Color image forming apparatus with density control

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BACKGROUND OF THE INVENTION

In the present invention, distribution of adjoining pixels is reflected in density distribution of objective recording pixels so that high quality recording can be conducted. The present invention relates to a color image forming apparatus in which: one pixel image data is divided into small pixels m.times.n (the width.times.the length) considering the adjoining pixel data, and after that, the center of gravity of each line is found; the phase of the reference wave is deviated according to the center of gravity; and dot recording composed of n small scanning lines is conducted by the modulated signal of the pixel density data modulated by the reference wave signal so that a character and a halftone image can be reproduced. The recording apparatus of the present invention is used for a printing apparatus or a displaying apparatus.

In the field of image forming apparatus using an electrophotographic method, a digital halftone image is reproduced in the following manner: an image signal of an original image is read by a scanner; and image density data in which the image signal is gradation-corrected, A/D converted, and shading-corrected, is modulated by a reference signal.

When an original image is read by the scanner, an edge portion of the image is read as halftone density due to the aperture of a solid state image pick-up element installed in the scanner. When a latent image is formed on a photoreceptor with the image density data obtained from the image signal, the image is recorded averagely in a recording pixel corresponding to the edge portion of the latent image in the case where the image density is intermediate, and thereby dot breakage is generated, and as a result, sharpness of the recorded image is lowered. Conventionally, it is widely known that MTF correction for sharpening the image can be conducted by a differential filter, a Laplacian filter, or the like in order to maintain the sharpness of the image. However, this emphasizes only the edge portion of the image, so that uniformity of the halftone image is relatively lowered.

On the other hand, even when an interpolated character or figure is formed from computer graphic (C.G.) data or font data, a similar problem is caused. That is, when the edge portion is interpolated smoothly by the intermediate density of the interpolation data, a recording pixel corresponding to the edge portion is recorded in pixels as average density, and thereby the resolution of the recorded image is lowered.

SUMMARY OF THE INVENTION

For the reasons mentioned above, intermediate density processing, which effectively operates on the edge portion of the image, is required.

Further, when intermediate density processing is conducted on each color in a color image forming apparatus, there occurs the problem in which color tone is varied, or characters become not sharp.

In view of the foregoing problems, an object of the present invention is to provide an image forming apparatus in which resolution of the image, which is formed from scanner data, C.G data, font data, or the like, is improved, and high quality recording is conducted.

The above described object is accomplished by a color image forming apparatus in which high density pixel recording is conducted according to the density data of a small pixel in the objective pixel which is determined corresponding to density data of the pixel adjoining the objective pixel, and the color image forming apparatus is characterized in that: when recording position of each color is modulated according to density distribution of the objective pixel and the adjoining pixel, the recording position of each color is determined according to density distribution of the pixel adjoining the specific color.

A preferable embodiment is a color image forming apparatus which is characterized in that: the specific color is green.

The aforementioned object is accomplished by an color image forming apparatus in which high density pixel recording is conducted according to density data of a small pixel in the objective pixel determined corresponding to density data of the pixel adjoining the objective pixel, which is characterized in that: when a recording position of each color is modulated according to density distribution of the objective pixel and the adjoining pixel, the recording position of each color is determined corresponding to density distribution of the adjoining pixel in the case where density of the objective pixel is not less than a predetermined density.

The aforementioned object is accomplished by a color image forming apparatus in which high density pixel recording is conducted according to density data of a small pixel in the objective pixel which is determined corresponding to density data of the pixel adjoining the objective pixel, and which is characterized in that: the apparatus is provided with means to image-discriminate the objective pixel; and means in which only the achromatic component is recording-position-modulated according to density distribution of the adjoining pixel when the objective pixel is discriminated to be in the halftone region by the image discrimination, and entire color components are recording-position-modulated when the pixel is discriminated to be in a character region.

The aforementioned object is accomplished by a color image forming apparatus in which high density pixel recording is conducted according to density data of a small pixel in the objective pixel which is determined corresponding to density data of the pixel adjoining the objective pixel, and which is characterized in that: a recording position of each color is determined according to density distribution of the pixel adjoining the objective pixel when the recording position of each color is modulated in the primary scanning direction and the subsidiary scanning direction according to the density of the objective pixel and density distribution of the adjoining pixel.

