Optical scanning apparatus

Abstract: The present invention is intended to reduce a f correction error by correcting an error of an object scanning speed which is produced because a reflecting point of a laser beam moves on a polygon mirror (12). In an inventive laser scanner (9), laser light emitted from a laser oscillator (10) is condensed into a beam by a condenser lens (11) at first. This laser beam is reflected by a polygon mirror (12) rotated by a scanning section (13) and is reflected again to a photoreceptor (4) by a curved reflecting mirror (14). A curved profile of a reflecting plane of the curved reflecting mirror (14) in the main scanning direction is formed so as to be asymmetrical on right and left bounding about a scanning center point (14c). Thereby, variation of speed for scanning the curved reflecting mirror (14) by the light reflected by the polygon mirror (12) may be canceled by the curved reflecting mirror (14) and a speed for scanning the object may be fixed almost at a predetermined speed.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning apparatus for one-dimensionally scanning an object by light and more particularly to an optical scanning apparatus for use in a laser bar code reader, a laser printer, a laser facsimile, a laser text reader, a digital copier and the like. 2. Description of Related Art A laser scanner is an apparatus for scanning an object by laser light. In general, the laser scanner is constructed so as to reflect laser light emitted from a laser oscillator by a scanning mirror which rotates or reciprocates and to one-dimensionally scan the surface of an object by the reflected light. A correcting lens or mirror is provided between a predetermined plane of the object and the scanning mirror in order to irradiate laser light in focus and to scan the object at a predetermined scanning speed. The direction in which the laser light is irradiated is changed and the scanning speed is corrected by causing the reflected light to pass through the correcting lens or by reflecting the reflected laser light again by the mirror. Or, thereby, a focal position is changed. FIG. 10 is a diagram showing a structure of a typical laser scanner according to a first prior art technology. The laser scanner comprises a laser oscillator 31, a condenser lens 32, a one plane mirror 33, imaging lenses 35, a light receiving element 36 and a return mirror 37. The one-plane mirror 33 is a scanning mirror for deflecting a laser beam so that it scans within a predetermined scanning range. It has a single reflecting plane and is provided with an axis of rotation within the reflecting plane so as to be angularly displaceable. The laser beam emitted from the laser oscillator 31 is condensed by the condenser lens 32 to be reflected in an arbitrary direction by the one plane mirror 33. The reflected laser beam is imaged on a scanning position 34 on a predetermined line segment within the plane by the imaging lens 35 which is a f lens. The return mirror 37 reflects the laser beam imaged out of the above-mentioned line segment among the laser beams which have passed through the imaging lens 35 to the light receiving element 36. The light receiving element 36 is disposed right before the scanning position 34 to detect the position of the laser beam which has passed through the imaging lens 35. FIG. 11 is a diagram showing a structure of a typical laser scanner according to a second prior art technology. The structure of this laser scanner is the same as that of the laser scanner shown in FIG. 10, except of that the imaging lens 35 is replaced by a curved reflecting mirror 38. The curved reflecting mirror 38 allows the same effect as that of the imaging lens 35 to be obtained and its reflecting surface is formed by a spherical plane whose radius of curvature in the main scanning direction of the laser beam is fixed. In this laser scanner, the laser beam reflected by the one plane mirror 33 is reflected again by the curved reflecting mirror 38 to image at the scanning position 34. A number of proposals have been made since the past as to the concrete structure of the lenses and mirrors composing the laser scanners shown in FIGs. 10 and 11. While an object scanning speed which is a speed at which the laser beam scans the surface of the object may be increased by increasing the speed of rotation of the one plane mirror 33 in the laser scanners shown in FIGs. 10 and 11, there is a limit in the increase of the speed of rotation because the faster the speed of the rotation is, the more complicated the structure of a mechanism for rotating the one plane mirror 33 becomes and the harder the control of the rotation thereof becomes. Therefore, a polygon mirror having a plurality of reflecting planes is normally used instead of the one-plane mirror for increasing the object scanning speed. When the polygon mirror is used, however, the position of reflecting point in the reflecting plane where the laser beam is reflected changes corresponding to an angle of rotation of the polygon mirror because the center of rotation of the polygon mirror is separated considerably from the reflecting planes. The variation of the position of the reflecting point varies depending on the size of an inscribed circle of the polygon mirror and the larger the inscribed circle, the greater the variation becomes. FIG. 12 is a diagrammatic view for explaining an angular displacement of the reflecting plane of the polygon mirror and changes of the reflecting point. The polygon mirror 39 is assumed to be rotated in the direction indicated by the arrow A about the center of rotation C. In a first state when the laser beam enters one reflecting plane of the polygon mirror 39, the reflecting plane is positioned so that the laser beam 20 condensed by the condenser lens 32 enters the reflecting plane with an incident angle of more than 45 degrees as indicated by a solid line 39a. In the first state, the laser beam 20 reflects at a reflecting point P1 and is deflected in the direction indicated by an arrow 21. When the polygon mirror 39 is rotated further from the first state, the reflecting plane is positioned so that the laser beam 20 enters the reflecting plane with an angle of 45 degrees as indicated by a two-dot chain line 39b. In the second state, the laser beam 20 reflects at a reflecting point P2 and is deflected in the direction indicated by an arrow 22. The reflecting point P2 is located on an imaginary axis containing the optical path of the laser beam 20 and behind the reflecting point P1 in the incident direction of the laser beam 20. Therefore, the reflecting point moves from the reflecting point P1 to the reflecting point P2 in the direction indicated by an arrow a1 when the polygon mirror 39 is rotated so as to put the reflecting plane into the second state from the first state. When the polygon mirror 39 rotates further from the second state, the reflecting plane is positioned so that the laser beam 20 enters the reflecting plane with an incident angle of less than 45 degrees as indicated by a broken line 39c. The laser beam 20 reflects again at the reflecting point P1 and is deflected in the direction indicated by an arrow 23 in this third state. Therefore, when the polygon mirror 39 rotates so as to put the reflecting plane into the third state from the second state, the reflecting point moves from the reflecting point P2 to the reflecting point P1 in the direction indicated by an arrow a2. When the polygon mirror 39 is rotated so as to put the reflecting plane into the first through third states sequentially, the reflecting point reciprocates on the imaginary axis. The reflecting point will not move as described above when the axis of rotation of the one plane mirror 33 is made to coincide with the reflecting plane in rotating the one plane mirror 33 because the reflecting point within the reflecting plane is always fixed. Therefore, the range in which the laser beam reflected by the one plane mirror can be deflected is bisected by the laser beam which is reflected by the one plane mirror 33 at 45 degrees. Because the reflecting point moves when the one plane mirror 33 of the laser scanner shown in FIGs. 10 and 11 is replaced by the polygon mirror 39 as described above, the linearity of scan at the scanning position 34 corrected by the imaging lens 35 or the curved reflecting mirror 38 decreases considerably as compared to the case of using the one plane mirror 33. Further, thereby, the range in which the laser beam reflected by the polygon mirror 39 can be deflected will not be bisected even if the laser beam which is reflected by the polygon