Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
1998-10-26
2001-02-27
Phan, James (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
C359S206100, C359S216100
Reexamination Certificate
active
06195189
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates a light beam scanning system which causes a light beam emitted from a light source and deflected by a deflector to scan a surface.
2. Description of the Related Art
A light beam scanning system is employed in an image read-out system and an image recording system in order to scan a recording medium by a light beam for reading out or recording an image on the recording medium. For example, in a radiation image read out system employing a stimulable phosphor sheet, the stimulable phosphor sheet is scanned by a stimulating light beam and light emitted from the stimulable phosphor sheet upon stimulation thereof is detected. As such a light beam scanning system, there has been known those comprising a light source, a deflector such as a rotary polygonal mirror for deflecting the light beam emitted from the light source, and a scanning/imaging optical system which causes the light beam deflected by the deflector to form an image on the surface of a recording medium and scan the same and is provided with an optical element such as an f&thgr; lens for causing the light to scan the surface at a constant speed and correcting curvature of field. An image is recorded on the recording medium or an image is read out from the recording medium by two-dimensionally scanning the surface of the recording medium by the light beam by causing the light beam to scan the surface at a constant speed in one direction (main scanning) while the recording medium is moved in a sub-scanning direction perpendicular to the main scanning direction.
In one type of such a light beam scanning system, the light beam is collimated by a beam shaper and the collimated light beam is caused to scan the surface as disclosed, for instance, in Japanese Unexamined Patent Publication No. 2(1990)-115814, and in another type, the light beam is converged to form a beam spot on the surface and the beam spot is caused to scan the surface in order to suppress curvature of field as disclosed, for instance, in Japanese Unexamined Patent Publication No. 4(1992)-361218.
However in the light beam scanning systems disclosed in the above identified patent publications where a collimated light beam or a converged light beam is caused to scan the surface, there has been a problem that when the light beam is deflected by an angle larger than 90° to scan a larger area of the surface, f&thgr; correction for ensuring that the light beam accurately scans the surface at a constant speed makes it impossible to accurately correct curvature of field. Though both the f&thgr; properties and the curvature of field can be accurately corrected by increasing the number of the lens elements in the scanning/imaging optical system, increase in the number of the lens elements increases the size of the system and adds to the cost.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object of the present invention is to provide a light beam scanning system in which both the f&thgr; properties and the curvature of field can be accurately corrected.
A light beam scanning system in accordance with the present invention comprises
a beam shaper means which shapes a light beam emitted from a light source,
a deflector means which deflects the light beam to scan a surface in a main scanning direction, and
a scanning/imaging optical system which causes the light beam deflected by the deflector to form an image on the surface and scan the same, and characterized in that
the beam shaper means comprises a diverging optical system which converts the light beam into divergent light.
In a preferred embodiment of the present invention, the scanning/imaging optical system consists of first and second lens elements arranged in this order from the side of the light source and having first to fourth surfaces as numbered from the side of the light source and satisfies the following formulae (1) to (3),
−1.42<f
all
/f
GF
<0 (1)
r
1GR
<0, r
2GR
<0, r
4GR
<0 (2)
f
GR1
>0, f
GR2
>0 (3)
wherein f
all
represents the composite focal length of the beam shaper means and the scanning/imaging optical system, f
GF
represents the focal length of the beam shaper means, r
1GR
represents the radius of curvature of the first surface of the scanning/imaging optical system, r
2GR
represents the radius of curvature of the second surface of the scanning/imaging optical system, r
4GR
represents the radius of curvature of the fourth surface of the scanning/imaging optical system, f
GR1
represents the focal length of the first lens element and f
GR2
represents the focal length of the second lens element.
Further it is preferred that the light beam scanning system of the present invention be provided with a surface tilt correction optical system for correcting fluctuation in pitches on the surface to be scanned due to tilt of the deflecting surface of the deflector means.
To correct distortion of an optical system is to cause the optical system to satisfy characteristics of f·tan &thgr; from definition of distortion. The relation between a scanning angle &thgr; and tan &thgr; is &thgr;<tan &thgr;, which naturally gives a relation of f·&thgr;<f·tan &thgr;. Accordingly in order to effect f&thgr; correction, it is necessary to intentionally undercorrect distortion by f·|&thgr;-tan &thgr;|.
When a lens system is used with an object point set nearer than a design criterion, there is generally a tendency for distortion to be undercorrected. Accordingly when divergent light is caused to enter the scanning/imaging optical system of the light beam scanning system, a state equivalent to setting nearer the object point, whereby distortion can be undercorrected. Thus by causing divergent light to enter the scanning/imaging optical system, f&thgr; properties can be corrected more accurately without increasing the number of lens elements in the scanning/imaging optical system.
Further when the beam shaper means and the scanning/imaging optical system satisfy formulae (1) to (3), curvature of field can be minimized with f&thgr; properties corrected accurately.
Further when the surface tilt correction optical system is provided, fluctuation in pitches on the surface to be scanned due to tilt of the deflecting surface of the deflector means can be corrected and the light beam can scan the surface at a high resolution.
REFERENCES:
patent: 3995110 (1976-11-01), Starkweather
patent: 5005928 (1991-04-01), Suzuki et al.
patent: 5276544 (1994-01-01), Maeda
patent: 5408095 (1995-04-01), Atsuumi et al.
patent: 2-115814 (1990-04-01), None
patent: 4-361218 (1992-12-01), None
Ishikawa Hiromi
Nishihata Sumihiro
Fuji Photo Film Co. , Ltd.
Phan James
Sughrue Mion Zinn Macpeak & Seas, PLLC
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