Multifaceted 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 polygon mirror and an optical scanning device including the polygon mirror, and is suitable for image forming apparatuses such as laser beam printers, digital copiers, and multifunction printers.

Background Art

[0002] Patent Document 1 discloses an optical scanning device that adjusts the light quantity distribution on a scanned surface by adjusting the reflectance of a plane mirror disposed between a polygon mirror and the scanned surface.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the optical scanning device disclosed in Patent Document 1, since it is necessary to adjust the reflectance of the plane mirror, the degree of freedom in the optical design of the entire device is low. Therefore, an object of the present invention is to provide a polygon mirror that can adjust the light quantity distribution on a scanned surface while improving the degree of freedom in optical design in an optical scanning device.

Means for Solving the Problems

[0005] The polygon mirror according to the present invention has a plurality of rectangular reflecting surfaces that reflect a light beam, and the reflecting surface includes a substrate made of a resin material and a multilayer film provided on the substrate. When the reflectance at the center of the reflecting surface with respect to a first light beam incident at a predetermined incident angle is A, and the reflectance at a predetermined point between the center and the end in the longitudinal direction of the reflecting surface with respect to the first light beam is B, 0.02 < |1 - B / A| < 0.10 satisfies the condition Furthermore, the reflectivity of the reflective surface for a light beam incident at a first angle of incidence decreases as you move from the center to a predetermined point in the longitudinal direction, while the reflectivity of the reflective surface for a light beam incident at a second angle of incidence increases as you move from the center to a predetermined point in the longitudinal direction. and is characterized by this. [Effects of the Invention]

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

[0007] [Figure 1] A schematic cross-sectional view of the main scanning area of ​​an optical scanning device equipped with a multifaceted mirror according to this embodiment. [Figure 2] A schematic diagram of a vacuum deposition apparatus for forming a film on a multifaceted mirror according to this embodiment. [Figure 3] This figure shows the longitudinal position dependence of the physical film thickness on the reflective surface of the polyhedron mirror according to the first embodiment. [Figure 4] A schematic perspective view of the polyhedron according to this embodiment. [Figure 5] This figure shows the dependence of the reflectance at the reflective surface of the polyhedron mirror according to the first embodiment on the angle of incidence. [Figure 6] This figure shows the longitudinal position dependence of the angle of incidence of the light beam onto the reflective surface in the polyhedron mirror according to this embodiment. [Figure 7] This figure shows the average reflectance for light beams incident at each longitudinal position on the reflective surface of the polyhedron mirror according to the first embodiment. [Figure 8] This figure shows the dependence of the reflectance on the angle of incidence and the dependence of the physical film thickness and average reflectance on the longitudinal position of the reflective surface of the polyhedron according to the second embodiment. [Figure 9] This figure shows the dependence of the reflectance on the angle of incidence and the dependence of the physical film thickness and average reflectance on the longitudinal position of the reflective surface of a polyhedron according to the third embodiment. [Figure 10] A sub-scanning cross-sectional view of the main part of the image forming apparatus according to the embodiment. [Figure 11] A schematic cross-sectional view of the main scanning area of ​​a conventional optical scanning device. [Modes for carrying out the invention]

[0008] The polygonal mirror according to this embodiment will be described in detail below based on the attached drawings. Note that the drawings shown below may be drawn to a different scale than the actual dimensions in order to facilitate understanding of this embodiment.

[0009] In optical scanning devices used in image forming equipment such as laser beam printers and digital copiers, the light beam emitted from the light source is guided to a deflector by the incident optical system. The light beam, deflected and scanned by the deflector, is then focused into a spot shape on the surface of the photosensitive drum positioned at the scanning surface by the imaging optical system, thereby causing the surface of the photosensitive drum to be optically scanned by this light beam.

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

[0011] Figure 11 shows a schematic cross-sectional view of the main scanning area of ​​a conventional optical scanning device 900. In the optical scanning device 900, the light source 901 is composed of, for example, a semiconductor laser, and the collimator lens 902 converts the 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 within the sub-scanning cross-section, and by focusing the light beam that has passed through the collimator lens 902 within the sub-scanning cross-section, a substantially linear image is formed on the deflection surface (deflection reflection surface) 905a of the deflector 905.

[0012] The aperture 904 is, for example, a slit member that limits the diameter of the light beam that passes through the cylindrical lens 903. The deflector 905 is constituted by, for example, a polygon mirror (rotating polygon 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 mainly composed of a first lens group 906a having a refractive power in the main scanning cross section and a second lens group 906b having a refractive power in the sub-scanning cross section. Then, the fθ lens group 906 guides the light beam deflected by the deflector 905 onto the scanned surface 907, and a spot is formed on the scanned surface 907. Also, in the fθ lens group 906, the deflection surface 905a and the scanned surface 907 are set in a substantially conjugate relationship in the sub-scanning cross section, and the image formation position shift due to the tilt of the deflection surface 905a is reduced, that is, a tilt correction optical system is configured.

[0014] Also, after being deflected in a predetermined direction by the deflector 905, the light beam that has passed through the first lens group 906a is reflected by the mirror 908 so as to travel toward the synchronous detection sensor 910. Then, the synchronous detection imaging lens 909 condenses the light beam reflected by the mirror 908 onto the synchronous detection sensor 910.

[0015] Also, in the optical scanning device 900 as described above, it is known to use a resin polygon mirror as the deflector 905. In the resin polygon mirror, there is a problem that it is difficult to manufacture a high-quality one with high environmental durability at a low cost. That is, in order to manufacture a high-quality resin polygon mirror, high-precision molding technology and high-quality film-forming technology are required.

