光学薄膜的反射率和透射率是其主要光学特性。透射率易于测量且精度较高，常用于光学薄膜光学常数和膜厚的拟合。许多光学薄膜工作在倾斜入射条件下，常需要测量其倾斜入射透射率（Ts和Tp）。在测量过程中，需要使用起偏器产生高偏振度的线偏振光。对于小型分光光度计来说，为其提供一个满足要求的起偏器，无疑将增加相当的成本。为解决这一问题，对旋转入射面法进行了进一步的研究和推广。考虑到旋转入射面法可使用具有一定偏振度的部分偏振光进行倾斜入射透射率的测量，且分光光度计的输出光束通常为具有一定偏振度的部分偏振光，旋转入射面法在满足一定条件的前提下可以不使用起偏器测量光学薄膜的倾斜入射透射率。研究表明：当偏振因子绝对值大于0.167时，旋转起偏器可以精确测量光学薄膜的倾斜入射透射率，且偏振因子绝对值越大测量精度越高。该方法所测平均透射率在整个测量波段内都具有很高的精度；而Ts和Tp在透射率对入射角较不敏感或随波长变化较平坦的波段具有较高的测量精度。

Reflectance and transmittance are main optical property of optical thin films. Transmittance is easier to be measured and has a higher accuracy than reflectance. Therefore, it′s frequently used to fit the optical constant and thickness of the film. Many coatings work in the oblique incidence case in application, and its oblique transmittances Ts and Tp are needed. A perfect polarizer with high extinction ratio is needed in this case. However, it costs too much for the compact spectrophotometer to provide such a polarizer. In order to solve this problem, rotation-incidence-plane (RIP) method is investigated deeply and generalized. Because RIP method makes measurement with a partially polarized beam, and the output beam of the spectrophotometer is partially polarized, RIP method can measure the oblique transmittance under certain condition. Advanced investigation found that if the absolute value of the polarization factor of the measurement beam is bigger than 0.167 the RIP method can make an accurate measurement about the oblique transmittance of optical coatings, and the polarization factor bigger the accuracy higher. This method has a good accuracy in the wavelength region, in which the transmittance is nearly flat or not sensitive very much to the incidence angle, when it′s used to measure the Ts and Tp. What′s more, the method has a better accuracy when it′s used to measure the average transmittance.