JPS633499B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a recording method in which recording is performed using a photoreceptor, and the photoreceptor is exposed to light using a video signal using electro-optical conversion means.

Generally, in an electrostatic recording method, a photoreceptor is uniformly charged and then exposed to light to form an electrostatic latent image, and this electrostatic latent image is developed, transferred to transfer paper, and fixed. However, in such an electrostatic recording method, the intermediate tones and so-called solid areas of the image are damaged by development and cannot be reproduced. Therefore, in the electrostatic recording method known from Japanese Patent Application Laid-Open No. 14261/1980, a method is used in which a photoreceptor is charged and then a halftone screen is used to expose a fine halftone dot pattern. However, this method requires a screen with fine dots and a high-resolution lens. Also, since charge is applied to the photoreceptor in a dot pattern,
In the electrostatic latent image formed by exposure, charges are applied to the white portions as well, resulting in background smearing. In order to prevent this background smearing, a means is required to detect only the image area and not apply electric charge to the white area.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a recording method capable of solving the above-mentioned drawbacks and reproducing halftones and solid areas satisfactorily using simple means.

Embodiments of the present invention will be described below with reference to the drawings.

The present invention is an electrostatic recording method in which a photoreceptor is exposed to light in accordance with a video signal using an OFF (optical fiber tube) or a laser, etc., in accordance with a video signal. It is characterized by improving the reproduction of halftones and solid areas by applying spatial halftone dots to the image and utilizing the spatial frequency characteristics of development. Figures 1 and 2 show an electrostatic recording method in which a photoreceptor formed on a drum is rotated and charged by a charging device.
After being uniformly charged to 1000V, this photoreceptor is exposed to light according to the video signal using an electro-optical conversion means, so that the highest potential is 1000V, the lowest potential is 200V, and the dot density is 1 dot/mm, 2 dots/mm, 4 dots/mm. Book/mm, 8
Photoreceptor when an electrostatic latent image of 1.0 mm/mm is formed and this electrostatic latent image is developed in a developing device with a distance between the developing bias electrode and the photoreceptor of 1.0 mm and a voltage of the developing bias electrode of 700 V. The electric field strength at the surface of the photoreceptor and the electric field strength at a distance of 5 μ from the photoreceptor are shown, respectively, where the vertical axis is the electric field strength and the horizontal axis is the distance in the photoreceptor rotation direction. As can be seen from this, the electric field strength near the photoreceptor increases as the spatial frequency of the electrostatic latent image increases. However, when a combination of a cathode ray tube (hereinafter referred to as CRT) and a focusing light guide, a so-called self-detection device, is used as an electro-optical conversion means, the resolution is limited to 8 lines/mm. Therefore, it is desirable that the above-mentioned frequency be lower than the value equivalent to 8 lines/mm, which is the resolution limit. A good recorded image was obtained by multiplying the dots.

To explain this embodiment specifically, an image reading device scans an original and performs photoelectric conversion to read an image from the original and outputs a video signal of the image in time series. FIG. 3 shows the image density of a document having a black part and a gradation pattern of density 1, where the horizontal axis is the distance over which the document is scanned by the image reading device, and the vertical axis is the image density. FIG. 4 shows an output signal of the image reading device when the image reading device reads the image of the original shown in FIG. For example, this image reading device
If the output voltage is V, the amount of light hitting the document is _I0 , and the amount of light reflected by the document is I, then the image density D is generally expressed as D=-10gI/ _I0 , and VocI. Therefore, the video signal from the image reading device is not linear. Therefore, the correction device shown in FIG.
A video signal as shown in the figure is gamma corrected and the above modulation is performed. That is, in the correction device, a circuit 11 comprising an input buffer amplifier and the like performs various corrections such as γ correction and 180° phase shift on the video signal from the image reading device. FIG. 9 is a timing chart of the correction device shown in FIG. 10. The gain of the variable gain amplifier 12 is controlled by the output signal b as shown in FIG. 9 of this circuit 11, and when the level of the output signal b is small, The gain is large and the output signal b
When the level of is large, the gain becomes small. The oscillator 13 oscillates at a frequency corresponding to 5 lines/mm as described above, and produces an output a consisting of a sine wave or a rectangular wave with a constant amplitude as shown in FIG. 9, and this output a is amplified by the variable gain amplifier 12. As shown in FIG. 9, the signal c is amplitude-modulated by the video signal b from the circuit 11. This signal c and circuit 11
The output signals b are added with a certain weight in the summing amplifier 14, and the output signal d has a waveform as shown in FIG. 9 and is amplified by the output amplifier 15. If the bias value of the output signal of the output amplifier 15 is e, the amplitude is W, and the level of the video signal b is V, it can be expressed as W=kV+e, k: coefficient. FIG. 5 shows the output signal of the output amplifier 15 when W=1/2V and the output of the oscillator 13 is a rectangular wave. Note that the output amplifier 15 is biased to cut out the input signal below a predetermined level so as not to modulate the low-density portion of the video signal.

