Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Reexamination Certificate
2002-09-30
2004-09-21
Cherry, Euncha P. (Department: 2872)
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
Reexamination Certificate
active
06795223
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to a scanning optical system, and more particularly to a scanning optical system comprising optical deflection means for deflecting light coming from a light source, so that the surface to be scanned is two-dimensionally scanned.
Exemplary prior scanning optical systems are shown in
FIGS. 10 and 11
. The scanning optical system shown in
FIG. 10
(JP-A 08-327926) uses a condensing optical system comprising collimator lens
52
, slit
53
and cylindrical lens
54
, through which light leaving light source
51
is collimated and guided to rotary polygon mirror
55
. The light reflected and deflected at rotary polygon mirror
44
is directed to image-formation lens
56
composed of two lens elements, so that image-formation surface
57
is subjected to one-dimensional scanning.
The scanning optical system shown in
FIG. 11
(JP-A 08-146320) uses collimator lens
62
for collimating light leaving light source
61
into a parallel light beam, which is then reflected and deflected by deflection means
63
, so that the surface
65
to be scanned is subjected to two-dimensional scanning by image-formation means
64
.
However, the optical system of
FIG. 10
, because of being constructed of a considerable number of optical elements, places strict limitations on the precision of assembling and adjustment to achieve the necessary optical performance, and incurs some added expenses as well. For the optical system of
FIG. 11
, on the other hand, nothing is disclosed about its specific arrangement.
SUMMARY OF THE INVENTION
Having been accomplished to provide a solution to such problems with the prior art as mentioned above, the present invention has for its object to provide a scanning optical system of small size, which is constructed of a reduced number of optical elements.
According to the first aspect of the present invention, the aforesaid object is achieved by the provision of a scanning optical system comprising optical deflection means for deflecting light from a light source to scan the surface to be scanned and an image-formation optical system for focusing the light deflected by said optical deflection means on the surface to be scanned, thereby forming an image thereon, characterized in that:
said image-formation optical system comprises an optical member wherein a surface thereof having optical power and located nearest to the surface to be scanned has a transmission function alone, and
said optical member comprises two or more reflecting surfaces, each of which has optical power and includes at least one rotationally asymmetric surface decentered with respect to an axial chief ray.
This scanning optical system is exemplified by Examples 1 to 6 given later.
The advantages (effects and actions) of the scanning optical system according to the first aspect of the invention are now explained. By allowing the optical member to comprise two or more reflecting surfaces, each of which has optical power and includes at least one rotationally asymmetric surface decentered with respect to an axial chief ray (hereinafter called the decentered, rotationally asymmetric surface), the “turn-back” effect is obtained so that the size of the optical system can be much more reduced than ever before. The reflecting surfaces of optical power, because of having both a lens action and a deflection action, contribute significantly to size reductions.
Referring here to an optical system comprising a rotationally symmetric reflecting surface having optical power and decentered with respect to an axial chief ray, light rays strike obliquely on that reflecting surface. Even with axial rays, accordingly, aberrations such as comas and astigmatisms are produced due to decentration. Such decentration aberrations may be corrected by configuring this reflecting surface in the form of a rotationally asymmetric surface as contemplated herein.
A problem with a general scanning optical system is that when light deflected by optical deflection means is entered on a decentered, rotationally symmetric surface, it is impossible to ensure any linear scan capability. However, this linear scan capability can be ensured by configuring the reflecting surface of an image-formation optical system in the form of a rotationally asymmetric reflecting surface.
Further, the use of the rotationally asymmetric surface enables the image-formation optical system to be formed of a two-dimensional f arcsine &thgr; lens or a two-dimensional f&thgr; lens. Consequently, the surface to be scanned can be easily subjected to constant-speed, two-dimensional scanning.
When optical deflection means with the angle of deflection changing linearly, such as a rotary polygon mirror, is used, an f&thgr; lens may be used as the image-formation optical system capable of producing minus distortions. Consequently, the surface to be scanned can be scanned at a constant speed. When optical deflection means with the angle of deflection changing sinusoidally, such as a galvanometer mirror, is used, the image-formation optical system may be configured as an f arcsine &thgr; lens by allowing it to produce distortions depending on the magnitude of the angle of deflection (plus distortion when the angle of deflection is small, and minus distortion when the angle of deflection is large). Consequently, the surface to be scanned can be subjected to constant-speed scanning.
In this case, the surface of the image-formation optical system, which has optical power and is located nearest to the surface to be scanned, is effective for correction of distortions because there is a large difference in light ray position between the angles of view, with a light beam of reduced diameter. It is noted that the function of this surface on correction of distortions becomes worse if this surface is designed to have a function of transmitting light and a function of reflecting light or to have a function of transmitting light and a function of transmitting light, because some restrictive conditions are placed on such a surface. It is noted that the surface formed of a single surface and designed to produce a plurality of optical functions will hereinafter called a combined surface. Thus, if that surface is designed to have a single optical function alone, i.e., only a transmission function as contemplated herein, it is then possible to make effective correction for distortions. It is also easy to ensure the angle of view.
According to the second aspect of the present invention, the scanning optical system of the first aspect is further characterized in that said optical member is configured in the form of a prism member.
This scanning optical system is exemplified by Examples 1-6 given later.
The advantages of the scanning optical system are now explained. Generally speaking, a reflecting surface must be more strictly controlled in terms of decentration errors than a refracting surface, and so its adjustment on assembling is an onerous task. However, if the reflecting surface of the optical member is configured as one surface of the prism member, then this problem can be solved because the whole positioning of the reflecting surface becomes easy.
Light rays incident from the deflection means on the prism member are refracted at the entrance surface of the prism member, so that the heights of off-axis light rays incident on the subsequent surfaces can be kept low. It is thus possible to reduce the size of the optical system and achieve a larger angle of view as well. In addition, the height of light rays depending on the off-axis light rays becomes so low that comas or the like can be reduced.
According to the third aspect of the invention, the scanning optical system of the first aspect is further characterized in that said optical member comprises at least one surface which has a function of transmitting light and a function of reflecting light. This surface should preferably be defined by a surface other than that located nearest to the surface to be scanned.
This scanning optical system is exemplified by Exampl
Cherry Euncha P.
Kenyon & Kenyon
Olympus Corporation
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