JP2007190788A - Line head and image forming apparatus using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a line head which enables simple correction of a quantity of light for every pixel, and to provide an image forming apparatus using the same. <P>SOLUTION: Fig. 1(a) shows one halftone dot in the case where a tone is expressed by a halftone dot screen. A density is expressed by a size of a plurality of regularly arranged halftone dots in the halftone dot screen. Exposure pixels d1 and d4 have two pixels arranged in a sub scanning direction, and exposure pixels d2 and d3 have four pixels arranged. In the embodiment, each exposure pixel can take four values of 0-3 because each exposure pixel has a 2-bit tone. Assuming that a tone value 3 is the quantity of light of 100%, a tone value 2 has the quantity of light of 66.7% and a tone value 1 has the quantity of light of 33.3%. A tone value 0 is a state not to light the exposure pixels. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a line head that can easily perform light amount correction for each pixel and an image forming apparatus using the line head.

  In general, an electrophotographic toner image forming unit includes a photosensitive member as an image bearing member having a photosensitive layer on an outer peripheral surface, a charging unit that uniformly charges the outer peripheral surface of the photosensitive member, and a uniform charging unit using the charging unit. An exposure unit that selectively exposes the outer peripheral surface charged to form an electrostatic latent image, and a toner as a developer is applied to the electrostatic latent image formed by the exposure unit to form a visible image ( Developing means for forming a toner image).

  As a tandem type image forming apparatus for forming a color image, a plurality (for example, four) of toner image forming means as described above are arranged on the intermediate transfer belt. The toner image on the photosensitive member by the single color toner image forming unit is sequentially transferred to the intermediate transfer belt, and the toner images of a plurality of colors (for example, yellow, cyan, magenta, and black (black)) are superimposed on the intermediate transfer belt. There is an intermediate transfer belt type that obtains a color image on an intermediate transfer belt.

  In the tandem type image forming apparatus having the above-described configuration, a line head using an LED or an organic EL element as a light emitting element is known. In such an optical writing line head using an LED or the like as a light source, since the light amounts of a plurality of light sources (light emitting portions) are not uniform, when writing is performed as it is, an image formed thereby is also displayed. There is a problem that shading (streaks) corresponding to the amount of light occurs.

  In order to avoid the occurrence of such a difference in density, conventionally, there has been a circuit for correcting the light quantity of each of the light sources provided corresponding to the pixels at the time of writing to make the density uniform. It was provided. Such light amount correction is performed by changing the lighting time of the light source and the drive current. To correct the light intensity, measure the light intensity of each light source at the time of shipment of the line head, and write the lighting time and drive current correction values corresponding to each pixel in the memory built in the line head. That is, at the time of image writing, the correction value is read to correct the lighting time and drive current.

  For example, Patent Document 1 discloses a method for independently controlling the gradation value and the correction value of each pixel. Patent Document 2 discloses a method for obtaining a correction value by performing Fourier transform on the light amount unevenness and inversely transforming a function that is corrected so that each frequency component is within an allowable value. Patent Document 2 shows that the higher the spatial frequency (that is, the shorter the period of fluctuation in light quantity), the larger the fluctuation in light quantity supplied from the light source.

  Further, Patent Document 3 discloses that one pixel is exposed by being divided into, for example, nine sub-pixels in the main scanning direction 3 × sub-scanning direction 3. The plurality of sub-pixels are turned on simultaneously regardless of the position. Although the light source of Patent Document 3 is described as “electroluminescence”, it is considered that an organic EL material is used for the light source because there is a description such as being weak in humidity. Patent Document 3 describes that “shading correction”, that is, light amount correction, is performed by controlling the number of light emitting units that emit light.

Japanese Patent Laid-Open No. 03-190765 Japanese Patent Laid-Open No. 06-155806 JP 2002-292922 A

  However, it is necessary to provide a correction circuit as disclosed in Patent Document 1 corresponding to each pixel separately from the lighting control of each pixel. In particular, when performing gradation control in which the light intensity of each pixel is changed according to the density of the image, the gradation control and the light amount correction must be performed independently, so that the circuit becomes complicated. was there.

  Particularly in recent years, such line heads are frequently used in color electrophotographic page printers. In color images, the demand for expressiveness and reproducibility of photographs and graphics is higher than that of monochrome images, and more precise light amount correction is required. The light amount correction as shown in the above-mentioned Patent Document 1 is performed digitally. In order to perform precise light amount correction, the correction value also requires a larger amount of information, that is, the number of bits, so the scale of the light amount correction circuit tends to become larger. In particular, in a line head using an LED as a light source, the light amount variation of the light source is ± 20% or more, while the light amount correction requires a resolution of ± 2% or less. An information amount of 6 bits is required for each pixel.

  In addition, the method disclosed in Patent Document 2 has a problem that a complicated calculation is required because Fourier transform is performed. Furthermore, the method disclosed in Patent Document 3 is configured by dividing one pixel into a plurality of light emitting units (nine in the embodiment), and is not a pixel controlled independently. Therefore, there is a problem that the resolution of the image is low, and gradation and smoothness of the outline cannot be obtained.

  The present invention has been made in view of such various problems of the prior art, and an object of the present invention is to provide a line head that can easily perform light amount correction for each pixel and an image forming apparatus using the same. It is to provide.

In the line head of the present invention that achieves the above object, a plurality of light sources are arranged in a line in the main scanning direction, and each light source is modulated in accordance with a predetermined gradation value with respect to image data. In line heads that perform tonal exposure,
The diameter of the imaging spot formed by imaging each light source on the exposure surface is set larger than the pixel pitch, and the gradation value of the exposure pixel of the light source that is turned on according to image data is increased or decreased, The light energy exposed from each of the light sources is corrected to be uniform. As described above, since the diameter of the imaging spot formed by imaging each light source on the exposure surface is set to be larger than the pitch of the pixels, a large number of light sources are arranged with a fine density. In addition, it is possible to perform good light amount correction control only by increasing / decreasing gradation values for a large number of light sources without providing a light amount correction circuit for each pixel.

