JP5082360B2 - Line head and image forming apparatus using the same - Google Patents

Description

  The present invention relates to a line head that can easily realize gradation expression and an image forming apparatus using the same.

  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 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 the line head having such a configuration, the exposure energy of each pixel is changed stepwise in order to improve the gradation of the image to be formed. In order to change the exposure energy, a method of changing the lighting time, that is, pulse width modulation (PWM) or intensity modulation (current modulation) for changing the exposure power is often used.

As an example of gradation control, for example, by arranging two pixel in the sub-scanning direction, on the photoreceptor by exposing while changing the timing of the image formed by overlapping pixel, there is performed a multiple exposure. In this example, gradation is expressed by combining lighting of overlapping pixels.

Although not an example of gradation control, Patent Document 1 describes an example in which one pixel (output image) is formed using a plurality of sub-pixels. In Patent Document 1 , 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 1 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. Further, Patent Documents 2 to 4 are examples in which a laser beam is used as a light source, but there is a description about setting a spot size with respect to a pixel pitch.

JP 2002-292922 A JP 2002-251023 A JP 60-154268 A Japanese Patent Laid-Open No. 03-004244

  However, the PWM or current modulation circuit for gradation control is required for each pixel, so that there is a problem that the drive circuit of each pixel becomes complicated and large. Especially in recent years, such line heads are often used for color electrophotographic page printers. However, color images require higher expressiveness and reproducibility of photographs and graphics than monochrome images. There is a need for more advanced gradation control. The gradation control as described above is performed digitally. However, since the gradation value also requires a larger amount of information, that is, the number of bits in order to perform the gradation control, the scale of the gradation control circuit is further increased. Since it tends to increase, there is a problem that the cost increases.

  Also, in order to improve the gradation of the image to be formed, the spot diameter (spot size when the light beam output from the light source forms an image on the surface of the image carrier through the lens array) is reduced according to the pixel density. It is difficult to do. Even if the spot diameter can be reduced, the variation in each pixel such as the spot size may increase due to the difference in the optical characteristics of each pixel such as the focus of the lens array, and the uniformity of the image may be impaired. There was a problem.

In an image forming apparatus in which two pixels are arranged in the above-described sub-scanning direction and exposure is performed at different timings so that the pixels are superimposed on each other to form an image, the two light emitting units emit light. Since the output light is superimposed at exactly the same position, there is a problem that the resolution and resolution are not improved even if the number of pixels is increased. Further, FIG. 8 of Patent Document 1 shows an example of light sources arranged in three rows in a zigzag pattern, but nine “light emitting portion groups” are formed, and the light is exposed as it is. Projected on the body. For this reason, there is a problem that gradation control is not performed. In addition, since the thing of patent document 2 -patent document 4 is all aimed at making the resolution high, when the pixel pitch is small, the spot size must be reduced accordingly. There is a problem that the control becomes complicated.

  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 capable of easily realizing gradation expression and an image forming apparatus using the same. is there.

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 to form a light emitting element line, and each light source is turned on or off according to image data, The light beam from the light source forms an imaging spot on the surface to be exposed through the lens array, and the image spot formed by the light beams from the plurality of light sources forming an image on the surface to be exposed is the main scanning direction or sub-scanning direction. In a line head that forms an image by overlaying with a slight shift in the scanning direction,
Among the images, the gradation image has a screen structure expressed by the area of lines or dots having a constant pitch, and the diameter of the imaging spot formed on the exposed surface is larger than the pitch of the pixels, and It is set to be smaller than the pitch of the lines or dots constituting the screen, and the gradation of the image is expressed by a combination of binary states of turning on and off each light source. With such a configuration, it is possible to perform good gradation control only by binary control of on / off without arranging a gradation control circuit for each pixel by arranging a large number of light sources at a fine density. Is possible.

  The line head of the present invention is characterized in that 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. With such a configuration, it is possible to easily superimpose the imaging spots in the main scanning direction.

  In the line head of the present invention, the lens array is a gradient index rod lens array in which rod lenses are arranged in a plurality of rows in the sub-scanning direction. With such a configuration, the imaging spot can be accurately formed on the exposed surface.

