| 摘要 |
PROBLEM TO BE SOLVED: To automatically detect that a pencil has been used or a document has been corrected with correction liquid. SOLUTION: The difference between the output image of a sensor 9, obtained with the light irradiating the top surface of a document 3 from a reflection light source 1, and the transmission output image of a sensor 10 obtained with the light irradiating the reverse surface of the document from a light source 2 are measured and calculated. When the ratio of a black image occupying the difference image is found to be larger than a specific value, it is decided that correction liquid has been applied.
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