JPS61283275A - Picture recorder - Google Patents

Abstract

PURPOSE:To obtain a half tone picture without having picture unevenness by converting the half tone picture signal of a low density component to a binary picture signal corresponded with a dot forming density pattern and converting the half tone picture signal of other that the low density component to the binary picture signal corresponded with a multi-valued density pattern. CONSTITUTION:The picture element data of a picture memory 62 assigned at an X address counter 63 and a Y address counter 64 is compared with the thresholds of matrix memories 65 and 66 assigned at a column counter 67 and a low counter 68 at each of digital comparator 69 and 70 and when it is larger than them, a black signal is outputted from the comparators 69 and 70. Either of the output signals from the comparators 69 or 70 is selected at a multiplexer 71 and it is sent to a laser printer 72 as the binary picture signal. When the low density level, for example, under 16 of a variable density picture signal component is converted to a binary picture out of the picture data inputted by the dot forming density pattern, so that it is converted to the binary picture at a density area prior to the generation of density unevenness, the density unevenness is suppressed to the utmost.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to an image that reproduces a pseudo halftone by transferring an image multiple times to the same recording medium according to an input halftone image signal. This relates to a recording device.

[Conventional technology]

Hitherto, dithering methods, density pattern methods, and the like have been proposed and put into practical use as methods for pseudo-reproducing tones using recording apparatuses whose halftone reproducibility is not very good. Regarding these methods, many applications have been filed including Japanese Patent Laid-Open No. 57-7!3977, and they are also described in detail in literature, so their explanation will be omitted. In these methods, attempts have been made to increase the number of gradations while the size of the matrix is limited.

An electrophotographic laser beam printer is an example of a recording device that applies these methods.

In this type of apparatus, a laser beam is used for main scanning using a rotating polygon mirror, and a photosensitive drum is rotated for sub-scanning. At this time, unevenness occurs in the main scanning direction due to the low processing accuracy and installation accuracy of the rotating polygon mirror, or rotational unevenness occurs due to the low rotational speed accuracy of the photosensitive drum, and density unevenness occurs in the sub-scanning direction of the output image, resulting in poor image quality. is deteriorating.

First, a process in which scanning unevenness becomes density unevenness in an electrophotographic laser beam printer will be described.

When the intensity of the laser beam and the main scanning time are TH1 and the average number of scanning lines per unit length in the sub-scanning direction is N, the average exposure per unit length in the sub-scanning direction is calculated by the following equation (1). Given.

E=I・THAN
・・・・・・ (1)
Here, if there is main scanning unevenness or rotational unevenness, the average number of scanning lines N will fluctuate, and if the variation in N is ΔN, then the variation in exposure amount ΔE can be calculated from the above equation (1) using the following equation (2). ) is given by the formula.

ΔE=I*TH・ΔN ・・・・・・(2
) When recording an image, one intensity factor is modulated, but if the intensity factor is kept constant to consider only unevenness, the main scanning time TH is constant, so the variation ΔE in the exposure amount is:
ΔEocΔN.

When a photosensitive drum is exposed to light, a potential latent image is obtained in an electrophotographic process, and the toner is developed, transferred to a sheet of paper, and fixed to give an image density, but generally the relationship between exposure amount and density is nonlinear.

Although the latitude of electrophotography is narrow, a part of it can be approximated by a straight line.For the sake of simplicity, let the density be DOCN, then the density variation ΔD will be ΔDocΔN.

Therefore, the relationship shown in equation (3) below is obtained.

Δl)/])ocΔN/N −・− (3) When considering the average value or actual value of the variation, the average value or the actual value ΔNRMS of ΔN is determined by the accuracy of the hardware and is considered to be approximately constant. . Also, since N is constant,
The ratio between the actual value ΔNRMS and N is constant, and this is defined as CV
Then, the actual value ΔDRMS of ΔD holds the relationship of the following equation (4), and ΔDRMSQCCV φD
--------(4) It can be seen that the density unevenness is proportional to the average density. In reality, since there is no proportional relationship between the exposure amount and the density, equation (4) does not strictly hold, but it can be seen that the larger the average density, the larger the density unevenness.

