| 摘要 |
PURPOSE:To facilitate the miniaturization of the equipment as a whole while smaller load of a rotating object to be inspected, by providing a reflecting means in which a reflecting surface is arranged to reflect specified order of diffraction light alone near a focal plane. CONSTITUTION:A convex lens 141, a reflecting mirror 151 and a mask 161 forms an optical system as a part of reflecting means and a luminous flux projected in parallel to the concave lens 141 is focused on the reflecting mirror 151 through the mask 161. The luminous flux thus focused is reflected to return to the original light path and irradiate the position M1 on the radiation grating 6 again. The optical system of the reflecting means thus arranged allows the removal of this diffraction lights other than specified order of diffraction light L1 such as zero order of one incident into the convex lens 141. Thus, this can reduce the distance between the radiation grating 6 and the reflecting means 141, 151 and 161.
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