  The line head of the present invention is characterized in that the gradation value is selectively increased or decreased for a plurality of exposure pixels arranged in the sub-scanning direction with respect to the correction target pixel. With such a configuration, it is possible to perform good light quantity correction control only by selectively increasing or decreasing the gradation values for a plurality of exposure pixels arranged in the sub-scanning direction.

  The line head according to the present invention is characterized in that the gradation value is selectively increased or decreased with respect to a plurality of exposure pixels arranged in the main scanning direction with respect to the correction target pixel. With such a configuration, it is possible to perform good light quantity correction control only by selectively increasing or decreasing the gradation value for a plurality of exposure pixels arranged in the main scanning direction.

  The line head according to the present invention is characterized in that the gradation value is selectively increased or decreased with respect to an exposure pixel within a certain distance in the vicinity of the correction target pixel. With such a configuration, it is possible to perform good light quantity correction control only by selectively increasing or decreasing the gradation value with respect to the exposure pixels within a certain distance in the vicinity of the correction target pixel.

  The line head of the present invention is characterized in that the gradation expression of the image by the plurality of light sources is a line screen process for expressing gradation by a line width. With such a configuration, the light amount correction can be performed simply even in the processing of the line screen.

  The line head according to the present invention is characterized in that the pixel density for forming the output image is different between the main scanning direction and the sub-scanning direction. With such a configuration, for example, the number of light sources provided in the line head can be reduced by making the pixel density in the sub-scanning direction larger than the pixel density in the main scanning direction. Even in this case, since the number of exposure pixels per unit area is the same in both the main scanning direction and the sub-scanning direction, there is no problem in light amount correction.

  In the line head of the present invention, the light source is formed on a glass substrate having a single organic EL element. Since it is set as such a structure, it is not necessary to make the diameter of a light emission part small, Therefore The optical power of a light emission part can be taken large. For this reason, it is possible to use an organic EL material having a low luminous efficiency. In addition, a large number of pixels can be formed on the glass substrate at a high density and with high accuracy at a time.

  The line head of the present invention is characterized in that the light source and a thin film transistor (TFT) circuit for driving the light source are formed on the glass substrate. According to this structure, since the organic EL element and TFT of a light source can be produced in the same process, manufacturing cost can be reduced.

  The image forming apparatus of the present invention is provided with at least two or more image forming stations in which each of the image forming units of the charging unit, any one of the line heads, the developing unit, and the transfer unit is arranged around the image carrier, and the transfer medium. The image forming is performed in a tandem manner by passing through each station. According to this configuration, the light amount correction can be easily performed in the tandem image forming apparatus.

  An image forming apparatus according to the present invention includes an image carrier configured to carry an electrostatic latent image, a rotary developing unit, and the line head according to any one of claims 1 to 7. The rotary developing unit carries toner stored in a plurality of toner cartridges on the surface thereof, and sequentially conveys different color toners to a position facing the image carrier by rotating in a predetermined rotation direction. A developing bias is applied between the image bearing member and the rotary developing unit, and the toner is moved from the rotary developing unit to the image bearing member, whereby the electrostatic latent image is visualized to form a toner image. It is characterized by forming. According to this configuration, the light amount correction can be easily performed in the rotary type image forming apparatus.

  In addition, the image forming apparatus of the present invention includes an intermediate transfer member. For this reason, the light amount correction can be easily performed in the image forming apparatus including the intermediate transfer member.

  As described above, the line head of the present invention and the image forming apparatus using the line head can perform light amount correction for each pixel without providing a light amount correction circuit for each pixel. For this reason, it is possible to obtain a good image without density unevenness while using a simple circuit.

A line head used in a normal page printer has pixels formed at a density of 600 dpi or 1200 dpi. On the other hand, in the embodiment of the present invention, a large number of light sources are arranged with a fine density to achieve good gradation control. As an example, the density of the number of pixels is 2400 dpi or 4800 dpi. FIG. 4 is an explanatory diagram schematically showing the basic technique of the present invention. In the example of FIG. 4, the density of the number of pixels is set to 2400 dpi. In FIG. 4A, 90 is a pixel of a light source, 91 is a spot diameter when a light beam formed by a light beam output from the light source is imaged on an exposed surface such as an image carrier through a lens array. It is. In this specification, for the sake of simplicity, the diameter of the imaging spot formed on the surface to be exposed will be abbreviated as the spot diameter, and light on the surface to be exposed will be described later with reference to FIG. It is defined as the width that becomes 1 / e ² intensity with respect to the peak value of the intensity profile. The X direction indicates the main scanning direction, and Y indicates the sub scanning direction. In this example, the spot diameter 91 is 50 μm, and the size of the light source forming the pixel (exposure pixel) 90 is 20 μm.

  Thus, although the spot diameter is large for one exposure pixel, the exposure energy is low, so that an image cannot be formed alone, and an actual image appears only after several tens of exposure pixels are exposed. That is, the spot diameter is formed several times the pixel pitch. As already described, it is difficult to reduce the size of the spot, and as described later, if the photosensitive member, the toner, and the developing system thereof cannot cope, the effect is small. In the case of an organic EL element, if the area of the light emitting portion is reduced in order to reduce the spot diameter, the power for forming an image is insufficient. Alternatively, the density of the gradation screen used for the expression of the gradation image is 100 to 300 LPI, and the halftone dots and lines are formed not by a single exposure pixel but by a large number of exposure pixels. There is no need to make it smaller.

  Therefore, in the embodiment of the present invention, as shown in FIG. 4B, a large number of exposure pixels are superimposed and exposed on the image carrier in the main scanning direction and the sub-scanning. In this example, the output image 93 is formed by superimposing 4 × 4 = 16 pixels, four each in the main scanning direction and in the sub-scanning. That is, an image is formed by superimposing the imaging spots formed on the surface to be exposed slightly shifted in the main scanning direction or the sub-scanning direction.

