JP4013601B2 - Image reading device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image reading apparatus that optically reads an image of a document.
[0002]
[Prior art]
In recent years, as an image reading apparatus used for a copying machine, a scanner apparatus, or the like, an apparatus capable of reading a color image is becoming widespread. This type of image reading apparatus uses a three-line CCD (Charge Coupled Device) sensor corresponding to each color component of R (red), G (green), and B (blue), which are the three primary colors of light, to provide a color image. That read both images and read black-and-white images are known.
[0003]
In general, when an image of an original is read by an image reading apparatus, high-quality reading quality is required for a color image, whereas high-speed reading is often required for a monochrome image. However, when reading a document image using a three-line CCD sensor as described above, monochrome image data is generated from the output signals of the respective color components obtained from the CCD sensor. Is a monochrome image, the reading speed is the same as when reading a color image. Therefore, it is not possible to sufficiently meet the demand for reading a monochrome image at high speed.
[0004]
Therefore, in recent years, a technique for speeding up reading of a monochrome image by adopting a CCD sensor having a four-line configuration in which one monochrome line sensor is provided in addition to three color line sensors corresponding to RGB color components. Has been proposed. As an example, in Japanese Patent Application Laid-Open No. 2001-144900, the above-described four-line image sensor is adopted, the number of pixels of the black-and-white line sensor is set to twice the number of pixels of the color line sensor, and the exposure cycle of the black-and-white line sensor. Has been proposed to read black-and-white images at high resolution and at high speed by setting ½ as the exposure cycle of the color line sensor.
[0005]
[Problems to be solved by the invention]
However, in the technique described in the above publication, since the exposure cycle of the black and white line sensor is ½ of the exposure cycle of the color line sensor, the signal charge of the black and white line sensor is approximately halfway through the exposure cycle of the color line sensor. It will be transferred vertically. For this reason, unnecessary noise from the monochrome line sensor side may wrap around the output signal from the color line sensor, which may degrade the color image reading quality.
[0006]
In order to read both a monochrome image and a color image at high speed using a CCD sensor having a 4-line configuration, as shown in FIG. 8, a charge readout shift pulse (vertical transfer pulse) corresponding to the monochrome line sensor is used. ) BW-SH and the charge readout shift pulse (vertical transfer pulse) CL-SH corresponding to the color line sensor may be output at individual timings. And the exposure cycle Tb of the color line sensor cannot be synchronized. Therefore, noise due to vertical transfer on the color line sensor side wraps around the output signal BW-OUT from the monochrome line sensor, or noise due to vertical transfer on the monochrome line sensor side affects the output signal CL-OUT from the color line sensor. There is a risk that both the reading quality of the black and white image and the reading quality of the color image may be lowered. In the timing chart of FIG. 8, the frequency F1 of the transfer clock BW-CLK corresponding to the monochrome line sensor and the frequency F2 of the transfer clock CL-CLK corresponding to the color line sensor are the same.
[0007]
The present invention has been made to solve the above-described problems, and an object of the present invention is to perform image reading capable of reading a monochrome image at a high speed without degrading the reading quality of a color image or a monochrome image. To provide an apparatus.
[0008]
[Means for Solving the Problems]
  An image reading apparatus according to the present invention reads a color image as a reading unit that picks up an image of a document using an imaging unit having a color line sensor and a monochrome line sensor, and as a reading color mode of an image by the reading unit. A reading color mode selecting means for selecting one of a color mode applied at the time and a black and white mode applied when reading a monochrome image, and a reading color mode selected by the reading color mode selecting meansThe color line sensor exposure period is fixed regardless of whether the color mode or the black and white mode.According to the reading color mode selected by the reading color mode selection means, the exposure cycle of the monochrome line sensor is set to the exposure cycle of the color line sensor.To the standardAnd an exposure cycle variable means for changingThe exposure cycle variable means makes the exposure cycle of the black and white line sensor the same as the exposure cycle of the color line sensor when the selected read color mode is the color mode, and when the selected read color mode is the black and white mode, By changing the exposure cycle of the monochrome line sensor so that the exposure cycle of the monochrome line sensor is ½ of the exposure cycle of the color line sensor, when the monochrome mode is selected, the same period as the vertical transfer period of the monochrome line sensor is set. The vertical transfer of the color line sensor is executed, and when the color mode is selected, the vertical transfer of the color line sensor is executed in synchronization with the vertical transfer period of the monochrome line sensor.
[0009]
In the image reading apparatus having the above configuration, the black and white mode is changed by changing the exposure cycle of the black and white line sensor by the exposure cycle changing unit according to the read color mode (black and white mode and color mode) selected by the read color mode selection unit. When selected, the vertical transfer of the color line sensor can be executed during the same period as the vertical transfer period of the monochrome line sensor. When the color mode is selected, the vertical transfer of the color line sensor can be executed in synchronization with the vertical transfer period of the monochrome line sensor. It becomes.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0011]
FIG. 1 is a schematic diagram illustrating a configuration example of an image reading apparatus according to an embodiment of the present invention. The illustrated image reading apparatus 1 includes an apparatus main body 2 incorporating a reading unit, and a document pressing portion 3 supported on the apparatus main body 2 by a hinge mechanism or the like so as to be opened and closed. A first document table (platen glass) 4 and a second document table 5 are arranged side by side on the upper surface of the apparatus body 2. These first and second document tables 4 and 5 are both made of light-transmitting glass (transparent glass or the like).
[0012]
The document pressing unit 3 is opened and closed by a user when pressing a document placed on the first document table 4 from above. The document pressing unit 3 is provided with a document setting unit 6, a document discharge unit 7, and a document transport unit 8. The document setting unit 6 is a portion where a document (document bundle) read according to a second reading method (described later) is set. The document discharge portion 7 is a portion where a document read by the second reading method is discharged. The document setting unit 6 and the document discharge unit 7 are arranged in a vertical positional relationship with the document pressing unit 3.
[0013]
The document transport unit 8 transports the document set in the document setting unit 6 one by one along the transport path R, and after moving the transported document on the second document table 5, finally A document is discharged toward the document discharge unit 7. The document conveying unit 8 is provided with a feeding roller 9, a first conveying roller 10, a registration roller 11, a second conveying roller 12, and a discharge roller 13.
