JPH0219668B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a halftone image output device, and particularly to a device for improving resolution and gradation in, for example, a laser beam printer that outputs halftone images.

In general, scanning systems using rotating polygon mirrors or vibrating mirrors are widely used in laser-based facsimile machines, various display devices, printing devices, etc. because they allow a large scanning angle and have little chromatic dispersion. In particular, when a rotating polygon mirror is used, it is widely used as a high-speed scanning device.

Conventionally, in such a scanning system, as a method of recording or displaying a halftone image, one pixel on the recording/display surface is constructed by a matrix structure consisting of n×n (n is a positive integer) fine pixels. However, there is a method of reproducing halftones by how each micropixel is filled in, that is, by the distribution of white micropixels and black micropixels. FIG. 1 is an explanatory diagram of the above-mentioned conventional recording method, and is an example in which one pixel is constituted by a matrix of 2×2 fine pixels. In the figure, 10 indicates a pixel, and 11 indicates a fine pixel. When attempting to reproduce halftones using such a pixel configuration, by sequentially filling in fine pixels as shown in 1 to 4 in Fig. 1, 0, 1, 2,
Five gradations of brightness (brigtness) of 3 and 4 are obtained.
Generally, by configuring pixels as an n×n fine pixel matrix, an image with n×n+1 gradations can be obtained.

The advantages of the halftone image method using this method are: (1) The system is simple because it is only necessary to record one fine pixel in two values, white or black, and (2) Therefore, even if the gamma of the photoreceptor is nonlinear, It does not depend on the type of photoreceptor, etc. On the other hand, according to this method, (1) the resolution deteriorates because one pixel is composed of multiple fine pixels, and (2) the more you try to increase the gradation level, the more
This results in disadvantages such as the pixel area becoming larger and the resolution becoming worse.

The present invention has been made in view of the above points, and it is an object of the present invention to provide a halftone image output device capable of reproducing stable halftone images with high resolution and high gradation.

That is, in the present invention, each pixel is n×n (n≧2)
Each fine pixel in the fine pixel matrix is reproduced in three or more gradations using a first density and a second density higher than the first density to produce halftones. A halftone image output device that outputs an image includes: a beam generating means for generating a beam to form the fine pixels on a photoreceptor; an input means for inputting a density signal representing the gradation of the fine pixels; The beam generating means modulates the beam generated by the beam generating means based on the concentration signal input from the input means and generates beams with first and second intensities corresponding to the first and second concentrations. modulating means for changing the driving current value of the means; means for forming an electrostatic latent image on the photoreceptor based on the beam modulated by the modulating means; and a detection means for detecting the potential of the image, the detection means corresponding to a first gradation pixel formed by the first density micropixels without using the second density micropixels. a potential of the first electrostatic latent image and a second gradation corresponding to a pixel of a second gradation that is formed by fine pixels of the second density and that is the next gradation of the first gradation; The configuration is such that the potential of the electrostatic latent image can be detected, and the first intensity of the beam is changed to
A drive current that determines the first intensity of the beam based on the potentials of the first and second electrostatic latent images detected by the detection means in order to correct the potential difference between the second electrostatic latent images. The present invention provides a halftone image output device provided with a calculation means for calculating and determining a value.

The present invention will be explained in detail below based on the drawings.

In order to simplify the explanation below, three levels of brightness are used: white, black, and intermediate brightness between white and black.
An example of obtaining a halftone image using the so-called ternary method will be described.

Figure 2 shows pixels with nine gradations of brightness that can be obtained in this example. It consists of

Figure 3 also shows pixels with nine gradations of brightness, and in this example, white, large black circles, and small black circles with an area half of the large black circle are each 1
By allocating it to one micropixel, the four micropixels constitute a pixel having nine gradations of brightness.

In this way, using fine pixels of three types of brightness,
When one pixel is composed of an n×n fine pixel matrix, an image having a gradation level of 2×n×n+1 can be obtained. Therefore, it is possible to obtain a gradation level that is approximately twice that of n x n + 1, which is the gradation level obtained when using an n x n micro pixel matrix using micro pixels displayed in binary format as in the past. . In addition, when outputting halftones using this method, the size of one pixel can be made smaller compared to the conventional method for the same gradation level, and therefore high resolution and sharp halftones can be output. You can get the image.

