Abstract:
A control method for controlling an optical head having a plurality of light emitting devices, includes: dividing the plurality of light emitting devices into a plurality of blocks by grouping adjacent light emitting devices of the plurality of light emitting devices; adjusting levels of driving signals supplied to the adjacent light emitting devices for each of the plurality of blocks so that emission luminances of the adjacent light emitting devices belonging to the each of the plurality of blocks become substantially equal to each other; and adjusting emission periods so that the equalized emission luminances of the plurality of blocks become substantially equal to each other.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a technique for correcting luminance of a plurality of light emitting devices. 
         [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 scan direction. Light emitting diodes may be used as the light emitting devices. 
         [0005]    In such an optical head, a plurality of semiconductor chips having light emitting diodes formed thereon, and a separated driving IC are mounted on a print substrate with patterned wire thereon. After manufacture, there exists nonuniformity of light emission luminance between different semiconductor chips or between individual light emitting diodes in the same semiconductor chip. Such nonuniformity of light emission luminance is not preferable for print quality since it may cause unevenness of vertical stripe shapes during printing, for example. Accordingly, driving conditions from the driving IC are finely adjusted depending on characteristics of the light emitting diodes. 
         [0006]    As correcting methods used for the optical head in the related art, the following methods are known. JP-A-6-297769 discloses a method of finely adjusting and correcting a period during which light emission current is applied. In addition, JP-A-8-39862 discloses a method of correcting a light emission driving current value using DAC. In the method disclosed in JP-A-8-39862, a plurality of correction memory circuits are provided using micromachining of a semiconductor process and a current value is finely adjusted depending on contents of a memory. 
         [0007]    Although the method disclosed in JP-A-6-297769 is based on the principle of adjusting an area of an electrostatic latent image formed on a photoconductor drum by finely adjusting a light emission period, it is necessary to operate a circuit with very high clock frequency to make such fine adjustment of the light emission period. However, it is difficult to apply this method to an optical head integrated with a driving circuit. This is because the driving circuit is formed on a substrate even larger than conventional driving ICs and thus time constant becomes large due to parasitic capacitance, thereby making it impossible to achieve high-speed operation. 
         [0008]    In addition, in the method disclosed in JP-A-8-39862, the machining precision of the process of forming a circuit on a large glass substrate is low compared with that of a semiconductor process on a silicon wafer, and thus it is difficult to mount equivalent correction memory circuits on the driving circuit-integrated optical head. 
       SUMMARY 
       [0009]    An advantage of some aspects of the invention is provide an optical head having a wide correction range with a simple structure, a control method thereof, and an image forming apparatus. 
         [0010]    According to an aspect of the invention, there is provided a control method for controlling an optical head having a plurality of light emitting devices, including: dividing the plurality of light emitting devices into a plurality of blocks by grouping adjacent light emitting devices of the plurality of light emitting devices; adjusting levels of driving signals supplied to the adjacent light emitting devices for each of the plurality of blocks so that emission luminances of the adjacent light emitting devices belonging to the each of the plurality of blocks become substantially equal to each other; and adjusting emission periods so that the equalized emission luminances of the plurality of blocks become substantially equal to each other. 
         [0011]    An optical head having a plurality of light emitting devices has a tendency that nonuniformity of emission luminance of light emitting devices becomes smaller as a distance between light emitting devices becomes smaller and becomes larger as the distance becomes larger. According to the first aspect of the invention, since the adjacent light emitting devices are grouped and divided into a plurality of blocks, intra-block nonuniformity of emission luminance is corrected by adjusting the level of driving signal, and inter-block nonuniformity of emission luminance is corrected by adjusting the emission period, it is possible to correct nonuniformity of emission luminance of the whole of the optical head. In addition, since the correction of the emission luminance is based on two factors, that is, the level of driving signal and the emission period, a range of change of the factors can be narrowed as compared to a case of correction by a single factor. 
         [0012]    Preferably, a range of change of emission luminance by adjusting the emission periods is larger than a range of change of emission luminance by adjusting the level of driving signals. As mentioned above, an optical head having a plurality of light emitting devices has a tendency that nonuniformity of emission luminance of light emitting devices becomes smaller as a distance between light emitting devices becomes smaller and becomes larger as the distance becomes larger. Accordingly, it is preferable that a range of change of emission luminance in intra-block correction is small and a range of change of emission luminance in inter-block correction is large. 