[0014]
The feeding roller 9 feeds the document set on the document setting unit 6 onto the conveyance path R. The first conveyance roller 10 conveys the document fed by the feeding roller 9 to the registration roller 11 side along the conveyance path R. The registration roller 11 sends out the document conveyed by the first conveyance roller 10 onto the second document table 5. The second transport roller 12 receives the document that has passed over the second document table 5 by the delivery of the registration roller 11 and transports the document to the discharge roller 13 side. The discharge roller 13 receives the document transported by the second transport roller 12 and discharges it to the document discharge unit 7.
[0015]
The document transport unit 8 configured using the plurality of rollers 9 to 13 transports a document by rotation of each roller, and moves the document on the second document table 5 during the transport. In addition to a typical transport function, a reversing function for reversing the front and back sides of the document surface for double-sided scanning, and an alignment function for returning the orientation of the document surface reversed by the reversing function to the original state and discharging it to the document discharge unit 7 are provided. ing. However, since the inversion function and the matching function are not directly related to the gist of the present invention, detailed description thereof is omitted.
[0016]
Inside the apparatus main body 2, an optical scanning system 14, an imaging lens 15, and a CCD sensor 16 are provided as reading means for optically reading an image of a document. The optical scanning system 14 includes a full-rate carriage 17 and a half-rate carriage 18 that are supported so as to be movable in the sub-scanning direction Y (left-right direction in the figure). A lamp 19 and a first mirror 20 are mounted on the full rate carriage 17, and a second mirror 21 and a third mirror 22 are mounted on the half rate carriage 18.
[0017]
The full-rate carriage 17 and the half-rate carriage 18 move in the sub-scanning direction Y using a common carriage moving motor as a drive source. At this time, the half-rate carriage 18 moves with a movement amount (moving speed) that is half that of the full-rate carriage 17, so that the first and second carriages 18 are moved to any position in the sub-scanning direction Y. The optical path length from the document surface on the document table 4 to the light receiving surface of the CCD sensor 16 is always kept constant.
[0018]
The lamp 19 serves as a light source for irradiating light (illumination light) toward a document to be read, and is configured by, for example, a xenon lamp that emits white light with three wavelengths. The light reflected from the document surface by the light irradiation from the lamp 19 is sequentially reflected by the first mirror 20, the second mirror 21, and the third mirror 22. The imaging lens 15 focuses the light reflected by the third mirror 22 on the light receiving surface of the CCD sensor 16 at a predetermined reduction magnification. The CCD sensor 16 is an image sensor (image sensor) for optically reading an image of a document.
[0019]
In the image reading apparatus 1 configured as described above, when reading an image of a document placed on the first document table 4 according to the first reading method, the optical scanning system 14 is set to a preset home position (FIG. 1). Is moved in the sub-scanning direction Y along the first document table 4 from the left edge of the first document table 4 so that the image of the document placed on the first document table 4 is converted into the optical scanning system 14, the imaging lens 15 and the Optically read and scanned by the CCD sensor 16. This first reading method is also called a “platen scan method” or a “flatbed scan method”.
[0020]
When reading an image of a document set on the document setting unit 6 according to the second reading method, the optical scanning system 14 is stopped at a predetermined position (home position or the vicinity thereof) in the sub-scanning direction Y. A document is conveyed one by one from the document setting unit 6 by the conveying operation of the document conveying unit 8 and sent onto the second document table 5, and an image of the document moving on the second document table 5 at a constant speed is transferred. The optical scanning system 14, the imaging lens 15, and the CCD sensor 16 optically read and scan. This second reading method is also referred to as “CVT (Constant Velocity Transfer) method” or “original flow reading method”. In this case, the position immediately above the first mirror 20 mounted on the full rate carriage 17 is the original image. Reading position.
[0021]
FIG. 2 is a schematic diagram showing a sensor configuration for image reading employed in the embodiment of the present invention. As illustrated, the CCD sensor 16 has a configuration (four-line configuration) including four line sensors. That is, the light receiving portion of the CCD sensor 16 is provided with one monochrome line sensor 23 and three color line sensors 24, 25, and 26. These four line sensors 23, 24, 25, and 26 are arranged in parallel to each other along the main scanning direction (the horizontal direction in the figure). The main scanning direction is a direction orthogonal to the sub-scanning direction.
[0022]
The monochrome line sensor 23 is a main sensor in the monochrome mode applied when reading a monochrome image. The color line sensors 24, 25, and 26 are main sensors in a color mode applied when reading a color image. That is, when the original image is read in the black and white mode, the black and white image data read by the black and white line sensor 23 is adopted as the read result, and when the original image is read in the color mode, the color line sensor 24, The color image data read at 25 and 26 is used as the read result. However, regardless of whether the reading color mode is the monochrome mode or the color mode, the image of the document is read almost simultaneously by both the monochrome line sensor 23 and the color line sensors 24, 25, and 26.
[0023]
The three color line sensors 24, 25, and 26 correspond to R (red), G (green), and B (blue) color components, respectively. Each of the line sensors 23, 24, 25, and 26 for monochrome and color is configured by arranging a plurality of pixels (photosensitive pixels) of a predetermined size in a straight line at a predetermined pitch. One pixel corresponds to one light receiving element (photodiode or the like).
[0024]
The three line sensors 24, 25, and 26 have different color separation characteristics. That is, the line sensor (hereinafter also referred to as R line sensor) 24 corresponding to the red component has a color filter (color separation filter) that transmits only the red component light on the pixel column, and the line sensor corresponds to the green component. A sensor (hereinafter also referred to as a G line sensor) 25 has a color filter that transmits only green component light on a pixel column, and a line sensor (hereinafter also referred to as a B line sensor) 26 corresponding to a blue component is A color filter that transmits only blue component light is provided on the pixel column. On the other hand, the monochrome line sensor 23 has a color filter that transmits only the green component light on the pixel column. That is, the monochrome line sensor 23 has the same color separation characteristics as the G line sensor 25.
[0025]
The reason why the monochrome line sensor 23 has the same color separation characteristics as the G line sensor 25 is as follows. First, when considering the monochromatic line sensor 23 alone, a configuration in which no color filter is provided is also conceivable. In such a case, the sensor sensitivity of the monochromatic line sensor 23 is higher than that of the other three line sensors 24, 25, and 26. For this reason, there is a possibility that only the output of the monochrome line sensor 23 is saturated with white illumination light required for reading a color image. If the amount of illumination light is reduced to avoid this, the amount of light received by the other three line sensors 24, 25, 26 is insufficient, and the RGB (Signal-to-Noise) of the RGB output deteriorates. For this reason, it is very difficult to design the sensor sensitivity of each of the line sensors 23, 24, 25, and 26.
