Measurement method of spatial straightness error using non-diffracting beam and moire-fringe technology

Based on the datum-line of the non-diffracting beam produced by an axicon, which is a thin lens with a weak conical surface on one face, a new measurement method of spatial straightness error has been developed in this paper, in combination with moire-fringe technology. When a ring grating is illuminated by a non-diffracting beam, a moire-fringe image is formed, from which the deviation of the centre of the ring grating to the centre of the Bessel fringe ring on the section of the non-diffracting beam, the offset, can be computed. The centre of the Bessel fringe ring embodies the non-diffracting beam datum-line, so the offset is also to the non-diffracting beam datum-line. Then, the spatial straightness error can be obtained from the set of the offsets corresponding to the section along the non-diffracting beam. In order to verify the performance of the new method, theoretical and experimental analyses were conducted. The results show that the presented method is effective and possesses a high system resolution in measuring the spatial straightness error. The measurement uncertainty is close to 0.28 mum in the measuring distance range 0.4-1.8 In behind the axicon.