Patent Publication Number: US-2006017800-A1

Title: Exposure system

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      This invention relates to an exposure system, and more particularly to an exposure system for exposing a color photosensitive material by the use of a plurality of kinds of light emitting element arrays emitting light in different wavelength ranges.  
      2. Description of the Related Art  
      As disclosed, for instance, in U.S. Pat. No. 6,731,322 and Japanese Unexamined Patent Publication No. 2001-260416, there has been known a system where a color photosensitive material is exposed to light by the use of an exposure head comprising a plurality of kinds of light emitting element arrays emitting light in different wavelength ranges, e.g., red, green and blue light.  
      Each of the light emitting element arrays generally comprises a plurality of organic EL (electroluminescence) elements which are arranged in one or more rows and emits light in the same wavelength range, and in the exposure head, a plurality of kinds of light emitting element arrays emitting light in different wavelength ranges are generally arranged in a direction substantially normal to the direction in which the light emitting elements are arranged in each of the light emitting element arrays and a lens array which converges light from each of the light emitting element arrays on the color photosensitive material is provided.  
      The exposure system using such an exposure head generally further comprises a sub-scanning means which holds the color photosensitive material in a position where the light from each of the light emitting element arrays is projected, and moves the color photosensitive material and the light emitting element arrays (together with the lens array when a lens array is provided) relatively to each other in the direction in which a plurality of light emitting element arrays are arranged.  
      Especially, in U.S. Pat. No. 6,731,322, there is disclosed a system in which the same place of the color photosensitive material can be exposed to light a multiple times by the use of a light emitting element array comprising a plurality of rows of the light emitting elements arranged side by side in the direction of the above-mentioned relative movement.  
      Further, as a light emitting element forming the light emitting element array in the exposure system of this type, there has been known a multi-layered type organic EL element where a plurality of light emitting structures are superposed one on another to form multiple layers as shown in Japanese Unexamined Patent Publication No. 2003-045676.  
      However, in the exposure system where a color photosensitive material is exposed to light by the use of a plurality of kinds of light emitting element arrays emitting light in different wavelength ranges, e.g., red, green and blue light, there has been a problem that the intensity ratio of light in the respective wavelength ranges fluctuate after a plurality of repeated exposures and color balance is shifted. When the color balance is shifted, a density unevenness extending in the sub-scanning direction can be generated in the exposed image at the worst.  
      Generation of the density unevenness can be prevented by discarding the exposure system immediately when the color balance is shifted. However this approach is disadvantageous in that the service life of the exposure system is governed by the service life of the light emitting element array which deteriorates at the highest speed in the parts of the exposure system.  
      Though problems in the exposure systems using arrays of self-luminous light emitting elements such as an organic EL element has been described, a similar problem can naturally arise in an exposure head using arrays of elements comprising a combination of a dimmer such as a liquid crystal or a PLZT and a light source. In this specification, the element comprising a combination of a dimmer and a light source will also be referred to as a “light emitting element” in view of that it emits the exposure light.  
     SUMMARY OF THE INVENTION  
      In view of the foregoing observations and description, an aspect of the present invention is to provide an exposure system which can prevent the shift in color balance and is long in service life.  
      In accordance with the present invention, there is provided an exposure system comprising a plurality of kinds of light emitting element arrays, each formed of a plurality of light emitting elements which are arranged in one row in one direction, which are arranged substantially normal to said one direction and emits light in different wavelength ranges, and a sub-scanning means which holds a color photosensitive material in a position where the light from each of the light emitting element arrays is projected, and moves the color photosensitive material and the light emitting element arrays relatively to each other in said one direction in which a plurality of light emitting element arrays are arranged, wherein the improvement comprises that  
      at least one of the plurality of kinds of light emitting element arrays is a multi-layered type light emitting element array where a plurality of light emitting structures are superposed one on another.  
      In the exposure system of the present invention, since at least one of the plurality of kinds of light emitting element arrays is a multi-layered type light emitting element array where a plurality of light emitting structures are superposed one on another (the number of light emitting structures is assumed to be N), the light emitting brightness of one light emitting structure may be 1/N as compared with the non-multi-layered type usual light emitting element array to provide a given amount of exposure, whereby the service life of the light emitting element arrays is elongated to substantially N times and the service life of the exposure system is elongated.  
