Patent Publication Number: US-8989644-B2

Title: Image forming apparatus having fixing unit with improved light emission, fixing device, and drying device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-246446 filed Nov. 8, 2012. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to an image forming apparatus, a fixing device, and a drying device. 
     2. Summary 
     According to an aspect of the invention, there is provided an image forming apparatus including an image forming device that forms an image on a recording medium; and a light source that, after the image has been formed by the image forming device, irradiates the recording medium with light to fix the image on the recording medium, while the recording medium moves relatively in a predetermined movement direction. The light source includes a current supply section, a light emitting section, and a current output section. Electric current for light emission is supplied to the current supply section. The light emitting section includes light emitting elements that are disposed two-dimensionally. At least the light emitting elements that are provided side by side in the movement direction among the light emitting elements are electrically connected in parallel. The current output section outputs the electric current that has passed through the light emitting section. The current supply section and the light emitting section are provided side by side in the movement direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic view of the structure of an exemplary image forming apparatus according to a first exemplary embodiment; 
         FIG. 2  is a schematic view of the structure of a fixing unit according to the first exemplary embodiment; 
         FIG. 3  is a plan view of a portion of the fixing unit shown in  FIG. 2  when seen from the top thereof (that is, from a light illuminating side); 
         FIG. 4  is a sectional view taken along line IV-IV of the fixing unit shown in  FIG. 3 ; 
         FIG. 5  is a plan view of a portion of a fixing unit according to a second exemplary embodiment when seen from the top thereof (that is, from a light illuminating side); 
         FIG. 6  is a plan view of a portion of a fixing unit according to a third exemplary embodiment when seen from the top thereof (that is, from a light illuminating side); 
         FIG. 7  is a plan view of a portion of a fixing unit according to a fourth exemplary embodiment when seen from the top thereof (that is, from a light illuminating side); 
         FIG. 8  is a plan view of a portion of a fixing unit according to a fifth exemplary embodiment when seen from the top thereof (that is, from a light illuminating side); 
         FIG. 9  is a sectional view taken along line IX-IX of the fixing unit shown in  FIG. 8 ; 
         FIG. 10  is a plan view of a portion of a fixing unit according to a sixth exemplary embodiment when seen from the top thereof (that is, from a light illuminating side); 
         FIG. 11  is a plan view of a portion of a fixing unit according to a seventh exemplary embodiment when seen from the top thereof (that is, from a light illuminating side); and 
         FIG. 12  is a schematic view of the structure of an image forming apparatus (inkjet recording apparatus) that forms an image using ink. 
     
    
    
     DETAILED DESCRIPTION 
     First Exemplary Embodiment 
     An exemplary embodiment of the present invention is hereunder described in detail with reference to the drawings. 
     Image Forming Apparatus 
     In the first exemplary embodiment shown in  FIG. 1  that is a schematic view of the structure of an exemplary image forming apparatus according to the first exemplary embodiment, application to an image forming apparatus  10  that forms an image using toner is described in detail. 
     As shown in  FIG. 1 , the image forming apparatus  10  according to the first exemplary embodiment forms an image on a sheet (recording medium)  28  while transporting the sheet  28  in the direction of arrow B. 
     The image forming apparatus  10  according to the first exemplary embodiment includes a photoconductor member  12  that rotates at a constant velocity in the direction of arrow A. A charging unit (charging device)  14 , a light source head (exposing device)  16 , a developing unit (developing device)  18 , a transfer member (transfer device)  20 , a cleaner  22 , and an erase lamp  24  are disposed in that order around the photoconductor member  12  in the direction of rotation of the photoconductor member  12 . The charging unit  14  charges the surface of the photoconductor member  12 . The light source head  16  performs exposure for forming an electrostatic latent image on the surface of the photoconductor member  12  that has been charged by the charging unit  14 . The developing unit  18  develops the electrostatic latent image using developer for forming a toner image. The transfer member transfers the toner image to the sheet (recording medium)  28 . The cleaner  22  removes residual toner remaining on the photoconductor member  12  after the transfer. The erase lamp  24  removes electricity from the photoconductor member  12  to equalize potential. 
     That is, after the surface of the photoconductor member  12  has been charged by the charging unit  14 , the light source head  16  irradiates the surface of the photoconductor member  14  with light to form a latent image on the photoconductor member  12 . The light source head  16  including light emitting elements is connected to a driving section (not shown). The driving section controls lighting of the light emitting elements, so that light is emitted on the basis of image data. 
     Toner is supplied to the formed latent image using the developing unit  18 , so that a toner image is formed on the photoconductor member  12 . The transfer member  20  transfers the toner image on the photoconductor member  12  onto the sheet  28  that has been transported. Residual toner on the photoconductor member  12  after the transfer is removed by the cleaner  22 . After residual electric charges on the surface of the photoconductor member  12  have been removed by illumination light emitted from the erase lamp  24 , the surface of the photoconductor member  12  is recharged by the charging unit  14 , and the same operations are repeated. 