  Here, since the individual pixels 90 for forming the output image 93 are different from the size of the pixels obtained as the output image 93, in this embodiment, the individual pixels 90 are defined as exposure pixels. In the present invention, each exposure pixel 90 further has a predetermined gradation.

  In the line head according to the embodiment of the present invention, as described above, the conventional output image composed of one pixel is formed by a large number of exposure pixels. For this reason, it is sufficient that the gradation value of each exposure pixel is not so large. For example, 2 bits, that is, 3 gradations of 0 to 3, is practically sufficient. That is, regarding the light source gradation control, on-off control results in 1-bit binary data, but in the embodiment of the present invention, a signal for increasing or decreasing gradation of 2 bits or more is formed as a light source control signal. Yes.

  Therefore, on the line head, gradation control can be performed with the minimum necessary modulation control circuit configuration for gradation control for each light source. For this reason, it becomes possible to mount a control element such as a TFT (light source driving thin film transistor) for performing light emitting element drive control on the same glass substrate as the light source, thereby simplifying the configuration of the control unit mounted on the line head. can do. A specific example of gradation control will be described later.

  FIG. 5 is an explanatory diagram illustrating an example of a pixel array in the embodiment of the present invention. FIG. 5A shows an example in which light emitting portions having a diameter of 20 μm are arranged at 2400 dpi. In this example, the spot diameter is 60 μm, and light emitting element lines 94 provided with a large number of exposure pixels 90 in the main scanning direction are arranged in three rows in the sub-scanning direction. The pixel pitch is 25.4 / 2400≈10.6 μm. The distance between the center lines at both ends in the sub-scanning direction of the light emitting element line 94 is 63.5 μm, which is about 6 times the pixel pitch. Thus, the ratio between the spot diameter and the pixel pitch is 60 / 10.6≈5.7, and the spot diameter is set larger than the pixel pitch.

  FIG. 5B shows an example in which light emitting portions having a diameter of 15 μm are arranged at 4800 dpi. In this example, the spot diameter is 55 μm, and five rows of light emitting element lines are formed in the sub-scanning direction. In this case, the pixel pitch is 25.4 / 4800≈5.3 μm. The distance between the center lines at both ends in the sub-scanning direction of the light emitting element line 94 is 105.8 μm, which is about 20 times the pixel pitch. In this example, the ratio of the spot diameter to the pixel pitch is 55 / 5.3≈10.4, and the spot diameter is set larger than the pixel pitch as compared with the example of FIG. 5A and 5B, the light emitting element lines are arranged in a plurality of lines in three or more rows in the sub-scanning direction, and are arranged so that the positions in the main scanning direction are different from each other. Yes. Therefore, it is possible to easily superimpose the imaging spots in the main scanning direction.

  The gradation of the original image is expressed as the sum of the number of lit pixels of the exposure pixel and the gradation value of each pixel. For example, when each exposure pixel has a 2-bit gradation with respect to a conventional pixel of 600 dpi, each exposure pixel can take three levels of light amounts of 1 to 3. Since 16 pixels correspond to 2400 dpi, 16 × 3 = 48 gradations, and 64 pixels correspond to 4800 dpi, so that there is a further sufficient gradation. Further, since the spot size is sufficiently large with respect to the pitch of the exposure pixels, the shape of the pixel formed by changing the number of lighting of the exposure pixels at the time of gradation recording is rarely deformed.

  Next, tone control when the spot diameter is set larger than the pixel pitch in the embodiment of the present invention will be described. FIG. 6 is an explanatory diagram showing changes in the surface potential distribution when the number of exposed exposure pixels is increased in the embodiment of the present invention. In this example, the exposure pixel density is 2400 dpi and the spot diameter is 60 μm. FIG. 7 and FIG. 8 are characteristic diagrams showing condition setting which is a premise of the configuration of FIG. FIG. 7 is a characteristic diagram showing the power distribution due to the spot of the light source imaged on the photosensitive member (image carrier).

When the spot imaged on the photosensitive member has a power (intensity) distribution as shown in FIG. 7, if the peak height is 1, 1 / e ² = 1 when e is a natural logarithm. /(2.72) ² ≈0.135. That is, the spot diameter of 60 μm indicates the width of the profile that is 13.5% of the power peak.

FIG. 8 is a characteristic diagram showing the slow current characteristic (PIDC) of the photoreceptor. The vertical axis represents the surface potential (V) of the photoreceptor, and the horizontal axis represents the exposure energy (μJ / cm ² ). In FIG. 12, the surface potential of the photoreceptor corresponding to the initial potential V0 is −600V. In this case, the exposure energy corresponding to the half exposure amount (surface potential is −300 V) is 0.08 μJ / cm ² .

Further, the state in which the surface potential hardly changes with respect to the exposure energy, that is, the state in which the surface potential is saturated is expressed as energy (saturation energy) when the entire surface of the photoreceptor is exposed. In the example of FIG. 8, this saturation energy is 0.3 μJ / cm ² .

In FIG. 6, as illustrated in FIGS. 7 and 8, the spot diameter of 1 / e ² ≈0.135 is 60 μm, the half-exposure amount of the photoreceptor is 0.08 μJ / cm ² , and the saturation energy is 0. The characteristics of the surface potential distribution in the case of ³ μJ / cm ² are shown. In FIG. 6A, the potential distribution Ew when the single exposure pixel 90w is turned on is formed in a substantially circular shape. In FIG. 6, the black circle in the figure indicates the center position of the exposure pixel.

  In FIG. 6B, another exposure pixel 90x is provided in the sub-scanning direction adjacent to the exposure pixel 90w described in FIG. 6A, and the potential when the exposure pixel 90w and the exposure pixel 90x are turned on simultaneously. Distribution Ex is shown. In this case, the potential distribution Ex is substantially circular. Here, the contour lines of the potential distribution Ex are formed at intervals of 50V.

