JP2015194588A - Optical scanning device and image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image scanning device which suppresses worsening of banding regardless of magnitude of dynamic range of a light source and prevents reduction in life of the light source.SOLUTION: An optical scanning device has an optical deflector 202b that deflects light emitted from a light source 101-1 comprising a VCSEL to scan a surface of a photosensitive drum 504. A shutter device 700, which functions as a light volume adjuster, is located between a prism beam splitter 203 and a 1/4 wavelength plate 204. The shutter device 700 is provided with an ND filter 700-7, or neutral density filter, which is inserted into a light path when a low line speed mode is used, which is the case when thick paper sheets or the like are used. Light volume is reduced when the light passes through the ND filter 700-7. Exposure energy can be thus adjusted to an appropriate level without having to alter intensity of the light source, which allows scanning using all mirror faces without having to skip faces even in the low line speed mode.

Description

  The present invention relates to an image of an optical scanning device that scans a surface to be scanned based on image information, a copying machine having the optical scanning device, a printer, a facsimile, a plotter, or a multifunction device having at least one of these. The present invention relates to a forming apparatus.

Electrons using a raster optical system that forms an electrostatic latent image on a photosensitive material (photoconductor) by deflecting and scanning light from a light source with a rotating polygon mirror (hereinafter referred to as “polygon” or “polygon mirror”). In a photographic image forming apparatus, banding of polygon cycles (such as band-shaped density unevenness and pitch unevenness) may occur.
Banding is a phenomenon caused by surface tilt that occurs in the production of polygon mirror surfaces.
In particular, in the case of a four-sided polygon having four mirror surfaces, if a so-called surface skipping method is employed, in which an image is formed only on a facing surface (1-3 surface or 2-4 surface) with a large difference in surface tilt, two rotation cycles of the polygon It is known that banding will occur.
In this type of image forming apparatus, if paper such as thick paper, which has extremely poor fixability, is passed, it takes more heat from fixing than plain paper, so to maintain continuous fixability (fixing temperature) Generally, the speed of image formation (linear speed) is decreased to reduce the speed of heat release from fixing.
In this case, the linear velocity is less than half of the standard linear velocity.

Generally, an energy target value to be exposed on the photosensitive member is determined, and feedback control is performed to increase or decrease the light amount due to deterioration of the photosensitive member sensitivity or the like.
If image formation is performed in the low linear velocity mode in the same manner as in the standard linear velocity mode, exposure energy is more than necessary, and the density is higher than the target image.
As a countermeasure for this, a so-called “surface skipping” method that uses the mirror surface of the polygon every other surface or every other surface without changing the rotational speed of the polygon is generally used.
In the surface skipping method, it is not necessary to change the intensity of the laser light according to the image forming speed.

When the surface skipping method is adopted, in the case of a four-sided polygon, image formation is performed only on the 1-3 side or 2-4 side as described above, but the combination of these sides has the largest difference in the top surface tilt. It is known to be.
In image formation by skipping the surface (low linear velocity mode), banding appears more conspicuously than in the standard linear velocity mode with one rotation cycle using the entire surface of the polygon.
In recent years, a vertical cavity surface emitting laser (VCSEL), which is a vertical surface emitting LD (laser diode), has been used as a light source.
Hereinafter, VCSEL is also referred to as “vertical surface emitting laser”. In an ordinary LD array, an output of 1 to 15 mW or the like is generally used for an output around 1 ch of a light emitting point. There is only a light emission range (dynamic range).

Since a normal LD array has a large dynamic range, it is possible to perform scanning by using the entire surface of the polygon by adjusting the light amount corresponding to the low linear velocity mode, and it is possible to avoid banding deterioration due to surface skipping.
On the other hand, since the dynamic range of the VCSEL is small, it is difficult to adjust the amount of light corresponding to the low linear velocity mode, and an appropriate image density cannot be obtained if scanning is performed by using the entire polygon as described above.
Further, even if scanning can be performed by using the entire surface of the polygon with a normal LD array, it is necessary to control the intensity of the laser light in the standard linear velocity mode and the low linear velocity mode, which leads to a reduction in the life of the light source.

