JPS6172260A - Gradation capacity correcting method of color electronic copying system - Google Patents

Abstract

PURPOSE:To obtain always color copies of good hue independently of originals by selecting a gradation reproducing curve in accordance with the color separation image of each color of a picture and minimum and maximum image densities of a neutral image and correcting an electrostatic image by digital erasing if necessary. CONSTITUTION:The processing is different by the combination between a maximum image density DM and a minimum image density DN. In case of DN=0 and DM=1.4, a relatively smaller exposure is selected to realize a gradation reproducing curve 3-2, and an electrostatic latent image is formed. In comparison with an ideal gradation reproducing curve 3-3, the electrostatic latent image according with the gradation reproducing curve 3-2 is underexposure in the density area except the highlight part. An erase rate is determined on a basis of the difference of exposure between gradation reproducing curves 3-2 and 3-3, and the electrostatic latent image is formed to realize the gradation reproducing curve 3-2, and an electrostatic latent image part corresponding to the neutral image of this electrostatic latenttimage is erased at the erase rate in accordance with an image density (x), and thus, the gradation reproducing curve 3-3 is realized, and erasing is performed to change the area.

Description

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

(Technical background) The original is color separated, and the electrostatic latent image corresponding to the color separation image of the color image is developed with toner colored in a complementary color to the color of the one-color separation, and then A color electronic copying system is intended in which an electrostatic latent image corresponding to an image is developed with black toner.

In the case of such a color electronic copying system, if the colors of color separation are red, green, and blue, one color copy consists of visual images of cyan, maze/yellow, and yellow, and a black visible image. However, in color copying, the best color tone is the lowest side I# of each color image! The density is the background density of the transfer paper; the maximum image density is the black visible image,
1.4 for cyan visible image. Maze/re visible image 1.2.
This is a case where the value is 1.0 in the Yellow Town visual image.

However, images obtained by color-separating the color image part of the original or two--
The lowest and highest image densities of toral images vary, and even if these are faithfully reproduced in color, it is not possible to obtain a color reproduction with good color tone as described above.

(Purpose) The present invention has been made in view of the above circumstances, and provides a novel gradation correction method that can always obtain color copies with good color tone, regardless of the type of original. The purpose is to provide.

(composition) The present invention will be explained below.

In the present invention, a color original to be copied in color is
Prior to the color electronic copying process, the colors are separated and read by a solid-state image sensor.

A color original has a chromatic image and an achromatic image. Among these images, the chromatic image is called a color image, and the remaining part is called a bi-tral image.

Reading by the solid-state image sensor may be performed using a color separation filter and three solid-state image sensors, or may be performed using a single-chip color solid-state image sensor described later.

Based on the read image information, the maximum and minimum density values in each color separation image and bi-toral image of the color image portion are detected.

Formation of electrostatic latent images corresponding to the individual visible images is performed so that the minimum image density of each of the visible images necessary to obtain one color copy is a predetermined value. At this time, the relative exposure amount is adjusted, and the formed electrostatic latent image is digitally erased as necessary. This digital erase is performed using area modulation. Note that it is also possible to perform this digital erase after charging the photoreceptor and before exposing the image. That is, it is also possible to erase the portion of the electrostatic latent image that should be erased in advance, prior to forming the latent image.

FIG. 1 shows a color electronic copying apparatus to which the present invention can be applied.
An example is shown. Since this figure is an explanatory diagram, the relative size relationships of the dimensions of each part are not necessarily accurate.

First, a brief description of the copying apparatus will be given, followed by a brief description of the color copying process, followed by a description of how the present invention is practiced.

In the first factor, 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, As1se
A photoconductive material having panchromatic spectral sensitivity, such as s, is used.

Around the photoreceptor 10, a charger 12, an eraser 18, developing devices 20, 22, 24, 26, a holder 28, . 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 1. The document is fixed on the document placement glass 16 in a flat manner.

144 and a filter device F.

Further, the reading optical system indicated by the reference numeral 40 is a mirror 40.
1.S/Z 402, single-chip color solid-state image sensor 403
The mirror 401 is configured to be able to selectively move between the attitude shown by the solid line and 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 a transfer paper.

Now, returning to the exposure optical system 14, when scanning the original O, the lamp 140 is made to emit light, the lamp 140 and the plane mirror 141 are integrally moved to the left, and at the same time, the roof mirror 142 is moved to the side of the plane mirror 141. Move to the left at a speed of μ of movement speed. 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 projected onto the photoreceptor 1 by the lens 144.
The image is formed on 0.

