JPS61189577A - Image forming device - Google Patents

Abstract

PURPOSE:To maintain the picture quality of a halftone image excellently by detecting the density of a formed visible image and controlling the pulse width of a modulating signal corresponding to halftone picture elements according to the detected density. CONSTITUTION:A CPU20 closes the switch of a variable pulse generating circuit 18 and a clock pulse generating circuit 17, on the other hand, is operated to apply a clock pulse F to the circuit 18. Then, a clock pulse C (reference modulating signal) is generated by the circuit 18 and then applied to a driving circuit 13 through an OR gate 15. Therefore, a laser 9 is modulated and driven corresponding to the signal C to flash and a projected laser beam 3 is scanned on a photosensitive body 1 for exposure. The density D of a sample visible image is detected by a sensor 21 and the CPU20 selects modulated pulse width for the formation of the halftone image with desired density corresponding to the detected sample visible image density D.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to an image forming apparatus that forms an image by exposing an electrophotographic photoreceptor to light modulated by a modulation signal corresponding to recorded information.

(Background of the invention) An image is formed only with the self-level pixel whose area average density is set to the lowest and the black level pixel whose area average density is set to the highest, that is, only with these non-halftone pixels. In this case, it is difficult to express both good resolution and halftones. Therefore, a halftone pixel whose area average density is set between that of the white level pixel and that of the black level pixel is used, and the halftone of the image is expressed by this halftone pixel.

One method of forming halftone pixels is to make the exposure time of each pixel of light to the photoreceptor shorter than that of a black level or white level pixel, that is, to modulate the pulse width of a signal that modulates the light. There is a method (pulse width modulation method), but as a characteristic of electrophotographic photoreceptors, the so-called gamma curve slopes steeply in the halftone region, so the output intensity of the light source changes and the sensitivity of the photoreceptor changes. If the characteristics of the developing device change, the image quality of the halftone image will deteriorate more easily than the image quality of the black level image or self-level image.
The entire image will deteriorate.

(Objective of the Invention) An object of the present invention is to enable an image forming apparatus employing a pulse width modulation method to maintain good image quality of a halftone image.

(Summary of the Invention) In the image forming apparatus of the present invention, the density of the formed visible image is detected, and the pulse width of the modulation signal corresponding to the halftone pixel is controlled in accordance with the detected density. .

(Embodiments of the Invention) FIG. 1 is a detailed explanatory diagram of the present invention. In FIG. The photoreceptor 1 is first uniformly charged by a charger 2, and then scanned and exposed in a direction substantially perpendicular to the rotational direction of the photoreceptor 1 with a laser beam 3 that is modulated on and off in response to a modulation signal. The electrostatic latent image thus formed on the photoreceptor 1 is developed and visualized by the developing device 4. In this embodiment, the developing device 4 is a developing device that performs so-called reversal development in which toner is attached to the portion of the photoreceptor l exposed to the laser beam, the so-called bright area. The toner supplied to the photoreceptor 1 is charged to the same polarity as the photoreceptor charging polarity by the charger 2 . Conversely, the laser beam 3 is modulated so as to expose the portion of the photoreceptor 1 to which toner is to be attached.

In any case, the visible toner image formed on the photoreceptor 1 is
The image is transferred onto a transfer material 6 by a transfer charger 5 . The visible toner image transferred to the transfer material 6 is fixed by a fixing device (not shown),
On the other hand, toner remaining on the photoreceptor l after transfer is removed and cleaned by a cleaning device 7. Thereafter, the charge remaining on the photoreceptor 1 is eliminated by the discharge light from the lamp 8, and the above steps are repeated again.

Laser beam 3 is emitted from semiconductor laser 9 .

Thus, the semiconductor laser 9 receives the modulation signal E at f
It is driven by the FI effect circuit 13 and emits a laser beam modulated on and off in accordance with the modulation signal E. When forming an image corresponding to recorded information on the photoreceptor 1 (image forming mode), the modulation signal E is a signal corresponding to the recorded information. In any case, semiconductor laser 9
The beam emitted from the rotating polygon mirror, galvano mirror
is scanned by a scanner 10 such as the following. Reference numeral 11 is a lens for forming a point-like image of the laser beam 3 on the photoreceptor l, and reference numeral 12 is a mirror for folding the optical path.

In the image forming mode, when the image to be formed includes a black level portion and a halftone level portion, the modulation signal applied to the drive circuit 13 includes a modulation signal corresponding to the black level and a modulation signal corresponding to the halftone level. It includes signals in chronological order. In the embodiment shown in FIG. 1, a black level signal A from a recorded information source 14 such as a computer, word processor, image reading device, information memory, etc. is applied to the drive circuit via an OR gate 15.

