Abstract:
An optical head represents gray scales of an image by expressing gray scales of pixels, which belong to a block, with binary values, the block being constituted by n pixels (n is a natural number of 2 or more) in a first direction and m pixels (m is a natural number) in a second direction. The optical head includes: a plurality of light emitting devices that extend in the first direction and emit light with luminance in accordance with driving current; a plurality of driving transistors that are provided corresponding to the plurality of light emitting devices and that supply the driving current; a potential line that applies source potential or gate potential to the plurality of driving transistors; and a plurality of driving circuits that are provided corresponding to the plurality of driving transistors and that supply a driving control signal to specify an ON state or an OFF state for gates of the driving transistors. The driving circuits each include: a line having an intersection at which the line intersects the potential line; and a logic circuit that generates the driving control signal based on image data to instruct turning-on or turning-off of the light emitting devices. The logic circuits of the plurality of driving circuits invert logic levels of the lines at the intersections every n natural number of intersections extending in the first direction corresponding to the block.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to an optical head driven by an area ratio gray-scale method, an exposure apparatus, and an image forming apparatus using the exposure apparatus. 
         [0003]    2. Related Art 
         [0004]    An image forming apparatus such as a printer includes an optical head used to form an electrostatic latent image on an image carrier such as a photoconductive drum. The optical head includes a plurality of light emitting devices arranged in the form of an array in a main scanning direction. Light emitting diodes may be used as the light emitting devices. 
         [0005]    An example of a method of representing gradation is an area ratio gray-scale method (for example, see JP-A-2004-249549). An area ratio gray-scale method represents gray scales in units of blocks by expressing pixels belonging to blocks, each of which is constituted by n pixels in a main scanning direction and m pixels in a sub scanning direction, with binary values. 
         [0006]    In the optical head using the light emitting diodes in the related art, a semiconductor chip and a driving IC are mounted on a print substrate with patterned wire thereon. It is important to realize basic print shading in an actual printing operation. In an optical head integrated with a driving circuit, since a head width is reduced and accordingly a circuit layout is extremely high in density, various signal lines have to intersect power lines of light emitting diodes. This produces parasitic capacitance at intersections of the signal lines and the power lines. Since the parasitic capacitance acts as coupling capacitance, when a signal of a signal line is changed from turn-on to turn-off and vice versa, noise is superimposed on a power line. As a result, current flowing in light emitting diodes turned on is varied, thereby temporarily varying luminance. Even a minute change in luminance may have an effect on the formation of an electrostatic latent image on a photoconductive drum, which may cause a user to recognize unevenness on a print sheet. 
         [0007]    This problem becomes serious in a so-called “solid coating” print. This is because several pixel circuits make the same logic shift simultaneously, and accordingly, larger noise is added to a power line in the solid coating print. For example, a head of an A3 sheet 600 dpi requires about 8000 pixels. In this case, although 8000 pixels are divided into blocks, several hundred pixel circuits have to operate simultaneously, which may result in a very high level of noise. 
       SUMMARY 
       [0008]    An advantage of some aspects of the invention is to provide an optical head, an exposure apparatus and an image forming apparatus, which are capable of restricting noise. 
         [0009]    According to an aspect of the invention, there is provided an optical head representing gray scales of an image by expressing gray scales of pixels, which belong to a block, with binary values, the block being constituted by n pixels (n is a natural number of 2 or more) in a first direction and m pixels (m is a natural number) in a second direction. The optical head includes: a plurality of light emitting devices that extend in the first direction and emit light with luminance in accordance with a driving current; a plurality of driving transistors that are provided corresponding to the plurality of light emitting devices and that supply the driving current; a potential line (for example, Lx shown in  FIG. 4  and Lz shown in  FIGS. 15A to 15D ) that applies a source potential or a gate potential to the plurality of driving transistors; and a plurality of driving circuits that are provided corresponding to the plurality of driving transistors and that supply a driving control signal to specify an ON state or an OFF state for gates of the driving transistors. The driving circuits each include: a line (for example, Ly shown in  FIG. 4 ) having an intersection at which the line intersects the potential line; and a logic circuit that generates the driving control signal based on image data to instruct turning-on or turning-off of the light emitting devices. The logic circuit of the plurality of driving circuits inverts a logic level of the line at the intersection every n natural number multiples extending in the first direction corresponding to the block. 
