对椭圆偏振光谱中的主角测量条件进行了分析。当入射角等于主角θp时，椭偏参数的相位为90°，据此给出了当材料的介电函数已知时对θp进行数值和解析计算的公式和方法，并可相应地计算出椭偏参数的幅值ρp0。在θp入射角下测量可获得较高的数据精度。主角条件下椭偏参数ρp0及其相位Δp的实验测量结果与计算值符合得很好。本工作给出的计算公式和方法可在其他光谱实验中获得应用。

The measuring condition for principle angle in spectroscopic ellipsometry is analyzed. As the incident angle is equal to principle angle θ p, the phase angle of ellipsometry parameters is 90°. It gives a way to obtain θ p from numerical calculation and analytic equation as the dielectric function of the material is known, and the amplitude ρ p0 of ellipsometry parameters can be calculated accordingly. Higher precision of data can be acquired as measuring at incident angle of θ p. The experimental results of Δ p and ρ p0 correspond very well with the calculated ones. The calculation formulas and methods given in this work can be applied to other spectroscopic experiments.