Image reading device

Abstract

To make it possible to adjust more precisely (seamlessly) the lighting time of an illumination light source on an image reading device.SOLUTION: When a lighting period of illumination means in each reading cycle on each line of a document to be read is divided into N sub-lighting periods, the number of sub-lighting periods N that constitutes a lighting period and start timing of each sub-lighting period are determined so that an average frequency of the clock signal in each sub-lighting period and an average frequency of the clock signal that is frequency modulated by spectrum spreading means are equal.SELECTED DRAWING: Figure 5

Description

Technical Field

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[0001] The present invention relates to an image reading apparatus provided with an image sensor.

Background Art

[0002] An image reading apparatus performs a series of processes such as driving an image sensor, controlling lighting of a light source, A / D conversion of an analog image signal obtained from the image sensor, and transferring digital data to an image processing unit in synchronization with a clock signal. In order to increase the reading speed of the image reading apparatus, it is necessary to increase the speed of the clock signal.

[0003] When the clock signal is increased in speed, the radiation noise tends to increase. Therefore, a technique using SSCG (Spectrum Spread Clock Generator) that modulates the clock signal at a predetermined modulation period to reduce the peak of the radiation noise is known. SSCG is an abbreviation for Spectrum Spread Clock Generator. However, when the clock signal modulated by SSCG is used for lighting control of an LED (light emitting diode) which is a light source for image reading, the lighting time fluctuates. The fluctuation of the lighting time may cause image unevenness or the like.

[0004] Patent Document 1 proposes to make the LED lighting time (light irradiation time) an integer multiple of the modulation period (frequency spread period) in order to make the LED lighting time constant.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, the invention described in Patent Document 1 only allows adjustment of the LED illumination time in integer multiples of the modulation period. Therefore, the present invention aims to enable more precise (seamless) adjustment of the illumination time of a light source in an image reading device. [Means for solving the problem]

[0007] In view of the above, an image reading device according to one aspect of the present invention is A lighting means that illuminates the document by turning on a light source, A reading means for reading a document illuminated by the aforementioned illumination means, A generating means for generating a clock signal that serves as a reference for the lighting time of the aforementioned lighting means, A spread spectrum means for frequency modulating the frequency of the clock signal with a predetermined modulation period, The system includes a control means that controls the illumination means and the reading means based on a clock signal frequency-modulated by the spread spectrum means, The control means is When the illumination period of the illumination means in each reading cycle when reading the document line by line is divided into N sub-illumination periods, the number of sub-illumination periods N constituting the illumination period and the start timing of each sub-illumination period are determined so that the average frequency of the clock signal in each sub-illumination period matches the average frequency of the clock signal frequency-modulated by the spread spectrum means. [Effects of the Invention]

[0008] According to the present invention, the illumination time of the illumination light source in an image reading device can be adjusted more precisely (seamlessly). [Brief explanation of the drawing]

[0009] [Figure 1] A cross-sectional diagram illustrating the configuration of the image reading device. [Figure 2] A block diagram showing the configuration of the main components related to the clock signal. [Figure 3]Figure showing other patterns of frequency fluctuations by SSCG. [Figure 4] Timing chart for one color when the lighting time is not divided. [Figure 5] Timing chart for one color when the lighting time is divided. [Figure 6] Timing chart showing an example of the division of the lighting time. [Figure 7] Timing chart showing an example of the division of the lighting time. [Figure 8] Timing chart showing an example of the division of the lighting time for three colors. [Figure 9] Timing chart showing the method of dividing the lighting time. [Figure 10] Flowchart showing the method of dividing the lighting time. [Figure 11] Figure showing the fluctuation of the lighting time. [Figure 12] Figure showing the fluctuation of the lighting time. [Figure 13] Flowchart showing the method of dividing the lighting time.

Mode for Carrying Out the Invention

[0010] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the invention according to the claims, and not all combinations of the features described in the embodiments are essential for the invention. Two or more of the features described in the embodiments may be arbitrarily combined. Also, the same or similar configurations are given the same reference numerals, and duplicate explanations are omitted.

[0011] [First Embodiment] <Image Reading Apparatus 100> FIG. 1 is a partial cross-sectional view schematically showing the configuration of an image reading apparatus including a document feeding apparatus.

[0012] In FIG. 1, the image reading apparatus 100 includes a sheet loading apparatus 101. A plurality of sheets (documents) can be loaded (placed) on the sheet loading table (document table) 1. The sheet loading table 1 is configured to be movable up and down. The loading table drive motor 2 moves the sheet loading table 1 up and down. An HP sensor (not shown) detects that the sheet loading table 1 is located at the home position (HP). The sheet sensor 3 detects that the sheet loaded on the sheet loading table 1 is at the sheet loading position. When the sheet loaded on the sheet loading table 1 is not at the sheet loading position, the loading table drive motor 2 is driven, and the sheet loading table 1 moves so that the uppermost surface of the sheet is located at the sheet loading position. The sheet loading sensor 12 detects that a sheet is loaded on the sheet loading surface 1a of the sheet loading table 1. The regulating plate 51 is movable in a direction intersecting the sheet conveyance direction and regulates the loading position in the sheet width direction.

[0013] The pickup roller 4 (pickup means) picks up the sheet on the sheet loading table 1 and feeds it to the feed roller 6. A pickup motor (not shown) rotates the pickup roller 4. In FIG. 1, the upper surface of the sheet is at the sheet loading position, and when the pickup roller 4 is rotated, the sheet pickup starts.

[0014] The feed roller 6 is located downstream of the pickup roller 4. The feed motor 8 rotates the feed roller 6 to feed the sheet downstream in the conveying direction. The feed roller 6 is connected to the feed motor 8 via a one-way clutch (not shown), and the driving force of the feed motor 8 is transmitted in only one direction. When the feed motor 8 performs feeding, the driving force is transmitted to the feed roller 6. However, when the sheet is conveyed by the resist rollers 17, 18, etc., at a conveying speed higher than the feeding speed of the feed motor 8, the driving force of the feed motor 8 is not transmitted via the one-way clutch. Therefore, the feed roller 6 rotates in a driven manner along with the conveying of the sheet. The separation roller 7, located opposite the feed roller 6 across the conveying path, receives rotational force from the separation motor 9 via a torque limiter (slip clutch) (not shown) that pushes the sheet back upstream in the conveying direction. When a single sheet is present between the feed roller 6 and the separation roller 7, the rotational force transmitted from the torque limiter to the separation roller 7 is a rotational force that pushes the sheet upstream. Furthermore, a rotational force in the opposite direction acts on the separation roller 7. This opposite rotational force is generated by the frictional force between the sheet, which is fed downstream by the feed roller 6, and the separation roller 7. Because the latter rotational force exceeds the former rotational force, the separation roller 7 rotates in accordance with the feed roller 6 (rotates together).

[0015] On the other hand, there may be multiple sheets between the feeding roller 6 and the separating roller 7. In this case, the separating roller 7 receives rotation from its roller shaft in a direction that pushes the sheets back to the upstream side, so that the remaining sheets, except for the top sheet, are not transported to the downstream side.