         [0010]    Parasitic capacitance is produced at the intersection of the line and the potential line. Since the parasitic capacitance acts as coupling capacitance, when the logic level of the line is shifted, noise is superimposed on the potential line. The invention employs an area ratio gray-scale method for representing gray scales. In this case, a pattern of logic levels of the line has blocks as basic units. In addition, since the logic circuits of the plurality of driving circuits invert the logic levels of the lines at the intersection every n natural number of intersections extending in the first direction corresponding to the block, noises superimposed on the potential line can cancel each other out. As a result, print unevenness can be reduced, thereby significantly improving print quality. 
         [0011]    The plurality of driving circuits may include a first driving circuit (for example, reference numeral  20 A of  FIG. 4 ) that inverts the image data an odd number of times until the image data reaches the intersection and a second driving circuit (for example, reference numeral  20 B of  FIG. 4 ) that inverts the image data an even number of times until the image data reaches the intersection. The first driving circuits and the second driving circuits may be alternately disposed every n natural number of intersections extending in the first direction corresponding to the block. In this case, since the logic levels at the intersections of the lines and the potential line are inverted in the first and second driving circuits, noises superimposed on the potential line can cancel each other out. 
         [0012]    According to another aspect of the invention, there is provided an exposure apparatus including: an optical head according to the first aspect; and a control circuit that generates image data representing gray scales of an image by expressing gray scales of pixels belonging to the block with binary values, and outputs the generated image data to the optical head. With the second aspect of the invention, it is possible to suppress noise and reduce print unevenness. 
         [0013]    According to still another aspect of the invention, there is provided an exposure apparatus including: an optical head including a plurality of light emitting devices extending in a first direction; and a control circuit that supplies image data to instruct turning-on or turning-off of the light emitting devices to the optical head. The control circuit generates the image data representing gray scales of an image by expressing gray scales of pixels, which belong to a block, with binary values, the block being constituted by n pixels (n is a natural number of 2 or more) in the first direction and m pixels (m is a natural number) in a second direction. The optical head includes: a plurality of driving transistors that supply a driving current to the plurality of light emitting devices; a potential line that supplies a source potential or a gate potential to the plurality of driving transistors; and a plurality of driving circuits that are provided corresponding to the plurality of driving transistors and that supply a driving control signal to specify an ON state or an OFF state for gates of the driving transistors. The driving circuits each include: a line having an intersection at which the line intersects the potential line; and a logic circuit that generates the driving control signal based on the image data. The control circuit generates the image data such that the logic circuits of the plurality of driving circuits invert logic levels of the lines at the intersections every n natural number of intersections extending in the first direction corresponding to the block. 
         [0014]    According to still another aspect of the invention, since the control circuit generates the image data such that the logic circuits invert logic levels of the lines at the intersections every n natural number of intersections extending in the first direction corresponding to the block, it is possible to suppress noises superimposed on the potential line and reduce print unevenness. 
         [0015]    The plurality of driving circuits each may include a first driving circuit (for example, reference numeral  20 A of  FIG. 12 ) and a second driving circuit (for example, reference numeral  20 C of  FIG. 12 ). The first driving circuit may include a latch circuit that latches the image data, a first inverting circuit that inverts an output signal of the latch circuit, and a second inverting circuit that inverts an output signal of the first inverting circuit and outputs the driving control signal, and the line having the intersection at which the line intersects the potential line connects an output terminal of the first inverting circuit and an input terminal of the second inverting circuit. The second driving circuit may include a latch circuit that latches the image data, and a first inverting circuit that inverts an output signal of the latch circuit and outputs the driving control signal, and the line having the intersection at which the line intersects the potential line connects an output terminal of the first inverting circuit and gates of the driving transistors. The first driving circuits and the second driving circuits may be alternately disposed every n natural number of intersections extending in the first direction corresponding to the block. 
         [0016]    In this case, since the control circuits invert the image data, the second inverting circuit in the second driving circuit of the optical head may be omitted. As a result, it is possible to simplify the optical head, thereby making it possible to make the optical head smaller. 
         [0017]    According to still another aspect of the invention, there is provided an image forming apparatus including: an exposure apparatus according to the second aspect; and an image carrier on which an image is formed by light emitted from the optical head. With this aspect, the effects of the above-described aspects can be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0019]      FIG. 1  is a perspective view showing a configuration of a portion of an image forming apparatus using an optical head according to a first embodiment of the on. 
           [0020]      FIG. 2  is a block diagram showing a configuration of an exposure apparatus. 
