JPS62139520A - Photoscanning device - Google Patents

Abstract

PURPOSE:To obtain a small-sized, high performance, and low-cost scanning device by using a single lens which has such distortion characteristics that luminous flux deflected by rotation characteristics of a deflector move on a scanned plane at an equal speed and also have aspherical surfaces as both surfaces. CONSTITUTION:The luminous flux emitted by a light source is reflected by a mirror surface SM at an angle theta of deflection corresponding to the rotation of a polygon mirror 5. A scanning lens 1 is so set as to form the image of the luminous flux on the scanned plane at a point T1 whose coordinate value Y is proportional to the angle theta of deflection. Then, the lens 1 has such distortion characteristics that the luminous flux deflected by the rotation characteristics of the polygon mirror 5 moves on the scanned plane at the equal speed and is the single lens whose surfaces are both made aspherical so that the curvature of image of the luminous flux at an optional position on the scanned plane is zero or almost zero.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an optical scanning device used in a laser beam printer or the like. More specifically, it relates to a scanning lens system.

[Technical background of the invention]

Laser beam printers, which record image information by deflecting and scanning laser beams at high speed, have excellent features such as high speed, high resolution, and low noise, and as they become smaller and cheaper, demand for them is rapidly increasing. It is increasing. Therefore, as an optical writing head, which is an important component of the optical scanning device, there is a strong demand for smaller size and lower cost. An optical scanning device is mainly composed of a light source, a deflector, and a scanning lens system, and simplifying the scanning lens system is effective in reducing the size and cost.

The scanning lens system is distorted so that the light spot moves at a constant speed on the scanning surface according to the rotational characteristics of the deflector. For example, when the deflector is a rotating polygon Q and the light beam is deflected at a constant angular velocity. It has a distortion such that the deflection angle θ and the image height Y are proportional,
It must also have the ability to uniformly image a light spot to a desired diameter everywhere on the scanning plane. Furthermore, in the case of a rotating polygon mirror deflector, a surface tilt correction function is also required to compensate for variations in the tilt of each surface of the polygon mirror (surface tilt error). Conventionally, high-performance scanning lenses with high resolution that combine these functions have inevitably been large, complicated, and expensive.

[Conventional technology]

Therefore, JP-A-54-98627 and JP-A-55-7727
, Japanese Unexamined Patent Publication No. 58-5706, attempts have been made to make the scanning lens into a single lens.

However, in Japanese Patent Application Laid-Open No. 54-98627, it is possible to effectively correct aberrations for various values of parameters such as shape by using the rotational characteristics of a deflector having sinusoidal vibration characteristics, but it is difficult to perform high-speed correction. For these reasons, the rotating polygonal mirror deflector, which is currently most widely used, has an aspherical surface to accommodate the constant angular velocity rotation characteristics, but it can only be used under extremely limited conditions as a special case. figure,
It is not possible to flexibly respond to various requirements such as the dimensions of the optical system, the light source, and the required dot diameter.

Further, in Japanese Patent Application Laid-Open No. 55-7727, an fθ lens is constructed with a plano-convex lens, but it cannot be said to have good imaging performance in terms of field curvature and the like.

In addition, in JP-A-58-5706, the fθ lens is constructed with a meniscus lens having positive power, but there is a problem in terms of spherical field curvature. A cylindrical lens that also serves as a lens must be added. Furthermore, in all of the above three examples, a new lens must be added in order to provide a surface tilt correction function, and in the end, it is no longer a single lens. Furthermore, it is possible to keep the aberrations within the evaluation range by increasing the optical axis length and narrowing the deflection angle, but this is not desirable because the entire optical system becomes larger.

Incidentally, the material of the lens is also an important issue when considering miniaturization and cost reduction. Conventionally, glass has been used as a material for scanning lenses, but since they are optical systems that require diffraction-limited performance and require high precision, manufacturing costs such as polishing are high. Therefore, polymethylmethylate (PMMA)
) If a plastic such as polycarbonate polystyrene is used as the lens medium, mass production by injection molding becomes possible, and the lens can be manufactured at an extremely low cost. However, there are only a few types of optical plastic materials, and none have a higher refractive index than glass. Therefore, it is more difficult to reduce the number of lenses and downsize the optical system than with glass.