  6C, in the configuration of FIG. 6B, an exposure pixel 90y is provided adjacent to the exposure pixel 90w in the main scanning direction, and the three exposure pixels of the exposure pixel 90w, the exposure pixel 90x, and the exposure pixel 90y are provided. Is a potential distribution Ey when is simultaneously turned on. Also in this case, the potential distribution Ey has a substantially circular shape.

  6D, in the configuration of FIG. 6C, another exposure pixel 90z is further provided adjacent to the exposure pixel 90x in the main scanning direction. The exposure pixel 90w, the exposure pixel 90x, the exposure pixel 90y, and the exposure A potential distribution Ez when the pixels 90z are simultaneously turned on is shown. In this case, the potential distribution Ez is formed in a substantially circular shape surrounding each of the exposure pixels 90w to 90z arranged in a rectangular shape. In the example described with reference to FIGS. 6A to 6D, each exposure pixel 90w-z has the maximum gradation, but each exposure pixel actually has a gradation.

  The reason for the potential distribution shape as described in FIG. 6 will be described. In the embodiment of the present invention, the diameter (spot diameter) of each pixel to be exposed is 60 μm, whereas the pixel pitch is about 10.6 μm, which is much smaller, and the exposure pixel position is shifted to perform multiple exposure. Even so, the circular shape is maintained. On the other hand, in the prior art, the pixel diameter is 20 μm, which is smaller than that of the embodiment of the present invention. Therefore, there is little overlap between the pixels, and the pixel arrangement appears in the potential distribution as it is.

  1-3 is explanatory drawing which shows embodiment of this invention. FIG. 1 (a) shows one halftone dot when a gray scale is expressed by a halftone screen. In the halftone screen, the density is expressed by the size of a plurality of regularly arranged halftone dots. In this embodiment, 12 exposure pixels having a pixel density of 2400 dpi are lit to form one pixel. FIG. 1A shows the position of the exposure pixel to be lit, and an output image 93a is formed.

  Referring to FIG. 1A, the exposure pixels d1 and d4 have two pixels aligned in the sub-scanning direction, and d2 and d3 have four pixels aligned. In this embodiment, since each exposure pixel has a 2-bit gradation, it can take four values from 0 to 3. If the gradation value 3 is 100%, the gradation value 2 is 66.7%, and the gradation value 1 is 33.3%. A gradation value of 0 is a state in which the exposure pixel is not lit.

  Four pixels in the center of the output image 93a have a gradation value of 3, and eight neighboring pixels have a gradation value of 2. Therefore, the total of the gradation values of all 12 pixels constituting the halftone dot is 3 × 4 + 2 × 8 = 28. Note that the light amounts of d1 to d4 have a certain variation. FIG. 1B shows an electrostatic latent image distribution Ea on the photoreceptor obtained under the following conditions. The interval between the contour lines of Ea is 50V.

The calculation conditions of the potential distribution Ea correspond to the characteristics described with reference to FIGS.
Exposure pixel pitch: 2400 dpi
Exposure pixel interval: 25.4 mm / 2400 = 10.58 μm
Spot diameter at which exposed pixels form an image on the photoreceptor: 60 μm
Sensitivity of photosensitive member to be exposed (half exposure amount): 0.08 μJ / cm ²
Light energy of exposed pixel: 0.3μJ / cm ² (average energy when all pixels are lit)
Initial charging potential: -600V or more

  In FIG. 2A, one gradation value at the upper right of the exposure pixel d4 is reduced from 2 to 1. That is, the sum of the gradation values is decreased by 1 to 27. Therefore, a light amount correction of −3.6% is performed. As is clear from comparison between FIG. 2B and FIG. 1B, the width of the peak portion (smallest contour line) of the distribution Eb of the electrostatic latent image and the right side portion of the bottom of the distribution are slightly narrowed. I can see that However, the overall distribution shape of Eb has not changed significantly from Ea.

In FIG. 3A, contrary to FIG. 2A, the gradation value is increased from 2 to 3 in the upper right one of the exposure pixel d4. That is, the light amount correction of + 3.6% is performed. Also in this case, the electrostatic latent image distribution Ec in FIG.
Compared with the electrostatic latent image distribution Ea in FIG. 1A, it slightly spreads to the right, but there is almost no difference in the overall potential distribution shape.

  In this way, the effect of multiple exposure due to the wide distribution of the amount of light of one exposure pixel results in a latent image distribution obtained by exposure even if the gradation value of one exposure pixel constituting the halftone dot is changed by one step. Shows that there is almost no difference. Actually, since there is a difference in light amount between d1 and d4, the light amount correction value can be finely adjusted depending on which exposure pixel gradation value is to be corrected. Conversely, when the correction width of one exposure pixel is insufficient, the number of exposure pixels to be corrected is increased to two or three, or when there is a pixel with a gradation value of 1, the gradation value is increased to 3. May be.

  The gradation control of the exposure pixel according to the present invention may be either pulse width control for controlling the lighting time or current value control for controlling the lighting light amount. The gradation control means is originally provided for control by the gradation of the image to be printed, and is not dedicated to light amount correction. That is, it means that the gradation can be controlled with the same accuracy as the light amount correction as described above.

  The contribution of one exposed pixel to image formation is small. That is, the tolerance of the light amount error of the pixel is larger than that in the conventional case. In order to form a pixel of one original image or a screen dot by overlapping spots of many exposure pixels, when correcting the amount of light of one exposure pixel, correction is performed at the exposure pixels around the corresponding exposure pixel. do it. Alternatively, even if the imaging spots do not overlap, they cannot be separated by human vision if they are close to each other. Therefore, correction may be performed including exposure pixels that do not overlap the imaging spots.

  Next, light amount correction of a gradation (line) screen that expresses gradation with a line width will be described. FIG. 9 is an explanatory diagram according to the embodiment of the present invention. In the example of FIG. 9, the spot diameter is 60 μm, and the same conditions are set as in the example described in FIG. FIG. 9A shows an example in which the exposure pixels 90r and 90s are juxtaposed in the main scanning direction and shifted in the sub scanning direction by two exposure pixels and arranged in an oblique direction. At this time, the lines Lr and Ls are formed by straight lines substantially parallel to the arrangement of the exposure pixels, and the gradation characteristics expressed by the line width are good characteristics.

