JPH04311158A - Image reader - Google Patents

Abstract

PURPOSE:To always stably and highly accurately execute shading correction without increasing cost by impressing a specific voltage as a reference voltage for an A/D conversion means. CONSTITUTION:The reading scanning of a white reference board 2 is executed and A/D conversion circuit 11 converts an image signal P into each digital signal value. A variable range detecting circuit 16 detects a maximum value and a minimum value in each line section from the diginal signal value. A CPU 10 allows a voltage generating circuit 14 to output the maximum and minimum voltage values. The minimum voltage value is impressed as the minus side reference voltage Vref (-) of the circuit 11 and the maximum voltage value is impressed as the plus side reference voltage Vref (+) through a selector 3. Since the level range of an analog image signal to be converted into a digital signal is narrowed to a minimum range to be required, the generation of quantizing errors is reduced. The number of quantizing bits is the same as a convensional method.

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 that performs shading correction on an analog image signal when reading an original image based on the results of reading a white reference plate.

0002

2. Description of the Related Art Image reading apparatuses are well known in which the surface of a document is illuminated with a light source such as a fluorescent lamp, and the reflected light is received by a line image sensor to sequentially read the document image line by line.

Generally, when a fluorescent lamp or the like is used as a light source, variations occur in the illuminance distribution, and in addition, in a line image sensor, there are variations in sensitivity for each pixel.

Therefore, in order to prevent the influence of these variations and to extract an image of a constant density at a constant image signal level, the image reading device described above performs shading correction on the image signal obtained from the line image sensor. I have to.

[0005] Normally, shading correction is performed by reading a white reference board that indicates the white reference density, storing the level change of the image signal at that time, and then reading the original image.
The obtained image signal is corrected based on the level change.

For example, suppose that a white reference plate is read and an image signal P whose level fluctuates as shown in FIG. 8 is obtained.

At this time, conventionally, the peak voltage Vp of the image signal P is set as the + side reference voltage of an A/D (analog-digital) conversion circuit for converting the image signal P into a digital signal, while the - As the side reference voltage, a voltage at a certain ratio to the peak voltage Vp, for example, Vp/2, was set. Then, in this setting state, the white reference plate was read again.

The A/D conversion circuit converts the range from the negative reference voltage to the positive reference voltage into a predetermined digital signal value. Therefore, if the number of quantization bits of the A/D conversion circuit is 4 bits, when the image signal level is voltage Vp/2, it is "0000", and the peak voltage Vp
When this happens, a digital signal value of "1111" is output.

[0009] Such digital signal values are stored in a memory for one line for each pixel, and then the original image is read. Now, assume that a document image is read and the image signal level of one pixel is Vi. Also assume that the stored digital signal value of that pixel is X. In this case, since the number of quantization bits is 4 bits, the number of quantization levels is "16", and the number of levels corresponds to a range of voltage Vp/2 or more, so from voltage "0" to peak voltage Vp
The range up to is equivalent to twice that number, ``32''. Therefore, in this case, the image signal level Vi is subjected to shading correction to the image signal level Vo expressed by the following equation.

0010

[Math 1] Vo={32/(16+X)}・Vi

001
1] By the way, in this case, the peak voltage Vp of the image signal is 1
00 percent, the quantization error e is e=100
/32, which is a large value of about 3.1%, making it impossible to perform seeding correction with high precision.

One method for reducing this quantization error e is to increase the number of quantization bits. However, this method increases the cost of the A/D conversion circuit.

Another method is to increase the negative reference voltage for the A/D conversion circuit to narrow the signal level range for converting the image signal into a digital signal when reading the white reference plate.

However, with this method, the image signal level may change due to changes in the characteristics of the light source and line image sensor over time.
If the voltage falls below the set negative reference voltage, the predetermined shading correction cannot be performed.

[0015]

[Problem to be solved by the invention] In this way, conventionally,
Attempts to improve the accuracy of shading correction have resulted in problems such as increased cost and inability to respond to changes in component characteristics, making stable operation impossible at all times.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image reading apparatus that can solve the above problems and always perform stable and highly accurate shading correction without increasing costs.

