JP5084600B2 - Scanning optical device and image forming apparatus using the same - Google Patents

Description

  The present invention relates to a scanning optical device and an image forming apparatus using the same. In particular, it is suitable for an image forming apparatus using an optical element made of resin, such as a laser beam printer (LBP) having an electrophotographic process, a digital copying machine, or a multifunction printer (multifunction printer).

  A plastic material is used for an optical element of an imaging optical system (scanning optical system) used in the scanning optical device. The optical performance of the scanning optical system has been greatly improved by improving the material technology, manufacturing technology, and design technology of the optical element.

  Plastic materials that replace glass materials are required to have low moisture absorption, low birefringence, low refractive index distribution, and the like. Examples of the plastic material include ZEONEX (registered trademark) of polyolefin resin manufactured by Nippon Zeon Co., Ltd. and trade name APEL manufactured by Mitsui Petrochemical Co., Ltd.

  The lens shape (the shape of the optical element) is premised on these plastic materials and is designed as an aspherical shape. The optical element is formed by injecting a resin into a designed mold and molding it. By securing the freedom of optical design, it is easy to reduce the size and increase the performance of an optical system that could not be realized with a conventional lens system using only a glass material.

  However, plastic lenses have problems that are not found in glass lenses, such as refractive index distribution, birefringence, refractive index dependence on ambient temperature, and molding stability. When using plastic lenses, it is required to avoid these designs well.

  For example, these problems cause variations in the spot diameter of the light beam on the surface to be scanned and increase the spot diameter. This is an adverse effect when the spot diameter is reduced or the spot is stabilized in response to the demand for higher image quality.

  Various countermeasures against the refractive index distribution which is one of the problems have been conventionally proposed (see Patent Documents 1 to 4).

  Patent Document 1 discloses a method of making the lens height of the optical element in the sub-scanning direction higher than the thickness. For example, the imaging lens constituting the imaging optical system is set so that h / t> 2 when the lens height in the sub-scanning direction is h and the thickness in the optical axis direction is t in the sub-scanning section. And formed.

  Patent Document 2 discloses a method of manufacturing an optical element by performing an annealing process after a molding process.

  Patent Documents 3 and 4 disclose a sink molding technique in which an optical element is molded at a low pressure and the sink is positively guided to other than the optical surface.

  Another issue, birefringence, is highly material-dependent, so the performance improvement largely depends on the development of the material manufacturer. Moreover, the problem with respect to environmental fluctuations can be addressed by temperature compensation design of an optical system that cancels out of focus. Moreover, the problem with respect to molding stability (difference between cavities, difference between shots) is a technique that can be dealt with by improving the manufacturing process.

  Various scanning optical devices and image forming apparatuses using plastic lenses have been proposed (see Patent Document 5).

Patent Document 5 discloses a scanning optical apparatus and a color image forming apparatus configured to form a color image using a plastic lens in an imaging optical system.
JP-A-8-201717 JP-A-11-77842 JP 2000-329908 A JP 2003-241083 A JP 2004-184657 A

  Conventionally, various countermeasures have been taken for the problem of refractive index distribution.

  However, in Patent Document 1, in the method in which the lens shape is set to h / t> 2 as described above, a predetermined lens height h is required according to the wall thickness t, and the lens is unnecessary even if the lens effective portion is small. There was a tendency for the height h to increase.

  Moreover, in patent document 2, since the annealing process was provided, there existed a tendency for manufacture to become complicated.

  In Patent Documents 3 and 4, since sink molding is performed, it is necessary to increase the lens height as a portion for guiding the sink portion, and there is a problem that downsizing becomes difficult. Further, when the annealing process of Patent Document 2 is provided, the manufacturing tends to be complicated.

  Further, in the case of sink molding shown in Patent Documents 3 and 4, a lens portion for guiding the “incomplete transfer portion” is necessary, and the guide portion (concave portion) is usually provided on the upper surface or the lower surface of the lens (see FIG. 3). 3, FIG. 1 of Patent Document 4).

  This guide part is set so that the lens height is sufficiently separated from the light beam passing through the lens so that the distortion of the guide part does not reach the effective part. It becomes higher than in the case of molding not performed).

  The molding time of a plastic lens produced by injection molding is determined by the smaller dimension of the thickness and height in the optical axis direction of the lens, and there is a tendency that the number of production per unit time decreases when the molding time is long. In the conventional lens, the thickness in the optical axis direction is shorter than the height, and the molding time is determined by the thickness of the lens. Since the thickness of the lens is a requirement for optical performance, it has been difficult to increase manufacturing efficiency.

  The lens height is proportional to the area of the mirror surface of the mold and also proportional to the clamping force required for the molding machine. The number of cavities that can be taken in large numbers is limited within the range of mold clamping force, which is an apparatus capability of the molding machine. As a result, it tends to be difficult to obtain a large number of cavities.

  As another reason, when the lens height is increased, the handling of the optical path of the scanning optical system is limited. For example, in a color image forming apparatus, a light beam guided to a plurality of photosensitive drums is configured to share a part of an optical deflector and an imaging optical system, and the optical path is preferably not restricted in order to reduce the size.

