Method and apparatus for image reading capable of detecting dust that disturbs image reading operation

[0002] 1. Field of the Invention

[0003] The present invention relates to a method and apparatus for image reading, and more particularly to a method and apparatus for image reading that detects foreign particles which disturb an image reading operation.

[0004] 2. Discussion of the Background

[0005] Many image reading apparatuses perform various operations for adjusting an image signal from an image sensor before starting the image reading operation. For such an adjusting operation, the image reading apparatus is generally provided with a reference white plate for an image sensor to read a reference white image. Based on the reference white image data for one main scanning length obtained by reading the reference white plate, the image reading apparatus adjusts gains of image signal in order to justify the level of the image signal from the image sensor, or performs a shading correction to correct for signal distortion due to inaccurate optical mechanisms employed therein or variations in optoelectronic conversion components such as a CCD (charge-coupled device), and so forth.

[0006] Also, the image reading apparatus is provided with a reference black plate for an image sensor to read a reference black image. Based on the reference black image data for one main scanning length obtained by reading the reference black plate, the image reading apparatus reduces an offset which is previously included in an image signal from the image sensor.

[0007] To perform the above-described adjustment and correction, the image reading apparatus is required to read the reference white and reference black plates in an accurate manner. However, even if the image reading apparatus reads the reference white and reference black plates, the reading accuracy may be disturbed by a foreign particle such as dust or the like which is put on the reference white and reference black plates.

[0008] More specifically, if a dust particle which is less white is put on the reference white image of the reference white plate, the level of the reference white image reading at the location of the foreign particle may erroneously be decreased. If a dust particle which is less black is put on the reference black image of the reference black plate, the level of the reference black image reading at the location of the foreign particle may erroneously be increased. The image reading apparatus will adjust the image signal based on such an erroneous reference white/black image data. When the image reading apparatus is connected to a printer, for example, the image data may be recorded on a recording sheet with an erroneous black line or the like.

[0009] Various solutions for the above-described problem have been introduced. For example, Japanese Patent Publication No. 60-251768(1985) describes a technique in which the image sensor reads a number of times reference white and reference black images in order to reduce an adverse influence to the reference white and reference black image signals by the foreign particle. However, this technique fails in speed, requiring a relatively long time period for completing the adjustment. In fact, causes of the erroneous image data may not be only the foreign particles put on the reference white/black image plate but also those on the interior of the image sensor. The former can be recovered by removing the foreign particles but the latter may need an exchange of the image sensor, for example.

[0010] Typically, these problems caused by the inner or outer foreign particles are found only after outputting the image data in a visual form such as a printout form. Yet, an abnormal print generally needs to be analyzed as to what the cause is. The foreign particle may be only one of the causes.

[0011] Therefore, at present, there is no image reading apparatus which is capable of detecting a foreign particle such as dust or the like which is put on a reference white/black plate or an image sensor, which disturbs an image reading operation.

[0012] Accordingly, an object of the present invention is to provide a novel image reading apparatus which is capable of detecting foreign particles such as dust or the like that may disturb an image reading operation, in an efficient manner and with a simple structure.

[0013] To achieve the above-mentioned object, one example of image reading apparatus according to the present invention includes a reference image plate, an image reading device, a peak value detector, a first memory, a calculator, and an error output controller. The reference image plate has a reference white image on the surface of the plate, and the first memory stores a first reference value. The image reading device reads such a reference white image for one line in a main scanning direction. The peak value detector detects the smallest data among the data which are included in the reference white image read by the image reading device. Then, the calculator compares the smallest data detected by the peak value detector with the first reference value. The error output controller outputs a white image error when the smallest data is smaller than the first reference value.

[0014] In one example, the image reading apparatus according to the present invention may includes a second memory for storing a second reference value. By including the second memory to store a second reference value, the image reading apparatus according to the present invention may perform the additional operations. For example, the image reading device may read the reference white image for one line in the main scanning direction at first and second positions which are different from each other. The peak value detector then detects the smallest data among the data which are included in the reference white image read at the first and second positions by the image reading device. Then, the calculator compares each of the smallest data read at the first and second positions with the first reference value, and calculates a difference between the smallest data read at the first and second positions with the second reference value. Finally, the error output controller outputs a white image error caused by an exterior error factor when at least one of the smallest data read at the first and second positions is smaller than the first reference value and when the difference between the smallest data is greater than the second reference value, and outputs a white image error caused by an interior error factor when at least one of the smallest data read at the first and second positions is smaller than the first reference value and when the difference between the smallest data is smaller than the second reference value.

[0015] In one example, the image reading apparatus according to the present invention may include a third memory for storing a third reference value. By including the third memory, the image reading apparatus may perform the additional operations. For example, the reference image plate has a reference black image on the surface of the plate. The image reading device reads the reference black image for one line in a main scanning direction in a first condition in which a light source illuminates the reference image plate and in a second condition in which no light source illuminates the reference image plate. The peak value detector detects the greatest data among the data included in the reference black image read in the first and second conditions, respectively, by the image reading device. Then, the calculator compares the greatest data read in the first condition with the greatest data read in the second condition, and calculates a difference between the greatest data read in the first and second conditions with the third reference value. Finally, the error output controller outputs a black image error when the difference is greater than the third reference value.

[0016] In one example of the image reading apparatus according to the present invention, the first memory may include a plurality of first reference values which are user selectable.

[0017] In another example of the image reading apparatus according to the present invention, the image reading device may be configured with a charge-coupled device.

[0018] Other objects, features, and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.

[0019] A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

[0020] FIG. 1 is a schematic block diagram of an image reading apparatus according to a first preferred embodiment of the present invention;

[0021] FIG. 2 is a diagram illustrating exemplary white and black output waveforms of an image signal generated by an image sensor of the image reading apparatus of FIG. 1;

[0022] FIG. 3 is a diagram illustrating four exemplary white output waveforms of the image signal with four different offset levels;

[0023] FIGS. 4(A) and 4(B) illustrate typical waveforms of the image signals for white and black outputs which is read for one line in the main scanning direction by the image sensor of FIG. 1;

[0024] FIGS. 5(A) and 5(B) illustrate exemplary waveforms made particularly when foreign particles such as dust or the like are put on a image reading surface of the image sensor or other places so as to disturb an image reading operation of the image reading apparatus of FIG. 1;

[0025] FIGS. 6A-6C show flowcharts which illustrate a procedure of an offset/gain adjusting operation of the image reading apparatus of FIG. 1;

[0026] FIGS. 7A-7B show flowcharts which explain a procedure of a peak white/black value collecting operation of the image reading apparatus of FIG. 1;

[0027] FIG. 8 is a table which includes a plurality of peak black and peak white values both corresponding to each of eight phases for sample clock signals and which is stored in a memory of the image reading apparatus of FIG. 1;

[0028] FIG. 9 is a schematic block diagram of another image reading apparatus according to a second preferred embodiment of the present invention;

[0029] FIGS. 10A-10D show flowcharts which illustrate a procedure of an offset/gain adjusting operation of the image reading apparatus of FIG. 9;

[0030] FIG. 11 is a schematic block diagram of still another image reading apparatus according to a third preferred embodiment according to the present invention; and

[0031] FIGS. 12A-12D show flowcharts which illustrate a procedure of an offset/gain adjusting operation of the image reading apparatus of FIG. 11;

[0032] In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the present invention is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents which operate in a similar manner.

