Image reading and forming device.

0001The present invention relates to an image reading device for reading an image on an original sheet, and an image forming device for forming an image based on digital image data read by the reading device, more specifically, a digital copying machine which blanks the background other than the original sheet by electrically judging the size of the original sheet from the data read out.

0002In conventional copying machines, there is provided a white reflection plate on the original sheet cover so as to avoid blackening of the section other than the original sheet when an original sheet having a size smaller than the set sheet size is copied. When copying an original sheet, the sheet is placed on the original sheet glass, and the front surface of the sheet glass is covered by the white reflection plate. However, with the convention technique, when a three-dimensional object is copied, the reflection plate cannot be set on properly on the original, rendering the background section of the original on a copying sheet gray or black.

0003Some conventional copying machines include a frame erasing function for the purpose of overcoming such a drawback. The frame erasing function detects the size of a sheet used for copy, and controls not to form any image in the sheet's periphery section of a certain width. The conventional frame erasing function has a member for erasing images of the unnecessary section in advance by exposing the section to light from its light source before developing the static latent image.

0004In other conventional copying machines, the position and the size of an original is detected before start of a copying machine, and only the detected region of the original is copied. More specifically, there is a known method in which a metal reflection plate is used for the original sheet press surface of the sheet cover, and pre-scanning is performed with a light beam so as to detect the four corners of the original based on the difference between the intensity of the reflection light from the original and that from the original press surface. There is also a known method in which the original press surface of the original cover is coated with a fluorescent paint, and pre-scanning is performed by use of a filter transmitting only the light having a certain wavelength so as to detect the four corners of the original based on the difference between the intensity of the reflection light from the original and that from the original press surface.

0005The frame erasing function using a light source is effective in the case where an original image is copied on a copy sheet at substantially the same magnification; however in the case where the original image is small, the periphery of the original sheet is inevitably blackened. In the method of detecting the position and size of an original before start of a copying operation, detection of the size is in many cases based on typical sizes such as A4 and B4. Further, when the original is placed inappropriately, it is judged to be a wrong size.

0006As described above, with the conventional copying machines, the automatic sheet size detection function cannot detect any sheets but rectangular types. Further, with the copying machines having an original press coated with fluorescent paint, the sheet size cannot be detected when the original press is not appropriately set on the original sheet.

0007The purpose of the invention is to provide an image reading device for reading an image, which can accurately detect or erase the region other than the original, regardless of the original size, or how the original is placed, and an image forming device for forming an image, which includes the reading device.

0008According to the invention, there is provided an image scanning apparatus, comprising:    means for scanning target areas from a first edge area to a second edge area, the target area including original document areas;    means for producing image signal representative of the shade of darkness of areas scanned by the scanning means, the image signals having dark image signals corresponding to the darkness of the target areas except the original document areas and bright image signals corresponding to the darkness of the target areas brighter than the target areas corresponding to the dark image area;    means for detecting a first bright image signal and a last bright image signal which are the first signal and the last signal in the bright image signals produced by the producing means in the scanning direction; and    means for eliminating the image signal from the image signals corresponding to the first edge areas to the first bright image signals and from the last bright image signal to the image signal corresponding the image signal corresponding to the second edge area.

0009According to the invention, there is also provided an image reading device comprising:    means for scanning target areas from a first edge area to a second edge area, the target area including original document areas;    means for producing image signal representative of the shade of darkness of areas scanned by the scanning means, the image signals having dark image signals corresponding to the darkness of the target areas except the original document areas and bright image signals corresponding to the darkness of the target areas brighter than the target areas corresponding to the dark image area;    means for detecting a first bright image signal and a last bright image signal which are the first signal and the last signal in the bright image signals produced by the producing means in the scanning direction;    means for eliminating the image signal from the image signals corresponding to the first edge areas to the first bright image signals and from the last bright image signal to the image signal corresponding the image signal corresponding to the second edge area; and    means for forming an image on an image forming medium in accordance with the image signal from the eliminating means.

0010In an image forming device, image data is read, a picture element of the image data is compared with a reference value so as to detect each of edges of an original sheet, and picture element signals having a value smaller than the reference value located outside the edges of the original sheet are replaced with white data, thereby to blank the section other than the image on the original by white.