However, the density saturates at a certain amount of exposure, so even if the exposure changes, the density does not change.Therefore, when expressing halftones with two values of white and black, the intensity of the laser light must be adjusted. If the density is increased and the density is saturated, density unevenness will not appear. However, the cross section of the laser beam intensity distribution is not rectangular but approximated by a Gaussian curve, and the laser exposure distribution in the main scanning direction is the integral of the Gaussian curve passing through that position, and has a long base. If the off time in the main scanning direction is short, the exposure distribution will not reach zero because its bases will overlap, and the density unevenness will appear.

FIGS. 5(a) to (e) and FIGS. 6(a) to (e) are diagrams for explaining density unevenness, and in these diagrams, (
a) shows the driving pulses applied to the laser, TI, T
2 represents 08 hours.

(b) shows the distribution (Gaussian distribution) of the intensity factor of the laser beam, and (C) shows the exposure amount distribution of the intensity factor distribution shown in (b) above. For example, the exposure amount at point R is This corresponds to the area of the shaded part in b). (d) shows the electric field intensity distribution at the exposure dose distribution shown in (C) above. (e) is the above (d)
The output concentration distribution at the electric field shown in is shown.

As shown in FIG. 5(a), when the ON time TI of the laser is short, no overlap occurs in the exposure dose distribution as shown in FIG. 5(C). Furthermore, as is well known, in electrophotography there is an electric field development effect, that is, an edge effect, in which edges are emphasized due to the potential distribution determined from the exposure distribution. Fifth
When the exposure distribution shown in figure (e) is obtained, the fifth
The electric field intensity distribution shown in FIG. 5(d) is obtained, and the output density distribution during the electric field shown in FIG. 5(e) is obtained, resulting in dots with fewer intermediate tones than the exposure dose distribution. Note that the latent image is developed using an image development method.

Moreover, as shown in FIG. 6(a), the laser ON time T
2 is long, and the exposure amount distribution intersects as shown in FIG.
The electric field strength distribution shown in Figure (d) also decreases to an intermediate concentration.

Furthermore, the electric field development effect is also reduced at the center of the dot, resulting in a decrease in density. The intermediate density during this laser OFF period is influenced by the exposure amount variation ΔE described above, and appears as a density variation ΔD.

[Problem that the invention seeks to solve]

Therefore, when expressing halftones with z values, when the area of black dots is small in image development, in other words, density unevenness hardly appears in highlight areas where the average density is low, but the area of black dots becomes large, When the average density becomes a high density part, density unevenness appears, and as mentioned above, there is a problem that the higher the average density is, the larger the density unevenness becomes. Furthermore, when expressing halftones with multi-values, density unevenness naturally appears, but if multi-values are used only for highlighted areas, the density unevenness becomes extremely small. Furthermore, in the case of background development, contrary to the above, there is a problem in that density unevenness appears in highlight areas.

FIG. 7 is a characteristic diagram schematically showing the relative relationship between the halftone image signal and the output image density, where the vertical axis shows the output image density and the horizontal axis shows the input image signal N.

Dp indicates the density level of the recording material.

As can be seen from this figure, as the output image density approaches the high density region, density unevenness occurs and the waveform begins to fluctuate, and the degree of this becomes larger as the density region approaches the high density region.

As described above, in the conventional image recording apparatus, when reproducing halftones, density unevenness occurs in high density areas, and a stable halftone image cannot be outputted.

This invention was made in order to solve the above problems, and it forms an image in a high density area of a halftone image by forming an image in a low density area, and outputs a halftone image without image unevenness in the high density area. The purpose is to obtain an image recording device that can

[Means for solving problems]

The image recording device according to the present invention converts the low density component of the halftone image signal into a binary image signal according to the dot formation density pattern, and converts the other components of the halftone image signal into a binary image signal according to the dot formation density pattern. is provided with a modulation means for converting into a z-value image signal according to a multi-value density pattern.

