Patent Publication Number: US-7710444-B2

Title: Image forming apparatus for forming a latent image on an image carrier

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application claims priority to Japanese Patent Application Nos. 2005-032777 filed in Japan on Feb. 9, 2005; 2005-087923 filed in Japan on Mar. 25, 2005; 2005-087927 filed in Japan on Mar. 25, 2005; and 2005-094778 filed in Japan on Mar. 29, 2005; which are hereby expressly incorporated by reference in their entireties. 
   BACKGROUND 
   The present invention relates to an image forming apparatus (image printing apparatus) in which a latent image is formed on an image carrier by light emitted from light emitting elements. 
   In general, an image forming apparatus includes an image carrier and a head facing the surface of the image carrier. A photoreceptor drum is generally used as the image carrier. A plurality of light emitting elements formed of a light emitting material, such as an organic EL (electroluminescent) material, is formed on a surface of the head facing the photoreceptor drum. A latent image is formed on the surface of the photoreceptor drum by light emitted from each light emitting element. 
   In the image forming apparatus according to the related art, when light emitted from each light emitting element reaches the surface of the photoreceptor drum to form a latent image (hereinafter, referred to as a ‘spot image’) in each region on the surface, a large variation in the area or shape of the latent image may occur. When a large variation occurs in the area or shape of the spot image, it is difficult to form a high-resolution latent image composed of fine pixels. The large variation in the area or shape of the spot image may occur due to a difference in distances between the surface of the photoreceptor drum and the light emitting elements. In order to solve this problem, JP-A-7-178956 discloses a structure in which a guide member coming into contact with the surface of a photoreceptor drum is fixed to a head, and JP-A-5-27563 discloses a structure in which end surfaces of light emitting elements come into contact with the surface of a photoreceptor drum. 
   However, in these structures, there is a fear that the head may be elastically deformed by friction force acting on the head (including the guide member and the light emitting elements) from the photoreceptor drum, which is generated by the rotation of the photoreceptor drum, and then restored to the original shape thereof when the stress of the head exceeds a threshold value, that is, the head may be vibrated. When the photoreceptor drum is driven at high speed, the vibration of the head becomes more remarkable. When the head is vibrated, a gap between the photoreceptor drum and each light emitting element is changed, which results in a large variation in the area or shape of the spot image with the passage of time. In addition, in these structures, there is a fear that the surfaces of the head and the photoreceptor drum may be rapidly worn away by friction therebetween, resulting in a large variation in gap between each light emitting element and the photoreceptor drum. That is, there is a fear that the area or shape of the spot image may be greatly varied with the passage of time. 
   SUMMARY 
   An advantage of some aspects of the invention is that it provides an image forming apparatus capable of reducing a variation in the area or shape of a spot image. 
   According to an aspect of the invention, an image forming apparatus includes an image carrier that has an image carrier surface moving in a direction; a supporting member that faces the image carrier; a plurality of light emitting elements which are provided on a surface of the supporting member facing the image carrier and emit light to form a latent image on the image carrier; a roller which is arranged on the surface of the supporting member facing the image carrier such that a rotational shaft thereof extends in a direction traversing an image carrier surface; and an urging unit which urges the supporting member against the image carrier so that the roller comes into contact with the image carrier. According to this structure, since the supporting member is urged against the image carrier so that the roller comes into contact with the image carrier, a gap between the image carrier and the light emitting elements formed on the supporting member is maintained at a predetermined value. In addition, since the roller having a rotational shaft extending in the direction traversing the image carrier surface comes into contact with the image carrier, the roller rotates with the movement of the image carrier surface. Therefore, it is possible to prevent the vibration of the roller and the supporting member or the light emitting elements formed on the supporting member, and to reduce the degree of abrasion of a contact portion between the image carrier and the head. Thus, it is possible to reduce a variation in the area or shape of a spot image. In addition, even when a foreign material is caught between the image carrier and the roller, it can be rapidly removed therebetween by the rotation of the roller. Accordingly, it is possible to prevent the damage of the image carrier surface due to the foreign material being continuously stuck to the image carrier surface. These effects contribute to forming (printing) a stable and high-quality image. 
   Further, in the above-mentioned structure, it is preferable that the urging unit include a plurality of elastic members that is provided on a surface of the supporting member opposite to the image carrier to press the supporting member against the image carrier. According to this structure, it is possible to uniformly urge the supporting member against the image carrier with a simple structure. 
   Furthermore, in the above-mentioned structure, it is preferable that the urging unit include a frame member which has surfaces facing side surfaces of the supporting member; and elastic members which are provided between the side surfaces of the supporting member and the frame member. According to this structure, it is possible to reduce a space on the side of the supporting member opposite to the image carrier. In addition, in this structure, the urging unit may include elastic members which press the frame member against the image carrier. According to this structure, it is possible to reliably urge the supporting member against the image carrier. 
   Moreover, in the above-mentioned structure, preferably, a groove is formed in the surface of the supporting member facing the image carrier so as to extend in a direction traversing the image carrier surface, and the roller is accommodated in the groove such that an outer circumferential surface thereof partially protrudes, toward the image carrier, from the surface of the supporting member facing the image carrier. According to this structure, since a portion of the roller is accommodated in the groove, the thickness of the head including the supporting member, the roller, and the light emitting elements can be reduced. However, in this structure, the supporting member may be bent toward the roller to cause the bottom of the groove to come into contact with the roller. In this case, the rotation of the roller may be interrupted by friction caused by the contact therebetween. Therefore, it is preferable that an auxiliary roller be arranged in the bottom of the groove such that an outer circumferential surface thereof comes into contact with the outer circumferential surface of the roller. According to this structure, since the auxiliary roller rotates with the rotation of the roller, it is possible to smoothly rotate the roller. When the auxiliary is not provided, it is possible to smoothly rotate the roller by reducing a friction coefficient of the image carrier surface facing the roller of the supporting member or by forming the supporting member with a high-rigidity material to prevent the deformation thereof. 
   Further, in the above-mentioned structure, it is preferable that a plurality of the rollers be arranged in the supporting member so as to be opposite to each other with the plurality of light emitting elements interposed therebetween. According to this structure, it is possible to uniformly press the supporting member against the image carrier, and thus to maintain a uniform gap between the light emitting elements and the image carrier. In this structure, it is also preferable that the plurality of light emitting elements be arranged in the direction traversing the image carrier surface, and that the rollers be arranged so as to face the surface of the image carrier over the whole width of the image carrier. According to this structure, it is possible to maintain a uniform gap between the light emitting elements and the image carrier, and to shield light emitted from the light emitting elements by using the roller positioned at both sides of each light emitting element. Therefore, even when the light emitted from the light emitting elements is diffused, it is possible to selectively radiate light emitted from the light emitting elements onto a region of the image carrier interposed between the rollers. 
   Furthermore, in the above-mentioned structure, it is preferable that the image forming apparatus further include optical elements which are provided between the image carrier and the light emitting elements. For example, condensing lenses for condensing light emitted from the light emitting elements are provided between the image carrier and the light emitting elements. According to this structure, it is possible to effectively emit light from the light emitting elements to the image carrier. In this structure, since the are of a spot image (the area of a region where light emitted from the light emitting element is incident) is markedly changed according to a gap between the image carrier and the light emitting element, an error of the gap between the image carrier and the light emitting element allowable to form a spot image having a predetermined area on the image carrier surface is small, compared with a structure in which the lenses are not provided. According to this aspect, as described above, since it is possible to maintain a substantially uniform gap between the image carrier and the light emitting element, the structure in which the lenses are provided between the light emitting element and the image carrier also makes it possible to accurately form a desired spot image on the image carrier surface. 
   According to another aspect of the invention, an image forming apparatus includes an image carrier which has a curved image carrier surface moving in a direction (sub-scanning direction); a transmissive sliding member which has a sliding surface coming into surface contact with the image carrier surface, the sliding surface having a curvature substantially equal to that of the image carrier surface; and light emitting elements which are fixed to a surface of the sliding member opposite to the image carrier and which emit light to the image carrier surface to form a latent image on the image carrier. In this structure, the image carrier surface means the surface of the image carrier on which light emitted from the light emitting elements is incident. For example, the image carrier surface is an outer circumferential surface of a cylinder or hollow cylinder or an inner circumferential surface of a hollow cylinder. According to this structure, the light emitting elements are fixed on one surface of the sliding member, and the sliding surface of the sliding member, having a curvature substantially equal to that of the image carrier surface, comes into contact with the image carrier surface. Therefore, as compared with the structure in which the surface of the guide member or the end surface of the light emitting element comes into line contact with the surface of the image carrier, it is possible to accurately arrange the sliding member in a desired posture and at a desired position, and to accurately maintain the desired posture and position of the sliding member by preventing the vibration of the head. That is, it is possible to adjust the gap between the light emitting elements and the image carrier with high accuracy. Thus, the above-mentioned structure makes it possible to reduce a variation in the area or shape of a spot image. As a result, a high-quality image can be stably formed (printed). 
   Further, in the above-mentioned structure, preferably, the sliding member is arranged on the outside of the image carrier such that the sliding surface thereof comes into surface contact with the image carrier surface, which is an outer circumferential surface of a substantially cylindrical member (that is, a surface of the cylindrical member opposite to a center line thereof). According to this structure, it is possible to easily arrange the sliding member. 
   Furthermore, in the above-mentioned structure, preferably, the image carrier surface is an inner circumferential surface of a substantially cylindrical member, and the sliding member is arranged on the inside of the image carrier such that the sliding surface thereof comes into surface contact with the image carrier surface. According to this structure, it is possible to reduce a space require for arranging the sliding member. 
   Moreover, in the above-mentioned structure, it is preferable that the image forming apparatus further include an urging unit which urges the sliding member against the image carrier. According to this structure, it is possible to reliably maintain the posture or position of the sliding member with respect to the image carrier. In this aspect, elastic members, such as springs or rubber, are used for the urging unit. In this structure, the sliding member may be directly urged by the elastic members, or it may be indirectly urged against the image carrier by pressing a member fixed to the sliding member. 
   Further, in the above-mentioned structure, it is preferable that the light emitting elements be formed on the surface of the sliding member opposite to the sliding surface thereof and that a sealing member be formed on the surface of the sliding member opposite to the sling surface thereof so as to cover the light emitting elements. That is, in a structure in which a transmissive board having the light emitting element formed thereon transmits light emitted from the light emitting elements (a so-called bottom emission type), the board having the light emitting elements formed thereon can be used as the sliding member. According to this structure, it is possible to reduce the number of components and thus to achieve a reduction in manufacturing costs and a decrease in the number of manufacturing processes, compared with a structure in which the sliding member and the board having the light emitting elements formed thereon are composed of different members. Further, in this structure, preferably, a sealing member is formed on the surface of the sliding member opposite to the sliding surface so as to cover the light emitting elements. This structure makes it possible to prevent the deterioration of the light emitting elements due to permeation of air or water. 
   Furthermore, in the above-mentioned structure, it is preferable that the sliding member have an inclined surface which is positioned between the sliding surface and a side surface thereof located on the upstream side in a rotational direction of the image carrier and that the inclined surface be tilted such that an elevation angle with respect to the image carrier surface is an acute angle. In the structure in which the sliding surface and the side surface of the sliding member positioned on the upstream side in the rotational direction of the image carrier intersect each other at an acute angle, the image carrier surface may be damaged by collision with the edge of the sliding member. In contrast, according to this aspect, since the inclined surface is provided between the side surface and the sliding surface of the sliding member, it is possible to prevent the damage of the image carrier surface due to collision with the sliding member. 
   Moreover, in the above-mentioned structure, it is preferable that the image forming apparatus further include lenses which are provided between the image carrier surface and the light emitting elements to condense light emitted from the light emitting elements. According to this structure, it is possible to improve the utilization efficiency of light emitted from the light emitting elements. That is, it is possible to more reduce the amount of light required for forming a latent image on the image carrier, compared with the structure in which the lenses are not provided. Thus, it is possible to reduce power consumption and to prevent the deterioration of the light emitting elements. 
   Further, in the above-mentioned structure, preferably, the sliding member has, on the surface thereof facing the image carrier surface, a first portion in which the light emitting elements are formed and second portions which are positioned at both sides of the first portion in a direction traversing the image carrier surface and which protrude from the first portion toward the image carrier surface. In addition, preferably, the sliding surface is surfaces of the second portions facing the image carrier surface. According to this structure, the light emitting elements and the image carrier surface are separated from each other at a gap corresponding to a step difference between the first and second portions, which makes it possible to prevent the damage or deterioration of the light emitting elements due to contact with the image carrier surface. 
   Furthermore, in the above-mentioned structure, preferably, the image forming apparatus further includes a substrate which has the light emitting elements formed on a surface thereof facing the image carrier surface, and the sliding member is fixed to the substrate so as to be interposed between the light emitting elements and the image carrier. According to this structure, the substrate and the light emitting elements formed thereon are fixed on one surface of the sliding member, and the sliding surface of the sliding member, having a curvature substantially equal to that of the image carrier surface, comes into surface contact with the image carrier surface. Therefore, as compared with the structure in which the surface of the guide member or the end surface of the light emitting element comes into line contact with the surface of the image carrier, it is possible to accurately arrange the sliding member and the substrate in desired postures and at desired positions, and to accurately maintain the desired postures and positions of the sliding member and substrate by preventing the vibration of the head. That is, it is possible to adjust the gap between the light emitting elements and the image carrier with high accuracy. Thus, in this structure, it is preferable that the sliding member be composed of a sealing member for covering the light emitting elements together with the substrate. The sealing member seals the light emitting elements to protect them from the air. 
   Moreover, according to still another aspect of the invention, an image forming apparatus includes an image carrier which has an image carrier surface moving in a predetermined direction; a main substrate; light emitting elements which are formed on the main substrate and emit light to form a latent image on the image carrier surface; and a sealing substrate which overlaps the main substrate to seal the light emitting elements. In the image forming apparatus, the main substrate or the sealing substrate constitutes a contact surface coming into contact with the image carrier surface. Cylindrical optical waveguides, each having an end surface constituting a portion of the contact surface, are provided in the substrate constituting the contact surface. The other end surfaces of the optical waveguides cover the light emitting elements. Each optical waveguide guides light incident on the other end surface thereof to the one end surface by specularly reflecting the light from an outer circumferential surface thereof. In the image forming apparatus according to this aspect, the optical waveguides are provided in the substrate facing the image carrier, and the one end surface of each optical waveguide constitutes a portion of the contact surface coming into contact with the image carrier surface. Therefore, the other end surface of each optical waveguide is closer to the light emitting element than the one end surface thereof. In addition, the other end surfaces of the optical waveguides cover the light emitting elements. Most of light components emitted from the light emitting element to the substrate are incident on the other end surface of the cylindrical optical waveguide. The incident light is guided to the one end surface of the optical waveguide without leaking to the outside of the outer circumferential surface of the optical waveguide. Thus, a spot image having the same shape and size as those of the end surface of the optical waveguide is formed on the contact surface. 
   In general, when the light emitting element formed of a light emitting material, such as an organic EL material, contacts the air, the life span thereof is considerably shortened. Therefore, sealing the light emitting elements is indispensable. The sealing is performed by overlapping the sealing substrate with the main substrate having the light emitting elements formed thereon. Light emitted from the light emitting elements reaches the surface of the image carrier through one of the substrates. The light emitting elements are surface-emitting elements, and light emitted from a light emitting surface (light emitting layer) is diffused while traveling. Meanwhile, in order to ensure a sealing function, the main substrate and the sealing substrate need to have predetermined thicknesses. In general, light emitted from the light emitting surface is largely diffused at a point of time when it is emitted from the substrate. For example, when the light emitting surface has a diameter of 50 μm and a distance from the light emitting surface to the light emission surface of the substrate is 50 μm, a spot image having a diameter of about 100 μm is formed on the light emission surface. When a spot image having a large diameter is formed, the brightness of the spot image is lowered. In order to improve the brightness of the spot image while maintaining the size thereof, it is necessary to improve the brightness of the light emitting surface. For example, in the above-mentioned structure, in order to raise the brightness level of the spot image to the brightness level of the light emitting surface, it is necessary to raise the brightness of the light emitting surface by four times, which causes the life span of the light emitting element to be shortened. In addition, the larger the diameter of the spot image is, the lower the resolution of the spot image becomes. 
   When a latent image is formed, the surface of the image carrier moves. Therefore, in contact exposure in which the head comes into contact with the image carrier, a portion of the head coming into contact with the image carrier is worn away. When the contact portion of the head is worn away, an optical path from the light emitting element to the surface of the image carrier varies. As described above, in general, the variation of the optical path causes a change of the size a spot image. That is, a variation in the area or shape of a spot image occurs due to abrasion. 
   In contrast, in the image forming apparatus according to this aspect, light emitted from the light emitting element is guided, without leaking to the outside of the outer circumferential surface of the optical waveguide, to form a spot image, which makes it possible to achieve a high-definition spot image. In addition, since a spot image having the same size and shape as those of the end surface of the optical waveguide is formed on the contact surface, it is possible to reduce a variation in the area or shape of a spot image. Further, even when the optical waveguide is shortened due to the abrasion of the contact portion, the shape and size of the spot image formed on the contact surface are hardly varied, since the optical waveguide is a cylindrical member which guides light by specularly reflecting the light from the outer circumferential surface thereof and the central axis thereof extends in a direction where the contact surface recedes due to abrasion. Thus, it is possible to reduce a variation in the area or shape of the spot image, and thus to stably form a high-definition spot image. As can be seen from the above description, according to the image forming apparatus of this aspect, regardless of the contact exposure in which the head comes into contact with the image carrier, it is possible to guide light emitted from the light emitting elements without largely diffusing light in a transmissive substrate, and thus stably form a high-definition spot image. As a result, it is possible to stably form (printing) a high-quality image. 
   Further, in the above-mentioned structure, it is preferable that the optical waveguides are provided so as to pass through the substrate constituting the contact surface. This structure more reliably prevents the diffusion of light emitted from the light emitting elements. 