mirror 35 at 45 degrees is set as a boundary. For instance, in the above-mentioned range, a first range to which the laser beam whose reflecting angle at the polygon mirror 39 is 45 degrees or more is reflected, i.e. a range from the direction indicated by the arrow 21 to the direction indicated by the arrow 22, is narrower than a range to which the laser beam whose reflecting angle at the one plane mirror 33 is 45 degrees or more is reflected. Conversely, a second range to which the laser beam whose reflecting angle at the polygon mirror 39 is 45 degree or less is emitted, i.e. a range from the direction indicated by the arrow 22 to the direction indicated by the arrow 23, is wider than the range to which the laser beam whose reflecting angle at the one plane mirror 33 is 45 degrees or less is reflected. Accordingly, if the speed of rotation of the reflecting plane of the polygon mirror 39 is equal to the speed of rotation of the reflecting plane of the one plane mirror 33, the speed for scanning the curved reflecting mirror 38 by the laser beam reflected in the first range is slower than the speed for scanning the curved reflecting mirror 38 by the laser beam reflected by the one plane mirror 33, the laser beam whose reflecting angle at the polygon mirror 39 is 45 degrees being set as boundary. Conversely, the speed for scanning the curved reflecting mirror 38 by the laser beam reflected to the second range is faster than the speed for scanning the curved reflecting mirror 38 by the laser beam reflected by the one plane mirror 33. The object scanning speed in scanning the object by reflecting those laser beams by the curved reflecting mirror is proportional to the scanning speed of the curved reflecting mirror 38. From these facts, when the polygon mirror is used as the scanning mirror, the object scanning speed is not kept at a predetermined speed, producing a large error in the scanning speed. Therefore, a f correction error may increase. As a prior art technology of this laser scanner, there has been an optical scanning apparatus using an optical scanning curved mirror as disclosed in Japanese Unexamined Patent Publication JP-A 62-253116 (1987). This optical scanning apparatus is constructed so as to deflect light emitted from a light source by a scanning mirror and to reflect this light by an optical scanning curved mirror to condense and image linearly on a photoreceptor drum. A curved profile of this optical scanning curved mirror in the direction orthogonal to a locus on which a reflecting point where the light deflected by the scanning mirror moves is slanted such that the incident angle of the light increases gradually from the middle part to the both ends in order to deflect the light deflected by the scanning mirror in the direction intersecting the plane of deflection of the light. Because the curved profile in the direction parallel to the locus in the reflecting surfaces of the optical scanning curved mirror is nearly parabolic, the radius of curvature thereof is almost fixed. Therefore, it is difficult to prevent the error of the object scanning speed described above. SUMMARY OF THE INVENTION It is hence an object of the present invention to provide an optical scanning apparatus which causes less scanning speed error, i.e. less f correction error, in an optical scanning apparatus using a single curved reflecting mirror and a scanning mirror whose center of rotation is separated from reflecting planes. From US-A 5,353,047 there is known an optical scanning apparatus comprising: a light source for emitting light; a scanning mirror having reflecting planes for reflecting the light emitted from the light source; rotating means for rotating the scanning mirror; and a curved reflecting mirror having a curved reflecting surface for reflecting the light reflected by the reflecting planes of the scanning mirror to an object, wherein a curved profile in the main scanning direction of the reflecting surface of the curved reflecting mirror along a locus on which a point to which the light reflected by a reflecting plane of the scanning mirror moves is formed such that the portion of the curved profile from a scanning center point of the curved reflecting mirror to one end of the locus and the portion of the curved profile from the scanning center point to the other end of the locus are asymmetrical about the scanning center point of the curved reflecting mirror, said scanning center point being the point of the reflecting surface of the curved reflecting mirror from which the light is reflected to the center of a scanning range. This known optical scanning apparatus reflects the light emitted from the light source by the reflecting plane of the scanning mirror which is rotated by the rotating means. Thereby, the light emitted from the light source is reflected in a different direction corresponding to an angle of rotation of the scanning mirror, so that the light is deflected in an arbitrary direction within the plane of rotation of the scanning mirror. This light is reflected at one point in the reflecting surface of the curved reflecting mirror and hits against the surface of the object. The direction in which this light is inputted to the curved reflecting mirror, i.e. the direction of this light reflected by the scanning mirror, is different corresponding to the angle of rotation of the scanning mirror, so that the angle of reflection on the curved reflecting mirror also changes corresponding to the angle of rotation of the scanning mirror. Accordingly, the point at which the light hits on the surface of the object changes along the rotation of the scanning mirror, so that the surface of the object is scanned by this light. The range to which this light can hit corresponding to the angle of rotation of the scanning mirror is the scanning range. At this time, the reflecting surface of the curved reflecting mirror in the main scanning direction is formed so as to be asymmetrical at right and left about the scanning center point. Thereby, even when the distribution of the points at which the light reflected by the scanning mirror hits on the surface of the curved reflecting mirror is biased by the move of the reflecting point on the scanning mirror, the points at which the light reflected by the curved reflecting mirror hits within the scanning range are uniformly distributed. Therefore, even when there is an error in the speed for scanning the curved reflecting mirror due to the movement of the reflecting point, the speed for scanning the object may be corrected to a fixed speed. Thereby, the surface of the object to be scanned may be scanned almost at a predetermined scanning speed. Accordingly, an optical scanning apparatus excellent in terms of the f correction can be provided. In a first aspect according to claim 1, the invention is characterized in that the curved profile in the main scanning direction of the reflecting surface of the curved reflecting mirror along the locus is formed such that the radius of curvature of one of the two portions of the curved profile extending on the side opposite to the light incident side on the scanning mirror with respect to a reference axis increases from the scanning center point to the end of said one portion of the reflecting surface and the radius of curvature of the other of the two portions of the curved profile extending on the light incident side on the scanning mirror with respect to the reference axis decreases from the scanning center point to the end of said other portion of the reflecting surface, said reference axis being defined to be parallel to the direction in which the light is reflected by the scanning mirror to the scanning center point, being further defined to pass through the reflecting point of the reflecting plane of the scanning mirror from which the light is reflected to the two ends of the locus, and being additionally defined to be perpendicular to the incidence direction of the light on the scanning mirror. According to the invention, the curved surface in the main scanning direction of the curved reflecting mirror is formed so that its radius of curvature is as described above. Thereby, when the radius of curvature of the curved reflecting mirror of the present invention at the scanning center point is equal to the radius of curvature of the prior art curved reflecting mirror which is constant, the radius of curvature is longer than the radius of curvature of the prior art curved reflecting mirror on one side of the curved reflecting mirror of the present invention and the radius of curvature of the curved reflecting mirror of the present