Polygon mirror

Abstract

To provide a polygon mirror that can adjust the light quantity distribution on a surface to be scanned, while improving the degree of freedom in optical design in an optical scanner.SOLUTION: A polygon mirror 5 according to the present invention has a plurality of rectangular reflection surfaces 5a that reflect a light beam. When the reflectance at the center of the reflection surface 5a relative to the light beam incident at a predetermined incident angle is defined as A, and the reflectance at a predetermined point between the center and an end in a Y direction of the reflection surface relative to the light beam incident at the predetermined incident angle as B, the following condition is satisfied. 0.02<|1-B / A|<0.10.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a polygonal mirror and an optical scanning device including the polygonal mirror, which is suitable for use in image forming devices such as laser beam printers, digital copiers, and multifunction printers. [Background technology]

[0002] Patent Document 1 discloses an optical scanning device that adjusts the light amount distribution on the scanned surface by adjusting the reflectance of a plane mirror that is placed between a polygon mirror and the scanned surface. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 62-134618 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in the optical scanning device disclosed in Patent Document 1, the reflectance of the plane mirror needs to be adjusted, so that the degree of freedom in the optical design of the entire device is low. SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to provide a polygon mirror that can adjust the light quantity distribution on the scanned surface while improving the degree of freedom in optical design in an optical scanning device. [Means for solving the problem]

[0005] The polygonal mirror according to the present invention has a plurality of rectangular reflecting surfaces that reflect light beams, and when the reflectance at the center of the reflecting surface for a light beam incident at a predetermined incident angle is A and the reflectance at a predetermined point between the center and an end of the reflecting surface in the longitudinal direction for a light beam incident at a predetermined incident angle is B, 0.02<|1-B / A|<0.10 The present invention is characterized in that it satisfies the following conditions. Effect of the Invention

[0006] According to the present invention, it is possible to provide a polygon mirror that can adjust the light amount distribution on the scanned surface while improving the degree of freedom in optical design in an optical scanning device. [Brief description of the drawings]

[0007] [Figure 1] 1 is a schematic main-scan sectional view of an optical scanning device including a polygon mirror according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram of a vacuum deposition apparatus for forming a film on the polygon mirror according to the embodiment. [Diagram 3] 5 is a diagram showing the longitudinal position dependency of the physical film thickness on the reflecting surface of the polygon mirror according to the first embodiment. FIG. [Figure 4] FIG. 1 is a schematic perspective view of a polygonal mirror according to an embodiment of the present invention. [Diagram 5] 4 is a diagram showing the incidence angle dependency of reflectance on a reflecting surface of the polygon mirror according to the first embodiment. FIG. [Figure 6] 5A and 5B are diagrams showing the longitudinal position dependency of the angle of incidence of a light beam on a reflecting surface of the polygon mirror according to the embodiment. [Figure 7] 4 is a diagram showing the average reflectance for a light beam incident on each longitudinal position on the reflecting surface of the polygon mirror according to the first embodiment. FIG. [Figure 8] 13A and 13B are diagrams showing the incidence angle dependence of reflectance on a reflecting surface of a polygon mirror according to the second embodiment, and the longitudinal position dependence of physical film thickness and average reflectance. [Figure 9] 13A and 13B are diagrams showing the incidence angle dependence of reflectance on a reflecting surface of a polygon mirror according to the third embodiment, and the longitudinal position dependence of physical film thickness and average reflectance. [Figure 10] 2 is a sub-scanning sectional view of a main part of the image forming apparatus according to the embodiment. FIG. [Figure 11] FIG. 1 is a schematic main-scanning sectional view of a conventional optical scanning device. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The polygon mirror according to the present embodiment will be described in detail below with reference to the accompanying drawings. Note that the drawings shown below may be drawn at a scale different from the actual scale in order to facilitate understanding of the present embodiment.

[0009] 2. Description of the Related Art In an optical scanning device used in an image forming apparatus such as a laser beam printer or a digital copier, a light beam emitted from a light source is guided to a deflector by an incident optical system. The light beam deflected and scanned by the deflector is then focused by an imaging optical system into a spot on a photosensitive drum surface disposed at the position of the surface to be scanned, and the photosensitive drum surface is optically scanned by the light beam.

[0010] In such an optical scanning device, a light beam emitted from a light source is converted into a substantially parallel light beam by a collimator lens or the like, and then the light is focused near a deflection surface of a deflector by a cylindrical lens to form a line image. The light beam deflected by the deflecting surface of the deflector is condensed by a scanning lens so as to form a spot on the surface of the photosensitive drum, and the photosensitive drum surface is scanned at a substantially constant speed by, for example, rotating the deflector.

[0011] FIG. 11 shows a schematic main-scanning sectional view of a conventional optical scanning device 900. As shown in FIG. In the optical scanning device 900, a light source 901 is composed of, for example, a semiconductor laser, and a collimator lens 902 converts a divergent light beam emitted from the light source 901 into a substantially parallel light beam. The cylindrical lens 903 has a predetermined refractive power only in the sub-scanning cross section, and by concentrating the light beam that has passed through the collimator lens 902 in the sub-scanning cross section, an approximately linear image is formed on the deflection surface (deflection reflection surface) 905a of the deflector 905.

[0012] The diaphragm 904 is, for example, a slit member, and limits the diameter of the light beam that has passed through the cylindrical lens 903 . Deflector 905 is configured, for example, by a polygon mirror (rotating polygonal mirror) having a plurality of deflection surfaces 905a, and is rotated in the direction of arrow R in the figure by a driving means (not shown) such as a motor.

[0013] The fθ lens group 906 is composed of a first lens group 906a having refractive power mainly in the main scanning cross section, and a second lens group 906b having refractive power mainly in the sub-scanning cross section. An fθ lens group 906 guides the light beam deflected by the deflector 905 onto a surface 907 to be scanned, thereby forming a spot on the surface 907 to be scanned. In addition, the fθ lens group 906 sets the deflecting surface 905a and the scanned surface 907 in a substantially conjugate relationship in the sub-scanning cross section, thereby reducing the image position shift caused by the tilt of the deflecting surface 905a, i.e., a surface tilt correction optical system is configured.

[0014] After being deflected in a predetermined direction by the deflector 905 , the light beam passes through the first lens group 906 a and is reflected by the mirror 908 toward the synchronization detection sensor 910 . Then, a synchronous detection imaging lens 909 focuses the light beam reflected by the mirror 908 onto a synchronous detection sensor 910 .

[0015] In addition, in the optical scanning device 900 as described above, it is known to use a resin polygon mirror as the deflector 905. A problem with resin polygon mirrors is that it is difficult to manufacture high-quality resin polygon mirrors that have high environmental durability at low cost. That is, in order to manufacture a high-quality resin polygon mirror, highly accurate molding technology and high-quality film formation technology are necessary.

[0016] In particular, problems arise when manufacturing a high-quality resin polygon mirror at low cost using deposition techniques. In other words, when depositing a film by arranging a large number of polygon mirrors in a film depositing apparatus, it is difficult to stabilize the difference in reflectance among multiple deflection surfaces and the distribution of reflectance within each deflection surface so that the reflectances of the light beams incident on each deflection point on the deflection surface in the installed optical scanning device are approximately constant, while maintaining the film adhesion strength.

