JP2000050019A - Image reading device - Google Patents

Abstract

(57) [Problem] To provide an image reading apparatus in which a color shift in reading in a sub-scanning direction caused by a difference in persistence characteristics of each color in a white light source for document irradiation is reduced. SOLUTION: (a) of FIG.
(B) is a time chart when the duty ratio is 60%. As shown in the figure, the influence of the afterglow characteristic is reduced by forming the PWM control pulse symmetrically in the time axis direction around the center C in one accumulation period of a line sensor (not shown) for reading the original image. However, it is possible to reduce the color shift at the time of reading in the sub-scanning direction.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus for forming image information of a document on a line sensor via an image forming optical system and reading an image.

[0002]

2. Description of the Related Art Conventionally, image information of a document or the like is formed on a plurality of line sensors (solid-state image pickup devices such as CCDs) via an image forming optical system, and a monochrome or black or white image is formed based on an output signal from the line sensor. Various image reading apparatuses for digitally reading color image information have been proposed.

FIG. 12 is a schematic view of a main part of an optical system of a conventional color image reading apparatus.

In FIG. 1, reference numeral 100 denotes an original platen glass on which a read image is arranged; 101, a rod-shaped light source for irradiating the original;
Reference numeral 102 denotes a reflection shade for improving the irradiation efficiency. Image information of a document (not shown) illuminated by the rod-shaped light source 101 and the reflection shade 102 is transmitted to mirrors 103-a and
The light is guided to the imaging optical system 104 via 3-b and 103-c, and the imaging optical system 104 forms image information of the document on the solid-state imaging device 105. The mirror 103-a moves at the scanning speed v in the sub-scanning direction A, and in synchronization with this, the mirror 103-a
-B and 103-c can read the image information by two-dimensional scanning in accordance with the arrangement direction (main scanning direction) of the line sensors of the solid-state imaging device 105 by moving at the speed v / 2.

In such a configuration, the solid-state imaging device 10
The image information focused on 5 is converted into an electric signal and sent to an output device (not shown), where the image information is output as a print output, or sent to a storage device or the like to store the input image information. May be performed, and are used as respective image reading devices.

As a light source of the image reading apparatus having such a configuration, a halogen lamp, a fluorescent lamp, a xenon lamp or the like is used. Halogen lamps have usually been used as light sources for this type of image reading apparatus. Halogen lamps have high brightness, but the temperature rise of the apparatus accompanying the temperature rise of the lamp is large, and power consumption of 200 to 300 W is required. This necessitates an increase in power consumption required for the entire apparatus.

In recent years, in order to avoid such problems, fluorescent lamps and xenon lamps having high luminance have been developed and are being used as light sources for image reading apparatuses.

[0008] A fluorescent lamp or a xenon lamp uses a small amount of mercury particles and several Torrs of Ar or Kr, X in a rod-shaped hollow tube.
In many cases, various phosphors are applied to the inner wall of the tube, and electrodes are arranged at both ends of the tube to seal the tube.

The discharge from the electrodes excites the phosphor coated on the inside of the tube with ultraviolet rays emitted from mercury or various gases, and emits visible light according to the emission characteristics of the phosphor.

As the phosphor, various phosphors are selected according to the spectral energy characteristics required as a light source. In particular, in a color image reading device, a light source in a wide wavelength range corresponding to RGB or the like is required. In particular, when a high-luminance light source is required, phosphors of a plurality of colors are mixed and applied to the inner wall of the tube. A technique such as coating is used.

In the case of controlling the amount of light emitted (the intensity of light emission), a fluorescent lamp or a xenon lamp does not use a method of controlling a lighting voltage like a halogen lamp, but a time for lighting with a constant current value. Generally, the amount of emitted light is controlled by a pulse width modulation method to be controlled. This is because a fluorescent lamp or a xenon lamp has a characteristic of emitting light when the current value exceeds a certain current value, and the method of controlling the light emission amount by controlling the current value does not allow a large range of controlling the light emission amount. It is due to

On the other hand, in an image reading apparatus using a fluorescent lamp or a xenon lamp, the above-described light amount control is omitted.
The gain setting of an amplifier or the like that electrically amplifies the output signal of the solid-state imaging device with respect to deterioration of the light amount due to durability is variable,
There has been proposed a method of changing the gain in accordance with the deterioration of the light amount to obtain an appropriate signal output. In such a method, a phenomenon that the S / N of the read signal fluctuates depending on the value of the gain may occur.

