JP3103569B2 - Image reading device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image reading apparatus that optically reads an image and converts the image into an image signal.

2. Description of the Related Art Conventionally, an image scanner that reads document image information such as a book or a document as image information in the form of an electric signal generally includes, as shown in FIG. Light source 4 for illuminating original 3 placed on top
And a line sensor 7 for performing photoelectric conversion of the reflected light 5 from the original 3 via the imaging lens 6, three reflecting mirrors 8 to 10 for guiding the line sensor 7, and processing the electric signal obtained by the photoelectric conversion. It also includes a circuit board 12 for controlling the entire operation and an I / F unit 13 for transmitting and receiving information to and from the outside. As the light source 4 for illuminating the document 3, a fluorescent lamp is often used because it has sufficient light intensity and is inexpensive.

However, while fluorescent lamps have many advantages, they also have many disadvantages.
For example, a preheating time is required to prevent electrode deterioration, and high-frequency lighting is required to eliminate illumination unevenness.
Among them, the biggest drawback is that the brightness varies many times depending on the ambient temperature and varies according to the lighting time as shown in FIG.

Usually, various measures are taken to avoid this disadvantage. As one of the methods, there is a method in which a fluorescent lamp is wrapped with a surface heater to warm the tube wall at low temperatures and at the start. However, the power for the heater does not become ridiculous, and there is a time lag for the effect to appear, and it is still insufficient when viewed comprehensively.

As another method, it is known to control the tube current while monitoring the light output of the fluorescent lamp. However, this method also requires a sensor, the brightness does not increase for the required power, and the increase in the price of a power supply or the like does not become ridiculous for the effect.

As another method, a method as shown in FIG. 4 in which the price increase is relatively small is also employed. That is, a first reference white plate 15 that is located in the main scanning direction and immediately before reading the original around the original reading area 2 shown in FIG. 5, and a second reference white plate 15 that is located in the sub-scanning direction and monitors while reading the original. This is a method using the reference white plate 16. First reference white plate
The read signal from 15 is as shown in FIG. Here, the reference signal from the second reference white plate 16 area indicated by A ₂ is present along the sub-scanning direction. Now, when the reference signal level A ₂ parts is assumed to be 1/2 of the maximum signal level of A ₁ part samples the signal A ₂ parts by analog switches 17 shown in FIG. 4, two times by the amplifier 18 And the voltage is sampled and held by a sample / hold circuit 19, and the output signal is converted to an A / D signal through an analog switch AS2.
The signal is input to the reference terminal of the converter, the input video signal from the first reference white plate 15 is quantized to a digital signal, and is temporarily stored in the memory 21. Then, while outputting the reference signal from the memory 21, the signal is returned to an analog signal by the D / A converter 22, and is input to the reference terminal of the A / D converter 20 via the analog switch AS3. Become Further, a reference signal that changes with scanning in the sub-scanning direction is input to the reference terminal of the D / A converter 22 to compensate for a time change.

[Problems to be solved by the invention]

However, when a fluorescent lamp is used as the light source, the signal level of the video input section changes by up to ten times (see FIG. 3). Accordingly, when the intensity of the light source is low, the reference voltage of the A / D converter 20 shown in FIG. 4 also becomes low, and quantization is not performed with higher accuracy than when a sufficient level of the reference voltage is applied. Further, since the shading correction is performed simultaneously using the same reference voltage, a decrease in the level on both sides of the fluorescent lamp also causes a decrease in accuracy.

In view of the above, an object of the present invention is to provide an image reading device that performs more detailed calculation processing for shading correction and suppresses the influence of a change in light amount of a light source on shading correction. It is in.

[Means for solving the problem]

In order to achieve the above object, an image reading apparatus according to the present invention includes an image sensor that reads an image and outputs an image signal, a light source that illuminates the image, and a first light source that is arranged in a main scanning direction. A white reference member, a second reference white member disposed in the sub-scanning direction, amplifying means for amplifying an image signal output from the image sensor, and shading correction for the image signal amplified by the amplifying means. A shading correction unit that performs A / D conversion on the image signal output from the shading correction unit.
A / D converter and controls the image sensor to acquire shading correction data by reading the first reference white member before starting reading the image, and the image sensor is configured to read the image after starting reading the image. A light amount change of the light source is detected by reading a second reference white member, and the amplifying unit amplifies the image signal with an amplification factor corresponding to the light amount change, and thereafter, the shading correction unit performs shading correction Control means for performing the control as described above. Here, the amplifying means may have a configuration in which a variable attenuator for digitally changing an attenuation rate and an amplifier having a fixed amplification rate are connected in cascade.

(Operation)

According to the present invention having the above configuration, it is possible to reduce the influence of a change in light amount during image reading on shading correction without interrupting image reading.

〔Example〕

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 is a circuit diagram showing one embodiment of the present invention. Assume that the maximum level of the video input signal is 4.8 V and the minimum level is 0.4 V, which is 1/12 of the maximum level, for convenience of explanation. Therefore, when the input level is 0.4 V or less, it is determined that the fluorescent lamp is turned on and reading is waited for a while, or that the fluorescent lamp is deteriorated and needs to be replaced because of its life.

