JPH0719092B2 - Image recording method - Google Patents

Description

The present invention relates to an image recording method in an optical scanning electrophotographic recording device.

[Conventional technology]

Generally, in an optical scanning electrophotographic recording apparatus such as a laser printer, while a beam optically modulated by image information is main-scanned, a photosensitive member that is sub-scanned is sequentially exposed to form an electrostatic latent image. The electrostatic latent image is formed and visualized by toner development or the like to obtain a recorded image.

For example, in the optical scanning electrophotographic recording apparatus disclosed in Japanese Patent Laid-Open No. 56-8112, a character code signal for one page read from a magnetic tape and written at a predetermined address of a page memory is converted into 32 × 32 dots. A character pattern (which can be reproduced by 32 beam scans) is applied to a stored character generator. Then, every time the beam is scanned, a modulation signal is taken out from the character generator and applied to the beam modulator, and the beam is main-scanned while the laser beam is modulated by the beam modulator, and the beam is scanned on the photoconductor. Form a latent image. Then, this electrostatic latent image is developed with toner.

In this type of image recording method, due to rise and fall of the modulated beam, an image portion in the latent image forming area that should be visualized by attaching toner or the like and a non-image that should not be visualized. It is difficult to obtain a desired potential distribution at the boundary with the part.
Therefore, it is difficult to improve the quality of the recorded image. For example, in positive-positive recording in which a non-exposed portion not irradiated with a beam is visualized as an image portion, it is known that the image quality is deteriorated by the development of "image thinning". Further, the present inventor has found that at the boundary between the image part and the non-image part, the resolution of the image obtained by visualization becomes insufficient because a sufficient potential difference or potential contrast between both parts cannot be obtained. In the case of a document image, we also found a problem that the sharpness of the visualized characters may be lost.

Incidentally, in order to solve the problem of "image thinning" in the positive-positive recording, the above-mentioned Japanese Patent Laid-Open No.
In the '8112 publication, the width of the modulation pulse obtained based on the 32 × 32 dot character pattern is changed so that the width of the unexposed portion on the photoconductor on the image portion in the main scanning direction is relatively wide. An image recording method has been proposed. According to this image recording method, the problem that "the image becomes thin" in the positive-positive recording can be solved. However, as will be described later in detail, the invention of this known image recording method does not exert the effect of obtaining the sufficient resolution.

[Problems to be Solved by the Invention]

The present invention has been made in view of the above background, and an object thereof is to provide an image recording method in an optical scanning electrophotographic recording apparatus capable of recording an image with high resolution. .

〔Constitution〕

FIG. 1 shows a schematic structure of an optical scanning electrophotographic recording apparatus to which the image recording method of the present invention can be applied. In FIG. 1, this optical scanning type electrophotographic recording apparatus is configured such that a light source 1 including a laser diode is turned on according to binary image information BS,
By turning off, the laser beam is directly modulated. The modulated laser beam is used as an optical scanning system and a correction optical system 2.
Main scanning is performed through the charging unit 3, the charging unit 3 uniformly charges the surface of the drum-shaped photosensitive member 4 that is sub-scanned, and is sequentially irradiated to perform exposure. The electrostatic latent image formed by this exposure is visualized by toner adhesion or the like by the developing device 5, and this visualized image is transferred to the recording paper 6 in the transfer section. Instead of directly modulating the beam by turning on and off the laser diode,
A gas laser may be used as the light source 1, and the output beam from this may be optically modulated according to the binary image information using an acousto-optic modulator or the like.

The image recording method of the present invention is an image recording method in the optical scanning electrophotographic recording apparatus as shown in FIG. 1 and corresponds to image information for modulating a light beam in the optical scanning electrophotographic recording apparatus. A means capable of changing the pulse width is used. Then, at the boundary between the image part and the non-image part, the power of the exposure beam is controlled by controlling the pulse width of the light modulation corresponding to the image information so that the latent image potential difference between both parts is sufficiently large and the potential contrast is sufficiently high. Is changed appropriately. Specifically, in the main scanning of the light beam,
At the boundary between the image part and the non-image part, unlike the part other than the boundary, the ratio of the light beam exposure time to the one-pixel scanning time in the main scanning direction (hereinafter referred to as pixel frequency duty).
To be within a certain appropriate range. This gives the optimum scanning conditions of the light beam so that a recorded image with sufficient resolution can be obtained.

