Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2000-02-02
2001-06-05
Sugarman, Scott J. (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S713000
Reexamination Certificate
active
06243211
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a projection lens system and particularly to a projection lens system with a wide field angle which provides a bright image having an excellent focus performance even in the marginal area, uses an inexpensive glass material, and has a short projection distance and a projection image display apparatus using the system which is excellent in cost performance.
Recently, a television set as an image display apparatus for home use is proceeding to a larger screen size as the wide aspect ratio increases. As an image display apparatus for home use, there are two types available a direct view type using a cathode ray tube and a so-called projection type for enlarging and projecting an image from a miniature projection tube, whose screen size is about 7 inches of (diagonal), via a projection lens system. Due to restrictions concerning compactness and weight of a TV set, for a screen size of more than about 37 inch of diagonal, a projection image display apparatus is mainly used.
At first, this projection image display apparatus was inferior to the direct view type in screen brightness and focus performance. However, recently, the performance of each of the components such as the projection lens system, screen, and projection tube is improved, so that both the screen brightness and focus performance are approaching those of the direct view type. In the performance improvement process of the projection image display apparatus, various arts have been developed in the projection lens system which is a key device. First, to obtain screen brightness equivalent to that of the direct view type or higher, as disclosed in U.S. Pat. No. 4,682,862, reduction of the F-number has been tried by using many plastic aspherical lens elements.
Second, a projection lens system has been developed for realizing improvement of screen brightness and improvement of focus performance at the same time. With respect to this projection lens system, as disclosed in Japanese Patent Application Laid-Open No. 3-137610, there is an example using a plastic aspherical lens and a doublet glass lens. As a result, in the current projection TV set, a projection lens system having an F-number of about f/1.1 is used and both brightness and focus performance are improved on the whole screen.
At the third step, a projection lens system with a wide field angle by which a compact set dimension can be realized on account of the short projection distance has been developed mainly. A reference describing an actual art for realizing a projection lens system with a wide field angle without reducing the brightness and focus performance in the marginal area and an actual projection lens system is disclosed in Japanese Patent Application Laid-Open No. 4- 5608. Hereinafter, the art disclosed in this patent is referred to as a first prior art.
In this first prior art, by combining plastic aspherical lens elements and glass lens elements effectively in a projection lens system of six lens groups, the aforementioned problem is solved. Furthermore, the projection lens system is structured so that almost all the positive refractive power of the projection lens system is shared by the glass lenses and the plastic aspherical lens elements have little refractive power, so that the peculiar drift of the focus performance due to a temperature change is reduced even if the plastic aspherical lens elements are used is reduced.
In this first prior art, the profile of the fluorescent face of projection tube has a curvature so that it is convex on the electron gun side. As a result, the projection lens system is structured so that the normal of the fluorescent face in the marginal area is in the direction of the entrance pupil of the projection lens system and can fetch more light fluxes in comparison with the case using a flat fluorescent face. Therefore, even if the field angle is widened, a relative illuminance of a level which is almost no problem practically can be obtained in the marginal area.
The curvature of field is corrected by the lens element of the sixth lens group (hereinafter referred to as sixth lens). However, if the fluorescent face of projection tube has a curvature so that it is convex on the electron gun side, the generation amount of curvature of field is reduced and the focus performance in the marginal area is improved.
Furthermore, a projection lens system with a wide field angle which realizes a more excellent focus performance without reducing the brightness in the marginal area and an actual art for realizing it are disclosed in U.S. Pat. No. 5,272,540. Hereinafter, the art is referred to as a second prior art.
In the second prior art, a projection lens system having a constitution of five groups by six elements is disclosed and the profile of the fluorescent face of projection tube which is an object is an aspherical profile which is convex on the electron gun side. The curvature of the profile in the marginal area is smaller than that in the neighborhood of the optical axis. By doing this, highly precise correction of the curvature of field and astigmatism are compatible with each other and the satisfactory focus performance and the light amount which is sufficient in practice are reserved in the marginal area of screen.
In this projection lens system, the lens element of the third lens group (hereinafter referred to as third lens) sharing almost all refractive power of the overall lens system, has a concave lens of large dispersion glass and a convex lens of small dispersion glass stuck together, and the chromatic aberration is corrected; the large aperture (the F-number is 0.96) and the high focus performance are compatible with each other. Furthermore, the combination of the lens element of the first lens group (hereinafter referred to as first lens) and the lens element of the second lens group (hereinafter referred to as second lens) offsets the lowering of the focus performance generated by deformation and expansion of each lens element due to temperature change and humidity change which is an intrinsic problem when plastic lens elements are used.
On the other hand, in a conventional projection lens system, as a lens barrel for assembling each lens element with high precision, a lens barrel having the constitution disclosed in, for example, Japanese Utility Model Application Laid- Open No. 2-51478 is often used. The lens barrel of the prior art has an outer barrel and an inner barrel which is installed inside the outer barrel and can slide in the direction of optical axis of the lens without axial shift. The inner barrel is constructed so that it can be divided into two parts longitudinally in the direction of diameter of the lens along the optical axis of the lens and it has slits for holding a plurality of lens elements at predetermined intervals with high precision on its inner surface.
In the aforementioned projection lens system having a constitution of six lens groups of the first prior art, there are several problems to be solved.
The first problem is caused by the lens constitution. In the aforementioned projection lens system, the third lens having negative refractive power is arranged on the screen side of the lens element of the fourth lens group (hereinafter referred to as fourth lens) sharing almost all the positive refractive power of the overall lens system. The spherical aberration and coma aberration are corrected by the third lens.
Therefore, the location of the entrance pupil of the overall lens system moves to the screen side from the center of the fourth lens. As a result, if an attempt is made to realize a wider field angle (reduction of the projection distance) in the aforementioned lens constitution, correction of the distortion and astigmatism becomes difficult.
Next, the second problem is a point that if an attempt is made to reduce the F-number or (increase the aperture ratio) of the projection lens system having this lens constitution and obtain a sufficient marginal light amount ratio, the apertures of the first, second, and third
Hirata Koji
Mori Shigeru
Nakagawa Kazunari
Ogura Naoyuki
Yoshida Takahiro
Evenson, McKeown, Edwards & Lenahan P.L.L.C.
Hitachi , Ltd.
Schwartz Jordan M.
Sugarman Scott J.
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