Abstract:
An image forming apparatus that forms an image by scanning a recording medium with a plurality of light beams that are modulated by individual image forming signals respectively corresponding to the light beams. A beam detecting device detects the plurality of light beams at preset positions and outputs corresponding detection signals. At least one delay device delays each of the detection signals from the beam detecting device by an arbitrary time period which is predetermined individually for each of the detection signals. A reference clock signal generating device generates a plurality of reference clock signals on the basis of delayed detection signals output from the at least one delay device. The image forming apparatus performs an image forming operation according to the plurality of reference clock signals.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus that uses a plurality of laser beams to generate marks on a recording medium and that records the marks as an image. 
     2. Discussion of the Background 
     In general, many image forming apparatuses that record information on a recording medium using light, such as laser light, have used a single light beam and are therefore required to write an image in sequence on a recording medium with the single light beam. These single-beam image forming apparatuses are typically required to transfer an information signal for modulating the single beam at a relatively high speed in an event of printing an image at a relatively high speed or with a relatively high density. At the same time, the single-beam image forming apparatuses are required to perform a main scanning at a relatively high speed. Accordingly, those apparatuses which use a rotary polygon mirror for scanning the recording medium are required to drive the rotary polygon mirror at a relatively high rotational speed which may reach tens of thousands r.p.m. (revolutions per minute). Therefore, such a single-beam image forming apparatus has a structural limitation to perform a relatively high speed recording operation. 
     For the reasons as described above, an image forming apparatus which uses a plurality of light beams (multi-beams) has been developed. In such a multi-beam image forming apparatus, the light beams are modulated by each of individual recording signals and simultaneously scan a recording medium. FIG. 14 illustrates an exemplary disposition of light beams in such a multi-beam image forming apparatus that uses a four-laser-beam optical writing device. In this multi-beam optical writing device, distances between adjacent beams among four laser beams B 1 , B 2 , B 3 , and B 4  are needed to be equal to a distance between adjacent pixels (hereinafter referred to as a pixel distance) to be formed on the recording medium. 
     On the other hand, a semi-conductor laser array (hereinafter referred to as a laser diode) that is popularly used as a light beam source for an optical writing device generally has a distance between light emission points (hereinafter referred to as a light emission point distance) which is not equal to, and which is generally greater than, a pixel distance required by a recording device which has a current mainstream resolution of 600 dpi (dots per inch), for example. In order to adjust this difference, as illustrated in FIG. 14, a disposition of four light emission points is inclined at a preset angle θ 1  relative to a line L-L′ that is orthogonal to a beam scanning direction SL, so that actual beam distances between adjacent beams become equal to a pixel distance Ps. 
     In the above-described optical writing device, the laser beams B 1  to B 4  scan a beam detector including at least a photosensor. The beam detector outputs beam detection signals when detecting the laser beams B 1  to B 4 . An exposure starting position (hereinafter referred to as an “image write start position”)on the recording medium for each of the laser beams B 1  to B 4  is determined on the basis of the beam detection signals from the beam detector. 
     Although various constructions of a beam detector are hitherto known, there are generally two types. One type of beam detector detects a plurality of laser beams with one detector (light detector for detecting the laser beams), and the other type of beam detector detects a plurality of laser beams using a same number of light detectors. 
     FIGS. 15 to  18  illustrate cases of using a light detector d which has a relatively long beam detecting surface. FIG. 15 illustrates a manner in which the beam detector d is positioned with an end face L-L′ thereof made orthogonal to the beam scanning direction SL. In this case, if the operation for writing an image to the recording medium is started at a certain time after each of the laser beams B 1  to B 4  is received by the beam detector d, a vertical line of the image can be formed as a straight line that is parallel to the aforementioned end surface L-L′. 
     Namely, when times t 1 , t 2 , t 3 , and t 4  at which each of the respective laser beams B 1  through B 4  reaches the image write start position S passing through the end surface L-L′ are set to a uniform time, the vertical line formed by performing a plurality of scanning operations can be configured to a straight line in parallel with the end surface L-L′, as shown in FIG.  16 . 
