JPH0440070A - image recording device - Google Patents

Abstract

PURPOSE:To realize a high-definition half-tone picture which is stable, has high resolution and reduces the uneveness of density by setting the size of a threshold value matrix in a sub-scanning direction intersecting orthogonally with a laser beam scanning direction at the 1/2 of the number of mirrors in a rotaty polygon mirror, and providing a means to detect the rotating position of the rotary polygon mirror. CONSTITUTION:The size of the threshold value matrix in the sub-scanning direction intersecting orthogonally with the laser beam scanning direction is set to the 1/2 of the number of mirrors in a rotary polygon mirror 3, and a detecting means 6 is provided to detect the rotating position of the rotary polygon mirror 3. Therefore, the reflection surface of the prescribed rotary polygon mirror can be used to the threshold value matrix so as to specify the mirror reflecting a laser beam during scanning and to reduce the fluctuation of a picture element size based on the fluctuation of a laser scanning position caused by the falling-down of the reflection surface, the uneveness of density can be reduced and the size of the threshold value matrix in the sub-scanning direction can be set small. Thus, the high-resolution and high-definition half-tone picture can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an image recording device that records images using laser light, and more specifically, to an image recording device that can reduce density unevenness and set a small matrix size in the sub-scanning direction. , relates to an image recording device that can record high-resolution, high-quality halftone images. For example, it can be applied to laser printers, digital copying machines, digital color copying machines, and the like.

Laser printers, which combine electrophotographic technology and laser scanning technology, can use plain paper, produce high-quality images at high speed, and have low operating noise, so they can be quickly used as computer output devices or digital copying devices. It is becoming popular as a machine. A typical laser printer reflects a laser beam using a rotating polygon mirror, exposes and scans a drum-shaped photoreceptor that is driven in a sub-scanning direction that is perpendicular to the main-scanning direction of the laser beam, and creates an electrostatic latent image. Form. After this electrostatic latent image is developed with toner, it is transferred and fixed onto paper to obtain an output image.

In order to digitally reproduce a half-tone image using this type of image recording device, the shading of the image is compared with a threshold matrix of a fixed size made up of multiple pixels, and the shading is calculated based on the area of the dot, that is, the area within the matrix. A pseudo-intermediate reproduction method, which is an area gradation method for converting to the number of recording pixels, is generally used.

For example, the shading of one pixel of an image or multiple pixels within a certain area is compared with a threshold matrix that has threshold values that gradually increase from the center outward, and the threshold value of the threshold matrix is determined. The part with a density higher than the value is output as black. Therefore, the shading of the image is reproduced as the size of the area. In the following explanation, an example will be given in which one pixel is reproduced with a plurality of dots.

FIG. 9 shows an example of a 4×4 threshold matrix. Using this threshold matrix and a six-sided rotating polygon mirror, an image having the density of the 12th gradation is schematically created using the pseudo halftone method. A graphical representation is shown in Fig. 10. 10th
In the figure, the horizontal axis is the main scanning direction in which the laser beam is scanned, the vertical axis is the sub-scanning direction in which the photoreceptor is moved, and the numbers 1 to 6 written on the left end are the rotating polygons in which the laser beam is scanned. Shows the number of the mirror surface of the mirror. The laser beam is reflected by the mirror surfaces 1 to 6 of the rotating polyhedron and forms an image on the photoreceptor.If the 6-turn polygon mirror is a regular hexahedral rigid body and rotates at a constant speed without axis tilt, the laser beam is uniform as shown in the figure. However, if the mirror surface of the rotating polygon mirror falls down, that is, if wobble occurs, the length of the halftone dot P in the cross-shaped pattern in the sub-scanning direction varies. Therefore, when the image is visually observed, density unevenness (bunching) occurs in the sub-scanning direction, which significantly deteriorates the image quality.

In order to solve this problem, for example, Japanese Patent Laid-Open No. 63-23702
Publication No. 2 has been proposed. This publication is
By setting the number of mirror reflecting surfaces of the rotating polygon mirror to an integral multiple of the threshold matrix size or repetition period in the sub-scanning direction, the scanning position error caused by the tilting of the mirror reflecting surface of the rotating polygon can be reduced. Uneven rotation (jitter)
, and reduce wobble. In other words, since the size of the threshold matrix in the sub-scanning direction is equal to the number of mirror reflection surfaces of the rotating polygon mirror, each matrix is similarly affected by scanning pitch unevenness, so there is no banding without variation in dot area. will no longer occur. However, since the size of the threshold matrix in the sub-scanning direction is limited by the number of surfaces of the rotating polygon mirror, for example, when the recording density is 400 DPI (Dots per In
In the optical system of an 8-sided rotating polygon mirror, halftone reproduction is performed at a pixel density in the sub-scanning direction of 400/8 = 50 DPI (0,508 m pitch), resulting in a rough (low resolution) image. There was a problem with it being put away.

l-15 The present invention was made in view of the above-mentioned circumstances.
The object of this invention is to provide an image recording apparatus that provides stable, high-quality halftone images with high resolution and no density irregularities.