[0013]

However, the above-mentioned prior art has the following problems.

In an image reading apparatus using a light source using a phosphor as a light source, such as a fluorescent lamp or a xenon lamp,
As in the above-mentioned conventional example, a method of controlling the light emission amount by controlling the pulse width corresponding to the lighting time while keeping the current value flowing through the lamp constant has been generally used.

FIG. 13 shows a dimming waveform for controlling the amount of light emitted from the light source.

In FIG. 1, the horizontal axis represents time, and the vertical axis represents a current value for controlling the amount of light emitted from the light source.

The section of Hsync on the horizontal axis indicates a time corresponding to one accumulation time of the solid-state imaging device, and the charge is changed according to the amount of light incident on the light receiving portion of the solid-state imaging device as is generally used. It corresponds to the time that can be stored.

In the case of performing the normal pulse width control, the control signal is synchronized with the rising or falling position of the trigger signal indicating the beginning of the accumulation time, and the control signal is synchronized once per accumulation time. Output. As described above, by performing light amount control while synchronizing with a signal corresponding to a trigger signal for one accumulation time, an image signal due to a beat generated by interference between pulse width control for controlling the light amount and the accumulation time Noise was removed.

On the other hand, in a fluorescent lamp or a xenon lamp using a phosphor as a light source, a phosphor of each color is mixed and applied as a light source for an image reading apparatus for reading color information, so that the entire range of visible light is applied. In many cases, a white light source having a light emission characteristic in a wide wavelength range over a wide range is used.

When such a white light source is used, a problem has arisen that the afterglow characteristics unique to the phosphors of each color are different.

The afterglow characteristic is a characteristic that is determined by the time during which the phosphor excited by ultraviolet rays stays at a high energy level, and generally decreases exponentially.

This phenomenon indicates that the light emission remains even if the current for controlling the light emission of the light source is instantaneously cut off, and is expressed by the following equation depending on the characteristics of the phosphor material. You.

T = e (τ-1) Here, τ is a characteristic determined by the material of the phosphor, and when a phosphor corresponding to RGB is mixed and used like a white light source used in a color image reader. Another problem is that the afterglow characteristics of each color are different. Generally, a material used as a phosphor is determined from the viewpoint of emission wavelength characteristics, emission efficiency, and life in each wavelength range of the material, but the following materials are often used.

Blue: BaMg2Al16O27 Central wavelength: 452 nm T = 2 μsec Red: Y2O3: Eu2 + Central wavelength: 611 nm T = 1.1 msec Green: LaPO4: Ce, Tb Central wavelength: 544 nm And the time required for the amount of emitted light to reach 1 / e due to the attenuation. Since the afterglow characteristics of each color are different as described above (particularly, Blu
(The decay time of e is short.) The phenomenon that the center of gravity of the reading position in the sub-scanning direction differs depending on the color occurred. This phenomenon will be described with reference to FIG.

The horizontal axis of the graph shown in FIG. 13 represents time, and the vertical axis represents the amount of current for driving the fluorescent lamp and the amount of light emitted from the fluorescent lamp. H corresponding to one accumulation time of the solid-state imaging device
During the sync period, the solid-state imaging device accumulates electric charge proportional to the amount of incident light.

On the other hand, the dimming section in the figure corresponds to the time during which the current for driving the fluorescent lamp is continuously supplied in an amount proportional to the dimming duty, and the current in that section is mainly switched to a high frequency. Has been used. After the time corresponding to the light control section has elapsed, the amount of emitted light attenuates.

The attenuation characteristic is determined by the following two factors. One is the attenuation characteristic of the emission line spectrum emitted from the fluorescent lamp, and the other is the attenuation characteristic of the phosphor described above.

Normally, one accumulation time corresponding to Hsync is:
Although the attenuation characteristic of the emission line spectrum is less than 1 μsec, while it is several hundred μsec, it hardly affects the attenuation characteristic. However, the attenuation characteristic of the phosphor has a great influence because it is on the order of msec. Therefore, the attenuation characteristic of the light emission amount is determined by the sum of the above two types of light emission amounts and the attenuation characteristic of each light emission.