[0017]

[Means for Solving the Problem] To this end, the first invention provides an A/D conversion means for converting an analog image signal into a digital signal, and a positive reference voltage corresponding to the maximum value of the digital signal value. , and a negative reference voltage corresponding to the zero level.First, a constant voltage is applied to each of these reference voltages, and in that state, the white reference plate is read and a digital signal is extracted. Detect the maximum value and minimum value within one line, apply the maximum value as the + side reference voltage and the minimum value as the - side reference voltage, respectively. Next, in that state, read the white reference plate and output the digital signal. Take out the
The data of the digital signal obtained by this is stored, and then the original image is read, and the level of the obtained analog image signal is corrected based on the stored data, thereby executing a predetermined shading correction. I have to.

Further, in the second invention, the peak value of the analog image signal is detected by reading the white reference plate, and the peak value is applied as a positive reference voltage, while various voltages are sequentially applied as a negative reference voltage. At the same time, every time one voltage is applied, the white reference plate is read to extract digital signals, and one of the digital signals when various voltages are applied.
A voltage with no zero signal value in the line and the maximum applied voltage is applied as the negative reference voltage, and in that state, the white reference plate is read to extract the digital signal.
The data of the digital signal thus obtained is stored and shading correction is performed in the same manner as above.

[0019]

[Operation] According to each of the above inventions, the conventional A/D conversion means can be used as is, and the reference voltages on the + side and - side are always set to the maximum value of the image signal when reading the white reference plate. and the minimum value are set correctly.

Therefore, even if the image signal level changes due to aging of the light source or line image sensor, the signal level range to be converted into a digital signal can be set to the narrowest according to the image signal at that time.

[0021] This makes it possible to always perform stable and highly accurate shading correction without increasing costs.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows a block diagram of an image reading apparatus according to a first embodiment of the present invention. In the figure, a light source 1 is connected to a white reference plate 2 and an original document 3 to be read passing through it.
It is intended to illuminate. Image light from the white reference plate 2 or the original to be read 3 is reflected by a mirror 4, condensed by a lens 5, and incident on a line image sensor 6.

The image signal output from the line image sensor 6 is input to the amplifier 7, and the output of the amplifier 7 is
It is input to the variable gain amplifier circuit 8. The variable gain amplifier circuit 8 can arbitrarily set the gain. This variable gain amplifier circuit 8 receives signals from the RAM 9 and the CPU.
A control signal from A/10 is input, and its output signal is
The signal is input to a D conversion circuit 11 and a peak hold circuit 12.

[0025] The output of the selector 13 is inputted to the A/D conversion circuit 11 as a + side reference voltage Vref (+), and one output of the voltage generation circuit 14 is inputted as a - side reference voltage Vref (-). . Selector 13
The output of the peak hold circuit 12 and the voltage generation circuit 1 are
The other output of 4 is input.

The output of the A/D conversion circuit 11 is input to the RAM 9, the digital image processing section 15, and the variation range detection circuit 16. The CPU 10 has a variation range detection circuit 1.
6 is input, and the variable gain amplifier circuit 8, selector 13, and voltage generation circuit 14 are controlled. In addition to these, the light source 1 may be turned on or off, and the reading document 3 (not shown) may be turned on or off.
It controls the transportation mechanism, etc.

Assume now that the image reading apparatus of this embodiment is started up with the above configuration.

When this image reading device is started, as shown in FIG. 2, first, the light source 1 is turned on (process 101). Next, the voltage generating circuit 14 generates a constant voltage Vr and a voltage of 0 volts. The 0 volt voltage is applied as the negative reference voltage Vref(-) of the A/D conversion circuit 11. Also, at this time, the selector 13 inputs the above-mentioned constant voltage Vr generated by the voltage generation circuit 14 and converts the voltage into the + side reference voltage Vr of the A/D conversion circuit 11.
It is applied as ef(+) (processing 102). Next, the gain of the variable gain amplifier circuit 8 is set to "1" (processing 1).
03).