      When there is a difference in time constant of deterioration between the plurality of kinds of the light emitting element arrays due to difference in element structure, it is possible to equalize the light emitting element arrays in time constant of deterioration by changing the number N of layers of the superposed light emitting structures of the light emitting elements in the light emitting element arrays. If so, the intensity ratio of light in the respective wavelength ranges can be held constant even after a plurality of repeated exposures and shift of color balance can be prevented.  
      The same exposure can be obtained even if the light emitting brightness of one light emitting structure is 1/S by increasing the light emitting area of the light emitting element to S times. In accordance with the second exposure system of the present invention, the light emitting area of the light emitting element is nonuniform between at least two kinds of light emitting element arrays. Accordingly, when there is a difference in time constant of deterioration between the plurality of kinds of the light emitting element arrays due to difference in element structure, it is possible to equalize the light emitting element arrays in time constant of deterioration by changing the area S of the light emitting elements in the light emitting element arrays. If so, the intensity ratio of light in the respective wavelength ranges can be held constant even after a plurality of repeated exposures and shift of color balance can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side view of an exposure system in accordance with an embodiment of the present invention,  
       FIG. 2  is a schematic plan view of the exposure head of the exposure system,  
       FIG. 3  is a plan view showing the arrangement of the electrodes in the exposure head,  
       FIG. 4  is a view for illustrating the layer structure of the light emitting element of the exposure system, and  
       FIG. 5  is a plan view showing another example of the arrangement of the electrodes. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
     First Embodiment  
      As shown in  FIG. 1 , an exposure system  5  in accordance with a first embodiment of the present invention has an exposure head  1 . The exposure head  1  comprises a transparent base  10 , a red emitting element array  6 R, a green emitting element array  6 G and blue emitting element arrays  6 B formed of number of organic EL elements  20  formed on the base  10  by deposition, refractive index profile type lens arrays  30  ( 30 R,  30 G and  30 B) which are a unit system for imaging on a color photosensitive material  40  an image generated by the light emitted from the organic EL elements  20 , and a support  50  which supports the base  10  and the refractive index profile type lens arrays  30 .  
      The exposure system  5  further comprises, in addition to the exposure head  1 , a sub-scanning means  51  in the form of, for instance, a pair of nip rollers which conveys the color photosensitive material  40  at a constant speed in a direction of arrow Y.  
      The organic EL elements  20  comprises an organic compound layer  22  and a metal cathode  23  formed in sequence by deposition on the transparent base  10  formed of, for instance, glass. The organic compound layer  22  includes a transparent anode  21  and a light emitting layer and patterned for each pixel. The elements forming the organic EL elements  20  are arranged in a sealing member  25  which may be, for instance, a can of a stainless steel. That is, the base  10  is bonded to the edge of the sealing member  25  by adhesive and the sealing member  25  is filled with dry nitrogen gas. The organic EL elements  20  are sealed in the sealing member  25 .  
      When a predetermined voltage is imparted between the transparent anode  21  and the metal cathode  23 , the light emitting layer included in the organic compound layer  22  emits light, which is taken out through the transparent anode  21  and the transparent base  10 . The organic EL element  20  is excellent in wavelength stability. The arrangement of the organic EL elements  20  will be described in detail later.  
      The transparent anode  21  is preferably not lower than 50% and more preferably not lower 70% in transmittance to visible light in the wavelength range of 400 nm to 700 nm, and may be of known material such as tin oxide, indium·tin oxide (ITO), indium·zinc oxide, and the like. Film of metal such gold, platinum or the like which is large in work function may be employed as the transparent anode. Further, the transparent anode may be of an organic compound such as polyaniline, polythiophene, polypyrrole or a derivative of these compounds. Transparent conductive films shown in “New development of transparent conductive material” supervised by Yutaka Sawada, CMC, 1999, may be applied to the present invention. Further, the transparent anode  21  maybe formed on the base  10  by vacuum deposition, sputtering, ion plating or the like.  
