Exposure head and image forming apparatus using the same

An exposure head is provided in which light beams from light emitting parts 63 pass through a transparent substrate 62 and are projected on an image carrier. The transparent substrate 62 has plain faces substantially parallel to each other, one of the faces being a face on which the light emitting parts 63 are formed and the other being a face from which light beams are projected. The transparent substrate 62 is provided, at position(s) other than the face on which the light emitting parts 63 are formed and than the face from which light beams are projected, with light quantity detecting means 100 for detecting the quantity of light emitted from said light emitting parts 63. In the exposure head in which the light emitting parts of organic EL light emitting elements are aligned into array configuration on the transparent substrate, the light quantity detecting means is disposed on an end face of the transparent substrate and light beams totally reflected at the projection-side face of the transparent substrate are introduced to the light quantity detecting means, thereby effectively detecting the quantity of light of the light emitting elements and thus providing improved accuracy of detecting the quantity of light.

BACKGROUND OF THE INVENTION

The present invention relates to an exposure head, and an image forming apparatus using the same and, for example, to an exposure head in which organic EL light emitting elements as light emitting parts are aligned into an array configuration on a transparent substrate and an image forming apparatus which is miniaturized by employing the exposure head.

In conventional image forming apparatus, such as copying machines, printers, and facsimile machines, utilizing electrophotographic technology, it is common practice to employ a laser scanning optical system as light writing means (exposure means). Under such circumstances, an exposure means has been proposed in Japanese Patent Unexamined Publication No. H11-138899, which employs a single chip on which organic EL light emitting elements are integrated, thereby eliminating variation in light emitting characteristics and reducing the cost. In Japanese Patent Unexamined Publication No. 2000-238333, an exposure head has been proposed in which organic EL light emitting elements and light quantity sensors for measuring the quantity of emitted light are aligned into an array configuration on a same substrate, thereby preventing unevenness of density due to reduction in the quantity of emitted light.

As shown and described in the aforementioned prior publications, light beam emitted from a light emitting part the organic EL light emitting element is incident on a transparent substrate so that projected light is projected from a surface opposite to the surface on which the light emitting part is disposed. As shown inFIG. 16, when light beams are projected from the light emitting part63through the transparent substrate62, light beams from the light emitting part63fall into light beams “a” projected from the surface102and light beams reflected at the surface102. The light beams reflected at the projection side surface102further fall into light beams “b”, which are incident on the projection side surface102at an angle greater than a critical angle “θC” and are thus totally reflected, and light beams “c”, which are reflected at an angle less than or equal to the critical angle “θC”. The reflectance of totally internally reflected light is 100%. Since the reflectance of light reflected at an angle less than the critical angle “θC” is 10% or less, the intensity of light beam “c” is significantly smaller than the intensity of the light beam “b”. After reflected on the projection side surface102, light beams “b” are incident on the opposite surface101on which the light emitting part63is disposed. If the opposite surface101and the projection side surface102are parallel to each other, the light beams are also incident on the opposite surface101at an angle greater than the critical angle “θC” and are thus totally internally reflected. In this manner, most of the light beams “b” are finally projected through the end faces of the transparent substrate62after being repeatedly totally reflected between the both surfaces101and102of the transparent substrate62.

In case of Japanese Patent Unexamined Publication No. 2000-238333 as a prior art, since the light quantity sensors for measuring the quantity of light emitted from light emitting parts are disposed on the same surface on which the light emitting parts are disposed, detectable light beams are only light beams “c”. Therefore, since the detected quantity of light may be poor, highly accurate detection is impossible. Accordingly, there is a problem that it is impossible to prevent unevenness of density with a high degree of accuracy.

SUMMERY OF THE INVENTION

The present invention has been made in consideration of these problems of the prior arts. Therefore, it is an object of the present invention to provide an exposure head, having light emitting parts such as organic EL light emitting elements which are aligned into an array configuration on a transparent substrate, in which light quantity detecting means are disposed on end faces of the transparent substrate such that light beams totally reflected at the projection side surface are introduced to the light quantity detecting means, thereby effectively detecting the quantity of light of the light emitting elements and thus improving the accuracy of detecting the quantity of light.

An exposure head of the first aspect of the present invention achieving the aforementioned object is an exposure head which has light emitting parts formed into an array configuration on a transparent substrate and projects modulated light beams from said light emitting parts to an image carrier so as to form a predetermined pattern on said image carrier, the light beams from said light emitting parts being projected on said image carrier side through said transparent substrate, wherein said transparent substrate has plane faces substantially parallel to each other, one of the faces being a face on which the light emitting parts are formed and the other being a face from which light beams are projected, and said transparent substrate is provided, at position(s) other than the face on which the light emitting parts are formed and than the face from which light beams are projected, with light quantity detecting means for detecting the quantity of light emitted from said light emitting parts.

In this case, it is advantageous that the light emitting parts are light emitting parts of organic EL light emitting elements.

The light quantity detecting means may be disposed on an end face, in the sub-scanning direction, of the transparent substrate.

Alternatively, the light quantity detecting means may be disposed on an end face, in the main scanning direction, of the transparent substrate.

In this case, light quantity detecting means are preferably disposed on both end faces, in the main scanning direction, of the transparent substrate.

Further, the light quantity detecting means may be disposed at a plurality of positions on the end faces of the transparent substrate.

Furthermore, it is preferable that end faces, on which no light quantity detecting means is disposed, of the transparent substrate have light reflectivity.

It is also preferable that the quantity of light emitted from each the light emitting part is corrected on the basis of a light quantity detected signal detected by the single light quantity detecting means or on the basis of a light quantity detected signal which is a total of light quantity detected signals detected by a plurality of the light quantity detecting means.

Moreover, it is preferable to provide a storage means of storing correction coefficients for correcting the quantities of light emitted from the light emitting parts.

The present invention includes a color image forming apparatus of a tandem type comprising at least two image forming stations each having a charging means, an exposure head as mentioned above, and a toner developing means, and a transfer means which are arranged around the image carrier, and forming a color image bypassing a transfer medium through the respective stations.

In case of comprising a fixing means of fusing and fixing a toner image transferred from the transfer medium to a recording medium, it is advantageous that the exposure head of the first aspect of the present invention is employed.

A storage means for storing correction coefficients for correcting the quantities of light emitted from the light emitting parts of the exposure head of the first aspect may be disposed on the image forming apparatus body side, not the exposure head side.