[0033] Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to FIG. 1 thereof, there is illustrated a block diagram for showing an overall configuration of an image reading apparatus 100 as a first embodiment according to the present invention.

[0034] The image reading apparatus 100 of FIG. 1 includes a white and black referencing mechanism 1 (referred to as a reference mechanism 1) which includes a reference white/black plate 1a,a light source 1b,and an image sensor 2 which includes a photoelectric conversion device such as a CCD (charge-coupled device) 2a.The image reading apparatus 100 further includes an offset/gain correction circuit 2b,an offset adjusting circuit 4, an A/D (analog-to-digital) converter 5, a sample clock generator 6, and a peak value detector 7. The image reading apparatus 100 further includes an image processing unit 8, a RAM (random access memory) 9, a control unit 10, a ROM (read only memory) 11, and an error display unit 12. The RAM 9 includes a first peak white value memory block 9aand a first peak black value memory block 9b.The RAM 9 further includes a second peak white value memory block 9cand a second peak black value memory block 9d.Since the former memory group of the first memory blocks 9aand 9band the latter memory group of the second memory blocks 9cand 9dare not used at the same time, it may be possible to combine the both memory groups combined into one. The control unit 10 includes a reference white value register 10a.The reference white value register 10amay alternatively be located in the RAM 9.

[0035] The reference mechanism 1 is configured to provide white and black color references to an image data processing mechanism of the image reading apparatus 100. The reference white/black plate 1ahas a surface having reference white and reference black line images thereon both in full length along a CCD scanning passage in the main scanning direction. The light source 1bmay be a xenon lamp or the like, and illuminates the reference white/black plate 1aso as to intensify the reference white and black line images. The light source may be combined with another light source (not shown) that illuminates a document to be read by the image sensor 2. Such a reference white/black plate 1aneeds to be maintained free from dust or the like. For this purpose, users are required to clean the reference white/black plate 1aoccasionally.

[0036] The image reading apparatus 100 is provided with a clock generator (not shown) to generate clock signals, which are referred to as pixel clocks, for synchronizing image pixels. When reading a document, the CCD 2aof the image sensor 2 which receives the pixel clocks scans the document line by line in synchronism with the pixel clocks. The CCD 2aaccordingly outputs a primary image signal which represents an image of the document. The primary image signal output from the CCD 2ais applied to the offset/gain correction circuit 2b.The offset/gain correction circuit 2bcorrects for the gain and offset of the primary image output signal to the respectively appropriate levels. This correction makes an output signal from the image sensor 2 appropriate. The signal output from the image sensor 2 is referred to as an image signal A (indicated by the letter A), and is applied to the offset adjusting circuit 4.

[0037] When the image sensor 2 is installed, the offset/gain correction circuit 2bis adjusted to output an appropriate image signal. In addition, dust and the like on the image reading surface of the CCD 2aneeds to be cleaned off.

[0038] The offset adjusting circuit 4 receives the image signal A from the image sensor 2 and an offset-level-change signal issued by the control unit 10. The offset adjusting circuit 4 generates an offset-counterbalancing signal (referred to as offset c/b signal) in response to the offset-level-change signal, and adds the offset c/b signal to the image signal A. After adding signals, the offset adjusting circuit 4 outputs a mixed signal of the image signal A and the offset-level-change signal. This output of the offset adjusting circuit 4 is referred to as an image signal B (indicated by the letter B). The image signal B is input to the A/D converter 5.

[0039] The A/D converter 5 which receives sample clock signals generated by the sample clock generator 6 performs an analog-to-digital data conversion in which the image signal B is sampled with the sample clock signals so that the analog information of the image signal is quantified as digital information. The A/D converter 5 outputs the thus-sampled signal, which is referred to as an image signal C (indicated by the letter C).

[0040] More specifically to the aforementioned quantification operation on the image signal B, the image reading apparatus 100 is provided with a reference voltage generating source (not shown) for supplying a reference voltage, i.e. 2 volts, for example. With this voltage, the offset adjusting circuit 4 is configured to output the image signal B variable in a 0- to 2-volts range. The analog information of the 0- to 2-volts range is divided into 256 stepping levels with an 8-bit resolution, for example. Such an 8-bit resolution of the A/D converter 5 may be other resolutions such as a 6-bit, 10-bit, 12-bit, or more-bit resolution.

[0041] The sample clock generator 6 receives the pixel clocks from the clock generator (not shown) and produces the aforementioned sample clock signals in synchronism with the pixel clocks. The sample clock generator 6 supplies the sample clock signals to the A/D converter 5, as described above. The sample clock generator 6 also receives a phase change signal for changing phases P(L) for the sample clock signals from the control unit 10.

[0042] The peak value detector 7 analyzes the image signal C line by line so as to detect the pixels which have the greatest and smallest values among pixels contained in each line of the image in the main scanning direction. When the CCD 2areads the reference white line image, the image sensor 2 outputs a signal referred to as a white output. The smallest value detected in the white output is specifically referred to as a peak white value W_P. When the CCD 2areads the reference black line image, the image sensor 2 outputs a signal referred to as a black output. The greatest value detected in the black output is specifically referred to as a peak black value B_P. Alternatively, the image sensor 2 may generate a peak dark value B_pby detecting the peak black value output by the CCD 2aas it reads the reference white line image while the reference image is not illuminated.

[0043] After the above-described detection operations, the peak value detector 7 stores the peak white and peak black values of each image line, which have been detected, into the RAM 9 at predetermined locations, explained later, so as to be later read by the control unit 10. Detection of the peak white and peak black values of each image line by the peak value detector 7 is performed using, for example, the following simple data comparisons. When a plurality of successive pixels that constitute an image line are input into the peak value detector 7, values of two adjacent pixels are compared and a greater or smaller value is held. The value held is then compared with a value of the next adjacent pixel. This comparison is repeated one after another, and the value that is last held is determined as the peak white or peak black value of the image line.