[0016] In particular, problems occur when manufacturing a high-quality resin polygon mirror at a low cost using film formation by vapor deposition. In other words, when performing film deposition by arranging a large number of polygon mirrors in a film deposition apparatus, it is difficult to stabilize the difference in reflectivity between multiple deflection surfaces and the distribution of reflectivity within each deflection surface while ensuring film adhesion strength, so that the reflectivity for the light beam incident on each deflection point on the deflection surface is approximately constant for the mounted optical scanning device.

[0017] Therefore, the inventors of this application have conducted extensive research, as described below, and have found a method for creating high-quality polygon mirrors that are highly mass-producible and inexpensive, thereby solving the above-mentioned problems. This makes it possible to suppress the occurrence of light intensity distribution on the scanned surface without using a folded mirror, and to provide an inexpensive polygon mirror for high-resolution optical scanning equipment that has a simple film structure that can be deposited using a film deposition apparatus.

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

[0019] The optical scanning device 100 includes a light source 1, an aperture 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] Light source 1 is composed of a semiconductor laser having 41 light-emitting points. Aperture 2 is an aperture diaphragm with an elliptical opening, which regulates the beam width of the light beam emitted from 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 aperture 2 into a weakly focused light beam in the main scanning direction and a focused light beam in the sub-scanning direction. In other words, the anamorphic collimator lens 3 is composed of anamorphic lenses with different power levels in the primary scanning plane and the secondary scanning plane. The anamorphic collimator lens 3 is also formed from plastic molding.

[0022] As described above, in the optical scanning device 100, the incident optical system 75 is composed of the aperture 2 and the anamorphic collimator lens 3.

[0023] In the optical scanning device 100, the deflector 5 is a four-sided polygon mirror with a circumscribed circle diameter of 20 mm. It rotates 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. The device consisting of this polygon mirror deflector 5 and the driving means (drive 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 scanning surface 7, thereby forming a spot in the image area of ​​the scanning surface 7. Furthermore, the scanning lens 6 maintains a substantially conjugate relationship between the deflection surface (reflection surface) 5a of the deflector 5 or its vicinity and the scanned surface 7 or its vicinity within the sub-scanning cross-section, thereby correcting the tilt of the deflection surface 5a of the deflector 5. In addition, in the optical scanning device 100, the imaging optical system 85 is formed by the scanning lens 6.

[0025] With the above configuration, in the optical scanning device 100, when the light beam emitted from the light source 1 after being modulated according to the 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 has passed through aperture 2 is then converted by anamorphic collimator lens 3 into a light beam having different degrees of convergence in the main scanning cross-section and sub-scanning cross-section, and is focused on the deflection surface 5a of deflector 5 so that a nearly focused image (a line image elongated in the main scanning direction) is formed.

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

[0027] Furthermore, when performing optical scanning on the surface to be scanned 7, it is necessary to determine the timing for starting the scan 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) detects the output signal from the BD sensor and uses the resulting synchronization signal (BD signal) to determine the timing of the light emission point of the light source 1 for starting scanning during image recording 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 a deflector 5 for the optical scanning device 100, is manufactured by injection molding using a plastic material (specifically, for example, K22R manufactured by Zeon Corporation). This makes it possible to create the reflective surface 5a of the polygon mirror 5 according to this embodiment with high precision and at low cost.

[0029] Furthermore, the film deposition on the reflective surface 5a of the polygon mirror 5 according to this embodiment is performed using vacuum deposition. This allows for simultaneous film deposition on a large number of polygon mirrors 5 at once, thereby reducing the film deposition cost per polygon mirror. Furthermore, since commercially available general-purpose deposition equipment can be used, investment costs can also be kept down.

[0030] Figures 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 reflective surface 5a of the polygon mirror 5 according to this embodiment.

[0031] The vacuum deposition apparatus 300 is equipped with a film deposition chamber 301 that can maintain a vacuum state inside, and an exhaust system 302 consisting of a vacuum pump and the like for exhausting the film deposition chamber 301. In the film deposition chamber 301, a revolving component 304 capable of revolving around the revolving axis 303 is positioned, and the revolving component 304 is driven by the drive mechanism 305 via the gear 306.

[0032] Multiple polygon mirrors 5 are stacked and arranged so that a rotating axis component 307, provided in the deposition chamber 301, passes through a hole formed in the center of each mirror. Then, the rotation axis component 307, on which multiple polygon mirrors 5 are arranged in this manner, is installed at an angle Sθ with respect to the revolving component 304, and the rotation of the rotation axis component 307 is driven by a drive mechanism (not shown). In this way, as the orbiting component 304 is driven to orbit and the rotating axis component 307 is driven to rotate, film deposition is performed on the reflective surface 5a while the polygon mirror 5 is driven to orbit and rotate.

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

[0034] Furthermore, as shown in Figure 2(b), the multiple polygon mirrors 5 are arranged along the axial direction of the rotation axis component 307, with surfaces 35 perpendicular to the multiple reflective surfaces 5a facing upward, and spaced apart from each other by a gap G via spacers 311. In the vacuum deposition apparatus 300, a gap G is formed using a spacer 311, but this is not limited to this; a step may also be provided in the polygon mirror 5. In this case, the spacer 311 becomes unnecessary, thus reducing the number of parts.

[0035] Figure 2(c) shows a view of the polygon mirror 5 attached to the rotation axis component 307 in the vacuum deposition apparatus 300, viewed from the axial front.

[0036] As shown in Figure 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. Furthermore, the shielding plates 120a and 120b are positioned in the deposition chamber 301 at a distance d2 from each other in a direction perpendicular to the reflective surface 5a facing the polygon mirror 5. Furthermore, the positional relationship between the rotation axis component 307 and the shielding plates 120a and 120b does not change during film formation.

