JP2702134B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus for forming a halftone image.

[Prior Art] Conventionally, as a method of forming a halftone image, a dither method and a density pattern method are well known. In these methods, if a threshold matrix having a small size is used, a sufficient gradation cannot be obtained. Therefore, if a large-sized threshold matrix is used, good gradation can be expressed, but there is a problem that the resolution is reduced this time.

On the other hand, another method has been proposed in which the density is changed in the depth direction for one dot, thereby forming one density expression element in units of one dot or a plurality of dots. In particular, in a printer to which an electrophotographic method is applied, when exposing a photoreceptor (photosensitive drum), by changing a light intensity and a lighting time of a light source, information in a depth direction can be further transmitted for one dot signal. The technique of giving is used.

Here, examples of output pixels formed by changes in light intensity and lighting time are shown in FIGS. 9 (a) to 9 (c).

FIG. 9A shows a case where the light intensity is changed in four steps for each pixel (dot). FIG. 3B shows the case where the lighting (exposure) time of the light beam scanning in the direction of the arrow is changed, and FIG. 3C is the same as FIG. As one density expression element
This shows a state in which the density is changed by four types in the sub-scanning direction.

When expressing a halftone in this manner, it is necessary to express the image by binarization as in the above-described dither method and density pattern method, that is, by expressing a group of dots having substantially the same size of one pixel and having the same density. Instead, the output image is represented by a set of dots having different shapes and densities of the dots, that is, information having a depth direction.

[Problems to be Solved by the Invention] However, there is a problem that a delicate difference or change in process conditions during image formation from the formation of an electrostatic latent image to the development is conspicuous in an output image. There was found. This is a new problem that has hardly become a problem in the conventional dither method and density pattern method. In particular, in the electrophotographic system, as is well known, a latent image is formed on a photosensitive member having a realistic blur amount by using a light beam having a finite size of at least several ~ Number 10
Since development is performed with toner particles having a size of μ, it is difficult to provide an area gradation effect when information in the depth direction is given to a size of 1 dot or less. That is, as shown in FIG. 9 (b), even if processing such as seemingly binary area gradation is performed at the signal level, the actual print result is as shown in FIG. 9 (a).
The result is a density gradation image such as

The reason will be described with reference to FIG. FIG. 10 shows an example in which an optical image of a laser beam scanned in the main scanning direction is reversely developed, in which laser beams of 2 dots, 1 dot and 1/2 dot are irradiated, respectively. As can be seen from this figure, the width of the optical image in the main scanning direction does not become sharp when the half dot is ON, but the peak light quantity decreases when it is ON at 1/2 dot compared to when the two dots are ON and 1 dot is ON. As a result, it can be understood that a halftone image as shown in FIG. 9A is printed.

Thus, in the case of applying the electrophotographic method, 1
In order to express density gradation with pixels, it is necessary to use a region which is extremely unstable as compared with area gradation. Therefore, as described above, it can be seen that a subtle difference in the process conditions at the time of image formation is particularly prominent in the output image.

By the way, in the electrophotographic device, the image quality is determined by such factors as lack of skin fogging, sufficient maximum density, and line images appearing sharp and sharp. There are many other important factors besides gender, and in order to satisfy these requirements, it is often necessary to adjust the process conditions during image formation. Specifically 2,
For example, to improve the fogging phenomenon, adjust the bias voltage applied to the developing device, or adjust the output of the light source or the developing bias to optimize the maximum density. Adjustment is often done in practice.

In a system (apparatus) in which density information is given in the depth direction by pulse width modulation to a dot having an actual dot size of 1 dot or less, halftone expression is performed as described above. It has been found that changing the process conditions has a significant effect on the gradation (for example, the tone of a light part is skipped or the black part is crushed).

The present invention has been made in view of such a problem, and an object of the present invention is to provide an image forming apparatus capable of forming a good output image even when conditions for forming an image are changed.

[Means for Solving the Problems] In order to solve the problems, the present invention has the following configuration.

