Optical scanning apparatus and system
BACKGROUND OF THE INVENTION 
     The present invention relates to an optical scanning apparatus and system for scanning a surface to be scanned optically, and more particularly, to an electronic photographic printer, a scanner for scanning a surface of a document to read it as an image, and a photographic exposure apparatus for scanning a photosensitive surface to expose it. 
     In the optically scanning technique, the scanning is made by changing the direction of light. For example, in an electronic photographic apparatus, laser light (optical beam) concentrated into a beam is irradiated on a surface of a rotating mirror to vary the direction of the optical beam for scanning. There occurs a disadvantage that a scanning speed of the optical beam and a shape of a light spot formed by incidence of the optical beam on a surface of an object to be scanned, for example, a photosensitive body are varied. More particularly, there occurs a phenomenon (distortion aberration) that intervals (pitch) of the spots formed on the surface of the photosensitive body in the scanning direction are varied at a middle portion and a peripheral portion thereof and a phenomenon (curvature of field) that a position on which the optical beam is focused is shifted at the peripheral portion on the photosensitive body even if the optical beam is focused at the middle portion on the sensitive body. 
     FIG. 18 illustrates a principle of occurrence of the distortion aberration and the curvature of field. When a reflection position of the optical beam on a rotating mirror is O and a position on an object to be scanned is A-A&#39;, a relation of a width Dy 0  on A-A&#39; upon scanning by the optical beam by Δθ in the direction of the point O from A&#39;, that is, in the direction of θ=0 and a width Dy 1  on A-A&#39; in the direction of θ=θ is geometrically calculated by the following equation: 
     
         Dy.sub.1 =Dy.sub.0 /cos θ                            (1) 
    
     Thus, Dy 1  is larger than Dy 0 . In other words, the equation (1) means that the width on the object to be scanned is varied at irregular intervals when the scanning angle is varied at regular intervals. This is the distortion aberration. 
     When light reflected from the point O is focused on the object on A-A&#39; in the direction of θ=0, the position of the focal point of the light is moved along a circular arc of A&#39;-B when the angle θ is changed and according the light is not focused on the surface A-A&#39;. Such deviation of the focal point from the surface A-A&#39; is named the curvature of field. 
     In the prior art, in order to correct the above distortion, at least two lenses, so-called fθ lenses, disposed between the rotating mirror and the photosensitive body are used. 
     Further, in order to reduce the distortion, if the scanning angle of the rotating mirror is made small, a necessary scanning width can not be ensured and hence it is necessary to increase a distance between the rotating mirror and the object to be scanned. In any case, there is a problem that an optical scanning apparatus itself is made large in size. In order to solve this problem, there is a technique that light is reflected plural times to lengthen an effective length of a light path as compared with an apparent length thereof. As this technique, there are known a technique of using a plurality of mirrors for reflecting light in the air plural times and a technique of using an optical block for the same purpose. A representative prior art relating to the former is described in Japanese Patent Unexamined Publication No. 59-198417 corresponding to U.S. Pat. No. 4,966,446. Representative prior art relating to the latter is described in Japanese Patent Unexamined Publication Nos. 2-301715 and 63-71824 and U.S. Pat. Nos. 4,867,547, 4,730,882 and 4,239,337. 
     On the other hand, there is a case where even a scanner having an opposite path to the optical path extending from the scanning mirror to the object to be scanned as described above, for example, a reading scanner having a reduced optical system utilizes an fθ lens disposed between an object to be read and a reading sensor for the same purpose and uses a plurality of mirrors to reflect light plural times, so that the apparatus is made small in size. 
     The above prior art has the following problems. 
     (1) When the expensive fθ lens is merely adopted only for correction of the optical distortion, costs of not only an optical scanning system and a device using this optical scanning system but also an optical scanning apparatus having the device mounted thereon are influenced and a low cost optical scanning apparatus can not be realized any more. 
     (2) In order to make small the optical system, even if the mirror or the optical block for reflecting light plural times is adopted, the expensive fθ lens is indispensable for reduction of the distortion. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to solve the above drawbacks and to provide an inexpensive and small optical scanning apparatus capable of being substituted for the function of an fθ lens substantially without use of the fθ lens. 