     As shown in FIG. 17, for example, in some cases the beam detector d may be mounted with the end surface L-L′ slightly inclined (at an angle θ 2 ) relative to a line orthogonal to the beam scanning direction SL due to a mechanical inaccuracy. In such a case, when each of the laser beams B 1  through B 4  starts writing an image on the recording medium at a certain time after each of the laser beams B 1  through B 4  is detected by the beam detector d, as illustrated in FIG. 17, the line formed by connecting at least four image write start positions S is not orthogonal to the beam scanning direction SL. As a result, when the scanning operation is repeated, a vertical line becomes a jagged line having a cycle of four laser beams B 1  through B 4 , as illustrated in FIG.  18 . 
     In a case that the beam detector d includes a plurality of light detectors that form beam detectors d 1 , d 2 , d 3 , and d 4  for detecting the laser beams B 1  to B 4 , it is hard to vertically align the beam detectors d 1 , d 2 , d 3 , and d 4  since a beam pitch distance is relatively narrow. Accordingly, the beam detectors d 1 , d 2 , d 3 , and d 4  may be positioned in a slanting manner as illustrated in FIG.  19 . If the beam detectors are disposed in such a manner, a straight line orthogonal to the beam scanning direction SL cannot be formed unless the time from detecting the beam by each of the beam detectors d 1  through d 4  to starting the operation of writing an image on the recording medium is individually set. 
     Namely, in such a situation it is required to adjust and set the timing to start writing images (hereinafter sometimes referred to as an “image write start timing”). The image write start timing for the laser beam B 1  is set to a time after a time period of t+t 1  from a time when the beam detector d 1  detects the laser beam B 1 . The image write start timing for the laser beam B 2  is set to a time after a time period of t+t 2  from a time when the beam detector d 2  detects the laser beam B 2 . The image write start timing for the laser beam B 2  is set to a time after a time period of t+t 3  from a time when the beam detector d 3  detects the laser beam B 3 . The image write start timing for the laser beam B 2  is set to a time after a time period of t+t 4  from a time when the beam detector d 4  detects the laser beam B 4 . 
     An exemplary method is described in Japanese Laid-Open Patent Publication No. 57-8887/1982 of a control of the image write start position S for an optical writing in a recording device that uses a plurality of laser beams to scan a recording medium to record an image on the recording medium. However, this exemplary control method uses clock signals which have a frequency n times faster than that of pixel clock signals to control the number of pulses of the beam detection signal. Therefore, this method requires a counting of extremely high frequency pulses and, at the same time, a setting of the image write start timing for each of the laser beams individually. Accordingly, in this method, setting values of all the image write start timings for the laser beams are required to be readjusted, even at only a slight adjustment of an image position. This causes a problem for users in adjusting a position of an image on a recording sheet. 
     Therefore, it is believed that there is no image forming apparatus available that allows a user to perform an easy adjustment of an image position, including a delicate image positioning, without having a jagged line on an image. 
     SUMMARY OF THE INVENTION 
     In light of the above and other problems, an object of the present invention is to provide a novel image forming apparatus capable of adjusting an image write start position by adjusting the same at a time difference shorter than a cycle of a reference clock signal with ease. 
     According to an aspect of the present invention, a novel image forming apparatus of the present invention forms an image by scanning a recording medium with a plurality of light beams that are modulated by individual image forming signals respectively corresponding to the light beams. In this novel image forming apparatus a beam detecting device detects the plurality of light beams at preset positions and outputs corresponding detection signals. Further, at least one delay device delays each of the detection signals from the beam detecting device by an arbitrary time period which is predetermined individually for each of the detection signals. Moreover, a reference clock signal generating device generates a plurality of reference clock signals on the basis of delayed detection signals output from the at least one delay device. The image forming apparatus performs an image forming operation according to the plurality of reference clock signals. 
     At least one delay device of the image forming apparatus may delay one of the light beams detected at a selected one of the preset locations closest to a predetermined image write start position by a predetermined and fixed delay time. 
     In the novel image forming apparatus a number of delay devices may be the same as a number of the light beams. 
     In the novel image forming apparatus one delay device may be provided for delaying each of the plurality of light beams. 
     The novel image forming apparatus may further include at least one delay device that delays each of the detection signals from the beam detecting device by an arbitrary time period which is predetermined individually for each of the detection signals, and that controls the image write start position of each of the light beams. 