[0026]
Further, in the image reading by the second reading method, if foreign matter such as dust adheres on the second document table 5, the foreign matter may be continuously read to cause black stripes. . In order to prevent such black streaks from occurring, the original image sent to the second original table 5 by the original conveying section 8 is read almost simultaneously by the monochrome line sensor 23 and the RGB line sensors 24, 25, and 26. It is conceivable that the read image data is relatively compared to generate black streaks and perform correction to remove them. In such a case, if the sensor sensitivities of the line sensors to be compared with the read image data are different, a difference occurs in the sensor output even if the same image is read, so that it is difficult to compare the relative levels.
[0027]
On the other hand, when a color filter that transmits green component light is provided in the monochrome line sensor 23, the sensor sensitivity (sensor output characteristics) is equal to that of the G line sensor 25, so that the amount of illumination light is not reduced. Even in this case, it is possible to avoid saturation of the output from the monochrome line sensor 23. Further, as a measure for preventing the occurrence of black streaks, since the relative level comparison can be performed using the image data read by the line sensors 23 and 25 having the same color separation characteristics, the black streak occurrence detection and correction processing are appropriately performed. Can be done.
[0028]
Further, among the color filters corresponding to the RGB color components, when the color filter corresponding to the green component is adopted for the black and white line sensor 23, the color filters corresponding to the other color components (red component and blue component) are compared. As a result, the spectral characteristic region is wide and a larger output signal can be obtained. For this reason, the image of the original can be read with a good S / N ratio over almost the entire visible light region (about 400 to 700 nm). However, when the level adjustment of the sensor sensitivity is performed appropriately, the configuration of the monochrome line sensor 23 is a configuration without a color filter or a configuration using color filters corresponding to other color components (red component, blue component). It doesn't matter.
[0029]
In the CCD sensor 16 having the four-line configuration, as shown in FIG. 3, a charge transfer register (for reading and transferring signal charges generated by photoelectric conversion in the respective line sensors 23, 24, 25, and 26) Hereinafter, a transfer register) is provided. Here, if the number of transfer registers corresponding to each of the RGB line sensors 24, 25, and 26 is M, the number of transfer registers of the monochrome line sensor 23 is 2M. That is, the monochrome line sensor 23 has twice as many transfer registers as the color line sensors 24, 25, and 26.
[0030]
Specifically, for the R line sensor 24, two transfer registers 27A and 27B are provided on one side thereof (between the black and white line sensor 23), and for the G line sensor 25, it is opposite to the R line sensor 24. Two transfer registers 28A, 28B are provided on the side via the B line sensor 26, and two transfer registers 29A, 28B are provided on the opposite side of the G line sensor 25 via the transfer registers 28A, 28B. 29B is attached. The monochrome line sensor 23 is provided with four transfer registers 30A, 30B, 30C, 30D on one side (on the opposite side to the R line sensor 24). In this case, the color line sensors 24, 25, and 26 are arranged adjacent to each other at intervals of one line, and the monochrome line sensor 23 and the G line sensor 25 are arranged at intervals of 12 lines.
[0031]
Thereby, in the R line sensor 24, the odd-numbered (ODD) pixels and the even-numbered (EVEN) pixels can be distributed to the two transfer registers 27A and 27B and output in parallel. Similarly, in the G line sensor 25, the odd-numbered pixels and the even-numbered pixels can be distributed to the two transfer registers 28A and 28B and output in parallel. The B-line sensor 26 also has the odd-numbered pixels. And even-numbered pixels can be distributed to two transfer registers 29A and 29B in parallel. On the other hand, in the monochrome line sensor 23, the odd-numbered pixels are distributed to two transfer registers (for example, transfer registers 30A and 30C), while the even-numbered pixels are allocated to the other two transfer registers (for example, transfer registers 30B and 30D). ) And output in parallel.
[0032]
At that time, about 10 types of shift pulses SH for shifting (reading) signal charges to the four transfer registers 30A, 30B, 30C, and 30D attached to the monochrome line sensor 23 are required. Further, about 10 types of shift pulses SH are required to shift the signal charges to the transfer registers 27A, 27B, 28A, 28B, 29A, and 29B attached to the color line sensors 24, 25, and 26, respectively.
[0033]
Furthermore, a transfer clock for sequentially transferring the signal charges read to the respective transfer registers 27A, 27B, 28A, 28B, 29A, 29B, 30A, 30B, 30C, and 30D in the line direction is also required. In accordance with this transfer clock, black and white output signals BW1, BW2, BW3, and BW4 are output in parallel from the transfer registers 30A, 30B, 30C, and 30D via an amplifier. Further, red component output signals R1 and R2 are output in parallel from the transfer registers 27A and 27B via an amplifier, and green component output signals G1 and G2 are output in parallel from the transfer registers 28A and 28B via an amplifier. Then, blue component output signals B1 and B2 are output in parallel from the transfer registers 29A and 28B through an amplifier.
[0034]
From the above, assuming that the number of pixels of the R line sensor 24 is N, the two transfer registers 27A and 27B attached thereto transfer the signal charges of N / 2 pixels per line, respectively. Become. This also applies to the G line sensor 25 and the B line sensor 26. On the other hand, if the number of pixels of the black and white line sensor 23 is N, which is the same as that of the R line sensor 24, the four transfer registers 30A, 30B, 30C, and 30D attached thereto are each N / 4 per line. The signal charge of each pixel is transferred. As a specific example, if the number of pixels of each of the line sensors 23, 24, 25, and 26 is 7600 pixels per line, the transfer registers 27A, 27B, 28A, 28B, 29A, and 29B corresponding to the RGB color components respectively. Signal charges for 3800 pixels are transferred per line, and signal charges for 1900 pixels are transferred per line in the transfer registers 30A, 30B, 30C, and 30D corresponding to black and white.
[0035]
In this way, by attaching a plurality of transfer registers to one line sensor, it is possible to read a document image at a higher speed than when one transfer register is attached to one line sensor. Further, when the number of transfer registers of the monochrome line sensor 23 is set to be twice the number of transfer registers of the RGB line sensors 23, 24, and 25, signal charges obtained from the monochrome line sensor 23 are transferred by the transfer register. The number of steps is reduced. For this reason, the transfer efficiency of the signal charge obtained from the monochrome line sensor 23 can be increased, and the monochrome image can be read at a higher speed.
