JPH0985982A - Exposure for laser beam printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimize the quantity of exposure light under simple control to obtain a clear-cut output image by printing data through the variation of the quantity of exposure light for a single data to be printed in accordance with the number of adjacent up/down and right/left clots to the single dot to be printed. SOLUTION: A circuit 12 for controlling the quantity of light of a laser diode is connected to a laser diode driver 11. This circuit has line buffers for three lines, i.e., one center line buffer for print dots and two buffers for the lines each before and after the center line buffer, and sends information of adjacent clots 1-4 to a print dot 5 to an address line on ROM. In addition, 8-bit print data 8 is output to a D/A converter from a data line on the ROM, and then is converted to an analog value equivalent to the quantity of light for printing of the laser diode driver 11. Further, this analog value is output as a signal for driving the laser diode driver 11. Thus, the quantity of exposure light is output by deciding whether the number of adjacent clots to the print clot should be large or small, so that the density of an output image is well balanced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a laser beam printer which forms an image by irradiating a photosensitive medium with scanning light by means of a polygon mirror which rotates using laser light. The present invention is an exposure method in a laser beam printer most suitable for use in a high gamma value photoconductor for electrophotography having a high resolution.

[0002]

2. Description of the Related Art In the image exposure of a laser beam printer, conventionally, each dot to be printed is generally printed with the same exposure amount. In some of the laser beam printers, the amount of exposure is changed according to the position information, so that the density of the image to be formed is uniform. The purpose of this kind of exposure control is to correct the variation in the light amount due to the position caused by the optical system for printing, or to make the shading of letters in the characters noticeable by changing the exposure amount to the image edge part. The purpose was to get rid of it. In another example, in order to express a photographic image, the amount of light of one dot is changed to 8 bits (256 levels), and the size of dots is changed from white to black in an analog manner regardless of halftone dots. There was something to express.

[0003]

SUMMARY OF THE INVENTION The present invention is effective when used in combination with a high gamma value photoreceptor when handling images, especially photographic images such as those used in printing plate making. Generally, a photographic image is divided into halftone dots and printed, but the halftone dots are composed of a collection of dots. Particularly, when the photographic image density range is wide, the size of the dots forming the halftone dots becomes finer. High-gamma-value photoconductors are suitable for forming fine dots, and are being used for high-definition image formation such as used in printing plate making.

The high gamma value photoconductor has a threshold value (threshold value) in light intensity due to its photosensitivity. Generally, a cross section obtained by cutting a beam spot of a light source showing a Gaussian distribution by the threshold value becomes a latent image, and an electrophotographic system is used. In the developing step, toner is attached to this latent image to form an image. FIG. 2 schematically illustrates the amount of printing light and the change in the size of the beam when the beam is cut at a certain threshold value. The laser beam image forming point 16 in a Gaussian distribution state by the threshold value 21
It is shown that when ˜20 (16 is the case where the light quantity is large and 20 is the case where the light quantity is small) is cut off, the spot diameter changes correspondingly to 22 to 26. The numbers 0 to 4 in the spot diameters 22 to 26 indicate the printing light amount values described with reference to FIG.

Next, how an image is formed according to the size of the printed dots will be described. Specifically, in general, in order to form a high-definition image, the scattering of toner is minimized, and since it is necessary to faithfully reproduce the latent image, the toner should be adhered to the latent image portion on the photosensitive member by about three layers. Often. As shown in FIG. 6, when the spherical toner 34 is used in this state, it has a bales structure of almost three layers. Note that reference numeral 32 in FIG. 6 is a paper for transferring toner, and reference numeral 33 is a photoconductor. From this, in the peripheral part of the image of the set of relatively large dots, the upper surface 33 of the photoreceptor is set to
5, an inclined portion is formed toward the upper side 36 of the three-layer structure, and the toner image has a trapezoidal shape. At a minute point, the inclined portion forms a triangular pyramid shape. The same applies to a toner image forming an isolated point, and toner adheres to the bottom surface 37 in a size substantially the same as the size of the latent image, but only a few toner particles adhere to the upper side 38 to form a triangular pyramid shape. become.

Next, when the toner image thus formed is transferred to paper, the trapezoidal portion 35 of the toner image is transferred.
A case where there is a minute point portion 37 between the trapezoid portion 35 'and the trapezoid portion 35' will be described. In this case, the paper is the upper edge 3 of the trapezoid
6 and 36 'are pressed against each other, and in the toner image portion formed on the minute points, the vicinity of the conical apex 38
Only a few toners (one in FIG. 6) can contact the paper. In particular, when the distance between the trapezoidal portion and the minute point is short, there is almost no bending of the paper, and this tendency is large. Therefore, minute point 3
In No. 8, the size of the transferred image becomes smaller than that at the bottom. This phenomenon becomes more remarkable as the size of the image becomes smaller, which becomes a major obstacle when expressing finer dots and fine lines.