Figure 4 shows one of the configurations of an output device that obtains halftone images.
Give an example. Here, 12 is a semiconductor laser,
An optical deflector 14 that converts the output beam emitted from the semiconductor laser 12 into parallel light beams using a collimator lens 13 and forms the parallel light beams using a rotating polygon mirror.
incident on the The light beam reflected and deflected by the optical deflector 14 reaches the photosensitive drum 16 via an imaging lens 15 such as an Fθ lens.
image on top. Here, one-dimensional scanning of the light beam is performed by rotating the rotary polygon mirror 14, and two-dimensional light scanning is performed over the entire circumferential surface of the photosensitive drum 16 by rotating the photosensitive drum 16. Here, by modulating the semiconductor laser 12 using a drive circuit to be described later, an image pattern including a desired halftone image can be formed on the photosensitive drum 16.

The image forming process in the output device shown in FIG. 4 will be explained based on FIG. Photosensitive drum 16
has a photoconductor coated on its surface, and in the so-called NP process used in this example, an insulating layer is provided on the photoconductor layer. First, as a first step, the primary charger 17 uniformly charges the insulating layer positively. Next, AC corona discharge is performed by the static eliminator 18 while irradiating the photosensitive drum 16 with a laser beam 21 modulated by a semiconductor laser. Further, by uniformly irradiating the photosensitive layer using the entire surface exposure lamp 22, image information can be accumulated on the photosensitive drum 16 as a surface potential distribution, that is, as an electrostatic latent image. Next, by attaching toner particles of opposite polarity in the developing device 19 onto the photosensitive drum 16, the electrostatic latent image described above is developed and the paper 2 is
Transferred to 0. Here, the surface potential of the electrostatic latent image is measured by the surface potential sensor 23, and a desired image is stably obtained as described later. The image forming process has been described above, and now the operation of the surface potential sensor 23 will be described.

Generally, the relationship between the exposure amount E and the surface potential V is as shown in FIG. 6, where the surface potential decreases where the amount of light is large. Here, an image pattern as shown in FIG. 7, that is, nine gradations of brightness, can be created by one pixel using fine pixels to which three gradations of brightness are respectively assigned, as shown in FIG. 2 or 3. When the aforementioned laser beam 21 is modulated so as to obtain an image pattern having the following characteristics, the surface potential changes in a stepwise manner as shown in FIG. Here, the horizontal axis r is the number of filled-in fine pixels.

FIG. 9 shows a drive circuit for the semiconductor laser 12 shown in FIG. In this example, the semiconductor laser 12 provides three different types of optical output including zero. Here, 12 is the aforementioned semiconductor laser, 31 is an input terminal for the image density signal S1 in the form of a binary signal, and 32 is an input terminal for the image density signal S2 in the form of a binary signal. As shown in FIG. 3, a pair of signals S1 and S2 for obtaining fine pixels with three levels of brightness according to the present invention are input to these input terminals 31 and 32.
and controls the semiconductor laser 12 based on these signals S1 and S2 to produce three-value optical output L0,
Obtain LM and LH. That is, by the two transistors 33 and 34 that constitute the differential switch,
The semiconductor laser 12 is activated in response to the image density signal S1.
to drive. At this time, the current flowing through the semiconductor laser 12 is determined by the collector current of the current source transistor 35. On the other hand, two transistors 36 and 37 forming a differential switch drive the semiconductor laser 12 in response to the image density signal S2. At this time, the current flowing through the semiconductor laser 12 is determined by the collector current of the current source transistor 38. Therefore, image density signals S1, S2
The semiconductor laser 12 can be driven with different current values depending on which of the corresponding terminals 31 and 32 is supplied with either of them.

In this example, from the viewpoint of protecting the semiconductor laser 12, when low-level image density signals S1 and S2 are simultaneously supplied to the terminals 31 and 32, the maximum current is caused to flow through the semiconductor laser 12, and light is emitted in accordance with this current value. It is preferable to set the circuit constants so that the image obtained by the laser beam obtained by the laser beam has the maximum density, and to turn off the image density signal S1 to form an intermediate output that obtains an intermediate density. That is, as shown in FIG. 10, when the image density signals S1 and S2 are both at high level, the optical output of the semiconductor laser 12 is set to L0.
When the signal S1 is low level and the signal S2 is high level, the optical output is set to the intermediate output LM, and when the signals S1 and S2 are both low level, the optical output is set to the maximum output LH.

In order to stably obtain such different light outputs L0, LM, and LH in the image forming process, the ninth
Preferably, the light output LM for obtaining the intermediate image is controlled by controlling the force signal from the terminal 39 shown, ie the base potential of the transistor 35. Here, the reason why the optical output LM in the intermediate state is controlled to stabilize the optical output is that it is superior to controlling the maximum output LH in the following points. Here, the relationship between the exposure amount E and the laser light output D shown in FIG. 11 will be described.

