JP2012242719A - Scanning optical system and image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a scanning optical system capable of compensating fluctuation in beam spot diameter and fluctuation in beam spot position on a scanning object surface by a compact and simple configuration.SOLUTION: The scanning optical system includes compensation means for compensating for the fluctuation in the beam spot diameter and the fluctuation in the beam spot position on the scanning object surface, in an optical path for deflecting a light flux radiated from light source means by deflection means and imaging onto the scanning surface. The compensation means includes: a plurality of optical elements constituting an optical system, disposed along an optical axis of the optical system, and including an optical surface having a shape plane-symmetric to a symmetry plane that includes the optical axis and a symmetry axis perpendicular to the optical axis; first drive means for changing the relative positions of the plurality of optical elements in the direction parallel to the symmetry axis; and second drive means for changing the positional relationship of the plurality of optical elements in the direction parallel to the symmetry axis without affecting the change in the relative positions of the plurality of optical elements.

Description

  The present invention relates to a scanning optical system and an image forming apparatus.

In recent years, optical scanners such as laser beam printers and digital copiers have become increasingly demanding devices that are resistant to various environmental fluctuations and suitable for high-definition printing as the price and size of image forming apparatuses are reduced. .
In this type of optical scanning device, a plastic material is often used as a material constituting the optical element from the viewpoint of cost reduction and downsizing.
An optical element (plastic lens) made of a plastic material has a property that the refractive index changes with a change in use environment, for example, a change in environmental temperature.
As a result, in an optical scanning device using a plastic lens, focal position fluctuations and spot position fluctuations in the main scanning direction and sub-scanning direction due to environmental fluctuations occur.

Conventionally, in order to cope with these problems, Patent Document 1 proposes an apparatus in which a light beam is focused on a photosensitive drum while always maintaining a good spot diameter.
This apparatus includes two light receivers for detecting the beam state on the drum and a position adjuster for adjusting the position of the collimator lens, and receives the focusing pattern image projected on the photosensitive drum. Read with.
The position adjuster is driven by a control signal based on the read signal to correct the spot diameter on the photosensitive drum to a good state.
Further, Patent Document 2 proposes an optical scanning device that corrects a beam spot position to a predetermined position so that a beam spot position shift on the surface to be scanned does not occur due to an environmental temperature rise.
In this apparatus, a vibrating body having a mirror part that changes the direction of the laser beam is disposed between the light source and the deflecting means, and the driving of the vibrating body is corrected to correct a beam spot position shift caused by surface tilt. It is configured as follows.

Japanese Patent Laid-Open No. 11-042813 JP 2004-029113 A

However, the above conventional example has the following problems.
That is, in the case of Patent Document 1, in principle, it is impossible to correct the beam spot position caused by surface tilt in the sub-scanning direction.
Moreover, in the thing of patent document 2, correction | amendment of the focus position of a light beam cannot be carried out in principle. Further, by combining the means of Patent Document 1 and Patent Document 2 to simultaneously correct the focal position deviation and the beam spot position deviation of the light beam, many components are provided between the light source and the light deflection means. Therefore, the control mechanism necessary for correction becomes complicated.

  In view of the above problems, the present invention provides a scanning optical system and an image forming apparatus capable of correcting variations in beam spot diameter and beam spot position on a surface to be scanned with a small and simple configuration. With the goal.

The scanning optical system of the present invention is
Compensation means for compensating for variations in the beam spot diameter and beam spot position on the scanned surface in the optical path for deflecting the light beam emitted from the light source means by the deflecting means and forming an image on the scanned surface. A scanning optical system comprising:
The compensation means includes
A plurality of optics having optical surfaces that are symmetrical with respect to a symmetry plane that includes a symmetry axis that is perpendicular to the optical axis and the optical axis that is disposed along the optical axis of the optical system that constitutes the optical system. Elements,
First driving means for changing relative positions of the plurality of optical elements in a direction parallel to the symmetry axis;
Second driving means for changing the positional relationship of the plurality of optical elements in a direction parallel to the symmetry axis without affecting the change in the relative positions of the plurality of optical elements,
In accordance with a change in the relative position of the plurality of optical elements by the first driving means, the power of the optical system is changed to compensate for the variation in the beam spot diameter on the scanned surface,
The beam spot position variation on the surface to be scanned is compensated according to the positional relationship of the entire optical element by the second driving unit.
An image forming apparatus according to the present invention includes the above-described scanning optical system.

