L&T: Lens Choices

May
2007

LET'S GO FOR A DRIVE

The Right Lens Makes For The Perfect Trip

By Barry Santini, ABOM

Photographs by NED MATURA; Drivewear Lenses courtesy of Younger Optics. Frame: Persol/Luxottica

Although the Departments of Motor Vehicles (DMVs) in each of the 50 states has a different protocol for driver testing, licensure and re-registration, they all share one thing in common: a person’s vision must meet a minimum standard of acuity. This fact alone should persuade every vision professional (whether optician, optometrist, ophthalmologist or support staff) that emphasizing the importance of having the sharpest vision possible when driving is not only good for each client, it’s also good for society as a whole. Can you name another activity where less-thanoptimal vision poses such a potential danger for both an individual and others around them? Driving with sub-optimal acuity can result in reduced depth perception, low-level contrast discrimination, accuracy in judging distances and negatively impact traffic signal recognition. All of these deficiencies affect a driver’s reaction time and will therefore significantly increase chances for an accident. And while having sharp vision will not guarantee avoiding an accident, driving without it will certainly increase your odds.

VISION CONSIDERATIONS

When considering how to optimize a driver’s vision, we should first evaluate the visual conditions affecting driving. The most important factors that influence these visual conditions are the time of day (daytime, dusk or night time) and the driver’s field-of-view.

DAYTIME DRIVING
Daytime driving is generally done under photopic conditions. Photopic conditions mean your pupils will constrict to 1.5 to 3mm in response to the elevated lumen levels. However, smaller pupils can effectively mask focus error because the rays limited by these diameters tend to produce a sharp image. As a result, many clients may have prescriptions that indicate a small correction is needed, even though the client may not complain of vision problems when driving. This is why a proper refraction is generally performed in a darkened room (where the pupil can open up to at least 5mm). These conditions will reveal any lack of optimal focus. With this in mind, it is obvious that most motor vehicle acuity tests are not done in appropriate conditions. The current testing environment in most DMV offices features both bright lighting and reduced testing distances. These two testing parameters further compromise the reduced acuity thresholds employed by the DMVs. (It is also ironic many states have, or are considering laws that mandate pre-schooler screening standards—along with subsequent doctor referral—that utilize acuity thresholds far more stringent than our DMVs currently allow.)

Besides the basics of full prescription correction, daytime drivers must contend with the direct glare of the sun. When recommending sunglasses, the proper selection of tints and densities should be reviewed and determined with each client. There are various gender considerations that can influence the choice of lens color (also know as “hue”). While we’re familiar with the spectral differences between the primary sunwear color families of gray, brown and green, it is also important to note there are some differences between male and female color vision that will influence their preference of lens hue. In general, most men’s color vision is red-sensitive and often prefer a neutral-transmittance lens such as gray. On the other hand, most women are red-deficient, and often find a gray tint “flat” or dull, while a brown-based tint appears to provide more “natural” vision. These gender guidelines are by no means universal. Individual demonstrations are recommended for each client. In addition, people will often become accustomed to a particular tint during years of use (visual habit). Demonstrating all the primary and secondary sun tints (rose, for example) is the only way to discover the optimal choices for your client.

Photochromic lenses are a popular lens option today. But for almost all types, photochromic lenses do not work optimally for driving. The UV rays that change the lens from light to dark are blocked by a car’s roof and window glass.

An exception is the new Drivewear lens from Younger Optics. Developed in conjunction with Transitions Optical, Drivewear lenses are polarized photochromic lenses that are activated by both UV and visible light. Drivewear lenses can darken in a car, with a response proportional to the brightness levels encountered. Along with their photochromic quality, Drivewear lenses also feature optimized hue-changing qualities that further enhance their glare reduction.

Daytime drivers can also encounter the debilitating glare effects of reflected light. Whether glare reflections originate off window glass, the dashboard or the road surface, polarizing lenses are the ideal solution. Through selectively blocking of reflections from horizontal surfaces, polarizing lenses deliver improved contrast sensitivity, and thereby, allow drivers to more quickly see a child, bicycle, pedestrian or car lurking in the shadows of trees or buildings.