     The delay device of the image forming apparatus may further include a first delay device that delays all of the detection signals from the beam detecting device by a same time period and a second delay device that delays each of the detection signals from the beam detecting device by an arbitrary time period which is predetermined individually for each of the detection signals. Further, the delay device can control each of image write start positions of the light beams according to each of delayed detection signals output from the second delay device. 
     Scanning operations of the plurality of light beams of the novel image forming apparatus can be performed by a rotary polygon mirror and the arbitrary time period of the delay device can be automatically set in response to a change of a rotation speed of the rotary polygon mirror. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 is a schematic diagram illustrating an optical writing section of a multi-beam type image forming apparatus according to an embodiment of the present invention; 
     FIG. 2 is a block diagram showing an exemplary configuration of an image write start timing setting circuit of the image forming apparatus of FIG. 1, including beam detectors and delay devices; 
     FIG. 3 is a part of the block diagram of FIG. 2 showing a circuit for a laser beam B 1 ; 
     FIG. 4 is a time chart showing exemplary timing signals during an image forming timing control operation performed by the circuit of FIG. 3 when an adjustment control of an image write timing by a delay device is not applied; 
     FIG. 5 is a time chart showing exemplary timing signals during an image forming timing control operation performed by the circuit of FIG. 3 when adjustment control of image write timing by a delay device is applied; 
     FIG. 6 is a diagram showing exemplary relationships among positions of generating laser beams B 1 -B 4 , locations of beam detectors, and an image write start position when an image write start timing setting circuit includes beam detectors; 
     FIG. 7 is a time chart showing relationships between beam detection timing signals and image write start timing signals by which an image write start position can be aligned straight when positions of generating the laser beams B 1 -B 4 , locations of beam detectors, and an image write start position have relationships of FIG. 6; 
     FIG. 8 is an electric diagram illustrating an exemplary configuration of the delay device of FIG. 2; 
     FIG. 9 is an electric diagram illustrating another exemplary configuration of the delay device of FIG. 2; 
     FIG. 10 is a time chart showing a relationship among a beam detection signal, a reference signal, and a timing signal to be input to the delay device in FIG. 9; 
     FIG. 11 is a block diagram showing an exemplary configuration of an image write start timing setting circuit of the image forming apparatus of FIG. 1, for detecting a scanning timing of a plurality of laser beams using one beam detector according to another embodiment of the present invention; 
     FIG. 12 is a block diagram showing an exemplary configuration of a beam detection signal separator in the image write start timing setting circuit in FIG. 11; 
     FIG. 13 is a timing chart showing an exemplary operation timing of each of beam detection signals and timing signals of the beam detection signal separator in the image write start timing setting circuit in FIG. 12; 
     FIG. 14 is an explanatory view showing a positional deviation of multi-beams in a scanning direction; 
     FIG. 15 is an explanatory view showing a relationship between a beam detector for detecting multi-beams and an image write start position in FIG. 14; 
     FIG. 16 is an explanatory view showing a state of a vertical line formed under the relationship between the beam detector for detecting multi-beams and the image write start position in FIG. 15; 
     FIG. 17 is an explanatory view showing a relationship between the beam detector for detecting multi-beams and the image write start position in a case that the beam detector is mounted in an inclined manner; 
     FIG. 18 is an explanatory view showing a state of a vertical line formed under the relationship between the beam detector for detecting multi-beams and the image write start position in FIG. 17; 
     FIG. 19 is a diagram showing an exemplary relationship among positions of generating the laser beams B 1 -B 4 , locations of the beam detectors, and the image write start position when the image write start timing setting circuit includes the beam detectors; 
     FIG. 20 is a block diagram showing an exemplary configuration of the image write start timing setting circuit, provided with a first delay device before the delay devices in FIG. 2, including beam detectors and delay devices; and 
     FIG. 21 is a part of a block diagram showing an exemplary configuration of the image write start timing setting circuit, provided with a first delay device before the delay devices in FIG.  11 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention are described in detail referring to the figures, wherein like reference numerals indicate identical or corresponding parts throughout the several views. 
