JP2003182152A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the yield of an LED array head by managing variations in exposure intensity distribution so that image density unevenness is hardly recognized, and to achieve gradation and color reproduction without vertical stripes. To provide an image forming apparatus having excellent performance. SOLUTION: A lens array for imaging an LED element array in which a large number of LED elements 31 are arranged in a plurality of rows or a row and light obtained by lighting the LED elements with a predetermined drive current based on an input signal. 33, an image forming apparatus that forms an image by an electrophotographic method with an LED array head facing an image carrier, for a certain characteristic value in the exposure intensity distribution of each LED element, a pixel in a main scanning direction of a dither matrix. It is characterized in that an average value for each number is set to be within a certain predetermined range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical writing device LE.
The present invention relates to an image forming apparatus such as a printer, a digital copying machine, and a facsimile apparatus which forms an image by an electrophotographic method using a D array print head.

[0002]

2. Description of the Related Art In an image forming apparatus for forming an image by electrophotography, a laser scanning optical system including a laser light source and a polygon mirror for deflecting and scanning the laser light is mainly used for an optical writing apparatus. In recent years, an LED array printer using an LED array head as an optical writing device has been attracting attention because of its compactness and simplification of the entire device and high-speed writing at high density. . The LED array head has a large number of LED elements arranged in the main scanning direction. By controlling lighting of each LED element based on an image signal, optical writing is performed on the photoconductor to form an electrostatic latent image. To be done. Here, a conventional general LED array head will be described with reference to FIG. The LED array head 3 is composed of an LED element array substrate 30 in which a large number of LED elements 31 are arranged linearly and a lens array section 33 in which a plurality of imaging lenses 32 are arranged in parallel. The light emitted from each LED element passes through a plurality of imaging lenses in the lens array and forms an image on the photoconductor 1. As the lens array 33, a SELFOC lens array (trade name;
(Hereinafter referred to as SLA) is widely used. On the other hand, LE
Since the D array head includes variations in the LED element array (variations in the light emitting portion shape and arrangement of each LED element) and variations in the lens array (variation in the optical performance of each lens and variations in the installation position), the D array head has On the body)
In the above, it is practically impossible to manufacture such that all the LED elements have uniform optical characteristics. Therefore,
The dot image that is formed also differs for each LED element,
In the area gradation method based on binary writing, variations in the dot image appear as density unevenness, which significantly deteriorates gradation expression. Particularly in the LED array printer, since the same LED element is used for exposure in the sub-scanning direction, the density unevenness occurs as a vertical stripe image in the sub-scanning direction. Therefore, a method has been proposed in which the amount of light of each LED element is measured and the driving current and the driving time are changed so as to correct all the exposure amounts (Japanese Patent Laid-Open No. 3-196070, etc.). reference). This is to equalize the amount of light by creating correction data that controls the drive current so that the amount of light of all LED elements falls within a certain standard range with respect to the average value of the amount of light of all LED elements. Is. Then, if there is an LED element whose light amount is out of the standard range that cannot be completely corrected at that time, the LED array head itself becomes defective.

[0003]

However, all LEs
It has been known that even when the D element is corrected so that the light amount becomes uniform, vertical stripes that are density unevenness occur, and gradation expression, graininess, and color reproducibility may be deteriorated. It was also found that density unevenness is particularly often generated when using a high density LED array head such as 1200 dpi. A possible cause of this is a variation in the shape of the exposure intensity distribution of each LED element. FIG. 15 shows the exposure intensity distribution in the main scanning direction of the LED elements at different positions.
These show that the light amount is made uniform by the conventional light amount correction method, but the shape of the exposure intensity distribution is greatly different. Particularly, in a high-density LED array head such as 1200 dpi, it is necessary to make the LED elements small in order to increase the array density, the precision in the manufacturing process becomes more severe, and the LED elements become smaller, the beam spot diameter becomes smaller. However, there is a problem that the variation in the shape of the exposure intensity distribution of each LED element becomes larger than that of the conventional one because the diameter is reduced. As a method of paying attention to this problem, as in Japanese Patent Laid-Open No. 11-227254, a correction method for paying attention to a characteristic point in the light emission intensity distribution of each LED element and for making the light emission intensity distribution uniform by parameters such as a beam diameter is disclosed. Are known. However, as described above, in the case of a high density LED array head such as 1200 dpi, the number of elements is doubled as compared with the conventional 600 dpi, and the variation of the exposure intensity distribution of each LED element becomes large, so that an effective image is obtained. It becomes difficult to make the beam diameter uniform over the entire width (for example, about 15,000 dots of A3 width). It has become difficult to keep the beam spot diameter of the LED element within the standard range, the number of defective LED array heads has increased, and the cost has increased due to a significant deterioration in yield. Therefore, the present invention improves the yield of the LED array head by managing the variation of the exposure intensity distribution so that the unevenness of the image density is less likely to be recognized, and the image is excellent in gradation and color reproducibility without vertical stripes. An object is to provide a forming device.

