JP2986851B2 - Line image forming lens in optical scanning device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a line image forming lens in an optical scanning device.

2. Description of the Related Art An optical scanning device that deflects a light beam using a rotating polygon mirror has a problem of so-called "plane tilting." Formed as a long line image in the main scanning corresponding direction near the deflecting reflection surface
An anamorphic lens system is used as an anamorphic lens system that forms an image of a light beam deflected by the rotating polygon mirror as a light spot on the surface to be scanned. Relationship "has been done.

The line image forming lens is a lens for forming a parallel light beam from the light source device as a line image long in the main scanning direction in the vicinity of the deflecting reflection surface of the rotary polygon mirror in the optical scanning device as described above.

[Problem to be Solved by the Invention] The linear image forming lens is usually formed by one convex cylinder lens.

In order for the line image forming lens to function properly, it must be arranged in a proper posture. If the arrangement state of the line image forming lens is incorrect, there are various problems. In particular, when there is an optical axis shift (a shift of the optical axis due to the tilt of the lens) in the sub-scanning corresponding direction (direction having power), the scanning locus of the light spot, the change of the optical scanning position, the change of the light spot diameter, etc. This has a serious adverse effect on optical scanning.

For this reason, conventionally, the mounting of the line image forming lens has been troublesome.

The present invention has been made in view of the above-described circumstances, and has as its object to provide a novel line image forming lens that has a small optical axis deviation even with a slight inclination.

[Means for Solving the Problems] Hereinafter, the present invention will be described.

The line image forming lens according to the present invention `` forms a parallel light beam from the light source device into a long line image in the main scanning corresponding direction, and converts the light beam by a rotating polygon mirror having a deflecting reflection surface near the image forming position of the line image. "Lens for forming a linear image" in an optical scanning device that performs optical scanning by deflecting at a uniform angular velocity and forming a deflected light beam as a light spot on a surface to be scanned by an imaging optical system.
It is.

This line image forming lens is formed as a single convex cylinder lens. The lens surface on the light source side is a convex cylinder surface and the lens surface on the rotating polygon mirror is a flat surface. The center thickness t, the refractive index n of the lens material, and the focal length f _CY are (I) (t / f _CY ) <0.2 (excluding t / f _CY = 0.026) (II) The condition of 1.5 <n <1.9 is satisfied. I do.

The above conditions (I) and (II) are conditions for suppressing the deviation of the optical axis in the sub-scanning corresponding direction due to the inclination of the line image forming lens in the sub-scanning corresponding direction.

[Operation] In FIG. 1, reference numeral 10 denotes a convex cylinder lens which is a line image forming lens.

As shown in the figure, the lens surface on the light source side is a convex cylinder surface, and the lens surface facing the rotating polygon mirror is a flat surface. t represents the center thickness of the convex cylinder lens 10, and n represents the refractive index of the lens material.

The front principal point H of the convex cylinder lens 10 is the intersection of the convex cylinder surface and the optical axis, and the rear principal point H 'is located at a position separated from the convex cylinder surface by t {1- (1 / n)}. is there.

The vertical direction in FIG. 1 corresponds to the sub-scanning direction of optical scanning, and when a parallel light beam from the light source device is incident on the convex cylinder lens 10, a long line in the main scanning corresponding direction, that is, the direction orthogonal to the drawing in FIG. That is, an image is formed.

FIG. 2 shows a state in which the convex cylinder lens 10 shown in FIG. 1 is inclined by an angle θ in a plane including the optical axis and the direction corresponding to the sub-scanning.

In this state, the optical axis ray emitted from the convex cylinder lens 10 is shifted from the incident optical axis ray by X = D · tan θ. This is the optical axis shift in the sub-scanning corresponding direction described above. Note that D represents the interval between the principal points H and H ', specifically, the above-mentioned t {1- (1 / n)}.

Therefore, in order to reduce the optical axis shift in the sub-scanning corresponding direction due to the inclination of the line image forming lens, D may be reduced. Or decrease t and increase n.

Specifically, the thickness t is a condition (I) with respect to the focal length _fCY .
Is satisfied, and the refractive index n satisfies the condition (II).

When the center thickness t is increased beyond the range of the condition (I), the optical axis shift becomes extremely large.

When the value exceeds the upper limit of the condition (II), uniformity of glass quality as a lens material cannot be secured. If the lower limit is exceeded, the optical axis deviation cannot be reduced sufficiently even if the center thickness t is reduced.

[Examples] Specific examples will be described below.

What matters in the present invention is an optical axis shift in the sub-scanning corresponding direction, and the optical axis shift is related to the center thickness and the refractive index n as described above. Therefore, in designing an actual line image forming lens, the above conditions (I) and
Free setting is possible as long as (II) is satisfied.

r ₁ represents the radius of curvature of the convex cylinder surface. The radius of curvature r ₂ of the lens surface of the rotary polygon mirror side is ∞ Example _{1 r 1 = 306.708, t =} 101,6, n = 1.51118, designed with _{f CY = 600 t / f CY} = 0.169 The condition In the case of the line image forming lens, the optical axis deviation is 0.1 for the inclination angle θ of the line image forming lens of 10 ′.

Example 2 r ₁ = 130.351, t = 50.8, n = 1.51118, f _CY = 255 t / f _CY = 0.199 In the case of a line image forming lens designed under these conditions, the inclination angle θ of the line image forming lens is: The optical axis deviation is 0.05 for 10 '.

From a comparison between Examples 1 and 2, it can be seen that when the refractive index is the same, the effect of suppressing the optical axis deviation is smaller as the center thickness is smaller.

[Effects of the Invention] As described above, according to the present invention, a novel line image forming lens can be provided.

Since this line image forming lens is configured as described above, the optical axis shift in the sub-scanning corresponding direction due to the tilt of the lens is small, so that the accuracy of the lens tilt at the time of mounting is eased and the mounting operation is facilitated. .

[Brief description of the drawings]

FIG. 1 is a diagram for explaining a line image forming lens, and FIG. 2 is a diagram for explaining an optical axis shift. 10 ... Line imaging lens

Claims (1)

(57) [Claims] 1. A parallel light beam from a light source device is formed into a long line image in the main scanning direction, and the light beam is deflected at a constant angular velocity by a rotary polygon mirror having a deflecting / reflecting surface near an image forming position of the line image. A lens for imaging the line image in an optical scanning device that performs optical scanning by imaging the deflected light beam as a light spot on a surface to be scanned by an imaging optical system, wherein the light source side has a convex shape. The lens is formed as a single convex cylinder lens having a flat surface on the rotating polygon mirror side on the cylinder surface, and the center thickness t, the refractive index n of the lens material, and the focal length f _CY are caused by the inclination of the lens in the sub-scanning corresponding direction. Conditions for minimizing optical axis deviation in the sub-scanning direction (I) (t / f _CY ) <0.2 (excluding t / f _CY = 0.026) (II) 1.5 <n <1.9 Line imaging lens.