Patent Publication Number: US-2021176377-A1

Title: Optical print head, image forming apparatus and light amount correction method of optical print head

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a Continuation of application Ser. No. 16/288,174 filed Feb. 28, 2019, which is a Continuation of application Ser. No. 15/955,945 filed Apr. 18, 2018, which is a Continuation of application Ser. No. 15/429,437 filed Feb. 10, 2017, now U.S. Pat. No. 9,979,856, which is a Continuation-in-Part of application Ser. No. 15/171,028 filed Jun. 2, 2016, now abandoned, the entire contents of all of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to a technology for suppressing dispersion of light from an optical print head. 
     BACKGROUND 
     Conventionally, there is an optical print head in which two rows of light emitting elements are arranged in parallel below a rod lens array in which two rows of rod lenses arranged in parallel are integrated. The two rows of the light emitting elements are positioned alternately in an extending direction of the light emitting element rows. 
     In the optical print head, there is a case in which undesirable dispersion of light of each light emitted through the rod lens array by each light emitting element occurs. As the main reason of the dispersion, there is dispersion of luminous efficiency of each light emitting element and dispersion of a drive circuit connected with each light emitting element. As the main reason of the dispersion, there is dispersion of the refractive index distribution of the rod lens array and dispersion of a positional relation of each light emitting element with respect to each of the rod lens. 
     In a case of incorporating the optical print head in an image forming apparatus, the light emitted by each light emitting element forms a beam spot corresponding to one dot on a photoconductor. If there is dispersion of light of each light emitting element, density unevenness of an image occurs and the image quality is degraded. Thus, at the time of shipping the optical print head or at the time of shipping the image forming apparatus incorporated with the optical print head, a light amount correction operation for reducing the dispersion of the light is executed in manufacturing lines. 
     The amount of light dispersed by the light emitting element depends on an applied current value and light emitting time. In light amount correction, first, currents with the same value are applied to each light emitting element, and the light amount of each light emitting element (light amount of each light emitted through the rod lens array by each light emitting element) is measured. Next, under the condition of the application of the currents with the same value, the light emitting time of each light emitting element is adjusted with a PWM (Pulse Width Modulation) control so that the amounts of the light of the light emitting elements become identical. Correction information serving as an adjustment amount of the light emitting time of each light emitting element is information unique to the optical print head. 
     In the light amount correction, next, the correction information is written into a built-in memory of the optical print head. Through reading the correction information from the optical print head, the dispersion of the light of each light emitting element can be suppressed. 
     Incidentally, if the incorporation position of the light emitting element rows and the rod lens array deviates from an ideal position, a difference occurs in light transmittance. Thus, there is a case in which the amounts of light from the light emitting element rows are greatly different. If the amounts of light from the light emitting element rows are greatly different, there is a problem that the dispersion of the light cannot be completely suppressed through the light amount correction according to the light emitting time. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating control components of an image forming apparatus; 
         FIG. 2  is a diagram illustrating the structure of a printer section; 
         FIG. 3  is a perspective view illustrating the structure of an optical print head; 
         FIG. 4  is a diagram illustrating a positional relation between light emitting element rows and rod lenses; 
         FIG. 5  is a diagram illustrating beam spots formed by light emitting elements on photoconductive drums; 
         FIG. 6  is a cross-sectional diagram illustrating the optical print head; 
         FIG. 7  is a block diagram illustrating components of an external device; 
         FIG. 8  is a flowchart illustrating a light amount correction method; and 
         FIG. 9  is a diagram illustrating a measurement result of amounts of light of light emitting elements. 
     
    
    
     DETAILED DESCRIPTION 
     Generally, in accordance with an embodiment, an optical print head comprises a first light emitting element row, a second light emitting element row, a lens array, a first drive circuit and a second drive circuit. The first light emitting element row refers to the arrangement of first light emitting elements. The second light emitting element row refers to the arrangement of second light emitting elements in parallel with the first light emitting element row. The lens array concentrates light emitted by the first light emitting elements and the second light emitting elements. The first drive circuit drives each first light emitting element with an identical first current value. The second drive circuit drives each second light emitting element with an identical second current value different from the first current value. 