  According to the present invention, it is possible to realize a scanning optical system and an image forming apparatus that can correct the variation in the beam spot diameter and the variation in the beam spot position on the surface to be scanned with a small and simple configuration. it can.

Schematic explaining the structural example of the scanning optical system in Example 1 of this invention. The graph showing the power variation | change_quantity with respect to the relative position displacement amount in Example 1 of this invention. The schematic diagram which shows a mode that the relative position of each optical element was changed by the rotation effect | action of the drive means in Example 1 of this invention. The figure which shows the method of transmitting force by friction, such as a gearwheel and a roller, as a connection method of the drive means and member in Example 1 of this invention. The figure which shows the method of using the lever which made the rotating shaft of the drive means a fulcrum and a power point, and made the connection site | part of each member into an action point as a connection method of the drive means and member in Example 1 of this invention. FIG. 5 is a schematic diagram showing a state when the relative position between the optical axis in the optical system and the sub optical axis that is the optical axis in the optical element group is changed by the rotational action of the driving unit in Embodiment 1 of the present invention. The figure which shows a mode that the incident parallel light is converted into the convergent light by the relative position of the optical element in Example 1 of this invention changing. The figure which shows a mode when the gear is translated in the Y-axis direction in Example 1 of this invention. FIG. 6 is a diagram illustrating a configuration example of a scanning optical system in Embodiment 2 of the present invention. The figure explaining the installation example of the photodetector in Example 2 of this invention. FIG. 10 is a schematic diagram illustrating a method of receiving, with a photodetector, reflected light from a scanned surface with respect to a light beam formed on the scanned surface via a scanning optical element and a folding mirror in Embodiment 2 of the present invention. FIG. 6 is a diagram illustrating a state in which a light beam from a deflecting unit in Embodiment 2 of the present invention forms an image on a surface to be scanned through a scanning optical element. The figure which shows the structural example of the light receiving element in Example 2 of this invention. The schematic diagram when the light beam (beam) in Example 2 of this invention hits a light receiving element. The figure showing the case where there is no position shift of the spot in Example 2 of this invention, and only a focus shifts | deviates. The figure showing the case where the spot position in Example 2 of this invention changes. The graph showing the relationship between the total of the light quantity detected with the photodetector in Example 2 of this invention, and the amount of displacement of the relative position between optical elements. The graph which shows the relationship between the light reception amount ratio of the light quantity detected with the photodetector in Example 2 of this invention, and the displacement amount of an optical element group. Schematic explaining the structural example of the image forming apparatus in Example 3 of this invention.

  The mode for carrying out the present invention will be described with reference to the following examples.

[Example 1]
As Example 1, in the optical path in which the light beam emitted from the light source to which the present invention is applied is deflected to form an image on the scanned surface, the variation of the beam spot diameter or the variation of the beam spot position on the scanned surface is performed. A configuration example of a scanning optical system having compensation means for compensating will be described with reference to FIG.
As shown in FIG. 1, the scanning optical system 1 of this embodiment includes a light source means 2 and deflecting means 4 (collimator lens), 5 (cylindrical lens), 6 (deflection mirror), 7 having a function of deflecting and scanning a light beam. (Scanning optical element), compensation means 9 (optical element group), and a scanned surface 8 on which the deflected light beam is imaged.
The main scanning direction of the optical system represents the XZ section in FIG. 1, and the sub-scanning direction represents the YZ section in FIG.