In order to solve the problem of a decrease in the amount of light due to the droop characteristic of the semiconductor laser, the semiconductor laser is used in a region where the light emission amount is equal to or greater than a predetermined light emission amount, and the transmittance adjustment device uses a first state and a lower second state. An image forming apparatus that obtains one optical efficiency is also known (for example, Patent Document 1).
However, if a semiconductor laser is used in an area exceeding the rated output, the lifetime of the light source is reduced at a stretch, and consequently an increase in cost cannot be avoided.

  The present invention was devised in view of such a current situation, and provides an optical scanning device capable of suppressing deterioration of banding and suppressing a decrease in the lifetime of the light source regardless of the dynamic range of the light source. With a purpose.

  In order to achieve the above object, an optical scanning device of the present invention is used in an image forming apparatus having a first linear velocity mode and a second linear velocity mode slower than the first linear velocity mode, and a light source An optical deflector that deflects the light from the light source, a scanning optical system that guides the light deflected by the optical deflector to the surface to be scanned, and exposure energy for the surface to be scanned And a light amount variable device that selectively varies so that the second linear velocity mode is lower than the first linear velocity mode, and the first linear velocity mode and the second linear velocity mode are provided. The light emission quantity of the light source is the same within the rated output of the light source, and the lighting control of the light source is performed by the optical deflector in both the first linear speed mode and the second linear speed mode. The optical scanning using all the deflection surfaces is performed.

  According to the present invention, it is possible to suppress the deterioration of banding regardless of the dynamic range of the light source.

1 is a schematic configuration diagram of a color printer as an image forming apparatus according to an embodiment of the present invention. It is a schematic block diagram in the main scanning cross section of an optical scanning device. It is a schematic block diagram in the sub-scan cross section of an optical scanning device. It is a schematic block diagram in the main scanning cross section of an optical deflector. It is a figure which shows the light beam separation function by a prism beam splitter. It is the perspective view which looked at the light source unit from the lower surface side. It is an optical path figure in a light source unit. It is a schematic plan view showing a positional relationship of a shutter device as a light amount variable device in the optical scanning device. It is an outline top view of a shutter device. It is a general | schematic side view of a shutter apparatus. It is a figure for demonstrating the surface tilt of the polygon mirror of an optical deflector, (a) is a top view which shows the positional relationship of a mirror surface, (b) is a side view of a polygon mirror, (c) is a surface tilt with respect to an ideal mirror surface FIG. 8D is a diagram showing the relationship between each mirror surface and the amount of surface tilt. It is a figure which shows the ideal dot positional relationship when there is no surface tilt. It is a figure which shows the dot position relationship (banding) in standard linear velocity mode in case there exists a surface tilt. It is a figure which shows the dot positional relationship (banding) in the low linear velocity mode (surface skipping) when there exists surface tilt.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
Based on FIG. 1, an outline of the configuration of a color printer as an image forming apparatus according to the present embodiment will be described.
The color printer 500 is a tandem multicolor printer that forms a full-color image by superimposing four colors (black, cyan, magenta, and yellow).
The color printer 500 includes an optical scanning device 100A, four photosensitive drums (501, 502, 503, 504) as image carriers, four cleaning units (605Y, 605M, 605C, 605Bk), and four charging devices (602Y). , 602M, 602C, 602Bk), four developing rollers (603Y, 603M, 603C, 603Bk), four developing devices (604Y, 604M, 604C, 604Bk), transfer belt 606, secondary transfer roller 613, fixing device 610, A paper feed roller 608, a registration roller pair 609, a paper discharge roller pair 612, a paper discharge tray 611, and the like are provided.

