Abstract:
In an apparatus for forming multi-color image provided with a rotatable image carrying member and a plurality of image forming means each having a charging device, a scanning exposure device and a developing device; each scanning exposure device includes a beam generator and a polygonal mirror. The apparatus is further provided with a first sensor for detecting the beam passing at a start reference position on a peripheral surface of the rotatable image carrying member and for outputting a start reference signal; a second sensor for detecting the beam passing at an end reference position on the peripheral surface and for outputting a stop reference signal; and a controller to measure a time between the start reference signal and the stop reference signal and to control a frequency of clock pulses to transmit image signals for each scanning exposure device on the basis of the measured time of each scanning exposure device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an image forming apparatus of a single pass color system (hereinafter referred to as SPC) in which color forming processes for plural colors are conducted while an image carrier makes one turn, and in particular, to an image forming apparatus wherein plural images each being of a different color can be superposed without positional deviation. 
     As an image forming apparatus of an SPC system in which the number of copies per a unit of time is increased, there has been proposed, for example, an image forming apparatus wherein four sets (equivalent to four colors) each being composed of a scorotron charger, a scanning optical system and a developing unit which are arranged at prescribed intervals around an image carrier (a photoreceptor drum or a photoreceptor belt) in its rotational direction are arranged, and a transfer unit, a separating unit, a cleaning unit and a fixing unit are further provided so that image forming processes for plural colors may be conducted while the image carrier makes one turn. 
     In the image forming apparatus of an SPC system wherein plural scanning optical systems are arranged around an image carrier in its rotational direction, it is necessary to prevent deviation in the main scanning direction or in the sub-scanning direction by correcting with various sensors mounted around the image carrier. 
     However, there are deviation in the mounting positions for various sensors and dispersion in exposure width in the main scanning direction for each color, and these are further subjected to the change with time. As a result, it has been impossible to prevent positional deviation in the main scanning direction or in the sub-scanning direction among plural images each being of a different color. 
     SUMMARY OF THE INVENTION 
     In view of the aforesaid technical problems, an object of the invention is to provide an image forming apparatus capable of conducting highly accurate positioning in the main scanning direction and in the sub-scanning direction on a real time basis for plural images each being of a different color. 
     The invention solving the problems mentioned above is represented by the following structures. 
     An apparatus for forming multi-color image, comprises: 
     a rotatable image carrying member having a peripheral surface; 
     a plurality of image forming means each having a charging device, a scanning exposure device and a developing device so that the plurality of image forming means form the multi-color image during a single rotation of the image carrying member on the peripheral surface; 
     each scanning exposure device of the plurality of image forming means including a beam generator and a polygonal mirror and controlled such that the polygonal mirror is rotated at the same rotational speed as that of the other scanning exposure devices and a beam scans the peripheral surface in an axial direction of the image carrying member; 
     a first sensor for detecting the beam passing at a start reference position on the peripheral surface and for outputting a start reference signal; 
     a second sensor for detecting the beam passing at an end reference position on the peripheral surface and for outputting a stop reference signal; 
     measuring means for receiving the start reference signal and the stop reference signal sequentially for each scanning exposure device and for measuring a time between the start reference signal and the stop reference signal; and 
     control means for controlling a frequency of clock pulses to transmit image signals for each scanning exposure device on the basis of the measured time of each scanning exposure device. 
     Further, the object of the present invention may be attained by the following preferable structures. 
     Structure (1) An image forming apparatus which conducts image forming processes for plural colors while an image carrier makes one turn, and has therein a start position locating sensor that detects a position to start writing on the surface of the image carrier, an end position locating sensor that detects a position to end writing on the surface of the image carrier, an image forming member composed of a scanning optical system which conducts raster scanning by controlling so that the rotational speed of a polygon mirror may be the same, a charging unit and a developing unit, all arranged around the image carrier in its rotational direction, a time measuring means that measures an interval of time between a detection signal from at least one of the start position locating sensor and the end position locating sensor and an index signal from an index sensor provided on the scanning optical system, and a timing control means that controls exposure timing of the scanning optical system based on measured time coming from the time measuring means. 
     In the image forming apparatus stated above, it is possible to conduct positioning of an image for each color by controlling exposure timing by the use of at least one of the result of detection of the start position and the result of detection of the end position. 
     Structure (2) The image forming apparatus of Structure (1) wherein the time measuring means measures time between a detection signal from the start position locating sensor and a detection signal from the end position locating sensor, and there is provided a clock frequency control means which controls the clock frequency for image processing for exposure based on measured time coming from the time measuring means. 
     In the image forming apparatus stated above, it is possible to conduct positioning in the main scanning direction on an image of each color accurately for each pixel, because a clock frequency for image forming is controlled based on the time of main scanning between the start position and the end position. 
