JP2756125B2 - Scanning line bending correction method, and scanning optical system in which scanning line bending is corrected by the method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION The present invention relates to a method for correcting a scanning line in a scanning optical system applied to various printers, facsimile machines, and the like, and a scanning optical system in which the scanning line is corrected by this method. About the system.

[Conventional technology]

A first imaging optical system that linearly forms an image of a light beam emitted from a light source, an optical deflector having a deflecting / reflecting surface set in the vicinity of an imaging position of the first imaging optical system, A medium to be scanned which is scanned by the light beam deflected by the deflector, and which is disposed on the optical path of the light beam and between the medium to be scanned and the light deflector, and is continuously deflected by the light deflector. The light beam is imaged on the medium to be scanned while maintaining the optically conjugate relationship between the deflecting reflection surface of the optical deflector and the medium to be scanned in a plane perpendicular to the deflecting surface that is the trajectory surface of the light beam to be scanned. There is known a scanning optical system having a second imaging optical system for causing the scanning.

In FIG. 8 illustrating an example of the scanning optical system, a light beam emitted from a light source 1 is linearly imaged by a cylindrical lens 2 as a first image forming optical system. On the other hand, one of the plurality of deflecting and reflecting surfaces of the optical deflector 3 is set so as to be sequentially located according to the rotation in the vicinity including the image forming position.

As described above, the line image formed on the deflecting reflection surface 4 is the second image.
The light is emitted through an fθ lens 51 and an fθ lens 52 as an imaging optical system to form an image as a point image on an image plane 7 on a medium to be scanned, and according to the rotation of the optical deflector 3 in the arrow direction. Is scanned on a straight line 8. This scanning is called main scanning, and the direction is called main scanning direction XX. Further, scanning in the direction orthogonal to the main scanning direction XX on the image plane 7 is referred to as sub-scanning, and that direction is referred to as sub-scanning direction YY.

If, for example, the fθ lens is decentered in such a scanning optical system, a scanning line that should be scanned like a straight line 8 is a curved scanning line as indicated by reference numeral 8 ′.

Here, eccentricity refers to the lens limb shown in FIG.
If the one shown by the broken line is the ideal lens limb without eccentricity, it means that the eccentricity S is shifted to the state shown by the solid line. The eccentricity that greatly affects the scanning line bending is mainly accompanied by a situation where the lens surface is shifted in the sub-scanning direction YY.

 In FIG. 9, reference numerals 0-0 indicate the optical axis of the lens.

[Problems to be solved by the invention]

In a conventional monochrome printer, facsimile, or the like, the scanning line bending is about 0.2 to 0.5 mm, but such a value is allowed.

However, in recent years, as the development of wide printers, high-precision plate making plotters, and color printers that perform writing with a plurality of beams has been performed, the scanning line bending has affected the image quality as a writing displacement.

For example, in a color printer using a scanning optical system for performing writing for each color, as shown in FIG.
Each of the writing scanning lines 8Re, 8G, and 8B containing each information of the blue image may be curved with respect to the ideal linear scanning line 8, and each may cause a so-called bending of the scanning line.

The scanning line 8Re is in the sub-scanning direction Y with respect to the reference scanning line 8.
The amount of bending dR upward in −Y, the scanning line 8B is in the sub-scanning direction Y
Since −Y has a downward bending amount dB, the maximum difference between them is dR + dB, and a color shift corresponding to this difference occurs.

FIG. 10B shows an example of a wide printer. In this example, two scanning lines by the scanning optical system for the A2 size width on the left and right are combined to synthesize the writing of the A0 size width.
Since the scanning lines 81 and 82 are curved in the directions d1 and d2 different from each other with respect to −Y, a total of d1 + d2 seam shifts occur in the virtual line NN corresponding to the seam of the two scanning lines. Impair.

To eliminate such color shifts and seam shifts,
It is necessary to make the eccentric intersection of the lens and the intersection of the mounting surface of the lens extremely severe.