The photosensitive drum 501, the cleaning unit 605 </ b> Y, the charging device 602 </ b> Y, the developing roller 603 </ b> Y, and the developing device 604 </ b> Y are used as a set to form an image station (hereinafter referred to as a Y station) that forms a yellow image.
The photosensitive drum 502, the cleaning unit 605M, the charging device 602M, the developing roller 603M, and the developing device 604M are used as a set, and constitute an image station (hereinafter referred to as M station) that forms a magenta image.
The photosensitive drum 503, the cleaning unit 605C, the charging device 602C, the developing roller 603C, and the developing device 604C are used as a set, and constitute an image station (hereinafter referred to as C station) that forms a cyan image.
The photosensitive drum 504, the cleaning unit 605Bk, the charging device 602Bk, the developing roller 603Bk, and the developing device 604Bk are used as a set, and constitute an image station (hereinafter referred to as K station) that forms a black image.

Each of the photosensitive drums (501, 502, 503, 504) has a photosensitive layer formed on the surface thereof, and is driven to rotate in the direction of the arrow by a rotation mechanism (not shown).
Each charging device (602Y, 602M, 602C, 602Bk) uniformly charges the surface of the corresponding photosensitive drum.
The optical scanning device 100A includes an MY unit 100-1 that performs exposure scanning on the photosensitive drums 501 and 502, and a Bk-C unit 100-2 that performs exposure scanning on the photosensitive drums 503 and 504.
The optical scanning device 100A irradiates the corresponding photosensitive drum surface with lighting control based on image information to form an electrostatic latent image.
The formed electrostatic latent image moves in the direction of the corresponding developing roller as the photosensitive drum rotates.

The toner stirred in each developing device is obtained as a developing roller as the photosensitive drum rotates, and is uniformly applied on the electrostatic latent image.
That is, the developing roller attaches toner to a latent image formed on the surface of each corresponding photosensitive drum to make it visible (visualize), and an image with the toner attached (hereinafter referred to as “toner image”). Moves to the transfer belt 606 side as the photosensitive drum rotates.
The yellow, magenta, cyan, and black toner images are sequentially transferred onto the transfer belt 606 at a predetermined timing, and are superimposed to form a multicolor image.

A recording sheet 510 as a recording medium is fed one by one toward a registration roller pair 609 by a sheet feeding roller 608.
The registration roller pair 609 conveys the recording paper 510 between the transfer belt 606 and the secondary transfer roller 613 at a predetermined timing.
As a result, the toner image superimposed on the transfer belt 606 is transferred to the recording paper, and the transferred recording paper is sent to the fixing device 610.
In the fixing device 610, the toner image on the recording paper is fixed on the recording paper by heat and pressure at the nip portion between the fixing roller 610a and the pressure roller 610b.
The fixed recording paper is discharged through a pair of paper discharge rollers 612 and sequentially stacked on a paper discharge tray 611.

Each cleaning unit removes residual toner remaining on the surface of the corresponding photosensitive drum. The surface of the photosensitive drum from which the residual toner has been removed returns again to a position facing the corresponding charging device.
The color printer 500 has a standard linear velocity mode as a first linear velocity mode and a low linear velocity as a second linear velocity mode that is slower than the standard linear velocity mode (in this case, 1/2 of the standard linear velocity mode). Mode.

The configuration of the optical scanning device 100A will be described.
The optical scanning device 100A includes an MY unit 100-1 that exposes and scans the photosensitive drums 501 and 502, and a Bk-C unit 100-2 that exposes and scans the photosensitive drums 503 and 504.
Since the MY unit 100-1 and the Bk-C unit 100-2 have the same optical axis layout, the Bk-C unit 100-2 will be described here.

FIG. 2 shows an optical system layout of the Bk-C unit 100-2.
A light source unit 101 that emits a laser beam having a beam profile includes a light source 101-1 that emits laser light with linearly polarized light, a quarter-wave plate 101-4 that converts the laser light into circularly polarized light, A collimating lens 101-5 that converts laser light polarized by the four-wavelength plate into parallel light and an aperture 101-6 that cuts out the parallelized laser light are provided.
These optical elements are integrally adjusted and assembled and held in a light source holder 101-2 shown in FIGS.
The light source 101-1 is composed of a VCSEL (vertical surface emitting laser).

