JPH0511463B2 - - Google Patents

Description

Detailed Description of the Invention "Field of Industrial Application" The present invention optically reads an image on a document,
The present invention relates to a copying apparatus that converts this image into an electrical image signal pixel by pixel and performs recording based on the image signal.

``Prior Art'' As shown in FIG. 11, a type of copying apparatus includes a reading section 1 that reads an image on a document, and an image signal processing section 2 that digitally processes the image signal obtained thereby. There is a system in which recording is performed using a thermal head 3 based on the image signal.

A one-dimensional solid-state imaging device such as an image line sensor is usually used in the reading section 1 of this device. When reading an image on a document, the document 4 is irradiated with a lamp 6 such as a fluorescent lamp as shown in FIG. 12, and the reflected light 7 is transmitted to an optical system 8 such as a lens as shown in FIG. is used to guide it to the reading surface of the image line sensor 9. At this time, the intensity of the reflected light 7 is strong at the center of the document 4 when viewed in the longitudinal direction of the lamp 6, and is slightly weaker at the portions near both ends of the lamp. FIG. 14a shows the intensity distribution of the reflected light. The horizontal axis of the figure corresponds to the longitudinal position of the lamp. In addition, the image line sensor 9 (13th
2) has a configuration in which a large number of photoelectric conversion elements are arranged in a line in the longitudinal direction, but there are variations in photoelectric conversion ability for each element. Figure b shows the state of the output electrical signal level when light of uniform intensity is incident in the longitudinal direction.

In order to correct such reading unevenness of the image line sensor, a shading correction circuit 11 as shown in FIG. 13 has conventionally been employed. This circuit amplifies the image signal read by the image line sensor 9 with an amplifier 12, converts it into a digital signal with an analog-to-digital converter 13, and then converts it into a digital signal to cancel the unevenness shown in FIG. 14a and b. This is a circuit that operates to multiply an input signal by a correction coefficient having a characteristic as shown in Figure c. In this way, the reading unevenness of the reading section is corrected in the shading correction circuit 11, and an output signal with flat characteristics as shown in d of the figure can be obtained. Normally, this correction coefficient is created by reading and calculating a reference white surface separately provided at a predetermined location of the device immediately before starting to read an image on a document. This reading unevenness is caused by factors that change over a relatively short period of time, such as changes in the amount of light from a lamp over time. Therefore, it is preferable to update the correction coefficient each time a document is read.

On the other hand, on the recording side of this copying apparatus, as shown in FIG.
7 is pressed against the thermal head 18 side by a back crawler 19 to perform recording. In such an apparatus, structural problems in the thermal head 18 and its peripheral devices may cause recording unevenness, that is, variations in recording density depending on the recording position of the thermal head. For example, as shown in FIG. 16a, the heating resistors ₁₈₁ arranged in a line in the longitudinal direction of the thermal head 18 are divided into a large number of sections ₁₈₁₁ in the longitudinal direction as shown in FIG. The structure is such that an electric pulse is applied to generate heat. This variation in the resistance value of the heating resistor for each section 18 ₁₁ causes recording unevenness. Also, as shown in Fig. 17a, the bat crawler 1
If the outer diameter r of the roller 9 is uneven in the longitudinal direction, when the back roller 19 is pressed against the thermal head 18 as shown in FIG. Similarly, when viewed from the side of this figure, as shown in Figure 18,
The position of the heating element 18 ₁ of the thermal head 18 does not match the plane passing through the axis 19 ₁ of the back roller 19,
If there is a deviation by the gap G, the pressing force of the back roller 19 will also become non-uniform in the longitudinal direction of the thermal head 18. This is called a nip position shift. Even with this uneven pressing force,
Recorded images become uneven.

``Problems to be solved by the invention'' Density unevenness caused by such recording conditions can be improved to some extent by improving the quality of the thermal head, improving the material and processing accuracy of the back roller, and improving assembly accuracy during the manufacturing process. can.

However, this reduces the yield of parts and
Additionally, advanced technology is required for assembly, both of which increase costs.

Furthermore, if the characteristics of a thermal head, back tracker, etc. change due to long-term use, similar recording irregularities will occur, but there is no solution to this other than replacing parts, and maintenance costs are high.

The present invention has been made focusing on the above points,
It is an object of the present invention to provide a copying apparatus that can correct such recording unevenness.

"Means for Solving the Problems" The copying apparatus of the present invention includes a reading unit that optically reads an image on a document and converts it into an electrical image signal in pixel units, and A device that has a thermal head that records an image on recording paper and an image signal processing section that processes the image signal obtained by the reading section and transfers it to the thermal head. A pattern generation circuit generates an image signal with a constant pattern to obtain information on fluctuations in recording density due to Based on the resulting image signal for each pixel in the reading unit, an image signal for recording correction is created for each pixel in the reading unit, and then the reading corresponding to the image on the document read by the reading unit is performed. The apparatus is characterized in that it is provided with a recording correction circuit that corrects an image signal for each pixel in the section using an image signal for recording correction.

