JPH10253915A - Optical scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize a new optical scanner with large latitude in optical system design. SOLUTION: In the optical scanner coupling luminous flux from a light source 1 with a coupling lens 2, deflecting the flux in a constant angular velocity with a polygon mirror 4, converging the deflected luminous flux toward a surface 6 to be scanned by a second optical system 5 containing a scan image forming element and scanning the surface 6 to be scanned at a nearly constant speed, when a direction of main light when the main light of the deflected luminous flux by the polygon mirror 4 orthogonally intersects the surface to be scanned is made a reference direction a, an angle: θ1 between the main light of the deflected luminous flux going toward one end A of an effective write-in area and the reference direction a is different from the angle: θ2 between the main light of the deflected luminous flux going toward the other end B of the effective write-in area and the reference direction a each other, and one or above of image forming elements of the scan image forming element are shifted and/or tilted for the reference direction a in a deflection plane so as to reduce the effect of a sag.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical scanning device.

[0002]

2. Description of the Related Art In an optical scanning apparatus, a scanning image forming element for converging a light beam deflected by an optical deflector on a surface to be scanned has conventionally been used when the principal ray of the deflected light beam is orthogonal to the surface to be scanned. The light beam is provided so as to coincide with the optical axis of the scanning imaging element, and the effective writing area is set so that the optical axis is centered.

[0003] In such a deployed state of the scanning imaging element,
The deflection angle of the deflected light beam toward one end of the effective writing area and the deflection angle of the deflected light beam toward the other end have the same magnitude. However, such an optical arrangement has a large effect on the design of an optical system in an optical scanning device. It is a constraint.

In general, a polygon mirror is generally used as an optical deflector for deflecting a light beam in an optical scanning device. However, in the polygon mirror, a rotation axis of a deflecting and reflecting surface is separated from the deflecting surface. There is a problem of `` sag '' in which the starting point of the deflection of the deflecting light beam deflected by the deflecting reflection surface fluctuates irregularly with the rotation of the polygon mirror, and the sag is asymmetric with respect to the optical axis of the scanning imaging element. Field curvature and f
θ characteristics, linearity, etc. also occur asymmetrically with respect to the optical axis. It is not easy to correct such asymmetric field curvature, fθ characteristics, linearity, and the like using a scanning imaging element symmetrical with respect to the optical axis.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to realize a novel optical scanning device having a high degree of freedom in designing an optical system. Another object of the present invention is to realize a novel optical scanning device which has a large degree of freedom in designing an optical system and can effectively reduce the influence of sag.

[0006]

An optical scanning device according to the present invention includes a light source, a first optical system, a polygon mirror, and a second optical system. The “light source” emits a light beam for optical writing. LD (semiconductor laser) or LED as light source
(Light emitting diode) can be suitably used. As the light source, a light source having a plurality of light-emitting sources such as an LD array can be used, and a multi-beam scanning in which a plurality of scanning lines are scanned at one time using such a light source can be performed. . The “first optical system” includes a coupling lens that couples a light beam from a light source. The “polygon mirror” is rotated by the drive motor and deflects the light beam from the first optical system at a constant angular velocity. The “second optical system” is an optical system that includes a “scanning imaging element” that converges a light beam deflected by a polygon mirror toward a surface to be scanned and scans the surface to be scanned at a substantially constant speed. The second optical system is a long lens (long toroidal lens or long cylindrical lens) for correcting surface tilt or correcting curvature of field, or an optical path bending for bending the optical path of the deflected light beam, in addition to the scanning image forming element. A mirror or the like can be included. `` Scanning imaging device ''
Is composed of one or more imaging elements. The optical scanning device according to claim 1, wherein when the direction of the principal ray of the light beam deflected by the polygon mirror is orthogonal to the scanning line on the surface to be scanned, the direction of the principal ray is one end of the effective writing area. angle formed between the principal ray is the reference direction of the deflected light beam proceeding: theta ₁ and the angular principal ray of the deflected beam toward the other end of the effective writing region forms the reference direction: Unlike the theta ₂ are together a scanning image forming element 1 of lens
The optical axis described above is shifted and / or tilted with respect to the reference direction in the deflection surface so as to reduce the influence of sag. In the "reference direction", "the principal ray of the light beam deflected by the polygon mirror is orthogonal to the scanning line on the surface to be scanned" means that the deflected light beam is not affected by the imaging action of the second optical system. This means that the light beam will be orthogonal to the scanning line on the surface to be scanned.

