JPS6172259A - Gradation capacity correcting method of color electronic copying system - Google Patents

Abstract

PURPOSE:To improve gradation capacity from a low image density part to a high image density part by forming preliminarily a relatively rather underexposure electrostatic latent image and correcting this electrostatic latent image in accordance with the image density by digital erasing. CONSTITUTION:For the purpose of obtaining an ideal electrostatic latent image, an ideal exposure is determined for each image density, and the output of a color solid-state image pickup element in case of read of an original is checked when this determined exposure is set, and relations of a curve 4-1 are stored in a microcomputer. Since the original is read before the copy process, relations between the density of the color picture part of each color separation picture and the signal output and relations between the density of a neutral picture part and the output are obtained in accordance with read picture information. When a curve 4-2 indicates relations between the density and the output, an erase rate for a density x is determined on a basis of curves 4-1 and 4-2; and if this erase rate is, for example, 40% for a density x1, the area of the latent image part is set to Z or the latent image in 0.4Z area part is erased. Digital erasing is performed in accordance with this determined erase rate.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a gradation correction method in a color electronic copying system.

(Prior Art) As is well known, in the electronic copying method, it is difficult to reproduce gradation. This is because the gradation reproduction curve, that is, the line that shows the correspondence between the image densities of the original and the image density of the copied image on the horizontal axis and the copy image on the vertical axis, does not form a straight line.

As described above, since accurate reproduction of gradation is difficult, especially in color electronic copying systems, there is a problem in that the color tone of the color copy image differs from that of the original.

Conventionally, various methods for correcting gradation have been proposed in order to improve the reproducibility of gradation. The method was to narrow down the image density and improve gradation reproducibility as much as possible within the target image density range.

(Purpose) The present invention has been made in view of the above-mentioned circumstances. The purpose of this paper is to provide a new gradation correction method for copying systems.

(composition) The present invention will be explained below.

The gradation correction method of the present invention includes a reading step, an erase rate determining step, and an erase step.

The reading step is a step of color-separating and reading a color document, and this step is performed prior to the color electronic copying process. Reading is performed using a solid-state imaging device, such as a CCD. This reading may be performed using a color separation filter and three solid-state image sensors, or may be performed using a single-plate color solid-state image sensor, which will be described later.

In the erase rate determination step, the digital erase rate is determined based on the image information read in the reading step.

A plurality of electrostatic latent images are required to obtain one color copy from a color image, and the digital erase rate is determined for each of these plurality of electrostatic latent images.

The erase step is a step in which the electrostatic latent images described above are digitally erased based on the erase rate determined as described above. Digital erasing is performed in an area modulated manner using an eraser using a dot light emitter array.

This erasing can also be performed after the photoreceptor is charged and prior to image exposure.

The dot light emitter array is an LED array, a dot fluorescent tube array, etc., and the eraser is a combination of the dot light emitter array and an imaging optical system. As the imaging optical system, a converging light transmitter array or a mirror lens array is used.

Incidentally, digital erasure will be explained in detail later.

Now, the electrostatic latent image is formed as an electrostatic latent image that is relatively underexposed due to predetermined charging conditions and/or exposure conditions.

The present invention will be described in detail below, but for the sake of concreteness, a color electronic copying apparatus as shown in FIG. 1 will be taken as an example. In addition,
Since FIG. 1 is an explanatory drawing, it should be noted that the relative size relationships of the dimensions of each part are not necessarily accurate.

In the figure, reference numeral 10 indicates a photoconductive photoreceptor. This photoreceptor 10 is drum-shaped and is rotatable in the direction of the arrow.

As the material of the photoconductive layer, for example, a photoconductive material having panchromatic photosensitivity, such as As2Se3, is used.

Around this photoreceptor 10, a charger 12, an eraser 18, a developing device 20, 22, 24, 26, a holder 2, etc.
8, a static eliminator 32 and a cleaner 34 are provided.

Reference numeral 16 indicates a document placement glass, on which the document O to be copied is placed.
is placed flatly on this document placement glass 16.

