JPH0354509B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a shading correction device that enables high-speed reading even if the access time of a processing circuit for calculating shading correction coefficients is long.

The document to be recorded is irradiated with a lamp, and the reflected light is passed through an optical system including a reflecting mirror and lenses.
Recording devices that capture an image on a photoelectric conversion element such as a solid-state image sensor or a photodiode array, convert it into an electrical signal, and then form an electrostatic charge image using a needle-shaped electrode or the like and develop it to create a recorded image are already known and widely used. ing.

In this type of recording device, even when reading a document surface with uniform density, the output of photoelectric conversion signals is uneven, and the output at the edges is particularly small compared to the center, resulting in a recorded image that is black in the areas where the signal output is small. A phenomenon in which the concentration becomes uneven, such as a cloudy appearance, is observed. This phenomenon is called shedding, and its causes may be as follows.

(b) Illuminance unevenness and illuminance changes of the document irradiation lamp For example, a fluorescent lamp is used as the document irradiation lamp, but the length of the lamp is finite and due to the light emitting mechanism, the luminance is lower at both ends than at the center, so the illuminance is low. . Further, as the fluorescent lamp is used, both ends thereof become black, and the illuminance distribution changes depending on how the lamp is installed.

(b) Light attenuation effect by the lens of the optical system The amount of light passing through the lens of the optical system is cosine 4
Due to the power law, it decreases at the periphery, for example, when the half angle of view is 20
degree, the amount of light at the periphery is 78% of that at the center.

(c) Non-uniform sensitivity of photoelectric conversion elements Photoelectric conversion elements such as solid-state image sensors such as charge-coupled devices (CCDs) and diode arrays may have non-uniform sensitivity due to manufacturing or manufacturing reasons.

Conventionally, various correction measures have been taken to correct this shading. For example, it is known to provide a light distribution plate to lower the light intensity at the center of the lamp to match the light intensity at the periphery, thereby making the light distribution characteristics of the lamp uniform over the entire length of the lamp.
Although this method is effective in the initial state, it has little effect on the blackening of the edges that occurs with use, and has the disadvantage that the light distribution plate must be adjusted each time to deal with this. Therefore, in order to perform accurate correction, a method has been considered in which a photoelectric conversion element that outputs a shading waveform is installed nearby in addition to the photoelectric conversion element for reading the original, and the image signal obtained by reading the original and the shading waveform are calculated. However, although this method can correct the shading of the light source, there is a problem in that it cannot correct non-uniform sensitivity of the photoelectric conversion element or sensitivity fluctuations due to changes in ambient temperature. Still another correction method is to A/D convert a signal obtained by photoelectrically converting a uniform illuminance surface and store it in a storage element, and read out the stored contents when reading an original to correct shading. The correction accuracy with this method is quite good, but in order to store the shading correction coefficients in the storage element, an A/D converter capable of processing data at very high speed is required.
Although arithmetic processing circuits and storage elements are required, there is a problem that such circuits and elements are expensive and require long access times, making high-speed reading impossible.

Therefore, in order to solve the above problem, we applied for a patent application in 1983.
In No. 124791, high-speed reading was enabled by using an interpolation method to correct shading when reading a document, but the present invention enables high-speed reading using a different method. It can be used even for long periods of time.

That is, before reading the original image, a shading correction coefficient is determined based on light information from a uniform reflective surface and stored in a memory element such as a RAM, and when the original is read, the content of the storage element is read out and the original image signal is corrected. The object of the present invention is to provide a shedding correction device that is effective against fluctuations in light distribution characteristics of lamps due to aging and temperature changes and non-uniformity in sensitivity of photoelectric conversion elements. However, determining the shading correction coefficient and storing it in the storage element requires very high speed processing circuits, which are expensive and whose access time is not sufficient for high speed reading.

Therefore, in the present invention, when determining a correction coefficient based on optical information from a uniform reflective surface, the clock of the photoelectric conversion element is set to have a cycle that is slower than the normal document reading timing and longer than the access time of the processing circuit, and is also shaded. By reading out the correction coefficients at high speed, it is possible to use the processing circuit that requires a long access time to obtain the correction coefficients. The present invention is also a shading correction device that can remove background fog by varying the reference voltage of a differential amplifier provided after the photoelectric conversion element depending on the type of background of the document.

The present invention will be explained below based on the drawings.

FIG. 1 schematically shows an example of the configuration of a document reading section of a copying machine having a document reading device with a movable document table. The reflected light from 2 is incident on a photoelectric conversion element 6 via a mirror 4 and a lens 5, and is converted into an electrical image signal. In this example, a white reflective surface 7 is provided in a non-image area in front of the document table 1.

Now, when the document table 1 moves in the direction of the white arrow during document reading, the waveform of the signal output from the photoelectric conversion element 6 due to the light reflected from the reflective surface 7 in one scan is as shown by A in FIG. become. This shows the shading waveform of the entire reader. in general,
Since the photoelectric conversion element is composed of n unit elements, the shedding waveform is composed of V ₁ , V ₂ , . . . V _o in minute units.

