By Dr. Palmer Cook; Photograph by Ned Matura
In the 17th century Galileo tried to “see further” with a telescope, and van Leeuwenhoek tried to “see smaller” with his microscope. Your patients in the 21st century want to see better in every way, and by combining two excellent spectacle lens technologies you can help them reach that goal. The combination of these technologies is easy and synergistic—in other words you can produce a total improvement like adding one plus one and getting a number greater than two. Of course we all tried that in the first grade, but this combination will garner appreciation from your patients instead of a glare (of the expressive sort) from your teacher and possibly a dunce cap.
In the latter half of the last century eyecare providers began to make significant use of two important technologies that improve lens performance and patient comfort. This dynamic duo—anti-reflective (AR) treatments (think Batman) and photochromic tints (think Robin)—were used to improve vision, and for those patients complaining of light sensitivity or photophobia. Unfortunately, unlike the tale of the two partnered comic book heroes, the ophthalmic dynamic duo, AR and Photochromic Tint, were not “officially acquainted” in eyecare except by happenstance, and the value of combining them was widely unrecognized or misunderstood. Indeed, why would anyone want to simultaneously lighten a lens and darken it?
TAPPING AR’S MULTIPLE BENEFITS
Ask almost anyone in the industry why anti-reflective lenses make things look brighter and clearer, and you will be told it’s because anti-reflective lenses transmit more light. Anti-reflective lenses do pass more useful light, however that is only one part of the whole story. A standard plastic lens (index 1.49 and reflectance 3.97 percent) passes about 92 percent of the light entering it. The transmission rises to about 99 percent when a good quality AR is used, but the increase in brightness and clarity that the patient experiences is greater than if the room illumination is increased about 7 percent. If you doubt it, use a light meter in a room with rheostat-controlled lighting and do a little informal experimentation.
Light from everything in view in our entire environment is constantly passing through our lenses unless we are in a totally dark environment. The light forming the image of objects of interest makes up the major ray bundle and forms the part of the retinal image that falls on the fovea, the area of the retina that allows us to perceive the finest detail. Of course we can direct our attention at something peripheral to whatever we are looking at (our object of regard). Sometimes we call this “looking out of the corner of our eye.” This can be very useful if we are monitoring a group of students, surreptitiously observing someone, or watching out for danger, which if you think about it, is the truly all-inclusive category.
If a lens wearer observes one or more bright lights in an otherwise dark environment, for example a candle in a darkened room or oncoming headlights at night, each lens forms an annoying “ghost image.” These images float above, below, or to the side of the primary image of the headlamp. They are caused by light reflecting from the back surfaces of the lenses toward the front, reflecting again from the front surfaces of the lenses and back into the eye (See Fig. 1). This reflected light forms an image that overlays the primary retinal image, which in this case is entirely dark except for the image of the headlights.
Ghost images are a special case of a phenomenon in which light from everything in our visual environment forms an overlying image (or veiling glare) that degrades our primary retinal image. The involved ray pathways for this internally reflected light are shown in Fig. 2. This overlying image degrades the primary retinal image in the same way the image projected on a screen is degraded if the room lights are turned on. The overlying image is easiest to detect if there are one or more objects in the field of view that are much brighter than the rest of the environment. The ghost images of candle flames in restaurants with subdued lighting are recognizable portions of the overlying image caused by internally reflected light (Fig. 1A). For the most part we are not able to easily view this overlying image. However, if we alternately look through the treated and untreated portion of a demo AR lens, we can immediately appreciate the benefit of AR, because AR reduces the intensity of this internally reflected light. Fig. 3 compares a clear image in a daylight environment with an image degraded by veiling glare.
The light causing this veiling glare is reflected twice within the lens, making AR especially effective in reducing this overlay. For this reason, except for sun lenses, AR should always be applied to both sides of ophthalmic lenses. For a standard plastic lens with AR, the brightness of the overlying image is decreased by about 98 percent. At the same time the transmission of the major ray bundle is increased from about 92 percent to about 99 percent (Fig. 4).
Not only does AR treatment reduce the veiling glare caused by the reflected light pathways above, it also decreases the negative effects of all internally and externally reflected light. These other reflection problems include the ring reflections of strong minus lenses, the annoying reflections caused by light coming from behind the patient, and the reflections that give lenses a glassy, camouflaging appearance.
PHOTOCHROMICS UNLOCK THE POWER OF LIGHT TINTS
Most eyecare providers (ECPs) look at light tints as simply darkening the major ray bundle a bit. Until the advent of practical photochromic technology, many ECPs prescribed lightly tinted lenses only for asthenopia that seemed to be related to “bright light.” As technology evolved, patients appreciated the “darkening according to conditions” aspect of photochromics as both a comfort and a convenience. The result is that today many more people have the benefits of light tints by virtue of the ability of photochromic lenses to adjust to the environment.