However, if the intersection is made strict, the processing cost is increased this time, and there is a problem in that the demand for lowering the price of various recording apparatuses using the main scanning optical system cannot be met.

According to the present invention, it is possible to easily correct the bending of the scanning line to such an extent that the image quality is not affected without strictly controlling the eccentric intersection of the lens, the intersection of the mounting surface, and the like, and without increasing the cost. It is an object of the present invention to provide a method for correcting a scan line.

The bending of the scanning line is caused not only by the eccentricity of the lens but also by tilting of the lens, that is, by rotating the lens about an arbitrary axis in the main scanning direction in the lens. However, the bending in this case can be easily eliminated by a combination of the eccentricity in the sub-scanning direction and the eccentricity in the optical axis direction (shift of the lens surface). Further, the eccentricity in the optical axis direction is not a problem because the influence on the bending of the scanning line is small.

[Means for solving the problem]

In order to achieve the above object, in the present invention,
(A) First for forming a light beam emitted from a light source into a linear image
An image forming optical system, an optical deflector having a deflecting / reflecting surface set near an image forming position by the first image forming optical system, and a medium to be scanned scanned by a light beam deflected by the optical deflector. ,
On the optical path of the light beam, disposed between the medium to be scanned and the light deflector, and in a plane perpendicular to a deflection surface as a trajectory surface of the light beam continuously deflected by the light deflector. A scanning optical system having a second imaging optical system that forms an image of the light beam on the scanning target medium while maintaining the deflection reflecting surface of the optical deflector and the scanning target medium in an optically conjugate relationship; (2) As a method of correcting the bending of the scanning line due to the eccentricity of the lens in the image forming optical system, the second image forming optical system has a magnification in a direction orthogonal to the scanning direction by the light beam deflected by the optical deflector. Among the lenses of the system, for the lens having the lens surface with the largest magnification, ΔS is the correction amount, K is a coefficient determined by the magnification of the lens surface, S is the eccentricity of the lens surface, and the subscript n
Is the lens number of the lens constituting the imaging optical system including the lens having the lens surface having the largest magnification in the second imaging optical system, and the subscript number is the lens surface number. The lens is decentered in the sub-scanning direction by the correction amount ΔS calculated by the formula obtained by (Kn ₁ · Sn ₁ + Kn ₂ · Sn ₂ ) / (Kn ₁ + Kn ₂ ), and is fixed. Item 1), (b). A first imaging optical system that linearly forms an image of a light beam emitted from a light source, an optical deflector having a deflecting / reflecting surface set in the vicinity of an imaging position of the first imaging optical system, A medium to be scanned which is scanned by the light beam deflected by the deflector, and which is disposed on the optical path of the light beam and between the medium to be scanned and the light deflector, and is continuously deflected by the light deflector. The light beam is imaged on the medium to be scanned while maintaining the optically conjugate relationship between the deflecting reflection surface of the optical deflector and the medium to be scanned in a plane perpendicular to the deflecting surface that is the trajectory surface of the light beam to be scanned. As a scanning optical system having a second imaging optical system, the bending of the scanning line caused by the eccentricity of the lens in the second imaging optical system is orthogonal to the scanning direction of the light beam deflected by the optical deflector. Direction, and the largest magnification among the lenses of the second imaging optical system. For a lens having a lens surface, ΔS is the correction amount, K is a coefficient determined by the magnification of the lens surface, S is the amount of eccentricity of the lens surface, and the suffix n is the second imaging optical system in which the magnification is the largest. When the lens number and the suffix of the lens constituting the imaging optical system including the lens having the lens surface are the lens surface number, ΔS = (Kn ₁ · Sn ₁ + Kn ₂ · Sn ₂ ) / (Kn ₁ + Kn ₂ ) The lens is decentered in the sub-scanning direction by the correction amount ΔS calculated by the calculation formula obtained in the above (2), and is fixed.