It is composed of a filter device F.

Further, the reading optical system indicated by the reference numeral 40 is a mirror 40.
1. Lens 402, single-plate color solid-state image sensor 403
The mirror 401 can selectively assume the attitude shown by the solid line or the attitude shown by the broken line by swinging.

A reference density plate 42 is provided at the right end of the document placement glass 16.

Further, in FIG. 1, reference numeral 30 indicates a transfer device, and reference numeral 36 indicates a fixing device. Further, the symbol S indicates plain paper (hereinafter referred to as transfer paper) which is the recording medium.

Now, returning to the exposure optical system 14, when illuminating and scanning the original O, the lamp 140 is emitted, the wrap 140 and the plane mirror 141 are integrally moved to the left, and at the same time, the roof mirror 142 is moved by the plane mirror +41. Move it to the left at 1/2 the speed of speed 1. Then, when the mirror 401 is in the position shown by the broken line, the image of the illuminated portion of the original O is formed on the photoreceptor 10 by the lens 1411.

If the mirror 401 of the reading optical system 40 is placed in the position shown by the solid line, the image of the illuminated portion of the original O is formed on the color solid-state image sensor 403.

- The filter device F has a red filter F1, a green filter F2, a blue filter F3, and a neutral denture filter F4 (hereinafter abbreviated as ND filter Fll), and each filter is selectively arranged in the optical path of the exposure optical system. It is now possible to do so.

The eraser 18 is composed of an LED array 181 and a convergent light transmission array 182.

The holder 28 is drum-shaped and serves to hold the transfer paper S, and rotates in the direction of the arrow as the photoreceptor 1O rotates.

Now, in the color solid-state image sensor 403, a large number of minute light receiving elements are closely arranged in a row in a direction orthogonal to each other in the drawing of FIG. corresponds to individual light reception, and therefore,
One light receiving element is frozen at a time.

Each photodetector is covered with a tiny filter. These filters come in three colors: red, green, and ■.

They are arranged cyclically in blue order. Therefore, when looking at three adjacent light receiving elements, one of them is equipped with a red filter, the other with a green filter, and the other with a back filter.

These three light receiving elements correspond to one pixel on the original, that is, an area of 125μ77L×125μm on the original. Therefore, if reading is performed while scanning original O, original 0 will have red and green for each pixel.

The color is separated into blue and read.

In FIG. 1, if the length of the document O in the left-right direction is ``It'' and the length in the direction perpendicular to the drawing is 22, it is read as a matrix in rows and N columns and separated into colors. In IJ Nox, the pixel at the position of m row and n column will be denoted as pixel i,,t,,).

On the other hand, the LED array 181 of the eraser 18 has 125
/zm x 125 μm small light emitting elements (LEDs) having a light emitting area are closely arranged in a direction perpendicular to the drawing in Fig. 1, and these light emitting elements (hereinafter referred to as LEDs) can be used in any combination to emit light. It's getting wet.

When one of the LEDs is made to emit light, the same-magnification image of the light-emitting LED is formed on the photoreceptor due to the imaging action of the converging light transmitter array 182. Therefore, the eraser 18 can erase the electrostatic latent image on the photoreceptor 10 pixel by pixel.

Here, regarding the color copying process when the present invention is not implemented in the color electronic copying apparatus shown in FIG.
Explain the outline.

When the original O to be copied is placed on the original placing glass 16 as shown in the figure and the apparatus is operated, the mirror 401 of the reading optical system 40 for reading the original 0 is placed in the position shown by the solid line, and then the original 0 is illumination scanning is performed, and the color original O is scanned by the color solid-state image sensor 403, which scans the three primary colors red, red, and red for each pixel.
The signal is separated into green and blue and converted into a signal.

By the way, prior to reading the original 0, the image of the standard density plate 42 is projected onto the color solid-state image sensor 403, and the contents of the standard density plate 42 are read.

The standard density plate 42 has a gray scale of 16 levels,
Its contents are as shown in Table IK.