(1) To obtain stable output, the maximum optical output must be E.
It is better to be at the saturated point A of the -D curve.

(2) In response to environmental changes, E- shown in Figure 11
Even if the D curve changes as shown by the dotted line, the position of the maximum output is hardly affected. On the other hand, the intermediate density position B is greatly affected.

Therefore, it is desirable to fix the maximum light output LH with a constant current and control the intermediate light output LM.

In this way, the three-value optical output L0, LM, LH
Therefore, when an image pattern as shown in FIG. 7 is drawn, if the fine pixels of the intermediate tone can be configured correctly, the amount of change in surface potential for each tone will be approximately constant. However, as shown in FIGS. 2 and 3, gradation levels 4 to 5 are obtained by changing the configuration of fine pixels.
In the transition to , the surface potential may change discontinuously as shown in FIGS. 1A and 1B. Therefore, the surface potential on the photosensitive drum 16 is measured using a surface potential sensor 23 shown in FIG. 5 to control the light output. Note that when measuring the surface potential, it is necessary to extract the potential on the photosensitive drum 16 as a macroscopic surface potential distribution in units of one pixel. It shall be made so that it cannot be imaged. Therefore, it is necessary to draw the image pattern over a relatively large area. 12th
The figure shows a block diagram for controlling the optical output described above, in which the surface potential output from the surface potential sensor 23 (see FIG. 5) is amplified by the amplifier 50, and then
The surface potential information is digitized by the converter 51 and taken into the memory 54 from an input port controlled by the central processing unit CPU53. This series of procedures is performed by a drum clock obtained from a drum clock generator 52 in synchronization with the rotation of the photosensitive drum 16, and surface potential information averaged for each gradation of the halftone image is sequentially taken into the memory 54. That is, the semiconductor laser 12 (see FIG. 9) is driven by a test pattern signal for image adjustment (a pattern that produces gradations), and the latent image potentials thereof are sequentially taken into the memory 54. Now, if the image spot is not appropriate, as shown in Figure 13A and B,
The continuity of each gradation becomes poor. FIG. 13A shows a case where the intermediate optical output LM is too weak, and the potential change from 0 to 4 is slight, but it changes rapidly from 4 to 5. Figure 13B shows the case where the intermediate optical output LM is too strong (but lower than the maximum optical output LH), and the
There is little difference between 5 and 5.

Therefore, in order to obtain the optimum light output, the optimum intermediate light output LM is obtained from the surface potential changes represented by 0 to 4 and the surface potential changes represented by 5 to 8 in FIGS. 13A and B. The base potential of the transistor 35 shown in FIG. 9 may be controlled as shown in FIG. This can be done by following the steps below.

FIG. 14 is a flowchart showing the procedure for making each gradation uniform. First, average values V ₀ , V ₁ , . . . , V _N of potentials measured for each gradation are determined. 2×
2, then V _N =V ₈ . Therefore, each difference ΔV _i (i=1, 2, . . . , 8) is determined. Next, the average of each of the first half and the second half, where fine pixels of intermediate brightness appear, is calculated. That is, ΔV=1/4 ₄ 〓 ⁱ⁼⁵ ΔV _i , ΔV=1/4 ₈ 〓 ⁱ⁼⁵ ΔV _i . Here, if the difference between ΔV _A and ΔV _B is minute and smaller than the allowable range ε, the difference between each gradation is relatively uniform, and the operation may be continued in that state. This tolerance range ε can be determined depending on the accuracy of the device.

On the other hand, if the difference between ΔV _A and ΔV _B is larger than the allowable range ε, it is necessary to correct it. The amount to be corrected is
When ΔV _A −ΔV _B >0, the intermediate optical output LM is decreased; when ΔV _A −ΔV _B <0, the intermediate optical output LM is increased; |ΔV _A −ΔV _B | It should be controlled so that it is proportional. That is, the current i _A supplied to the semiconductor laser 12 for the intermediate optical output LM is controlled so that it becomes i _A 〓i _A −α·(ΔV _A −ΔV _B ).

Here, α is a positive proportionality constant and may be determined according to the characteristics of the device.