invention is shorter than the radius of curvature of the prior art curved reflecting mirror on the other side of the curved reflecting mirror of the present invention. Thereby, when the light reflected to said one side , i.e. the light which has been reflected by the scanning mirror at more than 45 degrees, is reflected by the curved reflecting mirror of the present invention to scan the object, the speed for scanning the object becomes faster than the speed for scanning the object of the prior art. Further, when the light reflected to said other side of the curved reflecting mirror of the present invention, i.e. the light which has been reflected by the scanning mirror at less than 45 degrees, is reflected by the curved reflecting mirror of the present invention to scan the object, the speed for scanning the object becomes slower than the speed for scanning the object of the prior art. As explained with reference to the prior art, in the laser scanner in which the polygon mirror is used as scanning mirror, the speed for scanning the curved reflecting mirror by the light which has been reflected by the scanning mirror at more than 45 degrees is slower and the speed for scanning the curved reflecting mirror by the light which has been reflected by the scanning mirror at less than 45 degrees is faster as compared to the speed for scanning the curved reflecting mirror when an one-plane mirror is used as the scanning mirror when the radius of curvatures of the curved reflecting mirror are equal. Because the object scanning speed is corrected as described above by the curved reflecting mirror in the inventive optical scanning apparatus, the error of the scanning speed explained with reference to the prior art may be cancelled. Accordingly, the object is scanned almost at a fixed speed. Thus, the present invention provides a curved reflecting mirror which can scan with less f correction error during scanning than the prior art curved reflecting mirror. In a second aspect according to claim 2 the invention is characterized in that the curved profile in the main scanning direction of the reflecting surface of the curved reflecting mirror along the locus is formed such that the radius of curvature of one of the two portions of the curved profile extending on a side opposite to a light incident side on the scanning mirror with respect to a reference axis is constant from the scanning center point to the end of said one portion of the reflecting surface and the radius of curvature of the other of the two portions of the curved profile extending on the light incident side on the scanning mirror with respect to the reference axis decreases from the scanning center point to the end of said other portion of the reflecting surface, said reference axis being defined to be parallel to the direction in which the light is reflected by the scanning mirror to the scanning center point, being further defined to pass through the reflecting point of the reflecting plane of the scanning mirror from which the light is reflected to the two ends of the locus, and being additionally defined to be perpendicular to the incidence direction of the light on the scanning mirror. According to the second aspect of the invention, the curved surface in the main scanning direction of the curved reflecting mirror is formed so that its radius of curvature is as described above. Thereby, when the radius of curvature of the curved reflecting mirror of the present invention at the scanning center point is equal to the radius of curvature of the prior art curved reflecting mirror which is constant, the radius of curvature of the curved reflecting mirror of the present invention is equal to the radius of curvature of the prior art curved reflecting mirror on one side of the of the curved reflecting mirror of the present invention and the radius of curvature of the curved reflecting mirror of the present invention is shorter than the radius of curvature of the prior art curved reflecting mirror on the other side of the of the curved reflecting mirror of the present invention. Thereby, the object scanning speed in scanning the object by reflecting the light reflected to said one side of the curved reflecting mirror of the present invention is equal to the object scanning speed of the prior art. Further, when the light reflected to said other side of the curved reflecting mirror of the present invention is reflected by the curved reflecting mirror to scan the object, the object scanning speed is slower as compared to the object scanning speed of the prior art as described in the first aspect of the invention. It can be seen from the experimental results described later that in the prior art laser scanner, although the f correction error of the object scanning speed in scanning the object by reflecting the light reflected to said one side of the prior art curved reflecting mirror increases linearly in proportional to a deflection angle made between the direction in which the light is inputted to the curved reflecting mirror and the reference axis, the f correction error of the object scanning speed in scanning the object by reflecting the light reflected to said other of the side the prior art curved reflecting mirror increases/decreases regardless of the deflection angle. Therefore, when the curved surface is formed as described above, the error of the scanning speed explained in the prior art may be cancelled because the object scanning speed in scanning the object by reflecting the light reflected to the one side by the curved reflecting mirror of the present invention is corrected as explained in the first aspect of the invention. Accordingly, the object to be scanned may be scanned almost at a fixed speed. Thus, the present invention provides the curved reflecting mirror which can scan with less f correction error during scanning than the prior art curved reflecting mirror. In a third aspect according to claim 3 the invention is characterized in that the reflecting planes of the scanning mirror are separated from the center of rotation the scanning mirror. According to the third aspect of the invention, the scanning mirror is formed as described above. A so-called polygon mirror or a thick one-plane mirror may be used as such a mirror. It allows a surface of an object to be scanned almost at a fixed speed in conjunction with the correcting characteristics of the curved reflecting mirror described above. Accordingly, it allows the object to be scanned at high speed and at the same time the f correction error to be reduced. Further, since the scanning mirror is formed such that the axis which is the center of rotation thereof does not pass through the reflecting plane, the axis which becomes the direct center of rotation may be provided in the scanning mirror and the structure thereof and the rotating means may be simplified. In a fourth aspect according to claim 4 the invention is characterized in that the scanning mirror is a polygon mirror having a plurality of reflecting planes. According to the fourth aspect of the invention, the scanning mirror is formed as described above. In such a polygon mirror the reflecting surfaces are the side faces of a prism whose perimeter is polygonal. Such a polygon mirror allows the scanning speed to be increased by increasing the number of reflecting planes without changing the speed of rotation. Accordingly, it allows the scanning speed to be increased readily. Further, although such a polygon mirror produces an error in the scanning speed due to the movement of the reflecting point as described above because the center of rotation does not coincide with the reflecting plane, it may be corrected by the inventive curved reflecting mirror. Thereby, the surface of the object may be scanned while keeping a faster scanning speed than the scanning speed when the one-plane mirror is used as scanning mirror, in conjunction with the correcting characteristics of the curved reflecting mirror described above. Accordingly, the fast scan may be implemented with less f correction error. In a fifth aspect according to claim 5 the invention is characterized in that the curved profile of the reflecting surface of the curved reflecting mirror along an axis vertical to the above-mentioned locus is elliptical. According to the fifth aspect of the invention, the curved surface of the curved reflecting mirror is formed as described above. Thereby, when the light reflected by the scanning mirror deviates along the axis from the locus, the curved reflecting mirror reflects this light toward the center of the ellipse. Accordingly, because the curved reflecting