The aforementioned object is accomplished by a color image forming apparatus in which high density pixel recording is conducted according to density data of a small pixel in the objective pixel which is determined corresponding to density data of the pixel adjoining the objective pixel, and which is characterized in that: a recording position of each color is determined according to density distribution of the pixel adjoining the objective pixel when the recording position of each color is modulated in the primary scanning direction and the subsidiary scanning direction according to the density of the objective pixel and density distribution of the adjoining pixel. In this image formation, when a period of the reference wave can be variable corresponding to the image, preferable resolution and gradation can be realized corresponding to the image. Further, the object of the present invention is accomplished by an image forming apparatus which is characterized in that: the apparatus is provided with means to image-discriminate the objective pixel; and means by which the recording position modulation is conducted according to the density distribution of the adjoining pixel using a reference wave having a long period, when the pixel is discriminated to be in the halftone region, and the recording position modulation is conducted by a reference wave having a short period, when the pixel is discriminated to be in the character region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an image processing circuit of an example of the image forming apparatus of the present invention.

FIG. 2 is a block diagram showing an example of a phase of reference wave determination circuit in FIG. 1.

FIG. 3 is a block diagram showing an example of a modulation circuit in FIG. 1.

FIG. 4 is a perspective view showing a general structure of the image forming apparatus of the present invention.

FIGS. 5(a) and 5(b) are views for explaining RE processing used for determination of phases of reference waves.

FIGS. 6(a) and 6(b) are views showing an example in the case where an objective pixel for RE processing is divided into 3.times.3, and P=0.5.

FIGS. 7(a) and 7(b) are views showing an example in the case where the objective pixel is divided into 2.times.2.

FIGS. 8(a) and 8(b)are views showing another example in the case where the objective pixel for RE processing is divided into 2.times.2.

FIG. 9 is a view for explaining phase deviation of the reference wave in the case where the objective pixel exists in the character region.

FIGS. 10(a)-10(d) are timing charts showing respective signals in a modulation signal generating circuit of the example in FIG. 1 in the case where the objective pixel exists in the character region.

FIG. 11 is a view for explaining phase deviation of the reference wave in the case where the objective pixel exists in a halftone region.

FIGS. 12(a)-10(d) are timing charts showing a signal of each portion of the modulation signal generating circuit of the example in FIG. 1 in the case where the objective pixel exists in the halftone region.

FIG. 13 is a graph showing characteristics of a high .gamma. photoreceptor used for the present embodiment.

FIG. 14 is a sectional view showing an example of a specific structure of the high .gamma. photoreceptor used for the present embodiment.

FIG. 15 is a view showing a semiconductor laser array of the example in FIG. 4.

FIG. 16 is a view showing a scanning locus of laser spots using the semiconductor laser array in FIG. 13.

FIG. 17 is a graph showing the relation between a driving current and an emitted output of the semiconductor laser.

FIG. 18 is a graph showing an example in the case where the relation between the center of gravity in the primary scanning direction and a recording position of a small scanning line, is changed.

FIG. 19 is a graph showing the case where average density of the small scanning line in the subsidiary scanning direction is changed.

FIG. 20 is a block diagram showing an image processing circuit of another example of the present invention.

FIG. 21 is a block diagram showing a phase of the reference wave determination circuit in FIG. 20.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The structure of an image forming apparatus 400 which is an example of the present invention, will be described as follows. FIG. 4 is a perspective view showing the outline of the structure of the image forming apparatus of the present embodiment.

An image forming apparatus 400 has the following functions in which: the photoreceptor is uniformly charged; after that, a dot-like electrostatic latent image is formed by a spot light which is pulse-width-modulated or intensity-modulated according to a modulation signal obtained by binarizing an analog image density signal which is obtained by D/A converting digital image density data obtained from a computer or a scanner, and a reference wave signal after they are compared with each other, or by differentially amplifying them; a dot-like toner image is formed by reversal development of the latent image; charging, exposing, and developing are repeatedly conducted so that a color toner image can be formed on the photoreceptor; the color toner image is transferred onto a recording sheet; the recording sheet is separated from the photoreceptor; and then the image is fixed so that the color image can be obtained.

The image forming apparatus 400 is composed of: a photoreceptor which is a drum-like image forming body 401 rotated in the arrowed direction (hereinafter, it will be called simply a photoreceptor); a scorotron charger 402 by which the surface of the photoreceptor 401 is uniformly charged; an optical scanning system 430; developing units 441 to 444 in which yellow, magenta, cyan, and black toners are loaded; a transfer unit 462 composed of a scorotron charger; a separator 463; a fixing roller 464; a separator 463; a cleaning unit 470; a discharger 474; and the like.