0011This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: <ul id="ul0001" list-style="none"><li>Fig. 1 is a schematic diagram showing an optical system of an image reading device, for reading an image, the image reading device being built in an image forming device according to the present invention;</li><li>Fig. 2 is a block diagram showing a processing circuit for an image signal of an image reading device built in an image forming device according to the invention;</li><li>Fig. 3 is a cross section of the structure of an original sheet cover set on an image forming device according to the invention;</li><li>Figs. 4A and 4B are waveform diagrams respectively showing one line image signal after shading correction, and one line image signal after black/white replacing correction, in the circuit shown in Fig. 2;</li><li>Fig. 5 is a schematic diagram showing types of reflection light beams from the original sheet;</li><li>Fig. 6 is a circuit block diagram showing an example of the circuit structure of the image reading device of the invention;</li><li>Fig. 7 is a plan view showing an example of arrangement of an original sheet base and an original sheet, designed to explain the principle for detecting the top and tailing ends of an original sheet sued in the image forming device of the invention;</li><li>Fig. 8 is a waveform diagram showing one line image signal obtained when the original sheet cover of the image forming device of the invention is open; and</li><li>Fig. 9 is a circuit block diagram showing another example of the circuit structure of the image reading device of the invention.</li></ul>

0012An image reading device for reading an image, and an image forming device for forming an image in accordance with the read image data, according to an embodiment of the present invention, with reference to accompanying embodiment.

0013Fig. 1 is a schematic diagram showing an optical system of an image reading device, for reading an image, the image reading device incorporated in an image forming device, and Fig. 2 is a block diagram showing a processing circuit for an image signal of an image reading device.

0014As can be seen in Fig. 1, the first and second carriages C1 and C2 are arranged movably in a sub-scanning direction Ss, below an original sheet glass 1. The first carriage C1 includes a light source such as a fluorescent lamp La for irradiating light rays or beams on an original sheet 2 via the original sheet glass 1, and the inner surface of the first carriage C1 is made of a recess-shaped mirror.

0015With the described structure, a light beam from the fluorescent lamp La irradiates substantially uniformly on a target region such as a band-like region of the original sheet 2, the region being set along the main scanning direction Sm perpendicular to the sub-scanning direction Ss. In the carriage C1, there is provided a flat surface mirror 3 so as to transmit the reflection light beam from the original 2. The reflection light beam from the original 2 is directed in a direction opposite to the sub-scanning direction by means of the flat surface mirror 3, and reflects on a flat surface mirror 4 provided in the second carriage C2 so as to bend the light beam path at right angle.

0016In the second carriage C2, there is also provided a flat surface mirror 5, which is similar to the mirror 4, for bending the light beam path at right angle. A light beam made incident on the flat surface mirror 4 reflects on the flat surface mirror 5 so as to proceed along a light beam path in parallel with a light beam path running from the mirror 3 to mirror 4, in other words, to be direct in the sub-scanning direction.

0017In the proceeding direction of the light beam path, there is provided a focusing lens 6, by which the light beam proceeding along the main scanning direction and containing the image data of the original 2 is focused toward a CCD sensor 7. Between the focusing lens 6 and the CCD sensor 7, there are arranged a filter 8 for transmitting visible rays, a flat surface mirrors 9 and 10. With this structure, only the visible ray components of the focused light beam concentrated by the focusing lens 6, transmit through the filter 8, and the light beam of the visible components is directed to the flat surface mirrors 9 and 10. The light beam reflects on the two flat surface mirrors 9 and 10 each time at right angle, and therefore is directed to a direction opposite to the sub-scanning direction, and then made incident on the CCD sensor 7.

0018The first and second carriages C1 and C2 can be moved in the sub-scanning direction Ss as mentioned before, but a motion speed V1 of the first carriage C1 is set to be substantially two times as fast as the motion speed V2 of the second carriage C2. Consequently, even if the reading region of the original 2 in the sub-scanning direction Ss moves due to movement of the first carriage C1, the distance of the light path from the region to the focusing lens 6 is maintained at constant, thereby maintaining the distance to the CCD sensor 7 at constant. Therefore, the magnification of the optical system shown in Fig. 1 is always set at constant regardless of the motion of each of the first and second carriages C1 and C2. Thus, the relationship between an object point and an image forming point is always kept between the scanning region on the original 2 and the CCD sensor 7, and the image in the line-like scanning region of the original 2, the region being set along the main scanning region Sm, is formed on the CCD sensor 7.

