By Gary Heiting, OD
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Anti-reflective coating (also called AR coating or anti-glare coating) improves both your vision through your lenses and the appearance of your eyeglasses.
Both benefits are due to AR coating’s ability to eliminate reflections of light from the front and back surface of eyeglass lenses.
AR coating is especially beneficial when used on high-index lenses, which reflect more light than regular plastic lenses. Generally, the higher the index of refraction of the lens material, the more light that will be reflected from the surface of the lenses.
For example, regular plastic lenses reflect roughly 8 percent of light hitting the lenses, so only 92 percent of available light enters the eye for vision. High index plastic lenses can reflect up to 50 percent more light than regular plastic lenses (approximately 12 percent of available light), so even less light is available to the eye for vision. This can be particularly troublesome in low-light conditions, such as when driving at night.
Today’s modern anti-reflective coatings can virtually eliminate the reflection of light from eyeglass lenses, allowing 99.5 percent of available light to pass through the lenses and enter the eye for good vision.
Anti-reflective coating reduces the glare that you see, as well as the glare that others can see on your lenses.
By eliminating reflections, AR coating also makes your eyeglass lenses look nearly invisible so people can see your eyes and facial expressions more clearly. Anti-reflective glasses also are more attractive, so you can look your best in all lighting conditions.
The visual benefits of lenses with anti-reflective coating include sharper vision with less glare when driving at night and greater comfort during prolonged computer use (compared with wearing eyeglass lenses without AR coating).
Anti-reflective coating also is beneficial when applied to the back surface of lenses in sunglasses, because it eliminates reflections of sunlight into your eyes from the lenses when the sun is behind you.
Since the purpose of sunglasses is to reduce how much light enters your eyes and because you aren’t concerned about making eye contact with others when wearing shades, there is no benefit to having AR coating applied to the front surface of sunglass lenses.
Most premium anti-reflective coatings include a “hydrophobic” surface layer that prevents water spots from forming and makes the lenses easier to clean. Some AR coatings also include an “oleophobic” surface layer that repels skin oils and makes it easier to remove smudges from the lenses.
Some eyeglass lenses have factory-applied AR coating on both lens surfaces. Other lenses, particularly progressive lenses and other multifocal lenses (i.e., bifocals and trifocals), have the coating applied after the lenses have been customized to your eyeglass prescription by an optical lab.
How Anti-Reflective Coating Is Applied
Applying anti-reflective coating to eyeglass lenses is a highly technical process involving vacuum deposition technology.
The first step in the AR coating process is to meticulously clean the lens surface and to inspect the lens for even microscopic surface defects. Even a tiny smudge or piece of lint or hairline scratch on the lens surface during the coating process can cause a defective coating.
Typically, a production line includes multiple washing and rinsing baths, including ultrasonic cleaning to remove any traces of surface contaminants. This is followed by air drying and heating of the lenses in special ovens to further remove unwanted moisture and gases from the lens surface.
Modern vacuum coating machine for applying AR coating to eyeglass lenses (Image: SatisLoh.)
The lenses are then loaded into special metal racks with spring-loaded openings so the lenses are held securely but with virtually all lens surfaces exposed for the coating application. The racks are then loaded into the coating chamber. The door of the chamber is sealed, and the air is pumped out of the chamber to create a vacuum.
While the lens racks are rotating in the coating chamber, a power source within the machine focuses a beam of electrons onto a small crucible that contains a series of metal oxides in separate compartments. When bombarded by the beam of this electron “gun” in succession, the metal oxides are transformed into vapors that fill the coating chamber and adhere to the lenses in a specific order to form a precise multilayer AR coating.
Each AR coating manufacturer has its own proprietary formula, but generally all anti-reflective coatings consist of multiple microscopic layers of metallic oxides of alternating high and low index of refraction. Depending on the AR coating formula, most lenses with anti-reflective coating have a very faint residual color, usually green or blue, that is characteristic of that particular brand of coating.
Anti-reflective coatings are incredibly thin. The entire multilayer AR coating stack generally is only about 0.2 to 0.3 microns thick, or about 0.02 percent (two one-hundredths of 1 percent) of the thickness of a standard eyeglass lens.
Caring for Glasses With Anti-Reflective Lenses
When cleaning AR-coated lenses, use only products that your optician recommends. Lens cleaners with harsh chemicals may damage the anti-reflective coating.
Also, don’t ever attempt to clean AR-coated lenses without wetting them first. Using a dry cloth on a dry lens can cause lens scratches. And because anti-reflective coating eliminates light reflections that can mask lens surface defects, fine scratches often are more visible on AR-coated lenses than on uncoated lenses.