         [0013]    Preferably, the plurality of light emitting devices are arranged in one direction, and the number of the adjacent light emitting devices belonging to each of the plurality of blocks is the number added with weight on the basis of a relative position relationship of blocks in the optical head. Nonuniformity of luminance of light emitting devices is due to nonuniformity of a manufacture process. Accordingly, there exists nonuniformity depending on a relative position relationship in the plurality of light emitting devices constituting the optical head. According to this aspect of the invention, since the number of the adjacent light emitting devices belonging to each of the plurality of blocks is set depending on the relative position relationship in the optical head, width of correction for each block can become substantially uniform, thereby making it possible to reliably smooth the intra-block emission luminance while smoothing the emission luminance on the whole of the optical head. 
         [0014]    That is, in order to make the width of nonuniformity of light emitting devices in each block more uniform compared with a case where the number of light emitting devices belonging to a block is fixed, it is preferable that the number of light emitting devices belonging to the block is the number added with weight based on the relative position relationship of blocks in the optical head. 
         [0015]    For example, after manufacture, light emitting devices arranged in one direction have a tendency that nonuniformity of emission luminance becomes smaller toward the center and larger toward the end. In the mean time, a range of correctable intra-block emission luminance has a limitation. Accordingly, by increasing the number of light emitting devices belonging to a block in the center and decreasing the number of light emitting devices in the end, it is possible to reliably smooth the intra-block emission luminance while smoothing the emission luminance on the whole of the optical heads. 
         [0016]    An optical head having a plurality of light emitting devices according to a second aspect of the invention, includes: a first adjusting unit (for example, reference numerals DMP 1  to DMPn shown in  FIG. 4 ) that divides the plurality of light emitting devices into a plurality of blocks by grouping adjacent light emitting devices of the plurality of light emitting devices, and adjusts levels of driving signals supplied to the adjacent light emitting devices for each of the plurality of blocks so that emission luminances of the adjacent light emitting devices belonging to the each of the plurality of blocks become substantially equal to each other; and a second adjusting unit (for example, reference numerals U 1  to U 4  shown in  FIG. 4 ) that adjusts emission periods so that the equalized emission luminances of the plurality of blocks become substantially equal to each other. 
         [0017]    According to the second aspect of the invention, since the adjacent light emitting devices are grouped and divided into a plurality of blocks, intra-block nonuniformity of emission luminance is corrected by adjusting the level of driving signal using the first adjusting unit, and inter-block nonuniformity of emission luminance is corrected by adjusting the emission period using the second adjusting unit, it is possible to correct nonuniformity of emission luminance of the whole of the optical head. In addition, since the correction of the emission luminance is based on two factors, that is, the level of driving signal and the emission period, a range of change of the factors can be narrowed as compared to a case of correction by a single factor. 
         [0018]    Preferably, the first adjusting unit includes a plurality of unit circuits (for example, reference numerals U 1  to U 4  shown in  FIG. 4 ) provided on a one-to-one basis for the adjacent light emitting devices belonging to the each of the plurality of blocks, and the plurality of unit circuits generate driving signals (for example, reference numeral Iel shown in  FIG. 4 ) having levels defined such that luminances of the adjacent light emitting devices belonging to the each of the plurality of blocks become substantially equal to a target luminance in a time period during which driving control signals (for example, reference numerals d 11 ′ to d 14 ′ shown in  FIG. 4 ) are effective. The second adjusting unit includes a plurality of logic circuits (for example, reference numeral  22  shown in  FIG. 4 ) that supply the driving control signal to the plurality of unit circuits belonging to the each of the plurality of blocks, and each of the plurality of logic circuits calculates a logical product of a lighting control signal (for example, reference numerals d 11  to d 14  shown in  FIG. 4 ) to specify turning-on or turning-off of each of the adjacent light emitting devices belonging to the each of the plurality of blocks and an emission period control signal (for example, reference numeral E 1  shown in  FIG. 4 ) to specify the emission period set for the corresponding block and generates the driving control signal based on the calculated logical product. The emission period control signal specifies the emission period for each block of the plurality of blocks so that inter-block emission luminances become equal to each other by correcting the target luminance set for the each block of the plurality of blocks. According to this aspect of the invention, inter-block correction of emission luminance can be made by the logic circuits using the emission period control signal, and intra-block correction of emission luminance can be made by the unit circuits. 
