Abstract:
In an optical printer which performs image recording by using plural laser beams, there is a case where abnormality can not be normally confirmed even if a test pattern is recorded. In order to prevent such a problem, an electrophotographic apparatus is provided to drive each of the plural laser beams according to inputted image data, and to perform scanning on scan paths mutually different on an identical recording medium with the plural laser beams. In this apparatus, any one of the plural laser beams is driven within a predetermined area to record the test pattern.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic apparatus for performing image formation by using plural light beams, which apparatus records a test pattern to detect an abnormal state (or wrong state). 
     2. Related Background Art 
     In recent years, a so-called multibeam laser printer which performs image formation by using plural light beams, e.g., plural laser beams and obtains a desired image through an electrophotographic process has been studied. 
     FIG. 12 shows an example of such the multibeam laser printer, and FIGS. 13A to  13 I show operation timing of the printer. 
     In FIG. 12, a laser printer  1  is connected to an external equipment  31  such as a computer or the like, and performs image formation on a recording paper under the control of the equipment  31 . The external equipment  31  supplies various control signals and image information to a video controller  27 , and the controller  27  outputs a video signal. A print control unit  26  is a control circuit for controlling each unit in the printer  1 . When an RDY signal from the external equipment  31  becomes TRUE as shown in FIG. 13A, the video controller  27  sets a PRINT signal TRUE as shown in FIG.  13 B. When the PRINT signal becomes TRUE, the print control unit  26  starts to drive a main motor  23  and a polygonal motor  14  as shown in FIGS. 13F and 13G. When the motor  23  is driven, a photosensitive drum  17 , fixing rollers of a fixing unit  9  and paper discharge rollers  11  start rotation. Then, the print control unit  26  starts to control a light quantity of a semiconductor laser  13 , and also sequentially performs high-voltage driving of a primary charger  19 , a development unit  20  and a transfer charger  21 . 
     When a time T 1  elapses from a drive start of the polygonal motor  14  and thus rotation of the motor  14  becomes stable as shown in FIG. 13G, the print control unit  26  turns on a paper feed clutch  24  to drive a paper feed roller  5  as shown in FIG.  13 H. Thus, a recording paper sheet  3  within a paper feed cassette  2  is fed toward resist rollers  6 . At timing when the paper  3  reaches the rollers  6 , the unit  26  outputs a VSREQ signal to the video controller  27  as shown in FIG. 13C, and also turns off the clutch  24  to stop driving the roller  5  as shown in FIG.  13 H. On the other hand, after the controller  27  expands the image information sent from the external equipment  31  into a dot image and then completes preparation for outputting a VDO signal, the controller  27  confirms that the VSREQ signal in FIG. 13C is TRUE. Then, the controller  27  sets a VSYNC signal TRUE as shown in FIG.  13 D. In synchronism with such an operation, after elapsing a time Tv as shown in FIG. 13E, the controller  27  starts to output the VDO signal as image data corresponding to one page. 
     At this time, the print control unit  26  turns on a resist roller clutch  25  after elapsing a time T 3  from rise of the VSYNC signal as shown in FIG. 13I, and drives the resist rollers  6 . The rollers  6  are driven for a time T 4  as shown in FIG. 13I, i.e., until a trailing edge of the recording paper sheet  3  passes through the rollers  6 . During the time T 4 , the print control unit  26  drives the semiconductor laser  13  according to the VDO signal sent from the video controller  27 . 
     The semiconductor laser  13  comprises lasers A and B which emit two laser beams, i.e., laser beams A and B respectively. The print control unit  26  drives each laser according to each VDO signal. The two laser beams are reflected by a rotating polygonal mirror  15  and then inclined by a mirror  16  in a scanner unit  7 , and the inclined beams are guided onto each scan path of the photosensitive drum  17 . For example, it is assumed that odd-number lines on the drum  17  are scanned by the laser beam A, while even-number lines are scanned by the laser beam B. As above, when the two laser beams modulated by the respective VDO signals are simultaneously radiated onto the photosensitive drum  17 , a latent image is formed on the drum  17  such that two lines are formed by each beam. By repeating such an operation, the latent image of one page is formed on the drum  17 . A not-shown beam detector is provided on the scan paths of the laser beams A and B and out of an image formation area. The beam detector detects the beams A and B, and generates /BD1 signal and /BD2 signal respectively corresponding to the beams A and B. Modulation timing of the laser beams is controlled on the basis of these two /BD signals. 