      The organic compound layer  22  may either be of a single layer of the light emitting layer or may be provided with, in addition to the light emitting layer, a hole injecting layer, a hole transfer layer, an electron injecting layer and/or an electron transfer layer, as desired. For example, the organic compound layer  22  and the electrodes may comprise an anode/a hole injecting layer/a hole transfer layer/a light emitting layer/an electron transfer layer/a cathode, an anode/a light emitting layer/an electron transfer layer/a cathode, or an anode/a hole transfer layer/a light emitting layer/an electron transfer layer/a cathode. Further, each of the light emitting layer, the hole transfer layer, the hole injecting layer and the electron injecting layer may be provided in a plurality of layers.  
      The metal cathode  23  is preferably formed of metal material which is small in work function, e.g., alkaline metal such as Li or K, or alkaline earth metal such as Mg or Ca, or alloy or mixture of these metals with Ag or Al. In order for the shelf stability and the electron-injectability at the cathode to be compatible with each other, the electrode formed of material described above maybe coated with metal with is large in work function and high in conductivity, e.g., Ag, Al Au or the like. The cathode  23  may be formed by a known method such as vacuum deposition, sputtering, ion plating or the like as the transparent anode  21 .  
      Arrangement of the organic EL elements  20  will be described in detail, hereinbelow.  FIG. 2  is a view showing the arrangement of the transparent anodes  21  and the metal cathodes  23  in the exposure head  1  and  FIG. 3  is a view showing the arrangement in an enlarged scale. As shown in  FIGS. 2 and 3 , each of the transparent anodes  21  is patterned into a predetermined shape extending substantially in the sub-scanning direction and common to the organic EL elements  21  arranged in this direction. In this particular embodiment, 7800 (=260×30) of the transparent anodes  21  are arranged in the main scanning direction. Each of the metal cathodes  23  linearly extends in the main scanning direction and common to the organic EL elements  21  arranged in this direction. In this particular embodiment, 16 of the transparent anodes  21  are arranged in the sub-scanning direction.  
      The transparent anodes  21  and the metal cathodes  23  form column electrodes and row electrodes and a predetermined voltage is imparted by a drive circuit  80  between one of the transparent anodes  21  selected according to the image signal and one of the metal cathodes  23  which are driven in sequence. When a voltage is imparted between one of the transparent anodes  21  and one of the metal cathodes  23 , the light emitting layer included in the organic compound layer  22  disposed at the intersection of the transparent anode  21  and the metal cathode  23  applied with the voltage emits light and the light is taken out through the transparent base  10 . That is, in this embodiment, one organic EL element  20  is formed at each of the intersections of the transparent anode  21  and the metal cathode  23  and a plurality of organic EL elements  20  are arranged in the main scanning direction at predetermined pitches to form a linear light emitting element array.  
      As can be understood from the description above, a so-called passive matrix drive system is employed in this embodiment. Since the passive matrix drive system is known, it will not be described in detail, here. It is possible to employ an active matrix drive system in which a switching element such as a TFT (Thin Film Transistor) is employed.  
      In this particular embodiment, the color photosensitive material  40  is a negative silver halide color paper having a layer including a first photosensitive material which develops in cyan, a layer including a second photosensitive material which develops in magenta, and a layer including a third photosensitive material which develops in yellow. The exposure head  1  of this embodiment is adapted to exposure of a full color image to the color photosensitive material  40 . The arrangement for this purpose will be described in detail, hereinbelow.  
      The organic EL elements  20  comprises those emitting red light, green light and blue light according to the light emitting layer included in the organic compound layer  22 . In order to separate the organic EL elements according to the color of light emitted from the organic EL elements, those emitting red light, green light and blue light are sometimes referred to as “the organic EL element  20 R”, “the organic EL element  20 G”, and “the organic EL element  20 B”, respectively, hereinbelow. The first photosensitive material of the color photosensitive material  40  senses the red light emitted from the organic EL elements  20 , and develops in cyan, the second photosensitive material senses the green light emitted from the organic EL elements  20 , and develops in magenta, and the third photosensitive material senses the blue light emitted from the organic EL elements  20 , and develops in yellow.  
      In this particular embodiment, the organic EL element  20 R, the organic EL element  20 G, and the organic EL element  20 B are a multi-layered type element where a plurality of the organic compound layer  22  are superposed one on another. The arrangement of the organic EL element  20 B will be described with reference to  FIG. 4 , hereinbelow, as an example of the arrangement of the multi-layered type element. The element comprises, as described above, the transparent anode  21 , the organic compound layer  22  and the metal cathode  23  formed in sequence on the transparent base  10 , and the organic compound layer  22  comprises a pair of light emitting structures laminated together with an electric charge generating layer  22   d  intervening therebetween. Each of the light emitting structures comprises a hole transfer layer  22   a,  a light emitting layer  22   b  and an electron transfer layer  22   c.  With this arrangement, in this organic EL element  20 B, light is taken out from both the light emitting layers  22   b  when an electric current is flowed between the transparent anode  21  and the metal cathode  23  from a DC power source  24 .  
