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Abstract

An illumination light to be used for position measurement of a movable scale is spatially (or physically) split into a first illumination light and a second illumination light using a triangle prism, and the first and second illumination lights are made to be incident on the same position on the movable scale, so that positional information of the movable scale is detected by utilizing interference of the first and second illumination lights. The spatially split first and second illumination lights interfere with each other even when they are made to be incident on the same position of the movable scale and are completely overlapped with each other, which is different, for example, from the case of the ±1st order diffracted lights that are generated by an illumination light being ±1st order diffracted by a diffraction grating. Therefore, wasted illumination lights that do not contribute to the interference can be minimized and the use efficiency of illumination lights can be improved.

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