Abstract:
An optical scanning apparatus is capable of scanning a surface with a plurality of fluxes of light along a principal scanning direction. In the apparatus, a plurality of fluxes of light are emitted by a plurality of light sources arranged at intervals along an auxiliary scanning direction perpendicular to the principal scanning direction. The plurality of fluxes of light are lead to an optical deflector via an optical system for incident light and deflected by the optical deflector. Most part of the deflected fluxes of light are lead to the surface via an imaging optical system for scanning the surface, while small part of the deflected fluxes of light are lead to a photodetector to generate a synchronizing signal for controlling the timing of the scanning start of each of the fluxes of light. A slit is arranged between the optical deflector and the photodetector and the slit is inclined relative to the auxiliary scanning direction, whereby the scanning start timings for the plurality of fluxes of light are adjusted. The optical scanning apparatus is used in an image forming apparatus or an electrophotographic printer.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical scanning apparatus adapted to scan a surface to be scanned by means of a plurality of fluxes of light and also to an image forming apparatus such as a laser beam printer or a digital copying machine and an electrophotographic printer using such an optical scanning apparatus. 
     2. Related Background Art 
     In many conventional optical scanning apparatus that are used in image forming apparatus, the flux of light emitted from a light source is led to a deflection means by way of an optical means for incident light and the flux of light deflected by the deflection means is focussed on the surface to be scanned to form a spot of light there, which is then used to scan the surface by a scanning optical means. 
     In the course of technological development in recent years in the field of image forming apparatus, seeking for higher performances and higher functions, there has been and still is a strong demand for optical scanning apparatus that are compact and operate at high speed. An idea for meeting the demand for high speed operation is the use of a plurality of light sources. On the other hand, the demand for down-sizing may be met by converging the flux of light to be led to the deflection means along the principal scanning direction of the scanning optical means. 
     Japanese Patent Application Laid-Open No. 9-54263 discloses an optical scanning apparatus proposed to meet the demand for high speed operation. In the proposed optical scanning apparatus, a plurality of fluxes of light are emitted in a linear array from a single multi-beam laser chip. Japanese Patent Application Laid-Open No. 5-45580 discloses an optical scanning apparatus proposed to meet the demand for down-sizing. In the proposed optical scanning apparatus, the flux of light emitted from a light source is converged to a point of spontaneous convergence located behind the surface to be scanned by means of a focussing optical system. 
     However, optical scanning apparatus using a plurality of light sources are accompanied by the problem that the scanning start points of the fluxes of light from the light sources can be displaced from each other. While this problem may be dissolved by providing different time lags for the synchronizing signal to be applied to the light sources in order to cause the latter to start their respective scanning operations with the time lags. However, the time lags will have to be provided electrically or by means of software for each light source to make the overall configuration very complex and limit the scanning speed of the apparatus. 
     On the other hand, optical scanning apparatus designed to converge the fluxes of light led to the deflection means along the principal scanning direction utilize a rotary polyhedral mirror having its center of rotation located off the deflection plane as deflection means. If the fluxes of light from the optical means for incident light are convergent or divergent in such an arrangement, while the scanning start points can be aligned along the principal scanning direction, the deflection points may be shifted relative to the optical axis of the optical means for incident light to shift the points of spontaneous convergence as the rotary polyhedral mirror rotate. Thus, an image forming apparatus comprising such an optical scanning apparatus may have the spot images of the light sources displaced relative to the principal scanning direction as the rotary mirror rotates to make it impossible to form a high quality image. 
     SUMMARY OF THE INVENTION 
     In view of the technological problems of the prior art, it is therefore the object of the present invention to provide an optical scanning apparatus that can prevent any displacement of the scanning start points of the plurality of fluxes of light emitted from so many light sources from occurring by a simple means and also to provide an image forming apparatus and an electrophotographic printer comprising such an optical scanning apparatus. 