[0044] The image processing unit 8 receives the image signal C and performs image processing operations, such as a shading correction and so forth. After the image processing unit 8, the image signal is transmitted to another stage of image processing operations and may be treated differently depending upon a host apparatus to which the image reading apparatus 100 is connected. For example, in some host apparatus, the image signal may be treated as the print data to be recorded on a recording sheet, or the display data to be displayed on a monitoring display of a host computer, or the facsimile data to be transmitted to other terminals.

[0045] The control unit 10 is a microcomputer for controlling the entire operation of the image reading apparatus 100 in accordance with the control procedure stored in ROM 11, using RAM 9 as a working memory. The reference white value register 10aof the control unit 10 stores a reference white value W_R. A plurality of reference white value W_Rmay be pre-stored in the reference white value register 10afor user selection.

[0046] The RAM 9 receives the peak white and peak black values W_Pand B_Pfrom the peak value detector 7, and exchanges information with the image processing unit 8 and the control unit 10. The first peak white value memory block 9astores the peak white value W_Pand the first peak black value memory block 9bstores the peak black value B_P. The second peak white value memory block 9cstores a peak white value set W_P(L)and the second peak black value memory block 9astores a peak black value set B_P(L). The error display unit 12 includes a plurality of indicators such as LEDs (light-emitting diode), for example, for indicating various errors in accordance with the instructions from the control unit 10.

[0047] The thick data lines shown in FIG. 1 indicate the image data with multiple bits.

[0048] Next, the details of reference white and reference black values are explained with reference to FIG. 2. When the image signal A is sampled for a time period of a pixel cycle T_p, two different waveforms may be detected depending upon the image signal A, as illustrated in FIG. 2. A waveform A1 of FIG. 2 is typically generated when the image signal A is a white output signal, and a waveform A2 is typically generated when the image signal A is a black output signal. The white output signal is produced when the image sensor 2 reads the reference white line image of the reference white/black plate 1a.The black output signal is produced when the image sensor 2 reads the reference black line image of the reference white/black plate 1a.

[0049] When the dark output signal is used in place of the black output signal, a waveform thereof may be similar to the waveform A2. As described above, the dark output signal is produced by the image sensor 2 when the CCD 2areads the reference white line image of the reference white/black plate 1awith the light source 1bturned off.

[0050] FIG. 2 also indicates eight-step phases P(L), i.e. from P(0) through to P(7) generated by the sample clock generator 6. The eight-step phases determine the timing of the sample clock signals during each pixel cycle T_p. The timing of the sample clock signals can be set to one of these eight-step phases by the phase change signal output from the control unit 10. Various known techniques are available to perform this phase change operation. For example, a set of different-delay-time elements may be provided and selectively applied to the sample clock signals. For another example, a set of the equal-delay-time elements connected in series may be provided and the number of connected delay elements may changeably applied to the sample clock signals.

[0051] As shown in FIG. 2, the waveforms A1 and A2 at the phase P(0) and P(7) are of the greatest and smallest voltages, respectively.

[0052] Next, functions of the offset c/b signal generated in the offset adjusting circuit 4 is explained with reference to FIG. 3. The waveforms A1 and A2 of FIG. 2 can be adjusted to different voltage levels. FIG. 3 illustrates four differently-shifted waveforms A1, for example, for the pixel cycle T_p, although this voltage level shift is applied to both waveforms A1 and A2. Each of the four waveforms A1 of FIG. 3 is generated by adding the offset c/b signal to the waveform A1 of FIG. 2. As described above, the offset c/b signal is generated by the offset adjusting circuit 4 in response to the offset-level-change signal from the control unit 10 and varied in a range from “offset 0” through to “offset 3” in accordance with the offset-level-change signal from the control unit 10.

[0053] The shift of the waveforms A1 shown in FIG. 3 is performed in connection to the degree of influence to the image signal C output from the A/D converter 5. As described above, the image signal C is generated in a form of the 256-level gray scale by the A/D converter 5 which has the 8-bit resolution capability. In conjunction with this 256-level gray scale of the image signal C, the shifting degrees of the “offset 0” through to “offset 3” are defined in the following manner. When the offset c/b signal is set to “offset 0,” the signal does not shift the waveform A1. When the offset c/b signal is set to “offset 1,” the signal pulls the waveform A1 downward to such an extent that the A/D converter 5 outputs the image signal C with a reduction of 10 gray scale levels. When the offset c/b signal is set to the “offset 2,” the signal pulls the white output signal A1 downward to such an extent that the A/D converter 5 outputs the image signal C with a reduction of 20 gray scale levels. When the offset c/b signal is set at the “offset 3,” the signal pulls the white output signal A1 downward to such an extent that the A/D converter 5 outputs the image signal C with a reduction of 30 gray scale levels.

[0054] In the exemplary circuit being explained, the offset adjusting circuit 4 is configured to add the offset c/b signal, which is an analog signal, to the analog image signal A so as to output the image signal B which is also an analog signal. The image signal B changes in a range of 0 to 2 volts. The A/D converter 5 receives the image signal B and converts the image signal B into a 256-level gray scale. Accordingly, one-level of the 256-level gray scale corresponds to an approximate value of 8 mV. Further, 10-, 20-, and 30-levels of the 256-level gray scale correspond to approximate values of 80 mV, 160 mV, and 240 mV, respectively.

[0055] Thus, in accordance with the selections of “offset 0”, “offset 1”, “offset 2”, or “offset 3”, the offset adjusting circuit 4 adds the offset c/b signal, which has a value of 0, −80, −160, or −240 mV, to the image signal A.

[0056] As described above, the voltage levels of the offset c/b signal is changed in accordance with the offset-level-change signal from the control unit 10. This voltage level changing operation may easily be achieved by the circuits known per se. For example, the offset adjusting circuit 4 may be provided with an D/A (digital-to-analog) circuit (not shown) and an adding circuit (not shown). The offset c/b signal which is sent from the control unit 10 to the offset adjusting circuit 4 is input to the D/A converter, so that the digital offset c/b signal is converted into an analog signal which can be formed in a desired waveform in accordance with the digital offset c/b signal input.

[0057] The thus-generated analog signal is determined as the offset-level-change signal. The analog offset-level-change signal is then input to the adding circuit inside the offset adjusting circuit 4. At the same time, the image signal A which is sent from the image sensor 2 to the offset adjusting circuit 4 is also input to the adding circuit, so that the adding circuit adds the analog offset offset-level-change signal to the image signal A. In this way, the control unit 10 controls the voltage level shift operation of the image signal A so as to output the image signal B in the different voltage level shifts.

