Optical: systems and elements – Lens – With field curvature shaping
Reexamination Certificate
2002-03-28
2004-07-27
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With field curvature shaping
C359S650000
Reexamination Certificate
active
06768594
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a projection lens for making an extended projection of an image from a video generating source using a CRT or other projection tube on a screen and displaying the extended image on the screen and a projection type image display apparatus using the same, and more particularly to a projection lens apparatus preferable for obtaining a high-contrast and bright image superior in a focusing performance with very little image distortion and a rear projection type image display apparatus using the same.
In recent years, a tendency of horizontally widening of a screen of a television set as a home image display apparatus has brought with it a large-sized screen thereof. There are two types of this home image display apparatus; a direct-view type using a cathode-ray tube (CRT), and a rear projection type for making an extended projection of an image from a video generating source, a small projection tube (CRT) having a size of 5 to 7 inches or so, on a screen from the rear side using a projection lens apparatus. From a viewpoint of compactness, weight and cost of the set, a rear projection type image display apparatus is in the mainstream as one having a screen size in excess of 37 inches.
As a projection lens apparatus for use in a rear projection type image display apparatus, there are known projection lens apparatuses described in JP-A-7-159688 (hereinafter, referred to as a first conventional art) and in JP-A-9-159914 (hereinafter, referred to as a second conventional art), for example. In the first conventional art, there is disclosed a lens of construction 5-groups 5-elements having a spherical glass lens of a low dispersion and a high refractive index and four aspherical lenses, as a power lens having the strongest positive refracting power included in the third lens group
3
. In the second conventional art, there is disclosed a lens of construction 6-groups 6-elements having a spherical glass lens of a high dispersion and a low refractive index and five aspherical lenses, as a power lens.
The projection lens apparatus is required to have a short focal length in order to realize a compact set, to have a bright screen peripheral portion and a favorable focusing performance, and to be manufactured at low cost. In order to realize the low cost, it is the most effective to decrease the number of lenses in a lens construction to the minimum and to use an inexpensive optical glass lens as a power lens. In general, the higher the refractive index is and the lower the dispersion is, the more expensive the optical glass is.
The optical glass used for the power lens of the first conventional art is a low-dispersion glass having high refracting power, SK16. Regarding a price of this optical glass, the price of SK16 is twice or more times (2.1) that of SK5 as a reference (1.0), which is a typical optical glass for use in a projection lens apparatus. Therefore, in the first conventional art, the lens construction of 5-elements is applied to reduce the cost.
If an inexpensive spherical glass lens of a high dispersion and a low refractive index is used as a power lens for further cost reduction, however, not only it becomes harder to obtain desired refracting power, but a generated aberration amount also increases. Furthermore, since an aspherical plastic lens having a role of correcting aberrations has 10 or lower degree of an aspherical coefficient, an aberration correcting capability is insufficient due to restrictions of a permissible lens shape and the number of aspherical lens elements. As a result, while it is possible to realize a low cost, it becomes hard to favorably correct aberrations. It is hard to realize a low cost by using an expensive spherical glass lens of a low dispersion and a high refractive index.
A generated amount of an image distortion such as a distortion largely depends upon a positional relationship between a power lens (a glass lens) and an entrance pupil. The distortion is a phenomenon that an original image from a video generating source is projected in a form of a distorted image on a screen due to a difference between a magnifying power on an optical axis (paraxial) and a magnifying power in a peripheral portion. If the entrance pupil is located in the screen side of the power lens, a peripheral magnifying power is higher than the paraxial magnifying power and therefore it causes a pincushion distortion, while if the entrance pupil is located in the video generating source side of the power lens, the peripheral magnifying power is lower than the paraxial magnifying power and therefore it causes a barrel distortion. In other words, the distortion is caused by an increase of a difference between the peripheral magnifying power and the paraxial magnifying power since the chief ray passing through the center of the entrance pupil from each object point on the video generating source passes over a location apart from the optical axis of the power lens.
In the first conventional art, there is arranged a second group lens, which corrects a spherical aberration and a coma aberration, having weak negative refracting power in the screen side of the power lens. Therefore, if it is attempted to realize a further wide field angle (short projection distance), the position of the entrance pupil of the entire lens system shifts from the center of the third group lens toward the screen. This results in an increase of the distortion. Due to the aberration correcting capability of the aspherical plastic lens as set forth in the above, it is hard to correct the distortion favorably.
In the second conventional art, the cost reduction is possible to some extent due to a use of an inexpensive spherical glass lens having a high dispersion and a low refractive index as a power lens. The second conventional art, however, requires further aspherical lens for an aberration correction (one more element than the first conventional art) for favorably correcting the aberration increased by using the spherical glass lens having a high dispersion and a low refractive index. Therefore, it results in unsuccessful drastic cost reduction.
In addition, the projection lens apparatus is required to improve a contrast. Generally, importance is attached to an aberration correcting capability of a projection lens as a lens performance of a projection lens apparatus. As an image quality in a rear projection type image display apparatus, however, an improvement of a contrast indicating a white-and-black ratio of an image becomes an important element for determining whether or not the lens performance is good. To improve the contrast, there is a need for arranging respective lens groups spaced as far as possible from each other to prevent a reflected light (unnecessary light) in each lens group in the projection lens apparatus from returning to the original image. This results in an increase of aberrations in almost all cases, thereby requiring a higher correcting capability for correcting the aberration favorably. In the arrangements of the first and second conventional arts, it is hard to further improve the correcting capability, and therefore it is also hard to improve the contrast.
Generally, to satisfy conditions of a desired magnifying power and performance in such a case that a projection lens apparatus is used for a rear projection type image display apparatus having a wide angle of field, there is a need for arranging a glass lens having the strongest power among the lenses of the projection lens apparatus in the side of the video generating source. Accordingly, with a change of a relative position of the entrance pupil and the power lens (glass lens), a position of the chief ray from each object point on the video generating source changes and the image distortion and an astigmatism increases, by which it becomes hard to correct the aberration.
Furthermore, an arrangement of a power lens in the side of the video generating source extends a width of a luminous flux incident on a plastic lens having a role of correcting aberrations arra
Hirata Koji
Ikeda Hidehiro
Imafuku Daisuke
Kato Shuji
Nakagawa Kazunari
Epps Georgia
Hasan M.
Hitachi , Ltd.
Townsend and Townsend / and Crew LLP
LandOfFree
Projection lens apparatus and rear projection type image... 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 apparatus and rear projection type image..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Projection lens apparatus and rear projection type image... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3247331