Scanning optical system

Abstract: The present invention relates to a scanning optical system and a scanning optical apparatus in which the pivot axis and the mirror surface of the deflector are inclined to have a predetermined relationship so as to obtain practical planeness of a surface to be scanned. A scanning optical system according to the present invention comprises a light source, lens for converging a light beam from the light source and a deflector for deflecting the light beam in a predetermined direction. In the optical scanning system according to the present invention, the maximum deflection angle of the deflector, a relationship between a curvature of the deflection/­ reflection surface of the deflector and a distance between an object point of a light beam incident on the deflection/reflection surface of the deflector and the deflection/reflection surface for reflecting a central ray of the incident light beam when a deflection angle of the deflector is 0, are determined to satisfy with a pre­ determined condition. And further, a relationship between an angle defined by a predetermined pivot axis of the deflector and the deflection/reflection surface of the deflector and an incident angle of the light beam from the light source to the deflection/reflection surface of the deflector within a plane including the predetermined axis and perpendicular to the deflection plane when the light beam emerging from the deflector is present within a single plane, is determined to satisfy with a predetermined condition.

Description: BACKGROUND OF THE INVENTION : Field of the Invention The present invention relates to a scanning optical system and a scanning optical apparatus for performing data recording on a surface to be scanned using the scanning optical system. Related Background Art Conventionally, a deflector used in a scanning optical system adopts a polygonal mirror in which a deflection velocity of a light beam is equiangular or a galvano mirror (sinusoidal oscillation mirror) in which a deflection velocity of a light beam is not equiangular. GB-A-2185167 discloses an example of a system using a rotating polygonal mirror. US-A-4627685 and EP-A-0214863 disclose examples of systems using an oscillating mirror. Figs. 1 to 3 are schematic views showing general arrangements of a scanning optical apparatus using a scanning system. For the sake of simplicity, a description will be made while an optical system for inclination correction is not necessary. Fig. 1 shows an example wherein a galvano mirror is used as the deflector. A light beam emitted from a light source 101 is collimated to a parallel beam by a collimator lens 102, and the parallel beam is incident on a mirror surface of a galvano mirror 103. The beam is reflected by the mirror surface, passes through an arcsine lens 104, and forms an image on a surface 106 to be scanned through a cylindrical lens 105. A photosensitive drum 107 for recording the light source is located at the surface 106. The light beam becomes incident on the galvano mirror 103 within a plane perpendicular to the surface 106 and including a pivot axis 108 of the deflector (this plane will be referred to as a z-x plane under an assumption that a coordinate system shown in Fig. 1 is given for the sake of simplicity). The cylindrical lens 105 is inserted for the purpose of correcting a curve of an image surface to be scanned in a z direction during rotation of the mirror surface. Note that in Fig. 1, 109 indicates a driver for the galvano mirror 103. Fig. 2 shows an example wherein a polygonal mirror is used as the deflector. A light beam emitted from a light source 101 is collimated to a parallel beam by a collimator lens 102, and is incident on a mirror surface of a polygonal mirror 110. The parallel beam is reflected by the mirror surface, and forms an image on a surface 106 to be scanned through an f-ϑ lens 111. The light beam becomes incident on the polygonal mirror 110 within a plane perpendicular to the surface 106 and a pivot axis 108 of the deflector. This plane is called a y-x plane for the sake of descriptive convenience. Fig. 3 shows another example wherein a polygonal mirror is used as the deflector. A light beam emitted from a light source 101 is collimated into a parallel beam by a collimator lens 102, and is incident on the mirror surface of a polygonal mirror 110. The parallel beam is reflected by the mirror surface, and is imaged onto the surface 106 to be scanned through an f-ϑ lens 111 and a cylindrical lens 105. A laser beam becomes incident on the polygonal mirror 110 within the z-x plane. The cylindrical lens 105 is inserted for the purpose of correcting a curve of an image surface to be scanned in a z direction during rotation of the mirror surface. Note that in Fig. 3, 112 indicates a driver for the polygonal mirror 110. In the scanning optical apparatus with the above arrangement, a curve of an image surface caused by scanning