[0016] In this way, the combined action of the feed roller 6, which feeds the sheets downstream, and the action of the separation roller 7, which prevents the sheets from being transported downstream, eliminates double feeding. Therefore, the feed roller 6 and the separation roller 7 constitute a pair of separation rollers 42. The separation roller pair 42 functions as an example of a sheet separation unit for separating and transporting multiple sheets one by one.

[0017] The separation force of the sheet by the separation roller 7 can be freely adjusted. The sheet can be separated even by simply holding the separation motor 9 without rotating it. A stronger separation force can be obtained by driving the separation motor 9 so that the separation roller 7 rotates in the direction that returns the sheet to the upstream side.

[0018] In this embodiment, a pair of separation rollers 42 is used, but this is only one example. Instead of the pair of separation rollers 42, a pair of separation belt rollers may be used, in which either the separation roller or the feeding roller is replaced with a belt. Alternatively, the separation rollers may be replaced with separation pads, and the separation pads may contact the sheets to prevent multiple sheets from being transported downstream.

[0019] A double-feed sensor 30 may be installed at a position where the separated sheets pass, allowing for the detection of whether the sheets have been separated one by one by the sheet separation unit. A detection device equipped with an ultrasonic transmitting and receiving unit may be used as the double-feed sensor 30. In this case, double feeding is detected by the amount of ultrasonic attenuation between the transmitting and receiving unit that crosses the transport path.

[0020] The transport motor 10 drives the register roller 18 and the transport rollers 21 and 23. The register roller 18 and the transport rollers 21 and 23 transport the sheet after separation to the image reading position and then to the discharge position. In the image reading device, the image of the sheet (original) is read by the image reading sensors 14 and 15 (image reading unit). The register roller 18 and the transport rollers 21 and 23 each have a register roller 17 and transport rollers 20 and 22 on the opposite side, which are driven. The transport motor 10 drives each roller so that the transport speed of the sheet can be changed according to the optimal speed for reading the sheet and the settings of the sheet resolution, etc. The register clutch 19 drives or stops the register roller 18 by transmitting the rotational driving force of the transport motor 10 to the register roller 18 (original transport unit) or by blocking said transmission.

[0021] The nip gap adjustment motor 11 adjusts the gap between the feed roller 6 and the separation roller 7, or the pressure applied by the feed roller 6 to the separation roller 7 via the sheet. This adjusts the gap or pressure to match the thickness of the sheet, allowing the sheet to be separated.

[0022] The transport roller pairs consisting of transport rollers 20 and 21, the transport roller pair consisting of transport rollers 22 and 23, and the further downstream roller pair shown in Figure 1 transport the sheets to the discharge loading section 44. Two guide plates, the upper guide plate 40 and the lower guide plate 41, guide the sheets being transported by the separation roller pair 42, the resist roller pair, each transport roller pair, and the downstream roller pair.

[0023] The image reading device 100 is equipped with a control board 45 that controls the operation of the entire device, and is responsible for driving each actuator and controlling the image reading sensors 14 and 15.

[0024] <Signal System and Operation> Based on Figure 2, the signal systems and operation of the control board 45 and image reading sensors 14 and 15 will be explained. First, the control board 45 will be explained. The clock generation unit 60 generates a clock signal that serves as the reference for controlling all of the image reading devices 100. The clock generation unit 60 is composed of, for example, a crystal oscillator and an oscillation drive circuit that causes the crystal oscillator to oscillate and outputs a clock signal. When the clock signal generated by the clock generation unit 60 is input to the SSCG 61, it is modulated with a predetermined modulation period and fluctuation rate, and a spread-spectrum modulated clock signal is generated. Spread spectrum is achieved by multiplying the input signal by a chip signal with a shorter period (chip period) than the period of the input signal, which is a type of frequency modulation. The modulated clock signal is input to the PLL circuit 62, and a clock signal of the frequency required in the subsequent stage is generated. Because the clock signal input to the PLL circuit 62 is modulated by the SSCG 61, the output clock signal that passes through the PLL circuit 62 remains modulated with the same modulation period and fluctuation rate.

[0025] The clock signal generated by the PLL circuit 62 is input to the control unit 46, the CIS clock generation unit 63, the line synchronization signal generation unit 64, and the image processing unit 65, serving as the reference clock for each of their operations. CIS is an abbreviation for Contact Image Sensor.

[0026] The CIS clock generation unit 63 generates a CIS clock signal that serves as a reference for the operation of the image reading sensors 14 and 15. The line synchronization signal generation unit 64 generates a line synchronization signal SP that serves as a reference for the timing of when the image reading sensors 14 and 15 read one line. The line synchronization signal SP is a synchronization signal for each color. The line synchronization signal for reading one line of the document is called HSYNC. In other words, when one line synchronization signal HSYNC is output, three line synchronization signals SP are output. The control unit 46 performs settings, operation instructions, and various calculations for each unit. The control unit 46 includes, for example, a processor (e.g., CPU) and memory (ROM and RAM) that stores the program. CPU is an abbreviation for Central Processing Unit. ROM is an abbreviation for Read-Only Memory. RAM is an abbreviation for Random Access Memory. The control unit 46 sets the period of the CIS clock signal and gives instructions to the CIS clock generation unit 63 to start / stop output. The control unit 46 sets the period for reading one line image and instructs the line synchronization signal generation unit 64 to start / stop outputting the line synchronization signal SP.

[0027] Next, the image reading sensors 14 and 15 will be described. The timing signal generation unit 71 operates in synchronization with the CIS clock output by the CIS clock generation unit 63. The timing signal generation unit 71 outputs various timing signals for reading one line in synchronization with the line synchronization signal SP output by the line synchronization signal generation unit 64. The timing signal generation unit 71 outputs to the LED drive unit 78 the current value to flow to each light source of the LED 73, and a signal to control the on / off timing of each light source. The current value and the on / off timing of each light source within a line are predetermined by the control unit 46 and set for the timing signal generation unit 71. The timing signal generation unit 71 outputs control signals at predetermined timings each time the line synchronization signal SP is output. Each timing signal output by the timing signal generation unit 71 is output by transitioning between High and Low at timings set using the CIS clock as the minimum unit, based on the line synchronization signal SP from the line synchronization signal generation unit 64.

[0028] The LED driver unit 78 drives the LED 73 in accordance with the control signal output from the timing signal generation unit 71. When it is time to turn on the LED, the LED driver unit 78 supplies current to the light source at a preset current value to turn on the light source. When it is time to turn off the LED, the LED driver unit 78 stops supplying current to the light source to turn it off. The current value is, in principle, a common value for all light sources. LED 73 has three colored LED light sources. LED_R74 emits red light, LED_G75 emits green light, and LED_B76 emits blue light, each emitting light independently.

[0029] The light-receiving unit 77 has multiple light-receiving elements for reading one line of image parallel to the main scanning direction, and performs photoelectric conversion each time light is irradiated from each LED light source. Furthermore, the light-receiving unit 77 outputs the voltage obtained by photoelectric conversion of the line read at the previous timing as an analog image signal to the A / D conversion unit 72 in synchronization with the signal from the timing signal generation unit 71. The A / D conversion unit 72 performs A / D conversion on the analog image signal from the light-receiving unit 77 in synchronization with the signal from the timing signal generation unit 71 to convert it into digital image data, and outputs it to the image processing unit 65. The image processing unit 65 performs various image processing (such as image cropping and image compression) on the digital image data to generate the output image.