[0058] In the above-described image reading apparatus 100, when the image sensor 2 reads the reference white line image of the reference white/black plate 1a,a value range from 140 to 240 is an appropriate range of the peak white value W_Pwhich is the smallest value detected in the white output included in the image signal C. When the image sensor 2 reads the reference black line image of the reference white/black plate 1a,a value range from 11 to 20 is an appropriate range of the peak black value B_Pin the black output included in the image signal C. Also, when the image sensor 2 reads the reference white line image of the reference white/black plate 1awith the light source 1boff, a value range from 11 to 20 is an appropriate range of the peak dark value B_pin the dark output included in the image signal C.

[0059] FIGS. 4A and 4B illustrate the waveforms of the white and black output signals, respectively, for one image line when the reference white/black plate 1aand the image sensor 2 are in good conditions. More specifically, FIGS. 4A and 4B correspond to when the reference white/black plate 1aand the image sensor 2 has the CCD 2aof which image reading surface are clean, and the offset/gain correction circuit 2bis pre-adjusted to output an appropriate image signal. However, these waveforms will be distorted in a abnormal shape if the reference white/black plate 1aor the CCD 2aof the image sensor 2 is not clean, e.g. if a dust particle has adhered to, for example, a specific position relative to the image reading operation.

[0060] FIGS. 5A and 5B illustrate the waveforms of the white and black output signals, respectively, in a abnormal shape for one image line, which are formed when the reference white/black plate 1aor the CCD 2aof the image sensor 2 carries a dust particle, for example, on a specific position relative to the image reading operation. As illustrated in FIG. 5A, when the reference white/black plate 1aor the CCD 2acarries a dust particle on a specific position relative to the image reading operation and when the CCD 2areads the reference white line image of the reference white/black plate 1afor one image line, the white output signal output from the CCD 2amay have an abnormal drop of voltage around an area (indicated by the letters D1) corresponding to the specific position where the dust particle is located.

[0061] When the reference white/black plate 1aor the CCD 2acarries a dust particle and when the CCD 2areads the reference black line image of the reference white/black plate 1afor one image line, the black output signal output from the CCD 2amay have an abnormal rise of voltage around an area (indicated by the letters D2) corresponding to the specific position where the dust particle is put, as illustrated in FIG. 5B. Also, when the reference white/black plate 1aor the CCD 2acarries a dust particle and when the CCD 2areads the reference white line image of the reference white/black plate 1awith the light source 1boff for one image line, the dark output signal output from the CCD 2amay have an abnormal rise of voltage around an area (indicated by the letters D2) corresponding to the specific position where the dust particle is put, as illustrated in FIG. 5B.

[0062] When a waveform of the image signal output from the CCD 2ais so distorted in an abnormal shape as illustrated in FIGS. 5A and 5B, various important signal processing operations such as the offset/gain correction by the offset/gain correction circuit 2b,the shading operation by the image processing unit 8, and so forth may not be performed in an accurate manner. As a result, when the output image signal is used as the print data, an output image on a recording sheet may have an abnormal black line or white line which is not included in an original image. Therefore, such a dust particle which is put on a specific position of the reference white/black plate 1aor the CCD 2arelative to the image reading operation must be detected and removed before the image reading operation.

[0063] Next, an exemplary procedure of offset/gain adjusting operation of the image reading apparatus 100 is explained with reference to FIGS. 6A-6C. This offset/gain adjusting operation of the image reading apparatus 100 includes, at the beginning thereof, an operation for detecting a foreign particle of dust or the like located on a specific position of the reference white/black plate 1aor the CCD 2arelative to the image reading operation.

[0064] In FIG. 6A, through a NO loop formed in Step S101, the control unit 10 continuously watches for an input of an offset/gain adjustment instruction generated when an operator requests the offset/gain adjustment operation or when such is sent from a host apparatus (not shown) connected to the image reading apparatus 100.

[0065] When the control unit 10 detects an offset/gain adjustment instruction and Step S101 results in YES, the process proceeds to Step S102 in which the control unit 10 instructs a sub-scanning mechanism (not shown) to move the image sensor 2 to a reference white position at which the CCD 2acan read the reference white line image of the reference white/black plate 1a.Then, in Step S103, the control unit 10 instructs the light source 1bfor irradiating the reference white/black plate 1ato turn the light on and to remain on when the light is on.

[0066] In Step S104, the control unit 10 temporarily selects and sends one of the offset-level-change signals to the offset adjusting circuit 4 so as to set the offset c/b signal to the “offset 0” level, or 0 mV. Accordingly, the image signal A, as it is, becomes the image signal B and is input to the A/D converter 5. In Step S105, the control unit 10 instructs the sample clock generator 6 to set the sample clock signal to the phase P(2), at which stable and average white and black outputs can be sampled.

[0067] Then, the control unit 10 instructs the image sensor 2 to read the reference white line image in the main scanning direction, in Step S106. At this time, the image signal A from the image sensor 2 is the white output and, accordingly, the image signal C corresponds to the white output. The peak value detector 7, also in Step S106, detects the peak white value W_Pfrom this white output and stores the detected peak white value W_Pin the first peak white value memory block 9aof the RAM 9. Then, in Step S107, the control unit 10 reads the peak white value W_Pfrom the RAM 9.

[0068] The control unit 10 then determines, in Step S108, if the peak white value W_Pis smaller than or equal to the reference white value W_R. The reference white value W_Ris such a value (i.e., 120) that no normal white output may have when the image sensor 2 performs the image reading operation with a clean reference white/black plate 1aand CCD 2a. If the peak white value W_Pis smaller than or equal to the reference white value W_Rand Step S108 results in YES, the process proceeds to Step S109. In this case, the control unit 10 determines that the reference white/black plate 1athe CCD 2acarries a foreign particle such as dust or the like. Accordingly, the control unit 10 performs an error process for an excessively-small white output, in Step S109. The control unit 10 then ends the process.

[0069] During the error process for the excessively-small white output, the control unit 10 instructs the error displaying unit 12 to display an error indication of the excessively-small white output. With this indication, the user will be notified of an action for cleaning the reference white/black plate 1aand the CCD 2a. Accordingly, when the operation is repeated from Step S101aafter the cleaning, the Step S108 results in NO and the process proceeds to Step S110 which leads to the main portion of the offset/gain adjustment operation.

[0070] If the peak white value W_Pis not smaller than the reference white value W_Rand Step S108 results in NO, the process proceeds to Step S110. In this case, the control unit 10 determines that the reference white/black plate 1aand the CCD 2acarry no foreign particle such as dust or the like on the specific positions relative to the image reading operation. Then, the control unit 10 performs the image reading operation to read the reference white line image with the light source 1boff in order to generate a dark output, in Steps 110-112. More specifically, the control unit 10 controls the light source 1bto turn off in Step S110 and instructs the image sensor 2 to read the reference white line image in the main scanning direction in Step S111. At this time, the image signal A from the image sensor 2 is the dark output and, accordingly, the image signal C is the dark output. The peak value detector 7, also in Step S111, detects the peak black value B_Pfrom this dark output and stores the detected peak black value B_Pin the first peak black value memory block 9bof the RAM 9. Then, in Step S112, the control unit 10 reads the peak black value B_Pfrom the RAM 9.