(Operation)

The setting position of the lens is adjusted a posteriori with the greatest influence on the scan line bending.

〔Example〕

First, as a reference for comparison, FIG. 3 shows an imaging relationship when there is no eccentricity.

This figure shows the scanning optical system shown in FIG. 8 in a section (sub-scanning section) passing through the lens optical axis and in the sub-scanning direction.

As shown in the figure, the fθ lens has a two-element configuration as indicated by reference numerals 51 and 52. The first fθ lens 51 has a spherical surface 51-1 on one side and a cylinder surface 51-2 on the other side, and the second fθ lens 52 has a cylinder surface 52-1 on one side and a toroidal surface 52-1 on the other side. 2 is comprised.

The luminous flux imaged linearly on the deflecting reflection surface 4 (the direction of the line is the main scanning direction YY and a direction perpendicular to the paper surface in FIG. 3) is
After being deflected, it is expanded by the lens function of the spherical surface 51-1 and the toroidal surface 52-2 to form a virtual image at a virtual image position V1 in space. Note that a principal ray passing through the lens optical axis is denoted by a reference character CR.

In general, this light beam is converged by the toroidal surface 52-2 having the highest power in the sub-scanning direction YY and is imaged at a predetermined position 8F on the image surface 7.

When the deflecting and reflecting surface 4 rotates in response to the rotation of the optical deflector, the optical axis on the image plane 7 scans in a direction perpendicular to the paper surface through the predetermined position 8F, that is, in the main scanning direction Y-Y. However, since it is assumed here that the lens has no eccentricity, the scanning line does not bend, so that this scanning locus does not deviate from the predetermined position 8F when projected on the paper surface of FIG. .

Next, let us consider a case where the fθ lens 52 has no eccentricity on the cylinder surface 52-1 and is decentered only on the toroidal surface 52-2 by the amount of eccentricity S in the sub-scanning direction YY. FIG. 4 shows the image forming relationship in this case together with the case where there is no eccentricity. In FIG. 4, the toroidal surface without eccentricity
52-2 is indicated by a broken line, and the eccentric toroidal surface 52-2 'is indicated by a solid line.

In FIG. 4, considering that there is no eccentricity, the principal ray CR that has advanced on the lens optical axis continues straight after passing through the toroidal surface 52-2 as shown by the broken line.

On the other hand, considering the case of eccentricity, the principal ray CR passes through the toroidal surface 52-2 ', and then is orthogonal to the lens optical axis passing through a point on the lens optical axis away from the focal length f of the toroidal surface 52-2. It bends so as to advance to a position shifted in the sub-scanning direction Y-Y from the lens optical axis by the amount of eccentricity S on the plane.

The light beam after being bent only in the sub-scanning direction is referred to as a main light beam C.
In other words, the principal ray CR and the principal ray C form an angle φ on the sub-scanning cross section.

Assuming the situation shown in FIG. 4 above, the imaging relationship, including other optical systems, is shown on the main scanning section which is a section passing through the lens optical axis and orthogonal to the sub-scanning section. It is.

FIG. 5 shows a state in which the principal ray C forms an image at a point 8C on the image plane 7 with an image height ratio of 0 from the lens optical axis, and the image on the image plane 7 near the scanning end having an image height ratio of about 1. It is shown that the image is formed at the point 8R. The ray contributing to the formation of the latter point 8R is the principal ray R.

In FIG. 5, the distance from the toroidal surface 52-2, which is the final surface of the fθ lens 52, to the image surface 7 is denoted by LC. Depending on optical conditions, the principal ray C and the principal ray R are respectively toroidal surfaces.
The light exits at a right angle to 52-2. Here, the intersection between the toroidal surface 52-2 and the principal ray C is indicated by a ring portion 52-2C, and the intersection between the toroidal surface 52-2 and the principal ray R is indicated by a ring portion 52-2R.