mirror reflects the light while correcting so that the light heads to a focal point set on the object when the light reflected by the scanning mirror deviates along the axis, the out-of-focus of the light may be reduced. In a sixth aspect according to claim 6 the invention is characterized in that the optical scanning apparatus further comprises condenser lens means, disposed between the light source and the scanning mirror, for condensing light emitted from the light source. According to the sixth aspect of the invention, the light emitted from the light source of the optical scanning apparatus is condensed by the condenser lens means and is then reflected by the scanning mirror. Thereby, the area in which the light hits the object may be reduced. Accordingly, the use of the inventive optical scanning apparatus allows a clear image to be drawn by distributing spots where light hits and spots where light does not hit on a photoreceptor, like an electronic photograph for example. In a seventh aspect according to claim 7 the invention is characterized in that the center of radius of curvature of the reflecting surface of the curved reflecting mirror is set on the reference axis. According to the seventh aspect of the invention, because the center of the radius of curvature is set as described above, the point at which the curved reflecting mirror crosses with the reference axis does not coincide with the scanning center point. Thereby, the light reflected at the scanning center point of the reflecting surface of the curved reflecting mirror is reflected so as to approach to the reference axis . Thereby, although the size of the range in which the light reflected to the side opposite to the light incident side beyond the reference axis, i.e. the first range, was different from that of the range in which the light reflected to the light incident side beyond the reference axis, i.e. the second range, in the prior art, the inventive curved reflecting mirror of the present invention allows the sizes of these ranges to be corrected so that they become almost equal. Accordingly, it allows the object scanning speed to be corrected so that it becomes more uniform within the scanning range. BRIEF DESCRIPTION OF THE DRAWINGS Other and further objects, features, and advantages of the invention will be more explicit from the following detailed description taken with reference to the drawings wherein: FIG. 1 is a front view showing a structure of a laser printer containing a laser scanner 9 of one embodiment of the present invention; FIG. 2 is a perspective view showing a schematic structure of the laser scanner 9; FIG. 3 is a section view showing the schematic structure of the laser scanner 9; FIG. 4 is a diagrammatic view for explaining states of reflection of a laser beam exerted by a curved reflection mirror 14 of the laser scanner 9; FIG. 5 is a perspective view showing the curved reflection mirror 14; FIG. 6 is a characteristic graph showing the relationship between a f correction error and a deflection angle exerted by a curved reflection mirror 38 of a prior art laser scanner; FIG. 7 is a characteristic graph showing the relationship between a f correction error and a deflection angle exerted by the curved reflection mirror 14 of the laser scanner 9 according to one embodiment of the present invention; FIG. 8 is a section view showing a curved reflecting mirror 51 according to another embodiment of the present invention; FIG. 9 is a characteristic graph showing the relationship between a f correction error and a deflection angle exerted by the curved reflection mirror 51 of the laser scanner having the curved reflecting mirror 51 shown in FIG. 8; FIG. 10 is diagram showing a schematic structure of a first prior art laser scanner using imaging lenses 35; FIG. 11 is a diagram showing a schematic structure of a second prior art laser scanner using a curved reflecting mirror 38; and FIG. 12 is a diagrammatic view for explaining moves of a reflection point of a laser beam exerted by a polygon mirror 39 in the prior art laser scanner in which the polygon mirror 39 is adopted as a scanning mirror. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Now referring to the drawings, preferred embodiments of the invention are described below. FIG. 1 is a front view showing a structure of a laser printer containing a laser scanner 9 of the present invention. The laser printer has a sheet stacker 2 for stacking sheets of paper on the side of the main body 1 of the equipment. Included within the main body 1 are a pickup roller 3, a photoreceptor 4, a developer 5, a transferrer 6, primary and secondary chargers, a cleaner, a fixer 7, a discharge roller 8 and the laser scanner 9. The chargers, the developer 5, the transferrer 6 and the cleaner are disposed along a circumference of the cylindrical photoreceptor 4 in this order. The pickup roller 3, the photoreceptor 4, the fixer 7 and the discharge roller 8 are disposed in this order along a conveying path for conveying sheets of paper within the main body 1. In printing an image with this laser printer, the cylindrical photoreceptor 4 rotates at first in a predetermined direction in which the parts from the chargers to the cleaner are disposed in the above-mentioned order centering on a center axis of rotation which passes through the center of bottom of the cylinder. The charger charges uniformly the surface of the photoreceptor 4, i.e. the side face of the cylinder. The laser scanner 9 forms an electronic latent image by scanning the surface of the charged photoreceptor in the direction parallel to the center axis of rotation of the photoreceptor 4 as the main scanning direction and in the direction parallel to the circumference of the photoreceptor 4 as the sub-scanning direction by laser light emitted from a laser oscillator which is controlled whether to emit the laser light or not corresponding to bits of binary image signals representing the image. Then, the developer develops the electronic latent image on the surface of the photoreceptor 4 by toner stored therein to form a toner image. Sheets of paper stacked within the sheet stacker 2 are fed into the main body 1 one by one by the pickup roller 3 provided at a predetermined location downstream of the conveying path to be conveyed between the photoreceptor 4 on which the toner image has been formed and the transferrer 6. The transferrer 6 transfers the toner image on the surface of the photoreceptor 4 to the paper and conveys the transferred paper to the fixer 7 located at the downstream side of the conveying direction. The photoreceptor 4 from which the toner image has been transferred repeats the above-mentioned series of steps after removing toner remaining on the surface thereof by the cleaner. The fixer 7 fixes the toner image transferred to the paper by heating it. The paper on which the image has been fixed is discharged out of the main body 1 by the discharge roller 8 provided at the downstream side of the fixer 7 along the conveying direction. FIG. 2 is a perspective view showing a structure of the laser scanner 9 and FIG. 3 is a section view showing the structure of the laser scanner 9. The laser scanner 9 will be explained with reference to FIGs. 2 and 3. The laser scanner 9 comprises a laser oscillator 10, a condenser lens 11, a polygon mirror 12, a scanning section 13, a curved reflecting mirror 14, return mirrors 15 and 16. The return mirrors 15 and 16 are not shown in FIG. 2. In the drawings hereinbelow, the direction parallel to the center axis of rotation of the polygon mirror 12 will be indicated by an arrow z, the direction parallel to the reference axis 24 described later within a plane including the direction in which a laser beam is deflected by the polygon mirror 12 will be indicated by an arrow y and the direction vertical to the reference axis 24 within the plane will be indicated by an arrow x. The laser oscillator 10 is a light source realized by a laser diode for example and emits laser light. The condenser lens 11 is interposed between the laser oscillator 10 and the polygon mirror 12 to condense the laser light emitted from the laser oscillator 10 as a laser beam. The polygon mirror 12 is a scanning mirror for deflecting the laser beam as a scanning laser beam. In concrete, it is a rotational polygon mirror having a plurality of reflecting planes for reflecting the laser beam. For example, the polygon mirror 12 may be a prism whose perimeter is polygonal and which is rotated about an axis which passes through the center of an circle inscribed in said perimeter and which is parallel to the side faces i.e. the