      In this particular embodiment, though the organic EL element  20 B is a two-layered element, the other organic EL elements  20 R and  20 G are of different layers and are six-layered elements.  
      The organic EL elements  20 R are disposed in R area in  FIG. 2  and 7800 organic EL elements  20 R are arranged in the main scanning direction to form one linear red light emitting element array and  100  linear red light emitting element arrays are arranged in the sub-scanning direction to form the red light emitting element array  6 R. However, in  FIG. 1 , the number of the linear light emitting element arrays forming the red light emitting element array  6 R are shown for the purpose of simplicity.  
      The organic EL elements  20 G are disposed in G area in  FIG. 2  and 7800 organic EL elements  20 G are arranged in the main scanning direction to form one linear green light emitting element array and 5 linear green light emitting element arrays are arranged in the sub-scanning direction to form the green light emitting element array  6 G.  
      The organic EL elements  20 B are disposed in B area in  FIG. 2  and 7800 organic EL elements  20 B are arranged in the main scanning direction to form one linear blue light emitting element array and 1 linear blue light emitting element array forms the blue light emitting element array  6 B.  
      In this embodiment, the R, G and B areas are formed on one glass base to drive the R, G and B areas in the passive matrix drive independently from and simultaneously with each other. 30 anode drive ICs of 260 channels are provided in a cascade connection in series for driving the transparent anodes of the G area, and one cathode drive ICs of 16 channels is provided for driving the cathode of the G area.  
      Operation of the exposure system of this embodiment will be described, hereinbelow. In the exposure system  5  shown in  FIG. 1 , when the color photosensitive material  40  is to be image-wise exposed, the red light emitting element array  6 R, the green light emitting element array  6 G, and the blue light emitting element array  6 B of the exposure head  1  are selectively driven by the drive circuit  80  according respectively to cyan image data, magenta image data, and yellow image data while the sub-scanning means  51  conveys the color photosensitive material  40  in the sub-scanning direction shown by arrow Y at a constant speed.  
      At this time, an image by red light from the 10 linear red light emitting element arrays of the red light emitting array  6 R, an image by green light from the 5 linear green light emitting element arrays of the green light emitting array  6 G, and an image by blue light from the blue light emitting element arrays  6 B are respectively imaged on the color photosensitive material  40  in a unit magnification by the refractive index profile type lens arrays  30 R,  30 G and  30 B. With this, the areas exposed to the red light are then exposed to the green light and then exposed to the blue light.  
      As for the exposure to red light, the same place of the color photosensitive material  40  is exposed to red light 10 times by the 10 linear red light emitting element arrays of the red light emitting element array  6 R as the color photosensitive material  40  is moved in the sub-scanning direction, and the 10 exposures provide in total a predetermined exposure corresponding to the cyan image data to the place. As for the exposure to green light, the same place of the color photosensitive material  40  is exposed to green light 5 times by the 5 linear green light emitting element arrays of the green light emitting element array  6 G as the color photosensitive material  40  is moved in the sub-scanning direction, and the 5 exposures provide in total a predetermined exposure corresponding to the magenta image data to the place. As for the exposure to blue light, a given place of the color photosensitive material  40  is exposed to blue light only once by the blue light emitting element array  6 B, and the 1 exposure provides a predetermined exposure corresponding to the yellow image data to the place.  
      The full color main scanning lines each thus formed are arranged side by side in the sub-scanning direction, whereby the color photosensitive material  40  is recorded with a two-dimensional full color latent image. The latent image is developed to a visible image by a known development means not shown.  
      The organic EL elements  20 R of the red light emitting element array  6 R, the organic EL elements  20 G of the green light emitting element array  6 G, and the organic EL elements  20 B of the blue light emitting element array  6 B are driven to emit light in a pulse-like fashion, and for instance, by controlling the pulse width, gradation can be generated for each pixel and the color photosensitive material  40  can be recorded with a continuous gradation image.  
      Prevention of shift of the color balance to elongate the service life of the exposure system in this embodiment will be described, hereinbelow. In this embodiment, the resolution in image exposure is 600 dpi, and the pitches of the pixels in the main scanning direction are 42.3 μm. The light emitting brightness Ir, Ig and Ib required for the red light emitting element array  6 R, the green light emitting element array  6 G and the blue light emitting element array  6 B from the characteristics of the color photosensitive material are as follows.
 