  In the line head of the present invention, the distance between the two most separated lines in the sub-scanning direction of the plurality of light-emitting element lines is the distance between the two rod lenses most separated in the sub-scanning direction of the plurality of rows of rod lens arrays. It is smaller than the center interval. With such a configuration, good imaging characteristics can be obtained.

  The line head according to the present invention is characterized in that the gradation expression of the image by the plurality of light sources is a gradation screen process for expressing gradation by a line width. Since such a configuration is used, gradation expression can be simplified even in gradation screen processing.

  In the line head of the invention, the light source is composed of an organic EL element. According to such a configuration, it is not necessary to reduce the diameter of the light emitting unit, so that the optical power of the light emitting unit can be increased. For this reason, it is possible to use an organic EL material having a low luminous efficiency.

  In the line head of the present invention, the light source is formed on a single glass substrate. With such a configuration, 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, gradation expression 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, gradation expression can be easily performed in a rotary image forming apparatus.

  In addition, the image forming apparatus of the present invention includes an intermediate transfer member. Therefore, gradation expression can be easily performed in an image forming apparatus including an intermediate transfer member.

  As described above, the line head of the present invention and the image forming apparatus using the same can achieve the following effects. (1) By arranging a large number of light sources at a fine density with respect to the resolution of an image to be formed and the diameter of an imaging spot where a light beam from a light source is formed on an exposed surface through a lens array, Therefore, it is possible to perform good gradation control without providing a gradation control circuit. That is, despite the binary dot lighting control, intensity-modulated gradation expression is realized by largely overlapping adjacent exposure pixels. (2) Although the pixel density is high resolution, the imaging spot on the photoconductor is not so different from that of the conventional apparatus, so that the accuracy required for the optical system is eased and the manufacture becomes easy. The optical depth of focus also increases. (3) In the present invention, sufficient gradation can be obtained by making the diameter of the imaging spot formed by imaging the light flux from the light emitting portion smaller than the pitch of screen lines or dots expressing gradation. The characteristics can be obtained. (4) As described above, the present invention makes it difficult to configure an image forming system in order to achieve high resolution by arranging binary control exposure pixels at a high density compared to the spot diameter. In addition, the gradation and smoothness of the contour can be obtained with simple control.

  A line head used in a normal page printer has pixels formed at a density of 600 dpi or 1200 dpi. In the embodiment of the present invention, the conventional technique performs gradation control by providing a gradation control circuit for each pixel, but excellent gradation by arranging a large number of light sources at a fine density. The control is realized. As an example, the density of the number of pixels is 2400 dpi or 4800 dpi.

FIG. 2 is an explanatory view schematically showing the basic technique of the present invention. In the example of FIG. 2, the density of the number of pixels is set to 2400 dpi. In FIG. 2A, reference numeral 90 denotes a light source pixel, and reference numeral 91 denotes a spot diameter when a light beam formed by a light beam output from the light source forms an image 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 a 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 gives an intensity of 1 / e ² 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. 2B, a large number of exposure pixels are superimposed and exposed on the image carrier in the main scanning direction and in the sub scanning. In this example, the output image 93 is formed by superimposing 4 × 4 = 16 pixels, four in the main scanning direction and four 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. In the example described above, since one conventional 600 dpi pixel is formed by 16 exposure pixels of 2400 dpi, the energy of one exposure pixel can be reduced to 1/16 of the conventional one. Therefore, as will be described later, the present invention is particularly effective for a line head using an organic EL as a light source for which it is difficult to secure a light amount per unit area. This is because the area of the light emitting portion can be increased as described above, and a synergistic effect can be achieved due to the fact that less energy is required for the exposed pixels.

  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 example of FIG. 2B, the image density is expressed by 4 × 4 = 16 gradations. That is, 16 gray scales can be controlled by turning on (turning on) or turning off (off) each of the 16 light sources.

  Therefore, a complicated modulation control circuit configuration for gradation control is not required on the line head, and gradation control can be performed only by on / off control of the light emitting element. For this reason, it is only necessary to mount a switching element such as a TFT (light source driving thin film transistor) for performing on / off control of the light emitting element on the same glass substrate as the light source, and the configuration of the control unit mounted on the line head is simplified. Can be.

