JP2006025286A - Image reader and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image reader reducing occurrence of lateral stripes by carrying out proper black level correction processing without being minimally affected by noise components. <P>SOLUTION: The image reader provided is with a CCD 9 for converting image information resulting from optically reading an original 14 into an analog electric signal, and changes a line period in accordance with a read mode. A black level detection circuit 33 changes the detection range of a black level in the main scanning direction in accordance with change of the line period. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image reading apparatus using a sheet-through type document supply apparatus and an image forming apparatus including the image reading apparatus.

2. Description of the Related Art Conventionally, it is known that an image reading apparatus of an image forming apparatus uses a CCD line sensor in which a large number of photoelectric conversion elements are arranged in one line in the main scanning direction in order to read image information (for example, Patent Document 1). 2).
JP 2000-125093 A JP 2003-274115 A

In such a CCD sensor, a signal output from each photoelectric conversion element outputs a certain amount of output value even when no light is incident. The output voltage in this case is referred to as dark output voltage.
At the time of image reading, the dark output voltage of the photoelectric conversion element is output while being loaded on each image signal voltage. Therefore, to obtain the true image signal voltage, the dark output voltage is subtracted from the output voltage of the photoelectric conversion element. It is necessary to perform a correction process (black level correction process).
As a method relating to the black level correction processing, there are a line-by-line correction method using a light shield element and a pixel-by-pixel correction method using photoelectric conversion elements for all pixels.
The former line-by-line correction method that uses light shield elements is that a plurality of light shield elements (dummy conversion elements) are formed in a light-shielded state outside the effective image area of the image sensor, and a dark output voltage is always obtained. It is in a state that can be. Based on the dark output value obtained from these light shield elements, the dark output correction of the original photoelectric conversion element in the effective image area is performed in line units.
In the latter pixel unit correction method using photoelectric conversion elements for all pixels, a dark output value obtained from photoelectric conversion elements for all pixels is stored for each pixel when the light source (exposure lamp) is turned off. . The output voltage obtained from each photoelectric conversion element during actual image reading when the light source is turned on is corrected for each pixel based on the stored dark output value.

In the case of the pixel-based correction method, compared with the line-based correction method, there is an advantage that non-uniformity of dark output values for each pixel can be avoided, but a memory that stores dark output values for all pixels. It has the disadvantage of requiring capacity and high cost.
In addition, since a constant black level is subtracted for each pixel during reading, there is a disadvantage that the low-frequency black level fluctuation during reading cannot be corrected.
The black level method for each line obtains the black level to be subtracted using the black level of the light shield element for each line, but since there are not many elements of the light shield element, the black level subtraction value is There is a disadvantage that it varies.
In particular, in a color image reading apparatus, a 3-line CCD line sensor is often used. However, since the black level subtraction value is calculated for each RGB line, if the black level subtraction value varies in RGB, an achromatic color is obtained. Even when the original is read, there is a drawback that a line where the color arrives is generated and a horizontal stripe is generated.
SUMMARY OF THE INVENTION An object of the present invention is to provide an image reading apparatus capable of performing appropriate black level correction processing and reducing horizontal stripes without being affected by noise components as much as possible in consideration of the above-described actual situation, and an image forming apparatus provided with the image reading apparatus. Is to provide.

In order to solve the above problems, the invention according to claim 1 is an image sensor that converts image information obtained by optically reading a document into an analog electrical signal, and a line cycle that is changed according to a reading mode. A possible image reading apparatus includes a detection range changing unit that changes a black level main scanning direction detection range in accordance with a change in line cycle.
The invention described in claim 2 has two or more reading modes for switching the reading line cycle, and extends the black level detection range in the main scanning direction outside the effective image area of the image sensor corresponding to the line cycle. It is characterized by.
According to a third aspect of the present invention, when the line period is short, the black level detection range in the main scanning direction is set to the optical black portion of the image sensor, and when the line period exceeds a certain period, the black level is detected. The detection range is set in an empty transfer portion of the image sensor.
According to a fourth aspect of the present invention, there is provided the image reading apparatus according to any one of the first to third aspects.

