Abstract:
An image forming apparatus includes image carriers; an exposing unit that forms latent images on the image carriers; a developing unit that develops the latent images with toners of different colors from each other; a first transfer unit that forms a color image by superimposing and transferring the developed images onto a second image carrier; a test pattern forming unit that forms, on the image carriers, test patterns to be transferred onto the second image carrier; and test pattern detection units capable of detecting the test patterns transferred onto the second image carrier in different positions from each other in a main-scanning direction. The test pattern forming unit selectively switches, depending on a width of the test patterns in the main-scanning direction, whether to form each of the test patterns in a position detectable by the corresponding one of the test pattern detection units.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2011-159166 filed in Japan on Jul. 20, 2011. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an image forming apparatus, a method thereof, and a computer program product. 
         [0004]    2. Description of the Related Art 
         [0005]    In apparatuses such as a copying machine and a multifunction peripheral (MFP) that houses a plurality of functions such as copy, facsimile, and printer functions in one housing, image adjustments such as a color shift correction and a density correction are performed by forming test patterns on an intermediate transfer belt with a toner, and then by detecting the test patterns with a plurality of sensors. The sensors detecting the test patterns are placed at locations at which main-scanning positions of the sensors are different from each other, and the test patterns are formed at locations on the intermediate transfer belt where the sensors can detect the test patterns. 
         [0006]    In addition, in order to reduce time (down time) during which a printing operation cannot be performed due to the image adjustment, the image adjustment is performed by forming the test patterns at both ends outside a main-scanning image area concurrently with the printing operation. 
         [0007]    However, in the above-described method in which the test patterns are formed at both ends outside the main-scanning image area concurrently with the printing operation, the sensors for detecting the test patterns need to be placed at outer ends of the image area. In addition, in the case of forming test patterns having a large width in a main-scanning direction, a laser diode (LD) needs to emit light outside an optical properties guarantee area of an LD scanning optical system. Thus, there is a problem that operational trouble or image degradation is caused by, for example, exposure of an unintended place to flare light. 
         [0008]    Here, the term “optical properties guarantee area” refers to an area in a main-scanning direction in which a scanning beam is guaranteed to be emitted to a target area of a photosensitive element. Outside this area, lens properties of the scanning optical system are not guaranteed, and when the beam is turned on, exposure of an unintended place occurs. When the unintended exposure occurs in the photosensitive element, the exposure causes occurrence of troubles such as a failure in an adjusting operation and image degradation, or, when the unintended exposure occurs in a sensor such as a synchronization detector, the exposure causes occurrence of operational abnormalities. 
         [0009]    In order to reduce the down time due to the image quality adjustment, Japanese Patent Application Laid-open No. 2009-169031 discloses a method in which patterns are formed for sensors at the ends of the image. Furthermore, in order to detect patterns for detecting a positional deviation without fail, Japanese Patent Application Laid-open No. H11-102098 discloses a method in which a rough adjustment is first performed by roughly making correction with large shaped patterns, and then a fine adjustment is performed by finely making correction with small shaped patterns. 
         [0010]    However, the technique described in Japanese Patent Application Laid-open No. 2009-169031 merely forms the patterns at the outer ends of the image area at the same time when the image is formed, and does not form patterns for sensors inside the image area. Thus, the patterns formed for the sensors at the ends of the image are situated outside the optical properties guarantee area. That is, a light source emits light outside the optical properties guarantee area. In the technique described in Japanese Patent Application Laid-open No. H11-102098, too, the light source emits light outside the optical properties guarantee area, depending on the size of the patterns formed and the locations of the sensors. 
         [0011]    Therefore, there is a need for an image forming apparatus and a method thereof that are capable of suppressing the light source from emitting light outside the optical properties guarantee area. 
       SUMMARY OF THE INVENTION 
       [0012]    According to an embodiment, there is provided an image forming apparatus that includes a plurality of image carriers; a charging unit that charges the image carriers; an exposing unit that forms latent images on the image carriers; a developing unit that develops the latent images formed on the image carriers with toners of different colors from each other; a first transfer unit that forms a color image by superimposing and transferring the images developed on the image carriers onto a second image carrier moving in transfer positions facing the image carriers; a second transfer unit that transfers the images transferred to and formed on the second image carrier onto a transfer material; a test pattern forming unit that forms, on the image carriers, test patterns to be transferred onto the second image carrier; a plurality of test pattern detection units that are capable of detecting the test patterns transferred onto the second image carrier in different positions from each other in a main-scanning direction; and a control unit that changes image forming conditions depending on detection results of the test patterns. The test pattern forming unit selectively switches, depending on a width of the test patterns in the main-scanning direction, whether to form each of the test patterns in a position detectable by the corresponding one of the test pattern detection units. 
         [0013]    According to another embodiment, there is provided an image forming method performed in an image forming apparatus. The image forming apparatus includes a plurality of image carriers; a charging unit that charges the image carriers; an exposing unit that forms latent images on the image carriers; a developing unit that develops the latent images formed on the image carriers with toners of different colors from each other; a first transfer unit that forms a color image by superimposing and transferring the images developed on the image carriers onto a second image carrier moving in transfer positions facing the image carriers; a second transfer unit that transfers the images transferred to and formed on the second image carrier onto a transfer material; a test pattern forming unit that forms, on the image carriers, test patterns to be transferred onto the second image carrier; a plurality of test pattern detection units that are capable of detecting the test patterns transferred onto the second image carrier in different positions from each other in a main-scanning direction; and a control unit that changes image forming conditions depending on detection results of the test patterns. The image forming method includes determining a width of the test patterns in the main-scanning direction by the test pattern forming unit; and selectively switching, depending on the width of the test patterns, whether to form each of the test patterns in a position detectable by the corresponding one of the test pattern detection units, by the test pattern forming unit. 
