With the unprecedented developments of the intense laser and aerospace projects', the interferometer is widely used in detecting middle frequency indicators of the optical elements, which put forward very high request towards the interferometer system transfer function (ITF). Conventionally, the ITF is measured by comparing the power spectra of known phase objects such as high-quality phase step. However, the fabrication of phase step is complex and high-cost, especially in the measurement of large-aperture interferometer. In this paper, a new fringe method is proposed to measure the ITF without additional objects. The frequency was changed by adjusting the number of fringes, and the normalized transfer function value was measured at different frequencies. The ITF value measured by fringe method was consistent with the traditional phase step method, which confirms the feasibility of proposed method. Moreover, the measurement error caused by defocus was analyzed. The proposed method does not require the preparation of a step artifact, which greatly reduces the test cost, and is of great significance to the ITF measurement of large aperture interferometer.
The full-aperture and full-frequency absolute surfaces of optical flats are of great significant for industrial applications but hard to achieve. To measure them simultaneously, Kuechel proposed an absolute testing based on N-position rotations by adding a set of measurement data of N-position rotations, in comparison to the traditional four measurements. Algorithm simulation and absolute detection experiments have been conducted before, however, the influence of rotation angle error has not been analyzed, and the full-aperture contrast experiments have not been conducted. In this paper, the influence of rotation angle error was analyzed, and the measurement result is within acceptable range even when the angle error reaches 1°. Moreover, to verify the accuracy of this method, full-aperture contrast experiments were proposed innovatively besides the two linear profiles contrast experiments. The contrast experiments prove the accuracy of the full-aperture absolute measured results, other than the accuracy of the two linear profiles results.