In order to achieve the above object, the present invention includes: (1) comparing an image signal with a threshold matrix, modulating a laser beam based on the comparison output, and scanning the modulated laser beam with a rotating polygon mirror; In an image recording device that reproduces halftones, the size of the threshold matrix in the sub-scanning direction perpendicular to the laser beam scanning direction is set to 1 of the number of mirror surfaces of the polygon mirror per revolution.
/2 and has a detection means for detecting the rotational position of the rotating polygon mirror, or (2) compares the image signal with a threshold matrix, modulates the laser beam by the comparison output, and modulates the laser beam. In an image recording device that reproduces halftones by scanning a laser beam with a rotating polygon mirror, the size of the threshold matrix in the sub-scanning direction perpendicular to the laser beam scanning direction is set to 1 of the number of mirror surfaces of the rotating polygon mirror. /N(N
: an integer of 2 or more and a divisor of the number of mirror surfaces), and the reflecting surfaces used for recording of the rotating polygon mirror are every N-1 surfaces, and (3) in (2) above, a plurality of It has threshold matrices with different matrix sizes in the sub-scanning direction, selects the threshold matrix by an external signal, and has a switching means for switching N (N: an integer of 2 or more and a divisor of the number of mirror surfaces). or (4) in an image recording device that compares an image signal with a threshold matrix, modulates a laser beam based on the comparison output, and scans the modulated laser beam with a rotating polygon mirror to reproduce halftones. , the size of the threshold matrix in the sub-scanning direction is 1/2N of the number of mirror surfaces of the rotating polygon mirror (N: an integer of 2 or more and a divisor of the number of mirror surfaces), which is used for recording among the reflective surfaces of the rotating polygon mirror. (5) The reflecting surface is arranged every N-1 planes, and has a detection means for detecting the rotational position of the rotating polygon mirror.
In 4).

It is characterized by having a plurality of threshold matrices with different matrix sizes in the sub-scanning direction, selecting the threshold matrix by an external signal, and having a switching means for switching N (N: an integer of 2 or more). It is something. Hereinafter, the present invention will be explained based on examples.

FIG. 1 is a configuration diagram for explaining an embodiment of an image recording apparatus according to the present invention. In the figure, 1 is a semiconductor laser (LD) having a modulation function that emits a laser beam modulated according to an image signal. ), 2 is a lens, 3 is a rotating polygon mirror (polygon scanner), 3a is a mirror reflection surface of the rotating polygon mirror 3, and 4 is an imaging lens (fθ).

5 is a photoreceptor, 6 is a rotational position sensor, and 7 is a light receiving element (synchronization detection element). That is, the laser optical system is equipped with a rotational position sensor 6 that detects the rotational position of a rotating polygon mirror, and is capable of specifying the mirror reflecting the laser beam being scanned.

A laser beam modulated according to an image signal from a semiconductor laser (LD) is reflected by a mirror reflection surface 3a of a rotating polygon mirror (polygon scanner) 3 via a lens 2, and is reflected by an imaging lens (
An image is formed as a minute spot on the photoreceptor 5 by the fθ lens) 4. This minute spot moves in the main scanning direction by the rotation of the rotating polygon mirror 3, and the photoreceptor 5 rotates and moves in the sub-scanning direction of arrow B, thereby exposing the photoreceptor 5,
Forming an electrostatic latent image of the image. When starting main scanning, it is necessary to perform control to set a minute spot at the image writing start position. The light receiving element (synchronization detection element) 7 is installed outside the image range of the scan start example on the scanning line, and outputs a synchronization signal for controlling the image writing start position.

FIG. 2 is a perspective view of a rotating polygon mirror used in a laser scanning optical system, where 31 is a polygon mirror and 32 is a scanner motor. The polygon mirror 31 is.