     In an aspect of the present invention, the above object is achieved by providing an optical scanning apparatus for scanning a surface to be scanned with a plurality of fluxes of light along a principal scanning direction, the apparatus comprising: 
     a plurality of light sources arranged at intervals along an auxiliary scanning direction perpendicularly to the principal scanning direction; 
     an optical deflector for deflecting the plurality of fluxes of light emitted from the plurality of light sources; 
     an optical system for incident light adapted to lead the plurality of fluxes of light emitted from the plurality of light sources to the optical deflector; 
     an imaging optical system for forming images of the plurality of fluxes of light deflected by the optical deflector on the surface to be scanned; 
     a photodetector for detecting part of the plurality of fluxes of light deflected by the optical deflector and outputting a synchronizing signal for controlling the timing of the scanning start of each of the fluxes of light; and 
     a slit arranged between the optical deflector and the photodetector; 
     the slit being inclined relative to the auxiliary scanning direction. 
     In another aspect of the invention, there is provided an image forming apparatus for forming an image by scanning the surface to be imaged with a plurality of fluxes of light along a principal scanning direction, the apparatus comprising: 
     a plurality of light sources arranged at intervals along an auxiliary scanning direction perpendicular to the principal scanning direction; 
     a central processing unit for driving the plurality of light sources independently relative to each other according to an image signal; 
     an optical deflector for deflecting the plurality of fluxes of light emitted from the plurality of light sources; 
     an optical system for incident light adapted to lead the plurality of fluxes of light emitted from the plurality of light sources to the optical deflector; 
     an imaging optical system for forming images of the plurality of fluxes of light deflected by the optical deflector on the surface to be imaged; 
     a photodetector for detecting part of the plurality of fluxes of light deflected by the optical deflector and outputting a synchronizing signal for controlling the timing of the scanning start of each of the fluxes of light; and 
     a slit arranged between the optical deflector and the photodetector; 
     the slit being inclined relative to the auxiliary scanning direction. 
     In still another aspect of the invention, there is provided an electrophotographic printer for forming an image by scanning the surface of a photosensitive body with a plurality of fluxes of light along a principal scanning direction, the apparatus comprising: 
     a plurality of light sources arranged at intervals along an auxiliary scanning direction perpendicular to the principal scanning direction; 
     a central processing unit for driving the plurality of light sources independently relative to each other according to an image signal; 
     an optical deflector for deflecting the plurality of fluxes of light emitted from the plurality of light sources; 
     an optical system for incident light adapted to lead the plurality of fluxes of light emitted from the plurality of light sources to the optical deflector; 
     a photodetector for detecting part of the plurality of fluxes of light deflected by the optical deflector and outputting a synchronizing signal for controlling the timing of the scanning start of each of the fluxes of light; 
     a slit arranged between the optical deflector and the photodetector, the slit being inclined relative to the auxiliary scanning direction; 
     a developing unit for developing an electrostatic latent image formed by scanning the surface of the photosensitive body with the plurality of fluxes of light; 
     an image transfer unit for transferring the developed toner image onto paper; 
     a fixing unit for fixing the toner image transferred on the paper. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic plan view of an embodiment of optical scanning apparatus according to the invention. 
     FIG. 2 is a schematic illustration of the light source unit of the embodiment of FIG. 1, showing the positional arrangement of the light sources. 
     FIG. 3 is a schematic perspective view of the embodiment of FIG.  1 . 
     FIG. 4 is a schematic cross sectional view of an embodiment of electrophotographic printer according to the invention and comprising the embodiment of optical scanning apparatus of FIGS. 1 through 3. 
     FIG. 5 is a schematic perspective view of a slit means that can be used for an optical scanning apparatus according to the invention. 
     FIG. 6 is a schematic perspective view of the rotary plate of the slit means of FIG. 5, where the slit is not inclined. 
     FIG. 7 is a schematic chart of scanning start timings when the slit is held to the state of FIG.  6 . 
     FIG. 8 is a schematic perspective view of the rotary plate of the slit means of FIG. 5 where the slit is slightly inclined. 
     FIG. 9 is a schematic chart of scanning start timings when the slit is held to the state of FIG.  8 . 
     FIG. 10 is a schematic perspective view of the rotary plate of the slit means of FIG. 5 where the slit is further inclined. 
     FIG. 11 is a schematic chart of scanning start timings when the slit is held to the state of FIG.  10 . 
     FIG. 12 is a schematic illustration of a slit that can be used for the purpose of the invention, where the slit is not inclined. 