 Ir= 18750 cd/m 2 
 
 Ig= 25000 cd/m 2 
 
 Ib= 500 cd/m 2 
 
      As a result of an advance measurement, the deterioration time constant τr, τg and τb of the first stage of the superposed light emitting structures of the red light emitting element array  6 R, the green light emitting element array  6 G and the blue light emitting element array  6 B are as follows.
 
τr=100 h
 
τg=200 h
 
τb=3200 h
 
      Since the light emitting sizes of the organic EL elements  20 R, the organic EL elements  20 G, and the organic EL elements  20 B are 40×40 μm, 40×40 μm and 40×37.5 μm, the light emitting areas Sr, Sg and Sb are as follows.
 
Sr=1600 μm 2 
 
Sg=1600 μm 2 
 
Sb=1500 μm 2 
 
      Further, as described above, the numbers Nr, Ng and Nb of layers of the superposed light emitting structures of the organic EL elements  20 R, the organic EL elements  20 G, and the organic EL elements  20 B are Nr=6, Ng=6 and Nb=2, and the numbers Mr, Mg and Mb of the exposures by the organic EL elements  20 R, the organic EL elements  20 G, and the organic EL elements  20 B are Mr=10, Mg=5 and Mb=1.  
      Accordingly, the values of M×N×τ×S for the respective colors are as follows and the same.
 
 Mr×Nr×τr×Sr= 9600000 (h·μm2)
 
 Mg×Ng×τg×Sg= 9600000 (h·μm2)
 
 Mb×Nb×τb×Sb= 9600000 (h·μm2)
 
      Accordingly, even after a plurality of repeated use, the intensity ratio of light between the red light emitting element array  6 R, the green light emitting element array  6 G and the blue light emitting element array  6 B can be held substantially constant and shift of color balance can be prevented. The reason for this is as described above.  
      Further, in this embodiment, since the light emitting brightness of one light emitting structure is suppressed by employing a multi-layered structure, where a plurality of light emitting structures are superposed one on another, in each of the red light emitting element array  6 R, the green light emitting element array  6 G and the blue light emitting element array  6 B and at the same time the multiple exposure is carried out by forming the red light emitting element array  6 R and the green light emitting element array  6 G by a plurality of rows of the light emitting elements arranged side by side, the service life of the light emitting element arrays  6 R,  6 G and  6 B is elongated and the service life of the exposure system is elongated. The reason for this is also as described above.  
      The values in the first embodiment are shown in the following table 1.  
                                       TABLE 1                                   M   N   τ (h)   S (μm 2 )   M × N × τ × S (h · μm 2 )                                                            R   10   6   100   1600   9600000       G   5   6   200   1600   9600000       B   1   2   3200   1500   9600000                  
 
