JP2682684B2 - Optical scanning method - Google Patents

Description

The present invention relates to an optical scanning method.

[Prior Art] Optical scanning is performed for the purpose of writing and reading information,
Conventionally, laser printers and laser facsimiles,
Various systems are known in connection with digital copying machines, laser plate making machines, and the like.

One well-known one of such optical scanning methods is that "a light beam from a light source device is formed into a long line image in the main scanning corresponding direction by the first image forming optical system, and the above line image is formed. The second polygon is deflected by a rotary polygonal mirror having a deflecting / reflecting surface near the image position, and the starting point of deflection by the deflecting / reflecting surface and the scanned surface in the sub-scanning direction are geometrically-optically substantially conjugate. There is an optical scanning system in which a deflected light beam is formed into a spot on the surface to be scanned by an imaging optical system and the surface to be scanned is optically scanned.

This optical scanning system is a well-known optical scanning system in which the surface tilt of the rotary polygon mirror is corrected, and even if the surface tilt of the rotary polygon mirror occurs, the main scanning position hardly changes in the sub-scanning direction.

However, this optical scanning method is used as a second imaging optical system.
Since an anamorphic lens system in which the power in the sub-scanning direction is stronger than that in the main-scanning direction is used, a large field curvature is likely to occur in the sub-scanning direction. Further, the position of the deflecting / reflecting surface of the rotary polygon mirror fluctuates in the optical axis direction as the rotary polygon mirror rotates, which also causes the occurrence of field curvature in the sub-scanning direction. When strong field curvature occurs in the sub-scanning direction, the spot diameter in the sub-scanning direction of the imaging spot that scans the surface to be scanned varies depending on the scanning position. For this reason, it is difficult to realize high-density optical scanning that requires a high stability in the spot diameter.

Conventionally, the following two methods are known as methods for solving the above-mentioned problem of field curvature in the sub-scanning direction. The first is a method of correcting curvature of field by designing the lens of the second imaging optical system.

The second is a method of correcting the field curvature by displacing the light source and the first imaging optical system in the optical axis direction in synchronization with the deflection of the deflected light flux by the rotating polygon mirror.

[Problems to be Solved by the Invention] However, the above-mentioned first method is not practically easy, and there is a limit to the correction of the curvature of field in the frame of the requirement of low cost and compactness of the lens. . The second method is theoretically quite effective, but the light source and the first imaging optical system are required to have considerably high positional accuracy because of the configuration of the optical scanning device. When it is displaced, the positional accuracy required for the light source or the like may be distorted due to the displacement movement.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a novel optical scanning that can satisfactorily correct the field curvature in the sub-scanning direction and enable high-density optical scanning. It is in the provision of methods.

[Means for Solving the Problems] Hereinafter, the present invention will be described. The optical scanning method of the present invention,
"A light beam from the light source device is formed as a long line image in the main scanning corresponding direction by the first image forming optical system, and the light beam is formed by a rotary polygon mirror having a deflecting reflection surface near the image forming position of the line image. The deflected light beam is spotted on the surface to be scanned by the second imaging optical system that deflects the light and causes the starting point of the deflection by the deflecting reflection surface and the surface to be scanned to be in a substantially geometrically conjugate relationship with respect to the sub-scanning direction. An optical scanning method for forming an image on the surface to be optically scanned on the surface to be scanned ”, and its characteristic features are as follows.

That is, an optical path length changing device is provided between the first imaging optical system and the rotary polygon mirror.

The optical path length changing device includes a fixed part, a movable part, and a drive part. The fixed part and the movable part are a combination of a roof prism and / or a roof mirror.

The movable portion of this optical path length changing device is driven by the driving portion to be displaced by the piezoelectric element or the magnetostrictive element according to the field curvature in the sub-scanning direction by the second imaging optical system for each deflection of the deflected light beam. The optical scanning is performed while correcting the field curvature in the sub-scanning direction.

The fixed part and the movable part are the roof prism and the roof prism, the roof mirror and the roof mirror, or the combination of the roof prism and the roof mirror, as described above.

The displacement of the movable part is performed by the piezoelectric element or the magnetostrictive element as described above, and as a driving method by the piezoelectric element, a method using a piezoelectric actuator can be mentioned.

For the displacement by the magnetostrictive element, a whistle coil actuator method known in connection with focusing of an optical pickup can be used.

Further, in order to correct the field curvature in the sub-scanning direction by the displacement of the movable portion, the relationship between the field curvature and the displacement amount of the movable portion for correcting this is determined for each scanning position and stored. Alternatively, the displacement amount may be controlled according to the scanning position, or the amount of field curvature in the sub-scanning direction may be detected and the displacement amount of the movable portion may be feedback-controlled based on the detected amount.