That is, input means for inputting image data, pulse width modulation means for comparing the image data with a pattern signal having a predetermined period to form a pulse width modulation signal based on the image data, and a pulse width modulation signal based on the pulse width modulation signal. Image forming means for forming an image by changing the process conditions of the image forming means, changing means for changing the maximum density of the formed image, and the image data and the predetermined period according to the output of the changing means Adjusting means for adjusting the relative level with respect to the pattern signal of the pulse width modulation signal, and when the density is changed by the changing means in the direction of increasing the density, the adjusting means adjusts the pulse width modulation signal formed by the pulse width modulation means. The relative level is adjusted so that the pulse width becomes shorter than before the change.

Embodiment An embodiment according to the present invention will be described below in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram of an image forming apparatus according to the first embodiment, which is shown as applied to an electrophotographic digital copying machine. The entire apparatus is roughly divided into a reader section A for reading a document image and a printer section B for forming an output image.
It is composed of The reader unit A controls various components under the control of the CPU 6.
This is executed according to the program stored in the ROM 7. In this embodiment, as an example, a digital multi-valued image signal is converted into an analog level and compared with a triangular wave, thereby pulse width modulating the image signal to give information in the depth direction per dot. We will use the method.

 The normal copying operation according to the configuration shown in the figure is as follows.

First, the original 1 is read by the reading element (CCD) 2 of the reader unit A, and the original image is converted into an analog electric signal. This electric signal is amplified by an amplifier (AMP) 3, converted into an 8-bit (= 256 gradation) digital image signal by an A / D converter 4, and sent to a RAM 10 for use gradation correction via a latch 5. input. Further, the image data corrected by the RAM 10 is converted into an analog image signal via the D / A converter 13 of the printer unit B, and the signal is input to the pulse width modulation circuit 14.

Here, the operation of the pulse width modulation circuit 14 will be described with reference to FIG.

In the figure, “image signal” is an analog image signal output from the D / A converter 13, and “pattern signal” is a triangular wave signal generated in the pulse width modulation circuit 14. The "image signal" and the "pattern signal" are synchronized as shown in the figure, and their levels are compared by a comparator (not shown) in the circuit 14. As a result, the resulting pulse signal (pulse width) is obtained. The modulation signal is a PWM signal output from the pulse width modulation circuit 14.

This is an example of the pulse width modulation operation, and is suitable for relatively high-speed image signal processing. On the other hand, when the image signal is relatively slow, without using the D / A converter 13, for example, by generating a digital pattern signal sufficiently faster than the digital image signal and comparing these digitally, It is also possible to generate a pulse width modulation signal directly from a digital image signal.

Now, the PWM signal output from the output of the pulse width modulation circuit 14 is amplified by the laser driver 15 and then amplified by the laser generation circuit.
16 and this amplified signal turns on the laser beam.
Used for / OFF control. The laser light emitted from the laser generation circuit 16 is irradiated on a photoconductor (photosensitive drum) 19 via an optical system including a polygon mirror 17 and an f-θ lens 18 and the like. The photoreceptor 19 is uniformly charged by the corona charger 20, and then receives the above-mentioned laser exposure to form an electrostatic latent image on the surface. After this electrostatic latent image is visualized by the developing device 21, it is transferred onto the transfer material 27 by the transfer charger 24,
After the transfer material 27 is separated from the photoconductor 19 by the separation charger 25, it is fixed by the fixing device 26. On the other hand, the toner remaining on the photoreceptor 19 without being transferred is collected by the cleaner 22, and the electrical history of the photoreceptor 19 is erased by the pre-exposure lamp 23, so that the next print cycle is started again.

<Description of Gradation Correction (FIG. 3)> Next, the contents of the RAM 10 used as gradation correction means will be described.

The RAM 10 stores a gradation correction table for faithfully reproducing the original image data input from the CCD 2 in the printer unit B, and has 8 bits (= φφH to FFH).
(256 gradations) of the image signal is gradation-corrected and output at 8 bits.

FIG. 3D shows the conversion contents of the gradation correction table.

