Optical: systems and elements – Single channel simultaneously to or from plural channels – By surface composed of lenticular elements
Reexamination Certificate
2000-01-21
2002-04-02
Schwartz, Jordan M. (Department: 2873)
Optical: systems and elements
Single channel simultaneously to or from plural channels
By surface composed of lenticular elements
C359S619000, C359S621000, C359S654000
Reexamination Certificate
active
06366408
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an optical imaging system used in an image transmission portion of, for example, a facsimile device or a copier.
BACKGROUND OF THE INVENTION
Optical imaging systems which include a plurality of rod lenses with a refractive index distribution in a radial direction and are arranged in an array, and those that contain a homogeneous lens array of convex microlenses that are arranged regularly and have a predetermined curvature on their front and back sides are widely used in the image transmission portion of, for example, facsimile devices or copiers.
Lenses used for rod lens arrays often have diameters of 0.6 to 1.1 mm, and a resolving power demanded from such a rod lens array called for an MTF (modulation transfer function) of at least 60% when a pattern of spatial frequency of 4-6 line-pairs / mm(ca. 200 dpi-300 dpi) is imaged.
The refractive index distribution of such rod lenses can be expressed as:
n(r)
2
=n
0
2
·{1−(g·r)
2
+h
4
·(g·r)
4
+h
6
·(g·r)
6
} Eq. 1
wherein r is the radial distance from the optical axis of the rod lens, n(r) is the refractive index at the radial distance r from the optical axis of the rod lenses, n
0
is the refractive index at the optical axis of the rod lens (center refractive index), and g, h
4
and h
6
are coefficients for the refractive index distribution.
The brightness of the rod lenses depends mainly on the aperture angle &thgr; (°), which can be expressed by
&thgr;=(n
0
·g·r
0
)/(&pgr;/180). Eq. 2
wherein r
0
is the radius of the portion of the rod lenses functioning as a lens.
The larger the aperture angle &thgr; is, the brighter the achieved image and the shorter the time required for scanning. The largest aperture angle &thgr; found in available rod lenses is 23°.
In the case of rod lenses used for one-to-one imaging, spherical aberration and image surface curvature are the main cause of deterioration of the resolving power. Spherical aberration on the optical axis can be corrected by optimizing the refractive index distribution. However, since rod lenses consist basically only of convex lenses, the Petzval sum becomes large, and thus the image surface curvature cannot be corrected. In addition, because a plurality of lens images are superimposed in a rod lens array, blurred images are superimposed on each other when there is image surface curvature, which leads to a considerable deterioration of the resolving power. Because the image surface curvature is proportional to the square of the aperture angle &thgr;, as the aperture angle increases (that is, the brighter the lens is), the influence of image surface curvature increases.
Recently, because of the improved image quality of printers and scanner, a resolving power of at least 12 line-pair/mm (ca. 600 dpi) is demanded of such rod lens arrays. Therefore, it is necessary to suppress the image surface curvature to a minimum in order to improve the resolving power.
There are two methods for reducing the image surface curvature to improve the resolving power. A first method is to use rod lenses with a small aperture angle &thgr;. For example, if the diameter of the rod lenses is 0.6 mm, and the aperture angle is 10° or less, the influence of the image surface curvature is so small that it can be ignored. However, reducing the aperture angle &thgr; makes the image darker, which causes the problem of longer scanning times.
A second method for reducing the image surface curvature to improve the resolving power is to reduce the diameter of the rod lenses. The variation of the focal point due to image surface curvature becomes smaller in proportion with the diameter of the rod lenses, so that the resolving power can be improved even when bright rod lenses with a large aperture angle &thgr; are used. However, when the diameter of the rod lenses is reduced, the distance WD between the rod lenses and the image plane becomes small, so that there is the problem that there is too little space to arrange for an illumination system or a sensor device. Moreover, the precision required for assembling the rod lens array becomes extreme, which becomes a factor for rising costs.
Besides rod lens arrays, homogenous lens arrays, in which roof lens arrays of convex lenses aligned with a reflector or two lens array plates in which convex microlenses are arranged regularly in the front side and back side of a transparent plate with a uniform refractive index are aligned, are also known as optical elements for one-to-one imaging. However, since these optical elements as well consist basically only of convex lenses, the Petzval sum becomes large, as pointed out above, and there is the same problem of lower resolving power due to image surface curvature as in rod lens arrays with refractive index distribution.
Moreover, if an erect one-to-one image is to be attained by using a homogenous lens array, adjacent lenses have to be separated, so as to prevent the adverse effect of transfer images due to stray light from adjacent lenses entering the lens faces arranged on one optical axis. For this case, Publication of Unexamined Japanese Patent Application No. Sho 55-90908 discloses a method, in which the lenses arranged on the optical axis are rod-shaped, and separated by a different material disposed between the rod lenses.
However, homogenous erect one-to-one lens arrays using rod lenses have a complex configuration, and rising costs due to complex steps and performance variations caused by their assembly cannot be avoided.
SUMMARY OF THE INVENTION
The present invention has been achieved in order to solve the above-mentioned problems of the prior art, and its object is to provide an optical imaging system whose resolving power can be improved by eliminating image portions with large image surface curvature. It is a further object of the present invention to provide an optical imaging system for erect one-to-one imaging using a homogenous material that can be easily molded into a lens array, and in which stray light does not enter adjacent lenses.
In order to attain these objects, an optical imaging system for focusing light from an object plane onto an image plane in accordance with the present invention includes a lens array having a plurality of optical lens systems that are arranged in at least one row with optical axes of the lens systems in parallel; and further includes, at least in a longitudinal direction of the lens array, means for blocking light rays that pass through the lens faces of the optical lens systems at an angle larger than a predetermined angle with respect to the optical axes of the optical lens systems. With this configuration, light rays that pass through the lens faces of the optical lens systems at an angle larger than a predetermined angle with respect to the optical axes of the optical lens systems are blocked in a longitudinal direction of the lens array, so that the aperture angle becomes smaller. Thus, a portion of the image with large image surface curvature is eliminated, and the resolving power of the lens array is improved.
In this configuration of an optical imaging system, it is preferable that light rays that pass through the lens faces of the optical lens systems are not blocked in a direction that is perpendicular to the longitudinal direction of the lens array.
In this configuration of an optical imaging system, it is also preferable that the optical lens systems include rod lenses having a refractive index distribution in a radial direction. It is also preferable that both edge portions of the rod lenses in the longitudinal direction of the lens array are shaved off along the optical axis of the rod lenses. In this configuration, light rays that pass through the lens faces of the optical lens systems at an angle larger than a predetermined angle with respect to the optical axes of the optical lens systems are blocked in a longitudinal direction of the lens array, so that the aperture angle becomes smaller. Thus, a portion of the image with large image surface cur
Kawamoto Shinji
Kittaka Shigeo
Nemoto Hiroyuki
Nippon Sheet Glass Co. Ltd.
Schwartz Jordan M.
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