[0030] The modulation period of SSCG61 is fixed to a predetermined value. SSCG61 always modulates the input clock signal with a constant modulation period. In this embodiment, SSCG61 modulates the input clock signal so that it takes the shape of a triangular wave. This means that the frequency of the clock signal changes while changing its slope every half-period.

[0031] Figures 3(a) to 3(c) show the frequencies of spread spectrum modulated clock signals. The vertical axis represents frequency, and the horizontal axis represents time. The modulation period of SSCG61 is Tssc.

[0032] As shown in Figure 3(a), one period of the triangular wave is Tssc, and the frequency alternates between minimum and maximum every Tssc / 2. In Figure 3(a), the maximum frequency is the frequency f0 of the SSCG61 clock source, and a downspread method is employed in which the frequency of the output clock signal fluctuates below that frequency. However, other modulation methods may be employed as long as the frequency of the output clock signal fluctuates periodically and alternates between minimum and maximum every half period. For example, as shown in Figure 3(b), a centerspread method may be employed in which the frequency of the clock source is at the center of the frequency fluctuation, and the frequency fluctuates above and below the frequency f0 of the clock source. Also, it is not necessarily a triangular wave. For example, as shown in Figure 3(c), a modulation method in which the output clock signal changes like a sine wave may be employed.

[0033] <Influence of SSCG in LED Lighting> The on / off timing of the LED is determined based on the clock signal modulated by the SSCG61. Therefore, even if the length of the lighting period is set by a certain number of clocks, the actual lighting time fluctuates depending on the timing between the modulation period of the SSCG61 and the lighting period.

[0034] Therefore, when the LED is continuously lit once for a certain period of time, the influence on the lighting time of the LED lit based on the clock signal modulated by the SSCG61 will be explained based on FIG. 4.

[0035] FIGS. 4(a) to 4(h) show the average frequency fa (black dot plots) of the clock signal frequency f when the modulation timing is different by 1 / 8 cycle with respect to the rising timing (lighting start timing) of the line synchronization signal SP. The horizontal axis represents time. Here, the average frequency fa is the average frequency of the clock signal during the lighting time tg of the LED.

[0036] In FIG. 4, the clock frequency f is the frequency of the CIS clock input to the timing signal generation unit 71, and varies with the modulation period Tssc of the SSCG61 as the repetition period. The dotted line on the horizontal axis in the graph of the clock frequency f indicates the average frequency in the modulation period Tssc. The modulation period Tssc is represented by the number of CIS clocks. Over the number of clocks corresponding to Tssc / 2, the clock frequency f decreases from the maximum to the minimum of the frequency fluctuation range. Thereafter, over the number of clocks corresponding to Tssc / 2, the clock frequency f increases from the minimum to the maximum.

[0037] As already explained, the line synchronization signal SP is a line synchronization signal for each color generated by the timing signal generation unit 71. The line synchronization signal SP is input to the light receiving unit 77 and serves as the reference for the transfer of one color of analog image signal and the start of photoelectric conversion. Between one line synchronization signal SP and the next line synchronization signal SP, one color of LED lights up. As a result, the light receiving unit 77 performs photoelectric conversion for one color. LED_G indicates the on / off timing of LED_G75. Tg is the on / off period Tg, and this period is called the on / off period. The on / off period is the period from when the LED starts to light up until when it turns off. The on / off period is the length of time of the on / off period. However, the length of the on / off period Tg is expressed in CIS clock cycles. For simplicity of explanation, Tssc / 2 = Tg is set here.

[0038] The modulation of the CIS clock by SSCG61 and the timing of the LED illumination are asynchronous. Therefore, the clock frequency f of the CIS clock at the start of illumination varies within the range of the SSCG61's fluctuation rate. In Figure 4(a), the clock frequency f is highest when the LED is first illuminated and lowest when it is turned off. The average value of the clock frequency f during this illumination period is the midpoint of the frequency fluctuation range.

[0039] In Figure 4(a), the black circles plotted represent the midpoint of the LED's illumination period and indicate the average frequency fa during that period. The average frequency fa is the same as the average value f0 of the clock frequency f within the modulation period Tssc. However, the LED's illumination start and end times may differ from those shown in Figure 4(a).

[0040] Fig. 4(b) shows a case where the LED lighting start timing is advanced by 1 / 8 cycle with respect to the clock. In this case, the average frequency fa of the clock frequency f during the LED lighting period is lower than the average value f0 within the modulation period. Similarly, looking at Figs. 4(c) to (h) where the LED lighting start timing is advanced by 1 / 8 cycle each with respect to the clock, the average values fa of the clock frequency f during the LED lighting period are different. That is, although the LED lighting time Tg is defined by the same number of clock counts, it can be seen that the actual lighting time varies depending on the phase of the modulation period of the clock signal.

[0041] <Division of LED Lighting Period> Figs. 5(a) to 5(h) show a method of dividing the LED lighting period into N sub - lighting periods and keeping the average value of the clock frequency f constant throughout the lighting period consisting of the N sub - lighting periods. In this example, N is an even number, and in the following explanation, N = 2. Therefore, the sum of the lengths (sub - lighting times) of the sub - lighting periods coincides with the lighting time Tg. That is, the LED lights in N sub - lighting periods each with a length of Tg / N. The time difference (interval) between the start timing of the first sub - lighting period and the start timing of the second sub - lighting period is Tssc / N. The white circles plotted on the graph of the clock frequency f indicate the average value of the clock frequency f in each sub - lighting period. The black circle indicates the average value of the clock frequency f over the entire lighting time Tgd.

[0042] In Figure 5(a), the first sub-lighting period starts when the clock frequency f is at its maximum and ends when the clock frequency f is in the middle (the average value f0 in this example). Therefore, the average frequency fa1 of the first sub-lighting period is higher than the average value f0 in the modulation period. The second sub-lighting period starts when the clock frequency f is at its minimum and ends when the clock frequency f is in the middle. Therefore, the average frequency fa2 of the second sub-lighting period is lower than the average value f0 in the modulation period. The first and second sub-lighting periods are shifted by about half a period of the modulation period. Therefore, even if the average value fa1 of the clock frequency f in the first sub-lighting period is high, the average value fa2 of the second sub-lighting period will be low. Thus, when the average frequency fa is calculated by further averaging the two average values, the difference between the two average values ​​cancels out and it matches the average value f0 of the clock frequency f in the modulation period Tssc.

[0043] In Figures 5(b) to 5(h), the start timing of the first sub-lighting period is advanced by 1 / 8 of the modulation period each time. However, the average value fa of the clock frequency f over the total lighting time Tgd remains the same regardless of the timing. In other words, the total actual lighting time of the LEDs is the same.