     Structure (3)The image forming apparatus of Structure (1) wherein a detection window means composed of either a reflection member or a transmission member is provided on the surface of the image carrier, and the start position locating sensor and the end position locating sensor conduct detection by receiving light from the detection window means. 
     In the image forming apparatus stated above, it is possible, due to the detection window means provided on the surface of the image carrier, to conduct positioning of an image in the main scanning direction and the sub-scanning direction by using scanning light of image forming. 
     Structure (4) The image forming apparatus of Structure (1) wherein a detection window means composed of either a reflection member or a transmission member is provided on the surface extended from the surface of the image carrier, and the start position locating sensor and the end position locating sensor conduct detection by receiving light from the detection window means. 
     In the image forming apparatus stated above, it is possible, due to the detection window means provided on the surface of the image carrier, to conduct positioning of an image in the main scanning direction and the sub-scanning direction by using scanning light of image forming, and to utilize the surface of the image carrier effectively. 
     Structure (5) The image forming apparatus of Structure (1) wherein there is provided a detection window means composed of either a reflection member or a transmission member wherein an edge of the main scanning starting side is not in parallel with that of the main scanning ending side, and the timing control means controls timing in the sub-scanning direction in accordance with a signal width obtained from the portion where the edge of the main scanning starting side and that of the main scanning ending side both of the detection window means are not in parallel with each other. 
     In the image forming apparatus stated above, it is possible to detect deviation of plural colors in the main scanning direction and the sub-scanning direction by the use of the detection window means having the edges which are not in parallel with each other. 
     Structure (6) The image forming apparatus of Structure (1) wherein there is provided a detection window means composed of either a reflection member or a transmission member wherein an edge of the main scanning starting side is not in parallel, and the timing control means controls timing in the sub-scanning direction in accordance with a signal width obtained from the portion where the edge of the main scanning starting side of the detection window means is not in parallel with each other. 
     In the image forming apparatus stated above, it is possible to detect deviation of plural colors in the main scanning direction and the sub-scanning direction by the use of the detection window means having the edges which are not in parallel with each other. 
     Structure (7) The image forming apparatus of Structure (1) wherein the time measuring means is equipped with a time measuring circuit which measures time in accuracy within one pixel clock and with a delay circuit which generates a clock wherein a phase is changed by 1/n within one pixel clock. 
     In the image forming apparatus stated above, it is possible to measure even timing that is less than one pixel clock. 
     Structure (8) The image forming apparatus of Structure (1) wherein the timing control means is equipped with a FIFO memory which adjusts exposure timing at a unit of a pixel clock and with a clock selection circuit which adjusts a phase of the pixel clock. 
     In the image forming apparatus stated above, the timing control means changes sending-out timing of image data in the main scanning direction to cope with deviation of rotational phase of a polygon mirror driven under the same clock. Accordingly, it is not necessary to make rotational phases of plural rotating polygon mirrors to be the same. 
     Structure (9) The image forming apparatus of Structure (1) wherein the scanning optical system is equipped with a fine adjustment prism which conducts fine adjustment of the exposure position within a pixel clock in the sub-scanning direction, and the fine adjustment prism is controlled by the timing control means. 
     In the image forming apparatus stated above, an exposure position in the sub-scanning direction is adjusted by the fine adjustment prism that is controlled by the timing control means. Due to this, deviation in the sub-scanning direction can be overcome. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing each function block in the electric structure of an image forming apparatus used in an embodiment of the invention. 
     FIG. 2 is a structure diagram showing illustratively the sectional structure of an image forming apparatus used in an embodiment of the invention. 
     FIG. 3 is a perspective view showing illustratively the main portions of an image forming apparatus used in an embodiment of the invention. 
     FIG. 4 is a sectional view of an optical system including a prism for position adjustment in the sub-scanning direction in an embodiment of the invention. 
     FIG. 5 is a structure diagram showing the arrangement of a detection window section, a start position locating sensor and an end position locating sensor in an embodiment of the invention. 
     FIG. 6 is a structure diagram showing an arrangement of a detection window section in an embodiment of the invention. 
     FIGS.  7 ( a ) to  7 ( j ) are time charts showing the state of operations in an embodiment of the invention. 
     FIGS.  8 ( a ) to  8 ( e ) are time charts showing the state of operations in an embodiment of the invention. 
     FIGS.  9 ( a ) to  9 ( d ) are time charts showing the state of deviation in the sub-scanning direction in an embodiment of the invention. 
     FIGS.  10 ( a ) to  10 ( d ) are time charts showing the other state of deviation in the sub-scanning direction in an embodiment of the invention. 
     FIGS.  11 ( a ) to  11 ( d ) are time charts showing the state of operations in an embodiment of the invention. 
     FIGS.  12 ( a ) and  12 ( b ) are illustrations showing the state of operations in an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the invention will be explained in detail as follows. 