In the figure, the afterglow generated by the attenuation characteristics of R, G, and B colors is shown as a model.

In a fluorescent lamp which is lit with a substantially constant light amount by a substantially constant current in a light control section, the light quantity corresponding to the bright line spectrum is instantaneously attenuated when the light control section ends. That portion is a portion corresponding to L1 in the drawing, and afterglow occurs due to the attenuation characteristic of the fluorescent lamp with respect to the light amount corresponding to L2 in the drawing.

The afterglow characteristics of each color have the following problems in an image reading apparatus.

One accumulation time of the solid-state image sensor serves as a time reference when reading pixel information, and also serves as a reference of a reading position for reading in the sub-scanning direction. The pixel density when reading image information is determined by the pixel size of the solid-state imaging device in the main scanning direction, and the sub-scanning direction corresponds to a moving distance when reading an image scanned by mirror scanning or the like. Therefore, the phenomenon that the barycentric position of the light emission amount of each color with respect to the time between Hsync differs depending on the afterglow characteristic may be considered by replacing the horizontal axis of the graph of FIG. 13 with the position information. This indicates that the center of gravity of the reading position in the sub-scanning direction differs depending on the color. The fact that the center of gravity of the reading position in the sub-scanning direction differs depending on the color causes a color shift at the time of reading in the sub-scanning direction, thereby deteriorating the performance of the image reading apparatus.

The present invention has been made in view of such circumstances, and in performing dimming control of a white light source for illuminating a document, the present invention has a problem in the sub-scanning direction caused by the difference in the persistence characteristics of each color. It is an object of the present invention to provide an image reading apparatus in which color shift during reading is reduced.

[0034]

In order to achieve the above object, according to the present invention, an image reading apparatus includes the following (1) to (4).
Configure as follows.

(1) In an image reading apparatus which forms image information of a document on a plurality of line sensors via an image forming optical system and reads an image, a white light source for irradiating the document and a color light source for each color of the white light source are provided. An image having a center-of-gravity shift reducing unit configured to reduce the center-of-gravity shift of the reading position in the sub-scanning direction in each of the plurality of line sensors, which is generated depending on the afterglow characteristic; Reader.

(2) In the image reading apparatus described in (1), the center-of-gravity shift reducing means uses a pulse width modulation method, and the control pulses of the pulse width modulation method are always stored in one accumulation time of the plurality of line sensors. An image reading device which is a light amount control means of the white light source formed symmetrically in the time axis direction with respect to the center of the white light source.

(3) In the image reading device according to (2), the hardware uses a computing unit to determine the starting point of the required control pulse from the required width of the control pulse and the one accumulation time. An image reading device for calculating an end point.

(4) In the image reading apparatus described in (2), the hardware detects a time from the end of the output control pulse to the end of the one accumulation time, and based on the detected time. An image reading device for setting a time from a start time of the one accumulation time to a start time of a control pulse.

[0039]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of the present invention will be described below in detail with reference to embodiments of an image reading apparatus. Although the embodiment is for reading a reflective original, the present invention is not limited to this, and the present invention can be similarly implemented by reading a transparent original.

[0040]

(Embodiment 1) FIG. 1 is a diagram for explaining the operation of an "image reading apparatus" according to Embodiment 1.

In this figure, (a) is a diagram showing a lighting method of a fluorescent lamp according to the present embodiment, and has the following features as compared with (b) which is a conventional lighting method.

First, the dimming control signal does not start the control in synchronization with the rise or fall of the section signal representing the Hsync section, but as shown in FIG. Control is performed such that the temporal center coincides with the center (center) of Hsync.

In this case, even if the duty (also called duty ratio) of the dimming control signal changes, the center position of the control signal does not change, so that the rising position of the control signal is variable according to the duty. Controlled.

(D) shows the center-of-gravity shift amount d2 generated by the afterglow characteristic of the fluorescent lamp by the conventional control method, and the afterglow of each color has a large effect.

On the other hand, (c) shows the center-of-gravity shift d1 due to the afterglow characteristic in the control method of this embodiment.