[0036]
FIG. 4 is a block diagram showing a functional configuration of the image reading apparatus according to the embodiment of the present invention. In FIG. 4, a CPU (central processing unit) 31 controls processing operations of the entire image reading apparatus 1 according to a control program given in advance. The CPU 31 has a reading color mode selection function, an exposure cycle variable function, and a resolution variable function as main processing functions. Details of each function will be described in detail later. A plurality of sensors (not shown) are connected to the CPU 31, and a detection signal necessary for controlling the operation of the entire image reading apparatus 1 is given from each sensor.
[0037]
Among the plurality of sensors, for example, an open / close detection sensor that detects the open / close state of the document pressing unit 3, a document size detection sensor that detects the document size of the document to be read, and whether the document is set in the document setting unit 6 A first document presence / absence sensor (which can also be used as a document size detection sensor), and a second document presence / absence sensor (document size) that detects whether or not a document is placed on the first document table 4. Various sensors such as a carriage position detection sensor that detects the positions of the carriages 17 and 18 in the sub-scanning direction, and a jam sensor that detects document jamming in the document transport unit 8 are included.
[0038]
A UI (user interface) 32 is configured by an operation panel (control panel) including an input unit and a display unit using buttons, switches, and the like, and operation information for operating the image reading apparatus 1, for example, reading start This is used when the user designates the reading magnification, reading range, document size, document color, document color discrimination, and the like. The specification of the document color is a specification of whether the image of the document to be read is monochrome or color, and the specification of the document color determination is a specification of whether or not to execute the document color determination process. .
[0039]
The crystal oscillator 33 generates a reference clock that serves as a reference when the CCD sensor 16 is driven. The reference clock is a monochrome timing generator (hereinafter abbreviated as TG) 34 and a color timing generator (hereinafter referred to as a timing generator). (Abbreviated as TG) 35. The TGs 34 and 35 generate drive signals for driving the CCD sensor 16 using the reference clock provided from the crystal oscillator 33.
[0040]
The drive signal includes a timing signal for reading out signal charges accumulated in the monochrome line sensor 23, a timing signal for reading out signal charges accumulated in the RGB line sensors 24, 25, and 26, and further a signal. A switching signal for selecting an output signal obtained by reading out electric charges, a reset signal for resetting electric charges after output, and the like are included. The timing signals generated by the TGs 34 and 35 include the above-described shift pulse SH and transfer clock, respectively.
[0041]
The two TGs 34 and 35 are individually given control commands from the CPU 32, and the TGs 34 and 35 respectively generate drive signals in accordance with the control commands. As a result, the image reading operation by the monochrome line sensor 23 (a series of operations including signal charge accumulation, shift, transfer, output, etc.) and the image reading operation by the color line sensors 24, 25, 26 are independent by the CPU 31. It is the structure controlled by.
[0042]
From the CCD sensor 16, the above-described black and white output signals BW1, BW2, BW3, BW4 and color (RGB) output signals R1, R2, G1, G2, B1, B2 are output in different systems. Among these, the black and white output signals BW1, BW2, BW3B, and BW4 are analog processed by analog processing circuits 36-1 and 36-2, and then A / D converters (analog-to-digital converter) 37-1, 37. -2 to convert from an analog signal to a digital signal. Incidentally, in the analog processing circuits 36-1 and 36-2 and the A / D converters 37-1 and 37-2, output signals output by reading from even-numbered pixels in the pixel line of the monochrome line sensor 23. W1 and BW3 and output signals BW2 and BW4 output by reading from odd-numbered pixels are processed separately.
[0043]
On the other hand, the red component output signals R1 and R2 are analog processed by the analog processing circuit 38 and then converted from analog signals to digital signals by the A / D converter 39. Similarly, the green component output signals G1 and G2 are analog processed by the analog processing circuit 40, and then converted from an analog signal to a digital signal by the A / D converter 41. The blue component output signals B1 and B2 are analog processed. After analog processing by the circuit 42, the A / D converter 43 converts the analog signal into a digital signal. Each of the analog processing circuits 36, 38, 40, 42 includes a sample hold circuit, a gain control circuit, an offset control circuit, and the like.
[0044]
The black and white and color image signals digitized in this way are supplied to the image processing circuit 44. The image processing circuit 44 applies various image processing to the black and white and color image signals inputted thereto, and outputs the BW output constituting the black and white image signal and the R output, G output and B constituting the color image signal. Generate output. The image processing performed by the image processing circuit 44 includes shading correction for correcting variations in the output signal due to uneven sensitivity of the CCD sensor 16 and uneven illumination of the illumination system, and correction of the reading position corresponding to the line sensor interval of the CCD sensor 16. In addition to (gap correction), gamma correction using a lookup table, etc., it is determined whether the image of the document to be read is monochrome or color (in other words, the document to be read is a monochrome document) On the second platen 5 when the original image is read by the so-called original color discrimination (ACS; Auto Color Selection) process or the second reading method (CVT method). This includes processing for detecting the occurrence of black streaks caused by foreign matters such as dust adhering to the surface and correcting (removing) the black streaks.
[0045]
Here, the reading color mode selection function, the exposure cycle variable function, and the resolution variable function included in the CPU 31 will be described in order. First, the reading color mode selection function is a function for automatically selecting an image reading color mode in accordance with preset mode selection conditions. Image reading color modes include a color mode applied when reading a color image and a monochrome mode applied when reading a monochrome image. The color mode is a mode for reading a document image as a color image, and the monochrome mode is a mode for reading a document image as a monochrome image.
[0046]
Of these two reading color modes, the CPU 31 selects one of the reading color modes (monochrome mode or color mode) according to the mode selection condition input via the UI 32. Specifically, the monochrome mode is selected when the image of the document is designated as black and white by the input operation of the UI 32 by the user, and the color mode is selected when designated as the color. Further, when it is necessary to determine whether an image of a document to be read is monochrome or color by document color determination processing, that is, when the user designates execution of document color determination processing via the UI 32, Since the original image needs to be read on the premise of a color image, the CPU 31 selects the color mode and executes reading of the original image, and the image processing circuit 44 determines whether the original image is a color image or a monochrome image. Determine.
[0047]
The exposure cycle variable function is based on the exposure cycle of the color line sensors 24, 25, 26 among the exposure cycle of the monochrome line sensor 23 and the exposure cycle of the color line sensors 24, 25, 26 as a reference (fixed). This is a function of automatically changing the exposure cycle according to the reading color mode. The reading color mode in this case refers to the reading color mode (monochrome mode or color mode) selected by the reading color mode selection function. The CPU 31 variably controls the exposure cycle of the monochrome line sensor 23 to switch between the monochrome mode and the color mode using this exposure cycle variable function.