     First, the structure of an image forming apparatus used in an embodiment of the invention will be explained, referring to FIG.  2  and thereafter. 
     An image forming apparatus used in the present embodiment is of an electrophotographic system type, and it employs either a photoreceptor drum or a photoreceptor belt as an image carrier. An image forming apparatus employing the photoreceptor drum will be explained here. 
     As shown in FIG. 2, there is provided drum-shaped image carrier  1  which rotates in the arrowed direction, and four sets each being composed of scorotron charger  2 , scanning optical system  3  and developing unit  4  are arranged at prescribed intervals around the image carrier  1  in its rotational direction. What is shown here is an occasion where an image is formed with four colors representing Y (yellow), M (magenta), C (cyan) and K (black), and four sets of  2 Y- 3 Y- 4 Y,  2 M- 3 M- 4 M,  2 C- 3 C- 4 C and  2 K- 3 K- 4 K are arranged. There are further arranged transfer unit  5 , separation unit  6 , a cleaning unit (not shown) and a fixing unit (not shown) around image carrier  1 . 
     Then, after the surface of the image carrier  1  is charged evenly, dot-shaped electrostatic latent images are formed by a spot light (laser beam) whose pulse width is modulated based on record signals modulated in accordance with digital image density data coming from a computer or a scanner. Then, a color toner image is formed on the image carrier  1  when four basic processes are repeated for four colors while the image carrier  1  makes almost one turn, in which the basic process is represented by a process wherein the electrostatic latent is subjected to reversal development that employs toner and thereby a dot-shaped toner image is formed. 
     Then, the color toner image is transferred by the transfer unit  5  onto a recording sheet which is then separated from the image carrier  1  by separation unit  6 . After that, the toner image is fixed on the recording sheet by the fixing unit, thereby a color image is obtained. The foregoing is image forming carried out by an image forming apparatus of a single pass color (SPC) system. 
     Structures of the primary portions will be explained in detail as follows. 
     First, mechanical structures of the image forming apparatus in the present embodiment will be explained. 
     Image carrier  1  is composed of a conductive support, an intermediate layer and a photosensitive layer. A thickness of the photosensitive layer is about 5 μm-100 μm, and it preferably is 10-15 μm. The image carrier  1  employs a drum shaped conductive support which is made of aluminum and has a diameter of 150 mm, on which a 0.1 μm-thick intermediate layer made of ethylene-vinyl acetate copolymer is provided, a 35 μm-thick photosensitive layer is provided on the intermediate layer. 
     Since four scanning optical systems  3 Y,  3 M,  3 C and  3 K are of the same structure, these will be explained as scanning optical system  300 , with reference to FIG.  3  and thereafter. FIG. 3 is a perspective view showing the scanning optical system in the present embodiment. 
     The scanning optical system  300  is one which makes semiconductor laser  301  to emit light based on record signals as shown in FIG. 3, and thereby conducts line scanning on the image carrier  1  to form an electrostatic latent image. 
     Namely, the semiconductor laser  301  is made to oscillate by pulse-width-modulated record signals, then laser beam L emitted from the semiconductor laser  301  is collimated by collimator lens  302  to be a collimated beam which is then reflected on polygon mirror  305  rotating at the constant speed to be deflected, and is converged by fθ lens  306  and cylindrical lenses  304  and  307  to a fine spot on the image carrier  1  for scanning. Incidentally, the cylindrical lenses  304  and  307  are those to correct the fluctuation of spot positions which is caused by face slant of the polygon mirror  305 . 
     Considering that color toner images are superposed on the image carrier  1 , the semiconductor laser  301  is represented by a semiconductor laser having the spectral sensitivity on the infrared side so that a laser beam emitted from the scanning optical system  300  may not be intercepted by color toner images. 
     The polygon mirror  305  is one corresponding to the rotating polygon mirror described in Structure, and it is represented by an octahedral mirror in the present embodiment. Incidentally, it is also possible to use a polygon mirror which is different from the octahedral mirror in terms of the number of mirror faces. 
     Index sensor  312  senses a beam reflected on reflecting mirror  311  and outputs electric current which is then subjected to current/voltage conversion in index detection circuit  313  to be outputted as an index signal. 
     This index signal detects a position of a face of a polygon mirror rotating at the prescribed speed, whereby optical scanning by modulated signals is conducted by a cycle in the main scanning direction through the raster scanning system. The index signal is supplied to time measuring circuit  400  which controls optical scanning timing of the scanning optical system  300 . 
     Prism  315  represents a main part of the fine adjustment prism described in the Structure, and it primarily is a prism which compresses, in the prescribed direction, a laser beam representing a collimated beam obtained by converging with collimator lens  302 , and it can adjust the scanning position in the sub-scanning direction when it is structured as will be stated later in the present embodiment. 
