| 摘要 |
PURPOSE:To prevent not only a faulty working of a copier due to the dust on an original and other factors but the soil on the surface of a copy sheet, by detecting a part of an optical image which gives an exposure to a drum through a photosensor and then setting the bias value of development with the mean value of those higher than the reference value among the detection outputs of the above-mentioned photosensor. CONSTITUTION:An original dividing mirror 10 divides a part of an optical image of an original 1 that gives an exposure to a photosensitive drum 9, and a photosensor 11 receives the divided beams. In such constitution, the detection signal of the sensor 11 is supplied to a CPU14 via an amplifying circuit 12 and an A/D converter 13. The value less than the reference value B is cut at the CPU14 to calculate the mean value of the signals at the texture part, and this output controls the output of a bias power supply 17 via a D/A converter 15 and a bias potential control circuit 16. In such way, the low bias application is eliminated although an abnormal value like signal C is caused by the dust. Thus the soil can be prevented on the surface of a copy sheet. |