[0071] Then, in Step S113 of FIG. 6B, the control unit 10 determines whether the peak black value B_Pis equal to or smaller than 10. If the peak black value B_Pis equal to or smaller than 10 and Step S113 results in YES, the control unit 10 performs an error process for an excessively-small black output in Step S114. In the error process, the control unit 10 instructs the error displaying unit 12 to indicate an error. Then, the control unit 10 ends the process in an error state.

[0072] If the peak black value B_Pis greater than 10 and Step S113 results in NO, the process proceeds to Step S115 where the control unit 10 determines whether the peak black value B_Pis within the range including 11 through 20. If the peak black value B_Pis within that range and Step S115 results in YES, the process proceeds to Step S116 where the control unit 10 selects the offset-level-change signal so as to ultimately set the offset c/b signal to “offset 0”, or 0 mV. That is, the image signal C is not reduced in gray scale. Then, the control unit 10 proceeds to Step S124 of FIG. 6C, explained later.

[0073] If the peak black value B_Pis greater than 20 and Step S115 results in NO, the process proceeds to Step S117 where the control unit 10 determines whether the peak black value B_Pis within the range including 21 through 30. If the peak black value B_Pis within that range and Step S117 is YES and the process proceeds to Step S118 in which the control unit 10 selects the offset-level-change signal so as to ultimately set the offset c/b signal to “offset 1”, or 80 mV. That is, the image signal C is decreased 10 levels of gray scale. Then, the control unit 10 proceeds to Step S124 of FIG. 6C, explained later.

[0074] If the peak black value B_Pis greater than 30 and Step S117 results in NO, the process proceeds to Step S119 where the control unit 10 determines whether the peak black value B_Pis within the ranging including 31 through 40. If the peak black value B_Pis within that range and Step S119 is YES, the process proceeds to Step S120 in which the control unit 10 selects the offset-level-change signal so as to ultimately set the offset c/b signal to “offset 2”, or 160 mV. That is, the image signal C is decreased 20 levels of gray scale. Then, the control unit 10 proceeds to Step S124 of FIG. 6C, explained later.

[0075] If the peak black value B_Pis greater than 40 and Step S119 results in NO, the process proceeds to Step S121 where the control unit 10 determines whether the peak black value B_Pis within the range including 41 through 50. If the peak black value B_Pis within that range and Step S121 is YES, the process proceeds to Step S123 in which the control unit 10 selects the offset-level-change signal so as to ultimately set the offset c/b signal to “offset 3”, or 240 mV. That is, the image signal C is decreased 30 levels of gray scale. Then, the control unit 10 proceeds to Step S124 of FIG. 6C, explained later.

[0076] If the peak black value B_Pis greater than 50 and Step S121 results in NO, the control unit 10 performs an error process for an excessively-great black output in Step S122. In the error process, the control unit 10 instructs the error displaying unit 12 to indicate an error. Then, the control unit 10 ends the process in an error state.

[0077] After the offset c/b signal is to “offset 0” in Step S116, “offset 1” in Step S118, “offset 2” in Step S120, or “offset 3” in Step S123, the control unit 10 proceeds to Step S124. In Step S124, the control unit 10 performs a peak white/black value collecting operation for collecting the peak white and peak black values W_Pand B_Pfor each of the phases P(0)-P(7) and storing these values into the second peak white memory block 9cand the second peak black value memory block 9d,respectively. Further details of the peak white/black value collecting operation will be explained later.

[0078] After the peak white/black value collecting operation in Step S124, the control unit 10 proceeds to Step S125. In Step S125, the control unit 10 analyzes the peak white and peak black values W_P(L)and B_P(L)stored in the second peak white memory block 9cand the second peak black value memory block 9d,respectively, so as to check if at least one of phases P(0)-P(7) represents the peak white values W_Pwhich is equal to or greater than 140 or equal to or smaller than 240 and the peak black value B_Pwhich is equal to or greater than 11 or equal to or smaller than 20. Then, in Step S126, the control unit 10 determines the check result of Step S125.

[0079] If Step S126 results in NO, the control unit 10 performs an error process for an abnormal peak white or black output. In the error process, the control unit 10 instructs the error displaying unit 12 to indicate an error. Then, the control unit 10 ends the process in an error state. In this case, at least one of the peak white and peak black values W_Pand B_Pis out of the predetermined appropriate range to such an extent that the inappropriate value cannot be corrected even by changes of the offset-level-change signal levels of the offset adjustment circuit 4 or the phases of the sample clock generator 6. Accordingly, the error indication by the error displaying unit 12 indicates to the user that the user needs to take appropriate actions such as exchanging the image sensor 2 with a new one.

[0080] If Step S126 results in YES, the control unit 10 proceeds the process to Step S128. In Step S128, the control unit 10 selects one phase which represents the greatest white value W_Pamong all the peak white values W_Prepresented by all the phases found in Step S125. Further, the control unit 10 instructs the sample clock generator 6 by the phase change signal to generate the sample clock signals in synchronism with the phase selected in Step S128.

[0081] In this way, the image reading apparatus 100 performs the offset/gain adjusting operation, and can detect a foreign particle such as dust or the like which disturb the image reading operation if the dust or the like is put on a specific position of the reference white/black plate 1aor the CCD 2arelative to the image reading operation.

[0082] Next, details of the peak white/black value collecting operation in Step S124 of FIG. 6C is explained with reference to FIGS. 7A and 7B. In Step S1001 of FIG. 7A, the control unit 10 controls the light source 1bto turn off when the light is on and to remain off when the light is off. At this time, the control unit 10 controls the image sensor 2 to be located at the position for reading the reference white line image of the reference white/black plate 1a. Then, the control unit 10 sets a variant L for counting a number of performance times to 0 in Step S1002, and substitutes the variant L into the phase P(L) of the sample clock signals in Step S1003.

[0083] The control unit 10 instructs the image sensor 2 to read the reference white line image of the reference white/black plate 1awith the light source off, in Step S1004. Then, the control unit 10 instructs the peak value detector 7 to detect the peak black value B_Pin the dark output in Step S1005, and to store the detected value B_Pinto the peak black value set B_P(L)in Step S1006. Further, the control unit 10 stores the peak black value set B_P(L)into the second peak black value memory block 9dwith a link of P_(L), in Step S1007. The control unit 10 then increments the variant L by one in Step S1008.

