JP3155292B2 - Image forming device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus.
The laser printer is suitably applied to, for example, a digital copying machine, a digital color copying machine, and the like.

[0002]

2. Description of the Related Art Laser printers that combine electrophotographic technology and laser scanning technology can be used as plain paper and can produce high-speed, high-quality images. Have been doing. Under these circumstances, a recording method that achieves both resolution and gradation by a one-dot multi-value recording method is an effective method for obtaining a higher-quality image.

The multi-value recording method is roughly classified into a light intensity modulation method and a pulse width modulation method of a semiconductor laser. Since the pulse width modulation method is close to binary recording, it is relatively stable against external fluctuation factors. Can record. However, as the laser scanning speed increases (the write pixel clock increases), the time interval for changing the pulse width becomes very short. For example, when the pixel clock is 20 MHz, if the number of gradations expressed by one dot is 256 gradations, about 0.2n
The time interval of s. is required, which is very problematic in terms of accuracy and cost.

On the other hand, in a method of modulating the light intensity of a semiconductor laser, an intermediate exposure area (unsaturated area) of a photosensitive member is used.
However, this technique is realized by forming an optical / electrical negative feedback loop at high speed, although exposure energy control accuracy is required (see FIG. 3 described later). With this control technique, 256 gradations can be easily realized at a pixel clock of 20 MHz.

However, when an image is formed by an electrophotographic process using a method of changing the light intensity of a semiconductor laser, the following problems occur.
1. There is density fluctuation due to speed fluctuation of the photoconductor. 2. There is a density change due to the polygon tilting. 3. Since the photoconductor surface potential does not have a steep distribution in a low density portion, dot reproduction is reduced. 4.1 When tone reproduction is performed using a plurality of dots, the smoothness of an image in a uniform density region is improved, but resolution is reduced, as compared with the case where tone expression is performed using one dot.

Conventionally, as a method of compensating for the above-mentioned four disadvantages, in an image output apparatus for performing matrix multi-tone recording, the size of the matrix is small when the spatial frequency in the information of the original image is high, and when the spatial frequency is low. There has been proposed a method of changing the value to be larger (Japanese Patent Laid-Open No.
No. 159173), there is no specific method for detecting a spatial frequency and switching the matrix. Further, in both the pulse width modulation and the light intensity modulation, in a dry electrophotographic process which is widely used at present, the size of the toner is large due to a large toner particle size.
There is a disadvantage that minute dots smaller than the dots are not faithfully reproduced, resulting in a noisy image.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above situation, and has been made in particular to realize an image forming apparatus which provides a stable and high-quality halftone image. Solves the above problems in the pulse width modulation method, and also solves the problems in the light intensity modulation, thereby providing a high quality image by a laser printer combining laser scanning technology and electrophotographic technology. The purpose of the present invention is to provide a forming apparatus.

[0008]

To achieve the above object, the present invention provides (1)
In the image forming apparatus, based on input image information, one of the pulse widths set to a plurality of pulse widths in which the duty of the modulation pulse of the recording light is not 100% is selected for each pixel. First means for changing the amount of exposure light according to each pixel information, and second means for converting the input pixel information into image information of M × N patterns predetermined by M × N matrix pixel information of the input pixel information And a third means for selecting the first means and the second means in accordance with the input image information, the size N of the matrix in the sub-scanning direction (the feeding direction of the recording medium). Is 2 or more, the matrix size M in the main scanning direction is 1 or more, and the exposure pattern in the matrix is set to have the highest spatial frequency in the sub-scanning direction. (2) In the image forming apparatus of ( 1 ), the recording light source is a semiconductor laser, and the light output of the driven semiconductor laser is detected by a light receiving unit, and the light output is proportional to the light output of the semiconductor laser obtained from the light receiving unit. A photoelectric negative feedback loop that controls a forward current of the semiconductor laser so that a light receiving signal and a light emitting level command signal are equal,
Coupling coefficient between the light output / forward current characteristic of the semiconductor laser and the light output of the light receiving unit and the semiconductor laser so that the light receiving signal and the light emission level command signal are equal,
Converting means for converting the light emission level command signal into a forward current of the semiconductor laser based on light input / light receiving signal characteristics of the light receiving section, wherein a control current of the optical / electrical negative feedback loop and the converting means It is obtained by the characterized in that the semiconductor laser control unit by means for controlling said semiconductor laser by a current of a sum or difference between the generated current by, or, (3) an image forming apparatus, 1 A first means for changing at least one of the light output pulse width and the light intensity for each pixel based on the input pixel information; and a size N in a sub-scanning direction (a feeding direction of the recording medium) of a matrix including a plurality of pixels. And the matrix size M in the main scanning direction is 1 or more, and the exposure pattern in the matrix has the highest spatial frequency in the sub-scanning direction according to a plurality of pieces of image information. A second means for setting, and a third means for selecting the first means and the second means in accordance with a differential coefficient of the input K × L pieces of pixel information, and (4) In the image forming apparatus described in ( 3 ), the recording light source is a semiconductor laser, and the light output of the driven semiconductor laser is detected by a light receiving unit, and the light output of the semiconductor laser obtained from the light receiving unit is obtained. An optical / electrical negative feedback loop for controlling the forward current of the semiconductor laser so that the light receiving signal proportional to the light emitting level command signal becomes equal to the light emitting level command signal, so that the light receiving signal and the light emitting level command signal become equal. The light emitting level command signal is derived based on the light output / forward current characteristics of the semiconductor laser, the coupling coefficient between the light receiving unit and the light output of the semiconductor laser, and the light input / light receiving signal characteristics of the light receiving unit. Conversion means for converting the laser into a forward current, and means for controlling the semiconductor laser by a sum or difference current between the control current of the optical / electrical negative feedback loop and the current generated by the conversion means. (5) In the image forming apparatus, one pulse of pulse widths set to a plurality of pulse widths based on input image information. First means for selecting the width for each pixel and changing the amount of exposure light in accordance with each pixel information; and M × N number of pixels predetermined by M × N pieces of pixel information of the input pixel information. A second means for converting into pixel information of a pattern, and a third means for selecting the first means and the second means according to a differential coefficient of the inputted K × L pieces of pixel information. Constitution Things, furthermore, (6) The image forming apparatus of (5), the optical output of the driven semiconductor laser is detected by the light receiving portion, a light receiving signal proportional to the light output of the semiconductor laser obtained from the light receiving portion And a photoelectric negative feedback loop that controls the forward current of the semiconductor laser so that the light emission level command signal is equal to the light output level of the semiconductor laser so that the light reception signal and the light emission level command signal are equal.
A conversion for converting the light emission level command signal into a forward current of the semiconductor laser based on a forward current characteristic, a coupling coefficient between the light receiving unit and an optical output of the semiconductor laser, and a light input / light receiving signal characteristic of the light receiving unit. Means having a means for controlling the semiconductor laser by means of a sum or difference current between the control current of the optical / electrical negative feedback loop and the current generated by the conversion means. It is a characteristic. Hereinafter, a description will be given based on examples of the present invention.

