Abstract:
In a line head, a plurality of element arrays arranged in a first direction. Each array includes a plurality of light emission elements arrayed in a second direction which is perpendicularly to the first direction. The light emission elements emit light for forming an electrostatic talent image on a photosensitive surface of an image carrier. A switcher activates the light emission elements in at least one of the element arrays while deactivating the others. A developer develops the latent image as a visible image with toner.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to a line head in which a plurality of light emission elements are arrayed is used as an exposer for forming a latent image to be developed, which is incorporated in an image forming apparatus using an electrophotographic process. 
         [0002]    Japanese Patent Publication No. 4-363264A discloses a line head in which a plurality of electroluminescence (EL) elements are arrayed to form a single line, and teaches that an interval between pulses for driving the elements to emit light is controlled so as to elongate the life of the elements. Japanese Patent Publication No. 3-101366A discloses a line head in which a number of EL elements ere arrayed to form a single line, one auxiliary pulse for driving all the elements to emit light is necessarily incorporated irrespective of main driving pulses in each primary scanning operation, so that predetermined light intensity can be obtained in a short time even when the devices have not emitted light for a long time. 
         [0003]      FIG. 20  shows one example of a lime-to-luminance characteristic of an EL element. The abscissa is used for a time (minutes) and the ordinate is used for luminance. As is shown, luminance decreases with an elapse of time since the EL element has activated (by about 20% in five minutes). The luminance is restored if the EL element is deactivated for a predetermined time period to decrease the element temperature, and is activated again. To simplify the explanation, the deactivated period is omitted from  FIG. 20 . 
         [0004]    Each of the line heads disclosed In the above publications is provided with a single array of the EL elements. Hence, if such a line head is subjected to continuous activation for a long time period, the luminance decreases as shown in  FIG. 20 , thereby deteriorating the quality of an image to be obtained. In addition, since the degree of the luminance decrease in one element is actually different from another element, the deterioration of the obtained image quality becomes remarkable after the long-term activation. Further, since the life of the EL element is shortened by such a continuous activation, the replacement of the line head is hastened. 
         [0005]    Moreover, at the occurrence of an unexpected event due to a failure in any of the light emission elements or the like, the line head has to be replaced, which poses a problem that the replacement is tedious and the cost is thereby increased. 
       SUMMARY OF THF INVENTION 
       [0006]    It is therefore an object of the invention to provide a line head capable of addressing such an unexpected event, avoiding such image deterioration due to the long-term activation, and prolonging the lifetime thereof. 
         [0007]    It is also an object of the invention to provide an image forming apparatus incorporating such a line head. 
         [0008]    In order to achieve the above objects, according to the invention, there is provided a line head, comprising: 
         [0009]    a plurality of element arrays arranged in a first direction, each array including a plurality of light emission elements arrayed in a second direction which is perpendicularly to the first direction; and 
         [0010]    a first switcher, which activates the light emission elements in at least one of the element arrays while deactivating the others. 
         [0011]    Preferably, at least one of the element arrays is used for a backup purpose. In this case, the image forming operation can be continued without replacing the line head, even if a failure is occurred on one of the element arrays. 
         [0012]    Preferably, each of the element arrays is divided into a plurality of blocks; and the line head further comprises a second switcher which activates the light emission elements in at least one of the blocks in the at least one element array activated by the first switcher. 
         [0013]    In this case, since only the block in the element array can be activated in accordance with the required area of the image formation, economic light emission can be realized. 
         [0014]    Here, it is preferable that a third switcher activates at least one of the light emission elements in the at least one block activated by the second switcher. 
         [0015]    In this case, the activation/deactivation and the activated time period of the respective light emission elements can be independently controlled. 
         [0016]    Alternatively, the second switcher may activate at least one of the light emission elements in the at least one element array activated by the first switcher. 
         [0017]    Preferably, each of the light emission elements is provided as either an organic electroluminescence element or a light emitting diode. 
         [0018]    In the former case, since the organic electroluminescence element can be controlled statically, the control system can be simplified. In the latter case, the manufacturing process for the light emission element can be simplified. 
         [0019]    Here, it is preferable that the first switcher controls a potential at one of a cathode or an anode of each of the light emission elements. Here, a potential at the other one of the cathode or the anode is fixed. In this case, the switching operation can be simplified. 
         [0020]    Preferably, the first switcher comprises switching transistors, in this case, the switching between the element arrays can be executed rapidly and accurately. 
         [0021]    Here, it is preferable that: each of the element arrays is associated with one of the switching transistors; and the first switcher simultaneously generates a first signal for activating the switching transistor and a second signal for deactivating the switching transistor; so that the first signal is inputted to the switching transistor for the at least one element arrays to be deactivated, while the second signal is inputted to the switching transistor for the others to be deactivated. 
         [0022]    In this case, the signal generation for the switching between the element arrays can be simplified. 
         [0023]    Alternatively, it is preferable that the element arrays includes at least one first element array and at least one second element array; the switching transistors includes a first transistor associated with the first element array and a second transistor associated with the second element array and haying a conductive type which is opposite to the first transistor; and the first switcher inputs a common signal to the first transistor and the second transistor, so that one of the first transistor and the second transistor is activated while the other one is deactivated. 
         [0024]    In this case, the wiring for providing the signals for switching between the element arrays can be simplified. 
         [0025]    Preferably, the first switcher sequentially activate at least one of the element arrays when a predetermined requirement is satisfied. 
         [0026]    In this case, the imago forming operation can be continued without replacing the line head, even if the luminance of the light emission element in the activated element array decreases due to the long-term continuous activation. 
         [0027]    Preferably, the first switcher sequentially activate at least one of the element arrays with a predetermined order. 
         [0028]    In this case, since each of the element arrays intermittently activated, not only the luminance decrease of the light emission element due to the long-term continuous activation can be avoided, but also the lifetime of the line head can be prolonged. 
         [0029]    Preferably, the first switcher sequentially activate at least one of the element arrays in accordance with an image to be formed. 
         [0030]    In this case, for example, the element array in which the luminance of the light emission elements decreases can be used for an image with less data such as a binary data imago. Accordingly, not only the deterioration of the obtained image quality can be avoided, but also the lifetime of the line head can be prolonged. 
         [0031]    Here, it is preferable that the element arrays includes at least one first element array for forming a binary data image, and at least one second element array for forming a gradation data image. 
         [0032]    In this case, the element array in which the luminance of the light emission elements decreases can be used to form the binary data image to maintain the obtained image quality. 
         [0033]    Alternatively, it is preferable that the element arrays includes at least one first element array capable of forming both of a binary data image and a gradation image, and at least one second element array for forming only the gradation data image. 