  According to the present invention, it is possible to perform appropriate black level correction processing without being affected by the noise component as much as possible, and to reduce horizontal stripes.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic view showing a general image reading apparatus. First, a specific configuration and operation will be described with reference to FIG.
A reading unit A in the scanner body 13 includes a first carriage 6 including an illumination lamp 2 for document exposure and a first reflection mirror 3 for reading a document 14 placed on the contact glass 1 serving as a document table, A second carriage 7 including a second reflecting mirror 4 and a third reflecting mirror 5 is provided.
Further, in the scanner main body 13, a lens unit 8 for forming an image on a CCD linear image sensor 9 (hereinafter referred to as CCD), a sensor board substrate 10 on which the CCD 9 is mounted, and a signal processing substrate 12 are connected to the sensor board substrate 10. And a white reference plate 15 for correcting various distortions caused by the reading optical system and the like.
When the document 14 is placed and fixed on the contact glass 1 for reading, the first carriage 6 moves forward (in the direction of arrow B) at a constant speed, and the second carriage 7 is ½ of the first carriage 6. The document 14 on the contact glass 1 is optically scanned by following the first carriage 6 at a speed.
After the reading of the document is completed, the first carriage 6 and the second carriage 7 return to the home position. The position shown in FIG. 1 is the home position of the first carriage 6 and the second carriage 7. The reading unit A also has a motor drive system (not shown) for moving the first carriage 6 and the second carriage 7.

FIG. 2 is a block diagram illustrating a circuit configuration of an image forming apparatus including the image reading apparatus of FIG. Next, processing of read data will be described with reference to FIG.
The CCD 9 is a color 3-line CCD in which three rows of CCD sensors covered with RGB filters are arranged. The analog processing circuit 19 samples the signal portion of the analog waveform output from the CCD 9 and incorporates an amplifier to adjust the signal gain.
The A / D converter 20 outputs R, G, and B color analog image signals to the image Y processing unit 17 as 8-bit color digital image information. In the case of the 3-line CCD 9, the signal output from the CCD 9 has a positional shift of 4 lines at the same magnification.
That is, there is an 8-line misalignment between R and B, and the inter-line correction unit 21 accumulates the R signal by 8 lines and accumulates the G signal by 4 lines and delays the line misalignment. Is corrected.
The black level correction circuit 32 detects the black level and subtracts the black level from the image data. The shading correction unit 22 corrects each RGB signal with respect to density unevenness of the optical system and sensitivity variation of the CCD 9.
At the time of zooming, there may be a case where the positional deviation does not match only with the interline correction unit 21, so the dot correction unit 24 detects the positional deviation of one line or less that cannot be corrected by the interline correction unit 21 with surrounding pixels. Refer to and correct.

In the scanner γ section 25, data having linear characteristics with respect to the reflectance is converted into characteristics that improve the correction accuracy of the color correction by the color correction section 27 in the subsequent stage. The image area separation unit 23 determines whether the image is a character area or a picture area in order to perform optimum processing that matches the characteristics of the image in subsequent processing.
In the filter 26, an edge is emphasized to sharpen the character area, and a smoothing process is performed to smooth the pattern area. The color correction unit 27 converts RGB signals read by the CCD 9 into C, M, Y, and K signals.
Since the sensitivity and ink characteristics of the CCD 9 are different from the ideal, the color correction parameter is adjusted in the copying machine so as to correct the difference from the original. In the variable magnification 28, a variable magnification process in the main scanning direction is performed.
In this case, by using the convolution method, the scaling process is performed while maintaining the MTF of the reading optical system, and the resolution of the image data is maintained. The sub-scanning direction is performed by controlling the scanning speed in the sub-scanning direction.
As for the convolution method, the printer γ29 finally matches the original and the copy density. Processing is performed so that the difference in spectral characteristics between the document and the toner, the gray balance, and the top density are appropriate.
The gradation processing unit 30 binarizes or multi-values 8-bit density information. At this time, binarization or multi-level quantization is performed in the character area, dither processing or error diffusion processing is performed in the picture area, and the result is output to the printer 18. The printer 18 prints C, M, Y, and K image data on a sheet via the LD writing 31.

FIG. 3 is a block diagram showing a black level correction circuit. In FIG. 3, RGB and 10-bit image data output from the interline correction unit 21 are input to a black level detection circuit 33 and a subtraction circuit 34.
The black level detection circuit 33 detects the black level of a certain range of image data in the main scanning direction by a timing control circuit (not shown). As a method of obtaining a subtraction value from the detected black level, a simple addition averaging is generally performed in the main scanning direction, and a sub-calculation method is obtained in the sub-scanning direction.
The calculated black level subtraction value is output to the subtraction circuit 34. The subtraction circuit 34 subtracts the black level subtraction value from the image data and outputs it. This subtraction value is a constant value for one line.