     The latent image formed on the photosensitive drum  17  is developed by the development unit  20 , and then a toner image is transferred onto the recording paper sheet  3  by the transfer charger  21 . After the transfer terminates, the paper  3  is carried to the fixing unit  9 , and the toner image is fixed to the paper  3 . After then, the paper  3  is discharged outward by the paper discharge rollers  11 . In case of continuously printing an image of next page, the print control unit  26  again sets the PRINT signal TRUE after elapsing a time T5 as shown in FIG. 13B, and performs the same control as in the printing of the first-page image. 
     As a test pattern data generation circuit for such the multibeam laser printer, for example, a circuit for generating longitudinal-line test pattern data in a two-beam laser printer will be explained. FIG. 14 shows a structure of this circuit, and FIGS. 15A to  15 J show operation timing of this circuit. 
     Hereinafter, structure and operation of FIG. 14 will be explained. A mask signal generation timing setting register  101  is a register which stores therein timing (=counter value) for releasing a /MASK1 signal  124  and a /MASK2 signal  224  necessary in test printing and timing (=counter value) for generating these signals. A storage operation into the resister  101  is performed at the beginning of the test printing. 
     In FIG. 14, in order to obtain horizontal synchronism in the test printing, a /BD1 signal  120  has been inputted in a first phase sync oscillator  102  and a first main-scan counter  103 . 
     When the /BD1 signal  120  becomes TRUE as shown in FIG. 15A, the first main-scan counter  103  is initially reset. Subsequently, the first phase sync oscillator  102  generates an image clock signal (CLK1 signal)  121  in synchronism with the /BD1 signal  120  as shown in FIG.  15 B. The CLK1 signal  121  is inputted to the first main-scan counter  103  and also to a counter  106  for generating test pattern data. Since the counter  103  counts the number of clock pulses, a first main-scan counter value  122  increases as time elapses. By a first comparator  104 , the value  122  is compared with a counter value  123  for releasing a mask set in the mask signal generation timing setting register  101 . On the other hand, a value of the counter  106  at this time is kept “0”, because a /writing inhibition signal  126  is TRUE and thus the counter  106  is continued to be cleared. 
     Subsequent to the /BD1 signal  120 , a /BD2 signal  220  changes its state from FALSE to TRUE as shown in FIG.  15 F. Thus, in the same manner as in the above first main-scan counter  103 , a second main-scan counter  203  is reset, a second phase sync oscillator  202  generates a second image clock pulse signal (CLK2 signal)  221  as shown in FIG. 15G, and the counter  203  counts the number of clock pulses. Even in a second comparator  204 , a mask release value  223  of the laser B and a second main-scan counter value  222  are compared with each other. As a result, while the value  222  is smaller than the value  223 , the /MASK2 signal  224  is kept TRUE. 
     When the first main-scan counter value  122  reaches the mask release value, a mask of the laser A is released as shown in FIG. 15C, and the /MASK1 signal  124  is inputted to a gate  105 . 
     At this time, when a /TOPE signal  125  being FALSE is inputted to the gate  105 , the four-bit first counter  106  starts counting as shown in FIG.  15 D. The respective bits counted by the counter  106  are managed as input values into an NAND gate  107  to generate a /TEST PATTERN1 signal  127 . When the value of the first counter  106 =Fh, the signal  127  becomes TRUE as shown in FIG.  15 E. 
     Also, when the second main-scan counter value  222  reaches the mask release value, a /TEST PATTERN2 signal  227  is generated in the similar manner. 
     When the first main-scan counter value  122  reaches a mask generation value, the /TEST PATTERN1 signal  127  becomes FALSE. Similarly, the mask is generated for the laser B, and the writing is inhibited. Such a series of operations is repeated until the /TOPE signal  125  becomes TRUE. Thus, a longitudinal-line test pattern is printed on the paper sheet. 
     Subsequently, examples of abnormal (or wrong) states which are specific to the multibeam laser printer will be explained, and also problems of the above conventional structures will be indicated. 
     EXAMPLE 1 OF ABNORMAL STATE 
     Initially, as the abnormal state example being specific when the plural light beams are used, a case where one of the plural light beams is deteriorated and thus does not completely operate will be explained. In the above conventional structure, if the longitudinal lines are outputted to perform the test printing, the test pattern is printed as longitudinal-direction solid lines  53  shown in FIG.  16 . That is, in the image of FIG. 16, for example, although a broken line of one-dot space should be essentially formed in a longitudinal direction, such the broken line is not often reproduced completely due to a condition in an electrostatic photographic process. Even if the broken line is reproduced completely, it is very difficult for human eyes to confirm the broken line if recording density of the lines in a sub-scan direction is 600 dpi or so. It is still more impossible almost for the human eyes to specify which beam is abnormal. As above, even if one of the beams is deteriorated and thus does not completely operate, the test pattern is merely recognized as a longitudinal-line pattern of which density is slightly thin, and there is a case where the abnormal state is not detected. 
