| 摘要 |
PURPOSE:To accurately detect a signal, by providing auxiliary slits to both sides of a slit for detecting the signal. CONSTITUTION:Auxiliary slits 11, 12 in a rotary side and auxiliary slits 13, 14 in a fixed side are provided to both sides of slits 4', 7' for obtaining a zero point signal. Each of these auxiliary slits 11, 12, 13, 14 is set to a shape having a length (l/2) half that of each of the central slits 4', 7' and a width (2W) twice that of each of said slits 4', 7'. A light receiving element is at first irradiated with lights passing through the auxiliary slits 11-14 and subsequently irradiated with the lights passing through the central slits 4', 7'. Therefore, even if the opening areas of the central slits 4', 7' are small, the rising of the detection signal in the light receiving element becomes sufficient by the lights from the auxiliary slits and a signal is detected certainly.
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