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 document 0 will be formed on the color solid-state image sensor 403.

The filter MF has a red filter Fl, an At color filter F2, a color filter F3, and a neutral filter F4 (hereinafter abbreviated as ND filter F4), and selects each filter. It can be placed in the optical path of the exposure optical system.

The eraser 18 includes an LED array 181 and a convergent light transmission array 182.

The holder 28 is drum-shaped and is used to hold the transfer paper S for transferring a visible image, and rotates in the direction of the arrow as the photoreceptor 10 rotates. ing.

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 perpendicular to the drawing of FIG. Each light-receiving element corresponds to an image element of □μm×-μm on the original #IO, and therefore one light-receiving element signals the image element of the alluvium at a time.

Each individual light-receiving element is covered with a tiny filter. The colors of these filters are red,
There are three types, green and blue, and they are arranged cyclically in the order of red, green, and blue. Therefore, when looking at three adjacent light receiving elements, one of them is covered with a red filter, the other with a green filter, and the other with a blue filter.

These three light receiving elements correspond to one pixel on the original, that is, an area of 125 μm×125 μm on the original. Therefore, if the document O is scanned with illumination and read, the document O will be read by separating the colors into red, green, and blue for each pixel.

In Fig. 1, the length of the document O in the left and right direction is tlN =
As Zt, the original 0 is matline 125μ in M rows and N columns.
This means that the image is converted into a 3D image and read by color separation. The pixel located at the m row and n column in this pixel's Mad 1 node will be referred to as pixel (m, n).

On the other hand, the LED array 181 of the eraser 18 has 125
A minute light emitting element (
IJD)t-The light emitting elements (hereinafter referred to as LFJD) are closely arranged in a direction perpendicular to the drawing of FIG. 1, and can emit light in any combination.

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

An outline of the color copying process in the color electronic copying apparatus shown in FIG. 1 will be explained below. The present invention has not yet been applied to this process.

When an 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 that reads the original 0 is placed in the position shown by the solid line, and then the original O 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 403K, and the contents of the standard density plate 42 are read.

The standard density plate 42 has 11+ gray scales in 16 steps.
, the contents are as shown in Table 1.

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

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

Next, manuscript O is read, but manuscript O is
It has a chromatic image and an achromatic image on a white background. As mentioned above, the chromatic image portion is called the color image portion, and the portion other than the color image portion is called the neutral portion.

Now, the reading signal of the original O is composed of three types of signals for each pixel. In other words, it is a signal that is read after being separated into red, green, and blue colors. Therefore, pixel (m, n)
The signals separated into red, green, and blue are respectively R(m, n).

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

Now, the reading signal R (m, n) etc. of the original O is sent to the aforementioned microcomputer and compared with the contents of the reference density board 42 that have been inputted previously. Converted to crab. That is, the signal R (m, n) becomes the density number LR (m, n), and the signal G (m, n) becomes L G ("+
n), B(m,n) is converted to L B (''-+ n).

Next, it is determined for each pixel whether the pixel belongs to the color image area or the neutral area.

Achromatic images are the same no matter what color they are separated into, so
For pixel (m, n), if LR (m, n) =
LG (m, n), and t, G (m, n) = LB (m,
n), the pixel (m, n) belongs to the neutral part; otherwise, it belongs 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 device F is placed in the exposure optical path. Therefore, the electrostatic latent image formed on the photoreceptor 10 corresponds to an image separated into red. Therefore, this electrostatic latent image is converted into red color separation a of document 0.
(I'll call it 11.

When this color-separated latent image passes through the erase section of the eraser 18, the microcomputer
81, LED corresponding to the pixel belonging to the neutral part
to emit light. As a result, the latent image portion of the color separation latent image corresponding to the neutral portion is erased.

Thereafter, this electrostatic latent image is developed by the developing device 20 using a cyan toner, that is, a toner colored in 777 colors (complementary colors to red in the color separation) using a magnetic brush development method. 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 process question, and as the holder 28 rotates, the transfer paper S is held so as to wrap around the surface of the holder 28, and the cyan on the photoconductor IO is held. Superimposed on the visible image of color. 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 F, and transfers the visible image onto the transfer paper S using the 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 bi-tral 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.

Placement filter F4 of filter device F and developing device 2
A similar process is repeated using 4. Developing device 2
In No. 4, yellow toner is used.

