JPS6326161A - Reproducing method for halftone picture - Google Patents

Abstract

PURPOSE:To obtain a print with high quality picture without visible stripe pattern by grasping ralation between uneven rotation and picture quality quantitatively in reproducing a halftone picture by an optical printer with a high write density. CONSTITUTION:In expressing a spetial frequency of the vibration of a picture forming body in (x)cycle/mm, a value(y) (%pk-pk) expressed in equation I gives a decrease in the vibration of the picture forming body. An allowable limit curve obtained by taking the spetial frequency of the vibration of the picture forming body as an abscisa and plotting the magnitude of the uneven vibration at which the stripe pattern on printing is recognized on an ordinate as each dot number. Curves (a), (b), (c) show allowable limit curves corresponding respectively to pictures of 90-line, 140-line and 175-line. In designing a printer, the magnitude of the uneven vibration is converged into the allowable limit by revising the component of the printer or the process so as to obtain the printer satisfying the prescribed specification.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an image forming apparatus such as an optical printer that reproduces halftone images using halftone images, and particularly to a method for reproducing halftone images.

[Conventional technology]

First, optical printers, such as laser printers, are based on the mechanics of conventional analog copiers.

In an analog copying machine, an original image from an original on a platen is directly irradiated onto a photoreceptor through a lens system, and the photoreceptor is exposed to light. At this time, since the original image is exposed to the photoreceptor through a slit having a certain width, even if there is uneven rotation of the photoreceptor drum, no effect is seen on the print. That is, the influence of uneven rotation of the photoreceptor drum is averaged out by the width of the slit, and does not affect the original image.

On the other hand, with optical printers, even if the same base machine is used, exposure is performed using a thin line from a writing device (B light device) such as a laser, LED, or i-product shutter, so rotational unevenness itself causes an exposure position error. Easy to come out clearly. In particular, when halftone images are reproduced using halftone dots, the effects of uneven rotation may appear as striped patterns on the print. That is, if there is uneven rotation of the photosensitive drum, the spacing between halftone dots changes in density, which appears as a striped pattern on the print.

However, with the optical printers currently on the market, the dot density of the writing device is low and the degree of halftone expression is low, so the effect of rotational unevenness cannot be seen, so the problem has not surfaced.
Furthermore, even if striped patterns appear, they are still commercialized. In other words, in the past, the effects of rotational unevenness on image quality were not clearly understood, and LT7m was only used to the extent that the fewer rotational unevenness, the better. The relationship between the number of lines and the magnitude of drum rotation fluctuation is not clearly understood.

[Problem that the invention seeks to solve]

However, when recording at a high density of, for example, 600 dots/inch or more, striped patterns become noticeable because the density of the image is faithfully reproduced. In order to prevent this striped pattern from appearing, it is necessary to quantitatively understand to what extent the rotational unevenness of the photoreceptor drum needs to be suppressed. Also, in terms of quality control of printers, it is necessary to be aware of the effect that uneven rotation has on image quality.

The present invention enables high-density halftone images to be reproduced without deteriorating image quality by clearly understanding the influence of drum rotation unevenness on image quality when reproducing halftone images using an optical printer with high writing density. The purpose is to reproduce.

[Means for solving problems]

In order to achieve the above object, the halftone image reproducing method of the present invention has a resolution of 600 dots/inch or more and a wire number of 80.
In a device that outputs a halftone image of lines or more, when the spatial frequency of the vibration of the image forming body is expressed by X cycles/dragon, Y = (0,65x"-1,64x + a) ÷ (3,
06X10"-5,17X10-2x) However, a = -0,0142X b +3.79 (8
0≦b≦180) a = 31.3x b −” ”
(180< b ≦300) b = the value of y expressed by the number of mesh lines (% pk - pk), which is characterized in that the vibration of the image forming body is made smaller.

[Effect]

In the present invention, the spatial frequency of the vibration of the image forming body is plotted on the horizontal axis, and the magnitude of vibration unevenness at which a striped pattern on the print is recognized is plotted on the vertical axis, and a tolerance limit curve as shown in FIG. get. This allowable limit curve is determined for each number of mesh lines. In Figure 1, curve (a), Tol, (C
1 are tolerance limit curves corresponding to images of 90 lines, 140 lines, and 175 lines, respectively.