     Motor driver  316  sends out PLL-controlled driving clocks to polygon motor  317  for the purpose of stabilizing the rotational speed of polygon mirror  305 . Microprocessor  320  sends out clocks with the same cycle to four motor drivers  316 Y- 316 K so that polygon mirrors  305  forming four scanning optical systems  3 Y- 3 K may rotate at the same rotational speed. 
     In the present embodiment, a cycle of the driving clock is twice that of the rotational speed of polygon mirror  305  for the purpose of stabilizing the rotational speed of the polygon mirror  305 . 
     Incidentally, when the same driving clocks are given and thereby the polygon mirrors are rotated at the same rotational speed, rotary phase does not need to be the same. 
     FIG. 4 is a sectional view of an optical system including prism  315  for adjusting positions in the subscanning direction in the present embodiment. Incidentally, the structure shown in FIG. 4 corresponds to the fine adjustment prism described in the Structure. 
     The prism  315  is attached on prism mounting member  216  at the prescribed angle. The prism mounting member  216  is fixed on a cylindrical frame body (not shown), and the cylindrical frame body is mounted on prism mounting body  218  formed in casing  201 , to be capable of rotating in the direction for crossing light beam L. On a part of the cylindrical frame body, there are provided screw rods  219  and  220  which are screwed in the casing  201  to be symmetrical laterally, and the tip of the screw rod  219  is brought into direct contact with stepped portion  221  formed on the cylindrical frame body  217 , while the screw rod  220  on the other side is brought into contact with stepped portion  223  formed on the cylindrical frame body  217  through spring member  222  to be fixed. 
     As stated above, the screw rod  219  is rotated when rotating force of step motor  250  is transmitted through gears  151  and  252 , and thereby the prism  315  structured on the casing  201  is adjusted. In that case, the stepped portion  221  formed on the cylindrical frame body  217  is caused by spring member  222  to be constantly in contact with the tip of the screw rod  219 , and when the screw rod  219  rotates for adjustment, the prism  315  makes rotary adjustment for the sub-scanning direction while reducing light beam L to the prescribed width, through the cylindrical frame body  217  and prism mounting member  216 . After completion of adjustment, the tip of the screw rod  219  is constantly blocked by the stepped portion  221  formed on the cylindrical frame body  217 , thus, no deviation from the adjusted position is caused. 
     Next, detection window sections  330  and  340 , start position locating sensor  350  and end position locating sensor  360  in the present embodiment will be explained as follows, referring to FIGS. 5 and 6. In FIG. 5, various lenses of scanning optical system  300  are not shown, but surroundings of image carrier  1  are shown. 
     FIG. 5, the numeral  330  represents a detection window section structured on the surface of image carrier  1  with a reflection member or a transmission member,  340  represents a detection window section structured on the surface of image carrier  1  with a reflection member or a transmission member,  350  represents a start position locating sensor which detects the position of start writing on the surface of image carrier  1  and  360  represents an end position locating sensor which detects the position of the end of writing on the surface of image carrier  1 . 
     The start position locating sensor  350  is used as the first sensor to detect the beam passing as the start reference position and the end position locating sensor  360  is used as the second sensor to detect the beam passing as the end reference position. 
     Incidentally, even in the case of the start position locating sensor  350  and the end position locating sensor  360 , each of them is arranged to match scanning optical system  300  for each color of Y, M, C and K. 
     Each of the aforesaid detection window section and the locating sensor is one which detects scanning positions of beams projected on image carrier  1  from plural scanning optical systems. 
     The detection window sections  330  and  340  stated above may also be provided on the line extended from the surface of the photosensitive layer of image carrier  1 , for example, on frames provided on both sides of the photosensitive layer, in addition to providing on neighborhoods of edges of the surface of image carrier  1 . 
     FIG. 6 shows an example wherein the detection window section is composed of a reflection member, and detection window section  330  is composed of triangular reflection member  331  and triangular reflection member  332 . Further, detection window section  340  is composed of triangular reflection member  341  and triangular reflection member  342 . 
     In this case, edge  331   a  of the reflection member  331  on the part of a starting side for main scanning is formed to be in parallel with sub-scanning direction on the surface of image carrier  1 . Edge  331   b  of the reflection member  331  on the part of an ending side for main scanning is formed not to be in parallel with the aforesaid edge  331   a  on the surface of image carrier  1 . Edge  332   a  of the reflection member  332  on the part of a starting side for main scanning is formed not to be in parallel with edge  332   b  stated later and to be inclined from the sub-scanning direction, on the surface of image carrier  1 . Further, edge  332   b  of reflection member  332  on the part of an ending side for main scanning is formed on the surface of image carrier  1  to be in parallel with the sub-scanning direction. 