The photoreceptor 401 used in the present embodiment has a high .gamma. characteristic and FIG. 14 shows an example of its specific structure.

The photoreceptor 401 is formed by a conductive support 401A, an intermediate layer 401B, and a photosensitive layer 401C, as shown in FIG. 14. Thickness of the photosensitive layer 401C is about 5 to 100 .mu.m, and preferably 10 to 50 .mu.m. The photoreceptor 401 is structured in the following manner: a drum-like conductive support 401A, which is made of aluminum and has a diameter of 150 mm, is used for the photoreceptor; the intermediate layer 401B, which is made of ethylene-acetic acid vinyl copolymer and whose thickness is 0.1 .mu.m, is formed on the conductive support 401A; and the photosensitive layer 401C, whose thickness of a film is 35 .mu.m, is provided on the intermediate layer 401B.

As the conductive support 401A, a drum, which is made of aluminum, steel, copper, or the like, and has a diameter of about 150 mm, is used, however, a belt-like body in which a metal layer is laminated or vapor-deposited on a paper or a plastic film, or a metallic belt such as a nickel belt, which is made by the method of electroforming, may be used as the conductive support. The intermediate layer 401B is preferably provided with the following properties: it can resist high charging voltage of .+-.500V to .+-.2000V; for example, in the case of positive charging, injection of electrons from the conductive support 401A can be prevented; and hole mobility can be provided so that superior light decay characteristics due to an avalanche phenomenon can be obtained. For the aforementioned reasons, positive charging type electric charge conveyance material, for example, disclosed in Japanese Patent Application No. 188975/1986 which has been proposed by the inventors of the present invention, is preferably added by not more than 10 weight % to the intermediate layer 401B.

As the intermediate layer 401B, the following resins, for example, which are commonly used for a photosensitive layer of electrophotography, can be used.

(1) vinyl polymer such as polyvinyl alcohol (Poval), polyvinyl methyl ether, and polyvinyl ethyl ether,

(2) nitrogen vinyl polymer such as polyvinylamine, poly-N-vinyl imidazole, polyvinyl pyridine (quarternary salt), polyvinyl pyrrolidone, and vinyl pyrrolidone-vinyl acetate copolymer,

(3) polyether polymer such as polyethylene oxide, polyethylene glycol, and polypropylene glycol,

(4) acrylic acid polymer such as polyacrylic acid and its salt, polyacrylamide, poly-.beta.-hydroxy ethylacrylate,

(5) methacrylate polymer such as polymethacrylate and its salt, polymethacrylamide, and polyhydroxy propyl methacrylate,

(6) ether cellulose polymer such as methyl cellulose, ethyl cellulose, carboxy methyl cellulose, hydroxy ethyl cellulose, and hydroxy propyl methyl cellulose,

(7) polyethylene imine polymer such as polyethylene imine,

(8) polyamino acid such as polyalanine, polyserine, poly-L-glutamine acid, poly-(hydroxy ethyl)-L-glutamine, poly-.delta.-carboxy methyl-L-cysteine, polyproline, lysine-tyrosine copolymer, glutamic acid-lysine-alanine copolymer, silk fibroin, and casein,

(9) starch and its derivatives such as starch acetate, hydroxyl ethyl starch, starch acetate, hydroxy ethyl starch, amine starch, and phosphate starch,

(10) polymer which is soluble in mixed solvent of water and alcohol, such as soluble nylon, and methoxy methyl nylon (8 type nylon) which are polyamide.

The electric charge conveyance material is not used for the photosensitive layer 401C basically, and the photosensitive layer 401C is formed by the following manner: phthalocyanine minute particles, which are made of photoconductive pigment and whose diameter is 0.1 to 1 .mu.m, antioxidant and binder resin are mixed and dispersed by using a solvent for the binder resin so that a coating liquid is prepared; the coating liquid is coated on the intermediate layer; and it is dried and thermally processed.