[0044] Thus, starting from the first lighting start, the period therefrom is divided corresponding to periods of half the modulation period Tssc / 2 each. That is, the lighting periods divided into N (hereinafter referred to as intervals) are assigned to either the odd-numbered half periods Tssc / 2 or the even-numbered half periods Tssc / 2, N / 2 each. However, the lighting pattern assigned to the i-th (where i is a natural number not exceeding N) interval in the odd-numbered half period Tssc / 2 cycle is the same as the lighting pattern assigned to the i-th interval in the corresponding even-numbered half period Tssc / 2. That the lighting patterns match means that the total number of clocks in the lighting period in the i-th interval of the odd-numbered half period Tssc / 2 is the same as the total number of clocks in the lighting period in the i-th interval of the even-numbered half period Tssc / 2, and the lighting start timing within each half period Tssc / 2 cycle is the same. In FIGS. 5(a) to 5(h), the start of the interval and the lighting start coincide. Thereby, the influence of the fluctuation of the clock frequency f by the SSCG61 is canceled out, and the fluctuation of the lighting time due to modulation is reduced. Note that the total of the periods in the lighting state (lighting periods) is the same before and after the division. Therefore, the influence of the frequency fluctuation can be reduced without changing the received light amount at the light receiving unit 77. The reason for pairing the i-th intervals in the odd-numbered and even-numbered periods of the modulation period divided into Tssc / 2 periods is that their frequency fluctuation characteristics are opposite. That is, by pairing these intervals, the frequency fluctuation is canceled out.

[0045] <Method of Dividing LED Lighting Period> In FIG. 5, the lighting time Tg in FIG. 4 is divided into two sub-lighting times of Tg / 2 each. The sub-lighting time refers to the temporal length of the sub-lighting period. The start timing of each sub-lighting period is separated by Tssc / 2, which is half the modulation period. The length Tgd from the first lighting start to the last lighting end is at least longer than Tssc / 2.

[0046] The Tg in FIG. 4 represents the period from the start of lighting to the end of lighting in one lighting before dividing the lighting period. On the other hand, in FIG. 5, among the plurality of sub-lighting periods after dividing the lighting period Tg, the period from the start of the first sub-lighting period to the end of the last sub-lighting period is defined as Tgd. In FIG. 5, the lighting period is divided into two sub-lighting periods. Therefore, Tgd is from the start of the first sub-lighting period to the end of the second sub-lighting period.

[0047] In FIG. 5, the minimum value of Tgd is obtained as follows. Let n be the number of modulation periods of SSCG61 that fall within the lighting period Tg, and n is obtained by the following formula.

[0048] n = ceil(Tg / Tssc) ···· (1) Here, ceil() is a function that rounds up to the nearest integer. Looking at the relationship between Tg and Tssc in FIG. 4, n = 1. Tgd is obtained by the following formula. [[ID=ll]]

[0049] Tgd = (n - 1 / 2) × Tssc + Tg / (2 × n) ···· (2) Substituting n = 1 into Equation 2, Tgd = Tssc / 2 + Tg / 2 ···· (3) which is consistent with FIG. 5.

[0050] As another example, when Tssc < Tg ≤ 2 × Tssc, n = 2, and Tgd = 3 × Tssc / 2 + Tg / 4. That is, the magnitude of Tgd varies depending on the magnitude relationship between Tg and Tssc. As will be described later, any method can be used as long as sub-lighting periods can be ensured such that the lighting patterns in the sub-lighting periods are paired and the influence of frequency fluctuations is canceled out.

[0051] In Equation 2, Tg and Tgd represent the parameters of the green LED. By substituting Tg and Tgd in Equations 1 and 2 with Tr and Trd for red and Tb and Tbd for blue, similar equations can be obtained for red and blue.

[0052] <000022Q>Equation 2 calculates the period during which Tgd is minimized, but it is not necessarily required to be the minimum. As shown in Figure 6, sub-lighting times of Tg / 4 may be placed in each of the four half-period intervals. In other words, the original lighting time Tg is divided (distributed) into four parts.

[0053] In Figure 6, the period from one line synchronization signal SP to the next line synchronization signal SP is defined as the reading period Tsp for one color line. In the example in Figure 6, approximately 2.5 modulation periods Tssc are included within one reading period Tsp. In other words, the LED illumination period may be distributed over a longer period than in Figure 5.

[0054] In Figure 6(a), the start times of two adjacent sub-lighting periods are separated by Tssc / 2. Thus, the number of divisions does not necessarily have to be 2; any even number, such as 4, is acceptable.

[0055] In Figure 6(b), two sub-lighting periods are arranged within a single half-cycle interval. The interval between the start timing of the first sub-lighting period and the start timing of the third sub-lighting period is Tssc / 2. However, the start timing of two adjacent sub-lighting periods is Tssc / 4. Multiple sub-lighting periods may be arranged within a single half-cycle interval in this manner.

[0056] In Figures 5 and 6, the original illumination time is divided into an even number of sub-illumination times of equal length. In other words, an even number of sub-illumination periods are provided. Furthermore, the start times of each sub-illumination period are evenly spaced apart. However, this is not mandatory.

[0057] Figure 7 shows various division methods. In Figure 7(a), the number of divisions is 2, but a longer interval is used than in Figure 5. Based on the start timing of the first sub-lighting period, multiple intervals (half-period intervals) of length Tssc / 2 are established. If each interval is numbered, when a certain lighting pattern is set in an odd-numbered interval, the same lighting pattern is set in an even-numbered interval. In Figure 7(a), pattern A is set in the odd-numbered 1st interval. Therefore, the same pattern A is also set in the even-numbered 4th interval. This cancels out frequency fluctuations.

[0058] Two different patterns may be combined, as shown in Figure 7(b). Pattern A is assigned to the first interval, and pattern B is assigned to the second interval. Furthermore, pattern A is assigned to the fourth interval, which is an even-numbered interval corresponding to the first interval, and pattern B is assigned to the third interval, which is an odd-numbered interval corresponding to the second interval.

[0059] As shown in Figure 7(c), the lights may remain lit consecutively in two consecutive sections. Pattern A is a lighting pattern in which the lights start lit at the beginning of the section, turn off once, turn on again, and remain lit until the end of the section. If this Pattern A is assigned to the first and second sections, the lights will remain lit consecutively at the end of the first section and the start of the second section. Even in this case, the same Pattern A is assigned to the even-numbered and odd-numbered sections, so the above-mentioned effect is achieved.

[0060] The illuminated state may continue from the beginning to the end of the section, as shown in Pattern A in Figure 7(d). Pattern A is assigned to the odd-numbered (1st) and even-numbered (4th) positions, and Pattern B is assigned to the other even-numbered and odd-numbered pairs.

[0061] As shown in Figure 7(e), multiple lighting patterns may be assigned to the same half-cycle. Pattern A is assigned to the odd-numbered 1st interval, and pattern B is assigned to the other odd-numbered 3rd interval. Furthermore, pattern A+B, which is a combination of pattern A and pattern B, is assigned to the even-numbered 2nd interval.

[0062] <Timing chart for the entire line> Figure 8 shows the lighting control of one line using three-color LEDs. In Figure 8, HSYNC is the line synchronization signal (main scan synchronization signal) generated by the line synchronization signal generation unit 64 and output to the timing signal generation unit 71. The period of HSYNC is Th. Each time HSYNC is output, an image for all three colors is read. Therefore, the timing signal generation unit 71 outputs a line synchronization signal SP for one color three times at intervals of Tsp. Tsp is the interval between line synchronization signals SP and is expressed in clock cycles. Th is predetermined according to the sheet transport speed and reading resolution. Also, Tsp is determined as a value obtained by dividing Th into three equal parts.

