Patent Publication Number: US-7907162-B2

Title: Exposure head, image forming device, and image forming method

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to an exposure head adapted to image a light beam emitted from a light emitting element with a lens, an image forming device using the exposure head, and an image forming method. 
     2. Related Art 
     As such an exposure head, there is proposed a device using a light emitting element array composed of a plurality of light emitting elements arranged linearly as described in, for example, JP-A-2000-158705 (Document 1). In such a line head, a light beam emitted from each of the light emitting elements provided to the light emitting element array is imaged by a lens as a spot to form a spot latent image on an image plane. Thus, the line head in the Document 1 forms a plurality of spot latent images aligned in a main-scanning direction. 
     Incidentally, in order for forming a more preferable spot latent image, it is desirable to form the spot latent image with a sufficient amount of light using a larger sized light emitting element. However, in the configuration described above having a plurality of light emitting elements arranged linearly, it is not easy to use larger sized light emitting elements. Because, in the case in which the light emitting elements with a larger size are used, there is possibility of causing interference between the light emitting elements respectively forming spot latent images adjacent to each other in the main-scanning direction. In the case in which the pitches between the light emitting elements are reduced for higher resolution, it becomes even more difficult to increase the sizes of the light emitting elements. 
     SUMMARY 
     In view of the problem described above, the present invention has an advantage of providing a technology capable of forming a latent image with a sufficient amount of light even in high-resolution conditions. 
     An exposure head according to an aspect of the invention includes a substrate having a plurality of light emitting element rows each having a plurality of light emitting elements arranged in a first direction, the light emitting element rows being arranged in a second direction perpendicular or substantially perpendicular to the first direction, and an imaging optical system adapted to image light beams from the light emitting elements on an exposed surface to form respective light-collected sections, and two of the light emitting elements forming the light-collected sections adjacent to each other in the first direction are respectively disposed in the light emitting element rows different from each other, and one of the light emitting element rows is disposed so as to match or substantially match with the meridian plane of the imaging optical system. 
     Further, an image forming device according to another aspect of the invention includes a latent image carrier, and an exposure head having a substrate having two or more light emitting element rows each having a plurality of light emitting elements arranged in a first direction, the light emitting element rows being arranged in a second direction perpendicular or substantially perpendicular to the first direction, and an imaging optical system adapted to image light beams from the light emitting elements of the light emitting element rows on the latent image carrier to form a latent image formed of respective light-collected sections, and two of the light emitting elements forming the light-collected sections adjacent to each other in the first direction are respectively disposed in the light emitting element rows different from each other, and one of the light emitting element rows is disposed so as to match or substantially match with the meridian plane of the imaging optical system. 
     Further, an image forming method according to another aspect of the invention includes (a) providing the exposure head according to above aspect of the invention, (b) forming a latent image formed of a plurality of light-collected sections on a latent image carrier in the first direction by lighting the light emitting element row at timing corresponding to movement of the latent image carrier, (c) developing the latent image formed in step (b) to form an image, and (d) transferring the image developed in step (c). 
     In the above aspects (the exposure head, the image forming device, and the image forming method) of the invention, the two light emitting elements forming the light-collected sections adjacent to each other in the first direction are respectively disposed in the different light emitting element rows. Therefore, the size of the light emitting element can be increased, thus it becomes possible to increase the amount of light in the light-collected sections. Therefore, even in high resolution conditions, it becomes possible to form the light-collected sections with sufficient amount of light, thereby preferably performing the latent image formation. 
     Incidentally, in the exposure head as described above, the alignment between the light emitting element and the imaging optical system becomes important. Therefore, in the invention, one of the light emitting element rows is disposed so as to match with or substantially match with the meridian plane of the imaging optical system. Therefore, by performing the alignment using the light emitting element row matching with the meridian plane as a reference, the alignment between the light emitting elements and the imaging optical system can be executed with ease and high accuracy. Further, the preferable latent image formation becomes possible using the exposure head in which the alignment is performed with high accuracy. 
     Further, it is possible that one of the light emitting element rows has the light emitting elements arranged symmetrically with respect to the optical axis of the imaging optical system. In such a configuration, as described later, the alignment between the imaging optical system and the light emitting elements in the first direction can be executed with ease and high accuracy. 
     Alternatively, one of the light emitting elements can be disposed on the optical axis. In such a configuration, as described later, the alignment between the imaging optical system and the light emitting elements in the first direction can be executed with ease and high accuracy. 
     Further, the imaging optical system can be of anamorphic. The reason is that such an anamorphic imaging optical system is advantageous to designing the optimum optical system suitable to the arrangement forms of the light emitting elements in the light emitting element groups. 
     Further, the imaging optical system can be arranged to invert the light beams from the light emitting elements in imaging them on the exposed surface. Further, the imaging optical system can be arranged to reduce the light beams from the light emitting elements in imaging them on the exposed surface. 
     Further, the number of light emitting element rows can be an odd number. The reason is that, as described later, such configurations make it possible to image the light beams emitted from the light emitting elements with relatively preferable aberration and to perform the preferable latent image formation. 
     Further, it is also possible to configure to provide the light-shielding member provided with a plurality of light guide holes making the light beams emitted from the light emitting elements forming two or more light emitting element rows towards the imaging optical system. Since such a light-shielding member prevents the stray light from entering the imaging optical system, the preferable latent image formation becomes possible. 
     Further, this aspect of the invention is preferably applied to the configuration using the organic EL elements as the light emitting elements. This is because, the organic EL elements only emit light with low intensity. Therefore, from the viewpoint of forming the light-collected sections with a sufficient amount of light, it is preferable to apply this aspect of the invention advantageous to increasing the amount of light by increasing the size of the light emitting element to such a configuration. In particular, since the bottom emission organic EL elements emit light with lower intensity, it is preferable to apply the present aspect of the invention to the configurations using the bottom emission organic EL elements as the light emitting elements. 
     Further, in the image forming device according to another aspect of the invention, it is also possible to configure to include the developing section adapted to develop the latent image, which is formed on the latent image carrier by the exposure head, using a liquid developer. This is because, the development with relatively high resolution can be performed with the liquid developer, and it is suitable for preferable image formation. 
     Further, the exposure head according to another aspect of the invention includes a substrate having a plurality of light emitting elements divided into groups to form light emitting element groups, and a lens array having a plurality of lenses adapted to image the light beams emitted from the light emitting elements of the light emitting element groups as spots to form spot latent images on an image plane, the imaging optical systems being provided corresponding respectively to the light emitting element groups, an image plane moves in a second direction perpendicular or substantially perpendicular to a first direction, a plurality of spot latent images is formed so as to be aligned in the first direction by the light emitting elements emitting light at the timing corresponding to the movement of the image plane, in the light emitting element group, a plurality of light emitting element rows each having a plurality of light emitting elements aligned in a direction corresponding to the first direction is arranged side by side in a direction corresponding to the second direction, the light emitting element rows are shifted from each other in a direction corresponding to the first direction so that the two light emitting elements forming the spot latent images adjacent to each other in the fist direction belong respectively to the light emitting element rows different from each other, and one of the plurality of the light emitting element rows matches with the meridian plane parallel to the direction corresponding to the first direction and including the optical axis of the lens. 
     Further, an image forming device according to still another aspect of the invention includes a latent image carrier having a surface moving in a second direction perpendicular or substantially perpendicular to a first direction, an exposure head having a substrate having a plurality of light emitting elements divided into groups to form light emitting element groups, and a lens array having a plurality of lenses adapted to image the light beams emitted from the light emitting elements of the light emitting element groups as spots to form spot latent images on a surface of the latent image carrier, the lenses being provided corresponding respectively to the light emitting element groups, a plurality of spot latent images is formed so as to be aligned in the first direction by the light emitting elements emitting light at the timing corresponding to the movement of a surface of a latent image carrier, in the light emitting element group, a plurality of light emitting element rows each having a plurality of light emitting elements aligned in a direction corresponding to the first direction is arranged side by side in a direction corresponding to the second direction, the light emitting element rows are shifted from each other in a direction corresponding to the first direction so that the two light emitting elements forming the spot latent images adjacent to each other in the fist direction belong respectively to the light emitting element rows different from each other, and one of the plurality of the light emitting element rows matches with the meridian plane parallel to the direction corresponding to the first direction and including the optical axis of the lens. 
     In this aspect (the exposure head, the image forming device) of the invention configured as described above, the plurality of light emitting elements are disposed by being grouped into a plurality of light emitting element groups. In the light emitting element group, a plurality of light emitting element rows each having a plurality of light emitting elements aligned in a direction corresponding to the first direction is arranged side by side in a direction corresponding to the second direction. Moreover, in each of the light emitting element groups, the light emitting element rows are shifted from each other in a direction corresponding to the first direction so that the two light emitting elements forming the spot latent images adjacent to each other in the first direction belong respectively to the light emitting element rows different from each other. In other words, in this aspect of the invention, the two light emitting elements forming the spot latent images adjacent to each other in the first direction are shifted from each other in a direction corresponding to the second direction. Therefore, since the light emitting element can be formed in the relatively large space, the size of the light emitting element can be increased. Therefore, it becomes possible to form the spot latent image with a sufficient amount of light even in high-resolution conditions, thus preferable spot latent image formation becomes possible. 
     Incidentally, in the configuration of grouping the plurality of light emitting elements into a plurality of light emitting element groups and providing the lens for each light emitting element group as in the aspect of the invention described above, the alignment (hereinafter abbreviated as “alignment” according to needs) between the light emitting element groups and the optical axes of the corresponding lenses becomes important. In particular, in the above aspect of the invention, each of the light emitting element emits light at the timing corresponding to the movement of the image plane in the second direction, thereby forming the plurality of spot latent images aligned in the first direction. Therefore, from a viewpoint of forming these spot latent images at appropriate positions on the image plane, it is more severely required in the direction corresponding to the second direction that the alignment is performed with high accuracy. To cope with this requirement, in this aspect of the invention, one of the plurality of the light emitting element rows matches with the meridian plane parallel to the direction corresponding to the first direction and including the optical axis of the lens. Therefore, it becomes possible to execute the alignment in the direction corresponding to the second direction with ease and high accuracy by performing the alignment using the light emitting row matching with the meridian plane as a reference. Further, the preferable spot latent image formation becomes possible using the exposure head in which the alignment is performed with high accuracy. 