     FIG. 1 is a schematic diagram illustrating an optical writing section of a multi-beam type image forming apparatus according to an embodiment of the present invention. 
     In FIG. 1, the optical writing section is composed of a laser unit  1 , a condenser  2 , a rotary polygon mirror  3 , an fθ lens  4 , a photoconductive drum  5 , a reflection mirror  6 , and a beam detector  7 . 
     The laser unit  1  is composed of a plurality of semi-conductor laser emission devices (hereinafter referred to as laser diodes) located arow (in one line), and a laser array that emits, in this example, four laser beams. The condenser  2  refracts divergent beam light emitted from the laser unit  1  into parallel laser beams L 1 , L 2 , L 3 , and L 4 , and leads the laser beams L 1 -L 4  to the rotary polygon mirror  3 . The rotary polygon mirror  3  scans the incident laser beams L 1 , L 2 , L 3 , and L 4  from the condenser  2  onto a surface of the photoconductive drum  5 . The fθ lens  4  converts the laser beams deflected to a uniform angular velocity from the rotary polygon mirror  3  into a uniform scanning motion and makes an image on the photoconductive drum  5 . The reflection mirror  6  is disposed at a tip end portion of the scanning line so that four laser beams B 1 , B 2 , B 3 , and B 4  are led to the beam detector  7 . At the beam detector  7 , a signal corresponding to an incident laser beam when a laser beam is made incident to an edge of the beam detector  7  is output. 
     As explained in the discussion of the background with reference to FIGS. 15 and 19, for the beam detector  7  (shown in FIG.  1 ), there are generally two types thereof. One type of beam detector is constructed such that a plurality of laser beams are detected by one beam detector. The other type of beam detector is constructed such that the laser beams are detected by a plurality of beam detectors provided for each laser beam. First, a case in which the laser beams are detected using the plurality of beam detectors is explained. 
     FIG. 2 is a block diagram showing an exemplary configuration of an image write start timing setting circuit provided with a plurality of beam detectors  7   a ,  7   b ,  7   c ,  7   d  corresponding to the number of laser beams L 1 -L 4  in the image forming apparatus according to the first embodiment of the present invention. In FIG. 2, output signals from each of the beam detectors  7   a ,  7   b ,  7   c , and  7   d  are amplified at amplifiers  10   a ,  10   b ,  10   c , and  10   d . Further, each of the outputs of amplifers  10   a ,  10   b ,  10   c , and  10   d  are individually delayed by delay devices (delay circuits)  11   a ,  11   b ,  11   c , and  11   d  so that all the output signals are aligned at a virtual image write start position. 
     Reference clock signal generators  12   a ,  12   b ,  12   c , and  12   d  generate reference clock signals synchronized with the timing signals (the aforementioned output signals from delay circuits  11   a - 11   d ) using the timing signals as trigger signals. The reference clock signals are input to a timing control circuit  13 . The desired number of clock signals (corresponding to n clock signals in FIG. 3, as shown later) are counted at the timing control circuit  13 , and if the image data is transmitted to LD drive circuits  14   a ,  14   b ,  14   c , and  14   d , the laser diodes  15   a ,  15   b ,  15   c , and  15   d  are modulated. Thereafter, the image is formed by scanning the laser beams L 1 -L 4  over the photoconductive drum  5  to expose the same as described earlier. 
     Further, in the aforementioned FIG. 19, each of the beam detectors corresponds to that in FIG. 2, i.e., d 1  corresponds to  7   a , d 2  to  7   b , d 3  to  7   c , and d 4  to  7   d , respectively. In the example in FIG. 19, the beam detector d 1  is located furthermost from the image write start position S and the beam detectors are located closer to the image write start position S in the order of d 2 , d 3 , and d 4  (d 4  is the closest). In a case that the time when the laser beam B 4  detected by the beam detector d 4  is scanned by the rotary polygon mirror  3  and reaches the virtual image write start position S is defined as t, a delay amount t is set in the delay device  11   d  that corresponds to the beam detector d 4 . 
