JP4412948B2 - Scanning optical system and printer - Google Patents

Description

  The present invention relates to a scanning optical system for forming an electrostatic latent image on the surface of a photosensitive drum or the like, and a printer in which such a scanning optical system is incorporated.

  As is well known, a scanning optical system is incorporated in a printing apparatus such as a laser beam printer, a facsimile machine, or a copier. The scanning optical system dynamically deflects the laser beam modulated according to the image information by a deflector such as a rotating polygon mirror, and converges the dynamically deflected laser beam on the surface of the photosensitive drum by the imaging optical system. By doing so, the photosensitive drum is scanned. A plurality of dots are drawn as an electrostatic latent image on the surface of the scanned photosensitive drum (surface to be scanned).

In general, the intensity distribution of the laser beam incident on the scanning target surface is not a perfect Gaussian distribution, and it is around the main beam due to the diffraction phenomenon at the aperture (aperture) provided in the optical path of the laser beam. It is known to have several light rings (side lobes) that have a lower light intensity than the main beam. It is also known that when the intensity of the side lobe exceeds about 6% of the center intensity of the main beam, the side lobe sensitizes the photosensitive drum to cause printing defects called black stripes during halftone printing. (See Patent Document 1). However, when the imaging optical system is in an ideal state, the side lobe intensity is about 4% of the center intensity of the main beam, and therefore no black streak occurs.
JP 09-080333 A

  However, if the optical surface of the imaging optical system has microscopic undulations, the intensity of the side lobe changes when the laser beam passes through the undulations. If the intensity of the side lobe exceeds the threshold due to the change, there is a problem that black streaks occur during halftone printing.

  Accordingly, an object of the present invention is to provide a scanning optical that can suppress the side lobe intensity from exceeding a threshold value as much as possible even when a certain degree of microscopic undulation occurs on the optical surface of the imaging optical system. It is to provide a system and a printer in which such a scanning optical system is incorporated.

  In order to solve the above problems, the scanning optical system according to the present invention and the scanning optical system incorporated in the printer according to the present invention employ the following configurations.

That is, this scanning optical system dynamically deflects a laser beam emitted from a light source by a deflector and converges the dynamically deflected laser beam as a spot light on a scanning target surface by an imaging optical system. Thus, the scanning optical system scans the spot light along the main scanning direction on the scanning target surface, and includes an optical element on an optical path between the light source and the deflector, and the optical element Is a central region that transmits a light beam in the vicinity of the central axis of the laser beam emitted from the light source, a light shielding region that shields a part of the light beam that is incident on the outside of the central region, the central region, and possess a transmissive region that transmits part of the light flux excluding a light flux incident on the light-shielding region, it and the light flux which the light beam transmitted through the transmissive region transmitting the central region having the same phase It is characterized.

  If comprised in this way, if the magnitude | size of a center area | region, a light shielding area | region, and a permeation | transmission area | region is selected suitably, the intensity | strength of the side lobe of the laser beam which injects into a scanning object surface will be 2 of the center intensity | strength of a main beam. % Can be reduced to less than%. Therefore, even if the intensity of the side lobe increases by about several percent due to the microscopic undulation of the optical surface of the imaging optical system, the threshold value is not exceeded. Therefore, the occurrence of black streaks during halftone printing can be suppressed.

  In the scanning optical system according to the present invention and the scanning optical system inside the printer according to the present invention, the optical element may include one set of a light-shielding region and a plurality of transmission regions. When the optical element includes a set of a light shielding region and a transmission region as in the former case, the light shielding region can be arranged outside the central region and the transmission region can be arranged outside the light shielding region. In the latter case, when the optical element includes a plurality of sets of light shielding regions and transmission regions, the light shielding regions and the transmission regions can be alternately arranged in the direction away from the central region. As described above, when the light shielding areas and the transmission areas are alternately arranged, it is desirable that the transmission areas are arranged on the outermost side.

  In addition, even if the light shielding area and the transmission area provided in the optical element are either one set or a plurality of sets, the light shielding area and the transmission area can be arranged radially from the central area, and in the main scanning direction. It is also possible to arrange only toward. When the light shielding area and the transmission area are arranged in the main scanning direction as in the latter case, it is preferable to arrange the light shielding area and the transmission area on both sides of the central area. When a light shielding area and a transmission area are arranged on both sides of the central area in the main scanning direction, the light shielding areas (or transmission areas) on both sides of the light shielding area are polygonal rings such as a quadrangle and a hexagon, or a ring-shaped light shielding. It may be a part of the region (or transmission region) or may not be so.

Further, in the case where the optical element includes a pair of a light shielding region and a transmission region, when the light shielding region and the transmission region are respectively arranged at symmetrical positions sandwiching the central region in the main scanning direction, this scanning optical system Is the following conditional expression (1),
0.70 <((ha1 + ha2) / 2) / hmax <0.85 --- (1)
Is preferably satisfied. However, ha1 is the distance in the main scanning direction from the beam center axis of the laser beam incident on the optical element to the inner edge of the light shielding region, and ha2 is the outer edge (transmission) of the laser beam from the beam center axis. The distance in the main scanning direction to the inner edge of the region), and hmax is the radius in the main scanning direction in the cross-sectional shape of the laser beam. In the conditional expression (1), when the upper limit is exceeded, the effect of reducing the intensity of the side lobe near the main beam is reduced, and when the lower limit is exceeded, the intensity of the side lobe located at a position away from the main beam is increased.

Further, in the case where the optical element includes a plurality of sets of light shielding regions and transmission regions, the light shielding regions and the transmission regions are directed away from the central region at symmetrical positions sandwiching the central region in the main scanning direction. When alternately arranged, this scanning optical system has the following conditional expression (2),
0.85 <((hb1 + hb2) / 2) / hmax <0.95 --- (2)
Is preferably satisfied. However, hb1 is the distance in the main scanning direction from the beam center axis of the laser beam incident on the optical element to the inner edge of the outermost light shielding region, and hb2 is the outermost side from the beam center axis of the laser beam. Is the distance in the main scanning direction to reach the outer edge of the light shielding region in FIG. 2, and hmax is the radius in the main scanning direction in the cross-sectional shape of the laser beam. In the conditional expression (2), when the value exceeds the upper limit, the effect of reducing the intensity of the side lobe near the main beam is reduced, and when the value is below the lower limit, the intensity of the side lobe located away from the main beam is increased.