Therefore, the angle formed by the principal ray R with respect to the principal ray C on the principal scanning plane is defined as a scanning angle θ, and the position at which the principal ray C exits from the toroidal surface 52-2 on the same plane in the lens optical axis direction. If the amount is Δ, the distance LR from the time when the principal ray R is emitted from the toroidal surface 52-2 to the point 8R on the image plane 7 is LR +
(Δ + LC) / cosθ.

Next, the eccentric toroidal surface 52- shown in FIG.
FIG. 6 (a) shows the image forming relationship by 2 'projected onto the sub-scanning cross section with respect to the principal ray C and principal ray R in FIG.

FIG. 6B is an enlarged view of a main part of FIG. 6A. In FIG. 6 (a), of the eccentric toroidal surface 52-2 ', the contour of the surface emitting the principal ray C is denoted by reference numeral 52-2'C, and the contour of the surface emitting the principal ray R is denoted by reference numeral. Indicated by 52-2'R.

In FIGS. 6 (a) and 6 (b), the principal ray C is on the sub-scanning cross-section and is shown exactly as in FIG. 4, and is shown to be bent by an angle φ from the lens optical axis. here,
φ = tan ^-1 (S / f) (see FIG. 4).

On the other hand, the angle formed by the principal ray R with the main scanning plane is also the angle φ as with the principal ray C. This is because the chief ray always exits at right angles to the toroidal surface 52-2 in FIG. 5, and the lens power on that surface remains unchanged under the angle θ. However, when such a principal ray R is projected onto the sub-scan section, it appears as an angle ω that is larger than the angle φ as shown in FIG. 7A. This angle ω is ω = tan ⁻¹ (S / f · cos
θ). As described above, the principal ray R is more curved than the lens ray, and the contour 52-2'R of the lens surface is shifted by the shift amount Δ.

FIG. 7 (a) is a collective representation of the above-described FIGS. 4 to 6 on a sub-scanning cross section. Therefore, the seventh
Consider how the scanning line bends according to FIG.

In FIG. 7 (a), the symbol "R (rotation)" indicates the fifth point around the center of the arc of the toroidal surface 52-2 in FIG.
6 shows an imaginary ray portion that appears when the principal ray R in FIG. 6 is rotated and turned back toward the lens optical axis by an angle θ onto the sub-scanning cross section and projected.

When considering the rotation of the principal ray R, the limb 52-2'R that generates such a principal ray R is also shown in a rotated position in a similar manner, and this contour is denoted by a reference numeral "52-2'R ( Rotation) ". This contour "52-2'R (rotation)" is the contour
It overlaps with 52-2'C.

The image forming position when the above rotation is considered is a virtual image plane 9 shifted from the image plane 7 by (distance LR−distance LC). Since the principal ray C and the principal ray R both form an angle φ with the main scanning section, the image forming position of the principal ray C is a position 8 ′ that is LC · tan φ away from the main scanning section in the sub-scanning direction YY. C
The imaging position of the main ray R is in the sub-scanning direction Y-Y from the main scanning section.
At the position 8'R separated by LR · tanφ.

The difference between the distance between the position 8'C and the optical axis of the lens and the distance between the position 8'R and the optical axis of the lens is the amount of curve d of the scanning line, and is expressed as d = (LR-LC) tanφ. be able to.

The scanning line bending amount d is specified by determining the distances LR and LC in accordance with the determination of the scanning angle θ.

The contour 52-2'C and the contour "52-2'R" in FIG.
(B) in which the “(rotation)” portion is shown in an enlarged manner, where r is the radius of the toroidal surface 52-2 in the sub-scanning cross section, Becomes

Normally S≪r, And tanφ in the above equation is Becomes Therefore, since tan φ is proportional to the amount of eccentricity S, the amount of curve d of the scanning line is And becomes a value proportional to the amount of eccentricity S.

By the way, in FIG. 5, the principal ray R and the principal ray R generally do not always exit perpendicularly to the toroidal surface 52-2. Therefore, the case where the light is not emitted vertically is considered.

