JPH0648849B2 - Image forming device - Google Patents

Description

The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus capable of faithfully reproducing a halftone image.

[Prior Art] Among image forming apparatuses, one using a laser printer in which an image is exposed to a photosensitive member with a laser beam and the image is developed to obtain an image is well known. Such a laser printer has advantages such as high printing quality and high speed, and is widely used as an output device such as a word processor or an ordinary printer. The following two methods have been proposed as a method of reproducing a halftone image by a laser printer.

A method of modulating the laser emission brightness according to the image density signal.

Generally, the relationship between the drive current of a semiconductor laser and the emission intensity is shown by a curve as shown in FIG. Therefore, if the driving current of the laser is changed according to the image density signal, the laser light amount changes, and the output image density can be obtained corresponding to the image signal. However, in this method, as shown in FIG. 3, the slope of the current-emission intensity graph is large, and the emission intensity changes greatly with a slight change in the drive current, so there is the drawback that control is difficult.

A method of modulating the laser drive pulse width according to the image density signal.

In this method, the laser drive current is kept constant and the width of the laser drive pulse is changed to perform modulation. Since the laser light is turned on and off while scanning the photoconductor, the total sum of the laser light amount in a certain pixel is represented by the integrated value of the laser light amount distribution. For example, in FIG. 4, when the laser spot whose light amount distribution is represented by the curve 42 is scanned in the x direction, when the laser is turned on at 40 and turned off at 41, the total light amount distribution is shown by the curve 43.

Therefore, if the laser lighting time, that is, the laser drive pulse width is changed in accordance with the image density signal, the amount of irradiation light to the photoconductor is changed and halftone can be reproduced. This method has high stability and is widely used because image reproduction can be performed while maintaining a stable value without changing the drive current. However, when trying to reproduce multi-tone halftones,
Particularly, there is a problem that the gradation becomes unstable in a region where the laser emission time is short. This is probably because the light intensity distribution has a great influence on the total light intensity distribution after integration, but especially in the region where the light emission time is short, the shape of the curve indicating the light intensity distribution has a direct effect.

In general, the light intensity distribution is easily affected by the accuracy and vibration of the optical system and is unstable, and if the light intensity distribution is wide or not, even if the total light intensity is the same, The image densities are generally different, and the difference is large especially in a region where the total light amount is small. Therefore, the image density becomes unstable in the region where the light emission time is short,
There was a drawback that halftones could not be reproduced accurately.

[Problems to be Solved by the Invention] The present invention has been made in view of the above-described conventional example, and an object thereof is to provide an image forming apparatus capable of always reproducing a stable halftone image.

[Means for Solving the Problems] In order to solve the above problems, the present invention provides an image forming apparatus that modulates an image signal corresponding to image density to form an image, and sets the image signal to a predetermined value. Comparing means for comparing, converting means for converting the density level of the image signal, modulating means for pulse width modulation by comparing the image signal with a reference signal, and comparing the outputs of the modulating means and the converting means It comprises a selecting means for selecting and outputting corresponding to the means, and an image forming means for forming an image based on the output of the selecting means.

[Operation] In the above-described structure, the image forming apparatus of the present invention changes the image signal having the predetermined density or less by the converting means, for example, changing the amplitude or pulse width of the image signal, and the image signal having the predetermined density or more by the modulating means It operates so as to perform width modulation and image formation.

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

[Block Diagram of Laser Printer (FIGS. 1 to 3 and 5)] FIG. 1 is a block diagram of a laser printer according to an embodiment of the present invention.

An input image signal 1 is proportional to the image density. Reference numeral 2 is a pulse adjustment circuit, which inputs the image signal 1 and outputs it.
Adjust the pulse width, gain, etc. This pulse adjustment circuit 2
The offset adjustment by means of FIG. 3 offsets the drive current Io when the laser starts to emit light shown in FIG. Next, the pulse width of the image signal 1 is shortened for adjustment, but the amount of shortening the pulse width is determined based on at least the light amount distribution after integration being stable. In this embodiment, the pulse width is set to 1/2. Further, the gain adjustment is performed corresponding to the set pulse width.

As described above, when the image signal 1 is adjusted by the pulse adjusting circuit 2 and then used as the drive signal for the semiconductor laser 4, the relationship between the level of the image signal 1 and the light emission amount is as shown in FIG. Since the offset is provided, the semiconductor laser 4 starts emitting light from the minimum image signal level of the image signal 1. Further, since the pulse width is halved, it is difficult to emit light with high brightness, but the change in the luminescence intensity with respect to the change in the image signal level is halved as compared with FIG. The slope of the straight line in FIG. 5 is half that of the straight line in the figure),
Stable brightness modulation can be performed.