The optical elements 9a and 9b included in the compensation means 9 are arranged along the optical axis of the optical system.
Each of the optical elements 9a and 9b has an optical surface having an optical surface that is symmetrical with respect to a symmetry plane including a symmetry axis perpendicular to the optical axis and the optical axis arranged along the optical axis. doing.
For example, it has an optical surface that is symmetrical with respect to the XZ plane in FIG.
An example of a function that expresses an optical surface having a plane-symmetric shape with respect to the XZ plane is shown in Equation 1 below.

Z = a × X ³ + b × X × Y ² Formula 1
(However, a and b are arbitrary constants.)

An optical surface having a shape generally known as an Alvarez surface can be expressed by a function represented by the following Equation 2.

Z = c × (1/3 × X ³ + X × Y ² )
+ D * X < ² > + e * XY + f * X + g + h * F (y) ... Formula 2
(However, c, d, e, f, g, and h are arbitrary constants, and F (y) is a function that does not depend on X.)

The function shown in Expression 1 can be expressed by appropriately selecting an arbitrary constant in the function expressed in Expression 2.
The power of the optical element group 9 can be changed by displacing the relative positions of the optical elements 9a and 9b in the direction parallel to the symmetry axis.

FIG. 2 shows a graph showing the relative positional displacement amount of the optical elements 9a and 9b and the power change amount in the optical element group 9. In FIG.
The horizontal axis indicates the relative displacement of the optical elements 9a and 9b with respect to the optical axis, and the vertical axis indicates the power change amount in the optical element group 9. In this graph, the power change amount of the optical element group 9 is represented as zero at the point where the relative positional displacement amount of the optical elements 9a and 9b is zero.

Taking the coordinate system shown in FIG. 1 as an example, an arrangement configuration of this embodiment in which the optical axis and a plurality of optical elements are constituted by optical elements 9a and 9b will be described.
Regarding the relative position change amount between the optical axis and the optical element 9a, the relative position change amount when the X-axis minus direction is changed is plus, and the relative position change amount when the X-axis plus direction is changed is minus.
Similarly, regarding the relative position change between the optical axis and the optical element 9b, the relative position change when the X axis is changed in the positive direction is positive, and the relative position change when the X axis is changed in the negative direction is negative. .
The direction and the magnitude of the relative position change amount in the optical elements 9a and 9b are both the same.
From this graph, it can be seen that when the relative position displacement amount changes in the plus direction, the power change amount changes to plus. Similarly, when the relative position displacement amount changes in the minus direction, the power change amount changes to minus.
The optical elements 9a and 9b are connected to members 10a and 10b having a function of transmitting the action of the driving means (first driving means) 11a to the optical elements 9a and 9b.
It is comprised by the member 10a which connects the drive means 11a and the optical element 9a, and the member 10b which connects the drive means 11a and the optical element 9b.
The unit 12 is connected to a driving means (second driving means) 11b.
The unit 12 is displaced by the action of the driving means 11b, and the optical element group (the plurality of optical elements as a whole) 9 is in a direction perpendicular to the optical axis (without affecting the change in the relative position of the plurality of optical elements ( The positional relationship is displaced in a direction parallel to the symmetry axis.

FIG. 3 is a schematic diagram showing a state in which the relative positions of the optical elements 21a and 21b are changed by the rotating action of the driving means 23. FIG.
In FIG. 3, the parallel light flux that has traveled along the optical axis passes through the optical element group 21. As the driving means 23 rotates and the relative positions of the optical elements 21a and 21b with respect to the optical axis change, the power of the optical element group 21 changes.
As a result, the light beam that has passed through the optical element group 21 diverges or converges. Here, the power of the optical element group 21 is increased and the light beam is converged.
The members 22a and 22b are connected so that the optical elements 21a and 21b move along an axis in a direction perpendicular to the optical axis.
For example, this can be realized by arranging the members 22a and 22b along grooves or guides arranged in a direction perpendicular to the optical axis. The drive unit 23 can control the amount of rotation according to an electric signal input from the outside by applying an actuator such as a stepping motor, for example.

