Optical scanning apparatus

Abstract: In an optical scanning apparatus, there are arranged a light source, a first optical system for converting a light beam emitted from the light source to a convergent light beam, a deflector for deflecting the convergent light beam from the first optical system, and a second optical system for image-forming the convergent light beam deflected by the deflector as a spot on a surface to be scanned. The symmetrical axis of the second optical system with respect to a main-scanning direction is disposed so as to have an inclination relative to a normal to the scanned surface in a main-scan cross section. As a result, aberrations of the optical scanning apparatus, especially degradation of f ϑ characteristics due to backward and forward movement of a mirror surface caused by the rotation of the polygon mirror, which occur in an optical scanning system wherein a convergent light beam is caused to enter the polygon mirror, are reduced.

Description: The present invention relates to an optical scanning apparatus comprising the features listed in the preamble of claim 1, and particularly to an optical scanning apparatus usable in an apparatus with an electrophotographic process, such as a laser beam printer and a digital copying or duplicating machine, in which a light beam is scanningly deflected by a deflector onto a surface to be scanned through an f lens to record picture information on the scanned surface. Moreover, the present invention relates to a laser beam printer comprising such an optical scanning apparatus. An optical scanning apparatus comprising the features of the preamble part of claim 1 is known from EP-A-0 431 603. Related Background Art Conventionally, a light beam emitted from a light source means is modulated according to a modulation signal in an optical scanning apparatus such as a laser beam printer (LBP). The modulated light beam is periodically deflected by an optical deflector such as a rotating polygon mirror, the deflected beam is condensed into a spot by an image-forming optical system having f characteristics and a photosensitive recording medium is scanned with the condensed spot to record picture information thereon. Fig. 1 schematically shows a prior art optical scanning system, and this figure is a cross-sectional view taken along or with respect to a main-scanning direction. In Fig. 1, a diverging light beam emitted from a light source means 1 is collimated by a collimator lens 12, and the collimated light beam is restricted or limited by a stop 3 to be incident on a cylindrical lens 4. Along the main-scan direction, the collimated light beam incident on the cylindrical lens 4 passes therethrough as it is, i.e. without any change. Along the sub-scan direction which is perpendicular to the main-scan direction, the collimated light beam is converged by the cylindrical lens 4 and is imaged as a substantially linear image on a reflection surface 5a of an optical deflector which consists of a polygon mirror. The light beam is reflectively deflected by the reflection surface 5a of the deflector 5 and is directed to a surface 7 to be scanned through an image-forming optical system 16 which has f characteristics and is disposed so that the optical axis of its lens surface is coincident with the optical axis of the optical scanning system. The optical system 16 consists of lenses 16a and 16b made of glass. The scanned surface 7 is scanned with the imaged light beam by rotating the optical deflector 5 in a direction of an arrow A. In the prior art optical scanning apparatus, the scanning lens system 16 consisting of the two lenses 16a and 16b is used as the image-forming optical system for condensing the light beam deflected by the deflector 5 onto the scanned surface 7. In recent years, in order to reduce the size of the optical scanning apparatus and the manufacturing cost thereof, it has been considered to make up the above-discussed scanning lens system of a single lens of synthetic resin. Wile the synthetic resin lens is advantageous in reducing the cost and in other points, it is difficult to make accurate the surface precision of the lens when its thickness is large. Therefore, it is desired to thin the thickness of the lens. If the lens is thinned, however, the power of the lens is lessened to the point where the power may be insufficient to image the light beam on the surface to be scanned. Hence, it is desirable to cause a converging light beam, and not a collimated beam, to be incident on the lens for compensating for insufficient lens power. To satisfy such a requirement, it is considered effective to construct the optical scanning apparatus with a light source, a first optical system for converting a light beam from the light source to a convergent light beam, a deflector for deflecting a light beam from the first optical system, and a second optical system for imaging the convergent light beam deflected by the deflector into a spot on a scanned surface. When the convergent light beam is caused to enter the second optical system (scanning lens), the rotational center of the deflector is determined so that a zero-percent light beam (i.e., a light beam deflected by the deflector when a center of a recording area is scanned therewith) is coincident with the optical axis of the scanning lens. Figure 2 shows the reason for this desired relation. If the zero-percent light beam were to enter the