   Furthermore, in the above-mentioned structure, preferably, a concave portion is formed in a surface of the substrate constituting the contact surface which faces the image carrier, and an optical waveguide plate is fixed in the concave portion. In addition, preferably, the optical waveguides are formed in the optical waveguide plate. According to this structure, it is possible to more reduce the number of manufacturing processes and manufacturing costs, compare with the structure in which the optical waveguides are formed so as to pass through the substrate constituting the contact surface. As will be described below, this structure can ensure a sealing function. A member having the optical waveguides formed therein may be deformed due to a difference between thermal shrinkage (expansion) of the optical waveguides and thermal shrinkage (expansion) of peripheral portions thereof. In the structure in which the optical waveguides are formed so as to pass through the substrate, a bonding surface of the optical waveguide and the peripheral portion thereof extends from an internal space on the light emitting side to an external space. When a gap is formed along the bonding surface due to the deformation, the sealing function is deteriorated. In contrast, in the structure in which the optical waveguide plate is fixed in the concave portion, the optical waveguides are formed in a member other than the substrate. Therefore, even when a gap is formed along the bonding surface between the optical waveguide and a peripheral portion thereof, the sealing function is not deteriorated since the bonding surface does not extend to the internal space. In addition, since the optical waveguides are formed in the optical waveguide plate, not in the main substrate and the sealing substrate, it is possible to reduce a possibility that the main substrate or the sealing substrate will be deformed due to the difference. 
   Moreover, in the above-mentioned structure, it is preferable that the substrate constituting the contact surface be the sealing substrate. A method of cutting the substrate can be used to the optical waveguides in the substrate. However, in this aspect, when the method is used, it is preferable to cut the sealing member, not the main substrate requiring a high degree of utilization efficiency of light. Therefore, the utilization efficiency of the mains substrate is not lowered, which is effective in the mass production. 
   Further, in the above-mentioned structure, it is preferable that the substrate constituting the contact surface be the main substrate. According to this structure, it is possible to more reduce a distance from the light emitting layer to the optical waveguide, compared with the structure in which the substrate constituting the contact surface is the sealing substrate. This structure contributes to an improvement in brightness. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
       FIG. 1  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a first embodiment of the invention; 
       FIG. 2  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of the image forming apparatus according to the first embodiment; 
       FIG. 3  is a plan view illustrating a surface of the head facing a photoreceptor drum in the image forming apparatus according to the first embodiment; 
       FIG. 4  is a cross-sectional view taken along the line IV-IV of  FIG. 3 ; 
       FIG. 5  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a second embodiment of the invention; 
       FIG. 6  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of the image forming apparatus according to the second embodiment; 
       FIG. 7  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of another image forming apparatus according to the second embodiment; 
       FIG. 8  is a cross-sectional view illustrating the structure of an image forming apparatus according to a first modification of the first embodiment; 
       FIG. 9  is a cross-sectional view illustrating the structure of an image forming apparatus according to a second modification of the first embodiment; 
       FIG. 10  is a cross-sectional view illustrating the structure of a roller and a peripheral portion of the roller of an image forming apparatus according to a third modification of the first or second embodiment; 
       FIG. 11  is a cross-sectional view illustrating the structure of an image forming apparatus according to a fourth modification of the first embodiment; 
       FIG. 12  is a plan view illustrating a surface of a head facing a photoreceptor drum in an image forming apparatus according to a fifth embodiment of the first or second embodiment; 
       FIG. 13  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a third embodiment of the invention; 
       FIG. 14  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of an image forming apparatus according to the third embodiment; 
       FIG. 15  is a plan view illustrating an element forming surface of a substrate of the image forming apparatus according to the third embodiment; 
       FIG. 16  is a perspective view illustrating the appearance of the substrate of the image forming apparatus according to the third embodiment; 
       FIGS. 17A to 17C  are cross-sectional views illustrating the structure of an image forming apparatus compared with the image forming apparatus according to the third embodiment and the structural defect thereof; 
       FIG. 18  is a cross-sectional view illustrating the structure of a portion of an image forming apparatus according to a fourth embodiment of the invention; 
       FIG. 19  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of the image forming apparatus according to the fourth embodiment; 
       FIG. 20  is a perspective view illustrating the appearance of a substrate of the image forming apparatus according to the fourth embodiment; 
       FIG. 21  is a front view illustrating the structure of a portion of an image forming apparatus according to a fifth embodiment of the invention; 
       FIG. 22  is a cross-sectional view taken along the line XXII-XXII of  FIG. 21 ; 
       FIG. 23  is a cross-sectional view taken along the line XXIII-XXIII of  FIG. 21 ; 
       FIG. 24  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a sixth embodiment of the invention; 
       FIG. 25  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of the image forming apparatus according to the sixth embodiment; 
       FIG. 26  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of another image forming apparatus according to the sixth embodiment; 
       FIG. 27  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a first modification of the third or sixth embodiment; 
       FIG. 28  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a first modification of the fourth embodiment; 
       FIG. 29  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a second modification of the third or sixth embodiment; 
       FIG. 30  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a seventh embodiment of the invention; 
       FIG. 31  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of the image forming apparatus according to the seventh embodiment; 
       FIG. 32  is a plan view illustrating an element forming surface of a substrate of the image forming apparatus according to the seventh embodiment; 
       FIG. 33  is a perspective view illustrating the appearance of the substrate of the image forming apparatus according to the seventh embodiment; 
       FIGS. 34A to 34C  are cross-sectional views illustrating the structure of an image forming apparatus compared with the image forming apparatus according to the seventh embodiment and the structural defect thereof; 
       FIG. 35  is a cross-sectional view illustrating the structure of a portion of an image forming apparatus according to an eighth embodiment of the invention; 
       FIG. 36  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of the image forming apparatus according to the eighth embodiment; 
       FIG. 37  is a perspective view illustrating the appearance of a substrate of the image forming apparatus according to the eighth embodiment; 
       FIG. 38  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a ninth embodiment of the invention; 
       FIG. 39  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of the image forming apparatus according to the ninth embodiment; 
       FIG. 40  is a cross-sectional view illustrating the structure of a head and a peripheral portion of the head of another image forming apparatus according to the ninth embodiment; 
       FIG. 41  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a first modification of the seventh or ninth embodiment; 
       FIG. 42  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a first modification of the eighth embodiment; 
       FIG. 43  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a second modification of the seventh or ninth embodiment; 
       FIG. 44  is a cross-sectional view illustrating main parts of an image forming apparatus according to tenth to thirteenth embodiments of the invention; 
       FIG. 45  is a plan view illustrating the structure of a head  200  of an image forming apparatus according to the tenth embodiment of the invention; 
       FIG. 46  is a cross-sectional view taken along the line XLVI-XLVI of  FIG. 45 ; 
       FIG. 47  is a cross-sectional view illustrating an optical operation of the head  200 ; 
       FIG. 48  is a diagram illustrating a spot image formed by the head  200 ; 
       FIG. 49  is a diagram illustrating a first process of a method of manufacturing the head  200 ; 
       FIG. 50  is a diagram illustrating the next process of that shown in  FIG. 49 ; 
       FIG. 51  is a diagram illustrating the next process of that shown in  FIG. 50 ; 
       FIG. 52  is a diagram illustrating the next process of that shown in  FIG. 51 ; 
       FIG. 53  is a cross-sectional view illustrating the structure of a head  300  of an image forming apparatus according to the eleventh embodiment of the invention; 
       FIG. 54  is a cross-sectional view illustrating an optical operation of the head  300 ; 
       FIG. 55  is a cross-sectional view illustrating the structure of a head  201  of an image forming apparatus according to the twelfth embodiment of the invention; 
       FIG. 56  is a cross-sectional view taken along the line LVI-LVI of  FIG. 55 ; 
       FIG. 57  is a cross-sectional view illustrating an optical operation of the head  201 ; 
       FIG. 58  is a diagram illustrating a first process of a method of manufacturing the head  201 ; 
       FIG. 59  is a diagram illustrating the next process of that shown in  FIG. 58 ; 
       FIG. 60  is a diagram illustrating the next process of that shown in  FIG. 59 ; 
       FIG. 61  is a diagram illustrating the next process of that shown in  FIG. 60 ; 
       FIG. 62  is a cross-sectional view illustrating the structure of a head  301  of an image forming apparatus according to the thirteenth embodiment of the invention; 
       FIG. 63  is a cross-sectional view illustrating an optical operation of the head  301 ; 
       FIG. 64  is a diagram illustrating a spot image formed by an image forming apparatus according to a third embodiment of the tenth to thirteenth embodiments; 
       FIG. 65  is a longitudinal sectional view illustrating an example of the overall structure of the image forming apparatus according each embodiment of the invention; 
       FIG. 66  is a longitudinal sectional view illustrating another example of the overall structure of the image forming apparatus according each embodiment of the invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   In the drawings used for describing the following preferred embodiments of the invention, scales and dimensions of components are different from actual scales and dimensions thereof. 
   First Embodiment 
     FIG. 1  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a first embodiment of the invention. The image forming apparatus is used for a printing unit of, for example, a printer, a duplicating machine, or a facsimile. As shown in  FIG. 1 , the image forming apparatus includes a cylindrical photoreceptor drum  110  which is supported so as to rotate in the direction of arrow A 1  and a head  10  which is arranged to face the side surface of the photoreceptor drum  110 . When the photoreceptor drum  110  rotates, the surface thereof is advanced. Hereinafter, the direction of a rotational axis of the photoreceptor drum  110  (that is, the direction of a bus of the photoreceptor drum  110  (the main scanning direction)) is referred to as ‘a drum axis direction X’. 
     FIG. 2  is a cross-sectional view illustrating the sectional structure of the components shown in  FIG. 1 , taken along a direction perpendicular to the drum axis direction X.  FIG. 3  is a plan view illustrating an upper surface of the head  10  corresponding to the photoreceptor drum  110 . The cross-sectional view taken along the line II-II of  FIG. 3  corresponds to  FIG. 2 . As shown in  FIGS. 1 to 3 , the head  10  includes a supporting member  31  having a substantially rectangular shape, and a light-emitting device  33  and two rollers  35  which are arranged in portions of the supporting member  31  opposite to the photoreceptor drum  110 . The supporting member  31  is formed of, for example, plastic, and the longitudinal direction thereof extends along the drum axis direction X. 
   The light emitting device  33  emits light to form a latent image on the photoreceptor drum  110 . As shown in  FIG. 3 , the light emitting device  33  includes a substrate  331  having a substantially rectangular shape whose longitudinal direction is the drum axis direction X and a plurality of light emitting elements  332  arranged on the surface of the substrate  331  in an array shape. Each light emitting element  332  is formed by interposing a light emitting layer formed of an organic EL material between an anode and a cathode. As shown in  FIG. 3 , the light emitting elements  332  are arranged in two rows or in island shapes along the drum axis direction X, and selectively emit light corresponding to an image to be printed on a recording medium, such as a sheet. Light emitted from each light emitting element  332  is incident on the surface of the photoreceptor drum  110 . Then, the exposure causes a latent image corresponding to a desired image to be formed on the surface of the photoreceptor drum  110 . The arrangement pattern of the light emitting elements  332  is not limited to that shown in  FIG. 3 . For example, the light emitting elements may be arranged in other patterns, such as in one row or three or more rows. 
   Meanwhile, each roller  35  is composed of a cylindrical black member having a diameter of about 1 mm to 2 mm, and is formed of a resin material, such as plastic. The total length of the roller  35  is equal to or larger than the width of the photoreceptor drum  110  (dimensions in the drum axis direction X). 
   Further, grooves  311  are formed in a surface of the supporting member  31  opposite to the photoreceptor drum  110  at both sides of the light emitting device  33 . Each groove  311  is a concave portion having a width larger than the diameter of the roller  35  (about 1 mm to 2 mm), and extends in the drum axis direction X, corresponding to the arrangement of the light emitting device  33 . Each roller  35  is provided in the groove  311 . Therefore, the rotational axis of the roller  35  extends a direction traversing the surface of the photoreceptor drum  110  (that is, a direction parallel to the drum axis direction X). In addition, as described above, since the total length of the roller  35  is equal to or larger than the width of the photoreceptor drum  110 , the roller  35  is opposite to the entire surface of the photoreceptor drum  110  in the widthwise direction. 
     FIG. 4  is a cross-sectional view taken along the line IV-IV of  FIG. 3 . As shown in  FIGS. 3 and 4 , a shaft portion  351  extending along a center line of the roller  35  is formed at both end surfaces of each roller  35 . Meanwhile, cover portions  313  are formed at end portions of the supporting member  31  in the longitudinal direction of each groove  311  such that each of them is opposite to a bottom  312  of the groove  311  with the shaft portion  351  interposed therebetween. The cover portions  313  prevent the rollers  35  from falling off from the supporting member  31 . 
   As shown in  FIGS. 2 and 4 , the diameter of the roller  35  is larger than the depth of the groove  311 . Therefore, a portion of the outer circumferential surface of the roller  35  accommodated in the groove  311  which is opposite to the photoreceptor drum  110  protrudes from the supporting member  31  toward the photoreceptor drum  110 . In addition, a portion of the roller  35  opposite to the photoreceptor drum  110  faces the bottom  312  of the groove  311  at a narrow gap. 
   As shown in  FIGS. 1 and 2 , a plurality of elastic bodies  41  are arranged on the surface of the supporting member  31  opposite to the photoreceptor drum  110 . Each elastic body  41  is a unit for urging the supporting member  31  against the photoreceptor drum  110 , and is provided between the supporting member  31  and a case  50  of the image forming apparatus. In the first embodiment, as shown in  FIG. 1 , six elastic bodies  41  are arranged around four corners of the upper surface of the supporting member  31  and in a central portion thereof in the longitudinal direction. 
   For example, a coil spring having one end fixed to the supporting member  31  and the other end fixed to the case  50  is used as the elastic body  41 . However, the elastic body  41  may have any other shapes. That is, any members can be used as long as they can urge the supporting member  31  against the photoreceptor drum  110 . For example, various members, such as a leaf spring and rubber interposed between the supporting member  31  and the case  50 , can be used as the elastic bodies  41 . 
   As represented by arrow F 1  in  FIGS. 1 and 2 , the supporting member  31  is urged against the photoreceptor drum  110  by the elastic bodies  41 . The urging force causes the roller  35  to be pressed against the surface of the photoreceptor drum  110 , so that friction force is generated between the roller  35  and the photoreceptor drum  110 . As a result, the photoreceptor drum  110  rotates in the direction of arrow A 1  in  FIG. 2 , which causes the roller  35  to rotate in the direction of arrow A 2 . 
   As described above, in the first embodiment, the elastic bodies  41  press the supporting member  31  to allow the roller to come into contact with the surface of the photoreceptor drum  110 . Therefore, even when errors occur in the dimensions of the head  10  and the photoreceptor drum  110  or in the mounting positions thereof, or even when surface deflection occurs in the surface of the photoreceptor drum  110  due to insufficient circularity of the cross section of the photoreceptor drum  110  or an error in the drum axis direction X, each light emitting element  332  follows the surface of the photoreceptor drum  110 . Therefore, it is possible to maintain a predetermined gap between the light emitting elements  332  and the photoreceptor drum  110 . 
   Further, the rollers  35  of the head  10  come into contact with the surface of the photoreceptor drum  110  and rotate with the revolution of the photoreceptor drum  110 , which makes it possible to solve various problems due to contact between the head  10  and the photoreceptor drum  110 . 
   For example, the related art has a problem in that the head vibrates due to contact with the photoreceptor drum. However, in the first embodiment, friction force generated from the photoreceptor drum  110  is hardly applied to the head  10 , making it possible to prevent the vibration of the head  10  due to the rotation of the photoreceptor drum  110 . Therefore, it is possible to keep a predetermined gap between the light emitting elements  332  and the photoreceptor drum  110  with high accuracy. In addition, this structure has an advantage of reducing the friction of a contact portion between the head  10  and the photoreceptor drum  110 , compared with the structure in which the photoreceptor drum rotates with its surface coming into contact with the head. Therefore, it is possible to prevent a variation in the gap between the light emitting element  332  and the photoreceptor drum  110  with the passage of time, and thus to maintain a uniform gap therebetween. 
   In general, a variation in the area or shape of a spot image formed on the surface of the photoreceptor drum may be caused by, for example, an error in the dimensions of the head or the photoreceptor drum or an error in the mounting positions thereof. In contrast, the structure according to the first embodiment can absorb these errors, which makes it possible to maintain a predetermined gap between the light emitting element  332  and the photoreceptor drum  110 . 
   As described above, according to the first embodiment, it is possible to reduce a variation in the area or shape of the spot image formed on the surface of the photoreceptor drum  110 . In addition, according to the first embodiment, a predetermined amount of light is radiated onto a region where the spot image is formed, and thus a clear spot image can be obtained. Further, according to the first embodiment, even when a foreign matter is caught between the roller  35  and the photoreceptor drum  110 , the foreign matter can be rapidly removed by the rotation of the roller  35 , which prevents the surface of the photoreceptor drum  110  from being damaged due to the foreign matter. 
   Furthermore, according to the first embodiment, a plurality of elastic bodies  41  is evenly arranged on the upper surface of the supporting member  31 . This structure makes it possible to uniformly press, for example the supporting member  31 , compared with a structure in which the elastic bodies  41  are unevenly arranged in a predetermined region on the upper surface of the supporting member  31 . Therefore, it is possible to maintain a uniform gap between the light emitting elements  332  and the photoreceptor drum  110 . 
   As shown in  FIG. 1 , when the elastic bodies  41  are arranged at the central portion of the supporting member  31  in the longitudinal direction thereof, the supporting member  31  may be bent due to the pressing force by the elastic bodies  41  or the weight of the supporting member  31 . In this case, the bottom  312  of the groove  311  may come into contact with the roller  35  in a portion B shown in  FIG. 4 , and thus friction therebetween may interrupt the rotation of roller  35 . Therefore, it is preferable that the bottom  312  of the groove  311  formed in the supporting member  31  and the outer circumferential surface of the roller  35  have a small friction coefficient. Thus, the first embodiment adopts this structure. Accordingly, even when the bottom  312  of the groove  311  comes into contact with the roller  35 , friction force generated from a contact portion therebetween is sufficiently small, which results in smooth rotation of the roller  35 . Alternatively, the supporting member  31  may be formed of a material having sufficiently high rigidity. This structure prevents the supporting member  31  from being curved, which makes it possible to avoid the contact between the roller  35  and the bottom  312 . In addition, a structure in which the elastic bodies  41  are not arranged at the central portion of the supporting member  31  in the longitudinal direction thereof (for example, a structure in which the elastic bodies  41  are formed only in the vicinities of the four corners of the upper surface of the supporting member  31 ) may be used. In this case, it is also possible to prevent the curve of the supporting member  31  which interrupts the rotation of the roller  35 . 