In the exposure head of the first aspect of the present invention, the transparent substrate has plane faces substantially parallel to each other, one of the faces being a face on which the light emitting parts are formed and the other being a face from which light beams are projected, and the transparent substrate is provided, at position(s) other than the face on which the light emitting parts are formed and than the face from which light beams are projected, with light quantity detecting means for detecting the quantity of light emitted from the light emitting parts. Therefore, it is possible to detect, at the position of the light quantity detecting means, light beams introduced by total internal reflection within the transparent substrate, thereby increasing the quantity of detected light and enabling the high-precision measurement of light quantity. As a result of this, even when there is variation in light emitting characteristic among the respective light emitting parts and/or even when the light emitting part(s) is deteriorated, uniform distribution of light quantity can be obtained by control. In addition, this structure can reduce the number of light quantity detecting means which are conventionally disposed corresponding to the respective light emitting parts, thereby simplifying the structure of the exposure head and reducing the cost.

An exposure head of the second aspect of the present invention achieving the aforementioned object is an exposure head which has light emitting parts formed into an array configuration on a transparent substrate and projects modulated light beams from said light emitting parts to an image carrier so as to form a predetermined pattern on said image carrier, the light beams from said light emitting parts being projected on said image carrier side through said transparent substrate, wherein said transparent substrate has plane faces substantially parallel to each other, one of the faces being a face on which the light emitting parts are formed and the other being a face from which light beams are projected, and said transparent substrate is provided, on the face on which the light emitting parts are formed, with light quantity detecting means for detecting the quantity of light emitted from said light emitting parts,

said light quantity detecting means is positioned to satisfy a relation of the following equation:
L≧2t·tan θc(1)
wherein the thickness of said transparent substrate is “t”, the critical angle of said transparent substrate is “θc”, and the distance between the center of the light emitting part nearest to said light quantity detecting means and the center of said light quantity detecting means is “L”.

An exposure head of the third aspect of the present invention is an exposure head which has light emitting parts formed into an array configuration on a transparent substrate and projects modulated light beams from said light emitting parts to an image carrier so as to form a predetermined pattern on said image carrier, the light beams from said light emitting parts being projected on said image carrier side through said transparent substrate, wherein said transparent substrate has plane faces substantially parallel to each other, one of the faces being a face on which the light emitting parts are formed and the other being a face from which light beams are projected, and said transparent substrate is provided, on the face from which light beams are projected, with light quantity detecting means for detecting the quantity of light emitted from said light emitting parts,

said light quantity detecting means is positioned to satisfy a relation of the following equation:
L≧t·tan θc(2)
wherein the thickness of said transparent substrate is “t” the critical angle of said transparent substrate is “θc”, and the distance between the center of the light emitting part nearest to said light quantity detecting means and the center of said light quantity detecting means is “L”.

In these cases, it is advantageous that the light emitting parts are light emitting parts of organic EL light emitting elements.

The light quantity detecting means may be disposed at a plurality of positions on the face on which the light emitting parts are formed or the face from which light beams are projected of the transparent substrate.

It is preferable that the quantity of light emitted from each light emitting part is corrected on the basis of a light quantity detected signal detected by the single light quantity detecting means or on the basis of a light quantity detected signal which is a total of light quantity detected signals detected by a plurality of the light quantity detecting means.

Moreover, it is preferable to provide a storage means of storing correction coefficients for correcting the quantities of light emitted from the light emitting parts.

The present invention includes a color image forming apparatus of a tandem type comprising at least two image forming stations each having a charging means, an exposure head as mentioned above, and a toner developing means, and a transfer means which are arranged around the image carrier, and forming a color image bypassing a transfer medium through the respective stations.

In case of comprising a fixing means of fusing and fixing a toner image transferred from the transfer medium to a recording medium, it is advantageous that the exposure head of the second aspect or the third aspect of the present invention is employed.

A storage means for storing correction coefficients for correcting the quantities of light emitted from the light emitting parts of the exposure head of the second aspect or the third aspect may be disposed on the image forming apparatus body side, not the exposure head side.

In the exposure head of the second aspect or the third aspect of the present invention, the transparent substrate has plane faces substantially parallel to each other, one of the faces being a face on which the light emitting parts are formed and the other being a face from which light beams are projected, and the transparent substrate is provided, on the face on which the light emitting parts are formed, with light quantity detecting means for detecting the quantity of light emitted from the light emitting parts so that the light quantity detecting means is positioned to satisfy the relation of the equation (1), or the transparent substrate is provided, on the face from which light beams are projected, with light quantity detecting means for detecting the quantity of light emitted from the light emitting parts so that the light quantity detecting means is positioned to satisfy the relation of the equation (2). Therefore, it is possible to detect, at the position of the light quantity detecting means, light beams to be introduced by total internal reflection within the transparent substrate, thereby increasing the quantity of detected light and enabling the high-precision measurement of light quantity. As a result, even when there is a variation in light emitting characteristics among the light emitting parts and/or even when some light emitting parts are deteriorated, uniform distribution of light quantity can be obtained by control. In addition, this structure can reduce the number of light quantity detecting means which are conventionally disposed corresponding to the respective light emitting parts, thereby simplifying the structure of the exposure head and reducing the cost.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of an image forming apparatus and an exposure head to be used in the apparatus according to the present invention will be described with reference to the attached drawings.

FIG. 1is a schematic sectional view showing the entire structure of an embodiment of an image forming apparatus employing an exposure head (image writing means) of the present invention. This embodiment is of a type employing an intermediate transfer belt as a transfer belt.

InFIG. 1, the image forming apparatus1of this embodiment comprises a housing body2, a first door member3which is disposed on the front of the housing body2such that the first door member is openable and closable, and a second door member (also functioning as an outfeed tray)4which is disposed on the top of the housing body2such that the second door member is openable and closable. The first door member3is provided with a lid3′ which is disposed such that the lid3′ is openable and closable relative to the front of the housing body2. The lid3′ can be opened and closed in conjunction with or independently from the first door member3.

Disposed in the housing body2are an electrical component box5in which substrates for power source circuits and substrates for control circuits are housed, an image forming unit6, a blower fan7, a transfer belt unit9, and a paper feeding unit10. Disposed in the first door member3are a secondary transfer unit11, a fixing unit12, and a recording medium carrying means13. Expendable supplies in the image forming unit6and the paper feeding unit10are detachable relative to the body. In this case, as the transfer belt unit9is detached together with the expendable supplies, the maintenance and replacement are allowed.