Further, when this scanning optical system satisfies the conditional expression (2), the following conditional expression (3),
0.65 <((hc1 + hc2) / 2) / hmax <0.75 --- (3)
Is preferably satisfied. Where hc1 is the distance in the main scanning direction from the beam center axis of the laser beam incident on the optical element to the inner edge of the innermost light shielding region, and hc2 is the innermost side from the beam center axis of the laser beam. The distance in the main scanning direction until reaching the outer edge of the light-shielding region in FIG. In the conditional expression (3), when the value exceeds the upper limit, the effect of reducing the intensity of the side lobe near the main beam is reduced, and when the value is below the lower limit, the intensity of the side lobe located at a position away from the main beam is increased.

Further, when the scanning optical system satisfies the conditional expressions (2) and / or (3), the following conditional expressions (4),
0.20 <Sa / (Sa + Sb) <0.75 --- (4)
Is preferably satisfied. However, Sa is the sum of the areas of the regions where the laser beam is incident among the innermost light-shielding regions in each light-shielding region, and Sb is incident with the laser beam of the outermost light-shielding region. This is the sum of the area areas. In the conditional expression (4), when the upper limit is exceeded, the strength of the side lobe located away from the main beam is increased, and when the lower limit is surpassed, the effect of reducing the strength of the side lobe near the main beam is reduced.

By the way, in this scanning optical system, it is desirable that the area of the light shielding region of the optical element is appropriately set apart from the above-described conditions. For example, when the total area of the regions where the laser beam is incident in the light shielding region of the optical element is S ′ and the area of the cross section perpendicular to the beam central axis in the laser beam is S, the following conditional expression (5 ),
0.03 <S '/ S <0.30 --- (5)
The area of the light shielding region is preferably set so as to satisfy the above. Under such setting conditions, if the lower limit is exceeded, the effect of reducing the third lobe is reduced. If the upper limit is exceeded, the side lobe can be effectively reduced, but the amount of decrease in the center intensity of the main beam is increased.

  In the scanning optical system according to the present invention and the scanning optical system inside the printer according to the present invention, the imaging optical system may be a transmissive optical system using an fθ lens or the like, or an fθ mirror. A reflection type optical system using the above may be used. Further, the deflector may be a rotary polygon mirror or a galvanometer mirror.

  As described above, according to the present invention, even when a certain degree of microscopic undulation is generated on the optical surface of the imaging optical system, the intensity of the side lobe is suppressed from exceeding the threshold as much as possible. be able to. Moreover, the tolerance for microscopic waviness of the optical surface is increased, and the processing cost can be reduced.

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be described with reference to the drawings. The first to sixth embodiments described below show examples in which the scanning optical system according to the present invention is applied to a laser beam printer.

Embodiment 1

<Schematic configuration of laser beam printer>
First, a schematic configuration of this laser beam printer will be described based on a side configuration diagram of FIG. The laser beam printer is used by being connected to an external personal computer or the like, and prints print data (including image data) transmitted from the personal computer or the like on continuous paper (fanfold paper) P. Is.

In FIG. 1, a charging unit 13, a reflection mirror 11, a developing unit 14, and a transfer unit 15 are sequentially provided around the photosensitive drum 12 in the clockwise direction. When the photosensitive drum 12 rotates clockwise in the drawing, first, the charging unit 13 charges the surface of the photosensitive drum 12. Next, the reflection mirror 11 reflects the scanning light (modulated light) emitted from the laser scanning unit (LSU) 10 according to the print data toward the photosensitive drum 12, and electrostatically reflects on the surface of the photosensitive drum 12. A latent image is formed. Next, the developing unit 14 attaches toner to the electrostatic latent image and visualizes it as a toner image. Next, the transfer unit 15 transfers this toner image onto the fanfold paper P.

  The fanfold paper P is a continuous paper drawn from the supply port A to the discharge port B of the laser beam printer, and feed holes (not shown) are opened at a constant pitch on both side edges. The tractor 16 is a belt conveyor in which a large number of protrusions 16 a that fit into the feed holes are formed. The fan fold paper P is conveyed at the same speed as the rotational peripheral speed of the photosensitive drum 12 by the protrusions 16 a.

  On the downstream side of the fanfold paper P conveyed by the tractor 16, a heat roll 17 and a press roll 18 are provided that sandwich the fanfold paper P from both sides and press-contact it. The heat roll 17 incorporates a heat-generating halogen lamp 19 therein, and is rotated by a motor (not shown) at the same rotational peripheral speed as the conveyance speed of the fanfold paper P. On the other hand, the press roll 18 is in pressure contact with the heat roll 17 at a constant pressure, and is driven to rotate by the rotation of the heat roll 17. Accordingly, when the portion of the fanfold paper P onto which the toner image is transferred passes between the heat roll 17 and the press roll 18, the toner is crushed by heat and pressure and is fused onto the fanfold paper P. The toner image is fixed.

<Optical configuration of LSU>
Next, the scanning optical system built in the LSU 10 will be described. FIG. 2 is a schematic optical configuration diagram of the scanning optical system. As shown in FIG. 2, this scanning optical system includes a laser light source 1, a collimating lens (collimator) 2, a partial light shielding member 3, an aperture stop 4, a cylindrical lens 5, a polygon mirror 6, and an fθ lens group 7. I have.