     In order to achieve the above object, an optical block for reflecting light plural times therein is used instead of an expensive fθ lens and a ratio (ratio of optical path length) of the whole length of an optical path extending from an optical scanning portion such as a rotating mirror to an object to be scanned and the length of an optical path within the optical block is set to a predetermined value. 
     Further, part of the optical block is structured as a concave mirror against jitter in a sub-scanning direction on a photosensitive body occurring due to mechanical factors such as an accuracy of a surface of the rotating mirror and deflection or yaw of a rotating axis and a cylindrical lens is provided between the photosensitive body and the optical block. 
     In addition, in order to improve the positioning accuracy of the scanning, a single optical lens is used together with the optical block, and a radius of curvature of the lens surface as well as the optical path length and the ratio of optical path length of the optical block are set to predetermined values, respectively. 
     Since a moving distance on the scanning surface of light going out from the optical block is increased by a plurality of reflections of light within the optical block even if the scanning angle of light incident on the optical block is small and a substantial length of the optical path can be increased, the scanning angle can be made small. Consequently, it serves to reduce the distortion aberration and the curvature of field. At this time, when the ratio of optical path length is selected to be a predetermined value corresponding to a refractive index of the optical block, the refractive index of the optical block is operated to correct the curvature of field. 
     Further, when the single lens is used together with the optical block and the radius of curvature of the lens is selected to a predetermined value with respect to the ratio of optical path length, the refractive index of the optical block is operated to correct the curvature of field. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side view of an optical scanning apparatus showing a first embodiment of the present invention; 
     FIG. 2 is a plan view of the optical scanning apparatus showing the first embodiment of the present invention; 
     FIG. 3 is a graph showing a characteristic of the ratio of optical path length for correcting the curvature of field versus the refractive index of an optical block member in the first embodiment of the present invention; 
     FIG. 4 is a characteristic diagram showing correction effects of the curvature of field with a dimension ratio in the first embodiment of the present invention; 
     FIG. 5 is a side view showing a second embodiment of the present invention; 
     FIG. 6 is a side view showing a third embodiment of the present invention; 
     FIG. 7 is a side view showing a fourth embodiment of the present invention; 
     FIG. 8 is a plan view of the fourth embodiment of the present invention; 
     FIG. 9 is a side view showing a fifth embodiment of the present invention; 
     FIG. 10 is a side view showing a sixth embodiment of the present invention; 
     FIG. 11 is a side view showing a seventh embodiment of the present invention; 
     FIG. 12 is a plan view showing the seventh embodiment of the present invention; 
     FIG. 13 is a characteristic diagram showing a relation of the optical length of the optical block capable of correcting the curvature of field and the ratio of curvature of the optical lens in the seventh embodiment of the present invention; 
     FIG. 14 is a side view showing an eighth embodiment of the present invention; 
     FIG. 15 is a plan view showing the eighth embodiment of the present invention; 
     FIG. 16 is a side view showing a ninth embodiment of the present invention; 
     FIG. 17 is a side view showing a tenth embodiment of the present invention; and 
     FIG. 18 illustrates the distortion aberration and the curvature of field in the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of the present invention is now described with reference to FIGS. 1 and 2. 
     FIG. 1 is a side view of an optical scanning apparatus as viewed from the side thereof and FIG. 2 is a plan view of the apparatus as viewed from the top thereof. An optical block 1 is disposed between a rotating mirror 2 constituting scanning means and a photosensitive body 3 and includes specular surfaces 1 and 2 disposed near the rotating mirror 2 and the photosensitive body 3, respectively. The specular surfaces constitute part of principal surfaces of the optical block 1 and are a pair of specular surfaces for reflecting light toward the inside thereof and which are structured so that plane surfaces parallel to each other are opposed. An optical beam emitted from means 4 for generating laser beam passes through a lens 5 and impinges on the rotating mirror 2, which reflects the optical beam. The reflected optical beam is incident on the optical block 1 through a notch of the specular surface of the optical block 1. The incident optical beam is reflected by the specular surface 2 on the side of the photosensitive body 3 and the specular surface 1 on the side of the rotating mirror 2 of the optical block 1 alternately in succession and finally goes out from the optical block 1 through a notch of the specular surface on the side of the photosensitive body 3 to impinge on the photosensitive body 3. 