The first door member3is attached to the lower front portion of the housing body2via pivotal shafts3bdisposed on both sides of the housing body2so that the first door member3is openable and closable about the pivotal shafts3b.

In this embodiment, as will be described later, the respective units can be attached to and detached from the apparatus only by access from the front of the apparatus. This allows the apparatus to be placed in a narrow space.

InFIG. 1, the transfer belt unit9comprises a driving roller14which is disposed in a lower portion of the housing body2and is driven by a driving means (not shown) to rotate, a driven roller15which is disposed diagonally above the driving roller14, an intermediate transfer belt16which is laid around the two rollers14,15with some tension and is driven to circulate in a direction indicated by an arrow, and a cleaning means17which can abut on the surface of the intermediate transfer belt16. The driven roller15and the intermediate transfer belt16are arranged obliquely to the upper left of the driving roller14in the drawing. Accordingly, during the operation of the intermediate transfer belt16, a belt face16aof which traveling direction is downward takes a lower side. In this embodiment, the belt face16ais a tension side (side tensioned by the driving roller14) at the time of driving the intermediate transfer belt.

The driving roller14and the driven roller15are rotatably supported by a support frame9awhich has a pivotal portion9bformed at a lower end thereof. The pivotal portion9bis fitted to a pivot shaft2bdisposed in the housing body2, whereby the support frame9ais attached to the housing body2such that it is pivotally movable. In addition, the support frame9ahas a lock lever9cwhich is rotatably disposed at an upper end thereof. The lock lever9ccan latch a latch pin2cdisposed on the housing body2.

The driving roller14also functions as a back-up roller for a secondary transfer roller19composing the secondary transfer unit11. The driven roller15also functions as a back-up roller for the cleaning means17. The cleaning means17is located at the belt face16aside, of which traveling direction is downward.

On the back of the belt surface16a, of which traveling direction is downward, of the intermediate transfer belt16, primary transfer members21composed of leaf spring electrodes are disposed. The primary transfer members21are pressed into contact with the back of the intermediate transfer belt16by their elastic force at locations corresponding to image carriers20of respective image forming stations Y, M, C, and K, described later. A transfer bias is applied to each primary transfer member21.

In proximity to the driving roller14, a test pattern sensor18is attached to the support frame9aof the transfer belt unit9. The test pattern sensor18is a sensor for positioning of toner images of respective colors on the intermediate transfer belt16and for compensating color registration error and densities of images of the respective colors by detecting image density of toner images of the respective colors.

The image forming unit6comprises the image forming stations Y (for yellow), M (for magenta), C (for cyan), and K (for black) for forming multi-color images (in this embodiment, four-color images). Each image forming station Y, M, C, K has an image carrier20composed of a photosensitive drum, a charging means22, image writing means23, and developing means24which are arranged around the image carrier20. Reference numerals for the charging means22, the image writing means23, and the developing means24of the image forming station Y are indicated on the drawings and the indication of the reference numerals for the other image forming stations is omitted because the image forming stations have the same structure. It should be understood that the image forming stations Y, M, C, K may be arranged in any order.

The image forming stations Y, M, C, K are disposed such that the respective image carriers20are in contact with the belt face16a, of which traveling direction is downward, of the intermediate transfer belt16. As a result of this, the image forming stations Y, M, C, K are arranged in an obliquely leftward direction relative to the driving roller14in the drawing. Each image carrier20is driven to rotate in the traveling direction of the intermediate transfer belt16as indicated by arrows.

The charging means22is a conductive brush roller which is connected to a high-voltage source and rotates at a peripheral speed about twice to triple the speed of the image carrier20as a photoreceptor in opposite direction with being in contact with the surface of the image carrier20so as to uniformly charge the surface of the image carrier20. In case of an image forming apparatus of a cleaner-less type just like this embodiment, it is preferable that a bias of the same polarity as the polarity of charged toner is applied to the brush roller during non image forming, whereby residual toner adhering to the brush roller is emitted to the image carrier20, is transferred to the intermediate transfer belt16at the primary transfer portion, and is collected by the cleaning means17of the intermediate transfer belt16.

Since the charging means22enables charging of the surface of the image carrier with extremely small amount of electric current, the charging means never pollute inside and outside of the apparatus with large amount of ozone like in case of using a corona charging method. In addition, since the charging means22softly touch the image carrier20, adhesion of toner remaining after transfer onto a charging roller which easily occurs in case of using a roller charging method hardly occurs, thereby ensuring the stability of the image quality and the reliability of the apparatus.

The image writing means23employs an organic EL array exposure head in which organic EL light emitting elements are aligned in line(s) in the axial direction of the image carrier20, as will be described later. The organic EL array exposure head is more compact than a laser scanning optical system because of its short optical path length so that the organic EL array exposure head can be arranged in proximity to the image carrier20, thereby miniaturizing the entire apparatus. In this embodiment, the image carrier20, the charging means22, and the image writing means23of each image forming station Y, M, C, K are united together into an image carrier unit25such that the image carrier unit25can be attached to and detached from the support frame9atogether with the transfer belt unit9, thereby keeping the positions of the organic EL array exposure heads relative to the image carriers20. When the image carrier unit25is replaced, the organic EL array exposure heads are also replaced together.

Then, details of the developing means24will be described by taking the image forming station K as an example. In this embodiment, since the image forming stations Y, M, C, K are obliquely arranged and the image carriers20are disposed to be in contact with the belt face16a, of which traveling direction is downward, of the intermediate transfer belt16, toner storage containers26are arranged obliquely downward to the lower left of the image carriers20. For this, special structure is employed in the developing means24.

That is, the developing means24each comprises the toner storage container26storing toner (indicating by hatching), a toner storage area27formed in the toner storage container26, a toner agitating member29disposed inside the toner storage area27, a partition30defined in an upper portion of the toner storage area27, a toner supply roller31disposed above the partition30, a blade32attached to the partition30to abut the toner supply roller31, the development roller33arranged to abut both the toner supply roller31and the image carrier20, and a regulating blade34arranged to abut the development roller33.

The image carrier20is rotated in the traveling direction of the intermediate transfer belt16. The development roller33and the supply roller31are rotated in a direction opposite to the rotational direction of the image carrier20as shown by arrows. On the other hand, the agitating member29is rotated in a direction opposite to the rotational direction of the supply roller31. Toner agitated and scooped up by the agitating member29in the toner storage area27is supplied to the toner supply roller31along the upper surface of the partition30. Friction is caused between the toner and the blade32so that mechanical adhesive force and adhesive force by triboelectric charging are created relative to the rough surface of the supply roller31. By these adhesive forces, the toner is supplied to the surface of the development roller33. The toner supplied to the development roller33is regulated into a coating layer having a predetermined thickness by the regulating blade34. The toner layer as a thin layer is carried to the image carrier20so as to develop a latent image on the image carrier20at and near a nip portion which is a contact portion between the development roller33and the image carrier20.