Beyond attenuation of the primary retinal image, ECPs and manufacturers seem to pay little attention to any improvement in the primary retinal image that results from using a light tint. If a tint is used to decrease the transmission of a standard plastic lens (3.97 percent reflectance) to 80 percent, a drop of about 12 percent, the overlay of veiling glare is reduced by about 34 percent. The primary retinal image is reduced 12 percent in brightness by the tint, but the brightness of the overlying and unwanted veiling glare image is reduced by over two and a half times as much (Fig. 5). In fact, just as AR decreases the problems caused by internally reflected light, a tint will also decrease these problems, although not nearly as effectively.
A light tint works in a similar fashion with higher index lenses. Using a 1.60 material (5.32 percent reflectance) with neither tint nor AR, the emerging multi-reflected rays (veiling glare) are 75 percent brighter than standard plastic with no AR. By simply adding a tint to reduce the transmission to 80 percent—a drop of 9.6 percent—the veiling glare caused by internally reflected light is reduced by about 29 percent.
THE DYNAMIC DUO COMES TO THE RESCUE
If the transmission of a photochromic lens with AR is at 88 percent (due to UV exposure), the combination of AR and absorption of light within the lens reduces the overlying veiling glare due to internal reflection by approximately 30 percent more than just using AR alone. If the photochromic tint darkens to 80 percent transmission, the veiling glare is reduced by about 57 percent compared to just AR alone, even though the tint only darkens the major ray bundle by an additional 8 percent (Fig. 6).
At 80 percent transmission with AR, the reduction in the overlying veiling glare is 99.7 percent darker than the veiling glare from a non-tinted, non-AR standard plastic lens. For higher index, non-AR, clear lenses the veiling glare is increased and the importance of using AR and a photochromic tint is even greater.
Eyecare providers frequently ask about combining photochromic tints and AR, and occasionally a question arises about combining a conventional light tint with AR. The latter question usually comes up when a patient “has always had” a light tint, but the ECP wants to give the patient the improved optical performance of an anti-reflective lens, which passes more of the incident light. Many labs are reluctant to provide conventional light tints with AR because the tint, which must be applied first, effectively lightens after the AR processing. As a result, the final transmission of the lenses becomes uncertain. You should check with your lab before ordering light tints combined with AR.
The AR process does not bleach out the photochromic properties of a lens, however it does increase the transmission of the lens so there is a slight overall lightening. This effect is almost unnoticeable when the lens is fully darkened. In the lightened state the increased transmission tends to be somewhat more noticeable. For patients with higher index materials this lightening may be a benefit since lenses for night driving should be kept to a transmission of about 80 percent or higher.
A 1.66 lens with no tint transmits about 88 percent of incident light and a 1.74 index transmits about 87 percent. These transmission figures are for non-AR lenses. The reduced transmission of higher index materials is directly related to their higher reflectance. When AR is used, the reflectance and transmission shortcomings of these materials are essentially resolved, and transmission is increased to approximately 99 percent.
COLOR MAKES A DIFFERENCE
One way to rate the optical efficiency of AR products is to compare the color and apparent brightness of light reflected from the front surfaces of two or several lenses at the same time. When making this comparison, you should view the reflected light that is bouncing toward you at approximately 90 degrees. The brightness and color of light reflected at oblique angles will not fairly represent performance. Even if two different AR products reflect the same percentage of light across the entire spectrum, the one that has a color closer to yellow-green (555 nm) will not dampen reflected light quite as effectively as an AR with less of a yellow-green appearance. It is also important to realize that although the retina is not equally sensitive to all wavelengths, the AR product that appears to reflect the least light may not be the best choice for your patient. Other factors including durability, ease of cleaning, anti-static properties, smudge resistance and cost should be considered in choosing the best AR product for your patient.
The dynamic and particularly beneficial aspect of combining AR and photochromic lenses lies in their synergy, which gives a better retinal image than either could alone. This combination represents an excellent way to increase lens clarity and reduce ghost images, but patients should not be told that the ghost images will be completely eliminated. Like a minor scratch on a lens, once seen, even very faint ghost images can be annoying and difficult to ignore.
All spectacle lens wearers can benefit from the enhancement of the retinal image when AR is combined with photochromic properties. This combination is especially effective with higher index materials. Caution should be exercised when prescribing any higher index lenses without AR for elderly patients who drive after dark. These patients often need a brighter retinal image, and the transmission of non-AR lenses diminishes as the index is raised. Fortunately there are now high quality photochromic lenses that have very low amounts of residual tint when fully lightened.
The very best anti-reflective lenses are dispensed with proper instructions: Avoid thermal shock—flush with tepid water to remove grit before cleaning. Use only AR-recommended lens cleaners. Remind patients that their super clear, high performance lenses must be kept clean for best performance and appearance. It’s all part of providing the best in eyecare and the best in eyewear.
Dr. Palmer R. Cook, OD, is director of professional education at Diversified Ophthalmics in Cincinnati, Ohio.