DUSK AND NIGHT TIME
The most important difference between daytime and dusk or night time driving is a reduction in contrast. Lower lumen levels result in mesoscopic or scotopic conditions, and encourage your pupil to dilate between 4mm and 7mm. At these diameters, any lack of optimal eye focus will no longer be camouflaged. These are the conditions that will often be accompanied by some sort of complaint from your client. If their correction is not fully up to par, this is the environment where it will show. Even if their last refraction concluded with a “no correction change needed,” drivers often complain about a lack of visual comfort and confidence during dusk and night time conditions. These evening considerations are really completely different from daytime. Unfortunately, our current refraction protocol does not routinely include optimizing driver vision for dusk and night.

NIGHT MYOPIA—
THE “HIDDEN” NEARSIGHTEDNESS

Although still not yet fully understood, night myopia significantly influences vision sharpness and contrast discrimination. Primarily evolutionary in origin, night myopia is defined as the eye’s tendency to adopt a nearsighted focus at larger pupil openings (6mm to 8mm), and under low levels of illumination. It is postulated that evolution played a role in the development of this trait through natural selection. Amongst our small, mammalian ancestors, those who could more quickly recognize a predator at close range, such as when awoken from sleep at night, survived to propagate offspring. This genetic trait was favorably selected down millions of years of evolution and contributed to the development of night myopia.

The underlying optics of night myopia are an amalgam of various factors, all contributing to a more near-sighted focus at larger pupil diameters. For one thing, our dilated eyes exhibit an effect known as spherical aberration, wherein the rays at the out edge of our expanded pupil are focused (prima-rily by the cornea) further in front of the retina. In addition, there is also the “Purkinje effect,” wherein the eye’s peak sensitivity shifts into the shorter wavelength, blue end of the spectrum at low levels of illumination. This also makes the eye’s focus more near sighted. Compounding these two near-sighted factors is the eye’s tendency to adopt a focus of approximately three feet when “at rest,” i.e., either with the eyes closed or with no distinct object to focus upon (also know as “empty-field myopia”). These three factors are generally thought to all contribute to the phenomenon of night myopia, by reducing acuity and contrast sensitivity at night.

WHAT WE’VE LEARNED FROM REFRACTIVE SURGERY

Refractive surgery: Did it spell the end of eyewear and contact lenses? This was forecasted, but the current reality is much different. No doubt, our understanding of vision processing is much more advanced today, primarily as a result of the analysis of dissatisfied refractive surgery clients. Once a term that generated much concern in the eyewear industry, refractive surgery (which also encompasses the more popular “laser surgery”) has now taken its place as an alternate refractive-correction choice. Refractive surgery’s use of abberometry, which produces a more comprehensive model of the eye’s focus, has enlightened our understanding of many of the night-myopia effects described above. Aberration analysis can often yield a more accurate and precise determination of astigmatism and axis than traditional objective/subjective refraction techniques. Through a more precise and complete measurement of the eye’s total focus, abberometry corroborates that there are clinically significant focus differences between smaller and larger pupil openings. Therefore, there is now a clearly defined need to test whether different prescriptions for daytime versus dusk/evening driving are advisable.

Either the doctor or dispenser (or both) should discuss and recommend a separate eyewear prescribed and designed for anyone who places a premium on vision and night driving safety. Besides delivering a crisp distance focus, optimally focused vision delivers superior contrast discrimination. This is especially important at dusk and night, where contrast gradients are notoriously soft. Eyewear for night driving will employ even the smallest Rx correction. The lenses may be clear, or if the client is particularly light sensitive, utilize one of the new, specialized polarizing hues optimized for conditions besides bright sunlight. Whether clear or tinted, premium, hydrophobic antireflection coatings are mandatory for reduced ghosting and contrast enhancement.