0019An original sheet cover 11 as shown in Fig. 3 is provided on the housing of the image forming device such that the cover can be placed on an original sheet glass 1. Further, as shown in Fig. 1, an open/close sensor 32 for detecting opening/closing of the cover 11 is provided for the housing the image forming device. In the original sheet cover 11, a material for absorbing light rays, for example, a plate-like light absorbing member 13 made of black sponge, is adhered to an original sheet cover frame 12, and a transparent member 14 such as a transparent vinyl sheet for allowing light rays to transmit therethourgh, and which is brought into tight contact with original sheets, is adhered on the light absorbing member 13. By use of the original sheet cover 11, reflection of a light beam from the original sheet cover 11 to the optical system shown in Fig. 2 can be suppressed to a low level, and therefore the boundary of the original 2 can be accurately detected.

0020The light beam transmitting through the black ink portion of the sheet 2 into the cover 11 can be attenuated sufficiently such as to prevent the light beam from returning to the original 2, thus the image of the original 2 can be sharpened. More specifically, in the original sheet cover 11 shown in Fig. 3, the light beam entering into the cover 11 directly or via the original 2 is dispersed by the transparent member 14, and made incident on the light absorbing member 13, where the light is attenuated. Even if there are a slight amount of light components reflecting on the light absorbing material 13, the light components are dispersed once again by the transparent member 14, and therefore the amount of the light components reflecting as shown in Fig. 3? can be reduced to an extremely small level.

0021The processing of an image signal from the CCD 7 will now be described.

0022When the copy start key of the key input section (not shown) of the image forming device is pressed by the operator, the first and second carriages C1 and C2 start to move in the sub-scanning direction Ss. During this motion, the image on the sheet is transferred in a line-wise fashion one after another along the main scanning direction Sm, and line image signals corresponding to the target images of the document, 2 glass plate 1 and the cover 11 are consecutively output from the CCD sensor 7. The image signals represents the shade of darkness of the target areas. The line image signals thus output are amplified as serial data in an amplifier 15 shown in Fig. 2, and input to A/D converting and shading correction circuits 16-1 and 16-2, where the analog line image signals are converted to digital line image signals, and shading caused by an irregular irradiation of an exposure lamp La and an irregular sensitivity of the CCD sensor is corrected.

0023As shown in Fig. 2, the shading-corrected digital line image signals are input to a comparison processor 17 and one of input terminals of a selector 18 as the serial data. In the comparison processor 17, the image signals are compared with the first reference signal Th1 shown in Fig. 4B, and those having a level lower than the first reference signal Th1 are judged to be black. While the image signals having the black level are being input, the selector 18 connects the terminal 18B to which white data is input in accordance with a switching signal from the comparison processor 17, to the output terminal 18C. Consequently, the image signal of black is converted to a white image signal, and output from the output terminal 18C. In the case where an image signal having a level of the first reference signal Th1 or higher is detected, the position P0 of the signal is judged to be a leading edge of the original 2 on the main scanning line corresponding to the image signal, and a switching signal is supplied from the comparison processor 17 to the selector 18 so as to connect the terminal 18A, to which image signals are input, to the output terminal 18C. Therefore, image signals are output from the selector 18 as they are. In the case where an image signal having a level lower than the first reference signal Th1 is detected by the comparison processor 17, the position PL of the signal is judged to be a tailing edge of the original 2 on the main scanning line corresponding to the image signal, and a switching signal is supplied from the comparison processor 17 to the selector 18 so as to connect the terminal 18B, to which white image data is input, to the output terminal 18C. Therefore, white image signals obtained by converted from a black image are output from the selector 18.