[0037] In the vacuum deposition apparatus 300, shielding plates 120a and 120b are provided to suppress the formation of so-called oblique incidence films, in which deposition particles are deposited at a large incident angle with respect to the reflective surface 5a of the polygon mirror 5. If a grazing incidence film is formed on the reflective surface 5a of the polygon mirror 5, the film density will decrease, leading to increased corrosion of the metal film in the atmosphere and a reduction in environmental durability. Therefore, by reducing the spacing d1 and increasing the distance d2, the formation of an obliquely incident 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, if the interval d1 is made smaller and the distance d2 is made larger, more deposited particles will be shielded towards the ends (outermost edges) in the longitudinal direction of the reflective surface 5a of the polygon mirror 5. Therefore, the physical thickness of the reflective film formed on the reflective surface 5a becomes thinner towards the edges in the longitudinal direction, resulting in a distribution of the physical thickness of the reflective film in the longitudinal direction. Furthermore, the distribution of the physical film thickness of the reflective film generated in the longitudinal direction creates 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 becomes non-uniform.

[0039] In other words, with the conventional, inexpensive processing methods described above, it was difficult to achieve both environmental durability on the reflective surface 5a of the polygon mirror 5 and uniformity of the light intensity distribution on the scanned surface 7. Therefore, after diligent research, the inventors of this application have found a film structure that has high environmental durability and forms a high-quality reflective surface 5a, as well as a method for processing a polygon mirror 5 having such a reflective surface 5a, as shown below.

[0040] Specifically, when processing the polygon mirror 5 according to this embodiment, in order to ensure environmental durability by suppressing the formation of obliquely incident films on the reflective surface 5a, a multilayer film was deposited on the reflective surface 5a of the polygon mirror 5 while setting the spacing d1 and distance d2 to 16 mm and 5 mm, respectively.

[0041] Figure 3 shows the distribution of the physical film thickness of the multilayer film on the reflective surface 5a of the polygon mirror 5 in which the multilayer film was deposited in this manner, specifically the longitudinal position dependence of the physical film thickness of the multilayer film on the reflective surface 5a. Regarding the arrangement of the shielding plates 120a and 120b, a polygon mirror 5 was obtained in which the same high level of environmental durability was achieved on the reflective surface 5a regardless of whether the spacing d1 was 14 mm to 20 mm or the distance d2 was 0 mm to 10 mm. In Figure 3, the physical film thickness of the multilayer film at the longitudinal center (0 mm) of the reflective surface 5a is normalized to 1.

[0042] Furthermore, while the film materials used for the multilayer film on the reflective surface 5a of the polygon mirror 5 according to this embodiment are a metallic material Al, a low refractive index material SiO2, and a high refractive index material Ta2O5, the invention is not limited to these materials.

[0043] When depositing a multilayer film on the reflective surface 5a, the pressure in the deposition chamber 301 is 1.3 × 10⁻⁶ -2 After exhausting the system 302 to reduce the pressure to Pa, the above materials are deposited onto the reflective surface 5a using the liner 308. Furthermore, the film deposition rate is controlled by a quartz film thickness sensor 310, and is 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. Furthermore, when depositing low refractive index material SiO2 and high refractive index material Ta2O5, ion assistance is provided by the ion gun 309.

[0044] Next, Table 1 below shows the physical film thickness of each layer of the multilayer film at the longitudinal center (0 mm) of the reflective surface 5a of the polygon mirror 5 according to this embodiment. In this embodiment, the multilayer film formed on the reflective surface 5a of the polygon mirror 5 has a layer of low refractive index material SiO2 as an underlayer (adhesion layer) between the substrate and the first layer, but this is omitted here.

[0045] [Table 1]

[0046] As shown in Table 1, on the reflective surface 5a of the polygon mirror 5 according to this embodiment, a multilayer film is formed on a substrate made of resin (K22R manufactured by Nippon Zeon Co., Ltd.) in the following order: a first layer formed of the metallic material Al, a second layer formed of the low refractive index material SiO2, a third layer formed of the high refractive index material Ta2O5, and a fourth layer formed of the low refractive index material SiO2. The first to third layers described above constitute a reflective layer, and the fourth layer constitutes a protective layer. In other words, the reflective surface 5a of the polygon mirror 5 according to this embodiment has a multilayer film in which three or more layers, including a base layer, a reflective layer, and a protective layer, are laminated together.

[0047] Furthermore, the physical film thickness of each layer shown in Table 1 indicates the thickness of the film deposited on each layer. In the polygon mirror 5 according to this embodiment, the physical thickness of each layer of the multilayer film on the reflective surface 5a is set to be different in the vicinity of the center and the vicinity of the edges in the longitudinal direction, thereby making the reflectivity of each layer different.

[0048] Figure 4 shows a schematic perspective view of the polygon mirror 5 according to this embodiment. As shown in Figure 4, the polygon mirror 5 consists of four rectangular reflective surfaces 5a for reflecting (deflecting) the light beam, and an upper surface 5u and a lower surface 5d perpendicular to the reflective surfaces 5a. Furthermore, for convenience, the longitudinal direction of the reflective surface 5a will be referred to as the Y direction (Y axis), and the transverse direction as the Z direction (Z axis), with the center of the reflective surface 5a being the origin in both the Y and Z directions.

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

[0050] Furthermore, the dependence of reflectance on the angle of incidence is shown at longitudinal positions Y=0mm, Y=-5.3mm, and Y=-2.7mm. Here, the longitudinal position Y=0mm corresponds to the longitudinal center of the reflective surface 5a, i.e., its center. Furthermore, the longitudinal position Y = -5.3 mm corresponds to the reflection point (hereinafter referred to as the off-axis reflection point) when the marginal ray of the light beam (hereinafter referred to as the off-axis light beam) at the end of writing (end of scanning) on ​​the reflective surface 5a is reflected, as shown in Figure 6. In other words, the longitudinal position Y = -5.3 mm is the reflection point of the marginal ray of the off-axis light beam that is furthest from the center (Y = 0 mm) in the longitudinal direction, as shown in Figure 6.