By bringing the light-emitting area to the center of Hsync, the residual light amounts of green (G) and red (R) are as shown in FIG.
As shown in the figure, it is divided into the front and back of the light emitting area,
As a result of the averaging, the movement of the center of gravity due to the afterglow is d1, which is a very small amount, and can be at a level that does not cause any problem with respect to performance degradation of the image reading apparatus.

Next, a configuration for realizing the control method of this embodiment will be described.

In this type of image reading apparatus, the amount of light emitted from the fluorescent lamp is detected by an optical sensor, and the amount of light is controlled by a light amount controller so that the amount of light from the fluorescent lamp becomes constant.

FIG. 2 is a perspective view of a conventionally known fluorescent lamp.

Both ends of the fluorescent lamp 1 are supported by sockets 2a and 2b, and current is supplied from pins (not shown) of the sockets 2a and 2b.

An aperture (optical opening) 3 is provided in a predetermined area of the fluorescent lamp 1, and strong light is emitted in the direction of arrow a, and relatively weak light is emitted from an area other than the aperture 3. Is injected.

A light quantity sensor 4 composed of a photodiode or the like is provided at an appropriate position of the fluorescent lamp 1.
Current corresponding to the amount of light emitted from the device.

FIG. 3 is a block diagram of the light amount control system in this embodiment.

The light amount sensor 11 detects the light amount of the fluorescent lamp 10 and outputs a light amount signal corresponding to the light amount. The light amount signal is converted into a voltage value by the amplifier 12 and amplified.

After that, the amplified voltage value is compared with a predetermined reference voltage by the comparator 13, and the comparison result is input to the light quantity controller 14.

The light quantity controller 14 synchronizes the phase with the synchronizing signal (Sync) and performs pulse width modulation (Pulse-Width Modulation) as shown in FIG.
lation: Hereinafter referred to as “PWM”. ) A signal is output to the inverter 15 to perform duty control. That is, when the voltage value output from the amplifier 12 is higher than the reference voltage, the PW
When the M signal is output and the voltage value output from the amplifier 12 is smaller than the reference voltage, the PWM signal is output so that the duty ratio increases. Further, the light amount controller 14 sends an inverter 15 to the CPU 17.
The duty value to be set is output.

Next, in the inverter 15, when the PWM signal input to the inverter 15 is at a high level, PWM
An AC current, that is, a lamp current, is supplied to the fluorescent lamp 10 at a frequency sufficiently higher than the M signal (for example, 10 to 100 times the frequency of the PWM signal) so that the fluorescent lamp 10 is turned on. Is low level, the lamp current is interrupted and the fluorescent lamp 10 is turned off.

Then, the frequency of the PWM signal is
It is larger than the optical frequency of turning on and off of 0, and is turned on and off repeatedly according to the cycle of the PWM signal. However, the light is apparently turned on with a constant light amount corresponding to the current value obtained by averaging the lamp current.

The configuration of the image reading apparatus according to the present embodiment using the configuration of the light amount control block will be described below.

FIG. 4 is a block diagram showing the configuration of the image reading apparatus of this embodiment. As shown, a mirror table 21 for irradiating the original 20 with light, an image processing unit 22 for performing predetermined image processing on an optical signal from the original 20 and outputting the processed signal to a printer, and an output from the mirror table 21 An amplifier 24 for amplifying a signal, a comparator 25 for comparing an output signal from the amplifier 24 with a reference signal and outputting the comparison result,
An AS that controls the amount of light based on the output result of the comparator 25 and outputs a PWM signal in phase synchronization with a predetermined synchronization signal
A light amount controller 26 composed of an IC or the like, an inverter 27 for performing a lighting operation or the like based on a command from the light amount controller 26, a CPU 28 for controlling the entire device,
A backup memory 29 is provided for storing the calculation results of the PU 28 and the like. Reference numeral 44 denotes an A / D converter, and reference numeral 45 denotes a circuit (output SYNC1) for generating a free-running main scanning synchronization signal (SYNC) and selecting one of the main scanning synchronization signal BD and the printer main scanning synchronization signal BD.

