JP2007196524A - Image forming apparatus and its control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which suitably maintains a contrast in a high density region while suitably reducing a density unevenness in a low density region. <P>SOLUTION: The image forming apparatus comprises: an exposure unit for exposing an image carrier to a plurality of lights having different frequencies; the image carrier for carrying a latent image formed through exposure; and an exposure controller for controlling a light emitting ratio of the plurality of lights having different frequencies according to a density of an image to be formed. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus using an electrophotographic system.

  Reducing the beam spot diameter is an effective means for improving the image quality of an electrophotographic image forming apparatus. This is because the beam spot is reduced in diameter so that the recording density is improved and a higher-definition latent image can be formed. In addition, since the beam spot is reduced in diameter, the charge distribution of the latent image becomes sharper. This has the advantage that development can be stabilized even if noise such as unevenness of potential or mechanical vibration occurs in the photosensitive drum.

By the way, a method for forming an image using a plurality of lasers having different wavelengths has been proposed (Patent Documents 1 and 2). According to Patent Document 1, a method is proposed in which a latent image of a character image is formed by a short wavelength laser and a latent image of a multi-tone image is formed by a long wavelength laser. Further, according to Patent Document 2, a method of using a short wavelength laser only around a character is proposed.
JP-A-8-164634 JP-A-11-320960

  However, neither of Patent Documents 1 and 2 paid attention to the relationship between the photosensitive characteristics of the photosensitive drum and the wavelength of the laser beam. That is, if a member whose sensitivity decreases as the wavelength becomes shorter is used as the photosensitive layer of the photosensitive drum, there is a possibility that a necessary image density cannot be obtained. In this case, even if a short wavelength laser is used, the quality of the formed image is deteriorated.

  Therefore, an object of the present invention is to solve at least one of such problems and other problems. Other issues can be understood throughout the specification.

  The image forming apparatus of the present invention is realized in an image forming apparatus that forms an image by scanning an image carrier with a light beam subjected to light modulation output from a laser light source means. The laser light source means includes a plurality of laser light sources having different oscillation wavelengths. The exposure control unit controls the emission ratios of a plurality of laser light sources having different oscillation wavelengths according to the density of the image to be formed.

  According to the present invention, the emission ratios of a plurality of lights having different oscillation wavelengths are suitably controlled according to the density of an image to be formed. This makes it possible to favorably maintain the contrast in the high density region while suitably reducing density unevenness in the low density region.

  An embodiment of the present invention is shown below. Of course, the individual embodiments described below will be helpful in understanding various concepts such as the superordinate concept, intermediate concept and subordinate concept of the present invention. Further, the technical scope of the present invention is determined by the scope of the claims, and is not limited by the following individual embodiments.

[First Embodiment]
FIG. 1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. Here, portions of the image forming apparatus 100 that are less relevant to the present invention are omitted.

  The exposure apparatus 101 is an apparatus that exposes the uniformly charged surface of the photosensitive drum 102 using a plurality of lights having different oscillation wavelengths. The photosensitive drum 102 is a so-called image carrier and carries a latent image formed by exposure. The charger 103 charges the surface of the photosensitive drum 102 so as to have a desired potential uniformly. The developing device 104 develops the latent image formed on the surface of the photosensitive drum 102 with a developer such as toner. The transfer roller 105 transfers the developer image formed on the surface of the optical drum 102 onto the recording medium. The fixing device 106 heats and presses the unfixed developer image to fix it on the storage medium.

  FIG. 2 is a diagram illustrating an example of an exposure apparatus and an exposure control unit according to the embodiment. The CPU 201 functions as an exposure control unit that comprehensively controls the exposure apparatus 101. For example, the CPU 201 controls the emission ratios of a plurality of lights having different oscillation wavelengths in accordance with the density of an image to be formed.