0024The level of the first reference signal Th1 is determined based on the level of the dark current of the CCD sensor 7, and the intensity of the reflection light beam from the black ink portion of the original 2. More specifically, in the original sheet cover 11 having a structure shown in Fig. 3, the reflection light beam from the sheet cover 11 has a level lower than the sensitivity of the CCD sensor 7, and therefore the A/D converted value of the level should be theoretically 0. However, in reality, the value will not be 0 due to noise such as a dark current, which is produced in a CCD sensor. Consequently, the level of the first reference signal Th1 is set to be higher than the dark current level. A slight portion of the light beam made incident on black ink portion 2A of the original shown in Fig. 5 reflects on the surface. Otherwise, a portion of the light, the portion being depending on the density of the black ink 2A, transmits through the black ink portion of the original sheet 2 into the sheet, once again reflecting in the sheet or on the rear surface of the sheet 2. The reflection component L1 from the black ink portion 2A of the original sheet 2, the component L2 from the inside of the sheet 2, and the component L3 from the rear surface of the sheet 2 are directed to the CCD sensor 7, where they are detected. The level of the first reference signal Th1 is determined such that it is not lower than the level of the image signal thus detected. The leading and tailing edge of the original sheet 2 along the main scanning line can be accurately detected by setting the level of the first reference signal Th1 as above.

0025An example of the circuit structure of the device will not be described in more detail with reference to Fig. 6.

0026As shown in Fig. 6, a line memory 20 for consecutively storing digital line image signals each corresponding to one scanning line, is connected to the shading correction circuit 16, to which also connected is a signal switching circuit 21 for selectively and serially outputting either each picture element signal of the digital line image signal for one scanning line or white data picture element signal from the white data generating circuit 22.

0027Further, in the comparison processor 17, a comparator 24 for comparing a dot signal of the digital line image signal corresponding to one scanning line with the first reference value from the reference value generator 23, is connected to the shading correction circuit 16. To the reference value generator 23, connected is a cover open/close sensor 32 for detecting opening/closing of the cover. When the cover is closed, the first reference signal Th1 is generated from the reference value generator 23, whereas when the cover is open, the second reference signal Th2 is generated from the reference value generator 23. To the comparator 24, connected are a leading edge detection circuit 25 and a tailing edge detection circuit 27, which generate a leading edge detection signal and a tailing edge detection signal, respectively, by detecting the leading edge and the tailing edge of the original 2 from the comparison result from the comparator 24. To the leading end detection circuit 25 and the tailing end detection circuit 27, respectively connected are a leading end FF (flip-flop) 26 for generating a high-level signal in accordance with the leading edge detection signal, and a tailing edge FF (flip-flop) 28 for generating a high-level signal in accordance with the tailing edge detection signal, and the leading and tailing edge FFs 26 and 28 are further connected to an original sheet range judging circuit 29 for switching the contact point of the signal switching circuit 21 in accordance with the signal outputs.

0028In the circuit structure shown in Fig. 6, a dot signal is supplied to the line memory 20 and the comparator 24 in synchronous with the clock. The initial dot signal of one scanning line indicated by the broken line in Fig. 7 is of a black image, and therefore a low-level comparison result is output from the comparator 24. At this point, the leading and tailing edge FFs 26 and 28 are not set, generating a low-level output in a similar manner to the above. Consequently, the sheet range judging circuit 29 judges that a dot signal out of the sheet range is input, and connects the contact point of the signal switching circuit 21 to the white data generating circuit 22. Hence, even if a dot signal corresponding to a black image is supplied from the line memory 20 to the signal switching circuit 21, a dot signal corresponding to a white image is supplied from the signal switching circuit 21 to the processing circuit 30 in synchronous with the clock.

0029When a number of dot signals each corresponding to a white image of the leading edge P0 of the original 2 are started to be supplied one after another to the line memory 20 and the comparator 24 in synchronous with the clock, the line memory 20 stores the dot signals in the order of input, and the comparator 24 compares each of the dot signals consecutively with the first reference value Th1, and generates high-level dot signals as the comparison results. The comparison results are continuously supplied to the leading edge detection circuit 25. When dot signals having a level higher than the first reference value Th1 exceed a predetermined number, the leading end detection circuit 25 judges that the dot signal of the leading edge of the original 2 has been input, and supplies a leading edge detection signal to the leading edge FF (flip-flop) 26. A high-level signal is output from the leading edge FF 26, and in accordance with the high-level signal from the leading edge FF 26, the original sheet range judging circuit 29 switches the signal switching circuit 21, and outputs the dot signal from the line memory 20 to the processing circuit 30.

0030While the white image corresponding to the region P0-PL on the original 2 is being supplied to the line memory 20 and the comparator 24, the line memory 20 is connected to the processing circuit 30, and the dot signals of the original 2 are directly supplied to the processing circuit 30.