  The laser beam emitted as divergent light from the laser light source 1 is converted into a parallel light beam having an elliptical cross section by passing through the collimator lens 2, and then sequentially passes through the partial light shielding member 3, the aperture stop 4, and the cylindrical lens 5, It is deflected dynamically by the reflecting surface of the polygon mirror 6 that rotates at a constant angular velocity. The laser beam deflected by the polygon mirror 6 is sequentially transmitted through the first to third lenses 7a to 7c constituting the fθ lens group 7 (focal length 135.5 mm) which is an imaging optical system, so that the surface S to be scanned is scanned. The light is converged as spot light for exposure, and the surface (scanning target surface) S of the photosensitive drum 12 is scanned at a constant speed along the main scanning direction as the polygon mirror 6 rotates. The spot light draws a linear trajectory (scanning line) on the scanning target surface S, but the scanning target surface S itself is moved at a constant speed in the sub-scanning direction orthogonal to the main scanning direction. On S, a plurality of scanning lines are formed at equal intervals. Further, the laser beam repeatedly scanned on the scanning target surface S in this way is on-off modulated in accordance with image information by a modulator (or the laser light source 1 itself) (not shown). A two-dimensional image composed of a plurality of dots is drawn.

The laser beam transmitted through the cylindrical lens 5 is reflected by the polygon mirror 6 as a parallel light beam in the main scanning direction and converged on the scanning target surface S by the convergence power of the fθ lens group 7. In the scanning direction, the light is converged once in the vicinity of the reflection surface of the polygon mirror 6 by the convergence power of the cylindrical lens 5, enters the fθ lens group 7 as divergent light, and is again incident on the scanning target surface S by the convergence power of the fθ lens group 7. Converged. At this time, since the vicinity of the reflection surface of the polygon mirror 6 and the scanning target surface S are optically conjugate in the sub-scanning direction by the fθ lens group 7, a slight inclination of each reflection surface of the polygon mirror 6 (so-called so-called) A shift in the sub-scanning direction of the scanning position on the scanning target surface S due to “surface tilt”) is corrected.

<Partial light shielding member>
Next, the partial light shielding member 3 will be described. The partial light shielding member 3 is a rectangular flat plate-shaped optical element that transmits most of the laser beam emitted from the collimating lens 2.

  FIG. 3A is a front view of the partial light shielding member 3, and FIG. 3B is a side view of the partial light shielding member 3. As shown in FIG. 3A, the partial light shielding member 3 is formed by attaching a ring-shaped film having a transmittance of 0% to a transparent flat plate as a light shielding portion (corresponding to a light shielding region) 3a. This light shielding part 3a is arranged at the center of the flat plate. The partial light shielding member 3 is arranged so as to be orthogonal to the beam center axis of the laser beam. However, the arrangement position of the partial light shielding member 3 is that the beam center axis and the vicinity thereof are incident on the inner side of the light shielding portion 3a. To be adjusted.

  By the way, as described above, the cross section of the laser beam incident on the partial light shielding member 3 is shaped into an elliptical shape by the collimating lens 2 (see the broken line in FIG. 3A), and its major axis is the main axis. In the scanning direction, the minor axis is oriented in the sub-scanning direction. In the first embodiment, the radius of the major axis in the cross-sectional shape of the laser beam incident on the partial light shielding member 3 is set to 1.35 mm (= hmax), and the radius of the minor axis is set to 0.50 mm. Is set. Further, as shown in FIG. 3B, the inner diameter of the light-shielding portion 3a is 1.80 mm (= 2 × (ha1)), and the outer diameter is 2.00 mm (= 2 × (ha2)). Therefore, the width in the radial direction of the light shielding portion 3a is only 0.10 mm. For this reason, part of the laser beam incident on the partial light shielding member 3 is blocked by the light shielding portion 3 a, and most of the remaining part is transmitted through the partial light shielding member 3.

  Note that the light beam incident on the inner region (corresponding to the central region) of the light shielding portion 3a and the light beam incident on the outer region (corresponding to the transmission region) of the light shielding portion 3a are both transparent with the same optical thickness. Since they only pass through the region, they are in phase with each other even after passing through the member 3. Since ((ha1 + ha2) / 2) / hmax is 0.704, the partial light shielding member 3 of the first embodiment satisfies the conditional expression (1).

<Aperture stop>
Next, the aperture stop 4 will be described. As shown in FIG. 4, the aperture stop 4 is a flat plate in which a slit 4 a oriented in the longitudinal direction in the main scanning direction is formed as an aperture.

<Functions of First Embodiment>
Hereinafter, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system according to the first embodiment configured as described above will be described when there is no partial light shielding member 3 and when there is a partial light shielding member 3. The explanation will be made in comparison with the time.

  FIG. 5 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in a range from the beam central axis to 0.25 mm in the main scanning direction. In this graph, the intensity is displayed as a ratio to the center intensity of the beam. FIG. 6 is an enlarged graph showing the range of the intensity ratio from 0% to 10% in the graph of FIG. In FIGS. 5 and 6, the curve indicated by the broken line indicates the intensity distribution when the partial light shielding member 3 is not present, and the curve indicated by the solid line indicates the intensity distribution when the partial light shielding member 3 is present. Show.

When there is no partial light shielding member 3 (see the broken lines in FIGS. 5 and 6), the intensity of the side lobe gradually decreases as the distance from the main beam increases, and the intensity of the side lobe adjacent to the main beam is 4 It is slightly over%.

  On the other hand, when the partial light shielding member 3 is present (see the solid lines in FIGS. 5 and 6), the side lobes are almost average in intensity, and both are less than 2%.

  Accordingly, even if the lens surfaces of the lenses 7a to 7c of the fθ lens group 7 have some microscopic undulations and the strength of the side lobe is increased by only a few percent, the strength of the photosensitive drum 12 is sensitive. The threshold is rarely exceeded.

  By the way, when the partial light shielding member 3 is viewed from the front (FIG. 3A), the area S ′ of the area where the laser beam is incident in the light shielding part 3a is appropriate to the area S of the cross section of the laser beam. It is desirable to be set to. In the first embodiment, since S ′ is 0.16 and S is 2.12, S ′ / S is 0.08. Therefore, the partial light shielding member 3 of the first embodiment satisfies the conditional expression (5).

