Absolute testing of two flats based on even and odd functions without testing refractive index uniformity

A novel absolute measurement method by using two plates based on the odd-even functions is proposed. Compared to existing methods, our method requires only two plates and the refractive index uniformity of the transmitted flat is not to be measured. The principle of the proposed method is discussed. The corresponding simulations are carried out to prove the feasibility of the method. Experiments on a 100 mm Zygo-interferometer yielded the PV value of the rest error is less than 3 nm. Error term affecting the calculation accuracy was analyzed including the rotation angle and pixel misalignment errors of the coordinate axis. The proposed method was proven to effectively obtain the accurate surface error.