[0017] Therefore, the inventors of the present application have conducted extensive research as described below, and as a result have found a method for producing inexpensive, high-quality polygon mirrors that are suitable for mass production by solving the above problems. This makes it possible to suppress the occurrence of light quantity distribution on the scanned surface without using a folding mirror, and also makes it possible to provide an inexpensive polygon mirror for a high-definition optical scanning device having a simple film configuration that can be deposited using a film deposition apparatus.

[0018] FIG. 1 shows a schematic main-scanning cross-sectional view of an optical scanning device 100 including a polygon mirror 5 according to the first embodiment.

[0019] The optical scanning device 100 includes a light source 1, a diaphragm 2, an anamorphic collimator lens 3, a deflector 5, and a scanning lens 6. The optical scanning device 100 also includes a mirror (not shown).

[0020] The light source 1 is composed of a semiconductor laser having 41 light emitting points. The diaphragm 2 is an aperture diaphragm having an elliptical opening, and regulates the width of the light beam emitted from the light source 1 in the main scanning direction and the sub-scanning direction.

[0021] The anamorphic collimator lens 3 converts the light beam that has passed through the diaphragm 2 into a weakly convergent light beam in the main scanning direction and into a convergent light beam in the sub-scanning direction. That is, the anamorphic collimator lens 3 is configured as an anamorphic lens having different powers in the main scanning section and the sub-scanning section. The anamorphic collimator lens 3 is made of a plastic mold.

[0022] As described above, in the optical scanning device 100, the diaphragm 2 and the anamorphic collimator lens 3 form the incident optical system 75.

[0023] In the optical scanning device 100, the deflector 5 is a four-sided polygon mirror with a circumscribed circle diameter of 20 mm, and is rotated at a constant speed in the direction of arrow R by a driving means such as a motor (not shown), thereby deflecting and scanning the light beam that has passed through the incident optical system 75. Note that an apparatus composed of the deflector 5, which is a polygon mirror, and a driving means (driving unit) can also be called a deflection device.

[0024] The scanning lens 6 is an fθ lens having approximately fθ characteristics, and guides the light beam deflected by the deflector 5 onto the surface to be scanned , thereby forming a spot in the image area of ​​the surface to be scanned . In addition, the scanning lens 6 has a substantially conjugate relationship between the deflecting surface (reflecting surface) 5a of the deflector 5 or its vicinity and the scanned surface 7 or its vicinity in the sub-scanning cross section, thereby correcting the inclination of the deflecting surface 5a of the deflector 5. In the optical scanning device 100, the scanning lens 6 constitutes an imaging optical system 85.

[0025] With the above-described configuration, in the optical scanning device 100, when the light beam emitted from the light source 1 after being optically modulated in accordance with image information passes through the aperture 2, the width of the light beam in the main scanning direction and the sub-scanning direction is limited by the opening of the aperture 2. The light beam that passes through the aperture 2 is converted by the anamorphic collimator lens 3 into a light beam having different convergence degrees in the main scanning cross section and the sub-scanning cross section, and is condensed so that an approximately focal line image (a line image that is long in the main scanning direction) is formed on the deflection surface 5a of the deflector 5.

[0026] The light beam reflected and deflected by the deflection surface 5a of the deflector 5 is focused into a spot on the scanned surface 7 by the scanning lens 6, and by rotating the deflector 5 in the direction of arrow R, the scanned surface 7 is optically scanned at an approximately constant speed in the main scanning direction. As a result, an image is recorded on the surface of a photosensitive drum serving as a recording medium, which is disposed at the position of the surface 7 to be scanned.

[0027] When optically scanning the surface to be scanned 7, it is necessary to determine the timing for starting scanning on the surface to be scanned 7. Therefore, in the optical scanning device 100, the light beam reflected and deflected in a predetermined direction by the deflector 5 is guided onto a BD sensor (not shown) by a BD lens (not shown). Then, a control unit (not shown) uses a synchronization signal (BD signal) obtained by detecting the output signal from the BD sensor to determine the emission timing of the light emitting point of the light source 1 for starting scanning to record an image on the scanned surface 7.

[0028] Next, the configuration of the polygon mirror 5 according to this embodiment will be described. The polygon mirror 5 according to this embodiment used as the deflector 5 of the optical scanning device 100 is made by injection molding using a plastic material (specifically, for example, K22R manufactured by Zeon Corporation). This makes it possible to produce the reflecting surface 5a of the polygon mirror 5 according to this embodiment with high precision and at low cost.

[0029] Moreover, the film formation on the reflective surface 5a of the polygon mirror 5 according to this embodiment is performed by vacuum deposition, which allows film formation to be performed simultaneously on a large number of polygon mirrors 5 at once, thereby reducing the film formation processing cost per polygon mirror. Furthermore, since a commercially available general-purpose deposition device can be used, investment can be reduced.

[0030] 2(a) and (b) show a schematic cross-sectional view and a schematic partially enlarged cross-sectional view of a vacuum deposition apparatus 300 for forming a film on the reflecting surface 5a of the polygon mirror 5 according to this embodiment.

[0031] The vacuum deposition apparatus 300 is provided with a film formation chamber 301 capable of maintaining the inside in a vacuum state, and an exhaust system 302 including a vacuum pump and the like for exhausting the film formation chamber 301 . In the film formation chamber 301 , a revolving part 304 that can be driven to revolve around a revolution axis 303 is disposed, and the revolving part 304 is driven to revolve by a drive mechanism 305 via a gear 306 .

[0032] A plurality of polygon mirrors 5 are stacked and arranged so that a rotation axis part 307 provided in the film formation chamber 301 passes through a hole formed in the center. The rotation axis part 307 on which the multiple polygon mirrors 5 are thus arranged is installed inclined to form an angle Sθ with respect to the revolution part 304, and the rotation axis part 307 is driven to rotate by a drive mechanism (not shown). In this manner, the revolving part 304 is driven to revolve and the rotation axis part 307 is driven to rotate, so that a film is formed on the reflecting surface 5a while the polygon mirror 5 is driven to revolve.

[0033] The vacuum deposition apparatus 300 is also provided with a liner 308 , an ion gun 309 , and a quartz crystal film thickness sensor 310 . Furthermore, the film formation chamber 301 is provided with an argon introduction line and an oxygen introduction line (not shown) for introducing oxygen gas. The liner 308 is disposed at a position spaced a distance OFS from the revolution axis 303 and a height SL from the polygon mirror 5 .

[0034] As shown in FIG. 2(b), the multiple polygon mirrors 5 are arranged along the axial direction of the rotation axis part 307 such that the surface 35 perpendicular to the multiple reflecting surfaces 5a faces upward and are spaced apart from each other by a gap G via the spacer 311. In the vacuum deposition apparatus 300, the gap G is formed using the spacer 311, but the invention is not limited to this and a step may be provided in the polygon mirror 5. In this case, the spacer 311 is not required, and the number of parts can be reduced.

[0035] FIG. 2(c) shows the polygon mirror 5 attached to the rotation axis part 307 in the vacuum deposition apparatus 300 as viewed from the front in the axial direction.

