JPH0272972A - Picture forming device - Google Patents

Abstract

PURPOSE:To remove the thinning of a dot, by which an image is formed, while also regenerating a pulse-width modulated middle tone picture excellently by delaying the laser-OFF time in response to the delay time at the time of laser-ON. CONSTITUTION:When a picture signal 111 represents a signal repeating ON-OFF at every one dot, the ON time is reduced only for 20nsec as shown in a broken line in laser beams 113 according to a conventional laser driver when the delay time 201 of the rise of a driver circuit 107 is 20nsec. Accordingly, the delay time 202 of the OFF of a laser drive signal 112 is delayed only for a section corresponding to the rise time 201, 20nsec, by a delay circuit 105, thus equalizing the ratio of ON/OFF as compared to the picture signal 111 as shown in a solid line in laser beams 113 and accurately recording dots, then eliminating the thinning of a slender longitudinal section and improving the reproducibility of one dot.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an image forming apparatus that forms an image using a laser beam, such as a laser beam printer.

[Prior Art] -M, a laser beam printer irradiates a photoreceptor with a laser beam modulated in accordance with an image signal to form an electrostatic latent image on the photoreceptor corresponding to the image signal; Image formation is performed by developing the electrostatic latent image and transferring it to recording paper or the like. The amount of laser beam emission (hereinafter referred to as laser power) in such a device is determined by the characteristics of the photoreceptor. In addition, the irradiation time per pixel (equivalent to one cycle of the pixel clock) depends on the recording density and recording speed, which are usually expressed in dots per inch (dpi), and the maximum image width in the main scanning direction of the laser. It is uniquely defined. Here, for example, set the recording density to 30
Assuming 0 dp i, the recording paper conveyance speed (process speed) in the sub-scanning direction is 50 mm/sec, and the maximum image width in the main scanning direction is 210 mm (A4 vertical feed), the pixel clock is approximately 1.7 MHz (1 clock cycle). is about 550n
sec).

Next, a method of driving a semiconductor laser will be explained.

The drive current for driving the semiconductor laser is determined based on the amount of light emitted by the laser, that is, the laser power, and the on/off state of the laser is determined based on the image signal as described above. When the laser is turned on, if the drive current is raised to the target value, an overshoot will occur as shown by the broken line in Figure 10. This will cause the laser to turn on strongly when it starts up. Not only will this affect the image, but there is a risk that the laser itself will be destroyed or its lifespan will be shortened.

Conventionally, therefore, the maximum current is not allowed to flow all at once when the laser is turned on, thereby preventing overshoot from occurring as shown by the solid line in FIG. In this case, it takes about 20 nsec until the laser drive current reaches its maximum.

However, in the case of the aforementioned recording density of 300 dpi, process speed of 50 mm/sec, and maximum image width in the main scanning direction of 210 mm, even if the rise of the laser beam per pixel is delayed by 20 nsec,
Since only about 4% of the full pixel lighting time of 550 nsec was not output, there was no significant effect on image formation.

[Problems to be Solved by the Invention] However, in recent years, the performance of host computers and the like that output image data to such printers has improved, and there has been a demand for printers that can record images at higher speed and higher density. In particular, the above-mentioned laser beam printer, for example, 600 dpi.

When recording images at a density such as 800 dpi,
The pixel clock is twice and 8/3 times that when the recording density is 300 dpi. Since this is performed in both the main scanning direction and the sub-scanning direction, the effect is squared, which is 4 times and 64/9 (approximately 7) times, respectively. In other words, the time required to record one pixel is 300d.
1/4 times larger than p i at 00dpi, 800dpi
It becomes 9/64 times. In such a case, the aforementioned 20 nsec delay at the time of laser startup becomes a serious problem. That is, for example, the recording time for one pixel at 600 dpi is 138 nsec, but a delay of 20 nsec is about 15% of this time, and furthermore, at 800 dpi, it becomes about 26%.

