JP2001136386A - Halftone expression system by dot - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent occurrence of a density difference depending on the position of an image in the case of expressing a gradation by the size or the number of dots. SOLUTION: In the halftone expression system of an image forming device which draw an image by scanning the image carrier of a surface to be scanned to the right and left by using light beams and expresses a gradation image by the size or the umber of dots, when a spot obtained by image-forming the light beams on the surface to be scanned is elliptic or oblong and the direction of the long axis of the ellipse or the oblong is varied within a scanning range, the growing direction of the size of the dots or the increasing pattern of the number of the dots is symmetrical with respect to the center line.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a halftone expression method using halftone dots, and more particularly to a halftone expression in which the shape of a halftone dot is optimized when the size of the halftone dot is expressed by a set of scanning light beam spots. It is about the method.

[0002]

2. Description of the Related Art In an image forming apparatus for recording a gradation image based on multi-valued image data, such as a copying machine, a facsimile, a printer, etc. employing an electrophotographic system, the density of pixels is analogously changed from a minimum density to a maximum density. It is difficult to change. For this reason, area gradation recording in which the apparent density is changed by changing the size or the number of pixels (halftone dots) is often performed.

The shape or arrangement when increasing the size or the number of halftone dots may be random as in the error diffusion method, but the halftone dots are enlarged according to a certain regularity, or In many cases, dots are arranged (dither pattern, “Imaging Part 1”, page 33 (published by Photo Kogyo Publishing Co., Ltd., January 20, 1988).
Day)). Among them, Fa, which is gradually thickened with the center at the core
6 (1) to 6 (15) show the shapes of halftone dots called a tenting type (dot concentration type) in the order of density. In this case, the outline of the halftone dot having the highest density (dots in which the entire area is filled with dots) is a square surrounded by a thick straight line, the square is divided by a 4 × 4 matrix, and the center of the square is the kernel. In FIG. 6, the number in parentheses below each halftone dot corresponds to the number of spots, and the number of dots (spots) in FIG. 6 corresponds to the number of spots. The part is a spot area.

On the other hand, when a scanning light beam is used to write an image in an image forming apparatus, the shape of a spot formed on a surface to be scanned by a scanning optical system is preferably a perfect circle or an ellipse. In the case of an ellipse, the orientation of the spot is generally determined so that the minor axis coincides with the main scanning direction.

[0005]

However, when the scanning optical system is an eccentric optical system, the shape of an image spot (hatched portion in FIG. 5) on the surface to be scanned is as shown in FIG. In the vicinity of the scanning center, the short axis of the ellipse coincides with the main scanning direction, but the short axis of the elliptical spot does not coincide with the main scanning direction and tilts toward both ends of the scanning. This phenomenon occurs due to coma aberration of the scanning optical system. When such a phenomenon occurs, the inclination of the spot becomes bilaterally symmetric about the scanning center as shown in FIG. 5, that is, the directions of the inclinations are reversed on the left and right sides of the scanning range.

As described above, if the shape of the image spot is asymmetrical at the scanning end and the direction of growth or the arrangement direction of the halftone dots has a left-right directionality as shown in FIG. The distance between adjacent halftone dots differs between the left and right positions of the image.

[0007] This point will be shown in a concrete example. FIG. 7 shows FIG.
The halftone dots having the density of (8) are used as image spots having characteristics as shown in FIG.
(A) near the scanning center and the left end
FIG. 7B is a diagram showing a halftone dot (b) at the right and a halftone dot (c) at the right end.
You. The horizontal direction in the figure is the main scanning direction, and the vertical direction is
This is the sub-scanning direction. It is clear from comparing FIGS.
As shown, for example, the distance between upper and lower halftone dots of the same density (8)
The separation is d_s, D_l, D _rAnd d_l<D_s<D
_rBecomes This means that the shape of the imaging spot is left at the scanning end.
It is asymmetrical to the right and left and right directions in the direction of dot growth
Because there is.

As described above, if the distance between adjacent halftone dots differs at the position of the image, especially in the electrophotographic system, the potential distribution and the like between the halftone dots differs, resulting in a density difference. In the case of a scanning light beam spot having a shape as shown in FIG. 5, a density difference occurs between left and right positions of an image.