To find the correction coefficient for correcting the shading, as shown in Fig. 2, set the reference voltage V _R (indicated by straight line B) to the shading waveform.
When the reference voltage V _R is divided by the output values V ₁ to V _o forming the shading waveform, a value as shown by the broken line C in the figure is obtained. This broken line C
Using the value shown as the shading correction coefficient,
Stored in a memory element such as RAM.

FIG. 3 shows an embodiment of the shading correction circuit according to the present invention. In the figure, 8 is the clock φ ₁ of the photoelectric conversion element 6 during normal document reading.
9 is a clock generation circuit that generates the clock φ ₂ of the photoelectric conversion element 6 when storing the shading correction coefficient; 10 is the reference voltage e generated by the output e ₁ of the photoelectric conversion element 6 and the reference voltage generator 11; ₂ and amplify the difference signal e ₀ (= e ₁ − e ₂ )
This is a differential amplifier that outputs . The reference voltage generator 11 generates three different voltages e _a , e _b , and e _c by switching the switch S ₁ according to the type of document (for example, the density of the background color). 12 is a sample/hold circuit that holds the difference signal _e0 output from the differential amplifier 10 as an image information signal _VX ;
13 is a D/A converter that converts the shading correction coefficient calculated by the method described later into digital/analog; 14 is the image information signal _VX held in the sample/hold circuit 12 and the D/A converter 13;
An arithmetic circuit 15 multiplies the shading correction coefficient V _Y output from the arithmetic processing circuit 14, and an arithmetic circuit 15 compares the reference voltage V _R with the signal V _O processed by the arithmetic processing circuit 14 to determine whether it is an H (High) level signal or an L (Low) signal. A comparator that outputs a level signal, 16 is a clock generation circuit 8
A transfer control circuit ₁₇ outputs _a transfer control signal _φ
Upper (MSB) analog switch of A converter 11
18 is a D/D control circuit controlled by the control circuit 17;
A storage circuit such as a RAM that stores the switch state of the A converter 13, that is, a shading correction coefficient; 19 a timing circuit that provides timing for storing the shading correction coefficient in the storage circuit 18; 20 a timing circuit that stores the data stored in the storage circuit 18; This is a buffer memory that stores data once and then reads it out at high speed using a clock _φ1 . As shown in FIG. The multiplexer 20e takes out the data held in the circuits 20a to _20d in synchronization with the clock φ1. switch
S ₂ , S ₃ , S ₄ , and S ₅ are connected to contact a when storing the shading correction coefficient, and contact b when reading the original.
can be switched to

Next, the operation of the above-mentioned shading correction circuit is explained in the fifth section.
This will be explained using figures. Incidentally, FIG. 5 is a time chart of the clocks φ ₁ and φ ₂ of the photoelectric conversion element 6 and the transfer control signal for one cycle of the shading correction operation.

Figure 5 A shows the clock φ ₁ of the photoelectric conversion element 6 during normal document reading, and B shows the photoelectric conversion element when storing a shading correction coefficient with a period larger than the period of the clock φ ₂ obtained by dividing the clock φ ₁ . 6 shows the operation clock φ ₂ , _C shows _the transfer control signal _φ First, the storage operation of the shading correction coefficient will be explained. Prior to the storage operation, the switches _S2 to _S5 are switched to contact b, and when the transfer control signal _φXA is output from the transfer control circuit 16 at time t1 in _FIG . is opened, and if the photoelectric conversion element 6 is a CCD, the image information data of the light receiving section is transferred to the transfer section, but since this image information data is previously photoelectrically converted data or data generated by dark current, the clock φ ₁ until the subsequent time t ₂ .

When the transfer control signal φ _XA is output at time t ₂ ,
The transfer gate of the photoelectric conversion element 6 is opened again, but
At this time, the image information data output from the photoelectric conversion element 6 is the amount of shading. Therefore, at this time, the connections of switches S ₂ to _{S 5} are changed from b to a, and a clock φ ₂ with a cycle longer than the processing time of the D/A converter 13, arithmetic processing circuit 14, and memory circuit 18 is used as the data transfer clock. use During this time, the transfer control circuit 16 controls so that the transfer gate of the photoelectric conversion element 6 is not opened. The sample-and-hold circuit 12 photoelectrically converts the white reflective surface 7 (see FIG. 1) using the clock φ ₂ to obtain the shading waveform A shown in FIG. 2 (actually, one output value V ₁ forming the shading waveform A). ) is held for longer than the processing time and output as V _X.

On the other hand, the control circuit 17 starts operating in response to the clock φ ₂ and first turns on the MSB analog switch of the D/A converter 13. As a result, V _Y is output _from the D/A converter ₁₃ , and one _{signal V} is calculated. This signal V _O is compared with the reference voltage V _R in the comparator 15, and V _R <V _O
When V _R >V _O , an L level is output from the comparator 15. The control circuit 17 leaves the analog switch as it is when the output from the comparator 15 is at L level, and turns off the switch when it is at H level and proceeds to the next bit. Thereafter, similar operations are performed up to the LSB, and the state of the switch is stored in the memory circuit 18. By performing the above steps for all bits of the photoelectric conversion element, storage of the shading correction coefficient is completed.