     Generally, in accordance with the present embodiment, an image forming apparatus comprises a photoconductor, an optical print head and a developing device. The optical print head refers to the foregoing optical print head which forms an electrostatic latent image on the photoconductor. The developing device develops the electrostatic latent image to form a toner image on the photoconductor. 
     Generally, in accordance with the present embodiment, a light amount correction method is a light amount correction method of an optical print head which comprises a first light emitting element row including the arrangement of first light emitting elements, a second light emitting element row including the arrangement of second light emitting elements in parallel with the first light emitting element row, and a lens array for concentrating light emitted by the first light emitting elements and the second light emitting elements. The light amount correction method can include a first step, a second step and a third step. The first step refers to driving the first light emitting element with a first current value at a first light emitting time and measuring a first light amount of the first light emitting element through the lens array. The second step refers to driving the second light emitting element with the first current value at the first light emitting time and measuring a second light amount of the second light emitting element through the lens array. The third step refers to driving the first light emitting element with a second current value different from the first current value at the first light emitting time and measuring a third light amount of the first light emitting element through the lens array to calculate a third current value of current through which the light amount of the first light emitting element through the lens array becomes the second light amount when the first light emitting element is driven at the first light emitting time, or driving the second light emitting element with a fourth current value different from the first current value at the first light emitting time and measuring a fourth light amount of the second light emitting element through the lens array to calculate a fifth current value of current through which the light amount of the second light emitting element through the lens array becomes the first light amount when the second light emitting element is driven at the first light emitting time. 
     Hereinafter, embodiments are described with reference to the accompanying drawings. 
       FIG. 1  is a block diagram illustrating control components of an image forming apparatus  1 . 
     In the image forming apparatus  1 , a processor  94 , which is a CPU (Central Processing Unit), executes programs stored in a memory  95  to execute various processing of the image forming apparatus  1 . A display  92  displays setting information or operation status of the image forming apparatus  1 , log information and notification to a user. An input section  93  including a touch panel or buttons receives input of the user. The processor  94  first reads an image of a document with a scanner  91  in a copy processing. 
       FIG. 2  is a diagram illustrating the structure of a printer section  2 . 
     The processor  94  forms electrostatic latent images based on image data on photoconductive drums  21 Y- 21 K with an optical print head  3 . The  21 Y- 21 K refers to  21 Y,  21 M,  21 C and  21 K. Y is yellow, M is magenta, C is cyan, and K is black. The other reference signs are the same as described above. 
     The processor  94  develops the electrostatic latent images on the photoconductive drums  21 Y- 21 K with developing devices  22 Y- 22 K through Y-K toners. Y-K toner images are formed on the photoconductive drums  21 Y- 21 K. 
     The processor  94  transfers Y-K toner images on the photoconductive drums  21 Y- 21 K onto a transfer belt  23  in the order of Y, M, C and K in an overlapped manner. One color image is formed on the transfer belt  23 . The processor  94  transfers the color image from the transfer belt  23  to a sheet at a secondary transfer position U. The secondary transfer position U is a position at which a secondary transfer roller  24  and the transfer belt  23  together form a nip. 
     The processor  94  heats the sheet with a fixing device  25  and discharges the sheet to a tray (not shown) after the image is fixed on the sheet. 
       FIG. 3  is a perspective view illustrating the structure of the optical print head  3 . 
     The optical print head  3  is equipped with a first light emitting element row  41 , a second light emitting element row  42 , a first drive circuit  51 , a second drive circuit  52 , a memory  53  (refer to  FIG. 7 ) and a microlens array  6 . 
     The light emitting element rows  41  and  42  and the drive circuits  51  and  52  are arranged on a substrate  7  made from glass or resin. 
