JPS62179274A - Scanning device for laser beam printer - Google Patents

Abstract

PURPOSE:To prevent an irregular picture density resulting from an error in the fabrication or an error in the assembling of a rotary polygon mirror from outstanding by performing plural times of scannings based on the respective equal modulation signals from a modulation signal repetition output means by the use of different reflected surfaces of the same rotary polygon mirror every one line. CONSTITUTION:Based on the same picture information, the same modulation signal is in repetition outputted twice by the modulation signal repetition output means SR, the scanning is carried out by the use of the two adjacent reflected surfaces of the polygon mirror 3 and a photosensitive drum 4 is synchronously rotated by a scanning control means SC. Further, the density in the scanning line in a sub-scanning direction may be plural times above three times of the density of the dot in a main scanning direction. Namely, by the modulation signal repetition output means SR, the same modulation signals are outputted plural times above the three times, and the scanning control means SC is constituted so as to take the synchronization of the rotation of the photosensitive drum 4 in such a manner that a latent image of one line is formed by the plural scannings above the three times according thereto.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention is capable of scanning a photoreceptor for one line by reflecting and scanning a laser beam modulated based on image information for one line by a rotating polygon mirror. The present invention relates to a scanning device for a laser beam printer that forms a latent image.

[Conventional technology]

Rotating polygon mirrors are widely used as scanning means in scanning devices of laser beam printers because of their excellent high-speed scanning accuracy. A scanning device for a conventional laser beam printer is configured to scan line by line based on image information for one line using one reflective surface of one rotating polygon mirror.

[Problem that the invention seeks to solve]

However, the conventional configuration described above has the following problems.

That is, each rotation of the rotating polygon mirror forms a latent image of the same number of lines as the number of its reflecting surfaces. That is, a latent image for one line formed by scanning by reflection from the same reflecting surface is repeated for each number of reflecting surfaces forming the rotating polygon mirror. By the way, each reflective surface has slight uneven reflectance or inclination, and there are also errors during assembly, so the attenuation of the latent image formed by scanning using each reflective surface The potential varies in the direction in which each scanning line is lined up, that is, in the sub-scanning direction. Therefore, in the output image as a final result, variations in density occur in the sub-scanning direction.

As described above, since the number of reflective surfaces and the number of lines are the same, the variation occurs for every line of the same number as the number of reflective surfaces.

For example, a rotating polygon mirror is composed of 10 reflecting surfaces, and the scanning line density in the sub-scanning direction on the photoreceptor is 10 lines and 7 ms.
In the case of , the above-mentioned density variations occur repeatedly for each lta. If this variation is repeated approximately every 1 w, the resulting image will become noticeably uneven.

On the other hand, in order to make the above-mentioned image unevenness less noticeable, it is conceivable to scan using a plurality of reflecting surfaces of a rotating polygon mirror.In other words, in this case, the above-mentioned density variations may be repeated. , by appearing in a smaller pinch,
It becomes less noticeable. However, with this configuration, only the laser beams reflected by the reflective surfaces of the rotating polygon mirror are involved in latent image formation, so the energy efficiency of the output laser beams is low. Put it away.

In view of the above-mentioned circumstances, an object of the present invention is to make variations in image density caused by manufacturing errors, assembly errors, etc. of a rotating polygon mirror less noticeable while avoiding a decrease in the energy efficiency of a laser beam as much as possible. It is in.

[Means for solving problems]

The characteristic configuration of the scanning device of the laser beam printer according to the present invention is that the same modulation signal based on the same image information is transmitted to the laser beam, which forms a latent image on the photoreceptor by being reflected and scanned by a rotating polygon mirror. A modulation signal repetition output means is provided for repeatedly outputting a plurality of lines, and a plurality of scans based on the same modulation signals from the modulation signal repetition output means are performed on different reflecting surfaces of the same rotating polygon mirror for each line. The invention is based on the provision of a scanning control means that uses the .