a light beam upon pivotal movement of the mirror surface of the deflector must be corrected, and a scanning speed of an image on the surface to be scanned by pivotal movement of the deflector must be rendered constant. For this purpose, a plurality of lens systems are required. These lens systems are generally difficult to design, have complicated structures, and hence are expensive. For this reason, the direction of a light beam reflected by the mirror surface is preferably symmetrical within a single plane. In the example of Fig. 2 of the above-mentioned examples, since the light beam becomes incident on the mirror surface within a plane perpendicular to the surface 106 to be scanned and the pivot axis 108 of the deflector, the reflected light beam is deflected within the same plane. However, the direction of the light beam reflected upon rotation of the polygonal mirror 110 becomes asymmetrical, and the curve of the image surface to be scanned also becomes asymmetrical. Therefore, the structure of a lens system for correcting this is complicated. When the light beam is incident on the mirror surface from a point within the z-x plane like in Figs. 1 and 3, symmetricity of the light beam to be reflected can be assured. However, since the image surface of the light beam to be scanned is curved in the z direction upon rotation of the mirror surface, the cylindrical lens 105 must be arranged so as to correct the curve. The cylindrical lens is also used for correcting an inclination error of the mirror surface. The curve in the z direction is larger than the influence caused by an inclination of an angle of the mirror surface, design, manufacture, and adjustment of the cylindrical lens are generally difficult, and this also makes the f-ϑ lens and arcsinϑ lens difficult to design as well as the cylindrical lens. JP-A-58-189609 discloses a light beam scanner using a regular polygon mirror which rotates about an axis of rotation, the axis of the polygon mirror crossing obliquely the axis of rotation. SUMMARY OF THE INVENTION According to the present invention there is provided a scanning system as set out in claim 1, The remaining claims set out optional features. Embodiments of the invention, given by way of non-limiting example will now be described with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 to 3 are schematic views showing arrangements of conventional scanning optical apparatuses, respectively; Fig. 4 is a perspective view showing the principle of the present embodiments; Fig. 5 is a sectional view showing the principle of the present embodiments; and Figs. 6 to 13 are views showing other embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: In these embodiments, the pivot axis and the mirror surface of the deflector are inclined so that a curve of an image surface of a light beam falls within an allowable range from a plane, and the pivot axis of the deflector is inclined with respect to a normal (z direction) of the deflection plane, so that a curve of the image surface of the light beam falls within the allowable range from a plane. In this case, the scanning surface is parallel to the z-axis. This will be explained in detail below with reference to the drawings. Fig. 4 shows the overall arrangement for explaining the principle of the present embodiments. As shown in Fig. 4, an x-y-z orthogonal coordinate system is provided. Fig. 5 is a sectional view taken along a z-x plane in Fig. 4, and illustrates a state wherein the scanning angle (pivot angle) of a mirror is 0. That is, light beams incident on and emerging from the mirror are present within an identical plane. In Fig. 4, a light beam emitted from a light source is incident as an incident beam 11 at an incident angle α on a mirror surface 12 of a deflector through a collimator lens (not shown), is reflected by the mirror surface, and forms an image on a surface to be scanned. The light beam may pass through a lens system (not shown) used as needed between the light source and the surface to be scanned. The mirror surface 12 of the deflector is fixed to a pivot axis 13 of the deflector, and is pivoted about the pivot axis, thereby scanning a reflected beam 14 on the surface to be scanned. A photosensitive drum 16 for performing recording is placed at the surface 15 to be scanned. An angle defined between the pivot axis and the mirror surface will be described below with reference to Fig. 5. The incident beam is incident to define an angle 2α with the x axis. On the other hand, the mirror surface is fixed to define an angle β (not 0) with the pivot axis. When the scanning angle is 0, the mirror surface forms an angle α with the z axis. Therefore, the reflected beam is parallel to the x axis. If β is 0, the relationship between a direction vector of the incident beam and the normal vector of the mirror surface is changed as the mirror is pivoted, and a z component is generated in the direction vector of the