scanning lens with a hight H relative to the optical axis of the scanning lens, the zero-percent light beam is not imaged at the center of the recording area, but rather deviated from the center by distance h. This deviation results because the zero-percent light beam is a convergent light beam, and therefore the image of the beam is formed closer with regard to the focal length f of the scanning lens. However, despite the desirability of this relation, when the rotational center of the deflector is arranged so that the zero-percent light beam is coincident with the optical axis of the scanning lens, light beams that scan peripheries of the recording area and are deflected with the same angle  relative to the zero-percent light beam are not symmetrical with each other with respect to the zero-percent light beam, as shown in Fig. 3. Fig. 3 illustrates such situation in the main-scan cross section. Because of the non-symmetry, distances from the center of tile recording area to both opposite peripheries are different from each other, that is, in Fig. 3, E<E'. As a result, when an f lens designed to be symmetrical with respect to the optical axis in a main-scan cross section is used as a scanning lens, the f characteristic considered with the image-formed position of an on-axis or zero-percent light beam as a center will not be symmetrical with respect to a symmetrical axis of the f lens system in the main-scan cross section. In particular, since the f characteristic exhibits reverse characteristics between picture image record starting side and its opposite side, such asymmetry becomes a problem. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical scanning apparatus which addresses the above-discussed problem. It is another object of the present invention to provide a laser beam printer comprising such an optical scanning apparatus. According to the present invention, the above objects are achieved by an optical scanning apparatus as defined in claim 1 and by a laser beam printer as defined in claim 8, respectively. The preferred embodiments are specified in the dependent claims. According to the present invention, there are arranged a light source, a first optical system for converting a light beam emitted from the light source to a convergent light beam, a deflector for deflecting the convergent light beam from the first optical system, and a second optical system for imaging the convergent light beam deflected by the deflector as a spot on a surface to be scanned. The symmetrical axis of the second optical system with respect to a main-scanning direction is disposed so as to have an inclination relative to a normal to the scanned surface in a main-scan cross section. Here, the symmetrical axis with respect to the main-scanning direction is an optical axis. By virtue of this arrangement, asymmetry is greatly reduced. These advantages and others will be more readily understood in connection with the following detailed description of the preferred embodiments in conjunction with the drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic view showing the structure of a prior art optical scanning apparatus. Fig. 2 is a schematic view showing the situation in which a convergent light beam is caused to be incident on a scanning lens. Fig. 3 is a view showing a deflection manner of a polygon mirror due to its rotation at the time of incidence of a convergent light beam. Fig. 4 is a perspective view showing the structure of a first embodiment of an optical scanning apparatus according to the present invention. Fig. 5 is a main-scan cross-sectional view showing the first embodiment. Fig. 6 is a view showing the situation of a light beam deflected by a polygon mirror due to its rotation at the time of incidence of a convergent light beam. Fig. 7 is a view showing a deflection manner of a polygon mirror due to its rotation after an f lens is inclined. Fig. 8 is a graph illustrating f  characteristic of the first embodiment, wherein a solid line a indicates the characteristic obtained when the f  lens is not inclined and a dotted line indicates the characteristic when the f  lens is inclined in a main-scan cross section. Figs. 9 and 10 are views showing structures obtained when an optical scanning apparatus of the present invention is installed in a body of a laser beam printer. DESCRIPTION OF THE PREFERRED EMBODIMENT A first embodiment of an optical scanning apparatus according to the present invention will be explained with reference to Figs. 4 and 5 which are respectively a perspective view and a cross-sectional view (a section taken along a main-scanning direction or a main-scan cross section). Here, the main-scanning direction means a direction in which a light beam is deflection-scanned by a deflection-reflection surface of an optical deflector. In Figs. 4 and 5, reference numeral 1 designates a light source means which consists of, for example, a semiconductor laser. Reference numeral 2 denotes a condensing lens which converts a divergent light beam from the light source means 1 to a convergent light beam in the main-scan cross section. Reference numeral 3 denotes a stop for restricting a light beam or light amount. Reference numeral 4 denotes a cylindrical lens or cylinder that has a predetermined refractive power primarily only in a sub-scanning direction perpendicular to the main-scanning direction (i.e., a direction perpendicular to a sheet of