         [0014]    According to still another embodiment, there is provided a computer program product including a non-transitory computer-readable medium including a computer program executed on an image forming apparatus. The image forming apparatus includes a plurality of image carriers; a charging unit that charges the image carriers; an exposing unit that forms latent images on the image carriers; a developing unit that develops the latent images formed on the image carriers with toners of different colors from each other; a first transfer unit that forms a color image by superimposing and transferring the images developed on the image carriers onto a second image carrier moving in transfer positions facing the image carriers; a second transfer unit that transfers the images transferred to and formed on the second image carrier onto a transfer material; a test pattern forming unit that forms, on the image carriers, test patterns to be transferred onto the second image carrier; a plurality of test pattern detection units that are capable of detecting the test patterns transferred onto the second image carrier in different positions from each other in a main-scanning direction; and a control unit that changes image forming conditions depending on detection results of the test patterns. The program, when executed on the image forming apparatus, causes the image forming apparatus to perform determining a width of the test patterns in the main-scanning direction; and selectively switching, depending on the width of the test patterns, whether to form each of the test patterns in a position detectable by the corresponding one of the test pattern detection units. 
         [0015]    The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]      FIG. 1  is a diagram illustrating a configuration of an image forming apparatus according to an embodiment of the present invention; 
           [0017]      FIG. 2  is a diagram schematically illustrating a configuration of an inside of each of detection sensors of  FIG. 1 ; 
           [0018]      FIG. 3  is a block diagram illustrating internal configurations of the detection sensors and a configuration of functions in charge of processing data detected by the detection sensors; 
           [0019]      FIG. 4  is a diagram illustrating marks in pattern images for positional deviation correction and a waveform example of a detection signal of the marks detected by the detection sensor; 
           [0020]      FIG. 5  is a diagram illustrating one set of the marks scanned by the detection sensor; 
           [0021]      FIG. 6  is a diagram illustrating an intermediate transfer belt and the detection sensors in the case of forming the patterns for correction concurrently with image printing; 
           [0022]      FIG. 7A  is a diagram illustrating the intermediate transfer belt formed with the test patterns for correction and the detection sensors; 
           [0023]      FIG. 7B  is a diagram illustrating the intermediate transfer belt formed with the test patterns for correction and the detection sensors; 
           [0024]      FIG. 8A  is a diagram illustrating the intermediate transfer belt formed with the test patterns for correction and the detection sensors; 
           [0025]      FIG. 8B  is a diagram illustrating the intermediate transfer belt formed with the test patterns for correction and the detection sensors; 
           [0026]      FIG. 9A  is a diagram illustrating the intermediate transfer belt formed with the test patterns for correction and the detection sensors; 
           [0027]      FIG. 9B  is a diagram illustrating the intermediate transfer belt formed with the test patterns for correction and the detection sensors; 
           [0028]      FIG. 10  is a flow chart illustrating a color shift correction process according to the embodiment of the present invention; and 
           [0029]      FIG. 11  is a block diagram illustrating a hardware configuration of the image forming apparatus according to the embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0030]    An embodiment of an image forming apparatus according to the present invention will be described below in detail with reference to the accompanying drawings.  FIG. 1  is a diagram illustrating a configuration of the image forming apparatus according to the embodiment of the present invention. This image forming apparatus  100  is an image processing apparatus including, for example, a facsimile apparatus, a printing apparatus (printer), a copying machine and an MFP, and is configured to include an optical device  101  that includes optical elements such as semiconductor laser light sources and a polygonal mirror; an image forming unit  102  that includes, for example, drum-like photosensitive elements (also called “photosensitive drums”), chargers, and developing units; and a transfer unit  103  that includes, for example, an intermediate transfer belt. 
         [0031]    The optical device  101  deflects, with a polygonal mirror  110 , light beams BM emitted from a plurality of light sources (omitted from illustration) that are semiconductor laser light sources including laser diodes (LDs), and inputs the deflected light beams to scanning lenses  111   a  and  111   b  that include fθ lenses. The light beams are generated such that the number of light beams corresponds to the number of images of each color yellow (Y), black (K), magenta (M), and cyan (C). The respective light beams are reflected by reflecting mirrors  112   y,    112   k,    112   m,  and  112   c  after passing through the scanning lenses  111   a  and  111   b.  For example, the yellow light beam Y is transmitted through the scanning lens  111   a,  then is reflected by the reflecting mirror  112   y,  and then enters into a WTL lens  113   y.  The same applies to the light beams K, M, and C having the colors black, magenta, and cyan, respectively, and therefore, description thereof will be omitted. 
         [0032]    WTL lenses  113   y,    113   k,    113   m,  and  113   c  form the entered light beams Y, K, M, and C, respectively, and then, deflect the light beams Y, K, M, and C to reflecting mirrors  114   y,    114   k,    114   m,  and  114   c,  respectively. The light beams Y, K, M, and C are further reflected by reflecting mirrors  115   y,    115   k,    115   m,  and  115   c,  respectively, and projected in an image-like manner onto photosensitive drums (hereinafter called “photosensitive elements”)  120   y,    120   k,    120   m,  and  120   c  as the respective light beams Y, K, M, and C used for exposure. 
         [0033]    The light beams Y, K, M, and C are projected onto the photosensitive elements  120   y,    120   k,    120   m,  and  120   c  by using the multiple optical elements as described above. Therefore, timing synchronization is performed with respect to main-scanning directions and sub-scanning directions relative to the photosensitive elements  120   y,    120   k,    120   m,  and  120   c.  Hereinafter, the main-scanning directions relative to the photosensitive elements  120   y,    120   k,    120   m,  and  120   c  are defined as scanning directions of the light beams, and the sub-scanning directions relative to the photosensitive elements  120   y,    120   k,    120   m,  and  120   c  are defined as directions perpendicular to the main-scanning directions. That is, the sub-scanning direction is defined as the rotation direction of the photosensitive elements  120   y,    120   k,    120   m,  and  120   c.    