In this state, the white reference plate 2 is read and scanned. Thereby, the line image sensor 6 outputs one line of image signals. The amplifier 7 amplifies the image signal,
The variable gain amplifier circuit 8 receives the image signal as input, and in this case outputs the image signal P without changing the signal level.

By the way, the constant voltage Vr is set in advance to be higher than the maximum level of the image signal P output from the variable gain amplifier circuit 8 at this time. For this reason,
As shown in FIG. 3, the image signal P fluctuates within a certain level range below a certain voltage Vr.

The A/D conversion circuit 11 converts the range from the negative reference voltage Vref (-) to the positive reference voltage Vref (+) in the image signal P, that is, the range from 0 volts to a constant voltage Vr, to each digital signal. Convert to signal value. The variation range detection circuit 16 detects the maximum value Vmax and minimum value Vmin in one line section from the signal value of the digital signal (processing 104).

The CPU 10 calculates the fluctuation rate D from the maximum value Vmax and minimum value Vmin using the following formula.

[0033]

[Formula 2] D=(Vmax-Vmin)/Vmax

00
34] Then, the calculated fluctuation rate D is compared with a preset constant value T1 (judgment 105). Here, if the fluctuation rate D is less than the constant value T1 (N of judgment 105),
Furthermore, the maximum value Vmax and a preset constant value T
2 (decision 106).

Here, if the maximum value Vmax is greater than or equal to the constant value T2 (N in judgment 106), the voltage generation circuit 14
Then, each voltage value of the maximum value Vmax and minimum value Vmin is output. Then, the voltage value of the maximum value Vmax is set to A/
+ side reference voltage Vref(+) of the D conversion circuit 11
The voltage value of the minimum value Vmin is applied as the negative reference voltage Vref(-) (process 107
).

In this state, the white reference plate 2 is read and scanned again in the same manner as described above. Assume now that the number of quantization bits of the A/D conversion circuit 11 is, for example, 4 bits. In this case, as shown in FIG. 4, the A/D conversion circuit 11 divides the range from the minimum value Vmin to the maximum value Vmax of the input image signal P into 16 equal parts, and divides the range from "0000" to "1111" into 16 equal parts.
output each digital signal. The RAM 9 stores one line of digital signal values of each pixel thus output (processing 108).

After this, reading of the original image is started. That is, while conveying the original 3 to be read and performing sub-scanning, reading scanning is performed line by line (process 109).

At this time, the RAM 9 sequentially outputs the digital signal value of each stored pixel in synchronization with the reading scan of each line. The variable gain amplification circuit 8 amplifies the image signal of each line obtained by reading the original image while changing the gain based on the digital signal value (processing 11).
0). As a result, the image signal P output from the variable gain amplification circuit 8 has undergone shading correction.

The peak hold circuit 12 receives the image signal P.
The peak value Vp is detected for each line. At this time,
The selector 13 inputs the peak value Vp and selects the A/D
It is applied to the conversion circuit 11 as a + side reference voltage Vref (+). Further, the voltage generation circuit 14 applies 0 volts as the negative reference voltage Vref(-) (
Processing 111).

Thereby, the A/D conversion circuit 11 converts the image signal P from the 0 level to the peak value Vp into each predetermined digital signal. Then, the digital image processing section 1
5, for the image information changed to the digital signal,
For example, execute various processes such as display and recording (processing 1
12).

On the other hand, in the above, the fluctuation rate D is a constant value T
1 (Y in Judgment 105), or if the maximum value Vmax is less than the constant value T2 (Judgment 1
06 Y), an alarm to the effect of the abnormality is issued, for example, by an alarm sound or a lamp display (processing 113). Thereafter, similar processing is executed (proceed to processing 107).

By the way, in general, the fluctuation range of the image signal P when reading the white reference plate 2 is as small as 20 to 30 percent of the maximum value. For example, as shown in FIG. 4, it is assumed that the fluctuation width of the image signal P, that is, the width between the maximum value Vmax and the minimum value Vmin, is 25% of the maximum value Vmax.