As the recording density increases and the recording time for one pixel becomes shorter, one pixel cannot be sufficiently exposed due to the delay in the rise of the laser beam mentioned above, and reproduction of one dot becomes very unclear. Also, a horizontal line of one dot in the main scanning direction has no effect because the laser remains on, but a straight line (vertical line) of one dot in the sub-scanning direction connects the dots of each main scanning line vertically. Because they are formed in a combined shape, they become thinner.

Conventionally, when reproducing halftones or photographic images using such a binary printer, gradation is expressed by performing dither processing using an 8 x 8 or 4 x 4 dither matrix. Was. However, this method has long been known to have a drawback in that as the number of gradations increases, the resolution drops. In other words, when an 8 x 8 dither matrix is assembled, 8 x 8 = 64 gradations can be expressed, but in order to express one density in a large area of 8 dots x 8 dots (area gradation), for example, a 300 dpi dither matrix is used. Even if it has a pixel density, its resolution is 300/8 (dpi
) = 37.5 (dpf) resolution could only be obtained.

Therefore, recently there has been a demand for expressing gradation without reducing resolution. That is, there is a need for a so-called multi-value printer that can not only express binary values by turning one pixel on and off, but also express gradations with one pixel. Pulse width modulation is currently widely known as a means for realizing this multivalue. This is, for example, 0 when the laser driving time per pixel is T n5ec.
．． Gradation is expressed by driving a laser for 3 Tn sec or 0.7 Tn sec. In other words,
One pixel can be represented by four gradations: laser off, 0.3T on, 0.7T on, and T on.

According to this method, for example, 16
It is possible to express 6 gradations and 256 gradations.

Multi-value data outputted to such a printer usually has an increased number of bits of image data, and for example, data from 0 to 15 can be transmitted using a 4-bit signal. According to such multi-value data, the laser driver determines the time to turn on the laser, and turns on the laser with a pulse corresponding to the multi-value data. For example, when the multilevel data is 7 degrees, gradation data can be obtained by turning on only 7/15 Tnsec. In this way, pulse width modulation is a very effective method for expressing gradations without reducing resolution.

However, the problem that arises again here is the rise time of the laser mentioned earlier. For example, the 300d mentioned earlier
Input 4-bit multi-value data with p i and change from “0” to “
Considering the case where a laser is turned on in response to an image signal up to 15", if the image data is l", then 550 / l
5 (=36.6nsec) will be turned on,
As mentioned above, there is a rise time of laser drive of 20 nsecC, so it is 46% of the desired laser on time.
The laser can only be turned on to a certain extent.

FIG. 11 shows this relationship. Here, the horizontal axis indicates the density value of multivalued image data, and in the case of 4 bits, 0-
It is shown up to 15. On the other hand, the vertical axis shows the laser lighting time as a percentage, with the laser on time per dot being T being 100%. In this figure, the solid line shows the ideal ratio of lighting time to the input signal, and the dotted line shows the actual ratio of laser lighting when the laser driver rise time is 20 nsec. As described above, in the actual device, the laser lighting time relative to the image data deviates considerably from the ideal value, making it impossible to obtain a good reproduced image.

Furthermore, the rise time of the laser beam varies due to slight individual differences in laser drivers (variations in C and R) and variations in the I-L characteristics of the laser itself, especially when reproducing multilevel images. This effect becomes noticeable, and there is a problem that the reproduced image density differs for each printer.

The present invention has been made in view of the above-mentioned conventional example, and by delaying the laser off time in accordance with the delay time when the laser is on, it is possible to eliminate thinning of the image formed dots, and also to prevent halftone images that are pulse width modulated. An object of the present invention is to provide an image forming apparatus that can reproduce images well.

Further, it is an object of the present invention to provide an image forming apparatus that can reproduce an optimal image by delaying the delay time when the laser is turned off in accordance with the delay time of the laser drive circuit or the amount of light emitted by the laser. With the goal.