[Effect]

In this invention, the modulation means converts a halftone image signal of a low density component of the input halftone image signal into a binary image signal according to a dot formation density pattern, and Halftone image signals other than the above are converted into binary image signals corresponding to a multi-value density pattern.

〔Example〕

FIG. 1 is a cross-sectional view of an image recording apparatus showing an embodiment of the present invention, in which 1 is a photosensitive drum, 2 is a charger for single-component charging,
3 is a secondary charging charger, 4 is a transfer charger, 5 is a pre-neutralization charger, 6 is a developing cylinder, 7 is a developer, 8 is a toner, 9
1 is a separation roller, 10 is a separation unit, 11.12 is a fixing roller, 13 is a fixing heater, 14.15 is a registration roller, 1
6.17 is a paper ejection roller, 18 is a paper feed roller, 19 is a paper feed cassette, 2o is a recording paper serving as a recording medium, 21°22.
23 and 24 are conveyance rollers, 25 is a conveyance sheet, 26 is a recording paper tray, 27 is a rotating polygon mirror, 28 is an f-θ lens,
29 is a laser beam, 30.31 is a movable guide plate, 32
~ 41 is a fixed guide plate, 51 is a modulator, and a laser 52
Turn on.

Modulate off. 53 is a collimator lens, and the laser 52
The laser beam 52a emitted from the laser beam 52a is made into a parallel beam of light.

Note that the image forming operation is based on a known electrophotographic method, so a description thereof will be omitted.

Next, the recording paper conveyance operation according to the present invention will be explained.

The recording paper 2o in the paper feed cassette 19 is sent out from the paper feed cassette 19 by the rotation of the paper feed roller 18. At this time, the movable guide plate 30 is at the position shown by the solid line, and the recording paper 2o is on standby at this position. Registration roller 14,1
5 starts rotating in response to a start signal in synchronization with the output of the image signal by the modulator 51, and exposes the recording paper 20 to light.

The recording paper 12 fed into the ram 1 has toner transferred thereto by the transfer charger 4, and is then transferred to the fixing rollers 11, 12, . Fixing heater 1
Fixed in 3. At this time, the movable guide plate 31 is at the position shown by the solid line, and the recording paper 20 is sent out in the direction of the guide 4o. The recording paper 20 is conveyed in the original direction by a conveyance sheet 25. The recording paper 2o is transferred again to the registration rollers 14. Plugged into i5.

Next, toner transfer and fixing are repeated again to the recording paper 20 in the same manner as described above. At this time, the image position on the recording paper 2o is controlled by the timing at which the registration rollers 14 and 15 start rotating. Also, at this time, the modulation device 51 outputs an image signal different from the first time. In this way, after toner transfer and fixing are repeated a predetermined number of times,
The movable guide plate 31 is at the position indicated by the broken line, and the recording paper 2o is transferred to the recording paper tray 26 by the paper ejection rollers 16,17.
sent to. When another image is to be reproduced, the movable guide plate 30 returns to the position indicated by the solid line, and a new recording paper 20 is fed.

Next, referring to FIG. 2, the modulation device 51 shown in FIG.
The configuration of is explained below.

FIG. 2 is a block diagram showing the configuration of the modulation device 51 shown in FIG. 1, and 61 is a clock generator that generates a timing clock CLK.

62 is an image memory in which each pixel is divided into (X, Y) addresses designated by an X address counter 63° and an X address counter 64 and stored therein.