     EXAMPLE 2 OF ABNORMAL STATE 
     Subsequently, as the abnormal state example being specific when the plural light beams are used, a case where abnormality occurs in horizontal sync control will be explained. It should be noted that such the abnormality occurs when, e.g., the BD signal is delayed due to dust on a beam optical path, a scratch on a lens or the like. In the above conventional structure, if the longitudinal lines are outputted to perform the test printing, the test pattern is printed as longitudinal-direction solid lines  65  shown in FIG.  17 . In the image of FIG. 17, although it is possible to recognize that something abnormal occurs in the horizontal sync control, it is impossible to specify whether merely timing of the two beams is asynchronous or jitter influences any one of the two BD signals. 
     EXAMPLE 3 OF ABNORMAL STATE 
     Subsequently, as the abnormal state example being specific when the plural light beams are used, a case where the light quantities of the plural beams are not uniform will be explained. If the light quantities are not uniform, unevenness in density appears. If it is assumed that a halftone solid-color image is recorded as the test pattern by slightly modifying the above conventional structure, a halftone pattern  83  shown in FIG. 18 can be obtained. That is, merely the obtained pattern becomes slightly thinner as a whole. Therefore, like the above example 1 of abnormal state, it is difficult for the human eyes to discriminate that the density of only the printed result of the specific beam is thin. 
     Moreover, in a test pattern data generation circuit having such the conventional structure as above, it is necessary to provide a print pattern generation circuit for each of the plural lasers to independently turn on and off each beam, thereby anticipating cost increase. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the above-described problems, and an object thereof is to generate, in case of generating test pattern data for an electrophotographic apparatus which performs image formation by using plural light beams, the test pattern data by which an abnormal (or wrong) state of the light beam can be accurately detected. 
     Another object of the present invention is to generate the test pattern data at low cost. 
     Still another object of the present invention is to record a test pattern by which it is possible to detect which light beam the abnormal state occurs. 
     To address the above objects, the present invention is an electrophotographic apparatus which includes plural emission means each for emitting a light beam, a drive means for driving each of the plural light beams according to inputted image data, a scan means for performing scanning on scan paths mutually different on an identical recording medium, with the plural light beams, and a generation means for generating test pattern data, the drive means driving each light beam according to the test pattern data to record a test pattern, wherein the drive means drives any one of the plural light beams within a predetermined area to record the test pattern. 
     A second embodiment of the present invention is an electrophotographic apparatus which includes plural emission means each for emitting a light beam, a drive means for driving each of the plural light beams according to inputted image data, a scan means for performing scanning on scan paths mutually different on an identical recording medium, with the plural light beams, and a generation means for generating test pattern data, the drive means driving each light beam according to the test pattern data to record a test pattern, wherein the apparatus further comprises a selection means for selecting any one of the plural light beams, and the drive means drives, within a predetermined area, the light beam selected by the selection means to record the test pattern. 
     A third embodiment of the present invention is an electrophotographic apparatus which includes plural emission means for emitting a light beam, a drive means for driving each of the plural light beans according to inputted image data, a scan means for performing scanning on scan paths mutually different on an identical recording medium, with the plural light beams, and a generation means for generating test pattern data, the drive means driving each light beam according to the test pattern data to record a test pattern, wherein plural areas each corresponding to each of the plural light beams are provided, and the drive means drives, within each of the plural areas, the light beam corresponding to such the area to record the test pattern. 
     A fourth embodiment of the present invention is an electrophotographic apparatus which includes plural emission means each for emitting a light beam, a drive means for driving each of the plural light beams according to inputted image data, and a scan means for performing scanning on an identical recording medium with the plural light beams, wherein a test pattern is recorded in a predetermined area by using any one of the plural emission means. 
     A fifth embodiment of the present invention is an electrophotographic apparatus which includes plural emission means each for emitting a light beam, a drive means for driving each of the plural light beams according to inputted image data, and a scan means for performing scanning on an identical recording medium with the plural light beams, wherein a test pattern is recorded in each of plural areas by using any one of the plural emission means. 