     A first light emitting element  411  emits light upwards in  FIG. 3  (direction orthogonal to the substrate  7 ). The first light emitting elements  411  are arranged in a horizontal scanning direction to form the first light emitting element row  41 . The horizontal scanning direction refers to a direction in which a beam spot moves along an axial direction of the photoconductive drums  21 Y- 21 K when the first light emitting element row  41  emits light to the photoconductive drums  21 Y- 21 K. 
     A second light emitting element  421  emits the light towards the upside of  FIG. 3 . 
     The substrate  7  is a top emission type substrate on which the light is emitted from upper surfaces of the first light emitting element row  41  and the second light emitting element row  42  simultaneously. 
     The second light emitting elements  421  are arranged in a row in the horizontal scanning direction to form the second light emitting element row  42 . The second light emitting element row  42  is positioned at one side (right side in  FIG. 3 ) of the vertical scanning direction with respect to the first light emitting element row  41 . The vertical scanning direction refers to a direction orthogonal to the horizontal scanning direction. The second light emitting element row  42  is arranged in parallel with the first light emitting element row  41  in the vertical scanning direction. 
     The light emitting elements  411  and  421  are positioned alternately in the horizontal scanning direction. 
     The light emitting elements  411  and  421  can be organic electroluminescence elements. The light emitting elements  411  and  421  each at least include an anode which injects an electron hole, a light emitting layer having alight emitting area, and a cathode which injects an electron. 
     The first drive circuit  51  drives the first light emitting element row  41 . The first drive circuit  51  can set a current value for the first light emitting element row  41 . The first drive circuit  51  can execute the PWM control on the first light emitting element  411  individually through the set current value. The first drive circuit  51  can individually control the light emitting time of the first light emitting element  411 . The first drive circuit  51  is positioned at the one side (left side in  FIG. 3 ) of the vertical scanning direction with respect to the first light emitting element row  41 . The first drive circuit  51  is positioned at a location nearest to the first light emitting element  411  at the end of one side (front side in  FIG. 3 ) of the horizontal scanning direction among the first light emitting elements  411 . 
     The second drive circuit  52  drives the second light emitting element row  42 . The second drive circuit  52  can set a current value for the second light emitting element row  42 . The second drive circuit  52  can execute the PWM control on the second light emitting element  421  individually through the set current value. The second drive circuit  52  can control the light emitting time of the second light emitting element  421  individually. The second drive circuit  52  is positioned at the other side (right side in  FIG. 3 ) of the vertical scanning direction with respect to the second light emitting element row  42 . The second drive circuit  52  is positioned at a location nearest to the second light emitting element  421  at the end of one side (front side in  FIG. 3 ) of the horizontal scanning direction among the second light emitting elements  421 . 
     The drive circuits  51  and  52  are opposite to each other in the vertical scanning direction. 
     The first drive circuit  51  is positioned at the one side (left side in  FIG. 3 ) of the vertical scanning direction with respect to the first light emitting element row  41 . The second drive circuit  52  is positioned at the other side (right side in  FIG. 3 ) of the vertical scanning direction with respect to the second light emitting element row  42 . Thus, the wiring for connecting the first drive circuit  51  with the first light emitting element  411  and the wiring for connecting the second drive circuit  52  with the second light emitting element  421  are not overlapped. 
     The rod lens array  6  is equipped with a plurality of integrated columnar rod lenses  611  and  621 . The rod lenses  611  are arranged in a row in a scanning direction to form a rod lens row  61 . The rod lenses  621  are arranged in a row in the scanning direction to form a rod lens row  62 . The rod lens rows  61  and  62  are arranged in the vertical scanning direction in parallel. The rod lens array  6  is positioned at the upper side in  FIG. 3  of the light emitting element rows  41  and  42  and opposite to the light emitting element rows  41  and  42 . The rod lens array  6  enables the light emitted by each of the light emitting elements  411  and  421  to be imaged on the photoconductive drums  21 Y- 21 K as beam spots. 
