| 摘要 |
PURPOSE:To obtain the same operation as clamp processing with simple constitution by using a gray reference board adjusted toward the black level from the maximum white level of an original as a reference board scanned by an image pickup means. CONSTITUTION:A gray reference board 100 darker by several % than a white reference board 1 is used. A variable resistor VR1 is adjusted so that the output of an amplifier 6 is zero while the gray reference board 100 is being scanned. Further, a control circuit 80 gives adjustment that the output of an amplifier 70 is a reference white level with respect to a read level (white level of higher density by several % than the maximum white level) of the gray reference board 100. An output of the amplifier circuit 70 is provided with a clamp circuit 90 comprising diodes D2, D3, then even if the output of the amplifier circuit 70 is going to be the reference white level or over, the level is clamped in the reference white level and if the output is going to be zero level or below, the level is clamped in the zero level.
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