[Means for Solving the Problems] In order to achieve the above object, an image forming apparatus of the present invention has the following configuration. That is, an image forming apparatus that creates a hard copy by scanning a photoreceptor with a laser beam modulated by an image signal, comprising: a current delay means for delaying the rise of a laser drive current for a predetermined time; A corresponding delay means for delaying the fall time of the laser beam is provided.

Another claimed invention is an image forming apparatus that scans a photoreceptor with a laser beam modulated by an image signal to create a hard copy, the invention comprising a current delay that delays the rise of a laser drive current for a predetermined period of time. means, a plurality of delay circuits each having a variety of different delay times, and a delay means for selecting the delay circuit corresponding to the predetermined time or the delay time of the laser drive circuit to delay the fall time of the laser beam. Equipped with.

[Function] In the above configuration, the current delay means operates to delay the rise of the laser drive current for a predetermined time, and to delay the fall time of the laser light corresponding to the predetermined time.

According to another aspect of the present invention, the current delay means delays the rise of the laser drive current for a predetermined period of time. Corresponding to the predetermined time or the delay time of the laser drive circuit, one of a plurality of delay circuits each having a variety of different delay times is selected to delay the fall time of the laser beam.

According to still another aspect of the present invention, the signal generating means outputs an image signal of a predetermined period, and the detecting means detects the intensity of the laser beam. Thereby, the delay means operates to select one of the delay circuits in response to the image signal of the predetermined cycle so that the amount of laser light detected by the detection means becomes a predetermined light amount.

[First Embodiment] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

[Description of Laser Beam Printer (FIGS. 1 and 2)] FIG. 1 is a block diagram showing a schematic configuration of a laser beam printer of a first embodiment.

In the figure, 100 is an input unit for inputting image data to be recorded from an external device such as a host computer, and 101 is a page memory for storing input image data page by page.

A D/A converter 102 converts digital image data read from the page memory 101 into analog image data. A reference signal generator 103 generates a reference clock signal such as a triangular wave, and a comparator 104 outputs a pulse width modulated image signal to the laser driver 110 by comparing the reference signal and an analog image signal.

The laser driver 110 outputs a pulse width modulated image signal 1.
Here, the image signal 111 is output to the driver circuit 107 through the OR circuit 106, and on the other hand, the image signal 111 is inputted to drive the laser 108.
is input to the delay circuit 105, and this delay circuit 10
5, the signal is output to the OR circuit 106 with a delay corresponding to the delay at the rise of the laser beam described above. The driver circuit 107 delays the rise of the drive current of the laser 108 by a predetermined period of time, and drives the laser 1O8 according to the signal output from the OR circuit 106.

Figure 2 shows the timing of each signal at this time.
Here, the case is shown where the recording density is 800 dpi, the process speed is 50 mm/sec, and the maximum image width in the main scanning direction is 210 nm.

As shown in FIG. 2, if the image signal 111 is a signal that repeats on and off for each dot, if the delay time 201 of the rise of the driver circuit 107 is 20 nsec, then according to the conventional laser driver, , laser beam 11
3, the on time is reduced by 20 nsec as shown by the broken line. Therefore, by delaying the OFF delay time 202 of the laser drive signal 112 by the rise time 201, that is, 20 nsec here, by the delay circuit 105 in FIG. Compared to No. 111, the on and off ratios are the same, dots are recorded accurately, and, for example, there is no thinning of the vertical pongee, and the reproducibility of one dot is good.

The semiconductor laser 108 is driven in this manner, and the laser beam 113 output from the laser 108 is scanned over the photoreceptor by the image forming unit 109, forming an electrostatic latent image on the photoreceptor (not shown), and forming an electrostatic latent image on the photoreceptor (not shown). It is converted into a hard copy using a photographic process.

[Second embodiment (Figure 3)] Next, the case of multivalued image data will be explained.

In the case where the recording density mentioned above is 300 dpi, the process speed is 50 mm/sec, and the effective image width is 210 mm,
A case will be explained in which 4-bit multivalued image data (from 0 to 15) is input. In this case, as in the previous case,
By delaying the off-timing of the laser by 20 nsec, it is possible to make the image signal and the laser emission time correspond to each other, as shown in FIG.