When designing a printer, the first consideration is the amount of vibration unevenness.
By modifying the components or processes of the device to fall within the tolerance limits shown in the figures, a printer that meets predetermined specifications can be obtained.

〔Example〕

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be specifically described below based on actual examples without reference to the drawings.

FIG. 2 is a block diagram of an experimental system for obtaining basic data of the present invention.

In the figure, reference numeral 1 indicates a photosensitive drum of a laser printer, and the photosensitive drum 1 is directly driven by a highly accurate servo motor 2. As shown in FIG. Therefore, normally the photoreceptor drum 1
The surface rotates very smoothly. The rotational speed of the servomotor 2 is controlled by a motor driver 3, and a single frequency disturbance of a sine wave is applied from a noise superimposer 4 to the rotation control signal of the motor driver 3 to forcibly eliminate rotational unevenness. to occur. If the process speed of the printer is 80.17 seconds, the frequency is 10Hz to 160
Hz (expressed in MTF: 0.125 cycles/2 from Caoyuan
．． 0 cycles/l) at intervals of about 10 Hz. The magnitude of rotational unevenness is pk-pk (beak-peak) and is varied from 1% to 8% in 1% increments.

As for the characteristics of the servo motor 2, it is necessary that there is no specific noise component and that rotational unevenness is sufficiently smaller than 1% in terms of ρ and −pk. Furthermore, it must not resonate from 10 Hz to slightly above 160 Hz.

The laser optical system 7 uses a He-Ne gas laser and scans with a polygon mirror. A pattern generator 8 capable of forming an arbitrary image pattern is connected to the laser optical system 7.

In addition, in the experimental system shown in Fig. 2, the scanning width is 1oo
Although the width is narrower than the scanning width of a normal printer, it is sufficient for evaluating image quality. The feature of this laser optical system 7 is that the writing density is approximately 600°700.800.10.
Since the density can be varied from 0.00 to 1200 to 1400 dots/inch, the video clock of the polygon motor (not shown) and the pattern generator 8 can also be varied accordingly. The magnitude and frequency of rotational unevenness of the photoreceptor drum l are detected by a rotation detector 5, for example, a 5400 pulse/rotation optical rotary encoder, which is connected to the photoreceptor drum shaft through a flexible coupling. This can be determined by inputting the frequency signal of the output of step 5 into the rotational unevenness measuring device 6 and performing arithmetic processing. This allows you to check whether rotational unevenness is occurring according to the noise given.

Next, print samples of halftone images with line counts of 90 lines, 133 lines, and 175 lines are output while forcibly generating rotational unevenness using this experimental system. Note that the nxn matrix method was used for halftone dots.

By varying the frequency and magnitude of rotational unevenness for each level, approximately 100 samples of one type of image quality were obtained, and 13 engineers subjectively evaluated them.

Subjective evaluations are divided into five levels using a graded evaluation method, and each print sample is ranked. The average of the evaluation values of 13 people was taken to determine the acceptable limit for the striped pattern appearing on the print sample, and these were plotted as shown in FIG.

However, is the horizontal axis the spatial frequency? It is expressed in lTF. This tolerance limit indicates the boundary at which striped patterns caused by uneven rotation can be clearly recognized in image quality when viewed from the distance at which texts and books are generally read or viewed, that is, approximately 30 to 40 degrees. be.

In other words, above the allowable limit curve shown in FIG. 1, the striped pattern clearly appears in the image quality, and below it, the striped pattern becomes less noticeable. This tolerance limit curve can be expressed mathematically as y” (1,02x 10−
' x ''-2,05x 10-" x + a) +
(3,06X10-'-6.46X10-'x)
・ −・ −(11a = −0,0142X b +
3.79 ・ ・ ・ ・ ・ 12) However,
X is the vibration frequency (Hz), y is the magnitude of rotational unevenness (%pk - pk), and b is the number of mesh lines (80≦b≦180). First, the number of mesh lines to be reproduced is input into equation (2) to obtain the value of a, and this is substituted into equation (1). Then, in 2fil, by substituting the vibration frequency that is expected to occur into X, the magnitude y of rotational unevenness at that time is calculated. In an actual printer, by making the magnitude of the rotational unevenness smaller than the calculated value of y, it is possible to obtain a print sample with a hole image quality in which the striped pattern is not noticeable. The smaller the uneven rotation of the photoreceptor drum 1 of the printer, the better, but achieving this requires an expensive and large precision motor and precision transmission device, which can be unnecessarily costly depending on the type of image quality. . Here, in the present invention, the magnitude of rotational unevenness to be suppressed can be clarified from the vibration frequency by the above-mentioned 2(11 and 21), and unnecessary expensive parts can be avoided.