The control of the current value determined by the above procedure is carried out in the 12th
It is executed based on the output voltage value which is converted into analog by the D/A converter 55 shown in the figure and outputted from the terminal 39. This output voltage is input to terminal 39 in FIG. 9 to control the collector current of transistor 35. In addition, in the above-mentioned control, depending on the characteristics of the device and the visual characteristics of humans, etc.,
All you need to do is decide on the constant α. In the case of the above-mentioned electrophotography, various characteristics are nonlinear, and proximity effects may appear depending on the size of pixels, the size of micropixels, etc., so it is preferable to determine them based on experiments.

Note that the above-described control needs to be performed in advance before outputting the image. This control may be performed, for example, immediately after the power is turned on and when the drum rotation speed reaches a constant number, and does not need to be performed every time. Alternatively, it may be performed manually as needed. At this time, it is not necessarily necessary to transfer the image pattern shown in FIG. 7 onto paper and take it out, and the image pattern may be stored and controlled. Furthermore, in the above example, in order to obtain an intermediate optical output, the collector current of the current source transistor was automatically controlled by measuring the surface potential. The intermediate tone may be controlled by changing the base current of the transistor. In such a case, you can place a variable resistor or other knob for adjustment on the outer frame of the device in advance so that it can be operated from the outside, and if necessary, output a test chart image and make adjustments. .

As explained above, according to the present invention, the latent image potential of the photoreceptor is actually detected, the beam intensity is corrected, and the gradation is corrected. It is possible to reproduce halftone images.

Note that the above explanation was given using three values as an example, but
Of course, a desired halftone image can be obtained using a similar method using four or more values of brightness. Moreover, although the present invention has been described above mainly using a laser beam printer as an example, the present invention is not limited to this, and can be effectively applied to printing using a facsimile machine, a printing machine, etc.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the halftones of a pixel in the conventional case where one pixel is composed of a matrix of 2×2 fine pixels, and FIGS. FIG. 4 is a block diagram showing an example of the halftone image output device of the present invention, FIG. 5 is an explanatory diagram of the image forming process, and FIG. 6 is an exposure diagram. Quantity E
Fig. 7 is a diagram showing the standard image pattern showing 10 levels of brightness, Fig. 8 is a diagram showing the relationship between
Figures 13A and 13B are diagrams showing the relationship between the number of filled-in micropixels and the surface potential, Figure 9 is a circuit diagram for driving a semiconductor laser that obtains three-level optical output, and Figure 10
The figure is a timing chart of the waveforms of each part, No. 11.
The figure is a diagram showing the relationship between the exposure amount E and the laser light output D, Figure 12 is a block diagram for controlling the semiconductor laser shown in Figure 9, and Figure 14 is the procedure for making each tone of brightness uniform. This is a flowchart showing the following. 10... Pixel, 11... Fine pixel, 12... Semiconductor laser light source, 13... Collimator lens, 14
... Light deflector, 15 ... Imaging lens, 16 ... Photosensitive drum, 17 ... Primary charger, 18 ... Static eliminator, 19 ... Developing device, 20 ... Paper, 21 ... Laser beam, 22 ...Exposure lamp, 23...Sensor, 31, 32, 39...Terminal, 33-38...
Transistor, 50...Amplifier, 51...A/D
Converter, 52...Drum clock generator, 53...
...CPU, 54...Memory, 55...D/A converter.

Claims (1)

[Scope of Claims] 1. Each pixel is constituted by an n×n (n≧2) fine pixel matrix, and each fine pixel in the fine pixel matrix has a first density and a density higher than the first density. A halftone image output device that outputs a halftone image by reproducing a gradation of three or more values using a second density, comprising: a beam generating means for generating a beam to form the fine pixels on the photoreceptor; input means for inputting a density signal representing the gradation of a fine pixel; and modulating the beam generated by the beam generation means based on the density signal inputted from the input means, and corresponding to the first and second densities. modulating means for switching the drive current value of the beam generating means to generate beams of first and second intensities; and forming an electrostatic latent image on the photoreceptor based on the beam modulated by the modulating means. and a detection means for detecting the potential of an electrostatic latent image of a pixel formed on the photoreceptor, the detection means detecting the potential of the electrostatic latent image of the first density without using the fine pixels of the second density. The potential of the first electrostatic latent image corresponding to the pixel of the first gradation formed by the fine pixels of The configuration is such that the potential of the second electrostatic latent image corresponding to the pixel of the second gradation can be detected, and the first intensity of the beam is changed to detect the first,
A drive current that determines the first intensity of the beam based on the potentials of the first and second electrostatic latent images detected by the detection means in order to correct the potential difference between the second electrostatic latent images. What is claimed is: 1. A halftone image output device comprising: calculation means for calculating and determining a value.