| 摘要 |
PROBLEM TO BE SOLVED: To measure a gap between an original plate and a base board without affected by S/N ratio while the effect of multiple reflection and irregular reflection of the signal due to the reflected light from the upper surface of base board toward the lower surface of original plate is excluded. SOLUTION: A signal peak position due to the reflection light from the lower surface of an original plate is stored, the signal peak position due to the reflection light from the lower surface of the original plate is shielded by a first shielding circuit, only the signal peak due to the original reflection light from the upper surface of a base board is left out, and related to the multiple reflection light on the base board upper surface following it, the signal due to the reflection light which is the multiple reflection or irregular reflection from the base board upper surface is shielded by a second shielding circuit so that with the signal peak position due to the original reflection light from the base board upper surface stored, the gap between the original plate and the base board is measured based on the position information between the signal peak position due to the reflection light from the original plate lower surface and that due to the original reflection light from the base board upper surface.
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