           [0021]      FIG. 3  is an explanatory view explaining blocks used in an area ratio gray-scale method. 
           [0022]      FIG. 4  is a circuit diagram showing a configuration of an optical head. 
           [0023]      FIG. 5  is a timing chart showing an operation of a processing unit. 
           [0024]      FIG. 6  is an explanatory view explaining a relationship between an area gray scale and a logic level of a line. 
           [0025]      FIGS. 7A and 7B  are waveform diagrams showing a waveform of noise. 
           [0026]      FIG. 8  is an explanatory view explaining a relationship between an area gray scale and a logic level of a line in a comparative example. 
           [0027]      FIG. 9  is an explanatory view explaining another configuration of the processing unit and a relationship between an area gray scale and a logic level of a line. 
           [0028]      FIG. 10  is a block diagram showing a configuration of an exposure apparatus according to a second embodiment of the invention. 
           [0029]      FIG. 11  is a timing chart showing an operation of a control circuit. 
           [0030]      FIG. 12  is a circuit diagram showing a configuration of an optical head. 
           [0031]      FIG. 13  is a block diagram showing another configuration of a processing unit. 
           [0032]      FIG. 14  is a timing chart showing an operation of a control circuit. 
           [0033]      FIGS. 15A to 15D  are circuit diagrams showing a configuration of a driving circuit according to a modification of the invention. 
           [0034]      FIG. 16  is a longitudinal sectional view showing a configuration of an image forming apparatus using the optical head according to an embodiment of the invention. 
           [0035]      FIG. 17  is a longitudinal sectional view showing a configuration of another image forming apparatus using the optical head according to an embodiment of the invention. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0036]    Exemplary embodiments of the invention will be hereinafter described with reference to the accompanying drawings in which like elements are denoted by like reference numerals. 
       First Embodiment 
       [0037]      FIG. 1  is a perspective view showing a configuration of a portion of an image forming apparatus using an optical head according to a first embodiment of the invention. As shown in  FIG. 1 , the image forming apparatus includes an optical head  10 A, a condensing lens array  15  and a photoconductive drum (image carrier)  110 . The optical head  10 A includes a plurality of light emitting devices arranged in the form of an array. These light emitting devices emit light selectively depending on an image to be printed on a recording material such as a paper. A light emitting device may be any device as long as it can form an electrostatic latent image on the photoconductive drum  110 . In the embodiment, for example, an OLED (organic light emitting diode) device may be used as the light emitting device. The condensing lens array  15  is interposed between the optical head  10 A and the photoconductive drum  110 . The condensing lens array  15  includes a plurality of gradient index lenses arranged in the form of an array, with their respective optical axes oriented toward the optical head  10 A. The light emitted from the light emitting devices of the optical head  10 A passes through the gradient index lenses of the condensing lens array  15  and is focused on a surface of the photoconductive drum  110 . While the photoconductive drum  110  is rotated, an electrostatic latent image according to a desired image is formed at a predetermined exposure position on the surface of the photoconductive drum  110 . In this embodiment, the optical head  10 A include 8 k light emitting devices (k is a natural number) arranged in a main scanning direction (first direction). 
         [0038]      FIG. 2  is a block diagram showing a configuration of an exposure apparatus A using the optical head  10 A. As shown in  FIG. 2 , the exposure apparatus A includes a control circuit  50 A and the optical head  10 A. The control circuit  50 A generates output image data Dout based on input image data Din supplied from an upper level apparatus. The output image data Dout is data instructing pixels to turn-on/off according to an area ratio gray-scale method. The control circuit  50 A outputs various control signals for controlling the optical head  10 A. In this embodiment, as shown in  FIG. 3 , one block is constituted by 4×4 pixels (4 pixels in the main scanning direction (first direction) and 4 pixels in a sub scanning direction (second direction)) and represents one gray scale. 
         [0039]      FIG. 4  shows a block diagram of the optical head  10 A. The optical head  11 A includes k processing units U 1 , U 2 , . . . , Uk (k is a natural number) to which image data D 1 , D 2 , . . . , Dk as the output image data Dout are respectively supplied. The image data D 1  to Dk are respectively time-multiplexed into data d 1 , d 2 , . . . , d 8  indicating turn-on/off of 8 light emitting devices. Selection signals SEL 1  to SEL 8  are signals that become of a high level exclusively during periods in which the data d 1  to d 8  become respectively validated. 