In this case, the maximum value Vmax and the minimum value Vmi
Since the width between n is quantized by 4 bits, the number of quantization levels is 16. The maximum value Vmax is 4 times that value, "
64'', and the minimum value Vmin is three times that value, which is ``48''.
”.

Therefore, if the image signal level of one pixel input to the variable gain amplifier circuit 8 is Vi, and the signal value of the image signal stored in the RAM 9 is X, the variable gain amplifier circuit 8 can be calculated using the following formula. Shading correction is performed to the image signal level Vo shown in FIG.

[0045]

[Math 3] Vo={64/(48+X)}・Vi

004
6] In this case, assuming that the peak voltage Vp of the image signal is 100%, the quantization error e is e=100/64, which is approximately 1.6%.

As described above, in this embodiment, the white reference plate 2
When reading, first set a constant voltage to each reference voltage Vref (+), Vref (-) of the A/D conversion circuit 11, extract the image signal, and then set the maximum value Vmax of the image signal. The minimum value Vmin is set to each reference voltage Vref(+), Vref(-), and the digital signal value is extracted.

Therefore, the minimum value Vmin of the image signal can be correctly set to zero of the digital signal value, and the maximum value Vmax can be correctly set to the maximum value of the digital signal value.

[0049] As a result, shading correction can always be performed stably, quantization errors during digital conversion of image signals are reduced, and correction accuracy is also improved. Also,
In this case, the number of quantization bits is the same as before, and the device cost does not increase.

Furthermore, in this embodiment, an alarm is output when the fluctuation range of the image signal exceeds a certain value or the maximum value is below a certain value, so the operator can monitor the decrease in the light intensity of the light source 1 due to aging, etc. or the characteristic deterioration of the line image sensor 6.
It is easy to know.

Note that the + side reference voltage Vref (+) of the A/D conversion circuit 11 is set to a value slightly higher than the maximum value Vmax of the image signal, and the - side reference voltage Vref (-) is set to a value slightly higher than the maximum value Vmin. A slightly lower value may be set. Thereby, it is possible to deal with fluctuations in signal level after each voltage setting.

Furthermore, in the above embodiment, when the image signal level is low, the alarm is output immediately, but after waiting for a certain period of time, the image signal level is detected again, and if it is still low, the alarm is output. You can do it like this. This operation is effective when using a light source that takes time to increase in light intensity, such as a fluorescent lamp.

Next, a second embodiment of the present invention will be described.

FIG. 5 shows a block diagram of the image reading apparatus of this embodiment. In the figure, the same reference numerals as in FIG. 1 indicate the same blocks, and in the figure, each part of the optical system, the line image sensor 6, and the amplifier 7 are omitted.

In the figure, the output signal of the peak hold circuit 12 is applied as the + side reference voltage Vref (+) to the A/D conversion circuit 11 and is also input to the variable voltage dividing circuit 17. The variable voltage divider circuit 17 divides the input voltage at various voltage division ratios, and its output is applied as a negative reference voltage Vref (-) to the A/D conversion circuit 11. The zero data determination circuit 18 determines the presence or absence of a signal value "0" in the output signal of the A/D conversion circuit 11. The CPU 10 inputs the determination result and controls the variable voltage dividing circuit 17 and other parts.

With the above configuration, when the image reading apparatus of this embodiment is started, it first turns on the light source 1 (process 201) and sets the gain of the variable gain amplifier circuit 8 to "1", as shown in FIG. Set (process 202). Further, the CPU 10 sets the variable Div on the control program to Div=0.5 (process 203).

Then, the white reference plate 2 is read and scanned. As a result, the variable gain amplifier circuit 8 outputs the image signal P, and the peak hold circuit 12 detects the peak value Vp of the image signal P (processing 204).

Here, the variable Div is set in the variable voltage dividing circuit 17 as a voltage dividing ratio. As a result, the peak value Vp of the image signal P is applied as the + side reference voltage Vref(+) of the A/D conversion circuit 11, and the peak value Vp
A voltage corresponding to the product of p and the variable Div is applied as the negative reference voltage Vref(-) (process 205
).

