JP2658237B2 - Image reading device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image reading apparatus capable of reading image data of a document with multiple gradations used in a facsimile, a digital copying machine, and the like.

2. Description of the Related Art In recent years, an image reading apparatus, such as a digital copying machine, capable of digitally reading two-dimensional image data of a manuscript in multiple gradations and reproducing halftones has been widely used.

In order to accurately read image data of an original including a halftone in multiple gradations, for example, Japanese Patent Application Laid-Open No. 61-261295 discloses a method in which white shading correction and black shading correction are combined as described below to reduce the light amount variation of a light source. Increase the S / N ratio of image data by correcting variations in sensitivity of each pixel of the image sensor and variations in dark output voltage of each pixel due to dark current, and increase the dynamic range for the density of the input document A method has been proposed.

FIG. 6 is a functional block diagram of a conventional image reading apparatus, wherein 1 is a document, 2 is a light source, 3 is a lens, 4 is a self-scanning image sensor, and 5 is a light source 2, a lens 3, and an image sensor 4. The carriage is held and movable in the sub-scanning direction indicated by arrow A.

Reference numeral 6 denotes a white reference plate made of a material having a light source reflectance close to 1, and reference numeral 7 denotes a black reference plate made of a material whose light source reflectance is close to zero.

FIG. 5 is a circuit block diagram of a conventional image reading apparatus, wherein 101 is a drive circuit for supplying a scan start signal HSYNC and a scan clock signal VCLK to the image sensor 4, and 102 is a digital value of the image signal from the image sensor 4. An A / D converter 103 for converting the image data into certain image data is a data selector for branching the image data.

Reference numeral 104 denotes a white reference RAM that stores white reference data that is read image data of a white reference plate, 105 denotes a black reference RAM that stores black reference data obtained by reading a black reference plate, and 106 denotes a white reference RA.
An address counter 107 for giving a write / read address to the M and black reference RAMs is a first subtractor, and outputs a black reference RAM from the white reference data which is output data of the white reference RAM.
And outputs data obtained by subtracting the black reference data, which is the output data of.

Reference numeral 108 denotes a second subtracter, which outputs data obtained by subtracting black reference data from image data of a document. A subtractor 109 receives the output data of the first subtractor and the second subtractor, and outputs a result obtained by performing a predetermined operation as corrected image data.

The operation of the image reading apparatus configured as described above will be described below with reference to FIG.

First, the carriage 5 moves to the position of the white reference plate, the light from the light source 2 irradiates the white reference plate 6, the reflected light is collected by the lens 3, and the light receiving portion of the image sensor 4 (omitted in the figure) It is led to.)

The drive circuit 101 sends a scanning start signal HSYNC to the image sensor 4.
And a scan clock signal VCLK. A light receiving element of the image sensor 4 has a row of light receiving elements arranged in the main scanning direction of the document, and each light receiving element outputs an image signal proportional to the black and white gradation at each position (each pixel) of the document. It is configured to be.

Therefore, an image signal for one line in the main scanning direction that has read the white reference plate is output in synchronization with the scanning start signal HSYNC and the scanning clock signal.

The read image signal is converted into digitized image data by an A / D converter.

At this time, when the white shading signal WHT is set to “H”, the data selector 103 supplies the image data to the white reference RAM as write data. At this time, the white reference RAM is R / W
The signal becomes "L", and the address counter 10 described below will be described.
One line of image data is written according to the address of 6, and the white reference data Dw is stored. When the white shading process is completed, the white shading signal returns to “L”. The written white reference data Dw becomes the maximum level of the read image data.

Next, the carriage 5 is moved to the position of the black reference plate, and the black shading signal is set to “H”. In the same manner as in the white shading processing, the data selector 103 selects one of the black reference plates.
The image data for the line is given as write data of the black reference RAM, and stored as black reference data Db in the black reference RAM. When the black shading process is completed, the black shading signal BLK returns to “L”. This black reference data Db
Is data indicating the black level of the image data, and indicates the minimum level of the read image data.

Here, the white shading signal WHT and the black shading signal BLK are both set to "L", and the carriage 5 is moved to the leading end position to start reading the image data Df of the document. The image data for one line of the original is input to the second subtractor 108 by the data selector 103, and the black reference data
Data (Df-Db) from which Db has been subtracted is obtained. Further, the first subtracter 107 obtains data (Dw-Db) obtained by subtracting the black reference data Db from the white reference data Dw.

These two data are input to the arithmetic unit 109, and the arithmetic operation of the following formula is performed. The output of the arithmetic unit 109 is subjected to white shading correction and black shading correction to obtain normalized image data Dr normalized. .

