Abstract: The bireflectance and extinction properties of pyrocarbon are analyzed. The theoretical basis for the measurement of extinction angles is expounded, and an interpretation is provided for the measured extinction angle values of pyrocarbon in C/C composites prepared by chemical vapor infiltration.The results show that the theoretical expression of extinction angles obtained from the extinction graph method is different from that by the onequadrant intensity observation method.However,the extinction angle is always determined by the bireflectance ratio of pyrocarbon. The measured value of extinction angle for pyrocarbon is the averaged optical response on a large reflection area which may contain heteroatom content, flaws, and turbostratic stacking of the carbon layer. Therefore, it is different in content from that of graphite. Though the structure of graphite is considered to be the limit of the anisotropic degree of pyrocarbon, the extinction angle of pyrocarbon should not be limited by the theoretical value of extinction angle of graphite.

Key words: C/C composites, chemical vapor infiltration, optical microscopy, optical property, bireflectance, pyrocarbon, extinction angle