[0048]
Specifically, when the monochrome mode is selected by the reading color mode selection function, the exposure cycle of the monochrome line sensor 23 is set to the first cycle. When the color mode is selected by the reading color mode selection function, the exposure cycle of the monochrome line sensor 23 is set to a second cycle longer than the first cycle. In the exposure cycle variable function of the CPU 31, exposure is performed in advance so that the first cycle is ½ of the second cycle and the second cycle is the same as the exposure cycle of the color line sensors 24, 25, and 26. Period change conditions are set.
[0049]
Incidentally, the exposure cycle of the monochrome line sensor 23 is determined by the output timing of the charge readout (vertical transfer) shift pulse SH corresponding to the monochrome line sensor 23, and the exposure cycle of the color line sensors 24, 25, 26 is It is determined by the output timing of the charge readout (vertical transfer) shift pulse SH corresponding to the color line sensors 24, 25, and 26. Therefore, the CPU 31 variably controls the exposure cycle of the monochrome line sensor 23 by changing the output timing (output cycle) of the charge readout shift pulse SH corresponding to the monochrome line sensor 23.
[0050]
The variable resolution function automatically changes the reading resolution in the sub-scanning direction of the monochrome line sensor 23 (hereinafter referred to as sub-scanning resolution) and the reading resolution in the sub-scanning direction of the color line sensors 24, 25, and 26 according to the reading color mode. It is a function to switch automatically. The reading color mode in this case also refers to the reading color mode (monochrome mode or color mode) selected by the reading color mode selection function. The CPU 31 variably controls the sub-scanning resolution of the black-and-white line sensor 23 and the sub-scanning resolutions of the color line sensors 24, 25, and 26 using this resolution variable function so as to switch between the black-and-white mode and the color mode.
[0051]
Specifically, when the monochrome mode is selected by the reading color mode selection function, the sub-scanning resolution of the monochrome line sensor 23 is set to be higher than the sub-scanning resolution of the color line sensors 24, 25, and 26. When the color mode is selected by the reading color mode selection function, the sub-scanning resolution of the monochrome line sensor 23 and the sub-scanning resolution of the color line sensors 24, 25, and 26 are set to be the same. In the variable resolution function of the CPU 31, when the monochrome mode is selected, the sub-scanning resolution of the monochrome line sensor 23 is twice the sub-scanning resolution of the color line sensors 24, 25, 26, and when the color mode is selected. The resolution changing conditions are set in advance so that the sub-scanning resolution of the monochrome line sensor 23 is the same for both the monochrome line sensor and the color line sensor.
[0052]
Incidentally, the sub-scanning resolution refers to a physical scanning speed (hereinafter referred to as a reading scanning speed) when reading and scanning an image of a document in the sub-scanning direction by the optical scanning system 14, and a timing for reading a signal charge to the transfer register. It depends on. The interval of signal charge read timing to the transfer register corresponds to one line cycle. Further, since signal charges are accumulated in each pixel of the line sensor within this one line period, one line period corresponds to the exposure period of the line sensor. On the other hand, the reading scanning speed is determined when the image of the document placed on the first document table 4 is read, that is, in the case of the first reading method (platen scanning method), the carriages 17 and 18 of the optical scanning system 14. In the case of reading an image of a document set on the document setting unit 6, that is, in the case of the second reading method (CVT method), the document conveying unit 8 is driven to move on the second document table 5. It depends on the moving speed of the original to be printed.
[0053]
Therefore, in the case of the first reading method, the CPU 31 rotates the carriage movement motor serving as the drive source of the optical scanning system 14 together with the signal charge read timing (line sensor exposure cycle) to the transfer register. By changing the number, the sub-scanning resolution of each of the line sensors 23, 24, 25, and 26 is variably controlled. In the case of the second reading method, the signal charge read timing to the transfer register (line sensor exposure) The sub-scanning resolution of each of the line sensors 23, 24, 25, and 26 is variably controlled by changing the number of rotations of the document transport motor serving as the drive source of the document transport unit 8 in combination with the period.
[0054]
Next, the operation of the image reading apparatus 1 based on the control process of the CPU 31 will be described. First, when the user places a document on the first document table 4 and presses the reading start button of the UI 32, the CPU 31 receives this and selects the first reading method as a document image reading method. Is given to various driver circuits (for example, driver circuits for illumination control, carriage movement control, etc.). When the user sets a document on the document setting unit 6 and presses the reading start button of the UI 32, the CPU 31 selects the second reading method as the document image reading method and receives a control command based on the selected reading method. Is supplied to various driver circuits (driver circuits for document conveyance control, illumination control, carriage movement control, etc.).
[0055]
Further, the CPU 31 selects the reading color mode (monochrome mode, color mode) based on the input information from the UI 32 together with the selection of the reading method. That is, when the reading start button of the UI 32 is pressed, if the original color is designated in black and white by the user's input operation prior to that, the monochrome mode is selected, and the original color is designated in color. When execution of the document color discrimination process is designated, the color mode is selected. When the monochrome mode is selected, the drive of the CCD sensor 16 is controlled using the monochrome line sensor 23 as a main sensor and the color line sensors 24, 25, and 26 as sub-sensors. When the color mode is selected, the color line sensor 24 is controlled. , 25 and 26 are used as main sensors, and the monochrome line sensor 23 is used as a sub sensor to control the driving of the CCD sensor 16. When the monochrome mode is selected, sensor output signals from the color line sensors 24, 25, and 26 that are sub-sensors are used as sensor output signals for detecting and correcting black streaks. When the color mode is selected, the black-and-white lines that are sub-sensors are used. The sensor output signal from the sensor 23 is used as a sensor output signal for detecting and correcting the occurrence of black stripes.
[0056]
First, when the CPU 31 selects the monochrome mode, as shown in FIG. 5, the charge readout shift pulse BW-SH1 corresponding to the monochrome line sensor 23 is output from the TG 34 at a predetermined line period T1, and the monochrome line is also output. The transfer clock BW-CLK1 for charge transfer corresponding to the sensor 23 is output from the TG 34 at the frequency F1. Further, a charge read shift pulse CL-SH corresponding to the color line sensors 24, 25, and 26 is output from the TG 35 at a reference line period T2, and charge transfer corresponding to the color line sensors 24, 25, and 26 is performed. The transfer clock CL-CLK is output from the TG 35 at the frequency F2. Further, as the above-described reading method, when the first reading method is selected, the image reading / scanning speed by moving the carriages 17 and 18 is selected. When the second reading method is selected, the image reading / scanning by moving the document is performed. Each speed is set to a first speed.