      When color photosensitive material is exposed to pulse-width-modulated light as in this embodiment, it is preferred that the following fundamental drive method and the following fundamental exposure method be employed. That is, before shipment of the exposure system, the red light emitting element array  6 R, the green light emitting element array  6 G and the blue light emitting element array  6 B are respectively driven at a suitable constant electric current, and the intensities of the exposure light passing through the refractive index profile type lens arrays  30 R,  30 G and  30 B at this time are measured. And correction coefficients to correct the drive pulse widths to correct the fluctuation in the intensities of the exposure light are obtained. When the color photosensitive material  40  is actually exposed, the exposure light is pulse-width-modulated on the basis of the image data and the correction coefficients.  
      The transparent anodes  21  and the metal cathodes  23  of the organic EL element  20  may be of a shape shown in  FIG. 5  as well as a linear shape shown in  FIG. 3 . In  FIG. 5 , two rows of the organic EL elements  20  extending in the main scanning direction are formed per one row of metal cathode  23 , and the organic EL elements  20  in one row is not spaced from the organic EL elements  20  in the other row in the main scanning direction. In this case, one main scanning line can be exposed without spaces between the pixels, for instance, by driving the exposure system so that the pixels of the odd numbers on the main scanning line are exposed by the organic EL elements  20  in one row and the pixels of the even numbers on the main scanning line are exposed by the organic EL elements  20  in the other row.  
     Second Embodiment  
      An exposure system in accordance with a second embodiment of the present invention will be described, hereinbelow. The second embodiment is basically the same as the first embodiment except that the light emitting size of the organic EL element  20 B differs from that in the first embodiment. That is, in this embodiment, the organic EL elements  20 R, the organic EL elements  20 G, and the organic EL elements  20 B are all 40×40 μm in light emitting size. Accordingly, the light emitting areas Sr, Sg and Sb are as follows.
 
Sr=1600 μm 2 
 
Sg=1600 μm 2 
 
Sb=1600 μm 2 
 
      The values of M×N×τ×S for the respective colors are as follows.
 
 Mr×Nr×τr×Sr= 9600000 (h·μm2)
 
 Mg×Ng×τg×Sg= 9600000 (h·μm2)
 
 Mb×Nb×τb×Sb= 10240000 (h·μm2)
 
      Though the value of Mb×Nb×τb×Sb differs from the value of Mr×Nr×τr×Sr or Mg×Ng×τg×Sg, such a small difference is able to prevent color balance in the exposed image from being largely shifted. Generally, when the ratio of the values of M×N×τ×S of the colors is in about 1:2, it is possible to prevent color balance in the exposed image from being largely shifted.  
      In this case, the blue light emitting element array  6 B is driven to provide a brightness of 469 cd/m 2  (=500 cd/m 2 ×1500/1600), whereby the value of light emitting brightness×light emitting area is equal to that in the first embodiment.  
      The values in the second embodiment are shown in the following table 2.  
                                       TABLE 2                                   M   N   τ (h)   S (μm 2 )   M × N × τ × S (h · μm 2 )                                                            R   10   6   100   1600   9600000       G   5   6   200   1600   9600000       B   1   2   3200   1600   10240000                  
 
     Third Embodiment  
      An exposure system in accordance with a third embodiment of the present invention will be described, hereinbelow. The third embodiment is basically the same as the first embodiment except that the values of M, N, τ and S differ from those in the first embodiment.  
      The values in the third embodiment are shown in the following table 3.  
                                       TABLE 3                                   M   N   τ (h)   S (μm 2 )   M × N × τ × S (h · μm 2 )                                                            R   5   12   100   1600   9600000       G   10   3   200   1600   9600000       B   1   1   3200   3000   9600000                  
 
      In this embodiment, since the values of M×N×τ×S for the respective colors are the same as in the first embodiment, it is possible to strictly prevent color balance in the exposed image from being shifted.  
     Fourth Embodiment  
      An exposure system in accordance with a fourth embodiment of the present invention will be described, hereinbelow. The fourth embodiment is basically the same as the first embodiment except that the values of M, N, τand S differ from those in the first embodiment. In this embodiment, the resolution in image exposure is 400 dpi, and the pitches of the pixels in the main scanning direction are 63.5 μm. The light emitting brightness Ir, Ig and Ib required for the red light emitting element array  6 R, the green light emitting element array  6 G and the blue light emitting element array  6 B from the characteristics of the color photosensitive material are as follows.
 