  FIG. 1 is an explanatory diagram illustrating an example of a pixel array in the embodiment of the present invention. FIG. 1A 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. 1B 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 between the spot diameter and 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. As described above, in FIGS. 1A and 1B, 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 number of pixels that are lit by the exposure pixels. That is, individual exposure pixels are controlled by binary values. For example, since 16 pixels correspond to 1400 pixels of 600 dpi at 2400 dpi, 16 gradations correspond to 64 pixels at 4800 dpi, so that there is 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, in the embodiment of the present invention, it will be described that the gradation control when the spot diameter is set larger than the pixel pitch is superior to the conventional example. FIG. 3 is an explanatory diagram showing changes in the surface potential distribution when the number of exposed exposure pixels is increased in the conventional example. In this example, the exposure pixel density is 2400 dpi and the spot diameter is 20 μm or less. In FIG. 3A, the potential distribution Ea when the single exposure pixel 90a is turned on is formed in a substantially circular shape. Hereinafter, the black circles in the figure indicate the center positions of the exposure pixels.

  In FIG. 3B, another exposure pixel 90b is provided in the sub-scanning direction adjacent to the exposure pixel 90a described in FIG. 3A, and the potential when the exposure pixel 90a and the exposure pixel 90b are turned on simultaneously. Distribution Eb is shown. In this case, the potential distribution Eb has an elliptical shape with long sides formed in the sub-scanning direction.

  3C, in the configuration of FIG. 3B, an exposure pixel 90c is provided adjacent to the exposure pixel 90a in the main scanning direction, and the three exposure pixels of the exposure pixel 90a, the exposure pixel 90b, and the exposure pixel 90c are provided. Is a potential distribution Ec when the are simultaneously turned on. In this case, the potential distribution Ec has a substantially triangular shape.

  3D, in the configuration of FIG. 3C, another exposure pixel 90d is provided adjacent to the exposure pixel 90b in the main scanning direction, and the exposure pixel 90a, the exposure pixel 90b, the exposure pixel 90c, and the exposure pixel are exposed. A potential distribution Ed when the pixel 90d is simultaneously turned on is shown. In this case, the potential distribution Ed is formed in a shape surrounding each of the exposure pixels 90a to 90d arranged in a rectangular shape.

  As described above, in the configuration of the conventional example shown in FIG. 3, although the potential distribution is sharp, the distribution shape changes according to the gradation, so the density change is not proportional to the number of pixels, and the gradation control is not performed. It becomes difficult.

  FIG. 4 is an explanatory diagram showing changes in the surface potential distribution when the number of exposure pixels to be lit 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. 5 and FIG. 6 are characteristic diagrams showing condition setting which is a premise of the configuration of FIG. FIG. 5 is a characteristic diagram showing the power distribution due to the spot of the light source imaged on the photoconductor (image carrier).

When the spot imaged on the photosensitive member has a power (intensity) distribution as shown in FIG. 5, if the peak height is 1, and e is a natural logarithm, 1 / e ² = 1. /(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. 6 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. 6, 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. 6, this saturation energy is 0.3 μJ / cm ² .

In FIG. 4, as explained in FIGS. 5 and 6, 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. 4A, the potential distribution Ew when the single exposure pixel 90w is turned on is formed in a substantially circular shape. Also in FIG. 4, the black circle in the figure indicates the center position of the exposure pixel.

  In FIG. 4B, another exposure pixel 90x is provided in the sub-scanning direction adjacent to the exposure pixel 90w described in FIG. 4A, 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.

  4C, in the configuration of FIG. 4B, an exposure pixel 90y is provided adjacent to the exposure pixel 90w in the main scanning direction, and 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.

  4D, in the configuration of FIG. 4C, 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.

  As described with reference to FIGS. 3 and 4, the reason why the shape of the potential distribution is different between the conventional example and the embodiment of the present invention 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.

  Next, with respect to a gradation screen that expresses gradation by a line width, the prior art and the embodiment of the present invention are compared. FIG. 7 is an explanatory diagram of a conventional example. In this example, the spot diameter is 20 μm. 90e to 90i are exposure pixels, and Ef is a potential distribution. In this case, the line widths La and Lb fluctuate and meander, and the density change is large.