As shown in FIG. 2, in addition to the light source unit 101, the Bk-C unit 100-2 includes a prism beam splitter 203 that divides laser light emitted from the light source unit 101 into two directions in the sub-scan section, and 2 The quarter-wave plate 204 that converts the polarization of the laser beam divided in the direction from linearly polarized light to circularly polarized light, and the laser light that has power only in the sub-scan section and is converted to circularly polarized light are mounted on the optical deflector 202. And a cylindrical lens 205 that forms an image on the mirror surface (deflection surface) of the rotating polygonal mirror 202b.
Thus, an incident optical system that forms a beam profile is configured.
The laser beam formed by the incident optical system is scanned in the main scanning direction by the optical deflector 202 that stably drives the rotary polygon mirror 202b at a desired rotational speed.
As shown in FIG. 3, the scanned laser light passes through a lens 301 and a lens 302 as a scanning optical system, mirrors 303a and 303b for turning back the laser light, dustproof glass 305, and the like on the surface of the photosensitive drum 504. Fast scanning is performed to form an electrostatic latent image.
The laser beam scanned by the lower stage of the rotating polygon mirror 202b is scanned at a constant speed on the surface of the photosensitive drum 503 through the lens 301 and the lens 302, the mirror 304 for folding the laser beam, and the dust-proof glass 305. An electrostatic latent image is formed.

The incident optical system, the optical deflector, and the scanning optical system are integrally fixed by an optical housing 400 to ensure the characteristics of the optical scanning device.
The elements constituting the optical housing and the optical unit are made of resin.
As shown in FIG. 4, the optical deflector 202 has a rotary polygon mirror 202b mounted thereon and is assembled to the motor board 202a.
The rotating polygonal mirror 202b has a four-surface two-stage configuration, and the upper and lower stages are formed so as to be shifted by an angle θ in the rotation direction. Here, θ = 45 °.
The upper stage scans the photosensitive drum 504, and the lower stage scans the photosensitive drum 503. These are configured to perform exposure scanning based on image information corresponding to each station without geometrically scanning at the same time.

FIG. 5 is an explanatory diagram of the prism beam splitter 203 that divides the laser light into two parts.
Laser light emitted from the light source unit 101 is converted from linearly polarized light to circularly polarized light by the quarter wavelength plate 101-4 in the light source unit 101.
The light beam A has a circular polarization characteristic. When the laser beam having the circular polarization characteristic reaches the polarization separation surface of the prism beam splitter 203, it is a component perpendicular to the reflection surface of the rotating polygon mirror 202b among the circular polarization components. Only s-polarized light is transmitted.

The laser light reflected by the polarization separation surface is p-polarized light, which is a component parallel to the reflection surface of the rotating polygon mirror 202b, and is reflected toward the rotating polygon mirror 202b by the reflection mirror surface of the prism beam splitter 203.
At this point, one laser beam is separated into two laser beams having different polarization characteristics (hereinafter referred to as “beam splitting”).
Each of the divided laser beams passes through the prism beam splitter 203 and is converted again into circularly polarized light by the quarter wavelength plate 204.

FIG. 6 is a view of the light source unit 101 as seen from the lower surface side. For convenience, the direction in which the laser beam is emitted (optical axis direction) is X, the main scanning direction is Y, and the sub-scanning direction is Z.
As described above, the light source unit 101 includes the optical elements of the light source 101-1, the quarter-wave plate 101-4, the collimator lens 101-5, and the aperture 101-6.
In FIG. 6, reference numeral 101-3 denotes a laser modulation substrate.
FIG. 7 is an optical path diagram of laser light emitted from the light source 101-1 mounted on the laser modulation substrate 101-3.
The aperture 101-6 forms a beam profile through an opening (not shown) and emits the beam profile toward the optical deflector.