[Operation] As described above, in the present invention, the optical path length changing device corrects the field curvature in the sub-scanning direction by changing the optical path length between the first imaging optical system and the rotary polygon mirror.

Therefore, the light source device and the first imaging optical system may remain fixed, and their positional accuracy can be maintained at a high level.

[Example] Hereinafter, a description will be given according to a specific example.

 FIG. 1 shows only an essential part of one embodiment of the present invention.

Reference numeral 1 denotes a light source such as a laser or a light source device composed of a light source and a condenser. The substantially parallel light flux emitted from the light source device 1 enters the cylinder lens 2 which is the first imaging optical system.

The light flux transmitted through the cylinder lens 2 becomes a unidirectional focused light by the focusing action of the lens 2, and enters the rotary polygon mirror 3 through the optical path length changing device shown by the fixed portion 8a and the movable portion 8b. Then, an image is formed as a line image LI in the vicinity of the deflection reflection surface 4 of the rotary polygon mirror 3. The longitudinal direction of this line image LI corresponds to the main scanning direction.

The light beam is then reflected by the deflecting / reflecting surface 4 and enters the surface 7 to be scanned through the second imaging optical system.

The second image forming optical system is composed of optical scanning lenses 5 and 6, and forms a light beam on the surface to be scanned 7 in a spot shape. As the rotary polygon mirror 3 rotates, the imaging spot scans the surface 7 to be scanned.

FIG. 2 shows a state in which the optical path from the light source device 1 to the surface to be scanned 7 is developed and viewed from the main scanning corresponding direction. In FIG. 2, the vertical direction corresponds to the sub-scanning corresponding direction.
The upper part (a) of FIG. 2 shows a state in which the image forming position of the second image forming optical system is displaced from the surface 7 to be scanned by ΔX ′ corresponding to the amount of curvature of field. This shift amount ΔX ′ can be corrected by shifting the image forming position of the line image from the normal position by ΔX, as shown in the lower part of FIG.

The relationship between ΔX and ΔX ′ in this correction is ΔX ′ = β ^{2 as} is well known by using the lateral magnification β of the second imaging optical system.
-Given by ΔX.

In the present invention, the above ΔX is realized by changing the distance between the first imaging optical system and the deflective reflection surface.

 Here, the specifications of the second imaging optical system will be concretely given.

The lens surfaces of the lenses 5 and 6 are first to fourth surfaces from the rotary polygon mirror side toward the scanned surface side, and the curvature radius of the i-th surface is the curvature radius in the main scanning direction (by the rotary polygon mirror 3). The radius of curvature on the cross section of the deflecting surface formed by deflecting the optical axis ray of the ideal deflected light flux is ri, and the radius of curvature in the sub-scanning direction (passing the optical axis and orthogonal to the deflecting surface Radius of curvature on the cross section by ri ′, the i-th surface and the (i +
1) The surface spacing between the surface and the surface is di, and the refractive index of the j-th lens from the side of the rotary polygon mirror 3 is nj.

i ri ri ′ di j nj 1 −107.774 ∞ 5.672 1 1.71221 2 ∞ ∞ 10.966 3 ∞ −52.565 6.807 2 1.675 4 − 45.569 −12.052 The above value is the composite focal length fm in the main scanning direction.
Is the value when is standardized to 100.

At this time, the brightness in the main scanning direction is F / No = 54.7, the effective deflection angle is 2θ = 67.8 °, and the angle formed by the optical axis ray of the light beam incident on the rotary polygon mirror 3 and the optical axis of the second imaging optical system. α = 60 °, the ratio R / fm of the inscribed circle radius R of the rotary polygonal mirror 3 to fm is 0.132, the combined focal length fs in the sub-scanning direction fs = 22.698, and the lateral magnification β = −4.12.

The curvature of field when the deflected light beam is incident on the second imaging optical system is as shown in FIG. The broken line indicates the main scanning direction, and the solid line indicates the sub scanning direction.

The fixed portion 8a of the optical path length changing device is a convex roof mirror as shown in FIG. 1, and the movable portion 8b is a concave roof mirror as shown in FIG.

Since the angle formed by the mirror surface of each roof mirror is set to 90 °, the light flux traveling from the cylinder lens 2 to the deflecting / reflecting surface 4 of the rotary polygon mirror is fixed portion 8a and movable portion 8b as shown in FIG. By this, the optical path can be bent so as to form three sides of the rectangle.

As shown in FIG. 4, the movable portion 8b is fixed to the fixed portion 8a by Δa.
Cylinder lens 2 and deflection reflection surface 4
The optical path length between and changes by 2Δa.