When the photoconductive material is used with the electric charge conveyance material, the photosensitive layer is structured in the following manner: the photoconductive material which is composed of the photoconductive pigment and a small amount of the electric charge conveyance material whose weight % is not more than 1/5, and preferably 1/1000 to 1/110 (weight ratio) of the photoconductive pigment, and the antioxidant are dispersed into the binder resin. When a high .gamma. photoreceptor is used, a sharp latent image can be formed although the diameter of the laser beam is spread, and thereby recording can be effectively conducted with high resolution.

In the present example, since color toner images are superimposed on the photoreceptor 401, the photoreceptor, which has spectral sensitivity on the infrared side, and a laser diode, which emits an infrared ray, are used so that a laser beam emitted from the optical scanning system 430 is not shaded by the color toner images.

Next, light decay characteristics of the high .gamma. photoreceptor used in the present example, will be explained as follows.

FIG. 13 is a graph showing characteristics of the high .gamma. photoreceptor. In the drawing, V1 is a charging potential (V), V.sub.0 is an initial potential (V) before exposure, L.sub.1 is an amount of irradiation (.mu.J/cm.sup.2) of a laser beam which is necessary when the initial potential V.sub.0 is decayed to 4/5, and L.sub.2 is an amount of irradiation (.mu.J/cm.sup.2) of a laser beam which is necessary when the initial potential V.sub.0 is decayed to 1/5.

A preferable range of L.sub.2 /L.sub.1 is

1.0<L.sub.2 /L.sub.1 .ltoreq.1.5

In the example, V.sub.1 =1000 (V), V.sub.0 =950 (V), L.sub.2 /L.sub.1 =1.2, and the photoreceptor potential of the exposure section is 10V.

When light sensitivity in the position corresponding to a middle period of exposure at which the initial potential (V.sub.0) is decayed to 1/2 in the light decay curve is defined as E1/2, and that in the position corresponding to an initial period of the exposure at which the initial potential (V.sub.0) is decayed to 9/10 is defined as E9/10, a photoconductive semiconductor which gives the following relations is selected.

(E1/2)/(E9/10).gtoreq.2

and preferably,

(E1/2)/(E9/10).gtoreq.5

In the aforementioned, the light sensitivity is defined as the absolute value of an amount of potential lowering to a minute amount of exposure.

In the light decay curve of the photoreceptor 401, the absolute value of the differential factor of the potential characteristics, which means the light sensitivity, is small, as shown in FIG. 13, at the time of a small amount of light, and it sharply increases when an amount of light increases. Specifically, the light decay curve shows the following characteristics: it shows almost horizontal light decay characteristics because sensitivity characteristics are not good for a small period of time at the initial period L.sub.1 of exposure, as shown in FIG. 13; however, it shows a super high .gamma. characteristic which become lower almost linearly because it has super high sensitivity, ranging from the middle period of exposure to the latter period thereof. It is considered that the photoreceptor 401 has high .gamma. characteristics, making use of the avalanche phenomena under the high charging voltage of, specifically, +500 to +2000V. That is, it is considered that carriers generated on the surface of photoconductive pigment at the initial period of exposure are effectively trapped by an interface layer of the pigment and coating resin so that the light decay is positively prevented, and thereby extremely sudden avalanche phenomena are generated after the middle period of exposure.

Next, a color image forming apparatus according to the present invention will be explained as follows. In the color image forming apparatus, an objective pixel of the image density data is formed by small pixels m.times.n (width.times.length), and a distribution of the density data of adjoining pixels including the objective pixel, is replaced with the distribution of small pixels m.times.n in one pixel, and the image is formed by the following method: a position in which dots of n rows are written, is displaced when a phase of a reference wave in each row of small pixels is displaced according to image density data of small pixels obtained by distributing data of the objective pixel multiplied by constant P corresponding to the distribution. Displacement of the position in which dots are written will be referred to as recording position modulation, hereinafter. Further, processing to convert the image density data of the objective pixel into the image density data of small pixels obtained by dividing the objective pixel into m.times.n, will be referred to as resolution improving processing (RE processing), hereinafter. Due to RE processing, high density recording can be conducted. In this case, a high .gamma. photoreceptor is specifically effective in order to form a latent image corresponding exactly to the reference wave.

In this invention, RE processing is conducted when 1 the image density data of the objective pixel is not less than a first threshold value, that is, not less than the specific density, namely the first threshold value. In an area corresponding to a highlight portion, in many cases, RE processing is not conducted on a background portion of a document, and small pixels m.times.n are caused to have a uniform density. In the case of CRT, this data display can be conducted.