[0036] As shown in FIG. 2(c), slit-shaped shielding plates 120a and 120b are provided between the polygon mirror 5 and the liner 308, spaced apart from each other by a distance d1. The shielding plates 120a and 120b are disposed in the film forming chamber 301 at a distance d2 in a direction perpendicular to the reflecting surface 5a facing the polygon mirror 5. Furthermore, the positional relationship between the rotation axis part 307 and the shielding plates 120a and 120b does not change during film formation.

[0037] The vacuum deposition apparatus 300 is provided with shielding plates 120a and 120b in order to suppress the formation of a so-called oblique incidence film, which is formed when deposition particles are deposited on the reflecting surface 5a of the polygon mirror 5 at a large angle of incidence. If an oblique incidence film is formed on the reflecting surface 5a of the polygon mirror 5, the film density will be small, and the metal film will corrode in the atmosphere, lowering its environmental durability. Therefore, by decreasing the interval d1 and increasing the distance d2, the formation of the oblique incidence film can be suppressed, and a reflective film with high environmental durability can be formed on the reflective surface 5a of the polygon mirror 5.

[0038] On the other hand, when the interval d1 is reduced and the distance d2 is increased in this manner, the deposition particles are blocked more toward the ends (extreme ends) of the reflecting surface 5a of the polygon mirror 5 in the longitudinal direction. Therefore, the physical thickness of the reflective film formed on the reflective surface 5a becomes thinner toward the ends in the longitudinal direction, resulting in a distribution of the physical thickness of the reflective film in the longitudinal direction. The distribution of the physical film thickness of the reflective film in the longitudinal direction generates a reflectance distribution on the reflective surface 5a of the polygon mirror 5, and by mounting such a polygon mirror 5 on the optical scanning device 100, the amount of light at each image height on the scanned surface 7 will no longer be uniform.

[0039] That is, with the conventional inexpensive processing methods as described above, it has been difficult to achieve both environmental durability of the reflecting surface 5a of the polygon mirror 5 and uniformity in the light amount distribution on the surface to be scanned . As a result of extensive research, the inventors of the present application have discovered a film configuration for forming a high-quality reflective surface 5a that has high environmental durability, as well as a method for processing a polygon mirror 5 having such a reflective surface 5a, as described below.

[0040] Specifically, when processing the polygon mirror 5 according to this embodiment, in order to ensure environmental durability by suppressing the formation of an oblique incidence film on the reflecting surface 5a, a multilayer film was formed on the reflecting surface 5a of the polygon mirror 5 while setting the interval d1 and distance d2 to 16 mm and 5 mm, respectively.

[0041] FIG. 3 shows the distribution of the physical thickness of the multilayer film on the reflecting surface 5a of the polygon mirror 5 on which the multilayer film is formed in this manner, specifically, the longitudinal position dependency of the physical thickness of the multilayer film on the reflecting surface 5a. With regard to the arrangement of the shielding plates 120a and 120b, the polygon mirror 5 achieved the same high environmental durability on the reflecting surface 5a was obtained regardless of whether the interval d1 was 14 mm to 20 mm or the distance d2 was 0 mm to 10 mm. In addition, in FIG. 3, the physical thickness of the multilayer film at the center (0 mm) in the longitudinal direction of the reflecting surface 5a is standardized as 1.

[0042] In addition, the film materials of the multilayer film on the reflecting surface 5a of the polygon mirror 5 in this embodiment are, but are not limited to, the metallic material Al, the low refractive index material SiO2, and the high refractive index material Ta2O5.

[0043] When a multilayer film is formed on the reflecting surface 5a, the pressure in the film forming chamber 301 is 1.3×10 -2 After exhausting the inside of the chamber to a pressure of 10 Pa using the exhaust system 302, the above-mentioned materials are deposited on the reflecting surface 5a using the liner 308. The deposition rates are controlled by a quartz crystal thickness sensor 310, and are specifically set to 2.5 nm / sec, 1.6 nm / sec, and 0.4 nm / sec for the metallic material Al, the low refractive index material SiO2, and the high refractive index material Ta2O5, respectively. In addition, when depositing the low refractive index material SiO2 and the high refractive index material Ta2O5, ion assist is performed by the ion gun 309.

[0044] Next, the physical film thickness of each layer of the multilayer film at the center (0 mm) in the longitudinal direction of the reflecting surface 5a of the polygon mirror 5 according to this embodiment, that is, at the center, is shown in Table 1 below. In addition, in the multilayer film formed on the reflecting surface 5a of the polygon mirror 5 in this embodiment, a layer of low refractive index material SiO2 is provided between the substrate and the first layer as a base layer (adhesion layer), but this is omitted here.

[0045] [Table 1]

[0046] As shown in Table 1, on the reflecting surface 5a of the polygon mirror 5 in this embodiment, a multilayer film is formed on a substrate made of resin (K22R manufactured by Zeon Corporation), so that a first layer formed of a metal material Al, a second layer formed of a low refractive index material SiO2, a third layer formed of a high refractive index material Ta2O5, and a fourth layer formed of a low refractive index material SiO2 are stacked in this order. The first to third layers constitute a reflective layer, and the fourth layer constitutes a protective layer. That is, the reflecting surface 5a of the polygon mirror 5 according to this embodiment has a multilayer film in which a plurality of layers, three or more layers including a base layer, a reflecting layer, and a protective layer, are laminated.

[0047] The physical film thickness of each layer shown in Table 1 indicates the thickness of the film formed on each layer. In the polygon mirror 5 according to this embodiment, the physical film thickness of each layer of the multilayer film on the reflective surface 5a is set to be different between the center and the ends in the longitudinal direction, thereby making the reflectance different from each other.

[0048] FIG. 4 shows a schematic perspective view of the polygon mirror 5 according to the present embodiment. As shown in FIG. 4, the polygon mirror 5 is composed of four rectangular reflecting surfaces 5a for reflecting (deflecting) a light beam, and an upper surface 5u and a lower surface 5d perpendicular to the reflecting surfaces 5a. For convenience, the longitudinal direction of reflecting surface 5a is referred to as the Y direction (Y axis) and the lateral direction as the Z direction (Z axis), with the center of reflecting surface 5a being the origin for the Y and Z directions.

[0049] FIG. 5 shows the incidence angle dependency of the reflectance on the reflecting surface 5a of the polygon mirror 5 according to this embodiment. The incidence angle here refers to the angle that the incident direction of the light beam incident on the reflecting surface 5a of the polygon mirror 5 in the optical scanning device 100 makes with respect to the normal to the reflecting surface 5a in the main scanning cross section (within a plane parallel to the normal to the reflecting surface 5a and the longitudinal direction).

[0050] The incidence angle dependence of reflectance is shown at longitudinal positions of Y=0 mm, Y=-5.3 mm, and Y=-2.7 mm. Here, the longitudinal position of Y=0 mm corresponds to the longitudinal center, that is, the center, of the reflecting surface 5a. Also, the longitudinal position of Y=-5.3 mm corresponds to the reflection point (hereinafter referred to as the most off-axis reflection point) when reflecting the marginal ray on the writing end side of the light beam (hereinafter referred to as the most off-axis light beam) to the most off-axis image height on the writing end side (scanning end side) on the reflecting surface 5a, as shown in Fig. 6. In other words, the longitudinal position of Y=-5.3 mm is the reflection point of the marginal ray of the most off-axis light beam that is farthest from the center (Y=0 mm) in the longitudinal direction, as shown in Fig. 6.