  FIG. 9B shows an example in which three exposure pixels 90r, 90s, and 90t are juxtaposed in the main scanning direction and shifted in the sub scanning direction by two exposure pixels and arranged in an oblique direction. At this time, the lines Lp and Lt are formed by straight lines substantially parallel to the arrangement of the exposed pixels, and the gradation characteristics expressed by the line width are good characteristics.

  In FIG. 9C, three exposure pixels 90r, 90s, and 90t are juxtaposed in the main scanning direction, and two exposure pixels 90u and 90v are arranged at a position where two exposure pixels are separated in the sub-scanning direction. Yes. In this case, the potential distribution Eu is formed in an elliptical shape. Further, the lines Lu and Lv are formed by smooth oblique lines that have some unevenness but have practically no problem. As described above, in the embodiment of the present invention, even in a gradation (line) screen that expresses gradation by the line width, useful characteristics can be obtained as compared with the conventional example. In the example described with reference to FIGS. 9A to 9D, the exposure pixels 90r to 90v have the maximum gradation as in the case of FIG. 6, but each exposure pixel actually has a gradation.

  FIG. 10 and FIG. 11 are explanatory diagrams of such a correction method when the gradation is expressed by a line screen. As shown in FIG. 10 and FIG. 11, the line screen expresses the density with the thickness of the diagonal line. For this reason, there is no clear correction unit such as a halftone dot when correcting the light amount of the exposure pixel. Therefore, the exposure pixel is corrected so that the moving average of the light amounts of several pixels is taken and the value becomes constant.

  In FIG. 10, the width in the sub-scanning direction of the diagonal lines La and Lb constituting the parallel lines is exactly four exposure pixels. Of the four exposure pixels arranged in the sub-scanning direction, the upper and lower two exposure pixels 90s have a gradation value of 2, and the central exposure pixel 90r2 has a gradation value of 3. Therefore, a total of 10 gradation values is obtained in the sub-scanning direction. The total sum of the gradation values changes according to the density of the image to be realized, but here, for the sake of explanation, all of them have the same line width, that is, the same image density.

  FIG. 12 is a characteristic diagram showing the amount of light and the moving average of each pixel on such a line. Here, the moving average takes 7 pixels before and after the target pixel. In the characteristic Ga before correction, the light amount variation of each pixel is about ± 20%, and the moving average also has a variation of about +5% / − 3%.

  Gb is a characteristic after gradation correction. In the characteristic Gb, NO. The gradation values of the upper and lower exposure pixels 4, 7, and 13 are increased from 2 to 3. In addition, NO. The gradation value of one exposure pixel above or below 5 is reduced from 2 to 1. Furthermore, NO. The gradation values of the upper and lower exposure pixels 8, 10, 11, 18, and 21 are corrected so as to decrease from 2 to 1. By correcting the gradation value in this way, the change in the moving average of the light quantity can be reduced to about 3%.

  FIG. 11 shows the above correction in terms of actual pixels. 90r is an exposure pixel having a gradation value of 3, 90s is an exposure pixel having a gradation value of 2, and 90t is an exposure pixel having a gradation value of 12. The position in the sub-scanning direction of the exposure pixel for which gradation correction is performed may be any of the four pixels, but in the present embodiment, it is the top or bottom of the four pixels in the sub-scanning direction. . Thus, referring to FIG. 11, although the contour of the line is uneven, the spot formed by each exposure pixel is actually much larger than the pixel pitch, so that a large number of exposure pixels are multiplexed. Since it is exposed, it becomes a very smooth line.

  If the size of the moving average section is too small, minute correction cannot be performed, and if it is increased, minute fluctuations in the amount of light within the section can be visually recognized. The discrimination limit by the naked human eye is said to be about 50 μm, which is when the contrast of light and shade is large. In the case of density fluctuation accompanying minute light quantity fluctuation, which is a problem in the embodiment of the present invention, it cannot be recognized unless it has a width of about 100 to 150 μm.

  Therefore, even if the moving average is 7 pixels at 2400 dpi as described above, it is about 74 μm (pixel pitch 10.6 × 7 = 74), so even if there is some light amount fluctuation within that range, it is not recognized at all. That is, as long as the moving average light amount can be made uniform to some extent, there is no problem even if the variation in individual pixels is somewhat large.

  FIG. 13 is a block diagram illustrating a schematic configuration of a control unit according to the embodiment of the present invention. In FIG. 13, reference numeral 70 denotes a host computer using a personal computer (PC) or the like, which creates image data and transmits it to a printer controller 72 provided in the printer control unit 71. In addition to the printer controller 72, the printer control unit 71 is provided with a line head control board 73 and a line head control means 74. The line head control means 74 has a light amount memory 75.

  The printer controller 72 creates binary data, which is digital data, for each exposure pixel based on the image data transmitted from the host computer 70 and outputs the binary data to the line head control board 73. The line head control board 73 is provided with a calculation unit. The calculation unit of the line head control board 73 increases or decreases the gradation value for each exposure pixel based on the light amount data for each pixel stored in the light amount memory 75 and the binary data input from the printer controller 72. Formed data.

  The exposure pixel correction processing according to the present invention depends on the arrangement and position of the exposed pixels that are lit, so that it is difficult to perform the correction process using a circuit inside the line head. Therefore, it is desirable that the exposure pixel correction processing is performed by the control board 73 provided outside the line head as shown in FIG. 13 or the printer controller 72 that performs image processing. Since the light amount data of each exposure pixel is stored in the light amount memory 75 in the line head, the light amount data is read out, and the exposure pixel is appropriately corrected from the array of exposure pixels that are actually turned on during printing.

  Further, even in gradation printing, when the density is high, or when characters and line drawings are printed, the difference in the amount of light of the exposed pixels does not appear as the density difference. This is due to saturation characteristics and visual characteristics of the photoreceptor. In such a case, it is not necessary to perform exposure pixel correction processing as described above.