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

[0052] Furthermore, as shown in Figure 3, in the polygon mirror 5 according to this embodiment, the physical thickness of the multilayer film on the reflective surface 5a is symmetrical between the positive and negative sides in the longitudinal direction. In other words, the dependence of the reflectance at the longitudinal position on the positive side of the reflective surface 5a on the angle of incidence can be directly obtained from the dependence of the reflectance at the longitudinal position on the negative side. Therefore, Figure 5 only shows the dependence of the reflectance on the incident angle at longitudinal positions on the negative side from Y=0mm. In other words, in the polygon mirror 5 according to this embodiment, the change in reflectance with respect to the light beam incident on the reflective surface 5a from the center to one end in the longitudinal direction is the same as the change from the center to the other end.

[0053] As shown in Figure 5, the polygon mirror 5 according to this embodiment is configured such that the dependence of the reflectivity on the angle of incidence is different at the center of the reflective surface 5a (Y=0mm) and at the off-axis reflection point (Y=-5.3mm). This is achieved by making the physical film thickness of each layer of the multilayer film at the center of the reflective surface 5a of the polygon mirror 5 (Y=0mm) and the physical film thickness of each layer of the multilayer film at the furthest off-axis reflection point (Y=-5.3mm) different from each other.

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

[0055] [Table 2]

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

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

[0058] With the above configuration, the optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment can make the amount of light beam deflected and reflected towards the image edges and the image center on the scanned surface 7 substantially uniform.

[0059] In the polygon mirror 5 according to this embodiment, C and D are defined as the sum of the physical thicknesses of each layer of the multilayer film at the center of the reflective surface 5a and the sum of the physical thicknesses of each layer of the multilayer film at the far-off reflection point, respectively. At this time, the physical film thickness of each layer is configured to decrease along the longitudinal direction from the center of the reflective surface 5a toward the outermost reflection point, such that the following condition (1) is satisfied. 0.50 <D / C<0.98 ···(1)

[0060] Specifically, from Tables 1 and 2, we can determine that C = 488.5 nm and D = 440 nm, so D / C = 0.90, and condition (1) is satisfied.

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

[0062] Figure 6 shows the angle of incidence of the light beam incident on each longitudinal position of the reflective surface 5a in the optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment. The error bars in Figure 6 indicate the magnitude of the luminous beam width incident at each longitudinal position. In other words, the center of the error bar (black circle) indicates the longitudinal position where the principal ray of each luminous beam is incident, while the ends of the error bar indicate the longitudinal positions where the marginal ray of each luminous beam is incident. Furthermore, the deflection point when deflecting the light beam to the on-axis image height on the scanning surface 7 corresponds to the longitudinal position of the reflective surface 5a at Y = +1.3 mm, and the positive and negative sides of this position correspond to the groups of deflection points when deflecting the light beam towards the writing start side (scanning start side) and writing end side (scanning end side) on the scanning surface 7, respectively.

[0063] As shown in Figure 6, for the light beam deflected towards the end of the image (off-axis image height) on the scanned surface 7, the principal ray is deflected at a longitudinal position of Y = -2.2 mm on the reflective surface 5a of the polygon mirror 5. Furthermore, the marginal rays are deflected at longitudinal positions of Y=-5.5mm and Y=+0.9mm on the reflective surface 5a of the polygon mirror 5, and the angle of incidence of the light beam onto the reflective surface 5a is 70°.

[0064] Furthermore, with respect to the light beam deflected toward the image center (on-axis image height) on the scanned surface 7, the angle of incidence of the light beam toward the reflective surface 5a is 45°, and on the reflective surface 5a of the polygon mirror 5, it is deflected within the range of Y = -0.2 mm to +2.8 mm in the longitudinal direction. Similarly, for the light beam deflected towards the beginning of the image on the scanned surface 7 (the furthest off-axis image height), the angle of incidence of the light beam onto the reflective surface 5a is 20°, and on the reflective surface 5a of the polygon mirror 5, it is deflected within the range of Y = +2.7 mm to +4.9 mm in the longitudinal direction.

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

[0066] Specifically, the light beam incident at an incident angle of 70° (a predetermined incident angle) at the center of the reflective surface 5a of the polygon mirror 5 and at the off-axis reflection point. (First beam of light) Let A and B be the reflectances for each of the two elements. In this embodiment, the polygon mirror 5 satisfies the following condition (2). 0.02 < |1 - B / A| < 0.10 ... (2) In this embodiment, the polygon mirror 5 can be determined from Figure 5 to be A=85% and B=79%, so |1-B / A|=0.07, and condition (2) is satisfied.

[0067] Furthermore, in the polygon mirror 5 according to this embodiment, it is preferable that the following condition (2a) is satisfied. 0.05≦|1-B / A|≦0.09 (2a) In the polygon mirror 5 according to this embodiment, the degree of freedom in designing the multilayer film on the reflective surface 5a can be improved by satisfying condition (2a), and in particular, cost reduction can be achieved by making the physical film thickness of at least one layer thinner.

[0068] As shown in Figure 5, the polygon mirror 5 according to this embodiment has a film structure such that condition (2) is satisfied for a light beam incident on the reflective surface 5a with an incident angle of 70°, P polarization, and a wavelength of 790 nm. However, the film configuration is not limited to this; it is also possible to configure the film so that condition (2) is satisfied for light beams incident at an incident angle of 20° or 45° on the reflective surface 5a.