[0007] The "deflection surface" is a plane formed by sweeping the principal ray of the deflected light beam ideally deflected by the polygon mirror, and is orthogonal to the rotation axis of the polygon mirror. Note that, on a virtual optical path obtained by linearly developing the optical path from the light source to the surface to be scanned, a direction corresponding to the main scanning direction in parallel with the main scanning direction is referred to as a “main scanning corresponding direction”, and the sub-scanning direction is defined on the optical path. The direction corresponding in parallel to the above is referred to as a “sub-scanning corresponding direction”.
“Shift” means that the optical axis of the lens is shifted in parallel, and “Tilt” means that the optical axis is inclined with respect to the reference direction.

As described above, under the influence of the above-mentioned "sag", "the case where the field curvature is remarkable at one end in the reference direction" or "the constant velocity characteristic such as the fθ characteristic becomes one end in the reference direction". In the case where the image is significantly deteriorated in the portion, one or more of the image forming elements in the scanning image forming element of the second optical system are shifted in the main scanning corresponding direction, and the side where the field curvature becomes remarkable or the uniform velocity characteristic is reduced. By setting the angle on the side that is significantly deteriorated to be small and the angle on the opposite side to be large, an effective writing area is set in a portion where the field curvature or constant velocity characteristics are relatively good as a whole, and the influence of sag is set. Can be effectively reduced, and the degree of freedom in designing the optical scanning device can be increased, for example, it is easy to secure a space for disposing an optical system for detecting synchronous light. The tilt of the lens optical axis has the same effect as the above shift. The shift and the tilt can be given at the same time.

In addition to the coupling lens, the first optical system converges the light flux coupled by the coupling lens in the sub-scanning corresponding direction, and makes a long line in the main scanning corresponding direction near the deflecting and reflecting surface of the polygon mirror. The image forming apparatus may have a cylinder lens or a concave cylinder mirror for forming an image on the image. In this case, the second optical system may be configured such that "the deflecting reflection surface of the polygon mirror and the surface to be scanned are substantially conjugated with respect to the sub-scanning corresponding direction. (Claim 2). By doing so, it is possible to correct the "face tilt" of the polygon mirror.

The coupling lens of the first optical system is an optical system for "coupling the light beam from the light source to the subsequent optical system". The effect of the coupling lens is to convert the light beam from the light source to " The function may be a function of converting into a "parallel light beam" (claim 3) or a function of converting a light beam from a light source into a "weak convergence light beam or weak divergent light beam" (claim 4). In the optical scanning device according to any one of claims 1 to 4, the scanning image forming element may include a “reflecting mirror having an image forming function” as the image forming element.

The "scanning image forming element" of the second optical system can be formed as a single lens (claim 5) or as two or more lenses (claim 7). When the scanning imaging element is composed of two or more lenses, a lens that gives a shift and a lens that gives a tilt may be set separately.

When the scanning image forming element is a single lens, at least one surface thereof is referred to as a special lens in which a line connecting the center of curvature in the plane orthogonal to the deflection in the main scanning direction is non-circular and non-linear in the deflection plane. It may be a toric surface. Further, as described above, the scanning image forming element is provided with a scanning image forming mirror (alone or in cooperation with another optical element) to converge the deflected light beam as a light spot on the surface to be scanned, and scan the same. A concave mirror having a function of equalizing the speed can also be used. Further, the light detecting means for detecting the deflected light flux deflected by the polygon mirror and transmitted through at least a part of the scanning image forming element can be provided on the side where the angles: θ ₁ and θ ₂ are small (claim 8). ).

An optical scanning device according to a ninth aspect of the present invention includes a light source, a first optical system, a polygon mirror, and a second optical system. These are the same as those in the optical scanning device according to claim 1, and the second optical system is “one or more imaging elements”.
It consists of. The optical scanning device according to the ninth aspect has the following features. That is, when the direction of the principal ray when the principal ray of the deflected light beam by the polygon mirror is orthogonal to the scanning line on the surface to be scanned is set as the reference direction, the principal ray of the deflected light beam directed to one end of the effective writing area is The angle formed by the reference direction: θ ₁ and the angle formed by the principal ray of the deflected light beam directed to the other end of the effective writing area to the reference direction: θ ₂ are different from each other. The light detecting means for detecting the deflected light beam transmitted through at least a part of the above is disposed on the side where the angles θ ₁ and θ ₂ are smaller.