   Moreover, according to the first embodiment, the rollers  35  arranged at both sides of the light emitting elements  332  have a light shielding property. According to this structure, light which has been emitted from the light emitting elements  332  and then has been diffused at a wide angle is shielded by the roller  35 . Therefore, it is possible to selectively radiate light from the light emitting elements  332  onto only a region on the surface of the photoreceptor drum  110  between the rollers  35  (a region R shown in  FIG. 2 ). As a result, a high-definition image can be printed. In particular, in the first embodiment, since the rollers  35  are opposite to the entire surface of the photoreceptor drum  110  in the widthwise direction thereof, it is possible to prevent light emitted from the light emitting elements  332  from being diffused on the entire surface of the photoreceptor drum  110 . 
   Second Embodiment 
     FIG. 5  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a second embodiment of the invention. In the first embodiment, the elastic bodies  41  for pressing the supporting member  31  against the photoreceptor drum  110  is arranged on the upper surface of the supporting member  31 . However, in the second embodiment, elastic bodies provided on the side surfaces of the supporting member  31  urge the supporting member  31  against the photoreceptor drum  110 . 
     FIG. 6  is a cross-sectional view illustrating the sectional structure of the components shown in  FIG. 5 , taken along a direction perpendicular to the drum axis direction X, and corresponds to  FIG. 2  in the first embodiment. As shown in  FIGS. 5 and 6 , the image forming apparatus according to the second embodiment has a substantially rectangular frame member  53  surrounding the head  10 . An inner circumferential surface of the frame member  53  corresponds to a side surface of the supporting member  31 . 
   A plurality of elastic bodies  42  is provided between the inner circumferential surface of the frame member  53  and the side surface of supporting member  31 . An end of each elastic body  42  is fixed to the inner circumferential surface of the frame member  53 , and the other end thereof is fixed to the side surface of the supporting member  31 . More specifically, as shown in  FIG. 5 , three elastic bodies  42  are arranged at equal intervals on the side surface corresponding to each long side of the substantially rectangular supporting member  31  such that an end of each of the three elastic bodies  42  is fixed to the side surface of the supporting member  31  and the other end thereof is fixed to the inner surface of the frame member  53  opposite to the side surface of the supporting member  31 . In addition, an end of one elastic body  42  is fixed in the center of the side surface corresponding to each short side of the supporting member  31 , and the other end of the elastic body  42  is fixed to the inner circumferential surface of the frame member  53  opposite to the side surface of the supporting member  31 . 
   Further, a plurality of elastic bodies  43  is arranged on a surface of the frame member  53  opposite to the photoreceptor drum  10 . The elastic bodies  43  are members for urging the frame member  53  against the photoreceptor drum  110 , and are provided between the case  50  of the image forming apparatus and the frame member  53 . In the second embodiment, six elastic bodies  43  are arranged around four corners of the frame member  53  and in a central portion thereof in the longitudinal direction. The elastic bodies  42  and  43  are the same components as the elastic bodies  41  of the first embodiment. For example, coil springs or leaf springs are used as the elastic bodies  42  and  43 . 
   In the above-mentioned structure, when the elastic bodies  43  urge the frame member  53  against the photoreceptor drum  110 , the elastic bodies  42  are extended. Then, the supporting member  31  is urged against the photoreceptor drum  110  by elastic force generated by the elastic bodies  42 . Therefore, in the second embodiment, the rollers  35  is also pressed against the photoreceptor  110 , and thus the same effects as those in the first embodiment can be obtained. The frame member  53  can have any shapes as long as it has a portion opposite to the side surface of the supporting member  31 . Therefore, the shape of the frame member  53  is not limited to that shown in  FIG. 5 . 
   In the above-mentioned structure, the elastic bodies  43  press the frame member  53  against the photoreceptor drum  110 . However, as shown in  FIG. 7 , the supporting member  31  may be urged against the photoreceptor  110  according to the fixed position of the frame member  53 . That is, in this structure, the frame member  53  is fixed to the case  50  such that an end portion P 1  of each elastic body  42  fixed to the frame member  53  is positioned closer to the photoreceptor drum  110  than an end portion P 2  of the elastic body  42  fixed to the supporting member  31 . In this structure, the elastic force of the elastic bodies  42  causes the supporting member  31  to be pressed against the photoreceptor drum  110 , and thus the same effects as those in the first and second embodiment can be obtained. In addition, according to this structure, it is unnecessary to provide the elastic bodies  43  on the surfaces of the supporting member  31  and the frame member  53  opposite to the photoreceptor drum  110 , which results in a reduction in a space to be ensured on the upper side of the head  10 . 
   Modifications of First and Second Embodiments 
   Various modifications of the above-mentioned embodiments can be made. The modifications thereof will be described in detail. The following modifications may be appropriately combined. 
   First Modification 
   In the above-mentioned embodiments, the head  10  is arranged so as to face the outer circumferential surface of the cylindrical photoreceptor drum  110 . However, as shown in  FIG. 8 , the head  10  may be arranged inside the photoreceptor drum  110  such that the light emitting device  33  and the rollers  35  are opposite to the inner circumferential surface of the photoreceptor drum  110 . In this structure, the photoreceptor drum  110  is formed by laminating a photosensitive layer on a cylindrical outer circumferential surface having a transmissive property. According to this structure, it is possible to reduce a space required for arranging the head  10 , compared with the structures described in the above-mentioned embodiments. 
   Second Modification 
   In the above-mentioned embodiments, the cylindrical photoreceptor drum  110  is used as an image carrier. However, as shown in  FIG. 9 , an endless belt  80  which is wound around a plurality of rollers  551  and rotates in the direction of arrow A 1  may be used as the image carrier. In  FIG. 9 , the head  10  is arranged to face the outer circumferential surface of the endless belt  80 . However, similar to the structure shown in  FIG. 8 , the head  10  may be arranged to face the inner circumferential surface of the endless belt  80 . In  FIGS. 8 and 9 , the head  10  is urged by the structure according to the first embodiment. However, the structure of the second embodiment may be applied to this modification. As described above, in this modification, it is preferable that the rollers be provided in the supporting member such that rotational axes thereof is arranged to traverse the surface of the image carrier (in the direction of a bus of the image carrier), and the shape of the image carrier does not matter. 
   Third Embodiment 
   As described in the first embodiment, when the bottom  312  of the groove  311  formed in the supporting member  31  comes into contact with the roller  35 , the rotation of the roller  35  is interrupted. In order to solve the problem, in the first embodiment, the bottom  312  of the groove  311  having a small friction coefficient is used. However, the structure shown in  FIG. 10  can also smoothly rotate the rollers  35 . In the structure shown in  FIG. 10 , a concave portion  315  is formed in the bottom  312  of the groove  311  of the supporting member  31 , and an auxiliary roller  37  is arranged in the concave portion  315 . The auxiliary roller  37  is a cylindrical member whose rotational axis extends parallel to the roller  35 , and is provided in the supporting member  31  with its side surface coming into contact with the side surface of the roller  35 . When the roller  35  rotates with the revolution of the photoreceptor drum  110 , the auxiliary roller  37  rotates following the rotation of the roller  35 . According to this structure, the bottom  312  of the groove  311  does not contact the roller  35 , which makes it possible to smoothly rotate the roller  35 . 
   Fourth Modification 
   In the above-mentioned embodiments, the light emitting device  33  is opposite to the photoreceptor drum  110  without any members interposed therebetween (that is, any members are not provided between the light emitting device  33  and the photoreceptor drum  110 ). However, optical members may be provided between the light emitting device  33  and the photoreceptor drum  110 . For example, an optical waveguide (for example, an optical fiber) for guiding light emitted from the light emitting elements  332  to the surface of the photoreceptor drum  110  or a lens (for example, a condensing lens array) for condensing light emitted from the light emitting elements  332  may be provided between the light emitting device  33  and the photoreceptor drum  110 . 
   Further, as shown in  FIG. 11 , microlenses  74  for condensing light emitted from the light emitting elements  332  may be provided between the light emitting device  33  and the photoreceptor drum  110  (fourth modification). The microlenses  74  are arranged in an array shape so as to be opposite to the light emitting elements  332 . In the structure shown in  FIG. 11 , first and second substrates  71  and  72  bonded to each other are arranged in a space between the light emitting device  33  and the photoreceptor drum  110 . A plurality of curved-shaped concave portions  711  are formed in a surface of the first substrate  71  opposite to the second substrate  72  at positions corresponding to the light emitting elements  332 , and a plurality of curved-shaped concave portions  721  are formed in a surface of the second substrate  72  opposite to the second substrate  71 . A resin material having a different reflective index from those of the first and second substrates  71  and  72  is filled up into spaces formed by the concave portions  711  of the first substrate  71  and the concave portions  721  of the substrate  72 . The microlenses  74 , which are double-sided convex lenses, are formed by the resin material. As represented by two-dot chain lines in  FIG. 11 , the microlenses  74  condense light emitted from the corresponding light emitting elements  332 , so that an image is formed on the surface of the photoreceptor drum  110 . The shape and arrangement of the microlenses  74  are not limited to those shown in  FIG. 11 . For example, convex microlenses protruding toward the photoreceptor drum  110  may be formed on a substrate  331  of the light emitting device  33 . 
   In the structure in which light emitted from the light emitting elements  332  is condensed by the microlenses  74 , even when a distance between the light emitting element  332  and the photoreceptor drum  110  varies slightly, a variation in the diameter of a spot region on the surface of the photoreceptor drum  110  where light emitted from the light emitting element  332  is incident becomes remarkable. According to the fourth modification, the roller  35  makes it possible to maintain a uniform gap between the light emitting device  33  and the photoreceptor drum  110  with high accuracy. Therefore, in the structure capable of improving the utilization efficiency of light by using the microlenses  74 , it is also possible to accurately form a predetermined latent image on the surface of the photoreceptor drum  110 . That is, the effects of maintaining a uniform gap between the light emitting device  33  and the photoreceptor drum  110  can be more reliably obtained by the structure in which optical components, such as the microlenses  74 , are arranged between the light emitting device  33  and the photoreceptor drum  110 , as shown in  FIG. 11 . In addition, as described in the above-mentioned embodiment, the structure in which the light emitting device  33  is adjacent to the photoreceptor drum  110  without any members interposed therebetween makes it possible to improve the utilization efficiency of light emitted from the light emitting elements  332  and to reduce manufacturing costs. Further, it is possible to reduce the brightness of the light emitting elements required for emitting a sufficient amount of light to the surface of the photoreceptor drum  110 . As a result, the power consumption of the light emitting device  33  can be reduced, and the life span of the light emitting element  332  can be prolonged. 
   Fifth Modification 
   In the above-mentioned embodiments, two rollers  35  are arranged at both sides of the light emitting device  33 . However, the number of rollers  35  and the positions thereof can be changed. In the first embodiment, the light-shielding rollers  35  prevent the diffusion of light emitted from the light emitting elements  332 . The roller  35  may be formed of a transmissive material, or the total length of the roller  35  may be smaller than the width of the photoreceptor drum  110  as long as the diffusion of light emitted from the light emitting elements  332  does not matter particularly (for example, a member for shielding diffused light is separately provided from the roller  35 , or the diffusion of light is suppressed by an optical component such as a lens). In addition, as shown in  FIG. 12 , a plurality of rollers  35  may be arranged in straight lines at regular intervals. 
   Third Embodiment 
     FIG. 13  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a third embodiment of the invention. The image forming apparatus is used for a printing unit of, for example, a printer, a duplicating machine, or a facsimile. As shown in  FIG. 13 , the image forming apparatus includes a cylindrical photoreceptor drum  110  which is supported so as to rotate in the direction of arrow A (sub-scanning direction) and a head  10   a  which is arranged to face an outer circumferential surface  21  of the photoreceptor drum  110 . Hereinafter, the direction of a rotational axis of the photoreceptor drum  110  (that is, the direction of a bus of the photoreceptor drum  110  (the main scanning direction)) is referred to as a ‘drum axis direction X’. 
     FIG. 14  is a cross-sectional view illustrating a head of the image forming apparatus according to the third embodiment and the peripheral structure thereof, taken along a direction perpendicular to the drum axis direction X shown in  FIG. 13 . As shown in  FIGS. 13 and 14 , the head  10   a  includes a substantially rectangular substrate  31   a  whose longitudinal direction is the drum axis direction X and which is opposite to the outer circumferential surface  21  of the photoreceptor drum  110 , a plurality of light emitting elements  38  formed on an element forming surface Sa 2  (one surface of the substrate  31   a ) opposite to a drum opposing surface Sa 1  which faces the photoreceptor drum  110 , and a sealing member  35   a  which is formed on the element forming surface Sa 2  of the substrate  31   a  so as to cover the light emitting elements  38 . The sealing member  35   a  is a film formed of various resin material, such as an acryl-based resin and an epoxy-based resin. The sealing member  35   a  covering the light emitting elements  38  makes it possible to prevent the deterioration of the light emitting elements  38  due to permeation of air and water. 
   As shown in  FIG. 13 , a plurality of elastic bodies  631  is arranged on the surface of the sealing member  35   a . Each elastic body  631  is a unit for elastically urging the head  10   a  against the photoreceptor drum  110 , and is provided between the head  10   a  and a case  60  of the image forming apparatus. In the third embodiment, six elastic bodies  631  are arranged around four corners of the surface of the sealing member  35   a  and in a central portion thereof in the longitudinal direction. For example, a coil spring having one end fixed to the head  10   a  and the other end fixed to the case  60  is used as the elastic body  631 . However, the elastic body  631  may have any other shapes. For example, various members, such as a leaf spring and rubber interposed between the sealing member  35   a  and the case  60 , can be used as the elastic bodies  631 . 
   The substrate  31   a  shown in  FIGS. 13 and 14  is a plate member formed of a transmissive material, such as glass or plastic. Each light emitting element  38  is formed by interposing a light emitting layer formed of an organic EL material between an anode and a cathode, and emits light when electric energy is applied.  FIG. 15  is a plan view illustrating the element forming surface Sa 2  of the substrate  31   a . As shown in  FIG. 15 , the light emitting elements  38  are arranged in two rows or in island shapes along the drum axis direction X, and selectively emit light corresponding to an image to be printed on a recording medium, such as a sheet. Light emitted from each light emitting element  38  is incident on the surface of the photoreceptor drum  110  through the substrate  31   a . That is, the head  10   a  of the third embodiment is of a bottom emission type. Then, the exposure by the head  10   a  causes a latent image corresponding to a desired image to be formed on the surface of the photoreceptor drum  110 . The arrangement pattern of the light emitting elements  38  is not limited to that shown in  FIG. 15 . For example, the light emitting elements may be arranged in other patterns, such as in one row or three or more rows. 
   Next,  FIG. 16  is a perspective view illustrating the appearance of the substrate  31   a . In  FIG. 16 , the drum opposing surface Sa 1  is positioned on the upper side (that is, the position of the substrate  31   a  is reverse to the position of the substrate  31   a  shown in  FIG. 13  or  14  in the vertical direction). As shown in  FIG. 16 , the drum opposing surface Sa 1  of the substrate  31   a  includes a sliding surface  41   a  and an inclined surface  45   a . As shown in  FIGS. 14 and 16 , the sliding surface  41   a  is a curved surface (concave surface) which is recessed toward the opposite side to the photoreceptor drum  110  with a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110 . That is, the sliding surface  41   a  can be referred to as a portion of the inner surface (inner circumferential surface) of a cylinder which has a radius substantially equal to the outer circumferential surface  21  of the photoreceptor drum  110 . When the elastic bodies  631  urge the head  10   a  against the photoreceptor drum  110 , the entire sliding surface  41   a  of the substrate  31   a  comes into contact with the outer circumferential surface  21  of the photoreceptor drum without a gap therebetween. 
   Meanwhile, the inclined surface  45   a  is positioned between the sliding surface  41   a  and a side surface  47   a  located at the upstream side of the substrate  31   a  in a rotational direction A of the photoreceptor drum  110 . As shown in  FIG. 14 , the inclined surface  45   a  is tilted with respect to the outer circumferential surface  21  such that an elevation angle θ 1  with respect to the outer circumferential surface  21  of the photoreceptor drum  110  is an acute angle. The elevation angle θ 1  is an angle formed between the inclined surface  45   a  and a tangent line PL of the outer circumferential surface  21  at an edge E positioned on the upstream side of the sliding surface  41   a  in the rotational direction A of the photoreceptor drum  110 . As described above, a portion of the substrate  31   a  which is opposite to the photoreceptor drum  110  and is positioned on the upstream side in the rotational direction A is chamfered. Therefore, as shown in  FIG. 14 , the sliding surface  41   a  comes into contact with the outer circumferential surface  21  of the photoreceptor drum  110 , and the inclined surface  45   a  is opposite to the outer circumferential surface  21  of the photoreceptor drum  110  with a gap therebetween. The shape of the substrate  31   a  is formed by mechanically or chemically polishing the surface of a substantially rectangular plate. 
   As described above, in the third embodiment, the sliding surface  41   a  having a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110  comes into contact with the outer circumferential surface  21  of the photoreceptor drum  110 . Therefore, it is possible to accurately maintain a predetermined distance between the light emitting elements  38  and the outer circumferential surface  21  of the photoreceptor drum  110 , compared with the structure in which the drum opposing surface Sa 1  is flat. This effect will be described below. 