     In a manner as described above, the time differences corresponding to each of distances from the beam detectors d   1   , d   2   , and d   3   to the virtual image write start position S, that is, t+t 1 , t+t 2 , and t+t 3 , are set in the delay devices  11   a ,  11   b , and  11   c . The image write start positions S are accurately aligned by using each of the delayed signals as trigger signals for generating the reference clock signals of each of the reference clock signal generators  12   a - 12   d.    
     The above-described operation is explained in further detail referring to FIGS. 3 to  5 . FIG. 3 is a part of the block diagram of FIG. 2 showing a circuit for one laser beam B 1 . A method for adjusting the image write timing by use of the delay devices  11   a  through  11   d  and the reference clock signal is now explained referring to FIG.  3 . 
     In a case of an absence of the timing adjustment by the delay device  11   a , only n clock signals or n+1 clock signals can be selected since the timing of writing images is controlled by counting clock signals of the reference clock signal (e), as shown in the timing chart of FIG.  4 . Therefore, an error with a maximum value of one reference clock signal arises between the image write start positions to be written by the laser beams B 2  through B 4  and that to be written by the laser beam B 1 , and thereby, a deterioration of the image is caused due to the fluctuation of the vertical line within one reference clock signal, as shown in FIG.  18 . 
     On the contrary, if the delay devices  11   a  through  11   d  are provided, as shown in FIGS. 2 and 3, since a delay time Δt can arbitrarily be set, as shown in a timing chart of FIG. 5, an arbitrary and slight adjustment of image write timing within one reference clock signal can be enabled by use of these delay devices  11   a through  11   d . Consequently, the image write start position S of the other laser beams B 2  through B 4  can be adjusted with a good accuracy, and the image can be prevented from deteriorating. 
     To be able to adjust the image write position S as mentioned above can be a significant and important factor for a color image forming apparatus that forms an image by superimposing a plurality of color images upon one another as an accurate superimposing operation improves image formation. Further, in the related art, since an output level of the beam detector is used for adjusting the image write start position S, a power variation of the laser diode affects the beam detector output level, and a fluctuation of the image write start position S thereby tends to occur. 
     Furthermore, since an edge portion of a rise time of a variation of the beam detector output or a fall time of the same is used for detecting the timing of the output signal, a time range other than this time range cannot be used for adjusting the image write start position S. Moreover, since the reference clock signal with a significantly higher frequency than a pixel clock signal is used, electric noise tends to occur in the related art. 
     However, the present invention has an advantage such that an arbitrary slight adjustment of image write position can be performed with a digital signal, and that since the reference clock signal has a frequency higher than that of a pixel clock signal, occurrence of noise is decreased in the control circuit. 
     FIG. 6 is a diagram showing exemplary relationships among positions of generating the laser beams B 1 , B 2 , B 3 , and B 4 , and locations of the beam detectors, d 1 , d 2 , d 3 , and d 4  in a mechanical manner. When there are distances Δ 11  (delta el one), Δ 12 , Δ 13  between pairs of the laser beams B 1  and B 2 , B 1  and B 3 , and, B 1  and B 4 , respectively in the beam scanning direction SL, and there are also distances Δm 1 , Δm 2 , Δm 3  between the beam detectors d 1  and d 2 , d 1  and d 3 , and, d 1  and d 4 , the image write start position S can be aligned by setting delay timings indicated by a timing chart, as shown in FIG.  7 . 
     Namely, in a case that the distances between each of the laser beams and the distances between each of the beam detectors are configured to be in the aforementioned relationship, and the beam scanning speed is set to v, the laser beam B 2  reaches the beam detector d 2  after a time (Δ 11 +Δm 1 )/v from the time when the laser beam B 1  passes the beam detector d 1 . In a same manner, the laser beam B 3  reaches the beam detector d 3  after a time (Δ 12 +Δm 2 )/v from the time when the laser beam B 1  passes the beam detector d 1 , and the laser beam B 4  reaches the beam detector d 4  after a time (Δ 13 +Δm 3 )/v from the time when the laser beam B 1  passes the beam detector d 1 , respectively. The beam detectors d 2 , d 3 , and d 4  then output the beam detection signals, respectively. 