     FIG. 13 is a chart illustrating the waveforms of the outputs of the synchronizing signal sensor of the embodiment of optical scanning apparatus using the slit of FIG. 12 when the slit is held to the state of FIG.  12 . 
     FIG. 14 is a schematic illustration of the slit of FIG. 12, where the slit is inclined. 
     FIG. 15 is a chart illustrating the waveforms of the outputs of the synchronizing signal sensor of the embodiment of optical scanning apparatus using the slit of FIG. 12 when the slit is held to the state of FIG.  14 . 
     FIG. 16 is a schematic illustration of another slit that can be used for the purpose of the invention and has a notch, where the slit is not inclined. 
     FIG. 17 is a chart illustrating the waveforms of the outputs of the synchronizing signal sensor of the embodiment of optical scanning apparatus using the slit of FIG. 16 when the slit is held to the state of FIG.  16 . 
     FIG. 18 is a schematic illustration of the slit of FIG. 16, where the slit is inclined. 
     FIG. 19 is a chart illustrating the waveform of the outputs of the synchronizing signal sensor of the embodiment of optical scanning apparatus using the slit of FIG. 16 when the slit is held to the state of FIG.  18 . 
     FIGS. 20,  21  and  22  are schematic illustrations of possible positional relationships of a deflection plane and two fluxes of light that can appear in an optical scanning apparatus according to the invention. 
     FIGS. 23 and 24 are schematic illustrations showing how a point of spontaneous convergence is shifted. 
     FIG. 25 is a schematic chart illustrating the timings of the scanning starts of fluxes of light that are differentiated from each other. 
     FIG. 26 is a schematic illustration of a slit that can be used for the purpose of the invention, where the slit is inclined. 
     FIG. 27 is a schematic chart illustrating the timings of the scanning starts of fluxes of light that can be regulated by means of the slit of an optical scanning apparatus according to the invention. 
     FIG. 28 is a schematic perspective view of a slit means obtained by modifying that of FIG.  5 . 
     FIG. 29 is a schematic perspective view of another slit means obtained by modifying that of FIG.  5 . 
     FIG. 30 is a schematic perspective view of still another slit means obtained by modifying that of FIG.  5 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the present invention will be described in greater detail by referring to the accompanying drawings that illustrate preferred embodiments of the invention. 
     FIG. 1 is a schematic plan view of an embodiment of optical scanning apparatus according to the invention and comprising a light source unit  1  that may be a semiconductor laser array adapted to emit two fluxes of light, an optical system for incident light including a collimator lens  2 , an aperture control  3  and a cylindrical lens  4  that are arranged along the path of the fluxes of light and a light deflector that may be a rotary polyhedral mirror  5 . The rotary polyhedral mirror  5  is supported by a drive motor (not shown) and driven to rotate in the sense of arrow A at high speed. 
     A scanning optical system (focussing optical system) including fθ lenses  6   a  and  6   b  and a surface to be scanned  7  which may be that of a photosensitive drum are arranged on the path of most of the fluxes of light deflected by the deflecting plane  5   a  of the rotary polyhedral mirror  5 . Additionally, a mirror  8  is arranged on the path of part of the fluxes of light passing through an end of the fθ lens system including the lenses  6   a  and  6   b  and a slit means  9  and a synchronizing signal sensor (photodetector)  10  are arranged on the path of the fluxes of light reflected by the mirror  8 . The end of the fθ lens system including the lenses  6   a  and  6   b  and the slit means  9  form a synchronization detecting optical means whereas the synchronizing signal sensor  10  constitutes a synchronization detecting means. While the fluxes of light coming from the rotary polyhedral mirror  5  is partly focussed by an end of the fθ lens system including lenses  6   a  and  6   b  in the above description, an independent lens system may be used for focussing part of the fluxes of light coming from the rotary polyhedral mirror  5 . 
     As shown in FIG. 2 in enlarged dimensions, the light source unit  1  comprises a pair of light sources  1   a  and  1   b  that are separated from each other both in a principal scanning direction X and in an auxiliary scanning direction Y. The collimator lens  2  transforms the divergent fluxes of light from the light sources  1   a,    1   b  into convergent fluxes of light and the aperture control  3  trims and optimizes the profiles of the convergent fluxes of light coming from the collimator lens  2 . The cylindrical lens  4  shows a predetermined refracting effect only in the auxiliary scanning direction Y so that it focusses the convergent fluxes of light coming from the aperture control near the deflection plane  5   a  of the rotary polyhedral mirror  5  along the auxiliary scanning direction Y and produces a linear image along the principal scanning direction X. 