[0004]

In order to solve the above-mentioned problems, according to the invention of claim 1, a large number of LED elements are arranged in a plurality of rows or one row, and the LEDs are based on an input signal. LE having a lens array for forming an image of light obtained by lighting an element with a predetermined drive current
Each LED in an image forming apparatus that forms an image by an electrophotographic method with a D array head facing an image carrier
For a certain characteristic value in the exposure intensity distribution of the element, the average value for each number of pixels in the main scanning direction of the dither matrix is
An image forming apparatus that is set to fall within a certain predetermined range has the most main feature. Generally, the sensitivity of human visual sense is 1 [cycle / mm] with high sensitivity.
It is known that it is difficult to be recognized in the high frequency region in the vicinity of 600 dpi or even 1200 dpi. Further, the area gradation method such as the dither method, which is a gradation expression in a binary printer, also utilizes this human visual characteristic, and the gradation is expressed by a high screen frequency which is difficult to recognize the periodic structure. And promote the visual integration effect,
It is possible to express smooth gradations such as natural images. Therefore, the human visual characteristics are insensitive to the density change in the cycle of one pixel unit at the resolutions of 600 dpi and 1200 dpi. That is, depending on the degree of variation in the shape of the exposure intensity distribution of one pixel, it is expected that there will be no visual problem in the image density unevenness. Therefore, it is desirable that the variation in the exposure intensity distribution of the LED elements is not managed on a pixel-by-pixel basis, but is managed by the average value of the exposure intensity distribution in a certain cycle in consideration of visual characteristics. In the present invention, by using the LED array head in which the variation when averaging a certain characteristic value in the exposure intensity distribution of each LED element is suppressed within a predetermined range by using the number of pixels in the main scanning direction of the dither matrix as a cycle, It is possible to obtain an image without uneven density such as a streak image.

In the invention according to claim 2, the certain characteristic value in the exposure intensity distribution of each LED element is a beam spot area when the exposure intensity distribution of each LED element is sliced by a predetermined threshold value. The described image forming apparatus is a main feature. Experiments by the inventors have revealed that there is a correlation between the positional relationship between the variation of the beam spot area and the vertical stripe that is the uneven density. However, the variation of the beam spot area for each pixel is large, and it is difficult to manage the variation, and there is a portion where the variation for each pixel cannot correspond to the density unevenness in the image (the beam spot area is larger than the average). In pixels that are out of alignment,
In some cases, vertical streaks, which are uneven density, were not always observed). Therefore, by using a value obtained by averaging the beam spot areas for the pixels of the dither matrix as the characteristic value, it becomes easier to manage the variation, so that it is possible to obtain an image without density unevenness such as a vertical stripe image. In the invention according to claim 3, the certain characteristic value in the exposure intensity distribution of each LED element is a beam spot diameter which is a length in the main scanning direction when the exposure intensity distribution of each LED element is sliced by a predetermined threshold value. An image forming apparatus according to claim 1 is a main feature. It was found that the variation of the beam spot area due to the variation of the exposure intensity distribution has a strong correlation with the vertical stripes, which are density unevenness, especially in the variation of the length in the main scanning direction. Therefore, by controlling the beam spot diameter in the main scanning direction, the inspection of the exposure intensity distribution of each LED element can be simplified and the vertical stripes can be effectively suppressed.

According to a fourth aspect of the present invention, the image forming apparatus according to the second or third aspect is characterized in that the predetermined threshold value is set to 10% or less with respect to the average value of the exposure intensity peak values in all the LED elements. The main feature. By setting the predetermined threshold to 10% or less of the average value of the exposure intensity peak,
It becomes easy to correspond the position of the variation of the exposure intensity distribution of each LED element and the position of the vertical stripe which is the density unevenness in the output image. A fifth aspect of the present invention is characterized mainly in the image forming apparatus according to the first to fourth aspects, in which the beam spot diameter of each LED element is 40 μm or less. Regarding the shape of the exposure intensity distribution, if the spread is too large for the pixel, the characteristic value cannot be managed accurately. Therefore, the inspection is effectively performed under this condition. According to a sixth aspect of the present invention, the image forming apparatus is a so-called color image forming apparatus that forms an image with toner of at least four colors of yellow, cyan, magenta, and black.
The image forming apparatus described in any one of 1 to 5 is a main feature. Since the density variation of each color directly leads to the deterioration of color reproducibility, it is necessary to manage the variation of the exposure intensity distribution of each LED element by the present invention in order to improve the image quality. A seventh aspect of the invention is characterized mainly in the image forming apparatus according to the first to sixth aspects in which a different dither matrix is set for each color. By setting the screen angle for each color, it is possible to prevent the density unevenness of each single color due to the variation of the light amount from overlapping in the same cycle to emphasize the density unevenness. Claim 8
According to the invention described above, in a color image forming apparatus that forms an image of each color with the same LED array head, the characteristic value of the exposure intensity distribution averaged for each number of pixels in the main scanning direction of the dither matrix in black, and the LED The image forming apparatus according to any one of claims 1 to 7, characterized in that the ratio of the characteristic value of the entire array head to the average value is within a predetermined value range. By inspecting using a predetermined value of black with a large change in lightness, it is possible to prevent a vertical stripe image having uneven density. In a ninth aspect of the present invention, in the color image forming apparatus that forms an image of each color with a plurality of LED array heads, the predetermined value range is set for each LED array head of each color. The described image forming apparatus is a main feature. By setting the range of the predetermined value according to the change in the brightness of each color, it is possible to perform the inspection in stages and suppress the deterioration of the yield of the LED array head,
The cost can be reduced.