     In the present embodiment, the rod lens rows  61  and  62  are arranged corresponding to the first and the second light emitting element rows  41  and  42 . However, one rod lens row may be arranged corresponding to a plurality of (e.g., 2) light emitting element rows. 
       FIG. 4  is a diagram illustrating a positional relation between the light emitting element rows  41  and  42  and the rod lenses  611  and  621 . 
     The diameter of each of the rod lenses  611  and  621  is, for example, 900 μm. 
     The light emitting surface of each of the light emitting elements  411  and  421  is a rectangular shape and dimension of two sides (length and width) of the light emitting surface is 30 μm*30 μm, for example. 
     The light emitting elements  411  and  421  are arranged alternately in the horizontal scanning direction (right and left direction of  FIG. 4 ). If resolution in the horizontal scanning direction is 1200 dpi, for example, the interval of the central parts of the light emitting elements  411  and  421  adjacent to each other in the horizontal scanning direction is 21 μm (=25.4 mm/1200). The number of the light emitting elements  411  and  421  is 15360 in total. The interval of the central parts of the light emitting elements  411  and  421  in the vertical scanning direction (up and down direction of  FIG. 4 ) is 105 μm. 
     Light emitted by the first light emitting element  411  largely passes through the rod lens  611  or  621  positioned directly above such a first light emitting element  411 . Since the light emitted by the first light emitting element  411  is diverging light, the light also passes through other rod lenses  611  and  621 . The light emitted by the first light emitting element  411  is concentrated on a single spot on the photoconductive drums  21 Y- 21 K by the plurality of rod lenses  611  and  621 . Similarly, light emitted by the second light emitting element  421  is concentrated on a single spot on the photoconductive drums  21 Y- 21 K by the plurality of rod lenses  611  and  621 . 
     As shown in  FIG. 5 , on the photoconductive drums  21 Y- 21 K, beam spots  411 A formed by the respective first light emitting elements  411  are arranged in a row in the horizontal scanning direction. The interval of the beam spots  411 A is the same as the interval of the first light emitting elements  411 . Similarly, beam spots  421 A formed by the second light emitting elements  421  are arranged in a row in the horizontal scanning direction. The interval of the beam spots  421 A is the same as the interval of the second light emitting elements  421 . 
     In the image forming apparatus  1  of the present embodiment, after one of the light emitting elements,  411  (or  421 ), are lighted while rotating the photoconductive drums  21 Y- 21 K, the other one of the light emitting elements,  421  (or  411 ), can be lighted. Thus, the beam spots  421 A each formed by the other light emitting element  421  can be positioned between the beam spots  411 A each formed by the one light emitting element  411  as shown in  FIG. 5 . In other words, the beam spots  411 A and  421 A formed by the light emitting elements  411  and  421  can be arranged in a row in the horizontal scanning direction. 
     In the present embodiment, the light emitting element rows  41  and  42  are provided in two rows in the vertical scanning direction. Thus, by driving the light emitting elements  411  and  421  as described above, the resolution in the scanning direction can become twice as compared to a case in which there is one light emitting element row. 
     In the present embodiment, the light emitting elements  411  and  421  are arranged in two rows. Thus, the first light emitting elements  411  can be prevented from interfering with the second light emitting elements  421  even when the areas of the light emitting elements  411  and  421  are increased. Thus, in the present embodiment, the areas of the light emitting elements  411  and  421  can be increased without changing the positional relation of the central parts between the first light emitting elements  411  and between the second light emitting elements  421 , i.e., without changing the resolution. 
       FIG. 6  is a cross-sectional diagram illustrating the optical print head  3 . 
     A lid  82  blocks the internal space of a holder  81 . The lid  82  holds the substrate  7 . The light emitting elements  411  and  421  on the substrate  7  are sealed by a sealing glass  83 . The holder  81  positions the rod lens array  6  and positions the substrate  7  at an operating distance of the rod lens array  6 . 
