JP2008284854A - Optical beam scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To substantially prevent a density difference in an image, in an optical beam scanner of an image forming apparatus. <P>SOLUTION: On the occasion of exposing a target pixel by deflected scanning with an optical beam, exposure energy is compensated according to the degree of influence of an exposed pixel which is exposed before within the same line. That is, the exposure energy is adjusted according to the number of the exposed pixels, exposure time and an exposure rate (rate of the number of exposed pixels or rate of the exposure time) and a distance (number of pixels) from the exposed pixel to the target pixel or an elapsed time of exposure. For instance, the exposure energy is compensated by a method of changing an optical output by adjusting the PWM duty of a lighting current. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light beam scanning apparatus, and more particularly, to a light beam scanning apparatus in a multifunctional image forming apparatus (MFP apparatus) or a printer.

  In a conventional light beam scanning apparatus such as an MFP apparatus or a printer, if droop correction is not performed, a density difference may occur in an image. Therefore, droop correction is performed by various methods. Below are some examples of prior art related to this.

  The “image forming apparatus” disclosed in Patent Document 1 is an improvement in gradation reproducibility of an electrophotographic image forming apparatus. A correction signal for light emission level correction is generated on the basis of the time from when the semiconductor laser emits light at each of the n pixels immediately before the light emission at the pixel to be printed and the gradation of each pixel. The temperature compensation data is generated by adding the correction signal to the print target pixel data. The light beam energy is controlled based on the temperature compensation data.

  The “image forming apparatus” disclosed in Patent Document 2 stably obtains a high-quality image by stably controlling the amount of light of a laser having poor droop characteristics. The laser is driven by the sum of the first drive current and the second drive current. The first drive current of the laser during one scan is controlled to be constant during one scan. The second drive current corresponding to the pixel modulation signal is turned on / off. The second drive current of the laser during one scan is controlled according to the on time of the laser. The light quantity and light emission characteristics of the laser are detected and stored, and an image is formed according to the light emission characteristics.

The “image recording apparatus” disclosed in Patent Document 3 is an apparatus that adjusts the amount of light in one main scan based on image data to correct droop characteristics and generates an image having no density difference on the image. . The image data is output as pulse width modulation data with APC emission data and synchronization detection emission data added. Correction data generated from the image data of each channel is input to the LUT. In the LUT, a data table for correcting the droop characteristic of each laser diode is set. Corrected by the LUT and output to the D / A converter of each channel. During one main scan, each laser diode is simultaneously modulated based on the image data of each channel, and each laser diode sequentially emits APC during the ineffective scan period outside the image area to correct the light output.
JP 09-314908 A Japanese Patent Laid-Open No. 11-291547 JP 2002-086793 A

  However, the conventional droop correction method has the following problems. Although the light emission level is corrected according to the light emission time based on the pixel data, the influence of all the pixels is regarded as the same, and detailed correction cannot be made. Since the same correction is performed for pixel data with little influence, the density difference cannot be sufficiently reduced in the image.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-described conventional problems so that a density difference hardly occurs in an image in a light beam scanning device of an image forming apparatus.

  In order to solve the above problems, in the present invention, a light beam scanning apparatus including a control unit that controls lighting of a light source and a deflecting unit that scans a light beam from the light source is the same before the pixel of interest. A configuration is provided that includes a correction unit that corrects the exposure energy of the target pixel in accordance with the degree of influence of the exposed pixel exposed in the line. The correcting means corrects the exposure energy of the pixel of interest according to the number of exposed pixels and the degree of influence thereof, or the exposure energy of the pixel of interest according to the exposure time of the exposed pixel and the degree of influence thereof. Or a means for correcting the exposure energy of the target pixel in accordance with the exposure rate of the exposed pixel and its influence.