  FIG. 8 is an explanatory diagram according to the embodiment of the present invention. In the example of FIG. 8, the spot diameter is 60 μm, and the same conditions are set as in the example described in FIG. FIG. 8A 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. 8B 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. 8C, similarly to FIG. 7, three exposure pixels 90r, 90s, 90t are juxtaposed in the main scanning direction, and two exposure pixels 90u, 90v is arranged. 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. Thus, in the embodiment of the present invention, even in a gradation screen that expresses gradation by the line width, useful characteristics can be obtained as compared with the conventional example.

  9 and 10 are explanatory diagrams showing an embodiment of the present invention using a gradation screen. FIG. 9 is an example of a screen that expresses a density gradation by the thickness of an oblique line at an image resolution of 600 dpi, for example. In FIG. 9, one size 90 of the mesh indicates one pixel of 600 dpi, which is 42.3 μm. 91 is a spot diameter, and Lx and Ly are screens. The line pitch P between adjacent screens Lx and Ly is 3.13 pixels, that is, about 133 μm. The value of the line pitch P of the screen is 192 LPI when expressed in terms of the number of printed lines (LPI: 1 inch = 2 screen lines per 25.4 mm). In ordinary commercial printing, a screen line number of about 175 LPI is used. In the example of FIG. 9, since exposure with a line head is assumed, the position of the pixel can be controlled in the sub-scanning direction.

  The process of expressing the gradation on the original gradation image on such a gradation screen is performed by the printer controller of FIG. Converting an original image into screen data of area gradation is binarization, and various methods have been proposed. In the present invention, since the binarization method is not related to the essence, the binarization method will not be described in detail. For example, the binarization method is described in “Photographic industry separate volume / Imaging part 1 / Photographic industry publisher / 1988”.

  When the screen line pitch P is narrowed, the resolution is improved, but the interval between adjacent lines is narrowed, so that they interfere with each other. If the level of the image forming process is not high, the degree of interference is different due to the influence of the nonuniformity of the process system, and the nonuniformity of the image tends to occur. Thus, since it is difficult to reduce the pitch P of the gradation screen, the size of the spot of the light beam that exposes the gradation screen may be a diameter that can express the pitch of the gradation screen.

  FIG. 10 shows an example in which exposure pixels are superposed at a pitch of 2400 dpi with a spot diameter 91 having the same size as that in FIG. In contrast to FIG. 9, although the outline of the diagonal line is smooth in FIG. 10, the width of the diagonal line is widened. As described above, the embodiment of the present invention has a characteristic that the width of the latent image in the exposed portion is widened because the exposure pixels are overlapped at a minute pitch without reducing the spot size 91. This will be described in detail with reference to the characteristic diagrams of FIGS.

  FIG. 11 shows a potential distribution diagram (potential contrast characteristic) when the spot size is 40 μm and the cross section is perpendicular to the line. In this figure, the photosensitive member and its exposure conditions are the same as those shown in FIGS. A line (A) in FIG. 11 shows a case where exposure is performed once without overlapping with a spot having a diameter of 40 μm with a pixel having a pitch of 600 dpi according to the prior art. In this figure, the portion exposed to the initial charged potential of −600 V rises to about −60 V, while the non-exposed portion rises only to −590 V. On the other hand, in the present invention, four exposure pixels arranged at a pixel pitch of 2400 dpi are overlapped in the width direction of the line, so that the width of the exposed portion is increased as shown by the line (B) in FIG. As a result, the potential of the non-exposed portion increases to -380V. That is, the contrast between the white part and the black part is insufficient.

  Also, under such conditions, it means that the tail portions of the potential distributions of adjacent lines interfere with each other. For this reason, the degree of interference varies depending on a slight difference in the light amount distribution of the imaging spot, and density unevenness is likely to occur. Therefore, when a plurality of exposure pixels are shifted and overlapped as in the present invention, it is necessary to increase the screen pitch by the shift. In this example, if the line pitch is increased to about 280 LPI as shown in FIG. 11C, a potential difference equivalent to that in the conventional case where there is no overlap at a pixel pitch of 600 dpi can be secured.

  Similarly, FIG. 12 shows potential contrast characteristics when the spot size is 60 μm, and FIG. 13 shows potential contrast characteristics when the spot size is 80 μm. When the spot size and the reproducible screen line pitch in these cases are summarized, the relationship between the spot size and the line pitch as shown in FIG. 14 is obtained. In the above example, 4 pixels are overlapped in the width direction of the line at an exposure pixel pitch of 2400 dpi, but the number of exposure pixels for this overlap varies depending on the gradation value to be expressed.