In FIG. 5, the magnification in the direction perpendicular to the plane of FIG. 5 along the principal ray C and the principal ray R along the contour 52-2C and the contour 52-2R is β _52-2 (C) and β _{52- 2} When (R), Figure 7 position 8'C of (a), each distance between 8'R and lens optical axis _{S (1-β 52-2 (C} )), S (1-β 52 ₋₂ (R)), and the amount of curve d of the scanning line can be expressed as d = S {β _52-2 (R) −β _52-2 (C)} (2).

On the other hand, in FIG. 5, the magnification of the principal ray C is βC, and (LC−f) / = βC, and the magnification of the principal ray R is βR.
It can be considered that (LR−f) / f = βR (3)

Then, when d is solved from the equations (1) and (3), d = S (βR−βC) (4) When this equation (4) is applied to the toroidal surface 52-2, d = S {β _52-2 (R) −β _52-2 (C)} (5), and equations (2) and (5) are obtained. Are equal.

Therefore, the same conditional expression can be applied to the bending amount d of the scanning line both in the case of vertical emission and in the case of not.

Therefore, referring to FIG. 1, the amount of scan line bending considering the influence of each lens constituting the scanning optical system by applying the above conditional expression will be examined.

In FIG. 1, reference numeral VI denotes an optical system before the cylinder surface 52-1 when there is no eccentricity in each lens surface shown in FIG. 5, that is, the deflecting / reflecting surface 4, the fθ lens 51, and the fθ lens 52. Similarly, the virtual image position VI ′ indicates the virtual image position when there is eccentricity. Therefore, the virtual image position VI ′
Virtual image position VI 'as in the case of light CR "passing through and parallel to the lens optical axis.
When the optical ray passing through the optical system is decentered in the optical system before the cylinder surface 52-1, the principal ray CR has no decentered toroidal surface 52-2.
Is an imaging position 8'y. That is, the imaging position 8'y is a position where the light decentered before the toroidal surface 52-2 is imaged on the toroidal surface 52-2 without decentering.

Then, the change in the virtual image position due to the lens surface before the toroidal surface 52-2 which is the final surface of the lens is represented by the difference between the respective virtual image positions VI and VI ', and the amount of the change is the image height ratio y. The magnification of each optical surface is represented by a function S ′ (y), and the image height ratio y
The function β (y) can be expressed as

The relationship between the function S '(y) and the function β (y) is as follows.

S ′ (y) = β ′ ₂ (y) · S ₂ + β ′ _51-1 (y) · S _51-1 + β ′ _51-2 (y) · S _51-2 + β ′ _52-1 (y) · S _52-1 ... (6) The subscript in each term in the above equation (6) indicates the sign of the lens surface (lens surface number), and is specifically as follows.

In this example, since β ₂ = 0, Σβ′m (y) = 1, where m is the lens surface number.

From S ′ (y) in the above equation (6), the luminous flux when the optical axis before the cylinder surface 52-1 and the lens optical axis when the toroidal 52-2 is not decentered has a decentered toroidal surface If the distance from the lens optical axis in the sub-scanning direction of the position 8 '(y) formed by 52-2 in the sub-scanning direction is obtained as S "(y), S" (y) =-S' (y) ｛1 −β _52-2 (y)｝ + S ′ (y) = S ′ (y) · β _52-2 (y)

Further, considering the case where the eccentricity of the toroidal surface 52-2 is _S52-2 , S ″ (y) = { _S52-2− S ′ (y)} 1- _β52-2 (y) ｝ + S ′ (y) = S _52-2 ｛1−β _52-2 (y)｝ + S ′ (y) · β _52-2 (y)

As a result, when the difference between the image formation position by the principal ray C on the image plane 7 and the image formation position by the principal ray R, that is, the amount of curve d of the scanning line is obtained, from the equation (2), d = β _{52 -2} (R) ｛S '(R) -S _{52-2 ＝=} β _52-2 (C) ｛S' (C) -S _52-2 ｝ When transformed into
The following equation is obtained.

d = K ₂ · S ₂ + K _51-1 · S _51-1 + K _51-2 · S _51-2 + K _52-1 -S _52-1 + K _52-2 · S _52-2 … (8) In the equation, each value of the coefficient Km is as follows.