  FIG. 9 is a schematic diagram of a main part of a color image forming apparatus disclosed in Patent Document 5.

  In FIG. 9, four scanning light beams are simultaneously deflected and scanned using the opposite surfaces of the common optical deflector 5. In the station SK1 (SK2), the photosensitive drums 8a and 8b (each having a different luminous flux are passed through the first and second imaging lenses 63ab and 64a (64b) and (63cd and 64c (64d)) and the reflection mirror. 8c and 8d) are shown.

  The first imaging lens 63ab (63cd) is shared by the two light beams, and the second imaging lenses 64a and 64b (64c and 64d) are arranged for each light beam. Although the optical path lengths from the deflection surface of the optical deflector 5 to the photosensitive drums 8a to 8d are the same, the positions of the photosensitive drums 8a to 8d are different, so that they enter the photosensitive drums 8b and 8c that are physically close to the optical deflector 5. It is necessary to return the light beam twice by the reflecting mirror. At this time, since the second imaging lenses 64b and 64c are arranged so as to be surrounded by the light flux as shown in FIG. 9, if the lens height is high, the second imaging lenses 64b and 64c cannot be folded back compactly, and the degree of freedom of mirror arrangement is high. Less. Further, since the position of the final mirrors 81b and 81c optically closest to the photosensitive drums 8b and 8c determines the lower surface 91a of the optical box 91, the optical box 91 becomes thick and it is difficult to reduce the thickness.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a scanning optical device capable of forming a lens with a simple shape at a low cost without being affected by a focus fluctuation due to a refractive index distribution, and an image forming apparatus using the same.

The scanning optical apparatus according to the present invention scans a light source unit, a first optical system that guides a light beam emitted from the light source unit to a deflecting unit, and a light beam deflected and scanned by a deflection surface of the deflecting unit. in the scanning optical apparatus comprising a second optical system for imaging on a surface, wherein the second optical system have a imaging optical element of two or more, and, the two or more imaging optical the closest imaging optical element in the element sac Chi said deflecting means and have a refractive index distribution in the sub-scanning direction, the sub-scanning direction including the contour on the optical axis of the closest imaging optical device to the deflecting means the height H, the sub-scanning direction of the depth of the light beam imaged the thickness on the optical axis D, and before Symbol surface to be scanned by the second optical system nearest the imaging optical element to the deflecting means the width L, and the distance on the optical axis from the nearest imaging optical element to the deflecting means to the deflecting surface of said deflecting means S1, The serial sub-scanning direction scaling ratio of the second optical system β, △ GI (1 / mm ) the refracting power due to the refractive index distribution of the closest imaging optical device to the deflecting means, to time,
(H / D) = 0.0204 (△ GI) ^−0.4439 ≦ 0.9
1/10 × (L / (S1 × β) ² ) ≦ △ GI ≦ 2/3 × (L / (S1 × β) ² )
It satisfies the following condition.

  According to the present invention, it is possible to achieve a scanning optical device and an image forming apparatus using the same that can form a lens with a low-cost and simple shape without being affected by a focus fluctuation due to a refractive index distribution.

  The scanning optical device of the present invention includes a light source means, a first optical system for guiding a light beam emitted from the light source means to the deflection means, and a light beam deflected and scanned by the deflection surface of the deflection means on the surface to be scanned. A second optical system that forms an image.

  The second optical system has at least one imaging optical element having a refractive index distribution in the sub-scanning direction. The height in the sub-scanning direction including the outer shape on the optical axis of the imaging optical element closest to the scanned surface is H, and the thickness on the optical axis of the imaging optical element closest to the scanned surface is D. . Further, the depth width in the sub-scanning direction of the light beam formed on the scanning surface by the second optical system is L, and the distance on the optical axis from the imaging optical element closest to the scanning surface to the scanning surface is Lb. And At that time, at least one optical element is set to a flatness ratio (H / D) that satisfies the following conditional expression.

(H / D) ≦ 0.0204 {1/10 × (L / Lb ² )} ^−0.4439 ≦ 1.33
(H / D) ≧ 0.0204 {2/3 × (L / Lb ² )} − ^0.4439
In the present invention, the optical element having a refractive index distribution in the sub-scanning direction means that the refractive index of the material of the optical element is not uniform and non-uniform when molding the optical element by molding. Say.

Hereinafter, reference examples and embodiments of the present invention will be described with reference to the drawings.

(Reference example)
FIG. 1 is a sectional view (main scanning sectional view) of a main part in the main scanning direction of a scanning optical apparatus according to a reference example of the present invention, and FIG. 2 is a sectional view of main parts (sub scanning sectional view) in the sub scanning direction of FIG. .

  In the following description, the sub-scanning direction (Z direction) is a direction parallel to the rotation axis of the deflecting means. The main scanning section is a section whose normal is the sub-scanning direction (direction parallel to the rotation axis of the deflecting means). The main scanning direction (Y direction) is the direction in which the light beam deflected and scanned by the deflecting means is projected onto the main scanning section. The sub-scanning cross section is a cross section whose normal is the main scanning direction.

