JPH08240943A - Recording density correcting method for image recorder - Google Patents

Abstract

PURPOSE: To cope with the non-linearity of a density curve and to improve the quality of a recorded image by making plural density levels used in the case of recording the image at a spot previously decided as a patch ununiform. CONSTITUTION: Every time the data of one screen is transmitted after specifying M(magenta) first, a color specifying signal is changed in order of C(cyan), Y(yellow), and BK(black), and then a signal converted by a density conversion table in a density correction part 202 is inputted to a PWM(pulse width modulation) part 203. In the PWM part 203, eight bits of image signal is synchronized with the rising of an image clock VCLK 210 by a latch circuit 204, converted into analog voltage by a D/A converter 205 and inputted to an analog comparator 206. Meanwhile, triangular wave is generated in a triangular wave generation part 207 through the image clock 7 and inputted to the comparator 206. Two signals of the analog voltage and the triangular wave are compared, and the signal subjected to PWM processing is inputted from the output of the comparator 206, and inverted by an inverter 208 so as to obtain a PWM signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a recording density correction method for an image recording apparatus such as a laser printer,
In particular, the present invention relates to a recording density correction method of an image recording apparatus for recording a stable gradation image on a sheet in multi-gradation image recording.

[0002]

2. Description of the Related Art In a multi-gradation image recording apparatus, the relationship between an input density level signal and the density actually printed is linear, and the density printed for the same density level signal is the temperature or humidity. It needs to be constant regardless of the situation. However, especially in the case of a laser beam printer, it is difficult to maintain the linearity of the density and the consistency of the density due to the characteristics of toner, electrical, and physical causes, and therefore, the following density correction is generally performed. Target. That is, first, data of a plurality of density levels for density measurement is prepared, and each density level data is printed on a corresponding portion before printing on paper (hereinafter, printing is performed for each density level. The image area is called a patch), and then the density of each patch is measured by a sensor. As a result of the measurement, the deviation from the desired density is measured, and the density is corrected by the deviation.

As a correction method, there is a method of preparing a density conversion table called a look-up table (LUT) and converting a density level signal by the table, or a method of changing the voltage in an analog manner. However, for example, in a printer having 256 gradations, 256
It is difficult in practice to print all the density levels of different types (that is, hit 256 patches). Particularly, in a color image recording apparatus, yellow, magenta, cyan, and
Since the densities of the four colors of black must be measured, the number of patches per color is limited accordingly. In the conventional method, some density levels are selected so that the differences between the levels are uniform, the patch is printed and the density is measured. It was estimated and interpolated.

[0004]

However, in the above-mentioned conventional method, when the number of patches printed is limited to a certain number or less, the level difference between the measured density levels becomes large to some extent. The wider the level difference, the more inaccurate the interpolation estimation of the intermediate density level, which is not measured, and as a result the accurate correction cannot be performed. Above all, accurate correction cannot be performed in the high density region and the low density region where the slope of the density curve changes subtly. Especially, in the low density region where the density change is easily noticeable, there is a drawback that the non-linearity and instability of the density are noticeable due to the deviation of the correction.

Therefore, an object of the present invention is to provide a recording density correction method for an image recording apparatus, which has improved density correction performance.

[0006]

In order to achieve such an object, the invention according to claim 1 is defined for a plurality of preselected density levels before performing the entire recording on a recording medium. A recording density of an image recording apparatus for recording an image at a specific place on the recording medium, measuring the density of the recorded image, and correcting the density of the entire recording by referring to the measured density. In the correction method, the plurality of density levels are selected so that the differences between the levels are not uniform.

The invention of claim 2 is characterized in that, in addition to the invention of claim 1, the image recording apparatus records on the recording medium by means of a laser.

According to a third aspect of the invention, in addition to the first or second aspect of the invention, the image recording device performs color image recording.

According to a fourth aspect of the present invention, in addition to the third aspect of the invention, the image recording apparatus has a number of density levels for density measurement.
And the difference between each level is yellow, magenta, cyan,
The feature is that each color is uniquely determined according to the visual characteristics of each color of black.

According to a fifth aspect of the present invention, in addition to the third or fourth aspect of the invention, the image recording device is yellow, magenta,
The feature is that after recording all four colors of cyan and black, the density of each color is measured in order.

According to a sixth aspect of the present invention, in addition to the third or fourth aspect of the invention, the image recording apparatus is yellow, magenta,
It is characterized in that the density of each color of cyan and black is measured every time one color is recorded.

[0012]

According to the first aspect of the present invention, the non-linearity of the density curve is dealt with by making a plurality of density levels used when an image is recorded at a predetermined location as a patch nonuniform.