FIG. 2 is a diagram showing the relationship between the speed of the laser beam in the main scanning direction and the light output waveform of the recording light.
Numeral 10 denotes a laser beam, wherein (a) shows an optical output waveform in the prior art (duty 100%), and (b) shows an optical output waveform in the present invention (duty <100%). Normally, the recording light pulse T ₀ is given by T ₀ = d / v (where d: pixel pitch = 1 / recording density, V: recording light scanning speed), and pulse width T = T _0.
Is referred to as 100% duty modulation.

FIG. 1 is a diagram for explaining the operation principle of the present invention. In FIG. 1, A ₁ has a pulse width of 25%, A ₂ has a pulse width of 50%,
A ₃ is 75%, A ₄ represents a case of 100%, B ₁ ~B
₄ shows the light intensity modulation range in each case. The invention shown in FIG. 1A is configured by combining a recording method for modulating light intensity at each pulse width with respect to a plurality of pulse widths A _{1 to} A ₄ according to input image data. Also, in the invention shown in FIG. 1B, the duty is not 100% (100%
Hereinafter, a plurality of pulse widths A _{1 to} A ₃ are configured by combining a recording method for modulating light intensity in each pulse width according to input image data.

FIG. 3 is a diagram showing the relationship between the exposure amount and the image density in electrophotographic recording, where I is an unsaturated density region,
II is a saturated density area, and as shown in the figure, the image density is divided into an unsaturated area I in which the density increases in accordance with the exposure amount up to the exposure amount E _0, and a saturated area II in which the density is saturated above E _0. Have.

FIG. 4 shows a pulse width of 25% (curve A), 50%.
% (Curve B), 75% (curve C), 100% (curve D)
Power modulation and pulse width modulation (curve E)
FIG. 4 is a diagram showing the relationship between the relative density of one dot pixel and the unsaturated density area depending on the intermediate exposure area of the photoconductor in the case of FIG. Is steep, and the dot reproducibility is improved.

FIG. 5 is a view showing the above-mentioned potential well. Curve A shows the case of 100% duty, and curve B shows the case of the present invention. However, for example, when the duty is 25%, the density does not increase even if the exposure energy is increased.
Therefore, for example, from a pulse width of 25% duty to 5%
If the relative density is switched to a pulse width of 0.6 when the relative density is 0.6, the unsaturated density region is smaller than in the case where the pulse width modulation is performed. If the recording is performed by switching to a pulse width of 1.0% and the density is 1.0, the portion depending on the intermediate exposure area of the photoreceptor is kept small, and the pulse width is set to a small number. It is easy to increase the accuracy. The control speed of the semiconductor laser used as the light source is 10 nsec.
If the pixel clock is realized at about 20 MHz, the light output can be easily controlled even when the pixel clock is 20 MHz. Further, since the density can be detected by the value of the image data, the pulse width may be selected according to the image data.

FIG. 6 is a diagram showing a density fluctuation caused by a speed fluctuation (or a laser scanning position fluctuation) of the photosensitive member or the writing optical system. In the drawing, a curve A ₁ indicates power modulation, and a curve A ₂
The power modulation (duty 50%), curve A ₃ represents the power modulation (duty 25%), although the curve B shows the pulse width modulation, from this figure, effectiveness of changing the duty of the modulation pulse Is clearly seen. In the above description, only the pulse widths of 25, 50, 75, and 100% have been described. However, similar effects can be obtained even if the pulse widths have different values.

The above is the description of the first means of the present invention. However, only the above means is not suitable for a dry electrophotographic process which is widely used at present, because the toner particle size is large, etc. The dots are not faithfully reproduced, resulting in a noisy image. As a method of improving this in electrophotography, a pseudo halftone expression method using a matrix composed of a plurality of pixels is used.

FIG. 7 is a diagram for explaining an example of the pseudo halftone expression method. In the present invention, it has a configuration as shown in the block diagram of FIG. Reference numeral 12 denotes a pulse width and emission intensity setting unit which performs pseudo halftone expression using a matrix based on the input image information, or selects one of pulse widths set in advance. A choice is made as to whether to use a multi-level recording method in which gradation is expressed by one dot by modulating the optical output of the semiconductor laser.