[0069] In the polygon mirror 5 according to this embodiment, for a light beam incident at an incident angle of 35° on the reflective surface 5a, the reflectivity is configured to increase from the center to the outermost reflection point along the longitudinal direction. In other words, in the polygon mirror 5 according to this embodiment, the difference in reflectivity due to the difference in incident angle between the light beam incident at an incident angle of 45° and the light beams incident at incident angles of 20° and 70° is canceled out by the difference in reflectivity due to the difference in incident position. This makes it possible to make the reflectivity of the reflective surface 5a approximately uniform between a light beam incident at an incident angle of 45° on the reflective surface 5a of the polygon mirror 5 so as to be deflected and reflected toward the center of the image on the scanned surface 7, and light beams incident at incident angles of 20° and 70° on the reflective surface 5a of the polygon mirror 5 so as to be deflected and reflected toward both ends of the image, respectively.

[0070] In this embodiment, the polygon mirror 5 is configured such that the dependence of the reflectivity on the angle of incidence is different at the center of the reflective surface 5a and at the off-axis reflection point, as described above, but it is not limited to this configuration. In other words, if the reflectance A at the center of the reflective surface 5a with respect to a light beam incident at a predetermined angle of incidence, and the reflectance B at a predetermined point between the center and the end in the longitudinal direction of the reflective surface 5a, satisfy condition (2), then the same effect can be obtained.

[0071] Here, when L is the distance from the center to the end of the reflective surface 5a and s is the distance from the center to the predetermined point, the polygon mirror 5 according to this embodiment satisfies the following condition (3). 0.2 ≤ s / L ≤ 0.8 ···(3) Furthermore, when R is the diameter of the circumscribed circle of the polygon mirror 5 in the main scanning cross-section, and N is the number of reflective surfaces 5a that the polygon mirror 5 has, the above distance L can be expressed as R / 2 × sin(π / N).

[0072] If the value falls below the lower limit of condition (3), it becomes difficult to change the physical thickness of each layer along the longitudinal direction on the reflective surface 5a so as to satisfy condition (2) when the polygon mirror 5 is mounted on the optical scanning device 100. On the other hand, if the upper limit of condition (3) is exceeded, it becomes difficult to provide a deflection point on the reflective surface 5a for reflecting and deflecting the light beam to the BD sensor when the polygon mirror 5 is mounted on the optical scanning device 100.

[0073] Furthermore, in the polygon mirror 5 according to this embodiment, it is preferable that the following condition (3a) is satisfied. 0.5 ≤ s / L ≤ 0.8 ···(3a) In this embodiment, the polygon mirror 5 has R=20mm and N=4, so L=7.1mm, and as shown in Figure 6, s=5.5mm, so s / L=0.77, and condition (3) is satisfied.

[0074] Figure 7 shows the average reflectance for light beams incident at each longitudinal position on the reflective surface 5a of the optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment. The average reflectance referred to here is a value obtained by taking the average of the reflectances at each longitudinal position included in the beam width (see Figure 6) for the light beam incident at each longitudinal position on the reflective surface 5a of the polygon mirror 5. Furthermore, the incident light beam is assumed to be P-polarized and have a wavelength of 790 nm.

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

[0076] Specifically, as shown in Figure 7, the polygon mirror 5 is configured to have an average reflectivity of approximately uniform between 83% and 84% at each longitudinal position of the reflective surface 5a when deflecting and reflecting the light beam over the entire range from the furthest off-axis image height at the start of scanning to the furthest off-axis image height at the end of scanning.

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

[0078] [Second Embodiment] Next, the configuration of the polygon mirror 5 according to the second embodiment will be described. Since the polygon mirror 5 according to this embodiment is composed of the same components as the polygon mirror 5 according to the first embodiment, the same reference numerals will be used for the same components, and their description will be omitted.

[0079] As shown below, the physical thickness of each layer of the multilayer film on the reflective surface 5a differs between the polygon mirror 5 of this embodiment and the polygon mirror 5 of the first embodiment. Table 3 shows the physical film thickness of each layer of the multilayer film at the center of the reflective surface 5a of the polygon mirror 5 according to this embodiment. In this embodiment, the multilayer film formed on the reflective surface 5a of the polygon mirror 5 has a layer of low refractive index material SiO2 as an underlayer (adhesion layer) between the substrate and the first layer, but this is omitted here.

[0080] [Table 3]

[0081] Table 4 also shows the physical film thickness of each layer of the multilayer film at the furthest off-axis reflection point of the reflective 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 according to this embodiment, the physical film thickness of the third layer (high refractive index material Ta2O5), the second layer and the fourth layer (low refractive index material SiO2) on the reflective surface 5a is changed compared to the polygon mirror 5 according to the first embodiment. In particular, the polygon mirror 5 according to this embodiment achieves cost reduction by shortening the film deposition time by making the physical film thickness of the third layer (high refractive index material Ta2O5), which has a low film deposition rate, thinner compared to the polygon mirror 5 according to the first embodiment.

[0084] Figure 8(a) shows the dependence of the reflectance at the angle of incidence on the reflective surface 5a of the polygon mirror 5 according to this embodiment. Specifically, the dependence of reflectance on the angle of incidence is shown at Y=0mm, corresponding to the center of the reflective surface 5a; Y=-5.3mm, corresponding to the outermost reflection point; and Y=-2.7mm, corresponding to the middle section. Furthermore, the incident light beam is assumed to be P-polarized and have a wavelength of 790 nm.

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

[0086] Figure 8(b) shows the longitudinal position dependence of the physical thickness of the multilayer film on the reflective surface 5a of the polygon mirror 5 according to this embodiment. In Figure 8(b), the physical thickness of the multilayer film at the center of the reflective surface 5a (Y=0mm) is normalized to 1. Figure 8(c) also shows the average reflectance for light beams incident at each longitudinal position on the reflective 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 according to this embodiment, as shown in Figure 6, the incident position, incident angle, and beam width of the light beam deflected and reflected to each image printing position on the scanning surface 7 are different from each other on the reflective surface 5a of the polygon mirror 5. To compensate for these differences, the reflectivity is changed by varying the physical thickness of each layer of the multilayer film on the reflective surface 5a of the polygon mirror 5 according to its longitudinal position, as shown in Figure 8(b), thereby making the average reflectivity at the reflective surface 5a for each light beam approximately uniform.