According to the present invention, the image quality of the electrophotographic multi-tone color image recording apparatus is improved. By selectively using the inventions of claims 5 and 6, the density measurement order and the patch recording order can be made to correspond to a suitable order of the image forming apparatus.

[0014]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(Embodiment 1) In this embodiment, the construction of an image recording apparatus using a color electrophotographic technique is particularly shown. Further, in this embodiment, the input density level is assumed to be sent as an image signal in frame sequential with 8 bits for each color of M (magenta), C (cyan), Y (yellow), and BK (black).

FIG. 1 shows an example of the structure of a color image recording apparatus to which the present invention is applied. In FIG. 1, the photoconductor drum 100 is uniformly charged to a predetermined polarity by the charger 101, and a first latent image of magenta, for example, is formed on the photoconductor drum 100 by exposure with the laser beam light L. Then, in this case, the required developing bias voltage is applied only to the magenta developing unit D _m to develop the magenta latent image, and the magenta first toner image is formed on the photosensitive drum 100.

On the other hand, the transfer paper P is fed at a predetermined timing, and immediately before the leading end of the transfer paper P reaches the transfer start position, the transfer bias voltage (+1.8 KV) having the opposite polarity (eg, positive polarity) to the toner is applied to the transfer drum. The first toner image on the photosensitive drum 100 is transferred to the transfer paper P, and the transfer paper P is electrostatically attracted to the surface of the transfer drum 102. After that, the photosensitive drum 100 is cleaned by the cleaner 10.
The remaining magenta toner is removed by 3, and the process prepares for the next color latent image forming and developing process.

Next, a second latent image of cyan is formed on the photosensitive drum 100 by the laser beam L, and then the second latent image on the photosensitive drum 100 is formed by the cyan developing device D _c. It is developed to form a cyan second toner image. Then, the cyan second toner image is transferred to the transfer paper P in alignment with the position of the magenta first toner image previously transferred to the transfer paper P. In transferring the toner image of the second color, a bias voltage of +2.1 KV is applied to the transfer drum 102 immediately before the transfer paper reaches the transfer portion.

In the same manner, yellow, black third,
The fourth latent images are sequentially formed on the photosensitive drum 100,
Each of them is sequentially developed by the developing devices D _y and D _b , aligned with the toner images previously transferred to the transfer paper P, and the yellow, black third and fourth toner images are sequentially transferred, and thus, The four color toner images are formed on the transfer paper P in an overlapping state.

2 is a block diagram for receiving the RGB image signals and transmitting the print information to the laser drive section for the above print configuration. Hereinafter, yellow, magenta, cyan, and black are referred to as Y, M, C, and
Represented as BK. A color processing unit 201 receives an RGB 24-bit image signal. This image forming apparatus has Y, M,
Since one screen is printed for each color of C and BK, the image data is frame sequential, that is, data for one screen of M, data for one screen of C, data for one screen of Y, and data for one screen of BK. Send in order. Therefore, the color processing unit 201 first converts the image data of one screen of RGB pixel by pixel into an 8-bit VDO (video) signal of M pixels.

Then, the C signal, the Y signal and the BK signal are converted in this order. In the color processing unit 201, the input RGB
The color designation signal sent from the formatter unit determines which color data of M, C, Y and BK is to be converted.

That is, for the color designation signal, first, M is designated and 1
Every time the data for the screen is sent, the order of C, Y, BK changes. Thereafter, the signal converted by the density correction unit 202 by the density conversion table described later is input to the PWM (pulse width modulation) unit 203. In the PWM unit 203, the 8-bit image signal is latched by the latch circuit 204, and the image clock VCL
The D / A converter 20 is synchronized with the rising edge of K210.
Converted to analog voltage at 5 and analog comparator 2
Enter in 06. On the other hand, the triangular wave generator 207 generates a triangular wave by the image clock and inputs it to the analog comparator 206. The analog voltage and the triangular wave signal are compared, and the output of the analog comparator 206 outputs P
The WM-processed signal is output and inverted by the inverter 208 to obtain a PWM signal.

Next, a method of correcting the recording density of the image forming apparatus will be described. When the power is turned on to the printer body, first, as shown in FIG.
The image signal of the magenta area as shown in (A) is read from the memory. The numbers 1 to 8 in FIG. 3A represent patches (numbers 3 to 7 are omitted). Each patch 1
8 indicates the density level data (numerical values 0 to 25)
5), the density level of which is determined by the table of FIG. 3, and the numbers in FIG. 3 correspond to the patch numbers in FIG. It is assumed that L1 to L8 in FIG. 4 represent the density levels of each patch and are selected so that L1 ≦ L2 ≦ L3 ≦ L4 ≦ L5 ≦ L6 ≦ L7 ≦ L8. Further, nothing is printed in the area other than the patch in FIG. Then, a latent image is formed on the photosensitive drum corresponding to this area. First, a magenta latent image is formed, and a magenta toner image is formed through the above process. At this stage, since the transfer paper is not conveyed, the toner image is directly placed on the patch portion of FIG. 3 on the transfer drum. .