     Other objects, structures and effects of the present invention will be apparent from the following detailed description and the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing a circuit to generate test pattern data according to first and second embodiments of the present invention; 
     FIG. 2 is a flow chart of an operation to control an SEL signal  330  in the first embodiment; 
     FIG. 3 is a view showing a printed result of a test pattern in an ordinary state in the first embodiment; 
     FIG. 4 is a view showing a printed result of the test pattern in a state that one of plural laser beams is not emitted in the first embodiment; 
     FIG. 5 is a flow chart of an operation to control an SEL signal  330  in the second embodiment; 
     FIG. 6 is a view showing a printed result of a test pattern in an ordinary state in the second embodiment; 
     FIG. 7 is a view showing a printed result of the test pattern in a state that horizontal synchronism is displaced between two laser beams in the second embodiment; 
     FIG. 8 is a view showing a printed result of the test pattern in a state that jitter in a BD signal detection means is large in the second embodiment; 
     FIG. 9 is a diagram showing a circuit to generate test pattern data according to a third embodiment of the present invention; 
     FIG. 10 is a view showing a printed result of a test pattern in an ordinary state in the third embodiment; 
     FIG. 11 is a view showing a printed result of the test pattern in a state that intensity is not uniform between two laser beams in the third embodiment; 
     FIG. 12 is a sectional view showing a structure of a multibeam laser printer (common to prior art); 
     FIGS. 13A,  13 B,  13 C,  13 D,  13 E,  13 F,  13 G,  13 H and  13 I are time charts for explaining an operation of the multibeam laser printer shown in FIG. 12; 
     FIG. 14 is a diagram showing a circuit to generate test pattern data in a conventional multibeam laser printer; 
     FIGS. 15A,  15 B,  15 C,  15 D,  15 E,  15 F,  15 G,  15 H,  15 I and  15 J are time charts for explaining an operation of the test pattern data generation circuit of the conventional multibeam laser printer shown in FIG. 14; 
     FIG. 16 is a view showing a printed result of a conventional test pattern in a state that one of the laser beams is not emitted; 
     FIG. 17 is a view showing a printed result of the conventional test pattern in a state that abnormality occurs in horizontal sync control of the laser beam; and 
     FIG. 18 is a view showing a printed result of the conventional test pattern in a state that intensity is not uniform between the two laser beams. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (First Embodiment) 
     FIG. 1 shows an example of a test pattern data generation circuit by which the present invention is realized in, e.g., a two-beam laser printer using two laser beams. That is, in the first embodiment, it is assumed that a laser A is selected to drive a laser beam A and driving of a laser B is inhibited for, e.g., an upper-half area of a test image, while the laser B is being driven and driving of the laser A is being inhibited for a lower-half area being neighboring to the upper-half area in a sub-scan direction. 
     In the first embodiment, a longitudinal-line image is recorded by using any one of the lasers in each of the plural areas. It should be noted that an operation of an electrophotographic process in the first embodiment is the same as that in the related background art already explained in FIG.  12 . 
     In FIG. 1, an SEL signal  330  used to select the two lasers and the two laser beams is inputted to a multiplexer  310  and also to a BD signal sync circuit  311 . 
     The multiplexer  310  selects one of a /BD1 signal  331  and a /BD2 signal  332  according to the inputted SEL signal  330 , and then inputs a signal  320  to a phase sync oscillator  302 , a main-scan counter  303  and the BD signal sync circuit  311 . If it is assumed that the SEL signal  330  instructs to select the laser A, the multiplexer  310  selects the /BD1 signal  331 . Hereinafter, a case where the SEL signal  330  instructs to drive the laser A will be explained. 
     In such a state as the laser A is being selected, when the laser beam A passes through a beam detector, the /BD1 signal  331  becomes TRUE. Thus, by using this TRUE signal as a trigger, the phase sync oscillator  302  starts to generate an image clock signal (CLK signal)  321  in synchronism with the /BD1 signal  331 . The CLK signal  321  is then inputted to the main-scan counter  303  and also to a counter  306  for generating test pattern data. 