A signal selection circuit 3 outputs either the A or B input as a signal 7 depending on whether the selection signal 6 is turned on or off. Here, when the selection signal 6 is "0", the A input is output, and when it is "1", the B input is output as the signal 7. A triangular wave generator 8 outputs a triangular wave 9 having the same period as the image signal 1. 1
Reference numeral 0 denotes a comparator, which inputs a signal 12 obtained by inverting the image signal 1 by an inverter circuit 11 and a triangular wave 9 and compares them to output a pulse signal 13 having a pulse width proportional to the magnitude of the image signal 1. The signal is input to the B input of the signal selection circuit 3.

The comparator 14 compares the reference signal 15 with the image signal 1, and when the image signal 1 is less than or equal to the reference signal 15, “0”, the reference signal 1
When it is 6 or more, the selection signal 6 which becomes “1” is output and input as the selection signal 6 of the signal selection circuit 3. Reference numeral 4 is a semiconductor laser, and reference numeral 5 is an image forming unit for inputting laser light from the semiconductor laser 4 to form an image.

FIG. 2 is a diagram showing a schematic configuration of the image forming section 5.

The photoconductor 20 is rotating in the direction of the arrow C. First, the photoconductor 20 is uniformly charged by the charger 21, and then the image portion is exposed to the non-image portion by the laser light from the semiconductor laser 4. An electrostatic latent image is formed on the photoconductor 20 by being imagewise exposed by imagewise exposure without exposure. This electrostatic latent image is developed by the developing device 22, and a toner image is formed on the photoconductor 20. This image is transferred onto the transfer paper 24 by the transfer charger 23.
The image is transferred onto the sheet, is fixed by a fixing device (not shown), and is ejected. The transfer residual toner remaining on the photoconductor 20 is cleaned by the cleaner 25.

In the above configuration, the signal 7 input to the semiconductor laser 4 becomes an amplitude-modulated signal from the pulse adjusting circuit 2 when the level of the image signal 1 is below a predetermined value, and when it is above a predetermined value, the comparator 10 A pulse width modulated signal is obtained. In this way, when the image signal 1 is less than or equal to the predetermined value, the semiconductor laser 4 is brightness-modulated, and when the image signal 1 is greater than or equal to the predetermined value, the semiconductor laser 4 is pulse-width modulated. It can be reproduced.

In the present embodiment, the case of the laser printer has been described, but the same can be implemented with an LED printer, for example. However, since the laser beam in the laser printer scans in the main scanning direction, the light amount in the main scanning direction is modulated by pulse width modulation, but in the LED printer, the light amount in the sub scanning direction is modulated.

As described above, according to the present embodiment, by performing the laser brightness modulation in the region where the laser emission amount is small and by performing the pulse width modulation of the laser in the region where the laser emission amount is large,
There is an effect that a stable halftone image can always be reproduced.

Further, in the area where the brightness modulation is performed, by making the pulse width shorter than usual, it is possible to reduce the change in the laser emission amount due to the change in the image signal, and it is possible to reproduce a more stable halftone image. There is.

[Effect of the Invention] As described above, according to the present invention, by changing the modulation method of the image signal in accordance with the density level of the image signal,
This has the effect of always reproducing a stable halftone image.

[Brief description of drawings]

FIG. 1 is a block diagram of a laser printer according to an embodiment of the present invention, FIG. 2 is a schematic diagram of an image forming portion of the laser printer, FIG. 3 is a diagram showing a relationship between a driving current of a semiconductor laser and emission intensity, and FIG. FIG. 5 is a diagram for explaining the light amount distribution when the laser spot is scanned, and FIG. 5 is a diagram showing the relationship between the level of the image signal and the emission intensity of the semiconductor laser in this embodiment. In the figure, 1 ... Image signal, 2 ... Pulse adjusting circuit, 3 ...
Signal selection circuit, 4 ... Semiconductor laser, 5 ... Image forming unit,
6 ... Selection signal, 8 ... Triangle wave generation circuit, 9 ... Triangle wave, 10,14 ... Comparator, 11 ... Inverter, 20
...... Photoreceptor, 21 …… Charger, 22 …… Developer, 23 ・ ・ ・
… A transfer charger.

Claims (2)

[Claims] 1. An image forming apparatus for forming an image by modulating an image signal corresponding to image density, comprising: comparing means for comparing the image signal with a predetermined value; and converting means for converting the density level of the image signal. A modulator for comparing the image signal with a reference signal to perform pulse width modulation, a selector for selecting and outputting the outputs of the modulator and the converter in correspondence with the comparator, and the selector. And an image forming unit that forms an image based on the output of the image signal, the image signal having a predetermined density or less is converted by the converting unit, and the other image signals are modulated by the modulating unit to form an image. An image forming apparatus characterized by the above. 2. The image forming apparatus according to claim 1, wherein the converting means includes an image signal offset adjusting means, a pulse width shortening means, and a gain adjusting means.