We used spectroscopic ellipsometry to study the dielectric function of an azobenzene polymer called Poly(disperse Red 1 methacrylate) (PDRM) as it underwent a photoisomerization process. The polymer films were fabricated by dissolving PDRM in tetrahydrofuran, and by spin coating films on silicon substrates. After fabricating the films, we used UV-VIS spectroscopic ellipsometry to determine the index of refraction, extinction coefficient and the thickness of each film. Using a heating cell coupled to the ellipsometer, we obtained temperature dependent ellipsometry data from 34 ℃ to 180 ℃, spanning a spectral region between 300 nm and 1500 nm. Using the raw data, we determined that the glass transition temperature of PDRM was 85 ℃ ± 3 ℃. Employing a three-oscillator model, we deduced the temperature dependent dielectric functions of PDRM films. Subsequently, the films were photoisomerized at two polarizations using a visible laser. For the horizontal polarization, there is nearly a 15% a change in the extinction coefficient as the PDRM films undergo a photoisomerization process.