FIG. 8 shows a configuration example of a matrix. FIG. 8 shows a case where the matrix size is 2 in the main scanning direction and 2 in the sub-scanning direction (hereinafter referred to as 2 × 2). FIG. 10 is a diagram showing an example of the halftone output, and FIG. 10 shows an example of a 128-level halftone output with an optical output intensity of 8 and a pulse width of 4 values. The reason why the expression gradation is different in the same pattern is that the light output intensity of the semiconductor laser is changed.

FIG. 9 shows that the size N of the matrix in the sub-scanning direction (the feeding direction of the recording medium) is 2 or more, the matrix size M in the main scanning direction is 1 or more, and the exposure pattern in the matrix is FIG. 4 is a diagram for explaining an example in a case where the frequency is set to be the highest. In FIG. 4, the matrix size is 2 in the main scanning direction and 2 in the sub-scanning direction (hereinafter 2).
x2), an optical output intensity of eight values, and a pulse width of 128
It is an example of halftone output of a value. Until the halftone level 64, pixels adjacent in the sub-scanning direction are not exposed, and the pattern is filled in the main scanning direction (an exposure pattern in which the spatial frequency increases in the sub-scanning direction). The exposure energy distribution in the sub-scanning direction is as shown in FIG. When such an exposure pattern is used, as is apparent from FIG. 11, even when the exposure beam has a Gaussian distribution and a flared shape in the low-density portion, there is no overlap with the adjacent pixels, and the fluctuation of the feeding speed of the recording medium. Even if there is, exposure unevenness does not occur, and density unevenness hardly occurs.

FIG. 12 shows a conventional example (matrix size 1 ×
1, the exposure amount distribution at halftone level 8 at 16 light output intensities). Since there is overlap between adjacent pixels between pixels, exposure unevenness ΔE occurs due to fluctuations in the feeding speed of the recording medium, resulting in density unevenness. Would. The case where the matrix size is 2 × 2 and the pulse width is four has been described above. However, the same effect can be obtained by using a matrix having the same exposure pattern configuration even when the matrix size is different and the pulse width is other than four. As described above, the present invention makes it possible to configure a laser printer that is less susceptible to fluctuations in the speed of the photoreceptor or fluctuations in the laser scanning position, and that has good dot reproducibility, so that high-quality images can be obtained. A possible image forming apparatus can be provided.

FIG. 13 is a diagram showing a matrix configuration where the matrix size in the main scanning direction is M = 1 and the matrix size in the sub-scanning direction is N = 2 (hereinafter referred to as 1 × 2). FIG. FIG. 7 is a diagram showing the light output when the light output is 8;
This is an example of a 64-level halftone output having four values and pulse widths.
In FIG. 14, up to the halftone level 32, only the pixel I is exposed. Therefore, the exposure energy distribution in the sub-scanning direction on the recording medium does not overlap with adjacent pixels even if the exposure beam has a Gaussian distribution and a flared shape, as in the case of FIG. Even if there is a fluctuation, no exposure unevenness occurs, and density unevenness hardly occurs.

FIG. 15 is a diagram showing a density fluctuation caused by a speed fluctuation (or a laser scanning position fluctuation) of the photosensitive member or the writing optical system, wherein a curve A is 1 × 1 light intensity modulation, a curve B is 1 × 1 pulse width modulation, Curve C shows the case of a 1 × 2 matrix. From the figure, the matrix size is 1 × 2 (curve C).
Thus, it can be seen that the density fluctuation in the low density portion is reduced. As described above, the matrix size is 2x4, 1x
2, the case of four pulse widths has been described, but the same effect can be obtained by using a matrix having the same exposure pattern configuration in cases other than the above matrix size and four pulse widths. As described above, according to the present invention, it is possible to configure a laser printer that is less susceptible to fluctuations in the speed of the photoconductor or fluctuations in the laser scanning position and has good dot reproducibility. And an image forming apparatus capable of performing the above.

In FIG. 4, the smaller the unsaturated concentration ratio, the steeper the potential well formed in the photosensitive member, and the higher the dot reproducibility. However, for example, 2
In the case of 5% (curve A), the density does not decrease even if the exposure energy is increased. Thus, for example, a relative density of 0.5% from a pulse width of 25% (curve A) to a pulse width of 50% (curve B).
If switching is made at the point 6, it can be set to a portion where the unsaturated concentration region is smaller than when pulse width modulation (curve E) is performed.
Further, switching to a pulse width of 75% (curve C) at a density of 0.8, and switching to a pulse width of 100% (curve D) when the density is 1.1, a portion dependent on the intermediate exposure area of the photoconductor is reduced. Since the number of pulse width settings is small while the pulse width is kept small, it is easy to increase the pulse width setting accuracy.
Furthermore, in the control of the semiconductor laser, if the control speed of the optical / electrical negative feedback loop is realized at about 10 ns, the light control accuracy can be easily realized even when the pixel clock is 20 MHz.
Also, since the density can be detected by the value of the image data, the pulse width may be selected according to the image data. Figure 3 is a diagram showing a graph of the density fluctuations caused by photoreceptor or writing optical system speed variation (or variation of the laser scanning position), the curve A ₁ is power modulation, curve A ₂ is 50% duty power modulation , Curve A ₃
Represents power modulation with a duty of 25%, and curve B represents pulse width modulation. This figure clearly shows the effectiveness of the light intensity modulation in which the pulse width is changed according to the image density.

