JP2966858B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Purpose of the Invention] (Industrial application field) The present invention relates to an image forming apparatus such as a laser printer.

(Prior Art) In recent years, various laser printers have been put to practical use.

In a laser printer, a laser beam generated by a laser generator is deflected and scanned by a rotating polygon mirror, and the laser beam is guided to a photosensitive drum as a recording medium via an optical system such as a mirror and a condenser lens. As a result, an electrostatic latent image is formed in which the photosensitive drum previously charged by the charging unit is exposed. After the electrostatic latent image is visualized by a developing device, it is transferred and fixed on a predetermined sheet.

Conventionally, in this type of apparatus, the spot diameter of the laser beam has been constant with respect to the temperature change even though the sensitivity of the photoconductor changes due to the temperature change of the apparatus.

(Problems to be Solved by the Invention) As described above, in the related art, the spot diameter of the laser beam is constant with respect to the temperature change even though the sensitivity of the photoconductor changes due to the temperature change of the apparatus. When the temperature changes, the size of the electrostatic latent image changes and the image quality deteriorates.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide an image forming apparatus capable of preventing a decrease in image quality due to a temperature change of the apparatus and obtaining a stable image quality. It is.

[Configuration of the invention]

(Means for Solving the Problems) In order to solve the above problems, according to the present invention, when the temperature decreases at a predetermined rate from the reference temperature (t0) (t1), the sensitivity decreases at the first change amount. And an image carrier whose sensitivity increases by a second variation smaller than the first variation when the temperature rises at the predetermined rate (t2) from the reference temperature (t0); and emits laser light. A light source, a first optical member for converting a laser beam emitted from the light source into a parallel beam, and a rotating polygon mirror for deflecting the laser beam parallelized by the first optical member. A second optical member that guides the laser light deflected by the deflecting means to the image carrier and corrects the influence of variations in the mirror surface of the rotating polygon mirror of the deflecting means;
The first and second optical members are formed of a plastic material having a large change in shape and refractive index due to a temperature change, and the ambient temperature of the first and second optical members is set to the reference temperature. When the temperature changes further, the shape changes and the refractive index fluctuates, and when the temperature is at the reference temperature (t0), when the temperature decreases at the predetermined rate from the reference temperature (t1), and when the temperature is lower than the reference temperature. When the temperature rises at the rate of (t
2) The diameter of each laser beam on the image carrier is Dt
Provided is an image forming apparatus characterized in that the focal length changes so as to satisfy the relationship of Dt1>Dt2> Dt0, where 0, Dt1, and Dt2.

(Operation) That is, according to the present invention, even when the light sensitivity of the recording medium changes (for example, decreases) due to a change in the temperature of the image forming apparatus, the spot diameter of the laser beam also changes (for example, increases) at the same time. Accordingly, a change in the size of the formed electrostatic latent image can be reduced, and stable image quality can be obtained even when there is a temperature change.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.

FIG. 2 shows a laser printer as a recording apparatus according to the present invention. In FIG. 2, reference numeral 52 denotes a housing. A photosensitive drum (image carrier) 54, which is rotated in the direction of the arrow, is provided at a central portion in the housing 52, and an exposure device 56 is provided below the photosensitive drum 54. This exposure device 56
Exposes a predetermined portion on the photosensitive drum 54 based on image information, and sequentially from the exposed portion along the rotation direction of the photosensitive drum 54, a developing device 58, a transfer roller 60, a static elimination lamp 62, and A charging charger 64 is provided.
A detachable paper cassette 66 is provided at the bottom of the housing 52.
A fixing device 68 is provided in an upper part in the housing 52, respectively. First, the paper p in the paper feed cassette 66 is
70 and then aligned by aligning rollers 72. Next, the aligned paper p is
It sequentially passes through a transfer section between the photoconductor drum 54 and the transfer roller 60 and the fixing device 68 and is sent to a pair of paper discharge rollers 74,
The paper is discharged onto the paper discharge tray 6 above the housing 52 by the paper discharge rollers 74. Reference numeral 78 denotes a paper cassette 66 and a paper discharge tray.
It is a control device arranged between the control device and the control device.