  In the first embodiment, the partial light shielding member 3 and the aperture stop 4 are described as separate members. However, for example, as shown in FIG. 7, the partial light shielding member 3 and the aperture stop 4 are bonded. Thus, it may be configured integrally.

  In the above description, an example in which the present invention is applied to a so-called transmissive scanning optical system having the fθ lens group 7 as an imaging optical system has been shown. However, an fθ mirror 7 ′ as shown in FIG. The present invention can also be applied to a so-called reflection-type scanning optical system having an image forming optical system. Note that, in the reflection type scanning optical system, the amount of increase in side lobe intensity due to microscopic undulations of the optical surface of the imaging optical system is larger than in the transmission type. Therefore, in the reflection type scanning optical system, black streaks are more likely to occur during halftone printing. Therefore, by applying the present invention to a reflective scanning optical system, side lobes can be reduced and the occurrence of black stripes can be reduced.

Embodiment 2

  The second embodiment has the same configuration as the first embodiment except that the specific numerical values of the partial light shielding members are different. Therefore, only the differences from the first embodiment will be described below. FIG. 9A is a front view of the partial light shielding member 3 ′ of the second embodiment, and FIG. 9B is a side view of the partial light shielding member 3 ′.

  Also in the second embodiment, the radius of the major axis in the cross-sectional shape of the laser beam incident on the partial light shielding member 3 ′ is set to 1.35 mm (= hmax), and the radius of the minor axis is 0.50 mm. Is set to Further, as shown in FIG. 9B, the inner diameter of the light-shielding portion (corresponding to the light-shielding area) 3′a is 2.06 mm (= 2 × (ha1)), and the outer diameter is 2.36 mm (= 2 × (ha2)), the width in the radial direction of the light-shielding portion 3′a is only 0.15 mm. For this reason, a part of the laser beam incident on the partial light shielding member 3 ′ is shielded by the light shielding part 3′a, and the remaining most part is transmitted through the partial light shielding member 3 ′.

  It should be noted that both the light flux incident on the inner area (corresponding to the central area) of the light shielding part 3′a and the light flux incident on the outer area (corresponding to the transmission area) of the light shielding part 3′a are both optically thick. Are transmitted through the transparent region having the same value, so that they are in phase with each other even after passing through the member 3 ′. Since ((ha1 + ha2) / 2) / hmax is 0.82, the partial light shielding member 3 ′ of the second embodiment satisfies the conditional expression (1). Further, since S ′ is 0.19 and S is 2.12, S ′ / S is 0.09. Therefore, the partial light shielding member 3 ′ of the second embodiment also satisfies the conditional expression (5).

<Function of Second Embodiment>
Hereinafter, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system according to the second embodiment configured as described above is shown when there is no partial light shielding member 3 ′ and when the partial light shielding member 3 ′ is present. This will be explained in comparison with when there was.

  FIG. 10 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in the range from the beam central axis to 0.25 mm in the main scanning direction. FIG. 11 is an enlarged graph showing the range of the intensity ratio from 0% to 10% in the graph of FIG. 10 and 11, the curve indicated by the broken line indicates the intensity distribution when there is no partial light shielding member 3 ′, and the curve indicated by the solid line indicates the intensity when there is the partial light shielding member 3 ′. Show the distribution.

  When there is no partial light blocking member 3 ′ (see the broken lines in FIGS. 10 and 11), the intensity of the side lobe gradually decreases as the distance from the main beam increases, and the intensity of the side lobe adjacent to the main beam becomes It is over 4%.

  On the other hand, when the partial light shielding member 3 ′ is present (see the solid lines in FIGS. 10 and 11), the strength of the side lobes is both less than 2%.

  Accordingly, even if the lens surfaces of the lenses 7a to 7c of the fθ lens group 7 have some microscopic undulations and the strength of the side lobe is increased by only a few percent, the strength of the photosensitive drum 12 is sensitive. The threshold is rarely exceeded.

  The partial light shielding member 3 ′ in FIG. 9 may also be configured integrally with the aperture stop 4 in the same manner as the partial light shielding member 3 in FIG. 9 may be used in a reflective scanning optical system as shown in FIG.

Embodiment 3

  The third embodiment has the same configuration as the first embodiment except that the partial light shielding member has two light shielding portions. Therefore, only the differences from the first embodiment will be described below. FIG. 12A is a front view of the partial shielding member 8 of the third embodiment, and FIG. 12B is a side view of the partial shielding member 8.

  The partial shielding member 8 of the third embodiment is also a rectangular flat plate-shaped optical element arranged so as to be orthogonal to the beam center axis. As shown in FIG. 12A, the partial light shielding member 8 has a ring-shaped film having a transmittance of 0% attached to a transparent flat plate as a first light shielding portion (corresponding to a light shielding region) 8a. A ring-shaped film having an inner diameter larger than the outer diameter of the light-shielding portion 8a and having a transmittance of 0% is attached to the transparent flat plate as the second light-shielding portion (corresponding to the light-shielding region) 8b. The first and second light shielding portions 8a and 8b are arranged at the center of the flat plate.

  The partial light shielding member 8 is arranged so as to be orthogonal to the beam center axis of the laser beam. However, the arrangement position is such that the beam center axis and the vicinity thereof are inside the first light shielding portion 8a. It is adjusted so that it may enter.