     The sheet  28  to which the toner image has been transferred is transported to a fixing unit  30  (fixing device) including light-emitting-element array chips  60  including light emitting elements, and is irradiated with light from the light emitting elements of the fixing unit  30 , so that a fixing operation is performed. The fixing unit  30  including the light emitting elements is connected to a driving circuit (not shown). The driving circuit controls the lighting of the light emitting elements, so that light is emitted towards the sheet  28 . 
     In this way, the toner image is fixed, so that a desired image is formed on the sheet  28 . The sheet  28  on which the image has been formed is discharged outside the apparatus. 
     Fixing Unit 
     Next, the fixing unit according to the exemplary embodiment is described in detail. 
     The structure of the fixing unit  30  according to the exemplary embodiment is described.  FIG. 2  is a schematic view of the structure of the fixing unit  30  according to the first exemplary embodiment. 
     The fixing unit  30  according to the exemplary embodiment is provided in a direction that crosses a transport direction of the sheet  28  (hereunder referred to as the “widthwise direction of transport”). In the exemplary embodiment, the entire horizontal region (in the widthwise direction of transport) of the sheet  28  is capable of being irradiated with light at the same time or substantially at the same time. Therefore, the fixing unit  30  has a length that is substantially in correspondence with the horizontal width of the sheet  28  (that is, the width of the sheet  28  in the widthwise direction of transport). 
     The fixing unit  30  according to the exemplary embodiment includes positive wires  52 , a negative wire  54 , and light-emitting-element array chips  60 , which are provided on a printed wiring board  50 . The positive wires  52  form positive wire patterns. The negative wire  54  forms a negative wire pattern. The positive wires  52 , the negative wire  54 , and the light-emitting-element array chips  60  are disposed side by side along the widthwise direction of transport in a range that substantially corresponds to the horizontal width of the sheet  28 . The positive wires  52  are connected to positive electrodes  74  (anode electrodes) of the light-emitting-element array chips  60 , and electric current is supplied to the light-emitting-element array chips  60  from a driving circuit (not shown). The negative wire  54  is connected to negative electrodes  75  (cathode electrodes) of the light-emitting-element array chips  60 , and causes electric current that is output from the light-emitting-element array chips  60  to return to the driving circuit. In the exemplary embodiment, electric currents that are supplied to the positive wires  52  and the negative wire  54  from the driving circuit may be supplied from an end portion or a central portion of the printed wiring board  50  in a longitudinal direction thereof. The electric current supplied from the driving circuit may be supplied from either a constant voltage source or a constant current source. 
     Light emitting elements  62  of the light-emitting-element array chips  60  are two-dimensionally disposed. In the exemplary embodiment, vertical cavity surface emitting lasers (VCSELs) are used as the light emitting elements. In the exemplary embodiment, the light-emitting-element array chips  60  are disposed side by side on the printed wiring board  50  in the widthwise direction of transport (that is, in the direction that crosses the transport direction) in the range that substantially corresponds to the width of a recording medium. The positive electrodes  74  of the light-emitting-element array chips  60  according to the exemplary embodiment and the positive wires  52  are connected to each other using bonding wires  56 . Driving current that flows from the driving circuit (not shown) is supplied to the light-emitting-element array chips  60  from the positive wires  52  via the bonding wires  56 , and flows to the negative wire  54  from the negative electrodes  75  of the light-emitting-element array chips  60 . This causes the light emitting elements  62  to emit light, and the light to be directed towards the sheet  28 . In the exemplary embodiment, the width of a light emitting section including the light emitting elements  62  in the widthwise direction of transport substantially corresponds to the width of its corresponding light-emitting-element array chip  60  itself. 
     The fixing unit  30  according to the exemplary embodiment is described in more detail with reference to the drawings.  FIG. 3  is a plan view of a portion of the fixing unit  30  when seen from the top thereof (that is, from a light illuminating side).  FIG. 4  is a sectional view taken along line IV-IV of the fixing unit  30  shown in  FIG. 3 .  FIGS. 2 ,  3 , and  4  are each a schematic view of the fixing unit  30  according to the exemplary embodiment. For example, the bonding wires  56 , the light-emitting-element array chips  60 , the light emitting elements  62  are examples, and are not limited to those that are illustrated. 
     The light-emitting-element array chips  60  used in the fixing unit  30  according to the exemplary embodiment are VCSEL array chips in which the light emitting elements  62  are two-dimensionally disposed. The light-emitting-element array chips  60  are surface-emitting VCSEL array chips. A semiconductor layer  72  is formed on an n-type GaAs substrate  70 . The semiconductor layer  72  is provided with a distribution bragg reflector (DBR) or an active layer including a p-type or an n-type semiconductor layer. Examples of p-type semiconductor layers are an AlGaAs semiconductor layer or an InGaAs semiconductor layer. The light emitting sections including the light emitting elements  62  emit light that has been generated. 