Combining these points, we have created a lens that can effectively correct aberrations even with a single lens and short optical axis length, regardless of the refractive index of the material.
It can be seen that a lens shape with a large degree of freedom is desired.

[Problem to be solved by the invention]

The present invention was made in view of the above-mentioned problems.
The purpose is to provide a compact, low-cost, and high-performance optical scanning device, especially a scanning lens.

For the above purpose, the optical scanning device of the present invention includes a light source that emits a narrow light beam, a deflector that deflects and scans the light beam in a predetermined direction, and a light beam deflected by the deflector that directs the light beam onto a scanned plane. and a scanning lens for forming an image, the scanning lens comprising:
The light beam deflected by the inherent rotational characteristic of the deflector has a distortion characteristic that moves at a constant speed on the scanned plane, and the field curvature aberration of the light beam at any position on the scanned plane is zero or It is characterized by being a single lens with aspherical surfaces on both sides so that the angle is almost zero. Preferably, the narrow light beam emitted from the light source is a parallel light beam.

[Means for solving problems]

The optical scanning device of the present invention includes a light source that emits a narrow light beam, a deflector that deflects and scans the light beam in a predetermined direction, and a scanning lens that forms an image of the light beam deflected by the deflector on a scanned plane. The scanning lens has a distortion characteristic in which the light beam deflected by the inherent rotational characteristic of the deflector moves at a constant speed on the scanned plane, and The lens is characterized in that it is a single lens with aspherical surfaces on both surfaces so that the field curvature aberration of the light beam in the lens is zero or almost zero.

〔Example〕

The principle of the present invention will be explained below using FIGS. 1, 2, 5, and 4.

As mentioned above, the scanning lens forms an image of the light beam, which is deflected by the deflector at a constant angular velocity or rotational characteristic such as sinusoidal vibration, on the scanned plane without curvature of field, and also forms an image point on the scanned plane. It has the function of applying distortion so that it is scanned at a constant speed. For example, if the deflector is a rotating polygonal bell, the light beam emitted from the light source is reflected by both surfaces 8M at a deflection angle θ corresponding to the rotation of the polygonal mirror 5, as shown in FIG.

The scanning lens 1 is set so as to focus this light beam on the scanned plane at a point TI whose coordinate value Y is proportional to the deflection angle θ.

The scanning lens of the present invention is a single-lens lens that has few aberrations and is capable of wide-angle deflection, in which the features of aspherical surfaces are highly utilized on both sides of the 81s 82 shown in FIG. 1 based on the principle described below.

The first principle of construction of the lens surface shape according to the present invention is to assume that the scanned light beam is very thin, express the light beam only by the parameters of the position and direction of the principal ray, and the imaging distance, and One point is that the inclination and curvature are determined to change the direction or imaging distance only for the chief ray that passes through it. In terms of aberration correction, this means ignoring spherical aberration and coma and completely correcting field curvature and distortion, including higher-order terms. The above assumption generally holds true in scanning optical systems such as laser beam printers.

Furthermore, in a scanning lens system, the principal ray of a light beam deflected at a given polarization angle is always on the same plane (this is called a meridional plane), so in addition to the fact that the light beam is very narrow, it is possible to It can be seen that the curvature can be specified only on the curve where the meridian plane intersects the lens surface. Therefore, the second principle of construction of the present invention is to create a curve on the meridian plane, and at any point on the curve, the inclination and curvature in the meridian plane achieve the purpose of the scanning lens described above, and It is assumed that a surface can be formed if the curvature of a cross section containing the chief ray and perpendicular to the meridian plane (referred to as a truncated cross section) is given at any point on the curve.

However, since the inclination and curvature in the meridian direction are continuously connected to form the lens surface position in the meridian plane, they cannot be determined independently from each other, but the curvature of the spherical section can be treated independently from them. Therefore, it is clear that an optical system to which the first and second configuration principles described above are applied only to the lens surface shape in the meridian plane is also included within the scope of the present invention.

Hereinafter, the principle of construction of the lens according to the present invention will be specifically explained using the perspective view of FIG.