Finally, the ND filter F4 of the filter device F is placed in the exposure optical path, and an electrostatic latent image that is not color separated (referred to as J1 color separated latent image) corresponding to the original 81iO is formed on the photoreceptor 10. . In this electrostatic latent image, the latent image portion corresponding to the color image portion is erased by the eraser 18.

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

When the black visible image obtained by smelling 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 an 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 now be described in detail with respect to this example device. In the above example, the visible images necessary to obtain one color copy are the seven-color Noah, maze/white, and yellow color visual images, and the black visible image. The minimum image density is set to be the background Q degree of the transfer paper S. The maximum image density is 717 colors, and the visible black image is 1.
4, and 1 for the magenta colored town visual image.
2. The yellow color town visual image is set to 1.0. How this is done is explained below.

In electronic copying systems, generally the image density of the original is plotted on the horizontal axis, and the image density of the visible image on the transfer paper is plotted on the horizontal axis.
A curve plotting the relationship between both image densities is generally called a gradation reproduction curve.

Curves 2-1.2-2.2-3 shown in FIG. 2 are all gradation reproduction curves.

Although the gradation reproduction curve changes depending on the relative exposure amount, development conditions, transfer conditions, etc., the relative exposure amount will be explained here as an example.

The relative exposure amount refers to P/V 1t, which is obtained by dividing the photoreceptor exposure tP in mll1'*exposure value by the photoreceptor charging potential (■). Therefore, in order to increase (decrease) the relative exposure amount, the exposure amount P is increased (decreased).

Alternatively, the charging potential V may be decreased (increased), or the exposure amount P may be increased (smaller) and the charging potential V
You can make it smaller (larger).

In FIG. 2, a tone reproduction curve 2-1 is a case where electrostatic latent image formation is performed with a normal relative exposure amount.

As the relative exposure amount increases, the tone reproduction curve becomes curve 2.
-3, and when the relative exposure amount becomes small, the gradation reproduction curve becomes like curve 2-2.

In the document image density, a low-density area with a density of 0.5 or less is called a highlight part, and a high-density area with a density of 1.0 or more is called a /yad part.

Decreasing the relative exposure amount generally means increasing the slope of the highlight area in the gradation reproduction curve, and conversely, as the relative exposure amount becomes thicker, the slope of the gradation reproduction curve increases. Some slopes increase.

Now, taking the case of a neutral image as an example, in the visible image obtained by transferring this onto a transfer paper using black toner, the minimum image density is the background density of the transfer paper and the maximum image density is 1.4. Explain how to do this.

Note that the background density of transfer paper is generally about 0.07, but for the sake of simplicity, this will be explained as O.

The gradation reproduction curve in FIG. 2 is also drawn with the background density as O.

Now, as described above, the color original O is separated into the three primary colors of red, green, and spine and read by the reading optical system. The image is then separated into a color-1iiiI image area and a neutral image area.

The maximum value of the output of the solid-state image sensor in the neutral image portion thus separated corresponds to the lowest image density of the neutral image, and the minimum value of the output corresponds to the highest image density.

Thus, the highest image 4 degrees and lowest image density in the neutral image are detected. The correspondence between the output of the solid-state image sensor and the image density can be determined by comparing the output with the read output of the standard density plate 42.

DM the highest image gIa degree detected by the method described above.

Let the minimum image density be DN.

The following handling differs depending on the combination of DM and DN.

First, as shown in FIG. 3 (1), DM=0.7, DN=0.
In the case of 0, this corresponds to a case where the neutral image has an overall low density.

At this time, select a relative exposure amount (small) that gives gradation reproduction curve 3-1, and use this exposure condition to perform the copying process, that is, charging, image exposure, and latent image for color image area. Erasing, development with black toner, and transfer may be performed.

Next, as shown in FIG. 3 (■), DN=O, DM=1
．． Consider case 4. In this case, the gradation reproduction curve 3-
3 is ideal, but in reality such a tone reproduction curve does not exist. Therefore, by selecting a relatively small amount of exposure, the entire electrostatic latent image is formed so as to realize a gradation reproduction curve such as curve 3-2 in FIG. 3(It).

Now, when we compare the curve 3-2 with the ideal gradation reproduction curve 3-3, we can consider that the electrostatic latent image giving the gradation reproduction curve 3-2 is underexposed in the density region except for the highlight area. I can do it.

Now, considering the image density It is uniquely determined depending on degree 2 and song m3-3. Therefore, let this exposure amount be tentative KIo(z).