[0084] In the Step S1009, the control unit 10 determines if the value of the variant L equal to seven. If the value of the variant L is not equal to seven and Step S1009 results in NO, the process returns to Step S1003 to repeat the process of Step S1003-S1009 until the value of the variant L becomes equal to seven. When the value of the variant L becomes equal to seven and Step S1009 results in YES, the control unit 10 instructs the light source 1bto turn on in Step S1010 of FIG. 7B. By this time, the peak black value set B_P(L)which includes B_P(0)through to B_P(7)corresponding to the phases P(L) which includes P(0) through to P(7) are collected and stored in the second peak black memory block 9d.

[0085] The control unit 10 sets the variant L to 0 in Step S1011, and substitutes the variant L into the phase P(L) in Step S1012. The control unit 10 instructs the image sensor 2 to read the reference white line image of the reference white/black plate 1awith the light source on, in Step S1013. Then, the control unit 10 instructs the peak value detector 7 to detect the peak white value W_Pin the white output in Step S1014, and to store the detected value W_Pinto the peak BW_P(L)in Step S1015. Further, the control unit 10 stores the peak white value set W_P(L)into the second peak white value memory block 9cwith a link of P_(L), in Step S1016. The control unit 10 then increments the variant L by one in Step S1017.

[0086] In the Step S1018, the control unit 10 determines if the value of the variant L equal to seven. If the value of the variant L is not equal to seven and Step S1018 results in NO, the process returns to Step S1012 to repeat the process of Step S1012-S1018 until the value of the variant L becomes equal to seven. When the value of the variant L becomes equal to seven and Step S1018 results in YES, the control unit 10 end the process. By this time, the peak white value set W_P(L)which includes W_P(0)through to W_P(7)corresponding to the phases P(L) which includes P(0) through to P(7) are collected and stored in the second peak white memory block 9c,in addition to the peak black value set B_P(L)stored in the second peak black memory block 9d.FIG. 8 illustrates these peak white and peak black value sets W_P(L)and B_P(L)which are collected into the second peak white memory block 9cand the second peak black memory block 9d, respectively, in the way described above.

[0087] Next, an image reading apparatus 200 as a second embodiment according to the present invention is explained with reference to FIGS. 9, 10A-10D. FIG. 9 illustrates a block diagram of an image reading apparatus 200 as a second embodiment according to the present invention. The image reading apparatus 200 of FIG. 9 is similar to the image reading apparatus 100 of FIG. 1, except for a third peak white value memory block 9eof the RAM 9 and a reference differential value register 10bof the control unit 10. The reference differential value register 10bmay alternatively locate in the RAM 9. In the following description of the image reading apparatus 200, elements which correspond to those of the image reading apparatus 100 described with reference to FIG. 1 are provided with the same reference numerals, and the description for these elements are not repeated for the sake of conciseness.

[0088] In the image reading apparatus 200, the control unit 10 instructs the image sensor 2 to read, if necessary, twice at different positions the reference white line image of the reference white/black plate 1aduring the offset/gain adjusting operation. The reference white value from the first read white output is referred to as a reference white value W_PXand stored into the first reference white value memory block 9a.The reference white value from the second read white output is referred to as a reference white value W_PYand stored into the third reference white value memory block 9e.

[0089] With reference to FIGS. 10A-10D, an exemplary procedure of the offset/gain adjusting operation of the image reading apparatus 200 is explained. This offset/gain adjusting operation of the image reading apparatus 100 includes, at the beginning thereof, an operation for detecting a foreign particle of dust or the like which is located on a specific position of the reference white/black plate 1aor the CCD 2aor an interior of the image sensor 2, relative to the image reading operation. Significantly, when a dust particle or the like is located inside the image sensor 2, the user cannot remove the foreign particle by cleaning.

[0090] In FIG. 10A, through a NO loop which is formed in Step S201, the control unit 10 continuously watches for an input of an offset/gain adjustment instruction which is generated when an operator requests the offset/gain adjustment operation or when such is sent from a host apparatus (not shown) connected to the image reading apparatus 200.

[0091] When the control unit 10 detects the offset/gain adjustment instruction and Step S201 results in YES, the process proceeds to Step S202 in which the control unit 10 instructs a sub-scanning mechanism (not shown) to move the image sensor 2 to a first reference white reading position X at which the CCD 2acan read the reference white line image of the reference white/black plate 1a.Then, in Step S203, the control unit 10 instructs the light source 1bfor irradiating the reference white/black plate 1ato turn on when the light is off and to remain on when the light is on.

[0092] In Step S204, the control unit 10 temporarily selects and sends one of the offset-level-change signals to the offset adjusting circuit 4 so as to set the offset c/b signal to the “offset 0” level, or 0 mV. Accordingly, the image signal A, as it is, becomes the image signal B and is input to the A/D converter 5. In Step S205, the control unit 10 instructs the sample clock generator 6 to set the sample clock signal to the phase P(2), at which stable and average white and black outputs can be sampled.

[0093] Then, the control unit 10 instructs the image sensor 2 to read the reference white line image at the first reference white reading position X in the main scanning direction, in Step S206. At this time, the image signal A from the image sensor 2 is the white output and, accordingly, the image signal C is the white output. The peak value detector 7, also in Step S206, detects the peak white value_WPXfrom this white output and stores the detected peak white value W_PXin the first peak white value memory block 9aof the RAM 9. Then, in Step S207, the control unit 10 reads the peak white value W_PXfrom the RAM 9.

[0094] The control unit 10 then determines, in Step S208, if the peak white value W_PXis smaller than the reference white value W_R. The reference white value W_Ris such a value that no normal white output may have when the image sensor 2 performs the image reading operation when the reference white/black plate 1aand the interior and the exterior of the image sensor 2 are clean.

[0095] If the peak white value W_PXis not smaller than the reference white value W_Rand Step S208 results in NO, the process proceeds to Step S209. In this case, the control unit 10 determines that the reference white/black plate 1aand the interior and the exterior of the image sensor 2 carry no foreign particle such as dust or the like on the specific positions relative to the image reading operation. Then, the control unit 10 performs the image reading operation to read the reference white line image with the light source 1boff in order to generate a dark output, in Steps 209-211.

[0096] After the image reading operation in Steps S209-S211, the control unit 10 performs the main portion of the offset/gain adjusting operation in Step S212-S222 of FIG. 10B and Steps S223-S227 of FIG. 10C. Descriptions for these operation steps is omitted since they are similar to those of the image reading apparatus 100 explained in Steps S113-123 of FIG. 6B and Steps S124-S128 of FIG. 6C. Also, the peak white/black value collecting operation which is performed in Step S223 of FIG. 10C is similar to that of Step S124 of FIG. 6C.

