Image forming elements array, and optical printing head and...

Details Image forming elements array, and optical printing head and... Image forming elements array, and optical printing head and...

2000-02-18

2002-12-17

Pham, Hai (Department: 2861)

Incremental printing of symbolic information

Light or beam marking apparatus or processes

Scan of light

C347S258000, C359S626000

Reexamination Certificate

active

06496214

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an image forming elements array for use in a digital image writing optical system of image forming apparatuses such as digital copying machines, printers and facsimile machines, and an optical printing head and an image forming apparatus using the image forming elements array.
DISCUSSION OF THE BACKGROUND
Recently, because image forming apparatuses such as digital copying machines, printers and facsimile machines must be smaller, digital image writing apparatuses included in such image forming apparatuses must also be smaller. Generally, digital image writing systems are classified into two types: (1) an optical scanning system which scans a light flux emitted from a light source such as a semiconductor laser by deflecting the light flux with a deflector and form the deflected light flux into an optical spot with a scanning image forming lens; and (2) a solid scanning system which forms a light flux emitted from a light emitting elements array such as an LED array into an optical beam spot with an image forming elements array.
The optical scanning system requires a relatively long light path due to the use of an optical deflector to deflect a light flux from a light source so as to scan a scanned surface. Therefore, the size of an apparatus using a digital image writing apparatus including the optical scanning system is relatively large. On the other hand, the solid scanning system can make a light path relatively short, and thereby an apparatus using a digital image writing apparatus including the solid scanning system can be made relatively small. Further, the digital scanning system has advantage in that mechanical devices that have to be driven, such as an optical deflector, are not required.
Japanese Patent Laid-open Publication No. 10-153751 describes an example of such digital image writing apparatuses using a solid scanning system. As illustrated in FIG.
1
(A), an image forming elements array
90
included in the digital image writing apparatus includes a plurality of image forming elements which are arranged in an arrangement direction. Each image forming element has an incident surface
90
a
at the incident side, i.e., the light emitting elements array side, an emerging surface
90
b
at the emerging side, i.e., the scanned surface side, and a pair of reflective surfaces
90
c
, which are formed integrated with each other. The pair of reflective surfaces
90
c
are arranged at right angles relative to each other so as to define a roof-prism. The reflective surfaces
90
c
do not affect an imaging function and are slanted 45 degrees relative to the incident light axis.
A light flux emitted from a point of the surface of a light emitting element is transmitted to the incident surface
90
a
of the image forming elements array
90
, is sequentially reflected by the pair of reflective surfaces
90
c
and then emerges from the emerging surface
90
b
to reach a scanned surface of a photoconductor. The incident light axis and the emerging light axis are substantially at right angles to each other. An image at the point of the light emitting element surface is formed by the image forming function of the incident surface
90
a
and the emerging surface
90
b
at a corresponding point of the scanned surface of the photoconductor.
The image forming elements array
90
further includes, as illustrated in FIG.
1
(B), an aperture member
91
having a plurality openings
91
a
for optimizing the light quantity and for preventing cross talk between the adjacent image forming elements. The aperture member
91
has a plate-like shape having an L-shaped configuration in the cross-section, and as illustrated in FIG.
1
(C) the openings
91
a
are provided in both parts of the L-shaped plate at equal intervals in the direction in which the image forming elements are arranged so as to correspond to the incident surface
90
a
and the emerging surface
90
b
of each image forming element.
The aperture member
91
having an L-shaped plate configuration in the cross-section as described above shields a relatively large part of the light flux emitted from the light emitting elements array
90
, and as a result the transmission efficiency of the image forming elements array
90
is significantly decreased. Particularly, when the thickness of the aperture member
91
is about one n-th of the arrangement pitch of the image forming elements, or the diameter of the openings
91
a
of the aperture member
91
is smaller relative to the arrangement pitch of the image forming elements, the decrease of the transmission efficiency is remarkable.
The image forming elements array
90
is generally used as an imaging device to form an image of an original document, that is placed on an object plane, on a scanned surface of a photoconductor or surfaces of light receiving elements of a light receiving elements array. In order to form an high quality image of an original document on a scanned surface of a photoconductor, the scanned surface of the photoconductor is required to be appropriately lighted by the light flux reflected from the surface of the original document carrying information of the image. Therefore, when an aperture member is used with an image forming elements array for optimizing the light quantity and for preventing cross talk between the adjacent image forming elements of the image forming elements array, it is important to configure the aperture member so as not to decrease the transmission efficiency of the image forming elements array so that an appropriate quantity of light is maintained.
On the other hand, a light emitting elements array for use in a solid scanning system for a higher resolution, such as 600 dpi or higher, for example, can increase the quantity of an emitting light by increasing the inputting current to the light emitting elements array. However, an excessive inputting current over the limit may damage the light emitting element itself, and further, even when an increased inputting current does not exceed the limit, increase of a current causes increase of emitted heat and power consumption. Therefore, increasing the quantity of an emitting light of a light emitting elements array by increasing the inputting current has some limitation. Accordingly, it is desired that an image forming elements array effectively utilizes a light quantity allowed for a light emitting element of the image forming elements array so as not to decrease the transmission efficiency of the image forming elements array. When the aperture member
91
as described above is used, it is necessary to increase the diameter of the openings
91
a
of the aperture member
91
and at the same time to reduce the thickness of the aperture member
91
as much as possible. It may be ideal if the aperture member
91
can be eliminated, but, without the aperture member
91
, the image forming performance of an image forming elements array may be decreased and as a result, optical beam spots having a good circularity can not be formed reliably.
Further, a solid scanning system using an image forming elements array has a disadvantage in that it is difficult to obtain uniform density in a half-tone image. It has been known that non-uniform density in a half-tone image is caused periodically according to the arrangement pitch of image forming elements of the image forming elements array. A human being is most sensible to non-uniform density occurring in a low frequency area, for example, at about 0.2 to about 1 cycle/mm. The arrangement pitch of image forming elements corresponding to the low frequency cycle of about 0.2 to about 1 cycle/mm is about 1 mm to about 5 mm. Therefore, the arrangement pitch for image forming elements is desired to be made shorter than about 1 mm so that the above low frequency area is avoid.
The image forming elements array used in such a solid scanning system is classified into three types, a lens array having distributed refractive index, a lens array as described for example in Japanese Patent Laid-open Publication No. 6-3

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