JPH08228283A - Image reader - Google Patents

Abstract

PURPOSE: To correct shading distortion caused in the image reader without decreasing the number of gradations. CONSTITUTION: A shading distortion detection means 40 detects shading distortion and a reference voltage Vref of an A/D converter 13 is controlled by a reference voltage control means 50 based on shading distortion detection data 52. Thus, a sensor output signal 10 is subjected to A/D conversion and digital image data 14 whose shading distortion is corrected are obtained without decreasing the number of gradations.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to improvement of shading correction technology in an image reading apparatus.

[0002]

2. Description of the Related Art An image reading device is widely known as a device for converting an appropriate image to be read into an electric signal including photographs, printed materials, various documents, and other three-dimensional objects.

<Outline of Conventional Image Reading Device> FIG. 9 is a diagram showing the main components of a conventional image reading device. As shown in the figure, in this device, the illumination means 1 such as a fluorescent lamp is used.
The object 2 to be read is illuminated by the above, and its image is formed on the sensor 5 through the optical system such as the mirror 3 and the lens 4, so that the image is converted into the electrical signal 10.

An electric signal (hereinafter referred to as a sensor output signal) 10 output from the sensor 5 is an analog signal, so that a sample and hold circuit 11 and a gain controller 12 are provided for convenience of signal processing by digital technology.
To the A / D converter 13 and converted into a digital signal (hereinafter referred to as digital image data) 14A. Furthermore, this digital image data 14A is
The resulting digital image data 16 is stored in the image memory 17 through the shading correction circuit 15 using the technique known in Japanese Patent Publication No. 58909 or Japanese Patent Publication No. 61-56668. The A / D converter 13 has a voltage source 18
Is applied with a reference voltage V _FIXED fixed to a constant value.

The digital image data 16 stored in the image memory 17 is used as it is, or after being subjected to appropriate signal processing as necessary, a television monitor, a printer,
It is supplied to various other equipment.

<Image Reading Section and Reference Density Plate> Sensor 5
Is an aggregate of image pickup devices such as CCD, and includes a one-dimensional array of line sensors and a two-dimensional array of area sensors. Since a line sensor is usually used, the sensor 5
Together with the illumination means 1, the mirror 3 and the lens 4, an image reading unit 6 is configured, and the image reading unit 6 and the reading target 2 are driven by a driving mechanism (not shown) to form a sensor 5
Is relatively moved in a direction perpendicular to the longitudinal direction of (1) (the arrangement direction of the image sensor). By this movement, the sensor 5 can scan and read the entire image of the reading target 2 having a planar spread. Note that this relative movement direction is called the sub-scanning direction.

If the sensor 5 is an area sensor, it may not be necessary to perform the above-mentioned scan.

The object to be read 2 facing the image reading unit 6
Window 7 made of transparent material such as glass for mounting
And a plate having uniform density such as all-white as a reference for correcting shading distortion (hereinafter referred to as a reference density plate)
There are 8 and.

The reference density plate 8 has a size corresponding to the field of view of the sensor 5 and is arranged so as to face the mirror 3 at the starting point position in the sub-scanning direction.

<Conventional Shading Correction Technology> Basically, the shading distortion is caused by the fact that the light quantity distribution by the illumination means 1 is not uniform on the reading object 2, and therefore the sensor 5
This is because density unevenness occurs in the image read in step 3, and the reference density plate 8 is used to know the light quantity unevenness.

The technique disclosed in Japanese Examined Patent Publication No. 1-58909 aims at correcting not only the light amount unevenness of the illuminating means 1 but also the light amount unevenness of the optical system and the sensitivity variation between the image pickup elements of the sensor 5. The reference white plate is used as the reference density plate 8, the reference white plate is read by the sensor 5 before the scan of the reading target 2, and the digital image data 14B for one line at that time is passed through the shading correction circuit 15 and the image memory 17 is passed. To store.