     Similarly, in synchronism with timing when the /BD1 signal  331  becomes TRUE, the main-scan counter  303  is reset. The counter  303  is reset based on the /BD signal to count the CLK signal  321  and detect which position in a main-scan direction the noticeable (i.e., remarkable) laser beam is being currently scanned. By a comparator  304 , an output (i.e., main-scan counter value  322 ) of this counter  303  is compared with a value  322  previously set in a mask signal timing setting register  301 . Then, according to a compared result, a mask signal (/MASK signal)  324  is outputted from the comparator  304 . It should be noted that, in the mask signal timing setting register  301 , the two values respectively representing main-scan positions for mask release and mask generation have been previously set by a print control unit  26  (FIG.  12 ). These values and the position to which the laser beam is currently scanning are compared to output the mask signal, thereby controlling writing inhibition in a main-scan horizontal direction. On the other hand, a control signal for writing inhibition in the sub-scan direction is inputted by the print control unit  26  as a /top erase signal (/TOPE signal)  325 . These control signals for writing inhibition in the main- and sub-scan directions (i.e., /MASK signal  324  and /TOPE signal  325 ) are synthesized by a gate  305  to output a writing inhibition signal  326 . 
     The test pattern data is generated by the four-bit counter  306  and an NAND gate  307 . In this case, since a bit length of the counter  306  should be selected according to a print pattern, it is unnecessary to always select the four-bit length. When the writing inhibition signal  326  becomes FALSE and thus the writing inhibition is released, the counter  306  start to count the CLK signal  321 . Then, when the value obtained from the counter  306  reaches “Fh”, a /test print signal  327  becomes TRUE. On the other hand, when the writing inhibition signal  326  is FALSE, the counter  306  is cleared, and the /test print signal  327  surely becomes FALSE. The signal  327  is outputted as a /TEST PATTERN1 signal  334  through a demultiplexer  312 , and thus the laser A is turned on or off in response to the outputted signal  334 . The demultiplexer  312  outputs the /test print signal  327  to the laser A of a semiconductor laser  13  (FIG. 12) as the /TEST PATTERN1 signal  334 , in response to an SEL signal  333  being synchronous with the /BD1 signal  331 . 
     As above, the laser is surely OFF during the writing inhibition, while the /TEST PATTERN1 signal  334  is TRUE for one clock at a 16 clock period during release of the writing inhibition, whereby the laser A records black pixels at a certain interval in the main-scan direction. It should be noted that, while the laser A is being selected, the driving of laser B is inhibited. 
     On the other hand, during a period when it is being instructed by the SEL signal  330  to select the laser B, the laser B is turned on or off in response to a /TEST PATTERN2 signal  335  through the similar process, and the driving of laser A is inhibited. 
     In the structure to generate the test pattern data as described above, the plural lasers time-divisionally utilize the test pattern data generation circuit provided only one. Therefore, it is unnecessary to provide the plural test pattern data generation circuits for the respective lasers. 
     FIG. 2 is a flow chart showing an operation to control the SEL signal  330  shown in FIG.  1 . 
     Initially, in a step S 1 , it waits for a test print instruction. If there is the test print instruction, then the flow advances to a step S 2  to initialize a laser counter variable “n”, a scan variable “scan” and a laser switch value “scan1”. Then, the flow advances to a step S 3  to select the laser. As a result, the flow advances to a step S 4  or a step S 5 . In the step S 4 , the select signal (SEL signal)  330  to select the laser A is outputted, while in the step S 5 , the SEL signal  330  to select the laser B is outputted. After the SEL signal  330  is sent, the flow advances to a step S 6  to be on standby until the writing inhibition in the sub-scan direction based on the /TOPE signal  325  is released. If the inhibition is released, the flow advances to a step S 7  to be on standby until the /BD signal  320  becomes TRUE after it passes through the multiplexer  310 . When the /BD signal  320  becomes TRUE, the flow advances to a step S 8  to perform increment of the scan variable “scan” by one. Then, the flow advances to a step S 9  to compare the scan variable “scan” with the laser switch value “scan1”. If “scan”≠“scan1”, the flow advances to a step S 10 , while if “scan”≠“scan1”, the flow advances to a step S 11 . In the step S 10 , it performs increment of the laser counter variable “n” by one, and then the flow advances to the step S 3 . On the other hand, if it is judged in the step S 11  that the writing inhibition in the sub-scan direction based on the /TOPE signal  325  is released, the flow advances to the step S 7 . On the other hand, if the writing inhibition is not released, the process terminates. It should be noted that the laser switch value “scan1” can be arbitrarily set. For example, in case of switching or changing the laser at the center of the paper sheet, the value “scan1” becomes “the number of scan lines until the lines reach the center / 2”. 