On the other hand, the halftone signal B from the recorded information source 14 is applied to the AND gate 16. A pulse signal C' having a width (temporal width) that is controlled and set in the control mode is applied to the AND gate 16, as will be described later. Therefore, when a halftone signal B exists in the recorded information signal, the pulse signal C' passes through the AND gate 16 as a modulation signal (pulse signal) D corresponding to the halftone, and the OR gate 15
is applied to

The E-written pulse signal C' and the reference modulation signal pulse signal C, which will be described later, are generated by a variable pulse generation circuit 18 to which a clock pulse (for example, 18M1(z)F) from a clock pulse generation source 17 is applied.Clock pulse generation Source 1
In the image forming mode, 7 outputs a clock pulse F in reverse synchronization with the reverse output of signals A and B from the recorded information source 14;
On the other hand, in the control mode, the control microcomputer 20
The clock pulse F is reversely outputted based on a command from the controller. in any case.

The variable pulse generation circuit 18 uses the clock pulses to form pulse signals of various widths (that is, changes the pulse width per pixel), and a known type can be used, for example, TTL (TTL). The operation delay time of transistors (transistors, transistors, logic) was utilized. Something like the one shown in Figure 2 can also be used. In FIG. 2, clock pulse F is applied to OR gate 18' and T T L
It is guided to the element row l8'. Then switch S1. S
2. S3. The width of the pulse output from the OR gate 18'' is selected depending on which switch S4 is closed. Here, the width of the output pulse of the OR gate when the switch S1 is closed (pulse duration ) is Tl, T2 is the value when switch S2 is closed, T3 is the value when switch S3 is closed, T4 is the value when switch S4 is closed, and T4 is the value when switch S4 is closed. T when you are there
If it is 5.

T 1 < T 2 < T 3 < T 4 < T
There is a relationship of 5 < T O. In addition, each switch 31 to S
4 is selectively closed by the microcomputer 20. To is the pulse width of the black level signal corresponding to one pixel of the black level image.

Here, the pulse width T3 formed by the circuit 18 is
In this embodiment, the pulse signal is also used as a reference modulation signal in the control mode described later, and the pulse width of the halftone modulation signal to be used in the image forming mode is set to T3 depending on the control mode. If determined, it is also used as a halftone modulation signal in image forming mode.

If one variable pulse generation circuit is used for both the control mode and the image forming mode in this way, the device will be simplified, but the variable pulse generation circuit should be used only for the image forming mode, and a reference modulation signal forming circuit for the control mode should be provided. There is no problem,
In any case, if a visible image is formed using this as the reference modulation signal, if the density is in a standard state with the laser output intensity, developer characteristics, photoreceptor sensitivity, etc. It is preferable to have a pulse width that provides a preset desired halftone density, because this allows more accurate control in the control mode. Therefore, it is preferable to use a modulation signal having such a pulse width as a reference modulation signal in the following embodiments, but this is not absolutely necessary, and the pulse width per pixel of the modulation signal for black level is preferably used as a reference modulation signal. If it is short,
A signal with any pulse width can be used as the reference modulation signal. This is because, no matter what the pulse width is, there is a correspondence between it and the pulse width of the modulation signal necessary to obtain the desired halftone density.

In the embodiment shown in FIG. 1, the control mode operation is performed prior to the image forming mode operation. In the control mode, the photoreceptor l rotates and the charger 2. Developing device 4
is activated, and the scanner lO also rotates. However, in this mode, in order to detect the density of the visible toner image on the photoreceptor 1, the transfer charger 5 may be activated or not. In the device that detects the density of the toner image, the charger 5 also operates. Static elimination lamp 8
is preferably activated.

In the control mode, the microcomputer (cp
u) When the control signal is transmitted to the CPU 20, the CPU 20 transmits the output of the variable pulse generation circuit 18 directly to the OR gate 15 without passing through the AND gate 16.
The switch circuit 19 is switched. Then, the CPU 20 closes the switch S3 of the variable pulse generation circuit 18, and operates the Glock pulse generation circuit 17 to generate the clock pulse F.
is applied to the circuit 18. As a result, the pulse width T
3 (reference modulation signal) is generated by the circuit 18, which signal C is applied to the drive circuit 13 via the OR gate 15. As a result, the laser 9 blinks in response to this signal C, 7g1'! IA was moved,
The thus emitted laser beam 3 scans and exposes the photoreceptor 1. The sample latent image formed on the photoreceptor 1 by this scanning exposure is first developed by a developing device 4 to become a sample visible image. Then, the density D of this sample visible image is detected by the sensor 21. Then, the CPU 20 selects a modulation pulse width for forming a halftone image having a desired density in accordance with the detected sample visible image density.