In FIG. 1, a light beam (Li-1) is converted into a light beam (L2) by a surface S1. Imaging distance measured from Ti of luminous flux (Li) gmi for total meridional luminous flux, gsi for spherical luminous flux
shall be. Generally gmi and gsi are not equal. As mentioned above, the luminous flux is very thin, so the luminous flux (Li)
When dealing with , it is only necessary to consider the principal light #Lci and the imaging distances gmi and gsi of the meridian and the meridian, respectively. Now, the direction of the principal light 1Lxi after passing through the surface S1 is Tr of the surface Si.
, iv can be controlled by the normal direction e1 at . Also, the imaging distance gni after passing through the plane si.

gsi is the radius of curvature Rmi of the meridian section in Ti of surface si
It can be controlled by the radius of curvature Rsi of the spherical section. Therefore, it has the function of focusing a single beam of light deflected at a certain angle to a position on the scanning plane where uniform speed scanning can be realized, depending on the position of a single point on the lens surface and its differential amount (normal direction and curvature). Therefore, if we connect them in a row and give the above function to each point on the lens surface that corresponds to the light beam deflected at an arbitrary angle, the desired scanning lens shape can be determined. . This is the first configuration principle mentioned above.

Now, as mentioned above, since the chief ray Lci etc. do not leave the meridian plane, the normal direction vector ti of the surface si also corresponds to the optical axis shown in FIG. One degree of freedom is sufficient, which is the angle αi formed by the normal vector. Also, the curvature of the meridional section of the surface Si is the differential amount of the inclination αi of the surface, and the curve αi of the surface is the differential amount of the position of the surface si on the meridian plane. It can be seen that specifying has the same meaning as solving a differential equation and creating a two-dimensional curve on the meridian plane. In addition, since the spherical cross-sectional curvature is determined without affecting the above curve,
After the curve is created, each point on the curve is determined individually. This is the second construction principle.

A scanning lens can be realized based on the above-mentioned construction principle, and the fact that it can be realized with a single lens having both aspherical surfaces will be explained using the principle diagram in FIG. 3. In FIG. 6, the plane of the paper represents the meridian plane.

First, let's consider the meridian plane. What we want to constrain now is the principal ray Lc and the non-scanning plane S! Intersection 1. The coordinate value Y! and Tx
are the two degrees of freedom of being the imaging point.

For example, scanning ti- of a light beam deflected at an arbitrary angle θ
In order to constrain the position tYX, the inclination α1 of the surface is specified at all positions on the surface, and the shape in which the surfaces are smoothly connected according to the boundary condition (for example, the coordinate value Xl of the intersection point Pl with the optical axis and the inclination there) 0), it is not possible to specify the radius of curvature Rml on that surface with a constant value of 1 for each line like Sl, and the light beam will be converged at a point Tx that is not on the scanned plane. On the other hand, if you specify the curved half & Rm t of the surface at all positions on the surface in order to constrain the imaging point, you cannot specify the slope αl of the surface in the same way.In this way, the ray One surface is required to constrain one degree of freedom among the parameters possessed by an arbitrary value of the deflection angle θ. Therefore, in order to constrain the two degrees of freedom mentioned above, at least two surfaces are required. A lens surface is required.

Next, considering the spherical beam, what we want to constrain is one degree of freedom of the spherical direction imaging distance gst, which is constrained within the calf plane, that is, while preserving the shape of the curve,
Since it can be controlled by adding curvature to the curve on the meridian plane in a direction perpendicular to it, there is no need to add a new surface to the two surfaces mentioned above.

Therefore, it can be seen that only two lens surfaces are required, and a single lens is sufficient. Furthermore, since both surfaces are inclined at all positions of the lens surface and have specified curvature, a single lens must have both aspherical surfaces.

Now, let's consider the symmetry of the surface of the single aspherical lens configured as described above. If two curves created in the meridian plane are rotated about an axis such as the optical axis, the degree of freedom of the radius of curvature in the direction of the sphere will be lost. Therefore, if rotational symmetry is provided, the imaging of the spherical beam cannot be controlled and spherical field curvature aberration occurs.