[0088] Specifically, as shown in Figure 8(c), the polygon mirror 5 is configured to have an average reflectivity that is approximately uniform between 79.5% and 82% at each longitudinal position of the reflective surface 5a of the polygon mirror 5 when deflecting and reflecting the light beam over the entire range from the furthest off-axis image height at the start of scanning to the furthest off-axis image height at the end of scanning.

[0089] In the polygon mirror 5 according to this embodiment, as can be seen from Figure 8(b), the longitudinal position dependence of the physical thickness of the multilayer film is set such that the physical thickness of the multilayer film is about 10% thinner at the outermost reflection point (Y=-5.3mm) compared to the center (Y=0mm), that is, the physical thickness of the multilayer film decreases from the center towards the outermost reflection point.

[0090] From Tables 3 and 4, the sum of the physical thicknesses of each layer of the multilayer film at the center of the reflective surface 5a, C, and the sum of the physical thicknesses of each layer of the multilayer film at the furthest off-axis reflection point, D, are determined to be 486.4 nm and 438 nm, respectively. Therefore, since D / C = 0.9, condition (1) is also satisfied in the polygon mirror 5 according to this embodiment.

[0091] Furthermore, in the optical scanning device 100 equipped with the polygon mirror 5 according to this embodiment, as shown in Figure 8(a), the reflectivity of the reflective surface 5a of the polygon mirror 5 for a light beam incident at an incident angle of 70° is configured to decrease from the center towards the off-axis reflection point. Furthermore, the reflectances A and B for a light beam incident at an incident angle of 70° at the center and the furthest off-axis reflection point of the reflective surface 5a of the polygon mirror 5 are found to be 82% and 74.5%, respectively. Therefore, |1-B / A|=0.09, and condition (2) is satisfied.

[0092] As described above, the polygon mirror 5 according to this embodiment has a film structure such that condition (2) is satisfied for a light beam incident on the reflective surface 5a with an incident angle of 70°, P polarization, and a wavelength of 790 nm. However, the film configuration is not limited to this; it is also possible to configure the film so that condition (2) is satisfied for light beams incident at an incident angle of 20° or 45° on the reflective surface 5a.

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

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

[0095] [Third Embodiment] Next, the configuration of the polygon mirror 5 according to the third embodiment will be described. Since the polygon mirror 5 according to this embodiment is composed of the same components as the polygon mirror 5 according to the first embodiment, the same reference numerals will be used for the same components, and their description will be omitted.

[0096] As shown below, the physical thickness of each layer of the multilayer film on the reflective surface 5a differs between the polygon mirror 5 of this embodiment and the polygon mirror 5 of the first embodiment. Table 5 shows the physical thickness of each layer of the multilayer film at the center of the reflective surface 5a of the polygon mirror 5 according to this embodiment. In this embodiment, the multilayer film formed on the reflective surface 5a of the polygon mirror 5 has a layer of low refractive index material SiO2 as an underlayer (adhesion layer) between the substrate and the first layer, but this is omitted here.

[0097] [Table 5]

[0098] Table 6 also shows the physical film thickness of each layer of the multilayer film at the furthest off-axis reflection point of the reflective 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 according to this embodiment, the physical film thickness of the third layer (high refractive index material Ta2O5), the second layer and the fourth layer (low refractive index material SiO2) on the reflective surface 5a is changed compared to the polygon mirror 5 according to the first embodiment. In particular, the polygon mirror 5 according to this embodiment achieves higher durability by increasing the physical film thickness of the second and fourth layers (low refractive index material SiO2) compared to the polygon mirror 5 according to the first embodiment, in order to improve the environmental durability of the reflective surface 5a.

[0101] Figure 9(a) shows the dependence of the reflectance at the angle of incidence on the reflective surface 5a of the polygon mirror 5 according to this embodiment. Specifically, the dependence of reflectance on the angle of incidence is shown at Y=0mm, corresponding to the center of the reflective surface 5a; Y=-5.3mm, corresponding to the outermost reflection point; and Y=-2.7mm, corresponding to the middle section. Furthermore, the incident light beam is assumed to be P-polarized and have a wavelength of 790 nm.

[0102] In the polygon mirror 5 according to this embodiment, by setting the physical thickness of each layer of the multilayer film on the reflective surface 5a as shown in Tables 5 and 6, the reflectivity can be made different at the center of the reflective surface 5a (Y=0mm) and the off-axis reflection point (Y=-5.3mm), as shown in Figure 9(a).

[0103] Figure 9(b) shows the longitudinal position dependence of the physical thickness of the multilayer film on the reflective surface 5a of the polygon mirror 5 according to this embodiment. In Figure 9(b), the physical thickness of the multilayer film at the center of the reflective surface 5a is normalized to 1. Figure 9(c) also shows the average reflectance for light beams incident on each longitudinal position on the reflective 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 according to this embodiment, as shown in Figure 6, the incident position, incident angle, and beam width of the light beam deflected and reflected to each image printing position on the scanning surface 7 are different from each other on the reflective surface 5a of the polygon mirror 5. To compensate for such differences, the reflectivity is changed by varying the physical thickness of each layer of the multilayer film on the reflective surface 5a of the polygon mirror 5 according to its longitudinal position, as shown in Figure 9(b), thereby making the average reflectivity on the reflective surface 5a for each light beam approximately uniform.

[0105] Specifically, as shown in Figure 9(c), the polygon mirror 5 is configured such that the average reflectivity at each deflection point on the reflective surface 5a of the polygon mirror 5 is approximately uniform between 82.8% and 86% when deflecting and reflecting the light beam over the entire range from the furthest off-axis image height at the start of scanning to the furthest off-axis image height at the end of scanning.