Since the digital image data 14B after A / D conversion obtained here is obtained by reading the reference white board, it should have a uniform signal level between pixels, but the above-mentioned shading distortion is caused. Therefore, the signal level of each pixel varies. Therefore, the shading correction circuit 15
Is obtained in advance as a correction coefficient for each pixel from the digital image data 14B in proportion to the reciprocal of the signal level of each pixel, and when the reading target 2 is read by the sensor 5 by relatively moving the image reading unit 6, each line For each A / D-converted digital image data 14A, shading correction is performed by multiplying the signal level of each pixel by a corresponding correction coefficient, and the result 16 is stored in the image memory 17.

The technique disclosed in Japanese Examined Patent Publication No. 61-56668 aims at correcting the sensitivity variation between the elements of the sensor 5, and uses two types of reference density plates 8, for example, a reference white plate and a reference blackboard, The reference white plate is set before the scan of the reading target 2, and the digital image data 14B for one line at that time is set to the shading correction circuit 1.
5 is passed through and stored in the image memory 17, then the reference blackboard is replaced and the digital image data 14C for one line at that time is also passed through the shading correction circuit 15 and stored in the image memory 17.

After that, when reading the reading object 2, the shading correction circuit 15 uses the digital image data 1
4B and 14C, and regular data that read each black and white reference density plate separately with a sensor with no sensitivity variations between elements 1
4D and 14E are used to correct the digital image data 14A of each line for each pixel by a linear interpolation method or the like, and the result 16 is stored in the image memory 17.

[0015]

However, in any of the conventional techniques, since the shading correction is performed on the digital image signal 14A after the sensor output signal 10 is A / D converted, the number of gradations due to shading distortion. However, the image quality is deteriorated. The reason is as follows.

Sensor output signal 1 when the reference white plate is read
When 0 has shading distortion as shown in FIG. 10, the dynamic range of the A / D converter 13, in other words, the reference voltage that determines the interval of the quantization level is the constant value V _{FIXED as} described above. In both the digital image data 14A after A / D conversion and the digital image data 16 after shading correction, the number of gradations is not uniform among pixels as shown in FIG. This means that in the corrected digital image data 16, the quantization level interval becomes coarse depending on the pixel, which leads to the deterioration of the image quality.

Incidentally, in FIG. 11, the A / D converter 13 is of 8 bits, and a maximum of 255 gradations can be obtained, but the gradation is lowered to 170 gradations in the portion where the level of the sensor output signal 10 is lowered.

An object of the present invention is to provide an image reading apparatus capable of improving shading correction and reduction of the number of gradations in view of the above-mentioned prior art.

[0019]

Means for Solving the Problems A first method for solving the above problems is described below.
According to the configuration of the invention, a sensor for converting an image to be read into an electric signal, an A / D converter for converting an output signal of the sensor into digital image data, a detecting unit for detecting shading distortion, and a detection unit for detecting shading distortion are detected. Control means for controlling the reference voltage of the A / D converter based on the shading distortion, and the A / D converter outputs the digital image data in which the shading distortion is corrected. It is an image reading device.

According to a second aspect of the present invention, the control means includes a voltage source for generating a constant voltage, and a correction voltage generating means for generating a correction voltage whose value changes according to the detected shading distortion. It is characterized in that it comprises means for superposing a correction voltage on the constant voltage and using it as the reference voltage.

According to a third aspect of the invention, the detection means includes the sensor, the A / D converter, a reference density plate in addition to these, and an A / D conversion while the sensor reads the reference density plate. It is characterized in that it comprises means for maintaining the reference voltage of the container constant and uses the digital image data obtained when the reference density plate is read by the sensor as the shading distortion detection data.

[0022]

The dynamic range of the A / D converter is determined by the reference voltage. If the number of bits is the same, the interval between the quantization levels becomes wider when the reference voltage is increased, and conversely when the reference voltage is decreased. The space between the two becomes narrower.