Thus, at the time of image formation, first, the surface of the photosensitive drum 54 is uniformly charged by the charging charger 64. Next, the charged photosensitive drum 54 is exposed and scanned by the photosensitive device 56. As a result, an electrostatic latent image is formed on the photosensitive drum 54. Next, a developer is supplied to the electrostatic latent image by the developing device 58, whereby the electrostatic latent image is visualized and a developer image is formed. Next, the developer image is transferred onto the sheet p by the transfer roller 60 at the transfer section. The developer image transferred onto the sheet p is fixed on the sheet p by a fixing device 68, and thereafter, a discharge roller 74
Is discharged onto the discharge tray 76. On the other hand, the surface of the photosensitive drum 54 after the transfer of the developer image onto the sheet p is neutralized by the neutralization lamp 62, and is prepared for the next cycle.

As shown in FIGS. 3 and 4, a hole 80 is provided at the bottom of the housing 52, and the exposure device 56 is inserted through the hole 80.
Is attached and detached. L-shaped grooves 84 are provided on both side surfaces of the case 82 of the exposure device 56, and a concave portion 86 is provided on the other side surface. On the other hand, the housing 52 has a pair of pins 88 that engage with the grooves 84 and the recesses 86.
There is provided a projection 90 that fits into the projection. Exposure unit 56 with housing
When mounting to the housing 52, align the groove 84 with the pin 88 above.
The exposure device 56 is inserted from the lower part of the housing 52 and then slid horizontally, and when the pin 88 is located at the end of the groove 84, the housing 52 is
And the concave portion 86 of the case 82 are fitted. Accordingly, the movement of the exposure device 56 in the vertical direction and the rotation direction is regulated, and the exposure device 56 is positioned with respect to the photosensitive drum 54. The exposure device 56 is fixed to the housing 52 by screws 92 in this positioned state. As a result, the exposure device 56 can be independently attached and detached, and is held at a fixed position even when the screw 92 is removed, so that the assembling property is good and the positioning adjustment is unnecessary.

As shown in FIGS. 5 and 6, a laser unit 94 is provided on the upper surface of the case 82 of the exposure apparatus 56.
The laser unit 94 includes a semiconductor laser (light source) 96
And a collimator lens (first optical member) 98 are integrally set. The semiconductor laser 96 generates a laser beam L modulated in accordance with image information. The generated laser beam L is collimated by a collimator lens 98. The laser unit 94 is rotatably mounted around the optical axis of the laser beam L emitted from the semiconductor laser 96 (that is, the optical axis of the laser beam L collimated by the collimating lens 98). In addition, the laser unit 94
Is the laser beam L emitted from the semiconductor laser 96
The optical axis of which is described below is a rotating polygon mirror 100 and a first fθ lens 102.
And is attached to the case 82 so as to be substantially perpendicular to the light scanning surface between them.

As shown in FIG. 5, a prism unit 104 is provided inside the case. This prism unit 104
Has two triangular prisms 106. Both ends of these triangular prisms 106 are held by holding members 110 by one plate screw 108 so that their apical angles are in the same direction. The laser beam L collimated by the collimator lens 98 is reduced by the two triangular prisms 106 in only one direction of the cross-sectional shape so as to have an appropriate spot size on the photosensitive drum 54, and at the same time, substantially perpendicularly. Deflected in the direction. The holding member 110
Is rotatably fitted into a hole 112 provided in the case and is fixed at a desired position by a screw 114. Therefore, by finely adjusting the rotational position of the holding member 110 with the screw 114, the light emitted from the triangular prism 106 can be finely adjusted in a desired direction.

