Image forming apparatus featuring a custom formed scanned surface for effecting high definition images

Abstract: An image forming apparatus forms image information by introducing a light beam (6) emitted from a light source (1) and deflected by a light deflecting device (4) to the surface (7) of an image carrying member (10) through an optical image forming system (5) and then by scanning the surface of the image carrying member using that light beam. The surface (7) to be scanned of the image carrying member (10) is shaped into a form which corresponds to the optical characteristics of the optical forming system. Consequently, the resultant image forming apparatus has a simple structure and is capable of forming high definition images.

Description: BACKGROUND OF THE INVENTION The present invention relates to an image forming apparatus. More particularly, the present invention pertains to an image forming apparatus for forming image information, such as characters or figures, on the surface of an image carrying member by directing a modulated light beam emitted from a light source to the surface of the image carrying member having a predetermined shape, such as a photosensitive drum or a recording material, e.g., a silver salt film, through a light deflecting device, such as a rotating polygonal mirror, and by scanning the surface of the image carrying member using that light beam. Such an image forming apparatus can be suitably applied to, for example, a laser beam printer (LBP) or a process optical printer. Conventional image forming apparatuses, such as laser beam printers, form image information by scanning the surface of an image carrying member by means of an optical scanning system using a light beam modulated by and emitted from a laser beam source. Generally, in laser beam printers, the light modulated by and emitted from the light source is converged into a beam made up of parallel rays of light by means of a collimator lens, and then is made incident on a reflecting surface of a light deflecting device which may be a rotating polygonal mirror. The light beam reflected by the reflecting surface of the light deflecting device is gathered by means of a scanning lens exhibiting f - ϑ characteristics, and is then directed to the surface to be scanned which is the image carrying member . The surface to be scanned is scanned using that light beam by rotating the light deflecting device by means of a motor or the like. Recently there has been an increasing demand for an image forming apparatus capable of high density, i.e., high definition, recording of image information. To achieve high definition recording of the image information, scanning the surface to be scanned using a light beam having a small spot size is generally required. However, a reduction in the spot size of the light beam results in a reduction of the depth of focus on the surface to be scanned, thus necessitating an optical image forming system (f - ϑ lens) which exhibits a less curvature of the field. To achieve a high definition or high quality image by reducing the light beam spot size on the surface to be scanned, a reduction in the F-number (FNo) of the optical image forming system, such as a f - ϑ lens, is desired. However, a reduction in the FNo of the f - ϑ lens decreases the size of the light beam incident on the f - ϑ lens, and rapidly reduces the depth of focus. Hence, very high assembly accuracy of the surface to be scanned is required. This makes manufacture of the surface to be scanned very difficult. In the optical scanning system provided with a means for performing tilt correction of the light deflecting device, such as a rotating polygonal mirror, it is very difficult to reduce the FNo of the f - ϑ lens. In the case of an optical scanning system having, for example, a scanning width corresponding to the A4 to A3 paper size, the FNo is generally between 20 and 30. In the optical scanning apparatus which employs a laser beam as a light source, the diameter of the spot L and the depth of focus H on the surface to be scanned approximately have the following relationships: L = kλF H = ±2λF² where λ is the wavelength of the light beam, F is the FNo of the optical system, and k is a constant (k ≧ 1.64) which represents the intensity of light beam in the vicinity of the pupil of the f - ϑ lens. The spot diameter L is a value obtained when the intensity of light is reduced to 1/e² of the maximum peak intensity. Table 1 lists the spot diameter L and depth of focus H in the horizontal direction in which a light beam is moved. The values shown in Table 1 have been calculated using the wavelength λ = 0.78 µm and the constant k = 1.7. As can be seen from the Table 1, when the FNo of the f - ϑ lens is, for example, about 15 and the spot diameter is 20 µm or less, the depth