[0057]
In this monochrome mode, the exposure cycle of the monochrome line sensor 23 is set to the first cycle “T1” according to the output timing of the shift pulse BW-SH1, and the exposure cycle of the color line sensors 24, 25, 26 is set. The reference period “T2” is set according to the output timing of the shift pulse CL-SH. In this case, the relationship between the two is T2 = T1 × 2, that is, the exposure cycle T1 of the monochrome line sensor 23 is half the exposure cycle T2 of the color line sensors 24, 25, and 26. The exposure cycle T1 of the monochrome line sensor 23 and the exposure cycle T2 of the color line sensors 24, 25, and 26 are synchronized with each other. On the other hand, the charge transfer frequency (transfer clock BW-CLK1 frequency) F1 of the transfer registers 30A, 30B, 30C, 30D corresponding to the monochrome line sensor 23 is the transfer register 27A, 27B corresponding to the color line sensors 24, 25, 26. , 28A, 28B, 29A, 29B have the same relationship as the charge transfer frequency (transfer clock CL-CLK frequency) F2.
[0058]
Therefore, when the CPU 31 selects the monochrome mode, in the monochrome line sensor 23, the signal charges accumulated in each pixel within the exposure cycle T1 are transferred to the four transfer registers 30A and 30B according to the shift pulse BW-SH1. , 30C, 30D, and sequentially transferred in the line direction in accordance with the transfer clock BW-CLK1 in each transfer register 30A, 30B, 30C, 30D. Therefore, if the number of pixels of the black line sensor 23 is N, 1 / 4N signal charges are transferred by the transfer registers 30A, 30B, 30C, and 30D within the exposure cycle T1. Therefore, signal charges of a total of N pixels are output within the exposure cycle T1.
[0059]
In the color line sensor 24 corresponding to the red component, the signal charges accumulated in each pixel within the exposure cycle T2 are read out to the two transfer registers 27A and 27B according to the shift pulse CL-SH. The transfer registers 27A and 27B sequentially transfer in the line direction according to the transfer clock CL-CLK. Therefore, assuming that the number of pixels of the color line sensor 24 is N, 1 / 2N signal charges are transferred by the transfer registers 27A and 27B within the exposure cycle T2. Accordingly, signal charges of a total of N pixels are output within the exposure cycle T2. The same applies to the color line sensor 25 corresponding to the green component and the color line sensor 26 corresponding to the blue component.
[0060]
From the above, when the CPU 31 selects the monochrome mode, the two-line cycle (T1 × 2) of the monochrome line sensor 23 becomes the one-line cycle (T2) of the color line sensors 24, 25, and 26. During the period in which the signal charges for one line are output by 24, 25, and 26, the monochrome line sensor 23 outputs the signal charges for two lines.
[0061]
Further, when the level of the output signal BW-OUT1 of the monochrome line sensor 23 is “V1” and the level of the output signal CL-OUT of the color line sensor 25 corresponding to the green component is “V2” when the monochrome mode is selected. The relationship between the output levels (high and low) is the relationship of V1≈V2 / 2 due to the difference between the exposure cycles T1 and T2, that is, the output level V1 of the monochrome line sensor 23 is almost half of the output level V2 of the color line sensor 25. It becomes.
[0062]
Further, the sub-scanning resolution when the monochrome mode is selected is that the line cycle T1 of the monochrome line sensor 23 is ½ of the line cycle T2 of the color line sensors 24, 25, and 26. The resolution is twice the sub-scanning resolution of the color line sensors 24, 25, and 26. Therefore, when the monochrome mode is selected and the sub-scanning resolution of the monochrome line sensor 23 is 600 dpi, the sub-scanning resolution of the color line sensors 24, 25, and 26 is 300 dpi.
[0063]
As described above, when the CPU 31 selects the monochrome mode, the vertical transfer (reading of signal charges to the transfer register) of the monochrome line sensor 23 is performed at almost the middle of the exposure cycle T2 of the color line sensors 24, 25, and 26. However, the vertical transfer of the main monochrome line sensor 23 is performed in the same period as the vertical transfer of the color lines 24, 25, 26, and the color line sensors 24, 25, 26 are within the exposure cycle T 1 of the monochrome line sensor 23. Vertical transfer is not performed. For this reason, noise caused by vertical transfer on the color line sensors 24, 25, and 26 side does not wrap around the monochrome sensor output signal mainly for the monochrome line sensor 23. Therefore, when the monochrome mode is selected, the monochrome image can be read with high quality using the monochrome line sensor 23.
[0064]
Also, the monochrome line sensor 23 is provided with four transfer registers 30A, 30B, 30C, and 30D, and the signal charges read from the respective pixels are transferred in parallel, so that the monochrome image can be read at high speed. it can. In addition, the monochrome variable image can be read with high resolution by the variable resolution function of the CPU 31. However, since the output signal of the line sensor 25 corresponding to the green component having the same color separation characteristic as that of the black and white line sensor 23 is used in black line occurrence detection and correction processing, the output signal from the line sensor 25 is used. It is desirable to consider noise wraparound from the monochrome line sensor 23 side.
[0065]
On the other hand, when the CPU 31 selects the color mode, as shown in FIG. 6, the charge readout shift pulse BW-SH2 corresponding to the monochrome line sensor 23 is output from the TG 34 at a predetermined line period T3, and the monochrome line A transfer clock BW-CLK2 for charge transfer corresponding to the sensor 23 is output from the TG 34 at the frequency F1. Further, a charge read shift pulse CL-SH corresponding to the color line sensors 24, 25, and 26 is output from the TG 35 at a reference line period T2, and charge transfer corresponding to the color line sensors 24, 25, and 26 is performed. The transfer clock CL-CLK is output from the TG 35 at the frequency F2.
[0066]
Further, as the above-described reading method, when the first reading method is selected, the image reading and scanning speed by the movement of the carriages 17 and 18 is selected. When the second reading method is selected, the image reading and scanning by the document movement is performed. The speed is set to a second speed that is slower than that in the monochrome mode (first speed). Specifically, the second speed is set so as to be a half of the first speed.