[0063] LED_G75 lights up based on the first line synchronization signal SP for HSYNC. The original illumination time Tg is divided equally into two parts. The length of each sub-illumination period (illumination time) is Tg / 2. The start timing of the second sub-illumination period is shifted by Tssc / 2 from the start timing of the first sub-illumination period. When LED_G75 stops lighting up and the first Tsp period ends, the next line synchronization signal SP is emitted. As shown in Figure 8, the illumination time and timing of LED_B76 are controlled in the same way as LED_G75. Furthermore, LED_R74 is controlled in the same way. Tg and Tgd of LED_G75 are denoted as Tb and Tbd for LED_B and Tr and Trd for LED_R. Once the illumination control for all three colors is complete, the reading control for one line is completed. Then, the next HSYNC is output. This process is repeated in the same manner.

[0064] <Dividing the lighting period, including current changes> Next, if the illumination period before splitting accounts for a large proportion of Tsp, it may not be possible to split the illumination period. Figure 9(a) shows the illumination periods before splitting. Of the illumination periods of the three colors, the illumination period of LED_G is the longest, and the illumination period of LED_R is the shortest. The control unit 46 determines whether the illumination period of LED_G, which has the longest illumination period among the three colors, can be split. According to Equation 2, if Tgd is calculated using Tg, then Tgd > Tsp, and it can be seen that the required illumination period cannot be secured. Therefore, the current value flowing through the LED is increased, and instead the illumination time is made shorter than the original illumination time Tg, so that the illumination period fits within Tsp. The light intensity of the LED is proportional to the current value. Therefore, it is sufficient that the product of the LED current value and illumination period is the same before and after splitting. The illumination time of the LED is determined to be shorter than the original illumination time Tg and the longest time among the multiples of the modulation period Tssc.

[0065] As shown in Figure 9(b), the illumination period of LED_G is determined to be 1x the length of Tssc. This corresponds to 2 / 3 of the original illumination time Tg. Accordingly, the current value is multiplied by its reciprocal, 3 / 2. That is, if the original current value is 20mA, the new current value becomes 20 × 3 / 2 = 30mA. The same reduction rate of 2 / 3 for LED_G's illumination time and the increase rate of 3 / 2 for the current value are applied to the other two colors, LED_B and LED_R. As a result, as shown in Figure 9(b), the current value for all three colors becomes 30mA, and the illumination time becomes 2 / 3 of the original.

[0066] In the state shown in Figure 9(b), the illumination time of LED_G is a multiple of Tssc, and is not affected by frequency fluctuations caused by SSCG61. However, the illumination periods of LED_B and LED_R are affected by frequency fluctuations. Therefore, the above-described process of dividing the illumination period is applied to LED_B and LED_R.

[0067] Of the three colors, LED_G has the longest illumination time, and its illumination time matches that of Tssc, but the illumination times of the other two colors are shorter. Therefore, the illumination periods and illumination times of LED_B and LED_R can always be divided. Thus, when the illumination periods are divided using the division method described above, the result is as shown in Figure 9(c). This makes the illumination periods of LED_B and LED_R less susceptible to the effects of frequency fluctuations.

[0068] <How to determine the timing of illumination> Figure 10 is a flowchart showing the method for determining the lighting timing executed by the control unit 46.

[0069] In S1001, the control unit 46 determines Th and Tsp based on the sheet transport speed corresponding to the reading resolution. For example, a table holding pairs of Th and Tsp linked to the transport speed may be stored in the ROM within the control unit 46. The control unit 46 refers to this table and determines Th and Tsp from the transport speed.

[0070] In S1002, the control unit 46 determines the lighting times Tg, Tb, and Tr. The RAM of the control unit 46 may store the lighting times Tg, Tb, and Tr, which are the results of the light intensity adjustment for each color performed in advance by the image reading sensors 14 and 15. The control unit 46 retrieves the lighting times Tg, Tb, and Tr from the RAM. As is well known, automatic light intensity adjustment is performed in the image reading device. Automatic light intensity adjustment determines the lighting time that can achieve the target amount of light received.

[0071] In S1003, the control unit 46 determines the corresponding segmented lighting periods Tgd, Tbd, and Trd based on the lighting times Tg, Tb, and Tr.

[0072] In S1004, the control unit 46 determines whether the illumination time Tgd is less than or equal to Tsp. In other words, the control unit 46 determines whether the illumination time Tg is divisible. If the illumination time Tgd is less than or equal to Tsp, Tg is divisible, so the control unit 46 proceeds to S1005. In S1005, the control unit 46 stores in RAM that Tg is divisible. On the other hand, if the illumination time Tgd exceeds Tsp, Tg is not divisible, so the control unit 46 proceeds to S1006. In S1006, the control unit 46 stores in RAM that Tg is not divisible.

[0073] In S1007, the control unit 46 determines whether the illumination time Tbd is less than or equal to Tsp. In other words, the control unit 46 determines whether the illumination time Tb is divisible. If the illumination time Tbd is less than or equal to Tsp, Tb is divisible, so the control unit 46 proceeds to S1008. In S1008, the control unit 46 stores in RAM that Tb is divisible. On the other hand, if the illumination time Tbd exceeds Tsp, Tb is not divisible, so the control unit 46 proceeds to S1009. In S1009, the control unit 46 stores in RAM that Tb is not divisible.

[0074] In S1010, the control unit 46 determines whether the illumination time Trd is less than or equal to Tsp. In other words, the control unit 46 determines whether the illumination time Tr is divisible. If the illumination time Trd is less than or equal to Tsp, Tr is divisible, so the control unit 46 proceeds to S1011. In S1011, the control unit 46 stores in RAM that Tr is divisible. On the other hand, if the illumination time Trd exceeds Tsp, Tr is not divisible, so the control unit 46 proceeds to S1012. In S1012, the control unit 46 stores in RAM that Tr is not divisible.

[0075] In S1013, the control unit 46 determines, based on the judgment result stored in RAM, that the illumination time for all three colors can be divided. If the illumination time for all three colors can be divided, the control unit 46 proceeds to S1014. In S1014, the control unit 46 determines the division timing for each of the three colors. This determines the number of divisions for the illumination times Tg, Tb, and Tr, and which illumination pattern to assign to which interval. The specific division method has already been explained. On the other hand, if the control unit 46 determines in S1013 that there is an illumination time for even one color that cannot be divided, the control unit 46 proceeds to S1015.

[0076] In S1015, the control unit 46 determines the longest illumination time among the three illumination times Tg, Tr, and Tb, and reduces the longest illumination time. Let the reduction rate be X. For example, if Tg is the longest of the three colors, a new illumination time is determined to be shorter than Tg and to be the longest period that is a multiple of the modulation period Tssc, similar to the example described above.

[0077] In S1016, the control unit 46 increases the current value of the LED. As described above, the new current value is determined by multiplying the original current value by 1 / X. Here, it is assumed that all three colors have the same current value.

[0078] In S1017, the control unit 46 modifies the illumination time of the other two colors. As described above, the original illumination time of the other two colors is multiplied by a reduction rate X.

[0079] In S1018, the control unit 46 determines the division timing for the other two colors. According to the division method described above, the number of divisions in the lighting time and which lighting pattern to assign to which section are determined.

[0080] In S1019, the control unit 46 sets the division timing for the three colors to the timing signal generation unit 71.