In the above description, the pulse width is 25%,
Although only the cases of 50%, 75%, and 100% have been described, similar effects can be obtained by further changing the pulse width. The above is the matter relating to the first means in the present invention. However, in dry electrophotography, dot reproducibility is still lacking by the above method alone. The area where dot reproducibility is required is an area where the image density change is gradual (low spatial frequency), and in electrophotography,
In particular, since the dot reproducibility in a low density portion is poor, a smooth image can be expressed in such an image area by using a pseudo halftone expression method using a dot concentration type matrix.

Referring to FIG. 7, a description will be given of an example in which the present invention is applied to dry electrophotography, and a portion where a selection calculation is performed performs pseudo halftone expression using a matrix based on input image information. Alternatively, a selection is made as to whether to use a multi-level recording method in which gradation is expressed by one dot by modulating the optical output of the semiconductor laser after selecting a preset pulse width. FIGS. 16A and 16B show examples of dot patterns obtained in this manner. In FIG. 16, the expression gradation is different in the same pattern because the light output intensity of the semiconductor laser is changed. In both Examples 1 and 2, the pattern of writing dots in the main scanning direction is inverted by the odd-numbered and even-numbered matrices. Although the pulse width set in FIG. 16 is 4, it is clear that the effect of the present invention can be obtained even if the pulse width is not 4. As described above, the present invention makes it possible to configure a laser printer that is hardly affected by fluctuations in the speed of the photoconductor or fluctuations in the laser scanning position, has good dot reproducibility, and has high exposure energy control accuracy. An image forming apparatus capable of obtaining a high-quality image can be provided.

FIGS. 17 and 18 show another embodiment of the present invention. FIG. 17 (a) is a diagram showing a matrix configuration, FIG. 17 (b) is a diagram showing an optical output when pulse width modulation is used, and FIG.
FIG. 17C shows the light intensity in the case of light intensity modulation (power modulation). The matrix configuration is shown in FIG.
In the example shown in FIG. 18, the main scanning direction is 2 and the sub-scanning direction is 4 (hereinafter referred to as “1 × 2”). 2x4) and 1
This is an example of six-value halftone output (two types of pulse widths). In both of the examples of FIGS. 18B and 18C, up to the halftone level 8 (that is, the low density portion), the pixels adjacent in the sub-scanning direction are not exposed, and the pattern is filled in the main scanning direction (the sub-scanning direction). Since the exposure frequency is such that the spatial frequency increases in the scanning direction, the exposure energy distribution in the sub-scanning direction on the recording medium (photoconductor) is as shown in FIG. In such a case, even if the exposure beam has a Gaussian distribution and a flared shape, there is no overlap with adjacent pixels, and even if there is a change in the feeding speed of the recording medium, exposure unevenness does not occur, and thus density unevenness occurs. A high-quality image that hardly occurs can be obtained. 17 (b) and 17 (c), a high-quality image with almost no density unevenness up to the halftone level 4 can be obtained. On the other hand, in the case of the conventional example shown in FIG. 12, since the pixels overlap with the adjacent beam, the exposure unevenness ΔE occurs due to a change in the feeding speed of the recording medium and the density unevenness occurs.

The density fluctuation caused by the speed fluctuation of the photosensitive member or the writing optical system (or the fluctuation of the laser scanning position) is as follows.
In FIG. 15, curve A is 1 × 1 light intensity modulation, and curve B is 1 × 1
The pulse width modulation, curve C is a 1 × 2 matrix. As an example of a matrix size of 2 or more in the sub-scanning direction, when the matrix size is 1 × 2 (curve C), density fluctuation in a low density portion is reduced. Is clearly shown. The case where the matrix size is 1 × 2, 2 × 4 has been described above, but the same effect can be obtained in the case of 2 × 2, 1 × 4 by using the exposure patterns as shown in FIGS. In general, if the matrix size N in the sub-scanning direction is 2 or more, the effect of the present invention can be obtained. The present invention is also effective when a combination of pulse width modulation and light intensity modulation is used. The above is the matter for the second means of the invention. In this method, the smoothness of the image is improved in only the low-density portion or only in the high-density portion. Since writing is performed in a matrix, the resolution at the boundary portion is reduced. Therefore, it is possible to switch so as to use the pseudo-halftone expression using the matrix where the density change is small, and to use the multi-tone expression of only one dot, which is the first means of the present invention, where the density changes greatly. Thus, a higher resolution and higher quality image can be obtained. Here, the switching between the first means and the second means is performed according to the differential coefficients of K × L pieces of input pixel information.

FIG. 21 is a diagram for explaining an embodiment of the switching means constructed as described above. Reference numeral 11 denotes a matrix pattern generation unit, 12 denotes a pulse width and emission intensity setting unit, and 13 denotes a differential coefficient. The calculation unit 14 is a digital comparator, and the reason why the differential coefficient is used as the switching means is that the differential coefficient is the magnitude of the density change, so that the image density change portion can always be detected, Can be expressed in multiple gradations of one dot in a portion where .DELTA. Greatly changes, that is, in a portion where the differential coefficient is large, and in a region where the density does not change much, that is, in a region where the differential coefficient is small, a pseudo halftone expression using a matrix can be performed.