     FIG. 3 shows an example of a test pattern result which is outputted when the test pattern data generation circuit in the first embodiment operates in a case where every light beam is normal. In FIG. 3, longitudinal lines  51  are printed or drawn in an upper-half area on the paper sheet by the laser A, and longitudinal lines  52  are printed or drawn in a lower-half area by the laser B. The lines  51  and  52  are exposed on a photosensitive drum  17  (FIG. 12) as broken lines each having one-dot space and expanding in a longitudinal direction. However, through the electrostatic process, these lines are actually printed as the lines approximating to solid lines in the longitudinal direction. 
     On the other hand, as the abnormal state example being specific when the plural light beams are used, in the case where one of the plural light beams is deteriorated and thus does not completely operate (i.e., example 1 of abnormal state), a test pattern result shown in FIG. 4 can be obtained in the first embodiment. In FIG. 4, since a lower-half area  55  is blank, it can be easily judged that the laser B has been deteriorated. It should be noted that FIG. 4 shows the example in the case where the laser B has been completely deteriorated. That is, in a transitional state before the laser B is completely deteriorated, the area  55  is printed with thin longitudinal lines. By applying the present invention as above, it is possible to generate the test pattern data capable of being detected even in such the transitional state. 
     Further, as items inspectable by using the longitudinal-line pattern as the test pattern, e.g., possibility of printing, degree of an inclination, degree of jitter in a scanner motor, confirmation of a mask area and the like can be cited. In this case, it should be noted that the confirmation of the mask area can be inspected only in a case where the mask generation circuit is identical between the test printing and the printing based on a /VDO signal. In any case, these items can be confirmed or discriminated from the longitudinal lines  51  and  52  respectively drawn by the lasers A and B both obtained in the first embodiment. 
     Although the two-beam laser printer is explained by way of example in the first embodiment, the present invention is not limited to such the printer. Namely, the present invention is applicable to a multibeam laser printer in which plural beams are used. 
     Further, it is explained in the first embodiment the example that one face of one paper sheet is divided into two areas and the test pattern is drawn in each area by one beam. However, the present invention is not limited to such the operation as the test pattern is printed on one face of one sheet. For example, it is possible to draw the test pattern on a first sheet by a first beam and on a second sheet by a second beam, and also possible to draw the pattern on a front face of the sheet by the first beam and on a rear face thereof by the second beam. 
     Furthermore, an interval between the adjacent longitudinal lines in the longitudinal-line pattern is determined based on the number of bits of the counter  306  or the like. However, it is possible to make the interval variable to generate a longitudinal-line pattern arbitrarily designated by a user every time the test pattern is generated. 
     (Second Embodiment) 
     Subsequently, the second embodiment will be explained with reference to FIG.  5 . FIG. 5 is the flow chart showing a control method of the SEL signal  330  shown in FIG.  1 . In the second embodiment, an abnormality judgment function in horizontal sync control is added to the functions already explained in the first embodiment. 
     Initially, in a step S 21 , it waits for a test print instruction. If there is the test print instruction, then the flow advances to a step S 22  to initialize a laser counter variable “n”, a scan variable “scan” and a laser switch value “scan1”. Then, the flow advances to a step S 23  to select the laser. As a result, the flow advances to a step S 24  or a step S 25 . In the step S 24 , the select signal (SEL signal)  330  to select the laser A is outputted, while in the step S 25 , the SEL signal  330  to select the laser B is outputted. After the SEL signal  330  is sent, the flow advances to a step S 26  to be on standby until the writing inhibition in the sub-scan direction based on the /TOPE signal  325  is released. If the inhibition is released, the flow advances to a step S 27  to be on standby until the /BD signal  320  becomes TRUE after it passes through the multiplexer  310 . When the /BD signal  320  becomes TRUE, the flow advances to a step S 28  to perform increment of the scan variable “scan” by one. Then, the flow advances to a step S 29  to compare the scan variable “scan” with the laser switch value “scan1”. If “scan mod scan1”=0, the flow advances to a step S 30 , while if “scan mod scan1”≠0, the flow advances to a step S 31 . In the step S 30 , it performs increment of the laser counter variable “n” by one, and then the flow advances to the step S 23  again. On the other hand, if it is judged in the step S 31  that the writing inhibition in the sub-scan direction based on the /TOPE signal  325  is released, the flow advances to the step S 27 . On the other hand, if the writing inhibition is not released, the process terminates. Like the first embodiment, the laser switch value “scan1” can be arbitrarily set. For example, in FIG. 6, the total number of scanning during the printing of one sheet is assumed to be 7000 times, whereby the value “scan1” is set to be 1750. 