As shown in FIG. 6, a deflection scanning device (deflection means) 116 is provided inside the case 82. The deflection scanning device 116 has a motor 118, and the motor 118
It is attached to the upper wall. The shaft 120 of the motor 118 is projected downward and inclined.
Is equipped with a rotating polygon mirror 100. The rotary polygon mirror 100 deflects and scans the laser beam L deflected by the triangular prisms 106, 106 by a width corresponding to a recording area on the photosensitive drum 54. Here, the rotating polygon mirror 100
Is 6 mm on the side so that wide angle scanning of about ± 45 ° can be performed.
A hexagonal prism having two reflecting surfaces 122 is used.

That is, the distance from the rotary polygon mirror 100 to the photosensitive drum 54 is 258 mm, compared to 261 mm in the conventional apparatus shown in FIG. In addition, since the conventional device is for A4 size, the scanned portion having a width of 210 mm is set to a focal length f = 215 mm fθ
When scanning with a lens, θ = (1/2) x (210 / f) (rad)
= (1/2) × (210/215) × (180 / π) = ± 28 ゜
As the rotary polygon mirror, an octagonal prism having eight reflecting surfaces on the side surface is used. However, since the device of this embodiment is for A4 size, the effective focal length f is set to substantially the same 209 mm, and 2
Assuming that a scanned portion having a width of 97 mm is scanned, θ = (1/2) × (2
97/209) × (180 / π) = ± 41 °. For this reason, a hexagonal column is used.

In the case 82, the first and second fθ lenses 10
An fθ lens system (second optical member) 126 composed of 2,124 and first and second mirrors 128, 130 are provided. Second
Lens 124 is provided at the upper part, and the second mirror 130 is disposed near the motor 118. Rotating polygon mirror 100
The laser beam L deflected and scanned by the first f
After passing through the θ lens 102, the light is sequentially reflected by the first and second mirrors 128 and 130. The laser beam L reflected by the second mirror 130 passes between the rotary polygon mirror 100 and the first fθ lens 102, and further passes through the second fθ lens 124, and then on the photosensitive drum 54. The light is collected and scanned in the width direction (main scanning direction) of the photosensitive drum 54. Here, the first f
The width of the θ lens 102 is set so that the effective scanning angle is, for example, ± 40 °. Since the first fθ lens 102 has a positive refractive index, the width of the second fθ lens 84 is
The effective scanning angle is set to, for example, ± 32 °. A third mirror 132 is provided at one end of the second fθ lens 124.
(See FIG. 4), and the third mirror 132
The laser beam L having a scanning angle of + 32 ° or more is arranged so as to be reflected in a direction orthogonal to the shaft 120 of the motor 118, and a part of the laser beam L is detected by the photoelectric conversion element 134. The photoelectric conversion element 134 is for obtaining a signal for controlling the recording start position.

The light incidence surfaces 102a and 124a and the light emission surfaces 102b and 124b of the first and second fθ lenses 102 and 124 are respectively formed in aspherical shapes.

That is, as shown in FIG.
Reference numeral 2 denotes a lens having a positive refractive power in which the light incident surface 102a is concave and the light exit surface 102b is convex. The second fθ lens 124 is a lens having a concave optical surface portion 124a1 and a convex surface on both side portions 124a2 of both the light incident surface 124a and the light exit surface 124b, and having a negative refractive power at least on the optical axis. is there.

Here, the light entrance surfaces 102a and 124a and the light exit surfaces 102b and 124b of the first and second fθ lenses 102 and 124 respectively have a three-dimensional coordinate system in which a deflecting surface is a yz plane, and an optical axis is a z axis. Then, the height z of the lens surface on the yz plane
Relation between distance and distance y Is represented by

Where RD is the radius of curvature, AD, AE, AF, and AG are quaternary and 6 respectively.
| AD | + | AE | + | AF | +… ≠
0.

The first fθ lens 102 has a curvature of the light incident surface 102a of 1 / | RD ₁ | and a curvature of the light exit surface 102b of 1 / | RD ₂ | Is established. When the lens surface is concave with respect to the object side (semiconductor laser 96 side), RD is negative.

The second fθ lens 124 has a curvature of the light incident surface 124a of 1 / | RD ₃ | and a curvature of the light exit surface 124b of 1 / | RD ₄ | Is established.