     The fθ lenses  6   a  and  6   b  are adapted to form respective spot images of the fluxes of light coming from the rotary polyhedral mirror  5  on the surface to be scanned  7  and also actually scan the surface to be scanned  7  with the respective spots of light at a constant rate in the direction indicated by arrow B. During this operation, the fθ lenses  6   a  and  6   b  establish a conjugated relationship between the vicinity of the deflection plane  5   a  of the rotary polyhedral mirror  5  and the vicinity of the surface to be scanned  7  in the auxiliary scanning direction Y to provide the function of correcting any relative inclination thereof, while the end portions of the fθ lenses  6   a  and  6   b  focus the fluxes of light reflected by the rotary polyhedral mirror  5  with a predetermined angle on the synchronizing signal sensor  10  and the slit means  9  determines the positions along the principal scanning direction X of the fluxes of light focussed on the surface to be scanned  7 . The synchronizing signal sensor  10  outputs a synchronizing signal for controlling the timings of scanning starts of the respective fluxes of light. 
     FIG. 3 is a schematic perspective view of the embodiment of optical scanning apparatus of FIG.  1 . The components same as those of FIG. 1 are denoted respectively by the same reference symbols. In this embodiment, the light sources  1   a  and  1   b  are semiconductor lasers that are connected to a laser drive circuit  92 . Central processing unit (CPU)  91  drives the light sources  1   a  and  1   b  independently by way of the laser drive circuit  92  according to the input video signal. 
     The two fluxes of light that are emitted from the respective light sources  1   a  and  1   b  and modulated according to the input video signal are collimated by the collimator lens  2  and, after passing through the aperture control  3 , they are made to strike the deflection plane of the rotary polyhedral mirror (optical deflector)  5  as long linear images running along the principal scanning direction formed by the lens  4 . The rotary polyhedral mirror  5  is driven to rotate at high speed by a spindle motor  95  in the sense indicated by arrow A to deflect the two fluxes of light. These two fluxes of light are then made to pass through the fθ lenses  6   a  and  6   b , reflected by a reflector mirror  94  and then focussed on the surface to be scanned  7  of the photosensitive drum  101  as so many spots of light. The spots of light are made to scan the surface to be scanned  7  along the principal scanning direction as the two fluxes of light are deflected. On the other hand, the photosensitive drum  101  is driven to rotate in the sense indicated by arrow C by means of a drive motor  93  and move relative to the spots of light along the auxiliary scanning direction. Thus an electrostatic latent image is formed on the photosensitive drum  101  as a result of the scanning motions of the spots of light along the principal scanning direction and the movement of the photosensitive drum  101  along the auxiliary scanning direction. 
     Meanwhile, part of the two fluxes of light deflected by the rotary polyhedral mirror  5  are reflected by the mirror  8 , made to pass through the slit means  9  and received by the synchronizing signal sensor (photodetector)  10 . The synchronizing signal sensor  10  is connected to a synchronizing signal generating circuit  90 , which circuit  90  generates a synchronizing signal according to the output of the synchronizing signal sensor  10 . The synchronizing signal generated by the synchronizing signal generating circuit  90  is then input to the central processing unit  91 , which unit  91  then controls the timings of scanning starts of the spots of light on the surface to be scanned  7  and hence the timing of drawing an image. 
     FIG. 4 is a schematic cross sectional view in the auxiliary scanning direction of an embodiment of electrophotographic printer according to the invention and comprising the above described embodiment of optical scanning apparatus. In FIG. 4, there are shown an electrophotographic printer  117  and the embodiment of optical scanning apparatus  100  as described above by referring to FIGS. 1 through 3. In FIG. 4, reference numeral  101  denotes a photosensitive drum adapted to carry a electrostatic latent image and reference numeral  102  denotes a charging roller arranged above and held in contact with the photosensitive drum  101  in order to uniformly charge the surface of the photosensitive drum  101  with electricity. The surface of the photosensitive drum  101  is irradiated in an area downstream relative to the charging roller  102  with fluxes of light  103  coming from the optical scanning apparatus  100 . 