The tenth aspect of the invention is characterized mainly in the image forming apparatus according to the ninth aspect, in which the range of the predetermined value of black is smaller than that of the other colors. By controlling the variation in the exposure intensity distribution of the LED array head used for black, which has a large change in lightness, more strictly than in other colors, it is possible to prevent vertical stripe images having uneven density. In the invention of claim 11, the range of the predetermined value of the black is
The image forming apparatus according to claim 10 has a main characteristic of ± 10%. In the invention according to claim 12, the range of the predetermined value of yellow is larger than the predetermined values of the other colors.
The image forming apparatus described in any one of 1 to 11 is a main feature. By increasing the permissible level of the variation in the exposure intensity distribution of the LED array head used for yellow, which has a small change in lightness, compared to the other colors, L is outside the permissible range for other colors.
Since the ED array head can be used, the deterioration of the yield can be suppressed and the cost can be reduced. In the invention according to claim 13, the range of the predetermined value of the yellow is ± 20.
% Is the main feature of the image forming apparatus according to claim 12. According to a fourteenth aspect of the invention, the arrangement density of the LED elements in the LED array head in the main scanning direction is 1200 or more per inch, and the image forming apparatus according to the first aspect is a main feature. . According to a fifteenth aspect of the present invention, the LED element is mainly driven by the image forming apparatus according to any one of the first to fourteenth aspects, which is driven to be driven by binary image data per dot.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below in detail with reference to the drawings. First Embodiment FIG. 1 is a sectional view of a first embodiment of an image forming apparatus of the present invention. A photosensitive drum 1, which is a latent image carrier that is driven to rotate in the direction of arrow A, is provided in the center, and a charging device 2 that uniformly applies a charge to the surface of the photosensitive drum is provided around the photosensitive drum 1, and a photosensitive member is provided based on an image signal. An exposure device 3 for exposing to form an electrostatic latent image, and a developing unit (4) provided with toner of each color (black, yellow, cyan, magenta).
(K, 4Y, 4C, 4M) sequentially rotate to face the photoconductor, by a so-called revolver type developing mechanism,
By developing the electrostatic latent image on the photoconductor 1,
A developing device 4 for forming a toner image for each color thereon, and a toner image on the photoconductor 1 are transferred for each color to the intermediate transfer belt 5a, thereby obtaining a toner image in which the colors are superimposed on the intermediate transfer belt 5a. A transfer device 5, a cleaning device 6 for removing the toner remaining on the photoconductor after the transfer process to the intermediate transfer device, and the like are sequentially arranged. Further, a transfer device 8 for transferring the toner image formed on the intermediate transfer belt to a recording material 7 such as paper conveyed in the direction of arrow B, and a fixing device 9 for fixing the transferred recording material are also arranged as shown in the figure. To be done. The charging device 2 uses a so-called scorotron charger. By applying the grid voltage Vg to the mesh-shaped grid electrode facing the photoconductor, the charging potential on the photoconductor is controlled. In this embodiment, the grid voltage Vg is -700.
It can be varied with V as a reference. The configuration of the charger is not limited to this, and a charging roller or the like may be used. The LED array head that is the exposure device 3 has the configuration shown in FIG. 14 described above, and the lens array is a SLA2 of SELFOC lens array (trade name).
0D is used. The array density of the LED elements in the main scanning direction is 1200 dpi (1200 per inch), and the total number N of LED elements is 15360. The developing device 4 contains a two-component developer composed of a toner and a carrier therein, and the developer is conveyed by the conveying screw 41 to the front side and the back side in FIG. 1, and at the same time, the toner and the carrier are charged by frictional charging. To do. In this embodiment, a toner having an average particle size of 6.9 μm and a carrier having an average particle size of 50 μm were used as the developer. The developing roller 42 arranged to face the photoconductor is composed of a rotatable developing sleeve and a magnet fixed inside, and the magnet in the developing roller attracts the developer to the surface of the developing sleeve. After that, the developer adhering to the developing sleeve is the doctor 4 which is a developer regulating member.
After being made a thin layer by 3, the sheet is conveyed to the developing area. A developing bias is applied to the developing sleeve, and only the toner adheres to the latent image on the photoreceptor due to the developing electric field formed between the photoreceptor and the developing sleeve, so that the latent image becomes visible. Here, in this embodiment, the developing bias is a so-called AC bias in which an AC component is superimposed on a DC component,
DC voltage VB_DC = -500 V, peak-to-peak voltage Vpp = 0.8 kV, and frequency f = 2.4 kHz are applied under the condition of symmetrical rectangular wave.