     Further, in the light emitting element row matching with the meridian plane, it is possible to dispose the light emitting elements symmetrically around the optical axis. In the configuration of thus providing the light emitting elements, alignment in the direction corresponding to the first direction can also be executed with ease and high accuracy. 
     Further, in the light emitting element row matching with the meridian plane, it is possible to dispose one of the light emitting elements on the optical axis. The reason is that it becomes possible to execute the alignment in the direction corresponding to the first direction more easily and more accurately. 
     Further, it is possible that in the light emitting element group, an odd number of light emitting element rows are arranged in the direction corresponding to the second direction, and in the direction corresponding to the second direction, the light emitting element rows are arranged in both sides of the light emitting element row matching with the meridian plane disposed at the center thereof. The reason is that by thus configuring, it is possible to image the light beams emitted from the light emitting elements with relatively preferable aberrations, and to perform preferable spot latent image formation. 
     Further, the lenses can be anamorphic lenses. The reason is that such an anamorphic lens makes it possible to design the optimum optical system suitable to the arrangement forms of the light emitting elements in the light emitting element groups. 
     Further, in the image forming device applying this aspect of the invention capable of preferably forming the spot latent images as described above, the spot latent images can be developed using the liquid developer. In other words, by using the liquid developer, development of the latent images can be performed with high resolution. Therefore, it is preferable to perform the development of the spot latent images preferably formed by the aspects of the invention using the liquid developers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a diagram for explaining terms used in the present specification. 
         FIG. 2  is a diagram for explaining terms used in the present specification. 
         FIG. 3  is a diagram showing an example of an image forming device according to an embodiment of the invention. 
         FIG. 4  is a diagram showing an electrical configuration of the image forming device shown in  FIG. 3 . 
         FIG. 5  is a perspective view showing a schematic configuration of an example of a line head according to the embodiment of the invention. 
         FIG. 6  is a cross-sectional view of a configuration of the line head along the width direction thereof. 
         FIG. 7  is an exploded perspective view of the line head. 
         FIG. 8  is a plan view schematically showing a lens array. 
         FIG. 9  is a cross-sectional view of the lens array along the longitudinal direction LGD. 
         FIG. 10  is a diagram showing a configuration of the reverse side of a head substrate. 
         FIG. 11  is a diagram showing a configuration of a light emitting element group in the first embodiment. 
         FIG. 12  is a diagram showing a spot latent image forming operation by the line head. 
         FIG. 13  is a perspective view showing an array moving mechanism and an observation optical system provided to a position adjustment device. 
         FIG. 14  is a diagram of the position adjustment device viewed along the longitudinal direction. 
         FIG. 15  is a block diagram showing an electrical configuration of the position adjustment device. 
         FIG. 16  is a flowchart showing a position adjustment operation. 
         FIG. 17  is a diagram showing images reflected by the observation optical system in the position adjustment operation according to the first embodiment. 
         FIG. 18  is a diagram showing a configuration of a light emitting element group in the second embodiment. 
         FIG. 19  is a diagram showing images reflected by the observation optical system in the position adjustment operation according to the second embodiment. 
         FIG. 20  is a diagram for explaining the case in which the light emitting element row matches with a meridian plane. 
         FIG. 21  is a diagram for explaining the case in which the light emitting element row matches approximately with the meridian plane. 
         FIG. 22  is a perspective view showing a schematic configuration of a line head provided with a light shielding member. 
         FIG. 23  is a cross-sectional view along the width direction showing a schematic configuration of the line head provided with the light shielding member. 
         FIG. 24  is a plan view showing another arrangement form of light emitting elements. 
         FIG. 25  is a plan view showing a configuration in which the number of lens rows LSR is one. 
         FIG. 26  is a diagram schematically showing a device for performing development with a liquid developer. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     A. Explanations of Terms 
     Before explaining embodiments of the invention, the terms used in the present specification will be explained. 
       FIGS. 1 and 2  are diagrams for explaining the terms used in the present specification. The terms used in the present specification will hereinafter be organized with reference to these drawings. In the present specification, a conveying direction on a surface (image plane IP) of a photoconductor drum  21  is defined as a sub-scanning direction SD, and a direction perpendicular to or substantially perpendicular to the sub-scanning direction SD is defined as a main-scanning direction MD. Further, a line head  29  is disposed facing to the surface (the image plane IP) of the photoconductor drum  21  so that a longitudinal direction LGD thereof corresponds to the main-scanning direction MD, and a width direction LTD corresponds to the sub-scanning direction SD. 
     An aggregate of a plurality (eight in  FIGS. 1 and 2 ) of light emitting elements  2951 , which is disposed on a head substrate  293  in one-to-one correspondence with each of lenses LS included in a lens array  299 , is defined as a light emitting element group  295 . In other words, the light emitting element group  295  formed of a plurality of light emitting elements  2951  is disposed on the head substrate  293  corresponding to each of the plurality of lenses LS. Further, an aggregate of a plurality of spots SP formed on the image plane IP by imaging the light beams from the light emitting element group  295  on the image plane IP by the lens LS corresponding to the light emitting element group  295  is defined as a spot group SG. In other words, a plurality of spot groups SG can be formed in one-to-one correspondence with a plurality of light emitting groups  295 . Further, the spot located uppermost stream in both the main-scanning direction MD and the sub-scanning direction SD in each of the spot groups SG is specifically defined as the first spot. Further, the light emitting element  2951  corresponding to the first spot is specifically defined as the first light emitting element. 
     Further, spot group row SGR and spot group column SGC are defined as shown in the “SURFACE OF IMAGE PLANE” column in  FIG. 2 . In other words, a plurality of spot groups SG arranged in the main-scanning direction MD is defined as a spot group row SGR. Further, a plurality of spot group rows SGR is arranged side by side in the sub-scanning direction SD at a predetermined spot group row pitch Psgr. Further, a plurality (three in the drawing) of spot groups SG arranged consecutively at a pitch having a component of the sub-scanning direction SD equal to the spot group row pitch Psgr and a component of the main-scanning direction MD equal to a spot group pitch Psg is defined as a spot group column SGC. It should be noted that the spot group row pitch Psgr is a distance in the sub-scanning direction SD between the geometric centroids of the respective two spot group rows GSR adjacent to each other in the sub-scanning direction SD. Further, the spot group pitch Psg is a distance in the main-scanning direction MD between the geometric centroids of the respective two spot groups SG adjacent to each other in the main-scanning direction MD. 
     Lens row LSR and lens column LSC are defined as shown in the “LENS ARRAY” column in the drawing. Specifically, a plurality of lenses LS arranged in the longitudinal direction LGD is defined as the lens row LSR. Further, a plurality of lens rows LSR is arranged side by side in the width direction LTD at a predetermined lens row pitch Plsr. Further, a plurality (three in the drawing) of lenses LS arranged consecutively at a pitch having a component of the width direction LTD equal to the lens row pitch Plsr and a component of the longitudinal direction LGD equal to a lens pitch Pls is defined as a lens column LSC. It should be noted that the lens row pitch Plsr is a distance in the width direction LTD between the geometric centroids of the respective two lens rows LSR adjacent to each other in the width direction LTD. Further, the lens pitch Pls is a distance in the longitudinal direction LGD between the geometric centroids of the respective two lens LS adjacent to each other in the longitudinal direction LGD. 
     Light emitting element group row  295 R and light emitting element group column  295 C are defined as shown in the “HEAD SUBSTRATE” column in the drawing. Specifically, a plurality of light emitting element groups  295  arranged in the longitudinal direction LGD is defined as the light emitting element group row  295 R. Further, a plurality of light emitting group rows  295 R is arranged side by side in the width direction LTD at a predetermined light emitting element group row pitch Pegr. Further, a plurality (three in the drawing) of light emitting element groups arranged consecutively at a pitch having a component of the width direction LTD equal to the light emitting element group row pitch Pegr and a component of the longitudinal direction LGD equal to a light emitting element group pitch Peg is defined as a light emitting element group column  295 C. It should be noted that the light emitting element group row pitch Pegr is a distance in the width direction LTD between the geometric centroids of the respective two light emitting element group rows  295 R adjacent to each other in the width direction LTD. Further, the light emitting element group pitch Peg is a distance in the longitudinal direction LGD between the geometric centroids of the respective two light emitting element groups  295  adjacent to each other in the longitudinal direction LGD. 
     Light emitting element row  2951 R and light emitting element column  2951 C are defined as shown in the “LIGHT EMITTING ELEMENT GROUP” column in the drawing. Specifically, in each of the light emitting element groups  295 , a plurality of light emitting elements  2951  arranged in the longitudinal direction LGD is defined as the light emitting element row  2951 R. Further, a plurality of light emitting element rows  2951 R is arranged side by side in the width direction LTD at a predetermined light emitting element row pitch Pelr. Further, a plurality (two in the drawing) of light emitting elements  2951  arranged consecutively at a pitch having a component of the width direction LTD equal to the light emitting element row pitch Pelr and a component of the longitudinal direction LGD equal to a light emitting element pitch Pel is defined as a light emitting element column  2951 C. It should be noted that the light emitting element row pitch Pelr is a distance in the width direction LTD between the geometric centroids of the respective two light emitting element rows  2951 R adjacent to each other in the width direction LTD. Further, the light emitting element pitch Pel is a distance in the longitudinal direction LGD between the geometric centroids of the respective two light emitting elements  2951  adjacent to each other in the longitudinal direction LGD. 
     Spot row SPR and spot column SPC are defined as shown in the “SPOT GROUP” column in the drawing. Specifically, in each of the spot groups SG, a plurality of spots SP arranged in the longitudinal direction LGD is defined as the spot row SPR. Further, a plurality of spot rows SPR is arranged side by side in the width direction LTD at a predetermined spot row pitch Pspr. Further, a plurality (two in the drawing) of spots arranged consecutively at a pitch having a component of the width direction LTD equal to the spot row pitch Pspr and a component of the longitudinal direction LGD equal to a spot pitch Psp is defined as a spot column SPC. It should be noted that the spot row pitch Pspr is a distance in the sub-scanning direction SD between the geometric centroids of the respective two spot rows SPR adjacent to each other in the sub-scanning direction SD. Further, the spot pitch Psp is a distance in the main-scanning direction MD between the geometric centroids of the respective two spots SP adjacent to each other in the longitudinal direction LGD. 