0031When the dot signals corresponding to the tailing end PL of the original 2 are supplied to the line memory 20 and the comparator 24 in synchronous with the clock, the line memory 20 stores the dot signals, and the comparator 24 compares each of the dot signals consecutively with the first reference value, and outputs low-level dot signals as comparison results. Then, when a predetermined number of low-level comparison results are supplied to the tailing edge detection circuit 27, the tailing edge detection signal 27 judges that the dot signal of the tailing edge of the original 2 has been input, and supplies the tailing edge detection signal to the tailing edge FF (flip-flop) 28. A high-level signal is generated from the tailing edge FF 28, and in response to the high-level signal from the tailing edge FF 28, the original sheet range judging circuit 29 switches the signal switching circuit 21, and connects the white data generating circuit 23 to the processing circuit. Consequently, even if a dot signal corresponding to a black image is supplied from the line memory 20 to the signal switching circuit 21, a dot signal corresponding to a white image is supplied from the signal switching circuit 21 to the processing circuit 30 in synchronous with the clock.

0032The dot signals corresponding to one line are supplied to the processing process 30, the leading and tailing edge FFs 26 and 28 are reset. In other words, the leading and tailing edge FFs 26 and 28 are reset by a scanning signal.

0033In the circuit structure shown in Fig. 6, the line memory 20 serves as a type of buffer. More specifically, until the leading or tailing edge is detected, the dot signal corresponding to the leading or tailing edge is not supplied to the signal switching circuit 21. Consequently, the black dots located outside the original can be converted into white dots.

0034A dot signal supplied to the processing circuit 30, that is, an image signal, is treated with a filtering process, a gamma correction process, or enlarge/reduction process, and converted into an ink amount signal, which is further supplied to a printer 31. In the printer 31, printing is executed in accordance with the ink amount signal so as to form an image on an image forming device. As is clear from the above operation, in the image thus formed, the section other than the original is printed in white.

0035By means of the above process, even if the original 2 is placed obliquely, the leading edge P0 and the tailing edge PL of the original 2 can be detected for each scanning line. Therefore, the region other than the original can be converted into a white image, and the region of the image forming medium other than the original can be formed in white.

0036The following is an explanation of the case where an original sheet is three-dimensional, for example, a thick original such as a book. In the case where an image of a thick original such as a book is read, the original sheet cover cannot be closed properly. Therefore, the state of the original sheet cover being open is detected by the cover open/close sensor 32 shown in Fig. 6, and in response to the open signal from the sensor 32, the reference signal generator 32 outputs the second reference signal Th2 in place of the first reference signal Th1 to the comparator 24.

0037The level of the second reference signal Th2 is set to be higher than the level of the first reference signal Th1 as shown in Fig. 8. In the case where an image of a thick original sheet such as a book is read, the original sheet cover cannot be closed. Therefore, external light enters in the original sheet glass 1, and the light may be detected by the CCD 7 as of a lower density than that of the black portion of the original 2. This is the reason why the level of the second reference signal Th2 is set to be higher than that of the first reference signal Th1.

0038In the circuit structure shown in Fig. 6, the cover open/close sensor 32 detects that the cover is closed, the second reference signal Th2 is generated from the reference value generator 23 in place of the first reference signal Th1. When a dot signal is supplied to the line memory 20 and the comparator 24 in synchronous with the clock, the initial dot signal of one scanning line indicated by the broken line in Fig. 7 is a dot signal corresponding to a black or gray image having a level lower than the second reference signal Th2, and therefore a low-level comparison result is output from the comparator 24. At this point, the leading and tailing edge FFs 26 and 28 are not set, generating a low-level output in a similar manner to the above. Consequently, the sheet range judging circuit 29 judges that a dot signal out of the sheet range is input, and connects the contact point of the signal switching circuit 21 to the white data generating circuit 22. Hence, a dot signal corresponding to a white image is supplied to the processing circuit 30 in synchronous with the clock.