As shown in FIG. 8, a shutter device 700 as a light amount variable device is disposed between the prism beam splitter 203 and the quarter wavelength plate 204 (not shown in FIG. 2).
The shutter device 700 is integrally fixed to the optical housing 400.

FIG. 9 is a diagram illustrating a configuration of the shutter device 700.
The shutter device 700 includes a base 700-5, a motor bracket 700-4 fixed to the base 700-5, a motor 700-3 assembled to the motor bracket 700-4, and an output shaft of the motor 700-3. It has a link 700-2 and the like fixed together by press-fitting or other fixing means.

The base is provided with an integral pin configuration, and the shutter 700-1 is rotatable about the pin portion.
An ND filter (a neutral density filter) 700-7 as an optical filter that varies the exposure energy is fixed to the shutter 700-1.
When the shutter 700-1 is rotationally driven, the ND filter 700-7 is selectively positioned at a position F2 that enters the optical path and a position F1 that retracts from the optical path. The position of F2 is a position where the light beam passes through the ND filter.

The shutter 700-1 is formed with a slot-shaped slit, and a circular head (not shown) of the link 700-2 is inserted into the slit portion.
With this configuration, the rotational drive of the motor 700-3 is transmitted to the shutter 700-1.
B part in a figure is a connector part of the motor 700-3 and a driver board not shown, and is connected via a harness not shown, and supplies a signal and electric power.
C part is a connector part of the sensor 700-6, and performs signal output of the sensor and power supply from an internal / external substrate (not shown) like the B part.

The operation of the shutter device 700 will be described.
When the rotation axis of the motor 700-3 is rotated from P1 to P2, as indicated by the dotted arrow, the filter side of the link 700-2 and the filler part D of the sensor are counteracted according to the rotation axis operation of the motor 700-3. It operates in the clockwise direction as P1 → P2 indicated by a solid line.
Here, the link 700-2 is a single member that is integrally formed including a fixed portion with the rotation shaft of the motor 700-3 and a filler portion D.
When the link 700-2 rotates from P1 to P2 in the figure, a cylindrical portion (not shown) of the link 700-2 transmits the power, and the shutter 700-1 is centered on the pin portion installed on the base 700-5. And operate as P1 → P2.
As a result, the ND filter 700-7 operates as P1 → P2, and the light beam passes through the ND filter 700-7 (F1 position).
When the motor 700-3 rotates in the reverse direction, the ND filter 700-7 operates as P2 → P1, and the ND filter 700-7 is retracted (F2 position) from the optical path.

At the F1 position (CLOSE state), the output signal of the sensor 700-6 is Low, and at the F2 position (OPEN state), the output signal of the sensor 700-6 is High.
That is, the CLOSE state means a state in which the light beam passes through the ND filter and the light amount decreases, and the OPEN state means a state in which the ND filter retreats from the optical path and the light amount of the light beam does not decrease.

FIG. 10 is a perspective view of the ND filter of the shutter device 700 viewed vertically.
The ND filter installed in the shutter 700-1 is fixed to the shutter 700-1 using a surrounding area that excludes the area of the transmission part by means of adhesion or double-sided tape.
When the shutter device 700 is in the CLOSE state, the upper laser beam is transmitted through the upper part of the ND filter, and the lower laser beam is transmitted through the lower part.
As for the ND filter, a filter having the same transmittance is installed in both the upper stage and the lower stage.

The ND filter 700-7 in the present embodiment is formed using a glass material as a base material. In this case, since it is not a crystalline material, it can be manufactured at low cost.
A liquid crystal optical filter may be used as the ND filter 700-7. In this case, since the transmittance can be varied depending on the voltage, there is an advantage that the layout space can be greatly reduced as compared with the driving method using the actuator.
In the case of a driving method using an actuator, a solenoid may be used instead of the motor as the driving source.