reflecting planes. The scanning section 13 turns the polygon mirror 12 at a predetermined speed of rotation about the center of rotation. The curved reflecting mirror 14 reflects the laser beam reflected by the polygon mirror 12 to the surface of the photoreceptor 4. The return mirror 15 is disposed between the polygon mirror 12 and the curved reflecting mirror 14 to reflect and bend an optical path of the laser beam reflected by the polygon mirror 12 so as to guide the laser beam to the reflecting surface of the curved reflecting mirror 14. The return mirror 16 is disposed between the curved reflecting mirror 14 and the photoreceptor 4 to reflect and bend the optical path of the laser beam reflected by the curved reflecting mirror 14 to guide the laser beam to the surface of the photoreceptor 4. Thereby, because the laser beam emitted from the laser oscillator 10 is reflected by the polygon mirror 12, it scans a predetermined anglular range of the reflecting surface of the curved reflecting mirror 14 . Because this laser beam is also reflected by the curved reflecting mirror 14, it scans a predetermined scan line 4a on the surface of the photoreceptor 4 at a predetermined scanning speed. Because the scanning mirror is realized by the polygon mirror 12 here, the scanning speed may be readily increased. FIG. 4 is a diagrammatic view for explaining the profiles of the polygon mirror 12 and the curved reflecting mirror 14 as well as the positional relationship between the both mirrors 12 and 14. The polygon mirror 12 is rotated in the direction of an arrow A1 by the scanning section 13. With the rotation, the incident angle at which a laser beam 20a condensed by the condenser lens 11 enters the reflecting plane changes. Therefore, the laser beam 20a is deflected so as to scan a predetermined range of the reflecting surface of the curved reflecting mirror 14 by one reflecting plane of the polygon mirror 12. The laser beams which hit against one end and the other end of this range will be referred to as leftmost scanning laser beam 21a and rightmost scanning laser beam 23a, respectively. The leftmost scanning laser beam 21a is the laser beam produced when the laser beam 20a is reflected by the scanning mirror with an angle of reflection of more than 45 degrees and the rightmost scanning laser beam 23a is a laser beam produced when the laser beam 20a is reflected with an angle of reflection of less than 45 degrees. A reflected laser beam produced when the angle of reflection is 45 degrees will be referred to as a center laser beam 22a. The center of rotation C of the polygon mirror 12 is separated largely from each reflecting plane. Therefore, when the laser beam 20a condensed by the condenser lens 11 is reflected by one reflecting plane of the polygon mirror 12, the reflecting point at which the laser beam 20a is reflected by the reflecting plane moves in correspondence to the rotation of the polygon mirror 12. In concrete, in a first state in which the laser light enters one reflecting plane of the scanning mirror, the reflecting plane is positioned so that the laser beam 20a enters the reflecting plane with an incident angle of more than 45 degrees as indicated by a solid line 51. In this first state, the laser beam 20a reflects on the scanning mirror at a reflecting point P1 and is deflected to the side opposite from the side where the laser beam 20 has entered beyond the reference axis 24, which is the axis passing through the reflecting point P1 and crossing at right angles the incident direction of the laser beam 20a. In a second state the polygon mirror 12 is rotated further from the first state and the reflecting plane is positioned such that the laser beam 20a enters the reflecting plane with an incident angle of 45 degrees as indicated by a two-dotted chain line 52. In this second state, the laser beam 20a is reflected at a reflecting point P2 and is deflected in a direction parallel to the reference axis 24. The reflecting point P2 is located on an imaginary line containing the optical path of the laser beam 20a and behind the reflecting point P1 in the incident direction of the laser beam 20a. Therefore, the reflecting point moves in the same direction of the incident direction of the laser beam 20a from the reflecting point P1 to the reflecting point P2 when the polygon mirror 12 is rotated so that the reflecting plane is put into the second state from the first state. In a third state the polygon mirror 12 is rotated further from the second state and the reflecting plane is positioned such that the laser beam 20a enters the reflecting plane with an incident angle of less than 45 degrees as indicated by a broken line 53. In this third state, the laser beam 20a is reflected at the reflecting point P1 again and is deflected to the side where the laser beam 20 has entered beyond the reference axis 24. Therefore, the reflecting point moves in the direction opposite to the incident direction of the laser beam 20a from the reflecting point P2 to the reflecting point P1 when the polygon mirror 12 is rotated such that the reflecting plane is put into the third state from the second state. When the laser beam is deflected as described above, the angle made between the reference axis 24 and the reflected laser beam will be referred to as a deflection angle . The laser beam deflected to the side opposite to the side from which the laser beam 20a has been inputted, beyond the reference axis 24, will be assumed to have a deflection angle  of a positive value and the laser beam deflected to the side from which the laser beam 20a has been inputted, beyond the reference axis 24, will be assumed to have a deflection angle  of a negative value. The angle made between the leftmost scanning laser beam 21a and the reference axis 24 will be referred to as a left maximum deflection angle a and an angle made between the rightmost scanning laser beam 23a and the reference axis 24 will be referred to as a right maximum deflection angle b. On the reflecting surface of the curved reflecting mirror 14, the part reflecting the laser beam from the leftmost scanning laser beam 21a to the center scanning laser beam 22a will be referred to as a left part 14a and the part thereof reflecting the laser beam from the center scanning laser beam 22a to the rightmost scanning laser beam 23a will be referred to as a right part 14b. Further, in the reflecting surface of the curved reflecting mirror 14, a point at which the center scanning laser beam 22a hits will be referred to as a scanning center point 14c. The curved profile of the curved reflecting mirror 14 along a locus of a reflecting point reflecting the laser beam reflected by the polygon mirror 12, i.e. the curved profile in the main scanning direction, is asymmetrical at the left part 14a and the right part 14b bounded by the scanning center point 14c. The closer to the left end from the center of the curved reflecting mirror 14, the greater the radius of curvature of the left part 14a in the main scanning direction is. The closer to the end from the center of the curved reflecting mirror 14, the smaller the radius of curvature of the right part 14b in the main scanning direction. The reflecting surface of the curved reflecting mirror 14 is formed as described above to correct changes of the scanning speed which are caused by the movement of the reflecting point on the polygon mirror 12 as described later. The increase/decrease of the radius of curvature is set such that the f correction error which is produced in the laser scanner using the prior art curved reflecting mirror 38 whose radius of curvature is fixed approaches zero. The center Rc of the radius of curvatures R1 and R2 of the left part 14a and the right part 14b is provided on the reference axis 24. The reference axis 24 is parallel to the center scanning laser beam 22a and passes through the reflecting point P1. The reflecting direction of the laser beam may be corrected so that the laser beam reflected by the curved reflecting mirror 14 is irradiated uniformly to the surface of the photoreceptor 4. One example of a method for determining the radius of curvature of the curved reflecting mirror 14 in the main scanning direction will be described below. The radius of curvature R1 in the main scanning direction at one point in the left part 14a is a value obtained by adding the product of the deflection angle  of the laser beam entered so as to reflect at that point and a predetermined constant K to a predetermined reference radius of curvature R0 as represented by equation (1). It is noted that the units of this deflection angle are radians