Next, the control of the lighting drive of the LED array will be described. FIG. 2 is a block diagram showing the configuration of the LED array driving section 10. This LED array drive unit has a well-known configuration, and includes a shift register 11, a latch 12, and an A
It is composed of an ND gate 13 and an LED driver 14. The shift register 11 uses the clock signal CLOCK
By this, 1-dot binary image data of "0" or "1" is input in order from the dot 1, and internally operates to send each dot data to each register. When all the N dot data are sent, the latch 12 latches the data, and the strobe pulse STB becomes the AND gate 1
3 is input, only the dot (LED element) to which "1" of the image data is sent is basically emitted by the LED driver 15 by the width of the strobe pulse STB. Next, the configuration of the LED array control unit 20 will be described based on the block diagram shown in FIG. First, the RGB 8-bit data input from the scanner or the frame memory is subjected to color correction and halftone processing in the image processing unit, and CMYK1b
It is generated as it2 image data. Thereafter, a FIFO (First-In First-Out) memory 21 for fetching 1-bit binary image data for one line from the outside is provided on the input side of the LED array drive unit 10. The FIFO memory 21 is reset by the main scanning line synchronization signal / LSYNC from the controller unit 22,
Image data for one line of main scanning is captured. Then, the main scanning line synchronization signal /
The LED array driver 10 is reset by LSYNC,
By the clock signal CLOCK generated from the oscillator 23, the image data is sequentially sent from the FIFO memory to the LED array driving section from the dot 1 to the dot N. Also,
A strobe pulse generator 24 is connected to the LED array driver in the LED array controller. The strobe pulse generator 24 is composed of, for example, a counter, a comparator, etc., and generates a strobe pulse STB. LED array driver 10
Then, the dot whose image data is "1" emits light at the timing of the strobe pulse STB. These controls are shown in the timing chart of FIG.

Next, the dither matrix used in this embodiment will be described. As shown in FIG. 5, based on 200 lines, an individual dither matrix having a screen angle for each color is used. For example, the black dither matrix expresses gradation as a set of basic dithers as shown in FIG. Next, a method of inspecting the exposure intensity distribution of each LED element of the LED array head in the present invention will be described. In this embodiment, attention was paid to the beam spot area in the exposure intensity distribution of each LED element. First, acquisition of the beam spot area of each LED element will be described with reference to the flowchart of FIG. 7. The measurement of the exposure intensity distribution is performed with a two-dimensional CCD measuring device,
The exposure intensity is detected at a level of 8 bits for each pixel on the XY coordinates. Also. By mounting the LED array head on the stage of automatic feeding, the exposure intensity distribution can be automatically measured for each LED element. First i
The exposure intensity distribution I (i) of the th LED element is measured (#
1b), all LED elements are measured (i = 1 to N) ((# 1a) to (# 1d)). Next, a peak value Ip (i) is obtained from the obtained exposure intensity distribution I (i), and this is averaged for all LED elements to obtain an average peak intensity value Ip.
0 is calculated (Ip0 = (ΣIp (i)) / N) (# 1
e). Next, a threshold value T0 that determines the beam spot area is set (# 1f). In this embodiment, the value of 10% of the average peak intensity Ip0 is set as the threshold value T0. Next, the area equal to or greater than the threshold value T0 is calculated and set as the beam spot area S (i) (# 1g). A schematic view of the beam spot area at this time is shown in FIG. Then, as the average beam spot area, S (i) of all LED elements is averaged to obtain S
0 is calculated (S0 = (ΣS (i)) / N) (# 1
h). Further, it is determined whether or not the obtained average beam spot area S0 is smaller than a predetermined area St (# 1i).
Here, St = 1600 [μm ^ 2]. If the result is false, it is determined that the LED array head has a large variation in beam spot area, and uneven density is unacceptable, and the LED array head is not mounted in the apparatus as a defect (# 1j). The determination of the threshold value T0 for calculating the beam spot area is made in correspondence with the generation state of vertical stripes which are density unevenness in the actual image output result. In order to obtain the correlation between the beam spot area and the vertical stripe that is the density unevenness, it is particularly desirable to set the threshold value at a position of 10% or less of the peak intensity.