The actual banding will be described with reference to FIGS.
The banding handled here generally means banding caused by density deviation or pitch deviation (sub-scanning position fluctuation) in an image such as a halftone.
Here, the number of polygon mirror surfaces is four (plural) as shown in FIG. In the first place, polygon mirrors are mainly processed by NC milling machines.
Therefore, it goes without saying that machining errors and accuracy are remarkably generated.

As shown in FIG. 11B, the description here uses a two-stage polygon mirror.
As described above, when the four-phase retardation polygon (θ = 45 °) is taken as an example in both the upper and lower stages, these mirror parts are formed by processing one part.
Normally, it is processed with the end of one member having a mirror as the reference plane, but the polygon center axis is decentered depending on the workability and the position of the reference plane. The surface falls with respect to the mirror surface.
These surface tilts often have periodicity on the four surfaces, and the surface tilt on each of the four surfaces is as shown in the graph of FIG.
As shown in FIGS. 11C and 11D, the explanation here is that the surface tilt amount φ of the first and third surfaces is 0 ″, the second surface and the fourth surface when the surface tilt amount is φ. An extreme situation in which the surface tilt amount φ is L1 and L2 will be described.

When the optical scanning device 100 is an ideal optical system, it is possible to correct the optical axis due to the surface tilt due to the influence of the lens 301 and the lens 302.
FIG. 12 shows an ideal dot position relationship.
A case will be described where the number of laser beams scanned on one surface of the polygon is assumed to be 7ch. Each light emitting point scans the surface to be scanned at an ideal pitch interval. The figure shows the light emitting point position on the surface to be scanned (in this case, the photosensitive drums 501 to 504 are indicated).
After one scan on the first side of the polygon (white circle in the figure), the second side (hatched circle in the figure) is half the dot position scanned on the first side (4th / 5th / 6th / 7th dots) Are scanned so that the first, second, and third dots on the second surface are interposed therebetween (interlace).

Next, the dot position scanned on the second side is also shifted by half a dot on the third side (dots in the figure) from the second side (4th / 5th / 6th / 7th dots). Are scanned so that the 1st / 2nd / 3rd dots on the third side are interposed.
The same applies to the fourth side (horizontal line circle in the figure), and dots are continuously formed so as to continue again with the first side, the second side, and so on.

In the ideal case, the relative positions of the dots formed on each surface are evenly arranged as shown on the right side of FIG. 12 (in the figure, all the differences in the center position of each surface are separated by the ideal pitch d). Written), the pitch variation between dots (sub-scanning position variation) is equal and cannot be a banding image.
However, the actual lens performance with respect to an ideal optical system causes field curvature (focus shift with respect to the scanning medium at the lens transmission position), and thus surface tilt is not completely corrected.
Therefore, non-uniform pitch variation as shown in FIG. 13 is caused.
Specifically, it is assumed that L1 and L2 have a plane tilt of 45 ″ with respect to the graph of FIG.
That is, L1 = 45 ″ and L2 = −45 ″.

When the surface 1 and the surface 3 are tilted 0 ″, dots are formed on the surface to be scanned with a geometric optical variation with respect to the angle.
The center dot position (fourth dot) on each surface in FIG. 11 and the value numbered for each surface on which they are formed are used.
In the case of dots formed on the first and second surfaces, the distance between the centers is d1-2. In the case of dots formed on the second and third surfaces, the distance between the centers is d2-3, on the third and fourth surfaces. In the case of dots to be formed, the distance between centers is d3-4, and in the case of dots formed on the fourth and first surfaces, the distance between centers is d4-1.
Each pitch is displaced from the ideal pitch d by the geometric optical variation corresponding to the surface tilt amount.

Since the surface tilt amount φ of the first and third surfaces is 0 ″, it is assumed that the surface is located at an ideal position. For example, the second surface has a surface tilt of +45 ″ and is away from the center of the first surface. Since the center position is shifted, d1-2> d.
Therefore, the pitch becomes larger than the ideal pitch d.
On the contrary, since it becomes the direction which approaches the 3rd surface which exists in an ideal position, it is d2-3 <d.
Further, the fourth surface has −45 ″ surface tilt and is in a direction closer to the center of the third surface, so d3-4 <d.
Next, since the first surface is rotated once to perform dot formation, the center position of the fourth surface is away from the center position of the first surface at the ideal position, and therefore d4-1> d.

