JP3331707B2 - Optical scanning device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device used for a laser printer or the like.

[0002]

2. Description of the Related Art In an optical scanning device used in a laser printer, a laser beam is deflectively scanned on a photosensitive drum surface, which is a surface to be scanned, by a combination of a scanning lens and a rotary polygon mirror. The purpose of the optical scanning device is to narrow down the laser beam to a predetermined size corresponding to the resolution and scan the laser beam on the photosensitive drum surface at a constant speed. In recent years, demands for higher resolution and lower cost of laser printers have been increasing. When a single Fθ lens is used as the scanning lens, the cost can be reduced by using a plastic material.

[0003]

However, if a single plastic lens is used as the scanning lens, a curvature of field in the scanning vertical direction occurs with a change in the environmental temperature, and the imaging characteristics are degraded. This is because the change in the refractive index due to the temperature of the plastic material is larger than that of the glass, and the vertical magnification in the scanning vertical direction of the single lens is inevitably increased. Note that, here, field curvature including defocus on the optical axis is also referred to. Accordingly, it is an object of the present invention to provide a high-resolution and low-cost optical scanning device that eliminates the above-mentioned disadvantages of the conventional technology.

[0004]

In order to achieve the above object, the scanning lens of the present invention comprises two lenses, and the lens on the optical deflector side corresponds to each deflection position of the optical deflector. The aspherical lens has a shape in which the curvature in the scanning vertical direction changes continuously along the scanning direction, and the lens on the surface to be scanned is a cylindrical lens having power only in the scanning vertical direction. Reduced. The price of the aspherical lens was reduced by using a plastic material.

When the radius of curvature on the optical axis is R and the conic constant is K, the shape of the aspherical lens in the scanning direction is as follows:
The angle formed by the light beam giving the maximum scanning width with the optical axis on the incident side of the scanning lens and the optical axis on the output side of the scanning lens are represented by V, V ′, and SIG (K), respectively, where K is the sign of K. At this time, the formula (2) was satisfied.

[0006]

(Equation 1)

[0007]

(Equation 2)

[0008]

FIG. 2 shows the manner in which the scanning lens is composed of a single aspherical plastic lens, and when the ambient temperature changes from room temperature to a high temperature, scanning vertical field curvature occurs. In the figure, 1 is a rotating polygon mirror, 6 is a photosensitive drum surface, 43 is an aspherical lens, 92 is a beam at room temperature, 93 is a beam at high temperature, 62 is an image plane in the scanning vertical direction at room temperature, and 63 is a high temperature. Is the image plane in the scanning vertical direction. Hereinafter, the radius of curvature, the image plane, and the field curvature refer to the scanning vertical direction. Since the radius of curvature of the aspherical lens 43 changes continuously at the deflection position so that the curvature of field becomes zero at normal temperature, the image surface 62 coincides with the photosensitive drum surface 6 if there is no error in the radius of curvature. In particular, considering the entrance and exit of the reflecting surface of the rotating polygon mirror,
This effect is large in an aspheric lens having an asymmetric shape. On the other hand, at high temperatures, the refractive index of the plastic material decreases and the power of the aspherical lens decreases, so that the image surface 63 is separated from the photosensitive drum surface 6.

FIG. 3 shows an aspherical lens and a cylindrical lens, both of which are made of plastic material.
This figure shows how the image field curvature occurs when the ambient temperature changes from normal temperature to high temperature when the sheet is constituted by sheets. In the figure, 44
Is an aspheric lens, 45 is a cylindrical lens, 94 is a beam at room temperature, 95 is a beam at high temperature, 64 is an image surface at room temperature, and 65 is an image surface at high temperature. Aspheric lens 44
Since the radius of curvature changes continuously at the deflection position so that the curvature of field becomes zero at room temperature, the image surface 64 coincides with the photosensitive drum surface 6 if there is no error in the radius of curvature. on the other hand,
At a high temperature, the refractive index of the plastic material decreases, and the power of the aspherical lens 44 and the cylindrical lens 45 decreases, so that the image surface 65 is separated from the photosensitive drum surface 6. Smaller than the difference.

Hereinafter, a type in which the scanning lens is composed of one aspherical plastic lens is referred to as an A type, and a type in which the scanning lens is composed of two aspherical lenses and a cylindrical lens is referred to as a B type.

FIG. 4 shows an A-type image. FIG. 5 shows a B-type image. In the figure, 15 is an aspheric lens 4
4 is a plane perpendicular to the optical axis passing through the principal combination point of the cylindrical lens 45 and the cylindrical lens 45. For simplicity, the distance between the object-side principal point and the image-side principal point is disregarded and is indicated by a single principal point. Aspheric lens 4
3,44 and the power of the cylindrical lens 45, respectively P1, P _20, P _cyl, the combined power of P ₂₀ and P _cyl and P2. A and B type rotating polygon mirror 1
Is an object point and the photosensitive drum surface 6 is an image point. The distance between the A-type aspherical lens 43 and the B-type aspherical lens 44 from the rotating polygon mirror 1 is the same, and the position of the combining principal point 15 is approximately intermediate between the rotating polygon mirror 1 and the photosensitive drum surface 6. Are determined as in equations (5) to (9).