Therefore, according to the present invention, since the interval of the quantization level of the A / D converter changes according to the shading distortion, the gradation number does not decrease due to the shading distortion, and the A / D conversion is not performed. Shading correction can be performed at the same time. Therefore, there is no deterioration in image quality.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings with reference to the drawings. In the drawings, FIG. 1 shows a configuration of a main part of an image reading apparatus according to a first embodiment of the present invention, and waveform examples of each part for explaining the operation are shown in FIGS. Further, FIG. 6 shows a second embodiment of the present invention. Further, FIGS. 7 to 8 show control examples in the case of using a reference density plate having an arbitrary density other than white. In these figures, parts having the same functions as those of the conventional device shown in FIG. 9 are designated by the same reference numerals.

<First Embodiment> An image reading apparatus of the embodiment shown in FIGS. 1 to 5 will be described. In FIG. 1, the image reading apparatus of the present embodiment includes an illuminating means 1 such as a fluorescent lamp and a mirror 3.
And an optical system such as a lens 4, a sensor 5, a window 7 made of a transparent member, a 100% white reference density plate (hereinafter referred to as a reference white plate) 8, a sample and hold circuit 11, and a gain controller 12. , A / D converter 13 and image memory 1
7, a voltage source 19, a digital arithmetic circuit 20, a D / A
Converter 21, superposition circuit 22, and system control circuit 23
It consists of

<Outline of Control System> Although details will be described later, in this embodiment, the sensor 5, the reference white plate 8, and the A / D converter 1 are used.
3 and the voltage source 19 constitute a shading distortion detecting means 40.

Further, the voltage source 19 and the digital arithmetic circuit 20
, The D / A converter 21, and the superposition circuit 22 constitute the reference voltage control means 50, and the output voltage V ₁ of the voltage source 19 is D / A.
The superposition circuit 22 superimposes the output voltage V ₂ of the A converter 21 into a reference voltage V _{ref, which} is _supplied to the A / D converter 13. (V
_ref = V ₁ + V ₂ )

The voltage source 19 enhances the accuracy of shading correction, so that when the sensor 5 reads the reference white plate 8,
In order that the output of the A / D converter 13 can be supplied to the A / D converter 13 while maintaining a constant small reference voltage so as to reach a signal level of 100% white as much as possible in a range where the output is not saturated. Generate the required constant voltage V ₁ .

In the reference voltage control means 50, the digital operation circuit 20 and the D / A converter 21 constitute the shading correction voltage generation means 51, and the digital operation circuit 20 is converted from the shading distortion detection data 24 as will be described later. A shading correction value (digital amount) SD _i is obtained, and the D / A converter 21 converts this to an analog voltage. That is,
The output voltage V _{2 of the} D / A converter 21 is the shading correction voltage.

In addition to the overall control of the apparatus, the system control circuit 23 is in a mode for reading the reference white board 8 or
The mode information signal 25 notifies the image memory 17 and the D / A converter 21 of whether the reading target 2 is in the reading mode.

The D / A converter 21 is a system control circuit 23.
While the reference white plate 8 is being read, the output voltage V ₂ thereof is set to zero on the basis of the mode information signal 25 from

Therefore, while the reference white plate 8 is being read,
The reference voltage of the A / D converter 13 becomes the constant voltage V ₁ generated by the voltage source 19. (V _ref = V ₁ = constant)

Based on the mode information signal 25 from the system control circuit 23, the image memory 17 divides the digital image data 14B when the reference white board 8 is read and the digital image data 14 when the reading target 2 is read. Separately, they are stored in separate areas.

<Description of Image Reading Unit> The sensor 5, the sample and hold circuit 11 and the gain controller 12 have the same functions as those of the conventional device shown in FIG. 9, and the sensor 5 is illuminated by the illumination means 1 such as a fluorescent lamp. An image of the read object 2 or the reference white plate 8 is formed through the optical systems 3 and 4. The sensor 5 converts the image formed thereon into an electrical signal 10, the sensor output signal 10 is passed through a sample and hold circuit 11, amplified by a gain controller 12, and then given to an A / D converter 13.