     Now, edge  341   a  of the reflection member  341  on the part of a starting side for main scanning is formed to be in parallel with sub-scanning direction on the surface of image carrier  1 . Edge  341   b  of the reflection member  341  on the part of an ending side for main scanning is formed not to be in parallel with the aforesaid edge  341   a  on the surface of image carrier  1 . Edge  342   a  of the reflection member  342  on the part of a starting side for main scanning is formed not to be in parallel with edge  342   b  stated later and to be inclined from the sub-scanning direction, on the surface of image carrier  1 . Further, edge  342   b  of reflection member  342  on the part of an ending side for main scanning is formed on the surface of image carrier  1  to be in parallel with the sub-scanning direction. 
     Here, “a reflection member wherein an edge of the main scanning starting side is not in parallel with that of the main scanning ending side” means that edge  331   a  is not in parallel with edge  331   b  or edge  341   a  is not in parallel with edge  341   b  in FIG.  6 . 
     Again, “a reflection member wherein an edge of the main scanning starting side is not in parallel” means that edge  331   a  is not in parallel with edge  332   a  or edge  341   a  is not in parallel with edge  342   a  in FIG.  6 . 
     Incidentally, when a diameter of a laser beam for exposure is about 60 mm, as an example, it is preferable that each of the detection window sections  330  and  340  is in size of 10 mm in the main scanning direction and 5 mm in the sub-scanning direction. 
     Without being limited to the foregoing, a shape other than the above is acceptable provided that a reflection member is surrounded by edges which are not in parallel with each other. Even for a diameter of the laser beam and a size of the detection window section, sizes exceeding these sizes may also be acceptable. 
     In FIGS. 5 and 6, there is shown the structure wherein light reflected from detection window section  330  composed of reflection members is detected by start position locating sensor  350  which is assumed to be equipped with a photosensor having a sufficient size to pick up reflected light surely. In the same way, there is shown a structure wherein light reflected from detection window section  340  composed of reflection members is detected by end position locating sensor  360 , and the end position locating sensor  360  is assumed to be equipped with a photosensor which has a sufficient size to pick up reflected light surely. Incidentally, it is also possible to provide on the front side of the photosensor an optical system such as a converging lens which can pick up reflected light surely, without making the photosensor itself to be greater. In this case, it is necessary to consider divergence of reflected light caused by the reflection member and dispersion in mounting. 
     Though the structure in this case employs a detection window section composed of a reflection member and a locating sensor which detects reflected light, it is also possible to employ an arrangement wherein a through hole (transmission member) having a shape corresponding to the detection window section is provided on image carrier  1 , and a locating sensor provided in the image carrier  1  detects light transmitted through the transmission member. When using the transmission member as in the foregoing, one set of locating sensor is enough. 
     Here, an electrical structure of a scanning control circuit which controls scanning operation of the scanning optical system  300  stated above will be explained. 
     FIG. 1 is a block diagram showing an embodiment of a scanning control circuit used in an image forming apparatus in the present embodiment. 
     The scanning control circuit in the present embodiment is a circuit which controls scanning operations of plural scanning optical systems  300  which materialize an SPC system wherein no control is conducted for phase matching of plural polygon mirrors. 
     To be concrete, accurate registration for plural images each having a different color is conducted on a real time basis in both the main scanning direction and the sub-scanning direction, for each image of each color of Y, M, C and K, by the use of detection signals from start position locating sensors  350 Y- 350 K and end position locating sensors  360 Y- 360 K. 
     First, a phase difference of plural polygon mirrors  305 Y- 305 K which rotate at the same rotational speed through PLL control is measured by time measuring circuit  400 , then step motor  250  in FIG. 4 is driven by microprocessor  320  and motor driver  240  to conduct the control in the sub-scanning direction which is more fine than one line. Incidentally, this control in the sub-scanning direction is only needed when starting an apparatus or needed at regular intervals. 
     In addition, control in the main scanning direction which is more fine than one pixel clock is conducted by driving time measuring circuit  400 , timing control circuit  500 , clock selection circuit  600 , first-out first-read memory in (hereinafter referred to as FIFO)  700 , modulation circuit  800  and laser driver  900  by the use of results of detection at start position locating sensor  350  and end position locating sensor  360 . 
     The structure of each section will be explained as follows. 
     Since schematic structures of index sensors  312 Y- 312 K and index detection circuit  313  have been explained with reference to FIG. 3, repeating explanation will be omitted. 
     Since start position locating sensors  350  ( 350 Y- 350 K) and end position locating sensors  360  ( 360 Y- 360 K) have also been explained with reference to FIG. 5, repeating explanation will be omitted. 
     I/V conversion circuit  370  conducts I/V conversion (current/voltage conversion) for output current from start position locating sensors  350  ( 350 Y- 350 K) and end position locating sensors  360  ( 360 Y- 360 K), and sends them to time measuring circuit  400 . 