Hereinafter, with reference to FIGS. 2 to 5, the relationship between the pixel frequency duty and the image resolution at the boundary between the image part and the non-image part, and the appropriate range of the pixel frequency duty for obtaining a good image resolution Will be described.

FIG. 2 shows a latent image potential distribution on the photoconductor in the main scanning direction with respect to a pattern exposed every other pixel in the main scanning direction (a striped pattern repeating every black and white line extending in the sub scanning direction). It is a characteristic diagram. In this characteristic diagram, the horizontal axis represents the relative distance that is a value obtained by removing the distance from a certain reference position in the main scanning direction by the pixel pitch, and the vertical axis represents the relative distance on the photoconductor with respect to the potential of the photoconductor when the exposure energy is zero. The relative potential, which is the ratio of the latent image potential of each part, is taken. Then, as a parameter, the pixel frequency duty is changed in the range of 10 to 150%, and the latent image potential distribution is shown for the pattern exposed by using the beam of each pixel frequency duty.

This pattern is a striped pattern in which black-and-white lines (extending in the sub-scanning direction) alternate in the main scanning direction, and is a pattern that gives a minimum interval between black lines exposed every other pixel in the main scanning direction. . Therefore, it is the most difficult pattern to ensure good potential difference and potential contrast of both parts at the boundary between the image part and the non-image part, as well as the image resolution, and conversely, it is the best pattern for evaluating these. It can be said that the pattern.

As is clear from this characteristic diagram, the valley of the relative potential gradually decreases as the pixel frequency duty increases, and the crest of the relative potential gradually decreases accordingly. And, the difference between the valley and the peak of the relative potential that not only affects the image quality itself but also the potential contrast of both parts at the boundary between the image part and the non-image part,
It changes according to the pixel frequency duty. Therefore, the pixel frequency duty is one of the conditions for improving the potential difference and the potential contrast of both parts at the boundary, and by extension, the resolution of an image obtained by visualizing this pattern and the sharpness of characters. Recognize.

In FIG. 2, when a level a, which is an example of the relative potential of the visualization level (corresponding to the developing bias value used in the developing device) in positive-positive recording, is used and the pixel frequency duty is 100%. It is also understood that the black line width b and the white line width c are shown as an example depending on the pixel frequency duty (of course, each line width b, c also depends on the visualization level). . Therefore, the pixel frequency duty, together with the visualization level,
It can also be seen that this is one of the conditions for setting the black and white line widths to appropriate values.

Furthermore, in this characteristic diagram, the horizontal scanning direction relative distance on the horizontal axis corresponding to the boundary between pixels in the main scanning direction is an integer (for example,
2.0) corresponds to the inclination of the latent image potential on the photoconductor at the above-mentioned boundary, which corresponds to the density gradation of the boundary as will be described later with reference to FIGS. 4 and 5. Of the image portion and the sharpness of the edge of the image portion determines the potential difference between the image portion and the non-image portion and the magnitude of the potential contrast at the boundary. The inclination of the characteristic line in such a portion changes from an increasing tendency to a decreasing tendency as the pixel frequency duty increases in the range of 10% to 150%,
The pixel frequency duty that gives the peak is 10 to 150%
It can be seen from the fact that the proper value exists within the range of the pixel frequency duty.

Here, in selecting an appropriate value for the pixel frequency duty, from the viewpoint of exposure unevenness in the sub-scanning direction, as is well known, first, the ratio of the beam diameter to the pixel pitch in the sub-scanning direction (hereinafter referred to as the relative beam diameter). It is necessary to specify the appropriate range of That is, linear stripes generated in the main scanning direction of a recorded image due to exposure unevenness in the sub-scanning direction (in the method of visualizing the non-exposed portion, visualized matter (toner) on the non-imaged portion)
It is necessary to set the relative beam diameter so that the linear stripes due to the color of the recording paper and the linear stripes due to the color of the recording paper do not appear in the image area in the method of visualizing the exposed portion.