0039When a number of dot signals each corresponding to a white image of the leading edge P0 of the original 2 are supplied one after another to the line memory 20 and the comparator 24 in synchronous with the clock, the line memory 20 stores the dot signals in the order of input, and the comparator 24 compares each of the dot signals consecutively with the second reference value Th2, and generates high-level dot signals as the comparison results. The comparison results are continuously supplied to the leading edge detection circuit 25. When dot signals having a level higher than the first reference value Th1 exceed a predetermined number, the leading end detection circuit 25 judges that the dot signal of the leading edge of the original 2 has been input, and supplies a leading edge detection signal to the leading edge FF (flip-flop) 26. A high-level signal is output from the leading edge FF 26, and in response to the high-level signal from the leading edge FF 26, the original sheet range judging circuit 29 switches the signal switching circuit 21, and outputs the dot signal from the line memory 20 to the processing circuit 30.

0040While the white image corresponding to the region P0-PL on the original 2 is being supplied to the line memory 20 and the comparator 24, the line memory 20 is connected to the processing circuit 30, and the dot signals of the original 2 are directly supplied to the processing circuit 30.

0041When the dot signals corresponding to the tailing end PL of the original 2 are supplied to the line memory 20 and the comparator 24 in synchronous with the clock, the line memory 20 stores the dot signals, and the comparator 24 compares each of the dot signals consecutively with the second reference value Th2, and outputs low-level dot signals as comparison results. Then, when a predetermined number of low-level comparison results are supplied to the tailing edge detection circuit 27, the tailing edge detection signal 27 judges that the dot signal of the tailing edge of the original 2 has been input, and supplies the tailing edge detection signal to the tailing edge FF (flip-flop) 28. A high-level signal is generated from the tailing edge FF 28, and in response to the high-level signal from the tailing edge FF 28, the original sheet range judging circuit 29 switches the signal switching circuit 21, and connects the white data generating circuit 21 to the processing circuit 21. Consequently, a dot signal corresponding to a white image is supplied to the processing circuit 30 in synchronous with the clock.

0042The dot signals corresponding to one line are supplied to the processing process 30, the leading and tailing edge FFs 26 and 28 are reset. In other words, the leading and tailing edge FFs 26 and 28 are reset by a scanning signal.

0043Another example of the circuit structure of the image reading device will not be described with reference to Fig. 9.

0044In the circuit structure shown in Fig. 9, as in the case of the circuit shown in Fig. 2, the first comparator 36 and the switch terminal 36B of the first selector 36, which is switched by the first comparator 34, are connected to the shade correction circuit 16-2, to which an image signal is output. To an output fixed terminal of the first selector 36, connected is a fixed terminal 48C of the second selector 48. To switch terminals 48A and 48B of the second selector 48, connected are input sides of line memories 38 and 39. To the line memories 38 and 39, one line serial image data items are input alternately, but when the data items are output, they are read out in the reverse order to that of input by means of a register. More specifically, when one line serial image data items are input to each of the line memories 38 and 39, signal input starts from a digital signal corresponding to the first dot, and signals are input serially in the order, until the digital signal corresponding to the final dot.

0045In contrast, when the image data items are output from the line memories 38 and 39, data output does not start from the digital signal corresponding to the first dot, but starts from the digital signal corresponding to the final dot. Further, the digital signals are output from the digital signal corresponding to the final dot in the order, and finally the digital signal corresponding to the first dot is output. To output sides of the line memories 38 and 39, connected are switching terminals 50A and 50B of the third selector 50. To a fixed side output terminal of the third selector 50, connected are the second comparator 40 and the fourth selector 42 which can be switched over by the second comparator 40. The fourth selector 42 is connected to the processing circuit 30. The first and second comparators 34 and 40 are connected to the reference signal generator 23 for generating a reference signal, and to a horizontal synchronous signal generator 44 for generating a horizontal synchronous signal, with which timing for transferring one line serial image data generated by the CCD sensor 7 is obtained. The first and second comparators 34 and 40 receives the synchronous from the horizontal synchronous signal generator 44 as a reset signal, and are reset by the synchronous signal. Further, the horizontal synchronous signal generator 44 is connected to the F/F 46, and by the output signal from the F/F 46, the third and fourth selectors 48 and 50 are switched over. In other words, the third and fourth selectors 48 and 50 are switched over in response to the horizontal synchronous signal.

0046In the circuit structure shown in Fig. 9, a black dot is replaced with a white dot, and an image signal is corrected in the following manner. From the CCD sensor 7, line image signals are generated consecutively in synchronous with the horizontal synchronous signal from the horizontal synchronous signal generator 44. The line image signals generated are amplified as serial data, and input to the A/D converter 16-1, where analog line image signals are converted into digital line image signals. The shading of the digital signals caused by an irregular irradiation of an exposure lamp La and an irregular sensitivity of the CCD sensor is corrected by the shading correction circuit 16-2.