[For production]

In other words, the latent image for each line formed on the photoreceptor is formed by repeating scanning multiple times using different reflective surfaces of the same rotating polygon mirror based on the same image information. The number of lines of the latent image formed by one rotation of the rotating polygon mirror is less than half the number of reflective surfaces constituting the rotating polygon mirror. Therefore, compared to the conventional structure in which a latent image with the same number of lines as the number of reflecting surfaces is formed by one rotation of the rotating polygon mirror, it is possible to As a result, it is possible to reduce the pinch caused by variations in density in the sub-scanning direction to less than half.

Moreover, since the laser beam reflected from all the reflective surfaces of the rotating polygon mirror is used to form the latent image, the energy efficiency that would occur if the rotating polygon mirror was configured to use multiple reflective surfaces for each surface is reduced. The decline can be eliminated.

In particular, when a semiconductor laser is used, the cross-sectional shape of the laser beam is generally elliptical, so by aligning the short axis direction with the sub-scanning direction, a desired beam diameter that is wide in the main-scanning direction can be obtained. A laser beam with a circular cross-sectional shape can be obtained without a beam-shaping optical system, and by overlapping a plurality of them in the sub-scanning direction, it is possible to form a latent image that is almost the same as a laser beam with a circular cross-sectional shape. It becomes possible.

〔Example〕

The present invention will be described in detail below based on the drawings.

FIG. 1 shows a schematic configuration of a scanning device of a laser beam printer. The laser beam (B) modulated based on the input image impression and oscillated from the semiconductor laser (1) is
The collimator lens (2) converts the light into parallel light, which is reflected by the reflective surface of a polygon mirror (3) that rotates at high speed. By rotating this polygon mirror (3), which is an example of a rotating polygon mirror, the inclination of its reflecting surface with respect to the laser beam (B) changes, and accordingly, the laser beam (B) after reflection changes as follows:
The photoreceptor drum (4), which is an example of a photoreceptor, is scanned in the longitudinal direction. (This direction is the main scanning direction).

This laser beam (B) is focused by an fθ lens (5), forms an image on the uniformly charged photoreceptor drum (4), and reduces the charged potential at that position. By repeating this scanning as the photoreceptor drum (4) rotates at a constant speed, an electrostatic latent image is formed on the photoreceptor drum (4). Note that (6) in the figure is an optical sensor for laser beam detection that is used to align each scanning start position with respect to the rotational direction of the photosensitive drum (4), which is the sub-scanning direction.

Thereafter, although not shown in the drawings, toner, which is a colored pigment, is selectively attached to this electrostatic latent image area to create an image, and an output paper is adhered to the toner image surface to transfer the toner image onto the paper surface. By heating the paper, the toner is melted and fused to the paper, producing an output image.

In the scanning device of this laser beam printer, the scanning line density in the sub-scanning direction is set to twice the dot density in the main scanning direction. In addition, it is configured to form an electrostatic latent image for one line by repeating scanning twice using adjacent surfaces of the polygon mirror (3) based on the same image information for the line. be. This results in
This makes it possible to make variations in the density of the output image less noticeable.

In other words, the scanning of the laser beam (B) is controlled by the polygon mirror (
In the case of the configuration performed by rotation (3), density variations due to non-uniform characteristics of the reflective surfaces and tilt errors are repeated every scanning line of the same number as the number of reflective surfaces. This variation becomes less noticeable as it is repeated at shorter intervals. That is, according to the above-mentioned configuration, compared to a configuration in which an electrostatic latent image for one line is formed by scanning using one surface of the polygon mirror (3), the repetition interval of the density variation can be reduced. It can be halved.

On the other hand, in this configuration, one pixel is composed of two dots lined up in the sub-scanning direction, so a laser beam with a circular cross-section (e.g., a gas laser) is used to determine the potential of each dot. When an image is formed, each pixel becomes an ellipse long in the sub-scanning direction. Therefore,
The cross-sectional shape of the laser beam (B) is an ellipse long in the main scanning direction. The graphs in FIGS. 2(A) and 2(B) show this situation using the intensity distribution of the laser beam (B).

The laser beam (B) used here has a beam diameter in the sub-scanning direction defined by the half width of [70 μm] and a beam diameter in the main scanning direction of [100 μm]. Further, the scanning line interval in the sub-scanning direction is set to [50 μm].

The graph in Figure 2 (A) shows two laser beams (B).
The intensity distributions of each are shown separately, and the graph in FIG. 2 (b) shows a composite of them.

