| 摘要 |
PURPOSE:To attain an excellent read over a long period by changing the polarity of a voltage applied to a photodetecting element into positive or negative polarity periodically by a voltage polarity changeover circuit so as to reduce the secular deterioration of a contrast. CONSTITUTION:In applying a read for a 1st first line, a signal S1 goes to an H level. Thus, the state is obtained that a positive voltage +V is applied to the photodetecting element 5 via a power line L1 from a power supply 12. In this transition to the read of next line, the signal S1 is switched to an L level this time. A transistor 15 is turned on the voltage changeover circuit 9 through the inversion of the signal S1 and the voltage of -12V is applied to a power line L1 by a power supply 13. The circuit is operated similarly even with the polarity change over of the applied voltage and a voltage in response to the optical current of each photodetecting element 5 is extracted. Since the operation above is conducted alternately at every line, the increase in the secular dark current of the photodetecting element is reduced. Moreover, the secular deterioration of the contrast is reduced and excellent reading is applied over a long period.
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