         [0019]    Preferably, a data signal obtained by time-division multiplexing the lighting control signal for each block of the plurality of blocks is supplied. The emission period control signal is commonly supplied to the plurality of logic circuits belonging to the corresponding block. The second adjusting unit includes a plurality of latch circuits that are provided corresponding to the plurality of logic circuits belonging to the corresponding block and each generates the light control signal by latching the data signal. According to this aspect of the invention, since a common emission period control signal is supplied to the blocks and the logic circuits calculate the logical product of the lighting control signal and the emission period control signal to generate the driving control signal, it is possible to adjust the intra-block emission luminance. 
         [0020]    Preferably, a data signal including a plurality of the lighting control signals and obtained by time-division multiplexing the lighting control signal for each block of the plurality of blocks is supplied. The second adjusting unit includes a plurality of latch circuits that are provided corresponding to the plurality of logic circuits belonging to the corresponding block and generate the plurality of the lighting control signals by latching the data signal based on a plurality of selection signals for selecting the plurality of the lighting control signals included in the data signal. The emission period control signal corresponding to one of the plurality of blocks is constituted by individual emission period control signals corresponding to the plurality of unit circuits belonging to the one of the plurality of blocks. The plurality of individual emission period control signals (for example, reference numerals E 11  to E 14  shown in  FIG. 8 ) corresponding to one of the plurality of blocks (for example, reference numeral B 1  shown in  FIG. 8 ) have a substantially same length of specified emission period and have different startings of the emission period depending on corresponding selection signals (for example, reference numerals SEL 1 ′ to SEL 4 ′ shown in  FIG. 8 ). In this case, since the individual emission period control signals are used to set startings of the emission period according to the selection signals, when it is assumed that a period during which one gray scale is represented is a unit period, it is possible to increase a ratio of the emission period to the unit period. As a result, it is possible to decrease a level of driving signal required to obtain desired emission luminance, thereby making it possible to simplify a configuration of a unit circuit. 
         [0021]    An image forming apparatus according to a third aspect of the invention includes: an optical head according to the second aspect; and an image carrier on which an image is formed by light emitted from the optical head. With the third aspect, the effects of the first and second aspects can be achieved. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0023]      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. 
           [0024]      FIG. 2  is a plan view showing a configuration of the optical head. 
           [0025]      FIG. 3  is a block diagram showing a configuration of an exposure apparatus. 
           [0026]      FIG. 4  is a circuit diagram showing a configuration of the optical head. 
           [0027]      FIG. 5  is a timing chart showing an operation of the optical head. 
           [0028]      FIG. 6  is an explanatory view explaining a relationship between intra-block correction and inter-block correction. 
           [0029]      FIG. 7  is an explanatory view explaining intra-block correction in a comparative example. 
           [0030]      FIG. 8  is a block diagram showing a configuration of an exposure apparatus according to a second embodiment of the invention. 
           [0031]      FIG. 9  is a circuit diagram showing a configuration of an optical head. 
           [0032]      FIG. 10  is a timing chart showing an operation of the optical head. 
           [0033]      FIG. 11  is a longitudinal sectional view showing a configuration of an image forming apparatus using an optical head according to an embodiment of the invention. 
           [0034]      FIG. 12  is a longitudinal sectional view showing a configuration of another image forming apparatus using an optical head according to an embodiment of the invention. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0035]    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 
       [0036]      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 if only 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 directing to 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 . 
         [0037]      FIG. 2  shows a rough mechanical configuration of an optical head  10 A. As shown in  FIG. 2 , the optical head  10 A includes a plurality of light emitting devices  36  and a driving circuit  60  formed on a glass substrate  70 . The plurality of light emitting devices  36  is arranged in a main scan direction and is divided into n blocks (n is an integer of 2 or more). In this embodiment, the number of light emitting devices  36  belonging to each block B 1  to Bn. That is, in this embodiment, the optical head  10 A includes in light emitting devices (n is a natural number) arranged in the main scan direction. 
         [0038]      FIG. 3  is a block diagram of an exposure apparatus A using the optical head  10 A. As shown in  FIG. 3 , the exposure apparatus A includes a control circuit  50 A and the optical head  10 A. The control circuit  50 A generates signals D 1  to Dn, selection signals SEL 1  to SEL 4  and emission control signals E 1  to En corresponding to the blocks B 1  to Bn, respectively, based on input image data Din supplied from a high level apparatus. Assuming that 1≦i≦n (i is an integer), a data signal Di is a signal obtained by time-division multiplexing a lighting control signal to specify light-on or light-off of four light emitting devices  36  belonging to an i-th block Bi. 
