Hard and transparent diamond-like carbon (DLC) films have been:prepared on low temperature substrates. by using the C2H2-Ar plasma immersion ion processing (PIIP) method. The optical properties of the DLC films deposited on poly(methylmethacrylate) (PMMA), Silicon wafers, and glass plates were systematically examined. It was found that careful control of:substrate bias was needed for favorable growth of DLC films with low atomic hydrogen contents, high hardness and mass densities, and excellent optical properties. The resultant DLC films exhibited a high hardness of above 22 GPa, an optical gap energy larger than 1.9 eV, a low refractive index of 1.87, and very high: optical transmittance in the visible light range. The study confirmed that C2H2-Ar PIIP with low-negative bias voltages and suitable C2H2/Ar gas ratios: can process optically transparent and hard protective DLC films on PMMA plastics. [S0734-211X(99)02502-03].