A method and apparatus for determining substrate temperature and the mirror center of a film structure formed on the substrate utilizes a transmission intensity spectrum of light to determine mirror center and a normalized transmission intensity spectrum of light to determine substrate temperature. White light is transmitted through the substrate and the film structure during the film structure formation process. Also during the film formation process, a reflected light intensity spectrum is obtained using the same or another light source, for light reflected by the film structure. Substrate temperature and mirror center may be determined during formation of the film structure and, based on the measured temperature and mirror center, the mirror center may be adjusted by changing film formation conditions during the film formation process to vary the thickness of the films being formed. The method and apparatus find particular application in the in-situ monitoring and adjustment of distributed Bragg reflectors used as semiconductor mirrors.