  The image signal distributor 202 processes the image signal in accordance with the value of the image signal output from the CPU 201 and sends the processed signal to the light emission signal generators 203 and 204. Based on the processing signal sent from the image signal distributor 202, the emission signal generators 203 and 204 light-modulate the corresponding semiconductor lasers 205 and 206 by blinking (ON and OFF) at a predetermined timing, respectively. Yes.

  The emission center wavelengths of the first laser 205 and the second laser 206 are different. For example, the emission center wavelength of the laser 205 is 405 nm, and the emission center wavelength of the laser 206 is 680 nm. Therefore, as a relative concept, it can be said that the first laser 205 corresponds to a short wavelength laser and the second laser 206 corresponds to a long wavelength laser.

In a state where the laser 205 is continuously turned on and the scanning of the light beam is stopped, the amount of light on the surface of the photosensitive drum 102 by the laser 205 is, for example, 0.8 mW. Under the same conditions, the amount of light on the surface of the photosensitive drum 102 by the laser 206 is, for example, 0.6 mW. The spot diameter of the light beam is generally defined as the diameter at which the light amplitude is 1 / e of the spot center (1 / e ^{2 in} power). That is, the spot diameter of the light beam at a light amount that is 1 / e ² with respect to the light amount at the emission center wavelength is, for example, 5 × 37 μm for the laser 205 and 50 × 58 μm for the laser 206. Here, the spot diameter is a diameter of an isointensity line having an intensity of 1 / e ² of the maximum value of light intensity, that is, 13.5%.

  The collimator lenses 207 and 208 are optical elements that change the light beams from the corresponding lasers 205 and 206 into substantially parallel light beams. The reflection mirror 210 is an optical element that reflects the light beam emitted by the laser 206. The half mirror 209 is an optical element that reflects the light beam emitted from the laser 205 and transmits the light beam emitted from the laser 206. Instead of the half mirror 209, a rotatable reflecting mirror may be adopted.

  The optical deflector 220 functions as a light beam deflecting unit. For example, the optical deflector 220 is a rotating polygon mirror having a plurality of deflection surfaces (reflection surfaces). The optical deflector 220 is rotated at a predetermined angular velocity in the direction of arrow A by driving means such as a motor. As a result, the light beam is scanned.

  The imaging optical system 230 includes three lenses a, b, and c having f-θ characteristics. The imaging optical system 230 forms an image of the light beam deflected by the optical deflector 220 on the surface of the photosensitive drum 102.

  By the way, the surface layer portion of the photosensitive drum 102 can be composed of various members. For example, the photosensitive layer of the photosensitive drum 102 may be composed of a member whose sensitivity decreases as the wavelength becomes shorter. A typical example of such a member is an inorganic semiconductor containing amorphous silicon.

  In general, the outermost layer of the photosensitive drum 102 is provided with a protective layer for improving the abrasion resistance. For this protective layer, for example, amorphous silicon carbide or a material containing amorphous carbide is used from the viewpoint of film formability. The transmittance of these materials also decreases on the short wavelength side. Therefore, the presence of the protective layer is also a factor that lowers the sensitivity on the short wavelength side.

  FIG. 3 is a diagram showing an example of a surface layer portion of an amorphous silicon drum having a positive charging polarity. The conductive support 301 is, for example, a cylinder made of aluminum (AL). On the conductive support 301, a charge injection blocking layer 302, a photoconductive layer 303, and a surface layer 304 are formed.

  Here, the charge injection blocking layer 302 blocks the injection of electrons from the conductive support 301 to the photoconductive layer 303. The photoconductive layer 303 is made of, for example, an amorphous material mainly containing silicon atoms. The photoconductive layer 303 exhibits photoconductivity, and its film thickness is, for example, 30 μm.

  The surface layer 304 is made of a material containing silicon atoms and carbon atoms. The surface layer 304 holds an electronic latent image formed on the surface.

