1. Field of the Invention
The present invention relates to a laser light amount control method of a semiconductor laser scanner used for a writing head of a laser beam printer and a digital copying machine, particularly relates to a laser light amount control method in a tri-level developing system of forming a normal latent image and a reverse latent image by one exposure to execute two-color developing.
2. Description of the Related Art
In a conventional laser printer, a method of developing one color of toner by one laser scanning beam is used. Recently, for one of methods for the color laser printer, a tri-level developing method is developed, in which a method is used for forming by one laser scanning beam a normal developing latent image, a reverse developing latent image and an intermediate potential latent image in which neither a normal developing nor a reverse developing is executed, thereby developing two colors of toner at a time.
In an electrophotographic process, when an electrostatic latent image is formed on a photosensitive drum, an electric field polarized opposite to the polarity of the latent image is generated in the circumference of the latent image in addition to an electric field for highlighting developing generated at the end of the latent image. In the conventional one-color developing, when an image is formed, field reversing does not matter. However, in the tri-level process in which positive and negative electrostatic latent images are formed on the photosensitive drum and the developing of two colors of toner such as red toner and black toner polarized opposite to the polarity of the respective latent images is executed, special developing (hereinafter called fringe developing) occurs that red toner adheres around a black image and black toner adheres around a red image because of field reversing. Therefore, a problem arises that the quality of a two-color image is remarkably deteriorated.
The fringe phenomenon remarkably emerges in a solid image. Though the situation of the emergence is different at the top end and at the back end in the rotational direction of the photosensitive drum, the fringe phenomenon particularly arises at the back end. Heretofore, efforts to reduce a fringe at the back end have been made.
For a method of reducing the fringe, there is a method of reducing the fringe by using various exposure controls for trade-off relations among the fringe, the adhesion of a carrier and a printing density, which is a theme of tri-level developing. Specifically, there is a method of controlling an electric potential field in steps around an image by controlling the exposure amount to improve the surface of the photosensitive drum.
The fringe has been reduced by controlling the exposure amount under the aforementioned background. In case of the reduction of the fringe, as described above, the exposure amount control has been focused at the back end of the solid image in which the fringe phenomenon is remarkable.
However, as the experiments are made, it has proved that another correction is also necessary for a new fringe phenomenon arising between close vertical lines because the fringe phenomenon is remarkable not only at the back end of the solid image but between the close vertical lines.
Moreover, in case that a reverse image and a normal image are adjacent in the horizontal scanning direction, the correction control generates an unnecessary image turbulence in a portion to which the correction exposure is applied under a solid part under a part between the adjacent images.