         [0040]    Next, the processing unit U 1  will be described. Other processing units U 2  to Uk have the same structure as the processing unit U 1 . The processing unit U 1  includes two block units U 1   a  and U 1   b . Each of the block units U 1   a  and U 1   b  includes light emitting devices  32  of the same number as the number ( 4  in this embodiment) of pixels constituting the block in the main scanning direction. 
         [0041]    The block unit U 1   a  includes  4  light emitting devices  32 ,  4  driving transistors  31  and  4  driving circuits  20 A. A potential VCT is supplied to cathodes of the light emitting devices  32 , while their anodes are respectively electrically connected to drains of the driving transistors  31 . Sources of the driving transistors  31  are electrically connected to a power line Lx. The power line Lx is supplied with a power potential WEL from a power supply circuit (not shown). In this embodiment, VLL is greater than VCT. 
         [0042]    The driving circuits  20 A include first latch circuits  21 , second latch circuits  22  and inverters  23  and  24 . These circuits function as logic circuits for applying gate potential to the driving transistors  31 . This is equally applied to driving circuits  20 B of the block unit U 1   b . The selection signals SEL 1  to SEL 8  are signals that become active sequentially during a predetermined period T, as shown in  FIG. 5 . Accordingly, the output signals d 1  to d 8  of the first latch circuits  21  are in synchronization with the selection signals SEL 1  to SEL 8 . The second latch circuits  22  latch the output signals d 1  to d 8  of the first latch circuits  21  based on a latch signal LAT and generate output signals d 1 ′ to d 8 ′, respectively. 
         [0043]    The driving circuits  20 A of the block unit U 1   a  and the driving circuits  20 B of the block unit U 1   b  are the opposite of each other in terms of logic levels of signals supplied to lines Ly intersecting the power line Lx. That is, output signals of the inverters  23  are supplied to the lines Ly in the driving circuits  20 A while output signals of the inverters  24  are supplied to the lines Ly in the driving circuits  20 B. In other words, the driving circuits  20 A invert a logic level of image data an odd number of times until the image data reaches intersections of the lines Ly and the power line Lx while the driving circuits  20 B invert a logic level of image data an even number of times until the image data reaches the intersections. 
         [0044]    Parasitic capacitance C is generated in the intersections of the power line Lx and the lines Ly. Since the parasitic capacitance C acts as coupling capacitance, noise is superposed on the power line Lx in synchronization with the inverted logic level of the signal supplied the lines Ly. Here, luminance of the light emitting devices  32  depends on driving current flowing therein. Accordingly, when the noise is superimposed on the power line Lx through the parasitic capacitance C, the driving current is varied, thereby varying the luminance of the light emitting devices  32 . In this embodiment, the reason for the inversion of the logic level of the signals supplied to the lines Ly intersecting the power line Lx in the driving circuits  20 A and  20 B in the block units U 1   a  and U 1   b  is to cancel the noises superimposed on the power line Lx. 
         [0045]      FIG. 6  is an explanatory view explaining a relationship between an area gray scale and logic levels of the lines Ly in the intersections. In this figure, shaded portions represent pixels turned on by the light emitting devices  32 . As shown in this figure, one pixel in each block is turned on for area ratio gray scale  1 , while  6  pixels in each block are turned on for area ratio gray scale  6 . Here, for area ratio gray scale  1 , during a period T 2 , all logic levels of the lines Ly of the block unit U 1   a  are “L”, while all logic levels of the lines Ly of the block unit U 1   b  are “H”. During a period T 3 , one logic level of the lines Ly of the block unit U 1   a  is shifted from “L” to “H”, while one logic level of the lines Ly of the block unit U 1   b  is shifted from “H” to “L”. That is, in this embodiment, since the driving circuits  20 A and  20 B are configured in such a manner that the logic levels of the lines Ly are inverted in the units of blocks, the number of shifts of the logic levels of the lines Ly (logic levels of the intersections) from “L” to “H” becomes equal to the number of shifts of the logic levels of the lines Ly from “H” to “L”. For example, for area ratio gray scale  11 , when the period T 1  is shifted to the period T 2 , the number of shifts of the logic levels from “L” to “H” is 3, while the number of shifts of the logic levels from “H” to “L” is also 3. 
         [0046]    When the logic levels of the line Ly are shifted from “L” to “H”, pulse-shaped noise of positive polarity is generated as shown in  FIG. 7A , and, when the logic levels of the line Ly are shifted from “H” to “L”, pulse-shaped noise of negative polarity is generated as shown in  FIG. 7B . These noises cancel each other out on the power line Lx, which suppresses generation of noise. 