1 / S1 ′ = 1 / S1 + P1 (5) 1 / S2 ′ = 1 / S2 + P2 (6) −S1 + S1 ′ = − S2 + S2 ′ = L> 0 (7) S2 < S1 <0 (8) −S2 ≒ L / 2 (9) Here, S + S ′ = L (10) is obtained as S that gives the minimum value of P. From equation (5), P = 1 / (L + S) −1 / S (11) 1 / P = −S (L + S) / L (12) First and second derivatives of S are obtained. .

(1 / P) ′ = − 2S / L (13) (1 / P) ″ = − 2 / L (14) In order to obtain S having an extreme value (1 / P) ′ = 0 (15) S = −L / 2 (16) and takes a minimum value, and therefore, P1>P2> 0 from equations (7) and (8). From the definition of geometrical optics, U1 ′ = U1 + H · P1 (18) U2 ′ = U2 + H · P2 (19) Here, the materials of the aspheric lenses 43 and 44 and the cylindrical lens 45 Α = (N−ΔN) / N (0 <α <1) (20) where N is the refractive index of the plastic material at room temperature and (N−ΔN) is the refractive index at high temperature. 6 and 7 show images of the A type and the B type at a high temperature, respectively.
The optical conjugate relationship with the photosensitive drum surface 6 does not hold. Equations (18) and (19) are respectively (21) and (2)
2) It can be rewritten as in equation (2).

U1′b = U1 + H · αP1 (21) U2′b = U2 + H · αP2 (22) From equation (20), U1′b <U1 ′ (23) U2′b < U2 '(24) Here, assuming that U is small, and SinU = U (25), as is clear from FIGS. 4 to 7, S1 = H / U1 (26) S1' = H / U1 ′ (27) S2 = H / U2 (28) S2 ′ = H / U2 ′ (29) Since (7), (8), (27) to (29) From the equation, U1 <U2 <0 (30) 0 <U1 ′ <U2 ′ (31) Also, U2′−U1 = ΔU, and the ratio at normal temperature and high temperature is A.
Type and B type, take this as δ, (1
7) to (19), (21), and (22).

Δ = U2′b / U2′−U1′b / U1 ′> U2′b / U2 ′ − (U1′b + ΔU) / (U1 ′ + ΔU) = U2′b / U2 ′ − (U1′b + ΔU) /U2'=H(1-.alpha.)(P2-P1)/U2'>0 (32) Then, from equations (26) to (29) and (32), S2 '/ S2'b-S1' / S1'b> 0 (33) ΔS1 '= S1'b-S1' (34) ΔS2 '= S2'b-S2' (35) Substituting and deforming, S1'b / S1'-S2'b / S2 '> 0 (S1' +. DELTA.S1 ') / S1'-(S2 '+. DELTA.S2') / S2 '>0.DELTA.S1' / S1 '-. DELTA.S2' / S2 ′> 0 ΔS1′−ΔS2 ′> 0 (36) As a result, the image forming position of the B type is higher than that of the A type due to a change in environmental temperature. Shift it can be seen that is small.

Further, by setting the shape of the aspherical lens 44 in the scanning direction as in the above formulas (1) and (2), the laser light is narrowed down to a predetermined size corresponding to the resolution on the surface to be scanned. This is a condition for scanning at a constant speed. If the shape of one lens in the scanning direction is circular on both sides, the beam is bent more than necessary at the periphery compared to the optical axis, so the image plane in the scanning direction is curved to the rotating polygon mirror side (-), The height is large (+) at the intermediate image height. For this reason, it is necessary to suppress the beam from being bent more than necessary at the periphery of the lens as a conic constant; not as a circle with K = 0 but as a hyperbola with K <0. In particular, when the expression (2) is satisfied, the effect is great.
Beyond the lower limit, the curvature of field in the scanning direction becomes +, and the image height error becomes-at the intermediate image height. On the other hand, when the value exceeds the upper limit, the curvature of field in the scanning direction becomes minus, and the image height error becomes plus at the intermediate image height.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a configuration diagram showing an embodiment of the optical scanning device according to the present invention. In the figure, 1 is a rotating polygon mirror, 6 is a photosensitive drum surface, 8 is a cylindrical lens, 12 is a collimator lens, 22 is a laser light source, 44 is an aspheric lens,
45 is a cylindrical lens and 91 is a beam.