  The exposure rate is the ratio of the number of exposed pixels to the number of pixels in one line or the number of non-exposed pixels, or the ratio of the exposure time to the effective scanning time or non-exposure time in one line. The influence degree is an amount corresponding to the distance from the exposed pixel to the target pixel, that is, the number of pixels, or the elapsed time from the exposure to the exposed pixel until the target pixel is exposed. The correction means includes means for instructing the control unit to correct the light output of the light source, that is, to correct the PWM duty of the lighting current of the light source in order to correct the exposure energy.

  With the configuration described above, the droop characteristic can be corrected by adjusting the light quantity in the main scanning based on the degree of influence of the image data, so that an image having almost no density difference in the main scanning direction can be formed.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to FIGS.

  In the embodiment of the present invention, attention is paid to the influence of the already exposed pixels according to the number of pixels exposed in the same line before the target pixel, the exposure time, the exposure rate, the exposure interval, and the elapsed time. This is a light beam scanning device for correcting the exposure energy of a pixel.

  FIG. 1 is a conceptual diagram of a light beam scanning apparatus in an embodiment of the present invention. In FIG. 1, an LD unit 1 is a light source that generates a light beam by a laser diode. The polygon mirror 2 is a rotating polygon mirror for deflecting and scanning a light beam. The F-θ lens 3 is a lens that corrects the optical path of the light beam. The reflection mirror 4 is a mirror that directs the light beam toward the photosensitive drum. The photosensitive drum 5 is a drum on which a latent image is formed with a light beam. The synchronization detection mirror 6 is a mirror that guides the light beam to the light receiving element. The light receiving element 7 is an element that detects a light beam in order to detect a synchronization position.

  FIG. 2 is a functional block diagram showing the configuration of the droop correction circuit and an explanatory diagram showing the concept of the correction method. In FIG. 2, a pixel count circuit 8 is a counter that counts pixels of one line of image data. The LD light amount correction circuit 9 is a circuit that corrects the light amount of the laser diode. The lighting ratio calculation circuit 10 is a circuit that calculates the lighting ratio of the laser diode.

  The function and operation of the light beam scanning apparatus according to the embodiment of the present invention configured as described above will be described. First, the basic operation of the light beam scanning apparatus will be described with reference to FIG. In the LD (laser diode) unit 1, the laser beam emitted from the laser diode (LD) is converted into parallel rays by a collimating lens (not shown), and then deflected and scanned by a rotating polygon mirror (polygon mirror) 2. An image is formed on a charged surface of a drum-shaped photosensitive member (photosensitive drum) 5 by an imaging lens composed of an F-θ lens 3 and the like and a reflection mirror 4. This laser beam is modulated based on the image signal and is repeatedly turned on and off. At the same time as scanning is repeated in the main scanning direction indicated by the arrow in FIG. 1, the photosensitive drum 5 rotates and performs sub-scanning, thereby forming an electrostatic latent image on the photosensitive drum 5.

  The electrostatic latent image formed on the photosensitive drum 5 is developed with a charged developer (toner). Further, the transfer material, such as transfer paper, which is given a charge opposite to that of the developer at the transfer portion is brought into close contact with the photosensitive drum 5, whereby the developer is transferred to the transfer material. Then, after the transfer material is separated from the photosensitive drum 5, the developer is fused and fixed on the transfer material by being heated and pressurized in the fixing unit. Therefore, in this embodiment, the photosensitive drum 5, the developer, the transfer unit, and the fixing unit constitute an image forming unit, and image formation is performed by a known electrophotographic method. Further, the light receiving element 7 detects the laser beam by the reflected light from the synchronization detection mirror 6 arranged outside the scanning region on the photosensitive drum 5. The writing control unit has a timing generation circuit that performs various signal processing based on the synchronization detection signal obtained by the light receiving element 7.

  In the laser diode, the optical output characteristics with respect to the driving current greatly vary depending on the temperature. When it is driven with a constant current, it has a so-called “droop characteristic” in which the light output decreases due to heat generation. For this reason, the laser diode has a built-in PD (Photo Diode) for monitoring the optical output. An APC (Automatic Power Control) operation that uses the output of the PD to control the optical output at a constant level is required. In the case of an image forming apparatus, in general, the APC operation is performed in a non-effective scanning period outside the image area for each main scanning line.