Note that the concentration sensor 21 includes a light source and a photosensor that receives reflected light from the object illuminated by the light source and forms an electrical signal corresponding to the amount of light received.

To explain the example in detail, the above concentration information is
It is taken into the RAM part of 20.

be remembered.

Next, the CPU 20 determines whether the absolute value of the difference between the detected density and the target halftone density DH is less than or equal to ΔD. That is, it is determined whether the density of the sample image differs from the target density within an allowable range. If D differs from DH within an allowable range, the density of the halftone image formed by the laser beam modulated by the modulation signal of pulse width T3 is within the allowable range with respect to the target halftone density. They only differ internally. Therefore, in this case, the application of the clock pulse F to the circuit 18 is stopped and the switch 19 is switched to AN
Switch to the D gate 16 side and end the control mode.

(Incidentally, ΔD is set in relation to the characteristics of the photoreceptor, the set target density, the charging potential by the charger 2, etc., and corresponds to the accuracy 1 etc. required for the intermediate m density.

On the other hand, if ID-DHI is greater than ΔD, cp
u20 calculates the target pulse width T''H according to the following calculation formula stored in its ROM part.

"rH=T'3+α(D-DH) Here, the target pulse width TH means that when a visible toner image is formed using a modulation signal having a pulse width of this TH,
This is the pulse width at which the density of the visible image becomes the target density or a value extremely close to it. Also, α is the characteristics of the photoreceptor, laser, and developer, the width of the reference modulation signal (T3),
This is a constant that is set depending on, for example, at which point on the gamma curve the target halftone density is set, and can be determined through experiments.

In any case, since the target pulse width TH is not necessarily equal to any of the selectable pulse widths T1 to T5, in the embodiment of FIG. 1, the pulse width closest to TH among T1 to T5 is 1 This is selected as the pulse width of the halftone modulation signal in the image forming mode, thereby resulting in a halftone image having a density that differs only within an acceptable range from a predetermined halftone density. Then, the CPU 20 stops applying the clock pulse F to the circuit 18, and causes the switches 51 to S4 of the variable pulse generating circuit 18 to form a pulse having a width closest to T). Select a switch that allows this and close it.TH
is closest to T5, the CPU 20 switches each switch 51 to
Open all of S4.

After selecting each of the switches S1 to S4, the CPU 20 switches the switch 19 to switch the signal path so that the output (pulse signal C') of the variable pulse generation circuit 18 can be transmitted to the OR gate 16.

The control mode ends with the above steps. A flowchart of the operation of the CPU 20 in the control mode is shown in FIG. 3. Note that the arithmetic expression in step h is not limited to the one described above.

For example, T according to the formula TH=α・T3・(D/DH)
H may be calculated (α is a constant), and a preferred arithmetic expression to be used can be determined based on experiments. Furthermore, the concentration of the other party with which the detected concentration is compared is not limited to the target concentration I)H, but may be any appropriate value.
This is because there is a certain relationship between them. In short, the pulse width of the modulation pulse signal for forming the recorded halftone image is controlled in accordance with the difference between the predetermined reference value and the detected sample visible image density.

In any case, once the above control mode is finished, then
Alternatively, once an image forming command (print command) is input after entering the production state, the operation in the image forming mode is started.

The above control mode may be performed in conjunction with the operation of turning on the power switch (main switch) of the apparatus, or in conjunction with the operation of turning on the image forming mode operation command switch (print switch). Alternatively, the time corresponding to the so-called paper gap between the conveyance of the transfer material and the subsequent conveyance of the transfer material (in other words, the time between the end of laser beam exposure of the photoreceptor corresponding to the previous image and the time corresponding to the next image) Alternatively, a test area may be provided on the side of the area of the photoreceptor where the image to be transferred is formed (with respect to the scanning direction of the laser beam). The above-described control may be performed by forming a visible image of the sample in the test area. After the end of the scan or before the start of the scan, a laser beam modulated by signal C scans the test area. Therefore,
In this case, the control mode and the image forming mode are performed in parallel. Or 1: Since there is usually a blank area in the peripheral area of the transfer material where no image corresponding to the recorded information is formed, the sample visible toner image formed on the photoreceptor is transferred to this blank area using a reference modulation signal, The density of this image may be detected on the transfer material 6 by the density sensor 21'. In this case as well, the control mode and the image forming mode are performed in parallel as described above.