After this, the white reference plate 2 is read and scanned again. Thereby, the A/D conversion circuit 11 converts the voltage value range of 0.5·Vp to Vp of the image signal P into a digital signal. The zero data determination circuit 18 determines whether the signal value of the digital signal has become "0" within one line (determination 207).

Now, if the image signal P is fluctuating at a level higher than the voltage value 0.5·Vp as shown in FIG. 7, the signal value of the digital signal will be "
It will exceed 0. In this way, if there is no signal value "0" (N in judgment 207), the variable Div is set to "0.
1'' (process 208), and performs the same operation (proceed to process 205). Then, the signal value “
If there is no "0", the operation is further repeated.

[0061] As a result, the - side reference voltage Vre
f(-) increases by 0.1·Vp. Now, the negative side reference voltage Vref (-) has a voltage value of 0.7· in FIG.
Assume that the signal level of the image signal P is exceeded as shown in section a in Vp. Then, in that section, the signal value of the digital signal becomes "0".

When the signal value "0" is detected in this way (Y in judgment 207), the variable Div is subtracted by "0.1" to control the variable voltage dividing circuit 17. As a result, a voltage value before exceeding the signal level of the image signal P is applied as the negative reference voltage Vref(-) (processing 209
).

[0063] After this, the operations after process 108 described in FIG. 2 are executed in the same manner. That is, the white reference plate 2 is read and scanned, and the digital signal value of the image signal at that time is R.
Recorded at AM9. Then, reading scanning of the document and gain control of the variable gain amplifier circuit 8 based on the above signal value are started. In addition, the + side reference voltage Vr of the A/D conversion circuit 11
The peak value Vp of the image signal P is applied to ef(+), and 0 volt is applied to the negative reference voltage Vref(-). In this state, the original image is read and processed by the digital image processing section 15.

As described above, in this embodiment, the white reference plate 2
When reading, the + side reference voltage Vref (+) of the A/D conversion circuit 11 sets the peak value Vp of the image signal, while the - side reference voltage Vref (-) changes from the voltage value 0.5·Vp. While increasing the voltage by 0.1·Vp, the digital image signal obtained during that time is judged. and,
When the signal value "0" is detected, the - side reference voltage V
ref(-) is set to the immediately previous voltage value.

[0065] As a result, as in the embodiment described above, when reading the white reference plate, the image signal can be always converted into a digital signal within a level range corresponding to its fluctuation width, so that the quantization error during digital conversion can be reduced. The correction accuracy during shading correction also improves.

Furthermore, in this case, the zero data determination circuit 18 that checks the output signal of the A/D conversion circuit 11 detects the signal "
Since only the presence or absence of "0" is determined, the circuit is simpler than the variation range detection circuit 16 of FIG. In addition, since the circuit that applies the reference voltage to the A/D conversion circuit 11 only needs to be the peak hold circuit 12 and the variable voltage divider circuit 17, the circuit is simpler and the number of circuits is smaller than in the case of FIG. .

In the above embodiment, the negative reference voltage Vref(-) is increased by 0.1·Vp from a low value of 0.5·Vp, but the level of increase in one step is arbitrary. Of course, it can be set to Alternatively, the voltage may be lowered sequentially from a high voltage, and the voltage may be set to a voltage value that is less than the minimum value of the image signal first.

[0068]

As described above, according to the first invention of the present application, a constant voltage is first applied as a reference voltage of the A/D conversion means to read the white reference plate, and the image signal detected at that time is Since the maximum value and minimum value are applied as a reference voltage to perform the prescribed processing, the conventional A/D conversion means can be used as is, and the white reference plate is always read as the reference voltage. Since the maximum and minimum values of the image signal at the time are set correctly, it becomes possible to always perform stable and highly accurate shading correction without increasing costs.

According to the second invention, the peak value of the image signal detected by reading the white reference plate is applied as the + side reference voltage, while various voltages are sequentially applied as the - side reference voltage, and in the meantime, From the obtained digital image signal, determine one voltage corresponding to the minimum value of the image signal and -
Since it is applied as the side reference voltage, similarly to the above, it becomes possible to always perform stable and highly accurate shading correction without increasing the cost.