In this embodiment, the sensor 5 is moved in the sub-scanning direction to scan the entire image of the read object 2. Therefore, the illumination means 1, the optical systems 3, 4 and the sensor 5 are integrally configured as an image reading unit 6, the image reading unit 6 is moved in the sub-scanning direction, and the main scanning direction is perpendicular to the sub-scanning direction. Further, the elongated reference white plate 8 is arranged at the starting point position in the sub-scanning direction so as to face the image reading unit 6.

<Explanation of Shading Distortion Detection and Reference Voltage Control> The A / D converter 13 receives the reference voltage V _ref (= V ₁ +) _supplied from the superposition circuit 22 of the reference voltage control means 50.
In accordance with V ₂ or = V ₁ ) the sensor output signal 10 passing through the sample and hold circuit 11 and the gain controller 12 is converted into digital image data 14 or 14B and given to the image memory 17. Image memory 17 is A / D
The digital image data 14 and 14B from the converter 13 are stored in a predetermined area.

Here, in the mode for reading the reference white plate 8,
Digital image data 14 obtained from the A / D converter 13
Since B has a constant reference voltage of V _ref = V _1, it reflects shading distortion as illustrated in FIG. 2, and this digital image data 14B itself can be used as the shading distortion detection data 24.

In the example of FIG. 2, the horizontal axis indicates n pixels P _{1 to} P _n for one line in order, and the vertical axis indicates the signal level. The signal level at the i-th pixel P _i is D. _It is represented as _i . D _max is the maximum value of the digital image data after A / D conversion, and in an 8-bit A / D converter, D _max
max = 2 ⁸ −1 = 255.

The digital arithmetic circuit 20 includes an image memory 17
The shading distortion detection data 24 is taken in from, and the signal level D _i (i = 1 to n) of each pixel and the maximum value D
_By using _max and the constant voltage value V ₁ used as the reference voltage, the shading correction amount SD _{i for} each pixel P _i is calculated by digitally calculating the equation (1) given by the equation 1.
Is calculated as a digital value.

[0040]

[Expression 1] SD _i = − ((D _max −D _i ) / D _max ) V ₁ ……………… Equation (1)

In the mode for reading the reading object 2, the digital arithmetic circuit 20 synchronizes with the operation of the A / D converter 13 and for each line and in the pixel order, the shading correction amount column SD _i (i = 1). To n) are given to the D / A converter 21.

The D / A converter 21 converts the shading correction amount sequence SD _i (i = 1 to n) into an analog shading correction voltage V ₂ and supplies it to the superposition circuit 22. The output voltage waveform of the D / A converter 21 is the same as SD _i and V ₂ = −
Since ((D _max −D _i ) / D _max ) V ₁ (i = _{1 to} n), the digital image data 14B shown in FIG.

Reference voltage V output from the superposition circuit 22
_refIs V_ref= V₁+ V₂= V₁× D _i/ D_max(I =
1 to n), which is a valid pixel as shown in FIG.
P₁~ P_nIn between, it is quite similar to the waveform in Figure 2 and
Changes depending on the boarding distortion.

As a result, assuming that the reference white plate 8 is read using the reference voltage of V ₁ + V ₂ = V _ref, the digital image data obtained from the A / D converter 13 at that time is:
As shown by the waveform 26 in FIG. 5, there is no variation between pixels. (D _i → D _i × V ₁ / V _ref = D _i × V ₁ / (V
₁ x D _i / D _max ) = D _max )

Therefore, as is apparent from the comparison of the waveform 26 of FIG. 5 with the conventional waveform of FIG. 11, when the object 2 to be read is read, the sensor output signal 10 is A / D converted and at the same time.
It can be clearly seen that the shading correction is performed without decreasing the number of gradations.

<Second Embodiment> In the first embodiment, the reference voltage control means 50 is composed of the voltage source 19 and the shading correction voltage generation means 51 (digital arithmetic circuit 20, D / A converter 2).
Although it is configured by using 1) and the superposition circuit 22), it can be configured by the digital operation circuit 31 and the D / A converter 21 as shown in FIG. In FIG. 6, parts having the same functions as those in FIG. 1 are designated by the same reference numerals, and duplicate description will be omitted. Also in this embodiment, the reference white plate is used as the reference density plate 8.