       FIG. 7  is a block diagram illustrating components of an external device  100 . 
     In the manufacturing line of the image forming apparatus  1 , the external device  100  is connected with the optical print head  3 . The external device  100  is equipped with a processor  101 , a memory  102 , a light receiving device  103 , a display  104  and an input device  105 . The processor  101  acting as a CPU executes programs stored in the memory  102  to execute various processing of the external device  100 . Light emitted by the light emitting elements  411  and  421  through the rod lens array  6  is concentrated on a light receiving surface of the light receiving device  103 . The light receiving device  103  is configured so that the light amounts of the beam spots formed by the light emitting elements  411  and  421  on the light receiving surface of the light receiving device  103  are equal to the light amounts of the beam spots  411 A and  421 A on the photoconductive drums  21 Y- 21 K. The light receiving device  103  may examine the light emitting element rows  41  and  42  by one row at a time. Alternatively, the light receiving device  103  maybe configured to examine the two rows of the light emitting elements  411  and  421  simultaneously. The display  104  displays setting information or operation status of the external device  100 , log information and notification to the user. The input device  105  including a touch panel or buttons receives input of the user. 
     The external device  100  executes the following light amount correction processing for suppressing dispersion of light emitted by the light emitting elements  411  and  421  through the rod lens array  6 . 
       FIG. 8  is a flowchart illustrating the light amount correction method.  FIG. 9  shows a measurement result obtained by the external device  100  when the light emitting elements  411  and  421  are driven with a first current value α 1  at a first light emitting time T 1 . The measurement result shows a measurement result of light amounts L 1  and L 2  of the light emitting elements  411  and  421  through the rod lenses  611  and  621 . 
     The external device  100  drives the light emitting elements  411  and  421  with the first current value α 1  at the first light emitting time T 1  simultaneously with the drive circuits  51  and  52  (ACT  1 ). 
     The external device  100  measures the first light amount L 1  of each first light emitting element  411  through the rod lenses  611  and  621 , and the second light amount L 2  of each second light emitting element  421  through the rod lenses  611  and  621  (ACT  2 ). 
     Here, with reference to  FIG. 4 , as the light emitted by each of the light emitting elements  411  and  421  passes through a position closer to the central part of each of the rod lenses  611  and  621 , the light concentrating function by the rod lenses  611  and  621  is more intensified. Consequently, the light amount on the beam spot formed by such light on the photoconductive drums  21 Y- 21 K is increased. Thus, as indicated by a chain line in  FIG. 4 , ideal positions of the light emitting element rows  41  and  42  are located in a region between the rod lenses  611  and  621  in which distances from the central parts of the rod lenses  611  and  621  to the first light emitting element row  41  are the same as distances from the central parts of the rod lenses  611  and  621  to the second light emitting element row  42 . 
     However, the light emitting elements  411  and  421  is very small as compared to the diameter of each of the rod lenses  611  and  621 , and the interval between the light emitting element rows  41  and  42  is also very small. Thus, if the incorporation position of each component is deviated from the ideal position, a case in which the positions of the light emitting element rows  41  and  42  are both biased towards one of the rod lens rows  61  and  62  occurs. In the present embodiment, the positions of the light emitting element rows  41  and  42  are both biased towards the rod lens row  62 . 
     Moreover, in the present embodiment, the second light emitting element row  42  passes through a position closer to the central part of the rod lens  621  than the first light emitting element row  41 . 
     In the present embodiment, the second light emitting element row  42  passes through a position closer to the central part of the rod lens array  62  than the first light emitting element row  41 . Thus, the light concentrating function of the rod lens  621  is exerted more strongly on the second light emitting element row  42  than on the first light emitting element row  41 . The light concentrating function of the rod lens  611 , on the other hand, is exerted more strongly on the first light emitting element row  41  than on the second light emitting element row  42 . However, since the rod lens  611  is farther away from the light emitting element rows  41  and  42  than the rod lens  621 , the function of the rod lens  611  is weaker than that of the rod lens  621 . 