Regarding plane symmetry, since the light flux is always on the meridian plane, it is obviously symmetrical about the meridian plane, and the light flux passing through the optical axis has a deflection angle of O, and the light flux with a deflection angle of θ is the same as the light flux with a deflection angle of 10. Therefore, it is also symmetrical about the plane that includes the optical axis and is perpendicular to the meridian plane. As described above, since the scanning lens of the present invention has no symmetry except for the presence of two planes of symmetry, it is possible to completely correct spherical field curvature aberration, meridional plane curvature aberration, and distortion characteristic aberration. .

A specific method for realizing the shape of the single lens side aspherical lens for scanning according to the present invention will be explained below using the principle diagram of FIG. 4. First, a method for creating two curves on the meridian plane will be explained. As shown in FIG. 4, each lens surface 5h82 intersects with the optical axis P1.
．． Distance 81, 112 along the curve from P2 and inclination angle α1 from the direction perpendicular to the optical axis at that point. It is defined in relation to α2.

Reexpressing this in orthogonal coordinates, for the planes St and 82, P1. With P2 as the origin, the optical axis is the X axis,
If the height direction of the lens is the y-axis, the coordinate values of points Pl and P2 (X 1 * 71) p (x2 r 'l 2
) becomes.

Now, as shown in FIG. 4, a luminous flux Li(i-
0, 1, 2) are the planes St, 82 and Tl, respectively.

At T2, image plane S! and intersect at TI, and the emission position island and emission direction of the luminous flux are expressed as follows. That is to say. Furthermore, T of surface Sl, S2! , the radius of curvature of the meridional section at T2 is Rms and Rm2, respectively, and the luminous fluxes Ll and L
Let the meridian imaging distance of 2 be gm t + gm z.

According to the above description method, the principle of construction of the lens shape described above can be formulated. The formulation will be explained by dividing it into the following six items.

(2) The direction of the light beam is controlled by the inclination K of the surface at the intersection of the surface St, 82 and the light beam.

(2) The imaging distance of the light beam is controlled by the curvature of the surface at the intersection of the surfaces 81 and 82 with the light beam.

■ The coordinates of the intersection of the surface and the luminous flux are equal.

■ Each point on the surface is smoothly continuous.

■ The light beam forms an image on the scanning plane.

■ The imaging point is scanned at a constant speed on the scanning plane.

The relationship between the inclination of the refractive surface and the direction of the luminous flux in (2) can be determined by applying the well-known law of refraction to the intersections of the St, 82 planes and Ll, L2. :S
s-plane (3) n s in(α2-θt)=sin(α
2-02): S2 side It can be expressed as (4). However, n
is the refractive index of the lens medium.

The relationship between the curvature of the surface and the imaging distance of the light beam in (2) can be determined by applying the relational expression k S of the meridional imaging distance when a thin light beam is obliquely incident on a surface with a certain curvature to the surface 82 gm" 3°
o-go Rms 8, surface obtained.

Regarding (2), assuming that the orthogonal coordinate value of the surface position calculated using the above equation (1) and the orthogonal coordinate value of the refraction point of the ray calculated based on the above equation (2) are equal, then There is a relationship. However, the X coordinate value %X2 of the intersection of the X t l'j plane S1 and the original axis is the X coordinate value of the intersection of the plane S2 and the optical axis.

Regarding ■, the conditions that the surfaces are continuous are (7) to (10
) is possible. Further, the condition for the surface to be smooth is that the inclinations αl and α2 of the surface are differentiable, and the following relationship exists: dα1−knee o11 dingτ−帥l dα21 d82 Rxn2(13).

The condition for the image point to be scanned at a constant speed on the scanning plane (①) is that the intersection of the image plane and the light beam (X■, Xt) is
sθ u3Yt=lzsinθz+ 1.5in
There is a relationship of ol + 16 sin θ Q4, and the scanning rank fil Y I becomes αG Y r = K @F-” (θ) using the rotation characteristic θ-F(τ) αS of the deflector. However, Fl is an inverse function of F, τ is a time parameter, and K is an appropriate proportionality constant.For example, if the rotation characteristic is constant angular velocity deflection, F' (τ) = ωτ ω
: Angular velocity Since it is a surface, it can be written as = fθ f = -12 constant αlω. Also, X in equation 03 represents the optical axis length in the X coordinate of the scanning plane.