  Thus, if the spot size is at least small with respect to the pitch of the screen, the contrast of the image can be ensured even when the lines constituting the screen are sufficiently exposed. In other words, in the embodiment of the present invention, even with a relatively high-density exposure pixel pitch of 2400 dpi, sufficient gradation expression can be obtained if the spot size is smaller than the pitch of the line image constituting the gradation screen. It was shown to be possible. For this reason, it is not necessary to make the spot size smaller than necessary, and the demand for an optical system for imaging each pixel can be alleviated.

  In the above embodiment, in contrast to the case of forming pixels at a pixel pitch of 600 dpi, in the 2400 dpi image formation to which the present invention is applied, four exposure pixels are arranged. For this reason, the exposure light quantity distribution or the latent image spreads by three of the 10.4 μm pitch of 2400 dpi, that is, 31.8 μm. In the example of FIG. 12, it can be seen that the latent image spreads to almost this value. On the other hand, in the case of FIG. 14 where the spot size is originally large, the spread of the image is not so large.

  For example, in order to express a 192 LPI (133 μm pitch) line as shown in FIG. 12, it is only necessary to realize a spot size of 80 μm. The above description has been made from the viewpoint of the reproducibility of the gradation screen. When the reproducibility of fine lines is more important than the gradation, the spot size is appropriately reduced within the scope of the present invention. You only have to set it.

  In the description of FIGS. 12 to 14, the case where the exposure time of one pixel when there is no conventional superposition is sufficiently shorter than the movement time of the photosensitive member for one pixel is described. When the exposure time of one pixel is equal to the one-pixel movement time of the photosensitive member, that is, when the entire pixel period is lit, in the sub-scanning direction, the overlap exposure in the embodiment of the present invention shown in FIGS. Similar potential distribution is obtained. In the above embodiment, the case of using a line screen that expresses the density gradation by the thickness of the diagonal line has been described, but even when using a dot screen that expresses the density gradation by the area of the halftone dots. The same can be said. In the case of a dot screen, a spot size having a diameter smaller than the minimum dot pitch may be set.

  FIG. 16 is a block diagram illustrating a schematic configuration of a control unit according to the embodiment of the present invention. In FIG. 16, 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 performs gradation control 2 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. Create value data.

In the embodiment of the present invention, a SELFOC lens array (abbreviated as “SLA”, a trademark of Nippon Sheet Glass Co., Ltd.) is used for the imaging optical system as a gradient index rod lens array. Thus, by using SLA for the optical system, the imaging spot can be formed on the exposed surface with high accuracy.
17 and 18 are explanatory diagrams showing an example using such an SLA. Note that the arrangement of the light source portions in FIGS. 17 and 18 corresponds to FIGS. 1A and 1B described above. In FIG. 17, the rod lens array 65 has rod lenses 65a to 65d arranged in a staggered pattern in two rows in the sub-scanning direction. Reference numerals 128a to 128c denote light emitting element lines in which a plurality of light emitting elements (exposure pixels) are arranged in each line.

  In this example, light emitting element lines 128a to 128c made of light emitting elements of the same size are arranged at positions symmetrical to the center line (center axis) CL of the rod lens array 65. That is, the light emitting element lines 128a and 128c are arranged at symmetrical positions with respect to the central axis. As described above, in the example of FIG. 10, the light emitting element lines 128 a to 128 c are arranged in three rows in parallel in the sub-scanning direction.

  Further, the distances between the light emitting element lines 128a and 128b and between the light emitting element lines 128b and 128c are equally arranged. For this reason, when performing multiple exposure of pixels using each light emitting element line, the timing for moving the image carrier and the timing for emitting light by switching from the previously emitted light emitting element line to the next light emitting element line are all Since the same timing can be used for the light emitting element lines, the control can be easily performed. In the example of FIG. 17, among the plurality of light emitting element lines 128a to 128c, the distance between two lines (128a and 128c) that are most separated in the sub-scanning direction is the rod lens in the sub-scanning direction of a plurality of rows of rod lens arrays. It is smaller than the center interval. With such a configuration, the plurality of light emitting element lines are arranged within the range of the rod lens array in the sub-scanning direction, and good imaging characteristics can be obtained.