     The entire back surface of each light-emitting-element array chip  60  is a negative electrode (which is formed of a metallic material such as Au), is disposed on the negative wire  54 , and is connected to the negative wire  54  using, for example, a silver paste. Excluding a light exiting opening of each light-emitting-element array chip  60 , the entire front surface of each light-emitting-element array chip  60  is covered with a metallic material, such as Au, and constitutes a positive electrode. The positive wires  52  are disposed, respectively, at an upstream side and a downstream side of the negative wire  54  (light-emitting-element array chips  60 ) in the transport direction. The light-emitting-element array chips  60  are connected to the positive wires  52  using bonding wires  56 . The light emitting elements  62 , which are formed between the positive electrodes  74  and the corresponding negative electrodes  75 , are two-dimensionally disposed, and are electrically connected in parallel. Accordingly, each light-emitting-element array chip  60  according to the exemplary embodiment is an electrode in which the positive electrode  74  is continuously formed on both sides of the light emitting elements  62 , and is such that electric current is supplied in parallel to the light emitting elements  62  via the continuously formed electrode. Not all of the light emitting elements in each of the light-emitting-element array chips  60  need to be connected in parallel. The light emitting elements  62  may be connected in parallel with every plural number of light emitting elements  62 . 
     In the exemplary embodiment, each light-emitting-element array chip  60  is provided with a current supply section. The bonding wires  56  are connected to an upstream edge of each light-emitting-element array chips  60  along the widthwise direction of transport, to supply electric current to the corresponding current supply section. Each current supply section is a portion of its corresponding positive electrode  74 , and has a width that substantially corresponds to the width including the light emitting elements  62  (that is, a width substantially corresponding to the width of its corresponding light-emitting-element array chip  60 ). The bonding wires  56  are connected to the positive wire  52  that is disposed upstream while the bonding wires  56  are disposed side by side in the widthwise direction of transport. Similarly, in the exemplary embodiment, each light-emitting-element array chip  60  is provided with another current supply section that is provided at a downstream edge of its corresponding light-emitting-element array chip  60 . The light-emitting-element array chips  60  are connected to the positive wire  52  that is disposed downstream using the bonding wires  56 . Driving current supplied from the positive wires  52  is supplied to the current supply sections of the light-emitting-element array chips  60  via the bonding wires  56 , and flows along the positive electrodes  74  in the transport direction. Then, the driving current flows out to the negative wire  54  via the negative electrodes  75  functioning as current output sections via the light emitting elements  62 , so that the light emitting elements  62  emit light. In the exemplary embodiment, the positive wires  52  and the light-emitting-element array chips  60  are connected to each other using the bonding wires  56 . It is desirable that a plural number of bonding wires  56  be provided for each light-emitting-element array chip  60  substantially in correspondence with the width of the light emitting section in the widthwise direction of transport. When a plural number of bonding wires  56  are provided substantially in correspondence with the width of the light emitting section, electric current flows more in the transport direction than when bonding wires  56  are provided substantially in correspondence with only part of the width of the light emitting section. In addition, it is desirable that a plural number of bonding wires  56  be provided at substantially equal intervals along the widthwise direction of transport. 
     In the fixing unit  30  according to the exemplary embodiment, when electric current is supplied to the light-emitting-element array chips  60 , electric current is supplied downstream in the transport direction from the current supply sections that are disposed at the upstream side in the transport direction, and electric current is supplied upstream in the transport direction from the current supply sections that are disposed at the downstream side. 
     Here, since the positive electrodes  74  of the light-emitting-element array chips  60  according to the exemplary embodiment have a plural number of light-exiting openings, the paths for electric current flow are limited. In addition, a plural number of light emitting elements  62  are two-dimensionally disposed, and are electrically connected in parallel. Therefore, the longer the supply path from the corresponding current supply section to the light emitting elements through the corresponding positive electrode  74  whose path for electric current flow is limited, the smaller the amount of supplied electric current due to an increase in wire resistance. That is, in the exemplary embodiment, the light emission amount of the light emitting elements  62  that are disposed at the upstream side in the direction of electric current flow tends to be larger than the light emission amount of the light emitting elements  62  that are connected in parallel at the downstream side because a larger amount of electric current flows through the upstream light emitting elements  62 . 
     When the light-emitting-element array chips  60  having such a structure are used in the fixing unit  30 , if the fixing unit  30  is formed so that electric current flows in the widthwise direction of transport, differences between the exiting light amounts in the widthwise direction of transport occur. As a result, differences between the amounts of received light at regions on a sheet  28  in the widthwise direction of transport occur, as a result of which differences in fixing characteristics. 