FIG. 11 shows the recording device used in this example,
In the CRT 16, the output signal of the output amplifier 15 is applied to the cathode to modulate the brightness, the output of the power source 17 is applied to the grid to control the beam, and the output of the deflection power source 18 is applied to the deflection coil 19 to deflect the beam. By doing so, line scanning is performed, and the photoreceptor 21 is exposed through the self-offset 20. As is well known in the CRT16, if the potential difference between the grid and the cathode is large, the electron beam current becomes small and the emission intensity of the fluorescent surface becomes small.
If the potential difference between the grid cards is small, the electron beam current becomes large and the luminous intensity of the fluorescent surface becomes large. For example, if the output of the correction device becomes 10V, which corresponds to the black part of the document, the fluorescent surface of the CRT 16 will emit light, and the output of the correction device will correspond to the white part of the document.
When the voltage reaches 27V, the fluorescent surface of the CRT16 will no longer emit light. The photoreceptor 21 is a 50 μm film formed on a drum.
It is made of Se and is rotationally driven by a drive device. After the photoreceptor 21 is uniformly charged to about 1000 V by the charging device 22, an electrostatic latent image is formed by exposure using the CRT 16. This electrostatic latent image is developed by the developing device 23, and the transfer charger 2
4, the image is transferred onto a transfer paper 25 and fixed by a fixing device. FIG. 6 is a projected diagram of the relationship between the surface potential of the photoreceptor 21 after transfer and the distance scanned by the CRT 16 output light on the photoreceptor 21. Figure 7 shows the output of the image reading device on a CRT without using the above correction device.
Recording was carried out using a general electrostatic recording method using 16 cathodes. Figure 4 shows the results of scanning a recorded image corresponding to the output signal of the image reading device with a microdensitometer and measuring its density, where the horizontal axis represents the distance scanned by the microdensitometer on the recorded image. The vertical axis is the density of the recorded image. In this recorded image, halftones and solid areas are damaged. FIG. 8 shows the results of scanning a recorded image according to this embodiment and corresponding to the output signal of the image reading device shown in FIG. 4 with a microdensitometer and measuring its density. The vertical axis indicates the density of the recorded image based on the distance scanned by the microdensitometer above. In this recorded image, halftones and solid areas are well reproduced.

In the present invention, as described above, an exposed image is halftone dotted by superimposing a signal having a predetermined frequency and having an original corresponding to the potential difference between the video signal and the white level on the video signal, excluding at least the white portion. The reason why the amplitude of the signal superimposed on the video signal is set to the amplitude corresponding to the potential difference between the video signal and the white level will be described below. The solid line in FIG. 12 shows the general relationship between the surface potential of a photoreceptor and the image density of a recorded image in a recording apparatus. As is clear from the solid line in FIG. 12, the surface potential of the photoreceptor and the recorded image density do not have a linear relationship. In the case of a recording device (writing system), ideally the relationship between the potential of the input signal and the density of the recorded image should be linear, but at least
If the relationship between the surface potential of the photoreceptor and the recorded image density is linear as shown by the broken line in the figure, the reproducibility of halftones and solid areas will be improved. Differences in image density △a, △ for the same photoreceptor surface potential between the solid line and the broken line in FIG.
One of the essential requirements of the present invention is to correct b, Δc... by changing the amplitude of the signal to be superimposed on the video signal based on the difference between the potential of the video signal and the potential of the white level. It is one. For example, if we consider a case where the relationship between the surface potential of the photoreceptor and the distance scanned by the CRT16 output light on the photoreceptor is as shown in Figure 13, the image density in this case would be 14th in the conventional electrostatic recording method. The relationship between the distance on the transfer paper and the image density shown by the solid line in the figure shows that the image reproducibility is poor even in the writing system. Furthermore, the reproducibility for originals becomes even worse. 1st
In the image density of Figure 4, the relationship between the distance on the transfer paper and the image density shown by the solid line is △a,
By adding △b, △c, etc., the surface potential of the photoreceptor is corrected so as to fill in the shaded area, which improves the reproducibility of halftones and solid areas, and this is achieved in the present invention. ing. The present invention includes one means for filling the shaded area in FIG. In other words, the relationship between the video signal and the white level as shown in Figure 4, which is the basis of the relationship between the distance scanned by the CRT output light on the photoreceptor and the surface potential of the photoreceptor as shown in Figure 13, is A signal having an amplitude corresponding to the potential difference and a predetermined frequency is superimposed.
Therefore, the video signal as shown in FIG. 4 is corrected as shown in FIG. 5, the surface potential of the photoreceptor becomes as shown in FIG. 6, and the recorded image density is corrected from the solid line in FIG. 15 to the broken line. Ru. In FIG. 15, the solid line shows the relationship between the density of the recorded image by the conventional electrostatic recording method and the distance on the transfer paper, and the broken line shows the relationship between the density of the recorded image and the distance on the transfer paper according to this embodiment. . As is clear from the comparison of the above results between the solid line in Figure 4 -> Figure 13 -> Figure 14 and the broken line in Figure 5 -> Figure 6 -> Figure 15, the present invention first focused on recording image density and exposure. It can be seen that by correcting the video signal so that the relationship with the surface potential of the body is linear, the reproducibility of halftones and solid areas in recorded image density has improved. Correcting the recorded image density (correcting the shaded area in Figure 14) means correcting the electric field strength of the photoreceptor, which spatially corrects the surface potential of the photoreceptor. This will correct the video signal. From this relationship, in order to improve the reproducibility of halftones and solid areas of recorded images, it is sufficient to manipulate the video signal in the writing system.