and the deflection angle  is always positive in the left part 14a. The constant K is a value greater than zero, which changes mainly in connection to the diameter of the inscribed circle of the polygon mirror 12. The constant K may change depending on the number of reflecting planes of the polygon mirror. When the constant K is greater than zero, the radius of curvature R1 is greater than the reference radius of curvature. Thereby, the radius of curvature R1 increases proportionally to the deflection angle  from the middle of the curved reflecting mirror 14 to the end thereof. R1 = R0 + K ·  The radius of curvature R2 in the main scanning direction at one point in the right part 14b is a value obtained by adding the product of the deflection angle  of the laser beam entered so as to reflect at that point and the predetermined constant K to the predetermined reference radius of curvature R0 as represented by equation (2). Because the deflection angle  is always negative in the right part 14b, the product is always negative and the radius of curvature R2 is smaller than the reference radius of curvature R0 also in the following expression. Thereby, the radius of curvature R2 decreases proportionally to the deflection angle  from the middle of the curved reflecting mirror 14 to the end thereof. R2 = R0 + K ·  As described above, the reflecting surface of the curved reflecting mirror 14 is a curved surface which is asymmetrical on the right and left such that the magnitude and variation of the radius of curvature is different at the left part 14a and the right part 14b. Because the deflection angle of the center scanning laser beam is zero, both parts 14a and 14b are connected smoothly in the middle of the curved reflecting mirror. The constant K is determined empirically for example. A value greater than zero is selected as a candidate value of the constant K at first. Then, the candidate value is substituted into Expressions 1 and 2 to determine the radius of curvature of the curved reflecting mirror and to calculate a f correction error based on the radius of curvature. A plurality of f correction errors are calculated by changing the candidate value and the corresponding f correction values are compared to set a candidate value when the error is least as the value of the constant K. For instance, in the case where the reference radius of curvature R0 is 660 mm, and the diameter of the inscribed circle of the polygon mirror 12 is in a range of 30 to 40 mm, if an integer greater than zero is selected as the candidate value of the constant K, the f error gradually decreases in the case where the candidate value increases from 1 to 5, and increases in the case where the candidate value increases from 5, namely, the f error becomes a minimum when the constant K is set to 5. Accordingly it is preferable to set the constant k to 5. The radius of curvature of the curved reflecting mirror 14 in the main scanning direction may be increased or decreased gradually from the middle of the curved reflecting mirror 14 to the ends thereof. When the radius of curvature is proportional to the deflection angle  as described above, the variation of the radius of curvature meets the above-mentioned condition and becomes simplest and smooth. Since a curved surface whose radius of curvature changes as such may be readily formed, it is preferable to change the radius of curvature in proportion to the deflection angle . Further, when the radius of curvature is determined based on Expressions 1 and 2, the constant K is preferable to be an integer in order to simplify the calculation of the radius of curvature. The range in the scanning line 4a on the photoreceptor 4 from a point Q1 to which a leftmost reflected laser beam 21b hits to a point Q2 to which the rightmost reflected laser beam 23b hits will be referred to as a scanning range on the object. The reflected leftmost and rightmost laser beams 21b and 23b are laser beams which are obtained after reflecting the leftmost scanning laser beam 21a and the rightmost scanning laser beam 23a by the curved reflecting mirror 14. While the reflecting point of the center scanning laser beam 22a, when reflected by the polygon mirror 12, is shifted to the side opposite to the side from which the center scanning laser beam 22a is inputted beyond the reference axis 24, the reflected light of the center scanning laser beam 22a hits against the position shifted to the side opposite from the center Q3 of the scanning range on the object because the center scanning laser beam 22a is inputted vertically to the reflecting surface in the arrangement of the prior art curved reflecting mirror. According to the arrangement of the curved reflecting mirror 14 of the present embodiment, the center scanning laser beam 22a returns in the direction approaching to the reference axis as indicated by the line 22b after being reflected by the curved reflecting mirror 14 and hits against the center Q3 of the scanning range on the object. The curved profile of the curved reflecting mirror 14 on an imaginary axis vertical to the above-mentioned locus, i.e. the curved profile in the sub-scanning direction, is an ellipse whose major axis R3 is perpendicular to the reference axis 24, whose minor axis is equal to the above-mentioned reference radius of curvature R0 and whose center is the above-mentioned center Rc. Thereby, the laser beam is reflected toward the center Rc even if the laser beam reflected by the polygon mirror 12 deviates in the sub-scanning direction. It then allows an out-of-focus of the laser beam in the main scanning direction and in the sub-scanning direction to be reduced. The operation of the laser scanner 9 will be explained below in detail with reference to FIGs. 2 and 3. The laser light emitted from the laser oscillator 10 enters the condenser lens 11. The condenser lens 11 condenses this laser light as a convergent laser beam having a finite focal point. The laser beam enters one of the reflecting planes of the polygon mirror 12. Because the polygon mirror 12 is rotated by the scanning section 13 about the center of rotation C at a predetermined speed of rotation, the direction of the laser beam inputted to the polygon mirror 12 to be deflected corresponding to the rotation of the polygon mirror 12 is changed. Thereby, the laser beam is deflected within a predetermined angular range within one plane and becomes a scanning laser beam which scans an imaginary line segment on the reflecting surface of the curved reflecting mirror 14. This scanning laser beam is reflected by the curved reflecting mirror 14 and reaches the scanning line 4a on the photoreceptor 4. Because the photoreceptor 4 is rotated in the sub-scanning direction, the surface of the photoreceptor 4 is scanned by this laser beam. Because the curved profile of the curved reflecting mirror 14 in the main scanning direction is formed to be asymmetrical in the left and right parts, the object scanning speed, i.e. the speed of the laser beam in scanning the scanning line 4a, is corrected so that it settles at a predetermined fixed speed. That is, the reflecting point of the laser beam on the polygon mirror 12 moves from the reflecting point P1 to the reflecting point P2 in scanning from the leftmost scanning laser beam 21a to the center scanning laser beam 22a. In other words, the reflecting point of the laser beam moves to the side opposite to the side from which the laser beam 20 has been inputted from the reference axis 24 when the laser beam is reflected to the left part 14a by the polygon mirror 12. Therefore, as described with reference to the prior art, the speed for scanning the part 14a of the curved reflecting mirror by this scanning laser beam is slower than the speed for scanning the prior art curved reflecting mirror and the speed for scanning the object by the laser beam reflected by the curved reflecting mirror 14 becomes faster. The reflecting point of the laser beam moves also from the reflecting point P2 to the reflecting point P1 in scanning from the center scanning laser beam 22a to the rightmost scanning laser beam 23a. That is, the reflecting point of the laser beam moves to the side opposite to the side from which the laser beam 20 has been inputted beyond the reference axis 24 to the reference axis 24 when the laser beam is reflected to the right part 14b by the polygon mirror 12. Due to that, the speed for scanning the part 14b of the curved reflecting mirror by this scanning laser beam is faster than the speed for scanning the prior art curved reflecting mirror as explained in the Description of the Related Art and the object