[0067]
In this color mode, the exposure cycle of the monochrome line sensor 23 is set to the second cycle “T3” according to the output timing of the shift pulse BW-SH2, and the exposure cycle of the color line sensors 24, 25, and 26 is set. It is set to “T2” according to the output timing of the shift pulse CL-SH. In this case, the relationship between the two is T3 = T2, that is, the exposure cycle T3 of the monochrome line sensor 23 is the same as the exposure cycle T2 of the color line sensors 24, 25, and 26. The exposure cycle T3 of the monochrome line sensor 23 and the exposure cycle T2 of the color line sensors 24, 25, and 26 are synchronized with each other. Further, the exposure cycle (second cycle) T3 of the monochrome line sensor 23 in the color mode is longer than the exposure cycle (first cycle) T1 of the monochrome line sensor 23 in the monochrome mode described above, and the relationship between them is T1 = The exposure cycle T1 of the monochrome line sensor 23 in the monochrome mode, that is, T3 / 2, is a half of the exposure cycle T3 of the monochrome line sensor 23 in the color mode.
[0068]
In this case, the CPU 31 changes the exposure cycle of the monochrome line sensor 23 in the monochrome mode and the color mode by changing the number of transfer clocks in one line cycle corresponding to the monochrome line sensor 23. Specifically, the monochrome line sensor 23 is changed by changing the number of transfer clocks in one line period corresponding to the monochrome line sensor 23 so that the number in the color mode is twice that in the monochrome mode. The relationship between the exposure periods (the first period and the second period) is changed to T1 = T3 / 2.
[0069]
On the other hand, the charge transfer frequency F1 of the transfer registers 30A, 30B, 30C, 30D corresponding to the monochrome line sensor 23 is the same as that of the transfer registers 27A, 27B, 28A, 28B, 29A, 29B corresponding to the color line sensors 24, 25, 26. The relationship is the same as the charge transfer frequency F2. Further, the charge transfer frequencies F1, F2 of the line sensors 23, 24, 25, 26 in the color mode are the same as the charge transfer frequencies F1, F2 of the line sensors 23, 24, 25, 26 in the monochrome mode described above. ing.
[0070]
Therefore, when the CPU 31 selects the color mode, in the monochrome line sensor 23, the signal charges accumulated in each pixel within the exposure cycle T3 are transferred to the four transfer registers 30A and 30B according to the shift pulse BW-SH2. , 30C, 30D, and sequentially transferred in the line direction in accordance with the transfer clock BW-CLK2 in each transfer register 30A, 30B, 30C, 30D. At this time, since the charge transfer frequency F1 in the color mode is set to be the same as that in the monochrome mode, the signal charge read to each transfer register 30A, 30B, 30C, 30D is an intermediate part of the exposure cycle T3. All will be transferred with. For this reason, in the second half of the exposure cycle T3, a black level signal whose signal charge is substantially zero is output by idle transfer in the transfer register. If the number of pixels of the monochrome line sensor 23 is N, 1 / 4N signal charges (excluding dummy signals) are transferred by the transfer registers 30A, 30B, 30C, and 30D within the exposure cycle T3. The Accordingly, signal charges of a total of N pixels are output within the exposure cycle T3.
[0071]
Further, in the color line sensor 24 corresponding to the red component, the signal charges accumulated in each pixel within the exposure cycle T2 are transferred in accordance with the shift pulse CL-SH, as in the above-described monochrome mode. After being read out to the registers 27A and 27B, they are sequentially transferred in the line direction in accordance with the transfer clock CL-CLK in the respective transfer registers 27A and 27B. Therefore, assuming that the number of pixels of the color line sensor 24 is N, 1 / 2N signal charges are transferred by the transfer registers 27A and 27B within the exposure cycle T2. Therefore, a total of N signal charges (the same number of signal charges as in the case of the monochrome line sensor 23) are output within the exposure cycle T2. The same applies to the color line sensor 25 corresponding to the green component and the color line sensor 26 corresponding to the blue component.
[0072]
From the above, when the CPU 31 selects the color mode, one line period (T3) of the monochrome line sensor 23 becomes one line period (T2) of the color line sensors 24, 25, 26. During the period in which the signal charges for one line are output at 25 and 26, the signal charge for one line is also output by the monochrome line sensor 23.
[0073]
Further, when the level of the output signal BW-OUT2 of the monochrome line sensor 23 is “V3” and the level of the output signal CL-OUT of the color line sensor 25 corresponding to the green component is “V2” when the color mode is selected, The output level relationship (level relationship) is V3≈V2 from the commonality of the exposure cycles T3 and T2, that is, the output level V3 of the monochrome line sensor 23 is almost the same level as the output level V2 of the color line sensor 25. It becomes.
[0074]
Further, the output level V3 of the monochrome line sensor 23 when the color mode is selected is higher than the output level V1 of the monochrome line sensor 23 when the monochrome mode is selected, and the relationship of V3≈V1 × 2 is obtained in the relative comparison between the two. . Further, the sub-scanning resolution when the color mode is selected is equal to the sub-scanning resolution of the monochrome line sensor 23 because the exposure cycle T3 of the monochrome line sensor 23 is the same as the exposure cycle T2 of the color line sensors 24, 25, and 26. The sub-scanning resolutions of the color line sensors 24, 25, and 26 are also the same.
[0075]
Further, since the scanning speed is ½ that in the monochrome mode, the sub-scanning resolution of the color line sensors 24, 25, and 26 is twice that in the monochrome mode. For this reason, when the sub-scanning resolution of the monochrome line sensor 23 is set to 600 dpi when the color mode is selected, the sub-scanning resolution of the color line sensors 24, 25, and 26 is also 600 dpi.
[0076]
As described above, when the CPU 31 selects the color mode, the vertical transfer of the main color line sensors 24, 25, and 26 is performed in the same period as the vertical transfer of the monochrome line sensor 23, and the vertical transfer of both is synchronized. Therefore, the vertical transfer of the monochrome line sensor 23 is not performed within the exposure cycle T2 of the color line sensors 24, 25, and 26. For this reason, noise caused by vertical transfer on the monochrome line sensor 23 side does not wrap around the color sensor output signals mainly of the color line sensors 24, 25, and 26. Therefore, when the color mode is selected, the color image can be read with high quality using the color line sensors 24, 25, and 26.
[0077]
Also, the color image can be read with high resolution by the resolution variable function of the CPU 31. Further, when black line occurrence detection or correction processing is performed, an output signal of the monochrome line sensor 23 having color separation characteristics equivalent to the color line sensor 25 corresponding to the green component can be used. In this case, the output signal CL-OUT of the color line sensor 25 and the output signal BW-OUT2 of the monochrome line sensor 23 have the same output level (V3≈V1). In addition, the correction process can be performed with high accuracy.