The scanner motor 32 has a polygon mirror reflection surface (hereinafter referred to as a mirror reflection surface) 3a whose outer circumferential surface is precisely divided into equal parts by planes parallel to the axis, and the scanner motor 32 has a polygon mirror reference surface 3C on the polygon mounting surface 3C of the scanner motor 32. It is fixed by face-to-face contact with surface 3b. The rotating polygon l1 configured in this way
The mirror reflecting surface 3a of No. 13 is ideally a surface parallel to the rotation axis, but in reality there is a surface inclination due to machining errors. This surface inclination is the polygon mirror reference surface 3b of the mirror reflecting surface 3a. This is caused by variations in the perpendicularity to the polygon mirror 32 and deflection in the thrust direction of the polygon mounting surface 3c of the scanner motor 32, and surface tilt due to the clearance between the central axis of the inner diameter of the polygon mirror 31 and the processing axis during processing of the polygon mirror 31. The cause is the inclination of the chucking during lathe machining of the rotor. Such inclination of the mirror reflecting surface 3a results in sinusoidal fluctuations having a period of one rotation.

FIG. 5 shows an example of the inclination of the reflecting surfaces when the rotating polygon mirror 3 has eight mirror reflecting surfaces 3a. FIG. 5 shows the degree of inclination of the mirror reflecting surface 3a with respect to the reference plane 0-X in the order of the moving reflecting surface. 3.4 side and 5.6, 7.8
Since the surfaces are symmetrical, one threshold matrix is formed from surfaces 1.2 and 3.4, and the next threshold matrix is formed from surfaces 5.6 and 7.8 to create pseudo halftones. By reproducing the above, it is possible to cancel the pitch unevenness of the main scanning line caused by the surface tilt which is set to one rotation period with respect to the rotation axis of each mirror reflection surface 3a of the rotating polygon t/A3.

However, as shown in FIG. 6, for example, the first and fifth surfaces are on the reference plane ox, and the 2.3.4 and 6.7.
When the 8th plane is in a symmetrical position with respect to the reference plane O-X,
Although it is impossible to completely cancel the pitch unevenness of the main scanning line, one threshold matrix is used for surfaces 1.2 and 3.4, and the next threshold is used for surfaces 5.6 and 7.8. If the matrix is configured, the main scanning line pitch unevenness can be reduced.

FIG. 7 is a diagram showing a threshold matrix, and is a diagram showing the relationship between the surface number of the mirror reflective surface 3a and the threshold matrices 1 and 2 corresponding to each mirror reflective surface. As shown by this relationship, the size of the matrix in the sub-scanning direction is 172, which is the number of mirror surfaces of the rotating polygon mirror 3. In this way, the threshold matrix 1 or 2 corresponding to the surface number of the mirror reflection surface 3a needs to detect the surface number, for example, the first surface, which is a reference in the correspondence relationship. The rotational position sensor 6 detects the rotational position of the rotating polygon mirror 3, and is a position detection sensor such as a reflective optical sensor that detects marks placed on the polygon mirror 31. Incidentally, since the rotational position sensor 6 detects the reflective surface during reflective scanning, it is not limited to the above-mentioned reflective optical sensor, and may be attached to the motor side, for example.

FIG. 3 is a block diagram showing the signal system of the image recording apparatus according to the present invention. In the figure, 11 is a binarization circuit, 12 is a threshold matrix, 13 is a semiconductor laser (LD) modulation circuit, and 14 is a A semiconductor laser (LD) 15 is a surface selection circuit. The image signal is compared with the threshold value of the threshold matrix 12 in the binarization circuit 11, and is binarized.

The output of the binarization circuit 11 is output from a semiconductor laser (LD) 1 based on the binarization signal inputted by the laser modulation circuit 13.
4 is ON10FF controlled. At this time, the rotating polygonal mirror position signal (polygon position signal) from the rotational position sensor 6
A surface selection signal is output from the surface selection circuit 15 based on the synchronization signal from the synchronization detection element 7, and the surface selection signal is used to select the mirror reflection surface 3 used for writing out the threshold matrix.
a is selected. In the present invention, the mirror reflecting surface 3a
is selected to provide a threshold matrix configuration that reduces pitch unevenness of main scanning lines. In addition, the mirror reflection surface 3
As mentioned above, the inclination of a is due to machining errors and differs for each product, so the value of the inclination is measured in advance.