Taking these into consideration, pitch fluctuation occurs with one rotation period of the polygon, such that the pitch is large ⇒ small ⇒ small ⇒ large ⇒ large with respect to the ideal pitch d.
Since these dot position fluctuations generate dot density, they are visually regarded as banding images.
When dots are formed using all four polygon surfaces, the image is shifted by the amount of geometric optical variation relative to the relative difference between adjacent surface tilts of each surface.
This is a banding mechanism in dot formation in the standard linear velocity mode.

However, for paper such as thick paper whose fixing ability is remarkably deteriorated, it takes more heat from fixing than plain paper. Therefore, in order to maintain continuous fixing ability (fixing temperature), the speed of image formation (linear Generally, it is generally performed to reduce the speed of heat release from fixing.
In this case, the linear velocity is less than half the standard linear velocity (low linear velocity mode). The problem here is the amount of light of the VCSEL.
Generally, the target energy value for exposing the photoconductors 501 to 504 is determined, and feedback control is performed to increase or decrease the amount of light due to deterioration of the photoconductor sensitivity or the like.
However, the fundamental difference between VCSEL and normal LD is in the dynamic range. In general, an LD of 1 to 15 mW or the like is generally used for the output around 1 ch of the light emitting point.

However, the VCSEL has only a light emission range of about 1/10. For this reason, when an image is formed in the low linear velocity mode in the same manner as in the standard linear velocity mode (doubling), exposure energy more than necessary (here, doubled energy) is exposed.
As a result, the image becomes darker than the target image.
In order to optimize exposure energy and form an image using the same amount of emitted light, a so-called surface skipping method is employed in which an image is formed by skipping one polygon surface.
In this case, image formation is performed only on the 1-3 side, or image formation is performed only on the 2-4 side.

Here, since the surface 1-3 is tilted 0 ″, it is assumed that pitch fluctuation does not occur. Therefore, a case where dots are formed on the surface 2-4 will be described.
FIG. 14 shows a dot formation state when only 2-4 planes are used in skipping. Similar to the description in FIG. 13, the formed surface is indicated by a pitch d obtained by numbering.
As apparent from FIG. 14, the fluctuation occurs at a pitch twice that of the standard linear velocity mode shown in FIG.

The reason is as follows.
In the standard linear velocity mode in which dots are formed on the entire surface of the polygon as shown in FIG. 13, pitch variation has occurred due to the relative difference in the amount of surface tilt between adjacent surfaces.
The 1-2 plane is displaced by the geometric optical variation corresponding to the plane tilt difference 45 ″, and the 2-3 plane is displaced by the geometric optical variation corresponding to −45 ″. The surface was shifted by the amount of geometric optical variation corresponding to −45 ″, and the 4-1 surface was shifted by the amount of geometric optical variation corresponding to the surface tilt 45 ″.
However, in the case of surface skipping, the surface is shifted by an amount corresponding to the geometric optical variation corresponding to the surface tilt difference −90 ″ of the 2-4 surface, and the surface 4-2 is geometric optical variation corresponding to the surface tilt difference 90 ″. It will be shifted by the amount.
Therefore, the amount of pitch variation is larger than when writing over the entire polygon mirror. Therefore, the banding is worse than the standard linear velocity mode even if the same polygon is used in the low linear velocity mode.

When a VCSEL is used as the light source, the dynamic range is small, so that dots must be formed by skipping the surface in order to satisfy the exposure energy.
However, for the above reasons, banding is worse than in the standard linear velocity mode.

  In order to solve this problem, in this embodiment, as shown in Table 1, the exposure energy is intentionally changed without changing the amount of light emitted from the VCSEL by the shutter device 700 in the low linear velocity mode in which dot formation by skipping is necessary. To the same dot formation as in the standard linear velocity mode.