FIG. 22 and FIG. 23 are graphs showing the differential coefficient calculation.
FIG. 22A is a diagram for explaining a specific example.
2 × 2 or 3 × 3 image information as shown in FIG.
Think. Now, the pixel of interest is (i, j) and the pixel density is A
_i,_j, And the derivative is f (i, j). First, 2x2
One example of calculating a differential coefficient from pixel information is
The coefficient f (i, j) is calculated as
Equal root, that is, f (i, j) = [(A_i-1,_j-A_i,_j)^Two+ (A_i,_j-A_i,_j-1)^Two]¹/^Two  There is a method. Also, as shown in FIG.
As an example of calculating a differential coefficient in × 3 pixel information,
As shown in FIGS. 23 (a) and 23 (b),
Think about Yilta. These filters are shown in FIG.
Differential coefficient f (i, j) by summing over elementary information
Can be calculated. Here, the derivative is calculated
Matrix and matrix size for pseudo-halftone representation
Need not be identical. In addition, the area for detecting the derivative
In the above example, the area is a 2 × 2 or 3 × 3 matrix.
Can set any area, and furthermore,
There are many other ways to easily detect numbers.
That the present invention is effective even if those means are used.
Of course.

Next, a specific example of switching between the first means and the second means described above will be described. First, the absolute value of the value of f (i, j) calculated above is input to the digital comparator 14, and the magnitude of the value of f (i, j) and a certain reference value are determined. The switching between the first means and the second means is performed. Here, as an example of the setting of the reference value, the absolute value of the value of f (i, j) is 0 to 1
Considering the case of 00, the reference value is set to 50. If the absolute value of the value of f (i, j) is 0 to 50, pseudo halftone expression using a matrix is performed. If the absolute value of the value of f (i, j) is 51 to 100, one dot is used. Performs multi-tone expression. Here, the reference value is set to a half of the absolute value of the maximum value of the differential coefficient. However, since this setting involves a method of calculating the differential coefficient and various other factors, any value may be set according to the situation. . Further, when switching is performed, there may occur a case where one dot in a matrix for performing pseudo halftone expression must represent one dot in multiple gradations. In this case, any one dot in the matrix has one dot. If multi-tone expression is required, switching may be performed by a method such as using all dots in this matrix as one dot multi-tone expression.

It should be noted that the present invention described above and Japanese Patent Application Laid-Open
In comparison with the invention described in JP-A-159173, the publication does not describe a specific realization method of detecting a spatial frequency and performing matrix switching, but the present invention specifically shows this method. ing. Further, in the above publication, the size of the matrix is made larger or smaller, but in the present invention, one dot (1 × 1) and the matrix (M × N) are switched. In the above publication, the spatial frequency is detected, but in the present invention, the differential coefficient is calculated because it is sufficient to detect only the place where the density of the pixel information greatly changes. As described above, the present invention is less susceptible to fluctuations in the speed of the photoconductor, or fluctuations in the laser scanning position, and
Since a laser printer with good dot reproducibility can be configured,
A specific configuration of an image forming apparatus capable of obtaining a high-quality image can be provided.

As described above, in FIG.
A ₁ , A ₂ , A ₃ , and A ₄ are pulse widths, B ₁ , B ₂ , B ₃ , and B ₄ are light intensity modulation ranges in respective pulse widths, and the pulse widths are represented by% of pixel bits. a ₁ is 25%,
A ₂ is 50% A ₃ is 75% A ₄ shows the case of 100%, the image data input optical writing method of modulating the light intensity in each of the pulse widths for a plurality of pulse width And a pulse width modulation of 25% (curve A), 50% (curve B), 75% (curve C), 100% (curve D) and pulse width modulation (curve E). The relationship between the relative density of one dot pixel and the unsaturated density area depending on the intermediate exposure area of the photoconductor is represented as shown in FIG. Thus, FIG.
It can be seen that the smaller the unsaturated concentration ratio, the steeper the potential well formed on the photoreceptor and the higher the dot reproducibility. For example, in the case of 25% (curve A), the exposure energy is increased. The density also does not rise. Therefore, for example, if the relative density is switched from a pulse width of 25% (curve A) to a pulse width of 50% (curve B) at a relative density of 0.6, the unsaturated density is higher than when pulse width modulation (curve E) is performed. It can be set to a part with a small area. Further, the density is switched to a pulse width of 75% (curve C) at a density of 0.8, and the density is changed to 1.1.
In this case, if the pulse width is switched to 100%, the portion depending on the intermediate exposure area of the photosensitive member is kept small, and the pulse width setting accuracy can be easily increased because the number of pulse width settings is small. Furthermore, in the control of the semiconductor laser, if the control speed of the optical / electrical negative feedback loop is realized at about 10 ns, the light control accuracy can be easily realized even when the pixel clock is 20 MHz. Also, since the density can be detected by the value of the image data, the pulse width may be selected according to the image data.

FIG. 6 shows the density fluctuation caused by the speed fluctuation (or the fluctuation of the laser scanning position) of the photosensitive member or the writing optical system as described above. Curve A ₁ shows power modulation, and curve A ₂ shows duty 50. % of power modulation, curve a ₃ is duty 25% of the power modulation, although the curve B shows the pulse width modulation, from this figure, efficacy clearly min of the light intensity modulation by changing the pulse width by the image density You. In the above, only the case where the pulse width is 25%, 50%, 75%, and 100% has been described. However, similar effects can be obtained even if the pulse width is set to a different value.

