Incremental printing of symbolic information – Electric marking apparatus or processes – Electrostatic
Reexamination Certificate
2000-07-10
2002-10-15
Lee, Susan S. Y. (Department: 2852)
Incremental printing of symbolic information
Electric marking apparatus or processes
Electrostatic
C347S130000, C347S258000
Reexamination Certificate
active
06466245
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a rod lens array and also to an optical printer head and an optical printer using such rod lens array. A rod lens array according to the invention is adapted to cause a plurality of light emitting elements to emit light and form highly accurate and sharp images of the elements on the surface of a photosensitive member that operates as a recording medium. A rod lens array according to the invention can suitably be used in an optical printer head or an optical printer comprising an LCD (liquid crystal display elements) or LEDs (light emitting diodes) to name a few.
2. Related Background Art
Rod lens arrays are known as conventional oblong imaging element arrays that are used in liquid crystal printers and LED printers for forming a correct life-size image of an object. A rod lens array is prepared by arranging a plurality of rod lenses cyclically in a predetermined direction, which rod lenses radially exhibit a refractive index distribution.
FIG. 1
of the accompanying drawings shows a schematic cross sectional view of a known rod lens array taken along a direction perpendicular to the optical axes of the rod lenses. In
FIG. 1
, reference symbol
30
denotes a rod lens array and reference symbol
31
denotes a rod lens. A rod lens
31
is normally formed by subjecting a rod-shaped piece of glass to an ion-exchange process so as to make it exhibit a refractive index distribution. A plurality of rod lenses
31
are densely arranged in the horizontal direction (hereinafter referred to as main-scanning direction) in two rows that are disposed one on the other in the vertical direction (hereinafter referred to as sub-scanning direction). Each of the rod lenses
31
is adapted to form a correct life-size image of an object at a position separated from it by a predetermined distance.
The plurality of rod lenses
31
are densely arranged and aligned relative to each other by means of a pair of side panels
32
. Subsequently, the gaps separating the rod lenses
31
are filled with opaque resin
33
to securely hold the rod lenses in position.
Normally, the side panels
32
are made of fiber-reinforced plastic or FRP. Therefore, the surfaces of the lateral walls
32
generally show undulations of a dimension of 6 to 8 &mgr;m reflecting the profile of the interwoven fibers contained therein.
On the other hand, known rod lens arrays are often accompanied by a problem of disorder of lens arrangement as seen from
FIG. 1
that arises in the manufacturing process because so many rod lenses are put there.
FIG. 2
is a schematic cross sectional view of a principal portion of an optical printer realized by using such a rod lens array, illustrating the image forming effect of the rod lens array in an ideal state. In
FIG. 2
, the components the same as or similar to those in
FIG. 1
are denoted respectively by the same reference symbols and will not be described any further.
Referring to
FIG. 2
, light emitting means (an array of light emitting elements)
45
comprises a plurality of light emitting elements that are adapted to emit respective light beams according to the image signal applied thereto. Then, the light beams are focused to respective imaging spots
44
on the surface of a photosensitive drum
46
operating as a recording medium to form a correct life-size image of an object by means of a rod lens array (imaging device)
30
comprising a plurality of rod lenses
31
arranged in an array. With this arrangement, a latent image of the object can be formed on the surface of the photosensitive drum of the optical printer. Note that the optical printer head of the optical printer includes the light emitting element array
45
and the rod lens array
30
. It has been believed that such a rod lens array does not produce any disorder of imaging spots on the photosensitive drum
46
if the arrangement of the rod lenses involves disorder to a slight extent because the rod lenses form a correct life-size image.
Meanwhile, in response to the increased demand for high definition printers in recent years, optical printer heads and optical printers providing a high resolution of 600 dpi or 1,200 dpi are currently being marketed. At the same time, while high definition optical printers adapted to deal with binary data have so far been in the main stream, those adapted to produce pictorial color images are appearing in the market. In other words, halftone images are becoming more and more important to optical printers. In line with this trend, means for electrically correcting the problem of unevenness existing in the optical printer including that of an uneven distribution of the quantity of light of light-emitting dots have been devised to improve the image quality of optical printers particularly in terms of halftone images.
However, while efforts have been paid to accurately correct the problem of uneven distribution of the quantity of light, halftone images produced by optical printers have often encountered a problem of creasy marks to degrade the image quality, although the reason for this problem is still unknown. As a result of research efforts, the inventor of the present invention has found that the problem of creasy marks is different from that of the uneven distribution of the quantity of light and attributable to an inaccurate arrangement of rod lenses of the rod lens array of the optical printer.
However, if a perversive factor or a distortion that can partially damage the magnification is involved in any of the rod lenses of the array as shown in
FIG. 3
, the light beam emitted from the rod lens involving such a problem is not focused at the right spot to give rise to a phenomenon referred to as a broken dot or a plump dot, which can give rise to creasy marks on the produced image at the time of development.
FIG. 3
is a schematic cross sectional view of a principal portion of an optical printer realized by using a rod lens array, illustrating the image forming effect of the rod lens array when some of the rod lenses
51
involve a perversive factor. In
FIG. 3
, reference symbol
50
denotes a rod lens array and reference symbol
52
denotes a side panel, while reference symbols
55
and
54
denote a light emitting element array and a spot formed by a focused light beam, respectively.
Since known rod lens arrays are not required to have a high resolving power, the problem of a broken dot is not serious if it arises. However, in the case of an optical printer head or an optical printer required to have a resolution of 600 dpi or more, any cyclical changes in the formation of dots can result in an uneven density distribution of the obtained image that normally involves half tones to give rise to a serious problem of creasy marks.
The inventor of the present invention also found that, when the side panels are made of FRP that has a cyclical structure, the uneven density distribution of the obtained image is significantly influenced by cycle of the beat produced by the cycle of arrangement of the rod lens array and that of the cyclical structure of the side panels. This invention is based on these findings.
SUMMARY OF THE INVENTION
Therefore, it is an object of the present invention to solve the above-identified technological problems of the prior art and provide a rod lens array that is practically free from the problem of degraded image quality due to the uneven density distribution of the obtained image if it is used for an optical printer.
Another object of the present invention is to provide en optical printer head and an optical printer that can produce high quality images by using a rod lens array according to the invention.
According to the invention, the above objects are achieved by providing a rod lens array comprising:
a plurality of rod lenses arranged cyclically in a predetermined direction;
an opaque substance filled in the gaps separating the plurality of rod lenses; and
a pair of side panels sandwiching the plurality of rod lenses and the opaque substance, the side panels having a cyclic
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