Optical Flat To Pentaprism Absolute Inspection Method

Interferometer is widely used to achieve the flatness measurement. The reference flat works as an ideal flat which contributes its own flatness errors to the final result. Therefore, it is a comparative method and the precision is restricted to that of the reference flat. In order to obtain the absolute surface distributions, a novel method is presented to achieve the absolute flatness measurement based on two flats in the Fizeau interferometer. By using the three surfaces of the two flats, the vertical profiles of both low spatial frequencies and high spatial frequencies are obtained thoroughly. By adding one rotation measurement, the whole surface profiles can also be obtained. This method can be extended to apply to the occasions such as the large aperture surface flatness measurement or calibration for the reference surface. This absolute measurement idea is introduced to the measurement of the deflection angle of pentaprism. Two-prism method and three-prism method are designed to achieve the absolute measurement of the deflection angles in the Fizeau interferometer.Experiments are implemented to obtained the surface profiles by the two-flat absolute measurement. The experiment results are given and the errors are analyzed. The precision is better thanλ/500. Surfaces with aperture of 600mm are also measured by two flat test and the vertical profiles are obtained. With the three flat test application of Zygo, the experimental results are compared with that of the two flat test. It shows good coincidence that vertical results are differed at 0.001λ(rms) and the horizontal results are differed at 0.003λ(rms), which testifies the feasibility of the two flat test. The absolute deflection angles of the pentaprisms are carried out and the results are given by both the two-prism method and the three-prism method. It demonstrates that the difference between the two methods is less than 0.2".