Here, the reference voltage e ₂ of the differential amplifier 10 that is compared with the output of the photoelectric conversion element 6, ie, the output e 1 of the white reflective surface 7 _, will be explained.

The reference voltage generator 11 is configured to generate, for example, e _a , e _b , e _c (e _a > e _b > e _c ) in descending order of the density of the ground color of the original, so that the voltage from the photoelectric conversion element 6 is Based on the light output of the output white reflective surface 7, the reference voltage e ₂ (e.g. e _a ) increases as the density of the ground color increases.
, the S/N ratio is improved and ground fog is removed. This reference voltage e ₂ i.e.
e _a , e _b , and e _c may be set manually by the operator, or may be set automatically by automatic concentration detection. Through the above operations, the shading correction coefficient is determined.

Next, the shading correction coefficient correction when reading a document will be explained.

When _{the transfer control} signal _φ
It becomes as shown by the broken line in Figure D. The data accumulated in the light receiving portion of the photoelectric conversion element 6 while storing the coefficients (times t ₂ to _{t 3} ) is output by the clock φ ₁ until time t ₄ . During this time, the latch circuits 20a, 20
The contents of the memory circuit 18 are shown in b, 20c, and 20d.
Latched. Thereafter, at time _t4 , when the transfer control signal _φXA is output, the original is read.
All switches _S2 to _S5 are switched to contact b.
That is, the shading correction coefficients previously stored in the high-speed buffer memory 20 are sent to the D/A converter ₁₃ in synchronization with the document reading clock φ1. The shading correction coefficient is converted into an analog value by the D/A converter 13, outputted as _VY , and outputted from the sample-and-hold circuit 12. Further, the contents of the memory circuit 18 are stored in the buffer memory 20 in advance. Arithmetic processing circuit 1 along with document reading signal _V
In step 4, the above-mentioned calculations V _X and V _Y are performed,
The corrected signal is output as V _O.

By performing the above-described shading correction for each scan, shading can be completely corrected.

In the above embodiment, an example has been described in which the uniform reflection surface for determining the shading correction coefficient is white and provided in the non-image area, but the present invention is not limited to this.

As described above, the present invention calculates and stores a shading correction coefficient based on light information from a uniform reflective surface before reading an original image, and when reading an original image, uses the shading correction coefficient to create an image. In a shading correction device that corrects signal shading, the shading correction coefficient is calculated at a cycle longer than the timing of reading the original image and longer than the calculation processing time of the shading correction coefficient, so that data processing can be performed at high speed. High-speed reading can be performed without using expensive circuits or elements such as /D converters, arithmetic processing units, storage elements, etc., and it can also be used even if the access time of the processing circuit for determining the shading correction coefficient is long. Further, by correcting the optical information, which is the basis for calculating the shading correction coefficient, according to the type of document, such as the density of the background of the document, it is possible to record an image without background fog.

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram of an example of a document reading section of a copying machine, FIG. 2 is a diagram showing a shading waveform and a shading correction coefficient, and FIG. 3 is a block diagram of an embodiment of a shading correction device according to the present invention. , FIG. 4 is a circuit diagram of an embodiment of the buffer memory used in the shading correction device of FIG.
The figure is a time chart of clocks, etc. used in the shading correction operation. 1...Original table, 2...Original, 3...Lamp, 4
... Mirror, 5 ... Lens, 6 ... Photoelectric conversion element, 7 ... Reflection surface, 8, 9 ... Clock generation circuit, 10 ... Differential amplifier, 11 ... Reference voltage generator, 12 ... Sample・Hold circuits 12, 13
...D/A converter, 14... Arithmetic circuit, 15...
Comparator, 16...Transfer control circuit, 17...Control circuit, 18...Storage circuit, 19...Timing circuit, 20...Buffer memory.

Claims (1)

[Claims] 1 A reflective section with a uniform reflectance, a photoelectric conversion means that converts the reflected light from the reflective section into an electrical signal, and a reference voltage that is variable depending on the type of document. A differential amplifier, a D/A converter that generates a voltage that changes at a constant rate every certain time, and an output voltage value from the photoelectric conversion means and an output voltage value from the D/A converter. a calculation means for comparing the product with a predetermined reference value and calculating a shading correction coefficient based on the comparison value, a storage means for storing the shading correction coefficient calculated by the calculation means, and a subsequent stage of the storage means. a buffer memory for once storing the shading correction coefficient stored in the storage means and then reading it out at high speed; and a buffer memory for reading out the shading correction coefficient at high speed after once storing it; 1. A shading correction device comprising: timing means for outputting a timing signal having a period longer than a calculation processing time.