FIG. 6 is a perspective view of the transfer drum. 500 is a transfer drum, 501 is a photosensitive drum, 50
2 is a developing device. The density of each of the eight patches 503 placed on the transfer drum 500 is measured by a density sensor 504. After the magenta measurement is completed, the area on the transfer drum shown in FIG. 3B is cyan, the area shown in FIG. 5A is yellow, and the area shown in FIG. 5B is black. Put the patches in order and measure the density in the same way as magenta. Here, the density levels corresponding to the patch numbers in FIGS. 3 and 5 are determined by the table in FIG. 5, like magenta. Further, four-color patches are placed on the transfer drum after the fourth rotation, as shown in FIG. Y, M, C, and BK in FIG. 7 represent yellow, magenta, cyan, and black patches, respectively. It can be seen from FIG. 7 that the patches of each color are arranged in order and are placed so as not to overlap each other.

As a result of the density measurement in this way, assume that the magenta density is as shown in FIG. In FIG. 8, the horizontal axis is the input density level and the vertical axis is the measured density. The white dots in FIG. 8 are the eight measured densities.
From this measurement result, the concentration curve under this condition is 702.
It is presumed to be like the line of. The densities for the remaining 248 density level data not measured by this estimated density curve are estimated, and the densities for the respective density data are corrected so that the density curve approaches the desired curve of 701. Here, the eight density level data measured as in the prior art are taken to have a uniform level difference as shown in FIG. 9 (A). As can be seen from FIG. 9, the level difference between the density levels is 32. The measured concentration at this time is shown in Fig. 1.
It becomes like an outline point of 0, and the density curve is easy to estimate at the intermediate density level (50 to 200) where the density curve is almost linear, and the true density curve (solid line) and the estimated density curve (dotted line) It can be seen that the error is small. However, it can be seen that the error is large when viewed only in the low density region of the density levels 0 to 50 and the high density region of 200 to 255, so that the accuracy of the density correction deteriorates in these two regions. This is because the density curve changes subtly in these two areas and it is difficult to estimate the shape of the curve. Particularly in the low density region where the density change is apt to be noticeable, if the density correction is performed inaccurately and the corrected density curve deviates from the desired curve, it becomes conspicuous as density variation.

In the present invention, the eight density data to be estimated are reduced in level difference between the low density level and the high density level (level difference 10 to 30) as shown in FIG. Increase the difference (level difference 6
0) Take. If the differences between the levels are made non-uniform in this way, the measured density will look like the white dots in FIG.
When viewed only in the low concentration range of -50 and the high concentration range of concentration levels 200 to 255, the concentration can be measured in detail, so the error between the true concentration curve (solid line) and the estimated concentration curve (dotted line) can be small, Therefore, the density correction can be performed with high accuracy.

Since the shape of the curve can be easily estimated in the intermediate density range, the density curve can be estimated with the same level of accuracy as in the conventional example. As a result, the corrected concentration curve as a whole can be brought closer to the desired curve.
Thus, it is possible to obtain a natural image in which the density changes smoothly from the low density area to the high density area. In this embodiment, the patch shown in FIG. 9B is selected as the patch to be measured, but basically, the highest density level 255 and the lowest visible density (8 to 10 in this embodiment) are selected at both ends. The selection method in the meantime may be selected with the above-mentioned matter in mind.

(Embodiment 2) In this embodiment, the basic structure of the image recording apparatus is the same as that in Embodiment 1, and the number of patches printable on the transfer drum is also the same as in Embodiment 1. . In the first embodiment, eight patches are printed for each color of Y, M, C, and BK, so that a total of 32 patches are printed on the drum. In the first embodiment, the concentration level to be measured is Y,
Although the same density level was selected for each color of M, C, and BK, in this embodiment, the number of patches to be measured for each color and the density level of each patch are individually selected according to the visual characteristics of each color.