     B. First Embodiment 
       FIG. 3  is a diagram showing an example of an image forming device according to an embodiment of the invention. Further,  FIG. 4  is a diagram showing an electrical configuration of the image forming device shown in  FIG. 3 . The device is an image forming device capable of selectively performing a color mode in which a color image is formed by overlapping four colors of toners of black (K), cyan (C), magenta (M), and yellow (Y), and a monochrome mode in which a monochrome image is formed using only the black (K) toner. It should be noted that  FIG. 3  is a drawing corresponding to a state when performing the color mode. In the present image forming device, when an image formation command is provided to a main controller MC having a CPU, a memory, and so on from an external device such as a host computer, the main controller MC provides an engine controller EC with a control signal and so on, and a head controller HC with the video data VD corresponding to the image formation command. Further, the head controller HC controls line heads  29  in charge of respective colors based on the video data VD from the main controller MC and a vertical sync signal Vsync and parameter values from the engine controller EC. Thus, an engine section EG performs a prescribed image forming operation, thereby forming an image corresponding to the image formation command on a sheet such as copy paper, transfer paper, a form, or an OHP transparent sheet. 
     Inside a main housing  3  provided to the image forming device, there is disposed an electric component box  5  housing a power supply circuit board, the main controller MC, the engine controller EC, and the head controller HC. Further, an image forming unit  7 , a transfer belt unit  8 , and a paper feed unit  11  are also disposed inside the main housing  3 . Further, inside the main housing  3  and on the right side thereof in  FIG. 3 , there are disposed a secondary transfer unit  12 , a fixing unit  13 , and a sheet guide member  15 . It should be noted that the paper feed unit  11  is configured so as to be detachably mounted to a main body  1  of the device. Further, it is arranged that the paper feed unit  11  and the transfer belt unit  8  can separately be detached from the main body to be repaired or replaced. 
     The image forming unit  7  is provided with four image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) for forming images with respective colors different from each other Further, each of the image forming stations Y, M, C, and K is provided with a cylindrical photoconductor drum  21  having a surface with a predetermined length in the main-scanning direction MD. Further, each of the image forming stations Y, M, C, and K forms a toner image of the corresponding color on the surface of the photoconductor drum  21 . The photoconductor drum is disposed so as to have the axial direction thereof substantially parallel to the main-scanning direction MD. Further, each of the photoconductor drums  21  is connected to a dedicated drive motor, and is driven to rotate at a predetermined speed in a direction of the arrow D 21  in the drawing. Thus, the surface of the photoconductor drum  21  is moved in the sub-scanning direction SD perpendicular to or substantially perpendicular to the main-scanning direction MD. Further, around the photoconductor drum  21 , there are disposed along the rotational direction, a charging section  23 , the line head  29 , a developing section  25 , and a photoconductor cleaner  27 . Further, a charging operation, a latent image forming operation, and a toner developing operation are executed by these functional sections. Therefore, when executing the color mode, the toner images respectively formed by all of the image forming stations Y, M, C, and K are overlapped on a transfer belt  81  provided to a transfer belt unit  8  to form a color image, and when executing the monochrome mode, a monochrome image is formed using only the toner image formed by the image forming station K. It should be noted that in  FIG. 3 , since the image forming stations in the image forming unit  7  have the same configurations as each other, the reference numerals are only provided to some of the image forming stations, and are omitted in the rest of the image forming stations only for the sake of convenience of illustration. 
     The charging section  23  is provided with a charging roller having a surface made of elastic rubber. The charging roller is configured so as to be rotated by the contact with the surface of the photoconductor drum  21  at a charging position, and is rotated in association with the rotational operation of the photoconductor drum  21  in a driven direction with respect to the photoconductor drum  21  at a circumferential speed. Further, the charging roller is connected to a charging bias generating section (not shown), accepts the power supply for the charging bias from the charging bias generating section, and charges the surface of the photoconductor drum  21  at the charging position where the charging section  23  and the photoconductor drum  21  have contact with each other. 
     The line head  29  is disposed corresponding to the photoconductor drum  21  so that the longitudinal direction thereof corresponds to the main-scanning direction MD and the width direction thereof corresponds to the sub-scanning direction SD, and the longitudinal direction of the line head  29  is arranged to be substantially parallel to the main-scanning direction MD. The line head  29  is provided with a plurality of light emitting elements arranged in the longitudinal direction, and is disposed separately from the photoconductor drum  21 . Further, these light emitting elements emit light onto the surface of the photoconductor drum  21  charged by the charging section  23 , thereby forming an electrostatic latent image on the surface thereof. 
     The developing section  25  has a developing roller  251  with a surface holding the toner. Further, the charged toner is moved to the photoconductor drum  21  from the developing roller  251  by a developing bias applied to the developing roller  251  from a developing bias generating section (not shown) electrically connected to the developing roller  251  at the developing position where the developing roller  251  and the photoconductor drum  21  have contact with each other, thereby making the electrostatic latent image formed by the line head  29  visible. 
     The toner image thus made visible at the developing position is fed in the rotational direction D 21  of the photoconductor drum  21 , and then primary-transferred to the transfer belt  81  described in detail later at a primary transfer position TR 1  where the transfer belt  81  and each of the photoconductor drums  21  have contact with each other. 
     Further, the photoconductor cleaner  27  is disposed downstream of the primary transfer position TR 1  and upstream of the charging section  23  in the rotational direction D 21  of the photoconductor drum  21  so as to have contact with the surface of the photoconductor drum  21 . The photoconductor cleaner  27  remove the residual toner on the surface of the photoconductor drum  21  after the primary transfer to clean the surface thereof by having contact with the surface of the photoconductor drum  21 . 
     The transfer belt unit  8  is provided with a drive roller  82 , a driven roller  83  (hereinafter also referred to as a blade-opposed roller  83 ) disposed on the left of the drive roller  82  in  FIG. 3 , and the transfer belt  81  stretched across these rollers and circularly driven in the direction (a feeding direction) of the arrow D 81  shown in the drawing. Further, the transfer belt unit  8  is provided with four primary transfer rollers  85 Y,  85 M,  85 C, and  85 K disposed inside the transfer belt  81  respectively opposed one-on-one to the photoconductor drums  21  included in the image forming stations Y, M, C, and K when the photoconductor cartridges are mounted. These primary transfer rollers  85  are electrically connected to respective primary transfer bias generating sections (not shown). Further, as described later in detail, when executing the color mode, all of the primary transfer rollers  85 Y,  85 M,  85 C, and  85 K are positioned on the side of the image forming stations Y, M, C, and K as shown in  FIG. 3  to press the transfer belt  81  against the photoconductor drums  21  included in the respective image forming stations Y, M, C, and K, thereby forming the primary transfer position TR 1  between each of the photoconductor drums  21  and the transfer belt  81 . Then, by applying the primary transfer bias to the primary transfer rollers  85  from the primary transfer bias generating section with appropriate timing, the toner images formed on the surfaces of the photoconductor drums  21  are transferred to the surface of the transfer belt  81  at the respective primary transfer positions TR 1  to form a color image. 
     On the other hand, when executing the monochrome mode, the primary transfer rollers  85 Y,  85 M, and  85 C for color printing out of the four primary transfer rollers  85  are separated from the image forming stations Y.M.C respectively opposed thereto, while only the primary transfer roller  85 K mainly for monochrome printing is pressed against the image forming station K, thus making only the image forming station K mainly for monochrome printing have contact with the transfer belt  81 . As a result, the primary transfer position TR 1  is formed only between the primary transfer roller K mainly for monochrome printing and the corresponding image forming station K. Then, by applying the primary transfer bias to the primary transfer roller  85 K mainly for monochrome printing from the primary transfer bias generating section with appropriate timing, the toner image formed on the surface of the photoconductor drum  21  is transferred to the surface of the transfer belt  81  at the primary transfer position TR 1  to form a monochrome image. 
     Further, the transfer belt unit  8  is provided with a downstream guide roller  86  disposed on the downstream side of the primary transfer roller  85 K mainly for monochrome printing and on the upstream side of the drive roller  82 . Further, the downstream guide roller  86  is arranged to have contact with the transfer belt  81  on a common internal tangent of the primary transfer roller  85 K and the photoconductor drum  21  at the primary transfer position TR 1  formed by the primary transfer roller  85 K mainly for monochrome printing having contact with the photoconductor drum  21  of the image forming station K. 
     The drive roller  82  circularly drives the transfer belt  81  in the direction of the arrow D 81  shown in the drawing, and at the same time functions as a backup roller of a secondary transfer roller  121 . On the peripheral surface of the drive roller  82 , there is formed a rubber layer with a thickness of about 3 mm and a volume resistivity of no greater than 1000 kΩ·cm, which, when grounded via a metal shaft, serves as a conducting path for a secondary transfer bias supplied from a secondary transfer bias generating section, not shown, via the secondary transfer roller  121 . By thus providing the rubber layer having an abrasion resistance and a shock absorbing property to the drive roller  82 , the impact caused by a sheet entering the contact section (a secondary transfer position TR 2 ) between the drive roller  82  and the secondary transfer roller  121  is hardly transmitted to the transfer belt  81 , thus the degradation of the image quality can be prevented. 
     The paper feed unit  11  is provided with a paper feed section including a paper feed cassette  77  capable of holding a stack of sheets and a pickup roller  79  for feeding the sheet one-by-one from the paper feed cassette  77 . The sheet fed by the pickup roller  79  from the paper feed section is fed to the secondary transfer position TR 2  along the sheet guide member  15  after the feed timing thereof is adjusted by a pair of resist rollers  80 . 