of focus is ±0.35 mm or less. Generally, the curvature of the field on the surface to be scanned has different distribution characteristics in the horizontal (the meridional direction, indicated by a curve M in Fig. 1) and the vertical (the sagittal direction, indicated by a curve S in Fig. 1) directions, as shown in Fig. 1. In an optical scanning apparatus, because the light beam diameter is large with respect to the vertical direction (in the direction perpendicular to the direction in which scanning is performed by the scanning means), the allowable degree of aberration is large. However, with respect to the horizontal direction (the direction in which scanning is performed by the scanning means), the light beam diameter is about one half of that in the vertical direction, and the allowable degree of aberration is thus small. As can be seen from Table 1, when the spot diameter is about 20 µm, the depth of focus is ±0.35 mm. Therefore, when the curvature of the field is ±0.35 mm, the aberration allowance of the surface to be scanned, e.g., the image carrying member, is zero in terms of the shape or arrangement thereof. As a result, if the spot diameter of the light beam is 20 µm or less, solution may not be available for the design of an optical scanning system using conventional techniques. Accordingly, Japanese Patent Laid-Open No. 116603/1984 discloses an image forming apparatus which has a control system for correcting image formed position errors such that the position on the surface to be scanned at which the spot of a light beam is formed coincides with the focal point thereof. However, the optical system of such an image forming apparatus is complicated, and hence the apparatus including the control system is very expensive. In the case of an image forming apparatus having a post objective type optical scanning system, since the surface to be scanned of the image carrying member is cylindrical, the overall size of the image forming apparatus is large. Also, the construction of the conveying system is complicated. The preambles of independent claims 1, 8, 13 and 17 are based on an image forming apparatus, an optical scanning apparatus, an image information forming method and a scanning method as is disclosed in the EP-A-213 511, respectively. This known generic device and method comprises a curved image carrying member so that a size reduction of the apparatus is possible (see page 15, lines 9 to 13 of the EP-A-213 511). However, the curvature is not chosen in correspondence with the field characteristics for compensation of errors of the optical means. Therefore, a high definition and high resolution scanning can be problematic. SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus in which the surface to be scanned of an image carrying member is shaped into an adequate form so as to simplify the structure of the apparatus and to achieve high definition, high resolution formation of image information. Accordingly, the present invention provides an image forming apparatus according to claim 1 or an optical scanning apparatus according to claim 8. The invention provides in another aspect an image information forming method according to claim 13 or scanning method according to claim 17. The optical characteristics of the image forming optical system may be a curved surface which is made up of the points at which the curvature of the field or the diameter of the light beam from the optical image forming system is minimized. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates a curvature of the field; Fig. 2 is a schematic view, taken along the horizontal scanning section, of an optical scanning system employed in a first embodiment of an image forming apparatus according to the present invention; Figs. 3 to 6 are schematic views of an image carrying member, respectively showing second to fifth embodiments of the image forming apparatus according to the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 2 is a schematic view, taken along the horizontal scanning section, of an optical scanning system used in a first embodiment of an image forming apparatus according to the present invention. The horizontal scanning section is a plane which is formed by a light beam moved by the light deflecting device with time and which contains an optical axis. In Fig. 2, a light source 1 which may be a semiconductor laser emits a modulated light. A collimator lens 2 converts the light emitted from the light source 1 into a beam made up of parallel rays of light. A cylindrical lens 3 has a predetermined refractive power only in the vertical direction. A light deflecting device 4, comprising a rotary polygon mirror, rotates at a fixed speed in the direction indicated by an arrow 'a'. An optical image forming system 5 which is a f - ϑ lens is made