By the way, as described above, the cross section of the laser beam incident on the partial light shielding member 8 is shaped into an elliptical shape by the collimating lens 2 (see the broken line in FIG. 12A), and its major axis is the main axis. In the scanning direction, the minor axis is oriented in the sub-scanning direction. In the third embodiment, the radius of the major axis in the cross-sectional shape of the laser beam incident on the partial light shielding member 8 is set to 1.35 mm (= hmax), and the radius of the minor axis is set to 0.50 mm. Is set. As shown in FIG. 12B, the inner diameter of the first light-shielding portion 8a is 1.70 mm (= 2 × (hc1)), and the outer diameter is 1.80 mm (= 2 × (hc2)). It is. The inner diameter of the second light-shielding part 8b is 1.90 mm (= 2 × (hb1)), and the outer diameter is 2.00 mm (= 2 × (hb2)). That is, the first and second
The width in the radial direction of the light shielding portions 8a and 8b is only 0.05 mm. For this reason, a part of the laser beam incident on the partial light shielding member 8 is blocked by the first and second light shielding portions 8 a and 8 b, and the remaining most part is transmitted through the partial light shielding member 8.

  Note that the light beam incident on the inner region (corresponding to the central region) of the first light shielding portion 8a and the light beam incident on the region (corresponding to the transmission region) between the first light shielding portion 8a and the second light shielding portion 8b. Since the light beams incident on the outer region (corresponding to the transmission region) of the second light-shielding portion 8b only pass through the transparent region having the same optical thickness, the light beams can pass through the member 8 after transmission. It becomes the same phase. Further, ((hb1 + hb2) / 2) / hmax is 0.72, and ((hc1 + hc2) / 2) / hmax is 0.65. Further, since Sa is 0.09 and Sb is 0.08, Sa / (Sa + Sb) is 0.53. Therefore, the partial light shielding member 8 of the third embodiment satisfies the conditional expression (4). Furthermore, since S ′ is 0.17 and S is 2.12, S ′ / S is 0.08. Accordingly, the partial light shielding member 8 of the third embodiment also satisfies the conditional expression (5).

<Function of Third Embodiment>
Hereinafter, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system of the third embodiment configured as described above will be described when there is no partial light shielding member 8 and when there is a partial light shielding member 8. The explanation will be made in comparison with the time.

  FIG. 13 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in the range from the beam central axis to 0.25 mm in the main scanning direction. FIG. 14 is a graph showing the intensity ratio in the graph of FIG. 13 in an enlarged manner from 0% to 10%. In FIGS. 13 and 14, the curve indicated by the broken line indicates the intensity distribution when the partial light shielding member 8 is not present, and the curve indicated by the solid line indicates the intensity distribution when the partial light shielding member 8 is present. Show.

  When there is no partial light shielding member 8 (see the broken lines in FIGS. 13 and 14), the intensity of the side lobe gradually decreases as the distance from the main beam increases, and the intensity of the side lobe adjacent to the main beam is 4 It is slightly over%.

  On the other hand, when the partial light shielding member 8 is present (see the solid lines in FIGS. 13 and 14), the intensity of the side lobes is almost average, and both are less than 2%.

  Accordingly, even if the lens surfaces of the lenses 7a to 7c of the fθ lens group 7 have some microscopic undulations and the strength of the side lobe is increased by only a few percent, the strength of the photosensitive drum 12 is sensitive. The threshold is rarely exceeded.

  Note that the partial light shielding member 8 in FIG. 12 may also be configured integrally with the aperture stop 4 in the same manner as the partial light shielding member 3 in FIG. 12 may be used in a reflective scanning optical system as shown in FIG.

Embodiment 4

  The fourth embodiment has the same configuration as the third embodiment except that the specific numerical values of the partial light shielding members are different. Therefore, only the differences from the third embodiment will be described below. FIG. 15A is a front view of the partial light shielding member 8 ′ of the fourth embodiment, and FIG. 15B is a side view of the partial light shielding member 8 ′.

Also in the fourth embodiment, the radius of the major axis in the cross-sectional shape of the laser beam incident on the partial light shielding member 8 ′ is set to 1.35 mm (= hmax), and the radius of the minor axis is 0.50 mm. Is set to Further, as shown in FIG. 15B, the inner diameter of the first light-shielding portion (corresponding to the light-shielding region) 8'a is 1.74 mm (= 2 × (hb1)), and the outer diameter is 1.86 mm. (= 2 × (hb2)) and the inner diameter of the second light-shielding portion (corresponding to the light-shielding region) 8′b is 2.28 mm (= 2 × (hb1)), and the outer diameter is 2.52 mm. (= 2 × (hb2)). That is, the width in the radial direction of the first and second light shielding portions 8′a and 8′b is only 0.06 mm and 0.12 mm. For this reason, part of the laser beam incident on the partial light shielding member 8 ′ is shielded by the first and second light shielding portions 8′a and 8′b, and most of the remaining part is blocked by the partial light shielding member 8 ′. To Penetrate.

  In addition, the light beam incident on the inner region (corresponding to the central region) of the first light shielding portion 8′a and the region between the first light shielding portion 8′a and the second light shielding portion 8′b (corresponding to the transmission region). ) And a light beam incident on a region outside the second light-shielding portion 8′b (corresponding to a transmission region) both pass only through a transparent region having the same optical thickness. Even after passing through the member 8 ', they are in phase with each other. Further, ((hb1 + hb2) / 2) / hmax is 0.89, and ((hc1 + hc2) / 2) / hmax is 0.67. Further, since Sa is 0.10 and Sb is 0.12, Sa / (Sa + Sb) is 0.45. Therefore, the partial light shielding member 8 ′ of the fourth embodiment satisfies the conditional expressions (2) to (4). Furthermore, since S ′ is 0.22 and S is 2.12, S ′ / S is 0.10. Therefore, the partial light shielding member 8 ′ of the fourth embodiment also satisfies the conditional expression (5).

<Functions of Fourth Embodiment>
Hereinafter, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system according to the fourth embodiment configured as described above is shown when there is no partial light shielding member 8 ′ and when the partial light shielding member 8 ′ is present. This will be explained in comparison with when there was.

  FIG. 16 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in the range from the beam central axis to 0.25 mm in the main scanning direction. FIG. 17 is an enlarged graph showing the range of the intensity ratio in the graph of FIG. 16 from 0% to 10%. In FIGS. 16 and 17, the curve indicated by the broken line indicates the intensity distribution when there is no partial light shielding member 8 ′, and the curve indicated by the solid line indicates the intensity when there is the partial light shielding member 8 ′. Show the distribution.