In the above description, the density is represented by the size of a halftone dot, but the density is represented by the number of halftone dots of the same shape occupying a certain area (dither pattern, Baye
r-type (dot dispersion type): "Imaging Part1"
On page 33 (published by Photo Kogyo Publishing Co., Ltd., January 20, 1988), the halftone dots are asymmetrical at the scanning end, and the right and left directions are to increase the number of halftone dots. The same applies when there is a possibility.

The present invention has been made in view of the above-mentioned problems of the prior art, and an object of the present invention is to write a gradation image by scanning an image carrier on a surface to be scanned by using a scanning light beam. An object of the present invention is to prevent a density difference from occurring depending on the position of an image when an image forming apparatus that performs embedding expresses gradation by the size or number of halftone dots.

[0011]

In order to achieve the above object, a halftone expression system using halftone dots according to the present invention writes an image by scanning an image carrier on a surface to be scanned in the left-right direction using a light beam. In a halftone expression method of an image forming apparatus in which a gradation image is represented by the size or number of halftone dots, the shape of a spot formed by imaging the light beam on the surface to be scanned is elliptical or elliptical. And the direction of the major axis of the ellipse or ellipse changes within the scanning range, the growth direction of the halftone dot size or the increasing pattern of the number of halftone dots is symmetrical. It is characterized by having.

In this case, the scanning optical system of the light beam enters the beam at an angle with respect to the deflecting reflection surface in the sub-scanning section, and forms a spot on the surface to be scanned via the eccentric optical system. It is desirable that the present invention be applied to the case where the optical system is formed.

The present invention can be applied to a case where the image carrier is an electrophotographic electrostatic latent image carrier.

In the present invention, the spot formed by imaging the light beam on the surface to be scanned has an elliptical or elliptical shape, and the direction of the major axis of the ellipse or ellipse is within the scanning range. When forming a changing shape, since the growth direction of the size of the halftone dots or the increasing pattern of the number of halftone dots is symmetrical, as long as the density to be written is the same, either the left or right end of the image or the vicinity of the center There is no density difference even at the position.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle and embodiment of a halftone expression system using halftone dots according to the present invention will be described below.

FIG. 1 is a diagram corresponding to FIG.
(1) to (15) show the halftone dot shapes according to the present invention in the order of density. In the halftone expression method using the halftone dot, the halftone dot shape is sequentially thickened with the center as a core, and the outline of the halftone dot having the highest density is a square surrounded by a thick straight line in the figure. And
The point where the number of spots of the scanning light beam is sequentially increased to increase the density with the center of the square as a nucleus is the same as in the case of the conventional FIG. 6, and is shown below each halftone dot in FIG. The number in parentheses corresponds to the number of spots, and the hatched portion of each halftone dot is a spot area.

In this method, as can be seen from any of the halftone dot shapes shown in FIG. 1, halftone dots of any density are left and right with respect to the left and right center lines of a square frame representing the maximum outer shape of the halftone dots. A scanning light beam spot is written in a symmetrical shape. For example, at density (5), four spots are at 2 × 2 positions at the center of the 4 × 4 matrix, and one spot is not within the small frame of the 4 × 4 matrix, but at 2 × 2 at the center of the matrix. 2 is written at the center of one row above, that is, the position on the left and right center line of one row (the position between the first row and the second column and the first row and the third column). The difference is clear when compared with the conventional arrangement of density (5) in FIG. 6 (the remaining one spot is written at the position of 2 rows and 1 column of the 4 × 4 matrix). The same applies to halftone dots of other densities. As described above, when the growth direction of the size of the halftone dot is bilaterally symmetric, as is apparent from the example described later, even when the shape of the scanning light beam spot is bilaterally asymmetric at both scanning ends, writing is also possible. As long as the densities to be detected are the same, there is no density difference at any position near the left and right ends of the image and near the center.

Also, in the case where the density is represented by the number of halftone dots occupying a certain area (dot dispersion type), even if the shape of the scanning light beam spot forming the halftone dot is bilaterally asymmetric at both scanning ends. If the number of halftone dots is increased symmetrically with respect to the left and right center lines, similarly, as long as the density to be written is the same, a density difference occurs at any position on the left and right ends of the image and near the center. Never.

In the present invention, left and right are defined as the direction of the scanning line, that is, the main scanning direction.