For example, this recording correction circuit accepts a line memory element that stores an image signal for recording correction for one line, and an image signal read by a reading section from the output of this line memory element, and performs recording correction on this image signal. A table memory element that outputs data multiplied by a correction coefficient according to the image signal is provided.

In addition, the recording correction circuit accumulates recording correction image signals for one line or several lines read by the reading section, and partially averages them in the main scanning direction and sub-scanning direction to record one line. It is preferable to create a correction image signal.

"Operation" As described above, the copying apparatus of the present invention has a pattern generation circuit that generates an image signal of a certain pattern inside the image signal processing section. This constant pattern image signal corresponds to, for example, a signal obtained by reading an image having a constant, uniform density over the entire surface of the document with an ideal reading unit. If such a signal is sent to the thermal head for recording, a recorded image with uniform density over the entire surface of the recording paper should be obtained. However, the above-described unevenness in recording conditions causes corresponding recording unevenness. The image is read by the reading section of this device. The signal is used as a recording correction image signal, and, for example, one line is stored in a memory element.

Then, the signal obtained by actually reading the image on the document is multiplied by a correction coefficient corresponding to the reciprocal of the recording correction image signal. With this, unevenness in recording conditions can be corrected.

Here, in the case of a copying machine that can record images with halftone density using a thermal head,
When looking at the recorded image pixel by pixel, each individual pixel is recorded at halftone density, and some white and black pixels are combined to give an apparent halftone density. There is something. In the case of the latter type of image, even if the image appears to be of intermediate tone to the naked eye, the density is not uniform when viewed microscopically. At this time, the image signal for recording correction read by the reading unit cannot be used as is for correction, so it is accumulated for one line or several lines and partially averaged to record one line. Recreate the image signal for correction. This process ultimately corresponds to the process of cutting the image on the document into areas of appropriate area and calculating the apparent density of the area.

In this way, deterioration in image quality due to uneven recording conditions can be prevented, and deterioration in image quality due to aging of the device can be prevented by simple maintenance and inspection work.

"Conventional Example" (Description of Block) FIG. 1 is a block diagram showing an embodiment of a copying apparatus according to the present invention.

This copying apparatus includes a reading section 1 that optically reads an image on a document and converts it into an electrical image signal pixel by pixel, and an image on the document is transferred to recording paper based on the image signal obtained by the reading section 1. The thermal head 3 is provided with a thermal head 3 for recording images, and an image signal processing section 2 that processes an image signal obtained by the reading section 1 and transfers it to the thermal head 3.

This copying apparatus includes an analog-to-digital conversion circuit 13, a shading correction circuit 11, a recording correction circuit 22, a dither circuit 23, a print control circuit 24, and a pattern generation circuit 25 in its image signal processing section 2. ing.

The analog-to-digital conversion circuit 13 is a circuit similar to that described above with reference to FIG.
This is a circuit that outputs a dot digital signal. The shedding correction circuit 11 includes a logarithmic converter (LOG) 11 ₁ , a random access memory (RAM) element 11 ₂ , and a subtracter 11 ₃ . This circuit is a circuit that performs shading correction by logarithmically converting the input image signal and subtracting the correction coefficient stored in the RAM element 11 ₂ from the value. Its operation is as described above with reference to FIG. 14, and is a circuit that corrects reading unevenness of the reading section 1 and outputs an image signal with flat characteristics.

The recording correction circuit 22 includes a smoothing circuit 22 ₁ , a nonvolatile random access memory (RAM) element 22 ₂ , and a read only memory (ROM) element 2.
It is composed of 2 and ₃ . As will be explained in detail later, this circuit generates a recording correction image signal to obtain a correction coefficient for correcting recording unevenness in the thermal head 3.
This circuit stores the image signal in the RAM element ₂₂₂ , sends it together with the input image signal to the address terminal of the ROM element ₂₂₃ , and takes out the corrected image signal.

The dither circuit 23 is a circuit provided for recording halftones, and processes an 8-dot image signal to express density in 64 levels using, for example, a dot matrix composed of a certain number of recording dots. This is a circuit that selects and outputs. Since this circuit is well known, a detailed explanation of its configuration will be omitted. Although such a circuit is not necessarily necessary for the copying apparatus of the present invention, in a copying apparatus that performs halftone expression, it is important to maintain image quality that the density of all recorded dots is uniform and stable. , it can be said that applying the present invention to this is most effective. The print control circuit 24 is a circuit that sends the image signals thus obtained to the thermal head 3 in a predetermined order and at a predetermined timing, and is also a circuit with a known configuration, so detailed explanation thereof will be omitted.