     The secondary transfer roller  121  is provided so as to be able to be selectively contacted with and separated from the transfer belt  81 , and is driven to be selectively contacted with and separated from the transfer belt  81  by a secondary transfer roller drive mechanism (not shown). The fixing unit  13  has a rotatable heating roller  131  having a heater such as a halogen heater built-in and a pressing section  132  for biasing the heating roller  131  to be pressed against an object. Then, the sheet with the image, which is secondary-transferred on the surface thereof, is guided by the sheet guide member  15  to a nipping section formed of the heating roller  131  and a pressing belt  1323  of the pressing section  132 , and the image is thermally fixed in the nipping section at predetermined temperature. The pressing section  132  is composed of two rollers  1321 ,  1322  and the pressing belt  1323  stretched across the two rollers. Further, it is arranged that by pressing a tensioned part of the surface of the pressing belt  1323 , which is stretched by the two rollers  1321 ,  1322 , against the peripheral surface of the heating roller  131 , a large nipping section can be formed between the heating roller  131  and the pressing belt  1323 . Further, the sheet on which the fixing process is thus executed is fed to a paper catch tray  4  disposed on an upper surface of the main housing  3 . 
     Further, in the present device, a cleaner section  71  is disposed facing the blade-opposed roller  83 . The cleaner section  71  has a cleaner blade  711  and a waste toner box  713 . The cleaner blade  711  removes foreign matters such as the toner remaining on the transfer belt  81  after the secondary transfer process or paper dust by pressing a tip section thereof against the blade-opposed roller  83  via the transfer belt  81 . Then the foreign matters thus removed are collected into the waste toner box  713 . Further, the cleaner blade  711  and the waste toner box  713  are configured integrally with the blade-opposed roller  83 . Therefore, as described below, when the blade-opposed roller  83  moves, the cleaner blade  711  and the waste toner box  713  should also move together with the blade-opposed roller  83 . 
       FIG. 5  is a perspective view showing a schematic configuration of an example of the line head according to the embodiment of the invention. Further,  FIG. 6  is a cross-sectional view of a configuration of the line head along the width direction thereof.  FIG. 7  is an exploded perspective view of the line head. It should be noted that in  FIG. 7 , some members such as a case are omitted from the illustrations. The line head  29  is oriented so that the longitudinal direction LGD corresponds to the main-scanning direction MD, and the width direction LTD corresponds to the sub-scanning direction SD. Further, the line head  29  is provided with a case  291 , and each end of the case  291  is provided with a positioning pin  2911  and a screw hole  2912 . Further, by fitting the positioning pin  2911  into a positioning hole (not shown) provided to a photoconductor cover (not shown) covering the photoconductor drum  21  and positioned with respect to the photoconductor drum  21 , the line head  29  is positioned with respect to the photoconductor drum  21 . Further, set screws are screwed in and fixed to the screw holes (not shown) of the photoconductor cover via the screw holes  2912 , thereby positioning and fixing the line head  29  to the photoconductor drum  21 . 
     The case  291  holds a lens array  299  at a position opposed to the surface of the photoconductor drum  21 , and is provided with a spacer  297  and a head substrate  293  disposed inside thereof in this order from the lens array  299 . The spacer  297  has a function of regulating the distance between the lens array  299  and the head substrate  293 , and has a hollow section  2971  formed inside. Further, the head substrate  293  is a transparent glass substrate, and is provided with a plurality of light emitting element groups  295  disposed on the reverse side (the surface on the opposite side to the lens array  299  out of the two surfaces provided to the head substrate  293 ). Specifically, the plurality of light emitting element groups  295  is disposed on the reverse side surface of the head substrate  293  two-dimensionally with a predetermined distance in each of the longitudinal direction LGD and the width direction LTD from each other. Here, each of the light emitting element groups  295  is composed of a plurality of light emitting elements arranged as described later. In the line head  29 , organic electro-luminescence (EL) elements are used as the light emitting elements. Specifically, the organic EL elements are disposed on the reverse surface of the head substrate  293  as the light emitting elements. Further, the light beam emitted from each of the light emitting elements towards the photoconductor drum  21  proceeds to the lens array  299  passing through the hollow section  2971  of the spacer  297 . 
     As shown in  FIG. 6 , a retainer  2914  presses a back lid  2913  against the case  291  via the head substrate  293 . Specifically, the retainer  2914  has elastic force for pressing the back lid  2913  towards the case  291 , and seals the inside of the case  291  light-tightly (in other words, so that light does not leak from the inside of the case  291  and that light does not enter from the outside of the case  291 ) by pressing the back lid with such elastic force. It should be noted that the retainer  2914  is disposed in each of a plurality of positions in the longitudinal direction LGD. Further, the light emitting element groups  295  are covered by a seal member  294 . 
       FIG. 8  is a plan view schematically showing the lens array, and corresponds to the case of viewing the lens array from the image plane side (the photoconductor drum  21  side). As shown in the drawing, the lens array  299  is provided with a plurality of lenses LS in a manner as described below. In the lens array  299 , a plurality of lenses LS is aligned along the longitudinal direction LGD to form the lens row LSR, and the lens row is disposed along each of three rows arranged in the width direction LTD. These three lens rows LSR 1  through LSR 3  are shifted from each other in the longitudinal direction LGD so that the positions of the respective lenses LS are different from each other in the longitudinal direction LGD. 
       FIG. 9  is a cross-sectional view of the lens array along the longitudinal direction LGD. The drawing corresponds to the case of viewing the lens array in the cross-sectional plane including the optical axis OA of each of the lens. In the drawing, the upper side thereof corresponds to the image plane side, and the lower side corresponds to the light emitting element group side. The lens array  299  is provided with one lens substrate LB made of glass, and each of the lenses LS is composed of two lens surfaces LSF 1 , LSF 2  arranged in a direction of the optical axis OA so as to hold the lens substrate LB in between. These lens surfaces LSF 1 , LSF 2  can be formed with, for example, light-curing resin. The lens surface LSF 1  of the two lens surfaces is formed on the reverse surface LBF 1  of the lens substrate LB, and the lens surface LSF 2  is formed on the obverse surface LBF 2  of the lens substrate LB. In other words, the lens surface LSF 1  is formed on one plane of the lens substrate LB on the light emitting element group side, while the lens surface LSF 2  is formed on the other plane thereof. 
     The lens LS thus configured has an inverting reducing optical characteristic, and the light beams emitted from the light emitting element group  295  are reduced and imaged by the lens LS as an inverted image. Further, as explained later with reference to  FIG. 11 , the light emitting element group  295  opposed to the lens LS has a flat shape with a larger length in the longitudinal direction LGD than in the width direction LTD. Therefore, in order for imaging the light beams from the light emitting element group  295  with a preferable aberration corresponding to such a shape of the light emitting element group  295 , an anamorphic lens having an optical characteristic in the width direction LTD and an optical characteristic in the longitudinal direction LGD different from each other is adopted as the lens LS of the present embodiment. 
       FIG. 10  is a diagram showing a configuration of the reverse surface of the head substrate, corresponding to the case in which the reverse surface is viewed from the obverse surface of the head substrate. It should be noted that in FIG.  10 , although the lenses LS are illustrated with double-dashed lines, this does not denote that the lenses LS are disposed on the reverse surface of the substrate, but denotes that the light emitting element groups  295  are disposed corresponding one-on-one to the lenses LS. As shown in  FIG. 9 , a plurality of light emitting element group columns  295 C, each having three light emitting element groups  295  (e.g., the light emitting element groups  295 _ 1 ,  295 _ 2 , and  295 _ 3 ) disposed at positions different from each other in the width direction LTD, are arranged in the longitudinal direction LGD. In other words, the light emitting element group rows  295 R having a plurality of light emitting element groups  295  arranged along the longitudinal direction LGD are arranged in the width direction LTD in three rows ( 295 R_A,  295 R_B, and  295 R_C). In this case, the light emitting element group rows  295 R are shifted from each other in the longitudinal direction LGD so that the positions of the light emitting element groups  295  become different from each other in the longitudinal direction LGD. 
       FIG. 11  is a diagram showing a configuration of the light emitting element group in the first embodiment. As shown in the drawing, an odd number (three light emitting element rows  2951 R_ 1 ,  2951 R_ 2 , and  2951 R_ 3  in the drawing) of light emitting element rows  2951 R, each having four light emitting elements aligned along the longitudinal direction, are arranged in parallel in the width direction LTD. The light emitting element rows  2951 R_ 1 ,  2951 R_ 2 , and  2951 R_ 3  are arranged so as to be shifted the light emitting element pitch Pel from each other in the longitudinal direction LGD, and the light emitting elements  2951  are located at positions different from each other in the longitudinal direction LGD. As a result, the light emitting element group  295  has a flat shape having a larger length in the longitudinal direction LGD than in the width direction LTD. It should be noted that “the light emitting element pitch Pel” corresponds to a pitch between the two light emitting elements  2951 , which forms the spot latent images adjacent to each other in the main-scanning direction MD, and “the spot latent image” is a latent image formed on the surface of the photoconductor drum by the light beam from the light emitting element  2951  imaged as a spot. Therefore, as described later, the spot latent images adjacent to each other in the main-scanning direction MD are respectively formed by the light emitting elements  2951  (e.g., the light emitting elements  2951 _ 1 ,  2951 _ 2  in the drawing) belonging to the different light emitting element rows  2951 R from each other. 
     Further, as shown in the drawing, in the light emitting element group  295 , one light emitting element row  2951 R_ 2  of the three light emitting element rows  2951 R matches with the meridian plane PL_lgd. In other words, each of the light emitting elements  2951  of the light emitting element row  2951 R_ 2  intersects the meridian plane PL_lgd. Further, in the light emitting element row  2951 R_ 2  matching with the meridian plane PL_lgd, each of the light emitting elements  2951  is disposed symmetrically around the optical axis OA. It should be noted that the meridian plane PL_lgd is a plane parallel to the longitudinal direction LGD and including the optical axis OA of the lens LS. Further, in the drawing, the optical axis OA is represented as an intersection between the plane PL_ltd parallel to the width direction LTD and including the optical axis OA and the meridian plane PL_lgd. 