The condition (2) of forming an image on the scanning plane is satisfied if the meridional beam imaging distance gm2 in equation (6) is equal to 12 expressed by the equation αJ%a4. That is, gmz=12
19 As described above, the construction principle of the lens shape according to the present invention is (31(4) (5) (61(7) (8)(
91 (it) (itl (+213 (14 (IS)
Although it has been formulated using Equation 14 of Q9, it will be explained below that by calculating these, the shape of the lens surface can actually be directly expressed in some form. Among the variables appearing in the equation, the deflection angle θ and the initial meridional imaging distance gmo are given at the time of emission and are known. Also, the optical axis length Xry plane S1. The intersection positions X t and X z with the optical axis of S2 and constant speed scanning constants are constant values that are independent of the deflection angle θ.

Therefore, the unknown remains θ! , θ2. αl, α2g”11
S2 + gml * gmz +l O+ll IH
A 2 y'ml tRm2 gYl, and all of the above 14 equations are independent, so the simultaneous equations can be solved and the above 14 variables can be expressed as a function of the deflection angle θ, for example. Therefore, for example, when expressing S1, it is sufficient to make the slope C1 correspond to the distance sl along the plane from the optical axis as the deflection angle θr parameter.

By the way, since the above-mentioned 14-element simultaneous equations are nonlinear and include differential terms and integral terms, they cannot be solved directly and must be solved numerically.

As a numerical solution method, the present invention is not limited to this, but as an example, this equation is actually numerically calculated by the method of numerical integration in a differential vector field, and the solution is determined by the lens shape. Show what you can do.

Solving using a differential vector field means expressing all equations in differential form, assuming that all current variable values are known, and calculating the increment of each variable (differential variable) to find the value of the next variable. If we rearrange the above equation 14 and play with the differential form, we get % (3N41 equation is (dα1-dθ)cos(α!-
〇)=n(dα,-aθ,) cos(αl-θl) prisoner n
(dα2-dθ1)cos(C2-θ1)=(dα2
-dθ2)cos(C2-θ2) C2shif51
(C1-0
1) cos” (cll-〇) dst +gm, dS
”= gmo 1a (ncos(C1-θ)-Cos(C1-θmonthdα12
3cos”(C2-θ2) Cos2(C2-θ1
)ds□+gmz ”” gms-11 (cos(αz #z)-n(os(C2-θ1))d
α, a However, g+y11 eliminates @0 equation simultaneously.

Also, equations (7) to (10) are dlocosθ−1os 1nOdθ=sinα1d
sI(24dlosinθ+1ocosθdθ=cos
αldB, WdltcosθH−dl
sinθ1dθ++dlocosθ−d,)sinθd
θ=sinα2ds2 C1 dl+sinθ++1. cosθ1dθ1+dgos
inθ+l(, cosθdθ=cosα2ds2
@ +1314 formula is o=d72cosθ2-12s+nθ2dθ2+dl!
1coslJ+ -d, sinθ1dθ++dloco
sθ−dosinθdO@dY+−d12! l inθ
z+12cosθ2dθ2+dl+sinθl+1,c
aSθ1dθ++d6osinθ+1ocos(jdθ
(c) Equation 00 becomes dY+ = K (F'(θ))°dθ. Equation 09 can be simply substituted. ■The unknown differential variable in the equations is dθ2. dθ2. dαt, dα2゜ds1y
ds2+dlQ+djffl+d12+dY! , the above (J to CIQ equations are 1 (i
All out-of-order equations where the formula for M is a quadratic equation are 1.
Although the following is easily solved, for example, d fj s=pθl(θl, θ2.αl・C
2HSl r 32 H]o, ls, 112)・dθ
It can be expressed as (31). From this, for example, θl is
By integrating θ a,=Fo,dθ+θ01 (5?), the deflection angle θ can be expressed as a parameter. However, θ1 is an initial value. In the actual calculation, the initial value is θ! , θ2. α1
．． For α2.31.82, 0°1o, 11. For I2, using the Xt, X2, and Xr O values mentioned above, 1o=X 11=X2 -XI
2 = Xx-X2, it can be calculated by numerical integration.

Now, as described above, the curve on the meridian plane of the lens shape of the present invention is materialized, and the constant n Xi lX2 +X+ + gmo * K that appeared in the process of materialization.
This is the degree of freedom that the lens shape of the present invention can take. That is, some suitable set of constants (x"t, x:.