This table shows the input characteristics of the CCD (a), the output characteristics of the printer (c), and the desired gradation curve when the original is copied (b), as shown by the arrows in the figure. It is obtained by combining. That is, when the gradation correction table shown in FIG. 3D is used, the original image is converted as shown in FIG. 3B to form an output image.

<Description of Adjustment Process (FIGS. 4 to 8)> In this embodiment, since the image is formed by the electrophotographic method using the reversal development method, the portion exposed by the laser 16 is black and is not exposed. The part is copied and output as white. The density of the black background portion and the fog of the white character portion indicate the relative relationship between the bright portion as the exposed portion and the dark portion as the non-exposed portion on the photoreceptor 19 and the developing bias value applied to the developing device 21. It changes greatly by changing. The reason will be described with reference to FIG.

In the present embodiment, as the developing bias,
A component in which components are superimposed is used, and this DC level value (voltage value) is defined as V _DC . In addition, complete dark area (= laser always O
Let the potential of N) be _VL . In FIG. 4, V _D = 500 V, V _L = 50 V, and V _DC is maintained at about 400 V. Here, in order to reduce the head, it may be increasing the value of _{CVD (= V D -V DC)} . In other words, increase the V _D by limiting the amount of charge,
Alternatively, V _DC may be reduced. To increase the concentration, the value of C _VL (V _DC −V _L ) may be increased.
That is, _VL is reduced by limiting the amount of light and the amount of charge, or V _DC
Increasing the value increases the concentration.

By the way, when such an adjustment is performed, the density of the halftone (V in the illustrated example) is higher than the density of the completely white or completely black portion.
(Equivalent to _HT ) was found to be more affected. This is because C _HT (= V _DC −V _HT ) changes. The reason why this appears largely in the image is as follows. That is, the device used in the present embodiment changes the ON / OFF time of the laser finely by pulse width modulation.
Due to the spot diameter of the laser, the responsiveness of the laser driver, and the characteristics of the electrophotographic system, the density expression by the area gradation effect as shown in FIG. This results in an expression using various density gradation effects.
Therefore, a steep VD characteristic which is a development characteristic of electrophotography as shown in FIG. 6 appears as it is, and a change in halftone density appears remarkably. Actually, as an example, when the output characteristics of the printer were re-measured while changing the developing bias V _DC , as shown in FIG.
It has been found that the fogging direction changes as indicated by a dotted line A, and the density increases as indicated by B. Therefore, in this embodiment,
By adjusting the relationship between the image signal and the comparison (pattern signal) wave described with reference to FIG. 2, the deviation of the printer output characteristic is corrected, and the change in gradation is prevented.

For example, it is assumed that an image signal and a triangular wave which is a comparison wave are applied in a relationship as shown in FIG. On the other hand, as shown in FIG. 3B, the image becomes lighter as a whole, and conversely, if the relationship is as shown in FIG. 3C, the image becomes darker as a whole. Furthermore, in the case of (d), the white side jumps, the black side is crushed, and in the case of (e), the white side is fogged and the black side is light. In the case where the V _DC of the developing bias is changed, the change of the halftone is corrected by changing the relationship between the image signal and the triangular wave as shown in FIG. 8 (b) or (c). Is possible.

Specifically, using the configuration as shown in FIG.
When changing the V _DC output of the development eye power supply 28 with
Pulse width modulation circuit 14 by an amount corresponding to the change
The offset amount of the triangular wave (and the amplitude of the triangular wave if necessary) may be changed. For example, in order to reduce "fog", the bias of the developing
When the V _DC is set in a lowering direction, the entire output image (halftone image) changes in a lighter direction. In order to correct this, the offset of the triangular wave in the pulse width modulation circuit 14 is changed as shown in FIG. In this case, the halftone pixel itself can be prevented from changing. When the bias V _DC is set to increase in order to increase the density, it is possible to always form a good output image by increasing the offset of the triangular wave as shown in FIG. 8 (b).