         [0034]    The photosensitive elements  120   y,    120   k,    120   m,  and  120   c  are each provided with a photoconductive layer, including at least a charge generation layer and a charge transport layer on a conductive drum made of, for example, aluminum. The photoconductive layers are respectively arranged corresponding to the photosensitive elements  120   y,    120   k,    120   m,  and  120   c,  and surface charges are given thereto by chargers  122   y,    122   k,    122   m,  and  122   c  configured to include a corotron, a scorotron, a charging roller, and the like. 
         [0035]    The electrostatic charges provided on the photosensitive elements  120   y,    120   k,    120   m,  and  120   c  by the chargers  122   y,    122   k,    122   m,  and  122   c,  respectively, are exposed in an image-like manner to the light beams Y, K, M, and C, respectively. Accordingly, an electrostatic latent image is formed on a scanned surface of each of the photosensitive elements  120   y,    120   k,    120   m,  and  120   c.    
         [0036]    The electrostatic latent images respectively formed on the scanned surfaces the photosensitive elements  120   y,    120   k,    120   m,  and  120   c  are developed by developing units  121   y,    121   k,    121   m,  and  121   c,  respectively, each including, for example, a developing sleeve, a developer supplying roller, and a regulating blade. With this process, a developer image is formed on a scanned surface of each of the photosensitive elements  120   y,    120   k,    120   m,  and  120   c.    
         [0037]    The developers carried on the scanned surfaces of the photosensitive elements  120   y,    120   k,    120   m,  and  120   c  are transferred by primary transfer rollers  132   y,    132   k,    132   m,  and  132   c  corresponding to the photosensitive elements  120   y,    120   k,    120   m,  and  120   c,  respectively, onto an intermediate transfer belt  130  that is moved in the direction of arrow D by carriage rollers  131   a,    131   b,  and  131   c.    
         [0038]    The intermediate transfer belt  130  is conveyed to a secondary transfer unit while carrying the developers of Y, K, M, and C thereon that have been transferred from the scanned surfaces of the photosensitive elements  120   y,    120   k,    120   m,  and  120   c,  respectively. 
         [0039]    The secondary transfer unit is configured to include a secondary transfer belt  133  and carriage rollers  134   a  and  134   b.  The secondary transfer belt  133  is conveyed by the carriage rollers  134   a  and  134   b  in the direction of arrow E. The secondary transfer unit is supplied with an image receiving material sheet P such as high-quality paper or a plastic sheet by carriage rollers  135  from a sheet housing unit T such as a paper cassette. The secondary transfer unit transfers, by applying a secondary transfer bias, a multicolor developer image carried on the intermediate transfer belt  130  onto the sheet P that is attracted to and held on the secondary transfer belt  133 . The sheet P is supplied to a fixing device  136  as the secondary transfer belt  133  is conveyed. The fixing device  136  is configured to include fixing members  137  such as fixing rollers that include, for example, silicone rubber and fluoro-rubber, and pressurizes and heats the sheet P and the multicolor developer image. Then, the sheet P is discharged by discharging rollers  138  as a printed matter P′ out of the image forming apparatus  100 . 
         [0040]    After the multicolor developer image is transferred from the intermediate transfer belt  130 , the residual developer after the transfer is removed from the intermediate transfer belt  130  by a cleaning unit  139  that includes a cleaning blade. Then, the intermediate transfer belt  130  is provided to the next image forming process. 
         [0041]    Provided near the carriage roller  131   a  are three detection sensors  5   a,    5   b,  and  5   c  for detecting pattern images (including a “test pattern image for color shift correction” and a “test pattern image for density correction”) for correcting image forming conditions used when the color image is formed on the intermediate transfer belt  130 . As the detection sensors  5   a,    5   b,  and  5   c,  reflective detection sensors respectively including well-known reflective photosensors may be used. Based on detection results by the respective detection sensors  5   a,    5   b,  and  5   c,  various deviation amounts of each color relative to a reference color are calculated, including a skew (inclination), a main-scanning registration deviation amount, a sub-scanning registration deviation amount, and a main-scanning magnification error; then, based on the calculation results, the various deviation amounts related to image quality adjustment are corrected; the image forming conditions (positional deviation correction and density correction) used when the color image is formed on the intermediate transfer belt  130  are corrected; and various processes related to generation of the test pattern images during image adjustment are executed. 
         [0042]      FIG. 2  is a diagram schematically illustrating a configuration of an inside of each of the detection sensors  5   a,    5   b,  and  5   c  illustrated in  FIG. 1 . The detection sensors  5   a,    5   b,  and  5   c  have common internal configurations. Although  FIG. 2  illustrates the detection sensor  5   a,  the detection sensors  5   b  and  5   c  have the same internal configuration, and therefore, description thereof will be omitted. 
         [0043]    The detection sensor  5   a  has one light-emitting element  10   a,  two light-receiving elements  11   a  and  12   a,  and a condenser lens  13   a.  The light-emitting element  10   a  is a light-emitting device that generates light, and is, for example, an infrared light LED that generates infrared light. The light-receiving element  11   a  is, for example, a specular reflection light-receiving element, and the light-receiving element  12   a  is, for example, a diffuse reflection light-receiving element. 