         [0034]    In this case, the restoration of the decreased luminance of the light emission elements in the first element array can be secured without slopping the image forming operation because the first array may be used to form the binary data image until the luminance is restored to a level enough to form the gradation data image. 
         [0035]    According to the invention, there is also provided an image forming apparatus, comprising: 
         [0036]    an image carrier, having a photosensitive surface; 
         [0037]    a line head, comprising:
       a plurality of element arrays arranged in a first direction, each array including a plurality of light emission elements arrayed in a second direction which is perpendicularly to the first direction, to emit light for forming an electrostatic latent image on the photosensitive surface; and   a first switcher, which activates the light emission elements in at least one of the element arrays while deactivating the others; and       
 
         [0040]    a developer, which develops the latent image as a visible image with toner. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein: 
           [0042]      FIG. 1  is a diagram for explaining an operation of a line head according to a first embodiment of the invention; 
           [0043]      FIG. 2  is a block diagram of a peripheral configuration of the line head of  FIG. 1 ; 
           [0044]      FIG. 3  is a circuit diagram of the line head of  FIG. 1 ; 
           [0045]      FIG. 4  is a circuit diagram of a line heed according to a second embodiment of the invention; 
           [0046]      FIG. 5  is a circuit diagram of a line head according to a third embodiment of the invention; 
           [0047]      FIG. 6  is a circuit diagram of a line head according to a fourth embodiment of the invention; 
           [0048]      FIG. 7  is a circuit diagram of a line head according to a fifth embodiment of the invention; 
           [0049]      FIG. 8  is a circuit diagram of a line head according to a sixth embodiment of the invention; 
           [0050]      FIG. 9  is a circuit diagram of a line head according to a seventh embodiment of the invention; 
           [0051]      FIG. 10  is a diagram of a line head according to an eighth embodiment of the invention; 
           [0052]      FIG. 11  is a diagram for explaining an operation of the line head of  FIG. 10 ; 
           [0053]      FIG. 12  is a circuit diagram of a line head according to a ninth embodiment of the invention; 
           [0054]      FIG. 13  is a time chart for explaining an operation of the line head of  FIG. 12 ; 
           [0055]      FIG. 14  is a table for explaining an operation of the line head of  FIG. 12 ; 
           [0056]      FIG. 15  is a diagram for explaining an operation of a line head according to a tenth embodiment of the invention; 
           [0057]      FIG. 16  is a diagram for explaining an operation of a line head according to an eleventh embodiment of the invention; 
           [0058]      FIG. 17  is a block diagram of a peripheral configuration of the line head of  FIG. 15  or  16 ; 
           [0059]      FIGS. 16 and 19  are schematic views showing examples of an image forming apparatus incorporating the line head according to any one of the above embodiments; and 
           [0060]      FIG. 20  is a graph showing a time-luminance characteristic of an electroluminescence element. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0061]    Embodiments of the invention will be described below in detail with reference to the accompanying drawings. 
         [0062]    As shown in  FIG. 10 , in a line head  10  according to a first embodiment of the invention, a number of light emission elements La are arrayed in a primary scanning direction (X direction) to form an element array, and a plurality of elements arrays  1   a  through  1   d  are arranged in a secondary scanning direction (Y-direction). Beside the EL element, a light emitting diode (LED) can be adopted as the light emission element La. 
         [0063]    In this embodiment, the element arrays  1   b  through  1   d  are provided as the element arrays for a backup operation. When the light emission elements La in the element array  1   a  for a normal operation have been activated for a predetermined time, the element array  1   b  for a backup operation is caused to emit light. 
         [0064]    As described the above, the element array provided for the backup operation is not limited to one array. The element array  1   c  can be used as the backup array for the element array  1   b . Similarly, the element array  1   c  can be used as the backup array for the element array  1   c . The arrays  1   c  and  1   d  may be used for arrays for performing multiple exposure. 
         [0065]    The line head of the invention is configured such that at least one array among a plurality of arrays arranged in the secondary scanning direction is used for the backup operation. In a case where three arrays of EL elements are provided, at least one array is used for the backup operation. That is, one array may be used as a normal operation while two arrays may be used as the backup operation. Alternatively, two arrays may be used as the normal operation while one array may be used as the backup operation. 
         [0066]      FIG. 1  shows how to operate the line head  10  in this embodiment. In order to simplify the explanation, there will be described a case where only the two arrays  1   a  and  1   b  are operated. In this case, all the light emission elements in the element array  1   a  are activated from the time points  0  to ta. All the light emission elements in the element array  1   b  are deactivated. At the time point ta, the element array  1   a  is deactivated, and switching is made for the element array  1   b  to be activated. Thereafter, the element array  1   a  and the element array  1   b  are activated alternately at the time points tb, tc, td, te, and tf. 
         [0067]    Since the two element arrays are alternately activated at a predetermined cycle so that each element array is intermittently activated, the luminance decrease due to the long term continuous activation can be avoided. Hence, not only can deterioration of the obtained image quality be prevented, but also the life of the line head can be prolonged. 
         [0068]    The switching timings for the respective element arrays, that is, the activation time period and the deactivation time period, are set by taking various factors into account. In other words, optimal timings are set on the basis of a temperature characteristic of light emission element materials, a heat-releasing characteristic of a panel of the line head to which the light emission elements are attached, etc. 
         [0069]    As shown in  FIG. 2 , the line head  10  is incorporated in a control section  20  of an image forming apparatus. A host controller  23  for generating image data, which is constituted by a computer, for example, is connected to the image forming apparatus. The control section  20  comprises a switching circuit  21 , a counter  24 , and a memory  25  in addition to the line head  10 . The counter  24  counts the activated time period of each element array and stores the counted value into the memory  25 . The memory  25  stores the prescribed switching timing in advance. 
         [0070]    The host controller  23  judges whether the activated time period of one element array reaches the switching timing stored in the memory  25 , if it reaches, a switching signal is transmitted to the switching circuit  21  to deactivate the one element array while activating another element array in the line head  10 . 
         [0071]    In this embodiment, the host controller  23  controls the counter  24  and the memory  25 . However, a controller such as a CPU for controlling the counter  24  and the memory  25  may be provided in the control section  20 ; such that the controller is formed on a substrate on which the switching circuit  21  is provided. In this case, the configuration of the control system can be compact and the host controller  23  is not involved. Hence, the wiring can be shorter, which can in turn increase the processing speed. 
         [0072]      FIG. 3  shows a circuit for switching the element array to be activated, The line head  10  is provided with element arrays  1   a  and  1   b . In the element array  1   a  are arrayed light emission elements D 00  through D 23  each comprising, for example, an EL element. Also, in the element array  1   b  are arrayed light emission elements D 50  through D 73  each comprising an EL element. 