     Further, in the light emitting element group  295 , the light emitting element row  2951 R_ 2  matching with the meridian plane PL_lgd is centered, and the other light emitting element rows  2951 R are disposed on both sides thereof in the width direction LTD. In other words, in the width direction LTD, the same number (one in the first embodiment) of light emitting element rows  2951 R are disposed respectively on both sides of the light emitting element row  2951 R_ 2 . The reason to arrange the light emitting element rows  2951 R as described above is as follows. That is, if the light emitting element rows  2951 R are not disposed on both sides centering around the light emitting element row  2951 R_ 2 , namely in the case, for example, in which the light emitting element row  2951 R_ 3  is disposed above the light emitting element row  2951 R_ 1  in the drawing, the light emitting element row  2951 R_ 3  is disposed more distantly from the optical axis OA. As a result, there are some cases in which the aberration of the image obtained by imaging the light beams from the light emitting element row  2951 R_ 3  becomes worse. In contrast, in the first embodiment, the light emitting element rows are disposed on both sides centering around the light emitting element row  2951 R_ 2  matching with the meridian plane PL_lgd, and therefore, it becomes possible to dispose each of the light emitting element rows  2951 R relatively closer to the optical axis OA. Therefore, it becomes possible to image the light beam emitted from each of the light emitting elements  2951  by the lens LS with a preferable aberration. A latent image forming operation by the line head  29  described above will hereinafter be explained. 
       FIG. 12  is a diagram showing a spot latent image forming operation by the line head. Hereinafter, the spot latent image forming operation by the line head according to the first embodiment will be explained with reference to  FIGS. 10 through 12 . Further, for facilitating understanding of the invention, the case in which a plurality of spot latent images is aligned on a straight line extending in the main scanning direction MD to form a line latent image will be explained here. As a rough outline, in the latent image forming operation, the head control module  54  makes each of the light emitting elements  2951  at predetermined timing while conveying the surface of the photoconductor drum  21  in the sub-scanning direction SD, thereby forming a plurality of spots aligned in the main-scanning direction MD. Hereinafter, the detail of the operation will be explained. 
     Firstly, when the light emitting element row  2951 R_ 3  of each of the light emitting element groups  295  (e.g.,  295 _ 1 , and  295 _ 4 ) belonging to the light emitting element group row  295 R_A on the uppermost stream side in the width direction LTD emits light, the spot latent images represented by the hatching pattern shown in the “1ST” row of  FIG. 12 . It should be noted that the outline circles in  FIG. 12  each represent a spot latent image which has not yet been formed, and will be formed later. Further, in the drawing, the reference numerals  295 _ 1 ,  295 _ 2 ,  295 _ 3 , and  295 _ 4  denote that the spot latent images in the corresponding columns are formed by the light emitting element groups denoted with the respective reference numerals. Further, in  FIG. 12 , the reference symbols Lsp_ 1  through Lsp_ 3  are specifically provided to the spot latent images formed by the light emitting elements  2951 _ 1  through  2951 _ 3 , respectively. 
     The light emitting element row  2951 R_ 2  emits light subsequently to the light emission of the light emitting element row  2951 R_ 3  to form the spot latent images represented by the hatching pattern in the “2ND” row of  FIG. 12 . Subsequently, the light emitting element row  2951 R_ 1  emits light to form the spot latent images represented by the hatching pattern in the “3RD” row of  FIG. 12 . Here, the reason to sequentially emit light from the light emitting element row  2951 R on the downstream side in the width direction LTD is for coping with the inverting characteristic provided to the lens LS. 
     Subsequently, the light emitting element groups  295  (e.g.,  295 _ 2 ) belonging to the light emitting group row  295 R_B on the downstream side of the light emitting element group row  295 R_A in the width direction LTD performs the light emitting operation in the same manner as the light emitting element group row  295 R_A to form the spot latent images represented by the hatching patterns shown in the “4TH” through “6TH” rows of  FIG. 12 . Further, the light emitting element groups  295  (e.g.,  295 _ 3 ) belonging to the light emitting group row  295 R_C on the downstream side of the light emitting element group row  295 R_B in the width direction LTD performs the light emitting operation in the same manner as the light emitting element group row  295 R_A to form the spot latent images represented by the hatching patterns shown in the “7TH” through “9TH” rows of  FIG. 12 . As described above, by performing the light emitting operations corresponding to the first through ninth times, the plurality of spot latent images are formed in a line in the main-scanning direction MD. 
     As described above, in the first embodiment, the light emitting element rows  2951 R are shifted from each other in the longitudinal direction so that the two light emitting elements  2951  for forming the spot latent images adjacent to each other in the main-scanning direction MD belong to the light emitting element rows  2951 R different from each other. In a specific explanation, the two light emitting elements  2951 _ 1 ,  2951 _ 2  forming the spot latent images Lsp_ 1 , Lsp_ 2  adjacent to each other in the main-scanning direction MD belong respectively to the light emitting element rows  2951 R_ 1 ,  2951 R_ 2  different from each other, for example. In other words, in the first embodiment, the two light emitting elements  2951  (e.g.,  2951 _ 1 ,  2951 _ 2 ) forming the spot latent images adjacent to each other in the main-scanning direction MD are disposed so as to be shifted from each other in the width direction LTD. Therefore, since the light emitting element  2951  can be formed in the relatively large space, the size of the light emitting element  2951  can be increased. Therefore, it becomes possible to form the spot latent image with a sufficient amount of light even in high-resolution conditions, thus preferable spot latent image formation becomes possible. 
     Further, such a configuration of the light emitting element group  295  allowing the size of the light emitting element  2951  to grow is suitable for the line head  29  having organic EL elements as the light emitting elements  2951 . This is because, the organic EL elements only emit light with low intensity. Therefore, from the viewpoint of forming the spots SP with a sufficient amount of light, with respect to such a configuration, it is preferable to apply the configuration described above advantageous to increasing the amount of light of the light emitting element  2951  by increasing the size of the light emitting element  2951 . In particular, since the bottom emission organic EL elements emit light with lower intensity, it is preferable to apply the configuration described above to the configurations using the bottom emission organic EL elements as the light emitting elements  2951 . 
     Incidentally, in the configuration of providing the lens LS in correspondence with each of the light emitting element group  295  as in the line head  29  described above, the alignment between the light emitting element groups  295  and the optical axes OA of the respective lenses LS becomes important. In particular, in the embodiment described above, each of the light emitting elements  2951  emits light at the timing corresponding to the movement of the surface of the photoconductor drum  21  in the sub-scanning direction SD, thereby forming the plurality of spot latent images aligned in the main-scanning direction MD. Therefore, from a viewpoint of forming these spot latent images at appropriate positions on the surface of the photoconductor drum  21 , it is more severely required in the width direction LTD corresponding to the sub-scanning direction SD that the alignment is performed with high accuracy. In this regard, in the line head  29  described above, the light emitting element row  2951 R_ 2  matches with the meridian plane PL_lgd. Therefore, it becomes possible to execute the alignment in the width direction LTD with ease and high accuracy by performing the alignment using the light emitting element row  2951 R_ 2  as a reference. Further, in the first embodiment, each of the light emitting elements  2951  are disposed symmetrically around the optical axis OA in the light emitting element row  2951 R_ 2 , and it is arranged that the alignment thereof in the longitudinal direction LGD can also be performed with ease and high accuracy. Further, by forming the latent images with the line head  29  in which the alignment has thus been performed with high accuracy, the preferable spot latent image formation becomes possible. Therefore, the position adjustment operation for executing the alignment will hereinafter explained. 
       FIG. 13  is a perspective view showing an array moving mechanism and an observation optical system provided to the position adjustment device of the line head,  FIG. 14  is a diagram of the position adjustment device of the line head viewed along the longitudinal direction thereof, and  FIG. 15  is a block diagram showing an electrical configuration of the position adjustment device of the line head. Further, the position adjustment operation can be performed by the position adjustment device  9 . The position adjustment device  9  is provided with a position adjustment device controller  98 , and the position adjustment device controller  98  is capable of controlling each sections of the position adjustment device  9 . Further, the position adjustment device  9  of the line head is provided with a substrate holding section  91  adapted to hold the head substrate  293 , three array moving mechanisms  93 ,  95 ,  97 , and an observation optical system  99 . 
     The substrate holding section  91  is configured to be capable of holding the head substrate  293  provided with the light emitting element groups  295  on the reverse surface thereof. Specifically, the substrate holding section  91  is provided with two mounting stages  911 ,  912 , and a retraction space  913  between the both mounting stages  911 ,  912 . The two mounting stages  911 ,  912  are provided with L-shaped notch sections  9111 ,  9121 , respectively. Further, these notch sections  9111 ,  9121  are disposed so as to be opposed to each other. Further, when holding the head substrate  293  by the substrate holding section  91 , one end of the head substrate  293  in the width direction LTD thereof is mounted on the notch section  9111 , and then the other end of the head substrate  293  in the width direction LTD is mounted on the notch section  9121 . The distance between the notch sections  9111 ,  9121  is set so as to limit the movement of the head substrate  293  in the width direction LTD, and the head substrate  293  mounted on the substrate holding section  91  is limited by the notch sections  9111 ,  9121  in the movement in the width direction LTD. It should be noted that there is provided a similar mechanism for limiting the movement of the head substrate  293  mounted thereon to the substrate holding section  91  with respect also to the longitudinal direction LGD substantially perpendicular to the width direction LTD. As described above, the substrate holding section  91  holds the head substrate  293  so as to limit the movement of the head substrate  293  in both of the width direction LTD and the longitudinal direction LGD of the head substrate  293  mounted thereon. 
     Further, in the condition in which the head substrate  293  is mounted on the substrate holding section  91 , although the light emitting element groups  295  and the seal member  294  disposed on the reverse surface of the head substrate  293  protrude from the head substrate  293  towards a lower side in the direction of gravitational force, the retraction space  913  is provided to the substrate holding section  91  as described above. By thus providing the retraction space  913 , the light emitting element groups  295  and the seal member  294  are prevented from having contact with other members. 
     The first array moving mechanism  93  is provided with a micrometer head  931  and a biasing rod  932 . The micrometer head  931  is supported by a support member  933  fixed to the substrate holding section  91 . Further, a moving rod  9311  as a spindle of the micrometer head  931  moves forward and backward in a stroke direction SD 93  in accordance with rotational movement of a knob  9312 . The biasing rod  932  is disposed so as to be opposed to the moving rod  9311 . As shown in the drawing, a biasing rod  932  is fitted into a hole provided to the support member  934  so as to move forward and backward through the hole in the stroke direction SD 93 . It should be noted that the support member  934  is fixed with respect to the substrate holding section  91 . Further, the support member  935  fixed to the substrate holding section  91  and the biasing rod  932  are connected to each other via a biasing member  936 , thus the biasing rod  932  is biased in the stroke direction SD 93 . 