         [0044]    In the detection sensor  5   a,  light L 1  emitted from the light-emitting element  10   a  is transmitted through the condenser lens  13   a,  and then, arrives at a test pattern (omitted from illustration) on the intermediate transfer belt  130 . Then, a part of the light is specularly reflected on a test pattern forming area and toner layers of the test pattern forming area to become specular reflection light L 2 , and then, after being transmitted through the condenser lens  13   a  again, is received by the light-receiving element  11   a.  Moreover, another part of the light is diffusely reflected on the test pattern forming area and the toner layers of the test pattern forming area to become diffuse reflection light L 3 , and then, after being transmitted through the condenser lens  13   a  again, is received by the light-receiving element  12   a.    
         [0045]    Note that as a light-emitting device, for example, a laser light-emitting element can be used instead of the infrared light LED. Note also that, although a phototransistor is used as each of the light-receiving elements  11   a  and  12   a  (the specular reflection light-receiving element and the diffuse reflection light-receiving element), an element composed of, for example, a photodiode or an amplifying circuit can also be used. 
         [0046]      FIG. 3  is a block diagram illustrating the internal configurations of the detection sensors  5   a,    5   b,  and  5   c  of the image forming apparatus  100  and a configuration of functions in a controller of the image forming apparatus  100  that are in charge of processing data detected by the detection sensors  5   a,    5   b,  and  5   c.  The detection sensors  5   a,    5   b,  and  5   c  of the image forming apparatus  100  are provided with the light-emitting elements  10   a,    10   b,  and  10   c,  and the light-receiving elements  11   a  and  12   a,    11   b  and  12   b,  and  11   c  and  12   c,  respectively. Note that the condenser lenses  13   a,    13   b,  and  13   c  illustrated in  FIG. 2  are omitted from illustration in  FIG. 3 . 
         [0047]    The controller of the image forming apparatus  100  is provided, as functional units related to processing of the data detected by the detection sensors  5   a,    5   b,  and  5   c,  with a CPU  1 , a ROM  2 , a RAM  3 , an input/output (I/O) port  4 , light emission amount control units  14   a,    14   b,  and  14   c,  amplifiers (AMPs)  15   a,    15   b,  and  15   c,  filters  16   a,    16   b,  and  16   c,  analog/digital (A/D) converters  17   a,    17   b,    17   c,  first-in-first-out (FIFO) memories  18   a,    18   b,  and  18   c,  and sampling control units  19   a,    19   b,  and  19   c.    
         [0048]    The ROM  2  stores therein various computer programs for controlling the image forming apparatus  100 , including a computer program composed of procedures executed by the CPU  1  for performing various processes, including the correction process to correct the image forming conditions used when the color image is formed on the intermediate transfer belt  130 , a positional deviation calculating process to calculate a positional deviation amount in the main-scanning direction occurring when the color image is formed on the intermediate transfer belt  130 , and a pattern image correction process. 
         [0049]    The CPU  1  monitors detection signals from the light-receiving elements  11   a,    11   b,  and  11   c  at appropriate times, and the light emission amount control units  14   a,    14   b,  and  14   c  control light emission amounts so that the signals can be detected without fail even if a conveying belt or the light-emitting element  10   a,    10   b,  or  10   c  is, for example, degraded, thus keeping the levels of light-receiving signals from the light-receiving elements  11   a,    11   b,  and  11   c  constant at all times. The RAM  3  is, for example, an NVRAM, and stores therein various parameters. 
         [0050]    Next, the processing of the data detected by the detection sensors  5   a,    5   b,  and  5   c  will be described with reference to  FIG. 3 . The CPU  1  executes the programs stored in the ROM  2  using the RAM  3  as a work area, and when a test pattern image is detected as will be described later in detail, controls the light emission amount control units  14   a,    14   b,  and  14   c  via the I/O port  4 , so as to respectively project predetermined light intensities of light beams from the light-emitting elements  10   a,    10   b,  and  10   c  of the detection sensors  5   a,    5   b,  and  5   c,  respectively. 
         [0051]    First of all, the light beam emitted from the light-emitting element  10   a  of the detection sensor  5   a  will be described. The light beam is projected onto the test pattern image, and parts of light reflected therefrom are respectively received by the light-receiving elements  11   a  and  12   a  of the detection sensor  5   a.  The light-receiving elements  11   a  and  12   a  send data signals corresponding to respective light intensities of received light beams to the amplifier  15   a.  The amplifier  15   a  amplifies the data signals and sends them to the filter  16   a.  The filter  16   a  passes only a signal component of line detection out of the output signals from the amplifier  15   a,  and sends the passed signal component to the A/D converter  17   a.  The A/D converter  17   a  converts the output signal of the filter  16   a  from analog data to digital data. Then, the sampling control unit  19   a  samples the digital data converted in the A/D converter  17   a,  and stores the sampled data in the FIFO memory  18   a.    
         [0052]    In the same manner as described above, as for data signals obtained from the light-receiving elements  11   b  and  12   b  of the detection sensor  5   b,  sampled digital data is stored in the FIFO memory  18   b,  and as for data signals obtained from the light-receiving elements  11   c  and  12   c  of the detection sensor  5   c,  sampled digital data is stored in the FIFO memory  18   c.    
         [0053]    After the detection of the test pattern image is completed in this manner, the digital data items respectively stored in the FIFO memories  18   a,    18   b,  and  18   c  are loaded into the CPU  1  and the RAM  3  via the I/O port  4  and data bus. Then, by executing the programs stored in the ROM  2 , the CPU  1  performs a predetermined calculation process on the data items, and executes various processes, including the correction process to correct the image forming conditions used when the color image is formed on the intermediate transfer belt  130 , the positional deviation calculating process to calculate the positional deviation amount in the main-scanning direction occurring when the color image is formed on the intermediate transfer belt  130 , and the pattern image correction process. 
         [0054]    In this manner, the CPU  1  and the ROM  2  control overall operation of the image forming apparatus  100  while serving as a control unit in charge of the processing of the data detected by the detection sensors  5   a,    5   b,  and  5   c,  and also serving as a correcting unit, a positional deviation amount calculating unit, and a pattern image correcting unit. The CPU  1  and the ROM  2  also serve as a unit for disabling the pattern image correcting unit. 