     Therefore, in the exemplary embodiment, in the range of the width of the sheet  28 , the current supply sections, the plural number of light emitting elements  62 , and the negative electrodes  75 , which are provided at the light-emitting-element array chips  60 , are disposed side by side in the transport direction. By virtue of this structure, electric current flows in the transport direction of the sheet  28 , so that differences between the amounts of light that are emitted to the sheet  28  in the widthwise direction of transport are reduced. 
     In the transport direction, in accordance with the distances from the current supply sections to the light emitting elements  62  in the transport direction (that is, the wire resistances), differences between the supplied electric current amounts occur, as a result of which differences between the light emission amounts in the transport direction occur. However, even if differences between light emission amounts in the transport direction occur, since the amounts of light used to illuminate the regions of the sheet  28  in the widthwise direction of transport are determined on the basis of integrated values of light emission amounts in the transport direction, the effect of uneven light emission in the transport direction upon the fixing operation is smaller than that of uneven light emission in the widthwise direction of transport upon the fixing operation. 
     In addition, in the exemplary embodiment, electric current is supplied from both the upstream side and the downstream side in the transport direction. Therefore, compared to a structure in which electric current is supplied from only one side, the paths to the light emitting elements  62  are shorter. As a result, uneven light emission in the transport direction is suppressed, so that uneven light emission in the two-dimensional light emitting sections is suppressed as a whole. In addition, connection regions are provided at both the upstream side and the downstream side. Therefore, compared to a structure in which connection regions are provided at only one side, it is possible to use a larger number of bonding wires  56 . 
     Second Exemplary Embodiment 
     A fixing unit  30  according to another exemplary embodiment is described. Structural features and operations that are substantially the same as those according to the first exemplary embodiment are not described in detail. 
       FIG. 5  is a plan view of a portion of the fixing unit  30  according to the second exemplary embodiment when seen from the top thereof (that is, from a light illuminating side). In the fixing unit  30  according to the first exemplary embodiment, the current supply sections of the light-emitting-element array chips  60  are disposed at both the upstream side and the downstream side in the transport direction, and electric current is supplied from both the upstream side and the downstream side. In contrast, in the second exemplary embodiment, as shown in  FIG. 5 , current supply sections of the light-emitting-element array chips  60  are disposed on one side, and electric current is supplied from only one side. The other structural features of the second exemplary embodiment are substantially the same as those of the first exemplary embodiment.  FIG. 5  shows, as an example of disposing the current supply sections at one side, the case in which the current supply sections of the light-emitting-element array chips  60  are disposed at the downstream side in the transport direction. However, the current supply sections may be disposed at the upstream side in the transport direction. 
     Therefore, in the second exemplary embodiment, as in the first exemplary embodiment, uneven light emission in the widthwise direction of transport is suppressed, as a result of which uniformity in light amount distribution is increased. Consequently, uneven fixing of a toner image is also suppressed. 
     Third Exemplary Embodiment 
     A fixing unit  30  according to still another exemplary embodiment is described. Structural features and operations that are substantially the same as those according to the first exemplary embodiment are not described in detail. 
       FIG. 6  is a plan view of a portion of the fixing unit  30  according to the third exemplary embodiment when seen from the top thereof (that is, from a light illuminating side). In the fixing unit  30  according to the first exemplary embodiment, the current supply sections of the light-emitting-element array chips  60  are disposed at both the upstream side and the downstream side in the transport direction. In contrast, in the third exemplary embodiment, as shown in  FIG. 6 , current supply sections are alternately disposed on either side of the respective light-emitting-element array chips  60 . The other structural features of the third exemplary embodiment are substantially the same as those of the first exemplary embodiment. 
     Therefore, even in the third exemplary embodiment, as in the first exemplary embodiment, the light emitting elements  62  that are disposed side by side in the widthwise direction of transport are such that uneven light emission in the widthwise direction of transport is suppressed, so that uniformity in light amount distribution is increased. 
     Further, in the fixing unit  30  according to the third exemplary embodiment shown in  FIG. 6 , a light-emitting-element array chip  60 A and a light-emitting-element array chip  60 C are connected to a positive wire  52  that is disposed at a downstream side in the transport direction, and electric current is supplied to current supply sections that are disposed at the downstream side. A light-emitting-element array chip  60 B is connected to a positive wire  52  that is disposed at the upstream side in the transport direction, and electric current is supplied to a current supply section that is disposed at the upstream side. Therefore, in the light-emitting-element array chip  60 A and the light-emitting-element array chip  60 C, electric current is supplied to the downstream current supply sections. Consequently, the light amounts of downstream light emitting elements  62  are large, whereas the light amounts of light emitting elements  62  that are disposed at the upstream side in the transport direction are small. In contrast, in the light-emitting-element array chip  60 B, electric current is supplied to the upstream current supply section. Therefore, the light amounts of the upstream light emitting elements  62  are large, whereas the light amounts of light emitting elements  62  that are disposed at the downstream side in the transport direction are small. Therefore, when the fixing unit  30  is considered as a whole, uneven light emission in the widthwise direction of transport is suppressed, so that uniformity in light amount distribution is increased. 
