Optical: systems and elements – Lens – Including a nonspherical surface
Reexamination Certificate
2002-06-05
2004-12-07
Spector, David N. (Department: 2873)
Optical: systems and elements
Lens
Including a nonspherical surface
C359S668000, C359S207110
Reexamination Certificate
active
06829104
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a resin-made non-spherical optical element, an optical scanning device using the optical element, and an image forming apparatus using the optical scanning device.
2. Discussion of the Background
Optical elements such as plastic lenses formed by plastic molding of a resin material are widely used for reducing weight and cost of an optical system or for shaping a surface of the optical elements in a special shape.
A shape of a coaxial non-spherical surface (a non-spherical shape having a rotational symmetry with respect to an optical axis), which is typical as a special surface shape, is generally expressed by the following formula (1) using: a distance in the direction orthogonal to an optical axis, r; a depth in the optical axis direction, f(r); a radius of curvature on the optical axis, R; and constant numbers, K, A
1
, A
2
, A
3
, . . . ,
f
(
r
)=(
r
2
/R
)/[1+{(1−(1
+K
)(
r/R
)
2
}]+A
1
r+A
2
r
2
+A
3
r
3
+A
4
r
4
+A
5
r
5
+A
6
r
6
+ (1)
As the number of terms of higher degree on the right side of equation (1) increases, the shape of the non-spherical surface becomes more complicated, so that correction of wave-front aberration can be more finely made.
A complicated shape of a non-spherical surface can be formed in a precise manner by plastic molding. However, as known, in plastic molding a surface sink mark and an internal distortion tend to occur in the periphery of a molded piece.
Accordingly, in plastic molding, anticipating that a surface sink mark and/or an internal distortion occur in the periphery of a molded piece, it is necessary to prevent the surface sink mark and/or the internal distortion from extending toward an effective area (e.g., in a lens, an area within an effective lens diameter in which the design optical function can be assured) of the molded piece.
Therefore, for example, when manufacturing a lens having a non-spherical surface by plastic molding, a sufficient margin is provided to the outside of the effective diameter of the lens, i.e., the outside of the effective area of the lens, so that even if a surface sink mark or an internal distortion occurs at the periphery of the margin, the effective area is not affected by such a surface sink mark or an internal distortion.
As the degree of a term of higher degree of (r) on the right side of the above formula (1) is higher, when a lens is relatively large, a change in the depth in the axial direction of the lens is excessively large at the periphery of the lens, so that a difference in the thickness of the lens greatly differs between the vicinity of the optical axis and the periphery of the lens. In this case, when making a metal mold for plastic molding, due to constraint of a contact angle between a tip end of a cutting bite and a surface of the metal mold and of a resolution of bite moving steps, a desired surface shape cannot be obtained in the metal mold. As a result, a sufficient margin cannot be obtained at the outside of an effective area of the lens surface, thereby causing a surface sink mark and/or an internal distortion in the plastic molding.
Further, because a time period for cooling differs between the center part and the peripheral part of the lens because of the above-described relatively large thickness difference, the above-described surface sink mark and/or internal distortion tend to occur often.
Furthermore, depending upon the non-spherical shape, when (r) is relatively large, it may occur that {(1−(1+K)(r/R)
2
} on the right side of the above formula (1) is negative and {(1−(1+K)(r/R)
2
} is an imaginary number, such that the non-spherical surface itself cannot be expressed. In this case also, a sufficient margin cannot be obtained outside of the effective area of an optical surface.
For example, a lens constituting an f&thgr; lens as a scanning image forming lens system of an optical scanning device may be relatively large such that the lens diameter in the main scanning direction exceeds 200 mm. In such a case, the above-described problem tends to occur rather often.
As a method for avoiding the above-described problem, in optical designing, a design may be made for a range broader than an effective area that is actually needed. However, the optimum optical performance may be shifted toward an outside of an effective diameter of a light ray, and as a result, the optical performance in the effective area may deteriorate.
Further, a straight line part (i.e. a flat part) that is discontinuous from a shape of an effective area may arise outside of the effective area. In this case, however, because a boundary between the effective area and an area outside of the effective area is discontinuous, in molding, the surface accuracy deteriorates in the vicinity of the boundary.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above-discussed and other problems and addresses the above-discussed and other problems.
Preferred embodiments of the present invention provide a novel resin-made non-spherical optical element in which a relatively large area is provided outside of an effective area on its optical surface having a non-spherical shape so that an adverse affect of a surface sink mark and/or an interior distortion will not extend to the effective area.
Further, the preferred embodiments of the present invention provide an optical scanning device capable of performing a satisfactory optical scanning by using such a resin-made non-spherical optical element, and an image forming apparatus capable of performing a satisfactory image formation by using the optical scanning device.
A resin-made non-spherical optical element according to an embodiment of the present invention is a resin-made optical element made by plastic molding, in which at least one optical surface is formed in a non-spherical shape. The optical surface of the optical element having the non-spherical shape includes an effective area and an area located outside of the effective area.
Here, the non-spherical shape can include not only a coaxial non-spherical shape but also a non-arc shape in which the shape in the main scanning cross-section and/or the shape in the sub-scanning cross-section are non-arc. Further, the non-spherical shape includes a sub-scanning non-arc shape in which the non-arc shape in the sub-scanning cross-section changes in the main scanning direction. For example, a cylindrical surface and a toric surface have non-spherical shapes.
The optical surface here means a surface formed substantially in a mirror-surface-like surface by a metal mold with plastic molding. Rib parts and gate parts are excluded from the optical surface.
The effective area here is an area on an optical surface where the optical performance of the optical surface is assured. For example, in a lens for condensing a deflected light flux onto a scanned surface in optical scanning, the effective area is a combined area of an area where a light flux scanning the optical surface for forming an image on the scanned surface passes, an area where a light flux for synchronization detection passes, and an area into which a light flux may be shifted due to tolerances of the parts.
Thus, because the effective area of an optical surface is an area where an optical performance of the optical surface is assured, surface accuracy, internal distortion, and alien substances in the effective area must be managed so that the optical performance is assured in the effective area.
The area outside of the effective area is an area outside of the effective area on the above-described optical surface. The optical surface shape in the area outside of the effective area is set to smoothly continue to the optical surface shape in the effective area, and is formed in a shape different from the non-spherical shape in the effective area. Accordingly, in the area outside of the effective area, the above-described optical per
Atsuumi Hiromichi
Suzuki Seizo
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