Further, the light incident surface 102a and the light exit surface 102b of the first fθ lens 102, and the light incident surface 124a of the second fθ lens 124
Is formed in a shape obtained by rotating the curve z represented by the above relational expression around the optical axis (z axis).

The light exit surface 124b of the second fθ lens 124 is a straight line that is parallel to the y-axis and at a distance of z = cvx from the lens surface on the optical axis (z-axis). Is formed on the toric surface obtained by rotating the rotation about the rotation axis.

Here, the range of AD, AE, AF, AG is the maximum radius of the lens as A
If P, | AD | <100 / (AP) ⁴ , | AE | <100 / (AP) ⁶ , | AF | <100 / (AP) ⁸ , | AG | <100 / (AP) ¹⁰ is there. This range is a range where the coefficient is not too large with respect to the order and the lens does not have a very awkward shape.

The equation representing the toric surface can be obtained by assuming that the intersection of the curve z and the optical axis (z axis) is z = 0 and the position of the rotation axis is z = 1 / cvz in FIG. And x ² + (1 / cvx−z ₁ ) ² = (1 / cvx−z), Becomes Further, the curve z is in the yz plane, that is, in the beam deflection plane, but may be slightly shifted from the yz plane when stray light from the photosensitive drum 54 is considered.

Further, the distance l between the light incident surface 102a of the first fθ lens 102 and the reflection point of the laser beam L on the rotary polygon mirror 100
Is set in the range of l = f / 15 to f / 3, where f is the focal length obtained by combining the first and second fθ lenses 102 and 124. For example, this distance 1 is 30 mm, and f = 20
Assuming 9 mm, in this case, l / f = 30/209 = 1/7.

The first fθ lens 102 is set so that the distortion factor (y−fθ) / y [y is the beam position on the image plane] of the fθ characteristic is within 10%.

The distance d from the light exit surface 124b of the second fθ lens 124 to the image plane (photosensitive drum 54) is assumed to be f, the focal length obtained by combining the first and second fθ lenses 102 and 124.
d = f / 22 to f / 3. For example, if the distance d is 30 mm and f = 209 mm, in this case, d / f = 30/209 = 1/7.

Further, the second fθ lens 124 is set so that the correction amount of the field curvature created by the light beam in the deflection surface is about 0 to 30 mm.

The first and second fθ lenses 102 and 124 are formed in a strip shape from a plastic material such as acrylic, for example. As shown in FIGS. 8 to 11, the second fθ lens 124 has a concave (cross-sectional) section in which a portion (use portion) 140 excluding two opposing peripheral portions is recessed in a direction along the optical axis. Is formed. Then, a light incident surface 124a and a light exit surface 124b are formed in the use portion 140, and the peripheral portion thereof serves as a reinforcing portion 142. In addition, on the surface where the light incident surface 124a and the light exit surface 124b are not formed, a hollow portion having a concave cross section is formed.
144 are formed.

According to the above configuration, the following operation and effect can be obtained.

(1) Since the light entrance surfaces 102a, 124a and the light exit surfaces 120b, 124b of the first and second fθ lenses 102, 124 are aspherical, f is large for a scanning angle of about ± 45 °.
The θ characteristic can be satisfied, and the field curvature can be corrected. As a result, the size and cost of the device can be reduced.

That is, the modulation of the semiconductor laser 96 is normally performed in proportion to the time. On the other hand, the rotating polygon mirror 100 rotates at a constant angular velocity. Assuming that the parallelized laser beam L is incident on the rotating polygon mirror 100 as an object point at infinity, the height of the object point is proportional to tan θ with respect to the incident angle θ of the rotating polygon mirror 100. However, the reflected laser beam L
Is distorted by the first and second fθ lenses 102 and 124, and an image height y proportional to the incident angle θ is obtained. This image height y
Is represented by y = fθ (rad). Here, the first and second fθ lenses are formed on an aspheric surface where f is not a focal length but a constant as a proportional coefficient. Therefore, a sufficient fθ characteristic can be satisfied even for a wide scanning angle.

