This study employs the ray tracing method to analyze and develop mathematical formulae for the internal reflectance of a free standing thick polymer film. The proposed formulae are then used to simulate the diffuse reflectance and diffuse transmittance. Analysis results indicate that internal reflectance is not a constant and varies with film thickness and absorption property. Internal reflectance of the front surface (irradiated surface) surpasses that of the back surface. In addition, all the internal reflectances decrease with the film thickness and the absorption property. In light of the measured thickness and experimentally determined diffuse transmittance of a polymer film, the extinction coefficient to diffuse IR irradiation can be obtained by interpolating in the graphs provided in this study.