     Even in the third exemplary embodiment, uneven fixing of a toner image is also suppressed. 
       FIG. 6  shows the case in which the light-emitting-element array chips  60  are alternately connected to a positive wire  52  at the upstream side in the transport direction and to a positive wire  52  at the downstream side in the transport direction. However, the light-emitting-element array chips  60  may be connected so that two light-emitting-element array chips  60  are connected to the positive wire  52  at the upstream side in the transport direction, the next two light-emitting-element array chips  60  are connected to the positive wire  52  at the downstream side in the transport direction, and so forth. From the viewpoint of suppressing uneven light emission in the transport direction, it is desirable that the difference between the number of light-emitting-element array chips  60  that are connected to the upstream positive wire  52  and the number of light-emitting-element array chips  60  that are connected to the downstream positive wire  52  be small. As shown in  FIG. 6 , it is desirable that the light-emitting-element array chips  60  be alternately connected to the upstream positive wire  52  and the downstream positive wire  52 . 
     Fourth Exemplary Embodiment 
     A fixing unit  30  according to still another exemplary embodiment is described. Structural features and operations that are substantially the same as those according to the first exemplary embodiment are not described in detail. 
       FIG. 7  is a plan view of a portion of the fixing unit  30  according to a fourth exemplary embodiment when seen from the top thereof (that is, from a light illuminating side). In the fixing unit  30  according to the first exemplary embodiment, the light-emitting-element array chips  60  are connected in parallel. In contrast, in the fourth exemplary embodiment, as shown in  FIG. 7 , the light-emitting-element array chips  60  are connected in series via positive wires  52  and negative wires  54 . The other structural features of the fourth exemplary embodiment are substantially the same as those of the first exemplary embodiment. 
     In the fourth exemplary embodiment, since the positive wires  52 , the negative wires  54 , and the light-emitting-element array chips  60  are connected in series, one wire has the function of connecting the negative electrode  75  and the positive electrode  74  of the light-emitting-element array chip  60  that is adjacent thereto. In the fixing unit  30  shown in  FIG. 7 , electric current that has been supplied from the positive wire  52 A to a current supply section of the light-emitting-element array chip  60  via bonding wires  56  flows to a negative wire  54 B, and is supplied to a current supply section of the light-emitting-element array chip  60 B via the bonding wires  56  with the negative wire  54 B serving as a positive wire  52 B. Further, electric current that has flowed from the light-emitting-element array chip  60 B to a negative wire  54 C is supplied to a current supply section of a light-emitting-element array chip  60 C through the bonding wires  56  with the negative wire  54 C serving as a positive wire  52 C, and flows towards a negative wire  54 D. 
     Accordingly, in the fourth exemplary embodiment, the positive wires  52 , the negative wires  54 , and the light-emitting-element array chips  60  are connected in series. Therefore, compared to the case in which the light-emitting-element array chips  60  are connected in parallel (for example, as in the above-described embodiments), the amount of electric current that flows through the entire fixing unit  30  is reduced. 
     Even in the fourth exemplary embodiment, as in the first exemplary embodiment, uneven light emission in the widthwise direction of transport is suppressed, so that uniformity in light amount distribution is increased. Therefore, uneven fixing of a toner image is also suppressed. 
     Therefore, as in the third exemplary embodiment, when the fixing unit  30  is considered as a whole, uneven light emission in the transport direction is suppressed, so that uniformity in light amount distribution is increased. 
     Fifth Exemplary Embodiment 
     A fixing unit  30  according to still another exemplary embodiment is described. Structural features and operations that are substantially the same as those according to the first exemplary embodiment are not described in detail. 
       FIG. 8  is a plan view of a portion of the fixing unit  30  according to a fifth exemplary embodiment when seen from the top thereof (that is, from a light illuminating side).  FIG. 9  is a sectional view taken along line IX-IX of the fixing unit  30  shown in  FIG. 8 . In the fixing unit  30  according to the first exemplary embodiment, surface-emitting-type light-emitting-element array chips  60  are used. In contrast, in the fifth exemplary embodiment, as shown in  FIG. 9 , backside-emitting-type light-emitting-element array chips  60  are used. In addition, in the fixing unit  30  according to the fifth exemplary embodiment, the light-emitting-element array chips  60 , a positive wire  52 , and a negative wire  54  are connected to positive electrodes  74  and negative electrodes  74  without using bonding wires  56 , that is, by using, for example, a silver paste. 
     As shown in  FIG. 9 , in each light-emitting-element array chip  60 , an internal electrode  55 , a semiconductor layer  72 , a positive electrode  74 , and a negative electrode  75  are stacked upon each other on a semi-insulating (intrinsic), GaAs substrate  71 . The internal electrode  55  includes an n-type GaAs layer. The semiconductor layer  72  is provided with a distribution bragg reflector (DBR) or an active layer including a p-type or an n-type semiconductor layer. Examples of p-type semiconductor layers are an AlGaAs semiconductor layer or an InGaAs semiconductor layer. The stacked structure is turned upside down, and mounted on a printed wiring board  50 . The positive electrodes  74  according to the fifth exemplary embodiment each have a structure that is the same as that of each of the positive electrodes  74  according to the first exemplary embodiment except that current supply sections to which bonding wires  56  are connected are not provided. 