Next, the optical system used in the present invention will be described. In the present invention, as described later, it is suitable to use an organic EL material for the light emitting portion. Since the light emitting portion made of the organic EL material is formed by coating, a circular light emitting portion is preferable in order to prevent uneven coating within the light emitting portion. As described above, the SLA can be used in the imaging optical system of the line head of the present invention. FIG. 15 is a characteristic diagram showing the relationship between the diameter of the light emitting part and the imaging spot diameter according to the product number SLA-20D of Nippon Sheet Glass Co., Ltd. The SLA is an equal-magnification optical system, but in FIG. 15, the spot size is shown as a diameter of 1 / e ² , so it is larger than the diameter of the light emitting part. It has been found that if the spot size is defined as half the peak value of the light amount distribution, it is almost equal to the diameter of the light emitting portion.

  Therefore, the diameter of the light emitting portion necessary for realizing the spot size already shown can be obtained from FIG. For example, in order to obtain a spot size having a diameter of 60 μm or less, it is understood that the diameter of the light emitting portion may be set to 35 μm or less. Therefore, for example, the light emitting units cannot be arranged in a row at 2400 dpi, that is, a 10.6 μm pitch. Therefore, the light emitting units are arranged in multiple rows as shown in FIGS. 1, 17, and 18. Note that the relationship shown in FIG. 15 shows the smallest image formation by the SLA, that is, the most focused state. In actuality, the size of the light emitting unit is further increased by considering further defocusing. It is desirable to make it smaller. In the description of FIG. 1A or FIG. 1B, in consideration of this, the spot size is set smaller than that of FIG.

  FIG. 18 is an explanatory diagram according to another embodiment of the present invention. In this example, five rows of light emitting element lines 128d to 128h are arranged. In the example of FIG. 18, among the plurality of light emitting element lines 128d to 128h, the distance between the two lines (128d and 128h) that are most separated in the sub-scanning direction is the center distance in the sub-scanning direction of the two rows of rod lens arrays. It has become smaller. As shown in FIG. 18, in the present invention, the light emitting element lines are arranged at positions symmetrical with respect to the central axis of the rod lens array. The light emitting elements can be arranged in a staggered manner in addition to the two-dimensional parallel arrangement. In either case, the light emitting element line can be arranged on the central axis of the rod lens array. Further, the light emitting element lines can be arranged at different distances in addition to being arranged at equal distances.

  With the configuration in which two rows of SLA are arranged in the sub-scanning direction as shown in FIG. 17, good imaging characteristics can be obtained near the center of the two rows of SLA. In addition, the light emitting element lines are arranged in three or more columns at positions within the range of the SLA in the sub-scanning direction. In this case, the width (range in the sub-scanning direction) of the light emitting element lines of three or more columns is set to 100 μm or less.

  By the way, it is difficult to reproduce an image having the same size as that of the light source on the imaging plane, although the SLA is an equal-magnification optical system due to an aberrational problem. For example, even if the diameter of the light emitting portion is 20 μm, the spot size is only about 60 μm as described above. Further, even if a small imaging spot is obtained, the “layer” of the electrostatic latent image accompanying the movement of the charge occurs in the two-layer photoconductor. However, since the diameter of the light emitting parts is still much larger than the pitch of the light emitting parts, it is difficult to arrange them in one row. It must be arranged in a zigzag pattern in multiple rows above the row.

  In this type of image forming apparatus, the particle size of the toner to be developed cannot be so small. Depending on the development method, toner scattering occurs even at the stage where the toner adheres to the image carrier. Other factors such as scattering at the time of transfer and toner deformation at the time of fixing contribute only to the direction of lowering the resolution of the image. For this reason, finding a minute image spot unnecessarily makes the management of the focus of the optical system strict and is easily affected by errors in the optical system, and has little substantial merit. Therefore, in the embodiment of the present invention, gradation is expressed not by reducing the spot diameter but by reducing the exposure pixel density. For example, the diameter of the imaging spot formed by forming each light source (exposure pixel) on the exposure surface by using finer 2400 dpi or 4800 dpi compared to the conventional 600 dpi and 1200 dpi. However, it is formed so as to be larger than the pitch of the exposed pixels to express gradation.

