Projection lens and projector using the same

Details Projection lens and projector using the same Projection lens and projector using the same

1999-10-04

2001-02-13

Epps, Georgia (Department: 2873)

Optical: systems and elements

Lens

With field curvature shaping

C359S650000, C359S651000, C359S643000

Reexamination Certificate

active

06188522

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection lens, in particular, a projection lens for a projector that projects an image in a spatial optical modulator onto a screen in a larger size.
2. Description of the Prior Art
The following method conventionally is known to obtain large screen images. An optical image corresponding to an image signal is formed in a spatial optical modulator, and the optical image is irradiated with light so that the image is projected in a larger size onto a screen with a projection lens. Recently, a projector using a liquid crystal panel as the spatial optical modulator has been receiving attention. A projector using a reflection type liquid crystal panel as the spatial optical modulator can direct illumination light from a light source toward a projection lens more effectively.
As a method for obtaining color images, the following methods are known: a method where three liquid crystal panels corresponding to image signals of R (red), G (green) and B (blue) are used, and a dichroic prism is used to synthesize the images of the three liquid crystal panels; and a method where light from a light source is divided into R, G, and B based on the time.
The projector using a prism requires a large space between the lens and the spatial optical modulator, so that the projection lens is required to have a long back focal length.
The projector using a prism has large dependence on the angle of incidence from the prism. Therefore, the transmittance of a light ray that is incident at an angle other than a designated angle of incidence is changed, thus resulting in non-uniformity in color and brightness. Therefore, the projection lens requires telecentric properties.
In a projector for data display or graphics display that strictly requires high performance at the periphery of a screen, it is important to eliminate the distortion of the outlines of graphics or blur of colors. To meet this end, it is necessary to correct the distortion aberration and the magnification chromatic aberration sufficiently.
Brightness is required at the periphery, so that the light quantity ratio at the periphery is required to be 70% or more.
Japanese Laid-Open Patent Publication (Tbkkai-Hei) No. 2-228620 discloses a projection lens having a long back focal length and telecentric properties, where distortion aberrations are corrected. However, the following problem is caused.
The projection lens shown in the examples in this publication has a small half angle of view of 36°, so that the size of the set is too large to be used as a projection television set.
SUMMARY OF THE INVENTION
Therefore, with the foregoing in mind, it is an object of the present invention to provide a projection lens where the magnification chromatic aberration is corrected sufficiently, which is required strictly, especially for a lens for projection, by optimizing the lens configuration, and that can realize high quality images throughout a screen.
The present invention has achieved a projection lens that has a short projection distance and a long back focal length, and provides images with reduced blur of colors and distortion by optimizing the lens configuration, and thus achieved the above-described object.
More specifically, the projection lens of the present invention has the following features.
A projection lens according to a first embodiment of the present invention is a projection lens for projecting light from a light source onto a screen to form a magnified image, comprising two joined lenses, each of which includes a negative lens and a positive lens, and satisfying the following requirements:
(1) −0.04<ndI1-ndI2<−0.003
(2) 13.9<vdI1-vdI2<31
(3) −0.039<ndII1-ndII2<0.001
(4) 0.8<vdII1-vdII2<11.5
(5) −0.033<PgFII1-PgFII2<−0.009
where ndI1 represents the refractive index of the negative lens of the first joined lens;
ndI2 represents the refractive index of the positive lens of the first joined lens;
vdI1 represents the Abbe number of the negative lens of the first joined lens;
vdI2 represents the Abbe number of the positive lens of the first joined lens;
ndII1 represents the refractive index of the positive lens of the second joined lens;
ndII2 represents the refractive index of the negative lens of the second joined lens;
vdII1 represents the Abbe number of the positive lens of the second joined lens;
vdII2 represents the Abbe number of the negative lens of the second joined lens;
PgFII1 represents the partial dispersion ratio (ng-nF)/(nF-nC) with respect to the g-line and the F-line of the positive lens of the second joined lens; and
PgFII2 represents the partial dispersion ratio (ng-nF)/(nF-nC) of the negative lens of the second joined lens with respect to the g-line and the F-line.
This embodiment allows the first joined lens to correct the first-order magnification chromatic aberration and the second joined lens to correct the second-order magnification chromatic aberration.
In the projection lens of the first embodiment, it is preferable that no lens is present on the optical axis between the first joined lens and the second joined lens. This embodiment allows the principal ray heights of both of the first and second joined lenses to be low so as to facilitate the correction of the magnification chromatic aberration.
Furthermore, in the projection lens of the first embodiment, it is preferable that a projection magnification for projecting an optical image illuminated by a light source onto a screen is a factor of 10 to 300. With a factor of 10 to 300, projection can be performed on a large-scale screen with a compact set with a small spatial optical modulator.
Furthermore, in the projection lens of the first embodiment, it is preferable that an aperture stop is provided on the side opposite to the screen of the projection lens. This embodiment achieves a compact projection lens with a short total length of the lens and well-corrected magnification chromatic aberrations.
Furthermore, a projection lens including a 1
st
lens group with a negative power, a 2
nd
lens group with a weak power, and a 3
rd
lens group with a positive power are provided in this order from the screen side. An aperture stop is provided between the 2
nd
lens group and the 3
rd
lens group, and the projection lens according to the first embodiment is used as the 2
nd
lens group. This embodiment can provide a projection lens with a wide angle and well-corrected magnification chromatic aberration.
In the above-described projection lens, it is preferable to satisfy the following requirements:
(6) −0.35<f0/fI<−0.27
(7) 0.01<f0/fII<0.06
(8) 0.35<f0/fIII<0.38
where fI, fII and fIII represent the focal lengths f0 the 1
st
, 2
nd
, and 3
rd
lens groups, respectively, and of represents the focal length of the entire lens system.
This embodiment can provide a projection lens with a wide angle and well-corrected magnification chromatic aberration.
Furthermore, in the above-described projection lens, each of the 1
st
, 2
nd
and 3
rd
lens groups has at least one aspherical surface. This embodiment can correct distortion aberration, spherical aberration, and coma aberration satisfactorily, and provides a projection lens with a wide angle and well-corrected magnification chromatic aberration.
Furthermore, preferably, the projection lens constitutes an exit- side telecentric system. This embodiment provides a set without non-uniformity in colors, because the angle of incidence from the prism located between the projection lens and the spatial optical modulator is uniform regardless of the screen position.
Furthermore, preferably, the projection lens has an F number of not more than 3 and a half angle of view of not less than 40°. When the F number is not more than 3, a system can be bright to the maximum extent while suppressing the drop of the contrast, in the case where a liquid crystal panel or a reflection type mirror device is used as the spatial optical modulator. Furthermore, when the h

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