     The fluxes of light  103  are modulated according to the input image data so that an electrostatic latent image is formed on the surface of the photosensitive drum  101  as the latter is irradiated with the fluxes of light  103 . The electrostatic latent image is then developed into a toner image by means of developing unit  107  arranged to contact the photosensitive drum  101  at a position downstream relative to the spots where the surface of the photosensitive drum  101  is irradiated with the beams of light  103 . Thereafter, the toner image is transferred onto paper  112  by means of transfer roller  108  arranged vis-a-vis under the photosensitive drum  101 . While the paper  112  is stored in a paper cassette  109  located in front of the photosensitive drum  101  (and right to the drum  101  in FIG.  4 ), it may alternatively be fed with hand. A feed roller  110  is arranged at an end of the paper cassette  109  to feed the paper  112  in the paper cassette  109  to the paper feed path by way of register rollers  111 . 
     Thus, the paper  112  onto which the unfixed toner image is transferred is then moved further to fixing unit arranged behind the photosensitive drum  101  (and left to the drum  101  in FIG.  4 ). The fixing unit contains in the inside a fixing roller  113  having a fixing heater (not shown) and a press roller  114  pressed against the fixing roller  113  so that, as the paper  112  fed from the transfer section of the printer is pressed and heated between the fixing roller  113  and the press roller  114 , the unfixed toner image becomes fixed on the paper. The paper  112  now carrying the fixed image is then delivered to the outside of the printer by way of delivery rollers  116  provided behind the fixing roller  113 . 
     Now, the slit means  9  that constitutes the most characteristic aspect of the present invention will be described in detail below. 
     Referring to FIG. 5 showing a perspective view of the slit means  9 , it comprises a rotary plate  12  having a slit for allowing fluxes of light to pass therethrough, a holder  13  for rotatably holding the rotary plate  12  and a rotary mechanism  14  for driving the rotary plate  12  to rotate. The rotary mechanism includes a drive rod  15  linked to the rotary plate  12  by way of a horizontal hole  13   a  and exposed to the inside of a vertical hole  13   b  formed in the holder  13 . An adjuster screw  16  having a knob section  16   a  for adjusting the rotary position of the drive rod  15  is extending through a screw hole  13   c  bored in the holder  13  to abut the drive rod  15 , which drive rod  15  is urged toward the adjuster screw  16  by means of a coil spring  17 . 
     FIG. 6 is a schematic perspective view of the rotary plate of the slit means of FIG. 5, where fluxes of light La and Lb emitted from respective light sources  1   a  and  1   b  are made to pass through the slit  11  that is not inclined, whereas FIG. 7 is a schematic chart of the scanning start points of the fluxes of light that are not displaced but the scanning end points are displaced from each other by an amount of displacement Xe. The sign of Xe in FIG. 7 is indicated as plus when the position of scanning end point for the flux of light Lb is displaced with reference to the position of scanning end point for the flux of light La. If the slit  11  is inclined under this condition as shown in FIG. 8, the scanning start points are negatively displaced by an amount of displacement Xs as shown in FIG. 9 to reduce the amount of displacement Xe of the scanning end points. The sign of Xs in FIG. 9 is indicated in the same manner as mentioned above for Xe. As the slit  11  is further inclined as shown in FIG. 10, the amount of displacement Xs of the scanning start points is increased to by turn reduce the amount of displacement Xe of the scanning end points as shown in FIG.  11 . 
     Thus, it is possible to adjust the timings of detecting the fluxes of light La and Lb for the synchronizing signal sensor  10  and hence their scanning start points can be adjusted by appropriately inclining the slit  11  of the slit means  9  in the auxiliary scanning direction Y. This will be described further below. 