In this embodiment, the development roller33disposed facing the image carrier20, the toner supply roller31, and the contact portion of the regulating blade34relative to the development roller33are not submerged in the toner in the toner storage area27. This arrangement can prevent the contact pressure of the regulating blade34relative to the development roller33from being varied due to the decrease of the stored toner. In addition, since excess toner scraped from the development roller33by the regulating blade34spills onto the toner storage area27, thereby preventing filming of the development roller33.

The contact portion between the development roller33and the regulating blade34is positioned below the contact portion between the supply roller31and the development roller33. There is a passage for returning excess toner, which was supplied to the development roller33by the supply roller31but not transmitted to the development roller33, and excess toner, which was removed from the development roller33by the regulating operation of the regulating blade34, to the toner storage area27at the lower portion of the developing means. The toner returned to the toner storage area27is agitated with toner in the toner storage area27by the agitating member29, and is supplied to a toner inlet near the supply roller31again. Therefore, the excess toner is let down to the lower portion without clogging the friction portion between the supply roller31and the development roller33and the contact portion between the development roller33and the regulating blade34and is then agitated with toner in the toner storage area27, whereby the toner in the developing means deteriorates slowly so that portentous changes in image quality just after the replacement of the developing means is prevented.

The sheet supply unit10comprises a sheet cassette35in which a pile of recording media P are held, and a pick-up roller36for feeding the recording media P from the sheet cassette35one by one. The sheet cassette35and the pick-up roller36compose a paper feeding portion.

Arranged inside the first door member3are a pair of resist rollers37for regulating the feeding of a receiving medium P to the secondary transfer portion at the right time, a secondary transfer unit11as a secondary transfer means abutting on and pressed against the driving roller14and the intermediate transfer belt16, a fixing unit12, the recording medium carrying means13, a pair of outfeed rollers39, and a dual-side printing passage40.

The fixing unit12comprises a fuser roller45which has a built-in heating element such as a halogen heater and which is freely rotatable, a pressure roller46pressing the fuser roller45, a belt tensioning member47which is disposed to freely swing relative to the pressure roller46, and a heat resistant belt49which is lied around the pressure roller45and the belt tensioning member47. A color image secondarily transferred to a recording medium is fixed to the recording medium at the nip portion formed between the fuser roller45and the heat resistant belt49at a predetermined temperature. In this embodiment, the fixing unit12can be arranged in a space formed obliquely upward the intermediate transfer belt16, that is, a space formed on the opposite side of the image forming unit6relative to the intermediate transfer belt16. This arrangement enables the reduction in heat transfer to the electrical component box5, the image forming unit6, and the intermediate transfer belt16, and lessens the frequency of taking the action for correcting color registration error.

The actions of the image forming apparatus as a whole will be summarized as follows:

(1) As a printing command (image forming signal) is inputted into the control circuit(s) in the electric component box5from a host computer (personal computer) (not shown) or the like, the image carriers20and the respective rollers of the developing means24of the respective image forming stations Y, M, C, K, and the intermediate transfer belt16are driven to rotate.

(2) The outer surfaces of the image carriers20are uniformly charged by the charging means22.

(3) In the respective image forming stations Y, M, C, K, the outer surfaces of the image carriers20are exposed to selective light corresponding to image information for respective colors by the image writing means23, thereby forming electrostatic latent images for the respective colors.

(4) The electrostatic latent images formed on the image carriers20are developed by the developing means24to form toner images.

(5) The primary transfer voltage of the polarity opposite to the polarity of the toner is applied to the primary transfer members21of the intermediate transfer belt16, thereby transferring the toner images formed on the image carriers20onto the intermediate transfer belt16one by one at the primary transfer portions. According to the movement of the intermediate transfer belt16, the toner images are superposed on the intermediate transfer belt16.

(6) In synchronization with the movement of the intermediate transfer belt16on which primary images are primarily transferred, a receiving medium P accommodated in the sheet cassette35is fed to the secondary transfer roller19through the pair of resist rollers37.

(7) The primary-transferred image meets with the receiving medium at the secondary transfer portion. A bias of the polarity opposite to the polarity of the primary-transferred image is applied by the secondary transfer roller19which is pressed against the driving roller14for the intermediate transfer belt16by the pressing mechanism, whereby the primary-transferred image is secondarily transferred to the receiving medium fed in the synchronization manner.

(8) Residual toner after the secondary transfer is carried toward the driven roller15and is scraped by the cleaning means17disposed opposite to the roller15so as to refresh the intermediate transfer belt16to allow the above cycle to be repeated.

(9) The receiving medium passes through the fixing means12, whereby the toner image on the receiving medium is fixed. After that, the receiving medium is carried toward a predetermined position (toward the outfeed tray4in case of single-side printing, or toward the dual-side printing passage40in case of dual-side printing).

Now, with reference toFIG. 2andFIG. 3, the replacement of the expendable supplies and the removal of a jammed recording medium will be described.FIG. 2shows a state that the first door member3is pivotally moved downwards about the pivotal shafts3btogether with the lid3′ to expose the fixing unit12and the secondary transfer unit11. In addition, the lock lever9cprovided on the top of the frame9aof the transfer belt unit9is pivotally moved to disengage itself from the latch pin2cand the frame9ais pivotally moved to the right about the pivot shaft2bso as to expose the transfer belt unit9and the image carrier unit25which are supported by the frame9a. The developing means24supported on the housing body2side can be exposed by the aforementioned operation. In this state, as shown inFIG. 3, the image carrier unit25and the transfer belt unit9can be detached from the frame9afor replacement. In addition, the developing means24can be also independently and selectively replaced. Moreover, it is possible to remove recording media jammed within a feeding passage.