(Operation and reading of pattern generation circuit) The pattern generation circuit 25 is connected to the recording correction circuit 22.
This circuit operates only when storing an image signal for recording correction in the RAM element 22 ₂ , and operates so that the dither circuit 23 outputs an image signal with a uniform pattern. When this circuit 25 operates, the recording correction circuit 22 to the dither circuit 23
Output of the image signal to is stopped. When the pattern generating circuit 25 operates, a recorded image is obtained in which white dots 27 and black dots 28 are arranged to form a checkered pattern, as shown in FIG. 2, for example.

Alternatively, an image signal may be output in which all the recording dots 28 are halftone (gray). Specifically, this circuit is composed of a read-only memory element that stores image signals corresponding to white dots or black dots and outputs them in a predetermined order in synchronization with a clock signal.

In order to read an image on a document using the copying apparatus of the present invention, the pattern generation circuit 2
5 and stores an image signal for recording correction in the recording correction circuit 22. As explained above, recording unevenness is caused by the structure of each part of the apparatus used for recording, and the conditions do not change over a short period of time. Therefore, such processing may be performed at any time, such as during shipping inspection of the copying apparatus or during periodic inspection.

First, as explained earlier, the pattern generation circuit 2
5 (FIG. 1) to obtain a recorded image with a dot pattern as shown in FIG. 3, for example. This recorded image has a configuration in which one of the four recorded dots is a black dot, and when viewed with the naked eye, it appears as a gray image with a density of 0.5, for example.

For example, suppose that the axis of the back roller 19 is inclined with respect to the thermal head 18 by an angle θ, as shown in FIG. At this time, the density of each black dot in FIG. 3 has density unevenness with a constant slope in the longitudinal direction of the thermal head, as shown in FIG. 5a. The fine concentration fluctuations are due to non-uniformity in the heat generation of the heating resistor of the thermal head 18. First, for example, the reading unit 1 (FIG. 1) reads a recorded image as shown in FIG.
Suppose we obtain an image signal as shown in . At this time,
Recording correction circuit 22 (first
The image signal is stored in the RAM element 22 ₂ shown in the figure and used as an image signal for recording correction. This RAM element ₂₂₂ is a non-volatile element that has been backed up by a battery or the like, and its contents are saved until the image signal for recording correction is re-stored. In FIG. 1, when an image signal 31 to be recorded is input to the recording correction circuit 22, the image signal 31 and the image signal 32 for recording correction are
Input to the address terminal of the ROM element ₂₂₃ . this
A table has been written in _{the three} elements of the ROM 22 so that an image signal 33 having a value obtained by multiplying the input image signal 31 by a correction coefficient taking into account the characteristics of the recording correction image signal 32 is output. That is, as shown in FIG. 5b, an image signal corresponding to the product of the correction coefficient corresponding to the reciprocal of the recording correction image signal (FIG. 5a) and the input image signal is output. As a result, recording unevenness can be corrected, and flat recording characteristics as shown in FIG. 3C can be obtained.

(Evenness of read signal) On the other hand, if the recorded image shown in FIG.
If the pitch of the reading elements of the image line sensor of the reading section is sufficiently smaller than the pitch of 8, an image signal whose level periodically fluctuates in the longitudinal direction can be obtained from this image line sensor. Correction coefficients cannot be directly obtained from such signals.

Therefore, as shown in FIG. 6, the recorded image ₃₆₁ on the recording paper 36 is read five times while being sub-scanned in the direction of the arrow 37. That is, as shown in FIG. 7, this image is read through a window with an area that is a collection of 25 windows 38 large enough to be read by one reading element, and a smoothing circuit as shown in FIG. average value. By this,
Create a new recording correction image signal for one line.

FIG. 8 shows a detailed embodiment of the recording correction circuit 22 shown in FIG.
5, an adder 47 that extracts and adds the 25 image signals stored therein, and an adder 47 that extracts and adds the 25 image signals stored therein, and adds the image signals to "25".
It consists of a divider 48 that divides by , and outputs an average value, a line memory element 51 that stores one line of the image signal obtained as a result, and two table memory elements 52 and 53. .

In this figure, each shift register for one line is composed of a number of flip-flops equal to the number of image signals for one line, but only the last four flip-flops are shown independently. The remaining parts are shown together on the left side of the figure. For example, the shift register 41 stores image signals for one line using four flip-flops 41 ₂ to 41 ₅ and a remaining portion 41 ₁ . The last four flip-flops 41 ₂ -41 ₅ , 42 2 -42 5 , _{43 2 -4} of each of these shift registers 41 - 45
When the input image signals of ₃₅ , ₄₄₂ to ₄₄₅ , ₄₅₂ to ₄₅₅ and the final output image signals of each shift register are collected, they are output from 25 windows as shown in FIG. The read image signals can be collected and added. If this is divided by 25 and averaged, an image like the one shown in Figure 3 can be read with apparent density. In this way, one line of recording correction 32 is obtained and stored in the line memory element 51.