         [0073]    A positive power supply line  4  is commonly connected to anodes of the respective light emission elements in the element arrays  1   a  and  1   b . A negative power supply line  5  is connected to cathodes of the respective light emission elements in the element array  1   a , and a negative power supply line  6  is connected to cathodes of the respective light emission elements in the element array  1   b . In other words, the element array  1   a  is connected between the power supply lines  4  end  5 , so that a DC voltage is applied. Also, it is configured in such a manner that the element array  1   b  is connected between the power supply lines  4  and  6 , so that a DC voltage is applied. It should be noted that in the actual line head  10 , control circuits to activate individual light emission elements may be provided between the anodes of the respective light emission elements and the positive power supply line  4 . 
         [0074]    When a contact  3   c  of a switch  3  is placed on a contact point  3   a , a DC voltage is applied between the power supply lines  4  and  5 , which causes the respective light emission elements D 00  through D 23  in the element array  1   a  to activate. When the contact  3   c  of the switch  3  is placed on a contact point  3   b , a DC voltage is applied between the power supply lines  4  and  6 , which causes the respective light emission elements D 50  through D 73  in the element array  1   b  to activated. 
         [0075]    The element array  1   a  is provided for a normal operation, and the element array  1   b  is provided for a backup operation. In the event that luminance of the element array  1   a  decreases, a voltage is applied to the respective light emission elements D 50  through D 73  in the element array  1   b  by the switch  3  so that they are activated. That is, in this embodiment, the element arrays are switched by the switch  3 , by switching the power supply lines  5  and  6 , to which are commonly connected the cathodes of the light emission elements in the respective element arrays. In this instance, the positive power supply line  4  is commonly connected to the anodes of the light emission elements in the respective element arrays. In other words, because only the power supply lines connected to one polarity of the light emission elements are switched, the configuration of the switching circuit can be simpler than in a case where the power supply lines of the both polarities are switched. 
         [0076]    The switch  3  is embodied by a mechanical switch as described the above, however, it may be embodied by an electronic switch such as a transistor. Either one of the element array  1   a  and the element array  1   b  is used for the normal operation and the other for the backup operation, and it is therefore possible to use the element array  1   b  for the normal operation and the element array  1   a  for the backup operation. 
         [0077]      FIG. 4  shows a second embodiment of the invention. The same components are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0078]    In a line head  10   a , a drive transistor Tr 2  is connected to the anode of each light emission element, and the source of a control transistor Tr 1  is connected to the gate of the drive transistor Tr 2 . The drive transistor Tr 2  and the control transistor Tr 1  are, for example, FFTs (Field Effect Transistors). 
         [0079]    In a case where the contact  3   c  of the switch  3  is placed on the contact point  3   a  so that a voltage is thereby applied between the power supply lines  4  and  5 , when the control transistors Tr 1  are activated by a control signal sent from a control circuit  7 , the drive transistors Tr 2  become conductive, which causes the respective light emission elements D 00  through D 23  in the element array  1   a  to emit light. When the contact  3   c  of the switch  3  is placed on the contact point  3   b , switching is made for the respective light emission elements in the element array  1   b  to be activated. That is, as in the first embodiment, this embodiment also switches the element arrays by controlling the anodes of the light emission elements. 
         [0080]    Since the control transistors Tr 1  are connected in series to the drive transistors Tr 2  connected to the respective light emission elements, it is possible to select individual light emission elements in the respective element arrays  1   a  and  1   b  for emitting light. Therefore, it is possible to meet various needs for image formation. 
         [0081]      FIG. 5  shows a third embodiment of the invention. The same components are designated by the same reference numerals and repetitive explanations will he omitted. 
         [0082]    In a line head  10   b , switching transistors Tr 3  and Tr 4  for switching element arrays are connected in series to the drive transistors Tr 2  common to the element arrays  1   a  and  1   b . Signal lines  8  and  9  through which selection signals Se 11  and Se 12  for the element arrays are fed. An inverter INV outputs a selection signal Se 12 , which is an inverted selection signal Se 11 , to be fed to the signal line  9 . Herein, the selection signal Se 12 , which is an inverted selection signal Se 11 , may be externally fed to omit the inverter INV. 
         [0083]    The positive power supply line  4  is commonly connected to the anodes of the respective light emission elements in the element arrays  1   a  and  1   b . Also, the negative power supply line  5  is connected to the cathodes of the respective light emission elements in the element array  1   a , and the negative power supply line  6  is connected to the cathodes of the respective light emission elements in the element array  1   b . The negative power supply lines  5  and  6  are maintained in a connected state at a common potential. 
         [0084]    When the control transistors Tr 1  are activated by a control signal from the control circuit  7 , the drive transistors Tr 2  become conductive. When the selection signal Se 11  is fed to the gates of the transistors Tr 3  for switching the element arrays from the signal line  8  in this state, the respective light emission elements in the element array  1   a  are activated. In this instance, because no selection signal Se 12  is fed from the signal line  9 , the light emission elements in the element array  1   b  are kept deactivated. 
         [0085]    When the selection signal Se 11  from the signal line  8  is turned off and the selection signal Se 12  from the signal line  9  is turned on, the switching transistors Tr 3  are shut and transistors Tr 4  become conductive. The light emission elements in the element array  1   a  are thereby deactivated, and instead the light emission elements in the element array  1   b  are activated. In other words, the transistors Tr 3  and Tr 4  are activated by the selection signals Se 11  and Se 12  from the signal lines  8  and  9 , respectively, to control the switching of the respective element arrays  1   a  and  1   b  at the cathodes. 
         [0086]    Since the switching of the element arrays is performed by the transistors Tr 3  and Tr 4 , it is possible to achieve a rapid and reliable switching operation in comparison with a mechanical switch, in addition, since the element arrays are switched by the signals from the signal lines without switching the polarities of the power supply lines, it is possible to prevent the occurrence of an instantaneous voltage change associated with the switching, which can in turn prevent damages to the light emission elements. 
         [0087]    In a case where the light emission elements comprise organic EL elements and the switching transistors Tr 3  and Tr 4  comprise TFTs (Thin Film Transistors), it is possible to assemble the line head  10   b  using the same fabrication technique for both the switching transistors and the tight emission elements. The manufacturing costs, therefore, can be saved Also in this embodiment, it is possible to activate or deactivate the individual light emission elements in the respective element array, by controlling the operation timings of the control transistors Tr 1  with the use of the control signal from the control circuit  7 . 
         [0088]      FIG. 6  shows a fourth embodiment of the invention. The same components are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0089]    In a line head  10   c , a driver transistor Tr 5  is provided for each light emission element in the element array  1   a , a driver transistor Tr 6  is provided for each light emission element in the element array  1   b , and transistors Tr 7  and Tr 8  for switching the element arrays are connected in series to the control transistors Tr 1  common to the element arrays  1   a  and  1   b.    