  As described above, in the present invention, the light amount is corrected by correcting the gradation value of the exposure pixel. However, the size of the optical imaging spot is much larger than the arrangement pitch of each exposure pixel. Since the exposure energy is large and smaller than the level at which the image is developed as a pixel on the photoconductor, an image appears only when multiple exposure pixels are subjected to multiple exposure. Thus, the effects of correcting a small number of exposed pixels are also averaged (or “thinned”), so there is little effect on the contour shape of the image and only the density is changed. In the present invention, since the number of gradations of each exposure pixel is 2 bits as described above, the gradation control circuit for each pixel of the applied line head or image forming apparatus is simplified. can do.

  Next, a modified example of the present invention will be described. Since the resolution in the sub-scanning direction can be controlled only by timing, it may be higher than that in the main scanning direction. For example, the lighting of the exposure pixels may be controlled so that the pixel density in the main scanning direction is 1200 dpi and the pixel density in the sub-scanning direction is 4800 dpi. If the pixel density is set in this way, the number of light sources provided in the line head is half the number of exposure pixels compared to when the pixel density is set to 2400 dpi in both the main scanning direction and the sub-scanning direction. Even in this case, since the number of exposure pixels per unit area is the same as that when the pixel density is 2400 dpi in both the main scanning direction and the sub-scanning direction, the light amount correction effect is the same as that of the above-described embodiment.

  In the embodiment of the present invention, an organic EL element is used as a light source. In the case of using an organic EL element, in the present invention, it is not necessary to reduce the diameter of the light emitting part, so that the optical power of the light emitting part can be increased. For this reason, it is possible to use an organic EL material having a low luminous efficiency. In the present invention, since the exposure pixels are arranged at a higher density than in a normal line head, the number of pixels increases dramatically. Although it is possible to apply the present invention to a line head using an LED as a light source which has been conventionally used, an LED array chip provided with a large number of LEDs is mounted on a substrate with high positional accuracy, and the number of pixels is higher than usual. Therefore, the number of bondings connecting the chip and the substrate increases, which makes manufacturing difficult.

  On the other hand, when an organic EL element is used as a light source, a large number of pixels can be formed on a glass substrate at a high density and with high accuracy, which is optimal as an embodiment of the present invention. Further, in the present invention, since a gradation control circuit and a light amount correction circuit for each pixel are not necessary, and a drive circuit that only controls on / off of each pixel is required, the circuit configuration is simplified and the same as that of the light emitting unit. It becomes easy to make a driving circuit with a thin film transistor on a glass substrate. Various types of thin film transistors such as amorphous silicon, low-temperature polysilicon, high-temperature polysilicon, and organic transistors can be used.

  Since the line head of the present invention has an extremely large number of pixels, it is also useful to divide the pixels into several groups and perform time-division driving. Even in that case, it is only necessary to control each light source with binary values of on and off as described above, so that the circuit configuration can be extremely simplified.

  In the above example, the organic EL element has been described as the light source (exposure pixel) of the present invention. In the embodiment of the present invention, for example, an LED, a fluorescent tube, various shutter arrays, and the like can be applied as the light source (exposure pixel).

  In the embodiment of the present invention, the correction value can be determined so as to correct not only the light amount correction but also the image density unevenness due to the non-uniform imaging performance of the rod lens array. In this case, further improvement in image quality can be seen.

  In the embodiment of the present invention, a tandem color printer that exposes four photoconductors with four line heads, simultaneously forms four color images, and transfers them onto one endless intermediate transfer belt (intermediate transfer medium). The target is a line head used in (image forming apparatus). FIG. 14 is a vertical side view showing an example of a tandem image forming apparatus using an organic EL element as a light emitting element.

  This image forming apparatus includes four organic EL element array exposure heads 101K, 101C, 101M, and 101Y having a similar configuration, and four corresponding photosensitive drums (image carriers) 41K, 41C, Arranged at the exposure positions 41M and 41Y, respectively, is configured as a tandem image forming apparatus.

  As shown in FIG. 14, the image forming apparatus is provided with a driving roller 51x, a driven roller 52, and a tension roller 53, and is tensioned by the tension roller 53 and stretched in the direction indicated by the arrow (counterclockwise). ) Is circulated to the intermediate transfer belt (intermediate transfer medium) 50. Photosensitive members 41K, 41C, 41M, and 41Y having photosensitive layers are arranged on the outer peripheral surface as four image carriers arranged at predetermined intervals with respect to the intermediate transfer belt 50.

  K, C, M, and Y added after the reference sign mean black, cyan, magenta, and yellow, respectively, and indicate that the photoconductors are black, cyan, magenta, and yellow, respectively. The same applies to other members. The photoreceptors 41K, 41C, 41M, and 41Y are rotationally driven in the direction indicated by the arrow (clockwise) in synchronization with the driving of the intermediate transfer belt 50.

  Around each photoconductor 41 (K, C, M, Y), charging means (corona charger) 42 (K) for uniformly charging the outer peripheral surface of the photoconductor 41 (K, C, M, Y), respectively. , C, M, Y) and the outer peripheral surface uniformly charged by the charging means 42 (K, C, M, Y) are synchronized with the rotation of the photoconductor 41 (K, C, M, Y). The organic EL element array exposure head (line head) 101 (K, C, M, Y) as described above according to the present invention is provided.

  Further, a developing device 44 (K) which applies toner as a developer to the electrostatic latent image formed by the organic EL element exposure head 101 (K, C, M, Y) to form a visible image (toner image). , C, M, Y) and a primary transfer roller 45 as transfer means for sequentially transferring the toner image developed by the developing device 44 (K, C, M, Y) to the intermediate transfer belt 50 as a primary transfer target. (K, C, M, Y) and a cleaning device 46 (K, C, Y) as a cleaning unit for removing the toner remaining on the surface of the photoreceptor 41 (K, C, M, Y) after being transferred. M, Y).