  In the figure, reference numeral 1 denotes a light source means, for example, a semiconductor laser. A condensing lens (collimator lens) 2 converts the divergent light beam emitted from the light source means 1 into a substantially parallel light beam (or convergent light beam). Reference numeral 3 denotes an aperture stop which shapes the beam shape by limiting the passing light flux. A cylindrical lens 4 has power only in the sub-scanning direction, and the light beam that has passed through the aperture stop 3 is converted into a line image on a deflection surface (reflection surface) 5a of an optical deflector 5 to be described later in the sub-scan section. The image is formed.

  Each element such as the collimator lens 2, the aperture stop 3, and the cylindrical lens 4 constitutes one element of the incident optical system LA as the first optical system.

  An optical deflector 5 as a deflecting means is composed of, for example, a four-sided polygon mirror (rotating polygon mirror), and is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown) such as a motor. ing.

  Reference numeral 6 denotes an imaging optical system (fθ optical system) as a second optical system having a condensing function and fθ characteristics, and first and second imaging lenses (imaging optical elements made of plastic material). Toric lens) 61, 62.

  The second optical system 6 may have two or more imaging optical elements.

The first and second imaging lenses 61 and 62 in this reference example have a refractive index distribution in the sub-scanning direction, and are made of polyolefin resin.

  The first imaging lens 61 mainly has a refractive power in the main scanning direction, and is arranged close to the optical deflector 5 to contribute to downsizing of the scanning optical device. The second imaging lens 62 mainly has a refractive power in the sub-scanning direction, and is arranged far from the optical deflector 5 to reduce the manufacturing sensitivity of the lens.

  Both the first and second imaging lenses 61 and 62 have different refractive powers in the main scanning direction and the sub-scanning direction, and receive a light beam based on image information deflected and scanned by the optical deflector 5. An image is formed on the photosensitive drum surface 8 as a scanning surface. The first and second imaging lenses 61 and 62 have a tilt correction function by providing a conjugate relationship between the deflection surface 5a of the optical deflector 5 and the photosensitive drum surface 8 in the sub-scan section. Yes.

  In FIG. 2, the second imaging lens 62 has a lens shape having a height H in the sub-scanning direction and a thickness D on the optical axis.

  Reference numeral 8 denotes a photosensitive drum surface as a surface to be scanned.

In this reference example , the imaging optical system 6 is composed of two imaging lenses 61 and 62. However, the present invention is not limited to this. For example, as shown in FIG. 66 may be used.

In this reference example , the first and second imaging lenses 61 and 62 inject and fill the resin material heated and melted into a molding die set at a specific mold temperature at a specific pressure, for a specific time. After passing, it is molded so as not to cause sink marks through a step of taking out from the mold.

  In general, this molding process determines the mold temperature, pressure, and molding time so as not to cause “sinking”, so that it is distinguished from “sink molding” and hereinafter referred to as “normal molding”.

  As the cooling proceeds from the outside to the inside of the lens, from the thin part to the thick part, the refractive index of the part that has been cooled first is high, and the refractive index of the part that has been cooled later is low. A refractive index profile occurs.

  Further, since the imaging lens is long in the main scanning direction, the refractive index distribution is mainly a distribution in the sub-scanning direction which is the height direction. Such a refractive index distribution in the sub-scanning direction causes the effect of a concave lens, and the imaging position of the lens is shifted far away.

  The refractive index distribution is the density distribution of the resin in the molding process, and can be reduced by performing an annealing process, but also the density distribution gradually becomes uniform with time. This is also confirmed by the fact that the refractive index distribution becomes small when left in the high temperature test layer.

  What is important in the problem of the refractive index distribution of the imaging lens is not the magnitude of the initial refractive index distribution after molding, but the amount of change over time. Under the molding conditions in which the initial refractive index distribution is stably generated, the focus shift component due to the refractive index distribution can be corrected by correcting the lens shape.

  However, the temporal change gradually shifts the focus position of the imaging lens in the usage state, so that the spot is enlarged and the image is deteriorated.

  The present inventor has studied the amount of change over time of the refractive index distribution (hereinafter also referred to as “GI”), and during the device warranty period (about 3 to 5 years) used as an imaging optical system, the amount of change over time is It was found that it was about 30% of the initial GI. During this period, not all of the initial GI is released. Let ΔS be the amount of focus deviation allowed by the GI environmental variation. Here, when the GI corresponding to the focus shift amount ΔS is ΔGI, it was found that the initial GI can be allowed up to 3.3 × ΔGI.

FIG. 5 is a graph showing the allowable GI (ΔGI) of the imaging lens. Here, the allowable GI is defined in terms of power (1 / mm) as ΔGI that is equal to or less than the amount of defocus ΔS (mm) in the sub-scanning direction on the surface to be scanned.