[0014]

FIG. 1 shows a light source 1 which is an LD.
Divergent light beam emitted from the coupling lens 2
Into a "parallel light beam" or a "light beam having weak convergence or divergence". The light is converged by the cylinder lens 3 in a direction corresponding to the sub-scanning direction (a direction orthogonal to the drawing), and is deflected by the deflecting and reflecting surface 4 of the polygon mirror 4. A linear image long in the main scanning corresponding direction is formed in the vicinity of ', and the light flux reflected by the deflecting / reflecting surface 4 ′ is deflected at a constant angular velocity by the constant speed rotation of the polygon mirror 4. The deflected light beam is incident on the scanning imaging element 5 and is condensed toward the surface 6 to be scanned by the operation of the element 5 to form a light spot, and scans the surface 6 to be scanned at a substantially constant speed. Since a photoconductive photoconductor is actually provided at the position of the surface 6 to be scanned, the light spot substantially optically scans the photoconductor surface.

The direction of the principal ray when the principal ray of the light beam deflected by the polygon mirror 4 is "perpendicular to the scanning line on the surface to be scanned" is referred to as "reference direction". Figure 1 shows the "reference direction"
Is denoted by the symbol a. In other words, the reference direction is the main direction when the deflected light beam is “perpendicular to the scanning line (trajectory of the light spot) on the surface 6 to be scanned, assuming that the deflected light beam is not affected by the imaging effect of the scanning imaging element 5”. The direction of the light beam. In FIG. 1, an area between the positions indicated by reference signs A and B on the scanned surface 6 is referred to as an “effective writing area”.

The angle θ ₁ between the principal ray of the deflected light beam directed to one end A of the effective writing area and the reference direction a, and the principal ray of the deflected light beam directed to the other end B of the effective writing area corresponds to the reference direction a. Horn:
and θ ₂ different from each other. In the embodiment of FIG.
One end A of the effective writing area is the writing start side, and the angle on the writing start side: θ ₁ is made smaller than the angle on the other end B, the writing end side: θ ₂ , and is deflected by the polygon mirror 4. the light detecting means 7, 8 for detecting the deflected light flux transmitted through at least a portion of the scanning imaging device 5 (mirror 7 and the photodetector 8), square: deploying the side of the theta _1. Angle: θ ₁ and the corner:
By arranging the light detecting means on the side of the small angle θ ₁ without setting θ ₂ to be the same, a space for arranging the light detecting means can be secured without increasing the diameter of the scanning imaging element 5.
The degree of freedom in designing the optical system can be increased.

The optical system (coupling lens 2 and cylinder lens 3) closer to the light source 1 than the polygon mirror 4 is the "first optical system", and the polygon mirror 4 and the scanning surface 6
The optical system provided between the first optical system and the second optical system is a “second optical system”. In FIG. 1, optical elements other than the scanning image forming element 5 of the second optical system are not shown. In FIG. 1, the scanning image forming element 5 is a “single lens”, but it is needless to say that the scanning image forming element can be composed of two or more lenses. Here, the “shift” and “tilt” of the scanning imaging element 5 will be described. The optical axis of the scanning imaging element 5 is set so that the optical axis of the scanning imaging element 5 is within the reference direction a within the deflection surface (the surface shown in FIG. 1). Is referred to as a shift, and the magnitude thereof (in FIG. 1, the upward shift is defined as positive) is indicated by “Δ” as shown in the figure. Further, the fact that the optical axis of the scanning imaging element 5 forms a finite angle with respect to the reference direction a in the deflection plane (the counterclockwise direction in FIG. 1 is defined as positive) is referred to as tilt, and the magnitude thereof is referred to as “tilt”. β ”.

As described above, the scanning image forming element 5 includes one or more lenses of the scanning image forming element (when the scanning image forming element 5 is a single lens, the single lens, and the scanning image forming element 5 includes a plurality of lenses). When composed of one or more lenses)
Are deployed shifted and / or tilted.

