We report here a novel two stage method for growth of highly transparent nano-crystalline diamond films with very low surface roughness. The first stage involves achieving high nucleation density on the substrate by using low methane concentration in the source gas, while the second stage involves growth at higher methane concentrations in order to achieve a low degree of crystallinity in the films, and consequently low surface roughness. Greater than 1 mu m thick films with 84% transmittance at 700 nm and average surface roughness of 62 Angstrom are thus obtained on quartz substrates pre-treated with mu m-size diamond powder, while for the substrates pre-treated with nm-size diamond powder, the corresponding values are 81% and 65 Angstrom. (C) 1998 Elsevier Science B.V. All rights reserved.