     Although, unlike each electrode formed of a metallic material according to the first exemplary embodiment, each internal electrode  55  has a semiconductor layer, the internal electrodes  55  are similar to the positive electrodes  74  according to the first exemplary embodiment in that they are formed continuously on both sides of the light emitting elements  62 . In the interiors of the light-emitting-element array chips  60 , the internal electrodes  55  are connected to the negative electrodes  75  that extend from the back sides to the inner portions of the light-emitting-element array chips  60 , and cause electric current that has passed through the light emitting elements  62  to flow to the negative electrodes  75 . 
     Like the positive electrodes  74 , the negative electrodes  75  are formed of metallic materials, such as Au. However, unlike the negative electrodes  75  in the first exemplary embodiment, the negative electrodes  75  are provided on the same side of the printed wiring substrate  50  as the positive electrodes  74  are. Each negative electrode  75  has a continuous surface having a width that substantially corresponds to the width of the corresponding light-emitting-element array chip  60  in the widthwise direction of transport. 
     Each positive electrode  74  is connected to the positive wire  52  using, for example, a silver paste at an upstream side of the corresponding light-emitting-element array chip  60  in the transport direction. Each negative electrode  75  is connected to the negative wire  54  using, for example, a silver paste at a downstream side of the corresponding light-emitting-element array chip  60  in the transport direction. That is, in the fifth exemplary embodiment, each positive electrode  74 , which is connected to the positive wire  52  using a silver paste, constitutes a current supply section as a whole. Electric current that has been supplied from the positive wire  52  to the positive electrode  74  of each light-emitting-element array chip  60  flows to the internal electrode  55  via the light emitting elements  62 , flows through the internal electrode  55  along the transport direction, and flows to the negative wire  54  via the negative electrode  75 . The other structural features of the fifth exemplary embodiment are substantially the same as those of the first exemplary embodiment. 
     As shown in  FIG. 9 , the backside-emitting-type light-emitting-element array chips  60  differ from the surface-emitting-type light-emitting-element array chips  60  according to the first exemplary embodiment in that semiconductor layers are used as internal electrodes. Therefore, the resistance values of the electric current paths in the backside-emitting-type light-emitting-element array chips  60  tend to be larger than the resistance values of the electric current paths in the surface-emitting-type light-emitting-element array chips  60 . Consequently, light amount variations that are in correspondence with the electric current paths tend to occur. However, in the fifth exemplary embodiment, as in the first exemplary embodiment, electric current flows in the transport direction at the light emitting elements  62  that are disposed side by side in the widthwise direction of transport. Therefore, uneven light emission in the widthwise direction of transport is suppressed, as a result of which uniformity in light amount distribution is increased. Consequently, uneven fixing of a toner image is also suppressed. 
     Accordingly, when application is made to the backside-emitting-type light-emitting-element array chips  60 , uneven light emission in the widthwise direction of transport is more effectively suppressed. 
     As shown in  FIG. 9 , in each backside-emitting-type light-emitting-element array chip  60 , light that is emitted from the light emitting elements  62  is transmitted through the substrate  70 , and illuminates the sheet  28 . In each backside-emitting-type light-emitting-element array chip  60 , compared to each surface-emitting-type light-emitting-element array chip  60 , the light emitting elements  62  are provided closer to the printed wiring board  50 . Therefore, when a cooling section is provided at the printed wiring board  50 , heat of the light emitting elements  62  is more efficiently absorbed in the backside-emitting type than in the surface-emitting-type type. Therefore, stability is increased. 
     Sixth Exemplary Embodiment 
     A fixing unit  30  according to still another exemplary embodiment is described. Structural features and operations that are substantially the same as those according to the first exemplary embodiment are not described in detail. 
       FIG. 10  is a plan view of a portion of the fixing unit  30  according to a sixth exemplary embodiment when seen from the top thereof (that is, from a light illuminating side). In the sixth exemplary embodiment, a positive wire  52  is provided at an upstream side of light-emitting-element array chips  60  in the transport direction, and a negative wire  54  and negative electrodes  75  are provided at a downstream side of the light-emitting-element array chips  60  in the transport direction. In addition, in the sixth exemplary embodiment, electric current that has been supplied to positive electrodes  74  flows from the upstream side to the downstream side of the light-emitting-element array chips  60 . In  FIG. 10 , similarly to the fifth exemplary embodiment, for example, the light-emitting-element array chips  60 , the positive wire  52 , and the negative wire  54  are connected to each other without using bonding wires  56 , that is, they are connected to each other using, for example, a silver paste. The other structural features of the sixth exemplary embodiment are substantially the same as those of the first exemplary embodiment and the fifth exemplary embodiment. 