That is, the optical scanning device shown in the embodiment of FIG. 1 includes a light source 1, first optical systems 2 and 3 including a coupling lens 2 for coupling a light beam from the light source, and the first optical system 2.
A polygon mirror 4 for deflecting a light beam from the optical system at a constant angular velocity, and a scanning image element for converging the light beam deflected by the polygon mirror toward the surface 6 to be scanned and scanning the surface 6 to be scanned at a substantially constant speed. A second optical system including a sub-element 5, wherein the scanning image-forming element comprises one or more image-forming elements,
When the direction of the principal ray of the deflected light beam due to the above is perpendicular to the scanning line on the surface to be scanned, the principal direction of the deflected light beam directed to one end of the effective writing area is defined as the reference direction. The angle: θ ₁ is different from the angle: θ ₂ formed by the principal ray of the deflected light beam toward the other end of the effective writing area with the reference direction, and at least one of the imaging elements of the scanning imaging element 5 The light is shifted and / or tilted with respect to the reference direction a in the deflection surface so as to reduce the influence of sag (claim 1). Further, the first optical system has a cylinder lens 3 that converges the coupled light flux in the sub-scanning corresponding direction and forms a linear image long in the main scanning corresponding direction near the deflection reflection surface of the polygon mirror 4. The second optical system 5 makes the deflecting reflection surface of the polygon mirror and the surface to be scanned substantially conjugate with each other in the sub-scanning corresponding direction (Claim 2), and the coupling lens of the first optical system uses the light flux from the light source. Is converted into a parallel light beam (claim 3) or a light beam with weak focusing or weak divergence (claim 4). Further, light detecting means 7 and 8 for detecting deflected light fluxes deflected by the polygon mirror 4 and transmitted through at least a part of the scanning image forming element 5 have angles: θ ₁ and θ _2.
(Claim 8).

FIG. 7 shows an optical scanning device according to a ninth embodiment of the present invention. A light source (not shown), a first optical system (not shown) including a coupling lens for coupling a light beam from the light source, a polygon mirror 4A for deflecting the light beam from the first optical system at a constant angular velocity, and the polygon A second optical system 5 including a scanning imaging element that converges the light beam deflected by the mirror 4A toward the scanning surface 6 and scans the scanning surface 6 at a substantially constant speed. Is composed of one or more imaging elements, and when the direction of the principal ray when the principal ray of the light beam deflected by the polygon mirror 4A is orthogonal to the scanning line on the surface to be scanned is the reference direction a, the effective writing area corners principal ray of the deflected beam toward the one end A1 forms the reference direction a: the theta _1, angle between the principal ray of the deflected light beam toward the other end B1 of the effective writing region and the reference direction a: theta ₂ and are different from each other ,
The light detecting means 7 and 8 for detecting the deflected light flux deflected by the polygon mirror 4A and transmitted through at least a part of the scanning image forming element 5 are on the smaller side of the angles θ ₁ and θ ₂ (the side of the angle θ ₁ ).
Has been deployed.

The difference from the embodiment of FIG. 1 described above is that no shift / tilt is given to the scanning imaging element 5 in this embodiment. The light beam deflection by the polygon mirror is repeated for each deflection reflection surface of the polygon mirror. For example, when the size of the polygon mirror is reduced, depending on the arrangement of the optical system, the light beam deflection toward one end A1 of the effective writing area may be performed. The angle formed by the principal ray with the reference direction a: θ ₁ and the angle formed by the principal ray of the deflected light beam directed to the other end B ₁ of the effective writing area with the reference direction a: θ ₂ by “θ ₁ = θ ₂ ”.
In this case, as shown in FIG. 7, the deflected light beam directed to either the write start side or the end side of the effective write area is reflected by the "corner portion" of the polygon mirror 4A. It is possible.
In FIG. 7, when the deflecting light beam reflected at the corner portion of the polygon mirror 4A is directed toward the writing start side, in the portion scanned by the reflected light beam at the corner portion, the deflecting light beam is partially formed by the corner. However, there is a risk that normal optical scanning may not be performed in this portion.

In such a case, as shown in FIG. 7, by setting each of θ ₁ <θ ₂ , the center of the effective writing area is shifted toward the end position B 1 with respect to the reference direction a, and the position A 1 And B1, good optical scanning with an appropriate deflection light beam without vignetting at the corner is performed, and the light detecting means 7 and 8 are provided in the space generated on the writing start side. It is sufficient for the light detecting means to detect only the deflected light beam, so that even a deflected light beam partially vignetted at a corner can be sufficiently detected.

[0023]

EXAMPLES Five specific examples will be given. The optical scanning device of each embodiment described below has the optical arrangement described with reference to FIG. 1, and includes the center thickness of the cylinder lens 3, the refractive index of the material, and the sub-scanning of the first and second surfaces. The radii of curvature in the corresponding directions are “d ₁ , n ₁ , r _1s , r _2s ”, respectively, and the “reflection position (reflection when the principal ray of the deflected light beam matches the reference direction a)” by the polygon mirror 4 from the cylinder lens 3. the distance to the position) "," d ₂ ", the distance in the reference direction a from the reference reflecting position to the first surface of the scanning imaging device 5 and" d ₃ ". The refractive indices of the lens materials are all related to the wavelength used (the emission wavelength of the light source 1).