reflected beam. When the pivot center of the mirror is different from the position of the mirror surface of the incident beam, an incident position is offset in the z direction, and an image formation position is changed. In the present embodiments, since the mirror surface and the pivot axis form the angle β ≠ 0, a curve of an image formation surface in the z direction caused when the mirror is pivoted can fall within an allowable range. Light rays reflected by the mirror surface before they reach the surface to be scanned can fall within a range regarded as a plane. It was found that the value β took different values depending on an angle of the incident beam, a incident beam position, a curvature of the mirror surface, and a radius of pivotal movement of the mirror surface (a distance between the incident position of light rays onto the mirror surface when the scanning angle is 0° and the pivot axis). Tests were made by changing these conditions. It was found that β causing a curve of the image formation surface in the z direction to fall within the allowable range had the relationship of 0.5 x 2α < β < 1.2 x 2α with 2α. Tables 1 to 22 below show values β according to the present embodiments for some incident beam directions, positions of an image surface in the z direction at some scanning angles at corresponding β, and positions of the image surface in the z direction according to comparative examples. In each table, 2α means an angle formed by an incident light with the x axis, β is the angle formed by the pivot axis and the mirror surface of the deflector according to the present embodiments, and ϑ is the pivot angle of the mirror. In this case, the angle ϑ represents an angle of pivotal movement of the mirror surface pivoted about the pivot axis of the deflector. When ϑ = 0°, a laser beam is reflected within the z-x plane. An image position is separated from the pivot center of the mirror by 200 mm, and a position in the z direction (an offset from an ideal scanning line) is shown in units of mm. No lens system is inserted between the mirror surface and the image surface. Tables 1 to 4 show cases wherein the mirror surface is a plane, and the radius of pivotal movement of the mirror is 0. Tables 5 to 8 show cases wherein the mirror surface is of a flat mirror, and a radius a of pivotal movement of the mirror is changed. Tables 9 to 22 show cases wherein the mirror surface has a curvature r and is one of a spherical mirror and a cylindrical mirror, and the radius a of pivotal movement of the mirror is changed. In the case of the cylindrical mirror, the generatrix of the cylindrical surface corresponds to the perpendicular of the mirror. On the basis of the above-mentioned embodiments, the relationship between β and 2α causing a curve of an image surface in the z direction to satisfactorily fall within an allowable range was examined. As a result, the following condition is preferably used: 0.8 x 2α < β < 1.1 x 2α When the mirror surface is a flat surface, the following condition is preferably used: 0.8 x 2α < β < 1.1 x 2α When the mirror surface is a spherical surface, the following condition is preferably used: 0.5 x 2α < β < 1.1 x 2α When the mirror surface is a cylindrical surface, the following condition is preferably used: 0.9 x 2α < β < 1.1 x 2α In the above embodiments, since a light beam is incident on the deflector from a z-x plane (a plane including a pivot axis of the deflector and perpendicular to the deflection plane), the reflected rays of the light beam can be symmetrical. That is, the reflected rays from the deflector can be regarded to be symmetrical within a single plane. Fig. 6 shows still another embodiment of the present invention, which utilizes a galvano mirror as a deflector. Fig. 7 shows only the galvano mirror shown in Fig. 6. In Fig. 7, since a mirror 19 of the galvano mirror is inclined at an angle according to the present invention with respect to a pivot axis 13, it is fixed to the pivot axis 13 by a fork-like support member 22 shown in Fig. 7. The mirror 19 sinusoidally oscillates around the pivot axis 13 by a driver 21. A light beam emitted from a light source 17 is collimated into a parallel beam by a collimator lens 18, and is then incident on the mirror 19 of the galvano mirror. The beam is then reflected by the mirror surface, and forms an image on a surface to be scanned through an arcsinϑ lens 20. Unlike in the arrangement of Fig. 1, a curve of the image surface in the z direction can be reduced or eliminated in this embodiment. Therefore, no cylindrical lens for correcting this is necessary. Fig. 8 shows still another embodiment of the present invention, which utilizes a polygonal mirror as a deflector. Fig. 9 is an enlarged view of a polygonal mirror 23 shown in Fig. 8. As shown in Fig. 9, a mirror surface of the polygonal mirror 23 is not parallel to a pivot axis 13. Therefore, as shown in Fig. 8, the pivot shaft 13 of the polygonal