[0051] The longitudinal position of Y=-2.7 mm corresponds to a reflection point located approximately midway between the above two points in the longitudinal direction. The incident light beam is P-polarized light and has a wavelength of 790 nm.

[0052] As shown in FIG. 3, in the polygon mirror 5 according to this embodiment, the physical film thickness of the multilayer film on the reflecting surface 5a is symmetrical between the positive side and the negative side in the longitudinal direction. That is, for the incidence angle dependency of the reflectance at a longitudinal position on the positive side of the reflecting surface 5a, the incidence angle dependency of the reflectance at a longitudinal position on the negative side can be used as is. Therefore, FIG. 5 only shows the incidence angle dependency of the reflectance at a longitudinal position on the negative side from Y=0 mm. In other words, in the polygon mirror 5 according to this embodiment, the change in reflectance for a light beam incident on the reflecting surface 5a in the longitudinal direction from the center to one end is the same as the change from the center to the other end.

[0053] As shown in Figure 5, the polygon mirror 5 of this embodiment is configured so that the incidence angle dependence of the reflectance at the center (Y = 0 mm) of the reflective surface 5a is different from that at the most off-axis reflection point (Y = -5.3 mm). This is achieved by making the physical thickness of each layer of the multilayer film at the center (Y = 0 mm) of the reflecting surface 5a of the polygon mirror 5 shown in Table 1 different from the physical thickness of each layer of the multilayer film at the most off-axis reflection point (Y = -5.3 mm).

[0054] Table 2 shows the physical film thickness of each layer of the multilayer film at the most off-axis reflection point (Y=-5.3 mm) of the reflecting surface 5a of the polygon mirror 5 according to this embodiment.

[0055] [Table 2]

[0056] As shown in Tables 1 and 2, the physical film thickness of each layer of the multilayer film is configured to be approximately 10% thinner at the most off-axis reflection point (Y = -5.3 mm) on the reflecting surface 5a of the polygon mirror 5 compared to the center (Y = 0 mm). Specifically, as shown in FIG. 3, the configuration of vacuum deposition apparatus 300 is devised so that the physical film thickness of each layer is approximately 2% thinner at positions Y=±4 mm compared to the center (Y=0 mm), and approximately 9% thinner at positions Y=±5 mm. That is, in the polygon mirror 5 according to this embodiment, the film thickness of the multilayer film on the reflecting surface 5a becomes thinner from the center to the most off-axis reflecting point along the longitudinal direction.

[0057] In the polygon mirror 5 of this embodiment, as shown in Figure 5, the reflectance at the center of the reflective surface 5a (Y = 0 mm) is configured to be smaller than the reflectance at the most off-axis reflection point (Y = -5.3 mm) at an incident angle of 45°. On the other hand, at an incident angle of 70°, the reflectance at the center (Y=0 mm) of the reflecting surface 5a is configured to be greater than the reflectance at the most off-axis reflection point (Y=-5.3 mm). That is, in the polygon mirror 5 according to this embodiment, the reflectance of the reflective surface 5a for a light beam incident at an incident angle of 70° (first incident angle) decreases from the center to the most off-axis reflection point along the longitudinal direction, whereas the reflectance of the reflective surface 5a for a light beam incident at an incident angle of 45° (second incident angle) increases from the center to the most off-axis reflection point along the longitudinal direction.

[0058] With the above-described configuration, in the optical scanning device 100 equipped with the polygon mirror 5 of this embodiment, the light amounts of the light beams deflected and reflected respectively to the image ends and the image center on the scanned surface 7 can be made approximately uniform.

[0059] In the polygon mirror 5 according to this embodiment, the sum of the physical thicknesses of the layers in the multilayer film at the center of the reflecting surface 5a and the sum of the physical thicknesses of the layers in the multilayer film at the most off-axis reflection point are denoted by C and D, respectively. At this time, the physical film thickness of each layer is configured to decrease along the longitudinal direction from the center of the reflecting surface 5a toward the most off-axis reflecting point so that the following conditional formula (1) is satisfied. 0.50 <D / C<0.98 ···(1)

[0060] Specifically, it is found from Tables 1 and 2 that C=488.5 nm and D=440 nm, and therefore D / C=0.90, which satisfies conditional expression (1).

[0061] Moreover, it is preferable that the polygon mirror 5 according to this embodiment satisfies the following conditional formula (1a). 0.75 <D / C<0.93 ···(1a)

[0062] FIG. 6 shows the angles of incidence of light beams incident on the reflecting surface 5a at various positions in the longitudinal direction in an optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment. The error bars in FIG. 6 indicate the width of the light beam incident on each longitudinal position. That is, the center of the error bar (black circle) indicates the longitudinal position at which the principal ray of each light beam is incident, and both ends of the error bar indicate the longitudinal positions at which the marginal ray of each light beam is incident. In addition, the deflection point when the light beam is deflected to the axial image height on the scanned surface 7 corresponds to the longitudinal position of Y=+1.3 mm on the reflecting surface 5a, and the positive and negative sides of this position correspond to the groups of deflection points when the light beam is deflected to the writing start side (scanning start side) and writing end side (scanning end side) on the scanned surface 7, respectively.

[0063] As shown in Figure 6, for a light beam deflected to the image end (most off-axis image height) on the writing end side on the scanned surface 7, the chief ray is deflected at the longitudinal position Y = -2.2 mm of the reflecting surface 5a of the polygon mirror 5. The marginal rays are deflected at the longitudinal positions of Y=-5.5 mm and Y=+0.9 mm on the reflecting surface 5a of the polygon mirror 5, respectively, and the angle of incidence of the light beams on the reflecting surface 5a is 70°.

[0064] Furthermore, the light beam deflected to the image center (axial image height) on the scanned surface 7 has an incident angle of 45° on the reflecting surface 5a, and is deflected within the longitudinal position range of Y=-0.2 mm to +2.8 mm on the reflecting surface 5a of the polygon mirror 5. Similarly, the light beam deflected to the image end portion (the most off-axis image height) on the side where writing begins on the scanned surface 7 has an incident angle of 20° on the reflecting surface 5a, and is deflected within the longitudinal position range of Y=+2.7 mm to +4.9 mm on the reflecting surface 5a of the polygon mirror 5.

[0065] As shown in Figure 5, in an optical scanning device 100 equipped with a polygon mirror 5 according to this embodiment, the reflectance of a light beam incident on the reflective surface 5a of the polygon mirror 5 at an incident angle of 70° is configured to decrease from the center to the most off-axis reflection point along the longitudinal direction.

[0066] Specifically, the reflectances at the center and the most off-axis reflection point of the reflecting surface 5a of the polygon mirror 5 for a light beam incident at an incident angle of 70° (predetermined incident angle) are designated as A and B, respectively. At this time, the polygon mirror 5 according to this embodiment satisfies the following conditional expression (2). 0.02<|1-B / A|<0.10 (2) In the polygon mirror 5 according to this embodiment, it is found from FIG. 5 that A=85% and B=79%, so |1-B / A|=0.07 and conditional expression (2) is satisfied.