Table 1 Concentration numbers 1 to 16 are convenient indicators given to the corresponding concentrations.

A read signal from the reference density plate 42 is sent to a microcomputer (not shown).

Next, manuscript O is read, but manuscript 0 is
It has a chromatic image and an achromatic image on a white background. this house,
The chromatic image portion is called a color image portion, and the portion other than the color image portion is called a neutral portion.

Now, the reading signal of the original O is composed of three heel signals for each pixel. That is, the signal is read after being separated into red, green, and blue colors. Therefore, the signals separated into red, green, and W at pixel (m, n) are respectively R(m, n).

Let G (m, n) and B (m, n).

Now, the read signal R (m, n) etc. of the original O is sent to the aforementioned microcopy/peter, and is compared with the contents of the standard density plate 42 inputted earlier. 16. That is, the signal R (m, n) has a density number of 5 (m, i) and G (m, i). (m, n), B(m, n) is converted to LB(m, i).

Then, it is determined for each pixel whether the pixel belongs to the color image portion or the bi-tral portion.

Achromatic images are the same no matter what color they are separated into, so
For pixels (,,,z), if LB(m, n
) = Lg (m, n ), and LG (m, n
) = LB (m.

7Z), the pixels (m, n) belong to the bi-tral part; otherwise, they belong to the color image part.

Subsequently, the mirror 401 is placed in the position shown by the broken line, the photoreceptor 10 is rotated, and uniform charging is performed by the charger.

Subsequently, the original O is illuminated and the photoreceptor 10 is exposed. At this time, the red filter of filter unit #F is placed in the exposure optical path. Therefore, the electrostatic latent image formed on the photoreceptor IO corresponds to an image separated into red. Therefore, this electrostatic latent image will be referred to as a red color separation latent image of the original O.

When this color-separated latent image passes through the erase section of the eraser 18, the microcomputer 181 selects the LE corresponding to the pixel belonging to the neutral section of the L, ED array 181.
Make D emit light. As a result, the latent image portion of the color separation latent image corresponding to the bilateral portion is erased.

Thereafter, this electrostatic latent image is developed by the developing device 20 using a magnetic brush development method using a funder, that is, toner colored in 777 colors (based on the relationship between red and complementary colors in color separation). . Thus, a visible image of 777 colors is formed on the photoreceptor 10 and moves as the photoreceptor rotates.

The leading end of the transfer paper S is clamped to the holder 28 according to the sequence of the process, and as the holder 28 rotates, it is held so as to wrap around the circumferential surface of the holder 28, and the 777 possible colors on the photoreceptor 10 are held. superimposed on the visual image. At this time,
The transfer device 30 charges the holder 28 from the back side with a polar charge that electrically attracts the visible image, and transfers the visible image onto the transfer paper S using electric force. After the visible image has been transferred, the photoreceptor 10 is neutralized by a static eliminator 32, and residual toner is removed by a cleaner 34.

The green filter F2 of the filter device F is then placed in the exposure optical path and the same process is repeated. The color-separated latent image formed at this time corresponds to the original image color-separated into green, but the latent image portion corresponding to the neutral portion is erased by the eraser 18, and developed with magenta toner by the developing device 22. The resulting magenta visible image is transferred onto the transfer paper S so as to be superimposed on the cyan visible image.

Blue filter F4 of filter device F and developing device 2
A similar process is repeated using 4.

The developing device 24 uses yellow toner.

Finally, the ND filter F4 of the filter device F is placed in the exposure light mist, and an electrostatic latent image that is not color separated (referred to as a non-color separated latent image) corresponding to the document 0 is formed on the photoreceptor 10.
is formed. In this electrostatic latent image, the latent image portion corresponding to the color image portion is erased by the eraser 18.

This electrostatic latent image is developed by a developing device 26 using black toner.

When the black visible image thus obtained is transferred onto the transfer paper S, the transfer paper S is separated from the holder 28 and transferred to the fixing device 16.
The toner image is fixed on the paper, and the paper is ejected from the apparatus as a color copy.