In FIG. 6, the digital arithmetic circuit 31 has a register 32, and the voltage value V ₁ suitable for reading the reference white board 8 given from the system control circuit 23 is stored in the register 32. Then, according to the mode information signal 25 from the system control circuit 23, the voltage value V ₁ in the register 32 is changed to the D / A converter 2 in the mode in which the reference white board 8 is read.
Give to one. As a result, a constant reference voltage V ₁ is set to D
The A / D converter 21 supplies the A / D converter 13.

Next, the digital arithmetic circuit 31 uses the digital image data 14B for the reference white board 8 provided from the A / D converter 13 through the image memory 17 as the shading distortion detection data 24, and the voltage value V ₁ in the register 32 , the signal level D _i of each pixel P _i described above, a /
From the maximum value D _max of the digital image data after D conversion,
Equation (2) given by Equation 2 is digitally calculated for each pixel P _i . As a result, a digital reference voltage value sequence V _refi that changes according to the shading distortion D _i is obtained.

[0049]

## _EQU2 ## V _refi = V ₁ × D _i / D _max (i = 1 to n) (2)

Further, the digital arithmetic circuit 31 reads the object 2 to be read.
In the mode of reading, in synchronization with the operation of the A / D converter 13, the reference voltage value sequence V _{re fi} = V ₀ × D _i / D _max (i = 1 to n) is applied to the D / A converter 21. As a result, the D / A converter 21 converts the analog reference voltage V _ref to the A / D converter 13.

As a result, the reference voltage V _ref changes in accordance with the shading distortion as in FIG. 4, and the number of gray levels is reduced at the same time when the sensor output signal 10 is A / D converted as in the first embodiment. It is possible to perform shading correction without the need.

In any of the first and second embodiments, the voltage V ₁ may be set by the system control circuit 23 or inside the digital arithmetic circuits 20 and 31.

<When using an arbitrary reference density plate other than the reference white plate> In the first and second embodiments, the reference white plate is used as the reference density plate 8. However, as described below, the exposure time control or V By performing the control of ₁ , it is possible to use a reference density plate having any density other than white as long as the density is uniform.

<Exposure Time Control> A reference density plate 8 other than white is used.
When used, the voltage generated by the voltage source 19 corresponds to the reference white board
Value V₁Therefore, when the reference density plate 8 is read
The digital image data after A / D conversion is the waveform shown in FIG.
27, and the signal level is the maximum value D _maxThan
Also decreases. This is usually the message when reading the reference white board.
The maximum value of the issue level is D_maxExposure time in advance
Is set.

Therefore, in the mode for reading the reference density plate 8, the exposure time is controlled so that the maximum value of the signal level reaches D _max as shown by the waveform 14B in FIG.

The shading correction amount SD _i is obtained as described in the first or second embodiment by using the digital image data 14B for the reference density plate 8 after controlling the exposure time in this way. The shading distortion is shown in FIG.
As can be seen from the above, since the two waveforms 14B and 27 are similar, there is no change even if the exposure time is changed.

Then, in the mode of reading the reading object 2, the exposure time is returned to the original value and the reference voltage V _ref is controlled based on the shading correction amount SD _i as described in the first or second embodiment. , Perform shading correction.

<V₁Control> Instead of controlling the exposure time,
As shown in FIG. 8, the voltage generated by the voltage source 19 is V ₁From
V₁If it is changed to ', the A / D change with respect to the reference density plate 8
Maximum value of signal level of digital image data 14B after conversion
To D_maxCan be adjusted to.

[0059] Therefore, in the mode for reading the reference density plate 8, the maximum value of the signal level so that the D _max, changes the voltage of the voltage source 19 to V ₁ 'from V _1.

[0060] Using the digital image data 14B with respect to the reference density plate 8 after such control from V ₁ to V ₁ ', obtains the shading correction amount SD _i in accordance with the first or second embodiment.

