A new interferometric system for measuring straightness is developed. In the system, triangularprisms are used to generate two converging beams. A straightness reflector is used to reflect back theconverging beams along their respective paths to form a combined beam. The combined beam isreceived by a detector system to count the interference fringes. Being different from the traditionalstraightness reflector, the new straightness reflector consists of not only reflecting surfaces but alsorefractive surfaces. The refractive surfaces cause a change in optical path difference (OPD) when arelative lateral displacement between the triangular prisms and the straightness reflector takes place.The relationship between the OPD and the lateral displacement is derived. The expected results agreewith the results in the experiments. A straightness measurement with an accuracy of 0.1 μm isobtained over a travel range of 120 mm.
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