The case where the offset of the triangular wave of the pulse width modulation circuit 14 is changed when the developing bias is changed has been described above.However, as another example, the case where the light emission intensity and the charge amount of the image exposure light source are changed is described. the, for example, to change simultaneously offset and amplitude (gain) of the triangular wave may change the level of V _B + V _W instead of the triangular wave. The present invention is extremely effective for stabilizing the image quality when changing process elements that affect various image quality.

In particular, in the electrophotographic system, the process conditions are often changed in the market due to background fogging, adjustment of the maximum density, and other image quality issues. Therefore, the operability or serviceability can be significantly improved as well as the image quality is improved.

In the embodiment described above, the bias V _DC and the offset of the triangular wave are changed by hardware in accordance with the change amount of the volume 12, but the change amount of the volume 12 is detected by the CPU 6, and the CPU 6 These values may be changed under control.

In the embodiment, the embodiment of the reversal developing method has been described. However, it goes without saying that the principle of the present embodiment can be applied to a normal development method in which a background portion for exposing a background portion is exposed and then a dark portion is developed. No.

Furthermore, the same principle can be applied to various electrophotographic printers, such as a printer using a liquid crystal shutter added to an LED array or other light source, other than a laser, for example. Further, even in a printer of a type in which an electrostatic latent image is formed on a surface of an insulating pair similar to an electrophotographic type and an image forming method using a recording head by a recording head, a method capable of giving density information in the depth direction to one dot is used. If so, they can be used exactly the same way.

[Effects of the Invention] As described above, according to the present invention, even if the maximum density of the formed image is changed, the gradation can be maintained without the black portion being crushed, and the gradation can always be maintained. It is possible to form a stable image having excellent tonality.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of the configuration of this embodiment, FIG. 2 is a diagram for explaining the principle of pulse width modulation, and FIG. FIG. 4 shows the relationship between the bias level of the source imager and the output image in the embodiment, FIG. 5 shows the relationship between the laser spot and the output pixel, and FIG. FIG. 7 shows output characteristics of a photograph. FIG. 7 shows the characteristics when the bias level of the source imager is changed.
FIGS. 8 (a) to 8 (e) are diagrams for explaining a state in which the offset and the amplitude of the triangular wave in the pulse width modulation circuit in the embodiment are changed, and FIGS. 9 (a) to 9 (c) are diagrams showing changes in output pixels due to changes in exposure intensity and lighting time, and FIG. 10 is a diagram showing changes in density of output pixels based on the irradiation time of laser light in the main scanning direction. is there. In the figure, 1 ... original, 2 ... CCD, 3 ... amplifier, 4 ... A
/ D converter, 5 ... Latch, 6 ... CPU, 7 ... ROM, 8 ...
... Buffer, 9 ... Bidirectional buffer, 10 ... RAM (gradation correction table), 11 ... I / O port, 12 ... Volume, 1
3 D / A converter, 14 pulse width modulation circuit, 15 laser driver, 16 semiconductor laser, 17 polygon mirror, 18 f-θ lens, 19 photoconductor, 20 ... Corona charger, 21 ... Developer, 22 ... Cleaner, 23 ... Pre-exposure lamp, 24 ... Transfer charger, 25 ... Separation charger, 26 ...
Fixing roller 27 ... Transfer paper 28 ... Development bias power supply.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification number Agency reference number FI Technical display location G03G 21/00 376 B41J 3/00 A

Claims (1)

(57) [Claims] An input unit for inputting image data; comparing the image data with a pattern signal having a predetermined period;
A pulse width modulation unit that forms a pulse width modulation signal based on the image data; an image formation unit that forms an image based on the pulse width modulation signal; and an image formed by changing a process condition of the image formation unit. Changing means for changing the maximum density of the image data, and adjusting means for adjusting the relative level between the image data and the pattern signal of the predetermined period in accordance with the output of the changing means. The density is increased by the changing means. When the direction is changed, the adjusting unit adjusts the relative level so that the pulse width of the pulse width modulation signal formed by the pulse width modulation unit is shorter than before the change. .