When it is assumed that the coupled light beam is not affected by the optical system thereafter, the position where light is naturally condensed is called a "natural light condensing point", and the light naturally condenses from the reference reflection position. The distance to the point (positive toward the surface to be scanned) is defined as “S”. The angle between the principal ray of the light beam incident on the polygon mirror 4 from the first optical system side and the reference direction a is "α" as shown in FIG. Regarding the polygon mirror, the number of the deflecting and reflecting surfaces is "N", the radius of the inscribed circle is "R '", and the distance between the principal ray of the light beam incident from the first optical system side and the rotation center of the polygon mirror 4 is shown in FIG. Like "h"
And In addition, the unit having the "dimension of distance" is "m
m ”.

Example 1 Example 1 is an example in which the scanning image forming element 5 is composed of two lenses in the embodiment described with reference to FIG. 1 (claim 7). These two lenses are referred to as first and second lenses sequentially from the polygon mirror 4 side.

Following the distance from the reference reflection position to the first surface (incident side surface) of the first lens (the above “d ₃ ”), the center thickness and the refractive index of the first lens are respectively “d ₄ , n ₂ ”. The distance from the second surface of the first lens to the incident side surface of the second lens, the center thickness of the second lens, and the refractive index at the wavelength used are respectively “d ₅ , d ₆ , n ₃ ”. The distance from the emission side surface to the surface to be scanned is “d ₇ ”.

S = ∞ (Coupled into a “parallel beam” by a coupling lens) Angle: α = 60 degrees Polygon mirror: R ′ = 18, N = 6, h = 10.2 Angle: θ ₁ = 44 Degree, angle: θ ₂ = 46 degrees d ₁ = 3.0, n ₁ = 1.511176, r _1S = 71.5
(Cylinder surface), r _2S = ∞, d ₂ = 100, d ₃ = 40,
d ₄ = 13.8, n ₂ = 1.537, d ₅ = 8.6, d ₆ =
11.5, n ₃ = 1.537, d ₇ = 101.1.

Both the “incident side surface” of the first lens and the “incident side surface” of the second lens have a “non-arc shape” in the deflection surface (shape in the main scanning direction).

The non-arc shape has a paraxial radius of curvature: R, a conic constant: K, and higher-order coefficients: A, B, C, D,. . Using,
Coordinates in the optical axis direction: X and coordinates in the optical axis orthogonal direction (direction corresponding to main scanning when no shift / tilt is given): Y is expressed as follows: X = Y ² / [R + R√ ｛1− (1 + K) Y ² / R ² }] + AY ⁴ + BY ⁶ + CY ⁸ + DY ¹⁰ +. . . In the shape represented by (1), R, K, A, B, C,
D,. . And the shape specified. Higher order coefficients:
A, B, C, D,. Is determined such that “the power of Y increases sequentially by the square”.

"Shape of the incident side surface" of the first and second lenses
Is a "non-circular toroidal surface" obtained by rotating the non-circular shape around an axis parallel to the main scanning direction in the deflection surface, and "between the non-circular shape and the axis" on the X axis. Distance: R _S.

The “surface on the exit side” of each of the first and second lenses is a “normal toroidal surface”, and has a radius of curvature: _RM and a radius of curvature in the deflection surface determined by the optical axis direction and the main scanning direction. The shape is specified by giving the radius of curvature in the plane orthogonal to the deflection including the optical axis: R _S.

Note that, in the following numerical expressions, E and the numbers following them represent "powers". For example, "E-9"
Means 10 to the 9th power, and this number is over the previous number.

The "shape of the incidence side lens surface" first lens R K A B C D -310.0 27.65 -1.248E-6 5.487E-10 -7.024E-13 3.769E-16 E F G H R S 7.854E- 20 -4.369E-23 -2.755E-26 7.47E -30 -19.5 "shape of the exit-side lens surface" R _M R _M -87.5 -31.0.

The "shape of the incidence side lens surface" second lens R K A B C D -246.0 -15.44 7.637E-7 -6.719E-11 -5.312E-15 -9.740E-19 E F G H R S - 1.276E-23 -3.519E-25 2.322E- 28 -3.175-32 -135.0 "shape of the exit-side lens surface" R _M R _M -248.3 -19.94.

The first and second lenses are both given a shift and a tilt. First lens: Δ = 1.39, β = 0.55 degrees Second lens: Δ = 1.47, β = −0.7 degrees Field curvature in the first embodiment (main scanning direction: broken line, sub-scanning direction) : Solid line) and constant velocity characteristics (fθ characteristic: broken line and linearity: solid line) are shown in FIG.