     The time measuring circuit  400  is one which measures in accuracy that is more fine than one pixel clock by the use of pulses detected from start position locating sensor  350  and end position locating sensor  360  and pulses detected from four index sensors  312 Y- 312 K. 
     The time measuring circuit  400  generates delay clocks in “n” types by delaying pixel clock by 1/n cycles by the use of digital delay line (TOKUGANHEI 4-16552). Due to this, the time measuring circuit  400  outputs delay clock dli which is delayed at plural steps in one cycle to clock selection circuit  600 . 
     Timing control circuit  500  is one which determines selecting operations of clock selection circuit  600  and driving timing for FIFO  700  by the use of time difference signals from time measuring circuit  400 . 
     Clock selection circuit  600  sends out some delay clocks dli from delay clocks dl 1 -dln obtained by delaying pixel clocks sent out from time measuring circuit  400  in plural steps, in which the relation of 1≦i≦n is satisfied. 
     FIFO  700  is one which sends out digital image data to modulation circuit  800  by delaying with the timing established by timing control circuit  500  based on delay clock dli obtained by phase-adjusting within a pixel clock. 
     The modulation circuit  800  is one obtained by incorporating in one package the circuit structures for modulating a pulse width and by converting them into IC, and it sends to laser driver  900  the modulation signals (recording signals) obtained by differential-amplifying analog image density signals obtained by D/A-converting digital image density data and by differential-amplifying reference wave signals. 
     Laser driver  900  is one which makes a semiconductor laser to oscillate with modulated signals, and it drives so that quantity of light from the semiconductor laser may be constant when signals corresponding to quantity of beam light from the semiconductor laser are fed back, in which an electric current sent to the semiconductor laser can be changed. Due to this, it is possible to adjust latent image voltage. 
     Microprocessor  320  is one which gives an instruction to PLL control of a motor driver which will be stated later, based on signals from time measuring circuit  400  and from timing control circuit  500 . 
     Motor drivers  240 Y- 240 K represent one which gives PLL-controlled driving clocks to step motor  250  in FIG.  4  and thereby conducts controls in the sub-scanning direction which is more fine than one line. 
     Motor drivers  316 Y- 316 K represent one which sends out clocks with the same cycle for the purpose of rotating polygon mirrors  305 Y- 305 K respectively of four scanning optical systems  3 Y- 3 K. 
     FIG. 4 is a time chart illustrating the following operations. 
     Detection and adjustment of a main scanning length for each color 
     Detection and fine adjustment of a sub-scanning position deviation for each color 
     FIGS.  7 ( a )- 7 ( j ) show detection signals at index sensors  312 Y- 312 K, detection signals at start position locating sensor  350 Y, detection signals at end position locating sensor  350 Y, detection signals at start position locating sensor  350 M, detection signals at end position locating sensor  350 M, detection signals at end position locating sensor  350 M, and the state of operations of Y and M. 
     Now, two pulses shown in FIG.  7 ( e ) are obtained as start position Y by detection window section  330  and start position locating sensor  350 Y. Interval T 1  between risings of these two pulses is measured by time measuring circuit  400 . 
     Further, two pulses shown in FIG.  7 ( f ) are obtained as end position Y by detection window section  340 Y and end position locating sensor  360 Y. Now, interval T 1  between rising of a pulse in the first half of start position Y and rising of a pulse in the first half of end position Y is measured by time measuring circuit  400 . 
     In the same way, two pulses shown in FIG.  7 ( g ) are obtained as start position M by detection window section  330  and start position locating sensor  350 M. Interval T 2  between risings of these two pulses is measured by time measuring circuit  400 . 
     Further, two pulses shown in FIG.  7 ( h ) are obtained as end position M by detection window section  340 M and end position locating sensor  360 M. Now, interval t 2  between rising of a pulse in the first half of start position M and rising of a pulse in the first half of end position M is measured by time measuring circuit  400 . 
     In the same procedures, T 3 , t 3 , T 4  and t 4  are further obtained and are measured by time measuring circuit  400 , though these are not shown. 
     Incidentally, measurement by time measuring circuit  400  is conducted by the sum total of the number of clocks counted in that period and a phase difference which is less than one clock (to obtain by the number with agreed timing by using delay output in quantity of n each having different phase). 
     Time t 1 , t 2 , t 3  and t 4  thus obtained respectively have values each being proportional to a main scanning length for each color of Y, M, C and K. 
     Therefore, a clock having a desired frequency is selected from clock selection circuit  600  so that the clock number at t 1  may be the same as that at each of t 2 , t 3  and t 4 . Then, each clock is supplied to FIFO  700 Y- 700 K. 
     By doing as stated above, the problem of deviation in the main scanning direction can be solved, because each color of Y, M, C and K has the same clock number in a range from the end portion of detection window section  330  to the end portion of detection window section  340 . Namely, accurate registration can be conducted in spite of difference in main scanning length caused by various deviation of scanning optical system for each color. 