 Ir= 12000 cd/m 2 
 
 Ig= 16000 cd/m 2 
 
 Ib= 300 cd/m 2 
 
      The values in the fourth embodiment are shown in the following table 4. In this embodiment, the organic EL elements  20 R, the organic EL elements  20 G, and the organic EL elements  20 B are all 50×50 μm in light emitting size. Accordingly, the light emitting areas Sr, Sg and Sb are all 2500 μm 2 .  
                                       TABLE 4                                   M   N   τ (h)   S (μm 2 )   M × N × τ × S (h · μm 2 )                                                            R   1   7   300   2500   5250000       G   1   3   700   2500   5250000       B   1   1   2000   2500   5000000                  
 
      In this embodiment, since the values of M×N×τ×S for the respective colors are substantially the same, it is possible to strictly prevent color balance in the exposed image from being shifted.  
     Fifth Embodiment  
      An exposure system in accordance with a fifth embodiment of the present invention will be described, hereinbelow. The fifth embodiment is basically the same as the fourth embodiment except that the light emitting size of the organic EL elements  20 B differ from that in the fourth embodiment. That is, in the fifth embodiment, the organic EL elements  20 B is 50×52.5 μm in light emitting size and 2625 μm 2  in the light emitting areas Sb.  
      The values in the fifth embodiment are shown in the following table 5.  
                                       TABLE 5                                   M   N   τ (h)   S (μm 2 )   M × N × τ × S (h · μm 2 )                                                            R   1   7   300   2500   5250000       G   1   3   700   2500   5250000       B   1   1   2000   2625   5000000                  
 