         [0055]    Next, a case will be described in which pattern images for positional deviation correction are used as the test pattern images.  FIG. 4  is a diagram illustrating marks in the pattern images for positional deviation correction and a waveform example of a detection signal of the marks detected by the detection sensor. 
         [0056]    The pattern image for positional deviation correction is a collection of predetermined marks for position alignment for specular reflection light, and one set of such marks  30  includes, as illustrated in  FIG. 4 , a transverse line pattern (also called “horizontal pattern”) and a slanted line pattern (also called “slanted pattern”) that are formed in the order of Y, K, M, and C. Eight of such marks  30  are arranged in the sub-scanning direction, and three rows of the eight marks  30  are arranged in the main-scanning direction in a manner corresponding to the detection sensors  5   a,    5   b,  and  5   c,  respectively, thus forming the pattern image for positional deviation correction. 
         [0057]    The transverse line pattern is a pattern having four laterally directed lines that are parallel to the main-scanning direction of the photosensitive elements  120   y,    120   k,    120   m,  and  120   c,  and each have a predetermined width and a predetermined length. The slanted line pattern is a pattern having four diagonally directed lines each having a predetermined inclination angle with respect to the main-scanning direction of the photosensitive elements  120   y,    120   k,    120   m,  and  120   c,  and a predetermined width and a predetermined length. The pattern image for positional deviation correction is formed on the intermediate transfer belt  130  in the arrangement illustrated in  FIG. 4  by forming, on the photosensitive elements  120   y,    120   k,    120   m,  and  120   c,  the transverse line pattern and the slanted line pattern, with 8 sets and 3 rows, of their corresponding colors Y, K, M, and C, respectively, and by transferring the formed patterns onto the intermediate transfer belt  130 . 
         [0058]    Long-dashed-short-dashed lines  31   a,    31   b,  and  31   c  illustrated in  FIG. 4  indicate trajectories of centers of the detection sensors  5   a,    5   b,  and  5   c,  respectively, when the sensors scan in the sub-scanning direction on the intermediate transfer belt  130 .  FIG. 4  illustrates an example of ideal trajectories obtained when the centers of the detection sensors  5   a,    5   b,  and  5   c  pass through centers of the pattern image for positional deviation correction. 
         [0059]      FIG. 4  illustrates an example in which the transverse line patterns and the slanted line patterns are formed so as to be arranged in the order of Y, K, M, and C from the front in the conveying direction of the intermediate transfer belt  130 . However, the order of arrangement of the colors in the transverse line patterns and the slanted line patterns may be other than this order. 
         [0060]    The detection sensors  5   a,    5   b,  and  5   c  arranged in the main-scanning direction respectively detect the three rows of marks in the pattern image for positional deviation correction that is formed on the intermediate transfer belt  130 . 
         [0061]    A waveform  140  illustrated in  FIG. 4  illustrates an example of a change in the detection level (detection signal) when the detection sensor  5   a  detects the marks  30  in the pattern image for positional deviation correction illustrated in  FIG. 4 . The same waveform is obtained by the other detection sensors  5   b  and  5   c,  and therefore, illustration thereof is omitted. 
         [0062]    The detection sensors  5   a,    5   b,  and  5   c  detect the intermediate transfer belt  130  in a portion other than the transverse line patterns and the slanted line patterns. Therefore, if, for example, the intermediate transfer belt  130  has a white color and the detection level thereon is regarded as a reference level, the detection levels at the transverse line patterns and the slanted line pattern are lower than the reference level. 
         [0063]    A threshold voltage level (voltage value) indicated by a dashed line  141  is a threshold value for detecting, as the transverse line pattern or the slanted line pattern, a place where a drop in the detection level exceeds the threshold voltage value, even if the detection level is lowered due to, for example, dirt on the intermediate transfer belt  130 . 
         [0064]    Each of the detection sensors  5   a,    5   b,  and  5   c  detects positions of the eight lines of the transverse line pattern and the slanted line pattern in the pattern image for positional deviation correction, and based on the results of the detection, measurement is made of the skews, the main-scanning registration deviation amounts, the sub-scanning registration deviation amounts, and the main-scanning magnification errors of other colors (yellow Y, cyan C, and magenta M) with regard to the reference color (such as black K). Based on these measurement values, deviation amounts are obtained between the center positions of the detection sensors  5   a,    5   b,  and  5   c  and the center positions of pattern image for positional deviation correction, and stored as positional deviation amounts to be referred to when the pattern image for positional deviation correction is formed next time. Moreover, it is possible to obtain correction values for the various deviation amounts, including the skews, the main-scanning registration deviation amounts, the sub-scanning registration deviation amounts, and the main-scanning magnification errors. 
         [0065]    Further, by detecting the three rows of marks with the detection sensors  5   a,    5   b,  and  5   c,  and calculating a mean value of the detection results, the deviation amounts including the skews, the sub-scanning registration deviations, the main-scanning registration deviations, and the main-scanning magnification errors are obtained from the results of the calculation, and thus, the deviation amounts of each color can be obtained with high accuracy. By correcting the deviation amounts, a high-quality image can be formed with very low deviations of each color. 
         [0066]    A well-known correction amount calculating unit (omitted from illustration) commands the execution of calculation of various positional deviation amounts and correction amounts, and execution of correction of the deviations. Then, the pattern image for positional deviation correction after being detected is removed by the cleaning unit  139  of  FIG. 1 . 