     Next, phase adjustment operations in the main scanning direction will be explained. 
     As stated above, the number of revolutions for each of plural polygon mirrors is the same because they are driven by the same clock. Therefore, phase adjustment is conducted by changing the timing for sending image data in the main scanning direction in accordance with deviation of rotational phase, without making the rotational phases of plural polygon mirrors to be the same. 
     FIGS.  8 ( a )- 8 ( e ) are time charts showing operations of phase adjustment in the main scanning direction. 
     FIG.  8 ( a ) shows pulses obtained by detecting detection window section  330  shown in FIG. 5 with start position locating sensor  350 , and no detection is made actually after the scanning for several tens of lines for each color. FIG.  8 ( b ) shows index signals sent out from index sensor  312 Y. 
     FIG.  8 ( c ) shows data of phase difference between rising of the front end of the pulse measured by time measuring circuit  400  and shown in FIG.  8 ( a ) and rising of the pulse shown in FIG.  8 ( b ), wherein what is suited to the number of clocks is measured by the time measuring circuit  400 . 
     Incidentally, measurement by time measuring circuit  400  is conducted by the sum total of the number of clocks counted in that period and a phase difference which is less than one clock (to obtain by the number with agreed timing by using delay output in quantity of n each having different phase). 
     FIG.  8 ( d ) represents data showing the latch time at FIFO  700 . 
     FIG.  8 ( e ) shows data which designate selection operations of clock selection circuit  600 , and select adjustment time within one pixel clock. 
     Incidentally, in the present embodiment, it is preferable to use an average value obtained by excluding the maximum value and the minimum value from values in the results of plural measurement, in view of dispersion in measurement and dispersion of polygon surfaces. 
     By doing the foregoing, FIFO  700  sends digital image data to modulation circuit  800  by delaying with the timing established by timing control circuit  500  based on the delay clock which has been subjected to phase adjustment within a pixel clock. Accordingly, phase adjustment is conducted by changing the timing for sending image data in the main scanning direction in accordance with deviation of rotational phase, without making the rotational phases of plural polygon mirrors to be the same. 
     Next, there will be explained operations of fine adjustment for positional deviation of a beam in the sub-scanning direction in the scanning control circuit of the present embodiment. 
     The scanning control circuit in the present embodiment uses detection window section  330  and start position locating sensor  350  ( 350 Y- 350 K) shown in FIGS. 5 and 6 to obtain, as explained using FIGS.  7 ( a )- 7 ( j ) interval T 1  between risings of two pulses for start position Y, T 2  between risings of two pulses for start position M, T 3  between risings of two pulses for start position C and T 4  between risings of two pulses for start position K which are then measured respectively by time measurement circuit  400 , in terms of the number of clocks corresponding to each interval. 
     In this case, when a laser beam that passes through detection window section  330  first for each color is deviated to the sub-scanning direction, pulse widths for T 1 -T 4  are detected to be different from each other, because edges on the starting side of main scanning shown in FIG. 6 (edge  331   a  and edge  332   a ) are not in parallel with each other. 
     FIG.  9 ( a ) shows an example wherein a laser beam of Y passes through detection window section  330 , and T 1  obtained in that occasion is shown in FIG.  9 ( b ). FIG.  9 ( c ) shows how the laser beam of M deviated to the sub-scanning direction passes through detection window section  330 , and T 2  obtained in that occasion is shown in FIG.  9 ( d ). As stated above, deviation of a laser beam to the sub-scanning direction is detected as a pulse width. Incidentally, explanation for T 3  and T 4  will be omitted. 
     Namely, when T 1  is considered as a standard, T 2 -T 1  is a positional deviation of M from Y in the sub-scanning direction, T 3 -T 1  is a positional deviation of C from Y in the sub-scanning direction, and T 4 -T 1  is a positional deviation of K from Y in the sub-scanning direction. Therefore, it is necessary to adjust an amount of such positional deviation with the fine adjustment prism  315  mentioned above. 
     Results of detection by start position locating sensors  350 Y- 350 K are sent to time measurement circuit  400  through I/V conversion circuit  370 , and data of positional deviation in the sub-scanning direction described above are generated at the time measurement circuit  400 . The data of positional deviation are sent from the time measurement circuit  400  to timing control circuit  500 . The timing control circuit  500  generates data of fine adjustment for C and K, and sends them to microprocessor  320 . 
     Then, the microprocessor  320  drives motor driver  240  to make each fine adjustment prism for M, C and K to rotate through the structure explained above with reference to FIG.  4 . 
     As stated above, deviation in the sub-scanning direction is detected from a difference of a detection pulse width in the case where a laser beam passing through the detection window section first for each color is deviated to the subscanning direction, by the use of the detection window section where edges  331   a  and  332   a  on the starting side for main scanning are not in parallel with each other, and this deviation in the sub-scanning direction is corrected by the fine adjustment prism. Thus, deviation in the sub-scanning direction is solved and accurate registration can be conducted. 