Fig. 5). Reference numeral 5 is an optical deflector that consists of a polygon mirror which is rotated in a direction of an arrow A by a driving means such as a motor. Reference numeral 6 designates an image-forming optical system or f lens that is made up of synthetic resin and has f characteristic. The f lens 6 consists of a single lens having different radii of curvature between the main-scanning and sub-scanning directions. Reference numeral 7 designates an image surface that is a surface to be scanned. In this structure, the condensing lens 2 shares the refractive power with the f lens 6 so that the refractive power of the f lens 6 can be reduced. In the first embodiment, the divergent light beam emitted from the semiconductor laser (light source means) 1 is converted to the convergent light beam by the condensing lens 2, and its light amount is limited by the stop 3. Then, the convergent light beam enters the cylindrical lens 4. While the light beam in the main-scanning direction is caused to be incident on the polygon mirror (deflector) 5 as it is, the light beam in the sub-scanning direction is imaged at a position near a deflection-reflection surface 5a the polygon mirror 5, by the cylindrical lens 4. Therefore, the light beam incident on the polygon mirror 5 is a linear image extending in the main-scanning direction. The light beam incident on the polygon mirror 5 is deflected as shown in Fig. 6 due to the rotation of the polygon mirror 5 in the direction of the arrow A. Since the rotational center of the polygon mirror 5 is apart from the deflection point of the light beam, the reflection surface 5a of the polygon mirror 5 moves back and forth as the polygon mirror 5 rotates and hence the deflection point of the light beam fluctuates according to a change in the deflection angle. In Fig. 6, reference numeral 21 denotes an on-axis light beam (zero-percent light beam), reference numeral 22 denotes a positive (+) seventy-percent light beam and reference numeral 23 denotes a negative (-) seventy-percent light beam, and these beams 22 and 23 are deflected at the same angle relative to the zero-percent light beam 21 on either side thereof. In the first embodiment, in order to correct the asymmetry of the f characteristic explained in Fig. 3, the f lens 6 is disposed in a state in which the lens 6 is rotated by an angle of 17' in a direction of an arrow 6a in Fig. 5 in the main-scan cross section, with respect to a normal to the scanned surface 7, about a point (a point P in Fig. 5) whereas a symmetrical line (an optical axis) 11 with respect to the main-scanning direction intersects a first surface of the lens 6 facing the polygon mirror side. Due to such rotation of the f lens 6, distances between the center of the recording area and both opposite peripheries thereof are made equal to each other. Fig. 7 shows the situation in the main-scan cross section after the f lens (scanning lens) 6 is inclined. In Fig. 7, G=G' is established. Thus, the optical axis of the f lens 6 is inclined in the main-scan cross section relative to the normal to the scanned surface 7, so that the f characteristic is made symmetrical. The convergent light beam incident on the f lens 6 is imaged on the scanned surface 7 by the f lens 6, and the scanned surface 7 is scanned with this light beam. The convergent light beam deflected by the polygon mirror 5 enters the f lens 6. In the first embodiment, the f lens 6 consists of a single lens, anti its lens surface is shaped into an aspherical surface. The aspherical shape is as follows: Its generating line and meridional line directions which respectively correspond to the main-scanning and sub-scanning directions are represented by the following two formulae, X=(Y 2 /R 2 )/[1+{(1+K)(Y/R) 2 } 1/2 ]+B 4 Y 4 +B 6 Y 6 +B 8 Y 8 +B 10 Y 10 S=(Z 2 /r)/[1+{1-(Z/r') 2 } 1/2 ] where, for example, an intersection between the f lens 6 and the optical axis is an original point, the optical axis direction is an X axis, an axis perpendicular to the optical axis in the main-scan cross section is a Y axis an axis perpendicular to the optical axis in the sub-scan cross section is a Z axis, R is a radius of curvature, K, B 4 , B 6 , B 8 and B 10 are aspherical coefficients and r' =r(1+D 2 Y 2 +D 4 Y 4 +D 6 Y 6 +D 8 Y 8 +D 10 Y 10 ). The above formulae show that the generating line direction is an aspherical surface that can be represented by a function up to 10th order and that the meridional line direction is a toric surface whose curvature changes according to a value of Y. Data of an example of the f lens 6 of the first embodiment will be described hereinbelow. The f lens 6 is designed to be symmetrical in the main-scanning direction with respect to the optical axis. The working of a mold of the thus symmetrically designed f lens 6 is easy, and an inspection after the molding can be readily performed. Distance between polygon mirror surface and first surface of f lens 29.99mm Central thickness of f lens 6mm Distance between second surface of f lens and image surface 106.48mm Convergence position of light beam from condensing lens when f lens is lacking 243.02mm (measured from polygon mirror surface) Refractive index of f lens 1.572 at a wavelength of 780 nm) The shape of the f lens 6 is as follows: Main-scanning direction in X(Y 2 /R 2 )/[1+{(1+K)(Y/R) 2 } 1/2 ]+B 4 Y 4 +B 6 Y 6 +B 8 Y 8 +B 10 Y 10 first surface second surface R 2.55827E1 