Among the one or more imaging lenses constituting the imaging optical system 6, a second imaging lens (hereinafter also referred to as a final lens) 62 that is closest to the scanned surface 8 side receives the target lens from the final lens 62. If the distance on the optical axis (lens back) to the scanning plane 8 is Lb, the allowable ΔGI (GI) of the second imaging lens is ΔGI = ΔS / Lb ² (1)
It can be expressed as

The permissible value of the focus shift amount ΔS is 1/5 or less of the depth width L, where L is the depth width in the sub-scanning direction of the imaging optical system (second optical system) of the scanning optical device. Further, the lower limit value of the focus shift amount ΔS does not become 0 in normal molding, and there exists about 3/100 of the depth width L. Therefore,
3L / 100 ≦ △ S ≦ L / 5 (2)
Can be written in Therefore, from equations (1) and (2),
3L / (100 × Lb ² ) ≦ △ GI ≦ L / (5 × Lb ² ) (3)
It becomes. This is the case when the GI changes 100% in the environment.

Actually, the GI fluctuation amount due to the environment is about 30%, so it can be tolerated up to 10/3 times. Therefore, Equation (3) is
1/10 × (L / Lb ² ) ≦ △ GI ≦ 2/3 × (L / Lb ² ) (4)
It becomes. This is an acceptable amount for the initial GI.

  FIG. 4 shows the case where ΔS = 1 mm as the allowable ΔGI. This is a value when the initial GI allowable value changes by 30% when the GI corresponding to the focus deviation amount ΔS changes by 100%.

  On the other hand, the initial GI (ΔGI) is the ratio of the height H in the sub-scanning direction including the outer shape on the optical axis of the imaging lens to the thickness D on the optical axis of the imaging lens as shown in FIG. The flatness (H / D) is the following relationship:

(H / D) = 0.0204 (△ GI) ^-0.4439 (5)
Therefore, from equations (4) and (5),
(H / D) ≦ 0.0204 {1/10 × (L / Lb ² )} − ^0.4439 (6)
(H / D) ≧ 0.0204 {2/3 × (L / Lb ² )} ^−0.4439 (7)
It becomes.

  When the upper limit value of conditional expression (6) is exceeded, the height of the final lens in the sub-scanning direction becomes higher than necessary, the degree of freedom of optical path arrangement is limited, and the height of the optical system in the sub-scanning direction becomes high. When the upper limit exceeds 1.32, it becomes a problem. If the lower limit value of conditional expression (7) is exceeded, the focus in the sub-scanning direction is shifted due to the influence of environmental fluctuations in the refractive index distribution, so that the desired spot diameter cannot be reduced and image quality is deteriorated.

  If the final lens shape (lens shape of the lens closest to the scanned surface) is determined so that the flatness ratio (H / D) satisfies the above conditional expressions (6) and (7), the optical system is out of focus due to GI. Can be tolerated.

Here, in the scanning optical device comprising the two-system imaging optical system shown in FIGS. 1 and 2, the surface to be scanned by the lens back Lb of the imaging optical system and the imaging optical system (second optical system). The depth width L in the sub-scanning direction of the light beam formed on the top is
Lb = 100.85mm
L = 10mm
It is. Substituting this into the conditional expressions (6) and (7) above,
The right side of conditional expression (6) is
0.0204 {1/10 × (L / Lb ² )} − ^0.4439 = 1.23
The right side of conditional expression (7) is
0.0204 {2/3 × (L / Lb ² )} − ^0.4439 = 0.53
It becomes.

Further, in the scanning optical apparatus including the single imaging optical system shown in FIG. 3, the lens back Lb of the imaging optical system and the depth width L in the sub-scanning direction of the imaging optical system (second optical system). Is
Lb = 110.5mm
L = 10mm
It is. Substituting this into the conditional expressions (6) and (7) above,
The right side of conditional expression (6) is
0.0204 {1/10 × (L / Lb ² )} − ^0.4439 = 1.33
The right side of conditional expression (7) is
0.0204 {2/3 × (L / Lb ² )} − ^0.4439 = 0.57
It becomes.

Therefore, in the case of the two-lens reference example of FIG. 1, if the final lens shape is determined in the range of the flatness ratio (H / D) in the range of 0.53 to 1.23, it is possible to allow the focus shift due to GI.

  That is, the conditional expressions (6) and (7) should be set as follows.

(H / D) ≦ 0.0204 {1/10 × (L / Lb ² )} ^−0.4439 ≦ 1.23 (6a)
(H / D) ≧ 0.0204 {2/3 × (L / Lb ² )} ^−0.4439 ≧ 0.53 (7a)
In the case of the single lens system of FIG. 3, if the final lens shape is determined in the range of the flatness ratio (H / D) in the range of 0.57 to 1.33, it is possible to allow a focus shift due to GI.

  That is, the conditional expressions (6) and (7) should be set as follows.

(H / D) ≦ 0.0204 {1/10 × (L / Lb ² )} − ^0.4439 ≦ 1.33 (6a ′)
(H / D) ≧ 0.0204 {2/3 × (L / Lb ² )} ^−0.4439 ≧ 0.57 (7a ′)
Therefore, in an imaging optical system composed of one, two, or more optical systems, the optical element closest to the surface to be scanned is
(H / D) ≦ 0.0204 {1/10 × (L / Lb ² )} ^−0.4439 ≦ 1.33
(H / D) ≧ 0.0204 {2/3 × (L / Lb ² )} − ^0.4439
It is good to do.