     Further, the moving operation of the lens array  299  with the array moving mechanism  93  can be performed as follows. Firstly, subsequently to the mounting of the head substrate  293  on the substrate holding section  91 , the spacer  297  is mounted on the head substrate  293 . In this state, the lens array  299  is located between the moving rod  9311  and the biasing rod  932  of the array moving mechanism  93 . Further, by rotating the knob  9312  to adjust an amount of the movement of the moving rod  9311 , the lens array  299  is held between the moving rod  9311  and the biasing rod  932 . By moving the moving rod forward and backward in the state of holding the lens array  299  between the both rods  9311 ,  932 , the lens array  299  can be moved in the stroke direction SD 93 . It should be noted that on this occasion the biasing rod  932  is biased towards the moving rod  9311  in the stroke direction SD 93 . Therefore, the lens array  299  is moved while being held between the moving rod  9311  and the biasing rod  932  with biasing force thus applied. 
     The second array moving mechanism  95  is provided with a micrometer head  951  and a biasing rod  952 . Further, the lens array  299  can be moved in a stroke direction SD 95  by rotating a knob  9512  to move a moving rod  9511  as a spindle of the micrometer head  951  forward and backward. It should be noted that since the configuration and the operation of the second array moving mechanism  95  are substantially the same as those of the first array moving mechanism  93 , detailed explanations therefor will be omitted. 
     Further, the third array moving mechanism  97  is provided with a micrometer head  971  and a biasing rod  972 . The micrometer head  971  and the biasing rod  972  of the array moving mechanism  97  are different from those of the array moving mechanisms  93 ,  95  in that the lens array  299  is held therebetween in the longitudinal direction LGD. Further, the lens array  299  can be moved in a stroke direction SD 97  by rotating a knob  9712  to move a moving rod  9711  as a spindle of the micrometer head  971  forward and backward. It should be noted that since the configuration and the operation of the third array moving mechanism  97  are substantially the same as those of the first array moving mechanism  93 , detailed explanations therefor will be omitted. 
     As shown in  FIG. 13 , the stroke directions SD 93 , SD 95  are substantially parallel to the width direction LTD, and the stroke direction SD 97  is substantially parallel to the longitudinal direction LGD. Therefore, the first and second array moving mechanisms  93 ,  95  have a function of moving the lens array  299  in the width direction LTD, and the third array moving mechanism  97  has a function of moving the lens array  299  in the longitudinal direction LGD. 
     The observation optical system  99  is disposed so as to view one end area in the longitudinal direction LGD of the head substrate  293  or the lens array  299  from an upper side in the direction of gravitational force. In other words, the observation optical system  99  reflects various parts (e.g., the lens array  299 ) of the line head  29  from the direction of the optical axis OA of the lens LS, and the image taken by a camera  991  corresponds to an image projected on a plane perpendicular to the optical axis OA of the lens LS. Further, the image reflected by the observation optical system  99  is acquired in an image processing section  981 , and the image processing section  981  can execute various kinds of signal processing on the image thus acquired. Further, the position adjustment operation can be executed using the position adjustment device  9  described above in a manner as described below. 
       FIG. 16  is a flowchart showing the position adjustment operation of the line head.  FIG. 17  is a diagram showing images reflected by the observation optical system in the position adjustment operation according to the first embodiment. In the step S 101 , the head substrate  293  is disposed on the substrate holding section  91  (the substrate disposing step). In the step S 102  subsequent thereto, the observation optical system  99  observes the light emitting element group  295  of the head substrate  293  (the column “ 1 A” of  FIG. 17 ), and then the image of the light emitting element group  295  is transmitted to the image processing section  981 . In the image processing section  981 , the image of the light emitting element row  2951 R_ 2  is obtained based on the image thus transmitted (the step S 103 , the image obtaining step). In other words, the image processing section  981  obtains the image of the light emitting element row  2951 R_ 2  matching with the meridian plane PL_lgd in the condition in which the alignment has been executed. It should be noted that the light emitting element row matching with the meridian plane PL_lgd in the condition in which the alignment has been executed will hereinafter be specifically referred to as “a reference light emitting element row.” 
     In the step S 104  subsequent thereto, the lens array  299  is tentatively mounted. It should be noted that “tentatively mounting” denotes that the lens array  299  is disposed so as to be opposed to the head substrate  293  in the condition in which the lens array  299  is movable to the head substrate  293 . Specifically, in the step S 104 , the spacer  297  is mounted on the head substrate  293 , and the lens array  299  is subsequently mounted on the spacer  297  (the tentatively mounting step). On this occasion, the lens array  299  is mounted so that the lenses LS correspond respectively to the light emitting element groups  295 . 
     In the step S 105 , in the condition in which the lens array  299  is thus tentatively mounted, each of the light emitting elements  2951  of the reference light emitting element row  2951 R_ 2  emits light, and the light beam from each of the light emitting elements  2951  is imaged as the spot SP by the lens LS. Thus, the spot row SPR composed of a plurality of spots SP is formed (the column “ 1 B” of  FIG. 17 ). On this occasion, in correspondence with the fact that the light emitting elements  2951  of the reference light emitting element row  2951 R_ 2  are disposed symmetrically with respect to the symmetry center SC 1 , the spots SP of the spot row SPR are disposed symmetrically with respect to the symmetry center SC 2 . It should be noted that, as explained above with reference to  FIG. 11 , in the condition in which the alignment has been executed, the light emitting elements  2951  of the reference light emitting element row  2951 R_ 2  are disposed symmetrically around the optical axis OA. Therefore, in the condition in which the alignment has been executed, the symmetry center SC 1  matches with the optical axis OA, and at the same time, the symmetry center SC 2  corresponding to the image of the symmetry center SC 1  by the lens LS also matches with the optical axis OA. 
     As shown in the column “ 1 B” of  FIG. 17 , the alignment between the light emitting element groups  295  and the lenses LS has not been executed at the instant of the step S 105 , the reference light emitting element row  2951 R_ 2  and the spot row SPR are shifted from each other. It should be noted that the image of the reference light emitting element row  2951 R_ 2  obtained by the image processing section  981  in the step S 103  is reflected in the image of the observation optical system  99  with dashed circles ( FIG. 17 ). Further, there are drawn an additional line AL 1  passing through the centers of the light emitting elements  2951  of the reference light emitting element low  2951 R_ 2 , and an additional line AL 2  passing through the centers of the spots of the spot row SPR ( FIG. 17 ). These additional lines AL 1 , AL 2  are lines drawn by the image processing section  981  in order for making the alignment operation in the subsequent step S 106  easy. 
     In the step S 106 , the array moving mechanisms  93 ,  95 , and  97  move the lens array  299  to execute the alignment between the light emitting groups  295  and the lenses LS. Specifically, the image of the reference light emitting element row  2951 R_ 2  and the spot row SPR are firstly overlapped with each other in the width direction LTD to execute the alignment in the width direction LTD of the lens array  299  (the column “ 1 C” of  FIG. 17 ). On this occasion, the alignment in the width direction LTD can easily be executed by overlapping the additional lines AL 1 , AL 2  with each other. As described above, according to the first embodiment, it is possible to execute the alignment in the width direction LTD with ease and high accuracy by performing the alignment using the reference light emitting element row  2951 R_ 2  as a reference. Further, the alignment in the longitudinal direction LGD is executed as follows. Specifically, by overlapping the symmetry center SC 1  of the reference light emitting element row  2951 R_ 2  and the symmetry center SC 2  of the spot row SPR with each other, the alignment in the longitudinal direction LGD is executed (the column “ 1 D” of  FIG. 17 ). When the alignment in both of the width direction LTD and the longitudinal direction LGD is completed in a manner as described above, the lens array  299  is fixed to the head substrate  293  (the step S 107 ). 
     C. Second Embodiment 
     In the first embodiment, an odd number (three light emitting element rows  2951 R_ 1 ,  2951 R_ 2 , and  2951 R_ 3 ) of light emitting element rows  2951 R, each having an even number (four) of light emitting elements  2951  aligned along the longitudinal direction, are arranged in parallel in the width direction LTD in each of the light emitting element groups  295 . However, the number of light emitting element rows  2951 R and the number of light emitting elements  2951  forming each of the light emitting element rows  2951 R are not limited thereto, but it is also possible to constitute the light emitting element groups  295  as follows, for example. 
       FIG. 18  is a diagram showing a configuration of the light emitting element group in the second embodiment. As shown in the drawing, an odd number (five light emitting element rows  2951 R_ 1  through  2951 R_ 5  in the drawing) of light emitting element rows  2951 R, each having an odd number (five) of light emitting elements  2951  aligned along the longitudinal direction, are arranged in parallel in the width direction LTD. The light emitting element rows  2951 R_ 1  through  2951 R_ 5  are arranged so as to be shifted the light emitting element pitch Pel from each other in the longitudinal direction LGD, and the light emitting elements  2951  are located at positions different from each other in the longitudinal direction LGD. Therefore, the spot latent images adjacent to each other in the main-scanning direction MD are respectively formed by the light emitting elements  2951  (e.g., the light emitting elements  2951 _ 1 ,  2951 _ 2  in the drawing) belonging to the different light emitting element rows  2951 R from each other. 
     As described above, also in the second embodiment, the light emitting element rows  2951 R are shifted from each other in the longitudinal direction LGD so that the two light emitting elements  2951  for forming the spot latent images adjacent to each other in the main-scanning direction MD belong to the light emitting element rows  2951 R different from each other. Specifically, the two light emitting elements  2951 _ 1 ,  2951 _ 2  forming the spot latent image adjacent to each other in the main-scanning direction MD belong respectively to the light emitting element rows  2951 R_ 1 ,  2951 R_ 2  different from each other, for example. In this manner, the two light emitting elements  2951  (e.g.,  2951 _ 1 ,  2951 _ 2 ) forming the spot latent images adjacent to each other in the main-scanning direction MD are disposed so as to be shifted from each other in the width direction LTD, thus the light emitting elements  2951  can be formed in a relatively large space. As a result, it becomes possible to increase the size of the light emitting element  2951 , and therefore, it becomes possible to preferably form the spot latent images with a sufficient amount of light even in high-resolution conditions. 