FIG. 4 is a timing chart showing the reading timing in monochrome. FIG. 5 is a timing chart showing main scanning direction reading timing in color. In the present embodiment, description will be made on the assumption that the reading speed for color at the same magnification and the reading speed for monochrome are different.
In general, in the case of a color copying machine, since the color copying speed is structurally slower, the reading speed can be slowed accordingly. Since the reading line period is proportional to the reading speed, the line period becomes longer if the reading speed is slow. At this time, if the reading pixel frequency is not switched, the number of pixels in one line increases.
4 and 5, the TG signal is a signal for transferring the charge accumulated in the photodiode by the transfer gate signal of the CCD 9 to the CCD register unit, and is generated in a line cycle.
The φ1 signal is a drive pulse for transferring charges in the CCD register unit. The VOUT signal indicates an analog signal output output from the CCD 9, and includes a dummy signal empty transfer portion, an optical black portion where the light receiving portion is optically shielded, and an effective photocell portion that is actually read. The TG and φ1 signals are generated by a timing generation circuit (not shown).
The black reading detection timing setting will be described. In the black-and-white reading mode shown in FIG. 4, the timing generation circuit sets various timings so that the line period T1 is reached. At this time, the black reading detection range is set to the signal portion of the optical black portion.

Next, as shown in FIG. 5, in the color reading mode, various timings are set so that the line cycle becomes T2. Since the line cycle is long in the case of color, the number of empty transfer portions after the effective pixel area increases in the VOUT output.
By setting the black reading detection range by the black level detection circuit (detection range changing means) 33 for color in this empty transfer section as shown in FIG. 5, the number of detection pixels increases. The black level detection circuit 33 averages the detected pixels and sets a black subtraction value. Therefore, the black level subtraction value can be calculated without being affected by noise as the number of detection pixels increases.
In the above description, the case where the color mode and the monochrome mode have different normal scanning speeds has been described. However, when there are modes with different scanning speeds in each mode, switching between the monochrome mode and switching in the color mode is also possible. .
According to the present invention, an apparatus that changes the line cycle according to the reading speed of the image reading apparatus has two reading speeds, for example, a color reading mode and a black and white reading mode. The reading speed can be reduced due to restrictions on the processing system.
Since horizontal stripes are more noticeable in color, widening the black level range in the main scanning direction in the color mode compared to the black and white mode improves the black level detection accuracy especially in the color mode and reduces the horizontal stripes. To do. Further, by setting the black level detection range for color in the empty transfer portion of the CCD 9, it is possible to set the detection range in a wider range.
In the above, the case where there are two reading speeds in the color reading mode and the black and white reading mode has been described, but the same applies to the case where there are two or more reading speeds in color, or where there are two or more reading speeds in black and white. The horizontal streak can be reduced by controlling the black reading detection range as much as possible.

It is the schematic which shows a general image reading apparatus. FIG. 2 is a block diagram illustrating a circuit configuration of an image forming apparatus including the image reading apparatus of FIG. 1 for processing of read data. It is a block diagram which shows a black level correction circuit. It is a timing diagram which shows the reading timing at the time of monochrome. FIG. 6 is a timing diagram illustrating main scanning direction reading timing in color.

Explanation of symbols

  9 Image sensor (CCD, CCD linear image sensor), 13 scanner body, 14 document, 32 black level correction circuit, 33 black level detection circuit (detection range changing means), 34 subtraction circuit, A reading unit

Claims (4)

  In an image sensor that converts image information obtained by optically reading a document into an analog electrical signal, and an image reading apparatus that can change the line cycle according to the reading mode, a black level main scan according to the change in the line cycle An image reading apparatus comprising detection range changing means for changing a direction detection range.   2. The apparatus according to claim 1, further comprising two or more reading modes for switching a reading line cycle, and extending a black level detection range in the main scanning direction outside the effective image area of the image sensor corresponding to the line cycle. Image reading device.   When the line cycle is short, the black level detection range in the main scanning direction is set to the optical black portion of the image sensor, and when the line cycle exceeds a certain cycle, the black level detection range is idle transferred by the image sensor. The image reading apparatus according to claim 2, wherein the image reading apparatus is set to a portion.   An image forming apparatus comprising the image reading apparatus according to claim 1.

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