In recent years, there has been an increasing demand for high-precision reproducibility of image information by a digital copying machine or a printing machine which uses an electrophotographic technique. In response to this demand, methods and techniques for stabilizing image forming conditions or for realizing more precise gray-level image presentation have been developed.
Japanese Patent Application Laid-Open No. 1-297269 discloses a method in which the amount of light for black information recording by a light emitting device which forms an electrostatic latent image on a photosensitive member is set within the saturation range of the damping characteristics of the photosensitive member. Within the saturation range, the potential of the photosensitive member does not fluctuate greatly along with a fluctuation of the amount of light of the light emitting device. Thus, density unevenness due to a fluctuation of the amount of light emitted from the light emitting device can be prevented.
Japanese Patent Application Laid-Open NO. 4-25871 discloses a technique in which a high .gamma.-photosensitive member is employed, and the relationship between the maximum light amount I.sub.0 of the beam amount distribution on the high .gamma.-photosensitive member and the half exposure light amount P.sub.1/2 for reducing the potential of the photosensitive member by half is defined as: 1.2.times.P.sub.1/2.ltoreq.I.sub.0.ltoreq.2.5.times.P.sub.1/2. By satisfying the above conditions, clear image formation is carried out regardless of fluctuations in sensitivity of the photosensitive member.
Japanese Patent Application Laid-Open No. 8-160677 also discloses an image forming process called a simultaneous cleaning method or a cleanerless method, in which the exposure intensity in the exposure process is set in a predetermined range so that excellent image formation can be obtained, though residual toners on the photosensitive member are again charged and exposed. In this manner, ghost images can be prevented, and high-resolution and excellent gray-level images can be obtained.
In the above Japanese Patent Application Laid-Open Nos. 1-297269, 4-25871, and 8-160677, "saturation write" is carried out to irradiate the photosensitive member with light to form an electrostatic latent image, and the amount of light is in the saturation range of the photosensitive member. Thus, fluctuations in exposure amount and photosensitivity, and image deterioration caused by residual toners, can be reduced.
As described above, methods for stabilizing conditions of image formation by performing "saturation write" have been developed. A half-tone technique, which uses "saturation write", is also suitable for reducing density unevenness, called "banding", which is caused by small fluctuations of the write position in the sub-scan direction.
The above three Japanese Patent Application Laid-Open Nos. 1-297269, 4-25871, and 8-160677, however, do not specifically disclose means for carrying out high-precision and stable image formation when a large enough amount of light for performing "saturation write" is emitted to the photosensitive member. More specifically, enough consideration has not been given to the following: the relationship between the exposure diameter of the light beam and the layer thickness of the photosensitive member, the relationship between the maximum exposure and the differential sensitivity, and the relationship between the thickness of the photosensitivity and the light beam diameter of the write unit. These relationships should be taken into consideration when reducing charge diffusion and electric field expansion on the photosensitive layer so as to realize high precision in recorded images. Images formed by the conventional method described above are coarse, and for such images with low precision, it is not necessary to take into consideration the relationship between the thickness of the photosensitive layer and the light beam diameter.
The image formation using an electrophotographic technique is essentially an analog system, in which each dot can be easily multi-valued. By multi-valuing the output of a write unit (a laser diode, for instance), fine image formation can be carried out with each pixel having a gray-level. If a dot area modulation method, such as a dither method and an error diffusion method, is employed without multi-valuing each dot, obtained color images cannot be considered for practical use.
However, multi-valuing each dot in image formation using an electrophotographic technique results in deterioration of stability against various fluctuations. To solve such a problem, the following methods have been developed.
There is a method of pulse-width modulating an elliptical beam for improving gray-level reproducibility, especially reproducibility and stability in highlight images (disclosed in "A High Quality Color Printing Technology by Electrophotography" in Thesis Compilation Vol. J69-C No. 9 by the Institute of Electronics and Communication Engineers of Japan, 1986). By this pulse-width modulation method, gray-level presentation including density dot modulation and area dot modulation can be obtained, thus improving gray-level reproducibility.
As a technique for improving gray-level reproducibility, there is a method called HIEST, in which the number of lines in a specific direction depending on the gray level (disclosed in "New Halftone Screening Technology Focused on Highlight Image Reproduction", Japan Hardcopy, 1995). However, the pulse-width modulation method and the HIEST method are designed to improve stability against various fluctuations caused by multi-valuing in the electrophotographic process. In the case where the "saturation write" is performed to apply a large enough amount of light to the photosensitive member, those methods are not effective in carrying out high-precision and stable image formation.