Hereinafter, an image carrier unit (image carrier cartridge)25in which the image carriers20, the charging means22, and the image writing means23of the respective image forming stations Y, M, C, and K are united together will be described with reference toFIG. 4throughFIG. 8.FIG. 4is a perspective view of the image carrier unit25as seen from the developing means24side andFIG. 5is a sectional view of the image carrier unit. The image carrier unit25comprises a casing50made of an opaque metallic plate or the like and having openings on a side confronting the intermediate transfer belt16. In the casing50, four image carriers (photosensitive drums)20of the image forming stations Y, M, C, and K are rotatably supported parallel to each other at certain intervals, conductive brush rollers as the charging means22are supported such that each charging means22rotates with being in contact with a predetermined position of each image carrier20, organic EL array exposure heads as the image writing means23are positioned relative to the image carriers20and parallel to the image carriers20on downstream side than the charging means22. Openings51are formed in the wall of the casing50on downstream side than the image writing means23so as to allow the developing rollers33of the developing means24to be in contact with the image carriers20, respectively. Between each opening51and each image writing means23, a shielding portion52of the casing50remains. Between each charging means22and each image writing means23, a shielding portion53of the casing50remains. As will be described later, the shielding portions52,53, particularly the shielding portion52between the opening51and the image writing means23, prevent ultraviolet rays from reaching the light emitting parts made of organic EL material from outside.

FIG. 6is a sectional view of one of the image writing means23. The image writing means23comprises an opaque housing60having a gradient index type rod lens array65which is composed of gradient index type rod lenses65′ (FIG. 8) aligned in zigzag fashion and is disposed on the central portion to face the image carrier20to allow the passage of light inside and outside, an organic EL light emitting element array61mounted in the housing to face the back of the gradient index type rod lens array65, and an opaque cover66for shielding the organic EL light emitting element array61from the back of the housing60. The cover66is pressed against the back of the housing60by a tie spring67so as to light-tightly seal the inside of the housing60.

FIG. 7is a sectional view showing an example of a portion near a light emitting part63of the organic EL light emitting element array61of the image writing means23shown inFIG. 6. The organic EL light emitting element array61comprises a glass substrate62, for example, of 0.5 mm in thickness, and TFTs (thin film transistors)71each of which controls the light emission of each light emitting part63and is made of polysilicon to have a thickness of 50 nm. The TFTs71are provided corresponding to the light emitting parts63, respectively, which are aligned in two lines in zigzag fashion, at positions out of the light emitting parts63. Formed on the glass substrate62except contact holes above the TFTs71is an insulating film72made of SiO2to have a thickness of about 100 nm. Positive electrodes73having a thickness of 150 nm made of ITO are formed on portions corresponding to the light emitting parts63in such a manner that the positive electrode73can be in contact with the TFT71through the contact hole. Then, another insulating film74made of SiO2having a thickness of about 120 nm is formed on portions corresponding to positions other than the light emitting parts63. Formed on the insulating films74are banks75made of polyimide having a thickness of 2 μm and having holes76which are formed corresponding to the light emitting parts63. In each of the holes76of the bank75, starting from the positive electrode73, a hole injection layer77having a thickness of 50 nm and a light emitting layer78having a thickness of 50 nm are formed. Further, a first negative electrode layer79amade of Ca having a thickness of 100 nm and a second negative electrode layer79bmade of Al having a thickness of 200 nm are successively formed to cover the upper surfaces of the light emitting layers78, the inner surfaces of the holes76, and the outer surfaces of the banks75. Furthermore, a glass cover64having a thickness of about 1 mm is provided on the negative electrode layers via inert gas such as nitrogen gas80. In this manner, the light emitting parts63of the organic EL light emitting element array61are formed. Light emission of the light emitting parts63is carried out on the glass substrate62side.

Regarding materials used to form the light emitting layer78and the hole injection layer77, it is possible to use various publicly known materials, for example, those disclosed in Japanese Patent Unexamined Publications No. H10-12377 and No. 2000-323276. Detailed description thereof is omitted herein.

FIG. 8shows an example of a mechanism for positioning the image writing means23relative to the image carrier (photosensitive drum)20attached to the image carrier unit25. The image carrier20is rotatably attached to the casing50of the image carrier unit25by its shaft. On the other hand, the organic EL light emitting element array61is held in the housing60having a long rectangular shape as shown inFIG. 6. Positioning pins69which are disposed on both end portions of the long housing60are fitted in corresponding positioning holes of the casing50. Then, fixing screws are screwed into the screw holes of the casing50through holes68formed in the both end portions of the long housing60, thereby fixing the long housing60. In this manner, the image writing means23are fixed at the predetermined positions, respectively.

As shown inFIG. 2andFIG. 3, when the developing means24are detached from the image carrier unit25to expose the image carrier unit25to outside light for the purpose of replacement of the expendable supplies or process for removing a jammed paper, ultraviolet rays from fluorescent lights and/or sunlight enter into the casing50through the openings51of the image carrier unit25. Since the shielding portions52of the casing50remain between the openings51and the image writing means23, the ultraviolet rays are prevented from being directly incident on the exposing positions and reflected at the image carriers20and thus prevented from reaching the light emitting parts63of the organic EL light emitting array61in the image writing means23via the gradient index type rod lens array65. In addition, ultraviolet rays entering through openings on the side of the casing50confronting the intermediate transfer belt16are shielded by the charging means22and the shielding portions53of the casing50between the charging means22and the image writing means23, thereby also preventing the ultraviolet rays from reaching the light emitting parts63. By applying black paint, capable of absorbing ultraviolet rays, on the inner surfaces of the casing50, the aforementioned action of shielding ultraviolet rays can be ensured.

On the other hand, since the housing60of the image writing means23is opaque and the back of the housing60is covered by the opaque cover66, ultraviolet rays from fluorescent lights and/or sunlight being incident on the back of the organic EL light emitting element arrays61are prevented from reaching the light emitting parts63of the organic EL light emitting element arrays61.

Therefore, even when the image carrier unit25is exposed to ultraviolet rays for the purpose of replacing the expendable supplies or removing a jammed paper, ultraviolet rays are prevented from reaching the light emitting parts63of the organic EL light emitting element arrays61in the image writing means23which are united into the image carrier unit25, thereby preventing the organic EL light emitting elements from being deteriorated due to ultraviolet rays.

In the image writing means23, light quantity detecting means for detecting the quantity of light emitted from the light emitting part63of each organic EL light emitting element will be described.FIG. 9is a sectional view of the glass substrate62of the organic EL light emitting element array61of the image writing means23in the sub-scanning direction. Regarding to the organic EL light emitting element array61, only one light emitting part63is shown in this drawing. The sub-scanning direction means a direction perpendicular to the rotational axis of the image carrier20and the main scanning direction as will be described later means a direction parallel to the rotational axis of the image carrier20.FIG. 10is a perspective view thereof.