     FIG. 6 shows an example of a test pattern which is outputted when a test pattern data generation circuit according to the second embodiment operates in a case where every light beam is normal in the sync control. In the test pattern of FIG. 6, longitudinal lines  61  by the laser A, longitudinal lines  62  by the laser B, longitudinal lines  63  by the laser A and longitudinal lines  64  by the laser B are sequentially printed or drawn from the top. Then, the lines  61  to  64  are exposed on a photosensitive drum respectively as broken lines each having one-dot spaces and expanding in a longitudinal direction. However, through the electrostatic process, these lines are actually printed as the lines approximating to solid lines in the longitudinal direction. 
     On the other hand, as the abnormal state example being specific when the plural light beams are used, in the case where abnormality occurs in the horizontal sync control (i.e., example 2 of abnormal state), and further in a case where, e.g., writing timing of two beams in a main-scan direction is asynchronous, a test pattern result shown in FIG. 7 is obtained. Further, the jitters in a means for detecting the /BD signal (/BD1 signal  120  and /BD2 signal  220 ) are relatively large, a test pattern result shown in FIG. 8 is obtained. 
     As described above, according to the second embodiment, in addition to the effect derived in the first embodiment, a further specific effect can be derived by repeatedly providing an area on which the printing is performed by using only one beam. This further specific effect is that, when horizontal sync can not be obtained, it is possible to clearly specify the reason of such inconvenience, i.e., to judge whether the horizontal sync of one of the two beams can not be obtained or the jitters in the means for detecting the /BD signal (/BD1 signal  120  and /BD2 signal  220 ) are large. 
     Like the first embodiment, although the two-beam laser printer is explained by way of example in the second embodiment, the present invention is not limited to such the printer. Namely, the present invention is applicable to a multibeam laser printer in which plural beams are used. 
     Further, it is explained in the second embodiment the example that one face of one paper sheet is divided into two areas and the test pattern is drawn or printed in each area by one beam. However, the present invention is not limited to such the operation as the test pattern is printed on one face of one sheet. For example, it is possible to draw the test pattern on a first sheet by a first beam and on a second sheet by a second beam, and also possible to draw the pattern on a front face of the sheet by the first beam and on a rear face thereof by the second beam. 
     Furthermore, although a laser switch interval is determined by the laser switch value “scan1” in the second embodiment, the present invention is not always fixed to such a determination operation. That is, the laser switch interval may be designated by a user every time the test pattern is generated. Furthermore, as described in the first embodiment, it is possible to make variable the interval between the adjacent longitudinal lines in the longitudinal-line pattern, to generate a longitudinal-line pattern arbitrarily designated by the user every time the test pattern is generated. 
     (Third Embodiment) 
     In the third embodiment, a solid-color image such as a halftone image or the like is recorded in each of plural areas by one of plural lasers. 
     FIG. 9 is a block diagram showing a structure of a test pattern data generation circuit by which the third embodiment is realized. In FIG. 9, an SEL signal  430  is a signal for selecting the laser to which test printing is hereafter performed. The SEL signal  430  is inputted to a multiplexer  410  and also to a BD signal sync circuit  411 . The multiplexer  410  which received the SEL signal  430  acts to connect an input signal (i.e., /BD1 signal  431  or /BD2 signal  432 ) required for the laser to be driven hereafter, with the test pattern generation circuit. 
     Hereinafter, a case where the SEL signal  430  for driving a laser A was inputted to the multiplexer  410  will be explained. The /BD1 signal (signal  420 ) outputted from the multiplexer  410  is inputted to a phase sync oscillator  402 , a main-scan counter  403  and the BD signal sync circuit  411 . At timing when the /BD1 signal  431  becomes TRUE, the main-scan counter  403  is reset. Similarly, at timing when the /BD1 signal  431  becomes TRUE, the BD signal sync circuit  411  sends the held SEL signal  430  to a demultiplexer  412  as an SEL signal  433 . The demultiplexer  412  which received the SEL signal  433  synchronous with the /BD1 signal  431  outputs an inputted /test print signal  426  to the laser A as a /TEST PATTERN1 signal  434 . Further, by using as a trigger the change that the /BD1 signal  431  becomes TRUE, the oscillator  402  starts to generate an image clock signal (CLK signal)  421  synchronous with the /BD1 signal  431 . The CLK signal  421  is inputted to the main-scan counter  403  and a NOR gate  405 , and the counter  403  counts the number of pulses of the CLK signal  421 . By a comparator  404 , a main-scan counter value  422  is compared with a value  423  set in a mask signal generation timing setting register  401 . As a result of such comparison, a /mask signal (/MASK signal)  424  is outputted from the comparator  404 . By a print control unit, two counter values at mask release and mask generation have been previously set in the register  401 , whereby writing inhibition control in a horizontal direction is performed. On the other hand, writing inhibition control in a vertical direction is performed based on a /top erase signal (/TOPE signal)  425  sent from the print control unit. The /MASK signal  424 , the /TOPE signal  425  and the CLK signal  421  are inputted to the NOR gate  405 . When the writing inhibition is released, the NOR gate  405  outputs the /test pattern signal  426  obtained by inverting the CLK signal  421 . The signal  426  is then outputted through the demultiplexer  412  as the /TEST PATTERN1 signal  434  to turn on and off the laser A. While the laser A is being selected, driving of a laser B is inhibited. 