[0067] In the polygon mirror 5 according to this embodiment, it is preferable that the following conditional formula (2a) be satisfied. 0.05≦|1-B / A|≦0.09 (2a) In the polygon mirror 5 of this embodiment, conditional formula (2a) is satisfied, thereby improving the freedom in designing the multilayer film on the reflecting surface 5a, and in particular, reducing the physical film thickness of at least one layer can achieve cost reduction.

[0068] As shown in FIG. 5, the polygon mirror 5 according to this embodiment has a film configuration that satisfies conditional expression (2) for a light beam that is P-polarized and has a wavelength of 790 nm and has an incident angle of 70° on the reflecting surface 5a. However, the present invention is not limited to this, and the film configuration may be designed so that conditional expression (2) is satisfied for light beams incident on the reflecting surface 5a at angles of incidence of 20° and 45°.

[0069] The polygon mirror 5 according to this embodiment is configured so that, for a light beam incident on the reflecting surface 5a at an incident angle of 35°, the reflectance increases from the center to the most off-axis reflection point along the longitudinal direction. In other words, in the polygon mirror 5 of this embodiment, the difference in reflectance due to the difference in incident angle between the light beam incident on the reflective surface 5a at an incident angle of 45° and the light beams incident at incident angles of 20° and 70° is offset by the difference in reflectance due to the difference in incident position. This makes it possible to make the reflectance of the reflective surface 5a approximately uniform between the light beam that enters the reflective surface 5a of the polygon mirror 5 at an incidence angle of 45° so as to be deflected and reflected toward the center of the image on the scanned surface 7, and the light beam that enters the reflective surface 5a of the polygon mirror 5 at incidence angles of 20° and 70° so as to be deflected and reflected toward both ends of the image.

[0070] As described above, the polygon mirror 5 according to this embodiment is configured so that the incidence angle dependence of the reflectance differs between the center of the reflecting surface 5a and the most off-axis reflecting point, but this is not limiting. In other words, a similar effect can be obtained if the reflectance A at the center of the reflecting surface 5a for a light beam incident at a predetermined incident angle and the reflectance B at a predetermined point between the center and the end portion in the longitudinal direction of the reflecting surface 5a satisfy conditional formula (2).

[0071] Here, when the distance from the center to the end of the reflecting surface 5a is L and the distance from the center to the specified point is s, the polygon mirror 5 according to this embodiment satisfies the following conditional formula (3). 0.2≦s / L≦0.8 (3) When the diameter of the circumscribing circle of polygon mirror 5 in the main scanning section is R and the number of reflecting surfaces 5a of polygon mirror 5 is N, the distance L can be expressed as R / 2×sin(π / N).

[0072] If the lower limit of conditional formula (3) is not reached, when the polygon mirror 5 is mounted on the optical scanning device 100, it becomes difficult to change the physical film thickness of each layer along the longitudinal direction on the reflecting surface 5a so as to satisfy conditional formula (2). On the other hand, if the upper limit of conditional expression (3) is exceeded, when the polygon mirror 5 is mounted on the optical scanning device 100, it becomes difficult to provide a deflection point on the reflecting surface 5a for reflecting and deflecting the light beam to the BD sensor.

[0073] In the polygon mirror 5 according to this embodiment, it is preferable that the following conditional formula (3a) be satisfied. 0.5≦s / L≦0.8 (3a) In addition, in the polygon mirror 5 of this embodiment, R = 20 mm and N = 4, so that L = 7.1 mm is obtained, and as shown in Figure 6, s = 5.5 mm is obtained, so that s / L = 0.77, and conditional formula (3) is satisfied.

[0074] FIG. 7 shows the average reflectance for a light beam incident on each longitudinal position on the reflecting surface 5a of the optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment. The average reflectance here refers to a value obtained by averaging the reflectance at each longitudinal position included in the beam width (see Figure 6) for the beam incident on each longitudinal position on the reflecting surface 5a of the polygon mirror 5. The incident light beam is P-polarized light and has a wavelength of 790 nm.

[0075] As described above, in the optical scanning device 100 equipped with the polygon mirror 5 of this embodiment, the incident position, incident angle, and light beam width on the reflecting surface 5a of the polygon mirror 5 of the light beam deflected and reflected to each image printing position on the scanned surface 7 are different from each other. In order to offset such differences, the reflectance is changed by varying the physical film thickness of each layer of the multilayer film on the reflective surface 5a of the polygon mirror 5 according to the longitudinal position, thereby making the average reflectance for each light beam at each longitudinal position on the reflective surface 5a approximately uniform. In other words, in the polygon mirror 5 according to this embodiment, between two given reflection points on the reflecting surface 5a, the change in reflectance caused by the change in the angle of incidence of the light beam and the change in reflectance caused by the change in the position of incidence cancel each other out.

[0076] Specifically, as shown in FIG. 7, the average reflectance is configured to be approximately uniform between 83% and 84% at each longitudinal position on the reflecting surface 5a of the polygon mirror 5 when the light beam is deflected and reflected toward the entire area from the most off-axis image height on the scanning start side to the most off-axis image height on the scanning end side.

[0077] As described above, in the polygon mirror 5 of this embodiment, the film configuration of the multilayer film on the reflective surface 5a, specifically the physical film thickness of each layer of the multilayer film, is set to change in the longitudinal direction so that the average reflectance of the light beam deflected and reflected to each image printing position on the scanned surface 7 becomes approximately uniform when the polygon mirror 5 is mounted on the optical scanning device 100. This makes it possible to obtain a polygon mirror 5 that, when mounted on the optical scanning device 100, can achieve high deflection performance at low cost, which can uniform the light amount distribution in the main scanning direction on the scanned surface 7.

[0078] [Second embodiment] Next, the configuration of the polygon mirror 5 according to the second embodiment will be described. Note that since the polygon mirror 5 according to this embodiment is configured from the same members as the polygon mirror 5 according to the first embodiment, the same members are denoted by the same reference numerals and descriptions thereof will be omitted.

[0079] As will be described below, the physical film thickness of each layer of the multilayer film on the reflecting surface 5a is different between the polygon mirror 5 according to this embodiment and the polygon mirror 5 according to the first embodiment. Table 3 shows the physical film thickness of each layer of the multilayer film at the center of the reflecting surface 5a of the polygon mirror 5 according to this embodiment. In addition, in the multilayer film formed on the reflecting surface 5a of the polygon mirror 5 in this embodiment, a layer of low refractive index material SiO2 is provided between the substrate and the first layer as a base layer (adhesion layer), but this is omitted here.

[0080] [Table 3]

[0081] Table 4 shows the physical film thickness of each layer of the multilayer film at the most off-axis reflection point of the reflecting surface 5a of the polygon mirror 5 according to this embodiment.

[0082] [Table 4]

[0083] Specifically, as shown in Tables 3 and 4, in the polygon mirror 5 of this embodiment, the physical film thicknesses of the third layer (high refractive index material Ta2O5) and the second and fourth layers (low refractive index material SiO2) on the reflecting surface 5a are changed compared to the polygon mirror 5 of the first embodiment. In particular, in the polygon mirror 5 of this embodiment, the physical film thickness of the third layer (high refractive index material Ta2O5), which has a low film formation rate, is made thinner than that of the polygon mirror 5 of the first embodiment, thereby shortening the film formation time and thereby achieving low costs.