     In the sixth exemplary embodiment, in each backside-emitting-type light-emitting-element array chip  60  including an internal electrode  55  formed of a semiconductor layer, a larger amount of electric current flows at the light emitting elements  62  that are at the side that is close to the negative wire  54  (that is, the downstream side) than the light emitting elements  62  that are at the side that is far away from the negative wire  54  (that is, the upstream side) because a path extending through the internal electrode  55  having a high resistance becomes short. Therefore, the light emission amount becomes large. Since the peak of the light emission amount exists towards the downstream side in the transport direction, the amount of light that illuminates a predetermined region of the sheet  28  is at first small, and, then, gradually increases. 
     When a toner image is fixed in the image forming apparatus  10 , it is desirable that toner of the toner image be preliminarily heated, and heating required for fixing be performed after the preliminary heating. In the sixth exemplary embodiment, since the peak of the light emission amount exists at the downstream side in the transport direction, after performing the preliminary heating as a result of illuminating the toner with a small amount of light, the toner is illuminated with a large amount of light to perform the heating of the toner required for fixing the toner image. 
     Accordingly, when the fixing unit  30  that fixes a toner image is used, a larger effect is obtained by disposing the negative electrode  75 , the light emitting section, and the current supply section of each light-emitting-element array chip  60  so that the amount of light that illuminates a predetermined region of the sheet  28  is at first small and then gradually increases. The exemplary embodiment is not limited to a backside-emitting light source. A surface-emitting light source may also be used. For example, in a surface-emitting light source in the second exemplary embodiment, the distance that electric current flows through the positive electrode  74  is short at the light emitting elements  62  that are far away from the negative electrode  75  (that is, the light emitting elements  62  that are close to the current supply section). Since the light emission amounts are large at these light emitting elements  62 , the negative electrode  75  is disposed at the upstream side in the transport direction, and the positive electrode  74  is disposed at the downstream side in the transport direction. 
     Seventh Exemplary Embodiment 
     A fixing unit  30  according to still another exemplary embodiment is described. Structural features and operations that are substantially the same as those according to the first exemplary embodiment are not described in detail. 
       FIG. 11  is a plan view of a portion of the fixing unit  30  according to a seventh exemplary embodiment when seen from the top thereof (that is, from a light illuminating side). In the seventh exemplary embodiment, a positive wire  52  is provided at a downstream side of light-emitting-element array chips  60  in the transport direction, and electric current that has been supplied to positive electrodes  74  flow from the downstream side to an upstream side of the light-emitting-element array chips  60 . In  FIG. 11 , similarly to the fifth exemplary embodiment, for example, the light-emitting-element array chips  60 , the positive wire  52 , and the negative wire  54  are connected to each other without using bonding wires  56 , that is, they are connected to each other using, for example, a silver paste. The other structural features of the seventh exemplary embodiment are substantially the same as those of the first exemplary embodiment and the fifth exemplary embodiment. 
     In the seventh exemplary embodiment, contrary to the sixth exemplary embodiment, the peak of light emission amount exists at the upstream side in the transport direction. Therefore, the amount of light that illuminates a predetermined region of a sheet  28  is at first small, and, then, gradually increases. 
     Therefore, when it is desirable that the amount of light that illuminates the sheet  28  be gradually reduced, it is desirable to use the structure according to the seventh exemplary embodiment. As described in the sixth exemplary embodiment, the exemplary embodiment is not limited to a backside-emitting light source. A surface-emitting light source may also be used. 
     Although, in each of the exemplary embodiments, the image forming apparatus  10  (fixing unit  30 ) that forms an image using toner, it is particularly desirable that, in each of the exemplary embodiments, an image forming apparatus  10  (drying device) that forms an image using ink be used (described in detail below). 
     As described above, in each of the above-described exemplary embodiments, the negative electrodes  75 , the light emitting elements  62 , and the current supply sections that supply electric current to the light-emitting-element array chips  60  are provided side by side in the transport direction of a sheet  28 . Electric current that flows to the negative electrodes  75  from the current supply sections through the light emitting elements  62  flows in the transport direction in a range substantially corresponding to the width of a recording medium. Therefore, uneven light emission in the widthwise direction of transport is suppressed, as a result of which uniformity in light amount distribution is increased. Consequently, uneven fixing of a toner image is also suppressed. 
     The current supply sections, the light emitting elements  62 , and the negative electrodes  75  need not be provided completely along the transport direction. Any structure may be used as along as the electric current that flows in the light-emitting-element array chips  60  as a whole flows in the transport direction rather than in the widthwise direction of transport. It is desirable that the largest amount of electric current flow in a direction that substantially corresponds to the transport direction. For example, it is desirable that the current supply sections, the light emitting elements  62 , and the negative electrodes  75  be disposed linearly side by side in a direction that substantially corresponds to the transport direction. 