     On the other hand, since there are positional deviations Δm 1 , Δm 2 , and Δm 3  from each of the beam detectors d 1 , d 2 , d 3 , and d 4  to the virtual image write start position S, time differences corresponding to each of the positional deviations are required to be set so that the image write start positions S of the respective laser beams B 1 , B 2 , B 3 , and B 4  are aligned. Accordingly, the image starts to be written after a time L/v from a time when the laser beam B 4  is detected with the beam detector d 4 , wherein L is defined as the distance between the beam detector d 4 , which is located closest to the virtual image write start position S out of the beam detectors d 1  through d 4 . 
     The time L/v is set to the delay device  11   d  corresponding to the laser beam B 4  in FIG.  2 . In a same manner, the distances from each of the beam detectors d 1 , d 2 , and d 3  to the image write start position S are L+Δm 3 , L+(Δm 3 -Δm 1 ), and L+(Δm 3 -Δm 2 ), respectively. Further, the delay times are (L+Δm 3 )/v, {L+(Δm 3 -Δm 1  })/v, and {L+(Δm 3 -Δm 2 )}/v, respectively, as also shown in FIG.  7 . Furthermore, FIG.  6  and FIG. 19 correspond to each other in relation to the laser beams B 1  through B 4 , when each of the elements are replaced as follows: 
     L/v=t, 
     Δm 3 =t 1 , 
     (Δm 3 -Δm 1 )/v=t 2 , and 
     (Δm 3 -Δm 2 )/v=t 3   
     If the delay devices  11   a  through  11   d  are thus used, a time adjustment, namely, an adjustment of the image write start position S slightly less than one clock signal of the reference clock signal can be performed without using the reference signal with relatively high frequency to control the image write start position S as used in the background art. Thus, a correction for the slight deviation of the image write start timing caused by the deviation of the position of each of the beam detectors d 1 , d 2 , d 3 , and d 4  can be implemented by properly setting each of the values t, t 1 , t 2 , and t 3 . 
     FIG. 8 is an electric diagram illustrating an exemplary configuration of the delay devices  11  of FIG.  2 . The delay device  11  is composed of a delay element  21  and a switch  22 . Buffer elements  20  and  23  are respectively mounted at an input side of the delay element  21  where a beam detection signal BS is input and an output side of the switch  22  where the delayed timing signal is output. 
     FIG. 9 is an electric diagram illustrating another exemplary configuration of the delay devices  11  of FIG. 2, composed of a reference signal generator  24  that generates a reference signal, for example a triangular wave that arises by the beam detection signal BS as a trigger signal, a fundamental signal generator  25 , and a comparator  26 . 
     In this composition of FIG. 9, the reference wave generator  24  generates the reference wave with the beam detection signal BS as a trigger. The generated reference wave and the fundamental signal generated at the fundamental signal generator  25  are compared in the comparator  26 , as shown in the timing chart of FIG.  10 . The timing signal is generated only when the reference wave exceeds the fundamental signal. Hereupon, a fundamental signal voltage can be set so that a desired delay time is obtained. 
     When the delay device  11  is thus constructed, since the delay time can be set in an analog-like manner and a slight adjustment of the delay time can arbitrarily be performed, this type of delay device is advantageous to use for the slight adjustment of the delay time. Further, the image write start position S can be adjusted by adjusting the delay times t 1 , t 2 , and t 3 , by setting a time t, which is required for scanning an area from the beam detector d 4  to the image write start position S as a fixed value, depending on the distance between each of the four laser beams. This results because the beam detector d 4  is closest to the image write start position S from among the beam detectors d 1 , d 2 , d 3 , and d 4  shown in FIGS. 6 and 19, and the setting delay time t for d 4  is the shortest. 
     Thereby, the circuit with regard to setting the delay time for the beam detector d 4  can be omitted, and an entire circuit construction can be simplified. Furthermore, since the aforementioned values t 1 , t 2 , t 3 , and t 4  are approximately determined mainly depending on a positional location of the laser array, the beam detectors, and so forth, or on a rotational accuracy of the rotary polygon mirror, the value can be set before shipment. Furthermore, the aforementioned fixed value can include “0”. 
     FIG. 11 is a block diagram showing an embodiment in which a plurality of laser beams are detected with one beam detector. 