Next, an example in which the beam spot area, which is a characteristic value of the exposure intensity distribution of each LED element, is managed by an averaged value by the number of pixels in the main scanning direction of the dither matrix will be described with reference to the flowchart of FIG. To do. In this embodiment, since a single LED array head is used as the exposure device, the beam spot area is averaged using a black dither matrix, and the number of pixels of the dither matrix in the main scanning direction is As shown in FIG. 5, X = 32. First, the beam spot area is averaged for each number X of pixels of the dither matrix in the main scanning direction, and the average beam spot area S_ave (k) for each dither matrix is calculated (# 2b). Here, the average beam spot area S_ave (k) of the kth dither matrix is from (kX + 1) th to ((k + 1)) of the beam spot area S (i).
The average beam spot area of the (X) th pixel is represented. Next, whether the variation of the average beam spot area S_ave (k) in the obtained k-th dither matrix is within C_bk as a ratio with respect to the average beam spot area S0 in all the LED elements (| S_ave
(K) -S0 | / S0 ≦ C_bk) is determined (# 2)
c). If it is false, it is determined that the LED array head has a portion where the unevenness in density is unacceptable and the variation in the beam spot area is large, and the LED array head is not mounted in the apparatus as a defect (# 2d). In this embodiment, this predetermined value is C
_Bk = 0.10. If true, the above inspection process is repeated until the entire range of the LED array head is (k + 1).
The process is continued until X> N (# 2f), and if the above-mentioned inspection process is true, the LED array head can be mounted on the device with an allowable level of density unevenness (# 2g).

Next, a second embodiment of the present invention will be described.
FIG. 10 is a sectional view of a second embodiment of the image forming apparatus of the present invention. The color image forming apparatus shown in FIG. 10 is a so-called tandem system, and is based on an image signal, a photoconductor 1 that is a latent image bearing member that is rotationally driven in the direction of arrow C, a charging device 2 that uniformly charges the surface of the photoconductor drum. An exposure device 3 using an LED head that exposes a photoconductor to form an electrostatic latent image, a developing device 4 that develops the electrostatic latent image with toner to form a toner image on the photoconductor, which will be described later. An image forming unit including a cleaning device 6 for removing the toner remaining on the photoconductor after the transfer process is arranged in series for each color, and the toner image formed on each photoconductor is transferred to each transfer device 8. Is sequentially transferred to a recording material 7 such as paper fed by a conveyor belt 5, and a fixing device 9 heat-fixes the toner image on the recording material on which the respective colors are superimposed on the recording material to obtain an image.
The present embodiment is characterized in that an exposure device, which is an LED array head, is provided for each color, and the other L
The lighting drive of the ED element is basically the same as that of the first embodiment. The halftone processing for each color is as shown in FIG.

Each L of the LED array head in the present invention
An inspection method for the exposure intensity distribution of the ED element will be described. In this embodiment, attention was paid to the beam spot diameter in the main scanning direction in the exposure intensity distribution of each LED element. This simplifies the measurement as compared with the beam spot area and shortens the inspection time. First, regarding the acquisition of the beam spot diameter of each LED element,
This will be described with reference to the flowchart of FIG. The exposure intensity distribution is measured by a two-dimensional CCD measuring device, and the exposure intensity is detected at a level of 8 bits for each pixel of XY coordinates.
A one-dimensional exposure intensity distribution in the main scanning direction is obtained by data processing. Also. By mounting the LED array head on the stage of automatic feeding, the exposure intensity distribution can be automatically measured for each LED element. First i
The exposure intensity distribution Ix (i) of the th LED element is measured (# 3b), and this is calculated for all LED elements (i = 1 to 1).
N) Measure ((# 3a) to (# 3d)). Next, a peak value Ip (i) is obtained from the obtained exposure intensity distribution I (i), and this is averaged for all LED elements to calculate an average peak intensity value Ip0 (Ip0 = (ΣIp (i)) /
N) (# 3e). Next, a threshold value T0 that determines the beam spot diameter is set (# 3f). In this embodiment, the value of 10% of the average peak intensity Ip0 is set as the threshold value T0. Next, the length sliced at the threshold value T0 is calculated and set as the beam spot diameter Wx (i) (# 3g). FIG. 12 shows a schematic diagram of the beam spot diameter at this time. Furthermore, as the average beam spot diameter, Wx of all LED elements
(I) is averaged to calculate Wx0 (Wx0 = (ΣWx
(I)) / N) (# 3h). It is determined whether or not the obtained average beam spot diameter Wx0 is smaller than a predetermined value Wxt (# 3i). Here, Wxt = 40 [μm]. If the result is false, it is determined that the LED array head has a large variation in beam spot diameter and the density unevenness is unacceptable, and the LED array head is not mounted in the device as a defect (# 3).
j). The determination of the threshold value T0 for calculating the beam spot area is made in correspondence with the occurrence state of vertical stripes which are density unevenness of the actual image output result, as in the first embodiment. In order to obtain the correlation between the beam spot diameter and the vertical streak, which is uneven density, it is particularly desirable to set the threshold value at a position of 10% or less of the peak intensity. In this embodiment, a predetermined value defining the allowable variation in the beam spot diameter averaged by the dither matrix is set for each color. Specifically, a predetermined value is set for each color according to the magnitude of the lightness change, such as a small predetermined value for black with a large lightness change and a large predetermined value for yellow with a small lightness change. As a result, the black L
Even an ED array head is likely to be usable in yellow. That is, by inspecting the LED array head for each color stepwise, it is possible to significantly suppress the deterioration of the yield.

