Optical: systems and elements – Lens – With variable magnification
Reexamination Certificate
2002-02-06
2004-04-06
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With variable magnification
C359S618000, C359S662000, C359S740000, C359S823000
Reexamination Certificate
active
06717740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical system for scanning light beams, and an image recording exposure device for recording an image on an exposure surface by performing simultaneous multiple main-scans on the exposure surface.
2. Description of the Related Art
In a conventional image recording device, an image is recorded on a photosensitive material by disposing at a predetermined distance scan-units facing a portion of the circumferential surface of a cylindrical drum (outer drum), carrying out main-scanning on a surface of the photosensitive material by high speed rotation in a state in which the photosensitive material is wrapped around the circumferential surface of the cylindrical drum and, during this high speed rotation of the drum, carrying out sub-scanning on the surface of the photosensitive material by moving the scan-units in an axis direction of the drum while maintaining the distance of the scan-units. In this image recording device, it has been considered that a plurality of light sources (a light source having a broad light emission area structured by point light sources arranged in at least one direction) is arranged at the scan-units, and light beams transmitted from the light sources are arranged in a sub-scanning direction to thereby record an image on the photosensitive material while forming main-scan lines simultaneously. As a result, high speed processing is made possible.
Further, the aforementioned light sources are useful when a light source which emits light with high energy is needed, such as with a thermal photosensitive material.
In such an image recording device, when a distance between the spots of respective light beams is changed (switched) in accordance with a desired resolution, it becomes necessary to change the magnification of an exposure lens which is arranged at each scan-unit.
During this process, when the magnification of the exposure lens is made higher, a converging angle that is formed by light beams which are condensed (focused) on an image recording surface (scanning surface) becomes larger, and a depth of focus which is acceptable for the exposure decreases.
FIGS. 9A
to
9
D show an example of an optical system for explaining the change of the depth of focus.
As shown in
FIG. 9A
, a divergent light which is emitted from a light source
100
is changed to a parallel light at a first lens
102
, and is focused through a second lens
104
onto a scan surface
106
. At this point, the light beams form a converging angle &thgr;
1
.
FIG. 9B
shows an enlarged view of a focus point at this time. An allowable spot diameter (beam diameter) d1 is defined, and thus a width (focal depth L1) for obtaining this allowable spot diameter d1 is determined.
FIG. 9C
shows a state in which the magnification is made higher by moving the second lens
104
in the direction of an optical axis (the imaginary line position in
FIG. 9C
shows the state of FIG.
9
A).
At this time, it is noted that a converging angle &thgr;
2
formed by the light beams which are condensed by the second lens
104
is greater than the converging angle &thgr;
1
.
FIG. 9D
shows an enlarged view of the focus point at this time. A width (depth of focus L2) for obtaining a defined allowable spot diameter d2 (which equals d1) is smaller than L1.
In order to solve this problem, removing any other expected factors that may cause errors (even when the depth of focus is low) can be considered. The any other factors that may cause errors include eccentricity of the outer drum, an amount by which image surfaces of the scan-units are curved, and the like.
Mitigation of such errors depends on accuracy with which mechanical parts are manufactured, accuracy with which the same are assembled, and complicated control of the device, so that the manufacturing cost of the device becomes high. This is particularly unsuitable for image recording devices which are desired to be manufactured inexpensively.
Due to the introduction of AF (auto-focus) mechanisms, the aforementioned accuracy requirement can be eased. However, the AF mechanism is deficient in reliability because of a problem with responsiveness. Moreover, because control of the AF mechanism itself is complicated, it is difficult to solve the problem of high manufacturing costs.
SUMMARY OF THE INVENTION
In view of the aforementioned facts, an object of the present invention is to obtain a scanning optical system having a simple structure, capable of preventing depth of focus from decreasing when magnification of the optical system is changed, especially when the magnification is increased, and capable of scanning while maintaining a range of acceptable spot diameter for all magnifications.
Another object of the present invention is to obtain an image recording exposure device which has an optical system for simultaneously scanning multiple light beams and in which, even with a change of resolution, the depth of focus can be maintained without causing a reduction of light amount, such that a spot diameter being scanned on a recording surface is kept within an acceptable range.
A first aspect of the present invention is an optical system for use in scanning a surface, the optical system comprising: a light source which emits a light beam; a group of lenses which condenses the light beam emitted from the light source to a surface to be scanned; a magnification changer for changing magnification of the group of lenses; and an aperture provided on an optical path of the light beam and having an opening in which only a portion of the light beam is transmitted therethrough.
A second aspect of the present invention is an optical system for use in scanning a surface, the optical system comprising: a light source including a broad light emission area comprising point-like light sources arranged in at least one direction; a lens group which condenses a light beam emitted from the light source to a surface to be scanned; a magnification changer which moves at least one lens of the lens group along an optical axis direction of the light beam for changing magnification of the lens group; and an aperture provided on an optical path of the light beam and including an opening which transmits only a portion of the light beam therethrough.
In accordance with the first and second aspects of the present invention, an amount of the luminous flux corresponding to an amount of widening of a converging angle at a converging point, due to an increase of magnification, is shielded by the aperture. Thus, the luminous flux that reaches the scanning surface forms almost the same converging angle as before the increase of magnification. Consequently, depth of focus can be prevented from decreasing.
For this case, intensity distribution of a laser beam L at a position at which an aperture is arranged is shown in FIG.
1
. The intensity distribution has a mountain-like characteristic whose central portion is the highest intensity and which deteriorates toward a periphery (foot of the mountain). Thus, if a peripheral edge portion of this intensity distribution is shielded by an aperture, the total light amount does not decrease in direct proportion to the area shielded by the aperture.
As shown in
FIG. 2
, assuming a Gaussian distribution, a radius of an opening portion
28
of an aperture arranged on an optical path of r, a light amount of 1/e
2
, and the radius r and a light transmissivity T each normalized as 1, when the aperture opens at a radius r other than 1, a light amount ratio of the radius r to the light transmissivity T has a relationship close to a direct proportion. When the radius r is ½ (i.e., the area is ¼), the light amount is also ½.
If the magnification is increased, regardless of the shape of the opening portion, a total light amount in a broad area direction is subjected to basic geometry, a converging spot diameter becomes smaller, and the light amount in unit area increases. If the broad area direction corresponds to the sub-scanning direction, the converging s
Choi William
Epps Georgia
Fuji Photo Film Co. , Ltd.
Sughrue & Mion, PLLC
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