| 摘要 |
PURPOSE:To prevent erroneous retrieval due to the variation of density of a retrieval mark by detecting the density of the retrieval mark in plural positions by a light detecting member and obtaining a slice level of the retrieval mark based on the frequency distribution of the detected density. CONSTITUTION:The light which is emitted from a lamp 1 and is transmitted through a microfilm F is detected by a photosensor 2, and this detection signal is sent to a CPU 5 through an amplifier circuit 3 and an A/D converter 4. The output of the amplifier circuit 3 is binarized by a binarization circuit 7 and is sent to the CPU 5. A slice level SC in the binarization circuit 7 is set based on the frequency distribution of the detected density of a retrieval mark (m).
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