     As shown in the plan view of FIG. 2, since the rotating mirror 2 is rotated by a mirror motor 6, the photosensitive body 3 is scanned by the laser beam and is exposed. In this way, since the laser beam is reflected plural times within the optical block, an actual length of an optical path (effective length of optical path) from the rotating mirror 2 to the photosensitive body 3 is larger than a distance T from the rotating mirror to the photosensitive body as shown in FIGS. 1 and 2. It has been found from a result of a simulation in which the distance T, a length W between the specular surfaces 1 and 2 (substantially equal to a length of a side of the optical block) and the number of times m of the reflection are varied that the curvature of field can be corrected when a ratio α (ratio of optical path length) of the whole length of the optical path from the rotating mirror to the photosensitive body and the length of the optical path including the reflection within the optical block is set to a predetermined value. 
     The simulation for correcting the curvature of field is now described. 
     1 When optical beams emitted from two points on the optical lens 5 in a plane containing the optical beam shown in FIG. 2 is scanned in the central direction of the main scanning (in the perpendicular direction to the rotating axis of the photosensitive body), the focal distance of the optical lens 5 is determined so that the two optical beams are joined or combined (focused) on the photosensitive body. 
     2 The scanning angle of the rotating mirror 2 is then changed to track the two optical beams so that a position on the photosensitive body where both of the optical beams are joined is obtained. 
     3The distance T, the length W, the refractive index n of the optical block member and the number of times of reflection are changed and when scanning is made in the main scanning direction to at least a predetermined width of paper (for example, A3 size) on the photosensitive body 3, deviation of the focal point from the surface of the photosensitive body is calculated repeatedly in the trial and error manner. 
     4 A combination of deviation of the focal point from the surface of the photosensitive body which is smaller than the last calculated value is set as an initial value to implement the above calculation 3 and this calculation is repeatedly made until the deviation of the focal point is smaller than a predetermined allowable value. 
     FIG. 3 shows a ratio α of optical path length having the corrected curvature of field versus the refractive index n of the optical block, obtained by a simulation. α is the ratio of optical path length at the optical path in the central direction in case where the whole width of the surface to be scanned is scanned. The refractive index of the optical block manufactured of material which is currently available is 1.2 to 1.7 and FIG. 3 shows the ratio of optical path length versus the refractive index having 1.2 to 3. As apparent from FIG. 3, a value of α capable of correcting the curvature of field is varied in dependence on the refractive index, while it is understood that α may be selected to be at least 0.6 or more in order to correct the curvature of field. When the whole length of optical path of light traveling from the rotating mirror to the photosensitive body is L and a distance from the rotating mirror to the photosensitive body is A, a value α is expressed by: 
     
         α=A/L                                                (2) 
    
     where W is a length of the optical block in the direction of the optical path, and m is the number of times of reflection of light within the optical block. 
     In the case of FIG. 1, when the distance T from the rotating mirror to the body to be scanned, the length W of the optical block in the direction of the optical path and the number of times m of reflection of light within the optical block are used, the equation is expressed as follows: 
     
         A=W·(m+1)                                         (3) 
    
     
         L=T+W·m                                           (4) 
    
     
         α=W·(m+1)/(T+W·m)                  (5) 
    
     That is, by selecting W, T and m properly, the dimension ratio α capable of correcting the curvature of field can be obtained. 
     In the embodiment, a pair of specular surfaces are constituted by parallel planes, while the pair of specular surfaces are not limited to the parallel planes and one or both of them may be curved. In brief, as far as it is structured so that light is reflected plural times and light comes into the optical block from one side thereof and goes out from the optical block through the other side thereof, the essence of the present invention is not varied. 
     FIG. 4 shows variation of the curvature of field when the number of times of reflection within the optical block 2 is varied in the case where the distance T between the rotating mirror and the photosensitive body is 200 mm. It is understood that there is the dimension ratio W/T capable of reducing the curvature of field to zero. When the number of times of reflection is two, the dimension ratio W/T is 0.32 and a calculated value of the whole length of the optical path at this time is 328 mm. That is, the distance T which does not include the reflection can be reduced to about 60% (=200 mm) of the whole effective length of the optical path by using the optical block and at the same time the curvature of field can be removed. Similarly, when the number of times of reflection is three, the whole length of the optical path is 392 mm and a reduction ratio of the optical path is about 51%. It is understood that by increasing the number of times of reflection in this way a small optical scanning apparatus without the curvature of field can be obtained. Material of the optical block may be any material as far as a predetermined refractive index is obtained. For example, plastic is light, cheap and practical and generally has the water absorption property and the dimension and the refractive index which tend to vary as compared with glass, while there is an effect that influence due to variation of the dimension and the refractive index is small if plastic is used to structure as in the present invention. 