[0106] In the polygon mirror 5 according to this embodiment, as can be seen from Figure 9(b), the longitudinal position dependence of the physical thickness of the multilayer film is set such that the physical thickness of the multilayer film is about 10% thinner at the outermost reflection point (Y=-5.3mm) compared to the center (Y=0mm), that is, the physical thickness of the multilayer film decreases from the center towards the outermost reflection point.

[0107] From Tables 5 and 6, the sum of the physical thicknesses of each layer of the multilayer film at the center of the reflective surface 5a, C, and the sum of the physical thicknesses of each layer of the multilayer film at the off-axis reflection point, D, are determined to be 526.5 nm and 474 nm, respectively. Therefore, since D / C = 0.9, condition (1) is also satisfied in the polygon mirror 5 according to this embodiment.

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

[0109] As described above, the polygon mirror 5 according to this embodiment has a film structure such that condition (2) is satisfied for a light beam incident on the reflective surface 5a with an incident angle of 70°, P polarization, and a wavelength of 790 nm. However, the film configuration is not limited to this; it is also possible to configure the film so that condition (2) is satisfied for light beams incident at an incident angle of 20° or 45° on the reflective surface 5a.

[0110] In the polygon mirror 5 according to this embodiment, for a light beam incident at an incident angle of 70° on the reflective surface 5a, the reflectivity is configured such that it increases from the center to the middle along the longitudinal direction, and then decreases from the middle to the outermost reflection point. In other words, in the polygon mirror 5 according to this embodiment, the difference between the dependence of the reflectance on the incident angle at the center of the reflective surface 5a and at the outermost reflection point can be reduced, thereby making the amount of light at each image printing position on the scanned surface 7 substantially uniform.

[0111] As described above, in the polygon mirror 5 according to this embodiment, the film structure of the multilayer film on the reflective surface 5a, specifically the physical film thickness of each layer of the multilayer film, is set to vary in the longitudinal direction so that the average reflectance for the light beam deflected and reflected to each image printing position on the scanned surface 7 is substantially uniform when mounted on the optical scanning device 100. Furthermore, the physical film thickness of the layers that contribute to environmental durability, specifically the second and fourth layers (low refractive index material SiO2), has been increased. This makes it possible to obtain a polygon mirror 5 that has high deflection performance capable of making the light intensity distribution in the main scanning direction on the scanned surface 7 uniform when mounted on the optical scanning device 100, and also achieves higher durability.

[0112] [Monochrome Image Forming Apparatus] Figure 10(a) shows a sub-scanning cross-sectional view of the main part of a monochrome image forming apparatus 104 equipped with an optical scanning unit 100 that includes a polygon mirror 5 according to any of the first to third embodiments.

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

[0114] The photosensitive drum 101, which is an electrostatic latent image carrier (photoreceptor), is rotated clockwise by the motor 115. As it rotates, 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. Above the photosensitive drum 101, a charging roller 102 is provided in contact with the surface of the photosensitive drum 101 to uniformly charge the surface of the photosensitive drum 101. The light beam 103 scanned by the light scanning unit 100 is then irradiated onto the surface of the photosensitive drum 101, which has been charged by the charging roller 102.

[0115] As explained earlier, the light beam 103 is modulated based on the image data Di, and by irradiating the photosensitive drum 101 with this light beam 103, an electrostatic latent image is formed on the surface of the photosensitive drum 101. This electrostatic latent image is developed as a toner image by a developer 107, which is positioned to contact the photosensitive drum 101 further downstream in the rotational 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 the paper 112, which is the material to be transferred, by a transfer roller (transfer unit) 108, which is positioned below the photosensitive drum 101 and facing the photosensitive drum 101. The paper 112 is stored in a paper cassette 109 in front of the photosensitive drum 101 (on the right side in Figure 10(a)), but it can also be fed manually. A paper feed roller 110 is provided at the end of the paper cassette 109, which feeds the paper 112 from the paper cassette 109 into the transport path.

[0117] As described above, the paper 112 on which the unfixed toner image has been transferred is further transported to the fuser 150 located behind the photosensitive drum 101 (on the left in Figure 10(a)). The fuser 150 consists of a fuser roller 113 having a fuser heater (not shown) inside and a pressure roller 114 positioned to press against the fuser roller 113. The unfixed toner image on the paper 112 is fixed by heating it under pressure at the contact point between the fuser roller 113 and the pressure roller 114. Further behind the fuser 150 is a paper discharge roller 116, which discharges the fixed paper 112 to the outside of the monochrome image forming apparatus 104.

[0118] Furthermore, the printer controller 111 not only converts data, but also controls various parts within the monochrome image forming apparatus 104, including the motor 115, and the polygon motor within the optical scanning unit 100.

[0119] [Color Image Forming Apparatus] Figure 10(b) shows a sub-scanning cross-sectional view of the main part of an image forming apparatus 60 equipped with optical scanning devices 61 to 64 that include polygon mirrors 5 according to any 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, and each records image information on the surface of a photosensitive drum, which is an image carrier. The image forming apparatus 60 comprises optical scanning devices 61, 62, 63, 64 equipped with polygon mirrors 5 according to any of the first to third embodiments, and photoreceptor drums 81, 82, 83, 84 as image carriers. The image forming apparatus 60 also includes developing units 31, 32, 33, and 34, a transport belt 51, a printer controller 53, and a fuser 54.

[0121] The image forming apparatus 60 receives R (red), G (green), and B (blue) color signals (code data) 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 the printer controller 53 within the apparatus. These image data are input as image signals and image information to the optical scanning devices 61, 62, 63, and 64, respectively. These optical scanning devices 61, 62, 63, and 64 then emit modulated light beams 71, 72, 73, and 74 according to the image data of each color. These light beams scan the photosensitive surfaces of the photosensitive drums 81, 82, 83, and 84 in the main scanning direction.

