| 摘要 |
PURPOSE:To detect even a fine pattern which can not be detected conventionally by arranging the opening of a mask nearly at the intermediate position between a light projecting element and light receiving element, and designing the width (x) of the opening of the mask so that a specific expression holds. CONSTITUTION:In a figure, there are a margin (gap) between a segment R2 and a mask opening edge 4'' and a margin (gap) between a segment R1 and a mask opening edge 4', so luminous flux regarding a low reflection part is incident on the light receiving element 2 to lower the S/N slightly. For the improvement of the S/N, the inequality I holds preferably. When the distance lbetween the mask 4 and a pattern 3 is extremely small, on the other hand, a 1st term in the right member of the expression I is a small value, so as value (x) approximates (a) and when the opening width (x) of the mask 4 becomes smaller than (a), the pattern can not be detected, so that (a) should be larger then (x). For the purpose, the mask 4 is so designed that a <=I and then the fine pattern can be detected.
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