As described above, in the first embodiment, the coupling lens converts the light beam from the light source into a parallel light beam (claim 3), and the scanning image forming element of the second optical system is composed of two or more lenses. According to a seventh aspect of the present invention, at least one lens of the scanning image forming element is provided so that its optical axis is shifted and tilted in the deflection plane with respect to the reference direction (the first aspect).

Embodiment 2 Embodiment 2 is an embodiment corresponding to the invention of claim 9 described with reference to FIG. FIG. 9 is a diagram of field curvature and constant velocity characteristics when θ ₁ = θ ₂ = 45 degrees when the same optical system as in the first embodiment is used, and no shift or tilt is applied to each lens of the scanning image forming element. As shown in FIG. 3 and FIG. 2,
It can be seen that the curvature of field and the linearity in the sub-scanning direction on the writing start side (upper end in the drawing) of the optical scanning are deteriorated as compared with the first embodiment. This means that the shift / tilt in the first embodiment effectively reduces the influence of sag and improves the curvature of field and linearity satisfactorily.

However, when FIG. 2 and FIG. 3 are compared in detail, it is found that the linearity is greatly deteriorated on the write start side. Therefore, in the above case, θ ₁
= Instead of the theta ₂ = 45 degrees, by the theta ₁ <45 degrees, perform optical scanning with a good area of the curvature of field, or constant velocity characteristics, start "writing by the theta ₁ <45 degrees By installing light detection means in the space created on the side,
The optical scanning device can be effectively made compact, and the degree of freedom in design increases. On the writing end side, if θ ₂ = 45 degrees, it is inevitable that the effective writing area becomes narrow. In such a case, if the field curvature and the constant velocity characteristic are θ ₂
If it is good even in the region of> 45 degrees, the writing end side may be extended as θ ₂ > 45 to compensate for the decrease in the scanning area on the writing start side. Of course, the invention according to claim 9 is based on the premise that the "asymmetrical shape due to sag" of the curvature of field and the constant velocity characteristics is not so remarkable.

The figures of the curvature of field and the constant velocity characteristics shown in FIGS. 3 and 4 are drawn in accordance with FIG.
In the diagram of the field curvature, the broken line is the main scanning direction and the solid line is the sub-scanning direction. In the diagram of the constant velocity characteristic, the broken line is the fθ characteristic and the solid line is the linearity.

Examples 3 to 5 described below are claims 4 and
It is an Example of the invention described in 5 and 6. In the third to fifth embodiments, the scanning image forming element 5 shown in FIG. 1 is constituted by a “single lens” (Claim 5). The luminous flux coupled by the coupling lens has "weak convergence" in all of Examples 2 to 4 (claim 4), and S> 0.

As in the first embodiment, the center thickness, the refractive index, and the radii of curvature of the incident side surface and the exit side surface of the cylinder lens 3 in the sub-scanning corresponding direction are denoted by d ₁ , n ₁ , r _1S , r _2S , respectively.
Scanning and imaging device the distance from the cylinder lens 3 to the reference reflecting position "d _2", from the reference reflection position (single lens)
Is the distance in the reference direction to the entrance side lens surface of “d ₃ ”, the center thickness and the refractive index of the scanning imaging element are “d ₄ , n ₂ ”
The distance from the emission side surface of the scanning image forming element to the scanned surface 6 is “d ₅ ”.

The scanning imaging element has a non-circular shape in the deflection plane (shape in the main scanning direction) expressed by the above equation (1) on both surfaces, and each "symbol" in the equation (1).
With a suffix: n, and n = 1,
Assuming that n = 2 on the surface on the injection side, paraxial radius of curvature: R _n ,
Conic constant: K _n, higher order _{coefficients: A n, B n, C} n,
D _n,. . . These are given to specify the shape.

Also, both surfaces of the scanning image forming element are the above-mentioned "special toric surface (Claim 5)", and are located within the "deflection orthogonal plane" at a distance of Y from the optical axis in the main scanning direction. Curvature radius (curvature radius in the sub-scanning corresponding direction): R
_Sn is a _{polynomial: R Sn = R S0n + a} n Y 2 + b n Y 4 + c n Y 6 + d n y 8 + e n Y
¹⁰ + f _n Y ¹² + g _n Y ¹⁴ +. . . (N = 1 on the entrance side, n = 2 on the exit side)
, R _S0n , a _n , b _{n ,.} . . Given.

