JPH043312Y2 - - Google Patents

Description

[Detailed description of the invention] [Industrial field of application] This invention is applicable to devices that form images by image exposure using laser beam scanning, such as laser printers or laser copy printers, and in particular to devices that form images by performing image exposure using laser beam scanning, especially in the electrophotographic process. The present invention relates to a device that controls the amount of laser light in order to maintain the image density at a desired value in an image forming device that develops the area that is hit by the laser beam.

[Conventional technology]

Conventionally, when an image is formed using a laser as an exposure source in an electrophotographic method, the image density is affected by the amount of laser light and the sensitivity of the photoreceptor, and the image density changes due to fluctuations in the amount of laser light and the sensitivity of the photoreceptor. is influenced by this.

In particular, when reproducing halftones using a dither method or the like, which involves developing the areas hit by light, reproduction stability is lacking. In other words, when reproducing halftones as described above, it is necessary to make the 100% halftone area a certain desired density or higher and to reproduce the high halftone area area so that it is not crushed. The reproduction variation of the halftone dot area is large, and the high halftone dot area may be crushed, or conversely, the density of the 100% halftone dot area may be too low. As shown in the copy density-gradation step characteristics in Figure 7, with respect to the ideal tone curve a, the characteristics when the laser power fluctuates become tone curves b and c, and when the power is large, the tone curve b in,
If the laser power is too strong, the high halftone dot area will be crushed and contrast will not be obtained, and if the laser power is too low, the density of the 100% halftone dot area will be too low. That's what I do.

Accordingly, as a technique related to solving this problem, there is a technique disclosed in Japanese Patent Application Laid-open No. 136754/1983. In this Japanese Patent Application Publication No. 59-136754,
It detects the potential of the pattern latent image in which the on and off parts of the light beam are mixed, determines whether the difference from the target value is within the allowable value, and if it is outside the range, first the charger is Then, the charger and laser light amount are controlled by controlling the laser light amount according to the control formula (the sum of the difference from the target value multiplied by a certain coefficient). describes an image forming apparatus that has high sharpness and good reproduction of halftones.

Further, Japanese Patent Laid-Open No. 59-154468 describes an electrophotographic copying machine that controls the amount of exposure based on the developed density of a reference density area.

However, these publications require that the charger and laser light amount be controlled in order to bring the potential of the two types of pattern latent images within a tolerance value around the target value, or that the exposure amount be simply controlled based on the developed density of the reference density area. However, it is possible to reliably reproduce 100% halftone areas, so-called solid black areas.
Furthermore, it was not possible to reproduce even high-density halftone dot images while maintaining the gradation.

[Problem that the invention attempts to solve]

This idea is intended to solve the above-mentioned problems such as the density of 100% halftone dots being too low and the high halftone dots being crushed.

[Means and operations for solving the problem] This invention therefore uses a laser that forms solid black areas and high-density halftone image patches on the non-image area of the photoreceptor, detects the density, and performs image exposure. By controlling the amount of laser light to increase or decrease in accordance with the detected density, a tone curve that can maintain the image density at a desired value in density-gradation step characteristics is obtained.

In other words, this invention consists of a means for performing image exposure by laser beam scanning, a means for forming an image by developing the area hit by light using an electrophotographic process, and a means for forming a solid black area and high density in the non-image area of the photoreceptor. In a device having a means for forming a halftone image and a means for detecting the density and controlling the amount of laser light, the amount of laser light is increased when the density of a solid black area is below a first predetermined value; This method is characterized in that the amount of laser light is reduced when the difference between the solid black area and the high-density halftone dot area is equal to or greater than a predetermined value but less than a second predetermined value.

〔Example〕

An embodiment of this invention will be described below based on the drawings.

Figure 1 shows an embodiment of this invention. Image exposure is performed by laser beam scanning, and an image is formed by a method of developing the areas hit by light using an electrophotographic process. By gradation method
The case is shown when applied to a laser printer that forms 36 gray levels.

In the figure, reference numeral 1 denotes a photoreceptor, which in the illustrated case is a drum-shaped photoreceptor, and various mechanisms for carrying out a normal electrophotographic process are provided around the photoreceptor 1. That is, 2 is a charger that uniformly charges the photoreceptor 1 prior to exposure, 3 is a developer that performs development using toner, and 4 is used to transfer images onto paper that is conveyed along paper 5. A transfer device, 6 a cleaner for removing toner remaining on the photoconductor 1 after transfer, and 7 a static elimination light source for eliminating the potential on the photoconductor 1, these are arranged along the rotation direction r of the photoconductor 1. It is set up. This photoreceptor 1
The exposure system that forms a latent image on the image includes a modulator 8 to which an image signal is supplied, a laser 9 serving as an exposure source,
Lens 10, mirror 11 and polygon mirror 12
A latent image is formed by developing a portion of the image area 13 of the photoreceptor 1 that is irradiated with light in accordance with the image signal.