[0084] FIG. 8(a) shows the incidence angle dependency of the reflectance on the reflecting surface 5a of the polygon mirror 5 according to this embodiment. Specifically, the incidence angle dependency of the reflectance is shown at Y=0 mm corresponding to the center of the reflecting surface 5a, at Y=-5.3 mm corresponding to the most off-axis reflection point, and at Y=-2.7 mm corresponding to the intermediate portion. The incident light beam is P-polarized light and has a wavelength of 790 nm.

[0085] In the polygon mirror 5 of this embodiment, by setting the physical film thickness of each layer of the multilayer film on the reflecting surface 5a as shown in Tables 3 and 4, it is possible to make the incidence angle dependence of the reflectance different between the center (Y = 0 mm) of the reflecting surface 5a and the most off-axis reflection point (Y = -5.3 mm) as shown in Figure 8 (a).

[0086] 8(b) shows the longitudinal position dependency of the physical thickness of the multilayer film on the reflecting surface 5a of the polygon mirror 5 according to this embodiment. Note that in FIG. 8(b), the physical thickness of the multilayer film at the center (Y=0 mm) of the reflecting surface 5a is normalized to 1. FIG. 8(c) shows the average reflectance for a light beam incident on each longitudinal position on the reflecting surface 5a of the optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment.

[0087] In the optical scanning device 100 equipped with the polygon mirror 5 of this embodiment, as shown in Figure 6, the incident position, incident angle, and light beam width on the reflecting surface 5a of the polygon mirror 5 of the light beam deflected and reflected to each image printing position on the scanned surface 7 are different from each other. In order to offset such differences, the reflectance is changed by varying the physical film thickness of each layer of the multilayer film on the reflecting surface 5a of the polygon mirror 5 according to the longitudinal position as shown in Figure 8(b), thereby making the average reflectance on the reflecting surface 5a for each light beam approximately uniform.

[0088] Specifically, as shown in FIG. 8(c), the average reflectance is configured to be approximately uniform between 79.5% and 82% at each longitudinal position on the reflecting surface 5a of the polygon mirror 5 when the light beam is deflected and reflected toward the entire area from the most off-axis image height on the scanning start side to the most off-axis image height on the scanning end side.

[0089] In the polygon mirror 5 of this embodiment, as can be seen from Figure 8 (b), the longitudinal position dependence of the physical thickness of the multilayer film is set so that the physical thickness of the multilayer film is approximately 10% thinner at the most off-axis reflection point (Y = -5.3 mm) compared to the center (Y = 0 mm), i.e., so that the physical thickness of the multilayer film becomes thinner from the center to the most off-axis reflection point.

[0090] From Tables 3 and 4, the total value C of the physical thicknesses of the layers of the multilayer film at the center of the reflecting surface 5a and the total value D of the physical thicknesses of the layers of the multilayer film at the most off-axis reflection point are found to be 486.4 nm and 438 nm, respectively. Therefore, since D / C=0.9, the polygon mirror 5 according to this embodiment also satisfies conditional expression (1).

[0091] Furthermore, in the optical scanning device 100 equipped with the polygon mirror 5 of this embodiment, as shown in Figure 8(a), the reflectance of the light beam incident on the reflective surface 5a of the polygon mirror 5 at an incident angle of 70° is configured to decrease from the center to the most off-axis reflection point. The reflectivities A and B for a light beam incident at the center and the most off-axis reflection point of the reflective surface 5a of the polygon mirror 5 at an incident angle of 70° are calculated to be 82% and 74.5%, respectively, so |1-B / A|=0.09, and conditional expression (2) is satisfied.

[0092] As described above, the polygon mirror 5 according to this embodiment has a film configuration that satisfies conditional expression (2) for a light beam that is P-polarized and has a wavelength of 790 nm and that has an incident angle of 70° on the reflecting surface 5a. However, the present invention is not limited to this, and the film configuration may be designed so that conditional expression (2) is satisfied for light beams incident on the reflecting surface 5a at angles of incidence of 20° and 45°.

[0093] On the other hand, the polygon mirror 5 according to this embodiment is configured so that for a light beam incident on the reflecting surface 5a at an incident angle of 35°, the reflectance increases from the center to the most off-axis reflection point. In other words, in the polygon mirror 5 of this embodiment, the difference in reflectance due to the difference in incident angle between the light beam incident on the reflective surface 5a at an incident angle of 45° and the light beams incident at incident angles of 20° and 70° is offset by the difference in reflectance due to the difference in incident position. This makes it possible to make the reflectance of the reflective surface 5a approximately uniform between the light beam that enters the reflective surface 5a of the polygon mirror 5 at an incidence angle of 45° so as to be deflected and reflected toward the center of the image on the scanned surface 7, and the light beam that enters the reflective surface 5a of the polygon mirror 5 at incidence angles of 20° and 70° so as to be deflected and reflected toward both ends of the image.

[0094] As described above, in the polygon mirror 5 of this embodiment, the film configuration of the multilayer film on the reflective surface 5a, specifically the physical film thickness of each layer of the multilayer film, is set to change in the longitudinal direction so that the average reflectance of the light beam deflected and reflected to each image printing position on the scanned surface 7 becomes approximately uniform when the polygon mirror 5 is mounted on the optical scanning device 100. Furthermore, by reducing the physical thickness of a layer having a low deposition rate, the deposition time for that layer is shortened. This makes it possible to obtain a polygon mirror 5 that, when mounted on the optical scanning device 100, can achieve high deflection performance that can uniformize the light amount distribution in the main scanning direction on the scanned surface 7 at an even lower cost.

[0095] [Third embodiment] Next, the configuration of the polygon mirror 5 according to the third embodiment will be described. Note that since the polygon mirror 5 according to this embodiment is configured from the same members as the polygon mirror 5 according to the first embodiment, the same members are denoted by the same reference numerals and descriptions thereof will be omitted.

[0096] As will be described below, the physical film thickness of each layer of the multilayer film on the reflecting surface 5a is different between the polygon mirror 5 according to this embodiment and the polygon mirror 5 according to the first embodiment. Table 5 shows the physical film thickness of each layer of the multilayer film at the center of the reflecting surface 5a of the polygon mirror 5 according to this embodiment. In addition, in the multilayer film formed on the reflecting surface 5a of the polygon mirror 5 in this embodiment, a layer of low refractive index material SiO2 is provided between the substrate and the first layer as a base layer (adhesion layer), but this is omitted here.

[0097] [Table 5]

[0098] Table 6 shows the physical film thickness of each layer of the multilayer film at the most off-axis reflection point of the reflecting surface 5a of the polygon mirror 5 according to this embodiment.

[0099] [Table 6]

[0100] Specifically, as shown in Tables 5 and 6, in the polygon mirror 5 of this embodiment, the physical film thicknesses of the third layer (high refractive index material Ta2O5) and the second and fourth layers (low refractive index material SiO2) on the reflecting surface 5a are changed compared to the polygon mirror 5 of the first embodiment. In particular, in the polygon mirror 5 of this embodiment, the physical film thickness of the second and fourth layers (low refractive index material SiO2) is made thicker than that of the polygon mirror 5 of the first embodiment in order to improve the environmental durability of the reflective surface 5a, thereby achieving higher durability performance.

