To get high accuracy for measuring length, an interferometric measurement is usually used. In this case advancement of the mirror usually accompanies the rotational movement of the mirror, and this affects the proportionality between the advancement of the mirror and fringe displacement. If the rotational movement is small the deviation from proportionality is small, but if the multipass method is used, the angular deviation is also magnified, and the deviation becomes large. The best way to overcome the difficulty is to use a corner cube. A multipass method for a corner cube is described.
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