Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2003-11-24
2004-11-23
Nguyen, Thong Q (Department: 2872)
Optical: systems and elements
Lens
With field curvature shaping
C359S649000, C359S784000
Reexamination Certificate
active
06822804
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a projection lens, and more particularly, to a projection lens used for a liquid crystal projector that magnifies and then projects an image produced by liquid crystal valves of liquid crystal display (LCD) elements. The present invention is also related to a liquid crystal projector incorporating the projection lens therein.
2. Prior Art
A liquid crystal projector produces image data of three primary colors, B (blue), G (green) and R (red) on its respective light valves, and then, after letting light fluxes of the image data of the primaries pass through a cross-prism to refract the light fluxes into optically coaxial fluxes, directs them upon a single projection lens to have a projection image on a screen. Since the cross-prism is disposed between the liquid crystal panel and the projection lens, the projection lens of the liquid crystal projector is required to take a long back focus.
To produce a clear image from light beams of G, B and R in color entering and exiting a dichroic mirror, it is desired that the projection lens is strongly telecentric to propagate a principal light beam with merely slight inclination. Also to make a produced image clear, aberration, especially, astigmatism, abaxial spherical aberration (coma), distortion, and the like must be corrected.
When the liquid crystal projector is to serve as a rear projector to cast light from behind the screen, it is additionally desired that the liquid crystal projector is compact and particularly incorporates the projection lens of a short focal length that enables a wide-angle projection.
An exemplary projection lens in the prior art is 47.00 mm in focal length, 154.99 mm in back focus, and 71.48 degrees in angular field and is highly telecentric in a section of reduction (e.g., see, as a whole, Patent Document 1 as identified with Japanese Patent Laid-Open No. H09-68650).
Various features of the projection lens disclosed in the Patent Document 1 are generally unsatisfactory to make up a currently demanded compact liquid crystal projector capable of projecting a clear image, and therefore, a solution to this is a projection lens that especially has a longer back focus enabling a wide-angle projection, with astigmatism, abaxial spherical aberration (coma), distortion aberration and the like being sufficiently corrected.
The present invention is made in view of the aforementioned disadvantages of the prior art projection lens, and accordingly, it is an object of the present invention to provide a projection lens of back focus long enough to enable a wide-angle projection with astigmatism, abaxial spherical aberration (coma) , distortion, and the like being sufficiently corrected.
SUMMARY OF THE INVENTION
The present invention provides a projection lens that includes first to third groups of lenses G
1
, G
2
and G
3
. The first group of lenses G
1
consist of four pieces of lenses, and the first or foremost negative one of the lenses is an aspheric lens. The second group of lenses G
2
consist of two pieces of lenses, and the first or foremost negative one and the second positive one of the lenses are joined together. The third group of lenses G
3
consist of six pieces of lenses, and the second foremost positive one, the third negative one, and the fourth positive one of the lenses are joined together. The projection lens is characterized in that the following formulae are satisfied;
bf/f≧
2.8 (1)
1.1
≦f
2
/f
3
≦1.6, and (2)
1.65
≦|f
1
|/f≦
2.05, (3)
where f
1
, f
2
and f
3
are focal lengths unique to the first, second and third groups of lenses G
1
, G
2
and G
3
, respectively, and f and bf are total focal length and back focus of the whole optics, respectively.
The present invention also provides a liquid crystal projector having three liquid crystal panels for three primary colors in combination with the projection lens.
In the projection lens according to the present invention, a condition of (1) bf/f≧2.8, leads to a longer back focus that ensures a space occupied by optical elements such as a dichroic mirror (cross-prism) as well as a shorter focal length that enables a wide-angle projection of which short projection distance further enables the projection in a-restricted space. Unless the condition is satisfied, it is impossible to ensure a sufficient back focus.
Another condition of (2) 1.1≦f
2
/f
3
≦1.6 guarantees a sufficient back focus with various aberrations being well balanced. Under a condition of f
2
/f
3
≦
1
.
1
, G
2
is relatively greater in refractive index than G
3
, and this causes an insufficient back focus and an unsatisfactorily corrected spherical aberration. This also causes a negative field curvature and a positive distortion. When a condition of 1.6<f2/f3 is satisfied, G
2
is relatively weaker in refractive index than G
3
, and this brings about a sufficient back focus but instead causes a positive field curvature.
Another condition of (3) 1.65≦|f
1
|/f≦2.05 ensures a sufficient back focus and also guarantees satisfactory corrections of abaxial spherical aberration and astigmatism as well as a smaller aperture of the first group of lenses G
1
. A condition of |f
1
|/f<1.65 is advantageous in ensuring a sufficient back focus but has adverse effects of a greater refractive index of G
1
and a greater positive field curvature, especially, a positive transit of a sagittal image surface. Under a condition of 2.05<|f
1
|/f, the aperture of the lens group G
1
should be greater to space the lens groups G
1
and G
2
farther from each other for a longer back focus.
The first or foremost one of the four pieces of lenses in the first group G
1
has its surface shaped negatively aspheric to reduce distortion.
The first or foremost negative one and the next one of the two pieces of lenses in the second group G
2
are joined together to successfully correct chromatic aberration.
The second foremost positive one, the third negative one, and the fourth positive one of the six pieces of lenses in the third group G
3
are jointed together to successfully correct magnification aberration.
REFERENCES:
patent: 6317269 (2001-11-01), Yamamoto
patent: 6445511 (2002-09-01), Kitahara
patent: 6452728 (2002-09-01), Shikama
patent: 6560041 (2003-05-01), Ikeda et al.
patent: 09068650 (1997-03-01), None
Jacobson & Holman PLLC
Nguyen Thong Q
Tamron Co., Ltd.
LandOfFree
Projection lens and liquid crystal projector does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Projection lens and liquid crystal projector, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Projection lens and liquid crystal projector will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3283648