OPHTHALMIC LENS COATINGS

This paper reviews modern coatings on plastic ophthalmic lenses whereb y particular attention is given to multilayer dielectric antireflectio n (AR) coatings. The optimisation of coating performance, coating proc esses and coating characterisation methods continues to be a considera ble technical challenge for the manufacturer of coated ophthalmic lens es. Coatings are needed on polymer ophthalmic lenses to enhance both t he mechanical durability of the relatively soft plastic surface and th e optical performance of the lens. High value ophthalmic lenses are co ated with a multilayer system consisting of a primer coating, abrasion resistant hard coating and a multilayer AR coating stack. The physica l properties of the AR coating need to be carefully balanced and contr olled. Optimising the adhesion of the coating stack to the relatively soft substrate may entail some compromises in the achievable coating h ardness. During a deposition process, plastic substrates cannot be ele vated to nearly the same temperatures that are permissible for glass s ubstrates. This can restrict the achievable level of durability and ad hesion of a film deposited on a plastic lens. Raising the substrate te mperature to near the limit plastic lenses can withstand often results in crazed coatings. The main factor is the large differences in the t emperature expansion coefficient between the plastic substrate and the dielectric coating which leads to large stress differentials. One oth er major factor affecting the coatability of plastic substrate is the desorption of water vapour from the substrate during the deposition pr ocess. Some of the traditional restrictions on coating plastic substra tes have been overcome by modern ion beam assisted deposition processe s. Hard films are usually under compressive stress and there are indic ations that stress free coating slacks perform better in service even if the hardness of the him has been greatly compromised. Data from a l imited study suggest a correlation between film brittle failure during hardness measurement induced indenter penetration and in-service perf ormance. The brittle failure may indicate large stress differentials b etween layers in the film stack. The difficulty in the optimisation of multilayer coatings on plastic lenses is in the measurement and corre lation of the film parameters to the deposition process variables and in-service performance criteria.