[Brief explanation of drawings]

FIG. 1 is a block configuration diagram of an image reading device according to a first embodiment of the present invention.

FIG. 2 is an operation flowchart when reading an image.

FIG. 3 is an explanatory diagram of an image signal when reading a white reference plate for the first time.

FIG. 4 is an explanatory diagram of image signals when reading the white reference plate for the second time.

FIG. 5 is a block configuration diagram of an image reading device according to a second embodiment of the present invention.

FIG. 6 is an operation flowchart during image reading in the embodiment.

FIG. 7 is an explanatory diagram of an image signal when reading a white reference plate in the above embodiment.

FIG. 8 is an explanatory diagram of an image signal when reading a conventional white reference plate.

[Explanation of symbols]

1. Light source 2 White reference plate 3　Reading manuscript 4 Mirror 5 Lens 6 Line image sensor 7 Amplifier 8 Variable gain amplifier circuit 9 RAM 10 CPU 11 A/D conversion circuit 12 Peak hold circuit 13 Selector 14 Voltage generation circuit 15 Digital image processing section 16 Variation range detection circuit 17 Variable voltage divider circuit 18 Zero data judgment circuit

Claims (3)

[Claims] 1. A reading means for reading a white reference plate or an original image and extracting an analog image signal line by line, an A/D conversion means for converting the analog image signal into a digital image signal, and an A/D conversion means for the A/D conversion means. voltage application means for applying a + side reference voltage corresponding to the maximum value of the digital signal value and a - side reference voltage corresponding to the zero level, and based on the reading result of the white reference plate, the In an image reading device that performs shading correction on an analog image signal, constant voltage applying means applies constant voltages as the + side reference voltage and the − side reference voltage, respectively;
a white reference plate first reading means for reading the white reference plate and extracting a digital image signal with the constant voltage applied; and a maximum/minimum detection means for detecting the maximum value and minimum value within one line of the digital image signal. and a maximum final voltage applying means for applying the maximum voltage as the positive reference voltage and the minimum voltage as the negative reference voltage, and a state in which each of the maximum and minimum voltages is applied. a white reference plate second reading means for reading the white reference plate and extracting a digital image signal; a storage means for storing data of the digital image signal obtained by reading the white reference plate;
An image reading device comprising: a document reading device for reading a document image and extracting an analog image signal; and a shading correction device for level-correcting the analog image signal based on the data stored therein. 2. A reading means for reading a white reference plate or an original image and extracting an analog image signal line by line, an A/D conversion means for converting the analog image signal into a digital image signal, and an A/D conversion means for the A/D conversion means. voltage applying means for applying a + side reference voltage corresponding to the maximum value of the digital signal value and a - side reference voltage corresponding to the zero level, and based on the reading result of the white reference plate, the An image reading device that performs shading correction on an analog image signal includes a peak value detection means for reading a white reference plate and detecting a peak value of the analog image signal, and a peak value application means for applying the peak value as the above-mentioned + side reference voltage. , various voltage applying means for sequentially applying various voltages as the above-mentioned negative side reference voltage, and reference plate first reading means for reading the white reference plate and extracting a digital image signal every time one voltage is applied by the various voltage applying means, one voltage applying means for applying one voltage having no zero signal value in one line of each digital image signal corresponding to the various voltages and having the maximum applied voltage as the negative side reference voltage; a second reference plate reading means for reading the white reference plate in the applied state and extracting a digital signal; a storage means for storing data of the digital signal obtained by reading the white reference plate; and a storage means for reading the original image and extracting an analog image signal. An image reading device comprising: a document reading means for taking out the analog image signal; and a shading correction means for level-correcting the analog image signal based on the data stored therein. 3. The various voltage applying means includes variable voltage dividing means for dividing the peak value of the analog image signal detected by the peak value detecting means at various voltage dividing ratios and applying the divided voltage as the negative reference voltage; 3. The image reading apparatus according to claim 2, further comprising a partial pressure ratio control means for sequentially changing the partial pressure ratio.