FIG. 3 is a characteristic diagram showing the relationship between the exposure unevenness and the relative beam diameter in the sub-scanning direction when the beam is sub-scanned while exposing the entire line in the main scanning direction.
In this characteristic diagram, the horizontal axis represents the relative beam diameter, and the vertical axis represents the peak of the surface potential of the photoconductor when the above exposure is performed with respect to the surface potential of the photoconductor when the exposure energy is zero (potential of the underexposed portion). ) Is taken as the relative potential of each. From the viewpoint of preventing uneven exposure in the sub-scanning direction, it has been found that it is desirable to set the relative potential shown in this figure to about 0.2 or less. Therefore, it can be seen from the figure that it is desirable to set the relative beam diameter to a value of about 1.0 (lower limit value) or more.

Next, under the condition that the relative beam diameter is set to about 1.0 or more, a sufficient potential difference and potential contrast between the image portion and the non-image portion are obtained in the pattern of FIG. Can get good resolution of
An appropriate range of pixel frequency duty will be described.

FIG. 4 shows the ratio of the potential difference between the peak and the valley (hereinafter referred to as the relative potential difference) to the surface potential of the photoconductor when the exposure energy is zero and the exposure pattern of FIG. It is a characteristic view which shows the relationship with the pixel frequency duty of the existing beam. In this characteristic diagram, the vertical axis represents the relative potential difference and the horizontal axis represents the pixel frequency duty.
Further, as a parameter, the relative beam diameter of the beam used for exposure of the pattern is changed, and the above relationship is shown for the pattern exposed using the beam of each relative beam diameter. Regarding this relative potential difference, the greater this is, the greater the difference between the density of the visualized material (toner) in the image area and the density of the non-image area becomes, resulting in a clearer image (sharp image of light and shade). From this viewpoint, it has been found that it is desirable that the relative potential difference has a value of about 0.6 or more. Therefore, this about 0.
Since a relative potential difference of 6 or more is secured, and the relative beam diameter of the exposure beam is set to about 1.0 or more,
The shaded area in the figure, that is, 10 to 110% is the appropriate range of pixel frequency duty.

Further, FIG. 5 shows the exposure pattern of FIG.
Potential contrast (expressed as (potential of peak-potential of valley) / (potential of peak + potential of valley) from potential of peak and valley in FIG. 2)) and pixel of beam used for exposure of the above pattern It is a characteristic view which shows the relationship with a frequency duty. In this characteristic diagram, the vertical axis represents the potential contrast and the horizontal axis represents the pixel frequency duty. Also, as a parameter,
The above relationship is shown for the pattern exposed by using the beam of each relative beam diameter by changing the relative beam diameter of the beam used for the exposure of the pattern. Regarding this potential contrast, the larger this is, the larger the density gradient at the boundary between the image portion and the non-image portion becomes,
It is possible to obtain a sharp image of the image part edge (sharp image of the image part edge) in which the density changes more rapidly at this boundary part, and from this point of view, the value of the potential contrast can be about 60% or more. Proved to be desirable. Therefore, since the potential contrast of about 60% or more is secured and the relative beam diameter of the exposure beam is set to about 1.0 or more, the shaded portion in the figure, that is, 20 to 140% is the pixel frequency duty. It becomes the proper range of.

From the relationship between FIG. 4 and FIG. 5 described above, the pixel frequency duty in order to sufficiently secure the potential difference and the potential contrast between the image part and the non-image part at the boundary, and as a result, to obtain the good resolution of the image. It can be said that the range of 20 to 110%, that is, the range in which the pixel frequency duty Td satisfies the following equation.

0.2 ≦ Td ≦ 1.1 Therefore, in the image recording method of the present invention, in exposure while scanning the light beam modulated according to the pulse width corresponding to the image information in the main scanning direction, the relative beam diameter in the sub-scanning direction is almost equal. After setting the exposure beam diameter to a predetermined value so that it falls within the range of 1.0 or more, at the boundary between the image part and the non-image part, the pixel frequency duty Td corresponds to the image information so as to satisfy the above formula. Change the pulse width of light modulation. As a result, it is possible to perform image recording with a high contrast and a high resolution regardless of whether positive-positive recording that visualizes an unexposed portion or negative-positive recording that visualizes an exposed portion. Experimentally, in recording that visualizes the unexposed area, the charging potential was 800 V and the visualization potential was 15
At 0V, the best image could be obtained when the pixel frequency duty was 60 to 70%.