Dr = 255 × (Df−Db) / (Dw−Db) Using a PROM as an arithmetic unit, the upper address is (Df−Db)
b) is input, (Dw-Db) is input to the lower address, and the operation result for the input address is written in a PROM in advance in a table method using a ROM table method.

As described above, correction data Dr synchronized with the scanning start signal HSYNC and the scanning clock signal VCLK can be obtained for the image data of the first line of the document. Further, the image data of the original can be read two-dimensionally by moving the carriage 5 in the sub-scanning direction by a moving distance of one line each time image data of one line is obtained.

A specific example is shown in FIG. FIG. 7 shows the read image data for one line of the document, and the corresponding white reference data and black reference data. Assume that the black reference data corresponding to the element Vr1 of the image sensor is 10 and the black reference data corresponding to Vr2 is 5.

It is also assumed that the white reference data Dw also has data corresponding to Vr1 and Vr2 of 250 and 150, respectively.

Next, when the document is read, first, the image data Df of the element Vr2 is output. It is now assumed that the value of the image data Df is 130. At this time, the black reference data Db and the white reference data Dw corresponding to the element Vr2 are output from the white reference RAM 104 and the black reference RAM 105, the respective calculations are performed by the subtracters 107 and 108, and the value 120 is obtained as the Df-Db signal. 145 as the Dw-Db signal
Is output. These two values are input to the arithmetic unit 109, and the corresponding values 109 are output as corrected image data.

This calculation is sequentially performed for each image data of each element of the image sensor, and similarly for the element Vr1, the image data Df
Is 200, the value 190 is obtained as Df-Db signal i, and Dw-
The value 240 is output as the Db signal n, and the value 1 is output as the correction data Dr.
00 is output.

As described above, in the conventional example, in order to take white reference data and black reference data for each pixel and equally divide the difference into image data, unevenness in light amount of a light source, distortion of an optical system, and Factors that distort image data such as sensitivity variations for each light receiving element and dark output voltage due to dark current for each light receiving element of the image sensor can be corrected and normalized, improving the S / N ratio and reading It is possible to read multi-gradation image data in which the dynamic range of the black density of the document is expanded.

However, in the above-described conventional configuration, the voltage level of the image signal obtained by reading the black reference plate is low in reading the black reference data. As compared with, the S / N ratio is relatively close, and the black reference data written in the black reference RAM has low accuracy.

Assume that an image signal having a low signal level as shown in FIG. 8A is read. The low image signal level means that the black density of the read original is high and the reflectance of the light source is low. Therefore, the level is close to the level of the black reference signal obtained by reading the black reference. When the corrected image data is obtained from these signals, the corrected image data should have a constant small value as shown in FIG. 8B by the correction described above.

However, since the absolute value of the digital value data converted by the A / D converter is small, the quantization error of the A / D conversion and
A rounding error in the calculation of a digital value by a subtractor or an arithmetic unit becomes relatively large. FIG. 8 (c) shows the A / D conversion of the black reference signal.
The quantization error when converting and generating the black reference data is shown. As a result, the corrected image data actually obtained is image data in which an error appears due to a quantization error as shown in FIG.

For this reason, when reading image data of a document having a high black density as a whole, the average S / N ratio of the image data decreases. In addition, in order to print image data using a printer such as a thermal sublimation method with gradation, the image data must be converted from a reflectance system to a density system. When converted to density, the error is greatly magnified and is very noticeable when viewed visually.

As described above, when the document has a low reflectance or high density as a whole, the S / N ratio of the image data decreases, and the error is much more noticeable than when reading a low-density bright document. Become. In addition, since these noise components appear as vertical streaks of a fixed pattern regardless of the sub-scanning direction, the reading quality of image data is significantly impaired.

Means for Solving the Problems In addition to the conventional configuration, the present invention provides a reference voltage value which is a conversion reference value of an A / D converter when reading white reference data and when reading black reference data. Is changed, the white reference data and the black reference data are converted into digital values by the reference voltage and stored, and shading correction is performed by the stored data.

Operation According to the above configuration, the digital value of the black reference data can be relatively large, so that the quantization error of the A / D converter is relatively small, and the black reference data stored in the black reference RAM is sufficiently large. Reliable data.

In addition, since the black reference data input to the shading calculator becomes large, the rounding error due to digital calculation is reduced, and the read quality of image data in a high density range can be remarkably improved.