scanning speed of the laser beam reflected by the curved reflecting mirror 14 becomes slower. Thereby, the object scanning speed of the laser beam reflected by the curved reflecting mirror 14 becomes almost constant at the left and right parts 14a and 14b. Thereby, the object scanning speed may be kept always at a predetermined constant speed regardless of the movement of the reflecting point on the polygon mirror 12. Further, because the polygon mirror is used as scanning mirror, the scanning speed of the laser beam in the scanning mirror may be increased readily. Accordingly, since the surface of the photoreceptor may be scanned at a predetermined high speed by the laser scanner of the present embodiment by using the polygon mirror as the scanning mirror and by correcting the scanning speed by the curved reflecting mirror, fast scan may be carried out with less f correction error. A function for correcting the object scanning speed using the curved reflecting mirror 14 will be explained by comparing the prior art curved reflecting mirror 38 which is a spherical mirror whose reflecting surface is formed of a curved surface having a fixed radius of curvature with the curved reflecting mirror 14 of the present embodiment. Assume here that the prior art curved reflecting mirror 38 has a radius of curvature of 660 mm. Assume also that the curved reflecting mirror 14 of the present embodiment has a reference radius of curvature R0 of 660 mm and the constant K of 5 and that the left maximum deflection angle  a is 30 degrees (0.524 radians) and the right maximum deflection angle b is -30 degrees. Thereby, the radius of curvature R1 increases gradually from 660 mm to 662.62 mm from the middle to the end of the left part 14a of the curved reflecting mirror 14. The radius of curvature R2 decreases gradually from 660 mm to 657.38 mm from the middle to the end of the right part 14b of the curved reflecting mirror 14. Assume also in both laser scanners of the prior art and the present embodiment that the distance L1 on the reference axis 24 from the polygon mirror 12 to the curved reflecting mirrors 14 and 38 is 206 mm, the distance L2 from the curved reflecting mirrors 14 and 38 to the scanning line 4a is 198 mm and the angle of the laser beam deflected by the polygon mirror 12 is ±30 degrees centering on the reference axis 24. The scanning line 4a on the photoreceptor 4 was scanned by the laser scanner that satisfies the above conditions to find the relationship between the f correction error and the corresponding deflection angle of the laser beam at the same time. The f correction error represents a dislocation from an ideal scanning position which is the position to which a laser beam is irradiated when the reflecting point does not move on the polygon mirror 12 in scanning the scanning line 4a by reflecting the scanning laser beam having a certain deflection angle by the curved reflecting mirror. FIG. 6 is a graph showing the relationship between the f correction error and the deflection angle  when the prior art curved reflecting mirror 38 is used as curved reflecting mirror. It can be seen from this graph that when the curved reflecting mirror 38 is used, a f correction error as large as maximum ±0.2 mm is produced corresponding to the movement of the reflecting point on the polygon mirror 12. This f correction error increases almost linearly when the deflection angle is a positive value and when the deflection angle is a negative value, it increases when the deflection angle is 0 degree to about -8 degrees and decreases when the angle is less than about -8 degrees. FIG. 7 is a graph showing the relationship between the f correction error and the deflection angle when the curved reflecting mirror 14 of the present embodiment is used as the curved reflecting mirror. It can be seen from this graph that when the curved reflecting mirror 14 of the present embodiment is used, the f correction error is always less than ±0.1 mm regardless of that the reflecting point moves on the polygon mirror 12. Thereby, the f correction error may be improved to a half or less as compared to the f correction error of the prior art. It can be seen also from FIG. 6 that when the prior art curved reflecting mirror 38 is used, the f correction error linearly increases from a negative value as the deflection angle decreases from the left maximum deflection angle. Contrary to that, the f correction error increases first and then decreases sharply as the deflection angle decreases from zero degree. It can be seen from these facts that the degree of changes of the speed for scanning the laser light reflected by the right part is larger than that of the speed for scanning the laser light reflected by the left part. Thereby, the variation of the radius of curvature of the left part 14a may be increased more than the variation of the right part 14b in the curved reflecting mirror 14 of the present embodiment. For instance, when the radius of curvatures R1 and R2 are to be determined based on Expressions 1 and 2, the value of the constant in Expression 1 may be set at 5 and the value of the constant in Expression 2 at 10. Further, while the scanning speed keeps a fixed value when the f correction error linearly changes, it means that the value thereof is deviating from the predetermined scanning speed. Because the f correction error in the left part changes linearly as described above when the prior art curved reflecting mirror 38 is used, the speed for scanning the object by the laser light reflected by the left part is considered to be kept at a fixed value. From these facts, the speed for scanning the object by the laser light reflected by the curved reflecting mirror may be fixed even when the profile of the reflecting surface of only the right part is corrected such that the speed for scanning the object by the laser light reflected by the right part approaches the speed for scanning the object by the laser light reflected by the left part without correcting the profile of the reflecting surface of the left part. FIG. 8 is a section view showing a curved reflecting mirror 51 used in a laser scanner of another embodiment of the present invention. This curved reflecting mirror 51 is formed such that the left part of the curved profile in the main scanning direction is a curved surface whose radius of curvature is fixed similarly to the prior art and a radius of curvature of the right part is smaller than the fixed radius of curvature and decreases gradually in proportional to the deflection angle with reference to the curve shown in FIG. 6. The radius of curvature R1a of the left part 51a of the curved reflecting mirror 51 is set at 660 mm. The radius of curvature R2 of the right part 51b is determined by using Expression (2), by setting the reference radius of curvature at 660 mm and by setting the constant K at 10. This constant K was determined by the same method as that used for setting the constant K with respect to the curved reflecting mirror 14, assuming that the diameter of the circle inscribed in the perimeter of the polygon mirror is in a range of 30 mm to 40 mm. In this case the f error gradually decreases as the candidate K value increases from 1 to 10, and gradually increases as the candidate K value increases from 10. Thereby, the radius of curvature turned out to be 660 + 100 mm within the range in which the deflection angle  is positive. The right and left maximum deflection angles a and b are also set at 30 degrees (±0.524 radians). Thereby, the variation of the radius of curvature R2 becomes greater in the left part of the curved reflecting mirror 14 than in the right part of the curved reflecting mirror. In concrete, the radius of curvature R2 decreases gradually from 660 mm to 654.76 mm from the middle to the right end. FIG. 9 is a graph showing the relationship between the f correction error and the deflection angle of a laser beam when the curved reflecting mirror 51 of the present embodiment is used as curved reflecting mirror 51. It can be seen from this graph that the f correction error is always less than ±0.02 mm even though the reflecting point moves on the polygon mirror. Thereby, the f correction error may be improved considerably such that it is always less than ±0.02 mm . The invention may be embodied in other specific forms without departing from the scope of protection as laid down in the claims including interpretation thereof within the frame of the specification and the drawings. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and the range of equivalency of the claims are therefore intended to be embraced therein.