         [0090]    In this embodiment, while the control transistors Tr 1  remain conductive by a signal from the control circuit  7 , either of the selection signals Se 11  and Se 12  from the signal lines  8  and  9 , respectively, is fed to the transistor Tr 7  or Tr 8  for switching the element arrays in this instance, the driver transistors Tr 5  or Tr 6  connected to the transistor Tr 7  or Tr 8  are activated, which causes the respective light emission elements in the element array  1   a  or the light emission element  2  to be activated. 
         [0091]      FIG. 7  shows a fifth embodiment of the invention. The same components are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0092]    In a line head  10   d , transistors Tr 7  and Tr 8  for switching the element arrays are connected in series to the control transistors Tr 1  common to the element arrays  1   a  and  1   b . Also, transistors Tr 7  and Tr 8  are connected in series to the drive transistors Tr 5  and Tr 6  for the light emission elements in the respective element arrays. 
         [0093]    To the signal line  11  are fed selection signals Se 11  through Se 14  to select the respective light emission elements in the element array  1   a . To the signal line  12  are fed inverted signals of the selection signals Se 11  through Se 14  to select the respective light emission elements in the element array  1   b . When the selection signals Se 11  through Se 14  are turned on, the inverted signals are turned off, and vice versa. 
         [0094]    Any of the selection signals Se 11  through Se 14  is fed to the drains of the switching transistors Tr 7  corresponding to the respective light emission elements in the element array  1   a . Also, any of the inverted signals of the selection signals Se 11  through Se 14  is fed to the drains of the switching transistors Tr 8  corresponding to the respective light emission elements in the element array  1   b.    
         [0095]    While the control transistors Tr 1  are activated by a control signal from the control circuit  7 , the switching transistors Tr 7  and the drive transistors Tr 5  become conductive when the selection signals Se 11  through Se 14  from the signal line  11  are turned on. The respective light emission elements in the element array  1   a  are thereby activated. In this instance, because the inverted signals or the selection signals Se 11  through Se 14  in the signal line  12  are turned off, the respective light emission elements in the element array are kept deactivated. 
         [0096]    When the inverted signals of the selection signals Se 11  through Se 14  in the signal line  12  are turned on, the respective light emission elements in the element array  1   b  are activated, and instead the respective light emission elements in the element array  1   a  are deactivated. Also in this embodiment, by controlling the operation timings of the control transistors Tr 1  with the use of the control signal from the control circuit  7 , it is possible to select individual light emission elements in the element arrays  1   a  and  1   b  for emitting light. 
         [0097]      FIG. 8  shows a sixth embodiment of the invention. The same components are designated by the same reference numerals and repetitive b explanations will be omitted. 
         [0098]    In a line head  10   e , transistors Tr 9  and Tr 10  are for switching the element arrays. With the switching transistors Tr 9 , the gates are connected to the signal line  11  and the drains are connected to the sources of the control transistors Tr 1 . With the switching transistors Tr 10 , the gates are connected to the signal line  12  and the drains are connected to the sources of the control transistors Tr 1 . The transistors Tr 9  and Tr 10  comprise FETs of the same channel (N channel in this embodiment). 
         [0099]    The transistors Tr 9  and Tr 10  are connected in series to the control transistors Tr 1 . While the control transistors Tr 1  remain conductive by a control signal from the control circuit  7 , the respective light emission elements in the element array  1   a  or the element array  1   b  are activated by enabling either the signal line  11  or  12 . 
         [0100]    In this embodiment, it is possible to select individual light emission elements in the element arrays  1   a  and  1   b  for emitting light, by controlling the operation timings of the control transistors Tr 1  by the control circuit  7 . This embodiment is different from the fifth embodiment in that the output signals of the control transistors Tr 1  are fed to the drains of the transistors Tr 9  and Tr 10 , and that selection signals are fed to the gates of the switching transistors Tr 9  and Tr 10 . In short, these embodiments are different in the way by which the control transistors Tr 1  are connected to the transistors Ir 9  and Tr 10 . 
         [0101]      FIG. 9  shows a seventh embodiment of the invention. The same, components are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0102]    In a line head  10   f , the transistors Tr 9  and Tr 11  for switching the element arrays comprise FETs of different channels. In this embodiment, the transistors Tr 9  comprise N-channel FETs and the transistors Tr 11  comprise H-channel FETs. 
         [0103]    To the gates of a pair of the transistors Tr 9  and Tr 11  are fed signals Se 11  through Se 14  from the same data line  11 . Hence, when one transistor Tr 9  is turned on, the other transistor Tr 11  is turned off, and vice versa. In other words, when the signals Sell through Se 14  are turned on, all the light emission elements in the element array  1   a  are activated and all the light emission elements in the element array  1   b  are kept deactivated. When the signals Se 11  through Se 14  are turned off, all the fight emission elements in the element array  1   a  are kept deactivated and all the light emission elements in the element array  1   b  are activated. Therefore, the data line to feed the inverted signals of the signals Se 11  through Se 14  can be omitted. 
         [0104]      FIG. 11  shows an eighth embodiment of the invention. The same components are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0105]    In this embodiment, one of the four element arrays  1   a  through  1   d  shown in  FIG. 10  is subsequently activated. Specifically, all the light emission elements in the element array  1   a  are activated from the time points  0  to tx. All the light emission elements in the element arrays  1   b  through  1   d  are kept deactivated. At the time point tx, the element array  1   a  is deactivated, and switching is made for the element array  1   b  to be activated. Thereafter, at the time point ty, the element array  1   b  is deactivated and switching is made for the element array  1   c  to be activated. 
         [0106]    At the time point tz, the element array  1   c  is deactivated and switching is made for the light emission element  1   d  to be activated. At the time point tw, the element array  1   d  is deactivated, in a case where predetermined. luminance has been restored in the element array  1   a  due to a drop in temperature of the respective light emission elements, it is possible to activate the element array  1   a  again at the time point tw. Also, it is possible to activate the respective element arrays  1   b  through  1   d  again after a predetermined time period since they are respectively deactivated. 
         [0107]    Accordingly, it is possible to prevent the decrease in luminance caused when a single element array is kept activated over a long Lime period, thereby prolonging the lifetime of the line head. 
         [0108]    In this embodiment, an allowable continuous driving time period is stored in advance in the memory  25  shown in  FIG. 2 . 
         [0109]    The host controller  23  judges whether the activated time period of one element array exceeds the allowable time period stored in the memory  25 . Upon judging that the activated time period of the one element array has exceeded the allowable time period, the host controller  23  sends a signal to the switching circuit  21  to activate another element array formed in the line head  10 . 