  FIG. 4 is a diagram illustrating an example of the spectral sensitivity of the photosensitive drum 102. The horizontal axis represents the image exposure wavelength. The vertical axis indicates the sensitivity per unit photon. The spectral sensitivity refers to the reciprocal of the amount of light attenuated from a constant dark part potential to a constant bright part potential. Here, the sensitivity is measured while changing the exposure wavelength, and normalized by the peak value.

  According to FIG. 4, the spectral sensitivity starts to rise from around 800 nm located on the long wavelength side and peaks at 700 nm. On the shorter wavelength side than 700 nm, the sensitivity gradually decreases.

  FIG. 5 is a diagram illustrating an example of the relationship between the laser light quantity and the surface potential. The horizontal axis indicates the laser light quantity. The vertical axis represents the surface potential of the photosensitive drum. Here, first, the photosensitive drum 102 is charged to −500 V by the charger 103. Thereafter, the exposure device 101 exposes the surface of the photosensitive drum 102. When exposed with a laser 206 (wavelength 680 nm), the surface potential of the photosensitive drum 102 is brightly attenuated to 100V. On the other hand, when the exposure is performed with the laser 205 (wavelength 405 nm), it can be seen that the surface potential of the photosensitive drum 102 decreases only to about 300V.

  FIG. 6 is a diagram illustrating the relationship between the surface potential of the photosensitive drum and the image density according to the embodiment. The horizontal axis indicates development contrast. The vertical axis represents the reflection density of the output image.

  For example, the developing device 104 is applied with a developing bias in which an AC bias having a maximum AC voltage of 1.5 kVpp and a frequency of 2 kHz and a DC bias having a DC voltage of −350 V are superimposed. The latent image is developed by an electric field corresponding to the difference between the DC bias and the surface potential of the photosensitive drum 102. This potential difference corresponds to the development contrast.

  According to FIG. 6, in this image forming apparatus 100, it can be said that a development contrast of at least 200 V is necessary to obtain a sufficient image density. As described with reference to FIG. 5, the surface potential of the photosensitive drum 102 of the short wavelength laser 205 decreases only to about 300V. Therefore, if the exposure is performed only with the short wavelength laser 205, only an image with a low density will be obtained.

  That is, in the image forming apparatus that employs an inorganic semiconductor typified by amorphous silicon for the photosensitive drum 102, the sensitivity on the short wavelength side is lowered. Therefore, it can be said that there is a problem that it is difficult to obtain a sufficient image density even if the spot diameter is reduced by shortening the exposure wavelength.

  The present embodiment solves this problem by controlling the light emission ratios of a plurality of lights having different wavelengths according to the density of an image to be formed. For example, in a low density region where the image density is equal to or less than a threshold value, the photosensitive drum 102 is exposed using a laser 205 having a short wavelength (eg, wavelength 405 nm). On the other hand, in a high density region where the density of the image exceeds the threshold value, exposure is performed by switching to the laser 206 having a long wavelength (eg, wavelength 680 nm). It can be said that the high density region is a first region where a relatively high contrast is required. The low density region can be said to be a second region where a relatively high contrast is not required.

  FIG. 7 is a diagram illustrating an example of a potential distribution on the photosensitive drum 102 when an image is formed by using PWM modulation for a density that is an intermediate gradation. Here, with respect to the density of the intermediate gradation, the exposure apparatus 101 PWM modulates the light beam with a resolution of 200 lpi. By controlling the pulse width of PWM modulation with 8 bits, the number of gradations becomes 256 gradations.

  The left side of FIG. 7 shows the potential distribution on the photosensitive drum 102 when an image is formed using only the short wavelength laser 205 so as to obtain a predetermined image density (0.05 to 1.2). Yes. Further, the right side of FIG. 7 shows a potential distribution on the photosensitive drum 102 when an image is formed using only the long wavelength laser 206 so as to obtain a predetermined image density. The potential distribution is calculated based on the light amount distribution of each laser, the laser scanning time, and the sensitivity of the photosensitive drum 102. The potential distribution here is a potential distribution for one line as viewed from the sub-scanning direction of the photosensitive drum 102.