Further, in the present invention, the signal to be superimposed on the video signal is a signal of a predetermined frequency, and the reason for this will be explained. As can be seen from FIG. 2, when the amplitude of the video signal is constant, the higher the spatial frequency of the electrostatic latent image on the photoreceptor, the higher the electric field strength applied to the toner, the better the stability of the image and the better the image quality. However, the relationship between the electric field strength applied to the toner and the spatial frequency of the electrostatic latent image is as shown in Figure 16, and the higher the spatial frequency, the better.
is selected. Also, Figure 4 → Figure 7, Figure 5 →
As is clear from the correspondence shown in Figure 8, by superimposing a signal with a predetermined wave frequency on a video signal, it is possible to prevent the negative effects caused by the edge effect and improve the image quality. Reproduction also improves gradation.

As described above, in the recording method according to the present invention, the video signal applied to the electro-optical conversion means has an amplitude corresponding to the potential difference between the video signal and the white level and a predetermined frequency, excluding at least the white portion. By superimposing the signals, the exposed image is created with halftone dots, so halftones and solid areas can be reproduced well with a simple method, improving image quality and gradation. No need for a fine-dot screen or high-resolution lens. Moreover, background smear does not occur even if no means is used to detect only the image area and prevent electric charge from being applied to the white area on the photoreceptor.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the electric field strength on the surface of a photoreceptor on which an electrostatic latent image is formed by a general electrostatic recording method, and Figure 2 is a diagram showing the electric field strength at a distance of 5 μ from the photoreceptor. FIG. 3 is a diagram showing the relationship between the scanned distance on a document and the image density, FIG. 4 is a diagram showing the output of the image reading device, and FIG. 5 is a diagram showing the output of the correction device used in implementing the present invention. 6 is an expected diagram of the photoreceptor surface potential in the recording apparatus used in the implementation of the present invention, FIG. 7 is a diagram showing the density of an image recorded by a general electrostatic recording method, and FIG. FIG. 9 is a timing chart of the correction device; FIG. 10 is a block diagram showing the configuration of the correction device; FIG. 11 is a sectional view of the recording device; Figure 12 is a diagram showing the relationship between the photoreceptor surface potential and the density of the recorded image.
Figure 3 shows the relationship between the distance on the photoreceptor and the surface potential of the photoreceptor, Figures 14 and 15 show the relationship between the distance on the transfer paper and image density, and Figure 16 shows the electric field applied to the toner. FIG. 3 is a diagram showing the relationship between intensity and dot density of an electrostatic latent image on a photoreceptor. 12... Variable gain amplifier, 13... Oscillator, 14...
Summing amplifier, 15...output amplifier.

Claims (1)

[Claims] 1. In a recording method in which a photoreceptor is uniformly charged, the photoreceptor is exposed to light using a video signal using an electro-optical conversion means to form an electrostatic latent image, and this electrostatic latent image is developed, in which the photoreceptor is added to the electro-optical conversion means. By superimposing a signal having an amplitude corresponding to the potential difference between the video signal and the white level and a predetermined frequency on the video signal, excluding at least the white portion, the exposed image is brought into a halftone state. Characteristic recording method.