As a method of connecting the driving means 23 and the members 22a and 22b, for example, a method of transmitting force by friction of gears or rollers (FIG. 4) is possible.
Various methods such as a method using a lever (FIG. 5) using the rotation shaft of the driving means 23 as a fulcrum and a force point and using a connection portion between the members 22a and 22b as an action point can be applied.
As described above, the power in the optical element group 21 can be changed by displacing the relative positions of the optical elements 21a and 21b.
By controlling the power in the optical element group 21, it is possible to compensate for power fluctuations in the optical system and to control the imaging state (spot size) of the light beam on the surface to be scanned.

FIG. 6 is a schematic diagram when the relative position between the optical axis 32 in the optical system and the sub optical axis 33 that is the optical axis in the optical element group 31 is changed by the rotating action of the driving unit 34.
Here, the sub optical axis is defined as an axis that coincides with the principal ray of the light beam passing through the optical element group in the optical element arrangement when the power in the optical element group 31 is zero.
In FIG. 6, the optical element group 31 is displaced in a direction perpendicular to the optical axis 32 while the relative arrangement of the optical elements 31 a and 31 b in the optical element group 31 is fixed by the rotating action of the driving means 34. Yes.
As a result, the sub optical axis 33 moves in a direction perpendicular to the optical axis 32. When the light beam 35 and the sub optical axis 33 of the optical element group are shifted, the emission angle of the light beam 35 changes and the light beam 35 is deflected.
As described above, the emission angle of the light beam from the optical element group 31 can be changed by changing the relative position between the optical axis 32 and the sub optical axis 33.
With this action, the beam spot position on the surface to be scanned can be controlled.
Variations in the beam spot diameter and beam spot position on the surface to be scanned are caused by various factors such as a change in the refractive index of the material caused by a change in the environmental temperature and a distortion such as expansion and contraction of the casing.

As a specific example, a drive unit that controls the relative position of the optical element and a drive unit that controls both the optical axis and the sub optical axis can be driven by a common actuator. May be.
First, the compensation optical system (compensation means) shown in FIG. 7 includes an optical element group 41 composed of a plurality of optical elements 41a and 41b, a drive means 44, a drive shaft 45, a gear 43, members 42a and 42b, and a fixed member. 46.
The rotational action of the drive means 44 is transmitted to the gear 43 via the rotary shaft 45. The action of the gear 43 changes the arrangement of the optical elements 41a and 41b via the members 42a and 42b.
As a result, the power in the optical element group 41 changes and the imaging state of the light beam changes. FIG. 7 shows a state in which incident parallel light is converted into convergent light by changing the relative positions of the optical elements 41a and 41b.

Next, FIG. 8 shows a state when the gear 43 is translated in the Y-axis direction.
The Y coordinate of the gear 43 and the Y coordinate of the fixed member 46 are changed by translating the driving means 44 in the Y direction.
Due to the displacement in the Y direction, the gear 43 comes into contact with only the member 42a and transmits the power from the driving means 44 only to the member 42a.
At the same time, the fixing member 46 comes into contact with the members 42a and 42b and connects the members 42a and 42b. As a result, the members 42a and 42b are integrated. The action from the gear 43 is equally transmitted to the members 42a and 42b.
The optical elements 41a and 41b connected to the members 42a and 42b move parallel to the X axis and in the same direction, so that the sub optical axis of the optical element group 41 is in a direction perpendicular to the optical axis. Moving.

The sub-optical axis is moved in a direction perpendicular to the optical axis, so that the parallel light incident on the optical element group 41 is emitted with the emission angle changed.
The driving means for controlling the relative position of the optical element and the driving means for controlling the relative position between the optical axis and the sub optical axis were controlled by the same driving means.
As a result, it is possible to control both the beam spot size and the beam spot position on the surface to be scanned.
As described above, according to the configuration of the scanning optical system of this embodiment, it is possible to compensate for both the variation of the beam spot diameter and the variation of the beam spot position on the scanned surface with a simpler control mechanism. It becomes.