         [0047]    If the block unit U 1   b  is constituted by the same driving circuit  20 A as in the block unit U 1   a , a relationship between the logic levels of the lines Ly at the intersections and the area ratio gray scale is as shown in  FIG. 8 . In this case, an imbalance occurs between the logic levels of the lines Ly at portions enclosed by dotted lines. For example, during a period T 2  of area ratio gray scale  6 , “L” becomes “ 6 ”, and “H” becomes “12”. With such an imbalance, the number of shifts of the logic levels from “L” to “H” becomes unequal to the number of shifts of the logic levels from “H” to “L”, thereby increasing noise superimposed on the power line Lx. 
         [0048]    In this manner, since the optical head  10 A according to this embodiment can suppress the noise superimposed on the power line Lx, when gray scales are represented by the area ratio gray-scale method, luminance unevenness is reduced, which may result in significant improvement in print quality. 
         [0049]    In the above-described embodiment, the noise superimposed on the power line Lx is reduced by inverting the logic levels of the lines Ly in the unit of block. This is because noises cancel each other out by making the number of shifts of the logic levels from “H” to “L” equal to the number of shifts of the logic levels from “L” to “H”. When gray scales are represented by the area ratio gray-scale method, a pattern of logic levels of the lines Ly (combination of the logic levels) has blocks as basic units. From the standpoint of suppression of noise, noises may be cancelled out in any unit. Accordingly, a driving circuit may be configured in such a manner that the logic levels of the lines Ly are inverted for natural number multiples of blocks. Here, if each of the blocks includes n pixels (n is a natural number of 2 or more) in the main scanning direction (first direction) and m pixels (m is a natural number of 2 or more) in the sub-scanning direction (second direction), logic circuits of a plurality of driving circuits may be those that invert the logic levels of the signals applied to the lines Ly every n natural number of intersections extending in the main scanning direction corresponding to the blocks. For example, as shown in  FIG. 9 , the driving circuits  20 A and the driving circuits  20 B may be arranged by the unit of two blocks. In this case, noises cancel each other out by the unit of four blocks. 
       Second Embodiment 
       [0050]      FIG. 10  is a block diagram showing an exposure apparatus B according to a second embodiment of the invention. In the above-described first embodiment, the configuration to invert the logic levels of the lines Ly by the block is completed within the optical head  10 A. On the other hand, the exposure apparatus B according to the second embodiment generates output image data Dout′ whose logic levels are inverted in the unit of block in a control circuit  50 B. More specifically, as shown in  FIG. 11 , among d 1 , d 2 , d 3 , d 4 , . . . , d 8  that constitute i-th image data (1≦i≦k) in the output image data Dout of the first embodiment, the control circuit  50 B inverts d 5  to dB to generate the output image data Dout′. d 1  to d 4  that constitute image data Di′ are supplied to a block unit Uia corresponding to (2i-1)-th block, and d 5   a  to d 8   a  that constitute the image data Di′ are supplied to a block unit Uib corresponding to 2i-th block. Since d 1  to d 4  and d 5   a  to d 8   a  are data in the unit of block, the logic levels of the output image data Dout′ supplied to the optical head  10 B are inverted in the unit of block. In this case, d 1  to d 4  instruct turning-on of the light emitting devices  32  in “0” and instruct turning-off of the light emitting devices  32  in “1.” On the other hand, d 5   a  to d 8   a  instruct turning-on of the light emitting devices  32  in “1” and instruct turning-off of the light emitting devices  32  in “0”. 
         [0051]      FIG. 12  is a circuit diagram showing an optical head  10 B according to the second embodiment. The optical head  10 B has the same structure as the optical head  10 A of the first embodiment shown in  FIG. 4  except that the former uses driving circuits  20 C instead of the driving circuits  20 B that constitute the block unit U 1   b . The driving circuits  20 C have the configuration in which the inverters  23  are excluded from the driving circuits  20 B. Since the logic levels of d 5   a  to d 8   a  are the inversion of the logic levels of d 1  to d 4  as described above, the driving circuits  20 C can invert the logic levels of the lines Ly without the inverters  23 . This makes a relationship between the logic levels of the lines Ly at the intersections and the area ratio gray-scale equal to the relationship in the first embodiment shown in  FIG. 6 . 