  Since the short wavelength laser 205 has a small spot diameter of 25 × 37 μm, a relatively sharp potential distribution is formed. However, since the sensitivity of the photosensitive drum 102 is not sufficient, in a high density region of 0.60 or more, the potential distribution related to the short wavelength laser 205 is crushed at about 200V.

  On the other hand, since the long wavelength laser 206 has a spot diameter of 50 × 58 μm, it has a relatively large spot diameter. However, the wavelength of the long wavelength laser 206 is 680 nm and belongs to a wavelength region where the photosensitive drum 102 is highly sensitive. As a result, the surface potential drops to near 0V.

  Focusing on the spot shape of the light beam, the short wavelength laser 205 forms a sharper and deeper latent image up to an image density of 0.35. Therefore, it can be said that the short wavelength laser 205 can form a more stable image up to an image density of 0.35.

  On the other hand, when the image density becomes 0.6 or more, the short wavelength laser 205 is in a state where the surface potential does not drop and only the exposure area increases. That is, the spot shape of the short wavelength laser 205 is wider than the spot shape of the long wavelength laser 206.

  According to the present embodiment, the CPU 201 switches to the long wavelength laser 206 when the potential distribution of the short wavelength laser 205 starts to widen from the relationship between the laser scanning time and the potential distribution.

  FIG. 8 is a diagram showing the relationship between 256-level PWM levels and image density. The horizontal axis indicates the PWM level. The vertical axis represents the image density. Here, the image signal distributor 202 uses a short wavelength laser 205 for an image signal whose PWM level corresponding to the image density is 45 or less which is a threshold value. The light emission ratio between the light emission amount of the short wavelength laser 205 and the light emission amount of the long wavelength laser 206 is 1: 0.

  On the other hand, for an image signal whose PWM level exceeds 45, the image signal distributor 202 uses a long wavelength laser 206. The light emission ratio between the light emission amount of the short wavelength laser 205 and the light emission amount of the long wavelength laser 206 is 0: 1. Thus, the short wavelength laser 205 and the long wavelength laser 206 are switched according to the image density.

  FIG. 9 is a flowchart illustrating an example of exposure control according to the embodiment. In step S901, the CPU 201 inputs an image signal from a print controller (not shown). Generally, a print controller is a controller that executes print data expansion processing, color conversion processing, halftone processing, and the like.

  In step S902, the CPU 201 determines the image density of the attention area based on the input image signal. The attention area is composed of, for example, one or more pixels.

  In step S903, the CPU 201 determines each light emission ratio for a plurality of light sources having different wavelengths according to the determined image density. In the embodiment, the light source is the short wavelength laser 205 and the long wavelength laser 206. Of course, three or more lasers having different wavelengths may be used as the light source.

  In step S904, the CPU 201 performs exposure control according to the determined light emission ratio. For example, the CPU 201 outputs the light emission ratio to the image signal distributor 202. The image signal distributor 202 causes the short wavelength laser 205 and the long wavelength laser 206 to emit light in accordance with the emission ratio.

  In step S905, the CPU 201 determines whether image formation has been completed for all image signals. If not completed, the process returns to step S901.

  FIG. 10 is a flowchart illustrating an example of a light emission ratio determination process according to the embodiment. Here, in order to show an example of the determination process of the light emission ratio, step S903 is made into a subroutine.

  In step S1001, the CPU 201 determines whether the image density of the attention area exceeds a predetermined threshold. As described above, this threshold value is desirably determined in consideration of the characteristics of the short wavelength laser 205, the long wavelength laser 206, and the photosensitive drum 102. That is, the threshold value should be determined so that density unevenness and the like are reduced.