Such a phenomenon does not cause any problem when the size of the toner is sufficiently small with respect to the image. A problem occurs only when the size of the toner and the size of the image to be expressed are close to each other as in the present invention. Empirically, if the size of the toner is about ⅛ or less of the size of the image, this phenomenon will not be a problem. However, the printing printer is 1200
Since a resolution of DPI or higher is required, the minimum dot size is about 20 microns or less. On the other hand, the size of the toner is about 6 microns even if it is the smallest at present. Therefore, the number of toners forming an image becomes 3 to 6, and it becomes impossible to reproduce minute dots.

Next, the case where a cross line as shown in FIG. 7A is printed will be described. In this case, the crossing portion 41 of the cross line is a problem. In such a place, a large number of dots are adjacent to each other, and the dot at the intersection portion becomes a large dot due to the influence of adjacent dots. It becomes like a oyster.
As a result, the adhesion of toner becomes as shown in the sectional shape of FIG. More toner adheres to the intersecting portion 42 than other portions, and when the toner is attempted to be transferred onto the paper 32, the intersecting portion 42 pushes up the paper, and a portion 43 where the toner does not contact the paper occurs near the intersecting portion. As a result, the cross hairs are discontinuous as shown in FIG. 7 (C). In the case of such an image, the print quality is also deteriorated.

As described above, when outputting an image in which a large number of dots and a small number of dots exist,
Areas where a large number of dots are formed are influenced by adjacent dots, and areas where a small number of dots are formed are small in appearance because there are no adjacent dots. Such a phenomenon causes deterioration of a photographic image in a laser beam printer or cuts fine lines to make an output image dirty.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an exposure method in a laser beam printer capable of optimizing an exposure light amount with a simple control and obtaining a clear output image. To do.

[0011]

SUMMARY OF THE INVENTION An object of the present invention is to, when a laser beam is modulated dot by dot by an externally input image signal, output the modulation signal of the dot adjacent to the one dot to be printed. This is achieved by the exposure method of the laser beam printer in which printing is performed by changing the amount of light of one dot to be printed according to the number of.

That is, when there are four adjacent dots (printing pattern 5) as shown in FIG. 4, the light quantity of the printing dots is about 60% as compared with the case where there are no adjacent dots. Similarly, when three (printing pattern 4) is about 70%, when two (printing pattern 3) is about 80%, when one (printing pattern 2) is 90%, and when independent points are 100%. (Print pattern 1). This balances the image densities of a lot of dots forming a halftone dot and a lot of dots forming a halftone dot, and a photographic image can be output neatly. Reference numeral 27 indicates a print position, reference numeral 28 indicates a printed dot, and reference numeral 29 indicates a print light amount value.

According to the present invention, the amount of printing light is determined to be large or small according to the number of dots adjacent to the printing dot and the amount of exposure light is output, thereby maintaining a good balance of output image density and producing a clear photographic output or fine pattern. Can be output. The following three examples can be given as the printed matter that is particularly effective.

(1) In a place where a large number of dots are gathered, such as an intersection portion of a character, the dots are often overlapped and the toner is raised, and the line becomes thick and unsightly when compressed during transfer. Further, one line emanating from the intersection may become thin near the intersection, and may be cut off, which may impair the character design. According to the above-described exposure method, it is possible to prevent the character design from being destroyed.

(2) When very thin lines are printed in a grid pattern, the same phenomenon as that described for the cross-shaped printed matter in FIG. Occurs almost twice as much. In the electrophotographic printer, since the image portion is composed of toner, a large amount of toner is attached to a place where the amount of light is large, and a small amount of toner is attached to a place where the amount of light is small. As a result, unevenness is generated on the photoconductor to which the toner is attached. When transferring toner to paper, the distance between the paper and the photoconductor is determined along the irregularities on the photoconductor, but the paper is regulated at the places where much toner is present, and in the vicinity of that place where there is little toner (intersection point). (In the vicinity, etc.), the distance from the paper is large, and in many cases it is not transferred to the paper, and lines of printed matter are blurred. Even in such a case, if the above-described exposure method is used, it is possible to limit the amount of printing light at the intersection point, to evenly distribute the toner, and to draw a clean grid line.

(3) In the halftone dot for printing, the exposure method according to the present invention is applied especially to a portion such as a highlight portion and a shadow portion where the dot forming the dot is very small or very complicated. By using it, a good quality of dot gain can be obtained.

