| 摘要 |
PURPOSE:To prevent background stains by detecting a part of light reflected from the original to be copied with an image density detector at the time of preexposure scanning preceding to copying, comparing the detected value with a reference voltage, and controlling bias voltage for development. CONSTITUTION:In the preexposure scanning executed prior to copying operation, a part of light reflected from an original illuminated by exposure operation falls on an original image density detector composed of a photodetector 12 and a filter 13, and it is detected as change of current, and converted into voltage with a resistor 35. The resistor 35 is made variable so as to change voltage to a fixed level in accordance with the amt. of the reflected light. The reflected light amt. converted into voltage changes in accordance with the image density of the original, and compared with the first and second reference voltages V1, V2(V1> V2) at the first and second comparators 23, 24, respectively. |