  When there is no partial light shielding member 8 ′ (see the broken lines in FIGS. 16 and 17), the intensity of the side lobe gradually decreases as it moves away from the main beam, and the intensity of the side lobe adjacent to the main beam is It is over 4%.

  On the other hand, when the partial light shielding member 8 ′ is present (see the solid lines in FIGS. 16 and 17), the strength of the side lobes is both less than 2%. Further, since the conditional expressions (2) to (3) are satisfied, the side lobe intensity is smaller than that in the third embodiment.

  Accordingly, even if the lens surfaces of the lenses 7a to 7c of the fθ lens group 7 have some microscopic undulations and the strength of the side lobe is increased by only a few percent, the strength of the photosensitive drum 12 is sensitive. The threshold is rarely exceeded.

  Note that the partial light shielding member 8 ′ in FIG. 15 may also be configured integrally with the aperture stop 4 in the same manner as the partial light shielding member 3 in FIG. 15 may be used in a reflective scanning optical system as shown in FIG.

Embodiment 5

  The fifth embodiment also has the same configuration as the third embodiment except that the specific numerical values of the partial light shielding members are different. Therefore, only the differences from the third embodiment will be described below. FIG. 18A is a front view of the partial light shielding member 8 ″ of the fifth embodiment, and FIG. 18B is a side view of the partial light shielding member 8 ″.

Also in the fifth embodiment, the radius of the major axis in the cross-sectional shape of the laser beam incident on the partial light shielding member 8 "is set to 1.35 mm (= hmax), and the radius of the minor axis is 0.50 mm. 18B, the inner diameter of the first light-shielding portion (corresponding to the light-shielding region) 8 "a is 1.86 mm (= 2 × (hc1)), The outer diameter is 2.14 mm (= 2 × (hc2)). The inner diameter of the second light shielding portion (corresponding to the light shielding area) 8 ″ b is 2.48 mm (= 2 × (hb1)), and the outer diameter is 2.60 mm (= 2 × (hb2)). That is, the radial widths of the first and second light shielding portions 8 "a, 8" b are only 0.14 mm and 0.06 mm. Therefore, some of the laser beams incident on the partial light shielding member 8 " Is blocked by the first and second light shielding portions 8 "a, 8" b, and most of the remaining light passes through the partial light shielding member 8 ".

  It should be noted that the light beam incident on the inner area (corresponding to the central area) of the first light-shielding part 8 "a and the area between the first light-shielding part 8" a and the second light-shielding part 8 "b (corresponding to the transmission area) ) And a light beam incident on a region outside the second light-shielding portion 8 ″ b (corresponding to a transmission region) only pass through a transparent region having the same optical thickness. Even after passing through the member 8 ", the phases are the same. Further, ((hb1 + hb2) / 2) / hmax is 0.94, and ((hc1 + hc2) / 2) / hmax is 0.74. Further, since Sa is 0.21 and Sb is 0.04, Sa / (Sa + Sb) is 0.84. Therefore, the partial light-shielding member 8 ″ of the fifth embodiment has the conditional expression (2) and Satisfies (3). Furthermore, since S ′ is 0.25 and S is 2.12, S ′ / S is 0.12. Therefore, the partial light shielding member 8 ″ of the fifth embodiment also satisfies the conditional expression (5).

<Function of Fifth Embodiment>
Hereinafter, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system according to the fifth embodiment configured as described above will be described when there is no partial light shielding member 8 "and when the partial light shielding member 8". This will be explained in comparison with when there was.

  FIG. 19 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in the range from the beam central axis to 0.25 mm in the main scanning direction. FIG. 20 is a graph showing the intensity ratio in the graph of FIG. 19 in an enlarged manner from 0% to 10%. In FIGS. 19 and 20, the curve indicated by the broken line indicates the intensity distribution when the partial light shielding member 8 ″ is not present, and the curve indicated by the solid line indicates the intensity when the partial light shielding member 8 ″ is present. Show the distribution.

  When the partial light shielding member 8 ″ is not present (see the broken lines in FIGS. 19 and 20), the intensity of the side lobe gradually decreases as the distance from the main beam increases. The intensity of the side lobe adjacent to the main beam is It is over 4%.

  On the other hand, when the partial light shielding member 8 ″ is present (see the solid lines in FIGS. 19 and 20), the strength of the side lobes is less than 2%. Since (3) is satisfied, the side lobe strength is smaller than in the third embodiment.

  Accordingly, even if the lens surfaces of the lenses 7a to 7c of the fθ lens group 7 have some microscopic undulations and the strength of the side lobe is increased by only a few percent, the strength of the photosensitive drum 12 is sensitive. The threshold is rarely exceeded.

  18 may also be integrated with the aperture stop 4 in the same manner as the partial light shielding member 3 in FIG. 3. The partial light shielding member 8 ″ in FIG. It may be used for a reflection type scanning optical system.

Embodiment 6

  The sixth embodiment has the same configuration as the first embodiment except that the partial light shielding member has three light shielding portions. Therefore, only the differences from the first embodiment will be described below. FIG. 21A is a front view of the partial shielding member 9 of the sixth embodiment, and FIG. 21B is a side view of the partial shielding member 9.

  The partial shielding member 9 of the sixth embodiment is also a rectangular flat plate-shaped optical element disposed so as to be orthogonal to the beam center axis. As shown in FIG. 21A, the partial light shielding member 9 has a ring-shaped film having a transmittance of 0% attached to a transparent flat plate as a first light shielding portion (corresponding to a light shielding region) 9a. A ring-shaped film having an inner diameter larger than the outer diameter of the portion 9a and having a transmittance of 0% is attached to the transparent flat plate as a second light-shielding portion (corresponding to a light-shielding region) 9b, and the outer diameter of the second light-shielding portion 9b. A ring-shaped film having a larger inner diameter and having a transmittance of 0% is pasted on a transparent flat plate as a third light-shielding portion (corresponding to a light-shielding region) 9c. The first to third light shielding portions 9a to 9c are arranged at the center of the flat plate.