  If the image density exceeds the threshold value, the attention area corresponds to the above-described high density area or an area where a relatively high contrast is required. Therefore, the process proceeds to step S1002, and the CPU 201 relatively increases the light emission ratio of light having a relatively long wavelength. In an extreme case, the light emission ratio between the light emission amount of the short wavelength laser 205 and the light emission amount of the long wavelength laser 206 is set to 0: 1. In this case, the short wavelength laser 205 and the long wavelength laser 206 will be completely switched.

  On the other hand, if the image density is equal to or lower than the threshold value, the attention area corresponds to the above-described low density area or an area where a relatively high contrast is not required. Therefore, the process proceeds to step S1003, and the CPU 201 relatively increases the light emission ratio of light having a relatively short wavelength. In an extreme case, the light emission ratio between the light emission amount of the short wavelength laser 205 and the light emission amount of the long wavelength laser 206 is set to 1: 0.

  As described above, according to the present embodiment, the CPU 201 suitably controls the light emission ratios of a plurality of lights having different wavelengths according to the density of the image to be formed. As a result, density unevenness and the like can be suitably reduced in the low density region, and contrast can be suitably maintained in the high density region.

  More specifically, in a high density region where the density of the image exceeds the threshold value, the CPU 201 relatively increases the light emission ratio of light having a relatively long first wavelength. The high density area is an image area that requires a relatively high contrast.

  On the other hand, in the low density region where the density of the image is equal to or lower than the threshold, the CPU 201 relatively increases the emission ratio of light having the second wavelength shorter than the first wavelength. The low density area is an image area where a relatively high contrast is not required. As for the threshold value, it is desirable to set a suitable value for each image forming apparatus. If an area is determined using such a threshold value, the CPU 201 can execute exposure control by a relatively simple process.

  Note that the CPU 201 may perform control so as to gradually increase the light emission ratio of light having a relatively short wavelength in an intermediate gradation region that transitions from a high concentration region to a low concentration region. Further, the CPU 201 may control to gradually increase the light emission ratio of light having a relatively long wavelength in the region where the low concentration region transitions to the high concentration region. As a result, a more natural density change will be maintained even in a so-called intermediate gradation region.

  The CPU 201 performs exposure with light having the first wavelength in a high density region where the density of the image exceeds the first threshold value. In a low density region where the image density is equal to or lower than the second threshold value which is lower than the first threshold value, exposure is performed with light having a second wavelength shorter than the first wavelength. In this way, the light emission ratio of the light having the first wavelength and the light having the second wavelength may be controlled in the concentration region that is equal to or lower than the first threshold and equal to or higher than the second threshold. As a result, a more natural density change will be maintained even in a so-called intermediate gradation region.

  Note that the invention according to the present embodiment is particularly effective when the sensitivity of the photosensitive member employed in the image carrier such as the photosensitive drum 102 has wavelength dependency. The material having such characteristics is, for example, an inorganic semiconductor containing amorphous silicon. In addition, when the protective layer of the photosensitive drum 102 has the same wavelength-dependent characteristic, the invention according to this embodiment will be more effective.

[Second Embodiment]
The first embodiment is an invention for controlling the light emission ratios of a plurality of light sources having different wavelengths according to the density of an image. On the other hand, in order to increase the definition of characters and line images and improve sharpness, it is preferable that the potential gradient of the contour portion is steep.

  Therefore, in the present embodiment, a description will be given of a method of forming a latent image using the laser 205 with a short wavelength without depending on the density of the pixels constituting the outline portion of the character and line image.

  FIG. 11 is a flowchart illustrating an example of a light emission ratio determination process according to the embodiment. In addition, the description is simplified by attaching | subjecting the same referential mark to the already demonstrated location.

  In step S1101, the CPU 201 determines whether or not the target pixel constitutes a contour portion of a character and a line image. Note that the CPU 201 may acquire information necessary for determining the contour portion from the print controller. If the target pixel constitutes an outline, the process proceeds to step S1003, and the short wavelength laser 205 is emitted as a main component. On the other hand, if the target pixel does not constitute an outline, the process proceeds to step S1001 and the CPU 201 determines the light emission ratio according to the image density.

