| 摘要 |
PURPOSE:To stabilize the photoelectrically converted output of a photoelectric conversion circuit, by preventing the variation of white level due to a paper leaf, light source, and photoelectric conversion element and, at the same time, fixing the white level in accordance with a white level reference value. CONSTITUTION:Whenever a paper leaf 12 is set, the content of a peak value holding circuit 20 is updated. The gain of a variable gain amplifier 16 is controlled and fixed so that the white level of an AD converted output 104 outputted from an AD converter 18 coincides with the white level reference value 112 of a white level reference value setting circuit 30 irrespectively of the variation of thickness of the paper leaf 12 and characteristics of a light source 10 and photoelectric conversion element. Thereafter, the paper leaf 12 is scanned and digitized picture signals are supplied to a quantizing circuit from the AD converter 18. |