In the above description, only the pixel frequency duty that greatly affects the quality of the recorded image is focused, but the quality of the recorded image is determined by many other factors such as scanning speed, beam power, and visualization potential. Therefore, in order to obtain a higher quality image, it is necessary to select an appropriate value of the pixel frequency duty after considering each of these factors.

Although various means are conceivable as means for specifically carrying out the present invention, they are not particularly shown here, but the exposure time with respect to one pixel scanning time at the boundary between the image portion and the non-image portion according to the image information. It can be easily executed by changing the ratio of, that is, the pulse width of the image information.

〔The invention's effect〕

As described above, according to the image recording method of the present invention, the exposure beam diameter is set to a predetermined value so that the relative beam diameter in the sub-scanning direction is within a range of approximately 1.0 or more, and the image portion and the non-image portion are At the boundary, by changing the pulse width of the light modulation corresponding to the image information so that the ratio of the exposure time by the light beam to the one-pixel scanning time in the main scanning direction becomes a predetermined value within the range of 0.2 to 1.1, It is possible to sufficiently secure the potential difference and the potential contrast between the image portion and the non-image portion at the boundary.
Therefore, the resolution is high, and it is possible to prevent the edge portion of the image from being blurred and impairing the sharpness of the image as in the conventional case.

For example, even when the linear image portions are close to each other at a minute interval, such as the character “Todoroki” shown in FIG. 6, it is possible to satisfactorily record in a small size such as 6 points. That is, when recording a character of "Todoroki" with a size of 6 points at 300 dpi, only the "car" portion is enlarged in FIG. 6, and the grid is also shown to show the pixel size. A portion where white and black lines are lined up for each pixel occurs in the main scanning direction indicated by arrow d. According to the recording method of the present invention, since good resolution can be ensured even in the pattern in which the exposure is performed every other pixel in the main scanning direction as described above, and the pattern that is the most difficult to obtain, such characters can be recorded well. it can.

The invention of the image recording method disclosed in the above-mentioned Japanese Patent Laid-Open No. 56-8112 for solving the problem of "the image becomes thin" in the positive-positive recording is as follows. Even when the black lines are lined up, the effect that a good resolution can be obtained is not exerted.

Hereinafter, this point will be specifically described with reference to FIG. FIG. 7 shows the latent image potential distribution in the main scanning direction when the exposure pattern of FIG. 2 is formed by using the beams with pixel frequency duty of 100%, 60%, and 10%, respectively. This is shown in the same manner as in FIG. In addition, FIG. 7 also shows a pulse e for beam modulation (exposure with H, non-exposure with L) and beam spots f and g when image frequency duty is 100% in positive-positive recording. There is.

When the image frequency duty is 100%, the beam spot f generated at the point at the relative distance of 1.0 in the main scanning direction when the pulse e is turned on moves in the main scanning direction indicated by the arrow d, and the pulse e is turned off. The same relative distance of 2.0
Disappears at the point. Then, as the pulse e is turned on again, a beam spot occurs at the same relative distance of 3.0,
This moves to the same relative distance of 4.0 and disappears. By the movement of the beam spot during this period, the beam intensity distribution (Gaussian distribution) at each value of the moving beam spot is shown.
The energy to be exposed is spatially integrated as shown in FIG. The potential of the charged photoreceptor is attenuated according to the energy. As a result, as shown in FIG. 7, a relative potential distribution from the point of the same relative distance of 1.0 to the same relative distance of 4.0 at the image frequency duty of 100% is obtained.
8 (b) and 8 (c) schematically show the state of exposure by moving the beam spot as in FIG. 8 (a), for the case of image frequency duty of 10% and 60%, which will be described later. It is shown.

Here, when the pixel frequency duty is 100%, the visualization level corresponding to the relative potential of the level h shown in the characteristic diagram of the latent image potential distribution in FIG. 7 is set (corresponding to this relative potential. When the developing bias is set to), only the portion having the width i is visualized in the portion having the relative distance of 2.0 to 3.0 corresponding to the image portion. Therefore, the visible image pattern P1 as shown in FIG. 7 is obtained. The phenomenon in which the developed width is narrower than the width corresponding to the image portion of the pulse e is the above-mentioned phenomenon of "the image becomes thin".