         [0039]    The selection signals SEL 1  to SEL 4  are signals used for a demultiplexing process of the data signal Di. 
         [0040]    Lighting control signals corresponding to the four light emitting devices  36  are obtained by latching the data signal Di using the selection signals SEL 1  to SEL 4 . The emission control signals E 1  to En are signals to specify an emission period for each block. As will be described in detail, in this embodiment, a level of driving current of the light emitting devices  36  is adjusted to meet target luminance defined for each block, and the emission period for each block is controlled to make emission luminance between blocks equal. 
         [0041]      FIG. 4  is a block diagram of the optical head  10 A. The optical head  10 A includes n demultiplexers DMP 1  to DMPn corresponding to n blocks B 1  to Bn, and four unit circuits U 1  to U 4  for each block. Although  FIG. 4  shows details of the demultiplexer DMP 1  and the unit circuits U 1  to U 4  for the block B 1 , this may be equally applied to other blocks B 2  to Bn. 
         [0042]    The demultiplexer DMP 1  includes four latch circuits  21  and four NAND circuits  22 . The latch circuits  21  latch the data signal D 1  by the selection signals SEL 1  to SEL 4 , respectively. As shown in  FIG. 5 , the data signal D 1  is supplied to the optical head  10 A from the start of a unit period T, and the selection signals SEL 1  to SEL 4 , which become active sequentially in synchronization with the data signal D 1 , are supplied to the optical head  10 A. As the latch circuits  21  latch the data signal D 1  in a period during which the selection signals SEL 1  to SEL 4  become active, lighting control signals d 11  to d 14  are obtained as shown in  FIG. 5 . 
         [0043]    The lighting control signals d 11  to d 14  are respectively supplied to one input terminals of the four NAND circuits  22 , and an emission period control signal E 1  is commonly supplied to the other input terminals of the NAND circuits  22 . Accordingly, emission periods TEL of the four light emitting devices  36  belonging to the block B 1  become equal to each other. In addition, the emission period control signal E 1  becomes active in periods other that the period during which one of the selection signals SEL 1  to SEL 4  becomes active. In this embodiment, assuming that a period during which one gray scale is represented is T, the selection signals SEL 1  to SEL 4  are collected at the start of the unit period T as shown in  FIG. 5 . This is for preventing an error signal in a sampling period during which the selection signals SEL 1  to SEL 4  become active from being reflected on a lighting operation. That is, the emission period control signal E 1  provided for correction has an effect of prevention of erroneous lighting as well. 
         [0044]    The unit circuit U 1  includes a driving transistor  35 , a light emitting device  36  and a current digital-analog converter (current DAC). On/Off of the driving transistor  35  is controlled by a driving control signal d 11 ′ having an emission period limited by the emission period control signal E 1 . The effective emission luminance of the light emitting device  36  is defined by the product of an emission period and intensity of driving current Iel supplied from the driving transistor  35 . The intensity of driving current Iel is defined by the current DAC. 
         [0045]    The current DAC includes transistors  31  to  34  and memories M 1  to M 4 . In this embodiment, the size of the transistors  31  to  34  is set to be a ratio of 1:2:4:8. In the memories M 1  to M 4  are stored a potential turning the transistors  31  to  34  into an off state and a potential turning the transistors  31  to  34  into an on state, respectively. The potential turning the transistors  31  to  34  into the on state is common for the memories M 1  to M 4 . Accordingly, the intensity of driving current Iel is controlled with 4-bit correction data stored in the memories M 1  to M 4 . In this embodiment, the correction data are stored in the memories M 1  to M 4  to meet target luminance defined for each block. 
         [0046]    The reason for two-step correction of smoothness of intra-block emission luminance and smoothness of inter-block emission luminance is as follows. That is, although nonuniformity of the emission luminance of the light emitting devices  36  is large on the whole of the optical head  10 A, there is little possibility that maximum and minimum values of nonuniformity are locally concentrated. In other words, although nonuniformity of the optical head  10 A is large, nonuniformity in divided blocks is small. Accordingly, intra-block emission luminance nonuniformity is corrected using the current DAC having a relatively small number of bits, and inter-block emission luminance nonuniformity is corrected by adjusting an emission period. That is, relatively small intra-block emission luminance nonuniformity is corrected by adjusting the intensity of driving current, while relatively large inter-block emission luminance nonuniformity is corrected by adjusting the emission period. 