2.74984E1 K -4.54286 -5.61058 B 4 -4.13566E-6 -4.70116E-6 B 6 1.25573E-9 1.58817E-9 B 8 -1.52601E-13 -5.03924E-13 B 10 1.97096E-17 1.07228E-16 Sub-scanning direction in S=(Z 2 /r)/[1+{1-(Z/r') 2 } 1/2 ] r' =r(1+D 2 Y 2 +D 4 Y 4 +D 6 Y 6 +D 8 Y 8 +D 10 Y 10 ) first surface second surface Y≧0 T<0 R -4.85374E1 -1.20464E1 D 2 1.53430E-3 1.47111E-3 D 4 -3.19837E-6 -2.93460E-6 D 6 3.44549E-9 2.90126E-9 D 8 -1.87738E-12 -1.41851E-12 D 10 4.01982E-16 2.70372E-16 Inclination angle of f lens The f lens 6 is inclined by 17' in a counter-clockwise direction about the intersection P between the first surface of the f lens 6 and the optical axis, in the main-scan cross section. Fig. 8 shows the f characteristic of the optical scanning apparatus according to the present invention. A dotted line b indicates the f characteristic of the first embodiment, and the f lens 6 is inclined by 17 ' in the main-scan cross section. A solid line a indicates the f characteristic when the f lens is not inclined. It can be seen from this example that the f characteristic accompanies the inclination when no inclination of the f lens is effected and that this inclination of f characteristic is favorably corrected by appropriately inclining the f lens in the main-scan cross section. Fig. 9 shows the structure of an embodiment in which an optical scanning apparatus of the present invention is installed in a body of a laser beam printer. Fig. 9 is a main-scan cross sectional view. The optical scanning apparatus is mounted in an optical box 30. The optical scanning apparatus includes a semiconductor laser light source 31, a condensing lens 32 for converting a light beam emitted from the light source 31 to a convergent light beam, a cylindrical lens 33, a rotating polygon mirror 34, an f lens 35 and so forth. The light beam deflection-reflected by the rotating polygon mirror 34 ic caused to be incident on a reflector mirror 36 through the f lens 35, is reflected by the reflector mirror 36 and passes through an opening of the optical box 30. Thus, a photosensitive body (not shown) is illuminated. The optical box 30 is sealed by a lid (not shown). The basic structure and function of the optical system are the same as those of the above-described first embodiment, so explanation thereof is omitted here. A convergent light beam which is convergent in the main-scan cross section enters the f  lens 35. The rotating polygon mirror 34 is rotated at a constant speed by a motor (not shown), and hence a spot, which is formed by condensing the light beam from the light source 31 on the photosensitive body, is moved in the main-scanning direction. The cylindrical photosensitive body is rotated about its axis so that the sub-scanning is properly performed. Thus, an electrostatic latent image is formed on the surface of the photosensitive body. In the neighborhood of the photosensitive body, there are arranged a corona discharger for uniformly charging the surface of the photosensitive body, a developing device for converting the electrostatic latent image formed on the surface of the photosensitive body to an explicit toner image, a transfer corona discharger for transferring the toner image onto a recording paper and so forth. Due to their functions, recording information corresponding to the light beam generated by the light source 31 is printed on the recording paper. The optical box 30 is fixed to a support plate that is mounted to the laser beam printer body. In this embodiment, lens mounting portions 37a and 37b are arranged in the optical box 30 in order to position the f lens 35 of synthetic resin in an inclined state in which the f lens 35 is inclined relative to the normal to the scanned surface in the main-scan cross section. These mounting portions 37a and 37b are arranged on either side of the optical axis of the lens. When the f lens 35 is pushed against the mounting portions 37a and 37b and fixedly positioned, the optical axis of the f lens 35 which is a symmetrical axis with respect to the main-scanning direction is inclined by an angle of 17' in a counter-clockwise direction about an intersection P between the first surface of the f lens 35 and the optical axis, in the main-scan cross section. In this embodiment, the reflector mirror 36 is fixed parallel with the main-scanning direction that is a scan direction of a light beam on the scanned surface, and a line connecting the mounting portions 37a and 37b is inclined relative to a direction parallel with the main-scanning direction. Further, the line connecting the mounting portions 37a and 37b is positioned so that this line is inclined relative to the mirror surface of the reflector mirror 36 in the main-scan cross section. In the main-scan cross section, the angle formed between the optical axis of the f lens 35 and the mirror surface is a value other than 90 degrees. Fig. 10 shows the structure of another embodiment in which an optical scanning apparatus of the present invention is installed in a body of a laser beam printer. Fig. 10 is a main-scan cross sectional view. The basic structure and function of an optical system of this embodiment are the same as those of the embodiment shown in Fig. 9, so explanation thereof is omitted. In this embodiment, as an image-forming optical system ( f lens) for condensing a convergent light beam deflected by the rotating polygon mirror 34 onto the scanned surface, a lens system of a two-lens structure