[0097] In Step S208 of FIG. 10A, if the peak white value W_PXis smaller than the reference white value W_Rand the determination results in YES, the process proceeds to Step S228 of FIG. 10D. In this case, the control unit 10 determines that the reference white/black plate 1aor the interior or the exterior of the image sensor 2 carries a foreign particle such as dust or the like. Accordingly, the control unit 10 performs an error process for identifying whether an error is caused by a foreign particle such as dust or the like located on one of the reference white/black image plate 1a,the exterior of the image sensor 2, or on the interior of the image sensor 2.

[0098] In Step S228, the control unit 10 instructs the image sensor 2 to move to a second reference white reading position Y. Then, the control unit 10 further instructs the image sensor 2 to read the reference white line image at the second reference white reading position Y in the main scanning direction, in Step S229. At this time, the image signal A from the image sensor 2 is the white output and, accordingly, the image signal C is the white output. The peak value detector 7, also in Step S229, detects the peak white value W_PYfrom this white output and stores the detected peak white value W_PYin the third peak white value memory block 9eof the RAM 9. Then, in Step S230, the control unit 10 reads the peak white value W_PYfrom the RAM 9.

[0099] The control unit 10 then determines, in Step S231, if the difference between the peak white values W_PXand W_PYis smaller than or equal to the reference differential value W_d. If the difference between the peak white values W_PXand W_PYis smaller than or equal to the reference differential value W_dand Step S231 results in YES, the control unit 10 determines that the error which has occurred is caused by a foreign particle such as dust or the like which is put on the interior of the image sensor 2. Then, in Step S232, the control unit 10 performs an error process for an abnormal offset adjustment due to an internal cause.

[0100] During the error process in Step S232 for the abnormal offset adjustment due to an internal cause, the control unit 10 instructs the error displaying unit 12 to display an error of abnormal offset adjustment due to an internal cause. With this indication, the user will be instructed to exchange the image sensor 2 with a new one. Accordingly, when the operation is repeated from Step S201 after the exchange of the image sensor 2, the Step S208 may result in NO and the process proceeds to Step S209 which leads to the main portion of the offset/gain adjustment operation.

[0101] In Step S231, if the difference between the peak white values W_PXand W_PYis not smaller than or equal to the reference differential value W_dand Step S231 results in NO, the control unit 10 determines that the error which has occurred is caused by a foreign particle such as dust or the like located on one of the reference white/black image plate 1aand the exterior of the image sensor 2. Then, the control unit 10 proceeds the process to Step S232 where the control unit 10 performs an error process for an abnormal offset adjustment due to an external cause.

[0102] During the error process in Step S233 for the abnormal offset adjustment due to an external cause, the control unit 10 instructs the error displaying unit 12 to display an error of the abnormal offset adjustment due to an external cause. With this indication, the user will be notified of an action for cleaning off the components relative to the image reading operation, such as the reference white/black image plate 1a,the image reading surface of the CCD 2a,and the like. Accordingly, when the operation is repeated from Step S201 after the cleaning, the Step S208 may result in NO and the process proceeds to Step S209 which leads to the main portion of the offset/gain adjustment operation.

[0103] In this way, the image reading apparatus 100 performs the offset/gain adjusting operation, and can detect a foreign particle such as dust or the like which disturb the image reading operation when such foreign particle is put on a specific position relative to the image reading operation, such as one of the reference white/black plate 1aand the exterior of the image sensor 2, or the interior of the image sensor 2, or the like.

[0104] Next, an image reading apparatus 300 as a third embodiment according to the present invention is explained with reference to FIGS. 11, 12A-12D. FIG. 11 illustrates a block diagram of an image reading apparatus 300 as a third embodiment according to the present invention. The image reading apparatus 300 of FIG. 11 is similar to the image reading apparatus 100 of FIG. 1, except for a third peak white value memory block 9e,a third peak black value memory block 9f,a reference differential value register 10b,and a reference black value register 10c.The former two memory blocks 9eand 9fare included in the RAM 9 and the latter two registers 10band 10care included in the control unit 10. The latter two registers 10band 10cmay alternatively be included in the RAM 9. In the following description of the image reading apparatus 300, elements which correspond to those of the image reading apparatus 100 described with reference to FIG. 1 are provided with the same reference numerals, and the description for these elements are not repeated for the sake of conciseness.

[0105] In the image reading apparatus 300, the control unit 10 instructs the image sensor 2 to read in different ways the reference white and reference black line images of the reference white/black plate 1aduring the offset/gain adjusting operation. The reference white value from the first read white output at a reference white reading position X is referred to as a reference white value W_PXand stored into the first reference white value memory block 9a.The reference black value from the first read black output is referred to as a reference black value B_PXand stored into the first reference black value memory block 9b.The reference white value from the second read white output at another reference white reading position Y is referred to as a reference white value W_PYand stored into the third reference white value memory block 9e.The reference black value from the second read dark output, reading the reference black line image with the light source 1boff, is referred to as a reference black value B_PYand stored into the third reference black value memory block 9f.

[0106] As mentioned above, in the image reading apparatus 300, the control unit 10 instructs the image sensor 2 to read the reference black line image instead of the reference white line image with the light source 1boff, after reading the reference white line image, in order to increase the reading accuracy.

[0107] With reference to FIGS. 12A-12D, an exemplary procedure of the offset/gain adjusting operation of the image reading apparatus 300 is explained. This offset/gain adjusting operation of the image reading apparatus 300 includes, at the beginning thereof, an operation for detecting a foreign particle of dust or the like which is located on a specific position of the reference white line image or the reference black line image of the reference white/black plate 1aor the CCD 2aor an interior of the image sensor 2, relative to the image reading operation. Significantly, when a dust particle or the like is located inside the image sensor 2, the user cannot remove such foreign particle by cleaning.

[0108] In FIG. 12A, through a NO loop which is formed in Step S301, the control unit 10 continuously watches for an input of an offset/gain adjustment instruction which is generated when an operator requests the offset/gain adjustment operation or when such is sent from a host apparatus (not shown) connected to the image reading apparatus 300.

[0109] When the control unit 10 detects the offset/gain adjustment instruction and Step S301 results in YES, the process proceeds to Step S302 in which the control unit 10 instructs a sub-scanning mechanism (not shown) to move the image sensor 2 to a first reference white reading position X at which the CCD 2acan read the reference white line image of the reference white/black plate 1a.Then, in Step S303, the control unit 10 instructs the light source 1bfor irradiating the reference white/black plate 1ato turn on when the light is off and to remain on when the light is on.