FIELD-OF-VIEW As a driver, not only is it important how sharply you see, but also how much you can see counts with equal significance. A critical issue surrounding any optimized eyewear/sunwear for driving is field-ofview. Most states’ DMV vision regulations contain minimum standards for a driver’s angular field of view (FOV) and will restrict deficient candidates even when their central acuity is 20/20. In New York, for example, drivers must possess not less than 140 degrees field-of-view (monocular or binocular). Although recent fashion trends favor larger eye-sizes, it was not long ago that 45mm to 49mm eye-sizes were the popular norm for both eyewear and sunwear. As our clients have already discovered, smaller sized sunglasses deliver inadequate sun protection. But smaller eye-sizes also do not allow a truly safe field of view for driving. The traditional solution for sunwear has been to choose larger eye-sizes, say 56mm or greater, with a shape difference of 6mm to 9mm. This results in heavier, thicker and more cumbersome sunwear, and still doesn’t provide an optimal solution for non-sun driving eyewear

ENTER “WRAP-AROUND” EYEWEAR
Finding first acceptance in plano sunwear for sports, we are rapidly entering a paradigm shift in lens-form dispensing. Wrap-around eyewear significantly increases the driver’s unobstructed FOV, without a need for large “B” lens dimensions and without additional weight and thickness. Wrap-around designs work for both sunwear and regular eyewear. Up until recently, examination, lens and frame technology has been based on flatter-form lens curves. (This is because lens production techniques were derived from those first optimized for glass.) Note how most of the latest technology for our popular aspheric lenses is designed to make them flatter and thinner. But these flat lenses are unsuitable for use with newer frames that feature steeper wrap-angles (also known as “face-form” angle).

Using frame wrap angles of 10 to 25 degrees, along with lens base curves of six to eight diopters, it is finally possible to deliver eyewear that can be truly described as offering a “panoramic” field-of- view… literally, akin to having a prescription ground in your windshield. However, there are new vision considerations that accompany these curved lenses and this is where a paradigm shift occurs. With non-traditional values for vertex distance (nine to 11mm) and pantoscopic lens tilt, an “Rx compensator” program must be used to ensure sharp central acuity and binocular fusion with these wrap angles and curves. There may also be residual issues with peripheral sharpness and that’s where a whole new generation of lenses specifically-designed for wrap-around eyewear come in. Products such as SOLA/Zeiss Spazio, Shamir Attitude and KBCo’s Wrap Solutions offer not only an optimized periphery for wrap eyewear through the use of aspheric and atoric curves, they also possess a lens form factor that reduces excessive thickness and weight (for minus powers).

Vision care professionals should also be aware that wrap eyewear requires pupillary compensation for the de-centering effects of lens tilt, depending on degree of wrap angle. Success in wrap eyewear also depends on using a starting Rx that is fully corrected for maximum distance focus. In my practice, I have found that clients enthusiastically receive vision-optimized, wrap-around single-vision and progressive eyewear. Although there is an initial perceptual adjustment, clients love the improved field-of-view and comfortable fit of wrap eyewear. Starting with wrap around sunwear, customers eagerly desire the same “panoramic” vision for special night-driving eyewear made in the same wrap around form.

EYEWEAR OPTIMIZED FOR DRIVING— THE HIDDEN SALES POTENTIAL

As sunwear is often cited as the ideal second pair sale, the time is ripe for vision professionals to unlock the hidden sales potential of optimized eyewear for driving. When your client comes says “I’m uncomfortable with driving in general, but particularly at dusk and night,” your response should be “With today’s vision technology, we have a better understanding of the unique vision processes that affect your eyes when driving. We’ll be able to show you how an optimized pair of driving eyewear can restore your feeling of confidence and safety, particularly at night.” There has never been a better time, nor a more receptive audience, for that “third” pair of eyewear. Make your clients happy and safe, and unlock the profits hidden in every sale of optimized eyewear for driving.
LT


Barry Santini is a New York State licensed optician based in Seaford, N.Y.

 

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