[Example 2]
As a second embodiment, a configuration example of a scanning optical system including the scanning optical system and the beam detection unit in the first embodiment will be described with reference to FIG.
In the scanning optical system 51 of this embodiment, the light source means 52, the deflection means (collimator lens 54, cylindrical lens 55, deflection mirror 56, scanning optical element 57), the compensation means 60, and the deflected light beam form an image. A scanned surface 58 is provided.
The main scanning direction of the optical system represents the XZ section in FIG. 9, and the sub scanning direction represents the YZ section in FIG.
The optical elements 60a and 60b included in the compensation unit 60 are connected to the driving unit 62a via the members 61a and 61b.
It has a mechanism in which the relative positions of the optical elements 60a and 60b change as the driving means 62a rotates.
The optical element group 60 is connected to the driving means 62b through the unit 63.
The optical element group 60 has a mechanism in which the relative position with respect to the optical axis changes as the driving means 62b rotates.
The photodetector 59 has a function of detecting the size of the beam spot and the position of the beam spot on the surface to be scanned.
The photodetector signal detected by the photodetector 59 is transmitted to the signal control means 64. The signal control means 64 generates a control signal for driving the drive means 62a and 62b based on the light detection signal detected by the light detector 59.

FIG. 10 is a diagram for explaining an installation example of the photodetector in this embodiment.
FIG. 10 shows a part of the scanning optical system. The light beam that has passed through the scanning optical element 71 is reflected by the folding mirror 72 and forms an image on the scanned surface 73.
The folding mirror 72 has a reflecting surface having a function of transmitting a part of the light amount, and the light that has passed through the folding mirror 72 is received by the photodetector 74.
At this time, the optical arrangement of the folding mirror 72 and the scanned surface 73 and the optical arrangement of the folding mirror 72 and the photodetector 74 are set to optically match.
This makes it possible to detect the imaging state of the light beam on the surface to be scanned and the spot position of the light beam using the photodetector 74.

As other means, as shown in FIG. 11, the reflected light from the scanned surface 83 with respect to the light beam formed on the scanned surface 83 through the scanning optical element 81 and the folding mirror 82 is sent to the photodetector 84. And receiving light.
As shown in FIG. 12, when a light beam from a deflecting unit 91 such as a polygon mirror forms an image on a scanned surface 93 through a scanning optical element 92, a part of the light beam exceeding a desired scanning angle is detected. A method of receiving light by the instrument 94 may be used.
The photodetector can take various arrangements in order to acquire information on the beam spot diameter and beam spot position on the surface to be scanned.
A plurality of photodetectors may be arranged according to the application and required accuracy.
A method for detecting the size of the beam spot diameter and the beam spot position on the surface to be scanned in the light receiving element mounted on the photodetector will be described with an example.

The light receiving element 101 shown in FIG. 13 includes four light receiving surfaces 101a, 101b, 101c, and 101d.
Each light receiving surface has a photoelectric conversion function for generating an electric signal having a value corresponding to the energy of the received light. From the electrical signals photoelectrically converted by the four light receiving surfaces, the imaging state of the light beam and the amount of beam position deviation can be calculated. The number and size of the light receiving surfaces installed in the light receiving element can be freely selected according to the application and required accuracy.

FIG. 14 is a schematic diagram when the light beam (beam) 102 hits the light receiving element 101.
Here, as shown in the arrangement example of the photodetector, the optical arrangement is such that the light beam state on the surface to be scanned can be grasped by observing the light beam state on the surface of the light receiving element.
FIG. 14 shows a case where the imaging state of the light beam is ideal and the spot diameter is the smallest.
In addition, the image forming position of the light beam is ideal, and the positional deviation in the vertical and horizontal directions does not occur.
In the graph shown at the bottom of FIG. 14, I (a), I (b), I (c), and I (d) represent the amounts of light received at the light receiving surfaces 101a, 101b, 101c, and 101d, respectively.
By analyzing the amount of light received at each of the light receiving surfaces and the ratio of the amounts of light received between the light receiving surfaces, it is possible to obtain information on the imaging state of the light beam and the light beam position shift.
Paying attention to the total amount of received light, the total amount of received light (ΣI (k) = I (a) + I (b) + I (c) + I (d)) when the spot diameter is the smallest is minimum, plus or minus The spot diameter increases with defocusing toward the side, and the total amount of received light increases.
On the other hand, paying attention to the ratio of the amount of light received between the light receiving surfaces, by taking the ratio of (I (a) + I (b)) / (I (c) + I (d)), the sub scanning direction (YZ direction) The spot position deviation amount in the main scanning direction (XZ direction) is acquired by taking the ratio of (I (a) + I (d)) / (I (b) + I (c)) as the spot position deviation amount. be able to.
For example, FIG. 15 shows a case where there is no spot position shift and only the focus shifts. As the spot diameter increases, the amount of light received at each light receiving surface increases.