That is, by operating a print start button (not shown), the photoreceptor 1 is charged to a uniform potential by the charger 2, the laser 9 is modulated by the modulator 8 according to the image signal, and the lens 10, mirror 11, and A latent image is formed in accordance with an image signal by irradiating a laser beam with a method of developing a portion of the photoreceptor 1 that is irradiated with light via a polygon mirror 12.

The latent image formed in this way is developed by a developing device 3 as the photoreceptor 1 rotates, and the toner image obtained by the development is transferred onto paper by a transfer device 4. The toner remaining on the body 1 is removed by a cleaner 6, and the potential on the photoreceptor 1 is further eliminated by a discharge light source 7 in preparation for the next cycle. These operations are repeated to obtain a specified number of prints, but the laser printer shown in FIG. A halftone image patch is formed, and its density is detected to control the amount of laser light.

That is, in this example, between each image area 13, there is a 100% halftone dot patch 15 that becomes a solid black area on the non-image area 14 on the photoreceptor 1, and a high halftone dot patch 15 that does not irradiate laser light to only one pixel in a 6×6 matrix. A dot area patch 16 (high dot area first gradation step) is formed as shown in FIG. 2, and the exposure system is provided with a ROM 17 used therefor. This ROM1
Reference numeral 7 denotes an image signal ROM in which signals for forming the above-mentioned 100% halftone dot patch 15 and high halftone dot area patch 16 on the photoreceptor 1 are stored.

Furthermore, Fig. 3A shows a 100% dot patch as described above in the case of a 4x4 matrix, and Fig. 3B shows a high dot patch as described above in the case of a 4x4 matrix. The area patch is shown.

As shown in FIG. 2, a non-image area 1 on the photoreceptor 1
The two patches 15 and 16 formed in 4 are
It is used as a target patch in a laser light intensity control system as described below.

Further, the laser printer of FIG. 1 further includes a photoreceptor 1 located between the transfer device 4 and the cleaner 6.
A density sensor 18 is provided to detect the densities of the two patches 15 and 16 developed above, and a comparison circuit 19 which applies feedback to the laser light intensity based on the detected density is connected to a sensor PWB (printed wire board). It is set in. In this example, the concentration sensor 18 is an infrared light reflection type sensor, and as shown in FIG.
The patch density is detected by the amount of reflected light. In addition, the comparison circuit 19
constitutes a control section that increases or decreases the laser beam according to the above-mentioned detected concentration, and the above-mentioned laser 9 is controlled by this control section.

Next, control of the amount of laser light with the above configuration will be explained with reference to FIG. 5, which shows the state of the surface of the photoreceptor 1 in chronological order, and FIG. 6, which shows an example of the flow of controlling the amount of light.

When obtaining a print by repeating the steps of charging, exposure, development, transfer, and cleaning as described above, patches 15,
form 16. That is, as shown in FIG. 5, the first image 13-1 and the second image 13-2
The image signal ROM is stored in the non-image area 14 between the second image 13-2 and the third image 13-3.
The 100% halftone dot patch 15 and the high halftone dot patch 16 in which only one pixel in the 6.times.6 matrix is not irradiated with laser are formed according to the signals stored in 17, and developed by the developing device 3. Since the 100% halftone dot patch 15 and the high halftone dot area patch 16 formed in this way are in the non-image area 14, they are not transferred to the paper, and as the photoreceptor 1 rotates, the infrared light reflective type It moves to the position of the concentration sensor 18. When the density sensor 18 reaches the position, the patch density is detected based on the amount of reflected light received by the light receiving element 18b, and the comparison circuit 1
The laser 9 is controlled according to the flowchart of FIG. 6 based on the sensor output given to the laser 9.

First, if the density D _BK of the 100% halftone dot patch 15 and the density D _G of the high halftone dot area patch 16 are detected,
As shown in steps ~ in FIG. 6, each density is compared with a reference value, and the amount of light from the laser 9 is controlled according to the comparison result. When the density D _BK is less than the set value 1.4, it is determined that the laser light intensity is too low and the potential of the photoconductor 1 is not sufficiently removed, and the laser 9 is controlled to increase _the laser light intensity. Yes, but
When D _BK −D _G ≦0.04, it is determined that the laser light intensity is too strong and the gradation in the high halftone dot area is lost, and the laser light intensity is reduced. If the determination result in step , is YES, it is determined in step whether or not the set number of sheets has been completed, and if the determination result is YES, this program ends, but if it is NO, the program returns to the step, The above process is repeated until the specified number of sheets is reached. In this way, by providing feedback to the laser light intensity and increasing or decreasing it, you can always reach the desired 100%.
% solid density and an ideal tone curve that does not lose gradation in a high density area.