         [0110]    The counter  24  counts the time period that one element array is activated for the above judgment. Since the luminance decrease of the light emission element can be judged on the basis of the number of printed dots or print media, the counter  24  and the memory  25  can be configured so as to set various parameters, such as the number of print dots or print media, to be used for the above judgment. 
         [0111]    Specifically, in a case where the number of print dots is given as a parameter, the allowable number of the print dots is registered in the memory  25 . The counter  24  is formed as a dot counter, so that when the count number in the dot counter exceeds the allowable number registered in the memory  25 , the host controller  23  outputs a switching signal of the element arrays to the switching circuit  21 . 
         [0112]    When the number of print media is given as a switching parameter for the element arrays, a switching signal is outputted, for example, each time a sheet of print medium has been printed. Alternatively, when a vertical Sync signal (Vsync) is given as a switching parameter for the element arrays, the number of pulses is counted. It should be noted that in the case of color print, the counter  24  and the memory  25  are provided for each color. 
         [0113]      FIG. 12  shows a ninth embodiment of the invention. The same components are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0114]    the element array  1   a  is provided for the normal operation and the element array  1   b  is provided for the backup operation. In the event of a failure in any of the light emission elements D 00  through D 23  in the element array  1   a , a voltage is applied to the respective light emission elements D 50  through D 73  in the element array  1   b  by the switch  3  for light emitting operations to be performed. 
         [0115]    Shift registers  11 ,  12 ,  13  controls the light emission elements D 00  through D 23  block by block. An output signal C 0  of the shift register  11  controls a block A including the light emission elements D 00  through D 03 . An output signal C 1  of the shift register  12  controls a block  8  including the light emission elements D 10  through D 13 . An output signal C 2  of the shift register  13  controls a block C including the light emission elements D 20  through D 23 . 
         [0116]    A start pulse SP is inputted to a data terminal D of the shift register  11  from a signal line  1 A. A clock signal CK is inputted to each of the shift registers  11  through  13  from a signal line  10 . Data signals Dat 0  through Dat 3  are fed from a signal line  70  to the respective light emission elements. 
         [0117]    The output signal C 0 , outputted from an output terminal Q of the shift register  11 , is applied via a signal line C 0   a  to the gates of the respective control transistors Tr 1  connected to the light emission elements D 00  through D 03 . The output signal C 1 , outputted from an output terminal Q or the shift register  12 , is applied via a signal line C 1   a  to the gates of the respective control transistors Tr 1  connected to the light emission elements D 10  through D 13 . The output signal C 2 , outputted from an output terminal Q of the shift register  13 , is applied via a signal line C 2   a  to the gates of the respective control transistors Tr 1  connected to the light emission elements D 20  through U 23 . 
         [0118]    In short, when the output signals C 0  through C 2  are at the high level (hereinafter, referred to as the H level), signals are applied to the gates of the respective control transistors Tr 1  that control the light emission elements in the corresponding blocks. All the light emission elements are connected in parallel between the power supply line  4  to which a positive voltage VDD is applied and the negative power supply line  5 . Since the light emission elements in each element array are divided into blocks to be activated or deactivated, an area at an exposure operation is executed can be controlled as required. 
         [0119]    The data signals Dat 0  through Dat 3  are fed to the drains of the respective control transistors Tr 1 . Hence, when the data signals Dat 0  through Dat 3  are fed to the control transistors Tr 1  for the light emission elements selected by the block selection signals, the drive transistors Tr 2  connected to these control transistors Tr 1  become conductive, which causes the corresponding light emission elements to be activated. For example, in the case of the block A, the data signals Dat 0  through Dat 3  are fed to the control transistors Tr 1  that respectively control the light emission elements DOD through DQ 3 . In other words, the data signals Dat 0  through Dat 3  serve as selection signals to select individual light emission elements within one block. That is, it is possible to select at least one individual light emission element to be activated. 
         [0120]    Concrete operations of the respective light emission elements will now be described with reference to the timing chart of  FIG. 13 . Assume that a DC voltage is applied between the positive and negative power supply lines  4  and  5  connected to the respective light emission elements of  FIG. 12 . In this timing chart, an operation time of the light emission elements is divided into 8 segments of time period. The length of each segment is set, for example, to 10 μs. When the start pulse SP is inputted to the data terminal D of the shift register  11 , the start pulse SP is captured in synchronous with the rising of the clock signal CK, and transferred sequentially from the shift registers  11  to  13 . 
         [0121]    Herein, intervals of the signal SP are set In such a manner that an output of any one of the shift registers  11  to  13  alone is at the H level. For example, in the segment  1 , the start pulse SP Is inputted to the shift register  11  at the time point  0 , at which the output signal C 0  of the shift register  11  shifts to the II level because the clock signal CK is at the H level at the timing “u”. The start pulse SP is then transferred to the shift register  12  at the timing “v” at which the next clock signal CK shifts to the H level, and the output signal C 1  thereby shifts to the H level. Further, the start pulse SP is transferred to the shift register  13  at the timing “w” at which the next clock signal CK shifts to the H level, and the output signal C 2  thereby shirts to the H level 
         [0122]    Outputs from the respective shift registers are connected to the control transistors Tr 1  in each block. As is shown in  FIG. 13 , the signals C 0  through C 2  to select the respective blocks are applied to the control transistors Tr 1  with a time difference. Hence, when the switches of the control transistors Tr 1  in a given block are turned on, the switches of groups of the control transistors Tr 1  in the other blocks are kept off. 
         [0123]    In the selected block, the control transistors Tr 1  are turned on so as to establish a condition that the data signals Dat 0  through Dat 3  can be fed to the gates of the drive transistors Tr 2 . The potentials of the gates of the drive transistors Tr 2  are thus determined according to the states (H level or L level) of the data signals Dat 0  through Dat 3 , and in turn the on/off states of the drive transistors Tr 2  in the selective block are determined. 
         [0124]    When a given block that is selected will no longer be selected, the data signals Dat 0  through Dat 3  will not be fed to the gates of the drive transistors Tr 2  in the block that has been selected. However, because the potentials of the gates of the drive transistors Tr 2  are held at the potentials when the block was selected due to a parasitic capacitance, the on/off states of the drive transistors Tr 2  are maintained. This state of the drive transistors Tr 2  is maintained until the block is selected again. 
         [0125]    When the block in the deselected state is selected at the next timing, the on/off states of the drive transistors Tr 2  in the block are determined according to the states of the data signals Dat 0  through Dat 3 . The operations described as above are repeated sequentially for all the blocks. Hence, the activation time period can be controlled on the basis of this repetitive cycle. 