In this case, the first fθ lens 102 is an aspherical lens having a positive refractive power with the light incident surface 102a being a concave surface and the light emitting surface 102b being a convex surface, and has an fθ characteristic as shown in FIG. As well as field curvature. However, the second f
The θ lens 124 is a non-spherical lens having a negative refractive power on the optical axis, in which both the light incident surface 124a and the light exit surface 124b have a concave optical axis portion 124a1 and convex portions on both side portions 124a2. The optical axis portion has a negative refractive power, but has a positive refractive power as the distance from the optical axis increases. Therefore, it is possible to correct the curvature of the image plane formed by the fθ characteristic of the first fθ lens 102 and the light beam in the yz plane. That is, when the refractive power of the first fθ lens 102 is positive, the refractive power of the second fθ lens 124 is negative, so that the Petzval curvature can be approximated to 0 as much as possible. The fθ characteristics can be satisfied even for a scanning angle as wide as about degrees, and the curvature of the image plane created by the light beam in the yz plane can be prevented. Therefore, since the laser beam L can be scanned at a wide scanning angle, the optical path from the rotary polygon mirror 100 to the photosensitive drum 54 can be shortened, and the size of the apparatus can be reduced. Further, since the parallel laser beam L can be focused on the photosensitive drum 54 on the yz plane, the resolving power can be improved.

Further, since the light exit surface 124a of the second fθ lens 124 is a toric surface, an image formed by a light beam in a cross section orthogonal to the xz plane is obtained separately from the field curvature correction on the yz plane. Surface curvature, ie, x-axis direction (sub-scan direction)
Can be corrected. That is, even if the reflecting surface 122 of the rotary polygon mirror 100 is inclined due to machining accuracy or the like,
The laser beam L can be focused on the yz plane. That is, it is possible to make the plane tilt correction image plane in the sub-scanning direction coincide with the focal point of the parallel laser beam, that is, the image plane on the yz plane.

The reflection point on the reflection surface 122 also moves, for example, by about 2 mm with the rotation of the rotating polygon mirror 100. However, since the movement of this reflection point is along the optical axis of the incident laser beam L, the reflection is If it is perpendicular to the pupil, it can be treated as a movement of the pupil, and it is considered that the position of the image is not affected. However, the depth of focus corresponding to the diameter of the incident laser beam L is necessary for the surface tilt correction optical system. Here, the depth of focus is the second f which is the toric surface.
When the light exit surface 124b of the θ lens 124 is as close to the image side as possible, that is, as close to the photosensitive drum 54 as possible, the longitudinal magnification becomes smaller, so that the depth becomes deeper, which is advantageous for surface tilt correction. As the depth of focus, for example, a beam diameter of about ± 2.5 mm is required.

(2) The light incident surface 102a and the light exit surface 102b of the first fθ lens 102 have an aspherical shape, and the laser beam L on the light incident surface 102a of the first fθ lens 102 and the reflection surface 122 of the rotary polygon mirror 100. Since the distance l to the reflection point of l is l = f / 15 to f / 3, the laser beam L reflected by the second mirror 130 rotates between the rotating polygon mirror 100 and the first fθ lens 102. The spacing may be such that it can pass between the polygon mirror 100 and the first fθ lens 102.

That is, if l = f / 15 = 14 (mm), the rotating polygon mirror
The distance between the outer end of the lens 100 and the light incident surface 102a of the first fθ lens 102 is 2 mm.
The laser beam L reflected at 30 cannot be passed. Conversely, if l is increased, the first fθ lens 102 will hit the first mirror 128, and the first and second fθ lenses 102 and 124 must be long and thick, requiring a large amount of lens material. In addition, the molding time is long and the cost is high. For such a design reason, 1 is desirably f / 15 at the maximum (l <f / 15).

The first fθ lens 102 has a distortion ratio (y−fθ) / y [y is the beam position on the image plane] of the fθ characteristic of 10% or less. , And the curvature of the image plane created by the light beam in the deflection surface can be largely corrected.