  Next, an outline of functions of the light beam scanning apparatus will be described. Lighting control of the light emission source (LD unit 1) is performed by the control unit. The light beam from the light source is scanned by the deflecting means (polygon mirror 2, F-θ lens 3, and reflection mirror 4). At that time, the exposure energy of the target pixel is corrected according to the influence degree of the exposed pixel exposed in the same line before the target pixel. In other words, the exposure energy of the pixel of interest is corrected by changing the light output by adjusting the PWM duty of the lighting source lighting current according to the number of exposed pixels, exposure time, exposure rate, and degree of influence. To do. The exposure rate is the ratio of the number of exposed pixels to the number of pixels in one line or the ratio of the exposure time (lighting time) to the effective scanning time of one line. The influence degree is an amount corresponding to the distance (number of pixels) from the exposed pixel to the target pixel or the elapsed exposure time.

  Next, the first correction method will be described with reference to FIG. An image data signal and a clock are input to the pixel count circuit 8. The image data signal is captured in synchronization with the clock. If it is a high level signal, a count signal is generated and the counter is operated. The count value is held in the counter circuit. The count value is adjusted according to the influence level. The degree of influence is an amount corresponding to the distance (number of pixels) from the exposed pixel to the target pixel or the elapsed time from the exposure to the exposed pixel until the target pixel is exposed. As means for adjusting the count value, there are a circuit that adjusts a clock by a PLL according to the influence degree, and a circuit that accumulates by multiplying a coefficient according to the influence degree. The adjusted count value is multiplied by a coefficient determined according to the characteristics of the LD element and the electrophotographic process, and the result is output to the LD light amount correction circuit 9 as a correction amount.

  Instead of the pixel count circuit 8, a circuit for measuring time may be used. By this circuit, the accumulated time obtained by measuring the lighting time of the light source is held inside. Adjust the accumulated time according to the degree of influence. The coefficient determined in accordance with the characteristics of the LD element and the electrophotographic process is multiplied and output to the LD light amount correction circuit 9 as a correction amount.

  Next, the second correction method will be described with reference to FIG. The image data input from the previous stage is counted by the pixel count circuit 8, and the lighting ratio calculation circuit 10 calculates the lighting ratio. The pixel count circuit 8 takes in the image data signal in synchronization with the clock, generates a count signal if it is a high level signal, and operates the counter. In addition, the number of pixels that are not lit can be obtained by holding the value obtained by counting all clocks regardless of the image data and outputting the difference.

  As the lighting ratio, the number of lighting (exposure) pixels / the number of non-lighting (non-exposure) pixels or the number of lighting (exposure) pixels / the total number of pixels is used. Adjust the lighting ratio according to the degree of influence. A correction amount is determined from the adjusted lighting ratio. In the case of an image data transfer method in which no clock is accompanied with image data, each time of High level / Low level can be obtained by sampling with a separately prepared high-speed clock. As the lighting ratio, lighting time / non-lighting time or lighting time / (lighting time + non-lighting time) is used. Adjust the lighting ratio according to the degree of influence. A correction amount is determined from the adjusted lighting ratio.

  Next, a method for adjusting according to the influence degree will be described with reference to FIG. The count value is corrected according to how far each pixel already exposed in the same line as the target pixel is from the target pixel. As shown in FIG. 2 (c), the correction amount can be appropriately approximated by reducing the integration as the distance from the target pixel decreases. As the degree of influence, an amount corresponding to distance (number of pixels) or elapsed exposure time is used.