   As a structure compared with the third embodiment, a head  10   x  including a substrate  31   b  having a flat drum opposing surface Sa 1  is considered, as shown in  FIG. 17A . This structure has a problem in that the position or posture of the head  10   x  with respect to the photoreceptor drum  110  is unstable since the drum opposing surface Sa 1  comes into line contact with the outer circumferential surface  21  of the photoreceptor drum  110 . For example, as shown in  FIG. 17A , even when the head  10   x  is maintained such that the drum opposing surface Sa 1  is arranged in a substantially horizontal direction, the head  10   x  rotates on a contact line between the drum opposing surface Sa 1  and the outer circumferential surface  21 . As a result, the head  10   x  may be inclined, as shown in  FIG. 17B . In addition, for example, when the photoreceptor drum  110  rotates, the position of the head  10   x  may be changed from the original position to the horizontal position by friction force acting from the photoreceptor drum  110  to the head  10   x  or other factors (for example, external force), as shown in  FIG. 17C . In both cases shown in  FIGS. 17B and 17C , a distance between the light emitting element  38  and the outer circumferential surface  21  of the photoreceptor drum  110  is larger than that of the ideal structure shown in  FIG. 17A . When the distance between the light emitting element  38  and the photoreceptor drum  110  is changed, a variation in the area or shape of a region (spot) of the outer circumferential surface  21  of the photoreceptor drum  110  where light emitted from the light emitting elements  38  is incident occurs, which makes it difficult to form a high-definition image. 
   In contrast, in the third embodiment, the surface contact between the sliding surface  41   a  and the outer circumferential surface  21  of the photoreceptor drum  110  enables a stable posture or position of the head  10   a  with respect to the photoreceptor drum  110 . That is, the surface contact therebetween effectively prevents the inclination of the head  10   a  as shown in  FIG. 17B  or the displacement of the head  10   a  in the horizontal direction as shown in  FIG. 17C . Therefore, it is possible to maintain a predetermined distance between the photoreceptor drum  110  and the light emitting elements  38 , and thus to form a high-definition image. In particular, in the third embodiment, a plurality of elastic bodies  631  which are evenly formed on the surface of the sealing member  35   a  urge the head  10   a  against the photoreceptor drum  110  with uniform force. Thus, it is possible to reliably and stably maintain the posture or position of the head  10   a  by the surface contact between the sliding surface  41   a  and the outer circumferential surface  21  of the photoreceptor drum  110 . 
   Further, in the third embodiment, since a portion of the drum opposing surface Sa 1  of the substrate  31   a  arranged on the upstream side thereof in the rotational direction A of the photoreceptor drum  110  serves as the inclined surface  45   a , it is possible to prevent the damage of the outer circumferential surface  21  due to collision between the drum opposing surface Sa 1  and the outer circumferential surface  21  of the photoreceptor drum  110 . For example, a structure in which the drum opposing surface Sa 1  (that is, for example, as shown in  FIG. 27 , the sliding surface and the side surface intersect with each other at an acute angle formed therebetween) does not include the inclined surface  45   a  is assumed. In this structure, the outer circumferential surface  21  may be damaged by contact between the outer circumferential surface  21  and a corner corresponding to the intersection between the sliding surface and the side surface (hereinafter, referred to as a head corner). In particular, when the cross section of the photoreceptor drum  110  is not a circular shape due to errors in manufacture or deformation with the passage of time, or when the position of the photoreceptor drum  110  deviates from the original position thereof due to the same reasons, a portion of the outer circumferential surface  21  protruding from the original surface toward the outside may be damaged due to contact with the head corner. In contrast, in the head  10   a  of the third embodiment, since the drum opposing surface Sa 1  has the inclined surface  45   a  (that is, the edge of the head is chamfered), it is possible to prevent the damage of the outer circumferential surface  21 . Further, in the third embodiment, the inclined surface  45   a  is formed on only the upstream side of the drum opposing surface Sa 1  in the rotational direction A. However, the same inclined surface as the inclined surface  45   a  may be additionally formed on the downstream side of the drum opposing surface Sa 1  in the rotational direction A. 
   Fourth Embodiment 
     FIG. 18  is a cross-sectional view illustrating the structure of a portion of an image forming apparatus according to a fourth embodiment. In the third embodiment, the head  10   a  is opposite to the outer circumferential surface  21  of the photoreceptor drum  110 . On the other hand, in the fourth embodiment, a head  10   b  is opposite to the inner circumferential surface of the cylindrical photoreceptor drum  110 . In the fourth embodiment, the photoreceptor drum  110  is a cylindrical member formed by laminating a photosensitive layer on an outer circumferential surface of a transmissive cylinder. The head  10   b  is urged by a plurality of elastic bodies  631  arranged on the surface of a sealing member  35   a  so that a substrate  31   c  comes into contact with an inner circumferential surface  22  of the photoreceptor drum  110 . In the fourth embodiment, the same components as those in the third embodiment have the same reference numerals, and a description thereof will be omitted. 
     FIG. 19  is an enlarged cross-sectional view illustrating a portion of the structure shown in  FIG. 18 .  FIG. 20  is a perspective view illustrating the appearance of the substrate  31   c . As shown in  FIG. 19 , a drum opposing surface Sa 1  of the substrate  31   c  includes an inclined surface  45   b  and a sliding surface  41   c  which is shaped so as to come into surface contact with the inner circumferential surface  22  of the photoreceptor drum  110 . As shown in  FIGS. 19 and 20 , the sliding surface  41   c  is a curved surface (convex surface) which protrudes toward the photoreceptor drum  110  with a curvature substantially equal to that of the inner circumferential surface  22  of the photoreceptor drum  110 . That is, the sliding surface  41   c  can be referred to as a portion of the outer surface (outer circumferential surface) of a cylinder which has a radius substantially equal to the inner circumferential surface  22  of the photoreceptor drum  110 . The inclined surface  45   b  of the substrate  31   c  is positioned on the upstream side of the sliding surface  41   c  of the drum opposing surface Sa 1  in the rotational direction A of the photoreceptor drum  110 . As shown in  FIG. 19 , the inclined surface  45   b  is tilted with respect to the inner circumferential surface  22  such that an elevation angle θ 2  with respect to the inner circumferential surface  22  of the photoreceptor drum  110  is an acute angle. 
   In the fourth embodiment, the sliding surface  41   c  having a curvature substantially equal to that of the inner circumferential surface  22  of the photoreceptor drum  110  comes into surface contact with the inner circumferential surface  22 . Therefore, it is possible to maintain a predetermined distance between the light emitting elements  38  and the inner circumferential surface  22  of the photoreceptor drum  110  with high accuracy, similar to the third embodiment. Further, in the fourth embodiment, since the head  10   b  is accommodated inside the photoreceptor drum  110 , it is possible to reduce a space required for arranging the head, compared with the third embodiment in which the head  10   a  is arranged outside the photoreceptor drum  110 . 
   Fifth Embodiment 
     FIG. 21  is a front view illustrating a head  10   c  and a photoreceptor drum  110  of an image forming apparatus according to a fifth embodiment, as viewed from the horizontal direction (a direction perpendicular to the drum axis direction). In addition,  FIG. 22  is a cross-sectional view taken along the line XXII-XXII of  FIG. 21 .  FIG. 23  is a cross-sectional view taken along the line XXIII-XXIII of  FIG. 21 . As shown in  FIG. 21 , in the fifth embodiment, the head  10   c  (which is of a top emission type) includes a substrate  50   a  whose longitudinal direction is the drum axis direction X and a plurality of light emitting elements  38  formed on the substrate  50   a.    
   In the third embodiment, the sliding surface  41   a  is formed on the entire surface of the substrate  31   a  in the longitudinal direction (drum axis direction X) thereof. In contrast, in the fifth embodiment, only both ends of the substrate having the light emitting elements  38  formed thereon in the longitudinal direction (drum axis direction) serve as the sliding surface. In the fifth embodiment, the same components as those in the third embodiment have the same reference numerals, and a description thereof will be omitted. 
   The substrate  50   a  includes a first portion  51  having a plurality of light emitting elements  38  on a surface opposite to the photoreceptor drum  110  and second portions  52  arranged at both ends thereof in the longitudinal direction. Surfaces of the second portions  52  opposite to the photoreceptor drum  110  protrude from the surface of the first portion having the light emitting elements  38  formed thereon toward the photoreceptor drum  110 . Therefore, as shown in  FIGS. 21 and 23 , the first portion  51  and the light emitting elements  38  formed therein are opposite to the inner circumferential surface  21  of the photoreceptor drum  110  at a gap B 1  corresponding to a step difference (about several micrometers) between the first portion  51  and the second portion  52 . The substrate  50   a  having the above-mentioned shape is formed by mechanically or chemically polishing a portion of a substantially rectangular plate corresponding to the first portion  51 . 
   In the fifth embodiment, as shown in  FIGS. 21 and 22 , a part of the second portion  52  opposite to the outer circumferential surface  21  of the photoreceptor drum  110  serves as a sliding surface  521 . Similar to the sliding surface  41   a  in the third embodiment, the sliding surface  521  is a curved surface (concave surface) which is recessed toward the opposite side to the photoreceptor drum  110  with a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110 . When a plurality of elastic bodies  631  (not shown in  FIGS. 21 to 23 ) arranged on a surface of the substrate  50   a  opposite to the light emitting elements  38  and the sliding surface  521  urges the head  10   c  against the photoreceptor drum  110 , the sliding surface  521  of the second portion  52  comes into surface contact with the outer circumferential surface  21  of the photoreceptor drum without a gap therebetween. Thus, the fifth embodiment obtains the same effects as those in the third embodiment. Further, in the fifth embodiment, the first portion  51  of the substrate  50   a  and the light emitting elements  38  formed thereon are separated from the outer circumferential surface  21  of the photoreceptor drum  110 . This structure makes it possible to prevent the damage of the light emitting elements  38  due to contact with the photoreceptor drum  110 . 
   In the fifth embodiment, the head  10   c  is opposite to the outer circumferential surface  21  of the photoreceptor drum  110 . However, the head  10   c  may be opposite to the inner circumferential surface  22  of the photoreceptor drum  110 , as in the fourth embodiment. In this structure, the sliding surface  521  of the second portion  52  of the substrate  50   a  opposite to the inner circumferential surface  22  of the photoreceptor drum  110  is a curved surface (convex surface) which has a curvature substantially equal to that of the inner circumferential surface  22  and protrudes toward the photoreceptor drum  110 . In addition, in  FIGS. 21 to 23 , parts of the second portions  52  opposite to the photoreceptor drum  110  serve as the sling surface  521 . However, an inclined surface  45 , which is the same as that in the third or fourth embodiment, may be formed in the second portion  52 . 
   In  FIGS. 21 to 23 , the top-emission-type head  10   c  is used. However, the fifth embodiment can be applied to a bottom-emission-type head. That is, when both ends (second portions  52 ) of the drum opposing surface Sa 1  of the substrate shown in  FIG. 14  or  19  in the longitudinal direction protrude from the other portion (first portion  51 ) toward the photoreceptor drum  110  and serve as the sliding surface  521 , the same effects as those in the fifth embodiment can be obtained. 
   Further, in  FIGS. 21 to 23 , the substrate  50   a  is formed of a single plate. However, similar to the third embodiment or the fourth embodiment, a base member formed by laminating a plurality of substrates may be used. In addition, as shown in  FIGS. 31 and 36 , a sealing member may be provided so as to cover the element forming surface Sa 2  and side surfaces of the substrate, thereby forming a base member. The substrate is formed such that both ends of the surface thereof opposite to the photoreceptor drum in the longitudinal direction (drum axis direction X) protrude from the other portion toward the photoreceptor drum  110 . 
   Sixth Embodiment 
     FIG. 24  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a sixth embodiment of the invention.  FIG. 25  is a cross-sectional view illustrating components shown in  FIG. 24 , taken along a direction perpendicular to the drum axis direction X, and corresponds to  FIG. 14  in the third embodiment. As shown in  FIGS. 24 and 25 , the image forming apparatus according to the sixth embodiment has a substantially rectangular frame member  65  surrounding the head  10   a . An inner circumferential surface of the frame member  65  corresponds to side surfaces of the head  10   a . The frame member  65  can have any shapes as long as it has a portion opposite to the side surface of the head  10   a . Therefore, the shape of the frame member  65  is not limited to that shown in  FIG. 24 . 
   In the third embodiment, a plurality of elastic bodies  631  is arranged on the surface of the head  10   a  opposite to the photoreceptor drum  110 . In contrast, in the sixth embodiment, elastic bodies arranged on the side surfaces of the head  10   a  urge the head  10   a  against the photoreceptor drum  110 . In the sixth embodiment, the same components as those in the third embodiment have the same reference numerals, and a description thereof will be omitted. 
   A plurality of elastic bodies  632  is provided between the inner circumferential surface of the frame member  65  and the side surface of the head  10   a . Each elastic body  632  has one end fixed to the inner circumferential surface of the frame member  65  and the other end fixed to the side surface of the head  10   a . More specifically, as shown in  FIG. 24 , three elastic bodies  632  are arranged at equal intervals on the side surface corresponding to each long side of the substantially rectangular head  10   a  such that an end of each of the three elastic bodies  632  is fixed to the side surface of the head  10   a  and the other end thereof is fixed to the inner circumferential surface of the frame member  65  opposite to the side surface of the head  10   a . In addition, an end of one elastic body  632  is fixed in the center of the side surface corresponding to each short side of the head  10   a , and the other end of the elastic body  632  is fixed to the inner circumferential surface of the frame member  65  opposite to the side surface of the head  10   a.    
   Further, a plurality of elastic bodies  633  is arranged on a surface of the frame member  65  opposite to the photoreceptor drum  10 . The elastic bodies  633  are members for urging the frame member  65  against the photoreceptor drum  110 , and are provided between the case  60  of the image forming apparatus and the frame member  65 . In the sixth embodiment, six elastic bodies  633  are arranged around four corners of the frame member  65  and in a central portion thereof in the longitudinal direction. The elastic bodies  632  and  633  are the same components as the elastic bodies  631  of the third embodiment. For example, coil springs or leaf springs are used as the elastic bodies  632  and  633 . 
   In the above-mentioned structure, when the elastic bodies  633  press the frame member  65  against the photoreceptor drum  110 , the elastic bodies  632  are extended. Then, the head  10   a  is urged against the photoreceptor drum  110  by elastic force generated by the elastic bodies  632 . Therefore, the sixth embodiment can obtain the same effects as those in the third embodiment. 
   In the above-mentioned structure, the elastic bodies  633  press the frame member  65  against the photoreceptor drum  10 . However, as shown in  FIG. 26 , the head  10   a  may be urged against the photoreceptor  110  according to the fixed position of the frame member  65 . That is, in this structure, the frame member  65  is fixed to the case  60  such that an end portion P 1  of each elastic body  632  fixed to the frame member  65  is positioned closer to the photoreceptor drum  110  than an end portion P 2  of the elastic body  632  fixed to the head  10   a . In this structure, the elastic force of the elastic bodies  632  causes the head  10   a  to be pressed against the photoreceptor drum  110 , and thus the same effects as those in the sixth embodiment can be obtained. In addition, according to this structure, it is unnecessary to provide the elastic bodies  633  on the surfaces of the head  10   a  and the frame member  65  opposite to the photoreceptor drum  110 , which results in a reduction in a space to be ensured on the upper side of the head  10   a.    
   In  FIGS. 24 to 26 , the head  10   a  according to the third embodiment is urged. However, the fourth embodiment or the fifth embodiment can adopt the structure in which the head  10   a  is urged against the photoreceptor drum  110  by the components of the sixth embodiment, such as the frame member  65  and the elastic bodies  632 . 
   Modifications of Third to Sixth Embodiments 
   Various modifications of the third to sixth embodiments can be made. The modifications thereof will be described in detail. The following modifications may be appropriately combined. 
   First Modification 
   In the third, fourth, and sixth embodiments, the surface of the head opposite to the photoreceptor drum  110  includes an inclined surface. However, the inclined surface is not necessarily needed. For example, in the third or sixth embodiment, as shown in  FIG. 27 , instead of the substrate  31   a , a substrate  31   f  may be used in which the entire drum opposing surface Sa 1  serves as a sliding surface  41   f  having a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110 . In addition, as shown in  FIG. 28 , in the fourth embodiment, a substrate  31   g  may be used in which the entire drum opposing surface Sa 1  serves as a sliding surface  41   g  having a curvature substantially equal to that of the inner circumferential surface  22  of the photoreceptor drum  110 . 
   Second Modification 
   In the third to sixth embodiments, optical components may be provided between the light emitting elements  38  and the photoreceptor drum  110 . For example, an optical waveguide (for example, an optical fiber) for guiding light emitted from the light emitting elements  38  to the surface of the photoreceptor drum  110  or a lens for condensing light emitted from the light emitting elements  38  may be provided between the light emitting elements  38  and the photoreceptor drum  110 . 
     FIG. 29  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a second modification of the third or sixth embodiment. In this structure, microlenses  55   a  for condensing light emitted from the light emitting elements  38  are provided between the light emitting elements  38  and the photoreceptor drum  110 . The microlenses  55   a  are arranged in an array shape so as to be opposite to the light emitting elements  38 . In the structure shown in  FIG. 29 , a board  56   a  overlaps a surface of a substrate  31   h  having the light emitting elements  38  formed thereon which faces the photoreceptor drum  110 . The board  56   a  may be a thin film which is formed of a resin material on the substrate  31   h , or a plate member bonded to the substrate  31   h . A plurality of curved-shaped concave portions  31   h   1  are formed in a surface of the substrate  31   h  facing the board  56   a  at positions corresponding to the light emitting elements  38 . Similarly, a plurality of curved-shaped concave portions  56   a   1  are formed in a surface of the board  56   a  facing the substrate  31   h  at positions corresponding to the light emitting elements  38 . A surface of the board  56   a  facing the outer circumferential surface  21  of the photoreceptor drum  110  is composed of a sliding surface  41   a  having a curvature substantially equal to that of the outer circumferential surface  21 . 
   A resin material having a different reflective index from those of the substrate  31   h  and the board  56   a  is filled up into spaces formed by the concave portions  31   h   1  of the substrate  31   h  and the concave portions  56   a   1  of the substrate  56   a , thereby forming the microlenses  55   a , which are double-sided convex lenses. The microlenses  55   a  condense light emitted from the corresponding light emitting elements  38 , so that an image is formed on the surface of the photoreceptor drum  110 . The shape and arrangement of the microlenses  55   a  are not limited to those shown in  FIG. 29 . For example, convex microlenses protruding toward the photoreceptor drum  110  may be formed on the surface of the substrate. 