From the above, it can be seen that the sum of the influences of each surface is the value of the curvature of the entire scanning line.

When considering the actual lens, it is clear that if the eccentricity of each lens surface is set to 0, the scanning line will not bend, but since the lens has at least two or more surfaces,
Even if the adjustment in the sub-scanning direction is performed, it is impossible to make the eccentricity of both surfaces zero unless the eccentricities of the two surfaces coincide.

However, each lens is adjusted by ΔS in the following equation, and Sn ₁ + Δ
If the eccentricity of S is forcibly left, the following equation (10) is obtained.

dn = 0 = Kn ₁ · (Sn ₁ + ΔS) + Kn ₂ · (Sn ₂ + ΔS) (10) where n is a lens number.

When the above equation (10) is modified and solved for ΔS, the following equation (11) is obtained.

By applying this equation (11), it is possible to eliminate the bending of the scanning line for each lens. In addition, the above (11) for all lenses
By adjusting according to the equation, d = 0 can be obtained.

Next, it is examined which of the lenses constituting the scanning optical system is most effective to adjust.

From equation (8), Km represents the degree of influence of the scanning line bending. Therefore, considering the magnification β of each surface, each surface from the surface of the cylindrical lens 2 to the cylinder surface 52-1 has 0 ≦ β ≦ 1 (β ₂ = 0).

Therefore, β ′ in the equation (7) also becomes 0 <β ′ <1. On the other hand, the toroidal surface 52-2 has β≪−1. Looking at the difference between the magnifications β (R) and β (C),
| Β (R) −β (C) | ≪ | β (R) + β (C) | / 2. In general, the one having a large magnification βm is | β
(R) -β (C) | is also large.

From the above, it is apparent from the above equation (9) that the Km of the surface having the large magnification | βm | is large.

For this reason, it can be seen that the curvature of most scanning lines can be eliminated by adjustment by moving a lens including a large surface of | β |.

Next, regarding the scanning optical system of the embodiment, when the specifications of each lens are as shown in Table 1, the influence amount on the bending of the scanning line is considered.

However, in Table 1, the rotation angle of the deflecting / reflecting surface 4 is 0 ° for the principal ray C, and 16.6 ° for the principal ray R.

Then, the magnification of each lens surface in Table 1 is as follows.

β ₂ (R) = β ₂ (C) = 0, β _51-1 (C) = 0.952, β _51-1 (R) = 0.947, β _51-2 (C) = 0.887, β _51-2 (R ) = 0.871, β _52-1 (C) = 0.811, β _52-1 (R) = 0.772, β _52-2 (C) = -5.45, β _52-2 (R) = -0.604 7), (9) from the _{equation, K 2 = 0.11, K 51-1} = 0.03, K 51-2 = 0.10, K 52-1 = 0.35, a K _52-2 = 0.59.

Also in this example, the magnification of the surface 52-2 having the largest coefficient K, β _52-2
(C) = − 5.45 and β _52-2 (R) = − 6.04 are the largest, and it is preferable to adjust the fθ lens 52 including the surface 52-2.

In a scanning optical system having an fθ lens having an anamorphic surface, a toroidal surface is generally disposed on the final surface of the fθ lens, and the magnification of this surface is β≪−1. Also in this example, the toroidal surface 52-2 is placed on the final surface, and the magnification of the surface is β≪-1 as described above. Therefore,
It can be said that adjusting the fθ lens 52 having the toroidal surface is effective.