Next, when an image on a document to be actually copied is read and an image signal 31 is input to the recording correction circuit 22, the image signal 31 for recording correction is sent from the line memory element 51 to the table memory element 52.
is output. As shown in FIG. 9, this ROM element 52 stores a conversion table for outputting a correction coefficient 32 ₁ corresponding to the reciprocal of the image signal 32 for recording correction. The correction coefficient 32 ₁ thus obtained is now input to another table memory element 53. As shown in FIG. 10, this table memory element 53 inputs an input image signal 31 (horizontal axis in FIG. 10) inputted to the upper bits of the address and a correction coefficient 32 inputted to the lower bits.
Image signal 33 multiplied by (parameters 0.4 to 2 in the same figure)
(vertical axis in the figure) is created and output. thus,
The image signal 31 is corrected, and uneven recording can be prevented.

"Variation" The present invention is not limited to the above embodiments.

The pattern generation circuit may generate a dot matrix-like halftone image signal for recording, which is a set of a certain number of recording dots, using a known dither method or pattern method, for example. Furthermore, this pattern generation circuit may be configured to output image signals of not only one type of pattern but two or more types of patterns. Furthermore, although the recording correction circuit performs shading correction and recording correction separately, the correction coefficient obtained by the recording correction circuit is combined with the correction coefficient of the shading correction circuit to perform correction at one location. I don't mind.

``Effects of the Invention'' According to the copying apparatus of the present invention described above, it is possible to correct recording unevenness caused by uneven recording conditions of each apparatus, that is, fluctuations in recording density due to the recording position of the thermal head, and improve the image quality of recorded images. can always be kept above a certain level. Moreover, since it becomes possible to sufficiently correct some degree of unevenness in recording conditions, the performance and precision required of each component of the apparatus are relaxed, and assembly of the apparatus becomes easier. Furthermore, by resetting the correction conditions during maintenance and inspection, it is possible to prevent deterioration in recorded image quality due to aging.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the copying apparatus of the present invention, FIG. 2 is a partially enlarged view of an image recorded using the image signal generated by the pattern generation circuit, and FIG.
The figure is a partially enlarged view of another image generated by the pattern generation circuit, Figure 4 is an explanatory diagram of the recording conditions in this embodiment, Figure 5 is a graph showing the relationship between recording density and correction coefficient, Figures 6 and 7 are explanatory diagrams of averaging processing for recording correction, Figure 8 is a block diagram showing a detailed embodiment of the recording correction circuit, and Figure 9 is a graph showing the contents of the conversion table of the ROM element. , FIG. 10 is a graph showing the contents of the conversion table of the ROM element, FIG. 11 is a block diagram of a copying apparatus suitable for carrying out the present invention, FIG. 12 is a perspective view of the main part of the reading section, and FIG. Block diagram of the main parts of the processing section,
FIG. 14 is an explanatory diagram of the operation of the shading correction circuit, and FIG. 15 is a perspective view of the main part of the recording block.
FIG. 16 is a perspective view and a wiring diagram of the main parts of the thermal head, and FIGS. 17 and 18 are a front view and a side view showing the relationship between the thermal head and the back roller. DESCRIPTION OF SYMBOLS 1...reading section, 2...processing section, 3...thermal head, 22...recording correction circuit, 25...pattern generation circuit, 32...for recording correction.

Claims (1)

[Scope of Claims] 1. A reading unit that optically reads an image on a document and converts it into an electrical image signal pixel by pixel, and a thermal head that records the image on the document on recording paper based on this image signal. and an image signal processing section that processes the image signal obtained by the reading section and transfers it to the thermal head, and the image signal processing section is provided with information on fluctuations in recording density depending on the recording position of the thermal head. a pattern generation circuit that generates an image signal of a certain pattern to obtain an image; and a pattern generating circuit that generates an image signal of a certain pattern to obtain an image; Based on the image signal for each pixel in the reading section, an image signal for recording correction for each pixel in the reading section is created, and then an image signal for each pixel in the reading section corresponding to the image on the document read by the reading section is created. A copying apparatus comprising: a recording correction circuit for correcting the image signal for recording correction using the image signal for recording correction. 2 The recording correction circuit receives a line memory element that stores an image signal for recording correction for one line, the output of this line memory element and the image signal read by the reading section, and applies the image signal to this image signal for recording correction. 2. A copying apparatus according to claim 1, further comprising a table memory element that outputs data multiplied by a correction coefficient according to an image signal. 3. The recording correction circuit accumulates recording correction image signals for one line or several lines read by the reading section, and partially averages them in the main scanning direction and the sub-scanning direction to record one line. 3. A copying apparatus according to claim 1, wherein the copying apparatus generates a correction image signal.