Conventionally, the risk of reducing the flatness ratio (H / D) has been avoided, but according to this reference example , the flatness ratio (H / D) is particularly 1 by appropriately selecting the lens back Lb− and the depth width L. By selecting a lens shape (H ≦ D) that is less than or equal to 33, the degree of freedom in lens arrangement becomes possible.

In other words, in this reference example , by selecting a lens shape that satisfies the conditional expressions (6a ′) and (7), and desirably the conditional expressions (6a ′) and (7a), it is possible to allow a defocus due to GI. In addition, the degree of freedom of lens arrangement becomes possible.

Next, numerical examples of the imaging optical system of this reference example will be shown.

The surface shapes of the refractive surfaces of the first and second imaging lenses 61 and 62 in this reference example are expressed by the following shape expression. The intersection of each lens surface and the optical axis is the origin, the optical axis direction is the x axis, the axis orthogonal to the optical axis in the main scanning section is the y axis, and the axis orthogonal to the optical axis in the sub scanning section is the z axis The bus direction corresponding to the main scanning direction is

(Where R is the radius of curvature, and k, B ₄ , B ₆ , B ₈ , and B ₁₀ are aspheric coefficients)
The sub-scanning direction (the direction including the optical axis and orthogonal to the main scanning direction) and the sub-line direction are

Where r ′ = r ₀ (1 + D ₂ Y ² + D ₄ Y ⁴ + D ₆ Y ⁶ + D ₈ Y ⁸ )
(Where r ₀ is the radius of curvature on the optical axis on the optical axis, and D ₂ , D ₄ , D ₆ and D ₈ are coefficients)
Incidentally, the radius of curvature r ′ outside the optical axis is defined in a plane perpendicular to the main scanning plane, including the normal of the bus at each position.

In this reference example , the surface shape is defined by the above formula .

(Numerical example)
Table 1 shows the optical layout in the present reference example shown in FIGS. 1 and 2 and the surface shapes of the first and second imaging lenses 61 and 62. In this reference example , the height of the first imaging lens (G1) 61 is 1.2 mm, and the height of the second imaging lens (G2) 62 is 4.5 mm. The parameters related to the conditional expression of this reference example are listed below.
S1 = 16.50mm
G1 D = 6mm
G1 H = 1.2mm
G2 D = 4mm
G2 H = 4.5mm
L = 10mm
Lb = 100.85m
β = 2.0
Thus, the second imaging lens 62 as the imaging optical element satisfies H / D = 1.13, which satisfies the conditional expressions (6a) and (7a).

Lens height of the second imaging lens 62 from the conditional expression (7a) is can be lowered from the point of view of the first imaging lens 61 to 2.1 mm, the present reference example if you make a lower lens far Needless to say, the effects can be utilized to the fullest.

Table 2 shows an optical layout diagram and a surface shape of the imaging lens 66 in the reference example of the single imaging optical system shown in FIG. In this reference example , the height of the imaging lens (G1) 66 was 6.3 mm. The parameters related to the conditional expression of this reference example are listed below.
G1 D = 11mm
G1 H = 6.3mm
L = 10mm
Lb = 110.5m
β = 2.0
Thus, the imaging lens 66 has H / D = 0.57, which satisfies the conditional expressions (6a ′) and (7a ′).

  From the conditional expression (7a ′), the lens height of the imaging lens 66 can be increased up to 14.63 mm. If the priority is given to the enhancement of the performance of the lens, the higher lens is raised so far. Can reduce the effects of

As described above, in the present reference example , in the normal plastic lens having the refractive index distribution as described above, the flatness necessary to prevent the focus shift due to the environmental variation of the refractive index distribution is set according to the designed lens arrangement condition. It becomes possible. This makes it possible to obtain the desired imaging lens by simple molding at a low cost, without increasing the height of the imaging lens in the sub-scanning direction more than necessary and without requiring special processes such as annealing and sink molding. Can do.

In this reference example , the imaging optical system is composed of one or two imaging lenses. However, the present invention is not limited to this and may be composed of three or more.

Further, the collimator lens 2 and the cylindrical lens 4 may be constituted by one optical element (anamorphic lens).
(Example)

Next, examples of the present invention will be described.

The cross-sectional views of the optical system of the scanning optical apparatus in the present embodiment in the main scanning direction and the sub-scanning direction are the same as those of the reference example (FIGS. 1 and 2) described above.

This embodiment differs from the above-described reference example in the first imaging lens (imaging optical element) 61 that is closest to the optical deflector 5 side in the scanning optical device including the two-sheet imaging optical system. This is that the conditions for the GI permissible value are defined. Other configurations and optical actions are the same as those of the reference example , thereby obtaining the same effects.

In other words, in the present embodiment, in the first imaging lens 61 closest to the optical deflector 5 of the imaging optical system 6, from the main plane on the optical deflector side of the first imaging lens 61 to the deflection surface 5a. The distance on the optical axis is S1. A magnification in the sub-scanning direction (sub-scanning magnification) of the imaging optical system 6 is β. At this time, the allowable ΔGI (GI) is
△ GI = △ S / (S1 × β) ² (8)
It can be expressed as.