  Here, in each organic EL element array exposure head 101 (K, C, M, Y), the array direction of the organic EL element array exposure head 101 (K, C, M, Y) is the photosensitive drum 41 (K, C). , M, Y) along the bus. Then, the emission energy peak wavelength of each organic EL element array exposure head 101 (K, C, M, Y) and the sensitivity peak wavelength of the photoconductor 41 (K, C, M, Y) are set so as to substantially match. Has been.

  The developing device 44 (K, C, M, Y) uses, for example, a non-magnetic one-component toner as a developer, and the one-component developer is conveyed to the developing roller by a supply roller, for example, and adheres to the surface of the developing roller. The film thickness of the developed developer is regulated by a regulating blade, and the developing roller is brought into contact with or increased in thickness by the photosensitive body 41 (K, C, M, Y), whereby the photosensitive body 41 (K, C, M, Y). The toner is developed as a toner image by attaching a developer according to the potential level.

  The black, cyan, magenta, and yellow toner images formed by the four-color single-color toner image forming station are intermediated by the primary transfer bias applied to the primary transfer roller 45 (K, C, M, Y). The toner image, which is sequentially primary transferred onto the transfer belt 50 and sequentially superposed on the intermediate transfer belt 50 to become a full color, is secondarily transferred to a recording medium P such as paper by a secondary transfer roller 66, and serves as a fixing unit. The toner is fixed on the recording medium P by passing through the fixing roller pair 61, and is discharged onto a paper discharge tray 68 formed in the upper part of the apparatus by a paper discharge roller pair 62.

  In FIG. 14, reference numeral 63 denotes a paper feed cassette in which a large number of recording media P are stacked and held, 64 denotes a pickup roller for feeding the recording media P one by one from the paper feed cassette 63, and 65x denotes a secondary transfer roller. 66, a pair of gate rollers for defining the supply timing of the recording medium P to the secondary transfer portion 66; a secondary transfer roller 66 as a secondary transfer means for forming a secondary transfer portion with the intermediate transfer belt 50; Is a cleaning blade as a cleaning means for removing the toner remaining on the surface of the intermediate transfer belt 50 after the secondary transfer.

  FIG. 15 is a schematic perspective view showing the image writing unit 101 in an enlarged manner. In FIG. 15, the organic EL element array 81 is held in a long housing 80. The positioning pins 89 provided at both ends of the long housing 80 are fitted into the opposing positioning holes of the case, and fixing screws are screwed into the screw holes of the case through the screw insertion holes 88 provided at both ends of the long housing 80. By fixing, each image writing means 101 is fixed at a predetermined position.

  The image writing unit 101 mounts a light emitting element (organic EL element) 83 of an organic EL element array 81 on a glass substrate 82, and is driven by a drive circuit 81 formed on the same glass substrate 82. A gradient index rod lens array (SLA) 65 constitutes an imaging optical system, and has a gradient index rod lens 84 arranged in front of the light emitting element 83. As the rod lens array 85, the “selfoc lens array” (abbreviated as SLA, trade name of Nippon Sheet Glass Co., Ltd.) as described above is frequently used.

  The light beam emitted from the organic EL element array 81 is formed on the surface to be scanned by the SLA 65 as an equal-size erect image. Thus, since the organic EL elements 83 are arranged on the glass substrate 82, the image carrier can be irradiated without impairing the light amount of the light emitting elements. Further, since the organic EL element can be controlled statically, the control system of the line head can be simplified. In the present invention, the light quantity correction can be realized by a simple means in the tandem type image forming apparatus as shown in FIGS.

  FIG. 16 is a longitudinal side view of a different image forming apparatus. In FIG. 16, the image forming apparatus 160 includes, as main constituent members, a rotary developing device 161, a photosensitive drum 165 functioning as an image carrier, and an image writing means (line head) 167 provided with an organic EL array. In addition, an intermediate transfer belt 169, a paper conveyance path 174, a fixing roller heating roller 172, and a paper feed tray 178 are provided.

  In the developing device 161, the developing rotary 161a rotates in the arrow A direction about the shaft 161b. The inside of the development rotary 161a is divided into four, and image forming units for four colors of yellow (Y), cyan (C), magenta (M), and black (K) are provided. Reference numerals 162a to 162d are arranged in the image forming units for the four colors. The developing rollers rotate in the arrow B direction, and the toner supply rollers 163a to 163d rotate in the arrow C direction. Reference numerals 164a to 164d are regulating blades that regulate the toner to a predetermined thickness.

  As described above, reference numeral 165 denotes a photosensitive drum that functions as an image carrier, 166 denotes a primary transfer member, 168 denotes a charger, and 167 denotes an image writing unit, which is provided with an organic EL array. The photosensitive drum 165 is driven in the direction of arrow D opposite to the developing roller 162a by a drive motor (not shown), for example, a step motor. The intermediate transfer belt 169 is stretched between the driven roller 170b and the drive roller 170a, and the drive roller 170a is connected to the drive motor of the photosensitive drum 165 to transmit power to the intermediate transfer belt. By driving the drive motor, the drive roller 170 a of the intermediate transfer belt 169 is rotated in the arrow E direction opposite to the photosensitive drum 165.

  The paper conveyance path 174 is provided with a plurality of conveyance rollers, a pair of paper discharge rollers 176, and the like, and conveys the paper. An image (toner image) on one side carried on the intermediate transfer belt 169 is transferred to one side of the paper at the position of the secondary transfer roller 171. The secondary transfer roller 171 is separated from and brought into contact with the intermediate transfer belt 169 by a clutch, and is brought into contact with the intermediate transfer belt 169 when the clutch is turned on, so that an image is transferred onto the sheet.

