Surface microtopography measurement of a standard flat surface by multiple-beam interference fringes at reflection

This paper describes a multiple-beam interferometry technique at reflection for measuring the microtopography of an optical flat nominally of λ/20. The measured value is the result of a piezoelectric phase shifting that made it possible to cover the area between the orders. The exposure area of the tested optical flat covered by the collimated laser beam is about 2 cm. The images captured by the digital detector were corrected using dark/flat field method. The uncertainty in the measurement due to incomplete parallelism of the incident beam and the inhomogeneity of the reflecting layers were calculated from the departure of the fringe position produced with a written ray tracing program. The uncertainty budget was estimated to be of the order of λ/40.