Spectroscopic ellipsometry (SE) performed in real time using a multichannel instrument over the range 1.5-4.2 eV has been applied to investigate nanocrystalline diamond film growth by microwave plasma-enhanced chemical vapor deposition on silicon wafer substrates. Using the real-time SE measurement capability, we have established calibration procedures that provide the true near-surface substrate temperature under the actual conditions of diamond growth. In addition, we have developed procedures for the analysis of the real-time SE data that provide the deposition rates and film properties for a series of films prepared on the same substrate under different conditions. These procedures have enabled us to identify a new diamond film growth process based on GO-rich CO/H-2 gas mixtures that yields high deposition rates (2.5 mu m/h) at low temperatures (similar to 500 degrees C).