The thickness and surface roughness of diamond films grown on a Ti-6Al-4V alloy in a microwave plasma reactor was measured in situ using optical pyrometry. The growing film results in oscillations of the apparent temperature with time, which can be explained by interference effects caused by reflections from the film/air and film/substrate interfaces. The equation governing the transmittance of the diamond/metal system has been derived by taking into account the complex index of refraction of an absorbing substrate. The apparent temperature was modeled using this relation for the transmittance in order to extract the time dependence of film thickness, surface roughness, and the true temperature of the substrate. The growth rate was observed to exhibit two regimes: an initial period of slowly increasing growth followed by a growth rate that was about 50% higher. The surface roughness increased at a nearly uniform rate but quickly reached a saturation roughness for long deposition times (high surface roughness). The limitation of the model for films in the high surface roughness regime is discussed. © 1998 American Institute of Physics.