[Third Embodiment (FIG. 4)] In the cases shown in the first and second embodiments described above, the delay time of the laser beam 113 with respect to the laser drive signal 112 does not vary much, but there is still some variation. There will be variations.

This variation is usually not a problem, but
When the pixel density of image data increases or the number of gradations increases, for example, when 1200 dpi or 256 gradations are expressed by pulse width modulation, the effects of these variations become apparent.

This variation is mostly due to tolerances of the capacitor, resistor, etc. of the driver circuit 107.

FIG. 4 is a block diagram showing the configuration of a delay circuit 41 according to another embodiment, which includes n delay elements (44-1 to 44-n) each having a different delay time. 42.
43 are selectors for selecting desired delay elements;
By selecting one of these delay elements in accordance with the delay time of the driver circuit 107, the driver circuit 1
07 is eliminated, and stable reproduced images can be obtained.

[Fourth Embodiment (Figs. 5 to 8)] Fig. 5 is a block diagram showing a schematic configuration of a laser beam printer according to another embodiment of the present invention, and parts common to Fig. 1 are designated by the same symbols. , and their explanation will be omitted.

In the figure, 41 is the delay circuit shown in FIG. 4, which includes a plurality of delay elements each having a different delay time.
The configuration is such that a desired delay time can be selected by a selection signal 54 from a control section 53. 52 is the laser 108
A photosensor 51 attached to the back light emitting section of the laser 108 is a driver circuit for driving the laser 108, and is configured so that its output light amount can be adjusted by a control signal from a control section 53.

First, the power of the laser 108 is set. As shown in FIG. 6, the relationship between the laser drive current and the amount of light emitted by the laser (
It is known that the IL characteristics (IL characteristics) change. Therefore,
If the laser 108 is driven with a constant current value, the amount of light emitted by the laser will change due to temperature rise of the circuit or the laser, resulting in a difference in the density of the reproduced image.

Therefore, APC (Auto Power Control) has conventionally been used to correct this and obtain a constant amount of laser light emission. As shown in FIG.
At this time, when the laser 108 is caused to emit light with a fixed current before image formation, the laser 108 is
08, and performs feedback control so that it becomes a predetermined value (the amount of light determined by the characteristics of the photoreceptor).
The drive current of the laser 108 is controlled by the control signal 55.

Next, a pulse generator 56 and a changeover switch 58 are provided, and the switch 58 is turned on in response to a signal 57, and a pulse signal having an image clock period outputted from the pulse generator 56 is output to the laser driver 50. This pulse signal is an on/off signal for each dot, and is input to the laser driver 50 as an image signal. FIG. 7 shows the relationship among the pulse signal, laser drive current, and laser light at this time.

At this time, the amount of light emitted by the laser 108 is monitored by the photosensor 52, and the delay time when turning off the laser 108 is changed by selecting the delay element of the delay circuit 41 so that the average amount of light becomes a predetermined value.

The target value at this time is approximately 50 when fully lit.
%, but it may be set to approximately 55%, 60%, etc., depending on the shape of the laser spot, etc.

FIG. 8 shows the relationship between the pulse signal, laser drive current, and laser light after adjusting the delay time in this way.

By delaying the timing of turning off the laser 108 in this way, the on/off ratio of the laser 10B is reduced to 50%.
%, one dot can be reliably turned on for a predetermined period of time, the reproducibility of one dot can be improved, and, for example, thinning of vertical lines can be eliminated.

[Fifth Embodiment (FIG. 9)] Next, a multivalue case will be described. The recording density mentioned earlier is 300 dpi, the process speed is 50 mm/sec,
A case will be explained in the case of a printer that has an effective image width of 210 mm and inputs 4-bit multivalued image data. First, the laser power is determined by APC as in the first embodiment. Next, the image density is "10" by the pulse generator 56.
”, i.e. IOT/15 hours on, 5T/
Pulse signal that repeats off for 15 hours (however, T
is the ON time of one pixel).