The above is the matter relating to the first means in the present invention. However, in dry electrophotography, dot reproducibility is still lacking by the above method alone. The area where dot reproducibility is required is an area where the image density change is gradual (low spatial frequency), and in electrophotography, the dot reproducibility is particularly poor in low density areas. For a region, a smooth image can be expressed by using a pseudo halftone expression method using a dot concentration type matrix. This method relates to the second means of the present invention, and a high-quality image can be obtained by using the first means in combination with the second means. Here, the switching between the first means and the second means is performed according to the differential coefficients of K × L pieces of input pixel information.
FIG. 21 shows an embodiment configured as described above.

FIG. 21 is a view for explaining an example of means for switching between the first means and the second means. As described above, reference numeral 11 denotes a matrix pattern generating section (M × N dot pattern). , 12 is a pulse width and emission intensity setting section (1 × 1), 13 is a differential coefficient calculating section, and 14 is a digital comparator. The reason why the differential coefficient is used as the switching means is that the differential coefficient is the magnitude of the density change. Therefore, it is possible to always detect a portion where the image density changes, and in a portion where the density changes greatly, that is, a portion where the differential coefficient is large, one-dot multi-tone expression is performed, and a region where the density does not change much, that is, the differential coefficient is small This is because a pseudo halftone expression using a matrix can be performed in the region. The calculation of the differential coefficient is performed by the method described above with reference to FIGS.

Next, a specific example of switching between the first means and the second means described above will be described. First, the absolute value of the value of f (i, j) calculated above is input to a digital comparator, and the magnitude of the value of f (i, j) and a certain reference value are determined. Switching between solid first means and second means. Here, as an example of the setting of the reference value, the case where the absolute value of the value of f (i, j) is 0 to 100 is set, and the reference value is set to 50. And f
If the absolute value of the value of (i, j) is 0 to 50, pseudo halftone expression is performed using a matrix. If the absolute value of the value of f (i, j) is 51 to 100, multi-gradation is performed with one dot. Make an expression. In this case, the reference value is set to a half of the absolute value of the maximum value of the differential coefficient. However, since this setting also involves a method of calculating the differential coefficient and various other factors, how to set the reference value according to the situation. Is also good. Further, when switching is performed, there may occur a case where one dot in a matrix for performing pseudo halftone expression must represent one dot in multiple gradations. In this case, any one dot in the matrix has one dot. If multi-tone expression is required, switching may be performed by a method such as using all dots in this matrix as one dot multi-tone expression.

FIG. 24 shows an example of the dot pattern thus obtained. In FIG. 24, (a ₁ ), (a ₂ )
Are examples of pseudo-halftone representation, respectively, (b ₁ ),
(B ₂ ) shows an example in which one-dot multi-value recording is performed. In this figure, the gray scale is set so that 256 gray scales can be expressed both in multi-tone gray scale expression and pseudo halftone gray scale expression. Is also good. Further, in the drawing, the expression gradation is different in the same pattern because the light output intensity of the semiconductor laser is changed. Further, in FIG. 24, the set pulse width is 4, but it is apparent that the effect of the present invention can be obtained even if the pulse width is not 4. As described above, according to the present invention, it is possible to configure a laser printer that is hardly affected by fluctuations in the speed of the photosensitive member or fluctuations in the laser scanning position, has good dot reproducibility, and has high exposure energy control accuracy. Therefore, it is possible to provide a specific configuration of the image forming apparatus capable of obtaining a high-quality image.

FIG. 25 is a diagram showing an example of the semiconductor laser control unit. Hereinafter, the operation when the present invention is realized using this semiconductor laser control unit will be described. In FIG. 25, the light emission level command signal is input to the comparison amplifier 1 and the current converter 2, and a part of the light output of the driven semiconductor laser 3 is monitored by the light receiving element 4. The comparison amplifier 1, the semiconductor laser 3, and the light receiving element 4 form an optical / electrical negative feedback loop, and the comparison amplifier 1 is proportional to the photovoltaic current (in proportion to the optical output of the semiconductor laser 3) induced in the light receiving element 4. The light receiving signal is compared with the light emission level command signal, and based on the result, the forward current of the semiconductor laser 3 is controlled so that the light receiving signal and the light emission level command signal become equal. In addition, the current converter 2 sets a current (light output / forward current characteristic of the semiconductor laser 3 and the light-receiving element 4 and the semiconductor light-emitting element 4) in accordance with the light-emitting level command signal so that the light-receiving signal and the light-emitting level command signal become equal. Coupling coefficient with laser 3, light receiving element 4
(A current preset based on the light input / light receiving signal characteristics). The sum of the output current of the current converter 2 and the control current output from the comparison amplifier 1 is the forward current of the semiconductor laser 3. Here, the crossover frequency of the open loop of the optical and electrical negative feedback loop when the 10000 DC gain and f _0, step response characteristics of the optical output Pout of the semiconductor laser 3 can be approximated as follows. Pout = PL + (PS−PL) exp (−2πf ₀ t) (where PL: light output at t = ∞PS: current converter 2)
Since the DC gain in the open loop of the optical / electrical negative feedback loop is set to 10000, if the allowable range of the setting error is set to 0.1% or less, the PL is considered to be equal to the set light amount. Therefore, if the light amount PS set by the current converter 2 is equal to PL, the light output of the semiconductor laser 3 instantaneously becomes equal to PL. Further, even if PS fluctuates by 5% due to disturbance or the like, if f ₀ = about 40 MHz, the error of the optical output of the semiconductor laser 3 with respect to the set value becomes 0.4% or less after 10 ns. By using the high-speed, high-precision, high-resolution semiconductor laser control circuit realized in this way, the exposure light quantity can be controlled with high accuracy even if the pulse width becomes short, so that the influence of the speed fluctuation of the photoconductor or the fluctuation of the laser scanning position is obtained. Since a laser printer that is less susceptible to noise, has good dot reproducibility, and has high exposure energy control accuracy can be configured, an image forming apparatus that can obtain high-quality images can be provided.