Next, an embodiment of an image forming apparatus using such a halftone expression system will be described.

FIG. 2 shows an exposure optical system 3 comprising a scanning optical system.
FIG. 2 is a cross-sectional view taken perpendicular to the rotation axis 8 of the photosensitive drum 1 of an electrophotographic image forming apparatus (for example, an electrophotographic printer) provided with an image forming apparatus, and corresponds to a sub-scanning cross section of the scanning optical system 3.
FIG. 2 shows a schematic configuration of the image forming apparatus. FIG. 3 is a diagram mainly showing the scanning optical system 3 of FIG. 1 viewed from the double arrow direction. In this embodiment, a rotating polygon mirror 14 is used as an optical deflector, and a direction orthogonal to the rotation axis 17 of the rotation polygon mirror 14 and perpendicular to the optical axis of the scanning optical system 3 is the main scanning direction, and the rotation polygon mirror 14 is used. The direction perpendicular to the optical axis in the plane including the rotation axis 17 (the plane in FIG. 2) is the sub-scanning direction.

2 and 3, the image forming apparatus mainly includes a photosensitive drum 1 of an electrostatic latent image carrier, a charger 2 for charging the photosensitive drum 1, and a scan for scanning and exposing the charged photosensitive drum 1. An optical system 3, a developing device 4 for developing the photosensitive drum 1 on which a halftone dot image is exposed by the scanning optical system 3 and an electrostatic latent image is formed with a developer (toner), and a paper or the like conveyed by a conveyer (not shown) It comprises a transfer unit 5 for transferring the developed toner image onto the transfer medium P, a cleaner 6 for cleaning the photosensitive drum 1 after the transfer, and the like. In addition, since a fixing device and the like are provided, and the configuration itself of such an image forming apparatus is well known, other description is omitted.

As described above, the scanning optical system 3 writes the electrostatic latent image on the photosensitive drum 1 by scanning the light beam from the light source 11 such as a laser in the main scanning direction.
1. A collimator lens 12 that converts a light beam emitted from a light source 11 into a beam that is parallel in both the main scanning direction and the sub-scanning direction. The beam collimated by the collimator lens 12 is deflected and reflected by a rotary polygon mirror 14 in the sub-scanning direction. A cylindrical lens 13 having power only in the sub-scanning direction for converting into a beam focused on the surface, a rotary polygon mirror 14 for deflecting a beam focused parallel in the main scanning direction and in the sub-scanning direction,
An imaging lens 15 for imaging the beam deflected by the rotary polygon mirror 14 on a surface to be scanned that coincides with the surface of the photosensitive drum 1 in the main scanning direction, and an image of the beam passing through the imaging lens 15 in the sub-scanning direction It consists of a long lens 16 to be made.

In such a scanning optical system 3, the beam incident on the rotary polygon mirror 14 is a sub-scan section (the plane in FIG. 2).
In the figure, an angle is formed with respect to a plane orthogonal to the rotation axis 17 of the rotary polygon mirror 14 (referred to as skew incidence). On the other hand, in the main scanning section, the incident beam enters from the front of the deflecting / reflecting surface of the rotary polygon mirror 14 so as to intersect with the rotation axis 17. When the beam is incident on the deflecting and reflecting surface of the rotary polygon mirror 14 at an angle in the sub-scanning cross section, the deflected beam moves conically around the deflection point, so that the scanning line is curved. To avoid this, the optical axis of the imaging lens 15 or the long lens 16 is decentered with respect to the deflected beam (in FIG. 2, the long lens 16 is decentered in the direction of the arrow).

When some of the elements of the scanning optical system 3 are decentered in this manner, as shown in FIG. 5, near the center of the scanning range, an elliptical image spot (a hatched portion in FIG. 5) is formed. Although the short axis coincides with the main scanning direction, elliptical spots are inclined with respect to the main scanning direction at both scanning ends. This phenomenon is caused by the coma of the scanning optical system as described above. The direction of the inclination is reversed depending on whether the inclination is right or left in the main scanning direction.

As described above, the shape of the spot formed on the surface to be scanned is an ellipse or an ellipse, and the direction of the major axis of the ellipse or the ellipse changes within the scanning range. In the case of, as a halftone dot representing a gradation image, a halftone dot grown in a symmetrical shape from the center line or formed in a symmetrical shape with respect to the center line according to the method of FIG. 1 is used. I do.

