Fast evaluation of period deviation and flatness of a linear scale by using a Fizeau interferometer

A reflective-type linear scale with a pitch of 1.67 μm used in an interferential scanning-type 2-DOF linear encoder is evaluated by the Fizeau interferometer with a measurement range of 100 mm. The 2-DOF linear encoder produces 2-axis position signals based on the interference between the X-directional positive and negative first-order diffracted beams from the linear scale. Firstly, the Z-directional flatness eZ(x,y) of the linear scale is evaluated from the wavefront of the zeroorder diffracted beam reflected from the linear scale. The linear scale is then tilted to align the axes of the first-order diffracted beams with that of the interferometer so that the X-directional period deviation eX(x,y)of the linear scale can be evaluated from the wavefronts of the X-directional positive and negative first—order diffracted beams. Finally, the Zdirectional flatness eZ(x,y) and X-directional period deviation eX(x,y) were verified by comparing those with the nonlinear components of the 2-DOF linear encoder using the evaluated linear scale.