As is clear from the comparison with the graph in Figure 3 showing the intensity distribution of a laser beam (It) with a perfect circular cross-sectional shape and a beam diameter of [100 μm], the two shown in Figure 2 (El) Although the laser beams (B) are combined, the intensity distribution in the sub-scanning direction is almost the same as that of a beam whose cross-sectional shape is a perfect circle. On the other hand, in the main scanning direction, the beam diameter of each laser beam is [100μ
m], and the intensity distribution of the composite product also resembles that of a product with a perfect circular cross-sectional shape. That is, it can be said that the intensity distribution of the combined two laser beams (B) is almost equal to that of a single laser beam with a perfect circular cross-sectional shape.

In reality, these two laser beams (B) are irradiated onto the photoreceptor drum (4) at different times through two scans, but the charges on the photoreceptor drum (4) differ from each other. Since it is attenuated according to the intensity of the laser beam (B), as a result, a latent image that is almost the same as that produced by irradiation with a single laser beam is formed.

Note that the laser beam (B) in this example is composed of a half-fold laser (1), and as shown in FIG. 4, the far field pattern has an elliptical shape.
The laser beam (B) having an elliptical cross-sectional shape long in the main scanning direction is realized without using a beam shaping optical system. If a gas laser or the like is used, a beam shaping optical system may be provided as appropriate.

Next, scanning by the scanning device of this laser beam printer will be explained using the block diagram of the control device shown in FIG.

The signal (PCLIO) is a clock signal for each pixel, and the signal (SO3) is an exposure start signal input from the laser beam detection optical sensor (6) in FIG.

Image information for one page is written in the image memory (7). For example, its capacity is 8 bits per l dot,
As 4096 dots x 4096 dots per page,
It is L6MByte. Furthermore, the upper 12 bits of the address are set as a vertical address, and the lower 12 bits are set as a horizontal address.

When entering the print cycle, both the horizontal read counter (8) and the vertical read counter (9) are cleared and wait for input of the exposure start signal (SO5). First exposure start signal (
When SOS) is input, the horizontal timing generator (10
) starts counting the clock signal (PCLK), and when the counted value reaches a predetermined value, the horizontal read counter (8)
and gate buffer (11) to instruct the start of the image area.

In the image area, a horizontal readout counter (8) is counted up according to the clock signal (on PCL). on the other hand,
Image memory (7) according to the clock signal (PCLK)
The data at the corresponding address above is stored in the gate buffer (11
) and is sent to the DA converter (12) where it is converted into an analog signal. This analog signal is used by the laser drive circuit (
13), and output modulation of the semiconductor laser (1) is performed accordingly.

When the output of the modulation signal based on one line of image information is completed, the horizontal timing generator (10) instructs the horizontal read counter (8) and gate buffer (11) to end the image area. As a result, counting of the clock signal (PCLK) by the horizontal read counter (8) is stopped, the gate buffer (11) is closed, and the next exposure start signal (SOS) is waited for.

On the other hand, a frequency divider (14) for dividing the exposure start signal (SOS) into 1/2 is provided before inputting the signal to the vertical read counter (9). In other words, the exposure start signal (SOS) is input to the vertical read counter (9) every other time.

Therefore, after the above-mentioned first exposure start signal (SO3) is output from the laser beam detection optical sensor (6) and the first scan is performed, the second exposure start signal (SO3) is output to detect the laser beam. Even if the output is from the optical sensor (6),
The output address signal from the vertical read counter (9) to the image memory (7) is not counted up. Therefore, the same data as the previous one is read out from the image memory (7), and based on that, the second scan is performed using the reflective surface of the polygon mirror (3) that is adjacent to the previously used reflective surface. It has become.

That is, the modulation signal repetition output means (S) repeatedly outputs the same modulation signal based on the same image information for one line using the frequency divider (14) and the vertical readout counter (9).
R).

The scan control means (SC) shown in FIG. 1 controls the rotation of the photoreceptor drum (4) and the rotation of the polygon mirror (3) so that an electrostatic latent image for one line is formed by these two scans. It is designed to be synchronized with the rotation. Therefore, by repeating the above-mentioned two scans, an electrostatic latent image for one page is sequentially formed.

