The present invention relates to an image forming apparatus for forming an electrostatic latent image on an image carrier by the use of a laser beam and developing the latent image with toners, wherein the sharpness on a line image portion can be improved without sacrificing image density on a solid image portion when writing a latent image through exposure by means of a laser beam and thereby both preferable image density and sharpness are achieved.
As a measure for controlling initial excessive developability (rate of toner adhesion on the surface of a photoreceptor for developers to be supplied) that takes place when fresh developers are used, following measures are generally taken.
(1) To control power of a laser PA0 (2) To lower initial toner concentration in the developer . . . 2-component development PA0 (3) To lower charging voltage
In the invention, an MTF (analytical value for evaluating sharpness) for generated images can be improved. According to a definition of MTF shown in FIG. 1, the MTF is determined by a ratio of the difference between the maximum density (line image portion) and the minimum density (sheet portion) to the sum thereof, both the maximum and minimum densities being obtained when image density on a resolution chart on which a plurality of lines are arranged at constant intervals, for example, is measured in the direction (for example, arrowed direction in the figure) perpendicular to the lines on the resolution chart.
For the purpose of improving the MTF deteriorated by thickened line widths caused by the use of fresh developers, it is necessary to lower the developability on the line image portion. As a method therefor, the three methods mentioned above have so far been used. However, the methods (1)-(3) prove to be excessive measures resulting in deterioration of the developability for entire portions (both line images portion and solid images portion) exceeding the line images portion, though they have been planned to aim an improvement of the MTF on the line images portion. Therefore, the maximum image density of the solid images portion is sacrificed. Further, in all of the (1)-(3) methods, a development electric field tends to be small, which does not lead to an improvement of the MTF because a span (difference between the upper limit value and the lower limit value in the measurement range) between the maximum density and the minimum density in output images is narrowed down if the development electric field is made small.
An aim of the present method is to improve the MTF by changing the emission time of a laser beam for one pixel without changing charging voltage, toner concentration and laser power from their initial values set.