The above inspection method will be described with reference to the flowchart of FIG. First, the constant col is set to 0 (# 4a). Here, col is a constant representing a color, and represents 0: black, 1: cyan or magenta, and 2: yellow. Next, the number of pixels X_c of the dither matrix in the main scanning direction
The beam spot diameters are averaged for each ol, and the average beam spot diameter Wx_ave (k) _c for each dither matrix.
ol is calculated (# 4c). Here, the average beam spot diameter Wx_ave (k) _c of the k-th dither matrix
ol is (kX + 1) of the beam spot diameter Wx (i).
It represents the average beam spot diameter in the ((k + 1) X) th pixel from the th. Further, X_col is the number of pixels in the main scan of the dither matrix as shown in Table 5 in the color col at that time. Next, the above-mentioned average beam spot diameter calculation processing is repeated, and L
Perform on the entire range of the ED array head ((k + 1) X
> N) (# 4e). Next, with respect to the average beam spot diameter Wx0 of all the LED elements, the Wx_ave
(K) Ratio A_c of maximum value and minimum value of _col
ol, B_col are acquired. Here, A = MAX (Wx_ave (1) _col, Wx_av
e (2) _col, ..., Wx_ave (k) _col,
…) / Wx0 B = MIN (Wx_ave (1) _col, Wx_av
e (2) _col, ..., Wx_ave (k) _col,
…) / Wx0. In the above, AVE () is the average value, M
AX () represents the maximum value and MIN () represents the minimum value. Furthermore, it is determined whether or not the following formula is satisfied for the predetermined value C_col determined for each color of the constant col (# 4g). Formula: MAX (A_col-1, 1-B_col) ≦ C_
col At this time, C_col is set in the relationship of C_0 <C_1 <C_2, and the smaller the change in lightness of the color, the predetermined value C_
col is getting bigger. Specifically, the predetermined value C_0 = 0.10 for black, the predetermined value C_1 = 0.15 for cyan and magenta, and the predetermined value C_2 = 0.20 for yellow.
And When the process of (# 4g) is true, which color the LED array head is used is determined according to the value of the constant Col. When Col is 0, it is black (# 4
h) (# 4i), when 1 is cyan or magenta (#
4j) (# 4k), when 2, the LED array heads are selected as being mountable in yellow (# 4l). When the process of (# 4g) is false, the constant Col is incremented (# 4m) and the next color (# 4g) is calculated.
Return to processing from b). Here, when the constant Col exceeds 2 (# 4n), the light amount variation is not allowed L
It is determined that it is an ED array head, and it is not mounted in the device as a defect (# 4o). By the above inspection method, the LED array head used for each color can be set according to the level of variation in the characteristic value of the exposure intensity distribution, so that the yield can be significantly improved and the cost can be reduced. In this embodiment, the characteristic value is the beam spot diameter,
By performing the same process on the beam spot area, more accurate inspection becomes possible.

[0015]

As described above, according to the first aspect of the present invention, the LED elements are arrayed in a plurality of columns or in one row and the LED elements are driven with a predetermined drive current based on the input signal. In an image forming apparatus in which an LED array head having a lens array for forming an image of turned-on light is opposed to an image carrier to form an image by an electrophotographic method, a certain characteristic in the exposure intensity distribution of each LED element The image forming apparatus is characterized in that a value averaged for each number of pixels in the main scanning direction of the dither matrix is set to fall within a predetermined range.
When managing the variation of the exposure intensity distribution of each LED element after passing through the lens array, by paying attention to the value obtained by averaging certain characteristic values in consideration of visual characteristics for each number of pixels in the main scanning direction of the dither matrix, Correlation with vertical stripes that are density unevenness can be obtained, image quality with good gradation can be obtained without vertical stripes, and the yield of the LED array head can be improved and cost can be reduced. An image forming apparatus according to a second aspect of the present invention is the image forming apparatus according to the first aspect, wherein a certain characteristic value in the exposure intensity distribution of each LED element is a beam when the exposure intensity distribution of each LED element is sliced at a predetermined threshold value. The image forming apparatus is characterized by a spot area, and in an LED array having a different exposure intensity peak for each LED element, by defining a beam spot area at a certain threshold value, a vertical stripe that is uneven in density is generated. Positions can be easily handled, and by managing the variations in the beam spot area, it is possible to always obtain a good image that has both sharpness and gradation, and the yield of LED array heads is good, which reduces costs. Can be achieved.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, the certain characteristic value in the exposure intensity distribution of each LED element is a slice of the exposure intensity distribution of each LED element with a predetermined threshold value. The image forming apparatus is characterized in that the beam spot diameter is the length in the main scanning direction at that time, and by setting the characteristic value of each LED element to the beam spot diameter at a certain threshold, the vertical stripe Corresponding to the positions of and becomes easy, and the processing in the inspection becomes simple. Therefore, it is possible to always obtain a good image having both sharpness and gradation, and to improve the yield of the LED array head and reduce the cost. Claim 4
The image forming apparatus of the present invention according to claim 2 is characterized in that, in claims 2 and 3, the predetermined threshold value is set to 10% or less with respect to an average value of peak exposure intensity values in all the LED elements. Since this is an image forming apparatus, the exposure intensity distribution of each LED element can easily correspond to the density unevenness occurrence position in the output image, so that it is possible to perform an accurate inspection.
It is possible to prevent the generation of vertical stripes that are uneven in density and always obtain a good image having both sharpness and gradation. An image forming apparatus according to a fifth aspect of the present invention is the image forming apparatus according to any one of the first to fourth aspects, characterized in that the beam spot diameter of each LED element is 40 μm or less. This is a condition for accurate management, which prevents the occurrence of vertical stripes that are uneven density.
It is possible to always obtain a good image having both sharpness and gradation. An image forming apparatus according to a sixth aspect of the present invention is the image forming apparatus according to any one of the first to fifth aspects, wherein the image forming apparatus forms an image with at least four color toners of yellow, cyan, magenta, and black. It is an image forming apparatus characterized by being an apparatus, and can obtain image quality excellent in gradation and color reproducibility, which are important image quality items in color images, and the yield of LED array heads is improved, resulting in cost reduction. Can be achieved.