     In this embodiment of FIG. 11, the setting operation for the beam detection and timing of the beam detection signals is performed by a beam detector  31 , an amplifier  32 , a beam detection signal separator  33 , and delay devices  34 - 34   d  that output delay timing signals based on the beam detection signal that is separated by the beam detection signal separator  33 . 
     In this composition, when each of the laser beams B 1  to B 4  reaches the edge of the beam detector  31 , the output signal from the beam detector  31  is amplified by the amplifier  32 . In this instance, the beam detection signal in this embodiment is a progressive signal that progresses in a time difference according to the beam distance that is shown by a numeral  39 . This signal is input to the beam detection signal separator  33 , and is separated into the signals corresponding to each of the aforementioned laser beams B 1 , B 2 , B 3 , and B 4  by the beam detection signal separator  33 , to be used as a timing signal. 
     Each of the signals is input to the delay devices  34   a ,  34   b ,  34   c , and  34   d  in which desired delay times are set, and each of the timing signals are generated. Then, the reference clock signals that are synchronized with the aforementioned timing signals are generated at reference clock signal generating circuits  35   a ,  35   b ,  35   c , and  35   d  on the basis of the timing signals a, b, c, and d. Thereafter, each of the image write start positions S is determined on the basis of each of the reference clock signals. 
     Hereupon, the set time for the delay devices  34   a ,  34   b ,  34   c , and  34   d  is set, for example, in the aforementioned FIG. 17, by adding each of time differences caused by an inclining angle θ 2  of an edge of the beam detector  31  to a delay time for registration (the delay time for registration is the same value for each of the laser beams respectively). Thereby, the image write timing position is aligned for each of the beam detection signals. Further, a setting manner for the delay time after the beam detection signal separator  33  is basically constructed the same as in the case in FIGS. 8 and 9 in which the aforementioned plurality of beam detectors  7   a  through  7   d  are used. 
     In this embodiment of FIG. 11, the beam detection signal separator  33  is constructed in a manner as shown in FIG.  12 . That is, the beam detection signal separator  33  is composed of a counter  40 , alignment detection circuits  41   a ,  41   b ,  41   c , and  41   d , and an inversion element  42 . In this composition, when a beam detection signal  39  that is amplified by the amplifier  32  is input to the counter  40 , a count up operation is sequentially performed, and the result is output from each of the terminals O 1 , O 2 , and O 3 . 
     Each of these outputs are then input to each of terminals D 1 , D 2 , and D 3  of each of the alignment detection circuits  41   a ,  41   b ,  41   c , and  41   d , and if the result is aligned to each of the setting values C 1  through C 3 , an alignment signal is output from each of the output terminals OUT 1 , OUT 2 , OUT 3 , and OUT 4  as a timing signal. The output timing is shown in FIG.  13 . As is apparent from FIG. 13, in a case that the result of the counter  40  is “1”, the data is aligned with the set data of the alignment detection circuit  41   a , and the alignment detection circuit  41   a  outputs 1 (level “high”)to the output terminal OUT 1 . 
     If the next beam detection signal is input to the counter  40 , the count up operation is performed, and the result of the counter becomes “2”. At this time, the data aligns with the set data of the alignment detection circuit  41   b , and the alignment detection circuit  41   b  outputs 1 (level “high”)to the output terminal OUT 2 . Further, if the next beam detection signal is input to the counter  40 , the count up operation is performed, and the result of the counter becomes “3”. 
     At this time, the data aligns with the set data of the alignment detection circuit  41   c , and the alignment detection circuit  41   c  outputs  1  (level “high”)to the output terminal OUT 3 . The count up operation and the alignment detecting operation is performed, and when the fourth beam detection signal is finally input to the counter  40 , the alignment detection circuit  41   d  outputs 1 (level “high”)to the output terminal OUT 4 , and at the same time, an input-inversion output from the inversion element  42  is input to the clear terminal of the counter  40 . Thereby, the counter  40  is reset and awaits the next beam detection signal returning to the initial value 0 (level “low”). 
     As for the timing signal, since a rising edge of the beam detection output signal is accurately aligned with the beam detection signal, the rising edge is used for the timing signal, even though the widths of the signals are different from each other. Thus, a plurality of beam detection signals can be separated. 