   In the structure in which light emitted from the light emitting elements  38  is condensed by the microlenses  55   a , even when a distance between the light emitting elements  38  and the photoreceptor drum  110  varies slightly, a variation in the area of a spot on the surface of the photoreceptor drum  110  where light emitted from the light emitting element  38  is incident becomes remarkable. According to this modification, the surface contact between the photoreceptor drum  110  and the sliding surface having a curvature substantially equal to that of the surface of the photoreceptor drum  110  makes it possible to maintain a uniform distance between the light emitting elements  38  and the photoreceptor drum  110  with high accuracy. Therefore, in the structure capable of improving the utilization efficiency of light by using the microlenses  55   a , it is also possible to accurately form a predetermined latent image on the surface of the photoreceptor drum  110 . That is, the effects of maintaining the uniform distance between the light emitting elements  38  and the photoreceptor drum  110  can be more reliably obtained by the structure in which optical components, such as microlenses, are arranged between the light emitting elements  38  and the photoreceptor drum  110 , as shown in  FIG. 29 . 
   Seventh Embodiment 
     FIG. 30  is a perspective view illustrating the structure of a portion of an image forming apparatus according to a seventh embodiment of the invention. The image forming apparatus is used for a printing unit of, for example, a printer, a duplicating machine, or a facsimile. As shown in  FIG. 30 , the image forming apparatus includes a cylindrical photoreceptor drum  110  which is supported so as to rotate in the direction of arrow A (sub-scanning direction) and a head  10   d  which is arranged opposite to an outer circumferential surface  21  of the photoreceptor drum  110 . Hereinafter, the direction of a rotational axis of the photoreceptor drum  110  (that is, the direction of a bus of the photoreceptor drum  110  (the main scanning direction)) is referred to as a ‘drum axis direction X’. 
     FIG. 31  is a cross-sectional view illustrating the components shown in  FIG. 30 , taken along a direction perpendicular to the drum axis direction X. As shown in  FIGS. 30 and 31 , the head  10   d  includes a substantially rectangular substrate  32  whose longitudinal direction is the drum axis direction X and which is opposite to the outer circumferential surface  21  of the photoreceptor drum  110 , a plurality of light emitting elements  38  formed on an element forming surface Sa 2  (one surface of the substrate  32 ) opposite to a drum opposing surface Sa 1  which faces the photoreceptor drum  110 , and a sealing member  36   d  which is formed on the element forming surface Sa 2  of the substrate  32  so as to cover the light emitting elements  38 . The sealing member  36   d  is a substantially rectangular member formed so as to cover the element forming surface Sa 2  and side surfaces of the substrate  32 , and is formed of various resin material, such as an acryl-based resin and an epoxy-based resin. The sealing member  36   d  covering the light emitting elements  38  makes it possible to prevent the deterioration of the light emitting elements  38  due to permeation of air or water. The sealing member  36   d  of the seventh embodiment has a transmissive property. 
   As shown in  FIG. 30 , a plurality of elastic bodies  631  is arranged on a surface of the head  10   d  (the sealing member  36   d ) opposite to the photoreceptor drum  110 . Each elastic body  631  is a member for elastically urging the head  10   d  against the photoreceptor drum  110 , and is provided between the head  10   d  and a case  60  of the image forming apparatus. In the seventh embodiment, six elastic bodies  631  are arranged around four corners of the surface of the sealing member  36   d  opposite to the photoreceptor drum  110  and in a central portion thereof in the longitudinal direction. For example, a coil spring having one end fixed to the head  10   d  and the other end fixed to the case  60  is used as the elastic body  631 . However, the elastic body  631  may have any other shapes. For example, various members, such as a leaf spring and rubber interposed between the sealing member  36   d  and the case  60 , can be used as the elastic bodies  631 . 
   The substrate  32  shown in  FIGS. 30 and 31  is a plate member formed of a transmissive material, such as glass or plastic. Meanwhile, each light emitting element  38  is formed by interposing a light emitting layer formed of an organic EL material between an anode and a cathode, and emits light when electric energy is applied.  FIG. 32  is a plan view illustrating the element forming surface Sa 2  of the substrate  32 . As shown in  FIG. 32 , the light emitting elements  38  are arranged in two rows or in island shapes along the drum axis direction X, and selectively emit light corresponding to an image to be printed on a recording medium, such as a sheet. Light emitted from each light emitting element  38  is incident on the surface of the photoreceptor drum  110  through the sealing member  36   d . That is, the head  10   d  of the seventh embodiment is of a top emission type. Therefore, the substrate  32  does not need to have a transmissive property. Then, the exposure by the head  10   d  causes a latent image corresponding to a desired image to be formed on the surface of the photoreceptor drum  110 . The arrangement pattern of the light emitting elements  38  is not limited to that shown in  FIG. 32 . For example, the light emitting elements  38  may be arranged in other patterns, such as in one row or three or more rows. 
   Next,  FIG. 33  is a perspective view illustrating the appearance of a surface Sa 1  of the sealing member  36   d  opposite to the photoreceptor drum (hereinafter, referred to as a ‘drum opposing surface’). In  FIG. 33 , the drum opposing surface Sa 1  is positioned on the upper side (that is, the position of the sealing member  36   d  is reverse to the position thereof shown in  FIG. 30  or  31  in the vertical direction). As shown in  FIG. 33 , the drum opposing surface Sa 1  of the sealing member  36   d  includes a sliding surface  41   d  and an inclined surface  45   d . As shown in  FIGS. 31 and 33 , the sliding surface  41   d  is a curved surface (concave surface) which is recessed toward the opposite side to the photoreceptor drum  110  with a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110 . That is, the sliding surface  41   d  can be referred to as a portion of the inner surface (inner circumferential surface) of a cylinder which has a radius substantially equal to the outer circumferential surface  21  of the photoreceptor drum  110 . When the elastic bodies  631  urge the head  10   d  against the photoreceptor drum  110 , the entire sliding surface  41   d  of the sealing member  36   d  comes into contact with the outer circumferential surface  21  of the photoreceptor drum  110  without a gap therebetween, as shown in  FIG. 31 . 
   Meanwhile, the inclined surface  45   d  is positioned between the sliding surface  41   d  and a side surface  47   d  located at the upstream side of the sealing member  36   d  in a rotational direction A of the photoreceptor drum  110 . As shown in  FIG. 31 , the inclined surface  45   d  is tilted with respect to the outer circumferential surface  21  such that an elevation angle θ 1  with respect to the outer circumferential surface  21  of the photoreceptor drum  110  is an acute angle. The elevation angle θ 1  is an angle formed between the inclined surface  45   d  and a tangent line PL of the outer circumferential surface  21  at an edge E positioned on the upstream side of the sliding surface  41   d  in the rotational direction A of the photoreceptor drum  110 . As described above, a portion of the sealing member  36   d  which is opposite to the photoreceptor drum  110  and is positioned on the upstream side in the rotational direction A is chamfered. Therefore, as shown in  FIG. 31 , the sliding surface  41   d  comes into surface contact with the outer circumferential surface  21  of the photoreceptor drum  110 , and the inclined surface  45   d  is opposite to the outer circumferential surface  21  of the photoreceptor drum  110  with a gap therebetween. 
   The above-mentioned sealing member  36   d  is formed by a method (injection molding) of hardening an ultraviolet-curable or thermosetting resin material filled into a mold by heating or radiation of ultraviolet rays and of taking it out or by a method of mechanically or chemically polishing a board which is formed substantially in a rectangular shape so as to cover the element forming surface Sa 2  of the substrate  32 . The former shaping method has an advantage that an inexpensive sealing member  36   d  can be produced in large quantities. In addition, the substrate  32  is fit into a groove formed by mechanically or chemically polishing the surface of the sealing member  36   d  opposite to the drum opposing surface Sa 1  and is then fixed to the sealing member  36   d  by an adhesive. 
   As described above, in the seventh embodiment, the sliding surface  41   d  having a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110  comes into surface contact with the outer circumferential surface  21  of the photoreceptor drum  110 . Therefore, it is possible to accurately maintain a predetermined distance between each light emitting element  38  and the outer circumferential surface  21  of the photoreceptor drum  110 , compared with the structure in which the drum opposing surface Sa 1  is flat. This effect will be described below. 
   As a structure compared with the seventh embodiment, a head  10   y  including a sealing member  36  having a flat drum opposing surface Sa 1  is considered, as shown in  FIG. 34A . This structure has a problem in that the position or posture of the head  10   y  with respect to the photoreceptor drum  110  is unstable since the drum opposing surface Sa 1  comes into line contact with the outer circumferential surface  21  of the photoreceptor drum  110 . For example, as shown in  FIGS. 34A to 34C , even when the head  10   y  is maintained such that the drum opposing surface Sa 1  is arranged in a substantially horizontal direction, the head  10   y  rotates on a contact line between the drum opposing surface Sa 1  and the outer circumferential surface  21 . As a result, the head  10   y  may be inclined, as shown in  FIG. 34B . In addition, for example, when the photoreceptor drum  110  rotates, the position of the head  10   y  may be changed from the original position to the horizontal position by friction force acting from the photoreceptor drum  110  to the head  10   y  or other factors (for example, external force), as shown in  FIG. 34C . In both cases shown in  FIGS. 34B and 34C , a distance between the light emitting element  38  and the outer circumferential surface  21  of the photoreceptor drum  110  is larger than that in the ideal structure shown in  FIG. 34A . When the distance between the light emitting element  38  and the photoreceptor drum  110  is changed, a variation in the area or shape of a region (spot) of the outer circumferential surface  21  of the photoreceptor drum  110  where light emitted from the light emitting elements  38  is incident occurs, which makes it difficult to form a high-definition image. 
   In contrast, in the seventh embodiment, the surface contact between the sliding surface  41   d  and the outer circumferential surface  21  of the photoreceptor drum  110  enables the stale posture or position of the head  10   d  with respect to the photoreceptor drum  110 . That is, the surface contact therebetween effectively prevents the inclination of the head  10   d  as shown in  FIG. 34B  or the displacement of the head  10   d  in the horizontal direction as shown in  FIG. 34C . Therefore, it is possible to maintain a predetermined distance between the photoreceptor drum  110  and the light emitting elements  38 , and thus to form a high-definition image. In particular, in the seventh embodiment, a plurality of elastic bodies  631  urge the head  10   d  against the photoreceptor drum  110  with uniform force. Thus, it is possible to reliably and stably maintain the posture or position of the head  10   d  by the surface contact between the sliding surface  41   d  and the outer circumferential surface  21  of the photoreceptor drum  110 . 
   Further, in the seventh embodiment, since a portion of the drum opposing surface Sa 1  of the sealing member  36   d  arranged on the upstream side thereof in the rotational direction A of the photoreceptor drum  110  is the inclined surface  45   d , it is possible to prevent the damage of the outer circumferential surface  21  due to collision between the drum opposing surface Sa 1  and the outer circumferential surface  21  of the photoreceptor drum  110 . For example, a structure in which the drum opposing surface Sa 1  does not include the inclined surface  45   d  (that is, for example, as shown in  FIG. 42 , the sliding surface and the side surface intersect with each other at an acute angle formed therebetween) is assumed. In this structure, the outer circumferential surface  21  may be damaged by contact between the outer circumferential surface  21  and a corner corresponding to the intersection between the sliding surface and the side surface. In particular, when the cross section of the photoreceptor drum  110  is not a circular shape due to errors in manufacture or deformation with the passage of time, or when the position of the photoreceptor drum  110  deviates from the original position thereof due to the same reasons, a portion of the outer circumferential surface  21  protruding from the original surface toward the outside may be damaged due to contact with the corner. In contrast, in the head  10   d  of the seventh embodiment, since the drum opposing surface Sa 1  has the inclined surface  45   d  (that is, the edge of the head is chamfered), it is possible to prevent the damage of the outer circumferential surface  21 . Further, in the seventh embodiment, the inclined surface is formed on only the upstream side of the drum opposing surface Sa 1  in the rotational direction A. However, the same inclined surface (a chamfered portion) as the inclined surface  45   d  may be additionally formed on the downstream side of the drum opposing surface Sa 1  in the rotational direction A. 
   In the above-mentioned embodiment, the top-emission-type head  10   d  is used. However, as described above, the structure in which the sliding surface comes into surface contact with the photoreceptor drum  110  can be applied to a bottom-emission-type head, as shown in  FIG. 14 . In the structure shown in  FIG. 14 , the transmissive substrate  31   a  having a substantially rectangular shape constitutes the drum opposing surface Sa 1 . The drum opposing surface Sa 1  of the substrate  31   a  includes the sliding surface  41   a , which is a curved surface recessed toward the opposite side to the photoreceptor drum  110  with a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110 , and the inclined surface  45   a  which is tilted such that an elevation angle θ 1  with respect to the outer circumferential surface  21  is an acute angle. In this structure, since the sliding surface  41   a  of the substrate  31   a  comes into surface contact with the outer circumferential surface  21  of the photoreceptor drum  110 , it is possible to maintain the stable posture or position of the head, similar to the structure shown in  FIGS. 30 to 33 . However, in this structure, it is difficult to achieve both a reduction in manufacturing costs and a stable posture or position of the head. This problem will be described below in detail. 
   The substrate needs to have high flatness in order to form the light emitting elements  38  thereon. For example, the substrate is more expensive than the sealing member not requiring high flatness. Therefore, from the viewpoint of a reduction in manufacturing costs, it is preferable that the substrate have a small size. In addition, when a plurality of substrates is manufactured by dividing a large board (a so-called mother glass), the larger the number of substrates obtained from one board is, the lower the manufacturing costs thereof becomes. Therefore, from this point of view, it is preferable that the substrate have a small size. Meanwhile, as shown in  FIG. 14 , in the structure in which the sliding surface  41   a  of the substrate  31   a  comes into surface contact with the photoreceptor drum  110 , as the contact area of the sliding surface  41   a  with the photoreceptor drum  110  is larger, the effect of stabilizing the posture or position of the head (the effect of maintaining a predetermined distance between the light emitting element  38  and the photoreceptor drum  110 ) becomes more remarkable. As such, in the structure shown in  FIG. 14 , a reciprocal relationship between the postural or positional stabilization of the head and a reduction in the manufacturing costs thereof is established. That is, when the size of the substrate  32  is reduced in order to lower manufacturing costs, the positional or postural stabilization of the head is lowered (accuracy for maintaining the distance between the light emitting element  38  and the photoreceptor drum  110  is lowered). On the other hand, when the size of substrate increases in order to improve the stabilization of the head, the manufacturing costs are raised. 
   In contrast, in the seventh embodiment shown in  FIGS. 30 to 33 , since the sealing member  36   d , not the substrate  32 , comes into contact with the photoreceptor drum  110 , it is possible to reliably ensure a sufficient area of the sliding surface coming into contact with the photoreceptor drum  110 , regardless of the size of the substrate  32 . For example, in the structure shown in  FIG. 31 , it is possible to increase a width W 2  of the drum opposing surface Sa 1  of the sealing member  36   d , regardless of a width W 1  of the substrate  32  in a direction perpendicular to the drum axis direction X. Therefore, it is possible to reduce the manufacturing costs of the substrate  32  by setting the width W 1  of the substrate  32  to the minimum value required for forming the light emitting element  38 , and to effectively stabilize the posture or position of the head (to accurately maintain a distance between the light emitting element  38  and the photoreceptor drum  110 ) by ensuring a sufficient width W 2  of the sealing member  36   d . That is, the seventh embodiment can achieve both the postural or positional stabilization of the head and a reduction in manufacturing costs, compare with the structure shown in  FIG. 14 . 
   Further, in the structure shown in  FIG. 14 , the substrate should have both a property suitable for forming the light emitting elements  38  and compatibility with the outer circumferential surface  21  of the photoreceptor drum  110  (in particular, friction resistance to the outer circumferential surface  21 ). The selection of the shape or material of the substrate is restricted within narrow limits. In contrast, in the seventh embodiment, a material forming the substrate having the light emitting elements  38  thereon can be separately selected from a material forming the sealing member  36   d  coming into contact with the photoreceptor drum  110 , which makes it possible to improve the flexibility of the design of the head, compared with the structure shown in  FIG. 14 . 
   Eighth Embodiment 
     FIG. 35  is a cross-sectional view illustrating the structure of a portion of an image forming apparatus according to an eighth embodiment. In the seventh embodiment, the head  10   d  is opposite to the outer circumferential surface  21  of the photoreceptor drum  110 . In contrast, in the eighth embodiment, a head  10   e  is opposite to the inner circumferential surface of the cylindrical photoreceptor drum  110 . In the eighth embodiment, the photoreceptor drum  110  is a cylindrical member formed by laminating a photosensitive layer on an outer circumferential surface of a transmissive cylinder. The head  10   e  is urged against the photoreceptor drum  110  by a plurality of elastic bodies  631  so that a sealing member  36   e  comes into contact with an inner circumferential surface  22  of the photoreceptor drum  110 . In the eighth embodiment, the same components as those in the seventh embodiment have the same reference numerals, and a description thereof will be omitted. 
     FIG. 36  is an enlarged cross-sectional view illustrating the head  10   e  shown in  FIG. 35 .  FIG. 37  is a perspective view illustrating the appearance of the sealing member  36   e , paying attention to the drum opposing surface Sa 1 . As shown in  FIG. 36 , the drum opposing surface Sa 1  of the sealing member  36   e  includes an inclined surface  45   e  and a sliding surface  41   e  which is shaped so as to come into surface contact with the inner circumferential surface  22  of the photoreceptor drum  110 . That is, as shown in  FIGS. 36 and 37 , the sliding surface  41   e  is a curved surface (convex surface) which protrudes toward the photoreceptor drum  110  with a curvature substantially equal to that of the inner circumferential surface  22  of the photoreceptor drum  110 . In other words, the sliding surface  41   e  can be referred to as a portion of the outer surface (outer circumferential surface) of a cylinder which has a radius substantially equal to the inner circumferential surface  22  of the photoreceptor drum  110 . The inclined surface  45   e  of the sealing member  36   e  is positioned on the upstream side of the sliding surface  41   e  of the drum opposing surface Sa 1  in the rotational direction A of the photoreceptor drum  110 . As shown in  FIG. 36 , the inclined surface  45   e  is tilted with respect to the inner circumferential surface  22  such that an elevation angle θ 2  with respect to the inner circumferential surface  22  of the photoreceptor drum  110  is an acute angle. 
