Lens Coatings - Advancing the Utility of Spectacle Lenses

All optical materials, no matter how advanced or well-suited for a patient's needs, suffer from innate shortcomings specific to the lenses chemical makeup. Where crown glass is unquestionably superior in optical quality, it is heavier and could shatter upon impact, thus it must be treated to resist such damage. Polycarbonate has long held the title on most impact resistant and is lighter in weight. Unfortunately, polycarbonate is not as resistant to scratching and therefore must be treated to improve its resilience to abrasion. CR-39 as well as high-index plastics share their burden of material shortcomings also which can easily be addressed through the application of secondary coating processes.

Although not able to address all material limitations, lens coatings add utility and aid the dispenser in minimizing potential patient dissatisfaction. From reduction of glare, protection of the lens material, and the patient's eye, to minimizing eyestrain and fatigue during computer use and nighttime driving, coatings are continually being advanced to fill ever broader roles in improved visual outcomes for spectacle wearers.

Lab procedures, equipment, and materials are key to the quality if coating and other lens treatments. It is a good idea to ask your lab for the brand name on lenses and materials as well as the standards for each.

Scratch Resistant Coatings: Lens Armor

Scratch resistant coatings impact precisely that - heightened RESISTANCE to scratching during normal use and handling by the patient. Primarily used in CR-39, polycarbonate, and high-index plastic lenses, scratch resistant coatings work by bonding a thin layer of various protective compounds to the lens surface. These coatings do not penetrate into the lens material and therefore do not alter the material. Quality coatings will resist most minor incidental abrasions the lens may encounter. Scratch resistant coatings themselves are limited only by the integrity with which they can be bonded to a particular lens material. Routine cleaning of a patient's lenses will eventually remove a poorly applied coating. The integrity of a coating is a factor of the lens material, the formulation and quality of the coating, and the process by which it is applied. Polycarbonate lenses come from the manufacturer already coated for scratch resistance on the front surface. The lab will scratch-coat the back surface of the lens following processing.

Anti-Reflective Coatings: - Reflections on Optics

An optic's job is to bend light, but it is rarely as simple as light waves entering the lens and exiting the other side in a different direction. In all transparent or translucent polished surfaces- glass, plastic, or even water, light will to some degree be allowed to pass through. The amount of light that makes it through is an expression of the materials transmittance. But, what happens to the light it doesn't transmit? It is either absorbed by the material or it is reflected. Colorless spectacle lens materials, if they are properly processed and are free of any debris or damage, will absorb negligible amounts of light. These materials, however, no matter how well they are processed, will all tend to noticeably reflect light. The reflection of one's own eye in the backside of a spectacle lens illustrates this effect. Reflections in a lens surface can result in eye strain and fatigue.

The goal of AR coating is to minimize the reflections by improving the transmittance of the lens. To understand how this is accomplished, one must first look at the nature of a reflection. When light enters the lens, some is reflected. If the reflected light can be reflected 180 degrees out of phase with the incoming light, the incoming and reflected light waves will cancel each other out. This is precisely what AR coatings do. By applying a series of thin coating layers to both lens surfaces with a thickness equal to an odd multiple of one quarter of the wavelength of the incoming light, the resultant reflected light is effectively cancelled by the incoming light. The result is fewer reflections and higher transmittance through the lens material. In fact, AR coatings can increase light transmittance through a lens up to 99.5%. The increased transmittance carries many benefits to the wearer, such as improved cosmetics, reduction of glare, and improved clarity for night driving.

UV Coatings: Added Insurance

Over the past 20 years or more, increased emphasis has been placed on the potentially harmful effects of ultraviolet (UV) radiation on the skin and eyes. UV radiation is divided into three categories- UVA, UVB, and UVC. UVC is for the most part, filtered by the earth's protective ozone layer. UVB is the component of solar radiation responsible for sunburn and can be attributed to many ocular conditions which result from its absorption by the cornea. UVA is considered to be the most damaging of all because it is not absorbed by the cornea. Instead, UVA is transmitted through the cornea to the crystalline lens and retina. UVA is thought to be responsible for serious ocular conditions including cataracts.

Most lens materials absorb minimal amounts of UV naturally. Materials such as polycarbonate and some high-index plastics can filter most UV. When a lens material lacks inherent UV protection, coatings can be applied to close the gap.

UV coatings should not be confused with tinting. Tintings that block UV do so only because they minimize light transmittance across the entire spectrum including ultraviolet. True UV coatings can be colorless or used in tandem with a variety of standard and fashion tints. Like other coatings, UV treatments can eventually be degraded by routine lens cleaning and handling.
Article provided courtesy of Optical Services International

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