     Consequently, as shown in  FIG. 9 , the light amount of the second light emitting element  421  of the second light emitting element row  42  on which the light concentrating function of the rod lens  621  is strongly exerted is 10% on an average more than that of the first light emitting element  411  of the first light emitting element row  41 . Thus, when the dispersion of the light in the light emitting element rows  41  and  42  is large as described above, the dispersion of the light of each of the light emitting elements  411  and  421  cannot be sufficiently suppressed through the conventional light amount correction according to the light emitting time, which causes image degradation. 
     After ACT  2 , the external device  100  drives the second light emitting element row  42  having a larger light amount between the light emitting element rows  41  and  42  with a second current value α 2  smaller than the first current value α 1  at the first light emitting time T 1  (ACT  3 ). The second current value α 2  is set to a value so that a third light amount L 3  of the second light emitting element  421  at the time of applying a current with the second current value α 2  is smaller than the first light amount L 1  of the first light emitting element  411  corresponding to the second light emitting element  421 . Hereinafter, the first light emitting element  411  corresponding to the second light emitting element  421  refers to the first light emitting element  411  corresponding to the second light emitting element  421  in the vertical scanning direction. Further, the first light emitting element  411  corresponding to the second light emitting element  421  refers to the first light emitting element  411  having the same number as the second light emitting element  421  when the light emitting elements  411  and  421  are each numbered from one side of the scanning direction. 
     The external device  100  measures the third light amount L 3  of each second light emitting element  421  of the second light emitting element row  42  (ACT  4 ). 
     The relation between the current value and the light amount is a proportional relation. For example, in  FIG. 9 , the difference (L 2 −L 3 ) of the amounts of light of a certain second light emitting element  421  at the time of being driven with the first and the second current values α 1  and α 2  different from each other is obtained by multiplying a proportionality coefficient K by the difference (α 1 −α 2 ) of the current values and is indicated by the following formula (1). 
         L 2− L 3= K (α1−α2)   (1)
 
     The external device  100  calculates the proportionality coefficient K based on the formula (1) (ACT  5 ). 
     The external device  100  calculates a third current value α 3  of a certain second light emitting element  421  based on the following formula (2) when the light amount at the time of driving the second light emitting element  421  at the first light emitting time T 1  is equal to the first light amount L 1  at the time of driving the first light emitting element  411  corresponding to the second light emitting element  421  with the first current value α 1  at the first light emitting time T 1  (ACT  6 ). 
         L 2− L 1= K (α1−α3)   (2)
 
     By driving each of the second light emitting elements  421  with the third current value α 3 , a light amount L 3  of each second light emitting element  421  (e.g., LEDs No.  1 ˜ 50  in  FIG. 9 ) becomes substantially the same as the light amount L 1  of each of the first light emitting elements  411  (e.g., LEDs No.  1 ˜ 50  in  FIG. 9 ) corresponding to each second light emitting element  421  and driven with the first current value α 1 . 
     As described above, in this correction processing, the levels of the light amounts of the light emitting element rows  41  and  42  are first equalized by correcting the current values of the light emitting element rows  41  and  42 . Thereafter, the light emitting time of each of the light emitting elements  411  and  412  is corrected by the following processing ACT  7  to suppress the dispersion of the light of each of the light emitting elements  411  and  412 . 
     The external device  100  calculates a second light emitting time T 2  at which a target light amount (reference light amount) is obtained for each first light emitting element  411  at the time of executing the PWM control by taking the current value of the first light emitting element  411  as the first current value α 1 . 
     The external device  100  calculates a third light emitting time T 3  at which a target light amount is obtained for each second light emitting element  421  at the time of executing the PWM control by taking the current value of the second light emitting element  421  as the third current value α 3 . 