In the above embodiment, the scanning line density in the sub-scanning direction is twice the dot density in the main scanning direction.

In this case, one pixel was formed from two dots separated by half a line in the sub-scanning direction.

Alternatively, one pixel may be formed from two dots that overlap in the sub-scanning direction. Although not shown, the configuration for this purpose is to configure the polygon mirror (3) with an even number of reflective surfaces, and to make every other reflective surface inclined with respect to the adjacent reflective surface, the photoreceptor drum (4) It is conceivable to configure the irradiation areas of the laser beam (B) on the upper part to overlap with each other. That is, in this case, this polygon mirror (3) also becomes a component of the scan control means (SC).

Furthermore, the scanning line density in the sub-scanning direction may be made to be three or more times the dot density in the main scanning direction.

That is, in the previous embodiment, based on the same image information,
The modulation signal repetition output means (SR) outputs the same modulation signal twice, and scans twice using two adjacent reflecting surfaces of the polygon mirror (3), while
The photosensitive drums (4) were synchronously rotated by a scanning control means (SC). Instead of this, the modulation signal repetitive output means (SR) is configured to output the same modulation signal three or more times, and the scan control means (SC)
1 by three or more multiple scans accordingly.
The configuration may be such that the rotation of the photosensitive drum (4) is synchronized so that a line latent image is formed.

In addition, in order to reduce variations in output images in the main scanning direction, the configuration is such that an electrostatic latent image is formed by simultaneously irradiating the same area of the photoreceptor drum (4) with laser beams from two scanning devices. It is also possible to apply the configuration according to the present invention to each scanning device.

〔Effect of the invention〕

As described above, in the scanning device of the laser beam printer according to the present invention, the latent image for each line formed on the photoconductor is formed on different reflecting surfaces of the same rotating polygon mirror based on the same image information. The latent image is formed by repeating scanning multiple times, and the number of lines of the latent image formed by one rotation of the rotating polygon mirror is less than half the number of reflective surfaces that make up the rotating polygon mirror. Compared to the conventional structure in which a latent image with the same number of lines as the number of reflecting surfaces is formed by one rotation of the rotating polygon mirror, the latent image is caused by uneven reflectance or tilting of each reflecting surface of the rotating polygon mirror. Since the number of pinches caused by density variations in the sub-scanning direction can be reduced by more than half, unevenness in the final output image can be made less noticeable.

Moreover, since scanning using all the reflective surfaces of the rotating polygon mirror can be used to form a latent image on the photoreceptor, the latent image can be formed by scanning using every plurality of reflective surfaces of the rotating polygon mirror. Energy efficiency can be increased compared to when forming

In particular, when a semiconductor laser is used, it becomes possible to omit an optical system for beam shaping, and it is possible to make the scanning device more compact and reduce costs.

[Brief explanation of drawings]

The drawings show an embodiment of a scanning device for a laser beam printer according to the present invention, and FIG. 1 is a schematic diagram of the scanning device, and FIG. 2 (a) is a graph showing the intensity distribution of two laser beams in the sub-scanning direction. Figure 2 (b) is a graph showing the intensity distribution in the sub-scanning direction due to the overlap of two laser beams, Figure 3 is a graph showing the intensity distribution of a single laser beam, and Figure 4 is a graph showing the intensity distribution of a laser beam from a semiconductor laser. A perspective view showing oscillation, and FIG. 5 is a block diagram of a control device for a laser beam printer. (3)...Rotating polygon mirror, (4)...Photoreceptor, (B)...Laser beam, (SR)...
...Modulated signal repetition output means, (SC)...
Scanning control means.

Claims (1)

[Claims] In the scanning device of a laser beam printer, which forms a latent image for one line on a photoreceptor by scanning a laser beam modulated based on one line of image information by reflecting it on a rotating polygon mirror, the scanning device uses the same image information. A modulation signal repetition output means for repeatedly outputting the same modulation signal for the laser beam based on the laser beam a plurality of times for one line is provided, and the plurality of scans based on the respective same modulation signals from the modulation signal repetition output means are performed in one line. A scanning device for a laser beam printer that is equipped with a scanning control means that uses a different reflecting surface of the same rotating polygon mirror for each line.