         [0052]    According to the second embodiment, since the control circuit  50 B determines whether or not the logic levels are alternately inverted in the unit of block, it is possible to simplifying the configuration of the optical head  10 B and suppress superimposition of noise on the power line Lx, thereby significantly improving print quality. 
         [0053]    Alternatively, the control circuit  50 B may determine whether or not the logic levels are alternately inverted in the unit of natural number multiple of blocks. In this case, the driving circuits  20 C may be used in correspondence to the inversion of the logic levels. Here, if each of blocks includes n pixels (n is a natural number of 2 or more) in the main scanning direction (first direction) and m pixels (m is a natural number of 2 or more) in the sub scanning direction (second direction), the control circuit  50 B may generate the output image data Dout′ in such a manner that the logic levels of the line Ly are inverted every n natural number multiples extending in the main scanning direction corresponding to the blocks. 
         [0054]    For example, if the driving circuits  20 A and the driving circuits  20 C may be arranged by the two blocks as shown in  FIG. 13 , the control circuit  50 B may invert the logic levels by the two blocks to generate the output image data Dout′, as shown in  FIG. 14 . In this case, a relationship between the area ratio gray scale and the logic level of the lines Ly becomes equal to the relationship shown in  FIG. 9 . 
       Modification 
       [0055]    Although the above-described embodiments make an issue of parasitic capacitance at the intersections of the power line Lx and the lines Ly, the luminance of the light emitting devices  32  may depend on the gate potential of the driving transistors  31 . Accordingly, if there is provided a potential line Lz for applying the gate potential when the driving transistors  31  are turned on, parasitic capacitance at intersections of the potential line Lz and the lines Ly may be also problematic. 
         [0056]    For example, it is assumed that a light emitting device  32  is driven with a circuit configuration shown in  FIG. 15A . In this example, when the light emitting device  32  is turned on, a transistor  31  is turned on, and accordingly, a reference potential Vref is supplied to a gate of the transistor  31  through the potential line Lz, and a transistor  34  is turned off. On the other hand, when the light emitting device  32  is turned off, a transistor  33  is turned off and the transistor  34  is turned on, and accordingly, a power potential VEL is supplied to the gate of the transistor  31 . In addition, as shown in  FIG. 15B , assuming that powers to drive latch circuits  21  and  22  and inverters  23  and  24  are VDD and VSS, respectively, a relationship of VDD≧VEL≧Vref≧VSS is set. 
         [0057]    With this configuration, there exists parasitic capacitance Cl between the lines Ly and the power line Lx, and there exists parasitic capacitance C 2  between the lines Ly and the potential line Lz. Accordingly, if the logic levels of the lines Ly are changed, noise are superimposed on not only the power line Lx but also the potential line Lz. Therefore, the cancellation of noises on the power line Lx as described in the above embodiments may be equally applied to the potential line Lz. 
         [0058]    In more detail, driving circuits  20 A′  20 B′ and  20 C′ shown in  FIGS. 15B ,  15 C and  15 D, respectively, may be employed instead of the driving circuits  20 A,  20 B and  20 C described in the above embodiments, respectively. 
       Image Forming Apparatus 
       [0059]    The optical heads  10 A and  10 B according to the above-described embodiments and modification may be used as a line type optical head for forming an electrostatic latent image on an image carrier in an electrophotography-based image forming apparatus. An example of the image forming apparatus may include a printer, a printing unit of a copy machine, a printing unit of a facsimile machine, etc.  FIG. 16  is a longitudinal sectional view showing an example of an image forming apparatus using the optical head  10 A or  10 B as a line type optical head. The shown image forming apparatus is a tandem type full color image forming apparatus using a belt intermediate transfer body system. 
         [0060]    In this image forming apparatus, an array of 4 organic EL devices  10 K,  10 C,  10 M and  10 Y having the same structure are disposed at exposure oositions of 4 photoconductive drums (image carriers)  110 K,  110 C,  110 M and  110 Y having the same structure, respectively. The array of organic EL devices  10 K,  10 C,  10 M and  10 Y corresponds to the optical head  10 A or  10 B according to the above-described embodiments and modification. 
         [0061]    As shown in  FIG. 16 , this image forming apparatus is provided with a driving roller  121  and a driven roller  122 . An endless intermediate transfer belt  120  is wound on these rollers  121  and  122  and is revolved around the rollers  121  and  122  as indicated by an arrow. Although not shown, there may be provided a tension roller or the like for granting tension to the intermediate transfer belt  120 . 