  Here, 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, it is assumed that pixels are arranged at 1200 dpi in the main scanning direction and 4800 dpi are arranged in the sub-scanning direction. In this case, since 2 × 8 = 16 exposure pixels correspond to a 600 dpi pixel, a sufficient gradation of 16 gradations can be obtained.

  In the embodiment of the present invention, the spot size is set larger than the pixel pitch. For this reason, it is difficult to obtain image resolution corresponding to the pixel pitch. However, since the exposure pixel positioning resolution is high, the contour of the image can be smoothed.

  In the case where an organic EL element is used, in the present invention, it is not necessary to reduce the diameter of the light emitting portion, so that the optical power of the light emitting portion 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 such a case, since it is only necessary to control each pixel with binary values of on and off as described above, the circuit configuration can be extremely simplified.

  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).

  The “exposure pixel” of the present invention can also form an image for the first time by multiple exposure, but is an independent pixel driven by individual modulation information. Also in the present invention, the light emitting element lines are formed in a plurality of columns in the sub-scanning direction. By changing the lighting timing according to the position difference in the sub-scanning direction and the speed of the photoconductor, Then, control is performed so that the formed latent images are arranged in a line. That is, since it functions as a high-resolution pixel that is binary, the resolution of the image position and the smoothness of the contour are dramatically increased as compared with the conventional case.

  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. 19 is a longitudinal 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. 19, this image forming apparatus is provided with a drive roller 51x, a driven roller 52, and a tension roller 53. ) 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. 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. 19, 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 from the paper feed cassette 63 one by one, 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. 20 is a schematic perspective view showing the image writing unit 101 in an enlarged manner. In FIG. 20, 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 means 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 85 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 65 , the above-mentioned “Selfoc (registered trademark) lens array” (abbreviation SLA, trade name of Nippon Sheet Glass Co., Ltd.) 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, gradation expression can be realized by simple means in the tandem image forming apparatus as shown in FIGS.

  FIG. 21 is a longitudinal side view of a different image forming apparatus. In FIG. 21, 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, gradation expression can be realized by a simple means in the rotary type image forming apparatus as shown in FIG.

  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 a prior art example. 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 characteristic view of the line head concerning this invention. It is explanatory drawing which shows a prior art example. 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 explanatory drawing of the line head concerning 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 a characteristic view 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. It is explanatory drawing of the line head concerning this invention. It is explanatory drawing of the line head concerning 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, 94, 128a to 128d ... Light emitting element line, Lx, Ly ... Screen

Claims (11)

A plurality of light sources are arranged in a line in the main scanning direction to form a light emitting element line. Each light source is turned on or off in accordance with image data, and a light beam from the light source passes through the lens array to the exposed surface. An image is formed by forming an imaging spot and superimposing the imaging spots, which are created by imaging the light beams from the plurality of light sources on the exposed surface, slightly shifted in the main scanning direction or the sub-scanning direction. In the line head forming
Among the images, the gradation image has a screen structure expressed by the area of lines or dots having a constant pitch, and the diameter of the imaging spot formed on the exposed surface is larger than the pitch of the pixels, and A line head, wherein the line head is set to be smaller than a pitch of lines or dots constituting the screen, and represents a gradation of an image by a combination of binary states of turning on and off each light source. 2. The line head according to claim 1, wherein the light emitting element lines are arranged in a plurality of three or more lines in the sub-scanning direction, and are arranged so that the positions in the main scanning direction are different from each other. The line head according to claim 1, wherein the lens array is a gradient index rod lens array in which rod lenses are arranged in a plurality of rows in the sub-scanning direction. An interval between two lines that are most separated in the sub-scanning direction of the plurality of light emitting element lines is smaller than a center interval between two rod lenses that are most separated in the sub-scanning direction of the plurality of rows of rod lens arrays. The line head according to claim 3. The line head according to claim 1, wherein the gradation expression of the image by the plurality of light sources is a gradation screen process for expressing gradation by a line width. 6. The line head according to claim 1, wherein the light source is composed of an organic EL element. The line head according to claim 1, wherein the light source is 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 10, further comprising an intermediate transfer member.

Images