   In the eighth embodiment, the sliding surface  41   e  having a curvature substantially equal to that of the inner circumferential surface  22  of the photoreceptor drum  110  comes into surface contact with the inner circumferential surface  22 . Therefore, the same effects as those in the seventh embodiment can be obtained. Further, in the eighth embodiment, since the head  10   e  is accommodated inside the photoreceptor drum  110 , it is possible to reduce a space required for arranging the head, compared with the seventh embodiment in which the head  10   d  is arranged outside the photoreceptor drum  110 . 
   Ninth Embodiment 
     FIG. 38  is a perspective view illustrating the structure of the head  10   d  and the photoreceptor drum  110  of an image forming apparatus according to a ninth embodiment of the invention.  FIG. 39  is a cross-sectional view illustrating components shown in  FIG. 38 , taken along a direction perpendicular to the drum axis direction X, and corresponds to  FIG. 31  referred in the seventh embodiment. As shown in  FIGS. 38 and 39 , the image forming apparatus according to the ninth embodiment has a substantially rectangular frame member  65  surrounding the head  10   d . An inner circumferential surface of the frame member  65  corresponds to the side surface of the head  10   d . The frame member  65  can have any shapes as long as it has a portion opposite to the side surface of the head  10   d . Therefore, the shape of the frame member  65  is not limited to that shown in  FIG. 38 . 
   In the seventh embodiment, a plurality of elastic bodies  631  is arranged on the surface of the head  10   d  opposite to the photoreceptor drum  110 . In contrast, in the ninth embodiment, elastic bodies arranged on the side surfaces of the head  10   d  urge the head  10   d  against the photoreceptor drum  110 . In the ninth embodiment, the same components as those in the seventh embodiment have the same reference numerals, and a description thereof will be omitted. 
   A plurality of elastic bodies  632  is provided between the inner circumferential surface of the frame member  65  and the side surface of the head  10   d . Each elastic body  632  has one end fixed to the inner circumferential surface of the frame member  65  and the other end fixed to the side surface of the head  10   d . More specifically, as shown in  FIG. 38 , three elastic bodies  632  are arranged at equal intervals on the side surface corresponding to each long side of the substantially rectangular head  10   d  such that an end of each of the three elastic bodies  632  is fixed to the side surface of the head  10   d  and the other end thereof is fixed to the inner circumferential surface of the frame member  65  opposite to the side surface of the head  10   d . In addition, an end of one elastic body  632  is fixed in the center of the side surface corresponding to each short side of the head  10   d , and the other end of the elastic body  632  is fixed to the inner circumferential surface of the frame member  65  opposite to the side surface of the head  10   d.    
   Further, a plurality of elastic bodies  633  is arranged on a surface of the frame member  65  opposite to the photoreceptor drum  110 . The elastic bodies  633  are members for urging the frame member  65  against the photoreceptor drum  110 , and are provided between the case  60  of the image forming apparatus and the frame member  65 . In the ninth embodiment, six elastic bodies  633  are arranged around four corners of the frame member  65  and in a central portion thereof in the longitudinal direction. The elastic bodies  632  and  633  are the same components as the elastic bodies  631  of the seventh embodiment. For example, coil springs or leaf springs are used as the elastic bodies  632  and  633 . 
   In the above-mentioned structure, when the elastic bodies  633  press the frame member  65  against the photoreceptor drum  110 , the elastic bodies  632  are extended. Then, the head  10   d  is urged against the photoreceptor drum  110  by elastic force generated by the elastic bodies  632 . Therefore, the ninth embodiment can obtain the same effects as those in the seventh embodiment. 
   In the above-mentioned structure, the elastic bodies  633  press the frame member  65  against the photoreceptor drum  110 . However, as shown in  FIG. 40 , the head  10   d  may be urged against the photoreceptor  110  according to the fixed position of the frame member  65 . That is, in this structure, the frame member  65  is fixed to the case  60  such that an end portion P 1  of each elastic body  632  fixed to the frame member  65  is positioned closer to the photoreceptor drum  110  than an end portion P 2  of the elastic body  632  fixed to the head  10   d . In this structure, the elastic force of the elastic bodies  632  causes the head  10   d  to be pressed against the photoreceptor drum  110 , and thus the same effects as described above can be obtained. In addition, according to this structure, it is unnecessary to provide the elastic bodies  633  on the surfaces of the head  10   d  and the frame member  65  opposite to the photoreceptor drum  110 , which results in a reduction in a space to be ensured on the upper side of the head  10   d.    
   In  FIGS. 38 to 40 , the head  10   d  according to the seventh embodiment is urged. However, similar to the eighth embodiment, the head  10   d  is urged against the photoreceptor drum  110  by the components of the ninth embodiment, such as the frame member  65  and the elastic bodies  632 . 
   Modifications of Seventh to Ninth Embodiments 
   Various modifications of the seventh to ninth embodiments can be made. The modifications thereof will be described in detail. The following modifications may be appropriately combined. 
   First Modification 
   In the seventh to ninth embodiments, the drum opposing surface Sa 1  includes an inclined surface. However, the inclined surface is not necessarily needed. For example, in the seventh or ninth embodiment, as shown in  FIG. 41 , instead of the sealing member  36   d , a sealing member  36   a  may be used in which the entire drum opposing surface Sa 1  is a sliding surface  41   j  having a curvature substantially equal to that of the outer circumferential surface  21  of the photoreceptor drum  110 . In addition, as shown in  FIG. 42 , in the eighth embodiment, a sealing member  36   b  may be used in which the entire drum opposing surface Sa 1  is a sliding surface  41   k  having a curvature substantially equal to that of the inner circumferential surface  22  of the photoreceptor drum  110 . 
   Second Modification 
   In the seventh to ninth embodiments, optical components may be provided between the light emitting elements  38  and the photoreceptor drum  110 . For example, an optical waveguide (for example, an optical fiber) for guiding light emitted from the light emitting elements  38  to the surface of the photoreceptor drum  110  or a lens for condensing light emitted from the light emitting elements  38  may be provided between the light emitting elements  38  and the photoreceptor drum  110 . 
     FIG. 43  is a cross-sectional view illustrating the structure of a head of an image forming apparatus according to a second modification of the seventh or ninth embodiment. In this structure, microlenses  55   b  for condensing light emitted from the light emitting elements  38  are provided between the light emitting elements  38  and the photoreceptor drum  110 . The microlenses  55   b  are arranged in an array shape so as to be opposite to the light emitting elements  38 . In the structure shown in  FIG. 43 , a board  56   b  overlaps a surface of a sealing member  36   c  covering the light emitting elements  38  which faces the photoreceptor drum  110 . The board  56   b  may be a thin film which is formed of a resin material on the sealing member  36   c , or a plate member bonded to the sealing member  36   c . A plurality of curved-shaped concave portions  36   c   1  are formed in a surface of the sealing member  36   c  facing the board  56   b  at positions corresponding to the light emitting elements  38 . Similarly, a plurality of curved-shaped concave portions  56   b   1  are formed in a surface of the board  56   b  facing the sealing member  36   c  at positions corresponding to the light emitting elements  38 . A surface of the board  56   b  facing the outer circumferential surface  21  of the photoreceptor drum  110  is composed of a sliding surface  41   m  having a curvature substantially equal to that of the outer circumferential surface  21 . 
   A resin material having a different reflective index from those of the sealing member  36   c  and the board  56   b  is filled up into spaces formed by the concave portions  36   c   1  of the sealing member  36   c  and the concave portions  56   b   1  of the substrate  56   b , thereby forming the microlenses  55   b , which are double-sided convex lenses. The microlenses  55   b  condense light emitted from the corresponding light emitting elements  38 , so that an image is formed on the surface of the photoreceptor drum  110 . The shape and arrangement of the microlenses  55   b  are not limited to those shown in  FIG. 43 . For example, convex microlenses protruding toward the photoreceptor drum  110  may be formed on the surface of the sealing member. 
   In the structure in which light emitted from the light emitting elements  38  is condensed by the microlenses  55   b , even when a distance between the light emitting elements  38  and the photoreceptor drum  110  varies slightly, a variation in the area of a spot on the surface of the photoreceptor drum  110  where light emitted from the light emitting element  38  is incident becomes remarkable. According to this modification, the surface contact between the photoreceptor drum  110  and the sliding surface having a curvature substantially equal to that of the surface of the photoreceptor drum  110  makes it possible to maintain a uniform distance between the light emitting elements  38  and the photoreceptor drum  110  with high accuracy. Therefore, in the structure capable of improving the utilization efficiency of light by using the microlenses  55   b , it is also possible to accurately form a predetermined latent image on the surface of the photoreceptor drum  110 . That is, the effects of maintaining the uniform distance between the light emitting elements  38  and the photoreceptor drum  110  can be more reliably obtained by the structure in which optical components, such as the microlenses  55   b , are arranged between the light emitting elements  38  and the photoreceptor drum  110 , as shown in  FIG. 43 . 
   Third Modification 
   In the first to ninth embodiments, the light emitting element includes the light emitting layer formed of an organic EL material. However, functions related to the above-mentioned embodiments or modifications may be realized by using a head having light emitting elements arranged therein, each including a light emitting layer formed of an inorganic EL material, or a head having light emitting diodes (LEDs) as light emitting elements. That is, elements which emit light when electric energy is applied as well as the light emitting elements each including a light emitting layer formed of an organic EL material can be used. 
   Tenth to Thirteenth Embodiments 
     FIG. 44  is a cross-sectional view illustrating main parts of an image forming apparatus according to tenth to thirteenth embodiments. As shown in  FIG. 44 , each of the image forming apparatuses according to the tenth to thirteenth embodiments includes an image carrier  110   a , such as a photoreceptor drum or a photosensitive belt, and a head  10  for forming a latent image on the image carrier  110   a . The image carrier  110   a  is supported in the case of the image forming apparatus, and has an image carrier surface S 110  on which a latent image is formed. The image carrier surface S 110  is advanced in a direction A 3  represented by an arrow in  FIG. 44  when a latent image is formed. The head  10  is supported in the case of the image forming apparatus, and has a contact surface S 110  coming into contact with the image carrier surface S 110 . In addition, the head  10  emits light from the contact surface S 110  to the image carrier surface S 110 . A latent image is formed on the image carrier surface S 110  by the emission. Light emitted from the head  10  travels along a direction X 1  (a direction perpendicular to the plane of  FIG. 44 ) traversing the image carrier surface S 110  which is advanced in the direction A 3 , and the latent image formed on the image carrier surface S 110  has a two-dimensional pattern by the emission of light from the head  10  and the advance of the image carrier surface S 110 . The direction X 1  is the drum axis direction X when the image carrier  110   a  is a photoreceptor drum. 
   Hereinafter, the tenth to thirteenth embodiments according to the invention will be described in detail. In the following drawings used for the description, only the main parts are hatched. In addition, the image forming apparatuses of the tenth to thirteenth embodiments differ from each other in the structure of the head  10 , and thus a description of the above-mentioned embodiments will be made, centered on the structure of the head  10 . In the tenth embodiment, the head  10  is denoted by reference numeral  200 . In the eleventh embodiment, the head  10  is denoted by reference numeral  300 . In the twelfth embodiment, the head  10  is denoted by reference numeral  201 . In the thirteenth embodiment, the head  10  is denoted by reference numeral  301 . 
   Tenth Embodiment 
     FIG. 45  is a plan view illustrating the structure of the head  200  of the image forming apparatus according to the tenth embodiment of the invention. As shown in  FIG. 45 , in the head  200 , a plurality of light emitting elements  205  are arranged in two rows or in island shapes along the direction X 1 . These light emitting elements  205  are covered with a flat sealing substrate  230 . A front surface of the sealing substrate  230  serves as a light emission surface S 200 , and a rear surface thereof is opposite to the light emitting elements  205 . The light emission surface S 200  is the contact surface S 10  of  FIG. 44 . A cylindrical optical waveguide  235  is formed in each light emitting element  205  in a portion of the sealing substrate  230  overlapping the light emitting element  205 . 
     FIG. 46  is a cross-sectional view taken along the line XLVI-XLVI of  FIG. 45 . As shown in  FIG. 46 , the head  200  has a structure in which a plurality of light emitting elements  205  is provided between a plate-shaped main substrate  220  and a plate-shaped sealing substrate  230 . The main substrate  220  is formed of, for example, glass or plastic, and the light emitting elements  205  are formed on the main substrate  220 . Each light emitting element  205  is an organic EL element which emits light when electric energy is applied, and regions where the light emitting elements  205  are formed are divided by an oxide film  260  and partition walls (banks)  270  formed on the oxide film  260 . In each region, an electrode  240  serving as a cathode, a surface-emitting layer  210  which is formed of an organic EL material, a transmissive hole-injecting layer  250 , and a transparent electrode  280  serving as an anode are sequentially formed on the main substrate  220 . 
   Further, the sealing substrate  230  overlaps the main substrate  220  having the light emitting elements  205  formed thereon so as to seal the light emitting elements  205  together with the main substrate  220 . The sealing protects the light emitting elements  205  from the air (in particular, water and oxygen) and thus prevents the deterioration thereof. A transmissive adhesive  290  is used to fix the sealing substrate  230  to the main substrate  220 . For example, a thermosetting adhesive or an ultraviolet-curable adhesive is used as the adhesive  290 . 
   The sealing method used for this technical field includes a film sealing method in which the entire surface of the sealing substrate  230  is bonded to the main substrate  220  by the adhesive  290  and a gap sealing method in which the periphery of the sealing substrate  230  is bonded to the main substrate  220  by the adhesive  290  to form spaces defined by the sealing substrate  230  and the main substrate  220  around the light emitting elements  205 . A drying agent is arranged in the spaces in the gap sealing method. In the tenth embodiment, the film sealing method is used, but the gap sealing method can be used. 
   The sealing substrate  230  is formed by arranging a plurality of optical waveguides  235  in a plate  231 . The plate  231  is formed of, for example, glass, metal, ceramic, or plastic. Each optical waveguide  235  is provided so as to pass through the front and rear surfaces of the sealing substrate  230 , and the central axis thereof extends in the thickness direction of the sealing substrate  230 . In addition, the outer circumferential surface of each optical waveguide  235  is covered with the plate  231 . One end surface of the optical waveguide  235  facing the light emitting element  205  serves as a portion of the rear surface of the sealing substrate  230 , and the other end surface thereof serves as a portion of the front surface (the light emission surface S 200 ) of the sealing substrate  230 . The one end surface of the optical waveguide  235  facing the light emitting element  205  covers the light emitting layer  210  of the corresponding light emitting element  205 , as viewed from the light emission surface S 200 . 
   Further, the optical waveguides  235  are formed of a transmissive material. The material has a refractive index which is equal to that of the adhesive  290  and is higher than that of a material forming the plate  231 . 
   In addition, the optical waveguides  235  are fixed to the plate  231 . Any methods can be used to fix the optical waveguides  235  to the plate, but attentions should be paid when using a method in which the outer circumferential surface of the optical waveguide  235  does not contact the plate  231 . In this case, a method of fixing the optical waveguides  235  to the plate  231  using an adhesive is considered. In this method, an adhesive needs to have a refractive index lower than that of a material forming the optical waveguide  235 . That is, the outer circumferential surfaces of the optical waveguides  235  must be covered with a material having a refractive index lower than that of the material. 
     FIG. 47  is a cross-sectional view illustrating the optical operation of the head  200 . In the head  200 , when a voltage is applied by the electrode  240  and the transparent electrode  280 , the light emitting layer  210  interposed therebetween emits light. Most of light emitted from the light emitting layer  210  to the sealing substrate  230  travels substantially in a direction perpendicular to the sealing substrate  230  to reach the rear surface of the sealing substrate  230  through the hole injecting layer  250 , the transparent electrode  280 , and the adhesive  290 , more particularly, to reach the one end surface of the optical waveguide  235  facing the corresponding light emitting element  205 . Since the refractive index of the material forming the optical waveguides  235  is equal to or higher than that of the adhesive  290 , it is easy for all light components reached the one end surface to be incident on the optical waveguide  235 . Therefore, most of the reached light components are incident on the optical waveguides  235  and then travel in the optical waveguides  235 . 
   When the light components traveling in the optical waveguide  235  reach the outer circumferential surface of the optical waveguide  235 , most of them are specularly reflected therefrom. That is, the optical waveguide  235  functions as a core of an optical fiber having a large diameter to guide the incident light. The reason why the specular reflection occurs is that an angle between the outer circumferential surface of the optical waveguide  235  and the traveling direction of light reached the outer circumferential surface is generally very small. In order words, this is because, in general, the incident angle of light on the outer circumferential surface of the optical waveguide  235  is very large. More specifically, the reason is as follows. 
   Since the refractive index of the material forming the optical waveguide  235  is higher than that of a material covering the outer circumferential surface thereof (for example, an adhesive or a material forming the plate  231 ), most of the light components traveling in the optical waveguide  235  are specularly reflected. However, in order for the specular reflection, the incident angle should be larger than a threshold angle which is determined on the basis of the ratio of two reflective indexes. Therefore, as described above, since the incident angle of light with respect to the outer circumferential surface is generally very large, most of the light components reached the outer circumferential surface are specularly reflected therefrom as long as a material having an excessive threshold angle is not used. Therefore, it is preferable to select a material forming the optical waveguides  235  and a material covering the outer circumferential surfaces thereof such that a large number of light components are specular reflected. 
   In this way, light travels in the optical waveguide  235  and is then emitted from the end surface of the optical waveguide  235  on the side of the light emission surface S 200 . Therefore, a spot image (an optical image) is formed on the light emission surface S 200 . 
     FIG. 48  is a diagram illustrating the spot image formed by the head  200 . The shape, size, and forming position of the spot image are identical with the end surface of the optical waveguide  235  on the side of the light emission surface S 200 . In addition, the spot image has a substantially uniform distribution of brightness. Further, since light emitted from the light emitting layer  210  is not specularly reflected from an interface between the adhesive  290  and the optical waveguide  235 , the utilization efficiency of light emitted from the light emitting layer  210  is improved. 