Each pixel of the image memory 62 has gradation information of 32 gradations from 0 to 31. However, the number 31 is black. 65.
Reference numeral 66 denotes a matrix memory that stores a 16×2 threshold matrix, in which a dot formation density pattern shown in FIG. 3(a) and a multi-value density pattern shown in FIG. 3(b), which will be described later, are stored. There is. 67 is a hexadecimal column counter that specifies column addresses of matrix memories 65 and 66. 68 is a binary row counter that specifies the row address of matrix memory 65, 66. 69.7
0 is a digital comparator that compares the pixel data stored in the image memory 62 with the threshold values of each pattern stored in the matrix memories 65 and 66, and when pixel data exceeding the threshold value is input, a predetermined value is input. The black signal is outputted to the laser printer 72 via the multiplexer 71.

The laser printer 72 receives a start signal V and a horizontal synchronization signal H.
to occur. The horizontal synchronization signal H can be easily generated by detecting the scanning beam 29 outside the effective image area.

The start signal V is generated at an arbitrary timing, and if the registration rollers 14 and 15 are rotated as described above with a delay to this start signal V, the binary image signal input to the laser printer 72 and the recording paper 2o are can be synchronized with the position of

The x and Y address counters 63 and 64 are reset by the start signal V generated by the laser printer 72, and the X address counter 63 counts the timing clock CLK generated by the clock generator 61 to advance the X address of the image memory 62. , the timing clock CLK is simultaneously counted by the column counter 67,
Advance the column address of matrix memory 65 and 66.

The X address counter 64 counts the horizontal synchronizing signal H generated by the laser printer 72 to advance the Y address of the image memory 62, and the row address counter 28 counts and advances the row address of the matrix memory 85, 66.

The pixel data in the image memory 62 specified by the X and Y address counters 63 and 64 are compared with the threshold values in the matrix memory 65 and 68 specified by the column counter 27 and row counter 68, respectively, by digital comparators 69 and 70. If it is larger than the black signal, the comparator 69, 70
is output from. One of the output signals of the comparators 89 and 70 is selected by a multiplexer 71 and sent to a laser printer 72 as a binary image signal. This signal drives the laser 52 to expose the photosensitive drum 1 to a binary image, and a binary image is formed on the recording paper 2o. Here, different threshold matrices are stored in the matrix memory 65, 66, and each time the process described in FIG.
In addition, in FIG. 2, the multiplexer 71 is placed in front of the digital comparators 89 and 70, the digital comparators are combined into one, and the output signals of the matrix memories 65 and 66 are selected by the multiplexer 71. It can also be configured to be input to a digital comparator.

Next, the density patterns stored in the matrix memories 65 and 86 shown in FIG. 2 will be explained with reference to FIGS. 3(a) and 3(b).

3rd IN (a) and (b) are schematic diagrams for explaining the density patterns stored in the matrix memory 65.86 shown in FIG. Threshold values corresponding to one dot of the signal are stored subdivided in the main scanning direction. FIG. 2B shows a multi-level density pattern in which threshold values corresponding to one dot of pixel data are subdivided in the main scanning direction and stored. In addition, in the dot formation density pattern shown in FIG.
When the pixel data level is 7 or more, the density is the same, and in the multi-value density pattern shown in FIG.

Next, while referring to FIG. 4, FIG. 3(a).

The output image density according to the density pattern shown in Cb) will be explained.

FIG. 4 is a characteristic diagram illustrating the output image density according to the density patterns shown in FIGS. 3(a) and (b), where the vertical axis shows the output image density and the horizontal axis shows the level of input pixel data. Note that Dp indicates the density level of the recording material.

In this figure, characteristic (a) indicates the output image density characteristic based on the dot formation density pattern shown in FIG. Show characteristics.

Characteristic (c) indicates the overall density characteristic of image formation based on characteristics (a) and (b).

Among the pixel data input according to the dot formation density pattern shown in FIG. 3(a), when the gray level image signal components of low density level, for example, 16 or less, are converted into a binary image, the characteristics shown in FIG. 4 (a) are obtained. Output density can be obtained according to the output density. In this case, since the image is converted into a binary image in a density region before density unevenness occurs, density unevenness can be suppressed as much as possible. Furthermore, pixel data components of 16 or more are converted into binary image signals based on the multi-value density pattern shown in FIG. 3(b). In this case, an output density corresponding to the characteristic (b) shown in FIG. 4 can be obtained.