0047The shading-corrected digital line image signals are input to the first comparator 34 and one of input terminals of the first selector 36 as the serial data. In the first selector 36, a terminal 36B, to which white data is input, is usually connected to an output terminal 36C. With this structure, even if an image signal corresponding to the first black dot and having a level lower than that of the first reference signal Th1 is input, the selector 36 maintains its state in which the terminal 36B to which white data is input is connected to the output terminal 36C. Consequently, the black image signal is converted into a white image signal, and output from the output terminal 36C. When an image signal having a level higher than that of the first reference signal Th1 is detected in the comparator 36, the position P0 of the signal is judged to be a leading edge of the original 2 on the main scanning line corresponding to the image signal, and a switching signal is supplied from the comparator 36 to the selector 36 so as to connect the terminal 36A, to which image signals are input, to the output terminal 36C. The terminal 36A is continued to be connected to the terminal 36C until the comparator 36 is reset. Therefore, the converted white dots of the initial group, the following image dots, and one line serial image signal containing black dots of the final group are the second selector 48. In the selector 48, the fixed contact point 48C is connected to the first switching contact point 48, and therefore the one line serial image signal is input to the first memory from the beginning. When one horizontal scanning period is over, a horizontal synchronous signal is generated, and transfer of the next (second) one line serial image signal from the CCD sensor 7 is started. At the same time, the comparators 34 and 40 are reset, and the output of the F/F 46 is switched. Consequently, in the selector 36, the fixed contact point 36C is connected to the switching contact point 36A, whereas in the selector 48, the fixed contact point 48C is connected to the switching point 48A. Further, in the selector 50, the fixed contact point 50C is connected to the switching contact point 50B. Thus, in the initial group of the second one line serial image signal, the black dots are replaced with white data as in the case of the first one line serial image signal, and input to the second memory 39 consecutively from the first dot. After that, the first one line serial image signal stored in the first memory 38 is read via the third selector 50, starting from the edge dot, and is input to the switching terminal 42A of the fourth selector 42, and the comparator 40. In the fourth selector 40, a terminal 42B, to which white data is input, is usually connected to an output terminal 42C. With this structure, even if an image signal corresponding to the first black dot and having a level lower than that of the first reference signal Th1 is input, the selector 42 maintains its state in which the terminal 42B to which white data is input is connected to the output terminal 42C. Consequently, the black image signal is converted into a white image signal, and output from the output terminal 42C. When an image signal having a level higher than that of the first reference signal Th1 is detected in the comparator 40, the position Ph of the signal is judged to be a tailing edge of the original 2 on the main scanning line corresponding to the image signal, and a switching signal is supplied from the comparator 40 to the selector 42 so as to connect the terminal 42A, to which image signals are input, to the output terminal 42C. The terminal 42A is continued to be connected to the output terminal 42C until the comparator 40 is reset. Therefore, one line serial image signal is supplied to the processing circuit 30, from the tailing edge dot toward the leading edge dot, in the order of the converted white dots, the following image dots, and the converted white dots. Each time a horizontal synchronous signal is generated, the above-described operation is repeated. One-line serial image signals are alternately stored in the memories 38 and 39, and are alternately read out therefrom. Further, black dots are converted into white dots, and are supplied to the processing device 30, starting from the edge dot.

0048In the above-described embodiments, each dot signal is compared with a reference value, but some dots can be selected as samples to be compared. In other words, the comparison may be performed for only one of a predetermined number of dot signals, or only a plurality of dot signals.

0049As described, according to the present invention, the image data of the section where an original sheet is not present can be converted into white data during an image forming operation with a relatively simple structure. Further, detection of whether or not the original is present, and a black/white conversion are performed at real time for each picture element unit, and therefore even if the original is placed obliquely, or has an arbitrary shape, the image data of the section where an original sheet is not present can be converted into white data. Moreover, In the case where it is failed to place an original sheet, or to close the original sheet cover, or the original is three-dimensional and thick, only the original sheet section is read, and copied. Therefore, waste of copy sheets and developer due to erroneous copying can be significantly reduced, and a neat copy with the periphery of a sheet being whitened can be achieved.