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

[0123] As described above, the image forming apparatus 60 uses light beams based on the respective 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. Subsequently, the electrostatic latent images of each color are developed into toner images of each color by the developers 31, 32, 33, and 34, and the developed toner images of each color are transferred in multiple layers by the transfer unit to the transfer material conveyed by the transport belt 51. Then, the transferred toner images are fixed by the fuser 54 to form a single full-color image.

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

[0125] 5. Polygon Mirror (Multifaceted Mirror) 5a Reflective surface

Claims

1. It has multiple rectangular reflective surfaces that reflect light beams, The reflective surface includes a substrate made of a resin material and a multilayer film provided on the substrate. When A is the reflectance at the center of the reflecting surface for a first light beam incident at a predetermined angle of incidence, and B is the reflectance at a predetermined point between the center and the end of the reflecting surface in the longitudinal direction for the first light beam, 0.02<|1-B / A|<0.10 The following conditions must be met, The reflectance of the reflective surface with respect to a light beam incident at a first angle of incidence decreases in the longitudinal direction from the center to the predetermined point. A polyhedron characterized in that the reflectance of the reflective surface for a light beam incident at a second angle of incidence increases as you move from the center to the predetermined point in the longitudinal direction.

2. The polyhedron mirror according to claim 1, characterized in that the reflectance of the reflective surface with respect to the first luminous beam decreases as you move from the center to the predetermined point in the longitudinal direction.

3. The polyhedron according to claim 1 or 2, characterized in that the change in reflectance in the longitudinal direction from the center to one end and the change from the center to the other end are the same.

4. Having a plurality of rectangular reflective surfaces that reflect light beams, The reflective surface includes a substrate made of a resin material and a multilayer film provided on the substrate. When A is the reflectance at the center of the reflecting surface for a first light beam incident at a predetermined angle of incidence, and B is the reflectance at a predetermined point between the center and the end of the reflecting surface in the longitudinal direction for the first light beam, 0.02<|1-B / A|<0.10 The following conditions must be met, A polyhedron characterized in that the change in reflectance in the longitudinal direction from the center to one end and the change from the center to the other end are the same.

5. The polyhedron mirror according to any one of claims 1 to 4, characterized in that the multilayer film includes a layer made of a metallic material.

6. The multifaceted mirror according to any one of claims 1 to 5, characterized in that the multilayer film includes a reflective layer consisting of multiple layers and a protective layer provided on the reflective layer.

7. The polyhedron mirror according to claim 6, characterized in that the multilayer film includes an underlayer provided between the substrate and the reflective layer.

8. The polyhedron mirror according to any one of claims 1 to 7, characterized in that the refractive index of the uppermost layer furthest from the substrate is the lowest among the multilayer films, and the refractive index of the layer adjacent to the uppermost layer is the highest.

9. The multilayer film is provided in order from the substrate side, with a first layer containing Al, a SiO ２ The second layer, including Ta ２ O ５ The third layer, including SiO ２ A polyhedron according to any one of claims 1 to 8, characterized in having a fourth layer including the above.

10. It has multiple rectangular reflective surfaces that reflect light beams, The reflective surface includes a substrate made of a resin material and a multilayer film provided on the substrate. When C and D are the physical thicknesses of the multilayer film at the center of the reflective surface and at a predetermined point between the center and the end in the longitudinal direction, 0.50<D / C<0.98 The following conditions must be met, The reflectance of the reflective surface with respect to a light beam incident at a first angle of incidence decreases in the longitudinal direction from the center to the predetermined point. A polyhedron characterized in that the reflectance of the reflective surface for a light beam incident at a second angle of incidence increases as you move from the center to the predetermined point in the longitudinal direction.

11. Having a plurality of rectangular reflective surfaces that reflect a light beam, The reflective surface includes a substrate made of a resin material and a multilayer film provided on the substrate. When C and D are the physical thicknesses of the multilayer film at the center of the reflective surface and at a predetermined point between the center and the end in the longitudinal direction, 0.50<D / C<0.98 The following conditions must be met, A polyhedron characterized in that the change in reflectance of the reflective surface in the longitudinal direction from the center to one end and the change from the center to the other end are the same.

12. The polyhedron according to claim 10 or 11, characterized in that the physical film thickness becomes thinner in the longitudinal direction from the center to the predetermined point.

13. The polyhedron mirror according to any one of claims 1 to 12, characterized in that, between two predetermined points on the reflective surface, the change in reflectivity due to a change in the angle of incidence of the light beam and the change in reflectivity due to a change in the position of incidence cancel each other out.

14. When L is the distance from the center to the end of the reflective surface, and s is the distance from the center to the predetermined point, 0.2 ≤ s / L ≤ 0.8 A polyhedron according to any one of claims 1 to 13, characterized in that it satisfies the following conditions.

15. A polyhedron according to any one of claims 1 to 14, A deflection device characterized by comprising a drive unit for rotating the multifaceted mirror.

16. A polyhedron according to any one of claims 1 to 14, An optical scanning apparatus characterized by comprising an imaging optical system that guides a light beam deflected by the multifaceted mirror to a surface to be scanned.

17. The optical scanning apparatus according to claim 16, characterized in that the predetermined point is the reflection point of the marginal ray of the furthest off-axis beam that is furthest from the center in the longitudinal direction.

18. An image forming apparatus comprising: an optical scanning apparatus according to claim 16 or 17; and a developer for developing an electrostatic latent image formed on the surface to be scanned by the optical scanning apparatus.

19. An image forming apparatus comprising an optical scanning device according to claim 16 or 17, and a controller that converts a signal output from an external device into image data and inputs it to the optical scanning device.

Images