The allowable value of the focus shift amount ΔS is 1/5 or less of the depth width L in the sub-scanning direction of the imaging optical system of the scanning optical device. Further, the lower limit value of the focus shift amount ΔS does not become 0 in normal molding, and there exists about 3/100 of the depth width L. Therefore,
3L / 100 ≦ △ S ≦ L / 5 (9)
Can be written in Therefore, from equations (8) and (9),
3L / (100 × (S1 × β) ² ) ≦ △ GI ≦ L / (5 × (S1 × β) ² ) (10)
It becomes. This is the case when the GI changes 100% in the environment.

Actually, the GI fluctuation amount due to the environment is about 30%, so it can be tolerated up to 10/3 times. Therefore, equation (10) becomes
1/10 × (L / (S1 × β) ² ) ≦ △ GI ≦ 2/3 × (L / (S1 × β) ² ) (11)
It becomes. This is an acceptable amount for the initial GI.

  FIG. 6 shows a case where ΔS = 1 mm as an allowable value of allowable ΔGI.

  The table of FIG. 6 shows a case where the GI corresponding to the focus deviation amount ΔS changes by 100%. The value at which the initial GI allowable value changes by 30% (not shown) is the value obtained by multiplying the vertical axis of this graph by 3.33 as in FIG.

  On the other hand, the initial GI (ΔGI), as shown in FIG. 5, is a flatness that is the ratio of the height H in the sub-scanning direction including the outer shape on the optical axis of the imaging lens to the thickness D on the optical axis of the imaging lens. It has the following relationship with rate (H / D).

(H / D) = 0.0204 (△ GI) ^-0.4439 (12)
Therefore, from equations (11) and (12),
(H / D) ≦ 0.0204 {1/10 × (L / (S1 × β) ² )} ^−0.4439 (13)
(H / D) ≧ 0.0204 {2/3 × (L / (S1 × β) ² )} ^−0.4439 (14)
It becomes.

  Similarly to the conditional expression (6), when the upper limit value of the conditional expression (13) is exceeded, the height in the sub-scanning direction of the lens becomes higher than necessary, and the degree of freedom of optical path arrangement is limited, and the sub-scanning direction of the optical system When the height becomes high, especially when the upper limit exceeds 0.9, there is a problem. If the lower limit of conditional expression (14) is exceeded, the focus in the sub-scanning direction is shifted due to the influence of environmental fluctuations in the refractive index distribution, and the desired spot diameter cannot be narrowed down, degrading the image quality.

  If the lens shape is determined so that the oblateness ratio (H / D) satisfies the above conditional expressions (13) and (14), it is possible to allow a focus shift due to GI as an imaging optical system.

Here, in the scanning optical apparatus including the two-element imaging optical system in FIG. 1, the distance S1 on the optical axis from the main plane on the optical deflector side of the first imaging lens 61 to the deflection surface 5a and the scanning optics. The depth width L in the light beam sub-scanning direction formed on the surface to be scanned by the imaging optical system (second optical system) of the apparatus is:
S1 = 16.5mm
L = 10mm
It is. Substituting this into the conditional expressions (13) and (14) above,
The right side of conditional expression (13) is
0.0204 {1/10 × (L / (S1 × β) ² )} − ^0.4439 = 0.45
The right side of conditional expression (14) is
0.0204 {2/3 × (L / (S1 × β) ² )} − ^0.4439 = 0.20
It becomes.

  Therefore, if the lens shape is determined in the range of the flatness ratio (H / D) in the range of 0.20 to 0.45, the focus shift due to the GI can be allowed.

  In the present embodiment, the first imaging lens 61 closest to the optical deflector 5 is originally easily flattened because the beam width in the sub-scanning direction of the beam passing through the imaging lens 61 is small. Therefore, according to the present embodiment, the lens arrangement is selected by appropriately selecting the distance S1 and the depth width L, and particularly by selecting the lens shape (H ≦ D) in which the flatness ratio (H / D) is 0.9 or less. The degree of freedom becomes possible.

  That is, the conditional expressions (13) and (14) are preferably set as follows.

(H / D) ≦ 0.0204 {1/10 × (L / (S1 × β) ² )} − ^0.4439 ≦ 0.45 (13a)
(H / D) ≧ 0.0204 {2/3 × (L / (S1 × β) ² )} − ^0.4439 ≧ 0.20 (14a)
From the numerical example , the first lens has H / D = 0.2, which satisfies the conditional expressions (13a) and (14a).

  In this embodiment, by selecting a lens shape that satisfies the conditional expressions (13a) and (14), and desirably the conditional expressions (13a) and (14a), it is possible to allow defocus due to GI, and A degree of freedom is possible.

  In the present embodiment, the conditional expression of the present invention is applied to the scanning optical apparatus including the image forming optical system including the two image forming optical elements. The present invention may be applied to an image optical system or an image forming optical system including three image forming optical elements.

