为获取具有微米量级变化的蓝宝石基片的面形分布，采用斜入射法拓展数字干涉仪测量范围。根据斜入射测量原理，推导了斜入射角、平面面形偏差与系统波像差之间的关系。通过 Zemax构建了斜入射测量仿真光路，研究了面形偏差及入射角度对检测结果的影响。通过仿真实验得出最佳斜入射角度等关键参数，并在斐索干涉仪上测量了直径为 100 mm的蓝宝石基片，测量结果的 PV为 5.182 μm，RMS为 1.251 μm。研究了斜入射角对分辨率、灵敏度因子、条纹对比度的影响。研究表明，斜入射角为 70°时，适宜测量矢高值 ≤5 μm的蓝宝石基片，引入误差小于 0.1 μm，测量范围是正入射测量范围的 1.46倍，且能获得适宜的条纹对比度。

To determine the surface profile of a sapphire substrate with the change of micrometer scale, a test based on oblique incidence is proposed to extend the measurement range of a interferometer. After studying the theory of oblique incidence measurement principle, the relationship between oblique incidence angle, surface error and wave-front aberration is derived. A simulation model is constructed by Zemax. The influence of surface error and incidence angle on the detection results is investigated. The key parameters such as the best incidence angle are determined in the simulation. The results are used to guide the measurement of 100 mm sapphire substrate on the Fizeau interferometer, the root mean square (RMS) and peak-to-valley (PV) results of the sapphire substrate are 5.182 μm and 1.251 μm. The influence of incidence angle on resolution, sensitivity factor and fringe contrast is discussed. The proposed method can extend the measurement range of interferometer. The result shows the method is suitable for the sapphire substrate with vector height less than 5 μm when the measurement angle is 70°. The error is less than 0.1 μm. The measurement range is upgraded by 1.46 times, and the suitable fringe contrast can be also obtained.