Therefore, after forming an image according to the dot formation density pattern shown in FIG. 3(a), a desired image is formed on the same recording paper 2o according to the multi-value density pattern shown in FIG. 3(b). When an image is formed a number of times, the characteristics shown in Fig. 4 (c) are obtained.
An image with less density unevenness can be obtained with output image density characteristics corresponding to the image density. ``Actually, the characteristics of input pixel data and output image density are not linear as shown in FIG. 4, but if a lookup table is inserted after the image memory 62 shown in FIG. , a linear relationship shown in FIG. 4 can be obtained.

As described above, when an electrophotographic process is performed multiple times using a dot-forming density pattern, moiré between dots occurs due to a shift in image position, that is, a shift in registration, which greatly deteriorates the image quality. Therefore, in this invention,
The generation of moiré is prevented by forming an image once using a dot forming density pattern and then forming an image using another multi-value density pattern. In addition, when driving the laser 52 according to the multilevel density pattern shown in FIG. 3(b), the laser 52 is turned on for short periods of time at a pitch shorter than the laser beam diameter, and the exposure amount is changed to create intermediate tones. It is being reproduced. Furthermore, there are cases where it is not possible to obtain too many multi-values to express the multi-values with the multi-value density pattern shown in Fig. 3 (b), and the slope of the characteristic (c) shown in Fig. 4 becomes sloppy. There is. In that case, by repeating the electrophotographic process two or more times using the multi-value density pattern shown in FIG. 3(b), an image density having the characteristic (c) shown in FIG. 4 can be obtained.

In the above embodiment, a case was described in which the process including image transfer and fixing is repeated many times to output an image. If the images are combined on the drum and then transferred to the recording paper 2o, the refeeding mechanism for the recording paper 2o can be omitted and the recording time can be shortened.

Furthermore, if a plurality of photosensitive drums 1 are provided according to each density pattern and the image is transferred onto the recording paper 2o, the mechanism can be simplified and the recording time can be shortened.

〔Effect of the invention〕

As explained above, the present invention converts the low density component of the halftone image signal into a binary image signal according to the dot formation density pattern, and converts the other components of the halftone image signal. Since the system is equipped with a modulation means for converting into a binary image signal according to a multilevel density pattern, an image in a high density area of a halftone image can be obtained by forming an image in a low density area, thereby eliminating image unevenness in a high density area. It has the excellent advantage of being able to output halftone images without any color.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an image recording device showing an embodiment of the present invention, FIG. 2 is a block diagram showing the configuration of the modulation device shown in FIG. 1, and FIGS. 2 is a schematic diagram illustrating the density pattern stored in the matrix memory shown in FIG. 4; FIG. 4 is a characteristic diagram illustrating the output image density according to the density pattern shown in FIGS. Ca)～(
e) and FIGS. 6(a) to (e) are diagrams for explaining density unevenness, and FIG. 7 is a characteristic diagram schematically showing the relative relationship between the input image signal and the output image density. In the figure, 62 is an image memory, 65 and 66 are matrix memories, 69 and 70 are digital comparators, and 72 is a laser printer. Figure 3 Figure 4 111111φN Figure 5 Figure 6

Claims (1)

[Claims] It has a comparison means for comparing the halftone image signal and the threshold signal, and according to the output of the comparison means, a predetermined number of image transfers are repeated on the same recording medium surface to obtain a limited number of gradations of two or more values. In an image recording device that pseudo-reproduces halftones by converting the halftone image signal into multiple values, the low density component of the halftone image signal is converted into a binary image signal according to the dot formation density pattern, and the An image recording apparatus comprising a modulation means for converting other components of the toned image signal into binary image signals according to a multi-value density pattern.