The mirror base 21 includes a fluorescent lamp 32, a heater 33 mounted on the fluorescent lamp 32, a photodiode 35 attached to the fluorescent lamp 32 to detect the amount of light of the lamp 32, a photodiode 35 and the photodiode 3.
And a light quantity sensor 37 having a preamplifier 36 for converting the minute current detected in 5 into a voltage signal.

The amplifier 24 receives the voltage signal output from the preamplifier 36 and the voltage signal from the variable resistor 23, and amplifies the light amount signal to a predetermined value.

The comparator 25 performs an initial operation of the switch 38 based on a command from the CPU 28, for example, when it is desired to reduce the light amount when the reflectance of the read image is particularly high, thereby switching the reference voltage. Becomes possible.

The light quantity controller 26 synchronizes with a synchronizing signal and outputs a light quantity comparison signal from the comparator 25 by a flip-flop (F / F) circuit 39. The light quantity controller 26 synchronizes with a synchronization signal (SYNC) based on the light quantity comparison signal. Width determining means 40 for increasing or decreasing the counter to determine the PWM width
And a P which outputs a PWM signal having a determined PWM width at a desired position in synchronization with a synchronization signal (SYNC).
A WM signal generation unit 41, an arithmetic unit 61 for obtaining a set value to be set in the PWM signal generation unit 41 using an output value from the PWM width determination unit 40, that is, a PWM width value, A preheating control unit 42 for performing preheating is provided.

The PWM width determining means 40 is constituted by an up / down counter. When the input voltage value of the comparator 25 is larger than the reference voltage, it counts down and the count value, that is, the duty ratio is reduced. When the input voltage value of the comparator 25 is smaller than the reference voltage, it counts up and outputs the duty ratio so as to increase.

Here, the output value of the PWM width determination means 40 is input to the CPU 28, and the CPU
The WM value can be read.

As an operation of the light amount controller 26, when the light amount is higher than the specified value, the output of the comparator 25, that is, the output of the F / F 39 becomes 0, the output of the PWM width determining means 40 decreases by a predetermined value, and As a result, the PWM signal (pulse width) input to the inverter 27 is reduced. Conversely, if the value is smaller than the predetermined value, the output of the comparator 25, that is, the output of the F / F 39 becomes 1, the PWM width determining means 40 increases the predetermined value, the PWM width value increases, and the result is input to the inverter 27. Increase the PWM value (pulse width).

When the power is turned on, the PWM value is made equivalent to the full lighting of the fluorescent lamp, and is converged to a predetermined value.

In the inverter 27, when the PWM signal input to the inverter 27 is at a high level, the frequency is sufficiently higher than the PWM signal (for example, 10 times the frequency of the PWM signal).
An AC current, that is, a lamp current, is supplied to the fluorescent lamp 32 at a frequency of about 100 times to control the fluorescent lamp 32 to be turned on.
In the case of a low level, the lamp current is shut off and the fluorescent lamp 32
Is turned off. And electrically, PWM
Lighting and extinguishing are repeated according to the cycle of the signal, but apparently the light is turned on at a constant light amount corresponding to a current value obtained by averaging the lamp current.

The image processing unit 22 receives the optical signal from the document 20 and converts the optical signal into an electric signal.
It has an analog processor 43 to which an electric signal output from D58 is input and performs predetermined signal processing, and an A / D converter 44 for converting an analog signal output from the analog processor 43 into a digital signal. Note that the CCD 58 accumulates charges read during one scanning period, which is one cycle of the synchronization signal. Accordingly, the output from the CCD 58 has a magnitude obtained by integrating the light amount during one scanning period, and the required output can be obtained by synchronizing the blinking of the fluorescent lamp 32 and the scanning by the CCD 58 in the same cycle.

FIG. 5 is a block diagram of the arithmetic unit 61, and FIG. 6 is a time chart thereof. The time chart of FIG. 6 will be described. The desired pulse width (t) is obtained by the PWM width determination means 40, and the pulse is set to 1 as described above.
It is necessary to provide it at the center of the synchronization signal (during one SYNC interval: time T). Therefore, the computing unit 61 computes L1 = T / 2−t / 2 L2 = L1 + t and outputs the resulting L1 and L2.

FIG. 7 is a block diagram of the PWM signal generating means 41.

In the figure, 48 is an up counter, 49 and 50 are comparators, and 53 is a JK flip-flop.