     FIG. 5 illustrates a second embodiment (modification) of the present invention, in which an exit of light emerging from the optical block and the photosensitive body 3 are disposed on the side of the rotating mirror 2, so that the optical scanning apparatus is structured small. In this case, by selecting the above-mentioned α as shown in FIG. 3, there can be obtained the effect of correcting the curvature of field. 
     In the embodiments shown in FIGS. 1 and 2, light travels within the optical block in the direction of connecting the rotating mirror and the photosensitive body, while light may be reflected within the optical block in any direction in dependence on a structure of the specular surface of the optical block and if α is selected on the basis of the whole length of the optical path through which light passes, the curvature of field is corrected effectively. 
     FIG. 6 shows a third embodiment of the present invention. In FIG. 6, the rotating mirror 2 inclines in the direction perpendicular to the rotating axis thereof due to a manufacturing accuracy of mirror surfaces of the rotating mirror and deflection or yaw of the rotating axis, that is, has a so-called inclination of surface. This inclination of surface causes a positional deviation of the optical beam in the sub-scanning direction on the photosensitive body. In order to prevent influence of the inclination of surface, as shown in FIG. 6, the specular surface 2 of the optical block near the photosensitive body is curved (power) in the sub-scanning direction so that the outer surface of the optical block is convex toward the outside. Within the optical block, the inner surface of the specular surface 2 serves as a concave mirror and accordingly functions in the same manner that a convex lens is placed within the optical block. At this time, by selecting the curvature of the specular surface 2 so that light focused in the sub-scanning direction at the position of the rotating mirror is re-focused on the photosensitive body, that is, by selecting the curvature of the optical block so that the position of the focal point is conjugate, the inclination of surface can be corrected. In this case, there is a merit that the curvature of field and the inclination of surface can be corrected only by the optical block and the optical system can be simplified. 
     FIGS. 7 and 8 show a fourth embodiment of the present invention with FIG. 7 being a side view of the optical scanning apparatus and FIG. 8 being a plan view. In the embodiment, a cylindrical lens having power is disposed in the sub-scanning direction between the photosensitive body and the optical block instead of the provision of the curvature on the optical block. By selecting the focal distance of the cylindrical lens 7 so that the rotating mirror and the photosensitive body are conjugate to each other, there is an effect that the inclination of surface is corrected. The specular surfaces of the optical block 1 are formed into parallel planes which are easy to make, so that the curvature of field and the inclination of field can be corrected. 
     FIG. 9 shows a fifth embodiment of the present invention, in which a convex portion 8 is formed at the exit of light of the optical block in the sub-scanning direction and has an effect of correcting the inclination of surface in the same manner as FIG. 6. 
     FIG. 10 shows a sixth embodiment of the present invention, in which a cylindrical lens having one surface which is formed into a plane and the other surface which is formed into a convex surface is mounted instead of the convex portion 8 of FIG. 8 and has an effect of correcting the inclination of surface in the same manner as FIG. 9. 
     In the embodiments described above, any refractive member having power such as a lens is not disposed in the main scanning direction between the rotating mirror and the photosensitive body and light is focused on the photosensitive body by means of the optical lens disposed between the laser generating means 4 and the rotating mirror 2. Accordingly, it is necessary to lengthen the focal distance of the optical lens and hence it is an obstacle to miniaturization. In order to solve this problem, it is considered that one optical lens is disposed between the rotating mirror and the optical block. FIGS. 11 and 12 show a seventh embodiment of the present invention which attains the above idea and are a side view and a plan view of the optical scanning apparatus, respectively. An optical lens 10 is disposed between the rotating mirror 2 and the photosensitive body 3. In such a structure, it has been found from a result of a simulation for investigating a relation among the distance T, the length W and the curvature of surface of the optical lens 10 that there is a combination of values thereof capable of correcting the curvature of field. FIG. 13 is a graph showing a relation of a ratio of the radius of curvature (abscissa) of the optical lens 10 having the corrected curvature of field and the ratio α of optical path length (ordinate). As shown in FIG. 13, when the ratio α is selected to be 0.4 or more in consideration of the reduction ratio of the practical length of the optical path, there is shown that the curvature of field can be corrected within the range that R1/R2 is equal to or smaller than 0.4 where R1 and R2 are the radii of both surfaces of the optical lens 10, respectively. There is correction effect even if α is equal to or smaller than 0.4, while the optical system is large in size and a merit of using the optical block is reduced. 