[0081] In this embodiment, the LED driver unit 78 sets a common current value for all light sources of the LED 73, but this is merely an example. The LED driver unit 78 may set different current values ​​individually for each light source. Furthermore, for colors whose lighting period can be divided without changing the current value, the current value does not need to be changed. For example, in the case shown in Figure 9(a), since the lighting time Tr of LED_R can be divided, the current value may be maintained at 20mA.

[0082] In this embodiment, a color reading method using a one-line sensor that sequentially reads one color at a time from three color LEDs using one line of light-receiving elements was described, but this is merely one example. For example, a color reading method using a three-line sensor that reads three colors simultaneously using three lines of light-receiving elements may also be employed. With a three-line sensor, the light sources for all three colors can be lit simultaneously, but the ability to set the lighting time for each color is the same as with a one-line sensor. The same applies when performing grayscale reading with a one-line sensor. Grayscale reading can be achieved by litting only one color or by litting all three colors simultaneously, but again, the ability to set the lighting time for each color is the same.

[0083] [Second Embodiment] Next, an image reading device according to the second embodiment will be described. Note that the parts common to the first embodiment will be omitted from the description, and the differences from the first embodiment will be mainly explained.

[0084] In the first embodiment, the illumination time Tg is divided without considering the relationship between the illumination time Tg and the modulation period Tssc of the SSCG61. This is because, generally, it is assumed that the modulation period Tssc is long relative to the illumination time Tg. However, there are cases where the modulation period Tssc is short relative to the illumination time Tg. In this case, the actual illumination time variation caused by the frequency variation of the CIS clock by the SSCG61 may be small enough to be negligible. In this case, division of the illumination time is not necessary. Therefore, in the second embodiment, it is determined whether the clock frequency variation by the SSCG61 is acceptable, and if it is not acceptable, the illumination is divided.

[0085] <Determining whether segmented lighting is necessary> The control unit 46 determines that segmented lighting is necessary if the difference between the average value fa of the clock frequency f during the LED lighting period and the average value f0 of the clock frequency f during the modulation period Tssc is large.

[0086] Figure 11 considers the case where LED_G is lit for a duration Tg. As an example where the lit-time Tg is shorter than the modulation period Tssc, it is assumed that Tg = Tssc / 2. As shown in Figure 11(a), when the phase midway through the lit-time Tg is superimposed on the phase where the clock frequency f is at its maximum value, the average clock frequency fa during the lit-time period is at a frequency 3 / 4 of the fluctuation range d above the minimum frequency fmin (fa3 = fmin + d × 3 / 4). In other words, the average value fa is at the frequency at the position of the black circle in the figure. For example, if the fluctuation range d is 1% and the maximum frequency is 10MHz, the minimum frequency fmin is 9.9MHz. In this case, the position at 3 / 4 of the fluctuation range d is -0.25%, and the average frequency fa is 9.975MHz.

[0087] Similarly, the average value of the clock frequency fa is minimized when, symmetrically with the above case, the phase midway through the illumination time Tg coincides with the phase at which the clock frequency f is minimized. In this case, the average value of the clock frequency fa4 is fa4 = fmin + d × 1 / 4 = 9.925 Hz.

[0088] At this time, the difference between the average value f0 of the clock frequency during the modulation period and the average clock frequency fa during the illumination period becomes relatively large. The rate of change dp(|fa3-fa4| = |fmin + d×3 / 4 - (fmin + d×1 / 4)| = d×1 / 2) of the average clock frequency fa during the illumination period is 0.5%.

[0089] On the other hand, consider the case where Tg = Tssc × 3 / 4, as shown in Figure 11(b). In this case, calculating in the same way as above, the fluctuation range dp of the average clock frequency fa during the illumination period is d × 1 / 4. If the fluctuation range d is 1%, the fluctuation rate dp = 0.25%. Thus, as Tg approaches Tssc, the fluctuation rate dp of the clock frequency during the illumination period approaches 0, and when Tg = Tssc, dp = 0.

[0090] Based on the above, if the fluctuation rate dp is greater than or equal to a preset threshold for the determined lighting time Tg, the LEDs will be lit in segments. If the fluctuation rate dp is less than the threshold, the LEDs will not be lit in segments. For example, if the threshold is set to 0.3%, in Figure 11(a) the fluctuation rate dp is 0.5%, so segments lighting is necessary. In Figure 11(b) the fluctuation rate dp is 0.25%, so segments lighting is not necessary.

[0091] Similarly, if the illumination time Tg is longer than the modulation period Tssc, and the fluctuation rate dp is greater than or equal to a preset threshold for the determined illumination time Tg, the LEDs are illuminated in segments. On the other hand, if the fluctuation rate dp is less than the threshold, the LEDs are not illuminated in segments.

[0092] Furthermore, if the illumination time Tg is longer than the modulation period Tssc, the fluctuation rate dp may not decrease significantly. An example of this is explained below.

[0093] Figure 12 shows an example where the illumination time Tg of LED_G is longer than the modulation period Tssc. Here, it is assumed that Tg = Tssc × 1.5. As shown in Figure 12, when the phase midway through Tg coincides with the phase at which the clock frequency f reaches its maximum value fmax, the average value fa of the clock frequency during the illumination period will be 5 / 12 of the fluctuation range d above the minimum value fmin (fa5 = fmin + d × 5 / 12). This corresponds to the -0.583% position.

[0094] Similarly, due to the symmetry, when the phase at the middle of the lighting time Tg is overlapped with the phase at which the clock frequency f reaches its minimum value fmin, the average value fa of the clock frequency f during the lighting period becomes a frequency that is 7 / 12 of the fluctuation range d above the minimum value fmin (fa6 = fmin + d×7 / 12). This corresponds to the position of -0.417%. The actual lighting time fluctuates within these ranges, and the fluctuation rate dp (|fa5 - fa6| = d×1 / 6) becomes 0.166% when the fluctuation range d is 1%.

[0095] In the range where Tssc < Tg, the fluctuation width dp becomes maximum when Tg = Tssc×1.5, but this is also smaller compared to the above-mentioned threshold value of 0.3% as an example. Therefore, when Tssc < Tg, it may be determined that divided lighting is unnecessary without calculating the fluctuation rate dp.

[0096] Generally, when n is a natural number and n×Tssc = Tg, the fluctuation rate dp becomes 0%. In the range where n×Tssc < Tg < (n + 1)×Tssc, the larger n becomes, the smaller the fluctuation rate dp becomes. Therefore, when a preset threshold value is a smaller value than the above-mentioned, such as 0.1%, by calculating n that Tg satisfies, it may be determined whether it is below the threshold value or whether it satisfies the corresponding conditions, and then it may be determined whether to perform divided lighting.

[0097] In summary, regardless of the magnitude relationship between Tg and Tssc, it may be determined whether to perform divided lighting based on the fluctuation rate dp of the average frequency fa of the clock frequency during the lighting period. In that case, it may also be determined whether to perform divided lighting based on whether Tg = n×Tssc. Also, when n×Tssc < Tg < (n + 1)×Tssc, n that Tg should satisfy for a predetermined threshold value is determined in advance, and it may be determined whether to perform divided lighting based on whether Tg satisfies n×Tssc < Tg with respect to the threshold value.

[0098] <Method for determining lighting timing> The flow of the process for determining the LED segment lighting timing in the second embodiment will be explained with reference to Figure 13. In Figure 13, processes common to Figure 10 are given the same reference numerals, and their explanations will be carried over.

[0099] After executing S1001 and S1002, the control unit 46 proceeds to S1301.

[0100] In S1301, the control unit 46 calculates the rate of variation of the illumination time for each color based on Tg, Tb, and Tr. The calculation method is as described in relation to Figures 11 and 12.

[0101] In S1302, the control unit 46 determines whether or not to divide the lighting time for each of the three colors based on the fluctuation rate and threshold for each color. As described above, if the fluctuation rate is greater than or equal to the threshold, it is necessary to divide the lighting time. If the fluctuation rate is less than the threshold, it is not necessary to divide the lighting time. The control unit 46 stores whether or not to divide each color in RAM and proceeds to S1303.

[0102] In S1303, the control unit 46 determines whether any of the three colors need to be split based on the judgment result stored in RAM. If none of the three colors need to be split, the control unit 46 terminates the splitting process. If any of the colors need to be split, the control unit 46 proceeds to S1304.

[0103] In S1304, the control unit 46 executes S1003 to S1012 as described above.

[0104] In S1305, the control unit 46 reads the result of the division feasibility determination from RAM and determines whether the color to be divided is divisible. That is, if all three colors need to be divided, it is determined whether all three colors are divisible. If two of the three colors need to be divided, it is determined whether both of those colors are divisible. If one of the three colors needs to be divided, it is determined whether that one color is divisible. If all the colors to be divided are divisible, the control unit 46 proceeds to S1306.

[0105] In S1306, the control unit 46 determines the timing for dividing the colors that need to be divided. This is as explained in S1014. On the other hand, if there are any colors among those that need to be divided that cannot be divided, the control unit 46 proceeds to S1307.

[0106] In S1307, the control unit 46 reduces the illumination time of the longest color among those to be divided. This is the same process as in S1015.

[0107] In S1308, the control unit 46 increases the current value for the color to be divided. This is the same process as in S1016.

[0108] In S1309, the control unit 46 changes the illumination time of the remaining colors among the colors to be divided. This is the same process as in S1017. If there is only one color to be divided, this process is unnecessary.

[0109] In S1310, the control unit 46 determines the timing for dividing the remaining colors among the colors to be divided. This is the same process as in S1018. If there is only one color to be divided, this process is unnecessary.

[0110] In S1311, the control unit 46 sets the timing for dividing the colors that need to be divided. This is the same process as in S1019.

[0111] <Summary> [Perspective 1] LED 73 is an example of an illumination means that lights up a light source to illuminate the document. Light receiving unit 77 is an example of a reading means that reads the document illuminated by the illumination means. Clock generation unit 60 is an example of a generation means that generates a clock signal which serves as a reference for the illumination time of the illumination means. SSCG 61 is an example of a spread spectrum means that frequency modulates the frequency of the clock signal with a predetermined modulation period. Control unit 46, timing signal generation unit 71, and LED drive unit 78 are examples of control means that control the illumination means and the reading means based on the clock signal frequency modulated by the spread spectrum means. The control means determines the number of sub-illumination periods N that constitute the illumination period and the start timing of each sub-illumination period, so that when the illumination period of the illumination means in each reading cycle (e.g., Th, Tsp) when reading a document line by line is divided into N sub-illumination periods, the average frequency fa of the clock signal in each sub-illumination period matches the average frequency f0 of the clock signal frequency modulated by the spread spectrum means. This allows for more precise (seamless) adjustment of the illumination time of the light source in the image reading device 100.

[0112] [Perspective 2] The time from the start of the first sub-lighting period to the end of the Nth sub-lighting period (e.g., Tgd, Trd, Tbd) is longer than half a period of the modulation cycle.

[0113] [Perspective 3] As illustrated in Figures 6 and 7, one reading cycle may contain M half-cycles of the modulation cycle, with the start timing of the first sub-lighting period as the reference point (M≧N). Each of the N sub-lighting periods is contained within one of the M half-cycles. The same lighting pattern applied to the odd-numbered half-cycles is also applied to the even-numbered half-cycles. Here, the lighting pattern indicates the length of the sub-lighting period. The lighting pattern may also define the start and end timings of each sub-lighting period. Basically, each start timing coincides with the start timing of the half-cycle interval, but as illustrated in Figures 7(c) and 7(e), the start timing of a sub-lighting period may be placed in the middle of a half-cycle interval.

[0114] [Perspective 4] The number of sub-lighting periods N is even. That is, the original lighting time is divided into an even number of sub-lighting periods and distributed between odd-numbered intervals and even-numbered intervals. There is a certain periodicity in the start timing of each sub-lighting period. That is, the interval from the start timing of one sub-lighting period to the start timing of the next sub-lighting period may be a constant interval.

[0115] [Perspective 5] As illustrated in Figure 5, the time difference between the start timing of the i-th sub-lighting period and the start timing of the (i+1)-th sub-lighting period among the N sub-lighting periods may be equal to half a period of the modulation cycle.

[0116] [Perspective 6] As illustrated in Figure 7, the first lighting pattern applied to the first odd-numbered half-period of the M half-periods is also applied to the first even-numbered half-period. The second lighting pattern applied to the second odd-numbered half-period of the M half-periods is also applied to the second even-numbered half-period.

[0117] [perspective 7] As illustrated in Figure 7(c), the lighting pattern may be one in which the lighting means is lit multiple times within a half-cycle.

[0118] [Perspective 8] As illustrated in Figure 7(d), the first odd-numbered half-period and the second odd-numbered half-period may be adjacent. The first lighting pattern may be a pattern in which the lighting means is continuously lit for the entire first odd-numbered half-period. The second lighting pattern may be a pattern in which the lighting means is lit for a shorter time than the second odd-numbered half-period.

[0119] [Perspective 9] As illustrated in Figure 7(e), the first even-numbered half-period and the second even-numbered half-period may be the same half-period.

[0120] [Perspective 10] As explained using Figures 10 and 13, the control unit 46 may determine whether the lighting period can be divided based on the reading period, the modulation period, and the lighting period. If the lighting period can be divided, the control unit 46 divides the lighting period into N sub-lighting periods. If the lighting period cannot be divided, the control unit 46 may reduce (shorten) the lighting period to another lighting period that is an integer J times half a period of the modulation period, and increase the drive current supplied to the light source of the illumination means from the first drive current to the second drive current.

[0121] [Perspective 11] The integer J is the largest integer such that the shortened lighting period does not exceed the original lighting period. In the embodiment described above, the case where J=1 was explained, but this is only one example.

[0122] [Perspective 12] The product of the illumination period and the first drive current is equal to the product of another time and the second drive current. This ensures that even if the illumination period is reduced, the integrated value of the illumination light that lights up the document remains constant.

[0123] [Perspective 13] As explained in relation to S1004, etc., the time from the start of the first sub-lighting period to the end of the Nth sub-lighting period (e.g., Tgd) among the N sub-lighting periods may be longer than the reading cycle (e.g., Tsp). In this case, the control unit 46 may determine that division is not possible.