The counter 48 is reset by a main scanning synchronization signal 47 (SYNC) of the image processing section 22 or the like, and is a counter that counts up with a clock signal. The comparators 49 and 50 are means for determining rising and falling. JK F / F53 has two comparators 4
This is a means for generating a PWM signal in accordance with the outputs of 9, 50. The comparators 49 and 50 include the arithmetic unit 61.
Output values L1 and L2 are set respectively.

Each output signal on the block circuit shown in FIG. 4 obtained by performing such a control method is shown in FIG.
This will be described with reference to FIG.

As each output signal, a Sync signal, a PWM signal
The signal, control current waveform (tube current), and light amount will be described.

In FIG. 8, the horizontal axis represents time, and the vertical axis represents each output signal.

In FIG. 8, (a) shows an output signal when the duty ratio is about 25%, and (b) shows an output signal when the duty ratio is about 6%.
The output signal at 0% is shown.

Sync1 represents a Sync signal output from the Sync generator 16 in the block circuit diagram shown in FIG.

PW output from light intensity controller 14
The M signal continues to output a high-level signal only during a section of a predetermined duty value.

Based on the PWM signal, the inverter 1
5 from the fluorescent lamp 1 at a frequency sufficiently higher than the PWM signal.
Supply current to zero.

The tube current shown in FIG. 8 indicates the signal. By this tube current, the fluorescent lamp 10 is turned on with a constant light amount corresponding to a current value obtained by averaging the tube current.

At this time, the line C, which is the center of all the signals of the PWM signal, the tube current, and the light amount when the fluorescent lamp is turned on, has the rising edge of the Sync1 section signal representing Hync corresponding to one accumulation time of the solid-state imaging device 58. It coincides with the center of falling and falling.

In FIG. 4B, similarly, the center C of the PWM signal, the tube current, and the light amount signal coincides with the center of the section signal of Sync1. In (b), the duty ratio is about 60%. Thus, even when the duty ratio changes, the position of the center of the lighting control signal does not change with time, and the Hsync section signal always remains. , The center of gravity of the light amount is always located near the center of the Hsync section signal even when the afterglow characteristics of the phosphors are different for each color, and the phosphor is not lit due to the afterglow. By averaging the light amount in the section before and after the lighting section within one accumulation time, the change in the position of the center of gravity can be made small.

As described above, according to the present embodiment,
When a white light source having a phosphor with different afterglow characteristics for each color is used for a reading color corresponding to a plurality of line sensors, it occurs depending on the afterglow characteristics of the light source depending on a fluorescent lamp lighting method. By having a means for reducing or correcting the movement of the center of gravity of the reading position in the sub-scanning direction of each color, and using a pulse width modulation method as a light amount control means of the light source, the method of growing the control pulse is based on the Hsync section signal. The center of gravity of the light quantity is always located near the center of the section signal of Hsync even if the afterglow characteristics of the phosphors are different for each color by growing symmetrically in the time axis direction about the center. By averaging the amount of light in the non-lighting section due to afterglow before and after the lighting section within one accumulation time, the change in the center of gravity position can be made a very small amount. Kicking color shift can be reduced.

(Embodiment 2) FIG. 9 is a block diagram showing a configuration of an "image reading apparatus" according to Embodiment 2.

In FIG. 9, the light amount controller 260
A flip-flop circuit 39 that outputs a light amount comparison signal from the comparator 25 in synchronization with the synchronization signal,
A PWM width determining means 40 for increasing / decreasing a counter in synchronization with a synchronization signal (SYNC) based on the light quantity comparison signal to determine a PWM width, and a PWM determined at a desired position in phase synchronization with the synchronization signal (SYNC). It has a PWM signal generating means 72 for outputting a PWM signal having a width, a PWM position determining means 71 for determining the position of the PWM signal, and a preheating control unit 42 for preheating the fluorescent lamp 32 before lighting.

FIG. 10 is a block diagram showing the structure of the PWM signal generating means 72.

In the figure, 48 is an up counter, 49 and 50 are comparators, 53 is a JK flip-flop, and 55 is an adder.