     Further, in the second embodiment, in the light emitting element group  295 , one light emitting element row  2951 R_ 3  of the five light emitting element rows  2951 R matches with the meridian plane PL_lgd. In other words, each of the light emitting elements  2951  of the light emitting element row  2951 R_ 3  intersects the meridian plane PL_lgd. In the light emitting element row  2951 R_ 3 , the light emitting elements  2951  are disposed symmetrically around the optical axis OA, and a middle light emitting element ME at the center of the light emitting element row  2951 R_ 3  is located on the optical axis OA as the symmetry center. 
     Further, in the light emitting element group  295 , the light emitting element row  2951 R_ 3  matching with the meridian plane PL_lgd is centered, and the other light emitting element rows  2951 R are disposed on both sides thereof in the width direction LTD. In other words, in the width direction LTD, the same number (two in the second embodiment) of light emitting element rows  2951 R are disposed respectively on both sides of the light emitting element row  2951 R_ 3 . Therefore, also in the second embodiment, it becomes possible to image the light beam emitted from each of the light emitting elements  2951  by the lens LS with a preferable aberration. 
     As described above, in the second embodiment, the light emitting element row  2951 R_ 3  matches with the meridian plane PL_lgd. Therefore, it becomes possible to execute the alignment in the width direction LTD with ease and high accuracy by performing the alignment using the light emitting element row  2951 R_ 3  (i.e., the reference light emitting element row  2951 R_ 3 ) as a reference. In the second embodiment, the light emitting element ME of the light emitting element row  2951 R_ 3  is disposed on the optical axis OA, and the alignment in the longitudinal direction LGD can more easily and more accurately be executed. Further, by forming the latent images with the line head  29  in which the alignment has thus been performed with high accuracy, the preferable spot latent image formation becomes possible. Then, the position adjustment operation for executing the alignment will hereinafter explained. 
       FIG. 19  is a diagram showing images reflected by the observation optical system in the position adjustment operation according to the second embodiment. It should be noted that the flow of the position adjustment operation in the second embodiment is substantially the same as described in the flowchart shown in  FIG. 16  presented in the first embodiment. Therefore, hereinafter, the position adjustment operation according to the second embodiment will be explained with reference to  FIGS. 16 and 19 . Further, in the following description, the difference from the position adjustment operation in the first embodiment will mainly be explained, and the explanation of the portions common to each other will be omitted. 
     In the step S 102 , the image (the column “ 2 A” of  FIG. 19 ) of the light emitting element group  295  observed by the observation optical system  99  is transmitted to the image processing section  981 . In the image processing section  981 , the image of the reference light emitting element row  2951 R_ 3  is obtained based on the image thus transmitted (the step S 103 , the image obtaining step). Then, when the lens array  299  is tentatively mounted (the step S 104 ), the light emitting elements  2951  of the reference light emitting element row  2951 R_ 3  emit light subsequently to the step S 104  (the step S 105 ). The light beams from the light emitting elements  2951  are imaged as the spots SP by the lens LS, and the spot row SPR is formed (the column “ 2 B” of  FIG. 19 ). On this occasion, in correspondence with the fact that the light emitting elements  2951  of the reference light emitting element row  2951 R_ 3  are disposed symmetrically with respect to the symmetry center, the spots SP of the spot row SPR are disposed symmetrically with respect to the symmetry center. Particularly in the second embodiment, since the middle light emitting element ME at the center of the light emitting element row  2951 R_ 3  is located at the symmetry center, the middle spot MS, which is the image of the middle light emitting element ME, is located at the symmetry center of the spot row SPR. It should be noted that as is explained with reference to  FIG. 18 , in the condition in which the alignment in both of the width direction LTD and the longitudinal direction LGD has been executed, the middle light emitting element ME of the reference light emitting element row  2951 R_ 3  is located at the optical axis OA. Therefore, in the condition in which the alignment has thus been executed, the middle light emitting element ME and the middle spot MS as the image of the middle light emitting element ME by the lens LS both match with the optical axis OA. 
     As shown in the column “ 2 B” of  FIG. 19 , the alignment between the light emitting element groups  295  and the lenses LS has not been executed at the instant of the step S 105 , the reference light emitting element row  2951 R_ 3  and the spot row SPR are shifted from each other. In the step S 106  subsequent to the step S 105 , the array moving mechanisms  93 ,  95 , and  97  move the lens array  299  to execute the alignment between the light emitting groups  295  and the lenses LS. Specifically, the image of the reference light emitting element row  2951 R_ 3  and the spot row SPR are firstly overlapped with each other in the width direction LTD to execute the alignment in the width direction LTD of the lens array  299  (the column “ 2 C” of  FIG. 19 ). On this occasion, the alignment in the width direction LTD can easily be executed by overlapping the additional lines AL 1 , AL 2  with each other. As described above, according also to the second embodiment, it is possible to execute the alignment in the width direction LTD with ease and high accuracy by performing the alignment using the reference light emitting element row  2951 R_ 3  as a reference. Further, the alignment in the longitudinal direction LGD is executed as follows. specifically, by overlapping the middle light emitting element ME of the reference light emitting element row  2951 R_ 3  and the middle spot MS of the spot row SPR with each other, the alignment in the longitudinal direction LGD is executed (the column “ 2 D” of  FIG. 19 ). When the alignment in both of the width direction LTD and the longitudinal direction LGD is completed in a manner as described above, the lens array  299  is fixed to the head substrate  293  (the step S 107 ). 
     As described above, in the present embodiment, the main-scanning direction MD and the longitudinal direction LGD correspond to “a first direction” of the invention, and the sub-scanning direction SD and the width direction LTD correspond to “a second direction” of the invention. Further, the photoconductor drum  21  corresponds to “a latent image carrier” of the invention, the surface of the photoconductor drum  21  corresponds to “an exposed surface” or “an image plane” of the invention, and the head substrate  293  corresponds to “a substrate” of the invention. Further, the line head  29  corresponds to “an exposure head” of the invention, and the spot SP corresponds to “a light-collected section” of the invention. 
     Other Issues 
     It should be noted that the invention is not limited to the embodiment described above, but can variously be modified besides the embodiment described above within the scope of the invention. For example, the number of light emitting element rows  2951 R and the number of light emitting elements  2951  forming each of the light emitting rows  2951 R are not limited to the content shown in the embodiments described above, but can be modified according to needs. 
     Further, although in the embodiments described above the three light emitting element group rows  295 R are arranged in the width direction LTD, the number of light emitting element group rows  295 R is not limited to three, but can be modified according to needs. 
     Further, although in the embodiments described above the light emitting elements  2951  in the reference light emitting element row is disposed symmetrically with respect to the optical axis OA, it is not essential to the invention to arrange the light emitting elements  2951  in such a manner. In essence, by constituting the line head  29  so that the reference light emitting element row matches with the meridian plane PL_lgd, the advantage of the invention that the alignment in the width direction LTD is executed with ease and high accuracy can be obtained. 
     Further, in the second embodiment, there is provided the middle light emitting element ME matching with the optical axis OA. However, it is not essential to the invention to provide such a middle light emitting element ME. In essence, by constituting the line head  29  so that the reference light emitting element row matches with the meridian plane PL_lgd, the advantage of the invention that the alignment in the width direction LTD is executed with ease and high accuracy can be obtained. 
     Further, in the embodiments described above, the additional lines AL 1 , AL 2  are provided, and the alignment is executed by overlapping these additional lines AL 1 , AL 2  with each other. However, it is not essential to the invention to provide these additional lines AL 1 , AL 2 , and it is also possible to execute the alignment by overlapping the reference light emitting element row and the spot row instead of using such additional lines AL 1 , AL 2 . 
     Further, although in the embodiments described above anamorphic lenses are adopted as the lenses LS, it is also possible to adopt non-anamorphic lenses as the lenses LS. 
     Further, in the embodiments described above, the line head  29  having the reference light emitting element row matching with the meridian plane PL_lgd is explained. However, the advantage of the invention can be obtained not only with the line head  29  having the reference light emitting element row matching with the meridian plane PL_lgd but also with the line head  29  having the reference light emitting element row substantially matching with the meridian plane PL_lgd. The reason is that such alignment can be executed with ease and high accuracy by performing the alignment between the light emitting elements  2951  and the lens LS so that the reference light emitting element row  2951 R_ 2  substantially matches with the meridian plane PL_lgd. It should be noted that the phrase “the reference light emitting element row  2951 R substantially matches with the meridian plane PL_lgd” corresponds to the case in which one or more of the light emitting elements  2951  of the reference light emitting element row  2951 R intersect the meridian plane PL_lgd. Here, the extent in which the reference light emitting element row matches or substantially matches with the meridian plane PL_lgd will specifically be explained with reference to the accompanying drawings. 
       FIG. 20  is a diagram for explaining the case in which the light emitting element row matches with the meridian plane PL_lgd.  FIG. 21  is a diagram for explaining the case in which the light emitting element row substantially matches with the meridian plane PL_lgd. As described above, the meridian plane PL_lgd of the lens LS is parallel to the longitudinal direction LGD. Further, in  FIG. 20 , the meridian plane PL_lgd passes through the center of each of the light emitting elements  2951  of the light emitting element row  2951 R. This case corresponds to the phrase “the meridian plane PL_lgd matches with the light emitting element row  2951 R.” On the other hand, in  FIG. 21 , there are displayed in an overlapping manner two examples each showing the case in which the light emitting element row  2951 R substantially matches with the meridian plane PL_lgd. Here, the light emitting element row  2951 R_cr is explained as a representative case. As shown in the drawing, one or more of the light emitting elements  2951  of the light emitting element row  2951 R_cr intersect the meridian plane PL_lgd. As described above, in this specification, the phrase “the light emitting element row  2951 R matches of substantially matches with the meridian plane PL_lgd” denotes the state in which one or more of the light emitting elements  2951  of the light emitting element row  2951 R intersect the meridian plane PL_lgd. 