Also, in the case where a sample visible toner image is formed on the transfer material 6 and its density is detected on this transfer material by the sensor 21', the control mode is set independently of the image forming mode, and Alternatively, it may be performed later, in which case the sample visible toner image may be formed on any portion of the transfer material 6.

In any case, an example of a time chart of the signals A to H is shown in FIG. 4. In FIG. 4, the left side is the control mode, and the right side is the image forming mode. The semiconductor laser 9 lights up when the signal E is at level 1. In Figure 4, section p is a white level image portion of 3 pixels, sections r and t are each a 1 pixel white level image portion, section V is a 2 pixel white level image portion, and section q is a 1 pixel portion of the white level image. In the black level image portion, section fiff s corresponds to a black level image portion of 3 pixels, 1 section U corresponds to a halftone image portion of 1 pixel, and 1 section W corresponds to a halftone image portion of 3 pixels.

Furthermore, in one or more embodiments, a modulation signal is applied to the semiconductor laser drive circuit, but the laser is not modulated and driven, and the laser beam is incident on an acousto-optic element (AO element), and the ultrasonic wave applied to the AO element is A modulation signal may be applied to the drive circuit of the transducer, in which case the AO element forms a laser beam modulated in accordance with the information to be recorded. This is convenient when using gas lasers, etc.

Further, in one or more embodiments, one halftone level is provided between the self level and the black level, but two or more halftone levels may be provided.

In this case, in the control mode, an appropriate pulse width for each modulation signal for each halftone level may be determined by applying the same procedure as described above for each halftone level, or one common standard may be used. Form a visible image of the sample using a laser beam modulated by a modulating signal.

An appropriate pulse width for each halftone level modulation signal may be determined in accordance with the density of this sample visible image.
In this latter case, the respective pulse widths may be controlled in response to the difference between the detected density and a common reference density, or alternatively, the respective reference density specific to each halftone level and the detected The respective pulse widths may be controlled in response to each difference in density.

Furthermore, although the above example is a laser beam printer, it uses an LED array in which a large number of small light emitting diodes (LEDs) are lined up, and each LED in this array is controlled to blink in response to a modulation signal to produce electrophotographic exposure. An image forming device that forms an image by exposing the body to light, or an LCS array in which a large number of micro liquid crystal shutters (LC5) are arranged is placed between a light source and an electrophotographic photoreceptor, and each LC in this array
The present invention can also be applied to an image forming apparatus that forms an image by driving the photoreceptor 3 with a modulation signal to modulate light from a light source and expose a photoreceptor.

Furthermore, in the above example, so-called reversal development is employed in which toner is attached to the area of the photoreceptor exposed to the image light.
An image forming apparatus to which a positive development method is applied, in which toner is attached to the photoreceptor area that has not been exposed to image light, the so-called dark area (therefore, the toner is charged with the opposite polarity to the photoreceptor charging electrode by the charger 2). The present invention is also applicable to In this case, what is called the black level in the above embodiment can be read as the own level. Therefore, in this case, the pulse width per pixel of the modulation signal corresponding to the halftone is the same as that of the modulation signal corresponding to the white level. It is shorter than the pulse width per pixel.

The above embodiment is a printer that transfers a visible image formed on a photoreceptor to a transfer material, but the present invention transfers a visible image formed on a photoreceptor to a display section in an image forming mode. It can also be applied to an image forming apparatus that displays images by moving them.

In this case, since it is desirable that the photoreceptor in the display section be planar, the photoreceptor is preferably belt-shaped.

(Effects of the Invention) According to the present invention, since the pulse width of the modulation signal for halftones is controlled in accordance with the visible image density, the pulse width of the halftone modulation signal is controlled in accordance with the visible image density. It becomes possible to stabilize the image quality of the tone image.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram of one embodiment of the present invention, and FIG. 2 is a circuit 18.
FIG. 3 is a flowchart, and FIG. 4 is a signal waveform diagram. 1 is an electrophotographic photoreceptor, 9 is a laser, 18 is a variable pulse generation circuit, 20 is a microcomputer, 21.21"
is a concentration sensor.

Claims (1)

[Claims] Image formation in which an electrophotographic photoreceptor is exposed to light modulated by a modulation symbol corresponding to recorded information to form a latent image, and a visible image is formed by developing this latent image. The apparatus includes a detection means for detecting the density of the formed visible image, and a control means for controlling the pulse width per pixel of the modulation signal corresponding to the halftone image in accordance with the detected density. An image forming apparatus comprising: