JPH03155278A - Image reading device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to an image reading device such as a facsimile machine or a digital copying machine that reads an image using an image sensor.

Conventional technology In general, in devices that photoelectrically convert light reflected from a document using an image sensor such as a COD, the following characteristics are used: ■ The light-gathering characteristics of the lens (there is a so-called cosine fourth power law, which means that light gathers in the center of the lens) ■ Sensitivity correction called shading correction is performed due to the uneven sensitivity of each pixel of the image sensor. .

For example, when an image with white-like density is read by an image sensor, the output waveform for one line will be as shown in FIG. 5(a), and as described above,
The output is high in the center (due to ■■) and uneven (due to ■) at a fine period (pixel period). If shading is corrected for each pixel, a uniform output will be obtained as shown in FIG. 2(b).

Therefore, a conventional electrical shading correction method will be explained with reference to FIG.

In the figure, a solid line route indicates a route when reading an image, and a broken line route indicates a route when reading shading data.

This circuit consists of a D/A converter 1, an amplifier 2, an A/D converter 3, a ROM 4, and a RAM 5, all of which handle 8-bit data. First, when reading shading data, a white reference plate (not shown) provided outside the image area and on the side of the reading start position is read by an image sensor and input to the analog input terminal of the D/A converter l.

Here, the D/A converter 1 is a digital-analog multiplier, and between the analog input voltage vi, the 8-bit digital input value D, and the analog output voltage vo, vo=vi-D/255...・・・・・・・・・
・・・・・・・・・(1) There is a relationship. That is, vo is a value obtained by multiplying vi by D/255. By the way, when reading shading data, the digital input terminal of D/A converter 1 is pulled up to the power supply, so D=25
5, and vo=vi.

The image signal vo output from the D/A converter 1 is amplified by the amplifier 2, and then input to the analog input terminal V1 of the A/D converter 3, where it is converted into 8-bit digital data for each pixel. . When actually reading an image, this is outputted to the outside as image data, but when reading shading data, it is input to the address of the ROM 4. This R.O.
Shading correction data corresponding to input values is stored in the M4 in the form of a table, and the output correction data is stored in the RAM 5 for each pixel.

After the scanner scans the white reference plate and finishes reading it, actual image reading begins. At this time, this shading correction data is sent to the digital input terminal of the D/A converter 1 for each corresponding pixel. is input. Therefore, D/A
A calculation process for shading correction is performed in the converter l, and D
A corrected output is obtained from the /A converter 1 as shown in FIG. 5(b).

Second, consider the correction data stored in the ROM 4.

This correction data is set so that the output after correction is uniform, and the input and output of the ROM 4 have the following relationship when reading an image.

Dout=255 (0≦DIN<255 a
)・・・・・・・・・・・・・・・・・・(2) Do
ur=255 ” ・a / D IN (255a≦D
1N≦255) ・・・・・・・・・・・・・・・・・・(3) Second,
DIN: input value, DOLIT: output value.

Further, a defines the level to be uniformly outputted with respect to the peak value, and is 0×a≦1. Shading correction is
In other words, the output waveform is flattened, and the S/N ratio is better if the output waveform is flattened at the highest level possible, but the portions at both ends that are not corrected increase. For example, there is one in which the flattening ratio is 40%, that is, a=0.4, and the contents (in hexadecimal notation) of the correction data stored in the ROM 4 at this time are shown in FIG. FIG. 8 shows the outline of the ROM input/output characteristics at this time. That is, when the input is from O to 66.4, which is 0.4 times 255, the output is F F it from equation (2), and the D/A converter 1 output is (1)
From the formula, vo=vi. When the input is from 668 to FF, the correction value according to formula (3) is selected, and the input is the maximum value,
That is, in the case of FF, the output is 66H, and in the D/A converter l, vo=0.4vi from equation (1). With the shading correction as described above, an output that is 0.4 times the peak value is obtained, except for parts at both ends. Therefore, it is possible to correct the unevenness caused by the above-mentioned items 1 to 2, and to obtain an output in which each pixel output is determined only by the density of the original.

Problems to be Solved by the Invention However, the above shading correction method has the following drawbacks. That is, since only correction data with an averaging ratio of 40% of the peak value is stored in the ROMA, it is not known whether this ratio is an appropriate value for the sound of water. That is, in order to increase the S/N ratio, it is better to make this averaging ratio as large as possible, but if it is made too large, the portions at both ends that are not corrected will increase. FIG. 9 shows this situation for various averaging ratios, and in the figure, the solid line portions indicate the portions that can be corrected, and the broken line portions indicate the portions that cannot be corrected. Therefore, the averaging ratio may be larger than the value of 40% without any problem, or it may have to be smaller than the value of 40%, and 40% may not be appropriate.

Means for Solving the Problems In the invention as set forth in claim 1, an image sensor including a white reference plate for shading correction, a large number of elements for reading an image and the white reference plate, and an A/ D
An A/D converter for converting the image signal, and a RAM for storing a digital value obtained by converting the image signal read from the white reference plate by the A/D converter, and a RAM for storing the digital value of the image signal read from the white reference plate. a ROM that stores shading correction data groups corresponding to values for a plurality of sheets at different averaging ratios; a selection means for selecting one set of shading correction data groups in the ROM; A computing unit is provided which performs a correction calculation using the corresponding correction data in the selected shading correction data group for the image signal. In addition to the invention as claimed in claim 1, the invention according to claim 2 includes a maximum/minimum detector for detecting the maximum value and minimum value of the read image signal, and a ratio between the detected maximum value and minimum value. A ratio calculator is provided for calculating the ratio, and a selection means selects a set of shading correction data in the ROM according to the calculated ratio, and the selected shading correction data group is selected for the image signal of each element read from the image. Corresponding correction data in the shading correction data group is used to perform correction calculations by a computing unit.

According to the invention described in claim 1, a plurality of coarse shading correction data groups having different averaging ratios are stored in the ROM, and one set of shading correction data selected by the selection means is Since the shading correction calculation is performed using the data, data with an appropriate averaging ratio can be used, and the output can be adjusted without being unable to correct on both sides of the image effective reading range or with a poor S/N ratio. Optimization can be achieved.

At this time, according to the invention as claimed in claim 2, the maximum value and the minimum value of the read image signal are detected by the maximum/minimum detector, and the ratio of the maximum value and the minimum value is calculated by the ratio calculator, Since a set of shading correction data groups in the ROM are selected according to the calculated ratio, the selection of an appropriate averaging ratio can be automated without the need for selection operations using switches, etc. Furthermore, the illuminance distribution of the light source can be adjusted automatically. It will also be possible to deal with changes over time.

Embodiment An embodiment of the invention set forth in claim 1 will be described with reference to FIGS. 1 to 3. First, as before, the D/A converter (
) 11, an amplifier 12, and a D/A converter 13 are provided in this order, and a RAM 14 is followed by a ROM 1.
5 is provided.

Here, in the ROM 15 of this embodiment, as shown in FIG. 2, multiple sets of shading correction data groups with different averaging ratios a are stored for each page, and one set of these data groups is selected. He is looking down at the page selection input section 16, which serves as selection means.

Further, the D/A converter 11 of this embodiment is configured so that digital data from the ROM 15 is input both when reading shading data and when reading an image.

The contents of the ROM 15 will be explained. For example, four groups of shading correction data with averaging ratios of 30%, 40%, 50%, and 60% are prepared, and the output of the ROM 15 is initially a shading correction data group with an averaging ratio of 40%. It is set as follows. Therefore, depending on the output state based on such initial settings, if the correction on both sides of the image effective reading range is insufficient, the averaging ratio is switched to 30% by page selection and a group of 30% shading correction data is generated. On the other hand, if the S/N becomes worse, increase the averaging ratio to 50 by selecting the page.
% to use a 50% shading correction data group.

Further, in this embodiment, even when reading shading data, the output vO from the D/A converter 11 is output without pulling up the digital input terminal of the D/A converter 11 to the power supply.
A certain constant is input from the ROM 15 so that the value becomes small. Therefore, the ROM 15 also stores level increase/decrease data for reading shading data.

This point will be explained. First, in the 6th figure equation,
During image reading, the D/A converter specific power is flattened to, for example, 0.4 times the peak value, but as it is, the input voltage VIN of the A/D converter 3 is too low, so an amplifier with a gain of 2.5 is used. 2 is used to amplify the signal. However, when reading shading data, the digital input terminal of the D/A converter is pulled up to the power supply, so the digital data input value is 255 and vi=vo. However, if this is amplified 2.5 times by the amplifier 2, the voltage value is too large and may exceed the reference voltage or power supply voltage of the A/D converter 3.

If the reference voltage is exceeded, the A/D converted value is a full-scale FF, and as a result, A/D conversion cannot be performed. Even worse, if the voltage exceeds the power supply voltage, the A/D converter 3 may be destroyed.

Therefore, in this embodiment, the level increase/decrease data in the ROM 15 is used to adjust the input value to the digital data input terminal of the D/A converter 11 so that the input VIN to the A/D converter 13 is appropriate even when reading shading data. It is something to control. A certain constant as this level increase/decrease data is set to 668, which is 0.4 times the full scale FF, considering that the amplifier 12 has a gain of 2.5 times, for example. Then, even when reading shading data, the output of the D/A converter 11 is vo=
o, 4vi, and by multiplying it by 25 by the amplifier 12, the input voltage (2) of the A/D converter 13 is exactly the value of vi.

The level increase/decrease data 66 is an example, and as in the case of the averaging ratio, it is preferable to prepare a plurality of pages in the ROM 15 so that the data can be selected arbitrarily.

