Lens coatings or treatments if you prefer, are everywhere. Every manufacturer, every lab, every retailer has the latest and greatest coating ever produced. So how does the savvy ECP choose?
A BRIEF HISTORY
John William Strutt, 3rd Lord Rayleigh (1842-1919) was a British physicist and mathematician and Nobel Prize winner, who developed the mathematical explanation for the light scattering that gives the sky its blue color, the Rayleigh Scattering. Lord Rayleigh also observed that a thin film, such as tarnish on a glass surface can reduce reflectivity. The ray of light is reflected twice, first from air to the film surface and again from the film surface to the glass surface.
Alexander Smakula(1900-1983), a Ukrainian-born physicist at Carl Zeiss, received the first patent for an anti-reflective coating process in 1935. The coating identified as T Transparenz remained a military secret until early in World War II. The Zeiss “T” is a core process, rather than a specific formula, constantly redefined as new lens materials and applications are developed.
Katharine Burr Blodgett (1898-1979), an American-born chemist and physicist, was awarded the first US patent for thin films and surface preparation while at General Electric. Dr. Blodgett was able to precisely control the thickness of her films by building them one molecular layer at a time, resulting in a layer equal to ¼ the average wavelength of visible light. She also refined the composition of the film to adjust its index of refraction to enhance the reflection-canceling, eliminating almost all of the reflection.
H.R. Moulton, Assistant Research Director, American Optical Company, developed an improved coating film and application process in 1943. It was also restricted to military use for the duration of WW II.
And so the development of anti-reflective coatings has grown from a layer of tarnish on glass to the multi-layer coatings now readily and easily available from innumerable sources.
Why utilize anti-reflective coatings?
There are 5 types of unwanted reflections that can be reduced by the application of an anti-reflective coating.
Light reflected from the back surface of a lens
Light reflected from the front surface of a lens, common in low light conditions and a bright light source such as headlights.
Ghost images created by a bright light source reflected at varying angles.
Power rings. Reflections created by medium and high powered lens edges.
Reflections created by high- and mid-index resin materials such as polycarbonate, 1.60, 1.67, 1.70, 1.74, and 1.60, 1.70, 1.80 and 1.90 glass lenses.
Anti-reflective coatings like Crizal, Carat, DuraVision, EX3 and others are differentiated by the number of layers, substrate and hard coatings, hydrophobic properties, oleophobic properties and reflex colors. Anti-reflective coatings are not completely clear and will exhibit a reflex color based on the combinations of metals in the AR stack.
The current configurations for anti-reflective coatings include base or hard coats that are matched to substrates (the bare lens materials) to create the hard surface necessary for proper adhesion and are often included in the count or number of layers in a broadband or multiple layer coating. The count may also include a hydrophobic layer as well. A more in-depth discussion on the content of anti-reflective coatings and the methods used to apply them can be found in Ophthalmic Lenses & Dispensing, Second Edition by Mo Jalie.
The durability of any anti-reflective coating is determined by the preparation and application of the stack by the lab or coating facility AND the care taken in cleaning and storage by the wearer.
HEV REFLECTIVE/ABSORPTIVE TREATMENTS
HEV lens coatings such as Crizal Prevencia, Zeiss DuraVision BlueProtect, Hoya Recharge, Unity TechShield are anti-reflective coatings combined with either blue absorbing or blue reflecting properties. They absorb/reflect between 17% and 20% of potentially harmful HEV light. These coatings generally have a slight tint and a more noticeable blue reflex color. These lenses do not begin with a tinted lens base. The slight color and reflex are a result of the AR stack.
Why should we be concerned about HEV exposure? Realistically, we are surrounded by blue light, both indoors and outdoors. We need some blue light to regulate circadian rhythms and to discern colors. However, our increased use of digital devices, including flat screen monitors and televisions and hand held devices including tablets and smart phones has changed our visual environment. We are using these digital devices for longer periods of time per day and at closer viewing distances than ever. We are exposing ourselves to blue light when our innate circadian rhythms believe it should be dark and thus interrupting sleep patterns. We are using them at younger ages, which will increase the amount of exposure during our lifetimes. While there are few definitive studies on this potential hazard, the available findings indicate serious concern about the oxidative effect HEV light has on photoreceptor cells and its relationship to age-related macular degeneration (AMD) and sleep deprivation.
It’s worth noting that patients who are already developing cataracts have no real need for HEV light reflecting treatments since the developing cataract is already filtering that portion of the visible spectrum. However, post-cataract patients, even those with HEV filtering lens implants will need the additional protection of these treatments.
There are two products currently available that are not coatings, but rather lens materials with inherent HEV absorptive properties. BluTech lenses from Eye Solutions are available in a wide range of lens styles, both single vision and a wide variety of progressive addition lens designs. BluTech offers lenses in both indoor and outdoor color options. TheraBlue™ from Luzerne Optical Labs in Wilkes-Barre, PA is a new category of HEV protection with very little perceived color, available in both single vision and multifocals, including digital PALs, computer lenses and a digital round seg.
TO “AR” OR NOT TO “AR”
That’s a tough question. The vast majority of ECPs will advise patients to choose an anti-reflective coating for their eyeglass lenses. The considerations for selecting which product to use is a judgment call for both ECP and patient.
Is the coating:
Easy to maintain?
Reflex color complimentary?
Guaranteed with a warranty?
Right for the wearer’s environment?
Our duties as responsible Eye Care Professionals include being well educated in the finer points of the products we offer. Simply steering patients to the product that has the best marketing or the cheapest price is not good customer service. Understanding and responding to our patient’s needs and wants is a core skill to be maintained through healthy curiosity and constant education.