  The sheet on which the image has been transferred as described above is then subjected to a fixing process by a fixing device having a fixing heater H. The fixing device is provided with a heating roller 172 and a pressure roller 173. The sheet after the fixing process is drawn into the discharge roller pair 176 and proceeds in the arrow F direction. When the paper discharge roller pair 176 rotates in the opposite direction from this state, the paper reverses its direction and advances in the double-sided printing conveyance path 175 in the arrow G direction. 177 is an electrical component box, 178 is a paper feed tray for storing paper, and 179 is a pickup roller provided at the outlet of the paper feed tray 178. For example, a low-speed brushless motor is used as a drive motor for driving the transport roller in the paper transport path. The intermediate transfer belt 169 uses a step motor because it requires color misregistration correction. Each of these motors is controlled by a signal from a control means (not shown).

  In the state shown in the drawing, a yellow (Y) electrostatic latent image is formed on the photosensitive drum 165, and a high voltage is applied to the developing roller 62a, whereby a yellow image is formed on the photosensitive drum 165. When all of the yellow back side and front side images are carried on the intermediate transfer belt 169, the development rotary 161a rotates 90 degrees in the direction of arrow A. The intermediate transfer belt 169 rotates once and returns to the position of the photosensitive drum 165. Next, two images of cyan (C) are formed on the photosensitive drum 165, and this image is carried on the yellow image carried on the intermediate transfer belt 169. Thereafter, the 90-degree rotation of the development rotary 161 and the one-rotation process after the image is carried on the intermediate transfer belt 169 are repeated in the same manner.

  For carrying four color images, the intermediate transfer belt 169 rotates four times, and then the rotation position is further controlled to transfer the image onto the sheet at the position of the secondary transfer roller 171. The sheet fed from the sheet feed tray 178 is conveyed by the conveyance path 174, and the color image is transferred to one side of the sheet at the position of the secondary transfer roller 171. The sheet on which the image is transferred on one side is reversed by the discharge roller pair 176 as described above, and stands by on the conveyance path. Thereafter, the sheet is conveyed to the position of the secondary transfer roller 171 at an appropriate timing, and the color image is transferred to the other side. The housing 180 is provided with an exhaust fan 181. In the present invention, in the rotary type image forming apparatus as shown in FIG. 16, the light amount correction can be realized by a simple means.

  The line head of the present invention and the image forming apparatus using the same have been described based on the embodiments. However, the present invention is not limited to these embodiments, and various modifications can be made.

It is explanatory drawing which shows embodiment of this invention. It is explanatory drawing which shows embodiment of this invention. It is explanatory drawing which shows embodiment of this invention. It is explanatory drawing which shows embodiment of this invention. It is explanatory drawing which shows embodiment of this invention. It is explanatory drawing which shows embodiment of this invention. It is a characteristic view of the line head concerning this invention. It is a characteristic view of the line head concerning this invention. It is explanatory drawing of the line head concerning this invention. It is explanatory drawing of the line head concerning this invention. It is explanatory drawing of the line head concerning this invention. It is a characteristic view of the line head concerning this invention. It is a block diagram which shows embodiment of this invention. 1 is a longitudinal side view of an image forming apparatus showing an embodiment of the present invention. It is a perspective view of the line head concerning the present invention. It is a vertical side view of an image forming apparatus showing another embodiment of the present invention.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 6 ... Image forming unit, 9 ... Transfer belt unit, 16 ... Intermediate transfer belt, 17 ... Cleaning means, 20 ... Image carrier, 21 ... Primary transfer member, 22 ... Charging means, 23 ... Image book Incorporating means (line head), 24 ... developing means, 33 ... developing roller, 60 ... housing (holder), 62 ... substrate, 63 ... light-emitting element for image formation, 65 ... gradient index rod lens array (SLA), 74: Line head control means, 84: Refractive index distribution type rod lens, 90 ... Exposure pixel, 91 ... Spot diameter, 93 ... Output image, Ea to Ec ... Distribution of electrostatic latent image

Claims (11)

In a line head in which a plurality of light sources are arranged in a line, and each light source is modulated in accordance with a predetermined gradation value with respect to image data to perform image gradation exposure.
The diameter of the imaging spot formed by imaging each light source on the exposure surface is set larger than the pixel pitch, and the gradation value of the exposure pixel of the light source that is turned on according to image data is increased or decreased, A line head, wherein light energy exposed from each of the light sources is corrected to be uniform. The line head according to claim 1, wherein the increase / decrease of the gradation value is selectively performed with respect to a plurality of exposure pixels arranged in the sub-scanning direction with respect to the correction target pixel. The line head according to claim 1, wherein the increase / decrease of the gradation value is selectively performed with respect to a plurality of exposure pixels arranged in the main scanning direction with respect to the correction target pixel. The line head according to claim 1, wherein the increase / decrease of the gradation value is performed on an exposure pixel within a certain distance in the vicinity of the correction target pixel. 5. The line head according to claim 1, wherein the light amount correction by the plurality of light sources is processing of a line screen that expresses a gradation with a line width. 6. The line head according to claim 1, wherein a pixel density for forming the output image is different in a main scanning direction and a sub-scanning direction. The line head according to any one of claims 1 to 6, wherein the light source comprises an organic EL element formed on a single glass substrate. The line head according to claim 7, wherein the light source and a thin film transistor circuit for driving the light source are formed on the glass substrate. At least two image forming stations in which image forming units including a charging unit, a line head according to any one of claims 1 to 8, a developing unit, and a transfer unit are arranged around an image carrier. An image forming apparatus provided as described above, wherein a transfer medium passes through each station and forms an image by a tandem method. An image carrier configured to carry an electrostatic latent image, a rotary developing unit, and the line head according to claim 1, wherein the rotary developing unit includes a plurality of toner cartridges. The toner stored in the toner is carried on the surface, and toners of different colors are sequentially conveyed to a position facing the image carrier by rotating in a predetermined rotation direction, and the image carrier, the rotary developing unit, An image forming method characterized in that a developing bias is applied between the toner and the toner is moved from the rotary developing unit to the image carrier to visualize the electrostatic latent image to form a toner image. apparatus. The image forming apparatus according to claim 9, further comprising an intermediate transfer member.

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