In FIG. 1, when the function S ′ (y) is a value S ′ that does not depend on the image height ratio, the toroidal surface 52 is added by that amount.
-2 in the sub-scanning direction Y-Y (the toroidal surface after this adjustment is indicated by reference numeral 52-2 "), the image forming position on the image surface 7 becomes the predetermined position 8F which is the original image forming position. An image is formed at an image formation position 8 ″ y which is further away by a distance K, and the scanning line does not bend. This means that, when the toroidal surface has eccentricity, the bending of the scanning line can be eliminated by moving the fθ lens 52 by the amount of the eccentricity.

Note that, as described above, the imaging position is separated from the predetermined position 8F by the distance K. However, in this type of scanning optical system, an optical path bending mirror M is generally provided between the final lens surface and the image forming position, and a method of guiding light rays to the image forming position is employed. Therefore, the angle of the mirror M is finely adjusted. The image forming position 8 ″
y can be moved to the predetermined position 8F.

Due to the adjustment of the image forming position, the image forming point is slightly shifted in the lens optical axis direction, but the distance between the image plane 7 and the mirror M is sufficiently longer than the distance K between the image forming position 8'y and the predetermined position 8F. In this case, the shift of the imaging point in the optical axis direction can be ignored.

More specifically, in the above example, S _52-1 = 0.1 mm, S
_{When 52-2} = −0.08 mm, the above equation (11) and the above K
ΔS = 0.34 mm is obtained from _52-1 = 0.35 and K _52-2 = 0.59.

Therefore, if the fθ lens 52 is moved and adjusted in the sub-scanning direction by 0.34 nm from the ideal position when there is no eccentricity, d ₅₂ ≒ 0 can be obtained.

As a result, the bending of the scanning line is caused only by the effect of the cylindrical lens 2 and the fθ lens 51, and the amount thereof is almost negligible.
Only the adjustment of 52 can sufficiently solve the problem of the scanning line bending.

From the above, it can be seen that even if the eccentricity tolerance of the single lens and the eccentricity tolerance of the mounting surface are loosened, it is possible to adjust the bending amount of the scanning line to be small.

Next, an example of a specific adjustment method will be introduced with reference to FIG.

In FIG. 2A, reference numeral 520 denotes an ideal fθ lens without eccentricity.

The dimension from the main diameter line (generating line) to the outer diameter portion, that is, the mounting surface of the fθ lens is H. Then, it is assumed that the fθ lens 520 is mounted on the housing 400 in a state where the main diameter line coincides with the ideal optical axis 0-0. This state is assumed as a reference state.

By the way, in order to implement the present invention, the upper surface of the housing 4000 is set to be smaller than that of the housing 400 by the dimension h in advance. This dimension h is determined as follows. That is, when the maximum adjustment amount obtained from the maximum tolerance of the eccentricity of both surfaces of each lens constituting the scanning optical system is ± ΔS _MAX , h ≧
| ΔS _MAX | is an arbitrary value that satisfies | In this way, any adjustment can be dealt with in the range of the thickness of the spacer having the dimension h.

Further, as shown in FIG. 2 (b), the eccentricity of both surfaces of the specific fθ lens 520A to be set on the housing 4000 is Sa, Sb, and the adjustment amount at that time is in the ascending direction. To ΔS (however, ΔS <| ΔS _MAX |)
Is obtained, the thickness of the spacer 50 is SP = h
+ ΔS, and the curvature of the scanning line can be corrected by fixing the lens with a spacer of this thickness interposed.

FIG. 2C shows an example in which the adjustment amount of the fθ lens 520B is obtained as ΔS ′ (where ΔS ′ <| ΔS _MAX |) in the decreasing direction, and the thickness of the spacer 50 ′ at this time is , SP ′ = h−ΔS ′, and by performing the adjustment in accordance with the above, it is possible to correct the bending of the scanning line.

〔The invention's effect〕

According to the present invention, it is possible to easily correct the bending of the scanning line to such an extent that the image quality is not affected, without strictly controlling the eccentricity tolerance of the lens, the tolerance of the mounting surface, and the like, so that the cost is not increased. Can be.

