Protective transparent coatings have been deposited on polycarbonate and CR 39 by dual mode MW/r.f. plasma-enhanced chemical vapour deposition from silane. The layer sequence consists of appropriate adhesion-promoting pretreatment, an intermediate columnar silica layer and a hard, densified nitride layer with adjusted intrinsic compressive stress by means of a particular coupling of the MW and r.f, plasmas. The deposition rate was up to 250 nm min(-1) for silica. Optical filters fabricated by similar processes can be included in the coating structure. Superior abrasion resistance and thermal variation endurance are obtained without the need for any wet process step.