Now, at a certain scanning position, if the distance by which the image forming position of the line image by the cylinder lens 2 should be displaced in order to correct the field curvature in the sub-scanning direction is ΔX, this distance is shown in FIG. It is ΔX ′ / β ² with respect to ΔX ′ explained immediately, and if this is performed by the displacement of the movable portion 8b, the displacement Δa
May be ΔX ′ / 2β ² .

FIG. 5 is a diagram showing how the movable portion should be displaced together with the scanning position in order to correct the field curvature (solid curve) in the sub-scanning direction shown in FIG.

That is, the right side of the figure means to move the movable portion 8b so that the position of the line image is displaced to the image side, and the left side is to move the movable portion 8b so that the position of the line image is displaced to the light source side. Means That is, the displacement of the movable portion 8b may be controlled so as to reduce the distance between the fixed portion 8a and the movable portion 8b in the former case and increase the distance in the latter case.

When the displacement of the movable portion 8b is thus controlled for each deflection as shown in FIG. 5, the field curvature in the sub-scanning direction is almost completely corrected as shown in FIG. In addition, the field curvature in the main scanning direction is hardly affected by the correction action.

Therefore, the diameter of the image forming spot on the surface to be scanned in the sub-scanning direction is extremely constant regardless of the scanning position. Further, the fluctuation of the image forming spot diameter in the main scanning direction due to the curvature of field in the main scanning direction can be corrected by adjusting the light amount in the light source device 1. Therefore, high-density optical scanning becomes possible.

The displacement control of the movable portion is performed as follows in this embodiment.

That is, the movable portion 8b is configured to be displaced by a voice coil type displacement mechanism known in relation to focusing of optical pick-up.

Further, the relationship between the scanning position and the displacement amount Δa is as shown in FIG. 5, and this relationship is stored in the storage device of the drive unit. Then, the scanning position by the deflected light beam is detected in relation to the clock, and according to the detected scanning position,
The movable portion 8a is displaced so as to correct the field curvature in the sub-scanning direction.

The field curvature correction may also be performed in a feedback manner.

 To do this, for example:

That is, a semi-transparent mirror is arranged between the second imaging optical system and the surface to be scanned, a part of the deflected light beam is separated as a separated light beam and is guided to a surface equivalent to the surface to be scanned, and this surface is scanned. Let A plurality of image sensors such as CCDs are provided on this surface in the scanning direction. The light receiving element array direction in each sensor is
It is made to be orthogonal to the scanning direction of the equivalent surface. When the separated light flux passes through the position of each sensor, if the diameter of the spot of the separated light flux is detected by the output of the sensor, it is possible to detect the field curvature in the sub-scanning direction from this result. The displacement of the movable part of the optical path length changing device may be feedback-controlled so that the image spot diameter becomes a predetermined value. An LD with two light sources as the light source, using an ordinary mirror instead of using a semi-transparent mirror
An array may be used, and the light flux from one of the light emitting sources may be guided to a surface equivalent to the surface to be scanned.

[Effects of the Invention] As described above, according to the present invention, a novel optical scanning method can be provided. Since this method has the above-described configuration, the second imaging optical system having a considerable field curvature in the sub-scanning direction is used without moving the light source or the first imaging optical system. However, good high-density optical scanning can be achieved. The roof angles of the roof mirror and the roof prism of the optical path length changing device are not limited to 90 ° described in the embodiment, but can be set to other angles.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining an apparatus according to an embodiment of the present invention,
2 to 6 are views for explaining the present invention in relation to the above-mentioned embodiment. 1 ... Light source device, 2 ... Cylinder lens as first imaging optical system, 3 ... Rotating polygon mirror, 4 ... Deflection / reflection surface,
5, 6 ... Lens forming second imaging optical system, 7 ... Scanned surface, 8a ... Fixed part of optical path length changing device, 8b ... Movable part of optical path length changing device

Claims (1)

(57) [Claims] 1. A rotary polygonal mirror having a light beam from a light source device formed as a long line image in a direction corresponding to main scanning by a first image forming optical system, and having a deflective reflection surface near the image forming position of the line image. The deflected light beam is deflected by the second imaging optical system in which the origin of the deflection by the deflection reflection surface and the surface to be scanned are geometrically and optically substantially conjugate with respect to the sub-scanning direction. In the optical scanning method in which an image is formed in a spot shape on the upper surface and the surface to be scanned is optically scanned, a fixed part, a movable part, and a drive part are provided between the first imaging optical system and the rotary polygon mirror. , A roof prism and / or a roof mirror are combined to provide an optical path length changing device having the fixed part and the movable part, and the movable part of the optical path length changing device is provided by the driving part for each deflection of the deflected light beam. , Sub-scanning by the second imaging optical system Depending on the curvature of the direction is displaced by driving the piezoelectric element or magnetostrictive element, and performing optical scanning with correct field curvature of the sub-scanning direction, the optical scanning method.