   Furthermore, even when the thickness of the sealing substrate  230  is reduced due to the abrasion of a contact portion and thus the optical waveguide  235  is shortened, the shape and size of the end surface of the optical waveguide  235  exposed from the light emission surface S 200  are hardly varied, since the optical waveguide  235  is a cylindrical member which guides light by the specular reflection from the outer circumferential surface thereof and the central axis thereof extends in a direction where the contact surface recedes due to abrasion. Thus, the shape and size of the spot image formed on the light emission surface S 200  are also hardly changed. 
   As described above, according to the image forming apparatus of the tenth embodiment, in disregard of contact exposure in which the head  200  comes into contact with the image carrier  110   a , it is possible to guide light emitted from the light emitting layer  210  to the sealing substrate  230  without leakage and thus to stably form a high-definition image. That is, it is possible to reduce a variation in the area or shape of a spot image. This effect contributes to an improvement in the quality of printing and stabilization. In addition, since the optical waveguides are formed in the sealing substrate not requiring surface accuracy as high as the main substrate, an image forming apparatus can be more easily manufactured than an image forming apparatus having a head in which the optical waveguides are formed in the main substrate. 
   Next, an example of a method of manufacturing the head  200  will be described. 
     FIG. 49  is a diagram illustrating a first process of the method of manufacturing the head  200 . As shown in  FIG. 49 , first, a plurality of cylindrical holes is formed in the plate  231 . Since these holes are filled up in a subsequent process to serve as the optical waveguides  235 , they are formed such that end surfaces thereof can cover the light emitting layer  210 . A well-known method suitable for a material forming the plate  231  is used to form these holes. For example, when the plate  231  is formed of glass, a method of etching the plate  231  by using fluoric acid to form the holes can be used. In addition, when the plate  231  is formed of, for example, an ultraviolet-curable resin, a method of radiating ultraviolet rays on a portion of the plate  231  covered with a mask to harden it and of cutting non-hardened portions can be used. 
     FIG. 50  is a diagram illustrating the next process of that shown in  FIG. 49 . As shown in  FIG. 50 , the holes are filled up to form the optical waveguides  235  in the sealing substrate  230 . That is, a resin material forming the optical waveguides  235  is filled up into the holes so that both end surfaces of each optical waveguide  235  are flush with the front and rear surfaces of the sealing substrate  230 . The holes can be filled up by, for example, a method of filling up the resin material using a squeegee or an ink-jet method of applying the resin material using a dispenser. When the sealing substrate  230  is manufactured by the filling method, a material forming the optical waveguide  235  is limited to resin. However, when the sealing substrate  230  is manufactured by the other methods, the material is not limited to resin. 
     FIG. 51  is a diagram illustrating the next process of that shown in  FIG. 50 . As shown in  FIG. 51 , a plurality of light emitting elements  205  is formed on the main substrate  220 . 
     FIG. 52  is a diagram illustrating the next process of that shown in  FIG. 51 . As shown in  FIG. 52 , the adhesive  290  is coated on the surface of the main substrate  220  having the light emitting elements  205  formed thereon (or the rear surface of the sealing substrate  230 ), and the sealing substrate  230  is bonded and fixed to the main substrate  220  by the adhesive  290 . At that time, the main substrate  220  and the sealing substrate  230  are arranged such that an end surface of each optical waveguide  235  facing the main substrate  220  covers the light emitting layer  210  of the corresponding light emitting element  205 , thereby completing the head  200 . 
   As described above, in this manufacturing method, a process for cutting the substrate is needed. However, the sealing substrate, not the main substrate, is cut. Therefore, the utilization efficiency of the main substrate requiring a high degree of utilization efficiency is not lowered, which is effective in the mass production. 
   Eleventh Embodiment 
     FIG. 53  is a cross-sectional view illustrating the structure of the head  300  of the image forming apparatus according to the eleventh embodiment of the invention. The head  300  differs from the head  200  shown in  FIG. 46  in that the optical waveguides are formed in the main substrate, not in the sealing substrate. From the viewpoint of the difference, in the head  300 , light emitting elements  305  are used instead of the light emitting elements  205 , and a main substrate  320  is used instead of the main substrate  220 . However, the plate  231  is used as the sealing substrate as it is. 
   A surface of the main substrate  320  opposite to the surface thereof having the light emitting elements  305  formed thereon is a light emission surface S 300 . The light emission surface S 300  corresponds to the contact surface S 10  of  FIG. 44 . The light emitting element  305  differs from the light emitting element  205  in that a transparent electrode  340 , serving as an anode, is substituted for the electrode  240  serving as a cathode, and an electrode  380 , serving as a cathode, is substituted for the transparent electrode  280  serving as an anode. 
   The main substrate  320  is formed by arranging optical waveguides  323  in a plate  321  so as to correspond to light emitting elements  305 . Each optical waveguide  323  overlaps the corresponding light emitting element  305 . The plate  321  is formed of, for example, glass, quartz, or plastic. Each optical waveguide  323  is provided so as to pass through the front and rear surfaces of the main substrate  320 , and the central axis thereof extends in the thickness direction of the main substrate  320 . In addition, the outer circumferential surface of each optical waveguide  323  is covered with the plate  321 . One end surface of the optical waveguide  323  facing the light emitting element  305  serves as a portion of the rear surface of the main substrate  320 , and the other end surface thereof serves as a portion of the front surface (the light emission surface S 300 ) of the main substrate  320 . 
   The one end surface of the optical waveguide  323  facing the light emitting element  305  covers the light emitting layer  210  of the corresponding light emitting element  305 , as viewed from the light emission surface S 300 . Further, the optical waveguides  323  are formed of a transmissive material. The material has a refractive index which is higher than that of a material forming the plate  321 . That is, the optical waveguides  323  are fixed to the plate  321 , and the outer circumferential surfaces thereof are covered with a material having a refractive index lower than that of the material. 
     FIG. 54  is a cross-sectional view illustrating the optical operation of the head  300 . In the head  300 , when a voltage is applied by the transparent electrode  340  and the electrode  380 , the light emitting layer  210  interposed therebetween emits light. Most of light emitted from the light emitting layer  210  to the main substrate  320  travels substantially in a direction perpendicular to the main substrate  320  to reach the rear surface of the main substrate  320  through the transparent electrode  340 , more particularly, to reach the one end surface of the optical waveguide  323  facing the corresponding light emitting element  305 . The light reached the one end surface is incident on the optical waveguide  323  and then travels in the optical waveguides  323 . Since the optical waveguide  323  functions as a core of an optical fiber having a large diameter, most of the light incident on the optical waveguide  323  travels in the optical waveguide  323  and is then emitted from the end surface of the optical waveguide  323  on the side of the light emission surface S 300 . 
   The image forming apparatus of the eleventh embodiment can obtain the same effects as those obtained from the image forming apparatus of the tenth embodiment. However, since the optical waveguides are formed in the main substrate, the effects obtained by forming the optical waveguides in the sealing substrate are not obtained. 
   Further, in the image forming apparatus according to the eleventh embodiment, since the optical waveguides  323  are formed in the main substrate  320 , a distance between the light emitting layer and the optical waveguide is smaller than that in the image forming apparatus according to the tenth embodiment. This contributes to an improvement in the brightness of a spot image. 
   Twelfth Embodiment 
     FIG. 55  is a plan view illustrating the structure of the head  201  of the image forming apparatus according to the twelfth embodiment of the invention. As shown in  FIG. 55 , in the head  201 , a plurality of light emitting elements  205  are arranged in two rows or in island shapes along the direction X 1 . These light emitting elements  205  are covered with a sealing substrate  238 , and are further covered with a flat optical waveguide plate  236  formed in a groove (concave portion)  239  of the sealing substrate  238 . A front surface of the optical waveguide plate  236  serves as a light emission surface S 201 , and a rear surface thereof is opposite to the light emitting elements  205  with the sealing substrate  238  interposed therebetween. The light emission surface S 201  on which a spot image will be formed is a portion of the contact surface S 10  of  FIG. 44 . A cylindrical optical waveguide  233  is formed in each light emitting element  205  in a portion of the optical waveguide plate  236  overlapping the light emitting element  205 . 
     FIG. 56  is a cross-sectional view taken along the line LVI-LVI of  FIG. 55 . As shown in  FIG. 56 , the head  201  has a structure in which a plurality of light emitting elements  205  is provided between a plate-shaped main substrate  220  and a plate-shaped sealing substrate  238 . The sealing substrate  238  overlaps the main substrate  220  having the light emitting elements  205  formed thereon so as to seal the light emitting elements  205  together with the main substrate  220 . A transmissive adhesive  290  is used to fix the sealing substrate  238  to the main substrate  220 . 
   The sealing substrate  238  is a plate member having a groove  239  in one surface (front surface) thereof opposite to the other surface (rear surface) thereof facing the light emitting elements  205 . The groove  239  has a flat bottom. The sealing substrate is generally formed of, for example, glass, metal, ceramic, or plastic. However, since the sealing substrate  238  needs to transmit light emitted from the light emitting layer  210 , the sealing substrate  238  is formed of a transmissive material. In addition, the refractive index of a material forming the sealing substrate  238  is equal to or higher than that of the adhesive  290 . 
   The optical waveguide plate  236  is fixed to the sealing substrate  238  such that the rear surface thereof comes into contact with the bottom of the groove  239 . Any methods can be used to fix the optical waveguide plate  236  to the sealing substrate  238  as long as a light shielding material is not interposed between the rear surface of the optical waveguide plate  236  and the bottom of the groove  239 . The optical waveguide plate  236  is formed by arranging a plurality of optical waveguides  233  in a plate  237  having rectangular front and rear surfaces. 
   Each optical waveguide  233  for guiding light emitted from the light emitting layer  210  is provided so as to pass through the front and rear surfaces of the optical waveguide plate  236 , and the central axis thereof extends in the thickness direction of the optical waveguide plate  236 . In addition, the outer circumferential surface of each optical waveguide  233  is covered with the plate  237 . One end surface of the optical waveguide  233  facing the light emitting element  205  serves as a portion of the rear surface of the optical waveguide plate  236 , and the other end surface thereof serves as a portion of the front surface (the light emission surface S 201 ) of the optical waveguide plate  236 . The one end surface of the optical waveguide  233  facing the light emitting element  205  covers the light emitting layer  210  of the corresponding light emitting element  205 , as viewed from the light emission surface S 201 . 
   Further, the optical waveguides  233  are formed of a transmissive material. The material has a refractive index which is equal to or higher than that of a material forming the sealing substrate  238  and which is higher than that of a material forming the plate  237 . In addition, the optical waveguides  233  are fixed to the plate  237 . Any methods can be used to fix the optical waveguides  233  to the plate  237 , but attentions should be paid when using a method in which the outer circumferential surface of the optical waveguide  233  does not contact the plate  237 . In this case, a method of fixing the optical waveguides  233  to the plate  237  using an adhesive is considered. In this method, it is necessary to use an adhesive having a refractive index lower than that of a material forming the optical waveguide  233 . That is, the outer circumferential surfaces of the optical waveguides  233  must be covered with a material having a refractive index lower than that of the material. 
   The width, length, and depth of the groove  239  are set such that the optical waveguide plate  236  is flush with the sealing substrate  238  except for the groove  239 . That is, the width, length, and depth of the groove  239  depend on the width, length, and thickness of the optical waveguide plate  236 . The width and length of the optical waveguide plate  236  are set to the minimum values capable of causing a plurality of optical waveguides  233  to be arranged in the optical waveguide plate  236 . That is, the width and length of the optical waveguide plate  236  depend on the arrangement of the light emitting elements  205 . More specifically, the width of the groove  239  is about several hundreds of micrometers. 
   The thickness of the optical waveguide plate  236  depends on the thickness of the sealing substrate  238 . 
   Since the optical waveguide  233  functions to guide light emitted from the light emitting layer  210 , it is preferable that the end surface thereof facing the light emitting element  205  be close to the light emitting element  205 . Therefore, it is preferable that a thick optical waveguide plate  236  be used to improve the utilization efficiency of light emitted from the light emitting layer  210 . However, in order to increase the thickness of the optical waveguide plate  236 , it is necessary to reduce the thickness of a portion of the sealing substrate  238  where the groove  239  is formed. In this case, the rigidity of the sealing substrate  238  should be considered. In this structure, the width of the groove  239  is about several hundreds of micrometers, but the width of the sealing substrate  238  (the length of a short side in  FIG. 55 ) is in a range of about 15 mm to 20 mm. Therefore, even when the groove  239  of the sealing substrate  238  has a relatively small thickness, a sufficient degree of rigidity is obtained. However, when the thickness of the groove  239  is excessively small, a sufficient degree of rigidity is not obtained. In addition, in this case, a sealing function is also deteriorated. Therefore, the thickness of the optical waveguide plate  236  is set as large as possible in the range not causing these problems. 
     FIG. 57  is a cross-sectional view illustrating the optical operation of the head  201 . In the head  201 , when a voltage is applied by the electrode  240  and the transparent electrode  280 , the light emitting layer  210  interposed therebetween emits light. Most of light components emitted from the light emitting layer  210  to the sealing substrate  238  travels substantially in a direction perpendicular to the sealing substrate  238  to reach the rear surface of the sealing substrate  238  through the hole injecting layer  250 , the transparent electrode  280 , and the adhesive  290 . Since the refractive index of the material forming the sealing substrate  238  is equal to or higher than that of the adhesive  290 , it is easy for all the light components reached the rear surface of the sealing substrate  238  to be incident on the sealing substrate  238 . Therefore, most of the reached light components are incident on the sealing substrate  238 . 
   Since a portion of the sealing substrate  238  covering the light emitting elements  205  has a relatively small thickness, all the light components incident on the sealing substrate  238  reach the front surface of the sealing substrate  238  (the bottom of the groove  239 ). More specifically, the light components reach the end surfaces of the optical waveguides  233  facing the light emitting elements  205 . Since the refractive index of a material forming the optical waveguides  233  is equal to or higher than that of a material forming the sealing substrate  238 , it is easy for all light components reached the one end surface to be incident on the optical waveguide  233 . Therefore, most of the reached light components are incident on the optical waveguides  233  and then travel in the optical waveguides  233 . 
   When the light components traveling in the optical waveguide  233  reach the outer circumferential surface of the optical waveguide  233 , most of them are specularly reflected therefrom. That is, the optical waveguide  233  functions as a core of an optical fiber having a large diameter to guide the incident light. The reason why the specular reflection occurs is the same as described in the tenth embodiment. Preferably, a material forming the optical waveguides  233  and a material covering the outer circumferential surfaces of the optical waveguides  233  are selected such that a sufficient amount of light can be specularly reflected. 
   In this way, light travels in the optical waveguide  233  and is then emitted from the end surface of the optical waveguide  233  on the side of the light emission surface S 201 . Therefore, a spot image (an optical image) is formed on the light emission surface S 201 , as shown in  FIG. 48 . The shape, size, and forming position of the spot image are identical with the end surface of the optical waveguide  233  on the side of the light emission surface S 201 . In addition, the spot image has a substantially uniform distribution of brightness. 
   Furthermore, even when the thicknesses of the optical waveguide plate  236  and the sealing substrate  238  are reduced due to the abrasion of a closely adhering portion and thus the optical waveguide  233  is shortened, the shape and size of the end surface of the optical waveguide  233  exposed from the light emission surface S 201  are hardly changed, since the optical waveguide  233  is a cylindrical member which guides light by the specular reflection from the outer circumferential surface thereof and the central axis thereof extends in a direction where the contact surface recedes due to abrasion. Thus, the shape and size of the spot image formed on the light emission surface S 200  are also hardly changed. 
   As described above, according to the image forming apparatus of the twelfth embodiment, in disregard of contact exposure in which the head  201  comes into contact with the image carrier  110   a , it is possible to guide light emitted from the light emitting layer  210  to the sealing substrate  238  without leakage and thus to stably form a high-definition image. That is, it is possible to reduce a variation in the area or shape of a spot image. This effect contributes to an improvement in the quality of printing and stabilization. In addition, light emitted from the light emitting layer  210  is hardly reflected not only from an interface between the adhesive  290  and the sealing substrate  238  but also from an interface between the sealing substrate  238  and the optical waveguide  233 , which makes it possible to improve the utilization efficiency of light emitted from the light emitting layer  210 . 
   Further, one groove  239  is formed in the sealing substrate  238 . Therefore, it is possible to easily manufacture a sealing substrate, compared with the structure in which the optical waveguides passing through the sealing substrate are directly formed in the sealing substrate. In addition, it is possible to reduce manufacturing costs, compared with the structure in which the entire surface of the sealing substrate is cut. 
   Furthermore, since the optical waveguide plate  236  having the optical waveguides  233  formed therein does not need to have a sealing function, there is no restriction to select a material forming the plate  237 . In addition, the optical waveguide plate  236  is smaller than the sealing substrate  238  in size. Therefore, it is possible to easily form the optical waveguides  233 , compared with the structure in which the optical waveguides passing through the sealing substrate is directly formed in the sealing substrate. 
   Moreover, in the head  201 , a surface of the sealing substrate  238  facing the main substrate  220  has no seam, which makes it possible to reliably maintain the sealing function. In addition, the optical waveguides  233  are formed in the optical waveguide plate  236 , not in the sealing substrate  238  or the main substrate  220 . Therefore, the optical waveguide plate  236 , not the main substrate and the sealing substrate, is directly deformed due to a difference between thermal shrinkage (expansion) of the optical waveguide  233  and thermal shrinkage (expansion) of a circumferential portion thereof (the plate  237  or adhesive). That is, the deformation of the main substrate or the sealing member due to the optical waveguides  233  does not occur. 
   Next, an example of a method of manufacturing the head  201  will be described. 
     FIG. 58  is a diagram illustrating a first process of the method of manufacturing the head  201 . As shown in  FIG. 58 , first, a plurality of cylindrical holes is formed in the plate  237 . Since these holes are filled up in a subsequent process to serve as the optical waveguides  233 , they are formed such that end surfaces thereof can cover the light emitting layer  210 . The holes can be formed by, for example, the hole forming method used in the tenth embodiment. 