Claims: An optical scanning apparatus comprising: a light source (10) for emitting light; a scanning mirror (12) having reflecting planes for reflecting the light emitted from the light source (10); rotating means (13) for rotating the scanning mirror (12); and a curved reflecting mirror (14) having a curved reflecting surface for reflecting the light reflected by the reflecting planes of the scanning mirror (12) to an object. wherein a curved profile in the main scanning direction of the reflecting surface of the curved reflecting mirror (14) along a locus on which a point to which the light reflected by a reflecting plane of the scanning mirror (12) moves is formed such that the portion (14a) of the curved profile from a scanning center point (14c) of the curved reflecting mirror (14) to one end of the locus and the portion (14b) of the curved profile from the scanning center point (14c) to the other end of the locus are asymmetrical about the scanning center point (14c) of the curved reflecting mirror (14), said scanning center point (14c) being the point of the reflecting surface of the curved reflecting mirror (14) from which the light is reflected to the center (Q3) of a scanning range (4a), characterized in that the curved profile of the reflecting surface of the curved reflecting mirror (14) along the locus is formed such that the radius of curvature (R1) of one (14a) of the two portions of the curved profile extending on the side opposite to the light incident side on the scanning mirror (12) with respect to a reference axis (24) increases from the scanning center point (14c) to the end of said one portion (14a) of the reflecting surface and the radius (R2) of curvature of the other (14b) of the two portions of the curved profile extending on the light incident side on the scanning mirror (12) with respect to the reference axis (24) decreases from the scanning center point (14c) to the end of said other portion (14b) of the reflecting surface, said reference axis (24) being defined to be parallel to the direction (22a) in which the light is reflected by the scanning mirror (12) to the scanning center point (14c), being further defined to pass through the reflecting point (P1) of the reflecting plane of the scanning mirror (12) from which the light is reflected to the two ends of the locus, and being additionally defined to be perpendicular to the incidence direction (20a) of the light on the scanning mirror (12). An optical scanning apparatus comprising: a light source (10) for emitting light; a scanning mirror (12) having reflecting planes for reflecting the light emitted from the light source (10); rotating means (13) for rotating the scanning mirror (12); and a curved reflecting mirror (14) having a curved reflecting surface for reflecting the light reflected by the reflecting planes of the scanning mirror (12) to an object, wherein a curved profile in the main scanning direction of the reflecting surface of the curved reflecting mirror (14) along a locus on which a point to which the light reflected by a reflecting plane of the scanning mirror (12) moves is formed such that the portion (14a) of the curved profile from a scanning center point (14c) of the curved reflecting mirror (14) to one end of the locus and the portion (14b) of the curved profile from the scanning center point (14c) to the other end of the locus are asymmetrical about the scanning center point (14c) of the curved reflecting mirror (14), said scanning center point (14c) being the point of the reflecting surface of the curved reflecting mirror (14) at which the light is reflected to the center (Q3) of a scanning range (4a), characterized in that the curved profile of the reflecting surface of the curved reflecting mirror (14) along the locus is formed such that the radius of curvature (R1) of one (14a) of the two portions of the curved profile extending on a side opposite to a light incident side on the scanning mirror (12) with respect to a reference axis (24) is constant from the scanning center point (14c) to the end of said one portion (14a) of the reflecting surface and the radius (R2) of curvature of the other (14b) of the two portions of the curved profile extending on the light incident side on the scanning mirror (12) with respect to the reference axis (24) decreases from the scanning center point (14c) to the end of said other portion (14b) of the reflecting surface, said reference axis (24) being defined to be parallel to the direction (22a) in which the light is reflected by the scanning mirror (12) to the scanning center point (14c), being further defined to pass through the reflecting point (P1) of the reflecting plane of the scanning mirror (12) from which the light is reflected to the two ends of the locus, and being additionally defined to be perpendicular to the incidence direction (20a) of the light on the scanning mirror (12). The optical scanning apparatus of claim 1 or 2, wherein the reflecting planes of the scanning mirror (12) are separated from the center (C) of rotation the scanning mirror (12). The optical scanning apparatus of claim 1 or 2, wherein the scanning mirror (12) is a polygon mirror having a plurality of reflecting planes. The optical scanning apparatus of claim 1 or 2, wherein the curved profile of the reflecting surface of the curved reflecting mirror (14) along an axis vertical to the above-mentioned locus is elliptical. The optical scanning apparatus of claim 1 or 2, further comprising condenser lens means, disposed between the light source (10) and the scanning mirror (12), for condensing light emitted from the light source (10). The optical scanning apparatus of claim 1 or 2, wherein the center (Rc) of the radius (R1, R2) of curvature of the reflecting surface of the curved reflecting mirror (14) is set on the reference axis (24).

Inventors: FUJIMOTO OSAMU, MATSUDA HIDEO, OHNISHI KAZUYUKI, SAKAGAMI HIDEKAZU, SHIMAZAWA YOICHI, FUJIMOTO, OSAMU, MATSUDA, HIDEO, OHNISHI, KAZUYUKI, SAKAGAMI, HIDEKAZU, SHIMAZAWA, YOICHI
Applicants: SHARP KK, SHARP KABUSHIKI KAISHA
Application year: 1997
Main classification: G02B26
IPC codes: H04N1, G02B26