The above is a rough outline of the color copying process when the present invention is not implemented in the example of the apparatus shown in FIG.

The present invention will be explained in detail by taking this color copying process as an example.

In carrying out the present invention, each of a plurality of electrostatic latent images necessary to obtain one color copy is digitally erased in an area modulation manner at an erase rate based on image information.

In the above process example, the number of electrostatic latent images required to obtain one color copy is 4, and these are the latent I# portion corresponding to the neutral area removed from each color separation latent image. , and the non-color separated latent image, excluding the latent image portion corresponding to the color image.

Now, the gradation reproduction curve plotted at the beginning is generally the second
It becomes an S-shaped curve as shown in curve 2-1 in the figure. Therefore, in general, low image density areas of the original (so-called illite areas)
The reproducibility of gradation is poor in high image density areas (so-called shadow parts).

In the formation of an electrostatic latent image, if exposure is relatively insufficient, the gradation reproduction curve will be as shown by curve 2-3. In this gradation reproduction curve 2-3, the reproducibility of the highlight portion is good. On the other hand, if the exposure is relatively overexposed, the tone reproduction curve becomes curve 2-2, and the tone reproduction in a part of 7 yards becomes better.

In addition, although underexposure and overexposure are referred to as relative here, this means the following. What determines the electrostatic latent image is charging and exposure. When the amount of charging is large,
Even if the exposure amount is normal, the gradation reproduction curve will be as shown in curve 2-3, and even if the exposure amount is small, if the charging amount is made small, the gradation reproduction curve will be as shown in curve 2-2.

Therefore, the case where a gradation reproduction curve like curve 2-3 is given is called relatively underexposure.

Now, the tone reproduction curve is the relationship between image densities,
If we take the density of the original image on the horizontal axis and the electrostatic latent image potential on the vertical axis, the relationship between the original image density and the latent image potential is shown by the curve in Figure 3 for relatively underexposed electrostatic latent images. It will be something like 3-1. When this electrostatic latent image is developed, a tone reproduction curve 2-3 is obtained.

Incidentally, the ideal tone reproduction curve is the broken line in FIG. 2, that is, a straight line with an inclination of 45 degrees.

Therefore, if we consider an ideal electrostatic latent image that gives such an ideal gradation reproduction curve and plot the relationship between the original image density and the latent image potential, we will get something like the curve 3-2 in Figure 3 (
Since the amount of toner adhesion during development is not proportional to the latent image potential itself, this relationship is also not a straight line.)

For example, in FIG. 3, if we consider a document image density of X, the relatively underexposed electrostatic latent image has a t value of t corresponding to ΔV at the image density X compared to the ideal latent image. It can be considered that the exposure is insufficient only outside the phase difference.

Therefore, in the present invention, an electrostatic latent image that is relatively underexposed is formed in advance, and then this electrostatic latent image is digitally erased with an eraser, resulting in a gradual reduction from the highlight area to the negative area. The basic idea of the present invention is to obtain a visible image with good tonal reproducibility.

Hereinafter, a detailed explanation will be given based on the example of the apparatus shown in FIG.

(a) Formation of an electrostatic latent image that is relatively underexposed.

In order to form an electrostatic latent image that is relatively underexposed, the amount of light emitted by the lamp 1400 is reduced, but the charger 12
The amount of charge on the char/child 12 is increased, or the amount of light emitted from the lamp 140 is decreased to increase the amount of charge on the char/child 12. In any case, by setting one or both of the charging conditions and the exposure conditions as described above, an electrostatic latent image (referring to the relationship between image density and potential) as shown by curve 3-1 in FIG. 3 is formed. Do it like this.

(b) Determination of digital erase rate.

In order to obtain an ideal electrostatic latent image (curve 3-2 in FIG. 3), it is possible to know in advance the amount of exposure that should be used for each image density.