The amount of light emitted by the laser 108 at this time is measured by the sensor 52.
The delay circuit 41 monitors 2/2 when fully on.
The delay time when turning off the laser 108 is selected so that the light intensity becomes 3. The states of the pulse signal, laser drive current, and laser light at this time are shown in FIG. Note that by applying the delay time when turning off the laser drive current to all of the 4-bit image data from "1" to "15", it is possible to obtain the amount of light emission that is suitable for the multivalued image data.

In addition, in this example, 2 pulse width modulated by a triangular wave is used.
Although the processing has been explained as processing for digital signals, it is not limited to this. For example, in addition to triangular waves, it can also be used for binary image data whose pulse width is modulated by the output value of a counter, or high-density binary image data created by the dither method. It can also be applied to cases where laser driving is performed using a value code or the like.

As explained above, according to this embodiment, by delaying the time when the laser is turned off in accordance with the delay time at the start-up of the laser drive, the fineness and imperfections of the image-formed dots are eliminated, resulting in a high quality image. It has the effect of allowing you to play back images.

[Effects of the Invention] As explained above, according to the present invention, the laser off time is delayed in accordance with the delay time when the laser is on.

This eliminates the narrowing of the dots formed in the image,
Further, there is an effect that halftone images subjected to pulse width modulation can also be reproduced satisfactorily.

Further, by delaying the delay time when the laser is turned off in accordance with the delay time of the laser drive circuit or the amount of light emitted from the laser, an optimal image can be reproduced.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the schematic configuration of the laser beam printer of the embodiment, Fig. 2 is a diagram showing the image signal, laser drive current, and timing of laser light in the first embodiment, and Fig. 3 is a diagram showing the timing of the laser beam printer of the second embodiment. A diagram showing the timing of the image signal, laser drive current, and laser light in the embodiment, FIG. 4 is a diagram showing another configuration of the delay circuit, and FIG. 5 is a block diagram showing the schematic configuration of the laser beam printer of another embodiment. Figure 6 is a diagram showing the I-L characteristics of the laser, Figures 7 to 9 are diagrams showing the relationship between pulse signals, laser drive signals, and laser light, and Figure 10 is a diagram showing the laser drive current. FIG. 11 is a diagram showing the relationship between the image signal and the laser lighting time. In the figure, 41, 1.05...delay circuit, 42, 43°5
8... Selector switch, 44-1 to 44-n... Delay element, 50, 110... Laser driver, 51, 10
7... Driver circuit, 52... Photo sensor, 53
...Control section, 54...Selection signal, 55...Control signal, 56...Pulse generator, 100...Input section, 101...Page memory, 102...D/A
Converter, 103... Reference signal generator, 104... Comparator, 107... Driver circuit, 108... Semiconductor laser, 109... Image forming unit, 111... Image signal, 112... . . . Laser drive current, 113 . . . Laser light.

Claims (3)

[Claims] (1) An image forming apparatus that creates a hard copy by scanning a photoreceptor with a laser beam modulated by an image signal, comprising: a current delay means for delaying the rise of a laser drive current for a predetermined period of time; An image forming apparatus comprising: a delay means for delaying the fall time of a laser beam in accordance with the above. (2) An image forming apparatus that creates a hard copy by scanning a photoreceptor with a laser beam modulated by an image signal, and has a current delay means that delays the rise of the laser drive current for a predetermined period of time. a plurality of delay circuits having various delay times, and selecting one of the delay circuits corresponding to the predetermined time or the delay time of the laser drive circuit;
An image forming apparatus comprising: a delay means for delaying a fall time of a laser beam. (3) signal generating means for outputting an image signal of a predetermined period;
further comprising a detection means for detecting the intensity of the laser beam, the delay means corresponds to the image signal of the predetermined period, and one of the delay circuits is arranged so that the amount of laser light detected by the detection means becomes a predetermined amount of light. 3. The image forming apparatus according to claim 2, wherein one is selected.