Then, in the mode of reading the read object 2, the voltage V ₁ ′ is returned to the original voltage V ₁ , and the first or second
As described in the above embodiment, the shading correction amount SD _i
The shading correction is performed by controlling the reference voltage V _ref based on

However, the signal level D _i in the equations (1) and (2) should be read as the value when D _max is set to V ₁ ′.

<Others> In the above embodiments, the sensor 5 is described as a line sensor, but it is also possible to use a two-dimensional array area sensor. Further, the shading distortion detecting means 40 including the sensor 5, the reference density plate 8 and the A / D converter 13 is used for detecting the shading distortion, but other than this, at least the illumination means 1 such as the light quantity detector.
Any light source can be used as long as it can detect the light intensity distribution.

[0064]

As described above with reference to the embodiments, the image reading apparatus according to the present invention has the following effects.

According to the first aspect of the invention, the reference voltage of the A / D converter is controlled according to the shading distortion.
Simultaneously with the conversion, the shading distortion can be corrected without decreasing the number of gradations.

A second aspect of the present invention is a more specific form of the control of the reference voltage in the first aspect of the invention, in which a correction voltage that changes according to shading distortion is superimposed on a constant reference voltage that is normally used. By doing, A
Shading correction can be performed without decreasing the number of gradations during D / D conversion.

The invention of claim 3 is a more specific embodiment of the detection of shading distortion in the invention of claim 1, and since the shading distortion is detected by the sensor, the reference density plate and the A / D converter, The shading distortion can be detected not only by the light amount distribution of the means but also by including the light amount unevenness of the optical system and the sensitivity variation between the elements of the sensor.

[Brief description of drawings]

FIG. 1 is a diagram showing a main configuration of an image reading apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a waveform example of shading distortion detection data.

FIG. 3 is a diagram showing a waveform example of a shading correction voltage.

FIG. 4 is a diagram showing a waveform example of a reference voltage formed by superimposing a shading correction voltage on a constant voltage.

FIG. 5 is a diagram showing a waveform example of digital image data after shading correction.

FIG. 6 is a diagram showing a main configuration of an image reading apparatus according to a second embodiment of the present invention.

FIG. 7 is a diagram showing a control example (exposure time control) when a reference density plate other than white is used.

FIG. 8 is a diagram showing another control example (V ₁ control) when a reference density plate other than white is used.

FIG. 9 is a diagram showing a main configuration of a conventional image reading device.

FIG. 10 is a diagram showing a waveform example of a sensor output signal.

FIG. 11 is a diagram showing a reduction in the number of gradations in conventional shading correction.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Illumination means 2 Reading target 3 Mirror 4 Lens 5 Sensor 6 Image reading part 7 Window 8 Reference density plate (reference white plate) 10 Sensor output signal 11 Sample and hold circuit 12 Gain controller 13 A / D converter 14 Corresponding to the reading target Digital image data 14B Digital image data corresponding to the reference density plate 17 Image memory 19 Voltage source 20, 31 Digital arithmetic circuit 21 D / A converter 22 Superposition circuit 23 System control circuit 24 Shading distortion detection data 25 Mode information signal 32 Register 40 Shading distortion detection means 50 Reference voltage control means 51 Shading correction voltage generation means

Claims (3)

[Claims] 1. A sensor for converting an image to be read into an electric signal, an A / D converter for converting an output signal of the sensor into digital image data, a detecting means for detecting shading distortion, and a detected means. An image comprising: a control unit for controlling a reference voltage of the A / D converter based on shading distortion, wherein the A / D converter outputs digital image data in which the shading distortion is corrected. Reader. 2. The control means includes a voltage source for generating a constant voltage, a correction voltage generating means for generating a correction voltage whose value changes in accordance with the detected shading distortion, and the correction voltage to the constant voltage. The image reading apparatus according to claim 1, comprising means for superimposing the reference voltage. 3. The detection means includes the sensor and the A /
It consists of a D converter, a reference density plate in addition to these, and a means for maintaining the reference voltage of the A / D converter constant while the sensor reads the reference density plate. The image reading apparatus according to claim 1, wherein the obtained digital image data is used as shading distortion detection data.