     FIG. 59  is a diagram illustrating the next process of that shown in  FIG. 58 . As shown in  FIG. 59 , the holes are filled up to form the optical waveguides  233  in the optical waveguide plate  236 . That is, a resin material forming the optical waveguides  233  is filled up into the holes so that both end surfaces of each optical waveguide  233  are flush with the front and rear surfaces of the optical waveguide plate  236 . The holes can be filled up by, for example, the filling method used in the tenth embodiment. 
     FIG. 60  is a diagram illustrating the next process of that shown in  FIG. 59 . As shown in  FIG. 60 , the groove  239  is formed by cutting the sealing substrate  238 , and the optical waveguide plate  236  is fitted into the groove  239 . In this way, one end surface of the optical waveguide plate  236  comes into contact with the bottom of the groove  239 . Meanwhile, a plurality of light emitting elements  205  is formed on the main substrate  220 . 
     FIG. 61  is a diagram illustrating the next process of that shown in  FIG. 60 . As shown in  FIG. 61 , the adhesive  290  is coated on the surface of the main substrate  220  having the light emitting elements  205  formed thereon (or the rear surface of the sealing substrate  238 ), and the sealing substrate  238  is bonded and fixed to the main substrate  220  by the adhesive  290 . At that time, the main substrate  220  and the sealing substrate  238  are arranged such that an end surface of each optical waveguide  233  facing the main substrate  220  covers the light emitting layer  210  of the corresponding light emitting element  205 , thereby completing the head  201 . 
   As described above, in this manufacturing method, a process for cutting the substrate is needed. However, the sealing substrate  238  and the plate  237 , not the main substrate  220 , are cut. Therefore, the utilization efficiency of the main substrate requiring a high degree of utilization efficiency is not lowered, which is effective in the mass production. 
   In this manufacturing method, the optical waveguide plate  236  is provided in the groove  239  of the sealing substrate  238  before the sealing substrate  238  is fixed to the main substrate  220 . However, the optical waveguide plate  236  may be provided in the groove  239  of the sealing substrate  238  after the sealing substrate  238  is fixed to the main substrate  220 . 
   Thirteenth Embodiment 
     FIG. 62  is a cross-sectional view illustrating the structure of the head  301  of the image forming apparatus according to the thirteenth embodiment of the invention. The head  301  differs from the head  201  shown in  FIG. 56  in that the optical waveguides are formed in the main substrate, not in the sealing substrate. From the viewpoint of the difference, in the head  301 , light emitting elements  305  are used instead of the light emitting elements  205 , and a plate-shaped main substrate  328  having a groove  329  formed therein is used instead of the main substrate  220 . In addition, a plate  231  is used as a sealing substrate. 
   The plate  231  differs from the sealing substrate  238  shown in  FIG. 56  in that it does not have a groove formed therein and is formed of a light shielding material. The light emitting element  305  differs from the light emitting element  205  in that a transparent electrode  340 , serving as an anode, is substituted for the electrode  240  serving as a cathode, and an electrode  380 , serving as a cathode, is substituted for the transparent electrode  280  serving as an anode. 
   The light emitting elements  305  are covered with the main substrate  328  and are further covered with a flat optical waveguide plate  326  which is provided in the groove  329  of the main substrate  328  and is fixed to the main substrate  328 . The main substrate  328  should be formed of a transmissive material, such as glass, quartz, or plastic. The optical waveguide plate  326  is fixed to the main substrate  328  by the same method as that used for fixing the optical waveguide plate  236  to the sealing substrate  238  in  FIG. 56 . The front surface of the optical waveguide plate  326  serves as a light emission surface S 301  on which a spot image is formed, and constitutes a portion of the contact surface S 10  shown in  FIG. 44 . The rear surface of the optical waveguide plate  326  is opposite to the light emitting elements  305  with the main substrate  238  interposed therebetween. 
   The groove  329  is formed in one surface (front surface) of the main substrate  328  opposite to the other surface (rear surface) thereof facing the light emitting elements  305 . The groove  329  has a flat bottom, and the rear surface of the optical waveguide plate  236  comes into contact with the bottom. Similar to the groove  239 , the width, length, and depth of the groove  329  are set such that the surface (the light emission surface S 301 ) of the optical waveguide plate  326  is flush with the surface of the main substrate  328  except for the groove  329 . That is, the width, length, and depth of the groove  329  depend on the width, length, and thickness of the optical waveguide plate  326 . In addition, the width, length, and thickness of the optical waveguide plate  326  are set, similar to the optical waveguide plate  236 . 
   A cylindrical optical waveguide  323  is formed in each light emitting element  305  in a portion of the optical waveguide plate  326  overlapping the light emitting element  305 . Each optical waveguide  323  for guiding light emitted from the light emitting layer  210  is provided so as to pass through the front and rear surfaces of the optical waveguide plate  326 , and the central axis thereof extends in the thickness direction of the optical waveguide plate  326 . In addition, the outer circumferential surface of each optical waveguide  323  is covered with the plate  327 . One end surface of the optical waveguide  323  facing the light emitting element  305  serves as a portion of the rear surface of the optical waveguide plate  326 , and the other end surface thereof serves as a portion of the front surface (the light emission surface S 301 ) of the optical waveguide plate  326 . The one end surface of the optical waveguide  323  facing the light emitting element  305  covers the light emitting layer  210  of the corresponding light emitting element  305 , as viewed from the light emission surface S 301 . 
   Further, the optical waveguides  323  are formed of a transmissive material. The material has a refractive index which is equal to or higher than that of a material forming the main substrate  328  and which is higher than that of a material forming the plate  327 . The outer circumferential surface of each optical waveguide  233  should be covered with a material having a refractive index lower than that of the material in any methods. 
     FIG. 63  is a cross-sectional view illustrating the optical operation of the head  301 . In the head  301 , when a voltage is applied by the transparent electrode  340  and the electrode  380 , the light emitting layer  210  interposed therebetween emits light. Most of light components emitted from the light emitting layer  210  to the main substrate  328  travels substantially in a direction perpendicular to the main substrate  328  and is then incident on the main substrate  328  through the transparent electrode  340 . 
   Since a portion of the main substrate  328  covering the light emitting elements  305  has a relatively small thickness, all the light components incident on the main substrate  328  reach the front surface of the main substrate  328  (the bottom of the groove  329 ). More specifically, the light components reach the end surfaces of the optical waveguides  323  facing the light emitting elements  305 . Since the refractive index of a material forming the optical waveguides  323  is equal to or higher than that of a material forming the main substrate  328 , it is easy for all the light components reached the end surface to be incident on the optical waveguide  323 . Therefore, most of the reached light components are incident on the optical waveguide  323  and then travel in the optical waveguide  323 . Since the optical waveguide  323  functions as a core of an optical fiber having a large diameter, most of the light incident on the optical waveguide  323  travels in the optical waveguide  323  and is then emitted from the end surface of the optical waveguide  323  on the side of the light emission surface S 301 . 
   The image forming apparatus of the thirteenth embodiment can obtain the same effects as those obtained from the image forming apparatus of the twelfth embodiment. However, since the optical waveguides are formed in the main substrate, the effects obtained by forming the optical waveguides in the sealing substrate are not obtained. 
   Further, in the image forming apparatus according to the thirteenth embodiment, since the optical waveguides  323  are formed in the main substrate  328 , a distance between the light emitting layer and the optical waveguide is smaller than that in the image forming apparatus according to the twelfth embodiment. This contributes to an improvement in the brightness of a spot image. 
   Modifications of Tenth to Thirteenth Embodiments 
   Various modifications of the above-mentioned tenth to thirteenth embodiments can be made. The modifications thereof will be described in detail. The following modifications may be appropriately combined. 
   First Modification 
   In the tenth to thirteenth embodiments, a cylindrical optical waveguide is used, but the shape of the optical waveguide is not limited thereto. For example, the optical waveguide may be formed in a prismatic shape or a pillar shape having a hemispherical end surface. That is, the optical waveguide can have any pillar shapes. 
   Second Modification 
   In the tenth to thirteenth embodiments, organic EL elements are used as light emitting elements. However, inorganic EL elements may be used as light emitting elements. 
   Third Modification 
   In the tenth to thirteenth embodiments, the optical waveguide serving as an optical fiber is used. An optical fiber array composed of a bundle of optical fibers may be used as the optical waveguide. In this case, one end surface of the fiber array constitutes a portion of a light emission surface (contact surface), and the other end surface thereof covers the light emitting layer  210 . Light incident on one end of the optical fiber travels toward the other end of the optical fiber while being specularly reflected from the inner circumferential surface of the optical fiber. The one end of each optical fiber constitutes a portion of the one end surface of the fiber array, and the other end thereof constitutes a portion of the other end surface of the fiber array.  FIG. 64  shows a spot image obtained from this modification. As shown in  FIG. 64 , the spot image formed on the contact surface S 10  by using the fiber array slightly has a slightly different shape from that of the light emitting layer  210 , and has non-uniform distribution of brightness. However, in this modification, except this point, the same effects as those in the tenth to thirteenth embodiments are obtained. 
   Overall Structure of Image Forming Apparatus 
     FIG. 65  is a longitudinal cross-sectional view illustrating an example of the overall structure of an image forming apparatus according to the invention. This image forming apparatus is a tandem-type full color image forming apparatus using a belt intermediate transfer method. 
   In this image forming apparatus, four heads  10 K,  10 C,  10 M, and  10 Y having the same structure are arranged at exposure positions of four photoreceptor drums (image carriers)  110 K,  110 C,  110 M, and  110 Y having the same structure. The heads  10 K,  10 C,  10 M, and  10 Y correspond to any one of the heads of the image forming apparatuses according to the above-described embodiments, and are organic EL array exposure heads in which organic EL elements, each including a light emitting layer formed of an organic EL material, are arranged as light emitting elements. 
   As shown in  FIG. 65 , the image forming apparatus has a driving roller  121 , a driven roller  122 , an endless intermediate transfer belt  120  wound around the driving roller  121  and the driven roller  122 , and the intermediate transfer belt  122  circulates around the rollers  121  and  122  in the direction of arrow shown in  FIG. 65 . Although not shown in  FIG. 65 , a tension applying unit for applying tension to the intermediate transfer belt  120 , such as a tension roller, may be provided. 
   The four photoreceptor drums  110 K,  110 C,  110 M, and  110 Y are disposed at predetermined intervals around the intermediate transfer belt  120 . Each photoreceptor drum has a photosensitive layer on the outer peripheral surface thereof. These photoreceptor drums correspond to the photoreceptor drums (image carriers) of the image forming apparatuses according to the above-described embodiments, and suffixes ‘K’, ‘C’, ‘M’, and ‘Y’ added to reference numerals indicate black, cyan, magenta, and yellow which are used for forming a toner image, respectively. This is similarly applied to other members. The photoreceptor drums  110 K,  110 C,  110 M, and  110 Y are rotated in synchronism with the driving of the intermediate transfer belt  120 . 
   A corona charger  111  (K, C, M, and Y), the head  10  (K, C, M, and Y), and a developing device  114  (K, C, M, and Y) are arranged around each photoreceptor drum  110  (K, C, M, and Y). The corona charger  111  (K, C, M, and Y) uniformly charges the outer peripheral surface of the corresponding photoreceptor drum  110  (K, C, M, and Y). The head  10  (K, C, M, and Y) writes a latent image on the charged outer peripheral surface of the photoreceptor drum. Each head  10  (K, C, M, and Y) is arranged such that a plurality of light emitting elements is arranged along a bus (the main scanning direction) of the photoreceptor drum  110  (K, C, M, and Y). The writing of the latent image is performed by radiating light emitted from the plurality of light emitting elements on the photoreceptor drum. The developing device  114  (K, C, M, and Y) applies toner, as a developer, onto the latent image to form a toner image, that is, a visible image on the photoreceptor drum. 
   Black, cyan, magenta, and yellow toner images formed by single-color toner image forming stations for the four colors are sequentially primarily transferred onto the intermediate transfer belt  120  so as to be superimposed on the intermediate transfer belt  120 , thereby forming a full-color toner image. Four primary transfer corotrons (transfer devices)  112  (K, C, M, and Y) are arranged inside the intermediate transfer belt  120 . The primary transfer corotrons  112  (K, C, M, and Y) are arranged in the vicinities of the photoreceptor drums  110  (K, C, M, and Y), respectively, and electrostatically attract the toner images from the photoreceptor drums  110  (K, C, M, and Y) to transfer the toner images onto the intermediate transfer belt  120  passing between the photoreceptor drums and the primary transfer corotrons. 
   Finally, sheets  102 , which are image forming targets, are fed one by one from a paper feed cassette  101  to a nip between a secondary transfer roller  126  and the intermediate transfer belt  120  coming into contact with the driving roller  121  by a pick-up roller  103 . The full-color toner image on the intermediate transfer belt  120  are collectively secondary-transferred onto one surface of the sheet  102  by the secondary transfer roller  126  and are then fixed on the sheet  102  by a pair of fixing rollers  127  serving as a fixing unit. Then, the sheet  102  is discharged onto a paper discharge cassette formed on the upper side of the apparatus by a pair of paper discharge rollers  128 . 
     FIG. 66  is a longitudinal sectional view illustrating another example of the overall structure of the image forming apparatus according to any one of the above-described embodiments. This image forming apparatus is a rotary-development-type full color image forming apparatus using a belt intermediate transfer method. In the image forming apparatus shown in  FIG. 66 , a corona charger  168 , a rotary developing unit  161 , a head  167 , and an intermediate transfer belt  169  are provided around a photoreceptor drum  165 . 
   The corona charger  168  uniformly charges the outer peripheral surface of the photoreceptor drum  165 . The head  167  writes a latent image on the charged outer peripheral surface of the photoreceptor drum  165 . The photoreceptor drum  165  corresponds to any one of the photoreceptor drums (image carriers) of the image forming apparatuses according to the above-described embodiments. The head  167  corresponds to any one of the heads of the image forming apparatuses according to the above-described embodiments, and is an organic EL array exposure head in which organic EL elements, each including a light emitting layer formed of an organic EL material, are arranged as light emitting elements. The head  167  is provided such that a plurality of light emitting elements is arranged along a bus (the main scanning direction) of the photoreceptor drum  165 . The writing of the latent image is performed by radiating light emitted from the plurality of light emitting elements on the photoreceptor drum  165 . 
   The developing unit  161  is a drum including four developing devices  163 Y,  163 C,  163 M, and  163 K arranged at right angles to each other, and can be rotated on a shaft  161   a  in the counterclockwise direction. The developing devices  163 Y,  163 C,  163 M, and  163 K respectively supply yellow, cyan, magenta, and black toners to the photoreceptor drum  165  to attach the toners, as developing agents, onto the latent image, thereby forming a toner image, that is, a visible image on the photoreceptor drum  165 . 
   The endless intermediate transfer belt  169  is wound around a driving roller  170   a , a driven roller  170   b , a primary transfer roller  166 , and a tension roller, and circulates around these rollers in the direction of arrow shown in  FIG. 66 . The primary transfer roller  166  electrostatically attracts the toner image from the photoreceptor drum  165  to transfer the toner image onto the intermediate transfer belt  169  passing between the photoreceptor drum and the primary transfer roller  166 . 
   More specifically, at the first rotation of the photoreceptor drum  165 , a latent image for a yellow (Y) image is written by the head  167 , and a toner image having the same color is formed by the developing device  163 Y and is then transferred onto the intermediate transfer belt  169 . At the next rotation thereof, a latent image for a cyan (C) image is written by the head  167 , and a toner image having the same color is formed by the developing device  163 C and is then transferred onto the intermediate transfer belt  169  so as to overlap the yellow toner image. When the photoreceptor drum  165  makes four rotations in this way, yellow, cyan, magenta, and black toner images sequentially overlap each other on the intermediate transfer belt  169 , thereby forming a full-color toner image on the intermediate transfer belt  169 . Finally, when images are formed on both surfaces of a sheet, which is an image forming target, a toner image having a color common to the front and rear surfaces is transferred onto the intermediate transfer belt  169 , and then a toner image having the next color common to the front and rear surfaces is transferred thereon, thereby forming a full-color toner image on the intermediate transfer belt  169 . 
   A sheet transfer path  174  through which sheets pass is provided in the image forming apparatus. The sheets are fed one by one from the paper feed cassette  178  by the pick-up roller  179  and are then transferred along the sheet transfer path  174  by a transfer roller. Then, the sheets pass through a nip between a secondary transfer roller  171  and the intermediate transfer belt  169  coming into contact with the driving roller  170   a . The secondary transfer roller  171  collectively and electrostatically attracts the full-color toner image from the intermediate transfer belt  169  to transfer the toner image onto one surface of the sheet. The secondary transfer roller  171  approaches or is separated from the intermediate transfer belt  169  by a clutch (not shown). When a full-color toner image is transferred onto a sheet, the secondary transfer roller  171  abuts on the intermediate transfer belt  169 . On the other hand, when the toner images are superposed on the intermediate transfer belt  169 , the secondary transfer roller  171  is separated therefrom. 
   In this way, the sheet having an image thereon is transferred to a fixing device  172  and passes between a heating roller  172   a  and a pressing roller  172   b  of the fixing device  172 , thereby fixing a toner image on the sheet. The sheet having the fixed toner image is transferred to the pair of paper discharge rollers  176  to be carried in the direction of arrow F. In a case in which printing is performed on both sides of a sheet, after most of the sheet passes through the paper discharge rollers  176 , the pair of paper discharge rollers  176  is reversely rotated to transfer the sheet in a double-sided printing transfer path  175  as represented by an arrow G. Subsequently, a toner image is transferred onto the other surface of the sheet by the secondary transfer roller  171 , and is then fixed by the fixing device  172 . Then, the sheet is discharged to the outside by the pair of paper discharge rollers  176 . 
   In the image forming apparatuses shown in  FIGS. 65 and 66 , the organic EL elements are used as writing units (exposure units). Therefore, even when a laser scanning optical system is used, it is possible to reduce the size of an image forming apparatus. In addition, the head according to any one of the above-described embodiments can be applied to electrophotographic image forming apparatuses having structures other than the above-described structures. For example, these heads according to the above-described embodiments can be applied to an image forming apparatus which directly transfers a toner image on a photoreceptor drum without using the intermediate transfer belt, an image forming apparatus capable of forming monochrome images, and an image forming apparatus including a photoreceptor belt instead of the photoreceptor drum.