However, in the case of laser recording which will be described later, it is difficult to display the data uniformly, and therefore, a reference wave whose density center exists in the center of the image density, is selected. Due to the aforementioned, uniformity in the highlight portion can be kept, and a noisy image can be prevented from occurring.

2 In the case of high density, and a steep density gradient, when a reference wave whose density recording position does not exist in the center is selected, dots are formed in the manner that they overlap with the adjoining pixel.

In order to prevent a density change and recording dot blocking between pixels, when the image density data of the objective pixel is not less than a specific second threshold value, in a high density portion also, a reference wave whose density center exists in the center of the image density, is selected.

Since a uniform display can be conducted in the case of CRT, the densities of small pixels m.times.n are processed as a uniform density. That is, RE processing is not conducted.

In a color image forming apparatus in which high density image recording is conducted according to density distribution data in the objective pixel which is determined corresponding to density data of the pixel adjoining the objective pixel, a color image forming apparatus characterized in that: when a specific density data of the objective pixel is not less than the first threshold value, recording position modulation is conducted according to the determined density distribution, is preferable, or the apparatus characterized in that; when a specific density data of the pixel is not more than the second threshold value, recording position modulation is conducted according to the determined density distribution, is preferable.

FIG. 5(a) is a plan view in which the adjoining pixels including the objective or target pixel m5 are expressed as m1 to m9 when the objective pixel is defined as m5, and the objective pixel m5 is divided into 3.times.3 small pixels or image dots. FIG. 5(b) is an enlarged view in which each small pixel is expressed by s1 to s9 when the objective pixel is divided into small pixels 3.times.3. m1 to m9 and s1 to s9 also express the density of each portion.

RE processing will be explained in detail as follows. Taking the example of the case where the objective pixel m5 is divided into 3.times.3 small pixels, density of a small pixel si is determined by the following equation.

si=(9.times.m5.times.P.times.mi/A)+(1-P).times.m5

Where i=1, 2, . . . , 9, and P is a constant, which is called the strength for RE processing, and in which the range of 0.1 to 0.9 is used, A is the sum total of m1 to m9.

In the above equation, the term (9.times.m5.times.P.times.mi/A) expresses a density in which the density of the objective pixel m5 multiplied by P is distributed to each pixel according to the density ratio of the adjoining pixels, and the term (1-P).times.m5 expresses a density in which the residual density of the objective pixel m5 is distributed equally to each small pixel, so that an element of unsharpness is taken into the equation.

FIG. 6 is an illustration showing an example in which the objective pixel m5 is divided into 3.times.3 small pixels, and P=0.5. FIG. 6(a) is an illustration showing an example of the density distribution of the adjoining pixels including the objective pixel m5. FIG. 6(b) is an illustration showing the density distribution in the objective pixel m5 which is calculated by P=0.5.

FIG. 7 and FIG. 8 show an example in which the objective pixel m5 is divided into 2.times.2 small pixels.

FIG. 7(a) is an illustration showing an example in which the objective pixel m5 is divided into 2.times.2 small pixels. FIG. 7(b) is an illustration showing an example of the adjoining pixels relating to small pixels s1 to s4 in the objective pixel.

Density of s1, s2, s3, and s4 is calculated according to Equation 1. ##EQU1## where A is the total sum of m1 to m9.

FIG. 8 (a) is an illustration showing another example in which the objective pixel m5 is divided into 2.times.2 small pixels. FIG. 8(b) is an illustration showing another example of the adjoining pixels relating to small pixels s1 to s4 in the objective pixel. Density calculation of s1, s2, s3, and s4 is conducted according to Equation 2. ##EQU2## where A is the total sum of m1 to m9.

FIG. 1 is a block diagram showing an example of an image processing circuit which is used for a color image forming apparatus of the present invention (an example in which the objective pixel is divided into 3.times.3). FIG. 2 is a block diagram showing a phase of reference wave determination circuit of the present embodiment, and FIG. 3 is a block diagram showing a modulation circuit of the present embodiment.

An image processing circuit 1000 of the present embodiment, is a circuit which structures a driving circuit of an optical scanning system, and is composed of an image data processing circuit 100, a modulation signal generator 200 and a raster scanning circuit 300.

The image data processing circuit 100 is a circuit which interpolates an edge portion of font data and outputs it, and is composed of an input