Feb
2005

Lens Choices

Today’s anti-reflective lens technology is literally light years ahead of what we had in even the recent past. AR lenses are durable, smudge resistant, easy to clean and their optical performance is much improved by their multi-wavelength design. This is patient-pleasing technology that has come of age.
Although more doctors and dispensers than ever are recommending AR lenses to their patients, AR lenses accounted for just 21 percent of all lenses sold in the U.S. in 2004, according to Jobson Optical Research. There are several reasons for this. Despite the fact that AR lenses consistently outperform non-AR lenses, some eyecare practitioners, focused on health, basic vision and the pressures of providing care, are prone to overlook quality of life issues—the enhanced vision, comfort and appearance that the best AR technologies can offer.

Also, there are many patients who do not understand all the benefits of AR technology and hear only the cost. Worse, the doctor handed them a lens “prescription” and didn’t mention anti-reflective lenses—so why should they pay extra? Do they really want just bare bones minimum care? If these patients don’t appreciate the benefits of AR, they won’t perceive its value.

In order for ECPs to present the latest AR lenses effectively to patients, it’s essential they understand the basic properties of AR technology and principles of how it works. The following information provides a solid overview of the subject.

Reduced Reflections—Increased Transmission
It seems obvious that anti-reflective lenses reduce reflections. We all know that a thin film of the appropriate index applied to the surface of a lens can reflect a wave front that is out-of-step with the wave front reflected from the lens. These two out-of-step wave fronts cancel each other (destructive interference, see Fig. 1) and the optical performance of the lens is improved.

Out-of-phase waves that show destructive interference. The peak of Ray A coincides with the valley of Ray B and destructive interference occurs.

Another effect is that a second set of in-step wave fronts passing through the lens is in-step with the major ray bundle and this yields an amplification of transmitted light. The resultant amplification of the major ray bundle is another improvement in optical performance (see Fig. 2).
 

Light reflecting from the surface of a film of a quarter wavelength thickness is “out-of-step” with light reflecting from the surface just under it. This destructive interference is the reason that less light is reflected from the front of anti-reflective lenses. Reducing this front surface reflection makes the lens less visible and the eyes more visible.
Since energy cannot be “lost,” this wave cancellation for the reflected ray adds energy to the light passing through the lens and the transmission of the lens is actually increased.

Annoying ghost images and back surface reflections disappear when useless reflected light is eliminated and both vision and comfort are improved. Also, the amplification of the useful, transmitted light brightens the retinal image. A reduction in front surface reflections is an additional benefit because it improves appearance. Anti-reflective technology, with transmissions in the 99 percent range, and with its ability to eliminate useless reflected light is almost like taking the lenses away and leaving the prescription in place.

It is important to understand the physics of reflected light. Lay people often believe that darker lenses reflect more light. This is not the case. The amount of light that is reflected by any lens is directly related to the index of refraction (n) of the lens (reflectance at normal incidence r = (n-1/n+1)2). Lower-index lenses reflect less, and higher-index lenses reflect more. Because of this, if a higher-index lens material is used, it becomes more important for the lens to have anti-reflective properties. Dispensers and doctors often overlook this very important point —problems related to reflected light including ghost images (see Fig. 3), reduced transmission, compromised appearance and annoying “stray light” reflections are made progressively worse as the index of the lens increases.

Out-of-phase waves that show destructive interference. The peak of Ray A coincides with the valley of Ray B and destructive interference occurs.


The relationship between index of refraction and reflectance tells us that a standard plastic 1.50-index lens reflects 4 percent of the light falling on it, polycarbonate (1.59) reflects about 5.2 percent, high-index plastic (n =1.66) reflects about 6.2 percent. Interestingly, the tear film over the cornea (n = 1.338) reflects just 2.09 percent, yet this is enough to give a twinkle (called the first Purkinje image by scientists) to our eyes.

“Stray light” is commonly a problem causing reflections from the back surface of sunlenses and it is progressively worse with higher-index lenses. This is especially troubling, not because of the darkness of the sunlens reflecting more light—it really does not—but because the reflected light is relatively brighter than the light passing through the sunlens than it would be if compared to light passing through a non-tinted lens.

Choosing ‘The Right’ Anti-reflective
Originally anti-reflective single-layer films were designed for an optimal wavelength (usually 555nm) and reflected images formed by light of other wavelengths were only reduced proportionally. Today’s premium anti-reflectives are all multiwavelength which means they are multi-layered so that more than a single wavelength is maximally affected. Even these premium anti-reflectives are not all the same in their durability, smudge and dust particle resistance, and color. These properties should all be considered when selecting the best one for your patient.

Taking Care Of Anti-reflectives
Temperature fluctuations affect durability because the coefficients of expansion of the anti-reflective film and of the lens material are not the same. Extreme temperature changes have a similar effect on the anti-reflective film as one would find when putting more air into a balloon that has been painted. Crazing and cracking occurs. Manufacturers do their best to minimize this problem, but patients should be told to flush their lenses with tepid (not hot) water prior to cleaning. They should also avoid leaving their eyewear under the hot sun on the dashboard of their cars or in overheated glove compartments.

Dust build-up is another issue for ophthalmic lenses. It is often related to a static electricity on the surface of the lens that attracts dust particles. Dust filming can be a source of significant veiling glare. It is annoying, impacts visual performance and often leads patients to do frequent, improper cleanings.

Wetting grit with lens cleaner produces a primitive “grinding slurry” that contributes to scratching. A good rule is to tell patients to always flush dust from the lens surfaces before using lens cleaner. Some anti-reflective films actually have resistance to the build-up of static electric charges. This property alone may make them a good choice for patients who work in grain elevators, sawmills and other dusty environments.

Smudging is especially annoying for wearers of anti-reflective lenses. Smudge-resistant top layers reduce this problem. Patients with oily skin are especially prone to this problem. Oils build up in the groove of the eyewire, which acts as a reservoir. Less-than-careful cleaning then results in these oils being smeared over the lens surfaces. Careful frame design is especially important for these patients. These patients should also have frames that will not allow the eyebrows or the facial skin to contact the lenses.

Optical Choices & Weighted Decisions
Color is more than a cosmetic concern with anti-reflective lenses. Of course, appearance conscious patients may want an anti-reflective color that complements their frame or their complexion. They may also have a personal preference for a particular color.

Cosmetic concerns aside, the color-related issue hinges upon apparent brightness. Our eyes are more sensitive to some colors than others. Light-adapted eyes see light that is yellow-green, around the 555nm wavelength, as brighter because that is the peak sensitivity of our retinas. Practically speaking, because we want to avoid stimulating retinas (the patient’s own and those of the people who are looking at him) with reflected light, the anti-reflective films that appear darker tend to be the better optical performers.

It’s important to remember that durability, anti-static properties, craze resistance and smudge resistance must be matched to patient needs. As a clinician, only you can decide which properties will be most important over the two or three years that the patient will be using those lenses. A brighter anti-reflective lens that is more durable may be a better choice for a careless patient, especially one who will be in a lower- index lens. A meticulous patient in a high-index lens, on the other hand, may especially appreciate a darker anti-reflective even though it might be less durable. The final choice for each patient must always be weighted by your professional judgment. That final choice is the one that impacts the quality of life and vision, and it should always leave your patient with a custom combination of today’s best ophthalmic technologies.

 

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