This paper presents the method of the optical analysis of thin films with defects. The attention is devoted to the defects consisting in boundary roughness. This method is based on interpreting the spectral dependences of the diffuse reflectance of light [1-5]. Thin films are used in the optical and military industries and in military applications, for example for the creation of anti-reflective layers or laser mirrors [6]. The numerical analysis confirms the fundamental influence of the parameters of the defects of thin films on the diffuse reflectance.