     The external device  100  writes the correction information such as the first and the third current values α 1  and α 3  and the second light emitting time T 2  and the third light emitting time T 3  into the built-in memory  53  ( FIG. 7 ) of the optical print head  3  (ACT  7 ). 
     If the optical print head  3  is incorporated in the apparatus and receives an instruction for driving the light emitting elements  411  and  421 , the first and the second drive circuits  51  and  52  drives the light emitting elements  411  and  421  according to the correction information. 
     In this case, the first drive circuit  51  drives each first light emitting element  411  with the same first driving current value (e.g., the first current value α 1 ). The first drive circuit  51  drives, based on the second light emitting time T 2  calculated for each of the first light emitting elements  411 , each first light emitting element  411  at the light emitting time corresponding to each target gradation value, respectively. 
     The second drive circuit  52  drives each second light emitting element  421  with the same second driving current value (e.g., the third current value α 3 ). The second drive circuit  52  drives, based on the third light emitting time T 3  calculated for each of the second light emitting elements  421 , each second light emitting element  421  at the light emitting time corresponding to each target gradation value, respectively. 
     In the present embodiment, as the drive circuits  51  and  52  are provided for each of the light emitting element rows  41  and  42 , the light emitting element rows  41  and  42  can be driven with different current values and the dispersion of the light of the light emitting element rows  41  and  42  can be suppressed. 
     In the conventional technique, the dispersion of light among the light emitting elements  411  and  421  is adjusted by adjusting the light emitting time. When the optical print head  3  includes a plurality of light emitting element rows  41  and  42 , however, the dispersion of light of the light emitting element rows  41  and  42  may be large as shown in  FIG. 9 . In this case, the dispersion of the light of the light emitting element rows  41  and  42  cannot be sufficiently suppressed through the conventional light amount correction according to the light emitting time, which causes image degradation. 
     According to the present embodiment, however, the levels of the light amounts of the light emitting element rows  41  and  42  are equalized by correcting the current values of the light emitting element rows  41  and  42 . After that, the dispersion of the light of each of the light emitting elements  411  and  421  is corrected with the light emitting time in the present embodiment. Thus, even when the dispersion of the light of the light emitting element rows  41  and  42  is large, such dispersion can be sufficiently corrected. 
     The number of the first drive circuit  51  for driving the first light emitting element row  41  is not limited to one. The first light emitting elements  411  may be classified into several groups and a plurality of the first drive circuits  51  may be set respectively corresponding to the groups. The second drive circuit  52  is the same as the first drive circuit  51 . 
     The drive circuits  51  and  52  may be arranged at positions sandwiching the first and the second light emitting element rows  41  and  42  in the vertical scanning direction. 
     In the above embodiment, the external device  100  is configured to equalize the light amount of the second light emitting element row  42  with the light amount of the first light emitting element row  41  by decreasing the current value of the second light emitting element row  42  having a larger light amount. Alternatively, the light amount of the first light emitting element row  41  may be equalized with the light amount of the second light emitting element row  42  by increasing the current value of the first light emitting element row  41  having a smaller light amount. In the case of decreasing the current value, the light amounts of the light emitting elements  421  and  411  are more likely to decrease to the target value even when some of the light emitting elements  421  and  411  have inadequate quality. In the case of increasing the current value, however, when some of the light emitting elements  421  and  411  have inadequate quality, there is a possibility that the light amounts of such light emitting elements  421  and  411  cannot achieve the target value. Thus, in the light amount correction processing, it is more preferable that the light amount of the second light emitting element row  42  be equalized with the light amount of the first light emitting element row  41  by decreasing the current value of the second light emitting element row  42  having a larger light amount. 
     The lens array for concentrating the light emitted by the light emitting elements  411  and  421  may be a lens array other than the rod lens array  6 . 
     As described above in detail, according to the technology described in the specification, a technology for suppressing the dispersion of the light from the optical print head can be supplied. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the invention. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the invention. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.