      In this embodiment, the blue light emitting element array  6 B is driven to provide a brightness of 285.7 cd/m 2  (=300 cd/m 2 ×2500/2625), whereby the value of light emitting brightness×light emitting area is equal to that in the fourth embodiment.  
      Though no color filter is used in the embodiments described above, a color filter such as a band pass filter, a low pass filter, a high pass filter or the like may be installed in order to narrow the spectrum of the exposure light to prevent mixing of colors. As the deterioration time constant at this time for each color, the deterioration time constant of the first stage of the superposed light emitting structures under the condition under which the intensity of light after passing through the color filter conforms to the intensity of light necessary to exposure may be used.  
      An example of the procedure for determining the number M of exposures, the number N of layers of the superposed light emitting structures and the light emitting area S of the light emitting element will be described, hereinbelow.  
      (1) A light emitting element size is first temporarily determined on the basis of the resolution required to the exposure system (e.g., 600 dpi)  
      (2) Then exposure energy (light emitting brightness) necessary for each color is calculated taking into account the sensitivity of the photosensitive material, the transmittance of the lens, the exposure speed and the like.  
      (3) Light emitting elements of a single light emitting structure (N=1) are prepared for the respective colors, and the time constants τthereof at the light emitting brightness calculated in (2) are obtained.  
      (4) The combination of M and N is determined so that the values of M×N×τ×S for the respective colors are substantially the same.  
      (5) Further, the values of the light emitting areas S is finely adjusted so that the values of M×N×τ×S for the respective colors further approach.  
      Since the drive voltage of the organic EL element becomes as large as N times as the number of layers of the superposed light emitting structures becomes N, the value of N must be determined taking into account the withstand voltage of the drive IC in the step (4). Further, it requires a time N times as long as an organic EL element having a single light emitting structure to film a multi-layered organic EL element having N light emitting structures superposed one on another. When taking into account both the withstand voltage and the filming time, the number N of layers should be 10 at most. Further, since, as the number M of exposures is increased, the size of the exposure head in the sub-scanning direction is increased, it is preferred that the number of rows disposed side by side be as small as possible. Accordingly, it is preferred that the number N of layers of the light emitting structures be as large as possible in the range not larger than about 10, and the number M of exposures be as small as possible.  
      Though, in the embodiments described above, cyan, magenta and yellow are developed by red light, green light and blue light, respectively, it is possible to develop cyan, magenta and yellow by light in other wavelength ranges, for instance, by light in three wavelength ranges in an infrared region and the present invention can be applied also to a so structured exposure system. Further, the present invention can be applied also to an exposure system which is to expose color photosensitive materials other than the silver halide color paper.  
      Further, the light emitting element arrays may, of course, be formed by light emitting elements other than the organic EL elements and, for instance, elements comprising a combination of an LED and an aperture mask, liquid crystal elements, or PLZT elements may be employed.  
      While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be mad without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention.  
      The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description. All changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.  
      For example, an alternate exposure system may comprise a plurality of kinds of light emitting element arrays, each formed of a plurality of light emitting elements which are arranged in one row in one direction, which are arranged substantially normal to said one direction and emits light in different wavelength ranges, and a sub-scanning means which holds a color photosensitive material in a position where the light from each of the light emitting element arrays is projected, and moves the color photosensitive material and the light emitting element arrays relatively to each other in said one direction in which a plurality of light emitting element arrays are arranged, wherein the improvement comprises that  
      the light emitting area of the light emitting element is nonuniform between at least two kinds of light emitting element arrays.  
      It is preferred that at least one of the plurality of kinds of light emitting element arrays comprises a plurality of rows of the light emitting elements arranged side by side in the direction of the above-mentioned relative movement so that the same place of the color photosensitive material can be exposed to light a multiple times. When the system is able to carry out the multiple exposure, it is preferred that the value of M×N×τ×S is substantially the same in each of the plurality of kinds of light emitting element arrays wherein M represents the number of exposure of the same place of the color photosensitive material in each kind of the light emitting element array, N represents the number of light emitting structures in each kind of the light emitting element array, τ represents the deterioration time constant of the first stage of the superposed light emitting structures at the light emitting brightness upon exposure and S represents the light emitting area of the light emitting element in each kind of the light emitting element array.  
      Further, it is preferred that the plurality of kinds of light emitting element arrays be three kinds of light emitting element arrays respectively emitting light in wavelength ranges of red, green and blue. As such light emitting element arrays, organic EL element arrays are suitable.  
      Further, in the exposure system of the present invention, it is preferred that silver halide color paper be used as the color photosensitive material.  
      When at least one of the plurality of kinds of light emitting element arrays in the first or second exposure system comprises a plurality of rows of the light emitting elements arranged side by side in the direction of the above-mentioned relative movement so that the same place of the color photosensitive material can be exposed to light M times, the light emitting brightness of one light emitting element may be 1/M as compared with the case where one place of the color photosensitive material is exposed to light only once, whereby the service life of the light emitting element arrays is elongated to substantially M times and the service life of the exposure system is elongated.  
      When there is a difference in time constant of deterioration between the plurality of kinds of the light emitting element arrays due to difference in element structure, it is possible to equalize the light emitting element arrays in time constant of deterioration by changing the number M of the exposures in the light emitting element arrays. That is, as the number M of exposures increases, the light emitting brightness or the light emitting time of the light emitting element array can be reduced, whereby the deterioration speed of the light emitting element array can be lowered. If so, the intensity ratio of light in the respective wavelength ranges can be held constant even after a plurality of repeated exposures and shift of color balance can be prevented.  
      When the same place of the color photosensitive material can be exposed to light a multiple times, the light emitting brightness L can be represented by a formula L=L 0 exp (−t/τ) wherein M represents the number of exposure of the same place of the color photosensitive material in each kind of the light emitting element array, N represents the number of light emitting structures in each kind of the light emitting element array, τ represents the deterioration time constant of the first stage of the superposed light emitting structures at the light emitting brightness upon exposure, S represents light the emitting area of the light emitting element in each kind of the light emitting element array, L 0  represents the initial light emitting brightness and t represents the light emitting time. Since there is a relation described above between the values of M, N and S and the light emitting brightness of the light emitting element array, when the value of M×N×τ×S is substantially the same in each of the plurality of kinds of light emitting element arrays, the deterioration speeds of the kinds of light emitting element arrays can be equal to each other, whereby shift of color balance can be more strictly prevented.