FIG. 2 shows an example of how multiple sets of shading correction data groups and level increase/decrease data groups are stored in the ROM 15. That is, for the shading correction data group, the averaging ratio is 35%, 4
Four sets of 0%, 50%, and 60% are prepared, and for the level increase/decrease data group, the output of the D/A converter 11 is multiplied by 0.35,
Four sets of constants for multiplying by 0.4 times, 0.5 times, and 0.6 times are stored.

By the way, the ROM 15 for storing such data can be configured as shown in FIG. 3 using, for example, 64 bits and 27C64. Address AONA7 is RAM
The address A8 is for the white reference plate reading data input from 14, and the address A8 is for a signal for distinguishing between reading shading data and reading an image. Addresses A9 to A12 are addresses for the page selection input section 16 and for selecting the averaging ratio by the switch group 17 to select a page. In this embodiment, the selection of the averaging ratio corresponds to the selection of the magnification of the level increase/decrease data, but the selection is not limited to this, and eight types of selections may be made, for example. By the way,
The 27C64 ROM 15 shown in FIG. 3 can store up to 16 sets of averaging ratios and level increase/decrease data.

Next, an embodiment of the invention according to claim 2 will be described with reference to FIG. In this embodiment, selection and setting of the averaging ratio is automated. For this reason, first, a peak hold circuit and edge portion hold circuit 18 is provided as a maximum/minimum detector for detecting the maximum value and minimum value of the read image signal.

This peak hold circuit & edge part hold circuit 18 is used for reading the white reference plate when reading shading data.
A peak value in a line image signal is detected by a peak hold circuit, and an output at the end of the image reading effective range (this is the minimum value) is detected by an edge portion hold circuit. This output is input to the ratio calculator 19, and the ratio between the maximum value and the minimum value is calculated. This ratio determines the averaging ratio, and the calculated ratio is applied as a page selection signal to the page selection input section 16 of the ROM 15, and the corresponding page is selected. According to this, there is no need to go to the trouble of selecting and setting the switch group 17. In addition, even if the illuminance distribution of the light source changes over time and the amount of light between the center and both ends fluctuates, the averaging ratio etc. will be changed accordingly to ensure that the data is accurate. You will get it.

Effects of the Invention Since the present invention is configured as described above, according to the invention described in claim 1, a plurality of coarse shading correction data groups having different averaging ratios are stored in the ROM, and the selecting means Since shading correction calculations are performed by a computing unit using a set of shading correction data groups selected by It is possible to optimize the output without causing deterioration of the output, and in particular, according to the invention set forth in claim 2,
A minimum detector detects the maximum value and minimum value of the read image signal, a ratio calculator calculates the ratio between the maximum value and the minimum value, and a set of shading corrections in the ROM is performed according to the calculated ratio. Since data groups are selected, it is possible to select an appropriate averaging ratio automatically without the need for selection operations using switches, etc., and it is also possible to change the selection taking into account changes over time in the illuminance distribution of the light source. You can get more appropriate output.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the invention as claimed in claim 1, FIG. 2 is an explanatory diagram showing the contents stored in the ROM, FIG. 3 is a block diagram showing a specific example of the configuration near the ROM, and FIG. The figure is a block diagram showing an embodiment of the invention as claimed in claim 2,
The figure is a characteristic diagram showing an example of the output waveform before and after shading correction, Figure 6 is a block diagram showing the conventional configuration, and Figure 7 is R
FIG. 8 is an explanatory diagram showing the contents stored in the OM, FIG. 8 is a ROM input/output characteristic diagram, and FIG. 9 is a characteristic diagram showing whether correction is possible or not based on the averaging ratio. 11... Arithmetic unit, 13... A/D converter, 14...
-RAM, 15...ROM, 16...selection means, 1
8...Maximum/minimum value detector, 19...Ratio calculator %
:5 - Mao q Figure

Claims (1)

[Claims] 1. An image sensor including a white reference plate for shading correction, a large number of elements for reading an image and the white reference plate, and an A/D conversion for A/D converting the read image signal. and the image signal read from the white reference plate to the A/D.
A RAM that stores digital values converted by the converter;
a ROM storing a plurality of shading correction data groups corresponding to digital values of image signals read from a white reference plate with different averaging ratios; and a selection means for selecting one set of shading correction data groups in the ROM. and an arithmetic unit that performs a correction calculation using the corresponding correction data in the shading correction data group selected for the image signal of each element read from the image. 2. A white reference plate for shading correction, an image sensor including a large number of elements for reading an image and the white reference plate, an A/D converter for A/D converting the read image signal, and the white reference plate. The image signal read from the board is sent to the A/D
A RAM that stores digital values converted by the converter;
A ROM that stores a plurality of shading correction data groups with different averaging ratios corresponding to the digital value of the image signal read from the white reference plate, and a ROM that detects the maximum and minimum values of the image signal read from the white reference board. - a minimum detector, a ratio calculator that calculates the ratio between the detected maximum value and the minimum value, and selection means that selects one set of shading correction data in the ROM according to the calculated ratio; An image reading device comprising: an arithmetic unit that performs a correction calculation using corresponding correction data in a group of shading correction data selected for an image signal of each element read from an image.