At this time, the optical element closest to the deflecting means (optical deflector) is
(H / D) ≦ 0.0204 {1/10 × (L / (S1 × β) ² )} − ^0.4439 ≦ 0.9
(H / D) ≧ 0.0204 {2/3 × (L / (S1 × β) ² )} ^−0.4439
It is good to do.

Moreover, you may comprise combining the conditions of the Example mentioned above and the conditions of the reference example mentioned above.

  The wall thickness d of the imaging lens is usually designed to be in the range of 3 to 20 mm. The lower limit is necessary from the structural strength of the lens. The upper limit value is considered in order to suppress the absolute value of the initial GI and the absolute value of birefringence. When the upper limit is exceeded, a height of 10 mm is required even if the flatness ratio is 0.5 with respect to a normal light beam diameter of about 4 mm, and the height of the optical system cannot be lowered.

[Image forming apparatus]
FIG. 7 is a cross-sectional view of the main part in the sub-scanning direction showing an embodiment of the image forming apparatus of the present invention. In the figure, reference numeral 104 denotes an image forming apparatus. Code data Dc is input to the image forming apparatus 104 from an external device 117 such as a personal computer. The code data Dc is converted into image data (dot data) Di by a printer controller 111 in the apparatus. The image data Di is input to the optical scanning unit 100 having the configuration shown in the embodiment . The light scanning unit 100 emits a light beam 103 modulated in accordance with the image data Di, and the light beam 103 scans the photosensitive surface of the photosensitive drum 101 in the main scanning direction.

  The photosensitive drum 101 serving as an electrostatic latent image carrier (photoconductor) is rotated clockwise by a motor 115. With this rotation, the photosensitive surface of the photosensitive drum 101 moves in the sub-scanning direction perpendicular to the main scanning direction with respect to the light beam 103. Above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided so as to contact the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with the light beam 103 scanned by the optical scanning unit 100.

  As described above, the light beam 103 is modulated based on the image data Di, and by irradiating the light beam 103, an electrostatic latent image is formed on the surface of the photosensitive drum 101. The electrostatic latent image is developed as a toner image by a developing unit 107 disposed so as to contact the photosensitive drum 101 further downstream in the rotation direction of the photosensitive drum 101 than the irradiation position of the light beam 103.

  The toner image developed by the developing unit 107 is transferred onto a sheet 112 as a transfer material by a transfer roller 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101. The paper 112 is stored in a paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 7), but can be fed manually. A paper feed roller 110 is provided at the end of the paper cassette 109, and feeds the paper 112 in the paper cassette 109 into the transport path.

  As described above, the sheet 112 on which the unfixed toner image has been transferred is further conveyed to a fixing device behind the photosensitive drum 101 (left side in FIG. 7). The fixing device includes a fixing roller 113 having a fixing heater (not shown) therein and a pressure roller 114 disposed so as to be in pressure contact with the fixing roller 113. Then, the sheet 112 conveyed from the transfer unit is heated while being pressed by the pressure contact portion between the fixing roller 113 and the pressure roller 114 to fix the unfixed toner image on the sheet 112. Further, a paper discharge roller 116 is disposed behind the fixing roller 113, and the fixed paper 112 is discharged out of the image forming apparatus.

  Although not shown in FIG. 7, the print controller 111 controls not only the data conversion described above, but also controls each part in the image forming apparatus including the motor 115 and a polygon motor in the optical scanning unit described later. I do.

The recording density of the image forming apparatus used in the present invention is not particularly limited. However, considering that the higher the recording density, the higher the image quality is required, the configuration of the embodiment of the present invention is more effective in an image forming apparatus of 1200 dpi or more.

[Color image forming apparatus]
FIG. 8 is a schematic view of a main part of a color image forming apparatus according to an embodiment of the present invention. This embodiment is a tandem type color image forming apparatus in which four scanning optical devices (optical scanning optical systems) are arranged in parallel and image information is recorded on the surface of a photosensitive drum as an image carrier. In FIG. 8, 60 is a color image forming apparatus, 11, 12, 13 and 14 are scanning optical devices each having one of the configurations shown in Reference Examples 2 to 2, and 22, 22, 23 and 24 are image carriers. The photosensitive drums 31, 32, 33 and 34 are developing units, and 51 is a conveyor belt. In FIG. 8, there are a transfer device (not shown) for transferring the toner image developed by the developing device to the transfer material, and a fixing device (not shown) for fixing the transferred toner image to the transfer material. doing.

  In FIG. 8, the color image forming apparatus 60 receives R (red), G (green), and B (blue) color signals from an external device 52 such as a personal computer. These color signals are converted into C (cyan), M (magenta), Y (yellow), and B (black) image data (dot data) by a printer controller 53 in the apparatus. These image data are input to the scanning optical devices 11, 12, 13, and 14, respectively. From these scanning optical devices, light beams 41, 42, 43, and 44 modulated according to each image data are emitted, and the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24 are caused by these light beams. Scanned in the main scanning direction.