up of a spherical lens and a toric lens. A light beam 6 is reflected by light deflecting device 4. An image carrying member 10 on which an image is formed may be a photosensitive drum or a recording material, such as a silver salt film. The image carrying member 10 has a shape which substantially coincides with an image-formed curve 7 corresponding to the scanned surface made up of the points at which the spot diameter of the light beam gathered by the optical image forming system 5 is minimized. The photosensitive surface of the image carrying member 10 is located on this image-formed curve 7. In this embodiment, the light emitted from the light source 1 is converged in the vertical direction by the cylindrical lens 3. The converged light beam forms a line image on the reflecting surface of the rotating polygonal mirror 4. The light beam 6 reflected by the reflecting surface of the rotating polygonal mirror 4 passes through the image forming optical system 5 to the photosensitive surface 7 which is the surface to be scanned to form a light spot. The light spot is moved along the photosensitive surface to form image information thereon. As stated above, the photosensitive surface 7 of the image carrying member 10 is formed such that it substantially coincides with the image-formed curve made up of the points at which the light beam 6 directed by the image forming optical system 5 has a minimum spot size. Consequently, in this embodiment, the light beam 6 is always received on the photosensitive surface 7 with a minimum spot size so as to achieve formation of a high definition image. Figs. 3 (A) and 3 (B) are respectively schematic views, taken along the horizontal and vertical scanning sections, of the image carrying member of a second embodiment of the image forming apparatus according to the present invention. In the embodiment shown in Figs. 3 (A) and 3 (B), a photosensitive member (the surface to be scanned) 27 comprises a recording material which is a silver salt film. The photosensitive member 27 is mounted on the surface of a supporting member 8 which will be described later. The supporting member 8 is formed such that the surface to be scanned substantially coincides with the image-formed curve made up of the points at which the light beam 6 directed by the optical image forming system 5 has a minimum spot size. In this embodiment, the supporting member 8 and the photosensitive member 27, in combination, form an image carrying member 20. Feed members 9 convey the photosensitive member 27 such that it is in contact with the supporting member 8 in the direction indicated by an arrow 'b' in Fig. 3 during the scanning operation. In this embodiment, the photosensitive member 27 is conveyed in a contact state by means of the feed members 9 during the scanning operation while the light beam 6 which has passed through the optical image forming system (not shown) is converged on the photosensitive member 27 in a spot so as to perform image formation over the entire surface of the photosensitive member 27. In this embodiment, the photosensitive member 27 shown in Fig. 3 (A) is conveyed in a contact state by the feed members 9 in a direction perpendicular to the plane of the paper from the front surface thereof to the rear surface thereof during the scanning. The photosensitive member 27 shown in Fig. 3 (B) is conveyed in a contact state by the feed members 9 in the direction of the plane of the paper from the lower end to the upper end. Fig. 4 is a schematic view of an image carrying member, showing a third embodiment of the image forming apparatus according to the present invention. In Fig. 4, a reference numeral 37 denotes a photosensitive member, i.e., the surface to be scanned, which comprises a recording material, such as an organic photocoductor (OPC) for use in electrophotographic process or amorphous silicon (a-Si). The recording material 37 is supported by a photosensitive drum body 38. In this embodiment, the surface of the photosensitive drum body 38 is formed such that it substantially coincides with the image-formed curve made up of the points at which the light beam 6 directed by the image forming optical system 5 has a minimum spot size, similar to the aforementioned embodiment. In this embodiment, an image carrying member 30 is constructed by coating or depositing the recording material 37, such as the OPC for the electrophotographic process or a-Si, on the surface of the photosensitive drum body 38. During the scanning operation, the light beam 6 which has passed through the optical image forming system (not shown) is converged on the photosensitive member 37 of the photosensitive drum body 38 so as to scan the photosensitive member 37. Consequently, a high definition image is formed on an optimum image forming surface. Figs. 5 (A) and 5 (B) are respectively schematic views of the image carrying member, showing a fourth embodiment of the present invention. In Figs. 5 (A) and 5 (B), a reference numeral 47 denotes a photosensitive member, i.e., the surface to be scanned, which is in contact with the surface of a supporting member 48 which will be described below. In this embodiment, the supporting member 48 is made of a transparent glass which is subjected to surface processing. The surface of the transparent glass 48 which is in contact with the photosensitive member 47 is formed such that it substantially coincides with the image-formed curve made up of the points at which the light beam 6 directed by the image forming optical system (not shown) has a minimum spot size, similar to the aforementioned embodiments. Reference numerals 41 and 42 denote a feed member for conveying the photosensitive member 47. The feed member 41 comprises a rubber roller having a shape corresponding to the image-formed curve. The feed member 42 comprises a sponge roller which is cylindrical. The portion of the feed member 42 makes contact with the curved surface of the support member 48, i.e., the photosensitive member 47, to conform to the shape of the surface of the support member 48. In this embodiment, because the supporting member 48 is made of the transparent glass, the light beam 6 can be made to be incident on the photosensitive member 47 through the transparent glass to form an image. In this embodiment, the feed member is a rubber roller (Fig. 5 (A)) or a sponge roller (Fig. 5 (B)), and the photosensitive member 47, which is to be scanned using a light beam, is closely attached to the supporting member 48. Consequently, recording of high definition image information can be conducted more stably. Figs. 6 (A) and 6 (B) are schematic views of the image carrying member showing a fifth embodiment of the image forming apparatus according to the present invention. Generally, the image-formed curve made up of the points at which the light beam is received by the image forming optical system, such as a f - ϑ lens, having a minimum spot size is a complex curve. Hence, in this embodiment, the surface of the image carrying member is formed such that it coincides with a curve or with a straight line obtained by approximating this complex image-formed curve, as shown in Figs. 6 (A) and 6 (B). Fig. 6 (A) shows an image carrying member 51 in which a surface 51a to be scanned, is approximated in a sine wave form. Fig. 6 (B) shows an image carrying member 52 in which a surface 52a to be scanned, is approximated in a step-like form. The use of the image carrying member having the above-described form simplifies the structure of this embodiment and enables high-definition image information to be formed. However, the surface of the image carrying member may also be formed in a saw tooth wave or circumferential form, or approximate a curve corresponding to an arbitrary function. As will be understood from the foregoing description, according to the present invention, there is provided an image forming apparatus for forming image information by scanning the surface of an image carrying member using a light beam introduced to that surface through an optical image forming system from a light deflecting device after being emitted from a light source. The surface to be scanned of the image carrying member is shaped into a form which corresponds to the optical characteristics of the image forming optical system. The form of the surface to be scanned substantially coincides with the image-formed curve made up of the points at which the light beam directed by the image forming optical system has a minimum spot size or corresponds to the curvature of the field of the image forming optical system. Consequently, the resultant image forming apparatus has a simple structure and is capable of forming high-definition images. While the present invention has been described with respect to what presently are considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the scope of the claims. The following claims are to be accorded a broad interpretation, so as to encompass all such modifications and equivalent structures and functions. An image forming apparatus forms image information by introducing a light beam emitted from a light source and deflected by a light deflecting device to the surface of an image carrying member through an optical image forming system and then by scanning the surface of the image carrying member using that light beam. The surface to be scanned of the image carrying member is shaped into a form which corresponds to the optical characteristics of the optical forming system. Consequently, the resultant image forming apparatus has a simple structure and is capable of forming high definition images.