The counter 48 is a counter which is reset by a main scanning synchronization signal 47 (SYNC) of the image processing section 22 and the like and counts up by a clock signal.
Numeral 5 is for adding the output t of the PWM width setting means 40 and the output L3 of the PWM position determining means 71.
The comparators 49 and 50 are means for determining the rise and fall of the control pulse.
F53 is a means for generating a PWM signal in accordance with the outputs of the two comparators 49 and 50. The comparator 4
9 and 50, the output L of the PWM position determining means 71 is provided.
3. The output of the adder 55 is set.

The operation of this embodiment will be described with reference to the time chart of FIG. The PWM signal generating means 72
Is output in one synchronization signal signal (1 SYN
It is necessary to provide it in the center of the section T: time T). Therefore, the time from the end of the PWM signal to the start of the next SYNC (L3 in the figure) is changed to the PWM of the next SYNC cycle.
It is regarded as the start time (L4 in the figure).

Accordingly, the PWM position determining means 71
It becomes a counter for obtaining the time L3 from the output of the WM signal generation means 72 and the synchronization signal (SYNC). The output of the PWM position determining means 71 is the time L3, and by inputting it to the PWM signal generating means 72 as shown in FIG. 10, a desired PWM signal is obtained.

According to this embodiment, the phase of the control pulse fluctuates back and forth in the time axis direction with the center of the accumulation time T accompanying the fluctuation of the width t of the control pulse.
Since the control pulse is formed symmetrically in the longitudinal direction about the center of the accumulation time T in the time axis direction, the same effect as in the first embodiment can be obtained.

The output L3 of the PWM position determining means 71
Is not directly input to the comparator 49 and the adder 55, but the average value of L3 at a predetermined time is input.
Variations in the phase of the control pulse described above can be substantially eliminated.

[0095]

As described above, according to the present invention,
It is possible to reduce a color shift at the time of reading in the sub-scanning direction, which is caused by a difference in afterglow characteristics of each color in a white light source for document irradiation.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of an operation of a first embodiment.

FIG. 2 is a perspective view of a fluorescent lamp.

FIG. 3 is a block diagram of a light amount control system.

FIG. 4 is a block diagram showing the configuration of the first embodiment.

FIG. 5 is a block diagram showing a configuration of a computing unit 61;

FIG. 6 is a time chart of a computing unit 61;

FIG. 7 is a block diagram showing a configuration of a PWM signal generation unit 41;

FIG. 8 is a time chart illustrating the operation of the first embodiment.

FIG. 9 is a block diagram illustrating a configuration of a second embodiment.

FIG. 10 is a block diagram showing a configuration of a PWM signal generation unit 72;

FIG. 11 is a time chart illustrating the operation of the second embodiment.

FIG. 12 is a diagram schematically showing an optical system.

FIG. 13 is a diagram showing a dimming waveform and an afterglow characteristic;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Fluorescent lamp 14 Light quantity controller 40 PWM width determination means 41 PWM signal generation means 61 Computing unit

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Mitsuru Kurita 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Shigeo Yamagata 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Incorporated F term (reference) 5C072 AA01 BA19 CA04 CA14 EA05 FB19 MB01 QA17 UA13 UA14 UA18

Claims (4)

[Claims] An image reading apparatus that forms image information of a document on a plurality of line sensors via an image forming optical system and reads an image, a white light source for irradiating the document, and a residual of each color of the white light source. A center-of-gravity shift reducing unit configured to reduce the center-of-gravity shift of the reading position in the sub-scanning direction in each of the plurality of line sensors, which is generated depending on the optical characteristics. Characteristic image reading device. 2. The image reading apparatus according to claim 1, wherein the center-of-gravity shift reducing means uses a pulse width modulation method, and always controls the control pulse of the pulse width modulation method at the center of one accumulation time of the plurality of line sensors. An image reading device which is a light amount control means of the white light source which is formed symmetrically in the time axis direction with respect to the center. 3. The image reading apparatus according to claim 2, wherein the hardware uses a computing unit to start and end the required control pulse based on the required control pulse width and the one accumulation time. An image reading apparatus for calculating the following. 4. The image reading apparatus according to claim 2, wherein the hardware detects a time from an end point of the output control pulse to an end point of the one accumulation time, and based on this time, the hardware detects the time. An image reading apparatus for setting a time from a start time of an accumulation time to a start time of a control pulse.