  The color image forming apparatus in this embodiment has four scanning optical devices (11, 12, 13, and 14) arranged in each of C (cyan), M (magenta), Y (yellow), and B (black) colors. It corresponds. In parallel, image signals (image information) are recorded on the surfaces of the photosensitive drums 21, 22, 23, and 24, and color images are printed at high speed.

  As described above, the color image forming apparatus according to the present embodiment uses the four scanning optical devices 11, 12, 13, and 14 and the corresponding photosensitive drums 21 and 22 respectively corresponding to the latent images of the respective colors by using the light beams based on the respective image data. , 23, 24 on the surface. Thereafter, a single full color image is formed by multiple transfer onto a recording material.

  As the external device 52, for example, a color image reading device including a CCD sensor may be used. In this case, the color image reading apparatus and the color image forming apparatus 60 constitute a color digital copying machine.

Main scanning sectional view of the scanning optical apparatus of the present invention Sub-scan sectional view of the scanning optical apparatus of the present invention Main scanning sectional view of another scanning optical apparatus of the present invention The figure which shows the relationship between the allowable GI of the final imaging lens and the lens back The figure which shows the relationship between allowable GI of an imaging lens, and an oblateness ratio The figure which shows the relationship between the allowable GI of the first imaging lens and S1, β FIG. 3 is a sub-scan sectional view showing an embodiment of the image forming apparatus of the present invention. 1 is a schematic view of a main part of a color image forming apparatus according to an embodiment of the present invention. Sub-scan sectional view of a conventional color image forming apparatus

DESCRIPTION OF SYMBOLS 1 Light source means 2 Collimator lens 3 Aperture 4 Cylindrical lens LA 1st optical system (incident optical system)
5 Deflection means 6 Second optical system Imaging optical system 61, 62 Optical element (imaging lens)
65 Dust-proof glass 8 Scanned surfaces 11, 12, 13, 14 Scanning optical devices 21, 22, 23, 24 Image carrier (photosensitive drum)
31, 32, 33, 34 Developers 41, 42, 43, 44 Light beam 51 Conveying belt 52 External device 53 Printer controller 60 Color image forming apparatus 100 Scanning optical apparatus 101 Photosensitive drum 102 Charging roller 103 Light beam 104 Image forming apparatus 107 Developing device 108 Transfer roller 109 Paper cassette 110 Paper feed roller 111 Printer controller 112 Transfer material (paper)
113 Fixing roller 114 Pressure roller 115 Motor 116 Paper discharge roller 117 External device

Claims (7)

A light source means, a first optical system for guiding the light beam emitted from the light source means to the deflecting means, and a second light beam for deflecting and scanning the deflected surface of the deflecting means on the surface to be scanned. In a scanning optical device comprising an optical system,
Wherein the second optical system have a imaging optical element of two or more,
And the closest imaging optical element in the two or more imaging optical elements sac Chi said deflecting means is have a refractive index distribution in the sub-scanning direction,
Said deflecting means height in the sub-scanning direction comprising a contour on the optical axis of the closest imaging optical element in H, the thickness on the optical axis D of the nearest image-forming optical element to the deflecting means, before Symbol in the sub-scanning direction depth range of the light beam is imaged on a surface to be scanned by the second optical system L, the nearest imaging optical element to the deflecting means on the optical axis to the deflecting surface of said deflecting means When the distance is S1, the magnification in the sub-scanning direction of the second optical system is β , and the refractive power due to the refractive index distribution of the imaging optical element closest to the deflecting means is ΔGI (1 / mm) ,
(H / D) = 0.0204 (△ GI) ^−0.4439 ≦ 0.9
1/10 × (L / (S1 × β) ² ) ≦ △ GI ≦ 2/3 × (L / (S1 × β) ² )
A scanning optical device characterized by satisfying the following conditions: 2. The scanning optical device according to claim 1 , wherein a material of the imaging optical element having a refractive index distribution in the sub-scanning direction is a polyolefin resin. The imaging optical element having a refractive index distribution in the sub-scanning direction is injection-filled with a specific pressure into a molding die set at a specific mold temperature with a heat-melted resin material, and after a specific time has elapsed. The scanning optical apparatus according to claim 1 , wherein the scanning optical apparatus is molded through a step of taking out from a mold. A scanning optical apparatus according to any one of claims 1 to 3, wherein a photosensitive member disposed on the scanned surface, electrostatic formed on said photosensitive member by a light beam scanned by said scanning optical device A developing device that develops an electrostatic latent image as a toner image, a transfer device that transfers the developed toner image onto a transfer material, and a fixing device that fixes the transferred toner image onto the transfer material. Image forming apparatus. Wherein the scanning optical apparatus according to any one of claims 1 to 3, that converts code data inputted from an external device into an image signal and a printer controller for inputting the imagewise signal into said optical scanning device An image forming apparatus. Claimed disposed on the surface to be scanned of the scanning optical apparatus according to any one of claim 1 to 3, a color image forming apparatus characterized by having a plurality of image bearing members for forming a color image of mutually different. 7. The color image forming apparatus according to claim 6 , further comprising a printer controller that converts color signals input from an external device into image data of different colors and inputs the image data to each scanning optical device.