| 摘要 |
PURPOSE:To adjust image density with the same change width in both a gradation expression mode and a standard mode by adjusting the image density with the aid of exposure in an ordinary mode and controlling the difference of the surface potential of a photosensitive body from developing bias in a mode where a screen is used. CONSTITUTION:In the standard mode where the screen for controlling gradation characteristic is not put in an exposure optical path for the photosensitive body, the exposure is controlled 29 according to the instruction of an image density adjusting instruction means 22. In the gradation expressing mode where the screen for controlling the gradation characteristic is put in the exposure optical path for the photosensitive body to perform exposure, the difference of the surface potential of the photosensitive body from the developing bias is controlled 30 according to the instruction of the image density adjusting instruction means 22. By thus controlling the difference of the surface potential of the photosensitive body from the developing bias in the gradation expressing mode, the light quantity of a light source lamp in the gradation expressing mode need not be increased perticularly and the image density can be adjusted with the nearly same change width. Then, the screen for controlling the gradation characteristic can be selectively used even in a color copying machine in which the photosensitive body having comparatively low sensitivity is used. |