There is one lens shape for each one of X tr g mor K ), and the present invention obviously includes all of these shapes.

Note that the meridian initial imaging distance goo is set to infinity. In other words, if the meridional light flux before entering the scanning lens is made into a parallel light flux, the beam diameter etc. can be easily controlled, resulting in an optical system that is easy to handle. The scanning lens of the present invention is naturally applicable to parallel light beams as described above.

Now, next, the radius of curvature R of the spherical section that controls the spherical imaging distance
The entire method for determining 3s and RBz will be explained.

(5) (Equation 61 shows the relational expression for the meridional imaging distance when a narrow beam of light is incident obliquely, but for the spherical imaging distance, the following holds true: Image formation in the spherical direction on the scanned plane The condition that there is a point is gsz-12 (36).The radius of curvature Rgl + R82 of the spherical section is determined by equations (34), (35), and (3A), and in the equation, 1ovlLe12+αl, α2 .θ, θl
, θ2 are known because the meridional curve has already been determined by the method described above, and s gso is given, so there are four unknowns: gsltg82yR81, R, . Therefore, there are redundant degrees of freedom for the three equations, and one of the unknowns can be determined appropriately.

For example, in order to simplify the surface shape, if R31 is always set to infinity and the second term on the right side of equation (34) is set to 0, the first surface becomes a surface that has no curvature in the spherical direction.

Note that the initial spherical imaging distance gso may be arbitrarily given, but if the deflector is a rotating polygon mirror, g80 ``''.

In this case, the reflection point of the mirror surface and the scanning point become a conjugate image point, which has a tilt correction function.

〔Example〕

Examples of calculating lens surface shapes based on the principle of lens shape construction according to the present invention are shown in Tables 1 to 9 and FIGS. 5 to 12.

As mentioned above, the lens shape of the present invention has the refractive index of the lens medium, n1, initial imaging distance, gos, the position where the first and second surfaces of the lens intersect with the optical axis, x1. x2. The six parameters of the optical axis length X11 scanning speed constant can be changed independently, and one lens shape exists for one parameter value set. Therefore, there are an extremely large number of embodiments that seem to have completely different shapes at first glance, and since it is impossible to list them all, only representative embodiments will be shown here.

The calculation conditions common to the examples shown below are: refractive index of the O lens medium n = 1.486Q optical axis length from the deflection point to the scanned plane X! = 2 0 The deflector is a rotating polygonal deflector with constant angular velocity deflection O. The initial meridional imaging distance gmo is infinite. That is, the light beam before entering the scanning lens is a parallel light beam.

The curvature of the O-spherical cutaway cross section is applied only to the second surface.

The initial spherical defect forming distance gso is o0. Therefore, the reflection point and the scanning point of the rotating polygon mirror become conjugate image points, and a surface tilt correction function is provided.

It is.

Note that the lens shape according to the present invention is not expressed by simple numerical values or formulas, but results are obtained as numerical examples, for example. Therefore, for convenience, a formula using well-known aspheric coefficients is used for the curve shape on the meridian plane: where X is the X coordinate value when the origin is the intersection of the optical axis with the XMt1% plane.

The curvature of the spherical cross section of the second surface R32 is expressed as R52.
=Rsz+A3'10B)' -+-cy +Dy +
Represented by Ey. The error from the true shape when approximated in this way is about (LOO1ch~Q,01ch).

Tables 1, 2, and 3 show coefficients RmxyBt+C+°, D1. Et, the coefficient Rm2+B2+C2+D2+E2 showing the curve shape on the meridian plane of the second surface S2 in Table 4, Tables 5 and 6, and the curvature in the direction of the spherical section in Tables 7, 8 and 9. Coefficient R2s A S * B S t C3+ indicating radius change
D S rEs with parameters θB, X1. The values calculated by varying X2 are listed. However, the effective deflection angle θ.

is a parameter used in place of the scanning speed coefficient in equation (18) above, and assuming that the effective scanning width is 200 dragons. As mentioned above, X t and X 2 are the first surface S1
This is the position of the point where the second surface S2 intersects with the optical axis. Note that under the above-mentioned common calculation conditions, the parameter set θ. ,X
Lenses with the same values of l and Xl are the same lens.