     Further, in the aforementioned embodiment shown in FIG. 17, since the beam detector is located inclined at the angle θ 2  to the beam scanning direction LS, the distance between the position where the laser beam B 4  is detected by the beam detector and the image write start position S is brought to be shorter than the case of the other laser beams B 1  through B 3 , and the delay time to be set with the delay device  34   d  becomes minimum. Therefore, the image write start position S can be adjusted by only adjusting the delay times t 1 , t 2 , and t 3  by setting the setting time t of the delay device  34   d  to a fixed value (which includes 0). Thereby, the circuit relating to set the delay time of the delay device  34   d  can be omitted, and the entire circuit construction can be simplified. 
     On the other hand, recently, there have been some image forming apparatuses in which an image data output speed thereof can variably be set at manufacture, or the resolution thereof can be changed by a user setting. In this kind of image forming apparatus, the image output speed or the resolution can be changed by changing a process condition based on the change of the rotation speed of the rotary polygon mirror  3 . Accordingly, when the rotation speed of the rotary polygon mirror  3  is changed, since the scanning speed v of the laser beams B 1  through B 4  is changed, the set delay times t, t 1  , t 2 , and t 3  are required to be changed according to each of the conditions. 
     Namely, if the rotation speed of the rotary polygon mirror  3  is set 10% faster, the image write start position S can be aligned by changing the set delay times t, t 1 , t 2 , and t 3  to be 10% shorter. 
     Further, in the embodiment as mentioned above, the image write start position S of each of the laser beams B 1 , B 2 , B 3 , and B 4  is made available to be slightly adjusted. As another embodiment, each of the beam detection signals BS can also be configured to be set in a uniform delay time by providing a first delay device just after beam detector  7   a  to  7   d , and  31  (in FIG.  21 ). This configuration is shown in FIGS. 20 and 21. The configuration in FIG. 20 corresponds to that in FIG. 2, and also the configuration in FIG. 21 corresponds to that in FIG. 11, with the above-noted modification. 
     Namely, in the embodiment shown in FIG. 20, a delay device  50  is mounted between amplifiers  10   a  through  10   d , which amplify detection outputs from the laser beam detectors  7   a  through  7   d , and the delay devices  11   a  through  11   d . In addition, in the embodiment shown in FIG. 21, a first delay device  51  is provided between the amplifier  32  and the beam detection signal separator  33 . The rest of the elements in FIGS. 20 and 21 are configured in a similar manner as shown in the aforementioned FIGS. 2 and 11, and a redundant explanation thereof is omitted. 
     In these embodiments of FIGS. 20 and 21, a large positional alignment, such as adjusting the so-called registration and setting the delay time or the like, is roughly performed with the first delay devices  50  and  51 . A slight adjustment for the delay of the image write start position S due to the inclination of each of the laser beam detectors  7   a  through  7   d , and  31 , or the like, is performed with second delay devices  11   a  through  11   d . A circuit for aligning the image write timing with a simple and enough accuracy can be realized by separate operations by both of the above first and second devices. 
     In addition, the adjustment for registration of the image write start position S can be performed by giving one set value to the first delay device after the setting values of each of the laser beam positions are set to the second delay device regardless of positional deviation of each of the laser beams. 
     Further, even in the same machine as the aforementioned embodiment, in an image forming apparatus in which an image output speed is configured to be variable by a manufacture setting, or in an image forming apparatus in which the change of the resolution can be performed by a user setting, the image write start position S is aligned by changing the delay time depending on the change of the rotation speed of the rotary polygon mirror  3 , as mentioned above. 
     Furthermore, the same timing signals as the timing signals a through d, and a′ through d′ in FIG. 2 are labeled as identical reference numerals in these figures, and a detailed explanation is omitted. 
     The controller of this invention may be conveniently implemented using a conventional general purpose digital computer or microprocessor programmed according to the teachings of the present specification, as is apparent to those skilled in the computer technology. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. The invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art. 
     Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein. 
     This document is based on Japanese patent application JPAP10-000370 filed on Jan. 5, 1998, and JPAP10-001004 filed on Jan. 6, 1998, the entire contents of which are hereby incorporated by reference.