         [0062]    The four photoconductive drums  110 K,  110 C,  110 M and  110 Y having photosensitive layers formed on their respective circumference are disposed with a predetermined interval therebetween around the intermediate transfer belt  120 . Suffixes K, C, M and Y are used to mean black, cyan, magenta and yellow development, respectively. This is true of other members. The photoconductive drums  110 K,  110 C,  110 M and  110 Y are rotated in synchronization with the driving of the intermediate transfer belt  120 . 
         [0063]    Corona chargers  111 K,  111 C,  111 M and  111 Y, the organic EL devices  10 K,  10 C,  10 M and  10 Y, and developing devices  114 K,  114 C,  114 M and  114 Y are disposed around the photoconductive drums  110 K,  110 C,  110 M and  110 Y, respectively. The corona chargers  111 K,  111 C,  111 M and  111 Y charge circumferences of respective photoconductive drums  110 K,  110 C,  110 M and  110 Y uniformly. The organic ETL devices  10 K,  10 C,  10 M and  10 Y form electrostatic latent images on the charged circumference of the respective photoconductive drums. The organic EL devices  10 K,  10 C,  10 M and  10 Y are installed such that an arrangement direction of a plurality of light emitting devices P lies along a parent line (main scanning direction) of the respective photoconductive drums  110 K,  110 C,  110 M and  110 Y. The electrostatic latent images are formed by irradiating the photoconductive drums with light emitted from the plurality of light emitting devices P. The developing devices  114 K,  114 C,  114 M and  114 Y develop the photoconductive drums (that is, form visible images) by attaching toner as developer to the electrostatic latent images. 
         [0064]    Developments of black, cyan, magenta and yellow images formed by 4 monochromatic development formation stations are primarily transferred into the intermediate transfer belt  120  sequentially and thus are superimposed on the intermediate transfer belt  120 , thereby obtaining a full color development. Four primary transfer corotrons (transcribers)  112 K,  112 C,  112 M and  112 Y are disposed at an inner side of the intermediate transfer belt  120 . The primary transfer corotrons  112 K,  112 C,  112 M and  112 Y are disposed near the photoconductive drums  110 K,  110 C,  110 M and  110 Y, respectively, and transfer the developments into the intermediate transfer belt  120  passing between the photoconductor drums and the primary transfer corotrons by electrostatically absorbing the developments from the photoconductive drums  110 K,  110 C,  110 M and  110 Y. 
         [0065]    Sheets  102  as objects on which images are finally formed are fed one by one from a sheet supply cassette  101  by means of a pickup roller  103  and is sent to a nip between the intermediate transfer belt  120  contacting the driving roller  121  and a secondary transfer roller  126 . Full color development on the intermediate transfer belt  120  is collectively secondary transferred into one side of the sheet  102  by means of the secondary transfer roller  126  and is fixed on the sheet  102  when the sheet  102  passes through a pair of fixing rollers  127  as a fixing unit. Thereafter, the sheet  102  is discharged to a sheet discharge cassette formed on an upper side of the apparatus. 
         [0066]    Next, an image forming apparatus according to another embodiment of the invention will be described. 
         [0067]      FIG. 17  is a longitudinal sectional view of another image forming apparatus using the optical head  10 A or  10 B as a line type optical head. The shown image forming apparatus is a rotary development type full color image forming apparatus using a belt intermediate transfer body system. In the image forming apparatus shown in  FIG. 17 , a corona charger  168 , a rotary development unit  161 , an organic EL array  167  and an intermediate transfer belt  169  are disposed around a photoconductive drum  165 . 
         [0068]    The corona charger  168  charges the circumference of the photoconductive drum  165  uniformly. The organic EL array  167  forms an electrostatic latent image on the charged circumference of the photoconductive drum  165 . The organic EL array  167  is the optical head  10 A or  10 B and is installed such that an arrangement direction of a plurality of light emitting devices P lies along a parent line (main scanning direction) of the photoconductive drum  165 . The electrostatic latent image is formed by irradiating the photoconductive drum  165  with light emitted from the plurality of light emitting devices P. 
         [0069]    The developing unit  161  is a drum having four developing devices  163 Y,  163 C,  163 M and  163 K disposed with an angular interval of 90° therebetween and can be rotated counterclockwise around an axis  161   a . The developing devices  163 Y,  163 C,  163 M and  163 K supply yellow, cyan, magenta and black toner to the photoconductive drum  165 , respectively, to develop the photoconductive drum  165  (that is, form a visible image) by attaching the toner as developer to the electrostatic latent image. 