As described with regard toFIG. 16, when light beams are projected from the light emitting part63through the transparent substrate62, light beams from the light emitting part63fall into light beams “a” projected from the projection-side surface102of the transparent substrate62and light beams “b” which are incident on the projection side surface102at an angle greater than a critical angle “θC” and are thus totally reflected. The middle of the light beams “a” is used for exposing the image carrier20through the gradient index type rod lens array65(FIG. 6,FIG. 8). On the other hand, light beams “b” are repeatedly totally reflected between the both surfaces101and102of the transparent substrate62and become no use. Here, a light quantity sensor100composed of photo acceptance unit such as photodiode is attached to a predetermined location (substantially the center inFIG. 10) of the end face103, in the sub-scanning direction, of the glass substrate62of the organic EL light emitting element array61so that light beams “b” introduced within the transparent substrate62from the light emitting part63by the total internal reflection are incident on the light quantity sensor100, thereby detecting the relative quantity of light emitted from the light emitting part63.FIG. 11is a sectional view similar toFIG. 6, but showing the image writing means23in this case.FIG. 12is a sectional view of a portion from the vicinity of a light emitting part63of the organic EL light emitting element array61to the light quantity sensor100of the image writing means23shown inFIG. 11.

On another end face104, in the sub-scanning direction, opposite to the face on which the light quantity sensor100is disposed, a light reflective layer91made of a metal or the like is preferably disposed. The light reflective layer91reflects light beams introduced to the end face104in such a manner as to return the light beams toward the end face103, on which the light quantity sensor100is disposed, after repeatedly reflected, thereby increasing the quantity of light being incident on the light quantity sensor100.

Now, description will now be made as regard to an example of the method for preventing the unevenness of density of each light emitting part63by stably controlling the light quantity of each light emitting part63of the organic EL light emitting element array61based on the light quantity data detected by the light quantity sensor100.

First, before shipment of the image carrier unit25, the quantity of light reaching the position of the image carrier20from the organic EL light emitting element array61of the image writing means23through the gradient index type rod lens array65is measured for every light emitting part63. For this, the image writing means23is fixed to a test jig. Disposed on the test jig is a light quantity detecting device for detecting, at an image position corresponding to the image carrier20, the quantity of light emitted from the light emitting part63of each light emitting element of the organic EL light emitting element array61. The light quantity detecting device may comprise a single detector which sequentially detects the quantity of light emitted from the light emitting parts63, respectively, while moving along the organic EL light emitting element array61and may comprise detectors, of which number is the same as the light emitting parts63, disposed corresponding to the light emitting parts63. Then, the respective light emitting parts63are turned on to emit light one by one to obtain a value “Phn” (“n” means the n-th light emitting part63) detected by the light quantity sensor100of the image writing means23and a value “Pgn” detected by the light quantity detecting device fixed to the test jig. Then, the correction coefficient Pgn/Phn of each light emitting part63is calculated.

The aforementioned measurement of the light quantity and the calculation of the correction coefficient are conducted for every light emitting part63of light emitting elements, thus obtaining the respective correction coefficients Pgn/Phn of all light emitting elements.

The respective correction coefficients Pgn/Phn obtained in this manner are stored in a memory124arranged in the image writing means23as shown in a block diagram ofFIG. 13. The correction of the light emitting quantity of each light emitting part63is conducted in the image forming apparatus1as shown inFIG. 1by using the respective correction coefficients Pgn/Phn stored in the memory124. Hereinafter, one example of the correction method will be described.

The respective light emitting parts63of the organic EL light emitting element array61of each image writing means23are turned on to emit light via a control circuit122and a driving circuit123based on the initial value data stored in the memory124. The light quantity at this time is measured by the light quantity sensor100. The light quantity at the image position of each light emitting part is calculated by multiplying the measured light quantity by the correction coefficient Pgn/Phn.

A difference is obtained by comparing the calculated light quantity to the desired light quantity given by a controller121of the electrical component box5. Based on the difference, for example, the volume of current to be supplied to the light emitting element of the organic EL light emitting element array61is controlled so that the light emitting quantity of each light emitting part63is adjusted to be desired light quantity. This adjusting operation is repeatedly conducted for every light emitting element, whereby the light quantities of all of the light emitting elements are adjusted to the desired value.

The aforementioned light quantity correction operation can be conducted according to the command from the controller121at any point in time just after the start-up of the image forming apparatus1, just before printing operation, a period until the next paper.

Instead of obtaining and storing the correction coefficient Pgn/Phn into the memory124before the shipment of the image carrier unit25, each light emitting part63of each light emitting element may be turned on in such a manner that the light quantity emitted from the light emitting part63becomes the desired light quantity at the image position corresponding to the image carrier20, that is, in such a manner that the “Pgn” becomes a predetermined value, and the value “Phn” detected by the light quantity sensor100at this time may be stored. In this case, each light emitting part63is turned on according to the initial value data stored in the memory124so as to obtain the value measured by the light quantity sensor100. By controlling, for example, the volume of current to be supplied to the light emitting element of the organic EL light emitting element array61to eliminate a difference between the measured value and the “Phn” stored in the memory124, the quantity of light emitted by each light emitting part63is adjusted to be the desired quantity.

Though the memory124in which the correction coefficient Pgn/Phn of each light emitting part63or the value Phn detected by the light quantity sensor100when the light emitting parts emit light of desired quantity are stored is arranged in the image writing means23as the aforementioned embodiments, the memory124may be connected to the controller121of the electrical component box5of the apparatus body and arranged on the apparatus body side.

As the correction coefficient Pgn/Phn of each light emitting part63or the value Phn when the light emitting parts emit light of desired quantity are stored in the image writing means23or the apparatus body, even when there is variation in light emitting characteristic among the respective light emitting parts63and/or even when the organic EL light emitting element is deteriorated with ultraviolet rays of outside light or with heat such as from a heat source of the fixing unit12, uniform distribution of light quantity can be obtained by control.

In the aforementioned embodiments, the light quantity sensor100is composed of a single light quantity sensor100disposed on the end face103, in the sub-scanning direction, of the glass substrate62of the organic EL light emitting element array61. However, the light quantity sensor100may be composed a plurality of sensors disposed at difference locations. In the embodiment ofFIG. 14, the light quantity sensor100is composed of four light quantity sensors1001, through10044of which two are disposed on each of the both end faces103and104, in the sub-scanning direction.