[0017]

The present invention is intended to obtain good image quality by changing the control of the laser beam according to the number of dots adjacent to the print dot, and an example will be described below.

FIG. 1 shows a laser beam printer equipped with an exposure control apparatus according to an embodiment of the present invention. An exposure control circuit 12 of the present invention is attached to a circuit 11 that drives a laser diode 13 that can generate a laser beam. The laser diode 13 is turned on by the signal of the print data 14,
The laser light is condensed by the collimator lens 6, the diameter of the laser beam is converted into a light beam having a desired size by the beam expander lens 5, and the light beam is made incident on the cylindrical lens 4 and then made incident on the polygon mirror 1.

The polygon mirror 1 is a motor 2 that rotates at high speed.
Since it is fixed to the polygon mirror 1, the polygon mirror 1 also rotates at high speed as the motor rotates. The laser beams 15 ′ to 15 ″ that have entered the polygon mirror 1 are reflected by the polygon mirror 1 and are scanned at high speed, and enter the Fθ lens 3.
Upon receiving the lens action, the light passes through the mirror 7 and the toric lens 8 and is focused on the photoconductor 10 in the form of a thin line (scan beam). Further, the light ray 15 that has passed through the Fθ lens 3 is
The light reaches the origin element 9 and becomes the origin light that determines the printing start point. The photoreceptor in this example is a high gamma photoreceptor.

In the present embodiment, the spot diameter is set so as to be about 1.4 times the feed pitch of the laser beam printer. This 1.4 times value is a value determined by the usage conditions of the laser beam printer, so 1.4
It does not mean that it must be doubled. Therefore, it is of course possible to set different values according to the usage state.

When the spot diameter is set to 1.4 times the feed pitch, the dots come into contact with each other on the printed matter at a line of an angle of 45 degrees. Also, with horizontal lines and vertical lines, dots are overlapped by 1.4 times and printed. 3 and 5 schematically show such a state. FIG.
In the case of printing without changing the light quantity of the printed dots depending on the number of dots on the upper, lower, left and right sides of the dot to be printed, the printed dots have a large portion due to the influence of the overlapped dots, and the shape is deformed. 3A and 3B are printed by controlling the light amount according to an embodiment of the present invention, and are printed clearly.

3 and 4 will be described in detail below. FIG. 3 shows a print pattern schematically written. Each circle indicates a print dot, a black-painted portion 28 indicates that it is printed, and a non-black-painted portion 27 is not printed. The number 29 in the black circle indicates the value of the amount of light given to the print dot. The relationship between each number and the amount of light is as follows. The size of one dot is almost 2 ^1/2 times (1.4 times) the size of the print bitch. 0: 100% light intensity printed 1: 90% light intensity printed 2: 80% light intensity printed 3: 70% light intensity printed 4: 60% light intensity Printed with

FIG. 4 shows the print patterns shown in 0 to 4 above, and the relationship between the value of light quantity and the arrangement of dots will be described. Printing pattern 1 ... When the printing dots are independent, the printing light quantity is 100%. Print pattern 2 ... 90% when there is one dot next to the print dot. Print pattern 3 ... If there are two dots next to the print dot, it is 80%. Print pattern 4 ... If there are three dots next to the print dot, it is 70%. Print pattern 5 ... If there are 4 dots next to the print dot, it is 60%.

The one shown in FIG. 3 is printed according to such an exposure light amount, and reproducibility is extremely good. FIG. 5 shows a printing state when printing is performed without using the exposure method of the present invention. In (A), as compared with FIG. 3, the character design is deformed to the extent that it changes, and in the part where one black dot is adjacent to one white dot, the one white dot is crushed and the reproducibility is poor. Has become. (B) and (C) are print patterns 4 shown in FIG.
Also, with respect to the print pattern 5, when the exposure method of the present invention is not used, the size of the print dot varies and it cannot be said that the print is good.

In the case of a high gamma value photosensitive member used as a printing medium of a printing apparatus having an exposure amount control mechanism of the present invention, the size of the printing dot is often determined by the printing light amount, the reason for which will be explained below. To do.

Since the laser beam spot at the image forming point in this type of printing device has a Gaussian distribution, the size of the printing dot on the high gamma value photosensitive member is determined by
It is a threshold value with respect to the light amount allowed by the photoconductor. When the light amount is large, the threshold value decreases, and as a result, the spot size increases. Also, the threshold value increases as the light amount decreases, and as a result, the spot size decreases. From this, the size of the printed dot (spot size) greatly depends on the light amount, and the effect of the present invention can be sufficiently exhibited.