     Further, although in the embodiments described above, there is disposed the spacer  297  between the head substrate  293  and the lens array  299 , it is possible to dispose a light-shielding member instead of the spacer  297 .  FIG. 22  is a perspective view showing a schematic configuration of a line head provided with the light shielding member.  FIG. 23  is a perspective view showing a schematic configuration of the line head provided with the light shielding member. Hereinafter, different sections between an embodiment shown in  FIGS. 22 ,  23  and the embodiments described above will mainly be explained, while the common sections are denoted with the same reference numerals, and the explanations therefor will be omitted. As shown in these drawings, the light-shielding member  298  is disposed between the head substrate  293  and the lens array  299 . The light shielding member  298  is provided with a plurality of light guide holes  2981  penetrating the light shielding member corresponding one-on-one to the plurality of light emitting element groups  295 . The light guide holes  2981  penetrate the light-shielding member  298  from the light emitting element groups  295  to the lenses LS opposed to the light emitting element groups  295 , respectively. Therefore, the light beams from the light emitting element groups  295  pass through the light guide holes  2981 , and then imaged as the spots SP by the lenses LS, respectively. In contrast, the stray light, which is a part of each of the light beams from the light emitting element groups  295  and fails to enter the light guide hole  2981 , is shielded by the bottom surface of the light-shielding member  2981 , and therefore, does not reach the lens LS. As described above, the light-shielding member  298  has a function of preventing the stray light from entering the lenses LS. Therefore, the line head  29  provided with the light-shielding member  298  is capable of performing more preferable latent image formation. 
     Further, in the embodiment described above, the light emitting element groups  295 , clusters of a plurality of light emitting elements  2951 , are disposed discretely. However, the arrangement form of the light emitting elements  2951  is not limited thereto.  FIG. 24  is a plan view showing another arrangement form of light emitting elements. The drawing corresponds to the case of viewing perpendicularly the reverse side surface of the head substrate  293  from the obverse side of the head substrate  293 . Further, although the lenses LS are illustrated with double-dashed lines, this is for showing the positional relationship between the light emitting elements  2951  and the lens LS, but not for showing that the lenses LS are disposed on the head substrate  293 . In the drawing, the lens row LSR is formed by linearly arranging a plurality of lenses LS in the longitudinal direction LGD at a pitch twice as large as the lens pitch Pls. Further, two lens rows LSR are arranged at positions different from each other in the width direction LTD. Further, these two lens rows LSR are shifted the lens pitch Pls from each other in the longitudinal direction LGD. In such a manner, the lenses LS are disposed at positions different from each other in the longitudinal direction LGD. 
     Three light emitting element lines  2951 L are disposed at positions different from each other in the width direction LTD corresponding to each of the lens rows LSR. The light emitting element line  2951 L are each formed by linearly arranging a plurality of light emitting elements  2951  in the longitudinal direction LGD at a pitch twice as large as the light emitting element pitch Pel. The three light emitting element lines  2951 L provided to the same lens row LSR are shifted the light emitting element pitch Pel from each other in the longitudinal direction LGD. In such a manner, the light emitting elements  2951  provided to the same lens row LSR are disposed at positions different from each other in the longitudinal direction LGD. 
     In the drawing, the meridian planes PL_lgd of the lenses LS belonging to the same lens row LSR are illustrated with one broken line. It should be noted that similarly to the embodiments described above, the meridian planes PL_lgd are parallel to the longitudinal direction LGD. Further, the light emitting element line  2951 L positioned at the center of the three light emitting element lines  2951 L matches with the meridian plane PL_lgd. 
     In the line head  29  having the light emitting elements  2951  disposed as described above, all of the light emitting elements  2951  do not necessarily contribute to the latent image formation, but some of the light emitting elements  2951  contribute to the latent image formation. In other words, the light emitting elements DEL (the light emitting elements  2951  illustrated with filled circles) located relatively distantly from the optical axis of the lens LS do not contribute to the latent image formation, and the light emitting elements UEL (the light emitting elements  2951  illustrated with open circles) located relatively close to the optical axis of the lens LS contribute to the latent image formation. Specifically, 18 used light emitting elements UEL disposed symmetrically with respect to the optical axis of the lens LS contribute to the latent image formation. 
     Further, these 18 used light emitting elements UEL correspond to the light emitting element group  295  of the embodiments described above. Specifically, each of the light emitting element rows  2951 R is composed of 6 used light emitting elements UEL arranged in a line in the longitudinal direction LGD. Further, three light emitting element rows  2951 R_ 1 ,  2951 R_ 2 , and  2951 R_ 3  are disposed at positions different from each other in the width direction LTD, and moreover, the three light emitting element rows  2951 R_ 1 ,  2951 R_ 2 , and  2951 R_ 3  are shifted from each other in the longitudinal direction LGD. Therefore, the two used light emitting elements UEL forming the spots SP adjacent to each other in the main-scanning direction MD are disposed so as to be shifted from each other in the width direction LTD. Therefore, the size of the light emitting element  2951  can be increased, thus the spots SP can be formed with a sufficient amount of light, thereby making it possible to preferably perform the latent image formation. 
     Further, the light emitting element row  2951 R_ 2  of the three light emitting element rows matches with the meridian plane PL_lgd parallel to the longitudinal direction LGD. Therefore, by executing the alignment between the light emitting elements  2951  and the lenses LS using the light emitting element row  2951 R_ 2  as a reference, the alignment therebetween can be executed with ease and high accuracy. Further, the preferable latent image formation becomes possible using the line head  29  in which the alignment is performed with high accuracy. 
     Incidentally, although in the embodiments described above, the number of lens rows LSR is two or three, the number of lens rows is not limited thereto.  FIG. 25  is a plan view showing a configuration in which the number of lens rows LSR is one. The drawing corresponds to the case of viewing perpendicularly the reverse side surface of the head substrate  293  from the obverse side of the head substrate  293 . Further, although the lenses LS are illustrated with double-dashed lines, this is for showing the positional relationship between the light emitting elements  2951  and the lens LS, but not for showing that the lenses LS are disposed on the head substrate  293 . It should be noted that although the configuration shown in  FIG. 25  and the configuration shown in  FIG. 24  are different from each other in the number of lens rows LSR, these configurations are basically the same in the other points, and therefore, the configuration shown in  FIG. 25  is denoted with the corresponding reference numerals, and the explanations therefor will be omitted. 
     Further, although in the embodiments described above, the image formation is executed by developing the latent image using so-called dry toners, it is possible to develop the latent image using liquid developers.  FIG. 26  is a diagram schematically showing a device for performing development with a liquid developer. Since the device in the drawing and the device shown in  FIG. 3  are mainly different in the configuration of the developing unit, the developing unit will mainly be explained hereinafter, and other sections are denoted with the corresponding reference numerals, and the explanations therefor will be omitted. 
     There are disposed four developing units  60 Y (for yellow),  60 M (for magenta),  60 C (for cyan), and  60 K (for black) corresponding to the respective toner colors side by side along the conveying direction of the intermediate transfer belt  81 . Each of the developing units  60 Y,  60 M,  60 C, and  60 K is provided with an oil container  601  for containing a carrier oil, a toner container  602  for containing a high-concentration toner, and an agitator  603 . The agitator  603  agitates the carrier oil supplied from the oil container  601 , the high-concentration toner supplied from the toner container  602  to generate a liquid developer with adjusted concentration. The liquid developer thus generated is supplied to the developer container  604 . Inside the developer container  604 , there are disposed a supply roller  605  and an anilox roller  606 . The lower part of the supply roller  605  is dipped in the liquid developer inside the developer container  604 . The supply roller  605  rotates in the direction indicated by the arrow in the drawing to draw up the liquid developer to feed the liquid developer to the anilox roller  606 . The anilox roller  606  rotates in the direction indicated by the arrow in the drawing to apply the liquid developer fed from the supply roller  605  to a developing roller  607 . 
     The developing roller  607  has contact with the photoconductor drum  21  at the developing position. The developing roller  607  is rotatable in the direction indicated by the arrow in the drawing, and the liquid developer supplied from the anilox roller  606  is held on the surface of the developing roller  607 , and supplied to the developing position. The toner included in the liquid developer supplied as described above adheres to the latent image on the surface of the photoconductor drum, thus the development is executed. 
     On the downstream side of the developing position in the rotational direction of the developing roller  607 , a cleaner blade  608  has contact with the developing roller  607 . The cleaner blade  608  strips off the liquid developer from the surface of the developing roller  607 , and a recovery container  609  recovers the liquid developer thus tripped off. Further, the liquid developer recovered by the recovery container  609  is returned to the agitator  603 , and then reused. 
     On the downstream side of the developing position in the rotational direction D 21  of the photoconductor drum, there are disposed two photoconductor squeezing rollers  610  having contact with the surface of the photoconductor drum  21 . Further, the photoconductor squeezing rollers  610  strip off the carrier oil from the surface of the photoconductor drum  21 , thus an amount of carrier oil included in the liquid developer on the surface of the photoconductor drum  21  is adjusted Further, the carrier oil thus stripped off is once recovered by the recovery container  611 , and then returned to the agitator  603  to be reused. 
     The image obtained by developing the latent image at the developing position is transferred to the intermediate transfer belt  81  at a primary transfer position TR 1 . On the downstream side of the primary transfer position TR 1  in the conveying direction D 81  of the intermediate transfer belt  81 , a belt squeezing roller  612  has contact with the intermediate transfer belt  81 . Further, the belt squeezing rollers  612  strip off the carrier oil from the surface of the intermediate transfer belt  81 , thus an amount of carrier oil included in the liquid developer on the surface of the intermediate transfer belt  81  is adjusted. Further, the carrier oil thus stripped off is recovered by a recovery container  613 . 
     The image thus primary-transferred is secondary-transferred to a paper sheet. The secondary transfer operation is executed by two secondary transfer rollers  82  and the backup rollers  121  disposed so as to be opposed respectively to the two secondary transfer rollers  82 . Further, a cleaner blade  1211  is disposed so as to have contact with each of the backup rollers  121  to strip off the liquid developer remaining on each of the backup rollers  121 , and the liquid developer thus stripped off is recovered by each of recovery containers  1212 . 
     As described above, in the device shown in  FIG. 26 , the latent image is developed (liquid-developed) using the liquid developers. Incidentally, in general, according to the liquid development as described above, the development of the latent image can be executed with relatively high resolution. Therefore, it is preferable to perform the development of the spot latent images preferably formed by the embodiments of the invention using the liquid development process. 
     The entire disclosure of Japanese Patent Application Nos: 2007-299186, filed Nov. 19, 2007 and 2008-243004, filed Sep. 22, 2008 are expressly incorporated by reference herein.