Therefore, for each image density, this ideal exposure amount was determined, and when such an exposure amount was set, what the output of the color solid-state image sensor 4o3 would be when reading a document was investigated and plotted. Then, for example, a curve like the curve 4-1 in FIG. 4 is obtained.

For example, in the case of curve 4-1 in FIG. If the signal can be obtained, the image density of the visible image on the transfer paper will also be the density number 9.

Therefore, the relationship of curve 4-1 in FIG. 4 is stored in the microcomputer.

Since manuscript O is read prior to the copying process,
From the read image information, the relationship between the density of the color image portion of each color separated image and the signal output, and the relationship between the density output of the bi-tral image portion are also determined.

For example, assume that the relationship between density and output in a color image portion separated into red is as shown by curve 4-2 in FIG. 4.

Curves 4-1 and 4-2 are both exponential functions. Therefore, if curve 4-1 is e &12' (x is concentration) and curve 4-2 is ek2x, then (l - (gkl'' - gk22'
)) Let X 100 be the erase rate for density X. When this erase rate is, for example, 40% with respect to the density It erases at 40 inches. That is, if the area of the latent image portion is Z, then the latent image with an area of 0.42 is erased.

(c) Digital erase.

Digital erase is performed according to the erase rate determined as described above, but it is not necessary to perform this digital erase in 1% increments. 10% increments are sufficient.

Therefore, consider, for example, determining the erase rate with a pixel array of 3 rows and 3 columns as one unit, and performing digital erase at an erase rate in 11% increments.

Table 2 In Table 2, the erase rate is the erase rate determined as described above, and the actual erase rate is the erase rate when actual erasing is performed by the eraser.

3 rows and 3 columns) l) The number of pixels to be erased within this l unit is increased by one as the erase rate increases by 11% in the nox-shaped pixel array.

FIG. 5 shows the state of dental erase when the actual erase rate is 33 times. Three pixels marked with diagonal lines have been erased.

In this manner, digital erasure is performed on the plurality of electrostatic latent images necessary to obtain a color copy image. The erasure rate is determined and erased by a microcontroller.

Performed and controlled by Peter.

In terms of the apparatus shown in FIG. 1, in each color separated image, the eraser 18 erases the latent image portion corresponding to the bilateral image, and digitally erases the latent image portion corresponding to the color image at a predetermined erasing rate. and the non-color separated latent image is
While the latent image area corresponding to the color image is erased,
The latent image area corresponding to the traral image is digitally erased.

(Effects) As described above, according to the present invention, a novel gradation correction method in a color electronic copying system can be provided.

In the present invention, an electrostatic latent image that is relatively underexposed is formed in advance, and this electrostatic latent image is corrected according to the image density by digital erasing. The gradation can be improved, and the color gradation in a color copy image can be improved.

It goes without saying that the present invention can be applied to various color electronic copying systems such as the Carlnon system known in the art, and can also be applied to monochrome copying systems. Also,
In the above example, the case where the gradation line is 45 degrees is used as the standard for the relationship between the output of the solid-state image sensor and the density. You may choose.

[Brief explanation of the drawing]

FIG. 1 shows one example of a color electronic copying apparatus capable of implementing the present invention.
The explanatory drawings showing only the main parts of an example, and FIGS. 2 to 5 are only diagrams for explaining the present invention. 0...Original, 10...Photoconductor, 12...Charger, 18...Eraser. Figure Z

Claims (1)

[Claims] Prior to the color electronic copying process, there is a reading process in which a color document is separated into colors and read using a solid-state image sensor, and a reading process in which a color document is separated into colors and read by a solid-state image sensor, and each of a plurality of electrostatic latent images required to obtain one color copy is separated. a step of determining a digital erase rate based on the image information read in the reading step; and a step of determining a digital erase rate based on the image information read in the above reading step; Color electronic copying, characterized in that it has a step of digitally erasing it in an area-modulated manner using an eraser, and forms an electrostatic latent image with relatively underexposure under predetermined charging conditions and/or exposure conditions. A gradation correction method in the method.