consisting of a spherical single lens 38 made of glass and a single lens 39 that is made of synthetic resin and has a toric surface having different refractive powers in directions orthogonal to each other, is used. A convergent light beam in the main-scan cross section enters the f lens of the two-lens structure. In this embodiment, the optical axis of the single lens 39 with the toric surface is inclined relative to the optical axis of the spherical lens 38 in the main-scan cross section, and thus these optical axes are made non-parallel to each other. In this embodiment, lens mounting portions 40a and 40b are arranged in the optical box 30 in order to dispose the lens 39 of synthetic resin in an inclined state in which the lens 39 is inclined relative to the normal to the scanned surface in the main-scan cross section. These mounting portions 40a and 40b are arranged on either side of the optical axis of the lens. When the lens 39 is pushed against the mounting portions 40a and 40b and fixedly positioned, the optical axis of the lens 39 which is a symmetrical axis with respect to the main-scanning direction is inclined in a counter-clockwise direction about an intersection Q between the incidence surface of the lens 39 and the optical axis in the main-scan cross section. In this embodiment, the reflector mirror 36 is fixed parallel with the main-scanning direction that is a scan direction of a light beam on the scanned surface, and a line connecting the mounting portions 40a and 40b is inclined relative to a direction parallel with the main-scanning direction. Further, the line connecting the mounting portions 40a and 40b is disposed so that this line is inclined relative to the mirror surface of the reflector mirror 36 in the main-scan cross section. In the main-scan cross section, the angle formed between the optical axis of the lens 39 and the mirror surface is a value other than 90 degrees. As is described in the foregoing, according to the present invention, the symmetrical axis with respect to the main-scanning direction of a lens for condensing a light beam deflected by a deflector onto a scanned surface is inclined relative to a normal to the scanned surface in the main-scan cross section. As a result, aberrations of an optical scanning apparatus, especially degradation of f characteristics due to backward and forward movement of a mirror surface caused by the rotation of a polygon mirror, which occur in an optical scanning system wherein a convergent light beam is caused to enter the polygon mirror, are reduced. Thus, an optical scanning apparatus which is capable of performing a high-precision scanning is achieved. While the present invention has been described with respect to what is presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. The present invention is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.

Claims: An optical scanning apparatus comprising: a light source (1, 31) for emitting a light beam; a first optical system (2, 32) for converting the light beam from said light source to a convergent light beam; a deflector (5, 34) for deflecting the convergent light beam from said first optical system (2, 32); and a second optical system (6; 35; 38, 39) for imaging the convergent light beam deflected by said deflector (5, 34) on a scanned surface (7) as a spot, characterized in that the axis of symmetry (11) of said second optical system (6; 35; 38, 39) is inclined in the main-scan cross section relative to the normal with respect to the scanned surface (7) . An optical scanning apparatus according to claim 1 characterized by further comprising mounting portions (37a, 37b; 40a, 40b)for mounting said second optical system (35; 38, 39), said mounting portions (37a, 37b; 40a, 40b) being respectively disposed on either side of the optical axis of said second optical system (35; 38, 39), a line connecting said mounting portions (37a, 37b; 40a, 40b) being inclined relative to a direction parallel with the main-scanning direction. An optical scanning apparatus according to any of the preceding claims characterized by further comprising a reflector mirror (36) interposed between said second optical system (35; 38, 39) and the scanned surface, an angle formed between the symmetrical axis (11) of said second optical system (35; 38, 39) with respect to the main-scanning direction and a mirror surface of said reflector mirror (36) in the main-scan cross section having a value other than 90 degrees. An optical scanning apparatus according to any of the preceding claims, characterized in that the symmetrical axis (11) with respect to the main-scanning direction is the optical axis. An optical scanning apparatus according to any of the preceding claims, characterized in that said second optical system (6, 35) comprises a single lens. An optical scanning apparatus according to claim 5, characterized in that said single lens (6, 35) is a resin lens. An optical scanning apparatus according to claim 6, characterized in that a lens surface of said single lens (6, 35) is shaped as an aspherical surface. A laser beam printer comprising a recording medium; characterized in that it further comprises an optical scanning apparatus as defined in any of claims 1 to 7.

Inventors: KATO MANABU, KATO, MANABU, Kato, Manabu, c/o Canon Kabushiki Kaisha
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1993
Main classification: G02B27
IPC codes: G02B26