     FIGS. 14 and 15 show an eighth embodiment in which the present invention is applied to a reading and scanning system and are a side view and a plan view thereof, respectively. The configuration of the scanning system is similar to the case of FIG. 11, while the traveling direction of light is opposite. (Light from the surface 11 to be scanned is incident on the optical block.) The surface to be scanned is an object 11 to be read such as a document. The object 11 to be read is illuminated by an illuminating light source 12 and light reflected by the object comes into the optical block 1 in which the light is reflected back and forth plural times. The reflected light within the optical block then passes through an optical lens 13 and reaches a reading sensor 14. A relation of the ratio α of the whole length of the optical path from the object 11 to be read to the reading sensor 14 and the length of the optical path including the reflection within the optical block is as shown in FIG. 13 described before and selection of α and R1/R2 can correct the curvature of field. Even in this case, the optical lens 13 has a convex surface facing to the optical block and a concave surface facing to the reading sensor. In this way, utilization of one optical block and one optical lens can attain the reading and scanning apparatus which is small in size and has no curvature of field. 
     FIG. 16 shows a ninth embodiment of the present invention. The embodiment is applied to an exposure scanning for a photosensitive body using a laser beam in a printer of an electrophotographic system. Light reflected by the rotating mirror 15 comes into the optical block 16 in which the light is reflected back and forth plural times. Then the light passes through a mirror 17 and reaches a photosensitive body 18. A latent image formed on the photosensitive body is developed by a developing device 19, the image being transferred onto a recording paper by a transferring corotron 20 and fixed by a fixing device 21. The recording paper is contained in a cassette 22. The whole control is made by a controller 23. With such a configuration, the printer itself can be made small and an inexpensive printer can be attained. Further, the printer using the optical block is not influenced by dust and dirt containing toner, vibration from the outside in a place in which the apparatus is installed, vibration of a moving portion in the printer and variation of surroundings such as temperature, humidity, atmospheric pressure and the like. 
     FIG. 17 shows a tenth embodiment of the present invention. The embodiment is applied to a reading portion and a recording portion of a facsimile. A document to be read 24 is illuminated by an illuminating light source and the reflected light is further reflected by a mirror 26, the light coming into an optical block 27 and being reflected in the optical block 27 back and forth plural times. Then, the light reaches a reading sensor 28. An image signal read by the reading sensor is encoded to compress a redundancy thereof and is modulated by a modem so that the modulated signal is transmitted to a communication line. The image signal transmitted through the communication line is demodulated by the modem and is returned to an encoded image signal, the image signal being decoded to the original image signal to extend the redundancy, the signal being recorded in the recording portion. The recording portion is configured in the same manner as in FIG. 16. In this way, by using the optical system including the optical block in the reading portion and the recording portion, a small and inexpensive facsimile can be provided. Further, similarly to the case of FIG. 16, there can be attained the facsimile which is not influenced by dust, vibration and surroundings. 
     The present invention is not limited to the above embodiments, while it can be applied to a reading scanner and an optical system for a drawing apparatus for photomask and an exposure apparatus for photoresist used in a manufacturing process of LSI. Further, it is needless to say that the present invention can be applied to an information input/output system including a printer, a facsimile and the like provided with the above optical scanning apparatus, for example, a desk top publishing system, a printing system, a presentation system and the like. 
     In manufacturing the optical block, material may be poured into a mold or a curved portion and a plane portion may be manufactured separately to be joined or affixed to each other. 
     Since the optical block provided with the specular surfaces formed in part of the inside thereof is used to lengthen the length of the optical path, the distortion aberration can be reduced and the curvature of field and the inclination of surface can be corrected. Accordingly, the optical scanning apparatus which is small in size can be attained. Further, the expensive fθ lens is not required and hence the inexpensive optical scanning apparatus can be achieved.