On the other hand, when exposure is performed at an image frequency duty of 10%, for example, the beam spot f generated at the same relative distance of 1.0 moves to a point of the same relative distance of 1.1 (a point moved by 10% of one pixel from the same relative distance of 1.0). Disappears, then the same relative distance
It occurs at the point of 3.0, moves to the same relative distance of 3.1, and disappears.
Due to the movement of the beam spot during this period, the exposed energy is spatially integrated as shown in FIG. 8 (b), and the image frequency duty is 10% as shown in FIG.
In the case of, the relative potential distribution from the point of the same relative distance of 1.0 to the same relative distance of 4.0 is obtained.

As is clear from the latent image potential distribution when the image frequency duty is 10%, the visualization level is 100% above the duty.
In the case of%, the potential becomes higher than the relative potential corresponding to the visualization level at all points at the same relative distance, and all points are visualized. As a result, it is not the image in which the black and white lines are lined up, but the visualization pattern 2 in FIG.
The black is recorded as a continuous image as shown by.
That is, it is not possible to exhibit the resolution enough to record an image in which white and black lines are lined up for each pixel. Then, the image recording method disclosed in the above-mentioned Japanese Patent Laid-Open No. 56-8112, that is, by changing the width of the modulation pulse, the width in the main scanning direction of the non-exposed portion on the photosensitive member which becomes the image portion is made relatively small. The invention of the image recording method, which is only widened, includes such an embodiment that leads to the same result as the recording when the image frequency duty is 10%.

On the other hand, according to the image recording method of the present invention, good resolution can be obtained even in a pattern in which the best resolution is the most difficult to be obtained by exposing every other pixel in the main scanning direction as described above. For example, when the image frequency duty is 60%, for example, a portion indicated by the width j in FIG. 7 is visualized, and portions not visualized remain at both ends of the portion in the main scanning direction. Therefore, as shown as the visualization pattern 3 in FIG. 7, it is possible to record an image in which white and black lines are lined up.

[Brief description of drawings]

FIG. 1 is a simplified block diagram showing a general optical scanning type electrophotographic recording apparatus. FIG. 2 is a characteristic diagram of the relative potential with respect to the relative distance for the pattern exposed every other pixel in the main scanning direction. FIG. 3 is a characteristic diagram of the relative potential with respect to the relative beam diameter when all the lines in the main scanning direction are exposed. FIG. 4 is a characteristic diagram of relative potential difference with respect to pixel frequency duty for a pattern exposed every other pixel in the main scanning direction. FIG. 5 is a characteristic diagram of potential contrast with respect to pixel frequency duty. FIG. 6 is an explanatory diagram of an example of a recorded image. FIG. 7 is an explanatory diagram of the relationship between the movement of the beam spot and the latent image potential. FIG. 8 is an explanatory diagram of the state of exposure by moving the beam spot. 1 ... Light source 2 ... Optical scanning system and correction optical system 3 ... Charging device 4 ... Photosensitive member 5 ... Developing device 6 ... Recording paper

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Claims (1)

[Claims] 1. A method of modulating a light beam according to a pulse width corresponding to image information, and exposing the modulated light beam to a pre-charged photoreceptor surface which is sub-scanned while scanning in the main scanning direction. In an optical scanning type electrophotographic recording apparatus for performing recording according to image information, a relative beam diameter, which is the ratio of the diameter of the exposure beam to the pixel pitch, and a pattern in which exposed pixels and unexposed pixels are in the main scanning direction. From the three-way relationship of the relative potential difference, which is the ratio of the potential difference between the photoconductor surface potential and the valley of the photoconductor to the surface potential of the photoconductor when the exposure energy is zero, and the potential contrast between the peak and the valley. , The exposure beam diameter is set to a predetermined value so that the relative beam diameter in the sub-scanning direction is within a range of 1.0 or more, and at the boundary between the image part and the non-image part in the main scanning direction, By changing the pulse width of the light modulation corresponding to the image information so that the ratio of the exposure time by the light beam to the scanning time for one pixel in the scanning direction becomes a predetermined value within the range of 0.2 to 1.1, An image recording method characterized in that the boundary of the image portion is sharpened.