[0078]
In the above embodiment, the CPU 31 changes the exposure cycle of the monochrome line sensor 23 in the monochrome mode and the color mode by changing the number of transfer clocks in one line cycle corresponding to the monochrome line sensor 23. However, the present invention is not limited to this, and by changing the frequency of the transfer clock within one line period corresponding to the monochrome line sensor 23, the monochrome mode and the color mode can be changed to monochrome. The exposure cycle of the line sensor 23 may be changed. Specifically, when the CPU 31 selects the color mode, as shown in FIG. 7, the frequency F3 of the transfer clock BW-CLK3 within one line cycle in the monochrome line sensor 23 is 1 in the monochrome mode (frequency F1). By setting to / 2, the relationship between the exposure periods (first period and second period) of the monochrome line sensor 23 is changed to T1 = T3 / 2.
[0079]
Even when the CCD sensor 16 is driven according to the timing chart shown in FIG. 7, when the color mode is selected, the vertical transfer of the main color line sensors 24, 25, and 26 is the same as the vertical transfer of the monochrome line sensor 23. The vertical transfer of the monochrome line sensor 23 is not performed within the exposure period T2 of the color line sensors 24, 25, and 26 because the vertical transfer is performed in the same period and both are performed in synchronization. For this reason, noise caused by vertical transfer on the monochrome line sensor 23 side does not wrap around the color sensor output signals mainly of the color line sensors 24, 25, and 26. Therefore, when the color mode is selected, the color image can be read with high quality using the color line sensors 24, 25, and 26. Also, the color image can be read with high resolution by the resolution variable function of the CPU 31. Further, when black line occurrence detection or correction processing is performed, the output signal CL− of the color line sensor 25 is obtained by using the output signal of the monochrome line sensor 23 having the same color separation characteristics as the color line sensor 25 corresponding to the green component. Since the OUT and the output signal BW-OUT3 of the monochrome line sensor 23 have the same output level (V4≈V1), the black stripe occurrence detection and correction processing can be performed with high accuracy by comparing both of them.
[0080]
【The invention's effect】
As described above, according to the present invention, the monochrome line sensor is selected when the monochrome mode is selected by changing the exposure period of the monochrome line sensor by the exposure period variable means according to the reading color mode selected by the reading color mode selecting means. The vertical transfer of the color line sensor can be executed in the same period as the vertical transfer period of the color line sensor, and when the color mode is selected, the vertical transfer of the color line sensor can be executed in synchronization with the vertical transfer period of the monochrome line sensor. As a result, it is possible to speed up the reading of a monochrome image without degrading the reading quality of a color image or a monochrome image. As a result, it is possible to achieve both maintenance / improvement of reading quality when reading a color image and improvement of productivity when reading a monochrome image.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating a configuration example of an image reading apparatus according to an embodiment of the present invention.
FIG. 2 is a schematic diagram illustrating a sensor configuration for image reading employed in an embodiment of the present invention.
FIG. 3 is a diagram showing details of a sensor configuration employed in the embodiment of the present invention.
FIG. 4 is a block diagram illustrating a functional configuration of the image reading apparatus according to the embodiment of the present invention.
FIG. 5 is a timing chart showing an example of sensor drive timing and sensor output signal in the black and white mode.
FIG. 6 is a timing chart showing an example of sensor drive timing and sensor output signal in the color mode.
FIG. 7 is a timing chart showing another example of sensor drive timing and sensor output signal in the color mode.
FIG. 8 is a timing chart showing an example of conventional sensor driving timing and sensor output signal.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Image reading apparatus, 8 ... Document conveyance part, 14 ... Optical scanning system, 15 ... Imaging lens, 16 ... CCD sensor, 23 ... Monochrome line sensor, 24, 25, 26 ... Color line sensor, 27A, 27B, 28A , 28B, 29A, 29B, 30A, 30B, 30C, 30D ... transfer registers, 31 ... CPU, 32 ... UI, 34,35 ... TG (timing generator), 44 ... image processing circuit

Claims (7)

Reading means for picking up an image of a document using an imaging unit having a color line sensor and a monochrome line sensor;
Reading color mode selection means for selecting one of a color mode applied when reading a color image and a monochrome mode applied when reading a monochrome image as the reading color mode of the image by the reading means;
Regardless of whether the reading color mode selected by the reading color mode selection unit is the color mode or the black and white mode, the exposure cycle of the color line sensor is fixed, and the reading color mode selected by the reading color mode selection unit is set. In response, the exposure cycle variable means for changing the exposure cycle of the monochrome line sensor based on the exposure cycle of the color line sensor ,
When the selected reading color mode is a color mode, the exposure cycle varying means makes the exposure cycle of the monochrome line sensor the same as the exposure cycle of the color line sensor, and the selected reading color mode is a monochrome mode. In this case, by changing the exposure cycle of the monochrome line sensor so that the exposure cycle of the monochrome line sensor is ½ of the exposure cycle of the color line sensor, the monochrome line is selected when the monochrome mode is selected. The vertical transfer of the color line sensor is executed in the same period as the vertical transfer period of the sensor, and the vertical transfer of the color line sensor is executed in synchronization with the vertical transfer period of the monochrome line sensor when the color mode is selected. A characteristic image reading apparatus. The image reading apparatus according to claim 1, wherein the number of charge transfer registers attached to the monochrome line sensor is twice the number of charge transfer registers per line sensor attached to the color line sensor. Resolution changing means for switching the reading resolution in the sub-scanning direction of the monochrome line sensor and the reading resolution in the sub-scanning direction of the color line sensor according to the reading color mode selected by the reading color mode selection means. The image reading apparatus according to claim 1, wherein: The color line sensor comprises a plurality of line sensors each having different color separation characteristics,
The image reading apparatus according to claim 1, wherein the monochrome line sensor has the same color separation characteristics as one of the plurality of line sensors. The image reading apparatus according to claim 1, wherein the exposure cycle varying unit changes the exposure cycle of the monochrome line sensor by changing the number of clocks within one line cycle corresponding to the monochrome line sensor. The image reading apparatus according to claim 1, wherein the exposure cycle varying unit changes the exposure cycle of the monochrome line sensor by changing a clock frequency within one line cycle corresponding to the monochrome line sensor. The resolution variable means sets the reading resolution in the sub-scanning direction of the monochrome line sensor higher than the reading resolution in the sub-scanning direction of the color line sensor when the monochrome mode is selected by the reading color mode selection means. The reading resolution in the sub-scanning direction of the monochrome line sensor is set to be the same as the reading resolution in the sub-scanning direction of the color line sensor when the color mode is selected by the reading color mode selection unit. The image reading apparatus according to claim 3 .

Images