Embodiment 3 S = 1372.8 (Coupled to "weak convergent light beam" by a coupling lens) Angle: α = 60 degrees Polygon mirror: R ′ = 18, N = 6, h = 9.35 Angle: θ ₁ = 43.6 degrees, Angle: θ ₂ = 44.4 degrees d ₁ = 3.0, n ₁ = 1.51933, r _1S = 13.81
(Cylinder surface), r _2S = ∞, d ₂ = 25.0, d ₃ = 3
3.1, d ₄ = 13.5, n ₂ = 1.51933, d ₅ =
128.4,.

The "shape of the deflecting surface" _{n R n K n A n B} n C n D n 1 160.325 -58.38 -9.22923E-7 3.65515E-10 -8.34355E-14 1.113E-17 2 -139.26 4.83 - 9.71348E-7 2.37E-10 -8.06014E- 14 2.65E-17 "shape in the deflection plane perpendicular" _{n R S0n a n b n c} n d n e n 1 -108.6 7.803E-2 -3.15051E-4 8.16834E-7 -1.10138E-9 7.352E- 13 2 -15.2 -1.6873E-3 3.41942E-6 -4.2899E-9 5.634E-12 -4.189E-15 n f n 1 -1.8802E-16 2 1.2966 E-18.

The scanning imaging element is given "shift only". That is, Δ = + 0.74, β = 0 FIG. 4 shows the curvature of field in the main and sub-scanning directions and the constant velocity characteristics (fθ characteristics and linearity) in the third embodiment. Field curvature
Both constant velocity characteristics are well corrected. Since the deflected light beam has a weak convergence in the main scanning direction, the scanning image forming element is not strictly an fθ lens, but the fθ characteristic in the constant velocity characteristic was calculated in the same manner as the fθ characteristic for the fθ lens. The same applies to the following Examples 3 and 4.

Example 4 S = 311.1 (Coupled to "weak convergent light beam" by a coupling lens) Angle: α = 60 degrees Polygon mirror: R ′ = 18, N = 6, h = 9.6 Angle: θ ₁ = 44.2 degrees, Angle: θ ₂ = 45.7 degrees d ₁ = 3.0, n ₁ = 1.51933, r _1S = 44.68
(Cylinder surface), r _2S = ∞, d ₂ = 70.0, d ₃ = 4
8.1, d ₄ = 20.0, n ₂ = 1.51933, d ₅ =
106.9,.

[0048] "shapes in the deflection plane" _{n R n K n A n B} n C n D n 1 199.5 -35.1384 -1.9846E-7 2.1692E-11 1.9018E-15 -1.88E-19 2 -212.0 2.106 - 3.709E-7 1.7132E-11 -5.93E- 15 1.494E-18 "shape in the deflection plane perpendicular" _{n R S0n a n b n c} n d n e n 1 -40.03 -1.19E-2 1.678E-5 -1.7646E-8 9.9902E-12 -2.8335E- 15 2 -15.973 -8.58E-4 2.072E-7 1.505E-9 -1.77196E-12 9.1971E-16 n f n g n 1 3.154E-19 0.0 2-2.28E-19 2.18171E-23.

"Tilt only" is given to the scanning image forming element. That is, Δ = 0, β = −0.2 degrees FIG. 5 shows the curvature of field in the main and sub scanning directions and the constant velocity characteristics (fθ characteristics and linearity) in the fourth embodiment. Both the curvature of field and the constant velocity characteristics are well corrected.

Fifth Embodiment In the third embodiment, the shift of the scanning image forming element is Δ = +
From 0.74, change to Δ = + 0.80 and tilt = β = 0
(No tilt) was changed to β = 0.14 degrees. At this time, the field curvature / constant velocity characteristics are well corrected as shown in FIG.

In the first to fourth embodiments, the angle α is set to 60 degrees. However, the angle α is further reduced so that the optical elements of the first optical system and the second optical system interfere with each other. Also in the case of ( ₁₎ , by setting the smaller one of the angles: θ ₁ and θ ₂ to the interfering side, it is possible to avoid interference between the optical path of the first optical system and the optical path of the second optical system.

When the scanning image forming element is a single lens, a normal toric surface may be used in place of the special toric surface (claim 5).

[0053]

As described above, according to the present invention, a novel optical scanning device can be realized. Since the optical scanning device of the present invention is configured as described above, the degree of freedom in designing the optical scanning device is large, and by giving a shift (and tilt) to the scanning image forming element, the field curvature caused by sag or the like can be reduced. Deterioration of the constant velocity characteristic can be effectively corrected.