Note that the feedback to the laser light intensity is such that the next image has already been formed when the patch density is detected at the position of the density sensor 18, so the next image signal of the detected patch is output as shown in Fig. 5. I'll do it later. Also, after detecting the concentration, patch 1
5 and 16 are removed by a cleaner and sent to the next cycle.

In this manner, the apparatus having the above configuration has the function of forming solid black areas and high-density halftone image patches in the non-image area 14 of the photoreceptor 1, and detecting the density to control the amount of laser light. However, when an image is formed by performing image exposure using laser light scanning and developing the areas hit by the light using an electrophotographic process, even if there are factors that change the image density, the image will be 100% halftone. By detecting the density of the high halftone dot area and controlling the laser light intensity, the desired 100%
It is possible to obtain an ideal tone curve that has % halftone density and does not lose gradation in high density areas.
Prints with good image quality can be obtained. The reproduction stability is high, and unlike conventional methods, when the laser power is too strong, the high halftone dot areas are crushed and contrast cannot be obtained.
On the other hand, the laser power is not too weak and the density of the 100% halftone dot area is not too low, and the density of the 100% halftone dot area and the high halftone dot area formed on the non-image area of the photoconductor 1 is not too low. Laser 9 can detect and control the laser power to obtain the desired tone curve.
Even in a device that forms an image by performing image exposure on the photoreceptor 1 using the exposure source, the image density can be easily maintained at a desired value.

Note that this invention is not limited to the configuration of the embodiment described above; for example, in the embodiment, patches 15 and 16 are formed between each image area 13 to detect the density and provide feedback to the amount of laser light.
This may be performed only during the first printing, or every time a certain number of sheets are printed. Further, although an infrared light reflection type sensor is used as the concentration sensor 18, the density sensor used is not limited to the infrared light reflection type sensor, and other types of concentration sensors may be used.

Further, in order to compensate for an increase in the bright area potential (residual potential) or a change in chargeability due to fatigue of the photoreceptor 1, it may be used in combination with known charge amount control or development bias control.

[Effect of idea]

As described above, according to this invention, it is possible to control the amount of laser light by detecting the solid black area and the density of the high halftone dot area formed in the non-image area of the photoreceptor and providing feedback to the laser of the exposure source. Even with this type of equipment, which forms an image by exposing the photoreceptor using light scanning and developing the areas hit by the light, it is possible to always achieve the desired halftone density of 100%.
It also has the advantage of being able to produce prints of good image quality without losing gradation in high-density areas.

Furthermore, in image formation using an electrophotographic process using laser beam scanning to develop the areas hit by light, solid black areas (100% halftone dot areas) and high-density halftone dot images are formed. The laser light intensity is increased when the density of the black part (100% halftone dot part) is less than a first predetermined value, and even if the density of the black part (100% halftone dot part) is more than the first predetermined value, the difference between the solid black part and the high density halftone part is less than a second predetermined value. By reducing the laser light intensity when
By controlling the amount of laser light, solid black areas (100% halftone dots) are reliably reproduced, and high-density halftone areas are also reproduced while maintaining the gradation, maintaining the gradation of high-density images such as human hair. It can be formed by

[Brief explanation of the drawing]

Fig. 1 is a schematic configuration diagram of a laser printer showing an embodiment of this invention, Fig. 2 is an explanatory diagram of a target patch, and Figs. 3 A and B are a 100% dot patch and a high dot patch in the case of a 4 x 4 matrix. Figure 4 is an enlarged explanatory diagram of the concentration sensor part, Figure 5 is a diagram showing the point patch,
The figure is an explanatory diagram showing the image on the photoconductor and the state of the patch in chronological order. Figure 6 is a flow chart showing an example of laser light intensity control. Figure 7 is an ideal tone curve and tone when changing laser power. FIG. 3 is a copy density-gradation step characteristic diagram showing a curve. Explanation of symbols, 1...Photoreceptor, 3...Developer, 9...
...Laser, 14...Non-image area, 15...100%
Halftone dot patch, 16...High halftone dot area patch, 17...
...ROM, 18...concentration sensor, 19...comparison circuit.

Claims (1)

[Scope of Claim for Utility Model Registration] Means for performing image exposure using laser beam scanning, means for forming an image by developing the area hit by light using an electrophotographic process, and solid black areas and solid black areas on the non-image area of the photoreceptor. In a device having means for forming a high-density halftone image and means for detecting the density and controlling the amount of laser light, the amount of laser light is increased when the density of a solid black area is below a first predetermined value; A laser light amount control device characterized in that the laser light amount is reduced when the difference between a solid black area and a high-density halftone dot area is equal to or more than a second predetermined value but less than a second predetermined value.