FIG. 16 shows a case where the spot position has changed.
(I (a) + I (b)) / (I (c) + I (d)), which is the ratio of the amount of light received between the light receiving surfaces, is larger than 1, and the beam spot position is shifted in the sub-scanning direction (YZ direction). You can see that
By using the above-described means, the image formation position shift amount and the spot position shift amount are converted from the respective received light amount changes. Based on this value, the position change amount in the drive means can be determined by converting into the position change amount of the optical element and the optical element group in the compensation optical system (compensation optical means).

FIG. 17 is a graph showing the relationship between the total amount of light detected by the photodetector and the amount of displacement of the relative position between the optical elements.
The abscissa indicates the total amount of light detected by the photodetector (that is, the spot diameter on the surface to be scanned), and the ordinate indicates the displacement amount of the relative position between the optical elements (that is, the driving amount of the driving means). Here, the case where the spot diameter at the design value is the minimum, that is, the total light amount detected by the photodetector is illustrated as the minimum.
Factors that increase the spot diameter include a decrease in the refractive index of the lens material constituting the optical system due to a change in environmental temperature, and a decrease in the power of the entire optical system compared to the design value.
In this case, the power of the entire optical system can be corrected by changing the relative position between the optical elements according to the insufficient power.
From FIG. 17, the relationship between the total amount of light detected by the photodetector and the amount of displacement of the relative position between the optical elements can be uniquely determined.
That is, the driving amount of the driving means having a function of displacing the relative position between the optical elements can be determined according to the total amount of light detected by the photodetector.

FIG. 18 shows the relationship between the received light amount ratio (I (a) + I (b)) / (I (c) + I (d)) of the light amount detected by the photodetector and the displacement amount of the optical element group.
The ratio of the amount of received light detected by the photodetector corresponds to the amount of spot position deviation on the surface to be scanned. The deviation of the spot position on the surface to be scanned is caused by, for example, a change in the optical element arrangement due to the distortion of the polygon mirror surface or the distortion of the casing due to the environmental temperature change.
By shifting the optical element group in the direction perpendicular to the optical axis, it is possible to correct the spot position deviation on the surface to be scanned.
From FIG. 18, the amount of displacement of the optical element group can be uniquely determined from the ratio of the amount of received light detected by the photodetector.
That is, the drive amount of the drive means having the function of displacing the position of the optical element group can be determined in accordance with the received light amount ratio of the light amount detected by the photodetector.
In summary, the beam spot diameter on the surface to be scanned is controlled by controlling the driving means so that the sum of the amounts of light received detected by the photodetector is minimum and the ratio of the amounts of light received on the light receiving surface is 1. And the beam spot position is compensated.
As described above, according to the configuration of the scanning optical system of this embodiment and the scanning optical system composed of the driving unit and the beam detecting unit, the variation of the beam spot diameter on the surface to be scanned can be achieved with a simpler control mechanism. It is possible to compensate for both variations in beam spot position.