FIG. 4 shows the scanning center when the halftone dots having the density of FIG. 1 (8) are formed in such a manner that the inclination direction of the elliptical spot is opposite on the right and left sides in the main scanning direction. It is a figure which illustrates the halftone dot (a) in the vicinity, the halftone dot (b) at the left end, and the halftone dot (c) at the right end. As in FIG. 7, the horizontal direction in the figure is the main scanning direction, and the vertical direction is the sub-scanning direction. 1A, 1B, and 1C, the distance between the upper and lower halftone dots of the same density (8) is almost the same.

As described above, even if the inclination of the elliptical spot is reversed at the left and right ends of the image, the distance between the adjacent pixels is substantially equal at any of the left and right ends and near the center. The position is also equal.

Further, even when the density is represented by the number of halftone dots,
Since the arrangement of halftone dots representing each density is symmetrical, there is no difference in density between left and right, as in the case where one halftone dot grows as described above.

The halftone expression method using halftone dots of the present invention has been described based on the principle and description of the embodiment.
The present invention is not limited to these, and various modifications are possible.

[0031]

As is clear from the above description, according to the halftone expression method using halftone dots of the present invention, the spot formed by imaging the light beam on the surface to be scanned has an elliptical or elliptical shape. None, and when the direction of the major axis of the ellipse or ellipse changes in the scanning range, the growth direction of the halftone dot size or the increase pattern of the number of halftone dots is symmetrical. Therefore, as long as the density to be written is the same, there is no density difference at any position near the left and right ends of the image and near the center.

[Brief description of the drawings]

FIG. 1 is a diagram showing halftone dot shapes of one embodiment according to the present invention in order of density.

FIG. 2 is a cross-sectional view of an embodiment of an image forming apparatus using a halftone expression system of the present invention.

FIG. 3 is a view mainly showing the scanning optical system of FIG. 1 viewed from a double arrow direction.

FIG. 4 is a diagram illustrating a case where halftone dots having the density of FIG. 1 (8) are formed by spots of the apparatus of FIG. 2;

FIG. 5 is a diagram showing a difference in an imaging spot shape by a scanning optical system in FIG. 2 depending on a scanning position.

FIG. 6 is a diagram showing a conventional Fattening type halftone dot shape in order of density.

FIG. 7 is a diagram showing a case where halftone dots having the density of FIG. 6 (8) are formed by the spots of FIG. 5;

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 photosensitive drum 2 charger 3 scanning optical system 4 developing device 5 transfer device 6 cleaner 8 rotating shaft of photosensitive drum 11 light source 12 collimator lens 13 cylindrical lens 14 rotating polygon mirror 15 Image lens 16 ... Long lens 17 ... Rotating axis of rotating polygon mirror P ... Transfer medium

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) G06T 5/00 H04N 1/40 104 F term (reference) 2C262 AA17 AA24 AA26 AA27 AB05 BB01 BB06 BB14 BB19 BB20 BB22 BB23 2C362 AA03 AA36 BA04 CA02 CA03 2H076 AB05 AB09 AB12 AB75 5B057 CA08 CA12 CA16 CB08 CB12 CB16 CC01 CE13 5C077 LL19 MP02 NN04 PQ08 TT03

Claims (3)

[Claims] 1. An image forming apparatus for writing an image by scanning an image carrier on a surface to be scanned in a left-right direction using a light beam, and representing a gradation image by the size or number of halftone dots. In the tone expression system, the shape of a spot formed by imaging the light beam on the surface to be scanned is elliptical or elliptical, and the direction of the major axis of the ellipse or ellipse changes within the scanning range. A halftone expression method using halftone dots, characterized in that the growth direction of the size of the halftone dots or the increasing pattern of the number of halftone dots is bilaterally symmetric when the shape is formed. 2. A scanning optical system for the light beam, the beam is incident at an angle to a deflecting reflection surface in a sub-scanning cross section, and the spot is imaged on a surface to be scanned via an eccentric optical system. 2. A halftone expression system using halftone dots according to claim 1, wherein the halftone expression system comprises an optical system. 3. The halftone representation system according to claim 1, wherein said image carrier is an electrophotographic electrostatic latent image carrier.