In FIG. 1, reference numeral 24 denotes an integrating D / A converter.
An analog video input signal is supplied to the reference input terminal. On the other hand, an 8-bit digital signal is input from the first latch 25 to the digital input terminal. As a result, the D / A converter 24 operates as an attenuator. That is, a D / A converter (hereinafter referred to as DAC) 24 is used as a digital attenuator, and attenuated to a maximum of 1/12 according to the input signal level. Also DA
The amplifier connected after C24 has a gain of 12 times as a fixed gain.

Since the second DAC 27 is equivalent to the feedback resistance of the amplifier 29, the feedback resistance is changed in accordance with the 8-bit signal output from the second latch 28, and thereby the amplifier is changed.
The gain of 29 is changed.

Next, the operation of the present embodiment will be described with reference to the flowchart shown in FIG.

(1) First, in order to set the attenuation factor in the DAC 24 to 1/12, the CPU 33 gives "21" to "256" (decimal notation). Therefore, since it is amplified 12 times in the subsequent amplifier,
The level is returned to the original level (step S1).

(2) Next, to initialize the resistance value of DAC27,
An initial value is supplied to the CPU 8 from the CPU 33 (step S2).

(3) Turn on the fluorescent lamp, scan the first reference white plate 15, and input the video signal to the DAC 24. The digital video signal obtained as a result is stored in the memory 32 as shading correction data. Where the first
Since the DAC 24 is initially set to have an attenuation rate of 1/12 and the second DAC 27 is set to have an initial resistance value (steps S1 and S2), the maximum value of the shading correction data is Does not match 4.8V (peak value).

(4) Then, the CPU 33 controls the maximum value to be 4.8 V by changing the attenuation data by changing the latch data of the first latch 25 (step S4).

(5) The image sensor (not shown) is energized to start scanning the document (step S5).

(6) Each time a video signal for each pixel is input as the document is scanned, the CPU 33 reads the shading correction data from the memory 32 one pixel at a time and stores the reciprocal value in the second latch 28. Let it. As a result, the shading correction for one line is performed (step S
6).

(7) The image sensor is moved to the next line and the same shading correction is performed. At this time, the level of the reference white signal obtained by scanning the second reference white plate 16 (see FIG. 5) is detected, and it is checked whether or not the light amount has changed (step S7).

(8) If a change in the amount of light is detected in step S7, the first latch 2 is reset to reset the attenuation rate of the DAC 24.
Then, re-storing to 5 is performed (step S8).

In this way, by using the variable attenuator, the fixed gain amplifier, and the variable amplifier, even if the brightness of the light source changes, for example, by a little over ten times, the signal of the same level can be converted by the A / D converter ADC.
30 can be entered.

In the above description, the attenuation factor of the DAC 24 included in the attenuator is set to 1/12. However, in order to reduce the quantization error and distortion of the signal, the attenuation factor and the amplification factor here are set low (specifically, It is desirable that the upper limit be at most about 8 times for stable operation). In that case, the programmable amplifier at the subsequent stage is made to share the amount, and stable and accurate correction can be performed as a whole with an appropriate level of magnification. Of course, it goes without saying that the sharing ratio may be determined according to the situation according to the accuracy of the DAC or the like or the gain of the amplifier.

In the above-described embodiment, the level correction is performed by the attenuator and the shading correction is performed by the variable amplifier. However, it is also possible to perform the level correction by the variable amplifier by replacing the level correction and perform the shading correction by the attenuator.

〔The invention's effect〕

As described above, according to the present invention, it is possible to reduce the influence of a change in light amount during image reading on shading correction without interrupting image reading.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of the present invention, FIG. 2 is a diagram for explaining a configuration of a general image scanner, FIG. 3 is a diagram showing that the brightness of a fluorescent lamp changes, FIG. 4 is an electric circuit diagram showing the prior art, FIG. 5 is a diagram for explaining the arrangement of a reference white plate arranged for reading a reference signal, FIG. 6 is a diagram showing a read signal from the reference white plate, FIG. 7 is a flowchart showing the operation of this embodiment. 24: D / A converter (variable attenuator), 25: Latch circuit, 26: Fixed gain amplifier, 27: D / A converter (variable feedback resistor), 29: Amplifier, 30: A / D converter, 32 …… Memory, 33 …… CPU.

Continuation of front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 1/401 G06T 1/00 460

Claims (2)

(57) [Claims] An image sensor for reading an image and outputting an image signal; a light source for illuminating the image; a first reference white member arranged in a main scanning direction; and an image sensor arranged in a sub scanning direction. A second reference white member; an amplifying unit for amplifying an image signal output from the image sensor; a shading correction unit for performing shading correction on the image signal amplified by the amplifying unit; and an output from the shading correction unit. A / D conversion means for A / D converting the image signal obtained, and controlling the image sensor to acquire shading correction data by reading the first reference white member before the image reading is started. After the image reading is started, the image sensor reads the second reference white member to detect a change in the light amount of the light source. Control means for controlling the amplification means to amplify the image signal at an amplification factor corresponding to the change in the amount of light, and thereafter controlling the shading correction means to perform shading correction. . 2. The amplifying means according to claim 1, wherein said variable attenuator for digitally changing an attenuation factor is cascaded with an amplifier having a fixed amplification factor.
The image reading device according to claim 1.