[Example 3]
As a third embodiment, a configuration example of an image forming apparatus configured by applying the present invention will be described with reference to FIG.
An image forming apparatus applied to the scanning optical system according to the present invention will be described.
As shown in FIG. 19, the scanning optical system 111 of this embodiment includes a light source unit 112, a deflecting unit (collimator lens 114, cylindrical lens 115, polygon mirror 116, scanning optical element 117), and an optical element group (compensating optical system). 120).
Furthermore, a scanning surface (photosensitive drum) 118, a photodetector 119, a signal control means 124, and driving means 122a and 122b are provided.
The optical element group 120 includes optical elements 120a and 120b.
The optical elements 120a and 120b are connected to the driving means 122a via the members 121a and 121b. It has a mechanism in which the relative position of the optical elements 120a and 120b changes as the driving means 122a rotates.
The optical element group 120 is connected to the driving unit 122b via the unit 123.
The optical element group 120 has a mechanism in which the relative position with respect to the optical axis changes as the driving unit 122b rotates.
The photodetector 119 has a function of detecting the size of the beam diameter and the beam spot position on the surface to be scanned.
The signal detected by the photodetector 119 is transmitted to the signal controller 124. The signal control device 124 generates a signal for driving the driving means 122a and 122b based on the signal detected by the photodetector 119.

A case where the spot diameter changes and the spot position on the scanned surface change when the environmental temperature changes from 25 ° C. to 50 ° C. will be described.
In the following examples, description is made excluding the influence on the environmental temperature change in the scanning optical element, but it may be appropriately considered according to the application and required accuracy.
First, at an environmental temperature of 25 ° C., the focal length of the collimator lens 114 is 15 mm, and the refractive index of the material constituting the collimator lens 114 is n = 1.525.
The focal length of the cylindrical lens 115 in the main scanning section is 120 mm, and the focal length in the sub-scanning section is 25.5 mm.
The refractive index of the material constituting the cylindrical lens 115 is n = 1.525.
The materials constituting the collimator lens 114, the cylindrical lens 115, and the optical element group (compensation optical system) 120 are made of a cycloolefin polymer (linear expansion coefficient is 70 × 10 ⁻⁶ ).

When the environmental temperature is changed from 25 ° C. to 50 ° C., the refractive index of the material fluctuates, and the lens curvature radius changes due to the volume expansion of the lens itself corresponding to the linear expansion coefficient of the cycloolefin polymer material.
The refractive index of the material constituting the collimator lens 114 and the cylindrical lens 115 at an environmental temperature of 50 ° C. changes to n ′ = 1.522, and the radius of curvature of each is about 0.175% compared to that at 25 ° C. Change.
As a result, the focal length of the collimator lens 114 changes to 15.11 mm, the focal length of the cylindrical lens 115 in the main scanning section direction changes to 120.21 mm, and the focal length in the sub-scanning section direction changes to 25.54 mm.

As the focal length of the entire optical system increases, the spot diameter on the scanned surface (photosensitive drum) 118 increases. As a result, the amount of light detected by the photodetector 119 increases.
The power fluctuation amount in the entire system generated at this time is 1.055 × 10 ^{−1 with} respect to the main scanning cross-sectional direction and 5.605 × 10 ⁻² with respect to the sub-scanning cross-sectional direction.
The surface shapes of the optical elements 120a and 120b necessary to obtain the optical power correction amount are obtained.
With respect to the relative position δ between the optical element and the optical axis of the optical system, assuming that δ = 0 mm at 25 ° C. and δ ′ = 0.010 mm (= 10 μm) at 50 ° C., the optical surfaces of the optical elements 120a and 120b The function can be expressed by Equation 3 below.
The relative position change amount (δ′−δ) between the optical element and the optical axis of the optical system can be freely selected according to the application and configuration.

Z = 2.2.5899x ³ + 0.889488x × y ² Formula 3

When the environmental temperature is 25 ° C. and 50 ° C., the value of the electrical signal output from the photodetector 119 and the relative position change amount of the optical element (in this case, when the environmental temperature changes from 25 ° C. to 50 ° C. The driving amount of the driving unit 122a can be determined from the relationship with the fluctuation of 10 μm.
Based on this driving amount, the signal control means 124 controls the value of the output signal with respect to the input signal, whereby the spot diameter on the surface to be scanned can be corrected appropriately. Further, the spot position shift on the surface to be scanned occurs due to the influence of the distortion of the casing caused by the environmental change and the surface tilt caused by the production accuracy of the polygon mirror surface.
The surface of the polygon mirror surface is ideally polished with a high accuracy of about 5 "(seconds) to 10" (seconds).