  As described above, according to this embodiment, in addition to the effects described in the first embodiment, it is possible to increase the definition and sharpness of characters and line images.

1 is a schematic cross-sectional view of an image forming apparatus according to an embodiment. It is a figure which shows an example of the exposure apparatus and exposure control part which concern on embodiment. It is a figure which shows an example of the surface layer part of the amorphous silicon drum which has a positive charge polarity. It is a figure which shows an example of the spectral sensitivity of a photosensitive drum. It is a figure which shows an example of the relationship between a laser light quantity and surface potential. It is a figure which shows the relationship between the surface potential and image density of the photosensitive drum which concerns on embodiment. It is a figure which shows an example of the electric potential distribution on the photosensitive drum at the time of forming an image using PWM modulation about the density | concentration used as an intermediate gradation. It is a figure which shows the relationship between the PWM level of 256 gradations, and image density. It is a flowchart which shows an example of the exposure control which concerns on embodiment. It is a flowchart which shows an example of the determination process of the light emission ratio which concerns on 1st Embodiment. It is a flowchart which shows an example of the determination process of the light emission ratio which concerns on 2nd Embodiment.

Claims (9)

In an image forming apparatus for performing image formation by scanning an image carrier with a light beam modulated by light output from a laser light source means,
A plurality of laser light sources having different oscillation wavelengths included in the laser light source means;
An image forming apparatus comprising: an exposure control unit that controls a light emission ratio of the plurality of laser light sources in accordance with a density of an image to be formed.   The exposure control unit relatively increases the light emission ratio of light having the first wavelength in the high density region where the density of the image exceeds the threshold value, and in the low density region where the density of the image is equal to or less than the threshold value. The image forming apparatus according to claim 1, wherein a light emission ratio of light having a second wavelength shorter than the first wavelength is relatively increased.   The exposure control unit switches to perform exposure with light having the first wavelength in the high density region, and to perform exposure with light having the second wavelength in the low density region. The image forming apparatus according to claim 2.   The exposure control unit performs exposure with light having the first wavelength in a high density region where the density of the image exceeds the first threshold, and the second density of the image is lower than the first threshold. In a low density region that is less than or equal to the threshold value, switching is performed so that exposure is performed with light having a second wavelength shorter than the first wavelength, and a density region that is less than or equal to the first threshold value and greater than or equal to the second threshold value The image forming apparatus according to claim 1, wherein a light emission ratio of light having the first wavelength and light having the second wavelength is controlled.   The exposure control unit gradually increases the light emission ratio of the light having the second wavelength in the region transitioning from the high concentration region to the low concentration region, from the low concentration region to the high concentration region. 3. The image forming apparatus according to claim 2, wherein a light emission ratio of the light having the first wavelength is gradually increased in the transition region.   In the first region where a relatively high contrast is required, the exposure control unit relatively increases the emission ratio of light having the first wavelength, and the second region does not require a relatively high contrast. The image forming apparatus according to claim 1, wherein a light emission ratio of light having a second wavelength shorter than the first wavelength is relatively increased.   3. The exposure control unit according to claim 2, wherein when exposure is performed on a pixel constituting a contour of a character or a line image, exposure is performed with light of the second wavelength without depending on the density of the image. 7. The image forming apparatus according to any one of items 6 to 6.   7. The image forming apparatus according to claim 2, wherein the photosensitive layer of the image carrier includes an inorganic semiconductor containing amorphous silicon. An image forming apparatus control method comprising:
Controlling a light emission ratio of a plurality of lights having different wavelengths according to the density of an image to be formed;
And a step of exposing an image carrier of the image forming apparatus using the one or more lights whose emission ratio is controlled.

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