     FIG. 12 is a schematic illustration of a slit that can be used for the purpose of the invention, where the slit is not inclined, and FIG. 13 is a chart illustrating the waveforms of the outputs of the synchronizing signal sensor when the slit is held to the state of FIG.  12  and the rising edges of the fluxes of light La and Lb are made to agree with each other but the quantities of light of the fluxes of light La and Lb are differentiated. Note that, in FIG. 13, the solid curved line indicates the flux of light La, whereas the broken curved line indicates the flux of Lb. The reference line drawn at the height of the offset value of 0 is used to determine if the fluxes of light La and Lb have passed through the synchronizing signal sensor  10 . Thus, FIG. 13 illustrates that there is a time lag Δt between the flux of light La and the flux of light Lb that have gone over the offset value 0 and the scanning start of the flux of light La is behind that of the flux of light Lb by time Δt. 
     On the other hand, FIG. 14 is a schematic illustration of the slit same as that of FIG. 12, where the slit is inclined, and FIG. 15 is a chart illustrating the waveforms of the outputs of the synchronizing signal sensor when the slit is held to the state of FIG.  14 . Then, both the flux of light La and the flux of light Lb cross the offset value line  0  at the same time and the synchronizing signal sensor  10  issues a signal that makes the scanning start point of the flux of light La and that of the flux of light Lb agree with each other. 
     If the rotary plate  12  of the slit  11  has a notched section  12   a  as shown in FIG. 16, there is also a time lag Δt between the flux of light La and the flux of light Lb that have gone over the offset value 0 and the scanning start of the flux of light La is behind that of the flux of light Lb by time Δt as shown in FIG.  17 . However, if the slit  11  is inclined as shown in FIG. 18, the two fluxes of light La and Lb no longer show any time lag Δt and the synchronizing signal sensor issues a signal that makes the scanning start point of the flux of light La and that of the flux of light Lb agree with each other as shown in FIG.  19 . 
     Furthermore, if the time lag between the time when the synchronizing signal sensor  10  detects the flux of light La and the time when the sensor  10  detects the flux of light Lb and the time lag between the time when the flux of light La crosses the level of the image height for the scanning start point and the time when the flux of light Lb crosses the level of the image height for the scanning start point are attributable to the difference in the magnifying power or some other causes, the synchronizing signal sensor  10  is made to issue a signal for making the scanning start points of the two fluxes of light agree with each other by inclining the slit  11 . The same effect is obtained also when the fluxes of light La and Lb from the collimator lens are parallel or divergent fluxes. 
     On the other hand, since the fluxes of light La and Lb entering the rotary polyhedral mirror  5  are converging fluxes, the image point of the flux of light La and that of the flux of light Lb are displaced from each other along the principal scanning direction even if the scanning start points of the two fluxes agree with each other in the principal scanning direction X. In other words, the positional relationship between the points of spontaneous convergence of the fluxes of light La and Lb established by the optical means for incident light is modified as the rotary polyhedral mirror  5  rotates to move the reflection point along the optical axis of the optical means for incident light. 
     For instance, if the angle of deflection of the deflection plane  5   a  of the rotary polyhedral mirror  5  is equivalent for both of the fluxes of light La and Lb, the fluxes of light La and Lb scan different image heights. Then, if the angle of deflection of the flux of light La is differentiated from that of the flux of light Lb by providing a time lag between the time when the flux of light La starts scanning and the time when the flux of light Lb starts scanning so that the points of scanning start of the fluxes agree with each other, the deflection plane  5   a  is positionally shifted forwardly or rearwardly when the angle of deflection is modified because the center of rotation of the deflection plane  5   a  is not found on the deflection plane  5   a  itself. Additionally, the positional shift of the deflection plane  5   a  is not proportional to the angle of deflection but approximates a parabola that can be expressed by a quadratic function. 
     Therefore, if the fluxes of light La and Lb are located substantially at a same spot on the deflection plane  5   a  when the image height is equal to zero as shown in FIG.  20  and the fluxes of light La and Lb are deflected from the optical axis of the optical means for incident light by the deflection plane  5   a,  the deflection plane for the flux of light Lb is located closer to the light source  1   b  at the side of scanning start position as shown in FIG. 21, whereas the deflection plane for the flux of light La is located closer to the light source  1   a  at the side of scanning end position as shown in FIG.  22 . 