[0124] [Perspective 14] The lighting means may have multiple light sources that can be individually lit (e.g., LED_R74, LED_G75, LED_B76). The control unit 46 determines whether the lighting period of each of the multiple light sources can be divided. If the control unit 46 determines that the lighting period of the first light source, which is the longest lighting period among the multiple light sources, cannot be divided, it may reduce the longest lighting period to another lighting period that is an integer J times half a period, increase the drive current supplied to the light sources of the lighting means from the first drive current to the second drive current, reduce the lighting period of the second light source, which is different from the first light source, by the same reduction rate as the reduction rate of the lighting period of the first light source, and divide the reduced lighting period of the second light source into N sub-lighting periods.

[0125] [Perspective 15] As illustrated in Figures 11 and 12, when the illumination period is arranged such that its center is at the phase where the clock signal frequency is maximum, the frequency difference between the average frequency of the clock signal and when the illumination period is arranged such that its center is at the phase where the clock signal frequency is minimum may exceed the allowable value. In this case, the control unit 46 may divide the illumination period into N sub-illumination periods. Conversely, if the frequency difference does not exceed the allowable value, the illumination period does not need to be divided. The necessity of division may also be determined for each color.

[0126] [Perspective 16] As described above, a control method for the image reading device 100 is provided.

[0127] The invention is not limited to the embodiments described above, and various modifications and changes are possible within the scope of the gist of the invention.

[0128] For example, in the embodiment described above, when dividing the lighting period into N sub-lighting periods, N was described as an even number, but this is not limited to this. For example, it may be divided into N odd intervals, and the division should be such that the average frequency fa of the clock signal in the N sub-lighting periods is closer to the average value f0 of the clock frequency in the modulation period than the average frequency of the lighting period before division. In this case, the lighting pattern should be set so that the difference between the average value f0 and the average frequency fa is less than or equal to a predetermined threshold. [Explanation of symbols]

[0129] 73: LED, 77: Light receiving unit, 60: Clock generation unit, 61: Spread spectrum clock generator (SSCG), 46: Control unit

Claims

1. an illumination means for illuminating the document by turning on a light source; a reading means for reading the document illuminated by the illumination means; A generating means for generating a clock signal that is a reference for the lighting time of the lighting means; a spectrum spreading means for frequency-modulating the frequency of the clock signal at a predetermined modulation period; a control means for controlling the illumination means and the reading means based on a clock signal frequency-modulated by the spectrum spreading means, The control means an illumination means for detecting a start timing of each of the sub-illumination periods and a start timing of each of the sub-illumination periods, the number of the sub-illumination periods constituting the illumination period being determined so that an average frequency of the clock signal in each sub-illumination period coincides with an average frequency of a clock signal frequency-modulated by the spectrum spread means, when an illumination period of the illumination means in each reading cycle when the original is read line by line is divided into N sub-illumination periods.

2. 2. The image reading device according to claim 1, wherein a time from a start timing of a first sub-illumination period to an end timing of an Nth sub-illumination period among the N sub-illumination periods is longer than a half period of the modulation period.

3. One reading cycle includes M half cycles of the modulation cycle based on the start timing of the first sub-illumination period (M≧N), The N sub-lighting periods are included in any of the M half periods, 3. The image reading device according to claim 2, wherein the same lighting pattern as that applied to odd-numbered half periods among the M half periods is also applied to even-numbered half periods, and the lighting pattern indicates the length of the sub-lighting period.

4. 2. The image reading device according to claim 1, wherein the number N of said sub-lighting periods is an even number, and the start timing of each sub-lighting period has a certain periodicity.

5. 5. The image reading device according to claim 4, wherein a time difference between a start timing of an i-th sub-lighting period and a start timing of an i+1-th sub-lighting period among the N sub-lighting periods is equal to a half period of the modulation period.

6. A first lighting pattern applied to a first odd-numbered half period among the M half periods is also applied to a first even-numbered half period; 4. The image reading device according to claim 3, wherein a second lighting pattern applied to a second odd-numbered half period among the M half periods is also applied to a second even-numbered half period.

7. 4. The image reading device according to claim 3, wherein the lighting pattern is a pattern in which the illumination means is turned on a plurality of times within the half cycle.

8. the first odd-numbered half period and the second odd-numbered half period are adjacent to each other, the first lighting pattern is a pattern in which the lighting means is continuously turned on for the entirety of the first odd-numbered half cycle, 7. The image reading device according to claim 6, wherein the second lighting pattern is a pattern in which the illumination means is turned on in the second odd-numbered half cycle for a time shorter than the second odd-numbered half cycle.

9. 7. The image reading device according to claim 6, wherein the first even-numbered half period and the second even-numbered half period are the same half period.

10. The control means determining whether the lighting period can be divided based on the reading cycle, the modulation cycle, and the lighting period; If the lighting period is divisible, the lighting period is divided into the N sub-lighting periods; 10. The image reading device according to claim 1, further comprising: when the lighting period cannot be divided, the lighting period is reduced to another lighting period that is an integer multiple J of a half cycle of the modulation cycle, and a drive current supplied to the light source of the illumination means is increased from a first drive current to a second drive current.

11. 11. The image reading device according to claim 10, wherein the integer J is a maximum integer such that the different lighting period does not exceed the lighting period.

12. 11. The image reading device according to claim 10, wherein a product of the lighting period and the first driving current is equal to a product of the different time and the second driving current.

13. 11. The image reading device according to claim 10, wherein the control means determines that the sub-lighting period is not divisible when a time from a start timing of a first sub-lighting period to an end timing of an Nth sub-lighting period among the N sub-lighting periods is longer than the reading cycle.

14. The illumination means has a plurality of light sources that can be turned on individually, 11. The image reading device according to claim 10, wherein the control means determines whether the lighting period of each of the multiple light sources is divisible, and when it determines that the lighting period of a first light source, which is the longest lighting period among the lighting periods of the multiple light sources, is not divisible, the control means reduces the longest lighting period to another lighting period that is an integer multiple of J of the half cycle, increases a drive current supplied to the light source of the illumination means from the first drive current to the second drive current, reduces the lighting period of a second light source different from the first light source among the multiple light sources by the same reduction rate as a reduction rate of the lighting period of the first light source, and divides the reduced lighting period of the second light source into the N sub-lighting periods.

15. 2. The image reading device according to claim 1, wherein the control means divides the lighting period into the N sub-lighting periods when a frequency difference between an average frequency of the clock signal when the lighting period is arranged so that its center is located in a phase where the frequency of the clock signal is maximum, and an average frequency of the clock signal when the lighting period is arranged so that its center is located in a phase where the frequency of the clock signal is minimum, exceeds a tolerance value.

16. an illumination means for illuminating the document by turning on a light source; a reading means for reading the document illuminated by the illumination means; a clock supplying means for supplying a clock signal that is a reference for a lighting period of the lighting means; a spectrum spreading means for frequency-modulating the frequency of the clock signal at a predetermined modulation period; a control means for controlling the illumination means and the reading means based on a clock signal frequency-modulated by the spectrum spread means, a control method for an image reading device, comprising: determining, when the lighting period of the illumination means in each reading cycle when reading the original document line by line, the number N of the sub-lighting periods constituting the lighting period and a start timing of each sub-lighting period so that an average frequency of the clock signal in each sub-lighting period matches an average frequency of a clock signal frequency-modulated by the spectrum spread means.

Images