That is, the first fθ lens 102 is
In combination with the lens 24, the original performance is exhibited. However, unless the distortion is corrected to a considerable extent by the first fθ lens 102, both the distortion and the field curvature are greatly increased by the second fθ lens. Cannot correct. Therefore, by correcting distortion to a considerable extent by the first fθ lens 102, the second fθ lens 124 can be used mainly for correction of field curvature. The distortion of the first fθ lens 102 used here was (y−fθ) /y=8.7/164=5.3% at the maximum as shown in FIG. 12, for example.

(3) The light entrance surface 124a and the light exit surface 124b of the second fθ lens 124 have an aspherical shape such that the lens surface on the yz plane has a shape represented by the curve z, and
From the light exit surface 124b of the second fθ lens 124 to the photosensitive drum 5
Since the distance d to 4 is set to d = f / 22 to f / 3, sufficient surface tilt correction (correction of curvature of an image plane formed by a light beam in a cross section orthogonal to the deflecting surface) can be performed and design is possible. Does not cause any problems.

In other words, if the second fθ lens 124 is too difficult from the photosensitive drum 54, troubles such as contact with the motor 118 will occur. In addition, the refractive power of the light emission surface having the toric surface shape becomes weak, and the beam diameter in the sub-scanning direction (the direction along the xz plane) increases. Further, if d = f / 3 or more, sufficient tilt correction cannot be obtained. Conversely, it is necessary to increase the thickness of the lens as d decreases.
It is disadvantageous in production. For example, when d = f / 22 or less, the thickness of the lens becomes 25 mm or more. Therefore, it is desirable in design that d = f / 22 to f / 3.

In addition, the second fθ lens 124 is configured so that the amount of correction of the curvature of the image plane formed by the light beam in the deflection surface is set to about 0 to 30 mm, so that it is possible to perform more sufficient surface tilt correction.

That is, as described above, since the first fθ lens 102 mainly has fθ characteristics, the field curvature cannot be reduced. For this reason, the field curvature correcting ability of the second fθ lens 124 needs to be relatively large. However, since the second fθ lens 124 slightly corrects the fθ characteristic and also performs surface tilt correction, it is difficult to provide a considerably large correction amount. Further, the first fθ lens 102 Field curvature can be kept within 30 mm. Therefore,
The correction amount of the field curvature of only the second fθ lens 124 is 0 to 30 m
About m is good. Here, FIG. 13 shows the first fθ
The field curvature when only the lens 102 is used is shown. The curvature of the image plane created by the light beam in the deflection surface is about 7 mm, and this is corrected by the second fθ lens 124. Also,
The curvature (surface tilt) of the image plane formed by the light beam in the cross section orthogonal to the deflection surface is about 25 mm, but this is mainly corrected by the light exit surface 124b of the second fθ lens 124 having a toric surface shape. .

(4) Since the first and second fθ lenses 102 and 124 are formed in an aspherical shape using a plastic material, they can be provided with high accuracy and at low cost.

That is, the first and second fθ lenses 102 and 124
As shown in FIGS. 14 to 16, the aberration characteristic at is corrected very well at normal temperature (25 ° C.). Mainly, even a change due to a temperature due to a change in the refractive index of the plastic material is within the usage range.

Further, since both the first and second fθ lenses 102 and 124 are formed in a strip shape, the occupied volume is reduced, thereby shortening the molding time and reducing the cost.

Further, the second fθ lens 124 is formed such that a portion (used portion) 140 excluding the peripheral portions of the two opposing surfaces is recessed in a section along the optical axis, and a light incident surface is formed on the used portion 140. 124a
Since the light exit surface 124b is formed, the light entrance surface 124a and the light exit surface 124b can be individually formed, and the accuracy can be improved.

Furthermore, since the light-receiving surface 144a and the light-exiting surface 124b are not provided with the lightening portion 144 having a light-walled concave cross-section, the molding time can be shortened without lowering the strength.

In other words, precision plastic lens molding is usually
It takes 1 minute per 1mm thickness. Therefore, the thinner the lens is, the more advantageous in terms of accuracy and cost. However, simply shortening the lens will warp the lens. The used portion 140 of the lens is a central portion having a width of only 1 mm.