Furthermore, for some of the embodiments shown in detail, the outline of the curved shape on the meridian plane is shown in Figures 5 to 12 along with optical path diagrams. However, since the curve is symmetrical about the optical axis, the side opposite to the optical axis is omitted.

All of the embodiments published here follow the construction principles of the present invention, with spherical field curvature aberration and meridional field curvature aberration completely eliminated, and distortion characteristics such that the scanning point moves at a constant speed. completely defined.

However, perfection is an ideal state, and the actual lens shape has some degree of field curvature aberration and distortion characteristic aberration due to numerical calculation errors when calculating the shape or manufacturing errors. Of course, these aberrations have a certain tolerance range, and within that range, the lens is effective as a scanning lens, so the present invention does not exclude them.

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FIG. 13 is a perspective view showing the overall optical system of a laser beam printer using an embodiment of the lens shape according to the present invention. The light beam emitted from the semiconductor laser 2 is turned into a parallel light beam by the collimator lens 5, and is converged only in the spherical direction by the cylindrical lens 4, forming a linear image near the casting surface of the rotating polygonal mirror deflector 6. The light beam is deflected at a constant angular velocity within the meridian plane by the rotation of the polygon mirror 5, passes through the scanning lens 1 according to the present invention, and then forms an image on the photosensitive drum 7. In the direction of the spherical defect, the mirror surface and the photosensitive drum surface form a conjugate imaging point, forming a surface tilt correction system. The image point is set in the axial direction of the photosensitive drum 7 by the scanning lens 1 of the present invention.
It scans at a constant speed and forms an image on a straight line without curvature of field. The photosensitive drum rotates by one pitch for each scan, and by repeating this rotation, a latent image is formed on the photosensitive drum.

〔effect〕

As described above, in the optical scanning device of the present invention, the scanning lens has distortion characteristics such that the light flux moves at a constant speed on the scanned plane, and the field curvature of the light flux on the scanned plane Since it is a single lens with aspherical surfaces on both sides so that the aberration is zero or almost zero, even with a single lens, there is almost no aberration and an extremely good imaging spot can be obtained.In addition, it has a wide angle deflection and a short optical axis length. A scanning lens can be constructed. Furthermore, for the same reason, even if the lens medium has a low refractive index, it does not interfere with the design, and therefore it becomes possible to use plastic as the lens medium. Therefore, it is possible to provide a compact, low-cost, and high-performance optical scanning device. It has this effect.

[Brief explanation of drawings]

Fig. 1 is a principle diagram showing the general structure of the optical scanning device of the present invention, Fig. 2 is a principle diagram illustrating the principle of configuring the lens shape of the present invention, and Fig. 6 is a principle diagram of the scanning lens of the present invention. Fig. 4 is a principle diagram for explaining that this can be realized with a single double aspherical lens, and shows the shape of the scanning lens of the present invention.
W, 1 from Figure 5, a diagram of the principle of dam explaining the method of
2 to 2 are diagrams showing embodiments of lens shapes of the present invention, and FIG. 13 is a perspective view showing an embodiment of the entire optical scanning device of the present invention. In the figure 1... Scanning lens 2... Semiconductor laser 5...
・Multi-facet @ 6...Rotating polygon mirror deflector 7...Scanned surface (photosensitive drum) Applicant: Seven Epson Symposium Fig. 1 112 Fig. 3 Fig. 4 Fig. 5 ST Fig. 7 χ error × 1 χ nail 2 Fig. 9 ST Z9Xl :Cri X2 Fig. 10 Fig. 11 F Fig. 12

Claims (2)

[Claims] (1) A light source that emits a narrow light beam, a deflector that deflects and scans the light beam in a predetermined direction, and a scanning lens that forms an image of the light beam deflected by the deflector on a scanned plane; The scanning lens has a distortion characteristic in which the light beam deflected by the inherent rotational characteristic of the deflector moves at a constant speed on the scanned plane, and the image plane of the light beam at an arbitrary position on the scanned plane. An optical scanning device characterized in that it is a single lens having aspherical surfaces on both sides so that the curvature aberration is zero or almost zero. (2) The optical scanning device according to claim 1, wherein the narrow light beam emitted from the light source is a parallel light beam immediately before entering the scanning lens.