Ophthalmic lens coatings

Abstract

This paper reviews modern coatings on plastic ophthalmic lenses whereby particular attention is given to multilayer dielectric antireflection (AR) coatings. The optimisation of coating performance, coating processes and coating characterisation methods continues to be a considerable technical challenge for the manufacturer of coated ophthalmic lenses. Coatings are needed on polymer ophthalmic lenses to enhance both the mechanical durability of the relatively soft plastic surface and the optical performance of the lens. High value ophthalmic lenses are coated with a multilayer system consisting of a primer coating, abrasion resistant hard coating and a multilayer AR coating stack. The physical properties of the AR coating need to be carefully balanced and controlled. Optimising the adhesion of the coating stack to the relatively soft substrate may entail some compromises in the achievable coating hardness. During a deposition process, plastic substrates cannot be elevated to nearly the same temperatures that are permissible for glass substrates. This can restrict the achievable level of durability and adhesion of a film deposited on a plastic lens. Raising the substrate temperature to near the limit plastic lenses can withstand often results in crazed coatings. The main factor is the large differences in the temperature expansion coefficient between the plastic substrate and the dielectric coating which leads to large stress differentials. One other major factor affecting the coatability of plastic substrate is the desorption of water vapour from the substrate during the deposition process. Some of the traditional restrictions on coating plastic substrates have been overcome by modern ion beam assisted deposition processes. Hard films are usually under compressive stress and there are indications that stress free coating stacks perform better in service even if the hardness of the film has been greatly compromised. Data from a limited study suggest a correlation between film brittle failure during hardness measurement induced indenter penetration and in-service performance. The brittle failure may indicate large stress differentials between layers in the film stack. The difficulty in the optimisation of multilayer coatings on plastic lenses is in the measurement and correlation of the film parameters to the deposition process variables and in-service performance criteria.