Note that the above equation (1) is an equation that holds true when the process speed is 8 (bm/sec), and when the vibration frequency X is 80 Hz, the MTF is 1.0 cycles/layer. If the spatial frequencies are the same, the formula f
It has been experimentally confirmed that il holds true. Therefore, in order to provide versatility, the vibration frequency X is replaced with a spatial frequency.

Let X in equation (1) be 80x, and y = (0,65X”-1,64X + a) + (3,
06x to-' -5,17X 10-" x) ・
...Rewrite it as +31. At this time, X is the spatial frequency of vibration (cycle/■nawate), and y is the size of rotational unevenness, %pk.
-pk). And the constant term a is a = -0,0142x b +3.79 (80
≦b≦180) a = 31.3X b-”' (
180<b≦300) However, b is expressed by the number of M4 lines.

Figure 1 illustrates this equation (2), with 90 lines and 14
The curves corresponding to the images of 0 line and 175 line are +al, fb
l, tc+, and it can be seen that the curve and vertical position of the curve, the spatial frequency X and the number of mesh lines have already been determined.

As mentioned earlier, the striped pattern is not noticeable in the area below these curves, so for a 90-line image, it is sufficient to keep the rotation unevenness to about 6% or less, and for a 175-line image, it is It can be seen that it is sufficient to suppress it to about 1% or less. Note that the permissible range of these rotational irregularities is not fixed for each number of grid lines (the permissible range widens as the spatial frequency X deviates from 1 cycle/1 m). By changing the process speed etc.
Striped patterns can be made less noticeable without changing other conditions.

Note that although this embodiment has been explained using a laser printer as an example, the invention is not limited to this (laser, LED, etc.).
The present invention can also be applied to printers using other light sources such as liquid crystal shutters. Further, as the image forming body, it can be applied to a belt-shaped photoreceptor other than a drum type, and furthermore, it can be applied to a printer having a writing device other than an optical printer.

As described above, using this experimental system, we took print samples by forcibly adding rotational unevenness from the outside, subjectively evaluated them, and graphed the results to show the vibration frequency and rotation when reproducing halftone images. The relationship between the allowable level of unevenness has been clarified.

An example of a procedure for applying the halftone image reproduction method of the present invention to an actual product will be described.

First, the permissible limit value of vibration of the image forming body with respect to image quality is determined in advance from equation (11) or equation (3).

Next, the magnitude of the vibration of the image forming body of the apparatus to be adjusted is measured for each frequency, and the frequency of the vibration at which °ζ is maximum is detected.

Next, a vibration source having a natural frequency that matches the detected frequency is identified. As the source of this vibration, motors, gears, etc. can be considered.

Finally, the frequency and/or magnitude of the vibration of the vibration source is adjusted so that the vibration of the identified vibration source is below the allowable limit value. Specifically, by replacing the vibration source itself or applying vibration-proofing means, it is possible to manufacture a printer that outputs prints with inconspicuous striped patterns.

(Experimental Example 1) A semiconductor laser printer was prototyped by modifying a dry copying II (Fuji Xerox FX4700). The process speed is 80I/sec, and both main scanning and sub-scanning are 600
Dot density of dots/inch, beam diameter 67μ end x 8
Equipped with a 4 μm (1/e") laser optical system. The photoconductor rotation variation due to the photoconductor drive system used in the FX4700 copying machine is 20% pk - pk, and the space that provides the maximum i width. According to Fourier analysis, the frequency is 0.
It was 5 cycles/sho. This was clearly larger than the rotational unevenness defined by the first equation.

When this laser printer outputted a halftone halftone image of 100 lines and 36 gradations, a striped pattern corresponding to the spatial frequency having the maximum amplitude appeared over the entire print, significantly degrading the print quality. The drive system of this laser printer is D
When I changed it to direct drive with a C servo motor (R720T made by Izu Denki), the photoconductor rotation fluctuation was 4% in ρ -pk, and the spatial frequency that gave the maximum amplitude was o,
is cycle/1m. In addition, the magnitude of rotational unevenness including other spatial frequencies was smaller than that defined by equation (1). 1 with a laser printer with this drive system changed
When a halftone dot image of 00 lines and 36 gradations was output, a high quality image with no recognizable stripes was obtained.