The present invention is also applicable to a case where the scanning image forming element of the second optical system has an asymmetric shape in the main scanning direction, and the effect of sag countermeasures is great. According to the ninth aspect, the optical scanning device can be made compact.

[Brief description of the drawings]

FIG. 1 is a diagram for describing an embodiment of an optical scanning device according to the present invention.

FIG. 2 is a diagram illustrating field curvature and constant velocity characteristics according to the first embodiment.

FIG. 3 is a diagram illustrating field curvature and constant velocity characteristics according to a second embodiment.

FIG. 4 is a diagram illustrating field curvature and constant velocity characteristics according to a third embodiment.

FIG. 5 is a diagram illustrating field curvature and constant velocity characteristics according to a fourth embodiment.

FIG. 6 is a diagram illustrating field curvature and constant velocity characteristics according to a fifth embodiment.

FIG. 7 is a view for explaining an embodiment of the invention described in claim 9;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Coupling lens 3 Cylinder lens 4 Polygon mirror 5 Scanning imaging element 6 Scanning surface

Claims (9)

[Claims] 1. A light source; a first optical system including a coupling lens for coupling a light beam from the light source; a polygon mirror for deflecting the light beam from the first optical system at a constant angular velocity; A second optical system that includes a scanning imaging element that converges the deflected light beam toward the surface to be scanned and scans the surface to be scanned at a substantially constant speed; and the scanning imaging element includes one or more imaging elements. When the direction of the principal ray when the principal ray of the deflected light beam by the polygon mirror is orthogonal to the scanning line on the surface to be scanned is defined as a reference direction, the deflected light beam directed to one end of the effective writing area and theta _1, angular principal ray of the deflected beam toward the other end of the effective writing region makes with the reference direction: angle principal ray makes with the reference direction different theta ₂ and each other, the scanning image forming element One or more imaging elements reduce the effects of sag As described above, in the deflection plane, an optical scanning apparatus characterized by being shifted and / or tilted with respect to the reference direction. 2. The optical scanning device according to claim 1, wherein the first optical system converges the coupled light beam in a direction corresponding to the sub-scanning direction, and extends near the deflecting reflection surface of the polygon mirror in the direction corresponding to the main scanning direction. A cylinder lens for forming a line image, wherein the second optical system has a substantially conjugate relationship between the deflection reflecting surface of the polygon mirror and the surface to be scanned in the sub-scanning corresponding direction. Optical scanning device. 3. The optical scanning device according to claim 1, wherein the coupling lens of the first optical system converts a light beam from the light source into a parallel light beam. 4. The light scanning device according to claim 1, wherein the coupling lens of the first optical system converts a light beam from the light source into a light beam having a weak convergence or a weak divergence. Scanning device. 5. The optical scanning device according to claim 1, wherein the scanning image forming element of the second optical system is a single lens. 6. The optical scanning device according to claim 5, wherein at least one surface of the single lens serving as the scanning image forming element has a line extending in the main scanning corresponding direction with a center of curvature in a plane orthogonal to the deflection in the plane of deflection. An optical scanning device having a special toric surface that is non-circular and non-linear. 7. The optical scanning device according to claim 1, wherein the scanning image forming element of the second optical system comprises two or more lenses. 8. An optical scanning device according to claim 1, wherein a light detecting means for detecting a deflected light beam deflected by the polygon mirror and transmitted through at least a part of the scanning image forming element is provided.
Angle: An optical scanning device which is provided on the smaller side of θ ₁ and θ ₂ . 9. A light source; a first optical system including a coupling lens for coupling a light beam from the light source; a polygon mirror for deflecting the light beam from the first optical system at a constant angular velocity; A second optical system that includes a scanning imaging element that converges the deflected light beam toward the surface to be scanned and scans the surface to be scanned at a substantially constant speed; and the scanning imaging element includes one or more imaging elements. When the direction of the principal ray when the principal ray of the deflected light beam by the polygon mirror is orthogonal to the scanning line on the surface to be scanned is defined as a reference direction, the deflected light beam directed to one end of the effective writing area angle principal ray makes with the reference direction: the theta _1, the effective principal ray of the deflected beam toward the other end of the write region is formed with the reference direction angle: theta ₂ and are different from each other, are deflected by the polygon mirror, At least a part of the scanning imaging element Light detecting means for detecting the transmitted deflected light beam,
An optical scanning device, wherein the optical scanning device is provided on the side where the above-mentioned angles: θ ₁ and θ ₂ are small.