[0101] FIG. 9(a) shows the incidence angle dependency of the reflectance on the reflecting surface 5a of the polygon mirror 5 according to this embodiment. Specifically, the incidence angle dependency of the reflectance is shown at Y=0 mm corresponding to the center of the reflecting surface 5a, at Y=-5.3 mm corresponding to the most off-axis reflection point, and at Y=-2.7 mm corresponding to the intermediate portion. The incident light beam is P-polarized light and has a wavelength of 790 nm.

[0102] In the polygon mirror 5 of this embodiment, by setting the physical film thickness of each layer of the multilayer film on the reflective surface 5a as shown in Tables 5 and 6, it is possible to make the reflectance different between the center (Y = 0 mm) and the most off-axis reflection point (Y = -5.3 mm) of the reflective surface 5a, as shown in Figure 9(a).

[0103] 9(b) shows the longitudinal position dependency of the physical thickness of the multilayer film on the reflecting surface 5a of the polygon mirror 5 according to this embodiment. Note that in FIG. 9(b), the physical thickness of the multilayer film at the center of the reflecting surface 5a is normalized to 1. FIG. 9C shows the average reflectance for a light beam incident on each longitudinal position on the reflecting surface 5a of the optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment.

[0104] In the optical scanning device 100 equipped with the polygon mirror 5 of this embodiment, as shown in Figure 6, the incident position, incident angle, and light beam width on the reflecting surface 5a of the polygon mirror 5 of the light beam deflected and reflected to each image printing position on the scanned surface 7 are different from each other. In order to offset such differences, the reflectance is changed by varying the physical film thickness of each layer of the multilayer film on the reflecting surface 5a of the polygon mirror 5 according to the longitudinal position as shown in Figure 9(b), thereby making the average reflectance on the reflecting surface 5a for each light beam approximately uniform.

[0105] Specifically, as shown in FIG. 9(c), the average reflectance is configured to be approximately uniform between 82.8% and 86% at each deflection point on the reflecting surface 5a of the polygon mirror 5 when the light beam is deflected and reflected toward the entire area from the most off-axis image height on the scanning start side to the most off-axis image height on the scanning end side.

[0106] As can be seen from Figure 9(b), in the polygon mirror 5 of this embodiment, the longitudinal position dependence of the physical thickness of the multilayer film is set so that the physical thickness of the multilayer film is approximately 10% thinner at the most off-axis reflection point (Y = -5.3 mm) compared to the center (Y = 0 mm), i.e., so that the physical thickness of the multilayer film becomes thinner from the center to the most off-axis reflection point.

[0107] From Tables 5 and 6, the total value C of the physical thicknesses of the layers of the multilayer film at the center of the reflecting surface 5a and the total value D of the physical thicknesses of the layers of the multilayer film at the most off-axis reflection point are found to be 526.5 nm and 474 nm, respectively. Therefore, since D / C=0.9, the polygon mirror 5 according to this embodiment also satisfies conditional expression (1).

[0108] Furthermore, in the optical scanning device 100 equipped with the polygon mirror 5 of this embodiment, as shown in Figure 9(a), the reflectance of the reflective surface 5a of the polygon mirror 5 for a light beam incident at an incident angle of 70° is configured to increase from the center (Y = 0 mm) to the middle part (Y = -2.7 mm) along the longitudinal direction. The reflectivities A and B for a light beam incident at an incident angle of 70° at the center (Y = 0 mm) and intermediate portion (Y = -2.7 mm) of the reflective surface 5a of the polygon mirror 5 are calculated to be 84.6% and 86.6%, respectively, so that |1-B / A| = 0.024, and conditional formula (2) is satisfied.

[0109] As described above, the polygon mirror 5 according to this embodiment has a film configuration that satisfies conditional expression (2) for a light beam that is P-polarized and has a wavelength of 790 nm and that has an incident angle of 70° on the reflecting surface 5a. However, the present invention is not limited to this, and the film configuration may be designed so that conditional expression (2) is satisfied for light beams incident on the reflecting surface 5a at angles of incidence of 20° and 45°.

[0110] In the polygon mirror 5 of this embodiment, for a light beam incident on the reflective surface 5a at an incident angle of 70°, the reflectivity increases from the center to the middle part along the longitudinal direction, and then decreases from the middle part to the most off-axis reflection point. In other words, in the polygon mirror 5 of this embodiment, by reducing the difference between the incidence angle dependence of the reflectance at the center and the most off-axis reflection point of the reflective surface 5a, the amount of light at each image printing position on the scanned surface 7 can be made approximately uniform.

[0111] As described above, in the polygon mirror 5 of this embodiment, the film configuration of the multilayer film on the reflective surface 5a, specifically the physical film thickness of each layer of the multilayer film, is set to change in the longitudinal direction so that the average reflectance of the light beam deflected and reflected to each image printing position on the scanned surface 7 becomes approximately uniform when the polygon mirror 5 is mounted on the optical scanning device 100. Moreover, the layers that contribute to environmental durability, specifically the second and fourth layers (low refractive index material SiO2) have a large physical film thickness. This makes it possible to obtain a polygon mirror 5 that has high deflection performance that can uniformly distribute the light amount in the main scanning direction on the scanned surface 7 when mounted on the optical scanning device 100, and also achieves higher durability.

[0112] [Monochrome image forming device] FIG. 10A shows a sub-scanning sectional view of a main part of a monochrome image forming apparatus 104 equipped with an optical scanning unit 100 including a polygon mirror 5 according to any one of the first to third embodiments.

[0113] Code data Dc output from an external device 117 such as a personal computer is input to the monochrome image forming apparatus 104. This code data Dc is converted into image data (dot data) Di by a printer controller 111 in the image forming apparatus 104. This image data Di is input to an optical scanning unit 100. Then, a light beam 103 modulated according to the image data is emitted from the optical scanning unit 100, and the photosensitive surface of the photosensitive drum 101 is scanned in the main scanning direction by this light beam 103.

[0114] The photosensitive drum 101, which is an electrostatic latent image carrier (photosensitive member), is rotated clockwise by a motor 115. With this rotation, the photosensitive surface of the photosensitive drum 101 moves in a sub-scanning direction perpendicular to the main scanning direction relative to the light beam 103. A charging roller 102 that uniformly charges the surface of the photosensitive drum 101 is provided above the photosensitive drum 101 so as to come into contact with the surface. The surface of the photosensitive drum 101 charged by the charging roller 102 is irradiated with a light beam 103 scanned by an optical scanning unit 100.

[0115] As described above, the light beam 103 is modulated based on the image data Di, and an electrostatic latent image is formed on the surface of the photosensitive drum 101 by irradiating the photosensitive drum 101 with the light beam 103. This electrostatic latent image is developed into a toner image by a developing device 107 disposed so as to come into contact with the photosensitive drum 101 further downstream in the rotation direction of the photosensitive drum 101 than the irradiation position of the light beam 103.

[0116] The toner image developed by the developing unit 107 is transferred onto a sheet of paper 112, which is a transfer material, by a transfer roller (transfer unit) 108 disposed below the photosensitive drum 101 so as to face the photosensitive drum 101. The sheets of paper 112 are stored in a paper cassette 109 in front of the photosensitive drum 101 (on the right side in FIG. 10(a)), but can also be fed manually. A paper feed roller 110 is disposed at the end of the paper cassette 109, which feeds the sheets of paper 112 in the paper cassette 109 into the transport path.