         [0126]    Referring to  FIG. 13 , the block A is selected at the time point  0  in the segment  1 . The block D is selected next at the time point ta, and the block C at the time point tc For the segments  2  through ft, any of the corresponding blocks A through C is selected in the same manner as above when the output signals C 0  through C 2  of the shift registers  11  through  13 , respectively, are at the H level. Each of the vertically extending hatched areas indicates that the corresponding block is selected. Each of the horizontally extending hatched areas indicates that the corresponding light emission element is activated. 
         [0127]    The data signals Dat 0  through Dat 3  select individual light emission elements within the respective blocks A through C as described above. For example, within the block A, the correspondence between the data signals and the light emission elements is as follows: Dat 0 (D 00 ), Dat 1 (D 01 ). Dat 2 (D 02 ), and Dat 3 (D 03 ). Fine lines (F) indicate the correspondence between the data signals and the light emission elements in the respective blocks as described above. 
         [0128]    For the block A, in the segment  1 , when the block selection signal C 0  is at the H level at the time point  0 , the data signals Dat 0  and Dat 3  shift to the H level, and the light emission elements D 00  and D 03  are activated. When the block selection signal C 0  shifts to the low level (hereinafter, referred to as the L level) at the time point ta, the voltage is maintained due to a parasitic capacitance between the gate and the source in each corresponding control transistor Tr 1 . The light emission elements D 00  and D 03  thus remain activated. 
         [0129]    The block selection signal C 0  and the data signal Dat 3  shift to the L level at the time point tc, at which the block selection signal C 1  shifts to the H level and the block B is thereby selected. However, because the drive transistor Tr 2  remain conductive due to the parasitic capacitance between the gate and the source in the control transistor Tr 1 , the light emission element D 03  remains activated. 
         [0130]    The block selection signal C 0  shifts to the H level at the time point ts, and the on/off state of the drive transistor Tr 2  connected to the light emission element D 03  is determined according to the state of the data signal Dat 3 . At the time point ts, the control transistor Tr 1  is turned off because the data signal Dat 3  is at the L level, and the drive transistor Tr 2  is also turned off. The light emission element D 03  is thereby deactivated. 
         [0131]    The block selection signal C 0  and the data signal Dat 2  shift to the H level at the time point te, and the light emission element D 02  is thereby activated. The data signal Dat 2  shifts to the L level at the time point tr; however, the activated state is maintained until the time point ts, that is, the timing at which the block selection signal C 0  shifts to the H level again. The light emission element D 03  is deactivated at the time point ts as described above. 
         [0132]    The date signal Dat 0  shifts to the L level at the time point tu, and the light emission element D 00  is thereby deactivated. As is shown in an enlarged view of the portion (E), the switching of the on/off states of the light emission element D 00  takes place in the selection period of the block selection signal C 0 . The switching of the on/off states of the other light emission elements also takes place in the selection periods of the block selection signals (although not explicitly shown in the drawing). 
         [0133]    Now, by referring to the operation for the block B, because the data signal Dat 0  is at the H level when the block selection signal C 1  is at the H level at the time point ta in the segment  1 , the light omission element D 10  is activated. Although the light emission element D 10  is in an indefinite state from the time points  0  to ta, it is placed in the activated state at the time point ta as described above. 
         [0134]    In the segment  3 , the block selection signal C 1  shifts to the H level and the data signal Dat 1  shifts to the H level at the time point tf, and the light emission element D 11  is thereby activated. Also, because the data signal Dat 3  is at the H level at the time point tf, the light emission element D 13  is also activated. Because the block selection signal C 1  shifts to the H level at the time point, th, the light emission element D 13  is activated or deactivated according to the state of the data signal Dat 3 . Because the data signal Dat 3  is at the L level in this instance, the light emission element D 13  is deactivated. 
         [0135]    At the time point tr, because the block selection signal C 1  shifts to the H level and the data signal Dat 1  shifts to the L level, the light emission element D 11  is deactivated. Because the data signal Dat 3  is at the H level at the time point tr, the light emission element D 13  is activated. The block selection signal C 1  shifts to the L level and the data signal Dat 0  also shifts to the L level at the time point tu. However, as has been described, the drive transistor Tr 2  remains conductive, and the light emission element D 10  thus remains activated until the time point tv as is indicated by G in the drawing. Descriptions for the operations of the respective light emission elements in the block C are omitted. 
         [0136]      FIG. 14  is an explanatory view, showing the timing chart of  FIG. 3  in the form of a table. In this table, a circle indicates that the corresponding block is selected. In the “data” section, “1” indicates the H level, and “0” indicates the L level of the data signal Dat  0  through Dat  3 . 
         [0137]    In the “light emission element” section, each of the vertically extending hatched areas indicates that the corresponding light emission element is activated (i.e., corresponding to the horizontal hatched areas in  FIG. 13 ). An asterisk indicates an indefinite state of the light emission element. An arrow indicates that whether the light emission element is activated or deactivated is determined by the on/off state of the data signal. A blank cell indicates a state where the preceding state is maintained. In this manner, in this embodiment, each of the plural element arrays is divided into plural blocks, and individual light emission elements in the respective blocks are selected and controlled as long as necessary. In other words, because two selecting stages are provided for individual light emission elements in the element arrays, it is possible to address image formation processing of various modes. The applications or the line head, therefore, can be broadened. 
         [0138]    In this embodiment, since the shift registers  11  through  13  can be formed on a single substrate together with the element arrays  1   a  and  1   b , the control transistors Tr 1 , and the drive transistors Tr 2 . A compact line head can be thereby achieved. 
         [0139]      FIG. 15  shows a tenth embodiment of the invention. The same components are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0140]    In this embodiment, the element array  1   a  is used for forming a binary data image such as character information, and the element array  1   b  for forming a gradation data image such as photographic information. Specifically, all the light emission elements in the element array  1   a  are activated during the time period  0  to ta to form the binary data image. All the light emission elements in the element array  1   b  are kept deactivated. 
         [0141]    When luminance decreases below the predetermined value due the continuous activation of the element array  1   a , the element array  1   a  is deactivated at the time point ta. Incidentally, switching is made for the element array  1   b  to be activated, so that the gradation data image is formed. At the time point tb, the element array  1   b  Is deactivated while the element array  1   a  is activated to form the binary data image for a short time until the time point ta Because the activation time period or the element array  1   a  is short and only the binary data image is formed, deterioration of the obtained image quality poses no practical problem. 
         [0142]    Generally, the accuracy in image formation, when an image is formed with decreased luminance of the light emission elements, differs between a case where the binary data image is formed and a case where the gradation data image is formed. In the former case, even when luminance of the light emission elements decreases slightly, a degree of deterioration of the obtained image quality is smaller than in the latter case. 