FIG. 4 is a timing chart corresponding to the block diagram of FIG. 3. A is a position detection signal that is transmitted every rotation of the rotating polygon mirror 3a, B is a synchronization signal that is transmitted for each surface number of a mirror reflecting surface selected based on the position signal of the rotational position sensor 6, and C. starts main scanning with a synchronization signal,
This C image signal corresponds to the embodiment of claim 1, which is a semiconductor laser (LD) modulation signal that is output after being compared with a threshold matrix. The first to fourth surfaces of the mirror reflecting surface are compared with threshold matrix 1, and the fifth to eighth surfaces are compared with threshold matrix 2, respectively.

Next, other embodiments of the present invention will be described.

This is a system in which a threshold matrix is constructed with one rotation of a rotating polygon mirror when recording halftones.For example, when reproducing halftones with a 4x4 threshold matrix using an 8-sided rotating polygon mirror, , a rotating polygon mirror is used on every other surface, and the seventh
1 of the rotating polygon mirror as shown in the rightmost number of the surface number in the figure.
3.5.7 planes always constitute a threshold matrix. N
is an integer greater than or equal to 2 and is a divisor of the number of mirror reflective surfaces 3a, the size of the threshold matrix is set to 1/N of the number of mirror reflective surfaces 3a of the rotating polygon mirror 3, and the rotation used for recording is The mirror reflection surfaces 3a of the polygon mirror 3 are arranged every N-1 surfaces. As a result, all threshold matrices are affected by the same main scanning line pitch unevenness, and a high-quality intermediate image without banding can be obtained.

Note that since banding is less likely to occur when recording a binary image without using a threshold matrix, the mirror reflection surfaces 3a of all the rotating polygon mirrors 3 may be used for scanning.

Timing chart D in FIG. 4 is a timing chart when, as described above, the mirror reflecting surfaces are every other first and third 5.7-degree surfaces, and each mirror surface corresponds to threshold matrix 1. The above corresponds to claim 2 of the present invention.

Next, still another embodiment of the present invention will be described. Typically, in digital copying machines, the size of the threshold matrix is switched depending on the original to be copied. That is, for originals containing text and photographs, high resolution is obtained with a small threshold matrix, but for originals containing only photographs, the number of gradations is increased by using a large threshold matrix. There is. In the present invention, there are threshold matrices of multiple sizes, and the surfaces to be used of the rotating polygon mirror are selected according to the sub-scanning size of the threshold matrix so that one matrix is formed in one rotation of the rotating polygon mirror. It includes doing. As a result, all threshold matrices are affected by the same scanning line pitch unevenness, so a high-quality halftone image without banding can be realized. For example, if an 8-sided rotating polygon mirror 3 has two threshold matrices of 4X4 and 2X2, recording will be performed on every other reflective surface when the mirror is 4x4, and when it is 2x2, as shown in Figure 8. As shown at the left end of the surface number, the reflective surfaces of the rotating polygon mirror 3 are used every three times, for example, the first surface and the fifth surface. That is, a plurality of threshold matrices with different matrix sizes in the sub-scanning direction are selected by an external signal to switch the number N of the mirror reflection surface 3a (N: an integer of 2 or more and a divisor of the number of mirror surfaces). It has a switching means. As a result, all the threshold matrices are affected by the same scanning line pitch unevenness, so a high-quality halftone image without banding can be realized. In this case as well, when recording a binary image without using a threshold matrix, banding is less likely to occur, so the mirror reflection surfaces of all rotating polygon mirrors may be used for scanning.

Next, still another embodiment of the present invention will be described. As shown in FIG. 1, this has a rotational position sensor for a rotating polygon mirror, and selects the reflecting surface of the rotating polygon mirror to be used.

For example, if a rotating polygon mirror 3 with eight surfaces has a 2×2 threshold matrix and has a surface tilt characteristic as shown in FIG. , and form the next threshold matrix on the 6th and 8th surfaces, so that the reflective surface used for writing out the threshold matrix has a threshold matrix configuration that reduces pitch unevenness of the scanning line. 8, and use the mirror reflection surfaces 3a of the rotating polygon mirror 3 every other surface as shown at the right end of the surface numbers in FIG. In other words, the size of the threshold matrix in the sub-scanning direction is set to 1/2N of the number of rotating mirror surfaces.
(N: an integer of 2 or more, a divisor of the number of mirror surfaces), and detecting means for detecting the rotational position of the rotating polygon mirror by setting the reflecting surfaces used for recording every N-1 of the reflecting surfaces of the rotating polygon mirror. It is something that you have.