[0110] In Step S304, the control unit 10 temporarily selects and sends one of the offset-level-change signals to the offset adjusting circuit 4 so as to set the offset c/b signal to the “offset 0” level, or 0 mV. Accordingly, the image signal A, as it is, becomes the image signal B and is input to the A/D converter 5. In Step S305, the control unit 10 instructs the sample clock generator 6 to set the sample clock signal to the phase P(2), at which stable and average white and black outputs can be sampled.

[0111] Then, the control unit 10 instructs the image sensor 2 to read the reference white line image at the first reference white reading position X in the main scanning direction, in Step S306. At this time, the image signal A from the image sensor 2 is the white output and, accordingly, the image signal C is the white output. The peak value detector 7, also in Step S306, detects the peak white value W_PXfrom this white output and stores the detected peak white value W_PXin the first peak white value memory block 9aof the RAM 9. Then, in Step S307, the control unit 10 reads the peak white value W_PXfrom the RAM 9.

[0112] The control unit 10 then determines, in Step S308, if the peak white value W_PXis smaller than or equal to the reference white value W_R. The reference white value W_Ris such a value (i.e., 120) that no normal white output may have when the image sensor 2 performs the image reading operation when the reference white/black plate 1aand the interior and the exterior of the image sensor 2 are clean.

[0113] If the peak white value W_PXis not smaller than or equal to the reference white value W_Rand Step S308 results in NO, the process proceeds to Step S309. In this case, the control unit 10 determines that the reference white/black plate 1aand the interior and the exterior of the image sensor 2 carry no foreign particle such as dust or the like on the specific positions relative to the image reading operation.

[0114] In Step S309, the control unit 10 instructs the image sensor 2 to move to a reading position for the reference black line image of the reference white/black plate 1aand, in Step S310, to read the reference black line image in the main scanning direction. At this time, the image signal A from the image sensor 2 is the black output and, accordingly, the image signal C is the black output. The peak value detector 7, also in Step S310, detects the peak black value B_PXfrom this black output and stores the detected peak black value B_PXin the first peak black value memory block 9bof the RAM 9. Then, in Step S311, the control unit 10 reads the peak black value B_PXfrom the RAM 9.

[0115] Then, the control unit 10 controls the light source 1bto turn off in Step S312 and instructs the image sensor 2 to read the reference black line image in the main scanning direction in Step S313. At this time, the image signal A from the image sensor 2 is the dark output and, accordingly, the image signal C is the dark output. The peak value detector 7, also in Step S313, detects the peak black value B_PYfrom this dark output and stores the detected peak black value B_PYin the second peak black value memory block 9fof the RAM 9. Then, in Step S314, the control unit 10 reads the peak black value B_PYfrom the RAM 9.

[0116] The control unit 10 then determines, in Step S315, if the difference between the peak black values B_PXand B_PYis greater than or equal to the reference black value B_R. The reference black value B_Ris such a value that the control unit 10 can determine that a foreign particle such as dust or the like is located on the reference black line image of the reference white/black plate 1awhen the difference between the peak black values B_PXand B_PYis greater than or equal to the reference black value B_R. This determination uses a phenomenon that the reference black line image which has a dust particle or the like will indicate a great difference in blackness reading values in the cases when the reference black line image is irradiated with the light source on and off.

[0117] If the peak the difference between the peak black values B_PXand B_PYis greater than or equal to the reference black value B_Rand Step S315 results in YES, the process proceeds to Step S316. In this case, the control unit 10 determines that a foreign particle such as dust or the like is located on the reference black line image of the reference white/black plate 1a. In Step S316, the control unit 10 performs an error process for an excessively-great peak black output. During the error process for the excessively-great peak output in Step S316, the control unit 10 instructs the error displaying unit 12 to display an error indicating an excessively-great peak black output. With this indication, the user will be notified of an action for cleaning off the reference black line image of the reference white/black plate 1a.Accordingly, when the operation is repeated from Step S301 after the cleaning, the Step S108 results in NO and the process proceeds to the main portion of the offset/gain adjustment operation illustrated in Steps S318-S328 of FIG. 12B and Steps S329-S333 of FIG. 12C, using reference black value B_Rsimilar to the manner in which the reference peak black value B_Pis used in Steps S113-123 of FIG. 6B and Steps S124-S128 of FIG. 6C. Descriptions for these operation steps is omitted since they are similar to those of the image reading apparatus 100 explained in Steps S113-123 of FIG. 6B and Steps S124-S128 of FIG. 6C. Also, the peak white/black value collecting operation which is performed in Step S329 of FIG. 12C is similar to that of Step S124 of FIG. 6C.

[0118] In Step S308 of FIG. 12A, if the peak white value W_PXis smaller than or equal to the reference white value W_Rand the determination results in YES, the process proceeds to Step S334 of FIG. 12D. In this case, the control unit 10 determines that the reference white/black plate 1aor the interior or the exterior of the image sensor 2 carries a foreign particle such as dust or the like. Accordingly, the control unit 10 performs an error process for identifying whether an error is caused by a foreign particle such as dust or the like which is put on one of the reference white/black image plate 1aand the exterior of the image sensor 2, or on the interior of the image sensor 2. This error identification process is performed in Steps S334-S339, which are similar to the process explained in Steps S228-S233. Therefore, a description for the identification process is omitted to be repeated. However, after Steps S338 or S339 in the process of FIG. 12D, the control unit 10 does not end but returns the process back to Step S309. This is so in order for the user to automatically retry to check the peak black value after taking an appropriate action in accordance with the indication on the error displaying unit 12 in Steps S338 or S339.

[0119] In this way, the image reading apparatus 300 performs the offset/gain adjusting operation, and can detect a foreign particle such as dust or the like which disturb the image reading operation when such foreign particle is put on a specific position relative to the image reading operation, such as one of the reference white and reference black line images of the reference white/black plate 1aand the exterior of the image sensor 2, or the interior of the image sensor 2, or the like.

[0120] This invention may be conveniently implemented using a conventional general purpose digital computer programmed according to the teaching of the present specification, as will be apparent to those skilled in the computer art. Appropriate software coding can readily be prepared by skilled programmers based on the teachings of the present disclosure, as will be apparent to those skilled in the software art. The present invention may also be implemented by the preparation of application specific integrated circuits or by interconnecting an appropriate network of conventional component circuits, as will be readily apparent to those skilled in the art.

[0121] Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the present invention may be practiced otherwise than as specifically described herein.

[0122] This application is based on Japanese patent application No. JPAP09339517 filed in the Japanese Patent Office on Nov. 26, 1997, and the entire contents thereof are hereby incorporated by reference.