First, since strict correction accuracy is not required for yellow, which has low visibility, the number of patches is four, and the density level to be measured is selected as shown in FIG. Next, of the remaining three colors, the density curve is the most stable, that is, for black for which it is easy to estimate the density curve, the number of patches can be estimated relatively accurately without increasing the number of patches so much. 8 and the concentration levels to be measured are selected as shown in FIG. 9B as in the first embodiment. Finally, for cyan and magenta, which are the most noticeable and have unstable density curves, the number of patches is 10 each, and the density to be measured is selected as shown in FIG.
By selecting in this way, it is possible to estimate the density curves in the low density regions of cyan and magenta fairly accurately. The above 32 patches are displayed on the transfer drum, for example, as shown in FIG.
You can put it like 2. In FIG. 13, Y, M, C, and BK represent yellow, magenta, cyan, and black patches, respectively. For example, M1 represents the first magenta patch. As described above, the number of patches measured for each color,
A more natural image can be obtained by changing the density level to be measured.

As described above, in the first embodiment, patches for all four colors are transferred. Thereafter, the density of each color patch is measured, the density curve is estimated, and a density conversion table for correction is created (see steps S10 to S30 in FIG. 14). On the other hand, in the second embodiment, the processing of printing the patch and creating the density measurement density conversion table is executed for each color (see steps S110 to S150 in FIG. 15). Which processing procedure is used may be determined according to the image recording method. Needless to say, the order of colors to be patched is not limited to this embodiment.

[0031]

As described above, according to the first aspect of the invention, the plurality of density levels used when an image is recorded at a predetermined location as a patch are made non-uniform to thereby make the density curve non-linear. deal with.

According to the present invention, the image quality of the electrophotographic multi-tone color image recording apparatus is improved. By selectively using the inventions of claims 5 and 6, the density measurement order and the patch recording order can be made to correspond to a suitable order of the image forming apparatus.

[Brief description of drawings]

FIG. 1 is a configuration diagram illustrating a configuration of a printing unit of a color image recording apparatus according to first and second embodiments.

FIG. 2 is a block diagram showing a configuration of an image signal processing system.

FIG. 3 is an explanatory diagram showing the contents of magenta and cyan patches of the first embodiment.

FIG. 4 is an explanatory diagram showing a relationship between patch numbers and density levels according to the first embodiment.

FIG. 5 is an explanatory diagram showing contents of yellow and black patches of the first embodiment.

FIG. 6 is a perspective view showing the relationship between the transfer drum and the density sensor of the first and second embodiments.

FIG. 7 is an explanatory diagram showing the patch contents of each color on the transfer drum of the first embodiment.

FIG. 8 is an explanatory diagram illustrating an example of a measured density of magenta according to the first embodiment.

FIG. 9 is an explanatory diagram for explaining how to select a density level according to the conventional example of the first embodiment and the present invention.

FIG. 10 is an explanatory diagram showing an example of measured densities according to a conventional example in the first embodiment.

FIG. 11 is an explanatory diagram showing an example of measured concentrations according to the present invention in the first embodiment.

FIG. 12 is an explanatory diagram showing the relationship between the numbers of yellow, magenta, and cyan patches of the second embodiment and the density levels.

FIG. 13 is an explanatory diagram showing the contents of patches of each color on the transfer drum of the second embodiment.

FIG. 14 is a flowchart showing a density measurement sequence of the first embodiment.

FIG. 15 is a flowchart showing a density measurement sequence of the second embodiment.

[Explanation of symbols]

 100, 501 Photoreceptor drum 101 Charging device 102,500 Transfer drum 103 Cleaner 201 Color processing unit 202 Density correction unit 502 Developer 503 Patch 504 Density sensor

Claims (6)

[Claims] 1. An image is recorded at a specific location on the recording medium defined for a plurality of preselected density levels before the entire recording on the recording medium, and the recorded image is recorded. In a recording density correction method of an image recording apparatus for measuring the density and correcting the density of the entire recording by referring to the measured density, the plurality of density levels are set so that the difference between the levels becomes uneven. A recording density correction method for an image recording apparatus, characterized in that 2. The recording density correction method for an image recording apparatus according to claim 1, wherein the image recording apparatus records on the recording medium with a laser. 3. The recording density correction method for an image recording apparatus according to claim 1, wherein the image recording apparatus records a color image. 4. The image recording apparatus is to individually determine the number of density levels for density measurement and the difference between the levels according to the visual characteristics of each color of yellow, magenta, cyan and black. The recording density correction method for an image recording apparatus according to claim 3, characterized in that: 5. The image recording apparatus according to claim 3, wherein after recording all four colors of yellow, magenta, cyan and black, the density of each color is measured in order. A recording density correction method for an image recording apparatus as described. 6. The image recording apparatus according to claim 3, wherein the density of each color of yellow, magenta, cyan and black is measured every time one color is recorded. 5. A recording density correction method for an image recording apparatus according to any one of 1.