[0038]

According to the image forming apparatus EFFECT 請 Motomeko 1, since by performing the light intensity modulation is set to a pulse width of less than 100% duty, it becomes steep potential well formed on the photosensitive member, the dot Since the reproducibility is improved and a pseudo-intermediate expression method using a matrix can be selected based on image information, a smooth halftone expression is possible. In a low-density portion of the matrix, adjacent beams (dots) in the sub-scanning direction Because of the configuration in which the overlap of images is reduced, it is possible to provide an image forming apparatus capable of obtaining a high-quality image with less deterioration in image quality due to density unevenness caused by fluctuations in the speed of the photosensitive member and fluctuations in the laser scanning position. . According to the image forming apparatus according to claim 2, since the control of the semiconductor laser by a high-speed, high precision and high resolution semiconductor laser control circuit, that the control accuracy of the exposure energy to obtain a high quality image at a high speed for high An image forming apparatus that can be provided. Further, the matrix size M = 1 in the main scanning direction,
By setting the matrix size N in the sub-scanning direction to N = 2, it is possible to provide an image forming apparatus capable of obtaining a high-resolution, high-resolution image with a small matrix size, and a high-speed, high-precision, high-resolution semiconductor laser. By controlling the semiconductor laser by the control circuit, it is possible to provide an image forming apparatus that can control exposure energy with high accuracy and can obtain a high-quality image at high speed. According to the image forming apparatus according to 請 Motomeko 3, the low density portion, since there is less overlap in the sub scanning direction adjacent beams (dots), density unevenness occurs velocity fluctuation of the photosensitive member, the laser scan position variation as a cause Image quality degradation due to image processing, and has specific means for easily switching between a one-dot multi-tone expression method and a matrix half-tone expression method based on image information. And a specific configuration of the image forming apparatus capable of obtaining a high-quality image can be provided. According to the image forming apparatus of the fourth aspect ,
Since the semiconductor laser is controlled by a high-speed, high-precision, and high-resolution semiconductor laser control circuit, exposure energy control accuracy is high, and in low density areas, adjacent beams (dots) in the sub-scanning direction are less overlapped. Deterioration of image quality due to density unevenness caused by body speed fluctuations and laser scanning position fluctuations is reduced, and a one-dot multi-tone expression method based on image information and a pseudo-halftone expression method using a matrix are provided. Is provided, a specific configuration of the image forming apparatus capable of improving the resolution and obtaining a high-quality image can be provided. According to the image forming apparatus of the fifth aspect , the light intensity modulation set to a plurality of pulse widths is performed, and one pulse width is selected from the plurality of pulse widths based on image information, and then the amount of exposure light is reduced. Since it has a first means for changing and a specific means for easily switching between the one-dot multi-tone expression method and the pseudo-halftone expression method using a matrix based on the differential coefficient of image information, It is possible to obtain a smooth and high-resolution image, is less affected by fluctuations in the speed of the photoconductor, and fluctuations in the laser scanning position. A specific configuration of the image forming apparatus that can be obtained can be provided. According to the image forming apparatus according to claim 6, since the control of the semiconductor laser by a high-speed, high precision and high resolution semiconductor laser control circuit, the control accuracy of the exposure energy is high, also be set to a plurality of pulse width Since it has specific means for performing light intensity modulation and easily switching between a one-dot multi-tone expression method and a pseudo-halftone expression method using a matrix based on the differential coefficient of image information, Image formation that can obtain a high-resolution image, is less susceptible to fluctuations in the speed of the photoreceptor, and fluctuations in the laser scanning position, and can realize a laser printer with good exposure energy control accuracy and can obtain high-quality images. A specific configuration of the device can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram for explaining the operation principle of the present invention.

FIG. 2 is a diagram showing an optical output waveform of a light source in optical writing.

FIG. 3 is a diagram illustrating a relationship between an exposure amount and an image density in electrophotographic recording.

FIG. 4 is a diagram illustrating a relationship between a relative density of one dot pixel and an unsaturated density region in power modulation and pulse width modulation.

FIG. 5 is a diagram showing an example of a potential well.

FIG. 6 is a diagram illustrating a relationship between image density and density unevenness.

FIG. 7 is a diagram for explaining an example of a pseudo halftone expression method.

FIG. 8 is a diagram showing a configuration of a 2 × 2 matrix.

FIG. 9 is a diagram showing the gradation in that case.

FIG. 10 is a diagram showing another gradation characteristic.

FIG. 11 is a diagram showing an exposure energy distribution according to the present invention.

FIG. 12 is a diagram showing an exposure energy distribution according to the related art.

FIG. 13 is a diagram illustrating an example of a 1 × 2 matrix configuration.

FIG. 14 is a diagram showing gradation in that case.

FIG. 15 is a diagram illustrating an example of image density and density unevenness.

FIG. 16 is a diagram illustrating an example of a dot pattern in that case.

FIG. 17 is a diagram for explaining an example of the present invention.

FIG. 18 is a diagram for explaining an example of the present invention.

FIG. 19 is a diagram illustrating an example of a matrix configuration to which the present invention is applied.

FIG. 20 is a diagram illustrating an example of a matrix configuration to which the present invention is applied.

FIG. 21 is a diagram for explaining switching between 1-dot multi-tone expression and pseudo-halftone expression.