FIG. 8 shows a laser diode drive circuit for achieving the exposure method of the present invention. This circuit has a line buffer for three lines of image data, and sends information on dots adjacent to the print dots to the address lines on the ROM. 8-bit print data is output from the data line on the ROM to the D / A converter, converted into an analog value corresponding to the print light amount of the laser diode driver 11, and output as a signal for driving the laser diode driver. . With such a circuit configuration, the intensity of the laser beam can be controlled, and a clean printed matter can be obtained. Note that the line buffer has a total of three lines by having a buffer of lines before and after the line buffer of print dots as a center.

[0028]

As is apparent from the above description, the present invention provides an optimum exposure method for a laser beam printer or the like that uses a photosensitive medium having a high gamma value and forms an image thereon. The exposure method of the present invention is a relatively inexpensive and easy-to-control method, and is effective when an image, particularly a photographic image used for printing plate making is handled. Particularly, it is possible to improve the reproducibility of the dot gain of halftone dots, and it is possible to perform the reproducibility of fine lines in an electrophotographic printer.

[Brief description of drawings]

FIG. 1 is a schematic view of a laser beam printer using an exposure method of the present invention.

FIG. 2 is a diagram showing a relationship between a print light amount value, a print dot size, and a threshold value.

FIG. 3 is a diagram showing an exposure light amount value of each dot when a character “Z” is printed according to an embodiment of the present invention.

FIG. 4 is a diagram showing a concept of the present invention, showing a relationship between a printing dot and an adjacent dot as a printing pattern, and showing a relationship between the pattern and a light quantity value of the printing dot.

FIG. 5 is a diagram showing the thickening of dots when the exposure light amount of print dots is set according to the conventional exposure method.

FIG. 6 is a diagram showing a toner adhesion state on a photoconductor.

FIG. 7 is a diagram showing a transfer state of toner onto paper when a cross hair is printed.

FIG. 8 is a diagram showing an example of a laser diode drive circuit.

[Explanation of symbols]

 1 polygon mirror 2 polygon motor 3 Fθ lens 4 cylindrical lens 5 expander lens 6 collimator lens 7 mirror 8 toric lens 9 origin detection 10 photosensitive drum 11 laser diode driver 12 laser diode light quantity control circuit 13 laser diode 14 print data 15 origin detection light 15 ′ Printing light (TOP) 15 ″ Printing light (END) 16 Light intensity distribution when printing light intensity change is zero 17 Light intensity distribution when printing light intensity change 1 18 Light intensity distribution when printing light intensity change 2 19 Printing light intensity change 3 20 Light intensity distribution when printing light intensity change 4 21 Threshold level 22 Print dot size when print light intensity change is 0 23 Print dot size when print light intensity change 1 24 Print dot size when print light intensity change 2 25 Printing dot size when printing light amount change 3 26 Printing dot size when printing light amount change 4 27 Printing position 28 Printed dot 29 Printing light amount value 30 Printing column 31 Printing line 32 Paper 33 Photoconductor 34 Toner 35, 35 'Toner bottom 36, 36' Toner top 37 Micro dot bottom 38 Micro dot top 39 Print data horizontal line 39 'Printed horizontal line 40 Print data vertical line 40' Printed vertical line 41 Cross line intersection 41 'Printed Crossing part 42 Toner rising part 43 Toner untransferred part

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yoshikazu Suganuma 4-2-1 Motofujisawa, Fujisawa-shi, Kanagawa Inside the EBARA Research Institute, Inc. (72) Inventor Taro Takahashi 4-2-1, Fujisawa, Kanagawa No. 1 Incorporated EBARA Research Institute (72) Inventor Masatake Takashima 8-3-1 Chuo, Ami-machi, Inashiki-gun, Ibaraki Mitsubishi Chemical Corporation Tsukuba Research Institute (72) Hirofumi Nakayama Kamoshida, Aoba-ku, Yokohama-shi, Kanagawa 1000 town, Mitsubishi Chemical Co., Ltd. Yokohama Research Institute

Claims (3)

[Claims] 1. A laser beam printer for irradiating a photosensitive medium with a laser beam to attach toner to transfer it onto paper, wherein the printing is performed in accordance with the number of dots adjacent to the top, bottom, left and right of one dot to be printed. An exposure method for a laser beam printer, wherein printing is performed by changing an exposure light amount of one dot to be printed. 2. An exposure method in which the amount of exposure light is changed according to the number of dots adjacent to one dot to be printed, and the amount of exposure light decreases as the number of adjacent dots increases. The exposure method in a laser beam printer according to claim 1, wherein 3. The exposure method in a laser beam printer according to claim 1, wherein the photosensitive medium has a high gamma value.