Hard coatings are today a standard within an antireflection coating. Therefore lacquers are used which form an almost ideal buffer between the lens surface and the more brittle AR coating.
Due to their low density, plastic lenses are extremely light and hence very comfortable to wear. They do have one disadvantage, however: their relatively soft surface makes them prone to scratches. To remedy this drawback, plastic lenses can be sealed with a hard protective coating.
Scratches on the lens not only lead to diffuse scatter resulting in blurred, "milky" vision, but are also a cosmetic drawback. The approx. 2 µm-thick hard coating makes the surface more resistant to scratches and therefore increases the durability of the lens.
Benefits of a hard coating
The surface strength of high-index plastic lenses is so low that a hard coating is an absolute must. This is why we at ZEISS always provide our high index plastic lenses with a standard hard coating.
The hard coating of plastic lenses using a dip lacquer is a very young technology compared with the AR coating technique introduced in the field of ophthalmic lenses for the first time in 1959. Although the plastic lens Clarlet was launched on the market as long ago as 1963, the first hard dip coating was not introduced until two decades later. ZEISS offered the first Clarlet lenses with a hard dip coating in 1986. Ten years later, the hard coating had already become an indispensable standard coating for plastic lenses.
Today, up to 9000 plastic lenses are provided with a hard lacquer coating in the central production facility of ZEISS in Aalen, Germany every day.
Hard dip coatings are usually produced with polysiloxane lacquers. With their elastic properties, these lacquers form an almost ideal buffer between the lens surface and the more brittle AR coating. At ZEISS the dipping technique is used to apply all hard coatings to plastic lenses. After a cleaning and pre-treatment process, several lenses are placed in special racks and immersed in the polysiloxane lacquer.
The thickness of the hard coating is regulated both by the speed at which the lenses are immersed and removed, and by the viscosity of the lacquer. To guarantee that the required viscosity is always obtained, the ambient temperature must remain constant at 18 °C.
The immersion and removal speeds are computer-controlled, and, as for the viscosity, meticulous care must be taken to ensure that specifications are met if the quality of the coating is to be guaranteed. Clean-room conditions are vital to prevent contamination by dust.
To harden the dip lacquer, the lenses are passed through a tempering furnace. Depending on the lens material involved, the lenses remain for three to four hours in the furnace. The tempering process creates a stable connection between the lens and the lacquer, ruling out the possibility of subsequent detachment.
Apart from so-called "wet-chemical" techniques, vacuum deposition processes can also be used to seal lenses with a hard coating. However, the deposited material – quartz (SiO2 ) – is very brittle. Scratches often break open the lens coatings completely to form numerous hairline cracks over the lens surface. The lens soon assumes a "milky" appearance which impairs its imaging properties.
Scratches on a hard lacquer coating, on the other hand, cause much less damage to the surface of the sealed lens. Both the optical quality and the cosmetic appearance of the lens remain intact for a longer time. This is why only the "wet-chemical" technique is used at ZEISS to produce hard lacquer coatings.
Since the introduction of the first hard dip coating at ZEISS in 1986, the quality of the coatings has been constantly improved. The result: all hard-coated lenses from ZEISS now display a high quality standard.
The use of newly developed plastics for spectacle lenses means that new, high-performance lacquer systems are also required.
The development of lens materials is centred on increasing the refractive index. To achieve the same quality standard for hard coatings on every material, ZEISS uses hard lacquers which have been precisely matched to the material’s properties. The higher the refractive index of the material, the higher the index of the lacquer.
To guarantee the high quality standard of the hard coatings, cleanroom conditions are used at ZEISS.
After Carl Zeiss was Europe’s first manufacturer to succeed in polymerising a plastic lens with the super-high index 1.665 for batch production, the company focused its attention on enhancing the coating structures used for this material.
The result: a high index hard coating is also used for the high index lenses ZEISS Single Vision AS 1.67 and ZEISS Single Vision Sph 1.6. One of the benefits of this specially matched coating is its high adhesive strength and hence its outstanding durability. Another decisive advantage: annoying interference is avoided, resulting in a cosmetically attractive lens in every respect.