As shown inFIG. 10andFIG. 14, since the case that the light quantity sensor100or the light quantity sensors1001through1004are located on the end faces103,104, in the sub-scanning direction, of the glass substrate62allows the light quantity sensor100or the light quantity sensors1001through1004to be located nearer to the light emitting parts63, there is a merit that the detected light quantity can be increased. It should be noted that, in the case using the plural light quantity sensors1001through1004, various changes may be made. For example, a total of the detected values of the respective light quantity sensors1001through1004may be used the aforementioned detected value “Phn” or a detected value of the nearest sensor of the light quantity sensors1001through1004may be used as the value “Phn”.

The light quantity sensor100may be disposed on end face(s), in the main scanning direction, of the glass substrate62of the organic EL light emitting element array61. InFIG. 15, two light quantity sensors1001,1002are disposed on the both end faces105and106, in the main scanning direction, of the glass substrate62of the organic EL light emitting element array61. According to this structure and using a total of the detected values of the light quantity sensors1001and1002as the value “Phn”, the detection with substantially uniform detected light quantity can be achieved relative to all of the light emitting parts63. Also in this case, various changes may be made, for example, a detected value of the nearest sensor of the light quantity sensors1001and1002may be used as the value “Phn”. Of course, only one of the light quantity sensors1001,1002may be disposed on one of the end faces105,106, in the main scanning direction.

In case that the light quantity sensors1001and1002are located on the end faces105,106, in the main scanning direction, of the glass substrate62, the size in the sub-scanning direction of the image writing means23having a line shape can be reduced, thereby achieving a compact line head (exposure head)23.

As described above, according to the exposure head of the first embodiment of the present invention and the image forming apparatus employing the same, a transparent substrate has plain faces substantially parallel to each other. One of the faces is a face on which the light emitting parts are formed and the other is a face from which light beams are projected. The transparent substrate is provided, at position(s) other than the face on which the light emitting parts are formed and than the face from which light beams are projected, with light quantity detecting means for detecting the quantity of light emitted from the light emitting parts. Therefore, it is possible to detect, at the position of the light quantity detecting means, light beams introduced by total internal reflection within the transparent substrate, thereby increasing the quantity of detected light and enabling the high-precision measurement of light quantity. As a result, even when there is a variation in light emitting characteristics among the light emitting parts and/or even when some light emitting parts are deteriorated, uniform distribution of light quantity can be obtained by control. In addition, this structure can reduce the number of light quantity detecting means which are conventionally disposed corresponding to the respective light emitting parts, thereby simplifying the structure of the exposure head and reducing the cost.

FIG. 17throughFIG. 24show exposure heads of the second and third embodiments of the present invention. A difference from the exposure heads shown inFIG. 9throughFIG. 12,FIG. 14, andFIG. 15is the mounting position of the light quantity sensor100a.FIG. 17is a sectional view of a glass substrate of an organic EL light emitting element array in the sub-scanning direction,FIG. 18is a plan view of the glass substrate shown inFIG. 17. In the exposure head of the second embodiment, similar to the embodiment shown inFIG. 10, a light quantity sensor100acomposed of photo acceptance unit such as photodiode is attached to a predetermined location in the sub-scanning direction on the face101, on which the light emitting parts63are disposed, of the glass substrate62of the organic EL light emitting element array61so that light beams “b” introduced within the transparent substrate from the light emitting part63by the total internal reflection are incident on the light quantity sensor100a, thereby detecting the relative quantity of light emitted from the light emitting part63.

As for the exposure head of the second embodiment, a relation of the following equation is preferably satisfied:
L≧2t·tan θc(1)
wherein the thickness of the glass substrate62is “t”, the critical angle of the glass substrate62is “θc”, and the distance between the center of the light emitting part63nearest to the light quantity sensor100aand the center of the light quantity sensor100ais “L”. As the above relation is satisfied, light beams “b” from all of the light emitting parts63of the organic EL light emitting element array61can be detected because the light beams reach the light quantity sensor100aafter totally reflected at the projection-side face102once or more. For example, when the thickness “t” of the glass substrate62is 0.5 mm and the refractive index “n” of the glass substrate62is 1.52, the critical angle “θc” of the glass substrate62is 41.4°. According to the above equation (1), the light quantity sensor100ais disposed at a position L≧0.87 mm.FIG. 19is a sectional view similar toFIG. 6, but showing the image writing means23in the aforementioned case, andFIG. 20is a sectional view of a portion from the vicinity of a light emitting part63of the organic EL light emitting element array61to the light quantity sensor100aof the image writing means23shown inFIG. 19.

According to the light quantity data detected by the light quantity sensor100a, the quantity of light emitted by each light emitting part63of the organic EL light emitting element array61is stably controlled to a certain quantity, thereby preventing the unevenness of density of each light emitting part63. Also in the exposure head of the second embodiment, the aforementioned respective correction coefficients Pgn/Phn are obtained and stored in the memory124. The respective light emitting parts63of the organic EL light emitting element array61of each image writing means23are turned on to emit light via a control circuit122and a driving circuit123based on the initial value data stored in the memory124. The light quantity at this time is measured by the light quantity sensor100a. The light quantity at the image position of each light emitting part63is calculated by multiplying the measured light quantity by the correction coefficient Pgn/Phn.

A difference is obtained by comparing the calculated light quantity to the desired light quantity given by a controller121disposed in the electrical component box5. Based on the difference, the volume of current to be supplied to the light emitting element of the organic EL light emitting element array61is controlled so that the light emitting quantity of each light emitting part63is adjusted to be desired light quantity. This adjusting operation is repeatedly conducted for every light emitting element, whereby the light quantities of all of the light emitting elements are adjusted to the desired value.

The aforementioned light quantity correction operation can be conducted according to the command from the controller121at any point in time just after the start-up of the image forming apparatus1, just before printing operation, a period until the next paper.

Instead of obtaining and storing the correction coefficient Pgn/Phn into the memory124before the shipment of the image carrier unit25, each light emitting part63of each light emitting element may be turned on in such a manner that the light quantity emitted from the light emitting part63becomes the desired light quantity at the image position corresponding to the image carrier20, that is, in such a manner that the “Pgn” becomes a predetermined value, and the value “Phn” detected by the light quantity sensor100aat this time may be stored. In this case, each light emitting part63is turned on according to the initial value data stored in the memory124so as to obtain the value measured by the light quantity sensor100a. By comparing the measured value to the “Phn” stored in the memory124to obtain a difference therebetween and controlling the volume of current to be supplied to the light emitting element of the organic EL light emitting element array61to eliminate the difference, the quantity of light emitted by each light emitting part63is adjusted to be the desired quantity.