[Brief description of the drawings]

1 and 2 are views for explaining the present invention, FIG. 3 is a plan view of a scanning optical system, FIG. 4 is a view for explaining an image forming relationship when a lens has and does not have eccentricity, FIG. 5 is a diagram in which each scanning light is projected onto the main scanning section, FIGS. 6 and 7 are diagrams in which the scanning light in the above figure is projected in the sub-scanning cut, and FIG. 8 is a scanning optical system suitable for carrying out the present invention. FIG. 9 is a diagram illustrating the eccentricity of the lens, and FIG. 10 is a diagram illustrating the bending of the scanning line caused by the conventional scanning optical system. 51: fθ lens, 52-2: toroidal surface, XX:
... main scanning direction, Y-Y ... sub-scanning direction.

Claims (2)

(57) [Claims] A first image-forming optical system for linearly forming an image of a light beam emitted from a light source; and a light deflection surface having a deflecting / reflecting surface set near an image-forming position of the first image-forming optical system. An optical deflector disposed on the optical path of the light beam between the medium to be scanned and the optical deflector, and a scanning medium to be scanned by the light beam deflected by the optical deflector; The light beam is scanned by the light beam while maintaining the optically conjugate relationship between the deflecting reflection surface of the optical deflector and the medium to be scanned in a plane perpendicular to the deflecting surface which is the trajectory surface of the light beam continuously deflected by A scanning optical system having a second imaging optical system for forming an image on a medium,
A method of correcting a scan line bending caused by the eccentricity of a lens in the second imaging optical system, comprising: a magnification in a direction orthogonal to a scanning direction by a light beam deflected by the optical deflector; Among the lenses of the two imaging optical systems, for the lens having the lens surface with the largest magnification, ΔS is the correction amount, K is a coefficient determined by the magnification of the lens surface, and S is the eccentricity of the lens surface. Is the lens number of the lens constituting the imaging optical system including the lens having the lens surface with the largest magnification in the second imaging optical system, and the subscript number is the lens surface number. = (Kn ₁ · Sn ₁ + Kn ₂ · Sn ₂ ) / (Kn ₁ + Kn ₂ ) The lens is decentered in the sub-scanning direction by the correction amount ΔS calculated by the calculation formula, and then fixed. Correction method of the scanning line. 2. A first image forming optical system for forming a light beam emitted from a light source into a linear image, and a light deflecting surface having a deflecting / reflecting surface set near an image forming position of the first image forming optical system. An optical deflector disposed on the optical path of the light beam between the medium to be scanned and the optical deflector, and a scanning medium to be scanned by the light beam deflected by the optical deflector; The light beam is scanned by the light beam while maintaining the optically conjugate relationship between the deflecting reflection surface of the optical deflector and the medium to be scanned in a plane perpendicular to the deflecting surface which is the trajectory surface of the light beam continuously deflected by A scanning optical system having a second imaging optical system for forming an image on a medium, wherein a bending of a scanning line caused by eccentricity of a lens in the second imaging optical system is deflected by the optical deflector. The lens has a magnification in a direction orthogonal to the scanning direction by the light beam, and is located inside the lens of the second imaging optical system. For the lens having the lens surface with the largest magnification, ΔS is the correction amount, K is a coefficient determined by the magnification of the lens surface, S is the amount of eccentricity of the lens surface, and the subscript n is the value of the second imaging optical system. Where, the lens number of the lens forming the imaging optical system including the lens having the lens surface having the largest magnification, the subscript number is the lens surface number, and ΔS = (Kn ₁ · Sn ₁ + Kn ₂ · A scanning optical system characterized in that the lens is decentered in the sub-scanning direction by a correction amount ΔS calculated by a formula obtained from (Sn ₂ ) / (Kn ₁ + Kn ₂ ) and then fixed.