     Incidentally, FIGS.  10 ( a )- 10 ( d ) show occasions wherein a detection window section where edges  331   a  and  332   a  on the starting side for main scanning are not in parallel with each other is utilized. 
     FIG.  10 ( a ) shows an example of the occasion where a laser beam for Y passes through detection window section  330 , and FIG.  10 ( b ) shows T 1 ′ obtained in the aforesaid occasion. FIG.  10 ( c ) shows the state wherein the laser beam for M deviated to the sub-scanning direction passes through detection window section  330 , and FIG.  10 ( d ) shows T 2 ′ obtained in the aforesaid occasion. As stated above, deviation of a laser beam in the sub-scanning direction is detected as a pulse width. Incidentally, explanation for T 3 ′ and T 4 ′ will be omitted. 
     As stated above, even in the case where a detection window section in which edge  331   a  on the starting side for scanning is not in parallel with edge  331   b  on the ending side for scanning is utilized, deviation to the sub-scanning direction is solved and accurate registration can be conducted by detecting deviation to the sub-scanning direction from a difference of detection pulse width in the case of a laser beam which passes through the detection window section first for each color and is deviated to the sub-scanning direction, and by correcting the deviation in the sub-scanning direction with a fine adjustment prism. 
     Lastly, exposure operations in the scanning control circuit of the present embodiment will be explained. 
     FIGS.  11 ( a )- 11 ( d ) are time charts showing operations of FIFO. 
     FIG.  11 ( a ) is one showing read-set signals, and detection signals sent from read-set signal start position locating sensors  350  ( 350 Y- 350 K) are used. 
     FIG.  11 ( b ) is one showing delay clock dli having prescribed delay time at time measurement circuit  400 , and such clock represents read clock for FIFO  700 . FIG.  11 ( c ) is one showing read-enable signal, and such read-enable signal is prepared with an index signal which is sent out from first index sensor  312 Y and serves as a standard. 
     FIG.  11 ( d ) represents image data in a unit of a line read out from FIFO  700 , and such image data are sent to modulation circuit  800 . The foregoing represents exposure operations of scanning optical system  300  in the present embodiment, and the exposure operations are conducted for each color. 
     As explained in detail above, an image forming apparatus of the present embodiment can prevent fully, when it is equipped with the aforesaid scanning control circuit, the positional deviations in the main scanning direction and in the sub-scanning direction for images for plural colors. 
     Incidentally, though an object of the aforesaid embodiment is to prevent deviations in the main scanning direction and the sub-scanning direction, it is also possible to use it for other applications by using the aforesaid den end position locating sensor. 
     For example, let it be assumed that a scanning laser beam crossing detection window sections  330  and  340  is inclined as shown in FIG.  12 ( a ). 
     Now, two pulses shown on the left side in FIG.  12 ( b ) are obtained as start position Y by detection window section  330  and start position locating sensor  350 Y. Interval T 11  between risings of these two pulses is measured by time measurement circuit  400 . 
     Further, two pulses shown on the right side in FIG.  12 ( b ) are obtained as end position Y by detection window section  340  and end position locating sensor  360 Y. Interval T 12  between risings of these two pulses is measured by time measurement circuit  400 . 
     In this case, a difference between T 11  and T 12  represents an inclination of a laser beam. By detecting the inclination of a laser beam in the same way even for M, C and K, it is possible to obtain inclination of each color. When such inclination is caused, images each being of each color in four colors are deviated. 
     When such inclination is detected, therefore, the so-called “image processing with correction of inclination” is conducted. For this purpose, image data equivalent to plural lines are taken in RAM in advance. Then, in the case of exposure (writing), an appropriate address is calculated on a real time basis, and data of the address are read out to be image data for the exposure. 
     Conducting interpolation processing from plural data of addresses corresponding to plural pixels in the neighborhood without obtaining from one address is more preferable in terms of image quality. 
     In the case of factory shipments, or as a method of mechanical adjustment conducted by a service engineer, a rotation axis of a polygon motor can be tilted in accordance with detected inclination. Namely, it is considered to adjust by changing the degree of clamping several screws which fix the polygon motor on the apparatus. 
     Incidentally, it is preferable for adjustment to display the degree of inclination on the display panel. 
     If an arrangement is made to display a message for service call when inclination arrives at a certain extent in the course of actual operations, it is preferable because a user can find out deterioration of image quality. 
     An image forming apparatus of the invention equipped with the aforesaid structures makes it possible to materialize an image forming apparatus capable of conducting accurate registration of images each being of a different color out of plural colors both in the main scanning direction and the sub-scanning direction on a real time basis.