Though the memory124in which the correction coefficient Pgn/Phn of each light emitting part63or the value Phn detected by the light quantity sensor100awhen the light emitting parts emit light of desired quantity are stored is arranged in the image writing means23as the aforementioned embodiments, the memory124may be connected to the controller121of the electrical component box5of the apparatus body and arranged on the apparatus body side.

As the correction coefficient Pgn/Phn of each light emitting part63or the value Phn when the light emitting parts emit light of desired quantity are stored in the image writing means23or the apparatus body, even when there is variation in light emitting characteristic among the respective light emitting parts63and/or even when the organic EL light emitting element is deteriorated with ultraviolet rays of outside light or with heat such as from a heat source of the fixing unit12, uniform distribution of light quantity can be obtained by control.

In the aforementioned embodiments, the light quantity sensor100ais composed of a single light quantity sensor100adisposed at the predetermined position in the sub-scanning direction on the face101, on which the light emitting parts63are disposed, of the glass substrate62of the organic EL light emitting element array61. However, the light quantity sensor100amay be composed a plurality of sensors disposed at difference locations. In the embodiment ofFIG. 21, the light quantity sensor100ais composed of four light quantity sensors10011through10041of which two are disposed on each of both sides, relative to the light emitting parts63in the sub-scanning direction, of the face101on which the light emitting parts63are disposed.

As shown inFIG. 18andFIG. 21, since the case that the light quantity sensor100aor the light quantity sensors10011through10041are located at the predetermined positions in the sub-scanning direction on the face101on which the light emitting parts63are disposed, allows the light quantity sensor100aor the light quantity sensors10011through10041to be located nearer to the light emitting parts63, there is a merit that the detected light quantity can be increased. It should be noted that, in the case using the plural light quantity sensors10011through10041, various changes may be made. For example, a total of the detected values of the respective light quantity sensors10011through10041may be used the aforementioned detected value “Phn” or a detected value of the nearest sensor of the light quantity sensors10011through10041may be used as the value “Phn”.

The light quantity sensor may be disposed on end portions(s) in the main scanning direction on the face101, on which the light emitting parts63are disposed, of the glass substrate62of the organic EL light emitting element array61. InFIG. 22, two light quantity sensors10011,10021are disposed at positions, corresponding to the both end portions relative to the light emitting parts63in the main scanning direction, on the face101with the light emitting parts63of the glass substrate62of the organic EL light emitting element array61. According to this structure and using a total of the detected values of the light quantity sensors10011and10021as the value “Phn”, the detection with substantially uniform detected light quantity can be achieved relative to all of the light emitting parts63. Also in this case, various changes may be made, for example, a detected value of the nearest sensor of the light quantity sensors10011and10021may be used as the value “Phn”. Of course, only one of the light quantity sensors10011,10021may be disposed on one of the end portions in the main scanning direction.

In case that the light quantity sensors10011and10021are located at the end portions in the main scanning direction on the face101, on which the light emitting parts63are disposed, of the glass substrate62, the size in the sub-scanning direction of the image writing means23having a line shape can be reduced, thereby achieving a compact line head (exposure head)23.

The light quantity sensor may be disposed on the projection side face102opposite to the face101, on which the light emitting parts63are disposed, of the glass substrate62of the organic EL light emitting element array61to detect light beams “b” totally reflected within the glass substrate62. This case is shown inFIG. 23andFIG. 24which are similar toFIG. 17andFIG. 20, respectively. In the exposure head of this embodiment, a light quantity sensor100acomposed of photo acceptance unit such as photodiode is attached to a predetermined location, in the sub-scanning direction, of the projection-side face102of the glass substrate62of the organic EL light emitting element array61with optical adhesive so as to reduce the interface reflection at the location so that light beams “b” directly from the light emitting part63are incident on the light quantity sensor100a, thereby detecting the relative quantity of light emitted from the light emitting part63.

As for the exposure head of the third embodiment, the following relation is preferably satisfied:
L≧t·tan θc(2)
wherein the thickness of the glass substrate62is “t”, the critical angle of the glass substrate62is “θc”, and the distance between the center of the light emitting part63nearest to the light quantity sensor100aand the center of the light quantity sensor100ais “L”. As the above relation is satisfied, light beams “b” from all of the light emitting parts63of the organic EL light emitting element array61can be detected because the light beams reach the light quantity sensor100adirectly or after repetitions of total internal reflection.

In case employing a plurality of light quantity sensors, some of the light quantity sensors may be disposed on the face, on which the light emitting parts63are disposed, of the glass substrate62of the organic EL light emitting element array61and the rest may be the projection-side face102opposite to the face101, on which the light emitting parts63are disposed, of the glass substrate62. In this case, of course, the position of the light quantity sensor on the surface101, on which the light emitting parts63are disposed, may be defined to satisfy the equation (1) and the position of the light quantity sensor on the projection-side surface102may be defined to satisfy the equation (2).

As apparent from the aforementioned description, according to the exposure head of the second or third embodiment of the present invention and the image forming apparatus employing the same, a transparent substrate has plane faces substantially parallel to each other, one of the faces being a face on which the light emitting parts are formed and the other being a face from which light beams are projected, and is provided, on the face on which the light emitting parts are formed, with light quantity detecting means for detecting the quantity of light emitted from the light emitting parts such that the light quantity detecting means are positioned to satisfy the relation of the equation (1), or alternatively, is provided, on the face from which light beams are projected, with light quantity detecting means for detecting the quantity of light emitted from the light emitting parts such that the light quantity detecting means are positioned to satisfy the relation of the equation (2). Therefore, it is possible to detect, at the position of the light quantity detecting means, light beams introduced by total internal reflection within the transparent substrate, thereby increasing the quantity of detected light and enabling the high-precision measurement of light quantity. As a result, even when there is a variation in light emitting characteristics among the light emitting parts and/or even when some light emitting parts are deteriorated, uniform distribution of light quantity can be obtained by control. In addition, this structure can reduce the number of light quantity detecting means which are conventionally disposed corresponding to the respective light emitting parts, thereby simplifying the structure of the exposure head and reducing the cost.

Though the exposure head of the present invention and the image forming apparatus employing the same have been described with reference to some embodiments disclosed above, the present invention is not limited thereto and various changes may be made therein.