         [0143]    Thereafter, the gradation data image is formed by the element array  1   b  until the time point td, and the binary data image is formed by the element array  1   a  during the lime period td to te. Herein, because a certain time period has passed since the respective light emission elements in the element array  1   a  were deactivated at the time point ta, the temperature has dropped and luminance has been restored. Hence, the binary data image is formed during the time period td to te, which is longer than the time period tb to tc. From the time points te to tf, the gradation data image is formed by the element array  1   b . Because the temperature of the light emission element in the element array  1   a  has sufficiently dropped at the time point tf and the luminance thereof has been sufficiently restored, the binary data image is formed continuously. 
         [0144]    According to the configuration of this embodiment, it is possible to prevent deterioration of the print quality due to the decrease in luminance of the light emission elements in comparison with a case where both the binary data image and the gradation data image are formed continuously by a single element array. It is also possible to prolong the lifetime of the light emission elements and the line head. 
         [0145]    The switching timings of the element arrays as shown in  FIG. 15  are set by taking various factors into account. In other words, optimal timings are sat on the basis of a temperature characteristic of the light emission element materials, a heat-releasing characteristic of the panel of the line head to which the light emission elements are attached, etc. In this embodiment, two element arrays are switched according to image information including binary data and gradation data. However, it may be configured in such a manner that three or more element arrays are used by being switched at adequate timings. 
         [0146]      FIG. 16  shows an eleventh embodiment of the invention. The same components, are designated by the same reference numerals and repetitive explanations will be omitted. 
         [0147]    In this embodiment, the element array  1   a  is first activated to form both the binary data image and the gradation data image, and the element array  1   b  is not used. However, after a predetermined time has passed, the element array  1   a  is used to form only the binary data image while the element array  1   b  is used to form only the gradation data image. As in the tenth embodiment, it is configured that the element arrays  1   a  and  1   b  are activated alternately. 
         [0148]    Specifically, all the light emission elements in the element array  1   a  are activated during the time period  0  to tp to form the binary data image and the gradation data image. All the light emission elements in the element array  1   b  are kept deactivated. When luminance decreases below a predetermined value due to the continuous activation of the element array  1   a , the element array  1   a  is deactivated at the time point tp, and switching is made for the element array  1   b  to form the gradation data image. At the time point tq, the element array  1   b  is deactivated and the element array  1   a  is activated to form the binary data image for a short time until the time point tr. Because the activation time period of the element array  1   a  is short and only the binary data image is formed, deterioration of the obtained image quality poses no practical problem. 
         [0149]    Thereafter, the gradation data image is formed by tine element array  1   b  until the time point ts, the binary data image is formed by the element array  1   a  during the time period ts to tu, and the gradation data image is formed by the element array  1   b  during the time period tu to tv, in a repetitive manner. At the time point tv, because the temperature of the light emission elements in the element array  1   a  has sufficiently dropped and the luminance thereof has been b sufficiently restored, the binary data image and the gradation data image are formed again. 
         [0150]      FIG. 17  shows a peripheral configuration of the line head of the tenth and eleventh embodiments. In this embodiment, a counter  24   a  and a memory  25   a  are associated with the element array  1   a . A counter  24   b  and a memory  25   b  are associated with the element array  1   b.    
         [0151]    The counter  24   a  counts the activated time period of the element array  1   a , and stores the counted value into the memory  25   a . In the memory  25   a , switching timing data of the element array  1   a  is stored in advance. Similarly, the counter  24   b  counts the activated time period of the element array  1   b , and stores the counted value into the memory  25   b . In the memory  25   b , switching timing data of the element array  1   b  is stored in advance. The host controller  23  judges whether the activated time periods of the element arrays  1   a  and  1   b  reach the switching timings stored in the memories  25   a  and  25   b , respectively. Upon judging that the switching timing has reached for the element array that is activated, the host controller  23  sends a signal to the switching circuit  21  to switch to the activated element array to another one. 
         [0152]    In this case, the host controller  23  controls the counters  24   a  and  24   b  and the memories  25   a  and  25   b  However, a controller such as a CPU for controlling the counters  24   a ,  24   b  and the memories  25   a ,  25   b  may be provided in the control section  20 , such that the controller is farmed on a substrate on which the switching circuit  21  is provided. In this case, the configuration of the control system can be compact and the host controller  23  is not involved. Hence, the wiring can be shorter, which can in turn increase the processing speed. 
         [0153]    The line head according to any one of the above embodiments can be applied to not only a monochromatic printer, but also a four-cycle type full color printer, and a tandem type full color printer. There will be described cases where the line head is incorporated in the full color printer. 
         [0154]      FIG. 18  shows a tandem-type image forming apparatus in which tour exposure heads  10   k ,  10 C,  10 M and  10 Y having the same configuration comprising a number of organic EL elements are disposed so as to face exposure positions on four photosensitive drums (image carriers)  41   k ,  41 C,  41 M, and  41 Y having the same configuration 
         [0155]    In the image forming apparatus, an intermediate transfer belt  50  stretched over a driving roller  51 , a follower roller  52 , and a tension roller  53  and circulated in a direction indicated, by an arrow in the drawing (counterclockwise direction) white being tensed by the tension roller  53 . The photosensitive drums  41 K,  41 C,  41 M, and  41 Y, each having a photosensitive layer on the outer peripheral surface thereof, are placed at regular intervals with respect to the intermediate transfer belt  50 . Capitals K, C, M and Y appended to the reference numerals stand for black, cyan, magenta, and yellow, respectively, and thereby indicate that they are photosensitive drums for black, cyan, magenta, and yellow, respectively. The same can be said for other members. The photosensitive drums  41 K,  41 C,  41 M, and  41 Y are rotated in a direction indicated by arrows in the drawing (clockwise direction) in synchronous with the circulation of the intermediate transfer belt  50 , 
         [0156]    On the periphery of the respective photosensitive drums  41  (K, C, M, and Y) are provided corona chargers  42  (K, C, M, and Y) for uniformly charging the outer peripheral surfaces uf the respective photosensitive drums  41  (K, C, M, and Y), and the exposure heads  10  (K, C, M, and Y) for scanning the outer peripheral surfaces that have been charged uniformly by the corona chargers  42  (K, C, M, and Y), in synchronous with the rotations of the photosensitive drums  41  (K, C, M, and Y) to form electrostatic latent images thereon. 
         [0157]    The image forming apparatus further comprises: developers  44  (K, C, M, and Y) to develop the electrostatic latent images as visible images with toner; primary transfer rollers  45  (K, C, M, and Y) for sequentially transferring the toner images developed by the developers  44  (K, C, M, and Y) onto the Intermediate transfer belt  50  (primary transfer); and cleaners  46  (K, C, M, and Y) for removing toner remaining on the surfaces of the photosensitive drums  41  (K, C, M, and Y) after the primary transfer. 