FIG. 22 is a diagram for explaining switching between multi-tone 1-dot expression and pseudo-halftone expression.

FIG. 23 is a diagram for explaining switching between one-dot multi-tone expression and pseudo-halftone expression.

FIG. 24 is a diagram showing an example of a dot pattern in that case, where (a ₁ ) and (a ₂ ) show pseudo halftone expressions,
(B ₁ ) and (b ₂ ) are diagrams showing a case where one-dot multi-level recording is performed.

FIG. 25 is a diagram illustrating an example of a semiconductor laser control unit.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Comparative amplifier, 2 ... Current converter, 3 ... Semiconductor laser,
4 ... light receiving element, 11 ... matrix pattern generator, 12
... Pulse width and emission intensity setting unit, 13 ... Derivative coefficient calculation unit, 14 ... Digital comparator.

Continued on the front page (31) Priority claim number Japanese Patent Application No. 2-120879 (32) Priority date May 10, 1990 (1990.5.10) (33) Priority claim country Japan (JP) (31) Priority claim number Japanese Patent Application No. 2-119130 (32) Priority date May 9, 1990 (1990.5.9) (33) Priority claim country Japan (JP) (72) Inventor Jin Hattori Ota, Tokyo 1-3-6 Nakamagome-ku, Ricoh Ricoh Co., Ltd. (72) Inventor Masaaki Ishida 1-3-6 Nakamagome, Ota-ku, Tokyo Ricoh Co., Ltd. (56) References JP-A-2-78577 ( JP, A) JP-A-2-6161 (JP, A) JP-A-61-214662 (JP, A) JP-A-62-15971 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , (DB name) H04N 1/405 B41J 2/52

Claims (6)

(57) [Claims] 1. A on the duty of the modulation pulses of the recording light has selected one of the pulse width of the pulse width set to a plurality of pulse width not 100% per 1 pixel based on input image information First means for changing the amount of exposure light in accordance with each piece of pixel information;
M determined in advance by N matrix pixel information
A second means for converting into image information of × N patterns;
The first means and the second means in accordance with the input image information;
The size N of the matrix in the sub-scanning direction is 2 or more, and the size M of the matrix in the main scanning direction is 1 or more,
An image forming apparatus, wherein an exposure pattern in a matrix is set so that a spatial frequency in a sub-scanning direction becomes highest. 2. A semiconductor laser as a recording light source, wherein a light output of a driven semiconductor laser is detected by a light receiving portion, and a light receiving signal proportional to the light output of the semiconductor laser obtained from the light receiving portion, a light emitting level command signal, And a photoelectric negative feedback loop that controls the forward current of the semiconductor laser so that the light output signal and the light emission level command signal are equal. Coupling coefficient between a light receiving unit and an optical output of the semiconductor laser, a conversion unit for converting the light emission level command signal into a forward current of the semiconductor laser based on a light input / light receiving signal characteristic of the light receiving unit, Means for controlling the semiconductor laser by means of a sum or difference current between the control current of the optical / electrical negative feedback loop and the current generated by the conversion means; The image forming apparatus according to claim 1, wherein the configuring the control unit. 3. A first means for changing at least one of a light output pulse width and a light intensity for each pixel based on input pixel information, and a size N in a sub-scanning direction of a matrix including a plurality of pixels. A second means for setting the matrix size M in the main scanning direction to 2 or more and the exposure pattern in the matrix to have the highest spatial frequency in the sub-scanning direction in accordance with a plurality of pieces of image information; An image forming apparatus comprising: third means for selecting one of the first means and the second means in accordance with a differential coefficient of K × L pieces of input pixel information. 4. A recording light source is a semiconductor laser, wherein a light output of a driven semiconductor laser is detected by a light receiving section, and a light receiving signal proportional to a light output of the semiconductor laser obtained from the light receiving section, a light emission level command signal, and And an optical / electrical negative feedback loop for controlling the forward current of the semiconductor laser so that the light output signal and the light emission level command signal are equal. Conversion means for converting the light emission level command signal into a forward current of the semiconductor laser based on a coupling coefficient between the light receiving unit and the light output of the semiconductor laser, and a light input / light receiving signal characteristic of the light receiving unit. Means for controlling the semiconductor laser by means of a sum or difference current between the control current of the optical / electrical negative feedback loop and the current generated by the conversion means. The image forming apparatus according to claim 3, characterized in that to constitute a control unit. 5. A method according to claim 1, wherein one of the pulse widths set for the plurality of pulse widths is selected for each pixel based on the input image information, and the amount of exposure light is changed according to each pixel information. (1) means, and (2) means for converting the input pixel information into pixel information of M × N patterns predetermined by M × N pixel information, and (2) the input K × L pixel information. The third means for selecting the first means and the second means according to the differential coefficient of the pixel information
An image forming apparatus characterized by comprising: 6. An optical output of a driven semiconductor laser is detected by a light receiving section, and the light emitting level command signal is made equal to a light receiving signal proportional to the optical output of the semiconductor laser obtained from the light receiving section. A negative feedback loop for controlling the forward current of the semiconductor laser, and the light output / forward current characteristics of the semiconductor laser and the light of the light receiving section and the semiconductor laser so that the light receiving signal and the light emission level command signal are equal. A conversion means for converting the light emission level command signal into a forward current of the semiconductor laser based on a coupling coefficient with an output and a light input / light reception signal characteristic of the light receiving unit; A semiconductor laser control unit is configured by means for controlling the semiconductor laser by a current obtained by adding or subtracting a control current and a current generated by the conversion means. The image forming apparatus according to claim 5.