         [0067]    Using  FIG. 5 , description will be made of a specific method for calculation of the various positional deviation amounts when the pattern image for positional deviation correction of  FIG. 4  is detected.  FIG. 5  is a diagram illustrating the detection sensor  5   a  and one set of the marks scanned by the detection sensor  5   a.  Here, although description will be made of a case in which the detection sensor  5   a  detects the marks in the pattern image for positional deviation correction, the same description applies also to the other detection sensors  5   b  and  5   c.    
         [0068]    The detection sensor  5   a  detects the transverse line pattern and the slanted line pattern in the pattern image for positional deviation correction at predetermined constant sampling intervals, and notifies the detected data to the CPU  1  of  FIG. 3 . The CPU  1  receives in sequence the notifications of detection of the transverse line pattern and the slanted line pattern from the detection sensor  5   a,  and then, based on intervals of the notifications of detection and the sampling time intervals, calculates distances between lines in the transverse line pattern and between each line in the transverse line pattern and each corresponding line in the slanted line pattern. In this manner, the distances between lines in the transverse line pattern and between each line in the transverse line pattern and each corresponding line of the same color in the slanted line pattern are obtained in one set of the marks  30 , and the distances thus obtained are compared, thereby enabling to calculate the various positional deviation amounts. 
         [0069]    For calculating the sub-scanning registration deviation amounts (color shift amounts in the sub-scanning direction), the transverse line pattern is used. Distance values (yl, ml, and cl) between the reference color (K) and target colors (Y, M, and C), respectively, are calculated, and compared with ideal distance values (y 0 , m 0 , and c 0 ) stored in advance. The positional deviation amounts of the target colors (Y, M, and C) relative to the reference color (K) can be calculated from (distance value y 1 —ideal distance value y 0 ), (distance value m 1 —ideal distance value m 0 ), and (distance value c 1 —ideal distance value c 0 ), respectively. 
         [0070]    Moreover, for calculating the main-scanning registration deviation amounts (color shift amounts in the main-scanning direction), distance values (y 2 , k 2 , m 2 , and c 2 ) between the lines of the colors K, Y, M, and C in the transverse line pattern and the lines in the slanted line pattern are first calculated. The distance values thus calculated are used to calculate difference values between the distance value of the reference color (K) and the distance values of the non-reference colors. Those difference values correspond to the color shift amounts in the main-scanning direction. This is because that the slanted line pattern is made to be slanted at a predetermined angle relative to the main-scanning direction, and thus, if a deviation occurs in the main-scanning direction, the distance between the slanted line and the transverse line of the reference color is larger or smaller than the corresponding distances of the other colors. Specifically, the positional deviations in the main-scanning direction between black and yellow, between black and magenta, and between black and cyan are obtained from (distance value k 2 −distance value y 2 ), (distance value k 2 −distance value m 2 ), and (distance value k 2 −distance value c 2 ), respectively. In this manner, the registration deviation amounts in the sub-scanning direction and the main-scanning direction can be obtained. 
         [0071]    Further, based on differences between detection results of different sensors among the detection sensors  5   a,    5   b,  and  5   c,  the skews and the main-scanning magnification errors can be obtained. First, the skew components can be obtained by calculating the differences in the sub-scanning registration deviation amounts between those detected by the detection sensor  5   a  and those detected by the detection sensor  5   c.  Moreover, magnification error deviations can be obtained by calculating the differences in the main-scanning registration deviation amounts between those of the detection sensor  5   a  and those of the detection sensor  5   b  and between those of the detection sensor  5   b  and those of the detection sensor  5   c.  Then, based on the various positional deviation amounts obtained as described above, the correction process is executed to correct the image forming conditions used when the color image is formed on the intermediate transfer belt  130 . 
         [0072]    The correction process is executed, for example, by adjusting light emission timing of the light beams Y, K, M, and C to the photosensitive elements  120   y,    120   k,    120   m,  and  120   c  so that the positional deviation amounts almost coincide. Alternatively, the correction process can be executed by adjusting the tilt angles of the reflecting mirrors (omitted from illustration) that reflect the light beams. The tilt angles of the reflecting mirrors are adjusted by driving stepping motors (omitted from illustration). The positional deviation amounts can also be corrected by changing image data. In this manner, the registration deviation amounts in the sub-scanning direction and the main-scanning direction can be obtained. 
         [0073]      FIG. 6  is a diagram illustrating the intermediate transfer belt  130  and the detection sensors  5   a,    5   b,  and  5   c  in the case of forming the patterns for correction concurrently with image printing. In the case of forming the test patterns for correction concurrently with image printing, it is necessary to arrange one or more of the multiple test pattern detection sensors at one or more outer ends of the image area in the main-scanning direction of a printed image.  FIG. 6  illustrates a configuration in which, among the three detection sensors  5   a,    5   b,  and  5   c,  the detection sensors  5   a  and  5   c  at two locations right and left are arranged at the outer ends of the image area. In image forming apparatuses that do not form the test patterns for correction concurrently with image printing, all of the multiple detection sensors are often arranged in the printed image area to obtain adjusted values in the image area. 
         [0074]      FIGS. 7A and 7B  are diagrams each illustrating the intermediate transfer belt  130  and the detection sensors  5   a,    5   b,  and  5   c  in the case of changing a width of the test pattern for correction in a main-scanning direction. In some cases, the width of test pattern for correction in the main-scanning direction is changed, for example, when the adjustment is performed.  FIGS. 7A and 7B  illustrate examples of a fine adjustment and a rough adjustment during color shift correction. The fine adjustment illustrated in  FIG. 7A  is an adjustment that is performed when a color shift is expected to be small, and aims at correcting the color shift with higher accuracy. In this case, the width of the test pattern to be formed in the main-scanning direction can be made small in accordance with the expected color shift, and the adjustment is performed with high accuracy by forming, with respect to each of the sensors, multiple sets of marks, each set being composed of the lines of the respective colors in the transverse line pattern and the slanted line pattern. 