The beam 91 emitted from the laser light source 22 passes through the collimator lens 12 and becomes a substantially parallel beam. The cylindrical lens 8 is arranged so as to act only in the scanning vertical direction (Y direction).
The beam is focused in the direction. In the scanning vertical direction, the vicinity of the reflection surface and the photosensitive drum surface 6 have a geometric conjugate relationship. A scanning lens for beam scanning is composed of an aspheric lens 44 and a cylindrical lens 45 both made of a plastic material. Table 1 shows examples of the specifications of the scanning lens.

[0019]

[Table 1]

Is a rotary polygon mirror, is a lens surface, and is a photosensitive drum surface. R is the radius of curvature in the scanning direction, r is the radius of curvature in the scanning vertical direction, d is the surface interval, and n is the refractive index. The shape of the surface is given by equations (3) and (4) with the vertex of the surface as the origin.

[0021]

(Equation 3)

[0022]

(Equation 4)

Where a, b, c, d and K are constants,
K is called the conic constant. Further, e is an asymmetric term and is a sample point as shown in Table 2, and an arbitrary position not shown in this table is given by a polynomial approximation.

[0024]

[Table 2]

FIG. 8 shows the amount of field curvature on the scanning vertical direction (on the optical axis) with respect to a temperature change. In the figure, 50 is an embodiment of the present invention, and 60 is a conventional example. In this way, it is reduced to about 1/8.

In the above description, the case where both the aspherical lens and the cylindrical lens are made of a plastic material as the scanning lens has been described. However, the aspherical lens is made of a plastic material, and the cylindrical lens is made of another optically transparent material such as a glass material. It goes without saying that the present invention can also be applied to the case where each is completed.

[0027]

According to the present invention, the scanning lens is composed of two lenses, and the lens on the optical deflector side has a curvature in the scanning vertical direction along the scanning direction corresponding to each deflection position of the optical deflector. The lens on the surface to be scanned has a cylindrical lens, and even if a plastic material, which is less expensive than glass, is used, the curvature of field in the scanning vertical direction due to changes in environmental temperature can be reduced. An optical scanning device with a low generation and high resolution can be obtained.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing one embodiment of an optical scanning device according to the present invention.

FIG. 2 is a diagram illustrating the occurrence of field curvature due to a temperature change of an optical scanning device according to the related art.

FIG. 3 is a diagram illustrating occurrence of curvature of field due to a temperature change of the optical scanning device according to the present invention.

FIG. 4 is a diagram showing an image formed by an optical scanning device according to the related art at room temperature.

FIG. 5 is a diagram showing an image formed at a high temperature of the optical scanning device according to the present invention.

FIG. 6 is a diagram showing an image formed by an optical scanning device according to the related art at room temperature.

FIG. 7 is a diagram showing an image formed at a high temperature of the optical scanning device according to the present invention.

FIG. 8 is a diagram showing a relationship between a temperature change and a curvature of field on the optical axis.

[Explanation of symbols]

1 is a rotating polygon mirror, 6 is a photosensitive drum surface, 22 is a laser light source, 44 is an aspherical lens, 45 is a cylindrical lens, and 91 is a beam.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-2-23313 (JP, A) JP-A-6-186492 (JP, A) JP-A-64-38709 (JP, A) JP-A-63-63 218916 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02B 13/00 G02B 26/10

Claims (4)

(57) [Claims] 1. A light beam from a laser light source is scanned on a surface to be scanned via an optical deflector and a scanning lens, and the optical deflector and the surface to be scanned are scanned in a vertical scanning direction via the scanning lens. In the optical scanning device arranged in a substantially conjugate imaging relationship with respect to the above, the scanning lens is composed of two lenses, and the lens on the optical deflector side in the scanning vertical direction corresponding to each deflection position of the optical deflector. has a shape curvature changes continuously in the scanning direction, the lens surface to be scanned side, run by environmental temperature changes
An optical scanning device comprising a cylindrical lens for reducing a shift of an image forming position in a vertical direction . 2. The optical scanning device according to claim 1, wherein a change in curvature of the lens on the optical deflector side in the scanning vertical direction is asymmetric with respect to the optical axis along the scanning direction. 3. The shape of the lens on the optical deflector side in the scanning direction is as follows: R is a radius of curvature on the optical axis, and K is a conic constant.
The angles formed by the equation (1) and giving the maximum scanning width to the optical axis on the incident side and the optical axis on the output side of the scanning lens are denoted by V, V ', and SIG (K), respectively. 3. The optical scanning device according to claim 1, wherein when the code of K is satisfied, Expression (2) is satisfied. (Equation 1) (Equation 2) 4. The optical scanning device according to claim 1, wherein a material of the lens on the optical deflector side is a plastic material.