An image forming apparatus according to a seventh aspect of the present invention is the image forming apparatus according to any one of the first to sixth aspects, in which a different dither matrix is set for each color, and a screen is provided for each color. Since the angle can be set differently, the periodic structure for each color is inconspicuous, there is no density unevenness such as vertical stripes, stable image quality with good gradation and color reproducibility is obtained, and the yield of the LED array head is high. It can improve the cost and reduce the cost. An image forming apparatus according to an eighth aspect of the present invention is the same LE as in the first to seventh aspects.
In a color image forming apparatus that forms an image of each color with a D array head, a characteristic value of an exposure intensity distribution averaged for each pixel in the main scanning direction of a dither matrix in black and a characteristic value of the entire LED array head The image forming apparatus is characterized in that the ratio to the average value of is within a range of a predetermined value, and by performing inspection using a predetermined value of black with a large change in brightness, density unevenness such as vertical stripes can be obtained. Without
Stable image quality with good gradation and color reproducibility is obtained, and L
The yield of the ED array head is improved and the cost can be reduced. An image forming apparatus according to a ninth aspect of the present invention is the color image forming apparatus according to any one of the first to seventh aspects, in which an image of each color is formed by a plurality of LED array heads.
The image forming apparatus is characterized in that the range of the predetermined value is set for each LED array head of each color, and the inspection is made efficient by setting the predetermined value according to the magnitude of the change in brightness of each color, There is no density unevenness such as vertical stripes, stable image quality with good gradation and color reproducibility is obtained, and LED
The yield of the array head is improved and the cost can be reduced. An image forming apparatus according to a tenth aspect of the present invention is the image forming apparatus according to the ninth aspect, characterized in that the range of the predetermined value of black is smaller than that of the other colors. By making the value smaller than the other colors, the inspection is made efficient, there is no density unevenness such as vertical stripes, stable image quality with good gradation and color reproducibility is obtained, and the yield of the LED array head is high. Improves and costs can be reduced

The image forming apparatus of the present invention according to claim 11 is the image forming apparatus according to claim 10, characterized in that the range of the predetermined value of the black is ± 10%. It is possible to obtain stable image quality with good gradation and color reproducibility without such uneven density. An image forming apparatus according to a twelfth aspect of the present invention is the image forming apparatus according to any one of the ninth to eleventh aspects, in which the range of the predetermined value of yellow is larger than the predetermined values of the other colors. By making the predetermined value of yellow, which has a small change, larger than other colors, it is possible to use the LED array head that was out of the predetermined range for other colors for yellow, making the inspection efficient,
There is no density unevenness such as vertical stripes, stable image quality with good gradation and color reproducibility can be obtained, and the yield of the LED array head can be greatly improved to reduce the cost. The image forming apparatus of the present invention according to claim 13 is the image forming apparatus according to claim 12, characterized in that the range of the predetermined value of the yellow is ± 20%. By widening the allowable level of the variation in the exposure intensity distribution of the LED array head used compared to other colors, it is possible to use LED array heads that are outside the allowable range for other colors, thus suppressing yield deterioration. And the cost can be reduced. The image forming apparatus of the present invention according to claim 14 is the image forming apparatus according to any one of claims 1 to 13,
The image forming apparatus is characterized in that the array density of the LED elements in the D-array head in the main scanning direction is 1200 or more per inch, and even in a high-density LED array head, the variation of the exposure intensity distribution is high. Can be efficiently inspected, there is no density unevenness such as vertical stripes, stable image quality with good gradation and color reproducibility can be obtained without impairing sharpness, and the yield of LED array heads is good. The cost can be reduced. An image forming apparatus according to a fifteenth aspect of the present invention is the image forming apparatus according to any one of the first to fourteenth aspects, in which the LED element is driven to be turned on by binary image data per dot.
The inspection can be simplified, and stable image formation can be performed even if the exposure density distribution becomes large due to high density, so there is no density unevenness such as vertical stripes, and stable gradation and color reproduction. Image quality with good performance can be obtained, and the yield of the LED array head can be improved to reduce the cost.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a first embodiment of an image forming apparatus of the present invention.