         [0075]    On the other hand, the rough adjustment illustrated in  FIG. 7B  is performed when a color shift is expected to be large, and aims at, not correcting the color shift with higher accuracy, but adjusting the color shift in a sure manner even when a large color shift occurs. In this case, the width of the pattern to be formed in the main-scanning direction needs to made large in accordance with the expected color shift, and one set, or fewer sets than in the fine adjustment, of marks are formed with respect to each of the sensors, where each set is composed of the lines of the respective colors in the transverse line pattern and the slanted line pattern. Which of the fine adjustment and the rough adjustment is to be performed is determined depending, for example, on a temperature change from a previously performed color shift correction, elapsed time, and on the number of copies. 
         [0076]    In the case in which the detection sensors are configured as illustrated in  FIG. 6 , and the widths of test patterns in the main-scanning direction are changed as illustrated in  FIG. 7A  or  FIG. 7B , there is a possibility that the LDs would emit light outside an optical properties guarantee area that is set for each apparatus. Here, the term “optical properties guarantee area” refers to an area in the main-scanning direction in which a scanning beam is guaranteed to be emitted to a target area of the photosensitive element. Outside this area, lens properties of a scanning optical system are not guaranteed, and when the beam is turned on, exposure of an unintended place occurs. When the unintended exposure occurs in the photosensitive element, the exposure causes occurrence of troubles such as failure in the adjusting operation and image degradation, or, when the unintended exposure occurs in a sensor such as a synchronization detector, the exposure causes occurrence of operational abnormalities. 
         [0077]    Therefore, the image forming apparatus of this embodiment changes, depending on the width of the test pattern to be formed in the main-scanning direction, the sensors that detect the test patterns.  FIGS. 8A and 8B  are diagrams each illustrating the intermediate transfer belt  130  and the detection sensors  5   a,    5   b,  and  5   c  in the case of changing the widths of the test patterns for correction in the main-scanning direction. In the fine adjustment of the color shift illustrated in  FIG. 8A  in which the widths of the test patterns in the main-scanning direction are small, the test patterns are formed in positions corresponding to all of the sensors, while, on the other hand, in the rough adjustment of the color shift illustrated in  FIG. 8B  in which the widths of the test patterns are large, the test patterns extending out of the optical properties guarantee area are not formed. 
         [0078]    Here, whether to form the test pattern for each of the detection sensors can be judged based on “width of test pattern in main-scanning direction: a”, “distance from center of sensor to end of optical properties guarantee area in main-scanning direction: b”, and “expected color shift amount: c”. The test pattern is not formed in the position corresponding to the detection sensor if (a/2+c) b is satisfied, and is formed there if (a/2+c)&lt;b is satisfied. Here, the “expected color shift amount c” is a maximum expected color shift amount (=a moving amount of the test pattern) during the generation of the test pattern, and is a value determined by multiplying a property of the scanning beam optical system (an amount of change in main-scanning position of exposure beam for 1° C. change) by a temperature change from the previous color shift correction. The number of times of processing required for the judgment can be minimized by applying this judgment only to the detection sensors disposed at both ends in the main-scanning direction. 
         [0079]      FIGS. 9A and 9B  are diagrams illustrating a modification example of this embodiment in which, when more than three detection sensors are provided, locations at which the test patterns are formed are changed depending on the width of each of the test patterns for correction in the main-scanning direction. In  FIGS. 9A and 9B , five detection sensors  5   a,    5   d,    5   b,    5   e,  and  5   c  are arranged from the left to the right in the drawings. During the fine adjustment of the color shift, as illustrated in  FIG. 9A , the test patterns are formed in positions corresponding to the detection sensors  5   a,    5   b,  and  5   c,  whereas during the rough adjustment of the color shift, as illustrated in  FIG. 9B , the test patterns are formed in positions corresponding to the detection sensors  5   d,    5   b,  and  5   e.  Thereby, the LDs can be prevented from emitting light outside the optical properties guarantee area. 
         [0080]      FIG. 10  is a flow chart illustrating the color shift correction process according to this embodiment. The CPU  1  judges, as Step S 12 , whether (a/2+c)&lt;b is satisfied with respect to the detection sensor  5   a.  When it is judged at Step S 12  that (a/2+c)&lt;b is satisfied (Yes), the CPU  1 , as Step S 14 , turns on a flag that controls the test pattern formation with respect to the detection sensor  5   a.    
         [0081]    When it is judged at Step S 12  that (a/2+c)&lt;b is not satisfied (No) or when having executed Step S 14 , the CPU  1  judges, as Step S 16 , whether (a/2+c)&lt;b is satisfied with respect to the detection sensor  5   b.  When it is judged at Step S 16  that (a/2+c)&lt;b is satisfied (Yes), the CPU  1 , as Step S 18 , turns on a flag that controls the test pattern formation with respect to the detection sensor  5   b.    
         [0082]    When it is judged at Step S 16  that (a/2+c)&lt;b is not satisfied (No) or when having executed Step S 18 , the CPU  1  judges, as Step S 20 , whether (a/2+c)&lt;b is satisfied with respect to the detection sensor  5   c.  When it is judged at Step S 20  that (a/2+c)&lt;b is satisfied (Yes), the CPU  1 , as Step S 22 , turns on a flag that controls the test pattern formation with respect to the detection sensor  5   c.    
         [0083]    When it is judged at Step S 20  that (a/2+c)&lt;b is not satisfied (No) or when having executed Step S 22 , the CPU  1  forms, as Step S 24 , the test patterns in the positions corresponding to the detection sensors with respect to which the flags controlling the test pattern formation are turned on. The detection sensors corresponding to the positions in which the test patterns are formed detect the test patterns as Step S 26 . The CPU  1  calculates, as Step S 28 , the color shift correction amounts based on the test patterns detected at Step S 26 . 