  The partial light shielding member 9 is arranged so as to be orthogonal to the beam center axis of the laser beam. However, the arrangement position is such that the beam center axis and the vicinity thereof are inside the first light shielding portion 9a. It is adjusted so that it may enter.

  By the way, as described above, the cross section of the laser beam incident on the partial light shielding member 9 is shaped into an elliptical shape by the collimating lens 2 (see the broken line in FIG. 21A), and its major axis is the main axis. In the scanning direction, the minor axis is oriented in the sub-scanning direction. In the sixth embodiment, the radius of the major axis in the cross-sectional shape of the laser beam incident on the partial light shielding member 9 is set to 1.35 mm (= hmax), and the radius of the minor axis is set to 0.50 mm. Is set. In addition, as shown in FIG. 21B, the inner diameter of the first light-shielding portion 9a is 1.82 mm (= 2 × (hc1)), and the outer diameter is 1.98 mm (= 2 × (hc2)). It is. The inner diameter of the second light-shielding portion 9b is 2.14 mm, and the outer diameter is 2.26 mm. Furthermore, the inner diameter of the third light-shielding portion 9c is 2.40 mm (= 2 × (hb1)), and the outer diameter is 2.56 mm (= 2 × (hb2)). That is, the width in the radial direction of the first to third light shielding portions 9a to 9c is only 0.08 mm, 0.06 mm, and 0.08 mm. For this reason, part of the laser beam incident on the partial light shielding member 9 is blocked by the first to third light shielding portions 9 a to 9 c, and most of the remaining light passes through the partial light shielding member 9.

  Note that the light beam incident on the inner region (corresponding to the central region) of the first light shielding portion 9a and the light beam incident on the region (corresponding to the transmission region) between the first light shielding portion 9a and the second light shielding portion 9b. , The light beam incident on the region (corresponding to the transmission region) between the second light shielding unit 9b and the third light shielding unit 9c and the light beam incident on the region outside the third light shielding unit 9c (corresponding to the transmission region) are Since both only pass through transparent regions having the same optical thickness, they are in phase with each other even after passing through the member 9. Further, ((hb1 + hb2) / 2) / hmax is 0.92, and ((hc1 + hc2) / 2) / hmax is 0.70. Further, since Sa is 0.13 and Sb is 0.07, Sa / (Sa + Sb) is 0.65. Therefore, the partial light shielding member 9 of the sixth embodiment satisfies the conditional expressions (2) to (4). Furthermore, since S ′ is 0.28 and S is 2.12, S ′ / S is 0.13. Therefore, the partial light shielding member 9 of the sixth embodiment also satisfies the conditional expression (5).

<Functions of Sixth Embodiment>
Hereinafter, the intensity distribution of the laser beam scanned on the scanning target surface S by the scanning optical system of the sixth embodiment configured as described above will be described when there is no partial light shielding member 9 and when there is a partial light shielding member 9. The explanation will be made in comparison with the time.

  FIG. 22 is a graph showing the intensity distribution of the laser beam incident on the scanning target surface S in a range from the beam central axis to 0.25 mm in the main scanning direction. FIG. 23 is a graph showing the intensity ratio in the graph of FIG. 22 in an enlarged manner from 0% to 10%. 22 and 23, the curve indicated by the broken line indicates the intensity distribution when the partial light shielding member 9 is not present, and the curve indicated by the solid line indicates the intensity distribution when the partial light shielding member 9 is present. Show.

When there is no partial light blocking member 9 (see the broken lines in FIGS. 22 and 23), the intensity of the side lobe gradually decreases as the distance from the main beam increases, and the intensity of the side lobe adjacent to the main beam is 4 It is slightly over%.

  On the other hand, when the partial light shielding member 9 is present (see solid lines in FIGS. 22 and 23), the strength of the side lobes is less than 1%. Since the number of light shielding portions is increased to three as compared with the fifth embodiment, the side lobe intensity is further reduced by appropriately setting the sizes and positions of the light shielding portions 9a to 9c.

  Accordingly, even if the lens surfaces of the lenses 7a to 7c of the fθ lens group 7 have some microscopic undulations and the strength of the side lobe is increased by only a few percent, the strength of the photosensitive drum 12 is sensitive. The threshold is rarely exceeded.

  The partial light shielding member 9 in FIG. 21 may also be configured integrally with the aperture stop 4 in the same manner as the partial light shielding member 3 in FIG. 21 may be used in a reflective scanning optical system as shown in FIG.

Schematic configuration diagram of the laser beam printer of the first embodiment Schematic optical configuration diagram of scanning optical system (A) Front view and (b) Side view of partial light shielding member Front view of aperture stop Graph showing the intensity distribution of the laser beam with and without a partial light blocking member A graph enlarging a part of the graph of FIG. Explanatory drawing which shows the example which formed the partial light-shielding member and aperture stop integrally. Schematic configuration diagram of a reflective scanning optical system to which the present invention is applied (A) Front view and (b) Side view of the partial light shielding member of the second embodiment Graph showing the intensity distribution of the laser beam with and without a partial light blocking member The graph which expanded a part of graph of FIG. (A) Front view and (b) Side view of partial light shielding member of third embodiment Graph showing the intensity distribution of the laser beam with and without a partial light blocking member The graph which expanded a part of graph of FIG. (A) front view and (b) side view of a partial light shielding member of the fourth embodiment Graph showing the intensity distribution of the laser beam with and without a partial light blocking member The graph which expanded a part of graph of FIG. (A) Front view and (b) Side view of partial light shielding member of fifth embodiment Graph showing the intensity distribution of the laser beam with and without a partial light blocking member The graph which expanded a part of graph of FIG. (A) front view and (b) side view of partial light shielding member of sixth embodiment Graph showing the intensity distribution of the laser beam with and without a partial light blocking member The graph which expanded a part of graph of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Laser light source 2 Collimating lens 3, 3 'Partial light-shielding member 3a, 3'a Light-shielding part 4 Aperture stop 4a Slit (aperture)
DESCRIPTION OF SYMBOLS 5 Cylindrical lens 6 Polygon mirror 7 f (theta) lens group 8-8 "Partial light shielding member 8a-8" a 1st light shielding part 8b-8 "b 2nd light shielding part 9 Partial light shielding member 9a 1st light shielding part 9b 2nd light shielding part 9c 3rd shading part