Further, the light sensitivity characteristics of the recording medium in such an apparatus may change with temperature. For example, the relationship between the photosensitivity of the photosensitive drum 54 and the temperature in this embodiment is shown in FIG. In FIG. 21, the vertical axis indicates the residual potential on the photosensitive drum 54 when light of a certain energy is applied to the photosensitive drum 54 charged to a certain potential, and the horizontal axis indicates the temperature. It can be said that the smaller the value of the residual potential, the better the sensitivity. This figure shows that the sensitivity decreases as the temperature decreases.

In the case of the present embodiment, since the reversal development is performed in which the developer at the portion irradiated with light arrives, for example, when the temperature is lowered and the sensitivity is deteriorated, the electrostatic latent image is exposed at the same energy and beam diameter at normal temperature. It is smaller than the latent image.

Therefore, in order to form an image having the same size as at room temperature, the beam diameter must be large at low temperatures.

Means for changing the beam diameter with respect to temperature change include a collimator lens 98 and an fθ lens which is a condenser lens.
The collimator lens 98 and the fθ lenses 102 and 124, which are eight condensing lenses, are moved in accordance with the temperature change by using a member that changes in size and shape depending on temperature, such as bimetal, as a holding member that holds an optical element such as 102 and 124. Can be realized.

Further, it can also be realized by forming optical elements themselves such as the collimator lens 98 and the fθ lenses 102 and 124 as condensing lenses using a plastic material whose shape and refractive index change largely due to temperature change.

That is, since the focal length changes due to a change in shape and refractive index due to a change in temperature, the beam diameter on the photosensitive drum 54 as a recording medium at a fixed distance changes.

In particular, if the lens itself is made of a plastic material, no structural member for moving the lens is required, the structure and assembly are simple, and the cost is lower than that of a glass lens.

The change in the beam diameter does not always change uniformly in every direction of the beam cross section, and there are directions that are easy to change and directions that are hard to change depending on the configuration of the optical system.

Hereinafter, a change in the beam diameter will be considered in the in-section direction where the change is most likely to occur.

In the case of the photosensitive characteristics as shown in FIG. 21, the change in sensitivity is small on the high temperature side and large on the low temperature side. Therefore, the change in the beam diameter D on the low temperature side is important, and the change in the beam diameter D on the high temperature side does not need to be considered so much.

That is, within the temperature range normally used (18-28 ° C)
The beam diameter at 25 ° C is D ₂₅ , and the maximum value in the entire scanning range is D _25
25 max, likewise may be a temperature when the beam diameter at t ₁ and _{Dt 1 max Dt 1 max> D} 25 max cold side (15 ° C. or less).

Further, since a normal temperature region is usually used, the beam waist portion (focal point) where the beam diameter D is most stable is obtained at room temperature so that the best image quality can be obtained at room temperature.
Generally, it is set to be above 54. At this time, the beam diameter D increases at both low and high temperatures.

However, even in such a case, considering the characteristics of FIG. 21, it is better to make the change in the beam diameter D on the high temperature side smaller than the change in the beam diameter D on the low temperature side over a wide temperature range as shown in FIG. Good image quality is obtained.

That is to a _{Dt 1 max> Dt 2 max>} D 25 max case of the maximum beam diameter Dt ₂ max at t ₂ ° C. hot side (35 ° C. or higher). However, here, t ₁ and t ₂ should be compared at 25 ° C. and at the same temperature difference t ₂ −25 = 25−t ₁ .

Up to now, according to the embodiment, the case where the sensitivity is lowered at a low temperature in the reversal development has been described.However, in the case of the normal development in which the part that was not exposed to light is developed, or the recording in which the sensitivity increases at a low temperature. In the case of using a medium, the opposite is also true, such as decreasing the beam diameter as the temperature decreases.

〔The invention's effect〕

As described above, according to the embodiments of the present invention, it is possible to provide an image forming apparatus capable of obtaining stable image quality over a wide temperature range without deterioration of image quality due to a change in temperature of the apparatus.