(Experimental Example 2) A semiconductor laser printer with 800 dots/inch for both main and scanning was prototyped. The photoreceptor drive system is the same as in Experimental Example 1.
A direct drive was used with a C servo motor (R720T manufactured by Deki Denki). 133 using the dot matrix method
Halftone images of 36 line gradations and 100 lines and 64 gradations were output, but in both cases, prints of animal quality with no recognizable stripes were obtained.

(Experiment Example 3) Unlike the usual dot matrix method, each of the laser printers in Experiment Example 1.
A halftone image was output using the submatrix method that increases the number of cotton lines.

In Experimental Example 1, halftone images with 100 lines and 72 gradations and 140 lines and 36 gradations were output, and in Experimental Example 2, halftone images with 133 lines and 72 gradations were output.
The rotational fluctuation of the photoreceptor, which is directly driven by a DC servo motor (R720T made by Yagi Denki), is smaller than the value defined by equation (1), so a high-quality image with no visible stripes on the print was obtained. .

(Experimental Example 4) A laser printer with 1400 dots/inch in both main and sub-scanning was manufactured. The photoreceptor was driven in the same manner as in Experimental Example 1, and a halftone image of 175 lines and 64 gradations was output using the focus matrix method. In this halftone image, when applying the spatial frequency of 0.15 cycles/I of the rotational unevenness of the motor (Yagi Denki 1-720T) to the formula (11), the size of the rotational unevenness is 3.5%pk-ρ. However, since the actual rotational unevenness of the motor is 4% ρ and -pk, a slight striped pattern was recognized throughout the print.Therefore, the DC servo motor was changed to CN-4007 manufactured by Yagi Denki, which has a higher accumulation degree. The rotational unevenness of the photoreceptor drum at this time is 1% pk - pk, and when Fourier analysis is performed, the spatial frequency that gives the maximum amplitude is 0.
This DC servo motor (Yagi Electric Co., Ltd. C
N-4007) was used to output 175 lines, 64 gradations, and 200 lines, 49 gradations, and in both cases, a descendant-quality image quality with no visible stripes was obtained.

〔Effect of the invention〕

As described above, in the present invention, the influence of rotational unevenness on image quality is understood from the relationship between the spatial frequency of the vibration of the photoreceptor and the rotational unevenness, and a tolerance limit curve indicating the limit at which striped patterns can be identified is determined for each mesh. The number of lines is determined quantitatively in advance through subjective evaluation. Based on this tolerance limit curve, by changing each component and process of the printer so that rotational unevenness is within the tolerance range, it is possible to obtain helmet-quality prints with no visible stripes as output from the printer. Can be done.

That is, the relationship between rotational unevenness and image quality, which could only be determined empirically in the past, can now be determined in a fixed manner, making it possible to stably maintain product quality at a high level. In addition, since quality control can be performed accurately, it is also effective in terms of production control.

[Brief explanation of the drawings] Figure 1 is a graph showing the relationship between the spatial frequency and the level of rotational unevenness at which a striped pattern is recognized in each halftone image, and Figure 2 is a graph showing the relationship between the vibration frequency of the photoreceptor and the rotational unevenness level. 1 is a schematic diagram of an experimental system used to generate data showing the relationship between 1: Photoconductor drum 2: Servo motor 3: Motor driver 4: Noise superimposition device 5; Rotation detector 6: Rotation unevenness measuring device 7: Laser optical system 8: Pattern generator patent applicant Masu Tegori, agent for Fuji Xerox Co., Ltd. (2 others) Figure 1 Figure 2

Claims (1)

[Claims] 80 mesh lines with a resolution of 1,600 dots/inch or more
In a device that outputs halftone images of lines or more, when the spatial frequency of vibration of the image forming body is expressed in x cycles/mm, y = (0.65x^2-1.64x+a) ÷ (3.06x10^ -^1-5.17×10^-^2
x) However, a=-0.0142×b+3.79 (80≦b≦180
)a=31.3×b^-^0^. ^6^1 (180<b
≦300) b: A halftone image reproduction method characterized by making the vibration of the image forming body smaller than the value of y (%pk-pk) expressed by the number of mesh lines.