Claims: An image forming apparatus comprising: a light source unit (1) for emitting a light beam, deflection means (4) for deflecting said light beam, optical means (5) for directing said light beam deflected by said deflection means (4) and an image carrying member (10; 20; 30; 40; 51, 52) to be scanned by said light beam (6) directed by said optical means (5) having a surface being shaped in a form having a curvature, characterized in that the curvature of said surface corresponds to a field characteristic of said optical means (5) such that the form of said surface substantially corrects aberration of said optical means (5). An image forming apparatus according to claim 1, characterized in that the form of said surface substantially coincides with an image-formed curve made up of points at which the light beam is directed by said optical means (5) to have a minimum spot size. An image forming apparatus according to claim 1, characterized in that said image carrying member (10; 20; 30; 40; 51, 52) includes a photosensitive member (7; 27; 37, 47) and a support member (8; 38; 48) for supporting said photosensitive member (7; 27; 37; 47). An image forming apparatus according to claim 1, characterized in that said image carrying member (20) includes a photosensitive member (27), a support member (8) for supporting said photosensitive member (27) and a contact member (9) for bringing said photosensitive member (27) into contact with said support member (8). An image forming apparatus according to claim 1, characterized in that the form of said surface is in the shape of an arbitrary curve. An image forming apparatus according to claim 1, characterized in that the form of said surface is in the shape of a sawtooth wave. An image forming apparatus according to claim 1, characterized in that the form of said surface is in the shape of a plurality of steps. An optical scanning apparatus comprising: a light source unit (1) for emitting a light beam, deflection means (4) for deflecting the light beam, optical means (5) for directing the light beam deflected by said deflection means (4); and a surface to be scanned by the light beam directed by said optical means (4); said surface to be scanned being shaped in a form having a curvature, characterized in that the curvature of said surface corresponds to a field characteristic of said optical means (5) such that the form of said surface substantially corrects aberration of said optical means (5). An optical scanning apparatus according to claim 8, characterized in that said form of said surface substantially coincides with an image-formed curve made up of points at which the light beam is directed by said optical means to have a minimum spot size. An optical scanning apparatus according to claim 8, characterized in that the form of said surface is in the shape of an arbitrary curve. An optical scanning apparatus according to claim 8, characterized in that the form of said surface is in the shape of a sawtooth wave. An optical scanning apparatus according to claim 8, characterized in that the form of said surface is in the shape of a plurality of steps. An image information forming method in an image forming apparatus, comprising the steps of: deflecting a light beam from a light source unit (1), by deflection means (4), scanning an image carrying member (10; 20; 30; 40; 51, 52) with the light beam deflected by said deflection means (4) and directed by an optical means (5) and forming a surface to be scanned of said image carrying member (10; 20, 30, 40; 51, 52) into a shape having a curvature characterized in that the curvature of said surface corresponds to a field characteristic of said optical means (5) such that the form of said surface substantially corrects aberration of said optical means (5). An image information forming method according to claim 13, characterized in that the shape of said image carrying member (10; 20; 30, 40; 51, 52) substantially coincides with an image formed curve made up of points at which the light beam is directed by said optical means (4) to have a minimum spot size. An image information forming method according to claim 13, characterized in that said image carrying member (10; 20; 30, 40; 51, 52) includes a photosensitive member (7; 27; 37; 47), and a support member (8; 38; 48) for supporting said photosensitive member (7; 27; 37, 47). An image information forming method according to claim 13, characterized in that said image carrying member (20) includes a photosensitive member (27), a support member (8) for supporting said photosensitive member (27), and a contact member (9) for bringing said photosensitive member (27) into contact with said support member (8). A scanning method in an optical scanning apparatus, comprising the steps of: deflecting a light beam from a light source unit (1) by deflection means (4). scanning a surface to be scanned with the light beam deflected by said deflection means (4) and directed by an optical means; and forming the surface to be scanned into a shape having a curvature characterized in that the curvature of said surface corresponds to a field characteristic of said optical means (5) such that the form of said surface substantially corrects aberration of said optical means (5). A scanning method according to claim 17 characterized in that the formed surface shape substantially coincides with an image formed curve made up of points at which the light beam is directed by said optical means (4) to have a minimum spot size.

Inventors: ISAKA KAZUO, MOURI AKIHIRO, SUZUKI MASAYUKI, ISAKA, KAZUO, MOURI, AKIHIRO, SUZUKI, MASAYUKI
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1991
Main classification: G02B27
IPC codes: G02B26, B41J2, G02B27, G03G15, G03G21