     In each of the exemplary embodiments, the positive wire  52  and the negative wire  54  may be interchanged. In addition, the polarities of the light-emitting-element array chips  60  may be interchanged. For example, in each of the exemplary embodiments, the positive wire  52  may be a negative wire, and the negative wire  54  may be a positive wire. Further, the polarity of the p-type semiconductor and the polarity of the n-type semiconductor of each light-emitting-element array chip  60  may be interchanged. By the interchanging, the positions of the current supply sections and the positions of the current output sections are also interchanged. 
     Although, in each of the above-described exemplary embodiments, the case in which a sheet  28  is moved is described, the fixing unit  30  may be moved, or both the sheet  28  and the fixing unit  30  may be moved. Any structure may be used as long as the fixing unit  30  and the sheet  28  move relative to each other. 
     Although, in each of the above-described exemplary embodiments, VCSELs are used as light-emitting-element array chips  60 , other types of surface-emitting lasers or light-emitting elements may be used. From the viewpoint of reducing costs and increasing reliability and high-temperature stability due to increasing output of a light source, it is desirable to use surface-emitting lasers (VCSELs) as in the exemplary embodiments. 
     The image forming apparatus  10  is not limited to the above-described exemplary embodiments. For example, although each of the above-described exemplary embodiments is applied to the self-scanning electrophotographic image forming apparatus  10 , the image forming apparatus is not limited thereto. For example, the image forming apparatus  10  may be one that forms a monochromatic image, or one that forms an image of a plural number of colors. 
     In addition, for example, although, in each of the above-described exemplary embodiments, the image forming apparatus  10  (fixing unit  30 ) that forms an image using toner is described, the image forming apparatus  10  may be one that forms an image using ink. An exemplary image forming apparatus  10  that forms an image using ink is described. 
       FIG. 12  is a schematic view of the structure of an image forming apparatus (inkjet recording apparatus)  10  that forms an image using ink. The image forming apparatus  10  shown in  FIG. 12  includes four recording/drying combined heads  80  (discharging device/drying device) which are provided in correspondence with four colors, that is, yellow (Y), magenta (M), cyan (C), and black (K), and which are disposed in the transport direction. The image forming apparatus  10  shown in  FIG. 12  records a full-color image. The image forming apparatus  10  is also provided with ink tanks  82  that store inks of four colors, respectively. Ink drops are discharged from the recording/driving combined heads  80  of the corresponding colors. The method of discharging ink drops is not particularly limited. Publicly known systems, such as a thermal system or a piezoelectric system, are used. Various types of publicly known inks, such as aqueous ink, oil-based ink, and solvent ink, are used as the ink. 
     Sheets  28  that are stacked in a sheet-feed tray  84  are taken out, and are transported to the recording/drying combined heads  80  of the corresponding colors via a transport path  86  while being held by a transport belt. The recording/drying combined heads  80  discharge ink drops corresponding to pieces of image data onto a sheet  28 . The sheet  28  is irradiated with light to dry the ink drops on the sheet  28 . Similarly to the fixing unit  30  according to each of the exemplary embodiments, the structure that dries ink drops in a driving system of each of the recording/driving combined heads  80  of the corresponding color is one in which the sheet  28  is irradiated with light emitted from the light emitting elements  62  of the light-emitting-element array chips  60  that are disposed on the printed wiring substrate  50  in the widthwise direction of transport. In the order corresponding to the transport of the sheet  28 , images of the corresponding colors are successively formed using the recording/drying combined heads  80  of the corresponding colors. The sheet  28  to which a desired image has been formed is discharged to the outside of the image forming apparatus  10 . 
     Accordingly, in the image forming apparatus  10  that forms an image using ink, the sheet  28  is irradiated with light to dry the ink drops on the sheet  28 . Therefore, similarly to the fixing unit  30  that has been described in each of the exemplary embodiments, light amount variation in the widthwise direction of transport causes uneven drying of the ink drops. Consequently, when the structure for drying ink drops is made similar to that of the fixing unit  30  that has been described in each of the exemplary embodiments, uneven light emission in the widthwise direction of transport is suppressed, as a result of which uniformity in light amount distribution is increased. Consequently, uneven drying of ink drops is also suppressed. 
     If, after drying the ink drops by heating the ink drops as a result of irradiating the ink drops with light, the heating of the ink drops is continued, the sheet  28  may be excessively heated. Therefore, it is desirable to reduce the amount of light, used for illuminating the sheet  28  so as to suppress heating as the drying of the ink drops is continued, in accordance with the drying of the ink drops. 
     When a drying device that dries ink drops is used, it is desirable that the structure that has been described in the seventh exemplary embodiment be used. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. Obviously, various modifications may be made in accordance with the circumstances within a scope that does not depart from the gist of the present invention.