         [0047]    If the emission luminance of the whole of the optical head  10 A is to be uniformly corrected by adjusting only the intensity of driving current, it is required to significantly increase the number of bits of the current DAC, thereby needing a large scale circuit. If the emission luminance of the whole of the optical head  10 A is to be uniformly corrected by adjusting only the length of the emission period, it is required to perform a high speed sampling operation of the data signals or provide an emission period control signal for each light emitting device. However, since the length of the optical head  10 A is defined by the width of print sheet, the length becomes large as compared to semiconductor integrated circuits. Accordingly, it is required to draw various signal lines such as lines for the selection signals SEL 1  to SEL 4  by a long distance, thereby making it difficult to transfer data at a high speed. In addition, considering the number of terminals, it is actually impossible to provide the emission period control signal for each light emitting device. On the contrary, in the optical head  10 A of this embodiment, since two kinds of correction, that is, the intra-block correction and the inter-block correction, are made, it is possible to make the emission luminance of the whole of the optical head  10 A uniform with a simple structure. 
         [0048]    Now, a process of emission luminance correction will be described in more detail with reference to  FIG. 6 . In this embodiment, it is assumed that the number of blocks of the optical head  10 A is  4 . Assuming that an upper limit of an emission luminance distribution is 100, the 4-bit current DAC can correct an output current value up to about 66 to 100 or so. This is an example where a current source by a transistor having a channel width corresponding to 3.3% of a channel width of a transistor constituting a basic current source is the gray scale current unit. In general, it is preferable that emission luminance after correction falls within a range of ±2% or so of a target value for a printer light source. In this embodiment, the emission luminance nonuniformity before correction in the optical head  10 A is  50  in its minimum value and 100 in its maximum value. If the whole of the optical head  10 A is corrected by only the 4-bit current DAC with the emission period fixed, there exists uncorrectable place as shown in  FIG. 7 . 
         [0049]    In the mean time, in the configuration of this embodiment, as shown in  FIG. 6 , the maximum and minimum values of the emission luminance for the block B 1  are 75 and 50, respectively, and a correctable lower limit is about 49. Accordingly, intra-block smoothness is possible with the 4-bit current DAC. For the block B 2 , the maximum and minimum values of the emission luminance are 81 and 60, respectively, a correctable lower limit is about 53, and accordingly, intra-block smoothness is possible with the 4-bit current DAC. For the block B 3 , the maximum and minimum values of the emission luminance are 100 and 71, respectively, a correctable lower limit is about 66, and accordingly, intra-block smoothness is possible with the 4-bit current DAC. For the block B 4 , the maximum and minimum values of the emission luminance are 86 and 71, respectively, a correctable lower limit is about 55, and accordingly, intra-block smoothness is possible with the 4-bit current DAC. Here, it is assumed that a period corresponding to 50% of one unit period T in each block B 1  to B 4  is set to be the emission period TEL. In addition, for each block B 1  to B 4 , it is assumed that the driving current Iel is adjusted with the maximum value of the emission luminance distribution before correction as target luminance. 
         [0050]    Next, the inter-block emission period control signals E 1  to E 4  are adjusted and the emission period TEL is adjusted. 50% emission period TEL for the block B 1  is reset to be about 66.7%, 50% emission period TEL for the block B 2  is reset to be about 62.5%, 50% emission period TEL for the block B 3  remains unchanged, and 50% emission period TEL for the block B 4  is reset to be about 58.1%. This allows the effective emission luminance to be substantially uniformly corrected on the whole of the optical head  10 A. In addition, if the emission luminance of the whole of the optical head  10 A is to be varied by X %, the emission period TEL for each emission period control signal E 1  to En may be varied by X %. 
         [0051]    In this embodiment, a range of change of the emission luminance by the adjustment of the emission period TEL is set to increase as compared to a range of change of the emission luminance by the adjustment of the driving current Iel. This allows the relatively small nonuniformity of the intra-block emission luminance to be smoothed with relatively small number of bits and allows the relatively large nonuniformity of the inter-block emission luminance to be corrected by adjusting the emission period TEL. 