  When the correction according to the elapsed distance is not performed as in the conventional method, the exposure amount is uniformly integrated as shown in FIG. In order to perform correction according to the elapsed distance, as shown in FIGS. 2 (c-2) to (c-4), the exposure amount of the pixels far from the target pixel is integrated with a small amount. In this example, it is assumed that the degree of influence is linear, and is linearly reduced according to the distance. However, the degree of influence may be reduced stepwise instead of linearly. Alternatively, it may be reduced exponentially. Alternatively, a correction table may be created and used based on experimental results. In either method, the density difference can be almost eliminated by finely adjusting the count value reduction amount according to the degree of influence so that the density difference actually decreases.

  Since the exposure energy is proportional to the optical output of the LD, the exposure energy is corrected by changing the LD light amount setting voltage that controls the optical output of the LD. Since the reference voltage before the LD light amount correction is input to the LD light amount correction circuit 9 in FIGS. 2A and 2B, the correction amount is added to this and output as the LD light amount setting voltage. Since the exposure energy is proportional to the lighting time of one pixel when the light output is constant, the lighting time is changed and corrected by adjusting the PWM duty.

  As described above, in the embodiment of the present invention, the light beam scanning device is used for the target pixel according to the number of pixels exposed in the same line before the target pixel, the exposure time, the exposure rate, the exposure interval, and the elapsed time. Therefore, it is possible to form an image with almost no density difference in the main scanning direction.

  The light beam scanning apparatus of the present invention is optimal as a light beam scanning apparatus in an image forming apparatus such as an MFP apparatus or a printer.

It is a conceptual diagram of the light beam scanning apparatus in the Example of this invention. FIG. 2 is a functional block diagram showing a configuration of a droop correction circuit in the light beam scanning apparatus in the embodiment of the present invention and an explanatory diagram showing a concept of a correction method.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... LD unit, 2 ... Polygon mirror, 3 ... F-theta lens, 4 ... Reflection mirror, 5 ... Photosensitive drum, 6 ... Synchronization detection mirror, 7 ... Light receiving element, 8... Pixel count circuit, 9... LD light quantity correction circuit, 10.

Claims (12)

  In a light beam scanning device including a control unit that controls lighting of a light source and a deflecting unit that scans a light beam from the light source, the influence degree of an exposed pixel exposed in the same line before the target pixel is affected. Accordingly, a light beam scanning device comprising correction means for correcting the exposure energy of the pixel of interest.   The light beam scanning apparatus according to claim 1, wherein the correction unit is a unit that corrects the exposure energy of the pixel of interest according to the number of pixels of the exposed pixels and the degree of influence thereof.   The light beam scanning apparatus according to claim 1, wherein the correction unit is a unit that corrects the exposure energy of the pixel of interest according to an exposure time of the exposed pixel and an influence degree thereof.   2. The light beam scanning apparatus according to claim 1, wherein the correcting unit is a unit that corrects the exposure energy of the target pixel in accordance with an exposure rate of the exposed pixel and an influence degree thereof.   5. The light beam scanning apparatus according to claim 4, wherein the exposure rate is a ratio of the number of exposed pixels to the number of pixels in one line or the number of non-exposed pixels.   5. The light beam scanning apparatus according to claim 4, wherein the exposure rate is a ratio of an exposure time to an effective scanning time or non-exposure time of one line.   The light beam scanning apparatus according to claim 1, wherein the influence degree is an amount corresponding to a distance from the exposed pixel to a target pixel.   8. The light beam scanning apparatus according to claim 7, wherein the distance from the exposed pixel to the target pixel is the number of pixels from the exposed pixel to the target pixel.   The light beam scanning apparatus according to claim 1, wherein the influence degree is an amount corresponding to an elapsed time from exposure of the exposed pixel to exposure of the target pixel.   The light beam according to claim 1, wherein the correction unit includes a unit that instructs the control unit to correct a light output of the light emitting source in order to correct an exposure energy. Scanning device.   The said correction | amendment means is provided with a means to instruct | indicate the said control part to correct | amend the PWM duty of the lighting current of the said light emission source in order to correct | amend exposure energy. Light beam scanning device. An image forming apparatus equipped with the light beam scanning apparatus according to claim 1.

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