incident angle of around 45.6°, so that a prism-type Fresnel screen can be used for the entire screen9.

An aspherical surface expressed by an odd-order polynomial is used for the aspherical mirror12. The aspherical surface is formed with negative values for coefficients for odd orders, and in particular the third and fifth orders, so that the aspherical mirror as a whole has a negative power. At the same time, the curvature of the aspherical surface in the radial direction decreases from a center of the mirror toward the periphery (outer circumference), with the sign (plus/minus) of the curvature becoming inverted on the way from the center to the periphery. That is, the curvature of the aspherical mirror12changes positive to negative on the way from its center to periphery. Accordingly, by reflecting the projection light6at the peripheral area (an edge part)12rof the aspherical mirror12that is distant from the optical axis19of the lens system11, scattering of the projected light6in the upper part and at the edges of the screen9is suppressed, thereby preventing a reduction in the resolution of the projected images.

Accordingly, by using the optical system10, it is possible to project the projection light6onto the entire screen9with a suitable incident angle, and therefore a prism-type Fresnel screen can be used for the entire screen, which makes it possible to display bright images at low cost. Since it is also possible to prevent resolution from falling in the upper part and at the edges of the screen, clear images can be displayed across the entire screen.

In addition, by suppressing scattering of the projected light6, aberration is favorably corrected for edge parts of the screen9and as shown by the longitudinal aberration inFIG. 4, the occurrence of distortion can be suppressed across the entire range in the height direction. Taking this characteristic into account also, the optical system10is further suited to displaying clear images.

Since the aspherical mirror12whose curvature in the radial direction falls from a center of the mirror toward the periphery with the sign of the curvature becoming inverted on the way is used, it is possible to favorably correct aberration across the entire screen using an optical system with rotational symmetry that is easy to align instead of using an optical system that is asymmetrical about the optical axis, such as a system including an anamorphic aspherical surface or a free-form surface.

In addition, in the optical system10, the distance between the lens system11and the aspherical mirror12can be shortened to a range that satisfies the condition (A) given above where the projection light6reflected by the aspherical mirror12is not interfered with by the lenses of the lens system11, which makes it possible to make the entire system compact. This means that the rear projector1that uses the optical system10can be made extremely slim, thereby realizing a slim image display apparatus with a large screen9.

Although a projector that includes an image generator that has DVD has been described, the present invention can be applied to LCD-type projector. Also, additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.