     On the other hand, while it is being instructed by the SEL signal  430  to select the laser B, the laser B is turned on and off according to a /TEST PATTERN2 signal  435  and also driving of the laser A is being inhibited in the same manner as above. 
     The SEL signal shown in FIG. 9 is generated in an operation according to the flow chart of FIG. 2 to control the SEL signal (but substituting description of multiplexer  410  for that of multiplexer  310 ). 
     FIG. 10 shows an example of a test pattern which is outputted when the test pattern data generation circuit operates in the third embodiment in a case where all the light beams are controlled to be uniform in intensity. In FIG. 10, a halftone  81  by the laser A is printed on an upper area on the sheet, and a halftone  82  by the laser B is printed on a lower area thereon. 
     On the other hand, as the abnormal state example being specific when the plural light beams are used, it is supposed a case where the light quantities of the plural beams are not uniform (i.e., example 3 of abnormal state). For example, the intensity of the laser beam B is weaker than its reference value, a test pattern result shown in FIG. 11 is obtained in the third embodiment. That is, as shown in FIG. 11, since a density of a lower-half area  85  is thinner than that of an upper-half area  84 , it can be relatively detected that the intensity of the laser beam B becomes weak. 
     As items inspectable by using a halftone pattern as the test pattern, e.g., possibility of printing, confirmation of density unevenness, confirmation of a mask area and the like can be cited. In this case, it should be noted that the confirmation of the mask area is inspectable only in a case where the mask generation circuit is identical between the test printing and the printing based on a /VDO signal. In any case, these items can be also confirmed or discriminated from the halftone pattern  81  drawn by the laser A and the halftone pattern  82  drawn by the laser B. 
     As described above, according to the third embodiment, dispersion in the image density due to dispersion in the laser beam intensity can be detected from the halftone pattern drawn by one laser beam in a multibeam laser printer. 
     Like the above embodiments, although the two-beam laser printer is explained by way of example in the third embodiment, the present invention is not limited to such the printer. Namely, the present invention is applicable to the multibeam laser printer in which the plural beams are used. 
     Further, it is explained in the third embodiment the example that one face of one paper sheet is divided into two areas and the halftone pattern is printed in each area by one beam. However, the present invention is not limited to such the operation as the halftone pattern is printed on one face of one sheet. For example, it is possible to print the halftone pattern on a first sheet by a first beam and on a second sheet by a second beam, and also possible to print the pattern on a front face of the sheet by the first beam and on a rear face thereof by the second beam. 
     (Other Embodiments) 
     In the above first to third embodiments, it has been explained the structure that the test pattern data generated by one test pattern data generation circuit is inputted to any one of the plural lasers. However, it is possible to provide the test pattern data generation circuit corresponding to each of the plural lasers. Further, it is possible to previously store the test patterns shown in FIGS. 3,  6  and  10  in an image memory and then perform printing based on the stored patterns. 
     As above, there have been explained the examples in which the various test patterns are recorded according to the various structures. However, it is still more preferable to combine these structures to enable switching of generation of the various patterns according to an instruction signal externally inputted. 
     According to the above embodiments, in case of recording the test pattern for the optical printer which performs the image formation by using the plural light beams, it is possible to record the test pattern by which the abnormality state of the light beam can be correctly detected. Further, it is possible to record the test pattern at low cost. Furthermore, it is possible to record the test pattern allowing the user to detect which light beam the abnormal state occurs. 
     As above, the present invention has been explained with reference to the several preferred embodiments. However, the present invention is not limited to these embodiments, and various modifications and application are possible within the appended claims.