Claims (21)

A laser beam emitted from a light source is dynamically deflected by a deflector, and the dynamically deflected laser beam is converged as a spot light on a scanning target surface by an imaging optical system. A scanning optical system that scans along the main scanning direction on the surface to be scanned,
An optical element is provided on the optical path between the light source and the deflector,
The optical element includes a central region that transmits a light beam in the vicinity of a beam central axis of a laser beam emitted from the light source, a light shielding region that blocks a part of the light beam incident on the outside of the central region, and a transmissive region possess for transmitting light flux excluding a light flux entering the central region and the light shielding region, the light beam transmitted through the central region can <br/> as a light beam and the phase passing through the transmissive region A characteristic scanning optical system. The optical element according to claim 1 scanning optical system according to characterized in that it comprises a pair of the light shielding region and the transmissive region. The shading region is adjacent to the outside of the central region;
The scanning optical system according to claim 2 , wherein the transmission region is adjacent to the light shielding region outside the light shielding region. 4. The scanning optical system according to claim 3, wherein the light shielding area and the transmission area are respectively arranged on both sides of the central area in the main scanning direction. 5. The scanning optical system according to claim 4, wherein the light shielding area and the transmission area are respectively arranged at symmetrical positions with the central area in between in the main scanning direction. 6. The scanning optical system according to claim 2 , wherein a cross section perpendicular to a beam central axis of the laser beam incident on the optical element has an elliptical shape. The scanning optical system according to claim 6 , wherein the major axis of the ellipse is oriented in the main scanning direction. The distance in the main scanning direction from the beam center axis of the laser beam incident on the optical element to the inner edge of the light shielding region is ha1, and the distance from the beam center axis of the laser beam to the outer edge of the light shielding region Where ha2 and the radius in the main scanning direction in the cross-sectional shape of the laser beam is hmax, the following conditional expression (1),
0.70 <((ha1 + ha2) / 2) / hmax <0.85 --- (1)
The scanning optical system according to claim 2, wherein: The optical element according to claim 1 scanning optical system according to characterized in that it comprises a plurality of sets of the light shielding region and the transmissive region. The scanning optical system according to claim 9 , wherein the light shielding regions and the transmission regions are alternately arranged in a direction away from the central region. The scanning optical system according to claim 10 , wherein the light shielding regions and the transmission regions are alternately arranged along the main scanning direction. The scanning optical system according to claim 11, wherein each of the light shielding regions and each of the transmission regions is disposed at a symmetrical position across the central region in the main scanning direction. The scanning optical system according to any one of claims 9 to 12, wherein a cross section perpendicular to a beam central axis of a laser beam incident on the optical element has an elliptical shape. 14. The scanning optical system according to claim 13 , wherein the major axis of the ellipse is oriented in the main scanning direction. The distance in the main scanning direction from the beam central axis of the laser beam incident on the optical element to the inner edge of the outermost light shielding region is hb1, and the distance of the outermost light shielding region from the beam central axis of the laser beam. When the distance in the main scanning direction to the outer edge is hb2, and the radius in the main scanning direction in the cross-sectional shape of the laser beam is hmax, the following conditional expression (2),
0.85 <((hb1 + hb2) / 2) / hmax <0.95 --- (2)
The scanning optical system according to claim 9, wherein: The distance in the main scanning direction from the beam central axis of the laser beam incident on the optical element to the inner edge of the innermost light shielding region is hc1, and the innermost light shielding region of the laser beam from the beam central axis. When the distance in the main scanning direction to the outer edge is hc2, the following conditional expression (3),
0.65 <((hc1 + hc2) / 2) / hmax <0.75 --- (3)
The scanning optical system according to claim 15, wherein: The total area of the areas where the laser beam is incident among the innermost light-shielding areas among the respective light-shielding areas is Sa, and the area of the area where the laser beam is incident among the outermost light-shielding areas. When the sum is Sb, the following conditional expression (4),
0.20 <Sa / (Sa + Sb) <0.75 --- (4)
The scanning optical system according to claim 15 or 16, wherein: When the total area of the regions where the laser beam is incident in the light shielding region is S ′, and the area of the cross section perpendicular to the beam center axis in the laser beam is S, the following conditional expression (5),
0.03 <S '/ S <0.30 --- (5)
The scanning optical system according to any one of claims 1 to 17, characterized in that meet. The imaging optical system, the scanning optical system according to any one of claims 1 to 18, characterized in that an optical system comprising a reflecting surface. The optical element has a shielding part as an aperture stop,
Said central region, said light shielding region and the transmissive region is the opening portion of the optical element, the scanning optical system according to any one of claims 1 to 19, characterized in that it is arranged. A laser beam emitted from a light source is dynamically deflected by a deflector, and the dynamically deflected laser beam is converged as a spot light on a scanning target surface by an imaging optical system. A printer provided with a scanning optical system that scans along a main scanning direction on a surface to be scanned,
An optical element is provided on the optical path between the light source and the deflector,
The optical element includes a central region that transmits a light beam in the vicinity of a beam central axis of a laser beam emitted from the light source, and a light-shielding region that blocks a part of the light beam incident on the outside of the central transmission region, possess a transmissive region that transmits light fluxes except the light beam incident on the central region and the light shielding region, the light beam transmitted through the central region becomes the light beam and the phase passing through the transmissive region <br/> that Printer characterized by.

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