Embodiment FIG. 1 is a circuit block diagram of an image reading apparatus according to an embodiment of the present invention. Reference numeral 101 denotes a driving circuit that supplies a scanning start signal HSYNC and a scanning clock signal VCLK to an image sensor 4, and 102 denotes an image sensor. This is an A / D converter for converting the image signal from No. 4 into image data which is a digital value. This A / D converter uses a reference voltage described later as a conversion reference value. 103 is a data selector for branching image data.

110 is a reference voltage generator, 2V from the tap of A, B
Outputs 1/4 of 0.5V from the tap. Reference numeral 111 denotes a data selector that supplies the A / D converter 102 with a reference voltage serving as a reference for digital conversion.

Reference numeral 104 denotes a white reference RAM that stores white reference data that is read image data of a white reference plate, 105 denotes a black reference RAM that stores black reference data obtained by reading a black reference plate, and 106 denotes a white reference RA.
This is an address counter that gives write and read addresses to the M and black reference RAMs.

A first subtractor 107 outputs data obtained by subtracting black reference data, which is output data of the black reference RAM 105, from white reference data, which is output data of the white reference RAM.

Reference numeral 108 denotes a second computing unit which outputs data obtained by subtracting black reference data from image data of a document. A subtractor 109 receives the output data of the first subtractor 107 and the second arithmetic unit 108 and performs a predetermined operation to output the result as corrected image data.

The operation of the image reading apparatus configured as described above will be described in detail below with reference to FIG.

First, the carriage 5 moves to the position of the white reference plate, the light from the light source 2 irradiates the white reference plate 6, the reflected light is collected by the lens 3, and the light receiving portion of the image sensor 4 (omitted in the figure) It is led to.)

The drive circuit 101 sends a scanning start signal HSYNC to the image sensor 4.
And a scan clock signal VCLK. A light receiving element of the image sensor 4 has a row of light receiving elements arranged in the main scanning direction of the document, and each light receiving element outputs an image signal proportional to the black and white gradation at each position (each pixel) of the document. It is configured to be.

Therefore, an image signal for one line in the main scanning direction that has read the white reference plate is output in synchronization with the scanning start signal HSYNC and the scanning clock signal.

The read image signal is converted into digitized image data by an A / D converter.

At this time, when the white shading signal WHT is set to “H”, the data selector 103 supplies the image data to the white reference RAM as write data. At this time, the white reference RAM is R / W
The signal becomes "L", and the address counter 10 described below will be described.
One line of image data is written according to the address of 6, and the white reference data Dw is stored. When the white shading process is completed, the white shading signal returns to “L”. The written white reference data Dw becomes the maximum level of the read image data.

Next, the carriage 5 is moved to the position of the black reference plate, and the black shading signal is set to “H”. In the same manner as in the white shading processing, the data selector 103 selects one of the black reference plates.
The image data for the line is given as write data of the black reference RAM, and stored as black image data in the black reference RAM.

At this time, the data selector 111 controls the A / D converter
In order to provide a 1/4 level reference voltage,
The output level of the black reference signal read from the black reference plate is quadrupled. Then do the white shading in the same way,
The image data for one line of the black reference plate is given by the data selector 103 as write data for the black reference RAM, and is recorded as black image data quadrupled in the black reference RAM.

. When the black shading process is completed, the black shading signal BLK returns to “L”.

Here, the white shading signal WHT and the black shading signal BLK are both set to “L”, and the carriage 5 is set to the original 1
To start reading the image data of the original. At this time, the image data for the first line of the document is output in synchronization with the scanning start signal HSYNC and the scanning clock signal VCLK, and the white reference data written in the above-described white reference RAM and the black reference RAM are written. The obtained black image data is read out and output corresponding to the image data of the document.

The quadruple black reference data read from the black reference RAM is input to the divider 112, divided by 4, and converted into black reference data Db that matches the level of the image data at the time of reading the original. Thereafter, similarly to the conventional example, the image data Df for one line of the original is input to the second subtractor 108 by the data selector 103, and the data (Df-Dw) from which the black reference data Db is subtracted. Get. In the first subtractor 107,
Data (Dw-Db) is obtained by subtracting the black reference data Db from the white reference data Dw.

The two data are input to the arithmetic unit 109, and the arithmetic operation of the following equation is performed. The output of the arithmetic unit 109 is subjected to white shading correction and black shading correction, and is normalized to obtain corrected image data Dr.

Dr = 255 × (Df−Db) / (Dw−Db) Using a PROM as an arithmetic unit, the upper address is (Df−Db)
b) is input, (Dw-DDb) is input to the lower address, and the operation result for the input address is written in a PROM in advance in a table format in a ROM table system.