mirror is set at an angle different from those in Figs. 2 and 3. As shown in Fig. 8, a light beam emitted from a light source 17 is collimated into a parallel beam by a collimator lens 18, and is then incident on the mirror surface of the polygonal mirror 23. The beam is then reflected by the mirror surface, and forms an image on a surface to be scanned through an f-ϑ lens 24. Fig. 10 shows still another embodiment of the present invention, in which the flat mirror of the galvano mirror in the embodiment shown in Fig. 6 is replaced with a convex mirror, and the arcsinϑ lens is omitted. A light beam emitted from a light source 17 is collimated into a parallel beam by a collimator lens 18, and is incident as a focused beam on a mirror 27 of the galvano mirror through an image formation lens 26. The beam is reflected by the mirror 27, and forms an image on a surface 15 to be scanned. Fig. 11 shows still another embodiment of the present invention. In this embodiment, a cylindrical lens 28 is added to the embodiment shown in Fig. 10 so as to correct an inclination error of a mirror surface. In this embodiment, a cylindrical lens is used. However, since a curve of an image surface in a z direction is smaller than that in Fig. 1 causing a curve of the image surface which has a smaller radius of curvature in the z direction, the design, manufacture, and adjustment of the cylindrical lens can be facilitated as compared to Fig. 1. Fig. 12 shows still another embodiment of the present invention. In this embodiment, the flat mirror of the polygonal mirror of the embodiment shown in Fig. 8 is replaced with a convex mirror, and the f-ϑ lens is omitted. A light beam emitted from a light source 17 is collimated into a parallel beam by a collimator lens 18, and is incident as a focused beam on a mirror surface of a polygonal mirror 29 through an image formation lens 26. The beam is reflected by the mirror surface of the polygonal mirror 29, and forms an image on a surface 15 to be scanned. The polygonal mirror 29 is rotated about a pivot axis 13 by a mirror driver 25. Fig. 13 shows still another embodiment of the present invention. In this embodiment, a spherical lens 30 is added to the embodiment shown in Fig. 12 to correct degradation of f-ϑ characteristics. As described above, according to the present invention, the pivot axis and the mirror surface of the deflector are inclined to have a predetermined relationship, thus obtaining practical planeness of a surface to be scanned. As a result, the design of a lens system from the deflector to the surface to be scanned can be facilitated or omitted.

Claims: A scanning optical system comprising: a light source (17); a lens (18) for converging a light beam (11) from said light source; and a deflector (19, 23, 27, 29) for deflecting and scanning the light beam from said light source in a predetermined direction, said deflector having a reflective surface (12) and being pivoted about a predetermined axis (13) which is tilted relative to the reflective surface (12), and the predetermined axis (13) and the light beam incident on the deflector being present within a single plane, characterised in that at the moment in the scanning operation when the light beam (11) incident on said deflector and the light beam (14) reflected from said deflector are present within the single plane, the following condition is satisfied: 0.5 x 2α < β < 1.2 x 2α where β is the angle in said single plane between the predetermined axis (13) and the reflective surface (12) of said deflector, and α is the angle of incidence in said single plane of the light beam (11) from said light source (17) on the reflective surface (12) of said deflector. A scanning optical system according to claim 1, wherein said deflector comprises a polygonal mirror (23, 29). A scanning optical system according to claim 1, wherein said deflector comprises a sinusoidally oscillating mirror (19, 27). A scanning optical system according to any one of claims 1 to 3 in which β < 1.1 x 2α . A scanning optical system according to claim 4, wherein the following condition is further satisfied: 0.8 x 2α < β < 1.1 x 2α A scanning optical system according to claim 4, wherein the reflective surface (12) of said deflector (23, 29) comprises substantially a spherical surface. A scanning optical system according to claim 5, wherein the reflective surface (12) of said deflector (19, 27) comprises substantially a flat surface and the following condition is satisfied, 0.8 x 2α < β < 1.1 x 2α A scanning optical system according to claim 4, wherein the reflective surface (12) of said deflector (23, 29) comprises substantially a cylindrical surface and the following condition is satisfied, 0.9 x 2α < β < 1.1 x 2α

Inventors: MAKINO JUN CANON MOEGINORYO, TATEOKA MASAMICHI, MAKINO, JUN CANON MOEGINORYO, TATEOKA, MASAMICHI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1989
Main classification: G02B26
IPC codes: H04N3, G02B26