Next, still another embodiment of the present invention will be described. As shown in Figure 1, this system has a rotational position sensor for a rotating polygon mirror, has threshold matrices of multiple sizes, and adjusts the position of the rotating polygon mirror according to the sub-scanning size of the threshold matrix. This is to select the reflective surface to be used. For example, if an eight-sided rotating polygon mirror 3 has two threshold matrices of 4X4 and 2X2 and has surface tilt characteristics as shown in FIG. 2.4 and 6.8 to form one matrix, and in the case of a 2×2 threshold matrix, the second and second surfaces of the mirror reflection surface It is also possible to form one matrix with four surfaces and form the next matrix with the sixth and eighth surfaces. In this case, alternate reflecting surfaces of a rotating polygon mirror as shown at the right end of the surface numbers in FIG. 8 are used.

In the above explanation, 8-sided rotating polygon mirror, 4×4 or 2×
Although an example using a threshold matrix of 2 has been described, the present invention can also be applied to other numbers of surfaces and matrix sizes.

As is clear from the above description, the present invention has the following effects.

(1) In order to reduce fluctuations in pixel size due to changes in laser scanning position due to tilting of the reflection surface, the reflection surface of the rotating polygon mirror is used in a predetermined manner with respect to the threshold matrix. Since the threshold matrix size in the scanning direction can be set small, a high-resolution and high-quality halftone image can be obtained.

(2) Even if you reduce the matrix size in the sub-scanning direction,
In order to select and use the reflective surface of the rotating polygon mirror so that the threshold matrix size is recorded in one rotation of the rotating polygon mirror,
Fluctuations in pixel size due to fluctuations in laser scanning position due to tilting of the reflective surface are canceled, and a high-resolution, high-quality halftone image without density unevenness can be obtained.

(3) Since it has multiple threshold matrices, an appropriate matrix size can be selected for each type of image, and the reflecting surface of the rotating polygon mirror can be adjusted so that the threshold matrix is recorded in one rotation of the rotating polygon mirror. Since it is selectively used, fluctuations in pixel size due to fluctuations in laser scanning position due to tilting of the reflective surface are canceled, and it is possible to obtain a high-quality halftone image with uniform density and little deterioration in image quality.

(4) It is possible to reduce the pixel size due to fluctuations in laser scanning position due to reflection surface tilt, and to set the threshold matrix size in the sub-scanning direction small, creating high-resolution, high-quality halftone images with less density unevenness. Obtainable.

(5) Since it has multiple threshold matrices, an appropriate matrix size can be selected for each type of image, and the threshold matrix By using the reflective surface of a rotating polygon mirror with a predetermined value, density unevenness can be reduced, and the threshold matrix size in the sub-scanning direction can be set small, resulting in high-resolution, high-quality halftone images with less density unevenness. Obtainable.

[Brief explanation of drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of an image recording apparatus according to the present invention, FIG. 2 is a perspective view of a rotating polygon mirror, and FIG. 3 is a circuit block diagram of FIG. 1. 4 is a timing chart of the circuit shown in FIG. 3, FIGS. 5 and 6 are diagrams for explaining the inclination of the reflective surface of the polygon mirror, and FIG. 7 is a threshold matrix according to the present invention. 8 is a diagram showing another threshold matrix according to the present invention, FIG. 9 is a diagram showing a conventional threshold matrix, and FIG. 10 is a diagram schematically showing a halftone image. This is a diagram. 1... Semiconductor laser (LD), 2... Lens, 3...
... Rotating polygon mirror, 3a... Mirror reflecting surface, 4... Imaging lens, 5... Photoreceptor, 6... Rotating position sensor,
7... Light receiving element. Figure 1 Patent applicant: Rico Co., Ltd. Figure 1 Figure 10 Main scanning direction

Claims (1)

[Claims] 1. An image recording device that compares an image signal with a threshold matrix, modulates a laser beam based on the comparison output, and scans the modulated laser beam with a rotating polygon mirror to reproduce halftones. The size of the threshold matrix in the sub-scanning direction orthogonal to the laser beam scanning direction is set to 1/2 of the number of mirror surfaces of the rotating polygon mirror, and a detection means for detecting the rotational position of the rotating polygon mirror is provided. An image recording device comprising: 2. In an image recording device that compares an image signal with a threshold matrix, modulates a laser beam based on the comparison output, and scans the modulated laser beam with a rotating polygon mirror to reproduce halftones, the laser beam scans. The size of the threshold matrix in the sub-scanning direction perpendicular to the direction is set to 1/N of the number of mirror surfaces of the rotating polygon mirror (N: an integer of 2 or more and a divisor of the number of mirror surfaces), and An image recording device characterized in that reflective surfaces used for recording are every N-1 surfaces.