[0117] The paper 112 onto which the unfixed toner image has been transferred in the above manner is further transported to a fixing device 150 behind the photosensitive drum 101 (on the left side in FIG. 10(a)). The fixing device 150 is composed of a fixing roller 113 having an internal fixing heater (not shown) and a pressure roller 114 arranged so as to be in pressure contact with the fixing roller 113. The paper 112 transported from the transfer section is heated while being pressed at the pressure contact portion between the fixing roller 113 and the pressure roller 114, whereby the unfixed toner image on the paper 112 is fixed. Furthermore, a paper discharge roller 116 is arranged behind the fixing device 150, and the fixed paper 112 is discharged to the outside of the monochrome image forming apparatus 104.

[0118] The printer controller 111 not only converts data, but also controls various parts of the monochrome image forming apparatus 104 such as a motor 115, a polygon motor in the optical scanning unit 100, and the like.

[0119] [Color image forming equipment] FIG. 10B shows a sub-scanning sectional view of a main part of an image forming apparatus 60 equipped with optical scanning devices 61 to 64 each including a polygon mirror 5 according to any one of the first to third embodiments.

[0120] The image forming apparatus 60 is a tandem type color image forming apparatus in which four optical scanning devices are arranged in parallel, each of which records image information on a photosensitive drum surface serving as an image carrier. The image forming apparatus 60 includes optical scanning devices 61, 62, 63, and 64 each including a polygon mirror 5 according to any one of the first to third embodiments, and photosensitive drums 81, 82, 83, and 84 as image carriers. The image forming apparatus 60 also includes developing units 31, 32, 33, and 34, a conveyor belt 51, a printer controller 53, and a fixing unit .

[0121] To the image forming apparatus 60, R (red), G (green), and B (blue) color signals (code data) are input from an external device 52 such as a personal computer. These color signals are converted into C (cyan), M (magenta), Y (yellow), and K (black) image data by a printer controller 53 in the apparatus. These image data are input to optical scanning devices 61, 62, 63, and 64 as image signals and image information. Then, from these optical scanning devices 61, 62, 63, and 64, light beams 71, 72, 73, and 74 modulated according to the image data of each color are emitted. The photosensitive surfaces of photosensitive drums 81, 82, 83, and 84 are scanned in the main scanning direction by these light beams.

[0122] In image forming apparatus 60, for example, a C (cyan) image signal is input to optical scanning device 61, an M (magenta) image signal is input to optical scanning device 62, a Y (yellow) image signal is input to optical scanning device 63, and a K (black) image signal is input to optical scanning device 64. Then, the image signals are recorded in parallel on the photosensitive surfaces of photosensitive drums 81, 82, 83, and 84, respectively, to print a color image at high speed.

[0123] As described above, the image forming apparatus 60 uses light beams based on image data from the four optical scanning devices 61, 62, 63, and 64 to form electrostatic latent images of each color on the photosensitive surfaces of the corresponding photosensitive drums 81, 82, 83, and 84. Thereafter, the electrostatic latent images of each color are developed into toner images of each color by developing devices 31, 32, 33, and 34, and the developed toner images of each color are multi-transferred by a transfer device onto a transfer material conveyed by a conveyor belt 51. The transferred toner images are then fixed by a fixing device 54, forming a single full-color image.

[0124] Furthermore, for example, a color image reading device equipped with a CCD sensor may be used as the external device 52. In this case, the color image reading device and the color image forming device 60 constitute a color digital copying machine. Furthermore, the image forming apparatus 60 is not limited to a configuration of four optical scanning devices and four photosensitive drums. For example, the image forming apparatus 60 may be configured with only one optical scanning device and one photosensitive drum. The image forming apparatus 60 may also be configured with two, three, or five or more optical scanning devices and one photosensitive drum. [Explanation of symbols]

[0125] 5 Polygon mirror 5a Reflective surface

Claims

1. The reflecting surface has a rectangular shape and is adapted to reflect the light beam. When the reflectance at the center of the reflecting surface for a light beam incident at a predetermined incident angle is A, and the reflectance at a predetermined point between the center and an end portion in the longitudinal direction of the reflecting surface for a light beam incident at the predetermined incident angle is B, 0.02<|1-B / A|<0.10 A polygonal mirror characterized by satisfying the following conditions.

2. 2. The polygon mirror according to claim 1, wherein the reflectance of said reflecting surface for a light beam incident at said predetermined angle of incidence decreases from said center to said predetermined point along the longitudinal direction.

3. a reflectance of the reflecting surface for a light beam incident at a first incident angle decreases from the center to the predetermined point along a longitudinal direction; 3. The polygon mirror according to claim 1, wherein the reflectance of said reflecting surface for a light beam incident at the second incident angle increases from said center to said predetermined point along the longitudinal direction.

4. 4. The polygon mirror according to claim 1, wherein a change in the reflectance from the center to one end in the longitudinal direction is identical to a change in the reflectance from the center to the other end.

5. 5. The polygon mirror according to claim 1, wherein the reflecting surface is a surface in which a multilayer film is formed on a substrate.

6. The optical element has a plurality of rectangular reflecting surfaces each having a multilayer film formed on a substrate, the reflecting surfaces reflecting a light beam; When the physical thicknesses of the multilayer film at the center of the reflecting surface and at a predetermined point between the center and an end in the longitudinal direction are C and D, 0.50<D / C<0.98 A polygonal mirror characterized by satisfying the following conditions.

7. 7. The polygon mirror according to claim 6, wherein the physical film thickness decreases from the center to the predetermined point along the longitudinal direction.

8. 8. The polygon mirror according to claim 5, wherein the multilayer film includes an underlayer, a reflective layer, and a protective layer.

9. When the distance from the center of the reflecting surface to the end portion is L and the distance from the center of the reflecting surface to the predetermined point is s, 0.2≦s / L≦0.8 9. The polygon mirror according to claim 1, which satisfies the following condition:

10. A polygon mirror according to any one of claims 1 to 9; and a drive unit that rotates the polygon mirror.

11. a deflection device according to claim 10, which deflects a light beam to scan a surface to be scanned in a main scanning direction; an imaging optical system that guides the light beam deflected by the deflection device to the surface to be scanned;

12. 12. The optical scanning device according to claim 11, wherein the predetermined point is a reflection point of a marginal ray of the most off-axis light beam that is farthest from the center in the longitudinal direction.

13. An optical scanning device as described in claim 11 or 12, characterized in that, between two specified reflection points on the reflection surface, the change in reflectance due to a change in the angle of incidence of the light beam and the change in reflectance due to a change in the position of incidence are in a mutually cancelling relationship.

14. 14. An image forming apparatus comprising: an optical scanning device according to claim 11; a developing unit which develops an electrostatic latent image formed on the scanned surface by the optical scanning device into a toner image; a transfer unit which transfers the developed toner image onto a transfer material; and a fixing unit which fixes the transferred toner image onto the transfer material.

15. 14. An image forming apparatus comprising: the optical scanning device according to claim 11; and a printer controller that converts a signal output from an external device into image data and inputs the image data to the optical scanning device.

Images