As described above, correction data Dr synchronized with the scanning start signal HSYNC and the scanning clock signal VCLK can be obtained for the image data of the first line of the document. Further, the image data of the original can be read two-dimensionally by moving the carriage 5 in the sub-scanning direction by a moving distance of one line each time image data of one line is obtained.

A specific example is shown in FIG. FIG. 3 shows read image data for one line of a document, and corresponding white reference data and black reference data. Assume that the black reference signal corresponding to the element Vr1 of the image sensor is 10 and the black reference signal corresponding to Vr2 is 5. At this time, since the reference voltage of the A / D converter 102 has a voltage of 1/4 added thereto, the corresponding output data of the A / D converter is 40 and 20, respectively. Written to black reference RAM.

It is also assumed that the white reference data Dw also has data corresponding to Vr1 and Vr2 of 250 and 150, respectively.

Next, when the document is read, first, the image data Df of the element Vr2 is output. It is now assumed that the value of the image data Df is 130. At this time, the black reference data Db = 20 corresponding to the element Vr2
Is output from the black reference RAM.
And outputs a value of 5 which is 1/4. As the white reference data Dw, 150 is output from the white reference RAM 104. , The respective calculations are performed in subtracters 107 and 108, and Df
The value 125 is output as the -Db signal i, and the value 145 is output as the Dw-Db signal n. These two values are input to the arithmetic unit 109, and the corresponding values 109 are output as corrected image data.

This calculation is sequentially performed for each image data of each element of the image sensor, and similarly for the element Vr1, the image data Df
Is 200, the value 190 is obtained as Df-Db signal i, and Dw-
The value 240 is output as the Db signal n, and the value 1 is output as the correction data Dr.
00 is output. Up to this point, the same operation as in the conventional example is performed.

Next, the fourth case of reading a high-density original will be described.
This will be described with reference to the drawings. The reason why the error component of the corrected image data of the high-density document increases is that the accuracy of the black reference data Db decreases as described in the conventional example. The effect is
Formula for calculating corrected image data Dr: Dr = 255 × (Df−Db) /
In (Dw-Db), the denominator (Dw-Db) is negligible, and appears mainly in (Df-Db). Therefore, the following description will be made with (Df-Db) as corrected image data.

FIG. 4A shows black reference data in a conventional example.

Here, when the reference voltage of the A / D converter is reduced to 1/4 during the black reference reading as shown in the present embodiment, the read image data becomes quadruple output as shown in FIG. 4 (b). . Here, the black reference data is reduced to 1/4 by the divider to match the level with the image data at the time of document reading. In the black reference data obtained as shown in FIG. 4 (c), the quantization error is suppressed, and 1.5 (Dw−Db) <2.5. As shown in FIG. 4 (d), the corrected image data obtained by (Df-Db) is always 2 and the accuracy is improved.

As described above, in the present embodiment, when obtaining the black reference data for correcting the image data, the reference voltage which is the reference value for the digital conversion of the A / D converter is reduced to 1/4, so that the reference voltage is quadrupled. The black reference data can be obtained, and the quantization error due to the A / D conversion of the corrected image data obtained because the black reference data reduced to 1/4 by the divider can be suppressed.

Therefore, it is possible to accurately read a high-gradation image regardless of the black density of the document. Here, the reference voltage of the A / D converter for obtaining the black reference data is 1/4
However, this may be another value.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram of an image reading device according to an embodiment of the present invention, FIG. 2 is a functional block diagram of the image reading device shown in FIG. 1, and FIG. 3 is a relationship between read image data and black-and-white reference data. FIG. 4 is a diagram showing a relationship between an image signal and a reference signal, FIG. 5 is a circuit block diagram showing a configuration of a conventional image reading device, and FIG. 6 is a conventional image reading device shown in FIG. Functional block diagram of device, seventh
FIG. 8 is a graph showing the relationship between the conventional image data and the monochrome reference data, and FIG. 8 is a graph showing the relationship between the conventional image signal and the reference signal. 1 Document, 2 Light source, 3 Lens, 4 Image sensor, 5 Carriage, 6 White reference plate, 7
Black reference plate, 101 ... Drive circuit, 102 ... A / D converter, 103 ...
... Data selector, 104: White reference RAM, 105: Black reference R
AM, 110 …… Reference voltage generator

Claims (1)

(57) [Claims] An image reading apparatus for performing A / D conversion of an image signal based on a reference voltage and performing shading correction using the A / D converted image signal, wherein a reference as a reference of the A / D conversion is provided. Reference voltage generating means for generating a voltage, and a reference voltage for switching the reference voltage to a reference voltage smaller than a reference voltage for A / D conversion of a white reference image signal when A / D converting a black reference image signal An image reading device, comprising: switching means.