         [0158]    In each of the respective exposure heads  10  (K, C, M, and Y), the organic EL elements are arrayed in a direction parallel to a generatrix of the photosensitive drums  41  (K, C, M, and Y). The peak energy wavelengths of light emitted from the respective exposure heads  10  (K, C, M, and Y) are almost matched with the peak sensitivity wavelengths of the photosensitive drums  41  (K, C, M, and Y). 
         [0159]    In the developer  44  (K, C, M, and Y), non-magnetic one-component toner is transported to a developing roller by a feeding roller. The thickness of the toner adhering to the surface of the developing roller is restricted by a control blade. The developing roller is brought into contact with or pressed against the photosensitive drum  41  (K, C, M, and Y) to adhere the toner thereon according to the potential levels of the photosensitive drums  41  (K, C, M, and Y). A toner image is thus developed. 
         [0160]    The respective toner images of black, cyan, magenta, and yellow are sequentially transferred onto the intermediate transfer belt  50  by primary transfer biases applied to the primary transfer rollers  45  (K, C, M, and Y), and are sequentially superimposed on the intermediate transfer belt  50  to form a full color toner image, which is secondary transferred onto a recording medium P, such as paper, by a secondary transfer roller  66  at a secondary transfer position. The toner image is then fixed onto the recording medium P by passing through a pair of fixing rollers  61 . The recording medium P is finally ejected by a pair of ejection rollers  62  onto a tray  66  formed at the top portion of the apparatus. 
         [0161]    The image forming apparatus further comprises: a feeding cassette  63  in which a number of recording media P are held in a stacked manner; a pickup roller  64  to feed the recording media P from the feeding cassette  63  one by one; a pair of gate rollers  65  to regulate the feeding timing of a recording medium P to the secondary transfer position; and a cleaning blade  67  for removing toner remaining on the surface of the intermediate transfer belt  50  after the secondary transfer. 
         [0162]    In a case where the above-described line head  10  is incorporated in the image forming apparatus as the exposer, it is possible to downsize the apparatus in comparison with a case where a laser scanning optical system is used. 
         [0163]      FIG. 19  shows a four-cycle type image forming apparatus  160  roughly comprising a rotary-type developer  161 ; a photosensitive drum (Image carrier)  16   b ; an exposure head  10  comprising organic EL elements; an intermediate transfer belt  169 ; a sheet transportation path  174 ; and a sheet feeding tray  178 . 
         [0164]    In the developer  161 , a rotary  161   a  rotates about an axis  161   b  in a direction indicated by an arrow A. The interior of the rotary  161   a  is divided into four segments, in which arc respectively formed image forming units for four colors, including yellow (Y), cyan (C), magenta (M), and black (K). Developing rollers  162   a  through  162   d  are respectively placed in the image forming units for four colors, to rotate in a direction indicated by an arrow B. Toner supply rollers  163   a  through  163   d  is rotated in a direction indicated by an arrow C, Control blades  164   a  through  164   d  to restrict the thickness of toner on the developing rollers  162   a  through  162   d  to a predetermined thickness. 
         [0165]    The image forming apparatus  160  further comprises a primary transfer member  166 ; and a charger  168 . The photosensitive drum  165  is rotated by an unillustrated driving motor such as a step motor, in a direction indicated by an arrow D, which is opposite to the rotary direction of the developing roller  162   a.    
         [0166]    The intermediate transfer belt  169  is stretched over a driving roller  170   a  and a follower roller  170   b . The driving roller  170   a  is linked to a driving motor of the photosensitive drum  165 , so that the intermediate transfer belt  169  is circulated in a direction indicated by an arrow E, which is opposite to the direction of the photosensitive drum  165 . 
         [0167]    The sheet transportation path  174  is provided with a plurality of transportation rollers and a pair of ejection rollers  176 . The toner image carried by the intermediate transfer belt  169  is transferred onto one surface of a sheet of paper by a secondary transfer roller  171  at the secondary transfer position. 
         [0168]    The sheet of paper, onto which the toner image has been transferred as described above, is then subjected to fixing processing at a fuser. The fuser is provided with a heating roller  1 / 2  and a press roller  173 . After the fixing processing, the sheet of paper is pulled into the pair of ejection rollers  178  to travel in a direction indicated by an arrow F. When the pair of ejection rollers  176  rotates in an inverse direction from this state, the sheet of paper inverts the direction and travels in a direction indicated by an arrow G through a double-sided print transportation path  175 . Numeral  177  denotes an electrical equipment box, and numeral  179  denotes a pickup roller provided at the outlet of the feeding fray  178 . 
         [0169]    For the sheet transportation path, for example, a low-speed brushless motor is used as the driving motor to drive the transportation rollers. Also, a step motor is used for the intermediate transfer belt  169  due to a need to correct color shift or the like. These motors are controlled by signals from an unillustrated controller. 
         [0170]    In the state shown in the drawing, an electrostatic latent image of yellow (Y) is formed on the photosensitive drum  165 , and an image of yellow is formed on the photosensitive drum  165  when a high voltage is applied to the developing roller  162   a . When images of yellow on the back surface and the front surface are entirely carried over onto the intermediate transfer belt  169 , the developing rotary  161   a  rotates by 90 degrees in a direction indicated by the arrow A. 
         [0171]    The intermediate transfer belt  169  rotates once and returns to the position of the photosensitive drum  165 . Images of cyan (C) on two surfaces are then formed on the photosensitive drum  165 , which are carried over to be superimposed on the images of yellow being carried on the intermediate transfer belt  169 . Thereafter, the processing is repeated in the same manner, so that the developing rotary  161  rotates by 90 degrees and the intermediate transfer belt  169  rotates once after the images are carried over. 
         [0172]    For images of four colors to be carried over, the intermediate transfer belt  169  rotate four times, after which the rotational position is controlled for the image to be transferred onto a sheet of paper at the position of the secondary transfer roller  171 . A sheet of paper fed from the feeding tray  178  is transported through the transportation path  174 , and the color image is transferred onto one surface of the sheet of paper at the position of the secondary transfer roller  171 . The sheet of paper bearing the transferred image on one surface is inverted by the pair of ejection rollers  176  as described above, and stands by in the transportation path. Subsequently, the sheet of paper is transported to the position of the secondary transfer roller  1 / 1  at the adequate timing, and the color image is transferred onto the other surface. A housing  180  is provided with an exhaust fan  181 . 
         [0173]    Although the present invention has been shown and described with reference to specific preferred embodiments, various changes and modifications will be apparent to those skilled in the art from the teachings herein. Such changes and modifications as are obvious are deemed to come within the spirit, scope and contemplation of the invention as defined in the appended claims.