Patent Publication Number: US-2017351193-A1

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

Description:
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 cross-sectional diagram illustrating the optical print head; 
         FIG. 5  is a block diagram illustrating components of an external device; 
         FIG. 6  is a flowchart illustrating a light amount correction method; 
         FIG. 7  is a diagram illustrating a positional relation between light emitting element rows and rod lenses; and 
         FIG. 8  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 rod 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 in a row in a first direction. The second light emitting element row refers to the arrangement of second light emitting elements in a row in the first direction and is positioned at one side of a second direction orthogonal to the first direction with respect to the first light emitting element row. Light emitted by the first light emitting element and the second light emitting element passes through the rod lens array. The first drive circuit drives each first light emitting element with identical first current value and drives each first light emitting element at a light emitting time corresponding to each target gradation value respectively. The second drive circuit, in response to transmittance of a light passing position of the rod lens array, drives each second light emitting element with identical second current value and drives each second light emitting element at a light emitting time corresponding to each target gradation value. The second current value is typically 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 first light emitting elements arranged in a row in the first direction, second light emitting elements arranged in a row in the first direction and positioned in the second direction orthogonal to the first direction with respect to the first light emitting element and a rod lens array. 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 first light emitting time and measuring a first light amount of the light emitted by the first light emitting element through the rod 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 light emitted by the second light emitting element through the rod lens array. The third step refers to driving the first light emitting element with the second current value different from the first current value at the first light emitting time and measuring a third light amount of the light emitted by the first light emitting element through the rod lens array to calculate the second current value of the current through which the light amount of the first light emitting element 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 third current value different from the first current value at the first light emitting time and measuring a fourth light amount of the light emitted by the second light emitting element through the rod lens array to calculate a fourth current value of the current through which the light amount of the second light emitting element 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 sheet in the order of Y, M, C and K in an overlapped manner while conveying the sheet with a belt  23 . One color image is formed on the sheet. 
     The processor  94  heats the sheet with a fixing device  24  and discharges the sheet to a tray  25  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. 5 ) 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 in which the beam spot moves along a circumferential direction of the photoconductive drums  21 Y- 21 K when the second light emitting element row  41  emits the light to the photoconductive drums  21 Y- 21 K. 
     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 a light emitting area, and a cathode which injects an electron. 
     If resolution in the horizontal scanning direction is 1200 dpi, for example, the light emitting elements  411  and  421  are arranged at an interval of 21 μm (=25.4 mm/1200) in the horizontal scanning direction and the numbers thereof are 7680 in total respectively. 
     In the present embodiment, as there are two rows of the light emitting element rows  41  and  42  in the vertical scanning direction, the resolution can become twice than a case in which there is one row light emitting element row. In the present embodiment, it is possible to increase the areas of the light emitting elements  411  and  421  without changing the resolution in the present embodiment. 
     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 other 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 one 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 other 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 one 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 spot light. 
     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 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. 5  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 . The light receiving device  103  measures the amounts of light of the light emitted by the light emitting elements  411  and  421  through the rod lens array  6 . 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. 
       FIG. 6  is a flowchart illustrating the light amount correction method. 
     The external device  100  drives the light emitting element rows  41  and  42  with a first current value α 1  at a first light emitting time T 1  simultaneously with the drive circuits  51  and  52  (ACT  1 ). 
     The external device  100  measures a first light amount L 1  of the light emitted by each first light emitting element  411  of the first light emitting element row  41  passing through the rod lens array  6 . The external device  100  measures a second light amount L 2  of the light emitted by each second light emitting element  421  of the second light emitting element row  42  passing through the rod lens array  6  (ACT  2 ). 
       FIG. 7  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  can be the same or different, but in this case, for example, is 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 interval of the adjacent central parts of the light emitting element rows  41  and  42  in the vertical scanning direction (up and down direction of  FIG. 7 ) is 105 μm for example. Other dimensions for the aforementioned elements are possible. 
     With respect to the diameter of each of the rod lenses  611  and  621 , the diameter of each of the light emitting elements  411  and  421  is very small and the interval of 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 biased towards one of the rod lens rows  61  and  62  occurs. In the present embodiment, the second light emitting element row  42  passes through the central part of the rod lens  621  and the first light emitting element row  41  passes through a position away from the central part of the rod lens  621  with respect to the second light emitting element row  42 . 
       FIG. 8  is a diagram illustrating a measurement result of the light amount and L 1  and the light amount L 2  of the light emitting elements  411  and  421 . 
     Through the difference in the position with respect to the rod lens row  62 , a difference occurs in the light transmittance of the first light emitting element row  41  and the second light emitting element row  42  with respect to the rod lens rows  61  and  62 . Thus, the light amount of the second light emitting element  421  of the second light emitting element row  42  is 10% on an average more than that of the first light emitting element  411  of the first light emitting element row  41 . Thus, in the conventional light amount correction, dispersion of the light of each of the light emitting elements  41  and  42  cannot be completely suppressed, which causes image degradation. Thus, the external device  100  executes the processing in ACT  1 -ACT  6  before the conventional light amount correction. 
     The external device  100  drives the second light emitting element row  42  with a second current value α 2  smaller than the first current value α 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 first and the second light emitting elements  411  and  421  of the first and the second light emitting element rows  41  and  42  are 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. 7 , the difference (L 1 −L 3 ) of the amounts of light of the 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 1− 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 1− L 2= K (α1−α3)  (2)
 
     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. 5 ) of the optical print head  3  (ACT  7 ). 
     In the present embodiment, as the drive circuits  51  and  52  are arranged for each of the first and the second light emitting element rows  41  and  42 , the first and the second light emitting element rows  41  and  42  can be driven with different current values and the dispersion of the amounts of light of the light emitting element rows  41  and  42  can be suppressed. 
     If the optical print head  3  is incorporated in the apparatus and receives an instruction for driving the first and the second light emitting elements  411  and  421 , the first and the second drive circuits  51  and  52  drives the first and the second 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 ) and drives each first light emitting element  411  at the light emitting time corresponding to each target gradation value respectively. 
     The second drive circuit  52  drives, in response to the transmittance of the light passing position of the rod lens array  6 , each second light emitting element  421  with the same second driving current value (e.g., the third current value α 3 ) and drives each second light emitting element  421  at the light emitting time corresponding to each target gradation value respectively. 
     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 first and the second light emitting element rows  41  and  42  may not have 2 rows in total. In this case, the foregoing processing is executed between the first light emitting element row  41  of each other row and the second light emitting element row  42  of each other row. 
     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. 
     The external device  100 , if the light amount of the first light emitting element row  41  is greater than that of the second light emitting element row  42 , lowers the current value of the first light emitting element row  41  and measures the light amount thereof to calculate the proportionality coefficient K of the first light emitting element  411 . Then, the external device  100 , at the time of driving the first light emitting element  411  at the first light emitting time T 1 , calculates the current value when the light amount is equal to the second light amount L 2  of the second light emitting element. 
     The second current value α 2  at the time of calculating the proportionality coefficient K may be greater than the first current value α 1 . However, if a possibility that the light amount is not increased even if the current value is increased is taken into consideration, the second current value α 2  is preferably lower than the first current value α 1 . 
     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.