     Additionally, when one of the fluxes of light, e.g., flux of light La, is made to strike the deflection plane  5   a  and the latter is displaced from the position indicated by a solid line to the position indicated by a broken line in FIG. 23, the point of spontaneous convergence of the flux of light La is displaced too to the position also indicated by a broken line in FIG.  23 . Similarly, deflection plane  5   a  is rotated from the position indicated by a solid line to the position indicated by a broken line in FIG. 24, the point of spontaneous convergence of the flux of light La is also displaced too to the position also indicated by a broken line in FIG.  24 . Thus, the point of spontaneous convergence is moved depending on the angle of deflection along the principal scanning direction X by an amount that varies as a function of the image height. As a result, since the scanning start points of the two fluxes of light La and Lb are influenced by the displacement of the point of spontaneous convergence, their positional relationships relative to the principal scanning direction X are modified as a function of the movement of the deflection plane  5   a.    
     Therefore, even if the scanning start points of the two fluxes of light La and Lb are made to agree with each other in terms of the image height at which the scanning operation is started by arranging the slit  11  to an unrotated state under ideal conditions where no difference arises in the ratio of the quantities of light, the point of spontaneous convergence may be shifted subsequently in a different way for the fluxes of light La and Lb. Thus, even if the slit  11  is not inclined as shown in FIG. 12, the positional relationship of the image points of the two fluxes of light on the surface to be scanned  7  may be shifted along the principal scanning direction X as shown in FIG.  25 . If, on the other hand, the slit  11  is inclined in the auxiliary scanning direction Y as shown in FIG. 26, the shift can be minimized as shown in FIG.  27 . 
     Thus, in this embodiment, if the negative displacement of the scanning start points of the two fluxes of light La and Lb in the principal scanning direction X is expressed by ΔXs and the positive displacement of the scanning end points of the two fluxes of light La and Lb in the principal scanning direction X is expressed by ΔXe, it is desirable that the positional relationship of the image points of the two fluxes of light satisfies the requirement of formula (1) below; 
     
       
         Δ Xs.ΔXe&lt; 0  (1) 
       
     
     and also the requirement of formula (2) below; 
     
       
         −2&lt;Δ Xe/ΔXs&lt;− 0.5  (2). 
       
     
     Thus, both the scanning optical system of the above embodiment and an image forming apparatus using such an optical system can provide high quality images with a reduced level of jitter when the requirements of the above equations (1) and (2) are met. Additionally, the load for signal processing operations can be reduced to allow the use of a simply configured system for processing signals in order to produce images at high speed by making the lapse of time between the time when the synchronizing signal sensor  10  outputs a synchronizing signal and the time when a corresponding scanning operation is started is made equal for both of the fluxes of light La and Lb. 
     If it is not possible to use a rotary slit means  9  for some reasons such as manufacture-related problems and/or the requirements to be met for the arrangement of components, a slit means  21  having one of the lateral sides of the slit made tiltable as shown in FIG. 28 may alternatively be used. The slit means of FIG. 28 comprises a fixed slit member  22  providing a lateral side  22   a  of the slit, a rotary member  23  providing another lateral side  23   a  of the slit, a holder  24  for rotatably holding the rotary member  23 , an adjuster screw  25  for adjusting the rotary position of the rotary member  23  and a coil spring  26  for urging the rotary member counterclockwise, the slit  27  being defined by the two lateral sides  22   a  and  23   a.    
     If it is possible to know the angle of inclination of the slit  11  in advance, the slit means  9  may be replaced by a slit means  31  as shown in FIG.  29 . The slit means  31  comprises members  32  and  33  having respective inclined lateral sides  32   a,    33   a  that define the inclined slit  34 . 
     Still alternatively, a slit means  35  as shown in FIG. 30 may be used depending on the manufacturing conditions. The illustrated slit means  35  comprises members  36  and  37  having respective inclined lateral sides  36   a ,  37   a  that define a V-shaped slit  38 . With this arrangement, if the flux of light La or Lb that passes through a lower portion of the slit  38  has to be used for staring a scanning operation, a falling edge of the output signal of the synchronizing signal sensor  10  may be used for determining the timing of the start of the scanning operation, although it may be appreciated that the above description holds true for the slit means  21  shown in FIG.  28  and having an adjusting means. 
     The applicability of the present invention is by no means limited to the above embodiments, which may be modified in many various different ways without departing from the scope of the invention as defined by the claims that follow.