       Second Embodiment 
       [0052]    In the above-described first embodiment, the control circuit  50 A concentrates the period during which the selection signals SEL 1  to SEL 4  become active at the start of the unit period T representing one gray scale, and assigns the emission period TEL to the period during which the selection signals SEL 1  to SEL 4  become inactive during the unit period T. Accordingly, assuming that the period during which the selection signals SEL 1  to SEL 4  become active is ΔT, a ratio of the emission period TEL to the unit period T is limited to (T−4ΔT)/T at the maximum. In the second embodiment, the ratio of the emission period TEL to the unit period T is increased as compared to the first embodiment. 
         [0053]      FIG. 8  is a block diagram of an exposure apparatus according to the second embodiment. A control circuit  50 B generates selection signals SEL 1 ′ to SEL 4 ′ distributed in the unit period T and generates emission period control signals E 11  to E 14 , E 21  to E 24 , . . . , and En 1  to En 4  corresponding to the blocks B 1  to Bn. 
         [0054]      FIG. 9  is a block diagram of an optical head  10 B according to the second embodiment, and  FIG. 10  is a timing chart thereof. The optical head  10 B has the same structure as the optical head  10 A of the first embodiment shown in  FIG. 4 , except for detailed configuration of demultiplexers DMP 1 ′ to DMPn′. For the demultiplexer DMP 1 ′, the data d 11  to d 14  are respectively supplied to one input terminals of the NAND circuits  22 , and the emission period control signals E 11  to E 14  are respectively supplied to the other input terminals of the NAND circuits  22 . For these emission period control signals E 11  to E 14 , a start timing of the emission period TEL is set in synchronization with falling of the selection signals SEL 1 ′ to SEL 4 ′, and intervals from the start of the emission period TEL to the end thereof are set to be equal to each other. Here, assuming that a ratio of the period ΔT during which the selection signals SEL 1 ′ to SEL 4 ′ becomes active to the unit period T is 10%, the emission period TEL can be set to be 90% at the maximum as shown in  FIG. 10 . 
         [0055]    In the first embodiment, since the emission period TEL is assigned in the latter half portion of the unit period T, it was required to make the ratio of the emission period TEL to the unit period T small, and thus it was required for each light emitting device  36  to supply large current to obtain desired emission luminance. On the contrary, in the second embodiment, since the ratio of the emission period TEL to the unit period T can be increased, it is possible to make the intensity of driving current supplied to the light emitting devices  36  small to obtain the desired emission luminance. As a result, it is possible to make the size of the transistors  31  to  36  constituting the unit circuits U 1  to U 4  small. 
       Modification 
       [0056]    The invention is not limited to the above-described embodiment, but may be modified as follows. 
         [0057]    (1) Although the OLED devices were employed as an example of the light emitting devices  36  in the above-described embodiments, the light emitting devices  36  may be inorganic light emitting diodes. In a word, the light emitting devices  36  may be any devices as long as they can emit light with emission luminance depending on a level of driving signal. For example, the light emitting devices  36  may include field emission devices (FEDs), surface-conduction electron-emitter devices (SEDs), ballistic electron surface emitting devices (BSDs), etc. 
         [0058]    (2) Although it has been illustrated in the above-described embodiments that the number of light emitting devices  36  included in each block B 1  to Bn, the number of light emitting devices  36  may be two or more. In addition, the number of light emitting devices  36  included in each block B 1  to Bn may not be necessarily identical. Light emitting devices  36  arranged in one direction have a tendency that nonuniformity of emission luminance becomes smaller toward the center and larger toward the end. In the mean time, a range of correctable intra-block emission luminance has a limitation. Accordingly, by increasing the number of light emitting devices belonging to a block in the center and decreasing the number of light emitting devices in the end, it is possible to reliably smooth the intra-block emission luminance while smoothing the emission luminance on the whole of the optical heads  10 A and  10 B. That is, in two adjacent blocks of the plurality of blocks B 1  to Bn, it is preferable that the number of light emitting devices  36  belonging to a block at the center side is larger than the number of light emitting devices  36  belonging to a block at the end side. 
       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. 11  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 positions 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. 11 , 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  11 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 EL 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 scan 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. 12  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. 12 , 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 scan 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. 11 and 12  use the light emitting devices  36  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: 2006-306253, filed Nov. 13, 2006 is expressly incorporated by reference herein.