         [0070]    The endless intermediate transfer belt  169  is wound on a driving roller  170   a , a driven roller  170   b , a primary transfer roller  166  and a tension roller and is revolved around these rollers in a direction indicated by an arrow. The primary transfer roller  166  transfers the development into the intermediate transfer belt  169  passing between the photoconductor drum and the primary transfer roller  169  by electrostatically absorbing the development from the photoconductive drum  165 . 
         [0071]    Specifically, with the first one rotation of the photoconductive drum  165 , the organic EL array  167  forms an electrostatic latent image for a yellow (Y) image on the photoconductive drum  165 , the developing device  163 Y forms the yellow development, and the yellow development is transferred into the intermediate transfer belt  169 . In addition, with the next one rotation of the photoconductive drum  165 , the organic EL array  167  forms an electrostatic latent image for a cyan (C) image on the photoconductive drum  165 , the developing device  163 C forms the cyan development, and the cyan development is transferred into the intermediate transfer belt  169  to be superimposed on the yellow development. In this manner, during four rotations of the photoconductive drum  165 , yellow, cyan, magenta and black developments are sequentially superimposed each other on the intermediate transfer belt  169 , thereby forming a full color development on the intermediate transfer belt  169 . If an image is to be formed on both sides of a sheet as an object on which the image is finally formed, the same color development of front and rear sides of the sheet is transferred into the intermediate transfer belt  169 , and thereafter, the next color development of front and rear sides of the sheet is transferred into the intermediate transfer belt  169 . In this manner, a full color development is obtained on the intermediate transfer belt  169 . 
         [0072]    The image forming apparatus is provided with a sheet transport path  174  through which sheets pass. The sheet are picked up one by one by a pickup roller  179 , travel along the sheet transport path  174  by a transport roller, and pass through a nip between the intermediate transfer belt  169  contacting the driving roller  170   a  and a secondary transfer roller  171 . The secondary transfer roller  171  transfers the full color development into one side of sheet by electrostatically absorbing the full color development from the intermediate transfer belt  169  collectively. The secondary transfer roller  171  becomes close to or distant from the intermediate transfer belt  169  by a clutch (not shown). When the full color development is transferred into the sheet, the secondary transfer roller  171  contacts the intermediate transfer roller  171 . While the transfer of development into the intermediate transfer belt  169  continues, the intermediate transfer belt  169  is separated from the secondary transfer roller  171 . 
         [0073]    The sheet onto which the image is transferred is transported to a fixing unit  172 , and the development on the sheet is fixed when the sheet passes between a heating roller  172   a  and a pressing roller  172   b  of the fixing unit  172 . The sheet after the fixing treatment is led between a pair of sheet discharge rollers  176  and travels along an arrow F. In case of double-sided print, after the most part of the sheet passes through the pair of sheet discharge rollers  176 , the pair of sheet discharge rollers  176  is reversely rotated, and the sheet is introduced into a double-sided print transport path  175  as indicated by an arrow G. Then, development is transferred into the other side of the sheet by the secondary transfer roller  171  and then is fixed by the fixing unit  172 , and then the sheet is discharged to the pair of sheet discharge rollers  176 . 
         [0074]    Since the image forming apparatuses shown in  FIGS. 16 and 17  use the light emitting devices P as an exposure means, it is possible to make the apparatuses smaller than those using a laser scan optical system. In addition, the optical head of the invention can be employed for electrophotography-based image forming apparatuses other than those described and shown in the above. For example, the optical head of the invention can be applied to image forming apparatuses of a type that development is directly transferred into a sheet from a photoconductive drum without using an intermediate transfer belt or image forming apparatuses that form a monochromatic image. 
         [0075]    In addition, the optical head of the invention is not applied to only image forming apparatuses. For example, the optical head of the invention may be employed as illuminators used for various kinds of electronic apparatuses including, for example, a facsimile machine, a copy machine, a multifunction copier, a printer, etc. Optical heads having a plurality of light emitting devices arranged in the form of a plane may be adaptively employed for these electronic apparatuses. 
         [0076]    The entire disclosure of Japanese Patent Application No: 2006-298561, filed Nov. 2, 2006 is expressly incorporated by reference herein.