Next, regarding the tone reproduction curve 3-2, if the exposure amount for image density 2 is r l F, then I o (z) I
x (z) is the exposure difference between the tone reproduction curves 3-2 and 3-3 regarding the image density X.

Therefore, (L (z) I I (Z) ) Xro
The erase rate I S (z) (%
display), and once an electrostatic latent image is formed so as to realize the gradation reproduction curve 3-2, the electrostatic latent image portion corresponding to the bi-tral image of this electrostatic latent image is If erase is performed at an erase rate I S (Z) according to
It becomes something like -3.

Therefore, in the present invention, the above-mentioned erasing is performed in an area modulation manner as follows.

In addition, as a practical problem, this area modulation erase is
It is not necessary to perform the erase rate in 1-chip increments;
It is sufficient to do this in increments of about 0%. Therefore, here we will consider performing the erase in nine stages.

That is, when the erase rate is O~1.1%, it is 11%.
When the erase rate is 12 to 23%, an erase of 23% is performed, and below, the erase rate is set to 11.
The data is increased in increments, and when the erase rate is 89 to 100%, 100% erasure is performed.

To execute this, the erase rate is determined by taking 9 pixels in 3 rows and 3 columns as 1 unit, and when performing 111 erases, the latent image for 1 pixel is erased. The number of pixels to be erased is increased by 1 each time the number of pixels increases in 11-inch increments.

FIG. 4 shows the erase situation when the erase rate is 23 to 34 inches. Diagonal line f: The shaded area indicates the erased pixel-corresponding latent image.

This kind of erasing is called area modulation digital erasing.

Next, as shown in FIG. 3 (III), DM = 1.0
In the case of DN=0.5, the relative exposure amount is determined so as to realize a gradation reproduction curve such as curve 3-5, and the bi-tral image of the electrostatic latent image formed with this exposure amount is In the corresponding areas, the highlight areas and intermediate low density areas are digitally erased so that the final gradation reproduction curve becomes like curve 3-6.

Also, as shown in Figure 3 (IV), DM = 1.4,
When DN=0.5, the relative exposure amount is reduced to form an electrostatic latent image that gives gradation reproduction curve 3-7, and the low image density part and the high image density part of this electrostatic latent image are The final gradation reproduction curve becomes curve 3-8 by digitally erasing the area.

Once the combination of DM and DN is determined, it is determined what kind of gradation reproduction curve the electrostatic latent image should be formed to realize.

In other words, this means that the relative exposure amount is determined depending on the combination of DM and DN. Further, conditions for digital erasure can also be uniquely determined for an electrostatic latent image formed under exposure conditions determined according to the combination of DM and DN.

Therefore, the relative exposure amount and digital erase conditions corresponding to the combination of DM and DN (this may be a combination of density numbers corresponding to DM and DN) are stored in the microcomputer, and The charging and exposure conditions may be set according to the stored contents, and the eraser 18 may be controlled. Note that FIG. 3 only shows four typical examples of combinations of DM and DN, and in reality, various combinations exist. There are 120 combinations of density numbers 1 to 16.

Although the case of the bi-tral image part has been described above as an example, the same applies to the case of each color separated image of the color image part.

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

Note that a well-known dot fluorescent tube array can be used instead of the LED array of the eraser.

In this method, tone reproduction curves are selected according to the minimum and maximum image densities of each color separation image of a color image and a neat neutral image, and if necessary, the electrostatic latent image is corrected by digital erasing, so the original Regardless of the condition, color copies with good color tone can always be obtained.

Note that as the image forming optical number of the eraser, in addition to a converging light transmitting body array, a strip lens array or a roof mirror lens array can be used.

[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 4 are only diagrams for explaining the present invention. L...Photoreceptor, F...Filter device, 18...
Eraser, 40...reading optical system

Claims (1)

[Claims] In a color electronic copying method in which a document is separated into colors and a neutral image is developed with black toner, the color document is color separated and read using a solid-state image pickup device prior to the color electronic copying process. Based on the image information obtained, the highest and lowest values of density in each color separated image and neutral image of the color image area are detected, and the image density is determined in each of the visible images necessary to obtain one color copy. When forming electrostatic latent images corresponding to the individual visible images, the relative exposure amount is adjusted so that the lowest value is the background density of the transfer paper and the highest value is the predetermined value. A method for correcting gradation in a color electronic copying system, characterized by digitally erasing an electrostatic latent image in an area-modulated manner as necessary.