When spot position fluctuations occur on the surface to be scanned, the ratio of the amount of light detected by the photodetector 119, such as (I (a) + I (b)) / (I (c) + I (d)) Changes.
The spot position on the surface to be scanned can be corrected by changing the relative position of the optical element group 120 with respect to the optical axis of the optical system in accordance with the change amount of the electrical signal.
For example, the spot position on the surface to be scanned may be corrected by controlling the position of the optical element group so that the received light amount ratio is 1.
The relative positional displacement amount of the optical element group with respect to the spot positional deviation amount depends on the magnification relationship of the entire optical system. For example, in order to correct the spot positional deviation amount of 20 μm, the optical element group is About 5 μm is required as the amount of positional displacement.

The drive amount of the drive unit 122b can be determined from the relationship between the value of the amount of received light detected by the photodetector and the relative position displacement of the optical element group.
Based on this driving amount, the signal control means 124 controls the value of the output signal with respect to the input signal, whereby the spot position on the surface to be scanned can be appropriately corrected. As described above, it is possible to provide an image forming apparatus that has a simple mechanism and is resistant to environmental fluctuations and assembly errors with respect to fluctuations in the beam spot diameter and beam spot position on the surface to be scanned.
According to the above configuration of the present invention, it is possible to provide compensation means that can be realized with a simple configuration against variations in the beam spot diameter and beam spot position on the surface to be scanned that occur in the scanning optical system. it can.
In addition, a part of the deflecting unit may be combined with the compensating unit. For example, it is possible to adopt various configurations such that the cylindrical lens constituting the deflecting unit is configured by an adaptive optical system composed of a plurality of optical elements, and has the functions of the cylindrical lens and the adaptive optical system.
The compensation means of the present invention can be applied to various optical devices having a scanning optical system such as an image forming apparatus.

1: Scanning optical system 2: Light source means 4: Collimator lens 5: Cylindrical lens 6: Deflection means (deflection mirror)
7: scanning optical element 8: surface to be scanned (photosensitive drum)
9: Optical element group (compensation means)
9a, 9b: Optical elements 10a, 10b: Members 11a, 11b: Driving means 12: Unit

Claims (5)

Compensation means for compensating for variations in the beam spot diameter and beam spot position on the scanned surface in the optical path for deflecting the light beam emitted from the light source means by the deflecting means and forming an image on the scanned surface. A scanning optical system comprising:
The compensation means includes
A plurality of optics having optical surfaces that are symmetrical with respect to a symmetry plane that includes a symmetry axis that is perpendicular to the optical axis and the optical axis that is disposed along the optical axis of the optical system that constitutes the optical system. Elements,
First driving means for changing relative positions of the plurality of optical elements in a direction parallel to the symmetry axis;
Second driving means for changing the positional relationship of the plurality of optical elements in a direction parallel to the symmetry axis without affecting the change in the relative positions of the plurality of optical elements,
In accordance with a change in the relative position of the plurality of optical elements by the first driving means, the power of the optical system is changed to compensate for the variation in the beam spot diameter on the scanned surface,
A scanning optical system that compensates for variations in the beam spot position on the surface to be scanned in accordance with the positional relationship of the entire optical element by the second driving means.   The scanning optical system according to claim 1, wherein a part of the deflection unit also serves as the compensation unit.   The first and second driving means are controlled by a control means for outputting a control signal based on a signal detected by a photodetector signal for detecting a light beam state on the scanned surface. The scanning optical system according to claim 1, wherein the scanning optical system is characterized in that:   The first driving means and the second driving means are configured to be drivable by a common actuator that also serves as both of these driving means. The scanning optical system according to item.   An image forming apparatus comprising the scanning optical system according to claim 1.

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