FIG. 2 is a block diagram showing a configuration of an LED array driving section of the present invention.

FIG. 3 is a block diagram showing a configuration of an LED array control unit of the present invention.

FIG. 4 is a timing chart of the LED array control unit of the present invention.

FIG. 5 is a diagram showing a dither matrix used in the present invention.

FIG. 6 is a diagram showing a black dither matrix used in the present invention.

FIG. 7 is a flowchart for acquiring the beam spot area of each LED element used in Example 1 of the present invention.

FIG. 8 is a diagram showing a schematic view of a beam spot area of the present invention.

FIG. 9 is a flowchart for managing the beam spot area, which is a characteristic value of the exposure intensity distribution of each LED element of the present invention, with an averaged value.

FIG. 10 is a cross-sectional view of a second embodiment of the image forming apparatus of the present invention.

FIG. 11 is a flowchart for acquiring the beam spot diameter of each LED element used in Example 2 of the present invention.

FIG. 12 is a diagram showing a schematic view of a beam spot diameter of the present invention.

FIG. 13 is a flowchart for stepwise inspecting the LED array head for each color.

FIG. 14 is an explanatory diagram of a conventional general LED array head.

FIG. 15 is a diagram showing an exposure intensity distribution in the main scanning direction of conventional LED elements at different positions.

[Explanation of symbols]

1 photoconductor drum, 2 charging device, 3 exposure device (LE
D array head), 4 developing device, 5 intermediate transfer device, 6
Cleaning device, 7 recording material, 8 transfer device, 9
Fixing device, 10 LED array driver, 11 shift register, 12 latch, 13 AND gate, 15 LE
D driver, 20 LED array control unit, 21 FIF
O memory, 22 controller, 23 oscillator, 24
Strobe pulse generator, 30 LED element array substrate, 31 LED element, 32 imaging lens, 33 lens array section, 41 conveying screw, 42 developing roller,
43 Doctor

Claims (15)

[Claims] 1. An LED element array in which a large number of LED elements are arranged in a plurality of rows or one row, and a lens array for forming light for illuminating the LED elements with a predetermined drive current based on an input signal. In an image forming apparatus that forms an image by an electrophotographic system by facing an LED array head having an LED to an image carrier, a certain characteristic value in the exposure intensity distribution of each LED element is determined for each number of pixels in the main scanning direction of the dither matrix. The image forming apparatus is characterized in that the averaged value is set within a predetermined range. 2. The certain characteristic value in the exposure intensity distribution of each LED element is a beam spot area when the exposure intensity distribution of each LED element is sliced at a predetermined threshold value. Image forming device. 3. A certain characteristic value in the exposure intensity distribution of each LED element is a beam spot diameter that is a length in the main scanning direction when the exposure intensity distribution of each LED element is sliced at a predetermined threshold value. The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. 4. The image forming apparatus according to claim 2, wherein the predetermined threshold value is set to 10% or less with respect to an average value of peak values of exposure intensity in all the LED elements. 5. The beam spot diameter of each LED element is 40.
The image forming apparatus according to claim 1, wherein the image forming apparatus has a thickness of not more than μm. 6. The image forming apparatus according to claim 1, wherein the image forming apparatus is a so-called color image forming apparatus that forms an image with toner of at least four colors of yellow, cyan, magenta, and black. Image forming apparatus. 7. The image forming apparatus according to claim 1, wherein a different dither matrix is set for each color. 8. In a color image forming apparatus for forming an image of each color by the same LED array head, a characteristic value of an exposure intensity distribution averaged for each number of pixels of a dither matrix in black in the main scanning direction, and an LED 8. The image forming apparatus according to claim 1, wherein the ratio of the characteristic value of the entire array head to the average value is within a predetermined value range. 9. A color image forming apparatus for forming an image of each color by a plurality of LED array heads, wherein L of each color is formed.
The image forming apparatus according to claim 1, wherein the range of the predetermined value is set for each ED array head. 10. The image forming apparatus according to claim 9, wherein the predetermined value range of black is smaller than that of the other colors. 11. The predetermined value range of the black is ± 1.
The image forming apparatus according to claim 10, which is 0%. 12. The yellow predetermined value range is larger than the predetermined values of the other colors.
The image forming apparatus according to item 1. 13. The predetermined value range of the yellow is ± 2
It is 0%, The image forming apparatus of Claim 12 characterized by the above-mentioned. 14. The array density of the LED elements in the LED array head in the main scanning direction is 1200 per inch.
The image forming apparatus according to claim 1, wherein the number of the image forming apparatuses is one or more. 15. The LED element has 2 dots per dot.
The image forming apparatus according to any one of claims 1 to 14, wherein the image forming apparatus is driven to be turned on by image data of a value.