[Brief description of the drawings]

FIG. 1 shows an embodiment of the present invention. FIG. 1 is a diagram schematically showing the configuration of an optical system of an exposure apparatus, FIG. 2 is a cross-sectional view schematically showing a laser printer, and FIG. FIG. 4 is a cross-sectional plan view showing an attached state of the exposure apparatus, FIG. 5 is a view showing a laser unit and a prism unit, FIG. 6 is a longitudinal sectional view showing the exposure apparatus, FIG.
FIG. 8 is a view for explaining the shape of the lens, FIG. 8 is a front view showing the second fθ lens, FIG. 9 is a cross-sectional view of the fθ lens shown in FIG. Is f shown in FIG.
FIG. 11 is a sectional view taken along line CC of the fθ lens shown in FIG. 8, FIG. 12 is a view showing fθ characteristics of the first fθ lens, and FIG. FIG. 13 is a diagram showing the relationship between the field curvature and the scanning angle by the first fθ lens, FIG. 14 is a diagram showing the fθ characteristics of the first fθ lens, and FIG. 15 is formed by a light beam in the deflection surface. FIG. 16 is a diagram showing the relationship between the curvature of the image surface and the scanning angle, and FIG. 16 is a diagram showing the relationship between the curvature of the image surface and the scanning angle created by a light beam in a cross section orthogonal to the deflection surface.
17 is a view showing a modification of the second fθ lens, FIG. 18 is a view for explaining the shape of the second fθ lens in the modification, and FIG. 19 is another view of the second fθ lens. FIG. 20 is a diagram showing a modification, FIG. 20 is a diagram showing the relationship between the scanning angle and the spot size, FIG. 21 is a diagram showing the relationship between the photosensitivity of the photosensitive drum and the temperature characteristic, and FIG. FIG. 4 is a diagram showing a relationship between the temperature and the temperature characteristics. 54: recording medium (photosensitive drum), 96: light generating means (semiconductor laser), 116: deflecting means (deflection scanning device), 98: collimator lens, 102, 124: condenser lens (fθ lens), L: Laser beam.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-63-94223 (JP, A) JP-A-60-67921 (JP, A) JP-A-62-223070 (JP, A) JP-A-62-23070 242968 (JP, A) JP-A-61-277261 (JP, A) JP-A-60-95556 (JP, A) JP-A-3-58655 (JP, U) JP-A-60-90406 (JP, U) (58) Field surveyed (Int.Cl. ⁶ , DB name) G03G 15/04-15/04 120 B41J 2/44 G02B 9/00-17/08 G02B 21/02-21/04 G02B 25/02- 25/04 H04N 1/23-1/31

Claims (1)

(57) [Claims] When the temperature decreases at a predetermined rate from the reference temperature (t0) (t1), the sensitivity decreases at the first change amount, and the temperature decreases at the predetermined rate from the reference temperature (t0). An image carrier whose sensitivity is increased by a second variation smaller than the first variation when rising (t2); a light source that emits laser light; and a laser light emitted from this light source is converted into parallel light. A first optical member for converting, a deflecting means including a rotary polygon mirror for deflecting the laser light collimated by the first optical member, and an image carrier for deflecting the laser light deflected by the deflecting means. A second optical member that guides the body to the body and corrects the influence of the variation in the mirror surface of the rotating polygon mirror of the deflecting unit. The first and second optical members have shapes and For plastic materials with large changes in refractive index When the ambient temperature of the first and second optical members changes from the reference temperature, the shape changes and the refractive index fluctuates. At the time of the reference temperature (t0), the temperature changes from the reference temperature. When the temperature decreases at a predetermined rate (t1) and when the temperature increases at the predetermined rate from the reference temperature (t1)
2) The diameter of each laser beam on the image carrier is Dt
An image forming apparatus characterized in that the focal length changes so as to satisfy the relationship of Dt1>Dt2> Dt0, where 0, Dt1, and Dt2.