That is, in the low linear velocity mode, the shutter 700-1 is in the CLOSE state, and the light beam is transmitted through the ND filter 700-7 and the light amount is reduced.
In the CLOSE state of the shutter 700-1, dots are formed on the entire polygon surface as in the standard linear velocity mode.
In this way, the exposure energy per polygon surface is halved, and even if dots are formed using the polygon 4 surface, irradiation is performed with a desired exposure energy.
Accordingly, the banding is at the same level as the banding in the standard linear velocity mode.

In the present embodiment, the VCSEL as the light source is driven with the same output (exposure energy) in the standard linear velocity mode and the low linear velocity mode within the rated output.
Since it is used within the rated output, it is possible to suppress a decrease in the life of the light source, which in turn contributes to cost reduction.
When a condition such as thick paper is input or detected, a control unit (not shown) switches the image forming mode to the low linear velocity mode and sets the shutter 700-1 of the shutter device 700 in the optical scanning device to the CLOSE state.
The control means sets the shutter 700-1 of the shutter device 700 to the OPEN state in the standard linear velocity mode.
The low linear velocity mode is selected not only when feeding thick paper but also when increasing the glossiness of an image in the color image forming apparatus. In this case, the same shutter control as described above is performed.

Since VCSEL has a small dynamic range, it is difficult to suppress banding by using the entire surface of the polygon, but banding can be easily suppressed by reducing exposure energy with an ND filter.
Further, since it is not necessary to change the emission intensity of the VCSEL between the standard linear velocity mode and the low linear velocity mode, it is possible to suppress a reduction in the lifetime of the light source.

The preferred embodiments of the present invention have been described above. However, the present invention is not limited to such specific embodiments, and unless specifically limited by the above description, the present invention described in the claims is not limited. Various modifications and changes are possible within the scope of the gist.
The effects described in the embodiments of the present invention are merely examples of the most preferable effects resulting from the present invention, and the effects of the present invention are limited to those described in the embodiments of the present invention. is not.

101-1 Light source 202b Optical deflector 501, 502, 503, 504 Photosensitive drum as scanning surface 700 Shutter device as light quantity variable device 700-7 ND filter as optical filter F1 Position entering optical path F2 Retreat from optical path Position

JP 2012-11706 A

Claims (6)

Used in an image forming apparatus having a first linear velocity mode and a second linear velocity mode slower than the first linear velocity mode;
A light source;
An optical deflector having a plurality of deflecting surfaces and deflecting light from the light source;
A scanning optical system that guides the light deflected by the optical deflector to a surface to be scanned;
A light amount variable device that selectively varies the exposure energy for the surface to be scanned so that it is lower in the second linear velocity mode than in the first linear velocity mode;
With
The amount of light emitted by the light source in the first linear velocity mode and the second linear velocity mode is the same within the rated output of the light source,
The light source lighting control is performed so that light scanning using all the deflection surfaces of the optical deflector is performed in both the first linear velocity mode and the second linear velocity mode. apparatus. The optical scanning device according to claim 1,
The light quantity variable device includes an optical filter that varies exposure energy, and a mechanism that selectively positions the optical filter at a position that enters the optical path of light from the light source and a position that retreats from the optical path. apparatus. The optical scanning device according to claim 2,
An optical scanning device in which the optical filter is a liquid crystal optical filter. In the optical scanning device according to any one of claims 1 to 3,
An optical scanning device in which the light source is a vertical surface emitting laser. In the optical scanning device according to any one of claims 1 to 4,
The optical deflector includes a rotating polygon mirror having four deflecting surfaces. An image carrier;
An optical scanning device that forms an electrostatic latent image on the image carrier based on image information;
Developing means for visualizing the electrostatic latent image;
Means for transferring and fixing the visible image to a recording medium;
Have
An image forming apparatus, wherein the optical scanning device is the optical scanning device according to claim 1.

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