Here are presented the results of the development and application of methods for determining the optical constants and thickness of thin films. The generalized target model function is formed to determine the unmeasured parameters. The model is applied by using the least squares method and the steepest descent. Increased efficiency is achieved by using a three-step processing algorithm. The proposed method was applied to calculate the characteristics of the multi-alkali photocathode, which is a complex compound having in its composition antimonides of potassium, sodium and cesium. A comparison of the calculation results with the data given in the literature is presented.