         [0084]    According to this embodiment, it is possible to determine the widths of the test patterns in the main-scanning direction depending on conditions such as whether the fine adjustment or the rough adjustment, and, depending on a width of the test patterns in the main-scanning direction, to selectively switch whether to form the test pattern in the position corresponding to each of the detection sensors  5   a,    5   b,  and  5   c.  With this method, the beams are suppressed from being turned on outside the optical properties guarantee area. Accordingly, for example, failure in the adjusting operation, image degradation, and operational abnormalities can be suppressed. 
         [0085]      FIG. 11  is a block diagram illustrating a hardware configuration of the image forming apparatus according to this embodiment. As illustrated in this diagram, the image forming apparatus  100  has a configuration in which a controller  10  is connected to an engine unit (engine)  60  via a peripheral component interconnect (PCI) bus. The controller  10  controls the entire image forming apparatus  100  and also controls drawing, communication, and input from an operating unit (not illustrated). The engine unit  60  is, for example, a printer engine that is connectable to the PCI bus, and is, for example, a black-and-white plotter, a single-drum color plotter, a four-drum color plotter, a scanner, or a facsimile unit. This engine unit  60  includes, in addition to a so-called engine portion such as a plotter, a unit for image processing such as error diffusion and gamma transform. 
         [0086]    The controller  10  includes the CPU  1 , a northbridge (NB)  13 , a system memory (MEM-P)  12 , a southbridge (SB)  14 , a local memory (MEM-C)  17 , an application-specific integrated circuit (ASIC)  16 , and a hard disk drive (HDD)  18 , and has a configuration in which an accelerated graphics port (AGP) bus  15  connects between the northbridge (NB)  13  and the ASIC  16 . The MEM-P  12  further includes the read-only memory (ROM)  2  and the random access memory (RAM)  3 . 
         [0087]    The CPU  1  performs overall control of the image forming apparatus  100 , and has a chipset composed of the NB  13 , the MEM-P  12 , and the SB  14 . The CPU  1  is connected to other devices via this chipset. 
         [0088]    The NB  13  is a bridge for connecting the CPU  1  to the MEM-P  12 , the SB  14 , and the AGP  15 , and has a memory controller that controls read from and write to the MEM-P  12 , a PCI master, and an AGP target. 
         [0089]    The MEM-P  12  is a system memory that is used, for example, as a memory for storing therein computer programs and data, a memory for loading the programs and the data, and a memory for drawing by a printer, and is composed of the ROM  2  and the RAM  3 . The ROM  2  is a read-only memory that is used as the memory for storing therein computer programs and data, and the RAM  3  is a writable and readable memory that is used, for example, as the memory for loading the programs and the data and as the memory for drawing by a printer. 
         [0090]    The SB  14  is a bridge for connecting the NB  13  to PCI devices and peripheral devices. The SB  14  is connected to the NB  13  via the PCI bus, to which, for example, a network interface (I/F) unit is also connected. 
         [0091]    The ASIC  16  is an integrated circuit (IC) for use in image processing having a hardware component for the image processing, and plays a role as a bridge that connects the AGP  15 , the PCI bus, the HDD  18 , and the MEM-C  17  to each other. The ASIC  16  is composed of a PCI target, an AGP master, an arbiter (ARB) constituting the core of the ASIC  16 , a memory controller that controls the MEM-C  17 , a plurality of direct memory access controllers (DMACs) that perform, for example, rotation of image data by using a hardware logic or the like, and a PCI unit that transfers data to and from the engine unit  60  via the PCI bus. A facsimile control unit (FCU)  30 , a universal serial bus (USB)  40 , and an Institute of Electrical and Electronics Engineers 1394 (IEEE 1394) interface  50  are connected to the ASIC  16  via the PCI bus. An operation display unit  20  is directly connected to the ASIC  16 . 
         [0092]    The MEM-C  17  is a local memory that is used as an image buffer for copying and a code buffer. The hard disk drive (HDD)  18  is a storage for storing therein image data, computer programs, font data, and forms. 
         [0093]    The AGP bus  15  is a bus interface for a graphics accelerator card that has been proposed for accelerating graphics operations. The AGP bus  15  accelerates operations of the graphics accelerator card by directly accessing the MEM-P  12  with high throughput. 
         [0094]    The programs executed in the image forming apparatus of the present embodiment are provided by being built into a ROM or the like in advance. The programs executed in the image forming apparatus of the present embodiment may be configured to be provided by being recorded in a computer-readable recording medium such as a CD-ROM, a flexible disk (FD), a CD-R, or a digital versatile disc (DVD) as files in an installable or an executable format. 
         [0095]    The programs executed in the image forming apparatus of the present embodiment may alternatively be configured to be provided by being stored on a computer connected to a network such as the Internet and downloaded via the network. The programs executed in the image forming apparatus of the present embodiment may still alternatively be configured to be provided or distributed via a network such as the Internet. 
         [0096]    The programs executed in the image forming apparatus of the present embodiment are composed of modules including the units (control units) described above. As actual hardware, the CPU (processor) reads the programs from the ROM and executes the programs, whereby the units are loaded into a main memory and generated therein. 
         [0097]    In the above-described embodiment, the example has been described in which the image forming apparatus of the present invention is applied to the MFP that has at least two functions out of a copy function, a printer function, a scanner function, and a facsimile function. However, the present invention can be applied to any image forming apparatus such as a copying machine, a printer, a scanner, and a facsimile apparatus. 
         [0098]    According to the present invention, a light source can be suppressed from emitting light outside an optical properties guarantee area. Accordingly, occurrences of failure in an adjusting operation, image degradation, and operational abnormalities can be suppressed. 
         [0099]    Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.